GBH Enterprises, Ltd.

Process Engineering Guide:
GBHE-PEG-MAS-608

Control of Continuous Distillation
Columns
Information...
Process Engineering Guide:

Control of Continuous
Distillation Columns

CONTENTS

SECTION

0

INTRODUCTION/PURPOSE

4

1

...
8

MORE ADVANCED TOPICS

43

8.1
8.2
8.1
8.2
8.1
8.2

Temperature Position Control
Inferential Measurement
Floating Pressu...
FIGURES

1

DISTILLATION COLUMN SHOWING A SELECTION OF
POSSIBLE CONTROL AND REGULATION POINTS

4

2

SCHEMATIC OF TYPICAL ...
15

REFLUX DRUM MOUNTED LEVEL WITH OR ABOVE
CONDENSER BUT WITH REDUCTION OF PRESSURE
DROP ROUND REFLUX LOOP

25

INSERTING...
28

THE OVERRIDE CONTROL OF REBOIL TO PROTECT
BASE LEVEL DRYING OUT

39

EP OVERRIDE OF REBOIL TO PREVENT COLUMN
FLOODING
...
0

INTRODUCTION/PURPOSE

As an industrial separation process, distillation has been in worldwide use since
the 1920s. It i...
1

SCOPE

This Process Engineering Guide deals with the control of continuous distillation
columns which are used widely i...
FIGURE 1

DISTILLATION COLUMN SHOWING A SELECTION OF
POSSIBLE CONTROL AND REGULATION POINTS

Refinery Process Stream Purif...
4

GENERAL DESCRIPTION OF A DISTILLATION COLUMN

The main part of the column consists of a series of plates or a packed be...
Vapor thus produced gives up its latent heat to boil liquid in the plate above and
so on. The rising vapor becomes richer ...
5

REGULATORY CONTROL

5.1

Composition Control

Disturbances to column operation are most usually due to changes in feed-...
(b)

some short term effects are often in the opposite direction to the
long term steady state requirements.

Note: When t...
FIGURE 3

COLUMN TOP AND BOTTOM TEMPERATURE CONTROL

Refinery Process Stream Purification Refinery Process Catalysts Troub...
5.1.1.1

Temperature Profiles

The temperature profile (Figure 4) shows a ''sensitive region'' (X) for temperature
control...
FIGURE 4

TYPICAL TEMPERATURE

FIGURE 5 PROFILE FOR
A COLUMN WITH VERY PURE
TOP PRODUCT BUT MIXED
COMPOSITION BOTTOM
PRODU...
5.1.1.2

Choice of Reboiler or Reflux Regulation

If temperature control is to be used and if the temperature profile has ...
definitive about where the breakpoint should be between choice of reboil or
reflux. F G Shinskey (Ref.4), argues that refl...
(c)

Difference or double difference temperature measurements are
sometimes made which compensate for pressure variations....
All analyzers are more expensive to install and maintain than temperature
measurements. For this reason analyzer use is li...
(c)

Liquid balance in the reboil system.

Table 1 shows the symptoms of imbalance and the regulatory variables. The
situa...
The details of how the condensation rate can be adjusted in various types
of condenser and how the reboil rate can be adju...
FIGURE 8

(c)

MASS BALANCE CONTROL

Mass balance on the liquid in the reboil system.
Liquid enters this system from the c...
From the preceding discussion it will have been apparent that certain
variables affect more than one mass balance or compo...
5.2.1 Surge Capacity and Level Control
It is a general rule that most capacities on a distillation column and its peripher...
TABLE 3

COMPOSITION REGULATION

This leaves reflux uncontrolled. In these circumstances it is normal to flow
control refl...
In cases like this it is not uncommon to regulate the bottoms level by adjusting
reboil and to use the composition measure...
(a)

The partial pressure of the condensable fraction of the
vapor in contact with the cooling surface.

(b)

The temperat...
FIGURE 9 (a)

REGULATION OF VAPOR PURGE

FIGURE 9 (b)

REGULATION OF COOLANT

Refinery Process Stream Purification Refiner...
FIGURE 9 (c)

FLOODED CONDENSER

In this case the refrigerant is in equilibrium with its own vapor so its temperature
can ...
The speed of operation of the system shown in Figure 9(b) depends very
much on the thermal capacity of the tube wall separ...
Adjustment of cooling in the air condenser can be effected by:
(a)

flooding with condensate, or,

(b)

bypassing.

Most a...
5.4.4 Operation at Atmospheric Pressure with a Total Condenser
Many columns are designed to operate at atmospheric pressur...
FIGURE 10 A HOT>GAS BYPASS IS THE MOST COMMON MEANS OF
CONTROLLING AIR>COOLED CONDENSERS

Refinery Process Stream Purifica...
5.4.5 Operation at Sub-Atmospheric Pressure
When the column operates at below atmospheric pressure, the proportion
of iner...
5.4.6

System Using a Refrigerant as Cooling Medium
When distillation is carried out at low temperature, a boiling refrige...
FIGURE 12 CONDENSATION USING A BOILING REFRIGERANT IN
CONDENSER SHELL

When partial condensation is required, and the vapo...
FIGURE 13 CONDENSATION USING A BOILING REFRIGERANT IN
CONDENSER TUBES

The basic method of operation is shown in Figure 14...
FIGURE 14 REFLUX DRUM MOUNTED LEVEL WITH OR ABOVE
CONDENSER

The liquid entering the reflux drum is therefore cooled below...
FIGURE 15 REFLUX DRUM MOUNTED LEVEL WITH OR ABOVE
CONDENSER BUT WITH REDUCTION OF PRESSURE DROP
ROUND REFLUX LOOP

Experie...
internally. In either case the heating medium may be steam or liquid within
the tubes. Both achieve 100% vaporization.
FIG...
Natural circulation or "thermosyphon" reboilers may be mounted either
horizontally or vertically. Vertical thermosyphon re...
5.5.1 Steam Reboilers
Steam is the most common vapor used for heating. In some low
temperature applications a refrigerant ...
5.5.2 Hot Oil Reboilers
When a liquid stream is used to boil-up a column there arises the question of
how to measure and c...
FIGURE 18 HEAT INPUT CONTROL

Flow to the other reboilers may then be throttled to match their requirements.
This can be a...
5.5.3 Fired Heaters
A fired heater may also be used as a reboiler. This is often done for the
fractionation of crude oil a...
The highest valve signal is selected as the input position controller which adjusts
the set points of all flow controllers...
FIGURE 20 HEATER PASS CONTROL> HOW THE VPC ADJUSTS FLOWS
TO MAINTAIN ONE VALVE FULLY OPEN

Refinery Process Stream Purific...
5.5.5 Heat Integration
It is attractive, for energy efficiency reasons, to integrate heat sources and sinks
in distillatio...
Ratio control and simple computation can go some way towards providing an
efficient system but control based on convention...
FIGURE 22 CONVENTIONALLY CONTROLLED COLUMN

The most basic feed-forward system would ratio reflux to feed. A more
comprehe...
FIGURE 23 AN EXAMPLE OF MORE COMPREHENSIVE FEED>FORWARD

Refinery Process Stream Purification Refinery Process Catalysts T...
Feed composition changes are normally only compensated for by the standard
feedback control system but, if they are very s...
FIGURE 24 FEED-FORWARD CONTROL WITH COMPOSITION
CORRECTION

Refinery Process Stream Purification Refinery Process Catalyst...
6.1.1 Dynamic Compensation
It is often not sufficient that the slave flows be ratioed to the feed in a
steady state relati...
Control of Continuous Distillation Columns
Control of Continuous Distillation Columns
Control of Continuous Distillation Columns
Control of Continuous Distillation Columns
Control of Continuous Distillation Columns
Control of Continuous Distillation Columns
Control of Continuous Distillation Columns
Control of Continuous Distillation Columns
Control of Continuous Distillation Columns
Control of Continuous Distillation Columns
Control of Continuous Distillation Columns
Control of Continuous Distillation Columns
Control of Continuous Distillation Columns
Control of Continuous Distillation Columns
Control of Continuous Distillation Columns
Control of Continuous Distillation Columns
Control of Continuous Distillation Columns
Control of Continuous Distillation Columns
Control of Continuous Distillation Columns
Control of Continuous Distillation Columns
Control of Continuous Distillation Columns
Control of Continuous Distillation Columns
Control of Continuous Distillation Columns
Control of Continuous Distillation Columns
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Control of Continuous Distillation Columns

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Control of Continuous Distillation Columns

0 INTRODUCTION/PURPOSE
1 SCOPE
2 FIELD OF APPLICATION
3 DEFINITIONS
4 GENERAL DESCRIPTION OF A DISTILLATION COLUMN

5 REGULATORY CONTROL
5.1 Composition Control
5.2 Mass Balance Control
5.3 Design of Feedback Control Systems
5.4 Pressure and Condensation Control
5.5 Reboiler Control

6 DISTURBANCE COMPENSATION
6.1 Feed-forward Control
6.2 Cascade Control
6.3 Internal Reflux Control

7 CONSTRAINT CONTROL
7.1 Override Controls
7.2 Flooding
7.3 Limiting Control

8 MORE ADVANCED TOPICS
8.1 Temperature Position Control
8.2 Inferential Measurement
8.1 Floating Pressure Control
8.2 Model Based Predictive Control
8.1 Control of Side-streams
8.2 Extractive/Azeotropic Systems

9 REFERENCES

TABLES

1 SYMPTOMS OF IMBALANCE AND THE REGULATORY VARIABLES

2 PRACTICAL LINKAGES BETWEEN CONTROL
(P, R, B, C) AND REGULATION VARIABLES
(h, r, d, b, c, v)

3 COMPOSITION REGULATION

4 COMPOSITION REGULATION - VERY SMALL FLOWS

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Control of Continuous Distillation Columns

  1. 1. GBH Enterprises, Ltd. Process Engineering Guide: GBHE-PEG-MAS-608 Control of Continuous Distillation Columns Information contained in this publication or as otherwise supplied to Users is believed to be accurate and correct at time of going to press, and is given in good faith, but it is for the User to satisfy itself of the suitability of the information for its own particular purpose. GBHE gives no warranty as to the fitness of this information for any particular purpose and any implied warranty or condition (statutory or otherwise) is excluded except to the extent that exclusion is prevented by law. GBHE accepts no liability resulting from reliance on this information. Freedom under Patent, Copyright and Designs cannot be assumed. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  2. 2. Process Engineering Guide: Control of Continuous Distillation Columns CONTENTS SECTION 0 INTRODUCTION/PURPOSE 4 1 SCOPE 4 2 FIELD OF APPLICATION 4 3 DEFINITIONS 4 4 GENERAL DESCRIPTION OF A DISTILLATION COLUMN 5 5 REGULATORY CONTROL 7 5.1 5.2 5.3 5.4 5.5 Composition Control Mass Balance Control Design of Feedback Control Systems Pressure and Condensation Control Reboiler Control 7 12 15 17 25 6 DISTURBANCE COMPENSATION 32 6.1 6.2 6.3 Feed-forward Control Cascade Control Internal Reflux Control 32 36 37 7 CONSTRAINT CONTROL 38 7.1 7.2 7.3 Override Controls Flooding Limiting Control 39 39 40 Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  3. 3. 8 MORE ADVANCED TOPICS 43 8.1 8.2 8.1 8.2 8.1 8.2 Temperature Position Control Inferential Measurement Floating Pressure Control Model Based Predictive Control Control of Side-streams Extractive/Azeotropic Systems 43 44 45 46 47 50 9 REFERENCES 51 TABLES 1 SYMPTOMS OF IMBALANCE AND THE REGULATORY VARIABLES 12 PRACTICAL LINKAGES BETWEEN CONTROL (P, R, B, C) AND REGULATION VARIABLES (h, r, d, b, c, v) 15 3 COMPOSITION REGULATION 16 4 COMPOSITION REGULATION - VERY SMALL FLOWS 17 2 Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  4. 4. FIGURES 1 DISTILLATION COLUMN SHOWING A SELECTION OF POSSIBLE CONTROL AND REGULATION POINTS 4 2 SCHEMATIC OF TYPICAL SIEVE TRAY 6 3 COLUMN TOP AND BOTTOM TEMPERATURE CONTROL 8 4 TYPICAL TEMPERATURE PROFILE 9 5 PROFILE FOR A COLUMN WITH VERY PURE TOP PRODUCT BUT MIXED COMPOSITION BOTTOM PRODUCT 9 PROFILE FOR A COLUMN WITH VERY PURE BOTTOM PRODUCT BUT MIXED COMPOSITION TOP PRODUCT 10 TEMPERATURE PROFILE SHOWING MULTIPLE SENSITIVE REGIONS 10 8 MASS BALANCE CONTROL 14 9(a) REGULATION OF VAPOR PURGE 18 9(b) REGULATION OF COOLANT 18 9(c) FLOODED CONDENSER 18 10 A HOT-GAS BYPASS IS THE MOST COMMON MEANS OF CONTROLLING AIR-COOLED CONDENSERS 21 CONDENSATION CONTROL SYSTEMS FOR OPERATION UNDER VACUUM 22 CONDENSATION USING A BOILING REFRIGERANT IN CONDENSER SHELL 23 CONDENSATION USING A BOILING REFRIGERANT IN CONDENSER TUBES 24 REFLUX DRUM MOUNTED LEVEL WITH OR ABOVE CONDENSER 24 6 7 11 12 13 14 Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  5. 5. 15 REFLUX DRUM MOUNTED LEVEL WITH OR ABOVE CONDENSER BUT WITH REDUCTION OF PRESSURE DROP ROUND REFLUX LOOP 25 INSERTING A TUBE BUNDLE EITHER DIRECTLY IN THE COLUMN BASE OR IN AN EXTERNALLY MOUNTED KETTLE 26 ALTERNATIVE ARRANGEMENTS OF THERMOSYPHON REBOILERS 27 18 HEAT INPUT CONTROL 29 19 DIAGRAM SHOWING THE VALVE-POSITION CONTROLLER (VPC) ADJUSTING THE TEMPERATURE SO THAT ONE VALVE IS ALMOST FULLY OPEN 30 HEATER PASS CONTROL- HOW THE VPC ADJUSTS FLOWS TO MAINTAIN ONE VALVE FULLY OPEN 31 16 17 20 21 HOW ENERGY INTEGRATION FORCES THE BOIL-UP OF ONE COLUMN TO BE DEPENDENT UPON ANOTHER 32 22 CONVENTIONALLY CONTROLLED COLUMN 33 23 AN EXAMPLE OF MORE COMPREHENSIVE FEED-FORWARD 34 24 FEED-FORWARD CONTROL WITH COMPOSITION CORRECTION 35 25 A SUDDEN FEED CHANGE COULD CAUSE A REBOIL INVERSE RESPONSE 36 26 TEMPERATURE/FLOW CASCADE 37 27 TYPICAL SYSTEM FOR MAINTAINING INTERNAL REFLUX CONSTANT WHEN CONDENSER SUBCOOLING VARIES 38 Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  6. 6. 28 THE OVERRIDE CONTROL OF REBOIL TO PROTECT BASE LEVEL DRYING OUT 39 EP OVERRIDE OF REBOIL TO PREVENT COLUMN FLOODING 40 30 LOW FEED FORWARD REFLUX FLOW LIMIT 41 31 COMPUTER SET BOTTOM OFF-TAKE WITH LEVEL REGULATING FEED VALVE 42 FLOW CONTROLLER TUNED TO ENSURE THAT LONG TERM DESIRED BOTTOM TAKE OFF-TAKE IS OBTAINED 42 33 SHARP TEMPERATURE PROFILE 43 34 POSITIONAL TEMPERATURE CONTROLLER 44 35 DIAGRAM SHOWING HOW THE VALVE-POSITION CONTROLLER SLOWLY ADJUSTS THE PRESSURE SET POINT TO KEEP THE CONDENSER FULLY LOADED IN THE LONG TERM 46 DIAGRAM SHOWING HOW ALL THE HEAT USED IN CRUDE OIL DISTILLATION ENTERS WITH THE FEED 48 DIAGRAM SHOWING HOW VIRTUALLY ALL THE METHANE FROM THE BOTTOM OF THE DEMETHANISER LEAVES WITH THE LOW GRADE ETHYLENE 49 29 32 36 37 DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE 52 Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  7. 7. 0 INTRODUCTION/PURPOSE As an industrial separation process, distillation has been in worldwide use since the 1920s. It is one of the leading unit operations in process engineering. A simple distillation column (see Figure 1) with only feedback control has over 700 possible control arrangements from simple pairing of variables alone. When other factors are taken into account such as choice of temperature measurement point, direct composition control, different possible methods of condensation and pressure control, different reboil arrangements, feed forward control and the question of the need for more advanced control techniques, the number of possible control systems for continuous distillation columns can be measured in the thousands. The choice of any control system should ideally be based on a quantitative understanding of the process and the comparative performance of different control structures. Mathematical modeling, simulation and controllability assessment are often used to design control systems where the design is not obvious a priori. In the case of distillation, modeling is difficult and time consuming. In addition, there are many different possible control arrangements. Fortunately, by a logical approach and argument it is possible in most cases to reduce the realistic possibilities to very few. This is the purpose of this Guide and the problem is tackled in the following way. (a) After a general description of a column, regulatory control for steady state operation is addressed by considering the related questions of composition and material balance. (b) Next, enhancements to cope with constraints and for superior control during transients are described. (c) Finally, the relevance and application of more advanced techniques is discussed. The Guide concludes with a number of references for further study. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  8. 8. 1 SCOPE This Process Engineering Guide deals with the control of continuous distillation columns which are used widely in petrochemical and chemical manufacture and refining. It covers regulatory control, disturbance compensation, constraint control and more advanced topics. The purpose of the Guide is to display control options; it does not deal with control hardware. 2 FIELD OF APPLICATION This Guide applies to the process and control engineering communities in GBH Enterprises worldwide. 3 DEFINITIONS For the purposes of this Guide, the following definitions apply: VPC Valve Position Controller PIC Proportional, Integral and Derivative Control MBPC Model Based Predictive Control DMC Dynamic Matrix Control Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  9. 9. FIGURE 1 DISTILLATION COLUMN SHOWING A SELECTION OF POSSIBLE CONTROL AND REGULATION POINTS Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  10. 10. 4 GENERAL DESCRIPTION OF A DISTILLATION COLUMN The main part of the column consists of a series of plates or a packed bed in which fractionation takes place (see Figure 2). The feed enters the column at some point between its ends chosen to minimize the mismatch between the feed and the material being processed at that point. Vapor from the reboiler enters at the lower end and passes upward through the column where it mixes intimately with the descending liquid. This liquid is that part of the condensed vapor returned as reflux from the condenser at the top of the column. In this way the excess enthalpy of the vapor is given up to boil the liquid on the plates of the column. FIGURE 2 SCHEMATIC OF TYPICAL SIEVE TRAY Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  11. 11. Vapor thus produced gives up its latent heat to boil liquid in the plate above and so on. The rising vapor becomes richer in the more volatile component of the feed while the residual liquid becomes richer in the heavier component and is returned to the plate below. The overhead vapor is said to strip the more volatile component from the liquid while the reflux scrubs the less volatile component from the vapor. The action of a packed column is analogous with the distinction that the process is continuous in space. A pattern is thus set up such that the proportion of more volatile component increases with column height while the proportion of the heavier component decreases. If the column equilibrium conditions are upset the pattern is distorted and a measure of this distortion can be provided by composition analyses or temperature measurements within the column. In general, there are only two independent quantities which may be held constant within a column. The reflux and reboil rates are the two independent variables which have a significant effect on the separation and may be externally adjusted to compensate for disturbances in feed conditions. That part of the overhead vapor which is not returned in condensed form as the reflux is removed as top product or distillate to maintain a mass balance in the condensation system. A measure of a lack of mass balance may be detected from the pressure or the liquid level, depending on the particular form of condensation system used. If the top product is removed entirely in the liquid phase, the system is said to use a total condenser but if a substantial part of the top product is removed in the vapor phase it uses a partial condenser. Similarly, at the bottom of the column the down-flow liquid enters a reboil system and that part which is not returned to the column as reboiled vapor is removed as bottom product. The measurement which provides an indication of any lack of mass balance in the reboil system is usually the liquid level in the base of the column or in the reboiler itself. Some distillation columns feature multiple feeds and side-streams. Control of side-streams is covered in 8.5. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  12. 12. 5 REGULATORY CONTROL 5.1 Composition Control Disturbances to column operation are most usually due to changes in feed-rate, composition or enthalpy but atmospheric conditions can also affect the column particularly some condensation systems. There are normally only two quantities which can be independently adjusted by a control system to compensate for disturbances. Reference to Figure 1 should make this clear. Five independent adjustments (control valves) are shown. There are 3 mass balances to be maintained, viz: base liquid, tops liquid and vapor. This leaves two for composition control. This means that it is not possible to keep the composition profile in the column constant – only two quantities related to that profile. In practice, we are normally interested in the composition at the top and/or bottom of the column. The composition of an n-component mixture is determined by n-1 quantities and so, if n > 3 the column can never be controlled to give a constant top and bottom composition as feed conditions change. A degree of over-purification is always necessary in the steady state to ensure acceptable performance during transient conditions. 5.1.1 Temperature Control Temperature is most often used to infer composition. However, it should be remembered that while at constant pressure the composition of a multicomponent mixture determines the boiling point uniquely, the converse is not true. As we are usually interested in the composition of products at the top and bottom of the column, it is sometimes suggested that the bottom temperature could be used to regulate reboil and the top to regulate reflux (see Figure 3). This can give rise to problems of interaction. If an analysis is carried out of the short term effects on temperature of changes in feed-flow, composition and enthalpy then it can be shown (see Ref.1) that: (a) the relationships between the temperatures and the flows they might regulate change, dependent on the disturbance and the condition of the feed, and Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  13. 13. (b) some short term effects are often in the opposite direction to the long term steady state requirements. Note: When the words Ref.1 etc are shown in the text this refers to that number document in the References in Clause 9. This suggests that dual temperature control regulating both ends of the column is fraught with difficulties and is likely to be highly interactive. This form of control is not recommended, therefore, for columns with simple feedback control systems. Dual composition control is feasible under some circumstances with mixed temperature and direct analysis (see 6.1) and with model based predictive control (see 8.4) where the column interactions and dynamics are built into the control system. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  14. 14. FIGURE 3 COLUMN TOP AND BOTTOM TEMPERATURE CONTROL Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  15. 15. 5.1.1.1 Temperature Profiles The temperature profile (Figure 4) shows a ''sensitive region'' (X) for temperature control where the rate of change of temperature is relatively high. It is in this region where temperature should be controlled to provide the maximum sensitivity for composition control. In the region of maximum sensitivity pressure variations are also likely to have a correspondingly small effect on the measured temperature. Generally, for good temperature control, the effect on temperature of normal pressure changes should be at least 5 times less than the effect of normal composition changes. In some instances there is a very large temperature change over only a few trays in the column and under these circumstances simple temperature control may be too sensitive. This problem is addressed in 8.1. In practice the column temperature profile is likely to be very different from that shown in Figure 4. It is necessary to distinguish carefully between the concept of high product purity and a product containing a constant proportion of a particular component. In the situation where a column is producing a very pure top product but a bottom product with different components the profile may well be as shown in Figure 5 while the opposite case is shown in Figure 6. In some instances the column temperature control may show multiple sensitive regions (see Figure 7) which are characteristic of particular internal separations. In all these cases it is important to be clear about the control objectives. Which of them need to be met under all circumstances and which might be relaxed and by how much? What are the key heavy and light components? What are the design feed rates? What is the normal turndown? Furnished with this information it is usually possible: (a) to make a reasonable decision on where to site the temperature measurement point; (b) to consider whether on-line direct analysis is feasible for composition control or as a supplement to temperature control, and (c) to decide whether the composition controller should regulate the reflux or the reboil rate to the column. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  16. 16. FIGURE 4 TYPICAL TEMPERATURE FIGURE 5 PROFILE FOR A COLUMN WITH VERY PURE TOP PRODUCT BUT MIXED COMPOSITION BOTTOM PRODUCT . Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  17. 17. 5.1.1.2 Choice of Reboiler or Reflux Regulation If temperature control is to be used and if the temperature profile has multiple sensitive regions, it is important to determine which corresponds to the key separation in the column and this is where the control temperature should be measured. In more complex azeotropic or extractive distillations (see 8.6), the presence of such a region can be dependent on maintaining other conditions. The choice of reboil or reflux regulation is usually decided on the basis of dynamic response. It is then usual to fix the other variable on flow control at a value to allow the column to cope with disturbances. Feed-forward compensation is dealt with later. In general, reboil changes affect the column temperature (composition) much more quickly than corresponding changes in reflux. It follows then that reflux regulation should be used only when the control measurement can be made very close to the top of the column. This is likely to be the case where one is interested in obtaining a top product containing a constant proportion of a given component, not a pure product. It is difficult to be Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  18. 18. definitive about where the breakpoint should be between choice of reboil or reflux. F G Shinskey (Ref.4), argues that reflux should only be used if the control temperature is measured on the top tray. This is probably too restrictive but if the temperature has to be measured lower than, say, 20% from the top of the column then regulation by reflux is likely to be unsuitable. Because of uncertainty in the expected shape of the profile, and to provide flexibility for later changes in operation, there is a need in the design phase to provide an adequate number of optional temperature measurement points around the expected region of sensitivity. If analytical measuring instruments of sufficient reliability and speed are available, direct composition analysis is to be preferred. The control loop measurement delay and analyzer time constant should be short compared to the column dynamics. There is then no objection to controlling top product composition by varying the reflux rate when one is particularly interested in the composition of the top product or the bottom product composition by varying the reboil rate when one is interested in the composition of the bottom product. The control loop is very short in either case. The poor dynamics of reflux control are not then important. 5.1.1.3 Effect of Pressure As described above, temperature measurement used to infer composition is sensitive to pressure variations and this is why it is best to measure the temperature in a region of high sensitivity to composition changes. On occasions it is not possible completely to mask the pressure effect. A number of solutions are possible. (a) Column pressure may be controlled at the point of temperature measurement. This assumes that it is desirable to hold column pressure constant (standard practice) but modern thinking suggests that a control system that allows pressure to float can be more economical (see 8.3). (b) The temperature can be mathematically compensated for variations of pressure so that the compensated temperature is seen by the temperature controller. The objective is to reference a temperature made at a variable pressure to one at some base pressure, e.g. atmospheric. The compensated temperature Tc is related to the measured temperature Tm, pressure p and base pressure P b as: Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  19. 19. (c) Difference or double difference temperature measurements are sometimes made which compensate for pressure variations. They are all affected by pressure changes to approximately the same extent. However if these arrangements are to be successful they need to be carefully matched to column characteristics (Ref. 2 & 3). 5.1.2 Direct Composition Control Modern analytical devices are much more reliable than hitherto and have the capability to supplement temperature control in many applications. The advantage is that they can be sited to measure directly top or bottom compositions and to regulate reflux or reboil in fast loops as appropriate. In the case of azeotropic or extractive distillation, where it is necessary to maintain a minimum component composition at some point in the column, this concentration can be measured directly. Analyzers are regularly provided to monitor product quality in run down lines. There is an opportunity to use these analyzers to control quality directly, by siting them in the column, rather than to monitor the performance of a less effective inferential system. Unfortunately this option is rarely taken. The most common form of on-line analyzer is the chromatograph which has the advantage of being able to measure multiple components. It only performs an analysis at discrete intervals, however, and can be very slow. If it is possible to use IR or UV analyzers then their continuous output makes them much more suitable for control. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  20. 20. All analyzers are more expensive to install and maintain than temperature measurements. For this reason analyzer use is likely to be restricted to applications where there is a strong economic incentive to reduce costs, to minimize environmental pollution or to maximize recovery of a particularly high value product. Generally, but not exclusively, they will be used in conjunction with temperature based composition control. A particularly successful mode of operation is, for example, to trim reflux/feed ratio when the reboil is regulated by a conventional temperature control loop (see 6.1). In this way, a measure of dual composition control is possible. Care should be taken in the installation of all direct composition control loops to minimize the analyzer sample delay which is a pure dead time. The analyzer should be sited as close as possible to the column and a vapor sample is generally to be preferred since the higher velocity reduces the dead-time in the sample line. Chromatographs present additional problems since their analysis is produced at discrete intervals known as the "sample interval". When a new analysis is produced, the controller should apply immediate control action and then wait until a new analysis measurement is made. This sampled/data control has the benefit of introducing an integral action effect into the control loop without the normal disadvantages of increasing period. If a computer is used, performance will be improved if its sample period is keyed to the sample period of the analyzer so that the control can take immediate action on new information. 5.2 Mass Balance Control The choice of regulation variable to control composition has been discussed above. This leaves many other variables to be regulated from other measurements. The correct values of these can be obtained from mass balance considerations. Sufficient symptoms of mass imbalance are available to enable a conventional feedback control system to be set up. To reduce the options to a manageable level it is helpful to consider the various mass balances that it is necessary to achieve and those regulatory variables which can affect them directly or indirectly. The balances are: (a) Vapor balance in the column. (b) Liquid balance in the condensation system. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  21. 21. (c) Liquid balance in the reboil system. Table 1 shows the symptoms of imbalance and the regulatory variables. The situation is shown diagrammatically in Figure 8. TABLE 1: SYMPTOMS OF IMBALANCE AND THE REGULATORY VARIABLES Note: Not all of these measurements and regulatory variables may be available on any given column. (a) Vapor balance in the column Vapor is added to the system by the reboiler, by evaporation of liquid from the plates and possibly from the feed. It is removed by condensation in the condenser, by condensation on the plates and possibly as top product taken off in vapor form (non-condensables in the feed or air leakage in the case of a vacuum system may be additional contributors to the vapor balance). Any lack of balance between the sum of the rates of production and the sum of the rates of removal of vapor causes a change in the pressure in the system so pressure measurement at some point provides a symptom of lack of vapor balance. Inspection of Figure 8 shows that of the quantities mentioned above, the only ones available for pressure control are the rate of generation of vapor in the reboiler (h), the rate of condensation in the condenser (c) and the rate of removal of top product in the form of vapor (v). At least one of them should be left available to be adjusted either directly or indirectly by the column pressure controller. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  22. 22. The details of how the condensation rate can be adjusted in various types of condenser and how the reboil rate can be adjusted in various types of reboiler are covered in later sections of this Guide. The present indications are symbolic. (b) Mass balance of the liquid in the condensation system. The condensation system is that part of Figure 8 enclosed by the dotted lines and includes the condenser and reflux drum. Liquid enters this system by condensation of vapor in the condenser (C). It leaves as distillate in condensed form (d) and as reflux returned to the column (r). To maintain a liquid balance it is necessary, in the steady state, that (c) = (r) + (d). Any lack of balance is indicated by a rise in or fall in the reflux drum level (R). It is not necessary to hold a reflux drum level precisely. Indeed, it is usually best to use the reflux drum and the base of the column as surge capacities to smooth disturbances to downstream units (see 5.2.1). All three of the quantities (c), (d) and (r) are available for direct control of reflux drum level. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  23. 23. FIGURE 8 (c) MASS BALANCE CONTROL Mass balance on the liquid in the reboil system. Liquid enters this system from the column itself and leaves it as bottom product or by evaporation in the reboiler. The bottom product flow (b) and the rate of vaporization (h) are available to control the level directly. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  24. 24. From the preceding discussion it will have been apparent that certain variables affect more than one mass balance or composition. This is shown in the matrix in Table 2. Added to the matrix is an additional row showing possible linkages for composition control. Direct linkages between regulation variables and the symptoms of imbalance are shown by a cross in the appropriate location. An (x) shows an indirect, feasible but less desirable potential linkage. At least one, or preferably both, of bottoms off take (b) and distillate (d) should be controlled by either a symptom of imbalance or composition. The composition link should only be considered if a sloppy split is adequate. TABLE 2 PRACTICAL LINKAGES BETWEEN CONTROL (P, R, B, C) AND REGULATION VARIABLES (h, r, d, b, c, v) Sometimes adjustment of a column feed is used to control base level. This generally provides poor control due to excessive dead time and process delay before changes in flow can affect the level. All other flows in the column should also be adjusted in the same ratio to maintain separation conditions. This form of control should be avoided if possible. An example of how to design a feedback control system for a particular arrangement is given in 5.3. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  25. 25. 5.2.1 Surge Capacity and Level Control It is a general rule that most capacities on a distillation column and its peripheral equipment should be used to the maximum extent to smooth out flow fluctuations and thereby help protect downstream equipment from disturbances. An exception to this general rule may be when, for example, reflux drum level is regulating reflux and it is important to increase reflux to match any increase in reboil rate. The reasons for setting up such a form of control is discussed later. In all other cases the level control system should be tuned to maximize the use of surge capacity. This means that a proportional only controller should be used tuned so that the capacity never empties or overfills. Sometimes proportional (error)2 control is suggested for such applications so that little control action takes place at the mid measurement point but more drastic action takes place as the measurement reaches its limits. In the experience of the author properly tuned proportional only control is normally acceptable. It is simpler and is better understood. 5.3 Design of Feedback Control Systems (a) Design Example Consider a distillation column with the following specification: Liquid feed (entering about half way up the column) - 10 te/hr Distillate - 4 te/hr Reflux ratio - 2 : 1, i.e. r/d The column has a total condensation system and operates at 20 bar g. The sensitive region for temperature control is 20% of the height above the base. It can easily be deduced that the column reflux is 2 x 4 = 8 te/hr and the bottom off-take is 6 te/hr. The flow into the base will be approximately the feed rate + reflux = 18 te/hr. The bottom off take is 6 te/hr and so the reboil rate will be approximately 12 te/hr. There are no critical flows, so, from Table 2 first make a choice of composition regulation. This is fairly evidently reboil (h) and the other controls can be built up for the reasons given in Table 3. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  26. 26. TABLE 3 COMPOSITION REGULATION This leaves reflux uncontrolled. In these circumstances it is normal to flow control reflux. This is discussed later. We have not considered the type of condensation system provided. If, for example, it were a water cooled condenser it is quite possible that no regulation would be available because the necessary valves are likely to be very large and cooling water is relatively inexpensive. How should pressure be controlled? In this circumstance there might be a temptation to control pressure by regulating reflux flow. But then there would be no independent way of setting the energy input to the column and, if the pressure and temperatures are mismatched (as they always will be), the reboil and reflux would ramp up or down until a balance is found. This might be maximum (or minimum) energy input. In these circumstances the correct action is probably to let the pressure float to achieve the lowest possible value at any time. The problems of temperature control with the variable pressure are covered in 5.1.1.3. The reflux should be flow controlled or ratioed to feed. Then the reflux flow and its temperature sets the heat input to the column to balance the reflux flow to maintain steady compositions. 5.3.1 Very Small Flows On some occasions a top or bottom off-take will be very small since only trace impurities are being removed. Consider the following case, identical to the above example except that the top off-take is 9 te/hr with the bottom off-take 1 te/hr. Carrying out a similar calculation to previously, it can be shown that the ratio of reboil to bottoms off-take is 27:1. In this circumstance, it is very difficult to maintain a bottoms mass balance by regulating bottoms off-take; reboil has a much greater effect. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  27. 27. In cases like this it is not uncommon to regulate the bottoms level by adjusting reboil and to use the composition measurement, particularly if it is close to the bottom of the column, to regulate the bottoms off-take. Similar arguments apply to the control of distillate and reflux from a temperature control near the top of the column and the reflux drum level. Consider the following table. In this case the bottoms mass balance is the critical parameter to be measured so consider it first. TABLE 4 5.4 COMPOSITION REGULATION - VERY SMALL FLOWS Pressure and Condensation Control 5.4.1 Principles of Condensation Control The overhead vapor leaving the top of the column consists of a mixture of condensable vapors containing all the components of the feed in various proportions together with a certain amount of non-condensables which may be either true inerts or material of too high a vapor pressure to condense to any appreciable extent under the conditions prevailing in the condensation system. Overhead condensers may be either total condensers where the condensable part of overhead vapor should ideally be completely condensed leaving only the inerts to be purged from a suitable point in the condensation system. Alternatively, they may be partial condensers where only a fraction of the condensable components are condensed, the remainder passing out with the inerts as a vapor top product. In any condenser the total rate of condensation depends on: Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  28. 28. (a) The partial pressure of the condensable fraction of the vapor in contact with the cooling surface. (b) The temperature on the cold side of the condenser. (c) The overall heat transfer coefficient. (d) The available surface area for condensation. (e) The effective latent heat of the mixture. Condensation control methods usually vary one or more of the first four of these quantities. They are considered in turn. Typical condensation arrangements are shown in Figures 9(a) to 9(c). In Figures 9(b) and 9(c) the vapor top off-take may not be present if condensation is total but there is always likely to be some means of manually venting inerts which have built up in the system. (1) Consider Figure 9(a). If it is required to reduce the rate of condensation, the control valve closes and the fraction of non-condensables in the condenser will then build up. The particular system shown can only be used with columns operating above atmospheric pressure but alternatives using the same principle can be used when operating at below atmospheric pressure. This is considered later. Strictly speaking, adjustment of the vapor purge valve also varies the overall heat transfer coefficient as well as the partial pressure of the condensables since the vapor side heat transfer coefficient is dependent on the fraction of the non-condensables in the vapor. (2) Figure 9(b) shows a system where the flow of cooling medium is varied. At high flow rates the average temperature of the coolant is reduced so the rate of condensation is increased. Sometimes boiling refrigerant is used as the cooling medium, absorbing the latent heat of condensation as its own latent heat of vaporization. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  29. 29. FIGURE 9 (a) REGULATION OF VAPOR PURGE FIGURE 9 (b) REGULATION OF COOLANT Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  30. 30. FIGURE 9 (c) FLOODED CONDENSER In this case the refrigerant is in equilibrium with its own vapor so its temperature can be changed simply by changing the pressure in the vapor space above it. This can be used for condensation control (see 5.4.6). Manipulation of the coolant flow is rarely used to adjust heat transfer in water cooled condensers on distillation columns. Response of the condensing rate to water-flow variations is non-linear and slow. The speed of response also varies with coolant flow which can cause loop stability problems. Low velocity and also leads to high fouling and metal corrosion. For more details see (Ref.2). (3) The surface area available for condensation can be varied by operating the condenser partly flooded with liquid as shown in Figure 9(c) where a control valve is placed in the line carrying the liquid away from the condenser. In this case only the part of the surface which is not submerged is available for condensation of vapor and the liquid will leave the condenser sub-cooled. The basic condensation control systems differ in dynamic performance. For example, the speed of operation of the system shown in Figure 9(a) will depend entirely upon the proportion of inerts present in the overhead vapor from the column. If the proportion is very small then, even if the vent valve is closed, it could take a long time to build up the sufficient proportion of inerts in the condensation system to affect heat transfer. Sometimes a split range control system is provided where nitrogen can be added to the system to speed up the response. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  31. 31. The speed of operation of the system shown in Figure 9(b) depends very much on the thermal capacity of the tube wall separating the hot and cold sides of the condenser and on the residence time of the cooling medium when this is a cold liquid. The system in Figure 9(c) has slow dynamics. Movement of the control valve affects the rate of change of the flooding depth in the condenser and this in turn affects the rate of change of condensation and hence pressure 5.4.2 Air Condensers In this system the air is directed upward by a fan through a horizontal bundle of finned tubes. The temperature of the condensing vapor tends to be constant if the tubes are not flooded with liquid and the following equation applies: The rate of heat transfer is limited by the air film and so the 0.2 power in the exponent is justified. As a consequence, heat flow should be reasonably linear with air flow. Controlling air flow is another matter. Variable speed fans are rarely used because of the high cost of drive units. Most air coolers use multiple fans which can be energized in stages but this gives only incremental control. Some fans are equipped with variable pitch blades or adjustable louvers but they have a tendency not to work satisfactorily. A further problem with air condensers is the effect of ambient conditions. Rain tends to convert the dry condenser into a wet condenser which can have a marked affect on the temperature of the condensate and hence the internal reflux in the column. In critical conditions this may require the provision of internal reflux control computation. This is discussed later. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  32. 32. Adjustment of cooling in the air condenser can be effected by: (a) flooding with condensate, or, (b) bypassing. Most air cooled condensers are horizontal and hence flooding does not give smooth control. Sometimes the condenser is mounted at an angle and this helps but since the tubes are of large capacity and can contain a substantial amount of liquid, response can be slow. Bypassing hot vapors around the condenser is shown in Figure 10. Column or reflux drum pressure can be controlled by adjustment of the bypass valve. The sizing of the bypass valve is difficult but critical to the effective operation of the condensation system. In particular, it is necessary to decide what proportion of the vapor shall be bypassed under different operational circumstances, throughput and ambient conditions. For more information on air cooled heat exchangers see (Ref.3). 5.4.3 Internal Condensers In some columns a condenser is mounted inside the column above the top plate. There is then no reflux drum and the condensed liquid reflux falls directly from the condenser onto the top plate through some distributor system. Uncondensed vapor can be taken off directly as top product or an internal weir may be fitted, allowing the distillate to be withdrawn. This limits the options for control. Ideally, all condensate should be trapped and withdrawn for metering and control. If this is not done the system behaves as if the reflux flow were regulated via a very high gain reflux drum level controller. (R <-> r) should be assumed when examining Table 1. Large and sudden changes in top off-take can cause corresponding changes in reflux and, in extreme situations, reflux flow may be lost for a period. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  33. 33. 5.4.4 Operation at Atmospheric Pressure with a Total Condenser Many columns are designed to operate at atmospheric pressure by linking the condenser to atmosphere through a vent and scrubbing system with a small pressure drop. In principle, this balances the rate of condensation to the overhead vapor flow by altering the proportion of inerts (air) present in the condensing vapor. Pressure control is rapid but varies with atmospheric pressure. From the point of designing an overall control system it is as if the pressure is linked to the condensation rate and (P <-> c) has to be assumed in Table 1. When a column is one of a number in a distillation train and a partial condenser is used, then the vapor top product may form the vapor feed to the next column in the train. If some method of pressure control is used on the second column, the first may be maintained at very nearly the same pressure as the second simply by ensuring that the line carrying the vapor offers little resistance to flow. Referring to the Table 1, this effectively means that the pressure is related to the vent rate (v) although physically there is no controller or control valve. Assume (P <-> v). Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  34. 34. FIGURE 10 A HOT>GAS BYPASS IS THE MOST COMMON MEANS OF CONTROLLING AIR>COOLED CONDENSERS Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  35. 35. 5.4.5 Operation at Sub-Atmospheric Pressure When the column operates at below atmospheric pressure, the proportion of inerts in the vapor entering the condenser is likely to be significant because of leaks of air into the system. The purge line is often connected to the suction side of some form of vacuum pump or steam ejector. With this arrangement it is possible to use any of the basic condensation control systems but, since the proportion of inerts in the overhead vapor may well be high, a modification of the system shown in Figure 9(a) is normally used. Three common arrangements are shown in Figures 11(a) to 11(c). FIGURE 11 CONDENSATION CONTROL SYSTEMS FOR OPERATION UNDER VACUUM Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  36. 36. 5.4.6 System Using a Refrigerant as Cooling Medium When distillation is carried out at low temperature, a boiling refrigerant is frequently used as the cooling medium in the condenser. The temperature of the boiling refrigerant depends on the pressure maintained in the vapor space above it which may very easily be regulated by manipulating a control valve in the refrigerant exit line. The refrigerant is often contained in the shell of a condenser as shown in Figure 12 and condensation of the process vapor takes place in the tubes. A constant refrigerant level is maintained by admitting a controlled flow of liquid refrigerant to the shell. The vapor condensation rate can be varied to a certain extent by altering the refrigerant level but the temperature of refrigerant boiling is usually used as the principal method of condensation control. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  37. 37. FIGURE 12 CONDENSATION USING A BOILING REFRIGERANT IN CONDENSER SHELL When partial condensation is required, and the vapor velocity in the condenser tube is high, the condensing liquid is carried out of the tubes by the vapor flow. If the vapor velocity is low, as will be the case with a total condenser, it may not be easy to keep the tubes well drained. In this case, better heat transfer may be obtained by condensing the vapor in the shell with the refrigerant vaporizing in the tubes. A system of this type is shown in Figure 13. The refrigerant is held in equilibrium with its vapor in a separate accumulator to which a control flow of refrigerant liquid is added as again, the temperature of this liquid is determined by the pressure in the vapor space above it. 5.4.7 Systems Where the Reflux Drum is Mounted Level to or Above the Condenser Sometimes, with free standing distillation columns, both condensers and reflux drums are mounted at ground level to be easily accessible for maintenance and to avoid the need for expensive supporting steelwork. The reflux drum is then sited at the same level as the condenser or even slightly above it. Liquid formed by condensation cannot fall into the reflux drum under gravity. In practice, to avoid the need to pump liquid into the reflux drum, a pressure differential is maintained between condenser and reflux drum. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  38. 38. FIGURE 13 CONDENSATION USING A BOILING REFRIGERANT IN CONDENSER TUBES The basic method of operation is shown in Figure 14. The condenser works at almost the same pressure as the top of the column and the rate of condensation is varied by flooding part of the cooled surface with condensed liquid. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  39. 39. FIGURE 14 REFLUX DRUM MOUNTED LEVEL WITH OR ABOVE CONDENSER The liquid entering the reflux drum is therefore cooled below its condensation temperature from contact with the submerged tubes. Its vapor pressure, which is the reflux drum pressure, is therefore lower than the pressure in the condenser. This pressure difference drives the condensed liquid into the reflux drum against the hydrostatic head due to the difference in level and the frictional losses in the piping. Pressure control is effected as shown in Figure 15 by bypassing some vapor from the column to the reflux drum. This alters the EP between condenser and drum and so alters the flow of condensate and thus the level in the condenser. This alters the condensation rate and so the column pressure. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  40. 40. FIGURE 15 REFLUX DRUM MOUNTED LEVEL WITH OR ABOVE CONDENSER BUT WITH REDUCTION OF PRESSURE DROP ROUND REFLUX LOOP Experience suggests that there can be operational problems with these systems and great care needs to be taken with the hydraulic design. If the bypass valve is adjusted to control the column pressure there is an inverse response which can cause instability. Controlling the reflux drum pressure by adjusting the bypass appears to be more satisfactory. 5.5 Reboiler Control Reboilers can be classified into two groups depending on whether the mechanism for transfer of heat to the column is natural or forced circulation. The different physical construction or heat sources can affect control. Figure 16 shows an example of natural circulation where the reboiler tube bundle is immersed in liquid either in a kettle exterior to the column or Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  41. 41. internally. In either case the heating medium may be steam or liquid within the tubes. Both achieve 100% vaporization. FIGURE 16 INSERTING A TUBE BUNDLE EITHER (a) DIRECTLY IN THE COLUMN BASE OR (b) IN AN EXTERNALLY MOUNTED KETTLE Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  42. 42. Natural circulation or "thermosyphon" reboilers may be mounted either horizontally or vertically. Vertical thermosyphon reboilers are used primarily in the chemical industry and are typically steam heated (see Figure 17(a). Horizontal thermosyphon reboilers are more common in petroleum refining or similar operations in the chemical industry. They are usually heated with circulating oil, see Figure 17(b). Forced circulation is used with vacuum distillation or when the heat input is obtained from oil or gas furnaces. FIGURE 17 ALTERNATIVE ARRANGEMENTS OF THERMOSYPHON REBOILERS Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  43. 43. 5.5.1 Steam Reboilers Steam is the most common vapor used for heating. In some low temperature applications a refrigerant is used. Superheated steam is a relatively poor heat transfer medium, as are most gases. Saturated steam is an excellent medium because of good heat transfer and a high latent heat of vaporization. The boil-up rate may be controlled either by steam flow or shell pressure. The steam flow measurement is usually made upstream of the control valve where the pressure is normally constant. The steam rate to the reboiler may be controlled either by a valve in the steam line or in the condensate off-take. With the control valve on the inlet to the reboiler, the saturation pressure in the shell varies with heat load. Since the heat is being transferred between a condensing and a boiling fluid, neither changes temperature greatly in the process. A steam trap or similar condensate seal is necessary to drain condensate without releasing steam. If pressure rather than flow is controlled, the boil-up automatically increases if the process fluid boiling point falls. This action compensates correctly for changes in bottom product composition but not for changes in column pressure. However, the rate of boil-up is not linear with steam pressure nor is steam pressure zero at zero boil-up. For these reasons, steam flow control is preferred to pressure control to establish and maintain boil-up rate, prevent flooding and control product quality. There are advantages and disadvantages in placing the control valve in the condensate line. In the first place the valve can be smaller - typically one third of the line size of a steam valve used for the same service. In addition, the steam reaching the reboiler is at a higher pressure than with a valve in the steam line and hence the maximum heat transfer rate is higher. The dynamic response characteristics of the two approaches differ significantly. Manipulating the control valve in the steam line causes steam flow to change immediately. Shell pressure, and hence rate of heat transfer, will lag behind for only a few seconds. By contrast, the condensate valve has no direct effect on steam flow and as the condensate level determines steam flow this level takes time to change. The slow response of a system with a valve in the condensate line usually means that it is not suitable if a boil-up is being used for base level control. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  44. 44. 5.5.2 Hot Oil Reboilers When a liquid stream is used to boil-up a column there arises the question of how to measure and control the heat input. With steam heating, the rate of condensation and therefore heat input is directly proportional to the steam flow. With liquid media, however, the relationship is very non-linear since, as the flow of hot oil increases, the temperature difference between inlet and exit also changes because the heat transfer coefficient is dependent on flow rate. Heat input can be calculated from the equation: The calculation of actual heat transfer can be implemented either with digital or analogue components as shown in Figure 18. This heat transfer equation assumes a steady state. In practice, the indicated heat flow always leads the true heat flow by the residence time of the liquid in the reboiler. Hot oil reboilers are often provided with heat from a fired heater (see Figure 19). A low oil flow requires higher reboiler inlet temperatures to transfer given flows of heat to the columns. Additionally, higher inlet temperatures require a higher flue gas temperature in the heater which causes higher stack losses. Maximum efficiency is realized when oil flow is at maximum and oil temperature is at its minimum. Conventional heater controls always include a bypass, recirculating hot oil back to the cold oil line, to protect against loss in flow through the heater. The bypass valve is usually manipulated to control differential pressure between hot and cold oil lines and, as less heat is required by the reboilers, the bypass valve opens to maintain a constant flow through the heater. A better arrangement is to ensure that the reboiler demanding the most heat, receives full oil flow at all times, with the oil temperature set to deliver that heat. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  45. 45. FIGURE 18 HEAT INPUT CONTROL Flow to the other reboilers may then be throttled to match their requirements. This can be achieved by use of a valve position controller (VPC) as shown in Figure 19. Each column will have its own heat input controls manipulating hot oil flow. The valve position signals are compared in a high selector and the highest is sent to the VPC. This device then adjusts oil temperature until the highest valve signal is at or near full opening. The oil temperature will then be at its minimum acceptable value as will the hydraulic power loss through the control valves. The valves are free to be manipulated by the individual column controls for fast response in the short term while the slower acting VPC minimizes energy loss in the long term. If such as scheme is implemented bypassing is not normally required since one of the load control valves is always nearly full open. However, a bypass valve that fails open on high ΔP should still be used to protect the heater against controller failure. For more information on reboilers see (Ref.5). Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  46. 46. 5.5.3 Fired Heaters A fired heater may also be used as a reboiler. This is often done for the fractionation of crude oil and for other products having boiling points in excess of 150°C. The flow of bottoms product to the heater is nearly always controlled to maintain efficient heat transfer. In addition, the flow is usually conducted through several parallel passes in the furnace and some means of equalizing flow through these passes should be provided. This can easily be done by use of a VPC. FIGURE 19 DIAGRAM SHOWING THE VALVE POSITION CONTROLLER (VPC) ADJUSTING THE TEMPERATURE SO THAT ONE VALVE IS ALMOST FULLY OPEN Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  47. 47. The highest valve signal is selected as the input position controller which adjusts the set points of all flow controllers to hold that valve exactly at the fully open position (see Figure 20). This arrangement results in the minimum attainable pressure loss through the valves. 5.5.4 Inverse Response Inverse response can sometimes be a problem when heat input is used to control column base level. The percentage vaporization increases with heat input such that the volume of vapor bubbles in the reboiler liquid mass increases. This moves liquid from the reboiler to the column base or, in the case of an internal tube bundle, increases the "voidage" in the base. In either case this can cause the measured base level to rise. This is only a short term effect because the increased rate of boiling will drive off mass in the long term causing the level to fall. In these circumstances the base level controller should be tuned for the long term response where the level falls with increased heat input. The short term transient response in the opposite direction can cause severe stability problems. The situation is analyzed on page 165 of (Ref. 4). Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  48. 48. FIGURE 20 HEATER PASS CONTROL> HOW THE VPC ADJUSTS FLOWS TO MAINTAIN ONE VALVE FULLY OPEN Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  49. 49. 5.5.5 Heat Integration It is attractive, for energy efficiency reasons, to integrate heat sources and sinks in distillation trains. This can be simply the use of a cooler on a bottom product to heat the feed to the column itself or the use of the top or bottom streams of some other column to provide feed heating. Such apparently innocent systems can sometimes transmit disturbances which may exacerbate an already difficult control problem. The reader should be aware of this possibility. On some distillation trains the condensers of one column provide the majority, if not all, of the reboil load for another column (see Figure 21). Such systems are difficult to operate efficiently since there can be a number of competing operating objectives but a distinct shortage of effective degrees of freedom, even when heater bypasses are provided. In practice, the control system should be set up to rank the objectives and where there is a limitation in, say, heat input, it should be remembered that some of the objectives may not be met fully. FIGURE 21 HOW ENERGY INTEGRATION FORCES THE BOIL-UP OF ONE COLUMN TO BE DEPENDENT UPON ANOTHER Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  50. 50. Ratio control and simple computation can go some way towards providing an efficient system but control based on conventional PID controllers is unlikely to be entirely effective. Model based predictive control, which takes account of measured process interactions, may well provide a better form of control in these cases and should be considered (see 8.4). 6 DISTURBANCE COMPENSATION 6.1 Feed-forward Control Up to now in this Guide we have discussed only feedback control systems. These are systems which feedback a correction as a result of a deviation from set point - an error. In other words, the process has to be disturbed before a feedback system can begin to make a correction. Feed-forward control, on the other hand, takes account of disturbances which can upset quality and, if properly designed, can apply corrective action to minimize deviations of the control variable. Feed-forward is theoretically capable of perfect control but in practice errors will always remain. Nevertheless a feed-forward calculation accurate to only ± 10% will reduce the sensitivity of product quality to these measured disturbances by a factor of 10; a good return for modest accuracy. It is difficult to understand why feed-forward is so little used; it is usually only considered when feedback systems fail. To a first approximation, to achieve constant separation, all flows in a distillation column should increase or decrease in sympathy with the feed rate. Feed rate changes are a common disturbance in distillation columns and so it is logical to feed-forward changes in feed in such a way as to adjust other column inputs to compensate for the feed change. Figure 22 shows a conventionally controlled column. Without feed-forward, the temperature controller will make the appropriate adjustments to the reboil but this will take time and the column will be subject to a larger disturbance than is necessary. Where reflux is flow controlled it would be set at the highest rate required for normal operation which could be costly in energy consumption. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  51. 51. FIGURE 22 CONVENTIONALLY CONTROLLED COLUMN The most basic feed-forward system would ratio reflux to feed. A more comprehensive system would also ratio reboil to feed and compensate this ratio from the composition measurement (see Figure 23). As the feed rate changes, reflux and reboil will also change. This would speed up the column response to disturbances and minimize off specification product. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  52. 52. FIGURE 23 AN EXAMPLE OF MORE COMPREHENSIVE FEED>FORWARD Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  53. 53. Feed composition changes are normally only compensated for by the standard feedback control system but, if they are very severe, it may be possible to analyze the feed and to make the appropriate changes to column operation. This is rare. Feed-forward allows a measure of dual composition control on a column because it helps to decouple feedback loops. For example, if the main separation is effected by a temperature controller near the base of the column, but it is important to maintain a fixed purity level in a top product, a system as shown in Figure 24 may be an option where an analyzer placed near the top of the column and monitoring a key tops impurity is used to modify the reflux ratio. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  54. 54. FIGURE 24 FEED-FORWARD CONTROL WITH COMPOSITION CORRECTION Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  55. 55. 6.1.1 Dynamic Compensation It is often not sufficient that the slave flows be ratioed to the feed in a steady state relationship. The correct dynamic relationship should be established between the variables. Feed, reflux and reboil are physically located at different points in the column and their effect on product quality will differ in speed of response. When differences exist between the responses of the manipulated inputs, feed-forward correction by steady state calculation alone is likely to cause transient errors. If this is the case, particularly for rapid disturbances, a dynamic lag should be placed in the feed-forward loop to match as closely as possible the response of the feed forward loop to that of the column response to feed changes. The need for dynamic compensation is clearly illustrated by examining the response of a column to feed and boil up variations. Consider the scheme shown in Figure 25. Here the bottom product flow is ratioed to the liquid feed through a feed-forward loop and the level in the column base is controlled by regulating heat input. FIGURE 25 A SUDDEN FEED CHANGE COULD CAUSE A REBOIL INVERSE RESPONSE Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com

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