summary – basics of process control

1. ICE401: PROCESS INSTRUMENTATION
AND CONTROL
Class 14
Miscellaneous, Summary –
Basics of Process Control
Dr. S. Meenatchisundaram
Email: meenasundar@gmail.com
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015

2. Servo & Regulatory Control:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015
• Servo Control (or) Set-point change:
→ Implementing a change in the operating conditions.
→ The set-point signal is changed and the manipulated
variable is adjusted appropriately to achieve the new
operating conditions.
→ Also called servomechanism (or "servo") control.
• Regulatory Control (or) Disturbance change:
→ the process transient behavior when a disturbance enters,
also called regulatory control or load change.
→ A control system should be able to return each controlled
variable back to its set-point.

3. Continuous and Batch Process:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015
• In general terms, we can relate to a production process in the
chemical industry as the processing of inputs to outputs in a
defined series of actions.
• The inputs include reactants, auxiliary materials, energy etc.
• The outputs include products, by-products, energy, etc.
• The series of actions is actually the conditions of reaction and
activation of the chemical reaction.
• Production processes in the chemical industry can be carried
out in several ways: batch, continuous or semi-continuous (in
which certain parts are done continuously and some in batch
form).

4. Batch Process:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015
• The batch process is a single- or multi-stage process in which
a certain quantity of inputs (raw materials, auxiliary materials,
energy, etc.) are fed into the chemical reaction unit under
conditions suitable for obtaining the desired reaction
(temperature, pressure, required time, etc.).
• In the batch process, in the reactor and at any given period of
time, various actions take place in the wake of which a
concentration of reactants and products varies so long as the
reaction progresses.
• At the conclusion of the process the mixture is removed from
the reactor and it then undergoes the appropriate separation
and processing stages (either physical or chemical).

5. Batch Process:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015
• A good example of a process working in batches is the
washing machine, into which a certain quantity of dirty
washing is put.
• The required inputs are water, electrical energy, washing
powder, etc.
• A “batch” of laundry goes through various stages that are
programmed as required: soaking, washing, various rinses,
and extraction.
• All the actions take place in one receptacle and at the
conclusion of the process we obtain wet laundry that is clean
and ready for drying.

6. Batch Process:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015
• From the washing machine the wet laundry is transferred to
the drier, in which a “batch” process of separation/purification
of the “end product” from water is also carried out, until dry
laundry is obtained.
• This process can be illustrated with a flow chart showing the
series of actions:

7. Continuous Process:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015
• The continuous process is one in which inputs are fed into the
system at a constant rate and at preset ratios (raw materials,
auxiliary materials, energy, etc.), and at the same time a
constant extraction of outputs is done (products, by-products,
energy, etc.).
• This process is characterized by a constant process taking
place in each section of the facility and during the time of its
action a constant process takes place.
• Thus, the concentration of reactants and products at every
location in the system is in a durable state and control of the
process is done by maintaining these concentrations.

8. Continuous Process:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015
• In the continuous process, all the stages are carried out
simultaneously (although possibly in different parts of the
system), and so the overall time required for the process is
shortened.
• In contrast, the required volume of the tanks for a specific
batch process is greater than that required for a parallel
continuous process.
• The initial financial investment in the establishment of
continuous installations is generally higher than that of batch
installations.
• This is due to the automated control systems and despite the
fact that the reactors in the batch installation are bigger.

9. Comparisons between batch and continuous
processes :
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015
Batch Process Continuous Process
Types of
materials
Can be used with all types of
materials (with non-flow materials, it
is easier to use the batch process).
Easier for use with flowing materials
(today, almost any material can be
produced with the continuous process;
investment cost is the decisive factor).
Installation
size
Relatively large installations. Very
big investment in land and
installations.
Relatively small installations.
Significant savings in land and
installations.
Reactor
Changes occur in the concentrations
of materials over time.
At all locations, conditions are
constant over time (durable
conditions).
Feeding
raw
materials
Raw materials are fed before the start
of the reaction.
Constant feeding of raw materials
during the entire reaction process.

10. Comparisons between batch and continuous
processes :
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015
Batch Process Continuous Process
Control of
the set of
actions in
the system
Simple control. It is easier to control
reaction conditions (pH, pressure,
temperature). Manual control can
also be done.
Complex control. Automatic control
must be used. Control of reactor
conditions is more difficult. Control
must be exercised over the rate of
flow of the materials.
Product(s) Extraction of materials only after all
the actions are finished with the
conclusion of the reaction.
Continuous extraction of products at
all times during the reaction.
Trouble
shooting
A fault or dealing with a batch
requiring “repair” does not cause
problems in the other stages.
Appropriate tests are conducted after
each stage.
The installations are interconnected,
so a fault in one causes a stoppage in
all the others. Material that has been
damaged cannot be repaired under the
same working conditions. It must be
isolated and the process restarted.

11. Comparisons between batch and continuous
processes :
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015
Batch Process Continuous Process
Quantities
produced
Preferable when production of small
quantities of a specific material are
planned.
Preferable for large scale production.
Variety of
products in
the plant
Preferable when the plant produces a
wide variety of materials and when
the product is likely to be changed
now and again, while using the same
reactor.
Preferable for a central and permanent
product.
Product
developme
nt stage
Preferable when the process is
relatively new and still unfamiliar.
In this case the initial investment is
in a smaller batch reactor, and thus
the economic risk is smaller.
Preferable after the conclusion of all
the stages of grossing-up and
economic feasibility tests.

12. Continuous Process:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015

13. Batch Process:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015

14. Self Regulation and Non-Self Regulation:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015
Self-Regulating Process:
• Self-regulating processes are processes that are inherently
self-regulating.
• Self-regulated processes have built-in feedback characteristics
that cause the process to tend towards self-regulation.
• An example of a self-regulating process is a tank of water
with an input of water entering the tank and an output of water
leaving the tank.
• Let’s say the water level in the tank is constant at 10 inches.
Water enters the tank at a rate of 20 gallons per minute and
leaves the tank at a rate of 20 gallons per minute.

15. Self Regulation and Non-Self Regulation:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015
• As long as this balance is maintained, water level in the tank will
remain constant at 10 inches.
• What happens if the outlet valve is opened an 1/8 of a turn and
water leaving the tank changes to a rate of 25 gallons per minute?
• Since this is a self-regulating process, the level will actually
stabilize at a new position and maintain that position.
• Flow out of the tank is proportional to the square root of the
differential pressure across the output valve.
• As level decreases, the differential pressure will also decrease,
causing the rate of drainage to decrease.
• At some point, the drainage rate will once again equal the fill
rate, and the tank will reach a new equilibrium point.

16. Self Regulation and Non-Self Regulation:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015
Non Self-Regulating Process:
• A non self-regulating process is one where the process does not
tend towards self-regulation.
• These processes have no self-regulating feedback characteristics
and will tend towards being unstable if not controlled externally.
• Take, for example, the initial scenario. The water level in the tank
is constant at 10 inches. Water enters and leaves the tank at a rate
of 20 gallons per minute.
• In this process, instead of having a discharge valve on the tank, a
positive displacement pump is used to drain the water. As long as
the balance is maintained, water level in the tank will remain
constant at 10 inches.

17. Self Regulation and Non-Self Regulation:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015
• If we increase the discharge rate of the positive displacement
pump to 25 gallons per minute, what will happen?
• Initially, as with the self-regulating process shown before, we are
removing 5 more gallons per minute from the tank than we are
putting in the tank.
• This causes the level to drop. However, unlike the self-regulated
process, this drop in level does not affect the flow out of the tank.
• A positive displacement pump will discharge a set flow rate
regardless of head pressure.
• The pump will continue to discharge at a rate of 25 gallons per
minute until the tank is completely empty.

18. Comparison of Variables:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015
Electrical Level Hydraulic Thermal Mechanical
Voltage (V) Head (h) Pressure (P) Temperature (T) Force (f(t))
Current (i) Flow rate (q) Fluid Flow (q)
Heat Flow Rate
(q)
Velocity (u(t))
Resistance (R) Resistance (R)
Hydraulic
Resistance (R)
Thermal
Resistance (RT)
Viscous Friction
(D)
Capacitance (C)
Area of Cross
Section (A)
Area of Cross
Section (A)
Thermal
Capacitance
(CT)
Inverse Spring
Constant (1/K)