Chemical Engineering

1. CHEG 5161 –Computer
Aided Process Design And
Simulation
Department of Chemical Engineering
BiT-BDU
Lecture No. 2 – Fundamentals of steady
state flow sheeting
4/6/2022 1
by Addis L.

2. Brain storming
• What are the differences among the
architecture of simulation software?
• what is the difference between dynamic
and steady state simulation and their
application areas?
• What do we mean by integrated
simulation
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3. Lesson outline
• Introduction
• Steady state flow sheeting
• Fundamental issues in flow sheeting
• Simulation procedure
• Unit operations
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4. Lesson objective
The objective of this course is to:
 To provide the background needed by
the chemical engineers to carry out
computer-aided analyses of large-scale
chemical processes.
 To be familiar with general approaches in
the steady state flowsheeting.
 To be clear with the simulation of different
unit operations.
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5. Introduction
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• Flow sheeting is the key
activity, but not the only
one.
• The core of an
integrated system is the
database system and the
Graphical Use Interface
(GUI).
• The assembly can be
interfaced with simulation
packages, primarily for
physical property and
thermodynamic
computations, as well as
for steady state flow
sheeting.

6. Introduction (cont…)
• This chapter develops a general
approach in steady state flowsheeting.
• Flowsheeting problem explained by
means of an example, the HDA plant.
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7. What are process flow sheets?
• Process flowsheets are the language of
chemical processes. They describe an
existing process or a hypothetical
process in sufficient detail to convey
the essential features.
• A process flowsheetis a collection of
icons to represent process and arcs to
represent the flow of material to and
from the units. It emphasizes the flow
of material and energy in a chemical
process.
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8. Steady state Flow sheeting
The use of computer aids to perform
steady-state heat and mass balances,
sizing, costing calculation for a
chemical process.
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9. What is process simulation for?
1. To interpret process flow sheets,
2. To locate malfunctions, and
3. To predict the performance of
process.
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10. A Typical Process Flow Sheet
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11. The fundamental issues in flow sheeting:
 Process description
 Problem Analysis
The problem analysis takes into account the following
aspects:
1. Input/Output streams,
2. Reactor system,
3. Reactor-Separation-Recycle system,
4. Separation system,
5. Control of flow sheet specifications,
6. Transformation of real units in simulation units,
7. Degrees of freedom analysis,
8. Thermodynamic issues,
9. Tear streams and computational sequence.
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Fundamental issues in Flow sheeting

12. Example: HAD (hydrodealkylation of alkyl-
benzenes and alkyl naphtalenes) process.
1. Process description
The HDA process converts toluene to
benzene in the presence of a large
excess of hydrogen. A simplified approach
considers two reactions:
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Fundamental issues in Fl….(cont…)

13. Cont…
Proposed HDA PFD
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14. Cont…
1. Input/Output streams
 The input streams are toluene of 100% purity and hydrogen
with 5% CH4.
 Input/Output analysis must ensure that the material balance is
consistent.
A golden rule
 Any material entering or being created by chemical reactions
must leave the process, such as no accumulation takes place.
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15. Cont…
2. Reactor Analysis
 Two models that can model reactor:
1. Stoichiometric model
2. Kinetic model
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The stoichiometric model describe the formation of by-products
and impurities necessary for the accurate simulation of
separators.
The stoichiometric modeling requires:
(1) the conversion of the main reaction
(2) the selectivity of the secondary reaction.
 The kinetic model describe the interaction between reactant and
product.
 Account for main reaction rate that depend on reactor volume
and recycle flow rate and composition.
What do you think their modeling
consideration???????????

16. Cont…
3. Reactor-Separation-Recycle system
 structure of the Reactor-Separation-Recycle
system are placed by lamping same units together.
(cold side of heat exchanger and furnace named as
HX1 and hot side of heat exchanger, steam
generator and cooler named as HX2)
 The Flash is the place where gas and liquid
phases separate.
 Then the gas is recycled via a compressor
simulated by a Compressor unit.
 The simulation of the liquid separation system is
more complicated. The simplest approach is to
lump all the items in a black-box unit named
Separation. simulated by a Separator module.
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17. 4/6/2022 17
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18. Cont…
4. Separation system
 The simulation of the train of distillation columns may be studied in
a separate flow sheet.
 After pressure reduction through the valve V1, the liquid mixture
enters the stabilizer (Stab) where dissolved gases are removed.
 An appropriate model is Rigorous Distillation with vapor distillate.
 After a second pressure reduction through the valve V2, the
separation of benzene, toluene and Heavies takes place in a
second column (Dist), for which the same rigorous distillation model is
used.
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19. Cont…
5. Control of flow sheet specifications
 the molar ratio hydrogen/toluene at
the reactor inlet should be kept
strictly at 5:1.
 therefore the manipulated variable is
split ratio of the purge.
 can be realized by building-up a large
gas recycle.
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21. Cont…
6. Transformation of real units in simulation
units
 Some real unit operations can find direct
correspondence with the 'blocks' used in flow
sheeting, as flashes, distillation columns, heat
exchangers, etc.
 However, the equivalence could be difficult for
many others.
 In some cases, a simple model may be
satisfactory for a quite complex unit from
mechanical point of view.
 The modeling of real units can follow one of the
following possibilities:
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22. Cont…
1. Decomposition in elementary simulation blocks.
Example: an azeotropic distillation column may be
decomposed in reboiled stripping column, heat
exchanger, three-phase flash separator and reflux
splitter.
2. Aggregation of units. Example: a heat exchanger
and a flash vessel may be combined in a single
flash block.
3. Black box units. Examples: membranes, dryers,
special separations, etc.
4. Add-on user units. This possibility involves the
existence of a programming environment, including
the access to physical properties and other
routines.
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23. Cont…
 For instance, the Furnace, which in
practice is a sophisticated equipment
item, can be modeled as simple heater.
 The cross Heat Exchanger may be
described either as two-side heat
exchanger, or as single-side heater and
cooler coupled by common duty.
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24. Cont…
7. Degrees of Freedom Analysis
 The degrees of freedom analysis (DOF) allows
the user to determine the variables needed to
be specified to execute a simulation.
 In steady state simulation the degrees of
freedom are the number of variables that must
be assigned to solve the non-linear algebraic
system describing the operational unit.
 This is the number of variables that must
be set in order to solve the system of
equations describing the model.
 Therefore, the user should have at least an idea
about the type of equations and algorithms
associated with different modeling units.
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25. Cont…
8. Thermodynamic issues
 The appropriate selection of thermodynamic models is
probably the most important aspect of a simulation work.
 Sometimes preliminary work is necessary to
estimate physical properties for non-library
components, or to identify the parameters of
thermodynamic models from experimental data.
 Specific thermodynamic options at unit level will increase the
reliability of the results.
 For the HDA process we have the following possibilities:
1. Equation of state model, as for example Peng-Robinson, for
the whole flow sheet. 2. Equation of state model only for the
high-pressure section (gas loop), and specific model for
aromatic hydrocarbons, as BK10, for low-pressure
separations.
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26. Cont…
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27. Cont…
Fluid package
Fluid package in ASPEN HYSIS
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28. Cont…
9. Computational sequence
 The flow sheet must be decomposed in
computational sequences if there are
recycle loops and/or design specifications.
 The streams necessary to be initialized
are called tear streams.
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29. Final simulation of HDA process
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30. Cont…
• Three recycle loops may be identified.
(heat integration around the reactor, recycle of
hydrogen and recycle of toluene.)
• The last two loops have a common part from
the mixer up to the flash.
• As a result, the two loops may be solved by only
one tear stream.
• Hence, we have three loops but only two tear
streams,
• As for example the exit streams from mixer and
reactor.
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31. Simulation procedure
Once the PSD known, the following approach
can be followed to run a simulation:
1. Draw the flow sheet.
2. Input the components.
3. Select the thermodynamic options.
4. Analyze the recycles and identify the tear
streams.
5. Supply data for input and tear streams.
6. Supply specifications for the simulation units
(blocks).
7. Run and make converge the simulation.
8. Analyze the results.
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32. Unit operations
1. Mixer, Splitter and (black
box) Separators.
2. Flashes.
3. Heat exchangers.
4. Shortcut distillation.
5. Multistage separations.
6. Liquid-Liquid extraction.
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7. Chemical reactors.
8. Pressure change: pump,
compressor, valve.
9. Pipes and pressure drop units.
10. Special separation units:
membrane unit, crystallizer, dryer,
etc.
11. Controllers.
12. User added units.
A Process Simulation Diagram can be built-up by means of unit
operations or simulation blocks.
 The list below cites the most frequent types, available in any all
purpose simulator:

33. Cont…
1. Mixers and splitters
 Mixer is a unit that performs an adiabatic mixing of several
inlet streams in a single outlet stream.
 Heat or work may be considered, but not in combination with
material streams. The outlet stream is flashed.
 Splitter divides an inlet streams into more streams of the
same composition and state.
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Unit operations (cont…)

34. Cont…
2. Flash units
• The flash unit is a key tool in flow sheeting, particularly for simulating
operations based on phase equilibrium. The standard models are:
1. Vapor/liquid flash, optional with free water decanting
2. Three-phase flash vapor/liquid (1)/liquid (2).
•The specification of a flash is given by the degrees of freedom analysis.
• Note that simple flashes can simulate a number of simple equilibrium
devices, as evaporators, decanters or crystallizers.
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Unit operations (cont…)

35. Cont…
3. Heat exchangers
The basic models
a. Energy modification
A simple heater/cooler unit is used to model operations where only the change
of state variables of a stream is relevant (temperature, pressure), and not the
thermal design of the heat exchanger
b. Shell-and-tubes heat exchangers.
• This unit simulates a two-sides heat exchanger that may operate in counter
current or co-current.
• If computation is a simple thermal design limited to duty and exchange area
calculation, then the overall heat transfer coefficient must be known.
• If the computation is of rating type, then the exchanger geometry must be
supplied.
• Some simulators have as defaults the sizing characteristics of shell-and-tubes heat
exchangers.
• Rigorous simulation (rating) may include pressure drop computation and zone
analysis.
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Unit operations (cont…)

36. Cont…
c. Multiple-stream heat exchanger
• The multiple-stream heat exchanger unit, is used to simulate heat
transfer between multiple hot or cold streams.
• Typical applications are the simulation of compact heat exchangers as
the plate-type heat exchanger used extensively in gas processing.
• The simulation of this unit includes the automatic generation of
multiple interconnected heaters.
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Unit operations (cont…)

37. Cont…
4. Distillation
a) Shortcut distillation
• Shortcut distillation is used in
preliminary design to determine
the number of stages needed by
a given separation.
• The computation is based on the
classical Fenske-Gilliland-
Underwood procedure. adapted to
handle total or partial condensers.
NB: shortcut models in early stages
of flow sheeting for easier
convergence of recycles.
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Unit operations (cont…)

38. Cont…
b) Rigorous distillation
 Rigorous distillation is probably the most
sophisticated unit in flow sheeting.
 The modelling may be classified in two
categories: (a) Equilibrium stage based models,
and (b) Rate-based models
The equilibrium-stage :
• models are still the most applied. They consist
of rigorous solution of the MESH equations
(Mass, Equilibrium, Summation, and Enthalpy
balances) following different strategies
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Unit operations (cont…)

39. • Liquid-liquid extraction
unit (Fig. 3.16) simulates
a counter-current
extraction device
consisting of equilibrium
stages.
• Accurate modelling of
liquid-liquid equilibrium is
essential, particularly
when the unit is
involved in a recycle.
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Unit operations (cont…)

40. Cont…
7. Reactors
Basic flowsheeting reactors are the plug
flow reactor (PFR) and continuous stirred
tank reactor (CSTR).
 A practical alternative is the
combination of ideal flow models with
stoichiometric reactors.
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Unit operations (cont…)

41. Cont…
a. Stoichiometric reactor
The stoichiometric model describes the material balance of the reaction
network by means of stoichiometric equations by using conversion or
extent of reaction as reaction variable.
b. Equilibrium reactor
In the case of fast chemical reactions, as at high temperatures or
accelerated by catalysts, the hypothesis of chemical equilibrium can give a
realistic idea about the maximum achievable performance.
Deviations in temperature or conversion with respect to the true
equilibrium may be specified. Single-phase chemical equilibrium, or
simultaneous chemical and multi-phase equilibrium may be treated. Two
models available
• Equilibrium reactions.
The chemical reactions are given explicitly. Input of expressions for
equilibrium constants may be considered.
• Gibbs free energy minimization
This model does not need the specification of stoichiometry, but only of
the species taking part in reactions.
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Unit operations (cont…)

42. a. Kinetic models
Kinetic models can be used to link the reactor design
with its performance. The reaction rate may be
expressed by power law functions, by more complex
expressions.
There are two ideal models, continuous stirred tank
reactor (CSTR) or plug flow (PFR), available in rating
mode (reaction volume fixed) or design mode
(conversion specified).
d. Batch reactor
Batch or semi-batch reactors can be simulated as
stand-alone or coupled with a continuous process.
Specification may include cycle operation with buffer
tanks, as well as reactions in single and multi-phases.
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Unit operations (cont…)