Chemical Process Simulation
The objective of this course is to provide the
background needed by the chemical engineers to
carry out computer-aided analyses of large-scale
chemical processes. Major concern will fall on steadystate processes with hands on experiences on
ChemCad simulator (CC-5).
CAD and the Structure of Design Process
Societal needs
Chemical Process Synthesis
Flowsheet Synthesis
Initial
Flowsheet
Flowsheet
Synthesis
1) Rxn path selection
2) Material balancing and
species allocation
3) Separation task selection
and sequencing
Initial
Values
4) Auxiliary task assignment
and process integration
ANALYSIS
Design
Variables
5)Evolutionary improvement
of initial flowsheet
CAD
Material &Energy
balances
CAD
Equip. Sizing
and
Costing
Parameter
optimization
Economic
Evaluation
Final flowsheet
Structure
Optimization
ChE Process Analysis
What is process simulation for?
1. To interpret process flowsheets,
2. To locate malfunctions, and
3. To predict the performance of
process.
Commercial Process Simulators
• Aspen Engineering Suite of Aspen tech., Inc.
– http://www.aspentech.com
• CHEMCAD 5.xx of Chemstations
– http://www.chemstations.net
• Process Engineering Suite of Simulation
Sciences, Inc.
– http://www.simsci.com
• SUPERPRO DESIGNER 4.x of Intelligen, Inc.
– http://intelligen.com/SuperPro.htm
Flowsheeting
the use of computer aids to
perform steady-state heat and
mass balances, sizing, costing
calculation for a chemical
process.
To perform process simulation using
a process simulator
•
•
Convert from a process flowsheet
to a simulation flowsheet, i.e.,
replace the process units with
appropriate simulation unit.
Model and solve the process unit
equations – a subroutine is written
for each process unit.
What are process flowsheets?
• 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 flowsheet is 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.
A Typical Process Flow Sheet
A Hypothetical process Flow sheet
What is a simulation flowsheet?
A simulation flowsheet is a collection of
simulation units to represent computer
program (subroutines or models) that
simulate the process units and arcs to
represent the flow information among
the simulation units.
A typical simulation flow sheet
A typical process simulator
subroutines
Process Modeling and Simulation
Chemical Process
Simulation(I)
Chemical Process
Simulation(II)
Typical Process Equipments
Modeling and Simulation procedure
• Translating the description of a physical
system into an appropriate mathematical
form.
• Selecting a suitable computational technique.
• Implementing the computational technique in
the form of a computer program.
A model is the simplification
of reality used to
predict system behavior.
Physical
Model
Mathematical
Model
F = ma
Modeling and Simulation
Results
and
Interpretation
Mathematical
model
Physical
system
Laws of Nature
Mass
Equil.
Sum
H-energy
+
Rate
+
Others
Equation Solver
Matlab
MathCad
CC-5
Aspen Plus
Hysis
others
Define process
Develop Math model
Identify constraint
Flow chart of steps
in
simulation
Develop computer
program
Run simulation program
No
All parameters
covered?
Yes
Do
model and exp.
Agree?
Yes
Determine optimum
conditions
No
General Process Unit Analysis
1.
2.
3.
4.
5.
Define system variables.
Write simulation equations.
Check degrees of freedom.
Choose design variables.
Choose appropriate math solver.
Flash Analysis
An
example
Flash Drum in situ.
Flash Vessel (1)
A Flash Vessel (2)
A Flash Vessel (3)
A Flash Vessel (4)
Defining Process Variables
Math. Model
Information Flow
in
Flash Calculations
Information Flow (1)
Information Flow (2)
Information Flow (3)
Information Flow (4)
Information Flow (5)
Information Flow (6)
Information Flow (7)
Information Flow (8)
A typical flash example
Chemical Process Simulation
•Process Flowsheet
•Simulation Flow sheet
Process Flowsheet
Simulation Flowsheet
Nature of the Simulation Problem
1. The nature of the process streams
2. The nature of the material being
processed
3. The nature of each type of process unit
4. The specific process configuration
5. The feed stream property
Nature of the Process Streams
1.
2.
3.
4.
5.
Flow rate
Compositions
Temperature
Pressure
Others
Nature of the Material Being Processed
1. A set values of the pure component
properties of each chemical component
in the stream.
2. Values of the stream variables for the
particular stream of interest – to
determine the temperature, pressure,
and composition of the stream mixture.
3. A model for the thermodynamic and
transport behavior of the mixture.
A typical set of pure-component properties for the cal’n of thermo properties
Nature of the Process Unit
1.
2.
3.
4.
5.
The conservation laws: the principles of conservation
of mass, energy, and momentum.
The rate laws: relations between rate of flow, heat
transfer, mass transfer, chemical rxn, etc. and driving
forces of temperature, pressure, conc., etc.
Physical property relations: relations between the
thermodynamic and transport properties and the
intensive variables of temperature, pressure, and conc.
Principles of thermodynamic equilibrium: limitations on
the performance of physico-chemical systems imposed
by the 2nd law of thermodynamics.
Automatic control theory: relations governing the
transfer of information through the system.
Process Configuration
1. The topology of the process – the
description of which streams are
connected to which inlet and outlet ports
of which units.
2. The specifications of all design and
operating parameters that are under the
control of the designer.
Feed/product Streams
1. Feed stream could be treated as a
process unit with an outlet and no inlet
2. Product stream could be treated as a
process unit with an inlet and no outlet.
Process Simulation Techniques
1. Sequential Modular Approach
2. Equation Oriented Approach
3. Simultaneous Modular Approach
Methods of Analysis of System Structure –
the decomposition of large system
1. Partitioning and precedence ordering
•
units that must be solved together are
identified in the flowsheet;
• the sequence of computations of the
partitioned subsystem are determined
2. Tearing : resolving a cyclic partitioned unit to
a acyclic one.
3. Design variable selection : the best choice
of design variables is to render the equations
most acyclic.
Components of a Simulation Program
Numerical
Routines
Unit
Module Library
Physical
Property
Data Bank
Thermodynami
c
Package
Input
Executive
Program
Solution
Optimization
Economic
Analysis
Output
Sequential Modular Approach
1. Acyclic process – w/o recycles
-Processes are solved sequentially one module
at a time.
2. Cyclic process – with recycles
-need to cut streams to enable the procedure.
Typical Process Modules
1.
2.
MIX - Mix several inlet streams adiabatically to form one product
stream.
SPLIT – Split a single inlet stream into two or more product
streams with the same composition and temperature.
3.
COMPRESS – Raise the pressure of a gas by a specific amount.
4.
5.
PUMP - Raise the pressure of a liquid by a specific amount.
FLASH – Convert a liquid stream at one pressure to liquid and
vapor streams in equilibrium at low pressure.
REACT – Simulate a chemical reactor.
DISTILL, EXTRACT, CRYSTAL, ABSORB – Simulate the
separation processes of distillation, extraction, crystallization, and
absorption, respectively.
6.
7.
Aspen Subroutine Library(I)
Aspen Subroutine Library(II)
Simulation of an Acyclic Process
The flowsheet shown here depicts a hypothetical multi-unit separation process.
Three liquid streams are mixed adiabatically; The product stream is pumped
Through a heater to a distillation column, and the overhead product from the
column is partially condensed to yield liquid and vapor products. Using blocks
MIX, PUMP, HEAT, DISTILL, and CNDS, construct a block diagram for the
Simulation of this process.
Process for separation of ammonia and water (I)
Process for separation of ammonia and water (II)
Acyclic process Calculation sequence
Ethylchloride production process flow sheet(I)
Ethylchloride production process flow sheet(II)
Recycle process calculation sequence
Equation Oriented Approach
• All the equations of the whole process are
collected and solved as a large system of
nonlinear algebraic equations.
• Mathematically, the problem is formulated
as an optimization problem, i.e.,
• Minimize h(x,u)
– Subject to
f(x,u) = 0 ;process model eqn.
g(x,u) = 0 ;process constraints.
Where x is the vector of state (dep.) variables, and
u is the vector of decision (indep.) variables.
Demo of Equation-Oriented approach
Simulation of an equilibrium reaction/separation process
Degrees of Freedom Analysis
Simulation Equations
Numerical Solution
Simultaneous Modular Approach
Execute Rigorous
Models
Outside
Loop
Generate Simple
Model
Parameters
Solved Reduced
Optimization
Problem
Inside
Loop
Steps of simultaneous modular approach
1.
2.
3.
4.
5.
For the first iteration, make initial estimates of recycle stream
values.
Solve the problem using the sequential modular approach.
Having obtained the first estimate of input and output values for
each unit, construct a linear relationship between them, i.e.,
linearize the model equations.
Since the interconnection equations are already linear, solve the
whole system of model and interconnection equations
simultaneously using matrix method to obtain a new set of inlet
values.
If two successive iterates of assumed stream values converge
within a preset tolerance, the simulation is complete. Otherwise,
go back to step (2).
The scope of a process simulator
1.
2.
3.
4.
5.
Prepare process designs
Analyze design alternatives
Predict the effects of changes on plant
operating conditions
Optimize energy consumption
Eliminate bottlenecks and increase
throughput
Running a Simulator
1.
2.
3.
4.
5.
Setting up a problem
Creating a flowsheet
Specifying engineering data
Performing the simulation
Viewing and printing results
Aspen-plus demo
Process Simulation
with
ChemCad
Typical ChemCad Module Library
Scope of ChemCad
ChemCad Simulation Procedures
1.
2.
3.
4.
5.
6.
7.
Draw flowsheet,
Choose components,
Choose thermodynamic model,
Define feed streams,
Provide equipment parameters,
Run the program, and
View, plot and output the reports and PFD.
ChemCad simulator
Benzene Process Flow Sheet
Be aware of
GIGO
References:
1.
2.
3.
4.
5.
6.
7.
8.
Analysis, Synthesis, and Design of Chemical
Processes by Richard Turton, et al.
Chemical Process Simulation by Asghar Husain.
Computer Applications in ChE by H. Th. Bussemaker.
Modeling and Simulation in ChE by R. G. E. Franks.
Process Modeling, Simulation and Control for
Chemical Engineers by W. L. Luyben.
Chemical Process Computations by Raghu Raman.
Elementary Principles of Chemical Processes,
Chapter 10, by Richard M. Felder and Ronald W.
Rousseau, 2nd Ed.
CHEMCAD (CC-5) user guide.