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 steady-state processes with hands on experiences on ChemCad simulator (CC-5).

CAD and the Structure of Design Process

ANALYSIS

Material &Energybalances

Equip. Sizingand

Costing

EconomicEvaluation

Final flowsheet

Societal needs

InitialFlowsheet

Parameteroptimization

StructureOptimization

FlowsheetSynthesis

Flowsheet Synthesis

1) Rxn path selection

2) Material balancing and species allocation

3) Separation task selection and sequencing

4) Auxiliary task assignment and process integration

5)Evolutionary improvement of initial flowsheet

Chemical Process Synthesis

CA

D

CAD

DesignVariables

InitialValues

ChE Process Analysis

What is process simulation for?

1. To interpret process flowsheets,2. To locate malfunctions, and3. 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 flowsheetto 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 ProcessSimulation(I)

Chemical ProcessSimulation(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.

F = ma

PhysicalModelPhysicalModel

MathematicalModelMathematicalModel

Modeling and Simulation

Physicalsystem

Mathematical model

Resultsand

InterpretationLaws of Nature

MassEquil.Sum

H-energy+

Rate+

Others

Equation SolverMatlab

MathCadCC-5

Aspen PlusHysisothers

Define process

Develop Math model

Identify constraint

Develop computerprogram

Run simulation program

All parameterscovered?

Do model and exp.

Agree?

Determine optimumconditions

Yes

Yes

No

No

Flow chart of stepsin

simulation

General Process Unit Analysis1. Define system variables.2. Write simulation equations.3. Check degrees of freedom.4. Choose design variables.5. Choose appropriate math solver.

Flash Analysis

Anexample

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 Flowin

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 streams2. The nature of the material being

processed3. The nature of each type of process unit4. The specific process configuration5. The feed stream property

Nature of the Process Streams

1. Flow rate2. Compositions3. Temperature4. Pressure5. 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. The conservation laws: the principles of conservation of mass, energy, and momentum.

2. The rate laws: relations between rate of flow, heat transfer, mass transfer, chemical rxn, etc. and driving forces of temperature, pressure, conc., etc.

3. Physical property relations: relations between the thermodynamic and transport properties and the intensive variables of temperature, pressure, and conc.

4. Principles of thermodynamic equilibrium: limitations on the performance of physico-chemical systems imposed by the 2nd law of thermodynamics.

5. 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 Approach2. Equation Oriented Approach3. 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 determined2. 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

ExecutiveProgram

UnitModule Library

NumericalRoutines

PhysicalProperty

Data Bank

Thermodynamic

Package

Input Output

Solution

Optimization

EconomicAnalysis

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. MIX - Mix several inlet streams adiabatically to form one product stream.

2. 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. PUMP - Raise the pressure of a liquid by a specific amount.5. FLASH – Convert a liquid stream at one pressure to liquid and

vapor streams in equilibrium at low pressure.6. REACT – Simulate a chemical reactor.7. DISTILL, EXTRACT, CRYSTAL, ABSORB – Simulate the

separation processes of distillation, extraction, crystallization, and absorption, respectively.

Aspen Subroutine Library(I)

Aspen Subroutine Library(II)

Simulation of an Acyclic ProcessThe flowsheet shown here depicts a hypothetical multi-unit separation process.Three liquid streams are mixed adiabatically; The product stream is pumpedThrough a heater to a distillation column, and the overhead product from the column is partially condensed to yield liquid and vapor products. Using blocksMIX, PUMP, HEAT, DISTILL, and CNDS, construct a block diagram for theSimulation 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

Generate SimpleModel

Parameters

Solved ReducedOptimization

Problem

InsideLoop

OutsideLoop

Steps of simultaneous modular approach

1. For the first iteration, make initial estimates of recycle stream values.

2. Solve the problem using the sequential modular approach.3. 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.

4. 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.

5. 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. Prepare process designs2. Analyze design alternatives3. Predict the effects of changes on plant

operating conditions4. Optimize energy consumption5. Eliminate bottlenecks and increase

throughput

Running a Simulator

1. Setting up a problem2. Creating a flowsheet3. Specifying engineering data4. Performing the simulation5. Viewing and printing results

Aspen-plus demo

Process Simulation with

ChemCad

Typical ChemCad Module Library

Scope of ChemCad

ChemCad Simulation Procedures

1. Draw flowsheet,2. Choose components,3. Choose thermodynamic model,4. Define feed streams,5. Provide equipment parameters,6. Run the program, and7. View, plot and output the reports and PFD.

ChemCad simulator

Benzene Process Flow Sheet

Be aware ofGIGO

References:1. Analysis, Synthesis, and Design of Chemical

Processes by Richard Turton, et al. 2. Chemical Process Simulation by Asghar Husain.3. Computer Applications in ChE by H. Th. Bussemaker.4. Modeling and Simulation in ChE by R. G. E. Franks.5. Process Modeling, Simulation and Control for

Chemical Engineers by W. L. Luyben.6. Chemical Process Computations by Raghu Raman.7. Elementary Principles of Chemical Processes,

Chapter 10, by Richard M. Felder and Ronald W. Rousseau, 2nd Ed.

8. CHEMCAD (CC-5) user guide.