chemical process simulation - tunghai university
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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.
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