Chemical Process Simulation

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

CAD and the Structure of Design Process ANALYSIS Material &Energy balances Equip. Sizing and Costing Economic Evaluation Final flowsheet Societal needs Initial Flowsheet Parameter optimization Structure Optimization Flowsheet Synthesis 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

CAD CAD Design Variables Initial Values

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.

Flowsheeting

the use of computer aids to

perform steady-state heat and

mass balances, sizing, costing

calculation for a chemical

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.

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 process simulator

subroutines

Chemical Process

Chemical Process

Simulation(II)

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

A model is the simplification of reality used to

predict system behavior.

F = ma Physical Model Physical Model Mathematical Model Mathematical Model

Modeling and Simulation

Define process

Develop Math model

Identify constraint

Develop computer program

Run simulation program

All parameters covered?

Do model and exp.

Agree? Determine optimum conditions Yes Yes No No Flow chart of steps

in

General Process Unit Analysis

1. Define system variables. 2. Write simulation equations. 3. Check degrees of freedom. 4. Choose design variables.

Flash Analysis

An

Information Flow

in

Chemical Process Simulation

•Process 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

Nature of the Process Streams

1. Flow rate

2. Compositions

3. Temperature

4. Pressure

5. 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

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

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 Simulation Techniques

1. Sequential Modular Approach 2. Equation Oriented 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 :

3. Design variable selection :

Components of a Simulation Program Executive Program Unit Module Library Numerical Routines Physical Property Data Bank Thermodynami c Package Input Output Solution Optimization Economic Analysis

Sequential Modular Approach

1. Acyclic process – w/o recycles

-Processes are solved sequentially one module at a time.

2. Cyclic process – with recycles

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.

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.

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

Demo of Equation-Oriented approach

Simultaneous Modular Approach Execute Rigorous Models Generate Simple Model Parameters Solved Reduced Optimization Problem Inside Loop Outside Loop

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 designs

2. Analyze design alternatives

3. Predict the effects of changes on plant

operating conditions

4. Optimize energy consumption

5. Eliminate bottlenecks and increase

Running a Simulator

1. Setting up a problem

2. Creating a flowsheet

3. Specifying engineering data

4. Performing the simulation

Process Simulation

with

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, and

Be aware of

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.