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 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)








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.