Introduction

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What is Process Simulation?

• Simulation is the imitation of the operation of a real-world process

or system over time. The act of simulating something first requires

that a model be developed; this model represents the key

characteristics or behaviors/functions of the selected physical or

abstract system or process.

• A computer simulation is an attempt to model a real-life or

hypothetical situation on a computer so that it can be studied to see

how the system works. It is a tool to virtually investigate the behavior

of the system under study.

• Process simulation is a model-based representation of chemical,

physical, biological, and other technical processes and unit operations

in software that allows the calculation of a process in computers.

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Why Use Process Simulation?

• In this modern age of powerful computers, it often makes no sense

to put pencil to paper like in the old days.

• Imagine you are tasked to design a distillation column to produce a

90% benzene overhead product and a 95% toluene bottoms product.

� You look up the VLE data, construct your x-y diagram, draw the feed line,

and step off your McCabe-Thiele trays. Estimate tray efficiencies.

� Then you calculate your mass and energy balance, product and bottoms

dew points and bubble points, and estimate your overhead condenser

and column reboiler duties.

� Wrestle with those tedious hydraulics equations to determine the

column diameter. Then dust off the old heat transfer equations to size

your condenser and reboiler. Finally - the system is designed.

• And then they decide they want 95% benzene, and by the way, the

feed rate is 2,500 pounds per hour, not 1,500....

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The Advantage of Simulation

• New software can perform repetitive chemical engineering

calculations in a fraction of the time it takes to execute them by hand.

The speed and accuracy of process simulation begins to save

tremendous time and money.

• In the example above, you had to select your reflux rate based on

some old rule of thumb about the optimum reflux being some

multiple of the minimum reflux. Wouldn't it be better to be able to

plot reflux rate versus energy usage? Versus number of trays?

• Multiple runs - quickly.

Process optimization - quickly.

Such is the power of process simulation.

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Motivation (why develop a process model?)

• Three broad categories of employment(The Work of Nations, R. Reich, 1991)

- routine production service

- in-person service increasing educational requirement

- symbolic-analytic service

• Symbolic analysts solve, identify, and broker problems by

manipulating symbols.

Reality Abstract images Reality

• Engineers (process engineers) are symbolic analyst.

� mathematical models→ characterize the process behavior

� symbols→ represent physical variables (e.g., P, T)

� numerical algorithm→ solve the model (simulation)

� analyze results and make decision (design or operation)

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Models

• Definition: A process model is a set of equations

(including the necessary input data to solve

the equations) that allows us to predict the

behavior of a system.

Classification 1:

- fundamental or first-principle model

(based on physical-chemical relationships)

- empirical model

(e.g., a simple least-squares fit to experimental data)

(useful for interpolation but not for extrapolation)

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• The possible use of model

(given a set of input data, a model is used to predict

the output “response”)

- marketing: demand vs. price

- allocation: supply chain management

- synthesis: process flowsheet (sequence of equipments)

- design: equipment sizing

- operation: optimal operating condition

- control: maintain stable operation

- safety: failure propagation

- environmental: global warming and its impact

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Classification 2:

- based on steady-state behavior

- describe dynamics responses (variables change with time)

Classification 3:

- lumped parameter system: a variable changes only

with one independent variable (time, but not space)

(e.g. perfectly mixed stirred tank)

- distributed parameter system: has more than one

independent variable (e.g., space and time)

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• Example. A perfectly insulated, well-stirred tank

(lumped parameter system)

Thot=60 ˚C, Tcold=10 ˚C

Steady-state

Dynamic response

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• Example. Countercurrent heat exchanger

(distributed parameter system)

Temperature of water stream can change with time and position

Tsteam = 100 ˚CTwater = 25 ˚C

Steady-state temperature profile

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• Mathematical models consists of the following types of

equations (including combinations).

- algebraic equations

- ordinary differential equations

- partial differential equations

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Systems

• Definition: A combination of several pieces of equipment

integrated to perform a specific function; thus

a fire control system may include a tracking

radar, computer, and gun.

• Simulation:

- develop models and numerical techniques that allow

us to “simulate” the behavior of a process

- be careful when using computer simulation

Do the results of this simulation make sense?

(common sense and “back of the envelope” calculation

will tell us if the numerical results are in the ballpark)

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• Linear system analysis

Analysis means seeking a deeper understanding of

a process than simply performing a simulation.

- Laplace transform

- state space techniques

• A broader view: how the response of system variable

changes when a parameter or input changes

Ex: jacketed chemical reactor

Output Multiplicity

• Engineering problem solving can be a combination

of art and science.

• The complexity and accuracy of a solution will

depend on the information available or what is

desired in the final solution.

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Tools for Process Simulation

• Commercial Process Simulator: Aspen, ChemCAD, Pro/II,…

� Suitable for large-scale plant

� Less flexibility

• Computing and Simulation Software: MATLAB/Simulink

� More modeling and programming involved

� More flexibility

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