What is Process Simulation?

advertisement
Introduction
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.
2
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....
3
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.
4
Motivation (why develop a process model?)
• Three broad categories of employment
(The Work of Nations, R. Reich, 1991)
- routine production service
- in-person service
- symbolic-analytic service
increasing educational requirement
• 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)
5
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)
6
• 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
7
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)
8
• Example. A perfectly insulated, well-stirred tank
(lumped parameter system)
Thot=60 ˚C, Tcold=10 ˚C
Steady-state
Dynamic response
9
• Example. Countercurrent heat exchanger
(distributed parameter system)
Temperature of water stream can change with time and position
Tsteam = 100 ˚C
Twater = 25 ˚C
Steady-state temperature profile
10
• Mathematical models consists of the following types of
equations (including combinations).
- algebraic equations
- ordinary differential equations
- partial differential equations
11
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)
12
• 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
13
• 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.
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
15
16
?
?
?
?
?
17
vs.
18
Download