Modeling Basics: 1. Verbal modeling

By Peter WoolfUniversity of Michigan

Michigan Chemical Process Dynamics and Controls Open Textbook

version 1.0

Creative commons

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Verbal Modeling

The first step in controlling any process is understanding!

4 important things to understand include:

1) Process objectives, risks, and costs

2) Factors that influence those objectives

3) Factors that can be manipulated directly

4) Physical dynamics that underlie a process

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Specify a process

• What do you want the process to do?– How do you make that happen?

• What are things to watch out for?– How do you avoid these?

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Eight steps to verbal modeling

1. Describe in words how the process works

2. Define the primary goal of the process3. Identify secondary processes that impact

the primary goal4. Identify potential safety and

environmental risks5. Identify major costs associated with the

process

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6. Identify variables that you can and should directly manipulate

7. Identify significant sources of variation8. Describe in words how your control

system would work

Eight steps to verbal modeling

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Alternative 12 step process

Image from Controls Wiki http://controls.engin.umich.edu/wiki/index.php/VerbalModeling

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Distillation Example1. Describe in words

how the process works

A liquid mixture of X and Y enters the column under pump pressure. Vapor traveling upward is contacted with the liquid traveling downward. At the base, the accumulated liquid is heated in the reboiler and some of the liquid is drawn off. At the top of the column, the vapor is condensed and the resulting reflux is either recycled near the top of the column or sent out as a top product.

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Distillation Example2. Define the primary

goal of the process

For this particular process, obtaining the highest purity top product is the most important goal as the sale price of the product is high and strongly depends on the product purity.

Primary

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Distillation Example2. Define the primary

goal of the process

For this particular process, obtaining the highest purity top product is the most important goal as the sale price of the product is high and strongly depends on the product purity.

NOTE: For different processes, even distillation, other primary goals are possible!!

E.g. Maintain product concentration above a threshold with a minimum energy expenditure

OR

Prevent any of compound X from leaving through the bottom due to an environmental risk

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Distillation Example3. Identify secondary

processes that impact the primary goal

For this particular process, obtaining the highest purity top product is the most important goal as the sale price of the product is high and strongly depends on the product purity.

Primary

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Distillation Example3. Identify secondary

processes that impact the primary goal

Primary

T

Secondary

Tertiary

Secondary here is likely to be the temperature at the top of the column. VLE strongly depends on temperature, and the temp at the top of the column determines the VLE of the product.Temp is controlled by cooling and reflux flow.

T

P

Tertiary is the temperature and composition of the rest of the column. These tertiary effects will influence the temperature at the top of the column and hence product quality

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Distillation Example4. Identify potential

safety and environmental risks

Fire

Explosion

Corrosive

Poison

Environmental toxin

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Distillation Example4. Identify potential

safety and environmental risks

Fire

Fire risks strongly depend on the process, but are more likely in hot places, places with open sparks, and places with fuel and oxidizer

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Distillation Example4. Identify potential

safety and environmental risks

Explosion

Explosion risks are present in any closed area, particularly those that are pressurized and or contain potentially explosive mixtures

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Distillation Example4. Identify potential

safety and environmental risks

Explosion

Explosion risks are present in any closed area, particularly those that are pressurized and or contain potentially explosive mixtures

Safety reliefvalve

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Distillation Example4. Identify potential

safety and environmental risks

Corrosive

Poison

Environmental toxin

These properties depend on the physical and chemical properties of the materials in the system. A material safety data sheet (MSDS) can help identify these.

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Distillation Example5. Identify major costs

associated with the process

Often your control decisions will ultimately be driven by cost considerations. Possible costs include:• Reagents• Energy• Cooling

$$$

$

$

$

$$$$$

• Safety risks (insurance, possibility of fines and litigation)

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Distillation Example6. Identify variables

that you can directly manipulate

In most chemical processes you can only control 2 things:

1) Valves2) Switches

Everything else follows from these

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Distillation Example6. Identify variables that

you can and should directly manipulate

In most chemical processes you can only control 2 things:

1) Valves2) Switches

Everything else follows from these

Common error: Avoid placing valves everywhere! Valves have a cost and complexity, thus what ones are really needed?

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Distillation Example7. Identify significant

sources of variation

Variables that are not under your control will disrupt your system--some more significantly than others.

Possible sources of variation:• Adjoining process• Feedstock quality• Environment• Operators and control systems• Market forces

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Distillation Example

8. Describe in words how your control system would work

Before you start writing equations, be sure you have a qualitative idea of how you want your controller to work for future sanity checks.

•Maintain a constant temperature in the reboiler using the valve on the steam

line exit.•Maintain a constant temperature at the top of the column by controlling the reflux flow valve.•Maintain the level of the reflux drum by controlling the top product flow valve.•Maintain the level of the reboiler by controlling the bottom product flow valve•Maintain a constant flow into the column•Maintain the pressure of the column by controlling the cold water exit flow rate.

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Common Errors• Impossible direct manipulations

– E.g. change the concentration of salt in a tank

• Missing the forest for the trees– E.g. Sacrificing product quality for tight level

control on a tank

• Excessive or insufficient control– E.g. Control every variable because you can

or ignore the possibility of significant disturbances

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Next Steps

• Lay out your variables to identify interrelationships