Modeling Basics: 1. Verbal modeling By Peter Woolf University of Michigan Michigan Chemical Process Dynamics and Controls Open Textbook version 1.0 Creative commons

2 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

3 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?

4 Eight steps to verbal modeling 1.Describe in words how the process works 2.Define the primary goal of the process 3.Identify secondary processes that impact the primary goal 4.Identify potential safety and environmental risks 5.Identify major costs associated with the process

5 6.Identify variables that you can and should directly manipulate 7.Identify significant sources of variation 8.Describe in words how your control system would work Eight steps to verbal modeling

6 Alternative 12 step process Image from Controls Wiki

7 Distillation Example 1. 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.

8 Distillation Example 2. 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

9 Distillation Example 2. 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

10 Distillation Example 3. 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

11 Distillation Example 3. 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

12 Distillation Example 4. Identify potential safety and environmental risks Fire Explosion Corrosive Poison Environmental toxin

13 Distillation Example 4. 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

14 Distillation Example 4. 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

15 Distillation Example 4. 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 relief valve

16 Distillation Example 4. 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.

17 Distillation Example 5. 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)

18 Distillation Example 6. Identify variables that you can directly manipulate In most chemical processes you can only control 2 things: 1)Valves 2)Switches Everything else follows from these

19 Distillation Example 6. Identify variables that you can and should directly manipulate In most chemical processes you can only control 2 things: 1)Valves 2)Switches Everything else follows from these Common error: Avoid placing valves everywhere! Valves have a cost and complexity, thus what ones are really needed?

20 Distillation Example 7. 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

21 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.

22 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

23 Next Steps Lay out your variables to identify interrelationships