Chapter 15 Control Case Studies

1. Chapter 15Control Case Studies

2. Control Systems Considered • Temperature control for a heat exchanger • Temperature control of a CSTR • Composition control of a distillation column

3. Heat Exchangers • Exhibit process deadtime and process nonlinearity. • Deadtime and gain both increase as tubeside flow decreases. • Major disturbances are feed flow and enthalpy changes and changes in the enthalpy of the heating or cooling medium.

4. Inferior Configuration for a Steam Heated Heat Exchanger

5. Analysis of Inferior Configuration • This configuration must wait until the outlet product temperature changes before taking any corrective action for the disturbances listed.

6. Preferred Configuration for a Steam Heated Heat Exchanger

7. Analysis of Preferred Configuration • For the changes in the steam enthalpy and changes in the feed flow or feed enthalpy, they will cause a change in the heat transfer rate which will in turn change the steam pressure and the steam pressure controller will take corrective action. • There this configuration will respond to the major process disturbances before their effect shows up in the product temperature.

8. Modfication to Perferred Configuration

9. Analysis Modfication to Perferred Configuration • A smaller less expensive valve can be used for this approach, i.e., less capital to implement. • This configuration should be slower responding than the previous one since the MV depends on changing the level inside the heat exchanger in order to affect the process.

10. Scheduling of PI Controller Settings

11. Inferior Configuration for a Liquid/Liquid Heat Exchanger

12. Preferred Configuration for a Liquid/Liquid Heat Exchanger

13. Comparison of Configurations for Liquid/Liquid Heat Exchangers • For the inferior configuration, the process responds slowly to MV changes with significant process deadtime. Moreover, process gain and deadtime change significantly with the process feed rate. • For the preferred configuration, the system responds quickly with very small process deadtime. Process deadtime and gain changes appear as disturbances.

14. CSTR Temperature Control • Severe nonlinearity with variations in temperature. • Effective gain and time constant vary with temperature. • Disturbances include feed flow, composition, and enthalpy upsets, changes in the enthalpy of the heating or cooling mediums, and fouling of the heat transfer surfaces.

15. Preferred Configuration for Endothermic CSTR

16. Exothermic CSTR’s • Open loop unstable • Minimum and maximum controller gain for stability • Normal levels of integral action lead to unstable operation • PD controller required • Must keep qp/tp less than 0.1

17. Deadtime for an Exothermic CSTR • tmix- Vr divided by feed flow rate, pumping rate of agitator, and recirculation rate. • tht- MCp/UA • tcoolant- Vcoolant divided by coolant recirculation rate • ts- sensor system time constant (6-20 s)

18. Exothermic CSTR Temperature Control

19. Exothermic CSTR Temperature Control

20. Maximizing Production Rate

21. Using Boiling Coolant

22. Distillation Control • Distillation control affects- • Product quality • Process production rate • Utility usage • Bottom line- Distillation control is economically important

23. The Challenges Associated with Distillation Control • Process nonlinearity • Coupling • Severe disturbances • Nonstationary behavior

24. Material Balance Effects

25. Effect of D/F and Energy Input on Product Purities [Thin line larger V]

26. Combined Material and Energy Balance Effects • Energy input to a column generally determines the degree of separation that is afforded by the column while the material balance (i.e., D/F) determines how the separation will be allocated between the two products.

27. Vapor and Liquid Dynamics • Boilup rate changes reach the overhead in a few seconds. • Reflux changes take several minutes to reach the reboiler. • This difference in dynamic response can cause interesting composition dynamics.

28. Effect of Liquid and Vapor Dynamics [(D,V) configuration] • Consider +DV • L/V decrease causes impurity to increase initially • After DV reaches accumulator, L will increase which will reduce the impurity level. • Result: inverse action

29. Disturbances • Feed composition upsets • Feed flow rate upsets • Feed enthalpy upsets • Subcooled reflux • Loss of reboiler steam pressure • Column pressure swings

30. Regulatory Control • Flow controllers. Standard flow controllers on all controlled flow rates. • Level controllers. Standard level controllers applied to reboiler, accumulators, and internal accumulators • Pressure controllers. Examples follow

31. Minimum Pressure Operation

32. Manipulating Refrigerant Flow

33. Flooded Condenser

34. Venting for Pressure Control

35. Venting/Inert Injection

36. Inferential Temperature Control • Use pressure corrected temperature • Use CAD model to ID best tray temperature to use

37. Single Composition Control - y • L is fast responding and least sensitive to Dz. • No coupling present. • Manipulate L to control y with V fixed.

38. Single Composition Control - x • V is fast responding and least sensitive to Dz. • No coupling present. • Manipulate V to control x with L fixed

39. Dual Composition ControlLow L/D Columns • For columns with L/D < 5, use energy balance configurations: • (L,V) • (L,V/B) • (L/D,V) • (L/D,V/D)

40. Dual Composition ControlHigh L/D Columns • For columns with L/D > 8, use material balance configurations: • (D,B) • (D,V) • (D,V/B) • (L,B) • (L/D,B

41. When One Product is More Important than the Other • When x is important, use V as manipulated variable. • When y is important, use L as manipulated variable. • When L/D is low, use L, L/D, V, or V/B to control the less important product. • When L/D is high, use D, L/D, B, or V/B to control the less important product

42. Configuration Selection Examples • Consider C3 splitter: high L/D and overhead propylene product is most important: Use (L,B) or (L,V/B) • Consider low L/D column where the bottoms product is most important: Use (L,V) or (L/D,V).

43. When One Product is More Important than the Other • Tune the less important composition control loop loosely (e.g., critically damped) first. • Then tune the important composition control loop tightly (i.e., 1/6 decay ratio) • Provides dynamic decoupling

44. Typical Column Constraints • Maximum reboiler duty • Maximum condenser duty • Flooding • Weeping • Maximum reboiler temperature

45. Max T Constraint - y Important

46. Max T Constraint - x Important

47. Keys to Effective Distillation Control • Ensure that regulatory controls are functioning properly. • Check analyzer deadtime, accuracy, and reliability. • For inferential temperature control use RTD, pressure compensation, correct tray. • Use internal reflux control. • Ratio L, D, V, B to F. • Choose a good control configuration. • Implement proper tuning.