Chapter 18 Control Case Studies

Control Systems Considered Temperature control for a heat exchanger Temperature control of a CSTR Composition control of a distillation column pH control

Temperature Control for Heat Exchangers

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.

Inferior Configuration for a Steam Heated Heat Exchanger

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

Preferred Configuration for a Steam Heated Heat Exchanger

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.

Modfication to Perferred Configuration

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.

Scheduling of PI Controller Settings

Inferior Configuration for a Liquid/Liquid Heat Exchanger

Preferred Configuration for a Liquid/Liquid Heat Exchanger

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.

Temperature Control for CSTRs

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.

Preferred Configuration for Endothermic CSTR

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  p /  p less than 0.1

Deadtime for an Exothermic CSTR  mix - V r divided by feed flow rate, pumping rate of agitator, and recirculation rate.  ht - MC p /UA  coolant - V coolant divided by coolant recirculation rate  s - sensor system time constant (6-20 s)

Exothermic CSTR Temperature Control

Maximizing Production Rate

Using Boiling Coolant

Distillation Control

Distillation control affects- –Product quality –Process production rate –Utility usage Bottom line- Distillation control is economically important

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

Material Balance Effects

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

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.

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.

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

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

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

Minimum Pressure Operation

Manipulating Refrigerant Flow

Flooded Condenser

Venting for Pressure Control

Venting/Inert Injection

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

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

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

Dual Composition Control Low 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)

Dual Composition Control High 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

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

Configuration Selection Examples Consider C 3 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).

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

Typical Column Constraints Maximum reboiler duty Maximum condenser duty Flooding Weeping Maximum reboiler temperature

Max T Constraint - y Important

Max T Constraint - x Important

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.

pH Control

pH control is important to any process involving aqueous solutions, e.g., wastewater neutralization and pH control for a bio-reactor. pH control can be highly nonlinear and highly nonstationary. Titration curves are useful because they indicate the change in process gain with changes in the system pH or base-to-acid ratio.

Strong Acid and Weak Acid Titration Cures for a Weak Base Which is an easier control problem?

Effect of pK a on the Titration Curves for a Strong and Weak Base

Titration Curves The shape of a titration curve is determined from the pK a and pK b of the acid and the base, respectively.

Degree of Difficulty for pH Control Problems Easiest: relatively uniform feed rate, influent concentration and influent titration curve with a low to moderate process gain at neutrality. (Fixed gain PI controller or manual control) Relatively easy: variable feed rate with relatively uniform influent concentration and influent titration curve. (PI ratio control)

Degree of Difficulty for pH Control Problems More Difficult: variable feed rate and influent concentration, but relatively uniform titration curve. (A ratio controller that allows the user to enter the titration curve) MOST DIFFICULT: variable feed rate, influent concentration and titration curve. Truly a challenging problem. (An adaptive controller, see text for discussion of inline pH controllers).