Introduction to Process Control Prepared by; Mrs Azduwin Binti Khasri Chapter 1 ERT 321 PROCESS CONTROL & DYNAMICS

PROCESS DYNAMICS Refers to unsteady-state or transient behavior. Steady-state vs. unsteady-state behavior; Steady state: variables do not change with time Examples of transient behavior: i. Start up & shutdown ii. Grade changes iii. Major disturbance; – e.g: refinery during stormy or hurricane conditions – iv. Equipment or instrument failure (e.g., pump failure)

PROCESS CONTROL To maintain a process at the desired operating conditions, safely and efficiently, while satisfying environmental and product quality requirements. Controlled variables (CVs) - these are the variables which quantify the performance or quality of the final product, which are also called output variables (Set point). Manipulated variables (MVs) - these input variables are adjusted dynamically to keep the controlled variables at their set-points. Disturbance variables (DVs) - these are also called "load" variables and represent input variables that can cause the controlled variables to deviate from their respective set points (Cannot be manipulated).

PROCESS CONTROL PROBLEMS- CONTINUOUS PROCESS HEAT EXCHANGER Variations in the inlet temperature and the process fluid flow rate affect the heat exchanger operation. CONTINUOUS STIRRED TANK REACTOR (CSTR) The feed conditions (composition,flow rate and temperature) can be manipulated variables or disturbance variables.

THERMAL CRACKING FURNACE The crude oil composition and the heating quality of the fuel are common disturbance variables. DISTILLATION COLUMNS If the feed stream is supplied by an upstream process, the feed conditions will be disturbance variables. PROCESS CONTROL PROBLEMS- CONTINUOUS PROCESS

Batch processes i. Inherently unsteady-state operation ii. Example: Batch reactor 1.Composition changes with time 2. Other variables such as temperature could be constant. PROCESS CONTROL PROBLEMS- BATCH AND SEMI-BATCH PROCESS Batch/semi-batch reactor Wood chip digesterPlasma etcherKidney dialysis unit

BIOPROCESS CONTROL In the production of biopharmaceutical products (human therapeutics) and fermented foods, such as bread products and yogurt. Fermentation process needs to be maintained for acceptable operation. With modern technology, batch bioprocess go through a systematic events such as sterilization, filling a vessel, maintaining T, pH, DO concentration, emptying vessel & washing vessel. FDA regulations: cGMP, a basic principles, procedures and resources to ensure a manufacturing environment which is suitable for producing bio-pharmaceuticals of acceptable quality. This required good process sensors.

JUSTIFICATION OF PROCESS CONTROL Specific Objectives of Control –Increase product throughput –Increase yield of higher valued products –Decrease energy consumption –Decrease pollution –Decrease off-spec product –Increase Safety –Extended life of equipment –Improve Operability –Decrease production labor

PROCESS CONTROL STRATEGIES FEEDBACK CONTROLFEEDFORWARD CONTROL

FEEDBACK CONTROL Distinguishing feature: measure the controlled variable (CV). It is important to make a distinction between negative feedback and positive feedback. Negative Feedback – desirable situation where the corrective action taken by controller forces the controlled variable toward the set point Positive feedback – controller makes things worse by forcing the controlled variables farther away from the set point.

FEEDBACK CONTROL Advantages: – Corrective action is taken regardless of the source of the disturbance. – Reduces sensitivity of the controlled variable to disturbances and changes in the process. Disadvantages: – No corrective action occurs until after the disturbance has upset the process, that is, until after the CV deviates from SP. – Very oscillatory responses, or even instability

Distinguishing feature: measure a disturbance variable (DV). – Advantage: Correct for disturbance before it upsets the process. – Disadvantage: Must be able to measure the disturbance. No corrective action for unmeasured disturbances. Requiring process model FEEDFORWARD CONTROL

Illustrative Example: Blending system

Assumptions: 1. w 1 is constant 2. x 2 = constant = 1 (stream 2 is pure A) 3. Perfect mixing in the tank Assumptions: 1. w 1 is constant 2. x 2 = constant = 1 (stream 2 is pure A) 3. Perfect mixing in the tank (The overbars denote nominal steady-state design values.) Steady state

Terminology: Controlled variable (or “output variable”): Manipulated variable (or “input variable”): Disturbance variable (or “load variable”): x w2w2 x1x1

Design Question. What value of is required to have Overall balance: Component A balance:

Control Question. Suppose that the inlet concentration x 1 changes with time. How can we ensure that x remains at or near the set point, x sp ? Control Question. Suppose that the inlet concentration x 1 changes with time. How can we ensure that x remains at or near the set point, x sp ? METHOD 1. MEASURE X AND ADJUST W 2. If x is too high, w 2 should be reduced If x is too low, w 2 should be increased Can be implemented by a person (manual control) More convenient and economical using automatic control

Method 1 can be implemented as a simple control algorithm (or control law): Proportional feedback control law; where K c is called the controller gain. w 2 (t) and x(t) denote variables that change with time t. The change in the flow rate, is proportional to the deviation from the set point, x SP – x(t).

Blending system METHOD 1

Method 2. Measure x 1 and adjust w 2. Measure disturbance variable x 1 and adjust w 2 accordingly. Thus, if x 1 is greater than, we would decrease w 2 so that If x 1 is smaller than, we would increase w 2.

Blending system METHOD 2

Because Eq. (1-3) applies only at steady state, it is not clear how effective the control law in method 2 will be for transient conditions. Method 3. Measure x 1 and x, adjust w 2. This approach is a combination of Methods 1 and 2. Method 4. Use a larger tank. If a larger tank is used, fluctuations in x 1 will tend to be damped out due to the larger capacitance of the tank contents. However, a larger tank means an increased capital cost.

MethodMeasured Variable Manipulated Variable Category 1x w2w2 FB 2x1x1 w2w2 FF 3x 1 and xw2w2 FF/FB 4--Design change Classification of Control Strategies Control Strategies for the Blending System Feedback Control: Measure The Controlled Variable Feedforward Control: Measure The Disturbance Variable Feedback Control: Measure The Controlled Variable Feedforward Control: Measure The Disturbance Variable

Hierarchy of process control activities. Required for all manufacturing plants Optional but can be very profitable

Major steps in control system development

Control allows us to regulate the behavior of process systems. Good control performance has the potential to yield substantial benefits for safe and profitable plant operation. Expanded role of process control represents an integration of the traditional role with plant information management. Objectives of control include stability, performance and optimization. SUMMARY

CLASS EXERCISE 1 27

CLASS EXERCISE 2

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