1. ERT 210/4 Process Control & Dynamics
INTRODUCTION
Hairul Nazirah bt Abdul Halim
Office:

3. Process Control Systems
Dirk Thiele, Emerson Process Management

4. Process Control Large scale, continuous processes:
Oil refinery, ethylene plant, pulp mill
Typically, 1000 – 5000 process variables are measured.
Most of these variables are also controlled.
Examples: flow rate, T, P, liquid level, composition
Sampling rates:
Process variables: A few seconds to minutes
Quality variables: once per 8 hr shift, daily, or weekly

5. Manipulated variables
We implement “process control” by manipulating process variables, usually flow rates.
Examples: feed rate, cooling rate, product flow rate, etc.
Typically, several thousand manipulated variables in a large continuous plant

7. 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.

8. 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

9. Objectives of Control
1. Maintain the process at the desired operating point.
A process is expected to operate at the steady-state condition as prescribed by its design.
But, the process may be unstable, implying that process variables may not remain within their physical bounds
A process operating at steady-state experiences frequent upsets due to various changes in its operating environment.

10. 2. Move the process from one operating point to another
In many process operations, the plant personnel may want to change the operating point that a process is at for a variety of reasons.

11. Control Terminology
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).

12. Classification of Control Strategies
Feedback Control
Feedforward Control

13. Closed-loop Artificial Pancreas (Feedback Control)
glucose setpoint
controller
sensor
pump
patient u y r
measured glucose
Bob Parker has integrated this example throughout his course (different levels of modeling, etc.

14. FEEDBACK CONTROL
Distinguishing feature: measure the controlled variable
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.

15. 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 (shown later).

16. FEEDBACK CONTROL Disadvantages:
No corrective action occurs until after the disturbance has upset the process, that is, until after x differs from xsp.
Very oscillatory responses, or even instability…

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

18. 1.1 Illustrative Example: Blending system
Notation:
w1, w2 and w are mass flow rates
x1, x2 and x are mass fractions of component A

19. Assumptions:
w1 is constant
x2 = constant = 1 (stream 2 is pure A)
Perfect mixing in the tank
Control Objective:
Keep x at a desired value (or “set point”) xsp, despite variations in x1(t). Flow rate w2 can be adjusted for this purpose.

20. Terminology:
Controlled variable (or “output variable”): x
Manipulated variable (or “input variable”): w2
Disturbance variable (or “load variable”): x1

21. Some Possible Control Strategies:
Control Question.
Suppose that the inlet concentration x1 changes with time. How can we ensure that x remains at or near the set point, xsp ?
Some Possible Control Strategies:
Method 1. Measure x and adjust w2.
If x is too high, w2 should be reduced
If x is too low, w2 should be increased
Can be implemented by a person (manual control)
More convenient and economical using automatic control

23. Method 2. Measure x1 and adjust w2.
Measure disturbance variable x1 and adjust w2 accordingly
Thus, if x1 is greater than , we would decrease w2 so that
If x1 is smaller than , we would increase w2.

25. Method 3. Measure x1 and x, adjust w2.
This approach is a combination of Methods 1 and 2.
Method 4. Use a larger tank.
If a larger tank is used, fluctuations in x1 will tend to be damped out due to the larger capacitance of the tank contents.
However, a larger tank means an increased capital cost.

26. Summary 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.

27. By applying the fundamental process control principles, the engineer can design plants and implement control strategies that can achieve the control objectives set forth by plant operations.