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CONCEPTCONCEPTIntroduction to Process Introduction to Process

ControlControl

ERT 210/4ERT 210/4Process Control & DynamicsProcess Control & Dynamics

MISS. RAHIMAH BINTI OTHMAN(Email: rahimah@unimap.edu.my)

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Course Outcome 1 (CO1)Ability to derive and develop theoretical model of chemical processes, analyze Laplace transform techniques to simplify first order and second order processes and create transfer functions and state space models.

1. Introduction Concepts

• Introduction to Process Control: Basic concepts of process dynamics and process control.

• Theoretical Models of Chemical Processes: 2. Laplace Transform

3. Transfer Function Models

4. Dynamic Behavior of First-order and Second-order Processes

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Process Dynamics

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a) Refers to unsteady-state or transient behavior.

b)Steady-state vs. unsteady-state behavior;i.Steady state: variables do not change with time

c) Continuous processes : Examples of transient behavior:i. Start up & shutdownii. Grade changesiii. Major disturbance; e.g: refinery during stormy or hurricane conditionsiv. Equipment or instrument failure (e.g., pump failure)

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Process Dynamics

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* Example: Typical Continuous Processes

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Process Dynamics

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d) Batch processesi. Inherently unsteady-state operationii. Example: Batch reactor

1.Composition changes with time2.Other variables such as temperature could be constant.

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* Example: Typical processes whose operation in noncontinuous.

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Process Control1. Objective:Enables the process to be maintained at the desired operation conditions, safely and efficiently, while satisfying environmental and product quality requirements.

2. 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).

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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

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• 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

Process ControlC

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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.

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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

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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.

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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. C

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1Objectives of Control

Feedback Control

CLASSIFICATION OF CONTROL STRATEGIES

Feedforward-plus-Feedback Control

DESIRED OUTPUT

Feedforward Control

Ratio Control

Split Range Control

Cascade Control

Differential Control

Closed-loop Artificial Pancreas (Feedback Control)

glucose setpoint

controller sensorpump patient

u

yr

measured glucose

Bob Parker has integrated this example throughout his course (different levels of modeling, etc.

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.

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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).C

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• 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…C

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FEEDBACK CONTROL

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.

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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

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Assumptions:

1. w1 is constant

2. x2 = constant = 1

(stream 2 is pure A)

3. 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.

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Terminology:

• Controlled variable (or “output variable”): x

• Manipulated variable (or “input variable”): w2

• Disturbance variable (or “load variable”): x1

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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

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SPx x

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Design Question. What value of is required to have 2w?SPx x

Overall balance:

Component A balance:

1 20 (1-1)w w w

1 1 2 2 0 (1-2)w x w x wx

(The overbars denote nominal steady-state design values.)

• At the design conditions, . Substitute Eq. 1-2, and , then solve Eq. 1-2 for :SPx x

2 1x 2w

12 1 (1-3)

1SP

SP

x xw w

x

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Development of Dynamic ModelsIllustrative Example: A Blending Process

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An unsteady-state mass balance for the blending system:

rate of accumulation rate of rate of(2-1)

of mass in the tank mass in mass out

Figure 2.1. Stirred-tank blending process.

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• Equation 1-3 is the design equation for the blending system.

• If our assumptions are correct, then this value of will keep at . But what if conditions change?x SPx

Control Question. Suppose that the inlet concentration x1 changes with time. How can we ensure that x remains at or near the set point ?

As a specific example, if and , then x > xSP.1 1x x 2 2w w

Some Possible Control Strategies:

Method 1. Measure x and adjust w2.

• Intuitively, if x is too high, we should reduce w2;

2w

SPx

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• Manual control vs. automatic control

•Method 1 can be implemented as a simple control algorithm (or control law):

• Proportional feedback control law,

2 2 (1-4)c SPw t w K x x t

1. where Kc is called the controller gain.

2. w2(t) and x(t) denote variables that change with time t.

3. The change in the flow rate, is proportional to the deviation from the set point, xSP – x(t).

2 2 ,w t w

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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.

1x

2 2;w w

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• Because Eq. (1-3) applies only at steady state, it is not clear how effective the control law in (1-5) will be for transient conditions.

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.

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Method Measured Variable

Manipulated Variable

Category

1 x w2FBa

2 x1 w2 FF

3 x1 and x w2 FF/FB

4 - - Design change

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Classification of Control Strategies

Table. 1.1 Control Strategies for the Blending System

Feedback Control:

• Distinguishing feature: measure the controlled variable

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• It is important to make a distinction between negative feedback and positive feedback.

Engineering Usage vs. Social Sciences

• 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).

• 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…

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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.

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1. Measurement and Actuation

2. Safety, Environment and Equipment Protection

3a. Regulatory Control

4. Real-Time Optimization

5. Planning and Scheduling

Process

3b. Multivariable and Constraint Control

(days-months)

(< 1 second)

(< 1 second)

(seconds-minutes)

(minutes-hours)

(hours-days)

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Figure 1.7 Hierarchy of process control activities.

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Figure 1.9 Major steps in control

system development

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.

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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.

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Prepared by, MISS RAHIMAH OTHMAN

Thank you

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(a)Feedback and feedforward control both require measured variable.

(b)The process variable to be controlled is measured in feedback control.

(c)Feedforward control can be perfect in the theoretical sense that the controller can take action via the manipulated even while the controlled variable remains equal to its desired value.

(d)Feedforward control can provide perfect control; that is, the output can be kept at its desired value, even with an imperfect process model.

(e)Feedback control will always take action regardless of the accuracy of any process model that was used to design it and the source of a disturbance.

QUIZ1

False/True

False/True

False/True

False/True

False/True

Which of the following statements are true?

(a)Feedback and feedforward control both require measured variable.

(b)The process variable to be controlled is measured in feedback control.

(c)Feedforward control can be perfect in the theoretical sense that the controller can take action via the manipulated even while the controlled variable remains equal to its desired value.

(d)Feedforward control can provide perfect control; that is, the output can be kept at its desired value, even with an imperfect process model.

(e)Feedback control will always take action regardless of the accuracy of any process model that was used to design it and the source of a disturbance.

QUIZ1

False/True

False/True

False/True

False/True

False/True

Which of the following statements are true?