ChE 403 Process Control

Cha

pter

1

Dr. M. A. A. Shoukat Choudhury 1

ChE 403 Process Control3.00 credits, 3 hours/week

Basic concepts of chemical process control: incentives for process control; design aspects; hardware elements.

Modelling for control purposes; development of mathematical models; linearization of nonlinear systems; input-output model; transfer functions.

Dynamic and static behavior of chemical processes: first, second and higher order processes; transportation lag; systems in series.

Course Outline

Cha

pter

1


Course Outline (cont’d)Analysis and design of feedback control systems: concept of feedback control; feedback controllers and final control elements; block diagrams; closed loop responses; concept of stability; stability testing.

Frequency response analysis: Bode diagrams; Nyquist plots;Bode and Nyquist stability criteria; control system design by frequency response analysis.

Cha

pter

1


Course Outline (cont’d)Analysis and design of advanced control systems: control of system with large dead time or inverse response; multiple-loop control systems; feedforward and ratio control; adaptive and inferential control.

Design of control systems for multivariable processes: synthesis of alternative control configurations for multiple-input and multiple-output processes; interaction and decoupling of control loops.

Design of control systems for complete plants.

Cha

pter

1


Course InfoTeacher: Dr. M. A. A. Choudhury

Textbook: 1. Seborg, D. E., Edgar, T.F., Mellichamp, D. A., (2004), Process

Dynamics and Control, 2nd edition, John Wiley

Reference Books:1. Karim, M.N., Riggs, J. B. (2006), Chemical and Bio-Process

Control, 3rd edition, Prentice Hall2. Marlin, T. (2000), Process Control: Designing Processes and

Control Systems for Dynamic Performance, McGraw Hill

Course Website:http://teacher.buet.ac.bd/shoukat/Then click courses Process Control

http://teacher.buet.ac.bd/shoukat/

Cha

pter

1


Cha

pter

1


Authors

Dale SeborgUC, Santabara

Thomas EdgarUT, Austin

Duncan MellichampUC, Santabara

Cha

pter

1


A Career in Process Control- Requires that engineers use all of their chemical

engineering training (i.e., provides an excellent technical profession that can last an entire career)

- Allows engineers to work on projects that can result in significant savings for their companies (i.e., provides good visibility within a company)

- Provides professional mobility. - There is a shortage of experienced process control

engineers.- Is a well paid technical profession for chemical

engineers.

Cha

pter

1


What is a Process?

Process:- A Heat Exchanger (heating/cooling) - A Chemical/Biological Reactor (make

petrochemicals or recombinant drugs)- A Separator (Distillation column or a

chromatographic column for separating proteins)

- A Feed or holding tank- Human body- A Car- A Computer Drive

Cha

pter

1


Chemical Process Industries (CPI)

Hydrocarbon fuelsChemical productsPulp and paper productsAgrochemicalsMan-made fibers

Cha

pter

1


Bio-Process Industries

Use micro-organisms to produce useful productsPharmaceutical industryEthanol from grain industry

Cha

pter

1


Cha

pter

1


Cha

pter

1


Gas stream Gas stream

Empty vessel

P

CONTROL

Cha

pter

1


Adjusting valves: Do you believe in automation?

Do we run around the plant to adjust the valves when required?

Process pictures courtesy of Petro-Canada Products

Cha

pter

1


Adjusting valves: Do you believe in automation?

Central control room • Overview of entire process

• Make immediate adjustment anywhere

• Safe location

• History of past operation

Process pictures courtesy of Petro-Canada Products

Cha

pter

1


What is Process Control?

Process

Inputs(cause)

Outputs(effects)

Output:“off-specification”Consequence:Less profit!

Cha

pter

1


Benefits of Improved Control

Time

Impu

rity

Con

cent

ratio

n Limit

Time

Impu

rity

Con

cent

ratio

n LimitOld Controller

New Controller

Time

Impu

rity

Con

cent

ratio

n Limit

Improved Performance

Cha

pter

1


Objectives of Process Control– Maintain a stable process operation– Appropriate instruments/sensors are to be

implemented to operate under “fail/safe” conditions.

– Make sure no “disturbances” affect the process output(s).

– Make sure when we make desired changes (set point) to the process, it does achieve the desired goal.

– Make sure the process always remain within a “tight” specification.

– Maximize the profitability of the plant

Cha

pter

1


Process Dynamicsa) Refers to unsteady-state or transient behavior.b) Steady-state vs. unsteady-state behavior

i. Steady state: variables do not change with timeii. But on what scale? e.g., noisy measurement

c) ChE curriculum emphasizes steady-state or equilibrium situations:i. Examples: ChE 111, 201, 203, 405

d) 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)

Cha

pter

1


e) 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 ControlObjective: to maintain or operate a process at the desired operating conditions safely and efficiently, while satisfying environmental and product quality requirements.a) Large scale, continuous processes:

i. Oil refinery, ethylene plant, pulp mill

ii. Typically, 1000 – 5000 process variables are measured.

1. Most of these variables are also controlled.

Cha

pter

1


iii. Examples: flow rate, T, P, liquid level, composition

iv. Sampling rates:

1. Process variables: A few seconds to minutes

2. Quality variables: once per 8 hr shift, daily, or weekly

b) Manipulated variables

i. We implement “process control” by manipulating process variables, usually flow rates.

1. Examples: feed rate, cooling rate, product flow rate, etc.

ii. Typically, several thousand manipulated variables in a large continuous plant

Process Control (cont’d.)

Cha

pter

1


c) Batch plants:

i. Smaller plants in most industries

1. Exception: microelectronics (200 – 300 processing steps).

ii. But still large numbers of measured variables.

d) Question: How do we control processes?

i. We will consider an illustrative example.

Process Control (cont’d.)

Cha

pter

1


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

Cha

pter

1


Assumptions:

1. w1 is constant

2. x2 = constant = 1 (stream 2 is pure A)

3. Perfect mixing in the tankControl 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.

Terminology:

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

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

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

Cha

pter

1


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= SPx x=

2 1x = 2w

12 1 (1-3)

1SP

SP

x xw wx−

=−

Cha

pter

1


• 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?

xSPx

Control Question. Suppose that the inlet concentration x1changes with time. How can we ensure that x remains at or near the set point ?

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

SPx

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

Cha

pter

1


• Manual control vs. automatic control

• 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−

Cha

pter

1


Cha

pter

1


Method 2. Measure x1 and adjust w2.

• Thus, if x1 is greater than , we would decrease w2 so that

• One approach: Consider Eq. (1-3) and replace and with x1(t) and w2(t) to get a control law:

1x2 2;w w<

1x 2w

( ) ( )12 1 (1-5)

1SP

SP

x x tw t w

x−

=−

Cha

pter

1


Cha

pter

1


• 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.

Cha

pter

1


Cha

pter

1


1.2 Classification of Control Strategies

Method Measured Variable

Manipulated Variable

Category

1 x w2 FBa

2 x1 w2 FF

3 x1 and x w2 FF/FB

4 - - Design change

Table. 1.1 Control Strategies for the Blending System

Feedback Control:• Distinguishing feature: measure the controlled variable

Cha

pter

1


• 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…

Cha

pter

1


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.

Cha

pter

1

Dr. M. A. A. Shoukat Choudhury 36pump

L

valve

sensor

pump

valve

The key elements and principles of a feedback loop – Cause and Effect

Exercise: The key elements and principles of a feedback loop

What is being measured?

Is this a valid feedback control loop?

Cha

pter

1

Dr. M. A. A. Shoukat Choudhury 37pump

F

valve

sensor

pump

valve


Exercise: You want to control the level, but you can only measure the flow in. What is your strategy? Are you using feedback?

Cha

pter

1


Gas stream Gas stream

Empty vessel

P





Cha

pter

1


v1

Hot Oil

v2

v3

L1

v7

v5 v6

Hot Oil

F1 T1 T3

T2

F2

T4T5

F3 T6

T8

F4

L2

v8

T7

P1F5

F6T9

v4





Cha

pter

1


v1

Hot Oil

v2

v3

L1

v7

v5 v6

Hot Oil

F1 T1 T3

T2

F2

T4T5

F3 T6

T8

F4

L2

v8

T7

P1F5

F6T9

v4

2. The key elements and principles of a feedback loop – Cause and Effect




Cha

pter

1




Hot process fluid into shell

Cooling water into tubes We want to

control the hot outlet

temperature.

Add a sensor and a valve to make this possible.

Cha

pter

1




Hot process fluid into shellCooling water

into tubes

Add a sensor and a valve to make this possible.

TC

We want to control the hot

outlet temperature.

Cha

pter

1


Figure 1.7 Hierarchy of process control activities.

Cha

pter

1


Figure 1.9 Major steps in control system development

Cha

pter

1


Acknowledgement

1. Prof. Nazmul Karim2. Prof. T. Marlinfor providing some of the slides

Documents

ChE 403 Process Control