ERT 422/4 Control system instrumentation MISS. RAHIMAH BINTI OTHMAN (Email: [email protected])

ERT 422/4ERT 422/4

Control system Control system instrumentationinstrumentation

MISS. RAHIMAH BINTI OTHMAN(Email: [email protected])

COURSE OUTCOMESCOURSE OUTCOMES

COAPPLY basic concepts of process dynamics and process control in bioprocess plant system, IDENTIFY the needs of control system and JUSTIFY the types of controller required in the selected plant processes. INTRODUCE a case study as a reference.

Basic concepts of process dynamics and process control in bioprocess plant system.

The needs of control system.

Types of controller required in the selected plant processes.

A case study.

OUTLINESOUTLINES

CONTROL SYSTEM

Process Control

Process Dynamics

Refers to unsteady-state (or transient) process behavior.

1. Process control is the method by which the input flow of processing plants is automatically controlled and regulated by various output sensor measurements.

2. Process control can also describe the method of keeping processes within specified boundaries and minimising variation within a process.

BASIC CONCEPTS OF PROCESS DYNAMICS

1. Safety - industrial plants operate safely so as to promote the well-being of people and equipment within the plant and in the nearby communities.

2. Environmental Regulations - Industrial plants must comply with environmental regulations concerning the discharge of gases, liquids, and solids beyond the plant boundaries.

3. Product Specifications and Production Rate. In order to be profitable, a plant must make products that meet specifications concerning product quality and production rate.

Ch

apte

r 10 4. Economic Plant Operation - the plant operation

over long periods of time must be profitable. Thus, the control objectives must be consistent with the economic objectives.

5. Stable Plant Operation. The control system should facilitate smooth, stable plant operation without excessive oscillation in key process variables. Thus, it is desirable to have smooth, rapid set-point changes and rapid recovery from plant disturbances such as changes in feed composition.

Why we need the control system? –cont’

BASIC CONCEPTS OF PROCESS DYNAMICS

Justification of Process 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

TYPICAL BIOLOGICAL

PROCESS

1. Raw materials (eg. Media) preparation.

2. Preparation of the fermentation

inoculum (microbial cells).

3. Sterilization of the process.

4. Combine the media and the microbial cell

in the bioreactor (inoculation).

5. Implement the fermentation step.

6. Product recovery from

the fermentation broth.

7. Preparation the product

(packaging).

1. In each of these steps, certain process conditions need to be maintained for acceptable operation, and this is accomplished by process control techniques.

2. Fermentation process needs to be maintained for acceptable operation.

3. With modern technology, bioprocess system go

through a systematic events such as sterilization, filling a vessel, maintaining T, pH, DO concentration, emptying vessel & washing vessel.

THE NEEDS OF CONTROL SYSTEM

Summary;

STEPS IN CONTROL SYSTEM DESIGN

The design procedure consists of three main steps:

1. Select controlled, manipulated, and measured variables.

2. Choose the control strategy and the control structure

3. Specify controller settings & tuning

TYPES OF CONTROLLER IN PROCESS PLANT

Controlled Variables (CV) - these are the variables which quantify the performance or quality of the final product, which are also called output variables (Set point).

Manipulated Variables (MV) - these input variables are adjusted dynamically to keep the controlled variables at their set-points.

Disturbance Variables (DV) - 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).

Setpoint – the desired o specified value for the CV. Sensor – the device that measures a process variable. Final Control Element – the system that changes the level of

the MV. The final control element usually involves a control valve and associated equipment or a variable speed pump.

Controller – a unit which adjusts the MV level to keep the CV at or near its setpoint.

CONTROL TERMINOLOGY

Formulate control

objectives

Management objectives

Information from existing plants (if

available)

Develop process model

Physical and chemical principles

Process control theory

Experience with existing plants (if available)

Select control

hardware and

software

Vendor information

Install control system

Adjust controller settings

FINAL CONTROL SYSTEM

Devise control strategy

Plant data (if available)

Computer simulation

= Engineering activity

= Information baseNOTE: MAJOR STEPS IN

CONTROL SYSTEM DEVELOPMENT

Feedback Control

TYPE OF PROCESS CONTROL LOOP

Feedforward-plus-Feedback Control

DESIRED OUTPUT

Feedforward Control

Ratio Control

Split Range Control

Cascade Control

Differential Control

BACKGROUND Normally a chemical or biochemical process has numerous inputs and many outputs. Consider the diagram below:

The objective of a control system is to keep the cv’s at their desired values (or setpoints). This is achieved by manipulating the mv’s using a control algorithm.

Feedback Control



DESIRED OUTPUT

Feedforward Control

Ratio Control

Split Range Control

Cascade 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.

FEEDBACK CONTROL

FEEDBACK CONTROL SYSTEM; EXAMPLE

A basic feedback control system is shown in Figure (1).

The objective is to control the temperature of the outlet stream of the shell and tube heat exchanger.

The temperature is CV.

The MV is coolant flow.

Typical DV’s would include inlet temperature, inlet flow, ambient temperature, etc.

FEEDBACK CONTROL SYSTEM; EXAMPLE (cont’)

If the CV is not at setpoint then the objective of the controller is to adjust the MV to ensure that the desired level of operation is obtained. It is easier (believe it or not) to visualise the control system in terms of a block diagram. A possible block diagram for the feedback control system is;

Block Diagram For The Feedback Control System

Note that the feedback controller is ‘driven’ by the error between the actual process output and the setpoint. Generally, the feedback controller is of the Proportional-Integral- Derivative (PID) type.

* Notation:• w1, w2 and w are mass flow rates

• x1, x2 and x are mass fractions of component A

FEEDBACK CONTROL SYSTEM; (Example: Blending System)

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.

Terminology:• Controlled variable (or “output variable”): x• Manipulated variable (or “input variable”): w2

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

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

FEEDBACK CONTROL SYSTEM; (Example: Blending System)

FEEDBACK CONTROL SYSTEM; Advantages & Disadvantages

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 x differs from xsp.

Very oscillatory responses, or even instability…

Feedback Control



DESIRED OUTPUT

Feedforward Control

Ratio Control

Split Range Control

Cascade Control


FEEDFORWARD CONTROL

A feedforward control law is used to compensate for the effect that measured DV’s may have on the CV.

The basic idea is to measure a disturbance directly and take control action to eliminate its impact on the process output.

How well the scheme will work depends on the accuracy of the process and disturbance models used to describe the system dynamics.

Feedforward control actually offers the potential for perfect control.

However, because of Plant Model Mismatch (PMM) and unmeasured / unknown disturbances this is rarely achieved in practice.

Consequently, feedforward control is normally used in conjunction with feedback control.

The feedback controller is used to compensate for any model errors, unmeasured disturbances etc. and ensure offset free control.

FEEDFORWARD CONTROL SYSTEM; EXAMPLE

The objective is to maintain the temperature of the reaction mass at

the desired value when subjected to changes in inlet concentration

(Cin) and temperature (Tin).

CV is reactor liquid temperatureMV is the coolant flowrate to the heat exchanger DV’s are inlet concentration and inlet stream temperature. The feedforward control loop may be configured as follows;


Here, 'FF' represents the feedforward control algorithm, 'CT' and 'TT'

are symbols used to describe the

composition and the temperature transmitters.

So, the disturbances are measured and passed to a 'FF' device that calculates the necessary coolant flowrate to compensate for any CV moves when the measured DV deviates from it's nominal value.



Feedforward control: a block diagram description;

Gp(s) is a symbol used to represent the process dynamics. This is the relationship between the coolant flow (the MV) and the temperature (the CV). This could be a 1st order plus dead-time transfer function.

Gd(s) is a symbol used to describe the mathematical relationship between inlet concentration and reactor temperature.

The feedforward controller calculates the appropriate MV to ensure the CV remains at SP.

* Notation:• w1, w2 and w are mass flow rates

• x1, x2 and x are mass fractions of component A

FEEDFORWARD CONTROL SYSTEM; (Example: Blending System) – Feedforward system

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.

Terminology:• Controlled variable (or “output variable”): x• Manipulated variable (or “input variable”): w2

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

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

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

1x

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.

FEEDFORWARD CONTROL SYSTEM; Advantages & Disadvantages

Distinguishing feature: measure a disturbance variable

Advantage:Correct for disturbance before it upsets the

process.

Disadvantages:Must be able to measure the disturbance.No corrective action for unmeasured

disturbances.

Feedback Control



DESIRED OUTPUT

Feedforward Control

Ratio Control

Split Range Control

Cascade Control



Because of the difficulty of accounting for every possible load disturbance in a feedforward system, this system are often combined with feedback systems.

Controller with summing functions are used in these combined systems to total the input from both the feedforward loop and the feedback loop, and send a unified signal to the final control element.

LCV-100

FT

FC

Y

Steam

TT

Process variable need to be controlled =

TemperatureFluid in

Fluid out

TC

FEEDFORWARD-PLUS-FEEDBACK CONTROL

Example Figure below shows compressed gas vessel. Process variable that need to be controlled is pressure where the vessel should maintain pressure at 60 psi. By using pressure controlled through both the gas flow measurement into the vessel and vessel pressure itself, draw a feedforward-plus-feedback control loop system.

V-100FT Process variable need

to be controlled = Pressure

FC

Y

PT

PIC

FEEDFORWARD-PLUS-FEEDBACK CONTROL

Exercise

Figure below shows the boiler system that used to supply hot steam to a turbine. This system need to supply 100 psi hot steam to the turbine where the PCV-100 will be opened when the pressure reached that desired pressure. With using pressure control through temperature and pressure measurement in the boiler, draw a feedforward-plus-feedback control loop system.

BOILERProcess variable need

to be controlled = Pressure

Water Hot steam

PIPING AND INSTRUMENTATION DIAGRAM (P&ID)

Answer 2

BOILER

TT

Process variable need to be controlled =

Pressure

TIC

Y

Water Hot steam

PIC

Figure below shows the boiler system that used to supply hot steam to a turbine. This system need to supply 100 psi hot steam to the turbine where the PCV-100 will be opened when the pressure reached that desired pressure. With using pressure control through temperature and pressure measurement in the boiler, draw a feedforward-plus-feedback control loop system.

PT


Feedback Control



DESIRED OUTPUT

Feedforward Control

Ratio Control

Split Range Control

Cascade Control


CASCADE CONTROL• Cascade control is widely used within the process industries.• Conventional cascade schemes have two distinct features:1.There are two nested feedback control loops. There is a secondary control loop located inside a primary control loop.2.The primary loop controller is used to calculate the setpoint for the inner (secondary) control loop.• Cascade control is used to improve the response of a single feedback strategy. • The idea is similar to that of feedforward control: to take corrective action in response to DV's (which are not necessarily measured) before the CV deviates from setpoint. • The secondary control loop is located so that it recognises the upset condition sooner than the primary loop.

CASCADE CONTROL; EXAMPLE

The normal block diagram representation of a cascade control loop is shown below,

Cascade Control

Cascade Control uses the output of the primary controller to manipulate the set point of the secondary controller as if it were the final control element.

Reasons for cascade control: •Allow faster secondary controller to handle disturbances in the secondary loop. •Allow secondary controller to handle non-linear valve and other final control element problems. •Allow operator to directly control secondary loop during certain modes of operation (such as startup).

CASCADE CONTROL

Cascade Control (cont…) Requirements for cascade

control: - Secondary loop process dynamics must be at least four times as fast as primary loop process dynamics. - Secondary loop must have influence over the primary loop. - Secondary loop must be measured and controllable.

CASCADE CONTROL


Cascade control of a CSTR

• Figure (2) shows a conventional feedback control scheme on a CSTR.

• Here temperature is being controlled using coolant flowrate to a cooling jacket.


Figure(3) shows a cascade control scheme on the same CSTR.

The idea of the cascade strategy is to improve the control of temperature specifically with regard to changes in coolant temperature.

Thus the inner loop controller takes control action to mitigate the effect of coolant temperature disturbances on the temperature of the reaction mixture.

Feedback Control



DESIRED OUTPUT

Feedforward Control

Ratio Control

Split Range Control

Cascade Control


Ratio Control

The objective of a ratio control scheme is to keep the ratio of two variables at a specified value.

Thus, the ratio (R) of two variables (A and B);

Is controlled rather than controlling the individual variables.

Typical ratio control schemes include:• Maintaining the reflux ratio for a distillation column.• Maintaining the stoichiometric ratio of reactants to a reactor.• Maintaining air/fuel ratio to a furnace.

Ratio Control

Implementation: method I

•The flowrate of the two streams is measured and their ratio calculated using a 'divider' (just a piece of extra electronics).

•The output of the divider is sent to the ratio controller (which is actually a standard PI controller).

•The controller compares the actual ratio with that of the desired ratio and computes any necessary change in the manipulated variable.

Ratio ControlImplementation: method II

• Here one stream is under standard feedback control.

• The flow of the second stream is measured and sent to a 'multiplier' (again just a piece of extra electronics) which multiplies the signal by the desired ratio yielding the setpoint for the feedback control law.

Ratio Control

Ratio control is used to ensure that two or more flows are kept at the same ratio even if the flows are changing.

Water

Acid

2 part of water1 part

of acid

FTFT

FFFIC


Ratio Control (cont…)

Application: - Blending two or more flows to produce a mixture with specified composition.

- Blending two or more flows to produce a mixture with

specified physical properties. - Maintaining correct air and fuel mixture to

combustion.

Water

Acid

2 part of water

1 part of acid

FTFT

FFFIC


Ratio Control (Auto Adjusted)

- If the physical characteristic of the mixed flow is measured, a PID controller can be used to manipulate the ratio value.

- For example, a measurement of the density, gasoline octane rating, color, or other characteristic could be used to control that characteristic by manipulating the ratio.

Water

Acid

2 part of water

1 part of acid

FTFT

FF

FIC

AIC

Remote Ratio Adjustment

Remote Set Point

Physical Property Measurement


Feedback Control



DESIRED OUTPUT

Feedforward Control

Ratio Control

Split Range Control

Cascade Control


Split Range Control

FC

FTValve A

Valve B


Split Range Control

TK-100

(pH adjustment tank)

TK-101

(acid feed tank)

The diagram shows pH adjustment; part of waste water treatment process. The process shall maintained at pH 6. When the process liquid states below pH 6, CV-102 will be opened to dosing NaOH to the tank TK-100. When the process liquid states above pH 6, CV-101 will be operated to dosing HCl.

TK-102

(base feed tank)

CV-101

CV-102

pHT 1

pHIC


Feedback Control



DESIRED OUTPUT

Feedforward Control

Ratio Control

Split Range Control

Cascade Control


Meaning

Systems Theory: In differential control, control action is based on the change (derivative) of the control error. The control error is defined as the difference between the set point and the process output.

Explanation:Derivative control is used to provide anticipative action. This is done by using the current change (derivative) of the control error to estimate the error some time ahead and act anticipatively on this estimation.



Flow ControllerLevel Control in a tankAerobic Fermentation Process

Fed-Batch Bioreactor

INDUSTRIAL PROCESS CONTROL

EXAMPLES

Control objective: to maintain the desired flow rate The setpoint: desired flow rate Controlled variable (CV): the outlet flow rate Manipulated variable (MV): the inlet flow rate of the

process stream Disturbances variable (DV): changes in the

upstream pressure for the process stream Sensor: combination of an orifice plate and a device

that measure a pressure drop across the orifice, which directly related to the flow rate.

Final control element: the control valve in the line Controller : flow controller (FC) – compares the

measured flow rate with the specified flow rate (flow setpoint) and opens/closes the control valve accordingly.

Flow control loop

Control objective: to maintain the level in the tank The setpoint: desired level in the tank Controlled variable (CV): the level in the tank Manipulated variable (MV): exit flow from the

tank Disturbances variable (DV): changes in the inlet

flow rate Sensor: level indicator on the tank (LT) Final control element: the control valve on the

outflow line Controller – level controller (LC) – compares the

measured level with the setpoint for the level in the tank and makes a change to the control valve on the exit flow from the tank.

LEVEL CONTROL IN A TANK

Control Diagram Of A Tank With A Level Controller

Control objective: to maintain a specified dissolved oxygen (DO) conc. in the fermentation reactor so that the cells in the process have adequate oxygen levels.

The setpoint: desired DO concentration Controlled variable (CV): the DO conc. in the fermentor Manipulated variable (MV): air flow rate to the fermentor Disturbances variable (DV): changes in the rpm of the

mixer impeller Sensor: DO sensor-transmitter (AT) Controller – DO controller (AC) – compares the measured

DO with the setpoint value and sets the air flow rate to the fermentor.

Final control element: variable speed air compressor

AEROBIC FERMENTATION PROCESS

Schematic of a Bio-reactor with a Dissolved Oxygen Controller

Control obj: to maintain a specified cell concentration in the bioreactor.

What is the CV, MV, DV, sensor & final control element?

FED-BATCH BIOREACTOR

Control objective: to maintain a specified cell concentration in the bioreactor.

The setpoint: desired cell concentration Controlled variable (CV): the cell conc. in the bioreactor Manipulated variable (MV): the feed rate of glucose and

nutrients mixture to the bioreactor. Disturbances variable (DV): changes in the

concentration of glucose in the feed. Sensor: turbidity meter (AT) which provides a

measurement that correlates with the cell conc. in the broth.

Controller – controller (AC) – compares the measured cell conc. with the setpoint value and sets the glucose feed rate.

Final control element: variable speed pump

Process variables can be classified into input variables and output variables.

Definition of input variables: physical variables that affect the output variables.

Input variables can be divided into manipulated variables and disturbance variables.

Manipulated variables are typically flow rates Common disturbance variables include the feed

conditions to a process and the ambient temperature. The output variables are process variables that typically

are associated with exit streams (e.g. compositions, temperatures, levels and flow rates).

Selection of Controlled Variables

Guideline 1.

All variables that are not self-regulating must be controlled.

-Non self-regulating variable: an output variable that exhibits an unbounded response after a sustained disturbance.

-must be controlled in order for control process to be stable.

Guideline 2.

Choose output variables that must be kept within equipment and operating constraints (e.g., temperatures, pressures, and compositions).

Guideline 3.

Select output variables that are a direct measure of product quality (e.g., composition, refractive index) or that strongly affect it (e.g., temperature or pressure).

Guideline 4.

Choose output variables that seriously interact with other controlled variables.

Guideline 5.

Choose output variables that have favorable dynamic and static characteristics.

Guideline 6.

Select inputs that have large effects on controlled variables.

Guideline 7.

Choose inputs that rapidly affect the controlled variables.

Guideline 8.

The manipulated variables should affect the controlled variables directly rather than indirectly.

Guideline 9.

Avoid recycling of disturbances.

Selection of Manipulated Variables

Guideline 10.

Reliable, accurate measurements are essential for good control.

Guideline 11.

Select measurement points that have an adequate degree of sensitivity.

Guideline 12.

Select measurement points that minimize time delays and time constants

Selection of Measured Variables

Exercise 1

V-100

PCV-100

PCV-101

LT 1

TK-100

LIC 1

FC

FCWhere LT 1 and LIC 1 to control PCV-100 (failure close);

PCV-100 close when level reached L 3

PCV-100 open when level below L3

L1

L2

L3

LT 2 LIC 2

Where LT 2 and LIC 2 to control PCV-101 (failure close);

PCV-101 close when level reached L5

PCV-101 open when level below L5

L4

L5

QUIZ 1

Answer

V-100

PRV-100

PRV-101

LT 1

TK-100

LIC 1

FC

FC Where LT 1 and LIC 1 to control PRV-100 (failure close);

PRV-100 close when level reached L 3

PRV-100 open when level below L3

L1

L2

L3

LT 2 LIC 2

Where LT 1 and LIC 1 to control PRV-101 (failure close);

PRV-101 close when level reached L5

PRV-101 open when level below L5

L4

L5

LT 1

LIC 1

LT 2

LIC 2


Prepared by, MISS RAHIMAH OTHMAN

Documents

ERT 422/4 Control system instrumentation MISS. RAHIMAH BINTI OTHMAN (Email: [email protected])