Chapter 4 Process Control and Instrumentation

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CHAPTER 4

PROCESS CONTROL AND INSTRUMENTATION

4.1 INTRODUCTION

Control is the most important part for each plant. All processes are subject to

change in operating conditions, compositions and physical properties of the

streams such as temperature and flow rate. There will be some effect if those entire

parameters run out of control. In order to minimize the those effect that could result

disturbances, chemical plants are implemented with special type of instrumentation

and automatic control devices that can be easily controlled from control room where

all parameters are shown in control panel. In producing the product from the raw

material, plant usually consumes and generates heat and sometime undesired

chemical reaction that can cause damages and even fatality if it runs out of control.

With controlling and considering all parameters and variables, the unexpected

incident and accident can be avoided.

There are many type of control mechanism. All of them can be predicted

using mathematical calculation with several integration and formulas. In controlling

equipments, time respond also can give some effect to the reaction of the controller

if the variables change. Accordingly, feed forward control is supplemented in most

instances with feedback. In a well-designed system, typically 90% of the corrective

action is provide by feed forward and 10% by feedback with the result thatintegrated error is reduced by factor 10%. The main types parameter controlled are

flow, temperature, and pressure and level controller.



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4.2 OBJECTIVES OF CONTROL

The primary objectives of process control are to maintain a process at the desired

operating conditions, safety and efficiently, while satisfying environmental and

product quality requirement. There are also specific scheme when designing the

control system which are:

4.2.1 Safe Operation

i. To keep the process variables within known safety operating limits and

within allowable limits

ii. To detect dangerous situations as they develop and to provide alarms

and automatic shut down system.

iii. To provide interlocks and alarms to prevent unsafe operation.

4.2.2 Production Specification

i. To achieve the design product output.

ii. To produce the desired quality of the final product

iii. To keep the product composition within the specified quality standards.

4.2.3 Economics

i. To operate at the lowest production cost

ii. The operation of the plant must conform to the market condition, which

is availability of raw materials and demand of the final product

4.2.4 Environmental Constraint

i. Variable controlled must not exceed the allowable limits set by laws

4.2.5 Operational Constraint

i. Various type of equipment used in chemical plant have constraints

inherent to their operation. Such constraints should be satisfied

throughout the operation of the plant.ii. Control system is needed to satisfy all these operational constraints.



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In a typical chemical processing plant these objectives are achieved by

combination of automatic control, manual monitoring and laboratory analysis.

4.3 CLASSIFICATION OF PROCESS VARIABLES

Process output variables are those that give information about the state of the

process. They can be subdivided into two subgroups (manipulated variables and

disturbance variables). Manipulated variables are can be adjusted freely by

operator or a control mechanism. Disturbance variables are subject to the external

environment and thus cannot be controlled. In a control system, manipulated

variables cause changes to controlled variables.

There are several individual process variables that need to be considered. They

are:

i. Composition - The application of continuously measuring in line analyzers is

highly desirable

ii. Flow Rate - The flow measurement is transmitted to a controller which then

adjusts the opening of a control valve so as to maintain the desired

condition.

iii. Pressure - For any system where vapour is present, any accumulation of

moles of vapour will cause the pressure to rise and any depletion of vapour

moles cause the pressure to fall. This means that pressure control can be

achieved by controlling the rate of exit vapour from a process system.

iv. Temperature - Temperature is typically controlled by varying the flow of a

stream other than the one in which the temperature is being measured.

v. Level - The flow of any liquid leaving a vessel must be controlled to assure

that there is no loss of the liquid seal between the vessel and the next

destination of the fluid.vi. Flow Ratio - The primary controller will go to a ratio control device which

adjust the set point of the other controller

4.4 TYPES OF CONTROL

There is several control approach that has been applied in the production plant, the

basic concepts of these control are stated below.



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4.4.1 Feedback Control

The feedback control system function is to bring the measured quantity to its

required value or set point. The feedback control system uses direct measurements

of the controlled variables to adjust the values of the manipulated variables. The

main advantage of the feedback control is the corrective actions occur as soon as

the controlled variable deviates from the set point regardless of its source and the

type of disturbances. Minimal knowledge of the process is sufficient to set up this

type of control.

It is also both versatile and robust which means that if the process

condition changes, retuning will still give a satisfactory result. However, this type of

control also has certain disadvantages, which are, there is no corrective action

taken until after a deviation in the controlled variable occurs. In addition, it does not

provide a predictive control action to compensate for the effects of known or

measurable disturbances. If the process encounters large and frequent

disturbance, the action of the controller will be such that the process will operate

continually in a transient state and never attain the desired steady state.

4.4.2 Feedforward Control

The basic idea of the feedforward control is to measure the important load variables

and take the corrective actions before they upset the process. However there are

disadvantages of this control technique as the load disturbances must be measured

online and in many applications this is not feasible. For this technique to be

effective, we need to have some basic knowledge about the process to construct a

process model. Ideal feedforward control theoretically is capable of achieving

perfect control but in reality it may not be physically realizable. There are timeswhen the combination of both feedback and feedforward control strategies are

required such as in the level control.

4.4.3 Cascade Control

Cascade control uses more than one measurement and manipulated variables.

This 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. This controller hastwo distinguishing features:



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i. The output signal of the master controller servers as the set point for the

slave controller

ii. The two feedback control loops are nested, with the secondary control loop

(for the slave controller) located inside the primary control loop (for the

master controller)

This controller is widely used in process industries and is particularly useful

when the final control element exhibits nonlinear behaviour. Reasons for cascade

control:

i. Allow faster secondary controller to handle disturbances in the secondary

loop.

ii. Allow secondary controller to handle non-linear valve and other final control

element problems.

iii. Allow operator to directly control secondary loop during certain modes of

operation (such as startup)

4.4.4 Ratio Control

Ratio control is a type of feed forward control and has a wide application in

industries. The objective of the controller is to maintain the ratio of two process

variable at a specified value, for example the ratio of manipulated variables and

disturbances is being controlled rather than controlling each variable.

Typical application of ratio control:

i. To maintain a ratio control of feed flow rate and the steam in the reboiler of

a distillation column.

ii. Maintaining the stoichiometric ratio of reactant to a reactor.

iii. Maintaining the reflux ratio in distillation column.

iv. Hold the ratio of two blended streams, in order to maintain the compositionsof the blending.

v. Hold the ratio of purge stream to the recycle streams.

vi. Maintaining the ratio of the liquid floe rate to the stripper, in order to achieve

desired composition in the exit vapour stream.



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4.5 CONTROL SYSTEM FOR MAJOR EQUIPMENT

4.5.1 Esterification Reactor (R-101)

During normal operation the feed entering the reactors are from mixer (M-101) into

the esterification reactor 1 R-101 respectively. The flowrate out from the reactor 1

by overflow is maintained by using level control.

i. To maintain the reactor temperature at 1100C

ii. To control the steam inlet at 1100C so that the reactor will be operated

under isothermal condition

iii. To maintain the reactor pressure at 0.29bar

iv. To control the flow rate of product as it reach the indicating level in the

vessel by overflow

Cascade control is superior method which most suited for the reactor temperature

control. Here the controlled variables is reactor temperature, whose response is

slow to changes in the heat transfer medium flow that is manipulated variables is

allowed to adjust the set point of a secondary loop, whose response to hot fluid flow

changes is rapid. In this case, the reactor temperature controller varies the set point

of the steam temperature control loop. Temperature controller acts as primary

controller (master loop) and steam temperature controller acts as secondary

controller (slave loop). The purpose of the slave loop is to correct for all outside

disturbances, without allowing them to affect the reaction temperatures. The slave

would notice the resulting upset at the steam inlet and would correct for it before it

had a chance to upset the master. Cascade loops will not function properly if the

master is faster than the slave. The slave controller maintains the steam inlet

temperature.



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Table 4.1: Control System of Esterification Reactor (R-101)

Operation: To provide space for reaction of reactant entered towards product

formation at specific residence time

Controlled

Variable

Measured

variable

Manipulated

Variable Disturbance Control System

Vessel

temperature

Measure the

product

temperature

Flow rate of

steam through

the coil

Change of reactor

temperature

Cascade control

(reactor TC as

master and

steam TC as

slave)

Vessel

pressure

Measure the

reactor

pressure

Vapour outlet

flow rate (S4)

Change of reactor

pressure

Feedback

Flow Measure the

flow of the

reactant

Feed stream

flow rate (S3)

Liquid level inside

the reactor deviate

from the operating

value

Feedback

Level Measure the

level of the

reactor

Outlet stream

flow rate (S5)

Reactant not react

with sufficient

residence time

Feedback

Steam

3

LT

5

4

FT

LC

PT

PC

FT

FC

TC

FC

TT

Figure 4.1: Control System Design for R-101



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4.5.2 Esterification Reactor (R-102)

The purposes of system controller in Esterification Reactor, R-102 are:

i. To control the flow rate in the feed stream S6 which is coming from effluent

of R-101.

ii. To maintain the temperature inside the esterification reactor for operating

under isothermal condition.

iii. To maintain the pressure at inlet S6, and outlet S8 and S7 at 0.29 bar

Table 4.2: Control System of Esterification Reactor (R-102)

Controlled

Variable

Manipulated

VariableDisturbance Control System

Temperature Flow of cooling

water inlet

Change of reactor

temperature

Cascade control

Pressure Vapour outlet flow

rate (S7)

Change of reactor

pressure

Feedback control

Flow Feed stream flow

rate (S6)

Liquid level inside

the reactor deviate

from the operating

value

Feedback control

R-102

PT

FT

TT

Coolant

6

8

7

FC

TC

PC

FC FT

Figure 4.2: Control System Design for R-102



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4.5.3 Distillation Column (T-102)

The main objectives of designing control system on esterification column are:

1. To control level of fluid inside the column base and reflux drum to make

sure it is in an acceptable value.

2. To maintain the ratio of vapor distillate flowrate and reflux flowrate at a

specified value.

3. To control the distillate flow rate of the column that can affect the plant

performance.

4. To maintain a smooth operation in which charges are not made so

rapidly to cause the operation become unstable.

5. To control or maintain the purity of distillate and bottom products.

The most important variables need to be controlled on a distillation column

(T-102) is the concentration of the distillate and bottom product at stream 9 and

stream 10 respectively. This can be done by manipulating the valves that controls

the flowrate of the reflux stream and steam flowrate into the reboiler. The set point

of the flow controller is 2292 kg/h. If the flowrate of the reflux stream is lower or

higher than the set point, the transmitter will then transmit signal about thedisturbance to the controller. The controller will decide the action whether to open

or to close the valve.

Pressure is controlled using the flow rate of cooling water to the condenser.

The feedback controller is driven by the error between the actual process output

which is the pressure of top column and the set point that is 0.04 bars. The

pressure controller takes error signal from the transmitter and decides what action

should be taken by valve to compensate for and hence remove the error.

Since the vapor distillate is the feed to downstream unit the distillate flow

must be maintained at an acceptable value otherwise the variability in flowrate can

significantly disturb this unit and can result in poor plantwide control performance.

In order to maintain the ratio of vapor distillate flowrate and reflux flowrate at a

specified value, the set point of the flow controller controlling the flow of distillate

(controlled flow) as a function of the desired ratio and the measured flow of reflux

(wild flow) need to be manipulated. The ratio is calculated by:



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R =Vapor distillate flowrate

Reflux flowrate

The set point of distillate flow controller is calculated as:

Vapor distillate flowrate = R × Reflux flowrate

Table 4.3: Control System of Distillation Column (T-102)

Operation: To separate the components fraction of inlet stream of the distillation

column into two outlet stream with desired components fraction

Measured

variable

Manipulated

VariableDisturbance

Control

SystemSet Point

Pressure in the

top column

Flow rate of

cooling water

to the

condenser

Variations of

pressure in the

column

Feedback

control

Pressure:

0.04 bars

Column base

level

Bottom

flowrate

(stream 10) of

column

Variations of

bottom flowrate

of column

Feedback

control

85% of

flooding point

or drying of

the column

Reflux drums

level

Reflux flowrate

to the column

Variation of reflux

flowrate

Feedback

control

85% of

flooding point

or drying of

the column

Distillate

flowrate

Distillate

flowrate

(stream 9)

Variation of reflux

flowrate

Feedback

control

Flow rate:

2292 kg/h

Column base

temperature

Flowrate of

steam into the

reboiler

Variation of

steam flowrate

into the reboiler

Cascade

control

Temperature:

120oC



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Figure 4.3: Control System Design for T-102



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4.5.4 Finishing Column (T-103)

This column is installing to separate the main product, which is 2-EHA from some

other component, which about 83% pure. It was the finale process before the

product is send to the storage tank. It is operate under vacuum condition, thus the

most important variable to be taken care is the pressure drop. Slightly difference of

the pressure drop can make some disturbance to the main product, especially. In

order to prevent any flooding occur, the level and flow controller were also installed.

The purposes of the controller install are to:

i. Maintain the pressure drop at around 5 kPa.

ii. Maintain the liquid level in the column to prevent any flooding (both inlet flow

and level controller inside the column).

iii. Maintain the phase of the top outlet column (stream 15) in liquid phase.

Table 4.4: Control System of Finishing column (T-103)

Operation: To purify the product and meet the specification

Control

Variables

Manipulated

VariableDisturbances Type of Controller

Flow Inlet stream (stream

12 and 9)

The liquid level

entering the column

Ratio control

Pressure Flow rate of cooling

water to the

condenser

Variations of pressure

in the column

Feedback control

Column base level Bottom flowrate

(stream 13) of column

Variations of bottom

flowrate of column

Feedback control

Reflux drums level Reflux flowrate to the

column

Variation of reflux

flowrate

Feedback control



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Figure 4.4: Control System Design for T-103



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4.5.5 Heat Exchangers

4.5.5.1 Description of Control Mechanism for Heat Exchangers

In this plant, there are 3 types heat exchanger which are heater, condenser and

cooler. The principle is the same for all heat exchanger. The principle is the outlet

stream temperature will be compared with the set point. If the temperature unable

to achieve desired temperature, hence the controller will take action by

manipulating the flow rate of cooling agent stream.

4.5.5.2 Heater (E-101 and E-102)

Table 4.5: Control System of Heater (E-101)

Objective (E-101) : To increase temperature from 89.53 oC to 110 oC

Controlled

VariableDisturbance

Measured

Variable

Controller

ConfigurationSet point

Outlet

temperature

of stream

(S3)

Change in

inlet

temperature

of feed

stream (S2)

Control the flow

rate of steam at

inlet steam

stream

Cascade

Stream (S3)

= 110oC

Alarm

High: 112

o

C

E-101

TT

Steam inlet

Steam outlet

FC

2 3

TC

FT

Figure 4.5: Control System Design for E-101



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Table 4.6: Control System of Heater (E-102)

Objective ( E-102): To increase temperature from 110 oC to 120 oC

Controlled

VariableDisturbance

Measured

Variable

Controller


Outlet

temperature

of stream

(S6)

Change in

inlet

temperature

of feed

stream

(S5)

Control the flow

rate of steam at

inlet steam

stream

Cascade

Stream (S6)

= 120oC

Alarm

High: 122oC

E-102

TT

Steam inlet

Steam outlet

FC

5 6

TC

FT






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4.5.5.4 Cooler (E-104)

Table 4.8: Control System of Cooler (E-104)

Objective: To reduce temperature from 117.2 oC to 20 oC

To maintain the product stream (S14) in the liquid form

To avoid polymerization occurred which could stop the operation.

Controlled

VariableDisturbance

Measured

Variable

Controller


Outlet

temperature of

stream

(S13)

Change in inlet

temperature of

feed stream

(S14)

Control the

flow rate of

cooling water

at inlet cooling

water stream

Cascade

Stream

(S14) =

20 oC

Alarm

High:

35 oC

FC

E-104

TT

Cooling water

inlet

Cooling water outlet

13 14

TC

FT




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4.6 CONTROL SYSTEM FOR OTHER EQUIPMENT

4.6.1 Mixer (M-101)

M-101 should be operated to maintain the stoichiometric ratio of the reactants to be

fed into reactor (R-101). Before the reactants are fed to M-101, acrylic acid and 2-

Ethylhexanol streams; S1 and S17 respectively are passed through to reduce the

pressure in each stream. Therefore pressure controller is installed at the prescribed

streams. The ratio set point between acrylic acid and 2-Ethylhexanol is 1:2. Hence,

the flow of S17 will be controlled according to the flow of S1.

Table 4.9: Control System of Mixer (M-101)

Controlled

Variable

Manipulated

VariableDisturbance

Control

System

Composition Feed streams of the

mixer flow rate

(S1, S17)

Change in composition Ratio control

F T

F C

1 7

2

1

FTFC

X

Figure 4.9: Control System Design for M-101



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6.6.2 Splitter (S-101)

S-101 is to separate the heavy product from the desired product (2-Ethylhexyl

Acrylate) that coming from the distillation column (T-102). The effluent at stream 11

is send to the incinerator while product at stream 12 is flow through another column

for further process.

Table 4.10: Control System of Splitter (S-101)

Controlled

Variable

Manipulated

VariableDisturbance

Control

System

Composition Effluent streams of

the splitter flow rate

(S12 and S11)

Change in composition Ratio control

10

FT FC

FT

FC

X

11

12

Figure 4.10: Control System Design for S-101



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CONCLUSION

Designing control systems for a whole chemical plant is the definitive goal for a

control designer where the main intentions of designing the control system are for

the safety of the plant, to maintain the production rate and the product quality as

well as to plan the economics so that the plant is operated at its minimum costing.

Temperature, pressure, concentration, level and flowrate are all the

important variables that need to be controlled. All the variables are controlled to

prevent any possible accident that can harm the employees from occurs such as

explosion, fire, leakage, flooding and others.

The operation of a plant must conform to market a condition, which are the

availability of raw materials and the demand of the final products. Furthermore, it

must be as economical as possible in its utilization of raw materials, energy, capital,

and human labour. Thus it is required that the operating condition are controlled at

given optimum levels of minimum operating costs, maximum profit and other

related matters.

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Chapter 4 Process Control and Instrumentation

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