Various control loops for stirred tank reactor, furnaces and distillation columns

1. Various control loops for continuous  stirred tank reactor

Figure: Control loop configuration for the CSTR.

Figure: Control loop configuration for the CSTR.

Figure: Control loop configuration for the CSTR.

Figure: Control loop configuration for the CSTR.

Figure: Control loop configuration for the CSTR.


2. Control of furnaces.

Figure: Control system with three single loops.

Figure: Cascade control system to regulate the temperature of a process stream leaving the furnace.


3. Control of distillation columns.

Figure: Distillation column with six single-loop control systems.

Figure: Distillation column with single-loop and cascade control systems.


Note: Click on the image to get an enlarged view.


Article Source:: Dr. Alexander Badalyan, University of South Australia

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Control loop configurations for chemical process variables

The different control loop configurations for different process variables are given below.Its just the simplest configurations used in control systems.


1. Level control.

Figure: On-off level control loop.

Figure. Control loops for continuous level control.
a - by the control of an inlet stream; b - by the control of an outlet stream;
c - cascade control loop.

Figure:  Level control in evaporator.


2. Pressure control

Figure: Pressure control loop for evaporating unit.
1, 2 - evaporating units; 3 - barometric condenser.

Figure: Pressure control in the distillation column.
1 - distillation column; 2 - reflux column; 3 - reflux tank;


3. Temperature control

Figure: Single loop control system for a stirred tank.

Figure: Single loop control system for heat exchanger.

Figure: Cascade control system for heat exchanger.


4. Flow control

Figure: Flow control by throttling the stream after the centrifugal pump.

Figure:  Flow control using by-pass pipe-line in the case of the piston pump.


5. pH control

Figure: pH control system.


6. Composition control

Figure: Feedback system for composition control.

Figure:Inferential system for composition control.


Note: Click on the image to get an enlarged view.


Article Source:: Dr. Alexander Badalyan, University of South Australia

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Symbols for instrumentation layouts

For Chemical Engineers involved in either design or production activity, it is necessary to be able to distinguish various types of field and indoor instrumentation and control systems used for particular industrial applications. In order to make layouts uniform and to visualise the process control system a set of symbols was introduced. The following symbols were taken from: Australian Standard - AS 1101.6-1989, Graphical Symbols for general engineering. Part 6: Process measurement control functions and instrumentation, Standards Australia, 1989, 52 pp.

Figure :. presents symbols for industrial instrumentation.

Diameter of a circle for the symbol is equal to 10 mm. Meanings of identification letters for these symbols are presented in Table .

To better understand how these symbols are placed in process control and instrumentation flowcharts I decided to give several examples for measuring and control of process variables in a variety of industrial processes in the successive posts.

Table : Meanings of identification letters.

First Letter			Succeeding Letters
Measured or initiating variable		Modifier	Readout or passive function	Output function	Modifier
A	Analysis		Alarm
B	Burner flame		User’s choice	User’s choice	User’s choice
C	Conductivity (electrical)			Control
D	Density (mass), specific gravity	Differential
E	Voltage (e.m.f.)		Primary element
F	Flow rate	Ratio (fraction)
G	Gaging (dimensional)		Glass
H	Hand (manually initiated)				High
I	Current (electrical)		Indicate
J	Power	Scan
K	Time or time schedule			Control station
L	Level		Light		low
M	Moisture or humidity				Middle or intermediate
N	User’s choice		User’s choice	User’s choice	User’s choice
O	User’s choice		Orifice (restriction
P	Pressure or vacuum		Point (test connection)
Q	Quantity or event	Integrate or totalise
R	Radioactivity		Record or print
S	Speed or frequency	Safety		Switch
T	Temperature			Transmit
U	Multivariable		Multifunction	Multifunction	Multifunction
V	Viscosity			Valve, damper
W	Weight or force
X	Unclassified		Unclassified	Unclassified	Unclassified
Y	User’s choice			Relay or compute
Z	Position			Drive, actuate

                                                        Article Source:: Dr. Alexander Badalyan, University of South Australia

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Process Control Fundamentals

This PDF file Process Control Fundamentals shown below is published by PAControl.com. As I have gone through this e-book I found it very useful so as to understand the basics of process control.


It covers the following topics: 

• Introduction
• The Importance of Process Control
• Control Theory Basics
• Components of Control Loops and ISA Symbology
• Controller Algorithms and Tuning
• Process Control Loops

Process Control Fundamentals

 

This PDF is uploaded  because I found it meanigless or difficult for readers to go through a number of post or a lengthy post to get the complete fundamentals. This e-book that I have provided here can be downloaded easily by a simple click and you can refer on your own convenience. A great thanks to PAControl.com  to publish this e-book. Really helpful.  

Article Source: PAControl.com

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Elements of a Process Control System

INTRODUCTION


-The term automatic process control came into wide use when people learned to adapt automatic regulatory procedures to manufacture products or process material more efficiently. Such procedures are called automatic because no human (manual) intervention is required to regulate them.


-All process systems consist of three main factors or terms: the manipulated variables, disturbances, and the controlled variables. Typical manipulated variables are valve position, motor speed, damper position, or blade pitch. The controlled variables are those conditions, such as temperature, level, position, pressure, pH, density, moisture content, weight, and speed, that must be maintained at some desired value. For each controlled variable there is an associated manipulated variable. The control system must adjust the manipulated variables so the desired value or “set point” of the controlled variable is maintained despite any disturbances.

Elements of a Process Control System

-The following figure illustrates the essential elements of a process control system. In the system shown, a level transmitter (LT), a level controller (LC), and a control valve (LV) are used to control the liquid level in a process tank. The purpose of this control system is to maintain the liquid level at some prescribed height (H) above the bottom of the tank. It is assumed that the rate of flow into the tank is random. The level transmitter is a device that measures the fluid level in the tank and converts it into a useful measurement signal, which is sent to a level controller. The level controller evaluates the measurement, compares it with a desired set point (SP), and produces a series of corrective actions that are sent to the control valve. The valve controls the flow of fluid in the outlet pipe to maintain a level in the tank.

-Thus, a process control system consists of four essential elements: process, measurement, evaluation, and control. A block diagram of these elements is shown in the following figure. The diagram also shows the disturbances that enter or affect the process. If there were no upsets to a process, there would be no need for the control system. The figure also shows the input and output of the process and the set point used for control.

PROCESS

In general, a process consists of an assembly of equipment and material that is related to some manufacturing operation or sequence. In the example presented, the process whose liquid level is placed under control includes such components as a tank, the liquid in the tank, and the flow of liquid into and out of the tank, and the inlet and outlet piping. Any given process can involve many dynamic variables, and it may be desirable to control all of them. In most cases, however, controlling only one variable will be sufficient to control the process to within acceptable limits. One occasionally encounters a multivariable process in which many variables, some interrelated, require regulation.

MEASUREMENT

- To control a dynamic variable in a process, you must have information about the entity or variable itself. This information is obtained by measuring the variable.

- Measurement refers to the conversion of the process variable into an analog or digital signal that can be used by the control system. The device that performs the initial measurement is called a sensor or instrument. Typical measurements are pressure, level, temperature, flow, position, and speed. The result of any measurement is the conversion of a dynamic variable into some proportional information that is required by the other elements in the process control loop or sequence.

EVALUATION

- In the evaluation step of the process control sequence, the measurement value is examined, compared with the desired value or set point, and the amount of corrective action needed to maintain proper control is determined. A device called a controller performs this evaluation. The controller can be a pneumatic, electronic, or mechanical device mounted in a control panel or on the process equipment. It can also be part of a computer control system, in which case the control function is performed by software.

CONTROL

The control element in a control loop is the device that exerts a direct influence on the process or manufacturing sequence. This final control element accepts an input from the controller and transforms it into some proportional operation that is performed on the process. In most cases, this final control element will be a control valve that adjusts the flow of fluid in a process. Devices such as electrical motors, pumps, and dampers are also used as control elements.

Article Source:  www.science-hamza.blogspot.com

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