CLASSIFICATION OF VARIABLES IN CHEMICAL PROCESS CONTROL

Classification of variables in chemical process control

In controlling a process there exist two types of classes of variables.

Input Variable

This variable shows the effect of the surroundings on the process. It normally refers to those factors that influence the process.

• An example of this would be the flow rate of the steam through a heat exchanger that would change the amount of energy put into the process. There are effects of the surrounding that are controllable and some that are not.

There are two types of inputs.

Manipulated(or adjustable) inputs :

These are the variable in the surroundings can be control by an operator or the control system in place.

• The values of manipulated inputs can be adjusted freely by the human operator or a control mechanism.

Disturbances:

These are the input variable that can not be controlled by an operator or control system.

• There exist both measurable and immeasurable disturbances.
• The values disturbances are not the result of adjustment by an operator or a control system.

Output variable-

Output variable also known as the control variable.

These are the variables that are process outputs that effect the surroundings.

Measured output variables:-

The values of measured output variables are known by directly measuring them

Unmeasured output variables:-

The values of unmeasured output variables are not or cannot be measured directly

Example CSTR with cooling jacket

• Consider a continuous stirred tank reactor (CSTR) in which an irreversible exothermic reaction A ---> B takes place. The heat of reaction is removed by a coolant medium that flows through a jacket around the reactor

where

• Input variables: CAi ,Ti, Fi, Tci, Fc
• Output variables: Fc, Tc0, CA, T, F

Case 1:- If the inlet stream in the CSTR system comes from an upstream unit have no control.

• Disturbances: CAi, Fi, Ti

Case 2:- If the coolant flow rate is controlled by a control valve, then

• Manipulated variable: Fc
• Disturbance: Tci 

Case 3:- If the flow rate of the effluent stream is controlled by a valve

• Manipulated variable: F

PROCESS CONTROL

Process control

Process control is a mixture between the statistics and engineering discipline that deals with the mechanism, architectures, and algorithms for controlling a process.

• Structure of chemical process plant is very complex
• Any chemical plant consists of various process units which are inter connected with one another in a systematic manner
• The main objective of any plant is to convert certain raw materials into the desired product using available sources of energy
• Other objectives- safety, product specification, environmental regulations, operation constraints, economics
• These all these parameters are controlled by an arrangement of various equipment like measuring devices, valves, controller

Examples of controlled processes are:
1. Controlling the temperature of a water stream by controlling the amount of steam added to the shell of a heat exchanger.

2. Operating a jacketed reactor isothermally by controlling the mixture of cold water and steam that flows through the jacket of a jacketed reactor.

3. Controlling the height of fluid in a tank to ensure that it does not overflow.

THERMODYNAMIC SYSTEM

Thermodynamic system

Thermodynamics is the science relating heat and work transfers and the related changes in the properties of the working substance. The working substance is isolated from its surroundings in order to determine its properties.

 

SYSTEM

Thermodynamic system is defined as a quantity of matter or a region in space upon which attention is concerned in the analysis of a problem.

• A system may be either an open one, or a closed one, referring to whether mass transfer or does not take place the boundary.

SURROUNDING

Everything external to system is call surrounding or the environment.

BOUNDARY

A physical or imaginary surface, enveloping the system and separating it from the surroundings.

• The boundary may be either fixed or moving.

UNIVERSE

A system and surrounding together comprise a universe.

RATE OF REACTIONS

Rate of reactions

The rate of reaction tells us how fast a number of moles of one chemical species are being consumed to form another chemical species.

• The term chemical species refers to any chemical component or element with a given identity.
• The identity of a chemical species is determined by the kind, number, and configuration of that species atoms.

Chemical reaction has taken place when a detectable number of molecules of one or more species have lost their identity and assumed a new form by a change in the kind or number of atoms in the compound and/or by a change in structure or configuration of these atoms.

• In this classical approach to chemical change, it is assumed that the total mass is neither created nor destroyed when a chemical reaction occurs.
• The mass referred to is the total collective mass of all the different species in the system.
• However when considering the individual species involved in a particular reaction, we do speak of the rate of disappearance of mass of a particular species.
• The rate of disappearance of a species say species A is the number of A molecules that lose their chemical identity per unit time per unit volume through the breaking and subsequent re-forming of chemical bonds during the course of the reaction.
• In order for a particular species to "appear" in the system some prescribed fraction of another species must lose its chemical identity.
• There are three basic ways a species may lose its chemical identity:

1. Decomposition:- ln decomposition the molecule loses its identity by being broken down into smaller molecules. atoms. or atom fragments. 
2. Combination:- A molecule may lose its species identity is through combination with another molecule or atom.
3. Isomerization:- Here, although the molecule neither adds other molecules to itself nor breaks into smaller molecules. it still loses its identity through a change in configuration.