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.

EXAMPLES OF MASS TRANSFER

Example of mass transfer

Mass transfer operations

The operation carried out for separating the component of the mixture, involving the transfer of (mass) material from one homogeneous phase to another, due to the difference in vapor pressure, solubility or diffusivity and in which driving force for transfer is a concentration the difference is called mass transfer operations

Example:- Distillation, Gas absorption, Liquid Extraction, Drying

Example of mass transfer operations

• Depending upon phase contact between Gas, liquid and solid phase following are the classification of the mass transfer operations

Gas-gas phase contact

• The fragrance of incense sticks after lightning diffuse from one room to another room. 

Gas-liquid contact

• A cup of water left in a room eventually evaporate as a result water molecules diffusing into the air. This is liquid to gas diffusion.
• Gas absorption:- If a mixture of ammonia and air is contacted with liquid water, a large portion of ammonia will dissolve in the liquid, and in this way air ammonia mixture can be separated.
• Desorption or stripping:- If air is brought into contact with an ammonia-water solution, some of the ammonia leaves the liquid and enters the gas phase.
• If the liquid phase is a pure liquid, containing one component while the gas contains two or more, the operation is humidification or dehumidification depending upon the direction of transfer
• Humidification of the air: - When dry air contacted with liquid water result in evaporation of some water into the air
• Dehumidification: - When very moist air contacted with pure liquid water may result in condensation of part of moist air in the air. 

Gas-liquid contact

• A piece of solid CO2 ( dry ice ) will also get smaller and smaller in time as the CO2 molecules diffuse into the air. This is a solid to gas transfer example.
• Fractional sublimation: - If a solid solution is partially vaporized without the appearance of the liquid phase, the newly formed vapor phase and residual solid each contains all the original components, but in different propositions
• Drying or Desorption:- If solid which is moistened with a volatile liquid is exposed relatively dry gas, the liquid leaves the solid and diffuses into the gas
• Adsorption: - If diffusion takes place from solid to gas phase is called adsorption.
• For example:- If a mixture of water vapor and the air is brought into contact with activated silica gel, the water vapors diffuse to solid, which retains it strongly, and the air is thus dried.

Liquid-liquid contact

• Liquid extraction operations:- This separation involving contact of two insoluble liquid phases.
• for example - Acetone water solution is shaken in separating funnel with carbon-tetra chloride and the liquid allowed to settle, a large portion of acetone will be found in the carbon chloride-rich phase and will thus have been separated from the water.
• Fractional extraction:- a solution of acetic acid and acetone can be separated by adding it to the insoluble mixture of water and carbon tetrachloride. After shaking and setting, both acetone and acetic acid will be found in both liquid phases, but in different proportions.

Liquid-solid contact

• If a spoon of sugar is added in a cup of coffee will eventually sweeten the coffee because sugar molecule diffuses into the coffee.
• Leaching:- In this process diffusion is from solid to liquid phase takes place.
• For example- Leaching of gold from its ore by cyanide solutions. Another example is the leaching of cottonseed oil from the seeds by hexane.
• Adsorption:- In this process diffusion is from liquid to solid phase takes place
• For example:- The colored material which contaminates impure cane sugar solutions can be removed by contacting liquid solutions with activated carbon.

Solid-solid contact

• The diffusion rate is very slow
• If metallic gold and metallic are placed in intimate contact, over a period of time molecules of lead will diffuse into the gold and vise versa. but the rate of diffusion is very slow.

MASS TRANSFER AND MASS TRANSFER OPERATIONS

Mass transfer

• Mass transfer means the transfer of substance through another on a molecule scale.
• The net movement of mass from one location usually by a stream, phase, fraction, or component, to another.
• This mass transfer occurs due to the concentration difference or gradient.

Hence The process of transfer of mass as a result of the concentration difference of a component in a mixture or two-phase which are in contact is called mass transfer

• Example:- evaporation of water from a pool of water into a stream of air flowing over the water surface

Mass transfer operations

The operation carried out for separating the component of the mixture, involving the transfer of (mass) material from one homogeneous phase to another, due to the difference in vapor pressure, solubility or diffusivity and in which driving force for transfer is a concentration difference is called mass transfer operations

• In the mass transfer operations, neither equilibrium phase consists of only one component. 
• Hence when two-phase is initially contacted, they will not be of equilibrium compositions. 
• The system then attempts to reach equilibrium by a relatively slow diffusive movement of the constituents, which transfer in part between the phases in the process. 
• Separations are therefore never complete, although they can be brought as near completion as desired. 
• Mass transfer occurs in many processes such as absorption, evaporation, adsorption, drying, precipitation, membrane filtration, and distillation. 

In mass transfer operations, the Mass transfer may occur

• In one direction (gas absorption)
• In both direction (distillation)
• With simultaneous heat transfer( drying and crystallization)
• With simultaneous chemical reaction (Absorption of CO2 in an aqueous solution of KOH)
• With the exchange of one or more components
• Isothermally and non-isothermally

MODES OF HEAT TRNSFER

Modes of heat transfer

Heat transfer may be defined as the transmission of the energy from one region to another region as a result of the temperature gradient.

Heat transfer takes place in three modes.

1. Conduction

Conduction is the transfer of heat from one part of substance to another part of same substance or from one substance to another which is in physical contact with it. 

• While transferring heat there is no appreciable displacement of molecules forming that substance is occur.
• In solid, the flow of heat is result of vibrational energy.
• In liquid, the flow of heat is result of kinetic energy.

2. Convection

Convection is the transfer of heat within a fluid which is arises due to mixing of one portion of the fluid with another. 

• In a fluid, the flow of heat results in the microscopic motion of a fluid.
• There are two types of Convection.

1. Natural Convection:- Heat transfer is caused by differences in density arising from a temperature gradient
2. Forced Convection:- Heat transfer is caused by use of external means of agency

3. Radiation

Radiation is the transfer of heat through space or matter by means other than conduction or convection.

• Transfer of heat is occurred without any material medium or through a perfect vacuum.
• Transfer of heat is in the form of electromagnetic waves.
• When this wave falls on another body, a part of the energy is reflected, transmitted through the body and remaining absorbed by the body.

Figure of Conduction Convection and Radiation