Chemical Processing, Unit Operation & Unit Process

Chemical Process Selection, Design and Operation

Adequate and flexible initial design is essential for the promotion of a chemical plant organic product or inorganic product.

In older days it was classified as inorganic chemical technology and organic chemical technology. Subsequently the oxford university made it as chemical works organization and management.

Some factors that must be considered in planning a plant are discussed in this section. The Process Engineer is an expert in the current aspects of chemical process design. Practical experience is a must if the senior design engineer is able to foresee and solve the problems of production, such as maintenance, safety and obeying the government, environmental by loss and control.

Experience consultants either individuals or professional consulting firms are able to advise, design and for erection of chemical plants.

Chemical Process Control and Instrumentation

Automatic and Instrument control chemical processes are common and essential. Instruments should not be chosen simply to record a variables, of the process. But their function is to assure consistent quality by sensing controls, recording and maintaining desired operating conditions. Instruments are the essential tool for modern processes. They are classified as

            1. Indicating Instruments          2. Recording Instruments          3. Controlling Instruments

Two types of Instruments are currently used as analogue and digital.

Analogue Instruments such as pressure spring thermometers and Bourden Gauges shows results by mechanical moments of some type of device which is directly proportional to the quantity measured.

On the other hand, digital devices are converts the quantity measured into a signal and electric circuits converts the signal to read the numerical values forward by control. Now the computers can monitor and regulate outputs from both the analogue and digital devices according to a prearranged program, also general conventional digital inputs are required. Chemical analytical control has been used in day to day factory procedures for analysis of incoming raw material or outgoing products. Thus quality chemicals are produced more in these days reliably their when human analysed control were used.

The latest advancement are the chromatography system, many spectroscopy have been automated an install of on-line basis for the process to run continuously without the problems encountered manually before.

Role of Chemical Engineers

Chemical Engineers are trained primarily to work in chemical industries. some of the vital role of the chemical engineers are as follows;

Chemical Process Economics

Engineer are totally different from Scientist by their customers of cost of production and profit generator. Therefore the objective of engineer should be to deliver safely the best product or most efficient service at lower cost to the employer and the public who consumes the product.

Material Balance

Yield and conversion are the chemical prospects from the basis for the material balances which is useful for cost determination.

Materials and their quantities from the standard practices are tabulated in the flow charts, energy given are observed for the chemical reactions and energy is frequently a major cost in chemical plants but it often possible by altering the process procedures by using modern separation technologies like “RO” and “Advanced Separation Processes” to produce high quality chemicals with low energy consumption.

Plant Location

The location of the chemical plant is decided ourselves by the availability of raw materials, transportation, market and power. Now the environmental constituents, water supply, availability of efficient labor, cost of land and waste disposal facilities form the criteria for the plant location.

Construction of Plant

For small and large companies construction engineering organizations are available that will built a plant and participate in its design. Some large chemical companies have their own civil construction department and starts their own plants.

The advancement of this is the worker who is going to operate the equipment can be more intimately corrected to the constructions and be familiar themselves for the future alternatives, expansion or modifications.

In built-in plants the top engineers are chartered engineers qualified for the development activities. They have been trained and suitably examined to guarantee technical competency and owe personal responsibility. They are now called as functional consultants and registered firm for dealing with legal aspects with proper training.

Research and Development

adequate and skilled research with patent protection is necessary for future profits. In the chemical process industries one of the outstanding tactics is rapidly changing processes, new raw materials and new markets. Research creates these changes and the factory will have a competitive progress. This research brings about development and the adoption of ideas, concepts, methodologies form the production of the industry. The results and benefits of research establishes the developing coutry on the road of progress and raise the level of life of common man.

Chemical Engineer in coming years

  1. Resources particularly energy and feed back for the Fertilizers and Heavy Chemical Industries.
  2. Infrastructure for Transportation and Telecommunications.
  3. Protection of the Environment.
  4. Development of Agro Industries where utilization of waste from Agro industries and exploitation of value added products from wastes.
  5. Transformation of Rural Economy, Industrialization and Privatization where the profits are less and consumption is more.
  6. Problems of less Technical context are,  
    • The Centre Vs. States
    • Command Economy Vs. Liberalisation & Privatisation
    • Internal Budget and External Balances
    • World Trade Globalization and relevant to India
    • Problem of Indian competitiveness   

The latest research and development have classified the following new industries;

  1. Cryogenics in Chemical Technology
  2. Chemicals from Sea
  3. Air as a Chemical Raw Material
  4. NUPLEXES ( Nuclear Power Agro Industrial Complexes )
  5. Proteins from Petroleum Fermentation and Single Cell Proteins from Animal horns.
  6. Food Industries
  7. Coal Chemicals
  8. Newer Petrochemicals
  9. Pesticides
  10. Pharmaceuticals Industries
  11. Metallurgical Industries
  12. Water treatment & Air Pollution Control

The chemical process industry had its growth from pre scientific chemical industries followed by scientific chemical industry. The growth with restrains, green challenge to chemical industry and the modern separations process involved in the indian chemical industry seen today.

We define Chemical Engineering as a synthesis of chemistry and engineering. A Chemical Engineering therefore carries out on a large scale reactions developed in the laboratory by the chemist.

The Major Areas of Work within Chemical Engineering are,

  • Research
  • Process Development
  • Process Design
  • Evaluation of Design
  • Plant Design
  • Construction
  • Production Supervision
  • Plant Technical Services
  • Sales of the Product

The Research is divided into three categories like Fundamental Research, Exploratory Research and Process Research.


S.No.

Industry

Typical Products

End User

 

 

 

 

1

Inorganic Chemicals  

H2SO4

Fertilizers, Chemicals, Petroleum Refining, Paints, Pigments, Metal Processing and Explosives

HNO3

Explosives & Fertilizers

NaOH

Rayon, Film Processing, Petroleum Refining, Pulp & Paper Industry, Lye, Cleaners, Soap & Detergents, Metal Processing

2

Organic Chemicals

Acetic Anhydride

Resins, Plastics & Nylon

Ethyl Alcohol

Antifreeze agents, Cellophane, Dynamite & Syn. Fibres

Formaldehyde

Plastics

Methanol

Mfr. Of Formaldehyde, IMS(Industrial Methylated Solvent) & Antifreeze agent

3

Petroleum& Petrochemicals

Gasoline

Motor Fuels

Kerosene

Fuel

Oil

Lubrication & Heading purposes

Ammonia

Fertilizer & Chemicals

Ethanol

Acetaldehyde solvents & other miscellaneous chemicals

Alkyl Aryl Sulfonate

Detergents

Styrene

Syn. Rubber, Polymers & Plastics

4

Pulp & Paper

Paper

Books, Records & Newspaper

Cardboard

Boxes for packing

Fiber Board

Building materials

5

Pigments & Paints

Zinc Oxide (ZnO)

Pigments for paints, inks, plastic, rubber, ceramics and linoleum

TiO2

Carbon Blade

Drying Oil

Lead Chromate

Linseed Oil

Phenolic Resins

Basic kequer warmish & enamels

Alkyl Resins

Ion exchange resins and constituents of enamel

6

Rubber

Natural Rubber(Isoprene)

Automobile tyres, moulds, sheets, footwear and insulation

Syn. Rubber (Neoprene)

Butyl Rubber

7

Plastics

Phenol Formaldehyde

Various users in all areas of everyday life

Poly Styrene

Polymethyl methacrylate

PVC

Polyethylene

Polyster

8

Synthetic Fibers

Rayon

Clothing

Nylon

Acrylics

Polyster

9

Minerals

Glass & Ceramics

Windows, containers, bricks & pipe tubes

Cement

Concrete for construction of buildings, highways, etc.

Coal

Fuels, coke and its by-products

10

Cleansing Agents

Soaps & Detergents

House hold cleaning & Industrial cleaning. Sodium alkyl aryl sulfonate is also used as wetting agent.

11

Bio Chemicals

Pharmaceuticals & Drugs

Health & Medicine applications

Fermentation product like penicillin

Medical use

Ethyl Alcohol

Solvent and Beverages

Food Products

Human sustance

12

Metals

Steel, Cu, Al & Zirconium

All the Building materials, machinery etc.

Uranium

Nuclear fuel

 

 


The largest tonnage inorganic chemicals is H2SO4. It is consumed by industry in the manufacture of other products. Thereby it reaches the public knowledge vary scarely. Large quantities are consumed by petroleum and metal industries. The important organic chemical include alcohols, dyes, dye intermediates used to produce other chemicals. Ethyl alcohol was initially produced by bio chemical fermentation before the second world war.

But now it is produced primarily from petroleum on the latest discovery of natural gas. The important petroleum products are gasoline, lubricants, petrochemicals, other fuels and miscellaneous chemicals. Since the second world war petrochemicals have assumed a commander role in the economy. The largest petrochemical ammonia is produced by reaction of H2 from natural gas or petroleum with N2 available in the Air.

This Ammonia reacts with CO2 to produce Urea in a fertilizer plant. Normally there are five different units in the fertilizer manufacture from coal based mines.

  1. Oil & Gasification plant
  2. Benfield De-sulphurization plant
  3. Ammonia plant
  4. CO2 plant
  5. Urea plant

The tendency of Urea is to form BIURETS which are used as regenerator salts in the metallurgical applications. Many plastics and synthetic detergents are produced with the help of oil refineries.


Unit Operation 

The basic physical operations of chemical engineering in a chemical process plant, that is distillation, fluid transportation, heat and mass transfer, evaporation, extraction, drying, crystallization, filtration, mixing, size separation, crushing and grinding, and conveying. In simple terms, the operation which involves physical changes are known as Unit Operation.

  1. Distillation is a unit operation is used to purify or separate alcohol in the brewery industry.

  2. The same distillation separates the hydrocarbon in a petroleum industries.

  3. Dry grapes and other food products or similar drying of filter precipitate like rayon industry where yarn is produced.

  4. Absorption of oxygen from air in a fermentation process of a sewage treatment plant and half hydrogen gas in a process fr liquid hydrogenation of oil.

  5. Evaporation of salts solutions similar to evaporation of sugar solution in the industry.

  6. Settling and sedimentation of suspend solids similar to minimizing and sewage treatment plant.

  7. Flow of liquid hydrocarbon in a petroleum refinery and flow of milk in a daily plant for the solidification in spray dryer.


Classification of Unit Operations

  1. Fluid Flow : Concerns the principle that determine the flow or transformation of fluids from one point to another. The fluid can be a liquid or a gas. This unit is entirely based on Bernoulli e's equation followed by continuity correlation.
  2. Heat Transfer : Deals with principles that govern accumulation and transfer of heat and energy from one place to another. The three concepts followed here are conduction, convection and radiation.
  3. Evaporation : A special case of heat transfer which deals with the evaporation of volatile solvent such as waste from a non-volatile solute such as salt or any other material in the solution. The evaporation of trichloro-ethylene a cleaning agent in the automobile service industry and acetone in the case of glassware in a chemical process industries follow this unit operations.
  4. Drying : An operation in which volatile liquids (usually water) are removed from solid material.
  5. Distillation : An operation where a components of the liquid mixture are separated by boiling because of their difference in vapor pressure.
  6. Absorption : A process whereby a component is removed from gas mixture by treatment with liquid.
  7. Liq-Liq Extraction : A process in which a solute in a liquid solution is removed by contact with another liquid solvent that is relatively irreversible with solution.
  8. Liq-Solid Leaching: It involves treating a finely divided solid with a liquid that dissolves and removes a solute contain in the solid.
  9. Crystallization : The removal of a solute such as a salt from solution by precipitation in the industries for large scale operations, electrostatic precipitation is operated for this concept.
  10. Mechanical physical separation: This involves separation of solids, liquids or gases by mechanical means such as filtration, settling, size reduction which are classified as separate unit operations.     
    • The outline of unit operation defines the settling tanks for sedimentation, filter press for separations, pressurized spheres for ammonia storage, pellatising for fertilizer compounds, pneumatic conveyors for cement industry, bucket wheel  elevators for thermal power stations and belt conveyors for core industries and many more in operation.



Stacks

Gases are discharged into the ambient atmosphere by stacks (referred to as chimneys in industry) of several types.
The chemical process steps involved the following;
  1. Preparing the Reactors
  2. React them
  3. Separate the Products
  4. Purify the Products
The purpose of chemical industry is to start from one and other chemical raw material arrive at a consumer product through a group of physical and chemical products. Therefore it is called as a creative industry rather than assembly industry.

This mainly fall into inorganic, natural products, organic chemicals and metallurgical industry.




Unit Processes

Processes that involve making chemical changes to materials, as a result of chemical reaction taking place. For instance, in the combustion of coal, the entering and leaving materials are differ from each other chemically. Coal and Air enters, and flue gases and residues leave the combustion chamber. Combustion is therefore a unit process. Unit processes are also referred to as chemical conversions. In simple terms, the process which involves chemical changes are known as Unit Processes.

Together with unit operations (physical conversions), unit processes (chemical conversions) form the basic building blocks of a chemical manufacturing process. Most chemical processes consist of a combination of various unit operations and unit processes.


           1. Alkylation:

             Addition of alkyl radical (CH3) with side chain final product. This alkylation process is widely used in organic  chemicals and petroleum industries. The reaction is given as,
                C=C-C-C + C-C-C
                

 

        2. Amination by Ammonolysis:               
         
          Cl-CH2CH2Cl + 4NH3 ------->
NH2CH2CH2NH2
                          EDC                                                Ethylene Diamine

    
       
This reaction is used in manufacture of dye stuffs, organic chemicals and synthetic fibres.

        3. Amination by Reduction

          CH3CHNO2CH3 + 3H2 ------> CH3CHNH2CH3
            2 Nitro Paraffin                         Iso Propylamine

            This unit process is also used in the manufacture of dye stuffs and organic chemicals.

        4. Amino Oxidation
            
          CH3CH2CH3 + NH3 + 1.5 O2 -----
> CH2:CHCN + H20
             Propylene                                          Acrylonitrile

            This reaction is used in the manufacture of plastics and synthetic fibres.

         5. Calcination
           
          CaCO3 ---Heat---
> CaO + CO2
            Limestone                Lime
            
            This reaction is used in the cement industry.

        6. Carbonylation
            
          CH3OH + CO -----
> CH3COOH            
             Methanol                   Acetic Acid
            This is used in the manufacture of organic chemicals.

        7. Carboxylation



            This reaction is used in the organic chemical industry.

        8. Combustion:

            CH4 + O2 ------> CO2 + 2H2O   ( Process Heating )

        9. Condensation:

          C6H5CHO + CH3CHO ------> C6H5CH:CHCHO + H2O

                 Benzaldehyde+Acetaldehyde            Cinnamaldehyde

        10.Cracking or Pyrolysis:

             C-C-C-C-C-C-C --------> C-C-C + C=C-C-C

            This reaction is used in petroleum destruction and distillation of coal.



1. Fluid - Solid Contact:

    Represented by fixed bed reaction. It is most widely used in catalytic reactor used with precious metal catalyst to minimize attrition losses. The catalyst used in the form of pellets. It can represented by the following figure.


    This is used in the packed column. The design of the column is determined by the breakthrough curve, equilibrium line for the given system of adsorbent and adsorbate's. The volume of the reactant coming from the top and the volume of which the product leaves the column, residence time, distribution decides the dimensions of the column. It is contrary to the fluid bed reactor where the bed is fluidized. Once the minimum fluidized velocity is reached the porosity of the bed is faster in a fixed bed reactor but varies from the fluidized bed where the porosity changes according to the height of the bed.
            
2. Fluid - Solid Separation: (Centrifugation)

    This operation separates very finely divided solids from liquid or mixture of liquid and liquid emulsion.


3. Wet Scrubber:

    It is an effective means of removing suspended particles from gas string by contact with liquid shower. 


    When solids are used in the place of liquid the operation is called Dry Scrubber. In the manufacture of MEK, wet scrubber is used and in other selected process industries Dry Scrubbers are used, 
    Scrubber just washes away the impurities and separate the product for further purification.

4. Filter Press:

    It is the simplest type of pressure filtration. the two important parts of the filter press are plates & frames and fabric used in between the two are made of variety of corrosion resistant materials. In the laboratory scales asbestos cloth are used for filtration at different pressures.
    The operation decides the value of specific cake resistance, filter medium resistance and compressibility of the chemical namely Kieselghur  a specific compound in the nature of diatomacceous earth which are used in the application of bio-physics and cyrstallography.



5. Fluid Storage:

    Tanks are widely used for storage of liquids of all types and atmospheric pressure when the liquid is highly volatile there is a floating roof which acts as lid for chemicals as and when the vapour pressure at which signifying the boiling point of liquid the roof changes its position and deserves the liquid from going out to the atmosphere.



6. Pressurized Spheres:

    Pressurized spheres are used for pressurized storage of liquefied gases or high vapors. The pressure permits safe storage with no vapor losses. This is seen in the fertilizer plant where ammonia is stored in spheres.



7. Gas-Liquid Contact: (Absorption)

    The best example is Absorption. It is used for taking a soluble gas in a solvent liquid and producing a solution plus an exit gas. Hydrogen Sulphide is removed from hydrocarbon by the absorption process.



8. Adsorption:

    It is classified into physiorption and chemisorption according to the process applied. The former one is almost a physical change or physical transformation while a later represents a chemical reaction which is a irreversible one. the common effluent treatment plants of varies nature lied textile effluents, sewage treatment, ETP plants in chemical industry, removal of hazardous solid wastes, etc are dealt with adsorption method and the adsorbent is regenerated over a period of time and used again and again.

9. Heat Exchangers:

    The various cooling towers of natural draft and forced draft are example of industrially applied H.Es. These are common facilities in the thermal power stations and in chemical industries the application of shell & tube heat exchangers are widely used. this is an excellent application of heat transfer from one medium to the other.

10.Membrane Separation:

    Dialysis is used to separate metals in solution having widely different molecular weight. for example caustic from sugar solution or cellulose.

11.Size Reduction:

    This involves crushing, grinding, pelletizing and prilling. Pelletizing is used in pharmaceutical industries and prilling used the manufacture of Urea. 



Modern chemical processes are offer extremely complex operations involving 100s of pieces of equipment. without a systematic approach it would be impossible to analyses an existing process or to design equipment process. The typical chemical process is analyzed with the following interdependent considerations like, 
        
    - Mass & Energy Balance
    - Thermo chemistry
    - Unit Operations
    - Plant Equipment
    - Ancillary Equipment
    - Process Plant Diagram
    - Instrumentation Control
    - Economics 
            which deals with net profit before taxation profit after taxation dividend paid to the public and share holders. Once the process as been developed and completed attention can be made to access the various liabilities, resource and assets. 

Alternatives and the remaining choices can be ranked in the order of desirability. They are as follows;

    - Effectiveness for reducing waste
    - Technical Risk
    - Extended of current views in the facility
    - Industrial Precedent
    - Capital and Operating cost incurred
    - Effect of the Quality of the product
    - Impact of Plant Operations
    - Required time for Implementation
    - Other aspects important in the particular situation according to the industrial


Conservation of Energy:
    
    dE = Q - W     This is a steady state batch process.
    dH = Q - Ws    Thia is for flow process.
           
    Q--> Heat energy transfered across system boundary.
    W-->Work energy transfered across system boundary.
    Ws->Mechanical work energy transfered across system boundary.
    E--> Internal energy of the system.
    dE, dH--> Changes in Internal Energy & Enthalpy during the process.


we are already classified the various unit operations and below is a particular basic column of mass transfer equipment.




1. Distillation:

    It is classified into Batch and Continuous Fractionation.
    a. Batch Fractionation:



        Used for intermittent operation and handling of small volume of feed and products.

    b. Continuous Fractionation:




        These are used for high volume continous seperation of complex mistures such as petroleum fractions connected to appropriate pumps, re-boilers, condensers, scrubbers, strippers and finally automatic controls.

2. Drying of Solids:

    Spray Dryer , Rotary Dryer & Tunnel Dryer are some example of these types.

3. Evaporation:

    Open pan evaporators and multiple effect evaporators as used in sugar and salt industries for example. Among these halogen family we have technology to separate chlorine and fluride but production of bromine from the 'sea brine'is almost not put into practice as the bromine chemicals is highly corrosive and necessary precaution has to be laid out for practical purpose.

4. Extraction:
  • Liquid - Liquid Extraction
  • Solid - Liquid Leaching are examples for this process

5. Fluid Handling Equipments:
  • Centrifugal pumps
  • Reciprocating pumps
  • Jet ejectors
6. Fluid - Solid Contacting:
  • Fixed Bed
  • Fluidized Bed
  • Moving Bed, etc.
7. Fluid - Solid Separation:
  • Centrifugation
  • Settling Tank / Sedimentation
  • Wet Scrubber / Dry Scrubber
  • Crystallization
  • Rotary Filter
  • Filter Press
  • Cyclone Separator
  • Electro-static Preciptator
  • Bag Filter
  • Thickeners based on Kynch Theory
8. Fluid Storage:
  • Gas Holders
  • Tanks
  • Pressurized Spheres
  • Underground Caverns which are used for the purpose of Natural Gas Storage.
9. Gas - Liquid Contact:
  • Absorption
  • Stripping
10.Heat Exchangers:
  • Fired Heaters
  • Re-boilers
  • Condensers
  • Shell & Tube Heat Exchangers
  • Jacketed Kettle
  • Quenching applied in conventional Heat Transfer and Metallurgical Operations.
11.Membrane Separation:
  • Dialysis
  • Gaseous Diffusion
12.Mixing:
  • Agitation
  • Solids Blending
13.Size Reduction & Enlargement:
  • Crushing
  • Grinding
  • Pelletizing
14.Solids Handling:
  • Pneumatic Conveying - Juices transfered to 200 km in Brazil
  • Bucket Elevators - Coal Industries
  • Screw Conveyors - Tooth Paste, Turbine Liquids
  • Belt Conveyors
15.Solid - Solid Separation:
  • Screening
  • Elutriation
  • Froth Rotation
  • Jigging
  • Magnetic Separation


CHEMICAL REACTORS

The Reactor is the heart of the chemical process. The design of an industrial chemical reactor must satisfy the requirements in four main areas.
  1. Chemical Factors
  2. Mass Transfer Factors
  3. Heat Transfer Factors
  4. Safety Factors
1. Chemical Factors:

This involve the kinetics of the reaction weather it's first order or second order and based on this chemical reaction engineering is built on the design must provide sufficient residence time to proceed the reaction for the required degree of reaction and conversion to product.

2. Mass Transfer Factors:

The reaction rate of homogeneous reaction may be controlled by the rate of diffusion of reactants rather than the chemical kinetics of Langmuir isotherm and Frendlich isotherm.

3. Heat Transfer Factors:

These describes weather the reaction is exothermic or endothermic. In Exothermic, heat is released outside and In Endothermic, heat is absorbed by reactants. The value of heat of reaction is necessary to operate the chemical reactor.

4. Safety Factors:

This involve the confinement of any hazardous reactant and products as well as the control of reaction and process conditions. 

Based on these factors the Reactor Types as follows;
    a. Mode of Operation - Batch or Continuous
    b. Phases Types - Homogeneous or Heterogeneous
    c. Reactor Geometry - Flow Pattern & Process of contacting the phases.

The five major classes of Reactor;

    i.  Batch
    ii. Stirred
    iii.Tubular
    iv.Packed Bed (Fixed)
    v. Fludised Bed

Compounds like pigments, dye stuffs, pharmaceuticals and polymers are manufactured by Batch Processes.
The Latest Heat Exchangers are Direct or Contact Exchangers In addition to Double Pipe Exchanger, Shell & Tube Exchanger and Plated Frame Exchanger.
            

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