Basic Principles of Unit Processes and Unit Operations in Organic Chemical Industries (Part - 3) | Chemical Technology - Chemical Engineering PDF Download

Unit Operations in Chemical Industries 

Unit operations are very important in chemical industries for separation of various products formed during the reaction. Table M-I 3.3 give the details of unit operation in chemical process industries. 

Table M-I 3.3: Unit Operations in Chemical Process Industries

Distillation

Distillation has been the king of all the separation processes and most widely used separation technology and will continue as an important process for the foreseeable future. Distillation is used in petroleum refining and petrochemical manufacture Distillation is the heart of petroleum refining and all processes require distillation at various stages of operations.

Membrane Processes 

Membrane processes have emerged one of the major separation processes during the recent years and finding increasing application in desalination, wastewater treatment and gas separation and product purification. Membrane technology is vital to the process intensification strategy and has continued to advance rapidly with the development of membrane reactors, catalytic membrane reactor, membrane distillation, membrane bioreactors for wide and varied application.
Membrane process classified based on driving force. Various type of membrane process and driving force are given in Table M-I 3.4.

Table M-I 3.4: Membrane Processes

Based on lower operating costs, comparable capital cost and only slightly product loss (including fuel), membranes have demonstrated a flexible, cost, effective alternative to amine treating for some natural gas processing applications . Gas membrane and its application areas are mention in Table M-I 3.5.

Membrane distillation is a membrane separation process, which can overcome the limitation of more traditional membrane process. Membrane distillation has significant advantage over other processes, including low sensitivity to feed concentration and the ability to operate at low temperature . Various type of membrane processes are mention in Table M-I 3.6. 

Table M-I 3.5: Gas Membrane Application Areas

Table M-I 3.6: Various Types of Membrane Processes 

Absorption   

Absorption is the one of the most commonly used separation techniques for the gas cleaning purpose for removal of various gases like H₂S, CO₂, SO₂ and ammonia. Cleaning of solute gases is achieved by transferring into a liquid solvent by contacting the gas stream with liquids that offers specific or selectivity for the gases to be recovered. Unit operation and is mass transfer phenomena where the solute of a gas is removed from being placed in contact with a nonvolatile liquid solvent that removes the components from the gas.

Solvent: Liquid applied to remove the solute from a gas stream.

Solute: Components to be removed from entering streams.

 Some of the commonly used solvents are:

Chemical Absorption

Amine Processes: Mono-ethanol amine (MEA), di-ethanol amine (DEA), tri-ethanol amine (TEA), diglycol amine (DGA), methyl diethanol amine (MDEA)

Carbonate Process: K₂CO₃, K₂CO₃+MEA, K₂CO₃ +DEA, K₂CO₃+arsenic trioxide 

Physical Absorption 

Polyethylene Glycol Dimethyl Ether (Selexol), N-methyl pyrrolidine,NMP (Purisol), Methanol (Rectisol), Sulphonane mixed with an alkanolamine and water (sulfinol).  

Adsorption

Adsorption technology is now used very effectively in the separation and purification of many gas and liquid mixtures in chemical, petrochemical, biochemical and environmental industries and is often a much cheaper and easier option than distillation, absorption or extraction. Some of the major applications of adsorption are gas bulk separation, gas purifications, liquid bulk separation, liquid purifications. One of the most effective method for recovering and controlling emissions of volatile organic compounds is adsorption  Some of the commercial adsorbent s are silica gel, activated carbon, carbon molecular sieve, charcoal, zeolites molecular sieves, polymer and resins, clays, biosorbents.  some of the key properties of adsorbents are capacity, selectivity, regenerability, kinetics, compatibility and cost . Some of the methods used for regeneration of adsorbent are thermal swing, pressure swing, vacuum (special case of pressure swing), purge and gas stripping, steam stripping . Commercial adsorption processes is given in Table M-I 3.7. Some of the important criteria of good adsorbent are . 

(1) it must selectivity concentrate one or more components called adsorbate to from their fluid phase levels

(2) the ability to release adsorbate so that adsorbent can be reused,  

(3) as high as possible delta loading the change of weight of adsorbate per unit weight of adsorbent between adsorbing and desorbing steps over a reasonable range of pressure and temperature  

Table M-I 3.7: Commercial Adsorption Processes

Pressure swing adsorption (PSA) is based on the principle of relative adsorption strength, is a milestone in the science of gas separation. Some of the commercial application of PSA are air drying, hydrogen purification, bulk separation of paraffins, air separation for oxygen and nitrogen production,  

 Chromatography is a sorptive separation process. in choromatography feed is introduce in  column containing a selective adororbent( stationary phase) and separated over the length of the column by the action of a carrier fluid (mobile phase)that is continually supplied to the column following the introduction of the feed. The separation occurs as a result of the different partitioning of the feed solutes between the stationary phase. The separated solutes are recovered at different time in the effluent from the column .

Crystallization Process  

Crystallization processes are used in the petroleum industry for separation of wax. The process involves nucleation, growth, and agglomeration and gelling. Some of the applications of crystallization is in the separation of wax, separation of p-xylene from xylenes stream. Typical process of separation of p-xylene involves cooling the mixed xylene feed stock to a slightly higher than that of eutectic followed by separation of crystal by centrifugation or filtration.

Liquid –Liquid Extraction  

Liquid –liquid extraction has been commonly used in petroleum and petrochemical industry for separation of close boiling hydrocarbons. Some of the major applications are:

• Removal of sulphur compound from liquid hydrocarbons
• Recovery of aromatics from liquid hydrocarbon
• Separation of butadiene from C4 hydrocarbons
• Extraction of caprolactam
• Separation of homogenous aqueous azeotropes
• Extraction of acetic acid
• Removal of phenolic compounds from waste water
• Manufacture of rare earths
• Separation of asphaltic compounds from oil
• Recovery of copper from leach liquor
• Extraction of glycerides from vegetable oil

Some of the important property of a good solvent

• High solvent power/capacity
• High selectivity for desired component
• Sufficient difference in boiling points of the solvent and the feed for effective separation
• Low latent heat of evaporation an specific heat to reduce utility requirement
• high thermal an chemical stability
• Low melting point
• Relatively inexpensive
• Non toxic and non –corrosive 
• Low viscosity low interfacial tension 

Technological development in unit operations-

• Distillation, Azeotropic, extractive distillation, reactive distillation, membrane distillation
• Random packing to Structured Packing
• Single and two pass to Multiple down comer
• Rasching rings and berl saddles to Intalox sadles, pall rings, nutter rings, half rings, super rings,Fleximax
• Pan park to  Wire gauge packing, Goodloe, Mellpark, Flexipack, Gempack, Intalox
• Fixed bed to Fluidised bed reactor
•  Conventional reactor to Micro reactor
• Ball mill grinding to Vertical roller mill and press roll Mill
• Open circuit grinding to Closed circuit grinding
• Batch digester to continuous digester
• Low speed and low capacity cipper to High speed chipper and high capacity chipper
• Low speed Paper machine to high speed machine
• Drum displacer, Pressure diffuser, Displacement presses, Combined deknotting and Fine screening,
•  High temperature screening before washing, Reverse cleaners
• Adsorption(Olex, Parex and Molex), Crystallisation and Membrane separation processes
• Solvent extraction processes and New solvents
• Conventional distillation Short path distillation, divided wall column
• Conventional bubble cap, sieve plate to  valve tray · Random packing to structured packing
• Axial flow reactor to radial flow reacto
• Conventional instrumentation to smart (intelligent) instrumentation