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
Absorption and stripping
Membrane Process: Reverse osmosis, Ultrafiltration, Dialysis,
Adsorption and desorption
Pressure Swing adsorption Chromatography
Crushing Grinding, Pulverizing and
Distillation: Batch distillation
Flash distillation, Azeotropic distillation, Extractive distillation Reactive distillation
Solid liquid extraction
Liquid- Liquid extraction
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 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
Common Gas Separation
Generation oxygen enrichment, inert gas
refinery hydrogen recovery
Syn. gas adjustment
Ammonia purge gas
Acid gas removal from natural gas
Natural gas dehydration
Sour gas treating
Table M-I 3.6: Various Types of Membrane Processes
Liquid or Gas
Sieving And Sorption
Gas And Vapour Permeation
Liquid or Vapour
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:
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
Polyethylene Glycol Dimethyl Ether (Selexol), N-methyl pyrrolidine,NMP (Purisol), Methanol (Rectisol), Sulphonane mixed with an alkanolamine and water (sulfinol).
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
Separation of paraxylene from mixed C8 aromatics isomers
Meta xylene from mixed C8 aromatics isomers
Linear paraffins from branched and cyclic hydrocarbons
Olefins from paraffins
Para cresol or meta cresol isomers
Para cymene or meta cymene from cymene isomers
Fructose from mixed sugar
separation of normal paraffins from hydrocarbon mixture
For separation of straight chain normal paraffins from the kerosene range(C10-C18) used for detergent industry
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 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:
Some of the important property of a good solvent
Technological development in unit operations-