Propylene, Propylene Oxide And Isopropanol (Part - 1) | Chemical Technology - Chemical Engineering PDF Download

Catalytic Dehydrogenation 

Catalytic dehydrogenation of light paraffins is of increasing importance because of the growing demand of olefins such as propylene and isobutene and nbutenes. Propane dehydrogenation accounts for 2 percent of the total world propylene production. Some of the commercial processes available for dehydrogenation of propane and n-butane are :

• Oleflex (UOP).
• Catofin (ABB Lumus).
• FBD-4 (Snamprogetti SPA).
• Star (Phillips Petroleum Company).

              Catalytic dehydrogenation takes place at high temperature (650⁰C) using platinum based or chromium-alumina or Fe, Cr/Al2O3 as catalyst. Reactor effluent treatment for the separation of hydrogen, propylene, and propane is not simple and total investment is high. These production units can be installed only in areas where field propane is available at low costs. 

Methanol to Propylene:

This process produces propylene from natural gas via methanol by converting methanol to dimethyl ether in adiabatic reactor using high activity, high selectivity catalyst. The methanol, water, DME stream is then feed to series of MTP reactor where steam is added. The product stream is first processed for removal of traces of water, CO2 and DME, followed by further processing for yielding polymer grade propylene.  

Propylene Oxide, Propylene Glycol and Polyols 

Propylene oxide, propylene glycols and polyols are important derivatives of propylene. propylene oxide is used for the manufacture of propylene glycol and polyols. Major consumption of propylene oxide is manufacture of polyurethane and polyester resins. Propylene glycol find major application in the manufacture of unsaturated polyester resins, food additives, pharmaceuticals and personal care, tobacco humectants, cellophane, paints and coatings. Polyols major use is in the manufacture of polyurethane. 

Propylene Oxide

Various rote for making propylene oxide are There are two major processes for the manufacture of propylene oxide: Propylene chlorohydrin process and propylene oxidation process using peroxides.

Propylene Chlorohydrin Route: 

The chlorohydrination process consists of formation of propylene chlorohydrin by the reaction between hypochlorous acid and propylene. The propylene chlorohydrin is epoxidised to propylene oxide by a 10% solution of milk of lime or NaOH. Various steps involved are

• Propylene hypochlorination: Propylene is reaxted with aquous chlorine resulting in the formation of propylene chlorohydrins. Unreacted propylene is recyled.
• Neutralisation: Neutrialisation of propylene chlorohydrins containing hydrochloric acid which is formed during the process.
• Dehydrochlorination: Reaction of propylene chlorohydrin with milk of lime or caustic soda to produce propylene oxide
• Purification: Distillation of crude propylene oxide for separation heavy ends 

Reactions : 

Byproducts formed during the reaction are 1,2-dichloropropane and chlorinated di-isopropyl ether. Some of the disadvantages and major economic drawbacks of the process which led to the wide acceptability of epoxidation processes are use of costly chlorine, production of weak calcium chloride byproduct, and corrosion problem due to chlorine handling.

Oxidation Route using peroxide Compounds: In this process, propylene and peracetic acid (in ethyl acetate) which is produced by oxidation of acetaldehyde are reacted in a series of three specially designed reactors at 50-80⁰C and 90-120 MPa pressure. The reaction products are fed to the stripper where a mixture of propylene and propylene oxide are obtained as top product while mixture of ethyl acetate and acetic acid is obtained as bottom product. Both mixtures are fed to two separated columns where separation of propylene oxide, ethyl acetate, acetic acid, and heavy end takes place. 

Reaction

Peroxide from acetaldehyde 

Oxidation of propylene 

Propylene Glycol

Propylene glycol is made by hydrolysis of propylene oxide. The process steps involve are:

Reaction Section: Hydrolysis of propylene oxide resulting in formation of mono propylene glycols(MPG). Small amount of di propylene glycol (DPG) and tri propylene glycol (TPG) s are also formed

 Concentration Section: Concentration of glycol solution in multiple effect evaporator

Distillation Section: Separation of MPG,DIPG and TPG separated from MPG column. n series of distillation column where MPG is separated in first column.

Polyols 

Polyols are made by polymerization of propylene oxide/ethylene oxide using an proprietary catalysed chain starter. The process consist of  

• Raw material Preparation: Preparation of chain starter and addition in reactor along with EO/PO 
• Reaction: Polymerisation using catalysed cahin extender
• Purification: Purification of raw polyol by neutralization