Olefins from cracked gases, polymerization

Polypropylene is made by polymerizing high-purity propylene gas recovered from cracked gas streams in olefin plants and oil refineries. The polymerization reaction is a low-pressure process that utilizes Ziegler-Natta catalysts (aluminum alkyls and titanium halides). The catalyst may be slumed in a hydrocarbon mixture to facihtate heat transfer. The reaction is carried out in batch or continuous reactors operating at temperatures between 50 and 80 C and pressure in the range of 5 to 25 atm. 

Ideally this system would consist of three phases only. Two of these phases would be liquid, water and cooled hydrocarbons, and the third would be the cracked gas stream. Unfortunately, solids may accumulate in this vessel. Coke fines and small particles from the furnace may be entrained into this unit, and activated olefins such as butadiene or styrene may polymerize to form insoluble particles. All of this behavior promotes foaming. In this aqueous system, foam formation may be controlled by addition of a polyglycol. 

The feedstocks used ia the production of petroleum resias are obtaiaed mainly from the low pressure vapor-phase cracking (steam cracking) and subsequent fractionation of petroleum distillates ranging from light naphthas to gas oil fractions, which typically boil ia the 20—450°C range (16). Obtaiaed from this process are feedstreams composed of atiphatic, aromatic, and cycloatiphatic olefins and diolefins, which are subsequently polymerized to yield resias of various compositioas and physical properties. Typically, feedstocks are divided iato atiphatic, cycloatiphatic, and aromatic streams. Table 2 illustrates the predominant olefinic hydrocarbons obtained from steam cracking processes for petroleum resia synthesis (18). 

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