This column is based on the report “Polychloroprene Production – Cost Analysis,” published by Intratec. It can be found at the following URL: www.intratec.us/analysis/polychloroprene-production-cost.
The need to find a substitute for natural rubber during World War II boosted the development of polychloroprene (chloroprene rubber), the first synthetic rubber produced industrially. Today, it is one of the most important special rubbers, alongside butyl rubber, nitrile rubber (NBR) and ethylene propylene diene rubber (EPDM).
The following describes a process for polychloroprene production from butadiene and chlorine. Figure 1 presents a simplified flow diagram.
Chlorination. In the first step, butadiene, chlorine, recycled butadiene and hydrochloric acid are mixed and fed to the chlorination reactor, where butadiene is chlorinated in the vapor phase. A large excess of butadiene is used to prevent the formation of super-chlorinated species during this reaction. The two main products of the chlorination are 1,4-dichloro-2-butene and 3,4-dichloro-1-butene.
Isomerization. An isomerization step is used to convert 1,4-dichloro-2-butene into 3,4-dichloro-1-butene (the precursor to chloroprene). The isomerization is conducted in the reboiler of a distillation column. In this way, 3,4-dichloro-1-butene is continuously removed from the liquid reaction medium by evaporation. The 3,4-dichloro-1-butene is recovered from the top stream of the column, while the column’s bottom product is recycled to the process.
Dehydrochlorination and purification.Caustic soda is diluted in process water, mixed with the 3,4-dichloro-1-butene stream from the isomerization step and fed to a series of continuously stirred vessels. The gaseous effluent is cooled, condensed and then settled for separating the organic and aqueous phases. The aqueous phase is discarded, while the organic phase is stripped against steam. This column is designed to recover a gaseous chloroprene-rich stream from the top, while the unconverted 3,4-dichloro-1-butene is recovered in the column’s bottom.
The bottom product is then allowed to settle, in order to separate the organic and aqueous phases. Then the aqueous phase is discarded, while the organic phase is dried and directed to a falling-film evaporator. The evaporator is used to remove heavy-end byproducts.
The gaseous stream from the evaporator is mixed with the top stream from the stripping column, and this mixture is distilled for the recovery of 3,4-dichloro-1-butene (bottom) and chloroprene (top). Both column products are allowed to settle, and the resulting aqueous phases are discarded, while the organic phase is dried in molecular sieves. The dried column-bottom stream is recycled, while the dried crude-chloroprene stream is purified in a final distillation step and routed to the polymerization stage.
Polymerization. The polymerization is carried out in continuously stirred, jacketed tank reactors connected in series, and cooled by a brine solution. Chloroprene is first mixed with emulsifier, demineralized water and initiators (hydroperoxides/ferrous sulfates). The polymer molecular weight is controlled by means of a chain-transfer agent (tertiary dodecyl mercaptan). The reaction is terminated with the addition of shortstop agents (for example, sodium polysulfide).
Recovery and finishing. The latex produced is steam-stripped for the removal of residual chloroprene, which is recycled. The hot latex is mixed with antioxidants and fed to a freeze roll to be coagulated into a polymer sheet, which is fed to a wash belt for removing polymerization-reaction medium. The washed polychloroprene film is dried in squeeze rolls and continuous-belt dryers. After drying, the film is formed into a rope and cut into polychloroprene chips, which are packed in bags.
The total operating cost (including raw materials, utilities, fixed costs and depreciation costs) estimated to produce polychloroprene was about $2,400 per ton of polychloroprene in the fourth quarter of 2014. The analysis is based on a plant constructed in the U.S. with the capacity to produce 50,000 metric tons per year of polychloroprene.
Edited by Scott Jenkins
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