Distillation: processes – separatory – With chemical reaction
Patent
1990-08-10
1992-06-16
Bascomb, Jr., Wilbur
Distillation: processes, separatory
With chemical reaction
203 71, 203 73, 203 87, 203 98, 203DIG9, 570262, B01D 300, C07C 1738
Patent
active
051222353
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
1. Technical Field
The present invention relates to a process for removing chloroprene which may be contained as an impurity in 1,2-dichloroethane. More specifically, this invention is concerned with a process for effectively removing chloroprene upon purification of 1,2-dichloroethane by distillation, which process can be applied to the production technology of vinyl chloride by thermal cracking of 1,2-dichloroethane.
2. Background Art
Production of vinyl chloride (hereinafter referred to as "VCM") by the thermal cracking of high-purity 1,2-dichloroethane (hereinafter referred to as "EDC"), namely, synthesis of VCM by the so-called thermal cracking process is industrially practiced on a large scale.
In a conventional process for the production of VCM, crude EDC obtained by direct chlorination or oxychlorination of ethylene is subjected to a purification step which uses a dehydrating column, a low-boiling material separation column and a high-boiling material separation column, thereby removing firstly water, then low-boiling impurities, and further high-boiling impurities to obtain purified EDC. The purified EDC is thereafter fed to a VCM-producing thermal cracker.
About 50-60% of the purified EDC is cracked in the thermal cracker, so that a cracked gas containing VCM and hydrogen chloride is obtained. This cracked gas is quenched and condensed, thereby separating and recovering hydrogen chloride and then VCM. A residue which has been obtained after the separation and recovery of hydrogen chloride and VCM contains, in addition to unreacted EDC, side reaction products formed during the thermal cracking. Unreacted EDC with the side reaction products contained therein (hereinafter called the "unreacted EDC") is subjected to the aforementioned purification process and is then fed again as purified EDC to the thermal cracker.
The side reaction products in unreacted EDC, however, include chloroprene as a low-boiling impurity.
Inclusion of chloroprene in EDC will lead to two serious problems.
Firstly, when chloroprene is contained in purified EDC to be fed to the thermal cracker, accelerated coking of cracking tubes and suppression to the cracking reaction will be observed. To avoid them, it is essential to control the content of chloroprene below 100 ppm in purified EDC which has an EDC content of 99% or higher.
Secondly, chloroprene is relatively prone to polymerization. When chloroprene is concentrated in a low-boiling material separation column in a purification process of unreacted EDC, it may be polymerized inside the column and may induce blocking of the column. It is therefore difficult to improve the efficiency of separation of chloroprene by increasing the degree of its concentration, so that the distillate is discharged out of the system while containing EDC at a relatively high concentration. It is thus difficult to improve the efficiency of separation of chloroprene in a purification process. Further, the inclusion of chloroprene results in a reduction in the unit of EDC. To avoid blocking of the low-boiling material separation column, it is necessary to control below 5 wt.% the concentration of chloroprene in vapor to be discharged from the top.
In the prior art, it has therefore been difficult to overcome problems such as blocking of the low-boiling material separation column and to control below 100 ppm the content of chloroprene in purified EDC to be fed to the thermal gas cracker.
As methods for overcoming the above problems to improve the efficiency of separation of chloroprene, there have been disclosed processes in which, (1) by introducing chlorine into a low-boiling material separation column or (2) by introducing a portion of a distillate from a low-boiling material separation column, chloroprene is chlorinated to convert it into a material having a higher boiling point than EDC and the high-boiling material is then separated in a high-boiling material separation column (Japanese Patent Application Laid-Open Nos. 61105/1979, 48127/1982 and 24968/1984; Japanese Patent Publicat
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Nagae Seiji
Naito Tadashi
Shirai Atsushi
Shirai Kenji
Bascomb, Jr. Wilbur
Mitsui Toatsu Chemicals Incorporated
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