Organic compounds -- part of the class 532-570 series – Organic compounds – Halogen containing
Reexamination Certificate
2000-10-09
2002-08-27
Siegel, Alan (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Halogen containing
C570S226000
Reexamination Certificate
active
06441257
ABSTRACT:
The invention relates to a process for the treatment of a cracking gas produced in the cracking of 1,2-dichloroethane to give vinyl chloride.
In processes for the preparation of vinyl chloride by incomplete cracking of 1,2-dichloroethane (EDC), the EDC employed is usually evaporated in the first step, then, in a second step, the vapor formed is cracked pyrolytically at relatively high temperature, furthermore, in a third step, the entrained solids are separated off from the hot cracking gas produced in the second step, and subsequently the purified cracking gas is fed to distillative treatment.
The main products formed in the EDC cracking carried out in the second process step are hydrogen chloride (HCl) and vinyl chloride (VCM):
C
2
H
4
Cl
2
+heat→HCl+C
2
H
3
Cl
By-products formed in traces are soot, chlorinated and unsaturated hydrocarbons and benzene. In order to limit the formation of these undesired by-products, the cracking temperature is held at a level which results in incomplete reaction of the EDC. The hot cracking gas produced by cracking in the second process step therefore also contains unreacted 1,2-dichloroethane (EDC) in addition to the main products hydrogen chloride (HCl) and vinyl chloride (VCM) and said by-products.
The cracking of EDC to give VCM is an endothermic process. It takes place in the gas phase in the form of pyrolysis. In industry, the pyrolysis is carried out without a catalyst under high pressure of from 1 to 3 MPa and at a temperature of from 450 to 600° C. However, work is also being carried out on catalytic processes which allow the pyrolysis to be carried out at lower pressure and lower temperature. The hot cracking gas produced by means of pyrolysis is formed at the pyrolysis temperature. It has to be conditioned so that it takes on a form which is suitable for the actual substance separation.
Before the actual substance separation of the cracking gas, the cracking gas heat is therefore utilized economically in one or more heat exchangers. In the process, the temperature of the cracking gas in the case of catalyst-free pyrolysis drops from between 480 and 540° C. to between about 180 and 280° C.
In a company publication published by Uhde GmbH in June 1995 with the title “Vinyl chloride plants/Hoechst process”, the process usual hitherto for the treatment of the cracking gas is described.
In the process flow chart reproduced on page 11 of this company publication, it is shown that the precooled cracking gas is then cooled further and partially condensed in a quench. To this end, a cooled cracking gas condensate is introduced at the top of the quench zone. Two take-off streams containing product (VCM) are formed in the quench zone from the cracking gas and the cracking gas condensate introduced at the top:
the bottom discharge product is formed from the discharge product flowing out at the base of the quench zone;
the vapors flowing out at the top of the quench zone form the quench gas.
Only partial condensation of the cracking gas takes place in the quench zone. The liquid running out of the quench zone at the bottom, the so-called bottom discharge product, therefore contains as principal constituents 1,2-dichloroethane, vinyl chloride and high-boiling components and the solids, i.e. soot and/or coke.
The quench gas flowing out at the top of the quench zone as vapors contains hydrogen chloride, vinyl chloride and 1,2-dichloroethane as principal constituents. It is more or less free from the solids, i.e. free from soot and/or coke.
The cracking gas condensate employed as quench liquid is usually branched off from the condensed quench gas as a sub-stream, causing the formation of a quench liquid circuit.
The bottom discharge product and the partially condensed quench gas are then treated further.
EP-0 276 775-B1 gives the temperature of the catalyst-free pyrolysis as from 450 to 550° C. and pyrolysis pressures of from 0.5 to 3 MPa, but preferably from 1.6 to 2.6 MPa, and gentle measures for substance stream guidance.
DE-23 13 037-C3 describes a gentle arrangement of the evaporation.
Since it has proven advantageous to separate off the solids particles entrained by the cracking gas therefrom before distillative treatment of the cracking gas, this is usually carried out together with its cooling and partial condensation in the quench zone. In general, a quench device of simple design, which essentially consists of a vertical tank and a device for atomization of quench liquid in the interior of the tank, generally effects virtually complete purification of the cracking gas. The solids removed accumulate at the bottom of the quench zone.
U.S. Pat. No. 5,558,746 describes a quench column with plates which has a complex design and in which the solids are likewise removed from the circuit together with the bottom discharge product and then separated off. At the same time, the condensation of the cracking gas, which is usually carried out in an external device, is integrated into the quench column.
As is known, the pyrolysis of 1,2-dichloroethane is highly endothermic and characterized by the consumption of large amounts of thermal energy.
There has therefore been no lack of proposals for recovery of as much as possible of the heat present in the hot cracking gas from pyrolysis by means of heat transfer to other media:
EP-0 276 775-B1 indicates four variants for utilization of the cracking gas heat content for combinations of prewarming, evaporation and superheating of the EDC feed into the pyrolysis and for the generation of steam.
DE-31 47 310-C2 indicates the utilization of the heat content of the cracking gas for steam generation or for the heating of bottom forced-circulation evaporators in the distillative treatment in the preparation of VCM.
EP-0 180 995-B2 uses the product stream freed from solids in a quench column for generating steam and warming the EDC feed for the pyrolysis.
A common feature of the above-mentioned process variants is that the pyrolytically generated hot cracking gas is either cooled substantially to the vicinity of the dew point or even condensed at the dew point and therefore has to be re-heated in the subsequent distillative treatment. Even in the case of comprehensive use of the above-described heat recovery measures, there remains on the one hand a significant remainder of unutilized heat lost to the environment due to cooling and on the other hand a deficit of heat which has to be supplied again if necessary.
In the above-mentioned company publication with the title “Vinyl chloride plants/Hoechst process” published by Uhde GmbH in June 1995, the process usual hitherto for the distillative treatment of the cracking gas is described. In the process flow chart reproduced on page 13 of this company publication under the heading “VCM distillation”, it is shown how the cracking gas taken off from the quench zone and freed from solids is then treated to give VCM, the distillative treatment of the pyrolytically generated cracking gas principally being directed toward the actual substance separation of this three-substance system with the three principal components HCl, VCM and unreacted EDC.
Three-substance systems require a comparatively complex separation apparatus for their separation. In addition, the present three-substance system also has a very broad boiling range, which means that the separation task requires solutions with a greater energy requirement than a mixture having a narrower boiling range.
Between the boiling points of HCl (minus 85° C.) and EDC (plus 83.5° C.), there is a temperature difference of 168.5° C. Owing to the very low boiling point of hydrogen chloride (HCl), cooling, preferably with cooling media which supply cold, must be carried out for the separation of HCl. In industry, the separation of HCl is carried out, for example, at 1.3 MPa absolute and −24° C. The industrial generation of cooling media which release cold is significantly more complex and also more complex than the provision of a cooling medium which has to achieve heat dissipation from the temperature level o
Katten Muchin Zavis & Rosenman
Krupp Uhde GmbH
Siegel Alan
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