The invention relates to a method and an apparatus for utilizing the heat of reaction evolved in the preparation of 1,2-dichloroethane, hereinafter referred to as EDC. EDC serves predominantly as intermediate in the preparation of monomeric vinyl chloride, hereinafter referred to as VCM, from which polyvinyl chloride, PVC, is finally prepared. The conversion of EDC into VCM forms hydrogen chloride HCl. EDC is therefore preferably prepared from ethene C2H4 and chlorine Cl2 in such a way that a balance is achieved between the hydrogen chloride HCl produced and consumed in the reactions according to the following reaction equations:Cl2+C2H4→C2H4Cl2 (pure EDC)+218 kJ/Mol  (1)C2H4Cl2 (dissociation EDC)→C2H3Cl (VCM)+HCl−71 kJ/Mol  (2)C2H4+2 HCl+½O2→C2H4Cl2 (crude EDC)+H2O+238 kJ/Mol  (3)
The process for preparing VCM having an HCl balance, hereinafter referred to as “balanced VCM process” for short, comprises:                a direct chlorination in which a part of the EDC required is produced from ethene C2H4 and chlorine Cl2 and is passed on as pure EDC; the utilization of the heat of reaction produced in this direct chlorination is a central aspect of the invention;        an oxychlorination in which the other part of the EDC is produced from ethene C2H4, hydrogen chloride HCl and oxygen O2 and is passed on as crude EDC;        a fractionating EDC purification in which the crude EDC together with the recycle EDC recirculated from the VCM fractionation is freed of the by-products formed in the oxychlorination and in the EDC pyrolysis in order to obtain a feed EDC suitable for use in the EDC pyrolysis; the utilization of the heat of reaction of the direct chlorination in the EDC purification is a central aspect of the invention;        an EDC pyrolysis in which the pure EDC is combined with the feed EDC and in which the mixture then known as dissociation EDC is thermally dissociated; the dissociation gas obtained contains VCM, hydrogen chloride HCl and unreacted EDC and also by-products;        a VCM fractionation in which the pure VCM desired as product is separated off from the dissociation gas and the other significant dissociation gas constituents hydrogen chloride HCl and unreacted EDC are separately recovered as materials of value and are recirculated as reusable feed in the form of recycle HCl or recycle EDC in the balanced VCM process.        
Plants which serve for the preparation of pure EDC and do not operate according to the balanced VCM process are usually designed as pure direct chlorinations which operate only according to equation (1). The invention likewise relates to plants which are designed according to the balanced VCM process and also to those which have only a direct chlorination, and to mixed forms of the two types.
The chlorine Cl2 required in the direct chlorination is usually produced from sodium chloride NaCl in an electrolysis plant. Sodium hydroxide NaOH having a concentration of about 33% is formed here as coproduct. Owing to the high toxicity of the chlorine Cl2 produced, efforts are made to avoid transport over long distances if possible. For this reason, a plant for the direct chlorination of ethylene C2H4 in which the chlorine Cl2 is directly processed further is usually located in the immediate vicinity of a plant for preparing sodium hydroxide NaOH and chlorine Cl2.
The plant for the direct chlorination of ethylene does not have to be present in an integrated facility but can produce EDC in “stand-alone operation”, with the relatively nonhazardous “transport form” of the chlorine being transported to other sites in order to be processed into VCM there. The EDC prepared by the process described, for example, in WO 01/34542 A2, in particular, is so pure that it does not require further work-up by distillation. For this reason, when such a plant is operated in the “stand-alone mode”, there is no longer the opportunity of recovering heat by means of column heating of EDC distillation columns, for example those which were present in the integrated facility for the “balanced VCM process” which is made up of direct chlorination, oxychlorination and EDC dissociation.
In this set-up, the considerable heat of reaction therefore has to be removed by means of large amounts of cooling water and/or air coolers, but each of these are undesirable for economic reasons. It is therefore an object of the invention to pass the heat evolved in the direct chlorination to a use and to achieve a significant reduction in the cooling water requirement.
If the direct chlorination is used within a balanced VCM process, it has to be taken into account that the secondary components formed in the dissociation gas in the EDC pyrolysis reduce the product purity of the VCM. The purification of the VCM by removal of the secondary components is accordingly complicated. A dissociation EDC which has been largely freed of impurities is therefore used in the EDC pyrolysis. For the large number of techniques by means of which the corresponding and disadvantageous by-products and/or secondary components can be avoided or, if appropriate, removed, reference may again be made to the document WO 01/34542 A2, in particular the prior art acknowledged there. It was able to be shown here that the heat of reaction liberated in the direct chlorination process by reaction of ethene C2H4 and chlorine Cl2 to form liquid EDC is sufficient to operate the purification columns for EDC produced in the balanced VCM process.
However, a disadvantage of the process presented there is that the utilization of the heat present in the EDC can take place only at relatively high temperatures, i.e. predominantly above about 100° C. Although the operation of the apparatuses for purifying the EDC can be achieved by means of the evolved heat alone, the further cooling of the EDC produced, e.g. for later use, still has to be carried out by means of cooling water, for which large amounts of cooling water are still required.
A further disadvantage of the process presented there is that the heat of reaction used for heating the purification columns requires the removal of a corresponding quantity of heat of condensation of the vapour. This removal is, according to the conventional state of the art, usually likewise achieved by means of cooling water which has to be provided in large quantities.