Oxychlorination step

Figure 10.4 The oxychlorination step of the vinyl chloride process. (From Smith and Petela, Chem. Eng., 513 24, 1991 reproduced by permission of the Institution of Chemical Engineers.)...

The third most crucial stage in the balanced process is the oxychlorination step.188-190,272,273 In this reaction ethylene and HC1 are converted to 1,2-dichloroethane in an oxidative, catalytic process. The reaction proceeds at temperatures of 225-325°C and pressures 1-15 atm. Pure oxygen or air is used as oxidant.276-278 Numerous, somewhat different industrial processes were developed independently.272-274 However, the reaction is generally carried out in the vapor phase, in fixed-bed or fluidized-bed reactors. 

Most of the chlorinated waste is produced in the oxychlorination step. Therefore, employing only direct chlorination of ethylene is more beneficial from the envi-... 

The oxychlorination step is described by the following global reaction ... 

In the balanced process, all of the hydrochloric acid produced in the ethylene dichloride pyrolysis is used as feed to the oxychlorination step so that there is no net production or consumption of hydrochloric acid. The ethylene feedstock is split with about half used in the chlorination reaction and the other half in the oxychlorination reaction. A block flow diagram illustrating the major steps in the process is shown in Figure 13 [20]. 

The chlorination process is well known and has been thoroughly covered in the literature. Therefore, it will be given only this cursory treatment here. The use of industrial gas as a feedstock applies in the oxychlorination step where there is a choice between air and high purity oxygen [20]. 

EDC-water condensate to washing step of oxychlorination (step 3)... 

MeXTH [Methyl halides to Hydrocarbons] (X = Cl or Br) A general name for processes for converting methanol (from methane) to hydrocarbons via methyl halides. This has been proposed as an alternative to the MTO process, requiring less energy, but it has not yet been commercialized. An oxychlorination step is followed by reaction over a zeotype catalyst such as H-SAPO-34. 

The direct chlorination of ethylene usually is run in the liquid phase and is catalyzed with ferric chloride. High-purity ethylene normally is used to avoid product purification problems. The cracking (pyrolysis) of EDC to vinyl chloride typically is carried out at temperatures of 430 to 530°C over a catalyst. The hot gases are quenched and distilled to remove HCl and then VCM. The unconverted EDC is returned to the EDC purification train. The oxychlorination step is the heart of the process and has two major variables, the type of reactor and the oxidant. The reactor may be either a fixed bed or a fluidized bed, and the oxidant is either air or oxygen. The temperature is in the range of 225 to 275" C with a copper chloride-impregnated catalyst. 

Consider vinyl chloride production (see Example 2.1). In the oxychlorination reaction step of the process, ethylene, hydrogen chloride, and oxygen are reacted to form dichloroethane ... 

In a typical balanced plant producing vinyl chloride from EDC, all the HCl produced in EDC pyrolysis is used as the feed for oxychlorination. On this basis, EDC production is about evenly spHt between direct chlorination and oxychlorination, and there is no net production or consumption of HCl. The three principal operating steps used in the balanced process for ethylene-based vinyl chloride production are shown in the block flow diagram in Eigure 1, and a schematic of the overall process for a conventional plant is shown in Eigure 2 (76). A typical material balance for this process is given in Table 2. 

If the production of vinyl chloride could be reduced to a single step, such as dkect chlorine substitution for hydrogen in ethylene or oxychlorination/cracking of ethylene to vinyl chloride, a major improvement over the traditional balanced process would be realized. The Hterature is filled with a variety of catalysts and processes for single-step manufacture of vinyl chloride (136—138). None has been commercialized because of the high temperatures, corrosive environments, and insufficient reaction selectivities so far encountered. Substitution of lower cost ethane or methane for ethylene in the manufacture of vinyl chloride has also been investigated. The Lummus-Transcat process (139), for instance, proposes a molten oxychlorination catalyst at 450—500°C to react ethane with chlorine to make vinyl chloride dkecfly. However, ethane conversion and selectivity to vinyl chloride are too low (30% and less than 40%, respectively) to make this process competitive. Numerous other catalysts and processes have been patented as weU, but none has been commercialized owing to problems with temperature, corrosion, and/or product selectivity (140—144). Because of the potential payback, however, this is a very active area of research. 

A typical reactor operates at 600—900°C with no catalyst and a residence time of 10—12 s. It produces a 92—93% yield of carbon tetrachloride and tetrachloroethylene, based on the chlorine input. The principal steps in the process include (/) chlorination of the hydrocarbon (2) quenching of reactor effluents 3) separation of hydrogen chloride and chlorine (4) recycling of chlorine to the reactor and (i) distillation to separate reaction products from the hydrogen chloride by-product. Advantages of this process include the use of cheap raw materials, flexibiUty of the ratios of carbon tetrachloride and tetrachloroethylene produced, and utilization of waste chlorinated residues that are used as a feedstock to the reactor. The hydrogen chloride by-product can be recycled to an oxychlorination unit (30) or sold as anhydrous or aqueous hydrogen chloride. 

Hexachloroethane is formed in minor amounts in many industrial chlorination processes designed to produce lower chlorinated hydrocarbons, usually via a sequential chlorination step. Chlorination of tetrachloroethylene, in the presence of ferric chloride, at 100—140°C is one convenient method of preparing hexachloroethane (142). Oxychlorination of tetrachloroethylene, using a copper chloride catalyst (143) has also been used. Photochemical chlorination of tetrachloroethylene under pressure and below 60°C has been studied (144) and patented as a method of producing hexachloroethane (145), as has recovery of hexachloroethane from a mixture of other perchlorinated hydrocarbon derivatives via crystalH2ation in carbon tetrachloride. Chlorination of hexachlorobutadiene has also been used to produce hexachloroethane (146). 

The third step, the oxychlorination of ethylene, uses hy-product HCl from the previous step to produce more ethylene dichloride ... 

The reaction conditions are approximately 225 °C and 2-4 atmospheres. In practice the three steps, chlorination, oxychlorination, and dehydrochlorination, are integrated in one process so that no chlorine is lost. Figure 7-5 illustrates the process. 

Trichloroethylene is currently produced in the United States using ethylene dichloride (a produet of ethylene and chlorine feedstocks) (EPA 1985e). PPG Industries uses a single-step oxychlorination proeess, which yields triehloroethylene and tetraehloroethylene. In the PPG proeess, ethylene dichloride is reaeted with chlorine and/or hydrogen chloride and oxygen to form the triehloroethylene and tetraehloroethylene. DOW Chemical produces trichloroethylene by a direct chlorination process, in which ethylene dichloride is reacted with chlorine to form trichloroethylene and tetraehloroethylene. 

Since more than 60% of the EO production is converted directly to EG, the obvious question some macho chemist might ask is why don t we do an end run and just convert ethylene directly to EG Skip the oxidation step. Research starting 50 years ago led to several promising commercial processes, oxychlorination and acetoxylation. Exotic catalysts were used, and both avoided the EO step. But neither process was quite effective enough to replace the ethylene-to-EO-to-MEG route, which predominates today. 

It consists of three basic steps direct chlorination of ethylene to form 1,2-dichloro-ethane [Eq. (6.40)], cracking of 1,2-dichloroethane to vinyl chloride and HC1 [Eq. (6.41)], and oxychlorination of ethylene with HC1 [Eq. (6.42)] formed in the second step. The net reaction is the oxychlorination of ethylene to vinyl chloride [Eq. (6.43)] ... 

Commercialization of a new vinyl chloride process has been announced. Instead of the traditional three-step production (see Section 6.3.4), it is based on ethane oxy-chlorination using HC1, 02, and Cl2 carried out over a CuCl-based catalysts.285 Overchlorinated products are dehydrochlorinated and hydrogenated (together with overchlorinated alkenes) in separate reactors these product streams are then led back to the oxychlorination reactor. 

Figure 10.11 shows an integrated plant for producing EDC and vinyl chloride from ethylene, chlorine, and air. In this process, vinyl chloride (VCM) is produced by the thermal cracking of EDC. The feed EDC may be supplied from two sources. In the first source, ethylene and chlorine are reacted in essentially stoichiometric proportions to produce EDC by direct addition. In the second source, ethylene is reacted with air and HC1 by the oxychlorination process. Ideally, both processes are carried out in balance, and the oxychlorination process is used to consume the HC1 produced in the cracking and direct chlorination steps. The chemical reactions are... 

Application In the balanced vinyl chloride (VCM) process, pure oxygen and ethylene are used in the oxychlorination (also known as oxy) step to convert hydrogen chloride into ethylene dichloride (EDC) with minimum vent gas, and without any micro-pollutant or heavy metals contamination of wastewater, or costly catalyst sludge accumulation. 

This process involves four main steps. The first,1 conducted at elevated temperature (230 to 270 Q concerns tbe action of benzene on a mixture of hydrochloric acid gas and air in the presence of an oxychlorination catalyst, consisting of copper and iron chlorides on an inert support Once-through conversion is limited to between 10 and 15 per cent to prevent the excessive formation of polychlorobenzenes (10 to 12 molar per cent) This conversion is as high as 98 per oent in relation to the hydrochloric acid. Since the reaction is exothermic, the catalyst is distributed in several beds, between which benzene injections at a lower temperature than those of the reaction streams serve to control the overall temperature. 

This step is followed by the purification of monochlorobenzene by distillation. In its initial version, this operation first involves the partial condensation of the oxychlorination products, followed by their introduction into a brick-lined column, with separation of the water/benzene azeotrope at the top. which is recycled to the reactor after settling. The 1/1 mixture of benzene and chlorobenzenes obtained at the bottom is neutralized with caustic soda, washed with water, and distilled in two columns to separate the dichlorobenzenes, monochlorobenzene and benzene. 

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