In oxychlorination, ethylene reacts with dry HCl and either air or pure oxygen in a heterogeneous catalytic reaction to form EDC and water.
C2H4 + 2 HCl + ½O2 → C2H4Cl2 + H2O
While there are many different commercial oxychlorination processes, in each case the reaction is carried out in the vapor phase in either a fixed bed or fluid bed reactor containing a modified Deacon catalyst. Unlike the Deacon process, however, oxychlorination of ethylene occurs readily at temperatures well below those required for HCl oxidation. The catalyst typically contains cupric chloride (CuCl2) as the primary active ingredient, impregnated on a porous support such as alumina, and may also contain numerous additives.
Oxychlor® 8 catalyst is ideally suited for fluid bed reactor operation. This sixth-generation catalyst is the result of continuous research and development, which has led to increased efficiencies and ease of operability. Oxychlor® catalysts are extremely robust and highly tolerant to process upsets and diverse operating conditions. In addition to easy operability, the latest generation of catalyst operates at higher temperatures, facilitating heat removal and higher reactor productivity, along with improved feedstock efficiencies and reduced catalyst losses.
The oxychlorination reaction is exothermic (ΔHrxn = -239 kJ/mol EDC made) and requires heat removal for temperature control, which is essential for efficient production of EDC. Higher reactor temperatures lead to more by-products, mainly through increased ethylene oxidation to carbon oxides and increased EDC cracking. (Cracking of EDC yields VCM, and subsequent oxychlorination and cracking steps lead progressively to by-products with higher levels of chlorine substitution.) High temperatures (>300°C) can also deactivate the catalyst through increased sublimation of CuCl2.
Chlorinated by-products of ethylene oxychlorination typically include 1,1,2-trichloroethane, chloroform, carbon tetrachloride, ethyl chloride, chloral, 2-chloroethanol, all of the chloroethylene congeners, and higher boiling compounds. Of particular concern is chloral, because it polymerizes in the presence of strong acids. Chloral must be removed (by caustic washing) to prevent the formation of solids, which can foul and clog processing equipment downstream.
Oxychlorination process feed purity can also contribute to by-product formation, although the problem usually is only with the low levels of acetylene which are normally present in HCl from the EDC cracking process. Since any acetylene fed to the oxychlorination reactor will be converted to highly chlorinated C2 products, selective hydrogenation of this acetylene to ethylene and ethane is widely used as a preventive measure.
Fixed Bed Technology
Fixed bed reactors resemble multi-tube heat exchangers, with the catalyst packed in vertical tubes held in a tubesheet at top and bottom. Uniform packing of catalyst within the tubes is important to ensure uniform pressure drop, flow, and residence time through each tube. Reaction heat is removed by generating steam on the shell side of the reactor, or by flowing some other heat transfer fluid through it. However, temperature control is more difficult in a fixed bed than in a fluid bed reactor because localized hot spots can develop in the tubes. The tendency to develop hot spots can be minimized by packing the reactor tubes with active catalyst and inert diluent mixtures in proportions that vary along the length of the tubes, so that there is low catalyst activity at the inlet, but the activity steadily increases to a maximum at the outlet. Another remedy is to pack the tubes with catalysts having a progressively higher loading of cupric chloride so as to provide an activity gradient along the length of the tubes. Multiple reactors in series are also used in fixed bed oxychlorination, primarily to control heat release by staging the air or oxygen feed. Each successive reactor may also contain catalyst with a progressively higher loading of cupric chloride. These methods of staging the air or oxygen feed and of grading the catalyst activity work to flatten the temperature profile and allow improved temperature control. Compared with the fluid bed process, fixed bed oxychlorination generally operates at higher temperatures (230-300°C) and gauge pressures (150-1400 kPa or 22-203 psig).
Fluid Bed Technology
Fluid bed oxychlorination reactors typically are vertical cylindrical vessels equipped with a support grid and feed sparger system designed to provide good fluidization and feed distribution. They contain internal cooling coils for heat removal, and use either internal or external cyclones to minimize catalyst carryover. Fluidization of the catalyst assures intimate contact between feed and product vapors, catalyst, and heat transfer surfaces, and results in a uniform temperature within the reactor. Reaction heat is removed by generating steam within the cooling coils or by passing some other heat transfer medium through them. An operating temperature of 220-245°C and gauge pressure of 150-500 kPa (22-73 psig) are typical for oxychlorination with a fluid bed reactor.
While fluid bed oxychlorination reactors are generally well-behaved and operate predictably, under certain (usually upset) conditions, they are subject to a phenomenon known as stickiness. This can best be described as catalyst particle agglomeration characterized by declining fluidization quality and, in severe cases, a slumped or collapsed bed. Oxychlorination catalyst stickiness is brought on by adverse operating conditions that promote the formation of dendritic growths of cupric chloride on the surface of individual catalyst particles, which leads to increasing interparticle interactions and agglomeration. All fluid bed oxychlorination catalysts normally exhibit some level of catalyst particle agglomeration/deagglomeration dynamics, and the severity of stickiness depends on catalyst characteristics as well as process operating conditions. Stickiness can be largely avoided by using catalyst formulations that exhibit excellent fluidization characteristics over a wide range of operating conditions. OxyVinyls' Oxychlor® 8 catalyst is extremely resistant to the effects of stickiness.
The use of oxygen instead of air permits operation at lower temperatures and results in significantly improved operating efficiency and product yield. Ethylene is generally fed in somewhat larger excess over stoichiometric requirements than in the air-based process. The reactor effluent is cooled, purified from traces of unconverted HCl, separated from EDC and water by condensation, recompressed to the reactor inlet pressure, reheated, and ultimately recycled. This permits lower ethylene conversion per pass through the reactor (giving higher selectivity to EDC) with an increase in overall ethylene yield.
Another important advantage of oxygen-based oxychlorination over air-based operation is the drastic reduction in volume of the vent stream. Only a small fraction of the reactor off-gas, typically 2-5%, is continuously purged to prevent accumulation of impurities such as carbon oxides, nitrogen, argon and un-reacted light hydrocarbons, which either form in the reactor or enter the process with the feed streams. In the air-based process, a large vent flow rate is required because of the nitrogen which enters with the air feed stream. (Nitrogen is actually the single component with the highest molar feed rate in the air-based process.)
Oxygen-Based Oxychlorination Technology
In the air-based oxychlorination process, ethylene and air are fed in slight excess of stoichiometric requirements to ensure high conversion of HCl while minimizing the loss of excess ethylene in the vent stream. Under these conditions, typical feedstock conversions are 94-99 percent for ethylene and 98.0-99.5 percent for HCl, with EDC selectivities of 94-97 percent. Downstream product recovery involves cooling the reactor effluent by either direct quench or with a heat exchanger, and condensing the EDC and water, which are then separated in a decanter. The remaining gases still contain 1-5 percent EDC by volume and, therefore, are usually further processed in a secondary recovery system involving either solvent absorption or a refrigerated condenser. If ethylene conversion is high, the dilute ethylene remaining in the vent is generally incinerated, but if conversion is low enough to justify it, various schemes may first be used to recover the unconverted ethylene, usually by direct chlorination to EDC.
Air-Based Oxychlorination Technology