High purity ethylene

Ethjlben ne Synthesis. The synthesis of ethylbenzene for styrene production is another process in which ZSM-5 catalysts are employed. Although some ethylbenzene is obtained direcdy from petroleum, about 90% is synthetic. In earlier processes, benzene was alkylated with high purity ethylene in liquid-phase slurry reactors with promoted AlCl catalysts or the vapor-phase reaction of benzene with a dilute ethylene-containing feedstock with a BF catalyst supported on alumina. Both of these catalysts are corrosive and their handling presents problems. 

Polymerization. Very high purity ethylene (>99.9% plus) is polymerized under specific conditions of temperature and pressure in the presence of an initiator or catalyst. 

A more recent development in ethylene polymerization is the simplified low pressure LDPE process. The pressure range is 0.7—2.1 MPa with temperatures less than 100°C. The reaction takes place in the gas phase instead of Hquid phase as in the conventional LDPE technology. These new technologies demand ultra high purity ethylene. 

The following is a description of plants leading to specific light ends cuts. This includes producing LPG propane, and also high purity ethylene. 

Significant quantities of Cj and C, acetylenes are produced in cracking. They can be converted to olefins and paraffins. For the production of high purity ethylene and propylene, the contained Cj and C3 acetylenes and dienes are catalytically hydrogenated leaving only parts per million of acetylenes in the products. Careful operation is required to selectively hydrogenate the small concentrations of acetylenes only, and not downgrade too much of the wanted olefin products to saturates. 

Polyethylene (low-pressure) 50,000 22,000,000 440 0.70 High-purity ethylene required... 

As shown in Figure 16-2, high purity ethylene (99.7%) and oxygen (99.0%) are fed under pressure (100 psi) to a vertical reactor containing the aqueous catalyst solution. Reaction temperature is maintained at 250-275°F. Because the reaction is exothermic, heat liberated is partially removed by vaporizing the water present in the reactor. Malceup water is continuously fed to the reactor to maintain proper catalytic solution concentration. 

High purity ethylene gas plus recycle ethylene are fed to a compression chamber, compressed and then fed along with catalyst previously dissolved in a suitable solvent into, parallel horizontal reactors, as many- as eight in parallel. Each reactor consists of a water-filled shell containing a single pipe, coiled to give maximum contact with the water. Reaction conditions are 350-425 F and 2000-3000 psi. 

Most sidestream columnsnave a small flow dedicated to removing an off-key impurity entering the feed, and that stream must be manipulated to control its content in the major product. For example, an ethylene fractionator separates its feed into a high-purity ethylene sidestream, an ethane-rich bottom product, and a small flow of methane overhead. This small flow must be withdrawn to control the methane content in the ethylene product. The key impurities may then be controlled in the same way as in a two-product column. 

Separation of the C2 stream to produce high-purity ethylene and ethane requires a large tower, sometimes the largest one in the plant. Separation of the C3 stream to produce high-purity propylene and propane also requires a large tower, and in some plants it is the largest one. Separation of butadiene from the C4 stream, if performed, is usually accomplished by extractive distillation. Aromatics are frequently recovered and separated to obtain benzene, toluene, and xylenes, especially when heavy feedstocks are used. 

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 VCM typically is carried out at temperatures of 430-530°C without a catalyst. The hot gases are quenched and distilled to remove HC1 and then VCM. The unconverted EDC is returned to the EDC purification train. The... 

A mixture of ethylene and butene-1 is prepared by the dimerization of ethylene in the presence of organic aluminium compound AIR3 in a boiling solvent reaction zone. High purity ethylene is fed into the dimerization reactor operating at 27 atm. The dimerization takes place... 

The aqueous solution rich in ethylene oxide is sent to purification. It passes through a stripping column, which operates under vacuum and separates the ethylene oxide at the top. The aqueous effluent leaving at the bottom is recycled to the absorption stage. It can be treated in an auxiliary unit to recover the glycol it contains. The top effluent which, in addition to carbon dioxide, contains acetaldehyde and hydrocarbon traces, is sent to two distillation columns in series, one for dehydration ( = 20 trays), and the second for purification (S 50 trays). These columns produce high-purity ethylene oxide with a very low acetaldehyde content. The product is stored in liquid form in tanks under nitrogen pressure. 

In this version, high-purity ethylene (99.8 per cent volume) and oxygen (99.5 per cent volume), mixed with dilution steam, are introduced at different levels at the base.of a titanium reactor more than 20 m high, containing 10 to-45 perforated trays and holdup catalyst solution. Conversion takes place at 0J to 5. 106 Pa, absolute, at a temperature kept at around 120 to 130°C by the vaporization of a fraction of the reaction medium (especially water), which removes the heat liberated by the oxidation of ethylene. 

Two different sources for the ethylene sample gas were used. The Gulf Oil Company-U.S., a participant in the OSRD ethylene program, contributed a cylinder of high purity ethylene for this program from its Cedar Bayou olefin plant. This source was used for the isotherms from 0° to 100°C, which were determined in order of increasing temperature. For the higher temperature isotherms and for comparative information at 25° and 100°C, the source was changed to a cylinder of chemically pure... 

We are tempted to proceed a little bit further, and examine the development of the whole flowsheet in relation with the reaction system. Let s suppose that the feedstock is of high purity ethylene and benzene. Because recycling a gas is much more costly than a liquid, we consider as design decision the total conversion of ethylene. The benzene will be in excess in order to ensure higher conversion rate, but also to shift the equilibrium. The equilibrium calculation can predict with reasonable accuracy the composition of the product mixture for given reaction conditions. Then polyalkylates, mainly diethylbenzene can be reconverted to ethylbenzene in a second reactor. 

The primary advantage of the two-stage process is that it uses atmospheric air rather than high purity oxygen. Another possible advantage is that the process can use low purity ethylene (95%) if it happens to be available at an attractive price. This advantage is not very important for the modern petrochemical complex where high purity ethylene is readily available. 

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