Ethylene Conversion

In the one-stage process (Fig. 2), ethylene, oxygen, and recycle gas are directed to a vertical reactor for contact with the catalyst solution under slight pressure. The water evaporated during the reaction absorbs the heat evolved, and make-up water is fed as necessary to maintain the desired catalyst concentration. The gases are water-scmbbed and the resulting acetaldehyde solution is fed to a distUlation column. The tad-gas from the scmbber is recycled to the reactor. Inert materials are eliminated from the recycle gas in a bleed-stream which flows to an auxdiary reactor for additional ethylene conversion. 

Similar to IFP s Dimersol process, the Alphabutol process uses a Ziegler-Natta type soluble catalyst based on a titanium complex, with triethyl aluminum as a co-catalyst. This soluble catalyst system avoids the isomerization of 1-butene to 2-butene and thus eliminates the need for removing the isomers from the 1-butene. The process is composed of four sections reaction, co-catalyst injection, catalyst removal, and distillation. Reaction takes place at 50—55°C and 2.4—2.8 MPa (350—400 psig) for 5—6 h. The catalyst is continuously fed to the reactor ethylene conversion is about 80—85% per pass with a selectivity to 1-butene of 93%. The catalyst is removed by vaporizing Hquid withdrawn from the reactor in two steps classical exchanger and thin-film evaporator. The purity of the butene produced with this technology is 99.90%. IFP has Hcensed this technology in areas where there is no local supply of 1-butene from other sources, such as Saudi Arabia and the Far East. 

The quantity of catalyst used for a given plant capacity is related to the Hquid hourly space velocity (LHSV), ie, the volume of Hquid hydrocarbon feed per hour per volume of catalyst. To determine the optimal LHSV for a given design, several factors are considered ethylene conversion, styrene selectivity, temperature, pressure, pressure drop, SHR, and catalyst life and cost. In most cases, the LHSV is ia the range of 0.4—0.5 h/L. It corresponds to a large quantity of catalyst, approximately 120 m or 120—160 t depending on the density of the catalyst, for a plant of 300,000 t/yr capacity. 

Disproportionation of Olefins. Disproportionation or the metathesis reaction offers an opportunity to convert surplus olefins to other desirable olefins. Phillips Petroleum and Institut Fransais du Petrc le have pioneered this technology for the dimerization of light olefins. The original metathesis reaction of Phillips Petroleum was intended to convert propylene to 2-butene and ethylene (58). The reverse reaction that converts 2-butene in the presence of excess ethylene to propylene has also been demonstrated (59). A commercial unit with a capacity of about 136,000 t/yr of propylene from ethylene via 2-butene has been in operation in the Gulf Coast since 1985 (60,61). In this process, ethylene is first dimerized to 2-butene foUowed by metathesis to yield propylene. Since this is a two-stage process, 2-butene can be produced from the first stage, if needed. In the dimerization step, about 95% purity of 2-butene is achieved at 90% ethylene conversion. 

Most of the vinyl acetate produced in the United States is made by the vapor-phase ethylene process. In this process, a vapor-phase mixture of ethylene, acetic acid, and oxygen is passed at elevated temperature and pressures over a fixed-bed catalyst consisting of supported palladium (85). Less than 70% oxygen, acetic acid, and ethylene conversion is realized per pass. Therefore, these components have to be recovered and returned to the reaction zone. The vinyl acetate yield using this process is typically in the 91—95% range (86). Vinyl acetate can be manufactured also from acetylene, acetaldehyde, and the hquid-phase ethylene process (see Vinyl polymers). 

An optimum temperature exists at which the ethanol production rate is maximal. Ethylene conversion is limited by catalyst activity at lower temperatures and by equiUbrium considerations at higher temperatures. 

An optimum ethylene-to-water ratio exists that gives a maximum ethanol production rate. However, as expected, the highest ethylene conversion is obtained at the lowest ethylene-to-water mole ratio. 

To prevent further oxidation of ethylene oxide, the ethylene conversion of the commercial processes is typically between 10 and 20%. 

The per pass ethylene conversion in the primary reactors is maintained at 20—30% in order to ensure catalyst selectivities of 70—80%. Vapor-phase oxidation inhibitors such as ethylene dichloride or vinyl chloride or other halogenated compounds are added to the inlet of the reactors in ppm concentrations to retard carbon dioxide formation (107,120,121). The process stream exiting the reactor may contain 1—3 mol % ethylene oxide. This hot effluent gas is then cooled ia a shell-and-tube heat exchanger to around 35—40°C by usiag the cold recycle reactor feed stream gas from the primary absorber. The cooled cmde product gas is then compressed ia a centrifugal blower before entering the primary absorber. 

Reaction conditions for the free radical polymerization of ethylene are 100-200°C and 100-135 atmospheres. Ethylene conversion is kept to a low level (10-25%) to control the heat and the viscosity. However, overall conversion with recycle is over 95%. 

The composition of the gas mixture, which is introduced into the tube bundle reactor (tubes of 6-12 m length and 20-50 mm diameter, filled with the Ag catalyst) consists of 15-50 vol % ethylene, 5-9% oxygen, as much as 60% methane as dilution gas, and 10-15% carbon dioxide. The reaction therefore proceeds above the upper explosion limit. The ethylene conversion runs up to 10% per cycle through the reactor. The ethylene oxide selectivity amounts to 75-83 % maximum. The formed ethylene oxide is recovered by scrubbing with water and the newly formed carbon dioxide is separated from the cycle gas, e.g., by hot potash washing process. 

Figure 12.11. Temporal evolution of the ethylene conversion in the multiple-channel Ru02/YSZ cell during a potentiostatic step of UM =30 V. OC open-circuit. Feed composition C2H4 O2/0.2 12 kPa, Fv=175 cm3 STP/min, T=360°C.9 Reprinted with permission from the Electrochemical Society.

The reaction is exothermic and so to avoid serious temperature excursions the reactor consists of a bundle of narrow tubes, each a few centimeters in diameter, surrounded by a heat transfer medium. The catalyst consists of relatively large silver particles on an inert a-Al203 support. The surface area is below 1 m g". Promoters such as potassium and chlorine help to boost the selectivity from typically 60% for the unpromoted catalysts to around 90%, at ethylene conversion levels of the order of50%. 

Catalytic performances in ethylene ammoxidation as function of reaction temperature of the different catalysts are compiled in Table 2. Data were collected under stationary conditions after a transition period of one hour. All catalysts are active and selective toward acetonitrile. Wherein, Cr-Cl catalyst exhibits the higher ethylene conversion and the higher acetonitrile selectivity. Chromium with highly oxidation state (VI) seems to play a key role in the ammoxidation reaction as confirmed by TPR and DRS spectroscopy results. This idea is strongly supported by the difference between catalytic behaviour of Cr03 and Cr203 supported on ZSM-5. Nevertheless, Cr(III) oxide seems to... 

In the second step, the crude ethylene plus benzene are passed over a silica mordenite catalyst at 650—700° F and atmospheric pressure to produce EB at 91% selectivity and 23% ethylene conversion. 

The ethylene conversion to ethyl alcohol per pass through the reactor is only 4-6%, so most of the ethylene needs to be recycled. But first the reactor effluent is cooled and caustic washed to neutralize any vaporized H3PO4. As the effluent cools down, the ethyl alcohol liquifies, and the ethylene can easily be separated. The ethylene recycle stream is then scrubbed by sloshing it through water prior to recycle. 

Fig. 2. Ethylene conversion for Pd/V205, pure V2Os and Pd/Al203.

The role of Brpnsted acid sites in the oligomerization of ethylene over HZSM-5 has been studied.[30,31] Amin and Anggoro1311 concluded that dealumination of HZSM-5 led to higher ethylene conversion, but the gasoline selectivity was reduced compared with a nondealuminated HZSM-5 (Si/Al = 15) zeolite sample. 

In the ring-closing metathesis reaction, intramolecular metathesis closes a ring to form a small cyclic molecule with concurrent loss of a small molecule (ethylene). Conversely, in the case of the acyclic diene metathesis reaction, macromolecules are formed by successive intermolecular condensation of two olefinic molecules [1],... 

Fig. 3.44. Stereo- and regioselective formation of a bromohydrin from a trisubstituted ethylene conversion to an epoxide.

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