Ethylene products, propylene conversion over

In the chapter on olefms plants, in the section on propylene, a route to making propylene involved butene-2. In this process, called metathesis, ethylene and butene-1 are passed over a catalyst, and the atoms do a musical chair routine. When the music stops, the result is propylene. The conversion of ethylene to propylene is an attraction when the growth rate of ethylene demand is not keeping up with propylene. Then the olefins plants produce an unbalanced product slate, and producers wish they had an on-purpose propylene scheme instead of just a coproduct process. The ethylene/butene-2 metathesis process is attractive as long as the supply of butylenes holds out. Refineries are big consumers of these olefins in their alkylation plants, and so metathesis process has, in effect, to buy butylene stream away from the gasoline blending pool. 

The normal butenes were pyrolyzed in the presence of steam in a nonisothermal flow reactor at 730°-980°C and contact times between 0.04 and 0.15 sec to obtain conversion covering the range between 3% and 99%. Isomerization reactions accompanied the decomposition of these olefins however, the decomposition was the dominant reaction under these conditions. Pyrolysis of 1-butene is faster than that of either cis- or trans-2-butene. Methane, propylene, and butadiene are initial as well as major products from the pyrolysis of the n-butenes. Hydrogen is an initial product only from the 2-butenes. Ethylene appears to be an initial product only from 1-butene it becomes the most prominent product at high conversions. Over the range of conditions of potential practical interest, the experimental rate expressions for the disappearance of the respective butene isomers, have been derived. 

Ethylene oxidation over five CuO catalysts prepared with various amounts of KOH was found to have a rate equal to A(C2H4) by Koutsoukos and Nobe 172). They used 0.02-0.1% C2H4 in air at 217-340°, and observed activation energies of 20-27 kcal/mole. In related work, Accomazzo and Nobe 173) used a supported copper catalyst (50 % CuO, 50 % AlgOj) for the oxidation of ethylene, propylene, and other hydrocarbons at low concentrations in air. The rate was expressed as equal to l (olefin) . Activation energies for ethylene and propylene were 18 and 17.5 kcal/mole, respectively. Propylene could be oxidized to an equal extent at about 20° lower temperature than for ethylene. The products were essentially only COg and HgO. Diffusion effects and conversion equations were discussed. 

Attempts to use cheaper feed stocks led to the development of new processes. For example, the drive to replace olefins as reactants by parafSns led to the development of processes in which ethane rather ethylene is used to produce vinyl chloride, propane rather than propylene to produce acrylonitrile, and butane rather than benzene to produce maleic anhydride. The drive to produce more-economical synthesis gas from methane has motivated various novel process developments. Moreover, enviromnental regulations and needs caused modifications of many processes in order to minimize the production of pollutants. Most reactors are designed to handle a relatively narrow range of feed concentrations and space velocities. A different design approach has to be used if reactors are to destroy pollutants, as they have to operate at high conversion over a very wide range of feed compositions and feed rates. 

The best olefin yields were observed over Pt-coated monoliths. In the case of ethane/02 mixtures, selectivities to ethylene up to 65% at 70% ethane conversion and complete O2 conversion were reported." The oxidative dehydrogenation of propane and -butane produced total olefin select vies of about 60% (mixtures of ethylene and propylene) with high paraffin conversions." " Mixtures of ethylene, propylene and 1-butene were observed by the partial oxidation of -pentane and n-hexane ethylene, cyclohexene, butadiene and propylene were the most abundant products of the partial oxidation of cyclohexane." ... 

For both feeds, propylene and butadiene are the major products obtained. The order of their production, however, appears to depend on the structure of the feed olefin. Thus, butadiene is the single, most prevalent product from the 2-butenes while propylene is predominant from 1-butene. A similar reversal in relative yield can be noted for the Ci/C2 products. Over the entire conversion range, the combined yields of butadiene and ethylene are approximately equal to the combined yields of propylene and methane for both starting olefins. 

Figure 6 shows the product distribution of the reaction over HY at 723K with the conversion of n-hexane in the lower conversion region. The distribution was also obtained on the basis of the statistical treatment of the experimental data. The formation of methane was observed. The relative amounts of ethylene and ethane formed were much smaller than those of propylene and propane, and they were seemingly not varied much by the conversion. 

The data in Table 1 summarize catalytic activities for epoxidation of a variety of olefins over an unpromoted 5%Ag/Al203 catalyst. These data illustrate the preferential reactivity at the allylic position relative to addition of oxygen across the C=C bond. While the selectivity to ethylene oxide is typical for an unpromoted catalyst, the selectivities to propylene oxide and butylene oxides are non-existent for propylene, 1-butene, and 2-butene, respectively. In addition to small amounts of the selective allylic oxidation products (acrolein in the case of propylene and butadiene in the case of 1-butene), the only products are those of combustion. However, the results for butadiene reveal it is possible to epoxidize this non-allylic olefin at moderate selectivity and activity. What is not obvious from Table 1 is the short-lived nature of this activity. After 2-3 hours of reaction time, activity and selectivity typically decreased to approximately <1% conversion of C4H6 and approximately 50-75% selectivity to epoxybutene. A typical chromatogram of the activity of an... 

Many acids appear as minor by-products when the higher olefins are oxidized. Selective oxidations are rare. Ethylene can be oxidized rather weU to acetic acid. For example, Gurdjian 155) reported 74% selectivity to acetic acid at 39 % conversion of ethylene over phosphomolybdic acid on silica at 290°. Propylene or acrolein can be oxidized to acrylic acid with quite good selectivity, as is shown in a number of patents. Generally molybdenum catalysts are used, moderated with P, B, Bi, Te, or As. With a highly active catalyst composed of vanadium and molybdenum oxides, propylene is oxidized mainly to acetic acid, according to Aliev and co-workers 165). 

By far the largest outlet for benzene (approx. 60%) is styrene (phenyl-ethene), produced by the reaction of benzene with ethylene a variety of liquid and gas phase processes, with mineral or Lewis acid catalysts, are used. The ethylbenzene is then dehydrogenated to styrene at 600-650°C over iron or other metal oxide catalysts in over 90% selectivity. Co-production with propylene oxide (section 12.8.2) also requires ethylbenzene, but a route involving the cyclodimerization of 1,3-butadiene to 4-vinyl-(ethenyl-) cyclohexene, for (oxidative) dehydrogenation to styrene, is being developed by both DSM (in Holland) and Dow. 60-70% of all styrene is used for homopolymers, the remainder for co-polymer resins. Other major uses of benzene are cumene (20%, see phenol), cyclohexane (13%) and nitrobenzene (5%). Major outlets for toluene (over 2 5 Mt per annum) are for solvent use and conversion to dinitrotoluene. 

In the beginning of 90th, Tan and Davis [136] investigated the coreaction of ethylene and methanol over silicalite S-115 by the isotopic tracer method ( "C labeled or unlabeled methanol and unlabeled or labeled ethylene) and concluded that ethylene was converted by adding a Cj specie derived from methanol. However, the relative "C in the hydrocarbon products revealed that the alkylation of alkenes is more rapid than the formation of Cj" and alkenes from methanol oidy. For the conversion of ethylene only (in the absence of MeOH), the dimerization to form butenes was the dominant reaction. Adding a flow of water in an amount equimolar to ethylene significantly decreased the conversion of ethylene. The addition of methanol to the feed stream, in an amount equal to that of ethylene, increased the total conversion and altered the product distribution so that propylene is formed in about twice the amount of butenes. The labels on the C3-C5 number products were similar to that of ethylene. The authors concluded therefore that the C3-C5 products were formed by the successive addition of an unlabeled Cj species derived from methanol to labeled ethylene. The data clearly show that the formation of ethylene from methanol is a slow reaction compared to the addition of the Cj species to the products. Thus, the formation of ethylene is an important issue only for the reaction initiation. In those processes, where a small amount of alkenes are added to the methanol feed, the formation of ethylene directly from methanol represents a small part of the hydrocarbons produced from methanol. 

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