Isobutylene from isobutane

One possible advantage of the HF process in propylene/ butylene alkylation is the production of isobutylene from isobutane effected by the hydride-ion transfer to propylene. Isobutylene is the C4 olefinic isomer which produces a significantly higher octane alkylate with HF. This shift, however, converts as much as 22% of the propylene to propane and is a debit. 

Alkylation of isobutylene and isobutane in the presence of an acidic catalyst yields isooctane. This reaction proceeds through the same mechanism as dimerization except that during the last step, a proton is transferred from a surrounding alkane instead of one being abstracted by a base. The cation thus formed bonds with the base. Alkylation of aromatics with butylenes is another addition reaction and follows the same general rules with regard to relative rates and product stmcture. Thus 1- and 2-butenes yield j -butyl derivatives and isobutylene yields tert-huty derivatives. 

There are other commercial processes available for the production of butylenes. However, these are site or manufacturer specific, eg, the Oxirane process for the production of propylene oxide the disproportionation of higher olefins and the oligomerisation of ethylene. Any of these processes can become an important source in the future. More recentiy, the Coastal Isobutane process began commercialisation to produce isobutylene from butanes for meeting the expected demand for methyl-/ rZ-butyl ether (40). 

The various sources of isobutylene are C streams from fluid catalytic crackers, olefin steam crackers, isobutane dehydrogenation units, and isobutylene produced by Arco as a coproduct with propylene oxide. Isobutylene concentrations (weight basis) are 12 to 15% from fluid catalytic crackers, 45% from olefin steam crackers, 45 to 55% from isobutane dehydrogenation, and high purity isobutylene coproduced with propylene oxide. The etherification unit should be designed for the specific feedstock that will be processed. 

When -butenes are used, the initiation produces a secondary carbenium ion/butoxide. This species may isomerize via a methyl shift (Reaction (2)) or accept a hydride from isobutane to form the tertiary butyl cation (Reaction (3)). Isobutylene forms the tertiary cation directly. 

Isobutylene has had a tremendous increased production in the last few years because of the dynamic growth of the gasoline additive MTBE. About two thirds of it is made from isobutane by dehydrogenation in thermal cracking. 

In general, we have outlined how the conversion of isobutane on sohd acid catalysts takes place according to well-established carbenium ion transition state chemistry. The difficulty with using isobutane conversion as a probe of catalyst performance is that many combinations of oligomcrization// -scission processes with isomerization steps are possible, resulting in a wide variety of adsorbed species and observable reaction products. For example, the following products are observed from isobutane conversion in the presence of ultrastable Y zeolite at temperatures near 520 K (where the reaction is initiated by the addition of isobutylene to the feed) ... 

Direct evidence for stepwise reduction was obtained in the electrolytic reduction of t-butyl bromide at a mercury cathode in DMF18,19. The polarogram or cyclic voltammogram shows two waves [E1/2 = -1.23 V, -1.46 V (see)] indicating stepwise reduction. Labelling experiments indicate that isobutylene and isobutane arise from disproportionation of t-butyl radicals, and the 2,2,3,3-tetramethylbutane arises from a coupling reaction (equation 2). Reduction at the second wave leads to a carbanion, evidence of which is provided by... 

The probability that hydride transfer from Isobutane to carbonlum Ion Intermediates Is the kinetically slow step affecting product quality was raised by an experiment In which a stream of 96 percent H2SO4 was rapidly mixed with a stream of Isobutane plus Isobutylene In a mixing tee, after which the products were Immediately quenched In a large vessel containing cold caustic (13). The Cg fraction of the product contained trlmethylpentenes but no trlmethylpentanes. 

Schmerling (9,10) had originally postulated that a hydride Ion transfer from Isobutane was both the most Important method of hydride transfer and also part of the chain set of reactions (see Reaction C of Table I and Reaction M-2 of Table IV). Other hydride transfer steps that have now been suggested Include transfer with the acid-soluble hydrocarbons (RH), see Reaction M-1 of Table IV (8,11), and with Isobutylene, see Reaction M-3 (8). Reaction M-3 Is however considered to be of minor Importance since only trace amounts of free Isobutylene are likely ever present at the acid-hydrocarbon Interface (the probable location of alkylation reactions) Isobutylene quickly protonates to form t-CaHg . Reaction M-1 Is considered to be more Important than Reaction H-2 especially when sulfuric acid Is used as the catalyst for the reasons listed as follows ... 

Ethylene has been separated from ethane by a silver nitrate solution passing countercurrent in a hollow fiber poly-sulfone.165 This separation has also been performed with the silver nitrate solution between two sheets of a polysilox-ane.166 A hydrated silver ion-exchanged Nafion film separated 1,5-hexadiene from 1-hexene with separation factors of 50-80.167 Polyethylene, graft-polymerized with acrylic acid, then converted to its silver salt, favored isobutylene over isobutane by a factor of 10. Olefins, such as ethylene, can be separated from paraffins by electroinduced facilitated transport using a Nafion membrane containing copper ions and platinum.168 A carbon molecular sieve made by pyrolysis of a polyimide, followed by enlargement of the pores with water at 400 C selected propylene over propane with an a-valve greater than 100 at 35°C.169... 

We have seen that by 1973 catalytic cracking will only satisfy 2 to 4 billion lbs/year of a projected 11 billion lb/year propylene demand. Most of the balance will be produced as a by-product of ethylene manufacture. Shifting from ethane and propane to heavier stocks such as n-butane and gas oil will satisfy propylene needs. Some propylene will also be produced from isobutane steam crackers as an isobutylene co-product. 

Utilities (Referred to a feedstock from isobutane dehydrogenation at 50 wt% isobutylene cone.)... 

The estimated capital cost for a propylene oxide plant to produce 540 million pounds per year of propylene oxide by the isobutane peroxidation process is given in Table 6. This estimate includes the equipment and offsites to coproduce isobutylene from TBA for feed to an MTBE plant. 

The isomerization of n-butane to isobutane is of substantial importance because isobutane reacts under mild acidic conditions with olefins to give highly branched hydrocarbons in the gasoline range. A variety of useful products can be obtained from isobutane isobutylene, t-butyl alcohol, methyl t-butyl ether and t-butyl hydroperoxide. A number of methods involving solution as well as solid acid catalysts have been developed to achieve isomerization of n-butane as well as other linear higher alkanes to branched isomers. 

Efficient utilization of the olefin feedstock is critical in ether production because of the limited suppfy and cost of the olefin feedstock [133], The estimated order of magnitude cost of a 12,500 bpsd MTBE complex is about 200 million (1992 dollars), of which 3550% of the cost is associated with dehydrogenation costs for isobutylene synthesis from isobutane [130]. The source of olefins is a major issue in ether production [134136], and the interest in TAME and other ethers for fuel oxygenates stems from the fact that they can be produced from methanol and olefins other than isobutylene [134]. 

Products do not contain 2,2,3-trimethylbutane or 2,2,3,3-tetramethylbutane, which would be expected as the primary alkylation products of direct alkylation of isobutane with propylene and isobutylene, respectively. In fact, the process iavolves alkylation of the alkenes by the carbocations produced from the isoalkanes via intermolecular hydride abstraction. 

Oxirane Process. In Arco s Oxirane process, tert-huty alcohol is a by-product in the production of propylene oxide from a propjiene—isobutane mixture. Polymer-grade isobutylene can be obtained by dehydration of the alcohol. / fZ-Butyl alcohol [75-65-0] competes directly with methyl-/ fZ-butyl ether as a gasoline additive, but its potential is limited by its partial miscibility with gasoline. Current surplus dehydration capacity can be utilized to produce isobutylene as more methyl-/ fZ-butyl ether is diverted as high octane blending component. 

Dehydrogenation of isobutane to isobutylene is highly endothermic and the reactions are conducted at high temperatures (535—650°C) so the fuel consumption is sizeable. Eor the catalytic processes, the product separation section requires a compressor to facHitate the separation of hydrogen, methane, and other light hydrocarbons from-the paraffinic raw material and the olefinic product. An exceHent overview of butylenes is avaHable (81). 

A typical feed to a commercial process is a refinery stream or a steam cracker B—B stream (a stream from which butadiene has been removed by extraction and isobutylene by chemical reaction). The B—B stream is a mixture of 1-butene, 2-butene, butane, and isobutane. This feed is extracted with 75—85% sulfuric acid at 35—50°C to yield butyl hydrogen sulfate. This ester is diluted with water and stripped with steam to yield the alcohol. Both 1-butene and 2-butene give j -butyl alcohol. The sulfuric acid is generally concentrated and recycled (109) (see Butyl alcohols). 

The reaction between isobutylene (separated from C4 fractions from cracking units or from cracking isobutane to isobutene) and formaldehyde produces a cyclic ether (dimethyl dioxane). Pyrolysis of dioxane gives isoprene and formaldehyde. The formaldehyde is recovered and recycled to the reactor. 

The alkylation unit in a petroleum refinery is situated downstream of the fluid catalytic cracking (FCC) units. The C4 cut from the FCC unit contains linear butenes, isobutylene, n-butane, and isobutane. In some refineries, isobutylene is converted with methanol into MTBE. A typical modern refinery flow scheme showing the position of the alkylation together with an acid regeneration unit is displayed in Fig. 1. 

With propene, n-butene, and n-pentene, the alkanes formed are propane, n-butane, and n-pentane (plus isopentane), respectively. The production of considerable amounts of light -alkanes is a disadvantage of this reaction route. Furthermore, the yield of the desired alkylate is reduced relative to isobutane and alkene consumption (8). For example, propene alkylation with HF can give more than 15 vol% yield of propane (21). Aluminum chloride-ether complexes also catalyze self-alkylation. However, when acidity is moderated with metal chlorides, the self-alkylation activity is drastically reduced. Intuitively, the formation of isobutylene via proton transfer from an isobutyl cation should be more pronounced at a weaker acidity, but the opposite has been found (92). Other properties besides acidity may contribute to the self-alkylation activity. Earlier publications concerned with zeolites claimed this mechanism to be a source of hydrogen for saturating cracking products or dimerization products (69,93). However, as shown in reaction (10), only the feed alkene will be saturated, and dehydrogenation does not take place. 

Isobutylene supply initially came mainly from the cracked gas streams generated by the cat cracker, plus whatever other units fed gases into the cracked gas plants. The isobutylene market trades very thinly, so when the cracked gas plant supply is insufficient, a producer must turn to a dehydrogenation process for converting isobutane to isobutylene. 

Besides ethylene and propylene, the steam cracking of naphtha and catalytic cracking in the refinery produce appreciable amounts of C4 compounds. This C4 stream includes butane, isobutane, 1-butene (butylene), cis- and trans-2-hutene, isobutene (isobutylene), and butadiene. The C4 hydrocarbons can be used to alkylate gasoline. Of these, only butadiene and isobutylene appear in the top 50 chemicals as separate pure chemicals. The other C4 hydrocarbons have specific uses but are not as important as butadiene and isobutylene. A typical composition of a C4 stream from steam cracking of naphtha is given in Table 8.3. 

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