Alkylation isobutanes

Isobutane. Although other isoparaffins can be alkylated, isobutane [75-28-5] is the only paraffin commonly used as a commercial feedstock. 

In this work, the goal is to design a control function in such a manner that neither the reaction heat nor kinetic nor mass transfer terms are required for stabilizing temperature. The scheme provides an estimated value of the heat generation from energy balance. Alkylation isobutane/propylene using sulfuric... 

In broad terms, alkylation refers to any process, thermal or catalytic, whereby an alkyl radical is added to a compound. In the petroleum industry, however, the term alkylation generally refers to the catalytic process for alkylating isobutane with various light olefins to produce highly branched paraffins boiling in the gasoline range. This specific process will be discussed in this paper. 

The above transformations and reactions can be used to explain the formation, as an example, of different heptane isomers when alkylating isobutane with propylene. As the first step in the mechanism, a tertiary butyl carbonium ion derived from isobutane reacts with a propylene molecule to form a carbonium ion of seven carbon atoms as outlined in reaction 2. This ion may then react directly with a molecule of isobutane as in reaction 6 to form the expected heptane molecule and to convert the isobutane molecule to a tertiary butyl carbonium ion. This seven-carbon carbonium ion, however, may undergo isomerization by the mechanism outlined in reaction 3 or 4 before reacting with isobutane to form an isomeric heptane molecule. 

Temperature is an important variable in the alkylation process. When alkylating isobutane with butenes, a reaction temperature of 40° to 50° F. produces the highest quality alkylate with the lowest catalyst consumption. Commercial operation has been... 

The catalyst consumption for sulfuric acid alkylation is expressed in terms of pounds of fresh acid depleted per barrel of alkylate produced. When alkylating isobutane with butenes at 50° F. and maintaining an isobutane-olefin ratio of 5 to 1, the acid consumption will average 35 to 40 pounds per barrel when charging 98% acid and discarding 88% acid in a batchwise operation. 

Although the preceding discussion of the sulfuric and hydrofluoric acid processes has been confined to butene alkylation, isobutane has also been alkylated commercially with other olefins. Ethylene, propylene, pentenes, and dimers of butenes have been used for this purpose. It is also possible to use these olefins for the alkylation of isopentane. Such an operation, however, has not achieved commercial acceptance because it produces an inferior alkylate with a high catalyst consumption, and because isopentane is a satisfactory aviation gasoline component in its own right. 

C4 Alkenes. Several industrial processes have been developed for olefin production through catalytic dehydrogenation138 166 167 of C4 alkenes. All four butenes are valuable industrial intermediates used mostly for octane enhancement. Isobutylene, the most important isomer, and its dimer are used to alkylate isobutane to produce polymer and alkylate gasoline (see Section 5.5.1). Other important utilizations include oxidation to manufacture maleic anhydride (see Section 9.5.4) and hydroformylation (see Section 7.1.3). 

A two-step (two-reactor) process292,293,305,306 may be operated at lower operating costs (lower ratios of isobutane to n-butenes, lower levels of agitation and acid consumption). More importantly, it affords alkylates of higher quality (99-101 octane number). In the first step sec-butyl sulfate is produced using a limited amount of acid. This then is used to alkylate isobutane with additional acid added. The two-step alkylation can be carried out in the temperature range of —20 to 0°C with the second reactor usually operated at lower temperature. 

Alkylation catalysts may have the ability of breaking down certain olefins and alkylating isobutane with the resulting olefins. A useful example of this ability is the charging of diisobutylene to an alkylation unit, wherein the diisobutylene is broken down and reacts with two molecules of isobutane to form two molecules of trimethylpentane. 

The highest quality component in the alkylation product when making aviation or motor alkylate is a mixture of trimethylpentanes. Although some of the other components have high octane, most are of inferior octane number. It is therefore desirable to make as much of the trimethylpentanes as possible. On a pure component basis, isobutylene and butylene-2 will alkylate isobutane quite readily to form trimethylpentanes. However, butylene-1 has a tendency to form dimethylhexanes. Most of these dimethyl-hexanes are of lower octane number, and their production is to be avoided... 

The trimethylpentanes are easily produced by alkylating isobutane with isobutylene, but unfortunately, the content of isobutylene produced by catalytic cracking is only about one-third of the total butylenes in the C4 stream, the remaining butylenes being butylene-1 and butylene-2. Although most of the butylene-2 tends to form trimethylpentanes, the butylene-1 must be isomerized to butylene-2, either in the alkylation reaction or in a separate previous reaction, before it will form trimethylpentane. If not isomerized, the butylene-1 when alkylated forms the much lower-octane material, dimethylhexane. 

High isobutane recycle purity is not required on HF alkylation units as is required on many H2SO4 units because relatively high normal butane concentrations in the reaction zone do not appreciably affect the quality of the alkylate. Isobutane purities below 60% are usually avoided, however, since this purity definitely gives lower-quality alkylate and the cost of recycling the normal butane is considerable in heat requirements as well as fractionation equipment requirements. 

In this work, the preparation and characterisation of PFAS-SiC>2 are described along with the catalytic results for alkylating isobutane. 

When alkylating isobutane, chain tennination forms primarily, but not entirely, 2,2,4-trimethylpentane the alkylate from chain termination very closely resembles isobutene alkylate. The similarity of alkylate compositions, particularly their C0 fractions, originating from various olefins and the distance from thermodynamic equilibrium composition indicates that alkylate molecules, once formed, are relatively stable under alkylation conditions and undergo little isomerization. Undesirable side products, e.g., dimethylhexanes and residue, are probably formed by buter e isomerization and polymerization (rather than by isomerization of alkylate or by isomerization of the C3 carbonium Ion which subsequently becomes alkylate). 

Also shown in Table II is the effect of olefin space velocity. Comparison of Runs 4 and 6 shows that the Amberlyst-I5/BF3 catalyst can alkylate isobutane with butene in good yield at an olefin WHSV of 2.6 g olefin/g resin-hour. The alkylate yields are slightly lower than the theoretical value of 2,04 due to removal of some of the reactor contents via the on-line sampling system. The yields shown are based on the liquid... 

Isobutylene was also found to react In the presence of sulfuric acid to form acid-soluble hydrocarbons that reacted with Isobutane to form alkylate (5). Although the exact nature of these acid-soluble hydrocarbons is not known. It Is thought that they are in part at least t-butyl sulfates (see Reaction 1-2) or that they complex (or react) with the conjunct polymer cations (R" "), as shown in Reaction J. In both cases, isobutylene would be liberated by reverse reactions, and the isobutylene would then alkylate Isobutane. 

It was found in the early alkylation work that various olefin esters could be used in place of olefins to alkylate isobutane with a strong H2SO4 catalyst. It was also known that diethyl sulfate was an accepted ethylating agent in the chemical industry. 

Mixed butenes obtained by ethylene dimerization are used for paraffinic alkylation (isobutane + n-butene —> trimethylpentanes) or to make propene by a subsequent metathesis reaction (ethylene + 2-butene —> 1 propene cf. Section 2.3.3). Higher ethylene oligomers are also used as high-octane-number gasoline components. 

Any olefin-containing hydrocarbon stream may be used to alkylate isobutane. Butenes are the usual alkylating agents, but propylene is also used, and ethylene and pentenes are employed to a limited extent. 

The chief sources of olefins are cracking operations, especially catalytic cracking. However, olefins can be produced by the dehydrogenation of paraffins butanes are dehydrogenated commercially to provide feeds to alkylation. Isobutane is obtained from crude oils, cracking operations, catalytic reformers, and natural gas. To supplement these sources, n-butane is sometimes isomer-ized. Only small concentrations of diolefins are permissible in feeds to alkylation, particularly for sulfuric add catalyst. Diolefins increase the consumption of acid. 

Svlfuric Add Alkylation. Despite some disadvantages, such as acid-reGOLYery expense and refrigeration to minimize oxidation, about four fifths of the alkylate produced for motor fuels is based on sulfuric acid as a catalyst. As with HF alkylation, isobutane is alkylated with olefins (other than ethylene), and a flow diagram for such a process is ven in Fig. 14-5. 

Chlorinated alumina has been successfully applied in the laboratory to alkylate isobutane and ethylene at temperatures below 107°C (45). The catalyst, possessing essentially Lewis acidity, is prepared starting from y-alumina, which is... 

Plants that alkylate isobutane to produce gasoline alkylates in the United States currently have production capacities of approximately 1.12 million barrels per day (3). In other countries, mainly Europe and the Far East, production capacities total 0.64 million barrels per day. In the United States, alkylates form about 11-13% of the gasoline pool. These alhylates have octane numbers, often in the 94-97 range, and are the cleanest burning gasoline available. 

Four processes have been employed for isobutane alkylation units in the last 20-30 years. Prior to that, one unit was built to alkylate isobutane with ethylene a liquid AICI3 complex was then employed as the catalyst. The processes that are currently important are discussed next. 

For alkylating isobutane, considerable research has been conducted to develop processes that use solid catalysts. Safety problems would then be significantly reduced, but no known process has yet been commercialized. In all cases, the catalysts were rapidly deactivated in hours or even minutes. Conjunct polymers were obtained that diffuse most slowly from the pores (1,35). These polymers adsorb on the inner surfaces of the pores. 

After the reaction, a number of products are formed that require further processing to separate and clean the desired chemical streams. A separator and an alkaline substance are used to remove [strip] the acid. The stripped acid is sent back to the reactor, while the remaining reactor products are sent to a distillation tower. Alkylate, isobutane, and propane gas are fractionally separated in the tower. Isobutane is returned to the alkylation reactor for further processing. Alkylate is sent on to the gasoline blending unit. 

Refrigeration systems are widely used in a refinery. Some of the more common services are alkylation (isobutane refrigerant), ammonia production (ammonia refrigerant), and gas plant light ends absorber (propane refrigerant). 

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