Boiling isobutane alkylation

Excess Polymerization. A small amount of high-boiling heavy "tail" or residue Is formed in Isobutane alkylation, even urxJer the most favorable reaction conditions. The polymer molecule is in reality on isoparaffin formed from two or more molecules of olefin plus one molecule of Isobutane. Polymer is formed because of the inherent tendency of larger carbonium ions, e.g., Cj or C0 ions, to complete with tertiary butyl carbonium ions for addition of olefin molecules before abstracting hydride ions and becoming isoparaffin molecules. Reactions follow ... 

To obtain light ends conversion, alkylation and polymerization are used to increase the relative amounts of liquid fuel products manufactured. Alkylation converts olefins, (propylene, butylenes, amylenes, etc.), into high octane gasoline by reacting them with isobutane. Polymerization involves reaction of propylene and/or butylenes to produce an unsamrated hydrocarbon mixture in the motor gasoline boiling range. 

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 fractionation section of the alkylation plant consists of a deisobutanizer, a debutanizer, and a rerun tower in series, and a depropanizer. The deisobutanizer overhead, which contains about 90% isobutane, recycles to the reactor. The deisobutanizer bottoms stream passes to the debutanizer, which removes normal butane diluent as an overhead stream. The debutanizer bottoms or raw alkylate stream then goes to a rerun tower for removal of the high boiling alkylate bottoms or trimers. The rerun overhead requires no further treatment to be satisfactory as an aviation gasoline blending stock. The depropanizer removes propane diluent from a slip-stream portion of the recycle isobutane stream to prevent propane build-up in the reaction system. 

A catastrophic explosion and major fire occurred within a major refinery as operations prepared a system for valve maintenance. This refinery stored a flashing flammable fluid (isobutane with a boiling point of 11° F or —12° C) in two spherical tanks. The spheres connected to an alkylation unit via a 10-inch (25 cm) line. Operating line pressure was about 50 psig (345 kPa gauge) and one of the valves in this underground system was in an open pit. [9]... 

It is obvious that the production of trimethylpentane involves the formation of an equimolar quantity of a low-boiling paraffin corresponding to the olefin alkylating agent. Such was found to be at least semiquantita-tively the case. It will be noted also that 2 moles of isobutane are consumed for each mole of liquid paraffin produced this excessive consumption of the isobutane is usually undesirable in commercial alkylation processes. 

In an analogous manner, isopropyl chloride was converted to propane in 90% yield whenitwas heated at 40° with isopentane and aluminum chloride. Heating the isopentane with aluminum chloride and hydrogen chloride caused it to undergo autodestructive alkylation, yielding higher-boiling isoparafiins and isobutane but no propane less catalyst complex was formed than when isopropyl chloride was used. 

Alkylation of n-butane and isobutanc with methyl or ethyl bromide in the presence of aluminum bromide has been claimed (Heldman, 36). It was found, for example, that the action of aluminum bromide (0.00158 mole as Al2Br ) on a solution of 0.0392 mole methyl bromide in 0.0750 mole n-butane at 25° for 120 hours yielded 0.0052 mole isopentane (13% of the theoretical based on the methyl bromide) isobutane was also formed. In a similar experiment carried out at 78° for 65 hours, the yield of isopentane was 33%. The reaction of isobutane (0.0520 mole) with methyl bromide (0.0392 mole) in the presence of aluminum bromide (0.00142 mole AUBrj) at 25° for 283 hours yielded low boiling material (0.0009 mole), n-butane (0.0062 mole), unreacted isobutane (0.0410, 79% of that charged), isopentane (0.0034 mole, 9% of the theory) and higher boiling material (0.0024 mole). 

Similarly, alkylation of isopentane with i-butyl alcohol under the same conditions resulted in the formation of alkylate (boiling above pentane) to the extent of 305% by weight of the available isobutylene. The product contained about 21% by weight hexanes, 6% heptanes, 9% octanes, 27% nonanes, and 25% decanes. Nonanes were thus formed in 36% of the theoretical yield. Decanes and isobutane, respectively, were isolated in 30% and 111% of the yields obtainable if all of the <-butyl alcohol had undergone hydrogen transfer and none simple alkylation to form nonanes. Extensive destructive alkylation of the decanes apparently occurred. 

Tc and Ti, are the critical and normal boiling temperatures, respectively, expressed in kelvins. Values of Ay from Table 2-383 are summed for each part of the molecule to yield Z Ay (e.g., for isobutane, 3 x —CHs -I-1 X >CH—). A(Platt no.) is equal to the Platt number of any alkyl chains in the molecule minus the Platt number of the n-alkane with the same number of carbon atoms. The Platt number is defined as the number of pairs of carbon atoms that are separated by three carbon-carbon bonds and is an indicator of branching (e.g., for 2,2,3-trimethylpentane, the Platt number is 5). The Platt number of an n-alkane is the number of carbon atoms minus three (e.g., for n-octane, the Platt number is five). Errors in Tc average about 4 K for paraffins to C20 and other hydrocarbons to C14. 

The yield of liquid products, including all hydrocarbons in the boiling range of gasoline as well as neohexane, is approximately 70 per cent by weight of the net consumption of ethane-propane and isobutane consumed during decomposition and alkylation. As the normally liquid hydrocarbon content of the coil effluent is increased from 20 to 35 per cent by weight (conversion per pass), the neohexane content of these liquids decreases from about 40 to about 30 per cent. 

---