Page 97 - Synthetic Fuels Handbook

84 CHAPTER THREE

is derived from the gas produced in a catalytic cracker, while isobutane is recovered by
refinery gases or produced by catalytic butane isomerization. To accomplish this, either
ethylene or propylene is combined with isobutane at 50 to 280°C (122–536°F) and 300 to
1000 psi (20–68 atm) in the presence of metal halide catalysts such as aluminum chloride.
Conditions are less stringent in catalytic alkylation; olefins (propylene, butylene, or pen-
tylene) are combined with isobutane in the presence of an acid catalyst (sulfuric acid or
hydrofluoric acid) at low temperatures and pressures [1–40°C (34–104°F) and 14.8–150 psi
(1–10 atm)] (Fig. 3.14).

Recycle isobutane
Feedstock

Reactor Acid
settler
Caustic scrubber Deisobutanizer Alkylate

Recycle acid

Fresh acid Reject acid
FIGURE 3.14 An alkylation unit (sulfuric acid catalyst).
Sulfuric acid, hydrogen fluoride, and aluminum chloride are the general catalysts used
commercially. Sulfuric acid is used with propylene and higher boiling feeds, but not with
ethylene, because it reacts to form ethyl hydrogen sulfate. The acid is pumped through the
reactor and forms an air emulsion with reactants, and the emulsion is maintained at 50 percent
acid. The rate of deactivation varies with the feed and isobutane charge rate. Butene feeds
cause less acid consumption than the propylene feeds.
Aluminum chloride is not currently used as an alkylation catalyst but when employed,
hydrogen chloride is used as a promoter and water is injected to activate the catalyst as
an aluminum chloride/hydrocarbon complex. Hydrogen fluoride is used for alkylation of
higher boiling olefins and the advantage of hydrogen fluoride is that it is more readily
separated and recovered from the resulting product.

3.3.9 Polymerization Processes
Polymerization is the process by which olefin gases are converted to liquid products which
may be suitable for gasoline (polymer gasoline) or other liquid fuels. The feedstock usu-
ally consists of propylene and butylenes from cracking processes or may even be selective
olefins for dimer, trimer, or tetramer production.
Polymerization may be accomplished thermally or in the presence of a catalyst at lower
temperatures. Thermal polymerization is regarded as not being as effective as catalytic
polymerization but has the advantage that it can be used to “polymerize” saturated materials
that cannot be induced to react by catalysts. The process consists of vapor-phase cracking
of, for example, propane and butane followed by prolonged periods at the high temperature
[510–595°C (950–1103°F)] for the reactions to proceed to near completion.