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Alkylation Process |
UOP ALKYPLUS PROCESS
PROCESS DISCUSSION
The Alkylation process for motor fuel production catalytically combines light olefins, which are usually mixtures of propylene and butylenes, with isobutane to produce a branched-chain paraffinic fuel. The alkylation reaction takes place in the presence of hydrofluoric (HF) acid under conditions selected to maximize alkylate yield and quality. The alkylate product possesses excellent antiknock properties and high-octane because of its high content of highly branched paraffins. Alkylate is a clean-burning, low-sulfur, low- RVP gasoline blending component that does not contain olefinic or aromatic compounds.
The alkylation of olefins with isobutane is complex because it is characterized by simple addition as well as by numerous side reactions. Primary reaction products are the isomeric paraffins containing carbon atoms that are the sum of isobutane and the corresponding olefin. However, secondary reactions such as hydrogen transfer, polymerization, isomerization, and destructive alkylation also occur, resulting in the formation of secondary products both lighter and heavier than the primary products.
The factors that promote the primary and secondary reaction mechanisms differ, as does the response of each to changes in operating conditions or design options. Not all secondary reactions are undesirable; for example, they make possible the formation of isooctane from propylene or amylenes. In an ideally designed and operated system, primary reactions should predominate, but not to the complete exclusion of secondary ones. For the HF Alkylation process, the optimum combinations of plant economy, product yield, and quality are achieved with the reaction system operating at cooling-water temperature and an excess of isoparaffin and with contaminant-free feedstocks and vigorous, intimate acidhydrocarbon contact.
PROCESS FLOW
The reactor and distillation systems that UOP uses have evolved through many years of pilot-plant evaluation, engineering development, and commercial operation. The overall plant design has progressed through a number of variations, resulting in the present concepts in alkylation technology.
Reactor Section
In the design of the reactor, the following factors require particular attention:
● Removal of heat of reaction
● Generation of acid surface: mixing and acid/hydrocarbon ratio
● Acid composition
● Introduction of olefin feed
The proper control of these factors enhances the quality and yield of the alkylate product. Selecting a particular reaction system configuration requires careful consideration of the refiner’s production objectives and economics. The UOP reaction system optimizes processing conditions by the introduction of olefin feed through special distributors to provide the desired contact with the continuous-acid phase. Undesirable reactions are minimized by the continuous removal of the heat of reaction in the reaction zone itself. The removal of heat in the reaction zone is advantageous because peak reaction temperatures are reduced and effective use is made of the available cooling-water supply.
Acid Regeneration Section
The internal acid regeneration technique has virtually eliminated the need for an acid regenerator and, as a result, acid consumption has been greatly reduced. The acid regenerator has been retained in the UOP design only for start-ups or during periods when the feed has abnormally high levels of contaminants, such as sulfur and water. For most units, during normal operation, the acid regenerator is not in service. When the acid regenerator is in service, a drag stream off the acid circulation line at the settler is charged to the acid regenerator, which is refluxed on the top tray with isobutane.
The source of heat to the bottom of the regenerator for a C3-C4 HF Alkylation unit is superheated isobutane from the depropanizer sidecut vapors. For a C4 HF Alkylation unit, the stripping medium to the acid regenerator is sidecut vapors from the HF stripper bottoms. The regenerated HF acid is combined with the overhead vapor from the isostripper and sent to the cooler.
Neutralization Section
UOP has designed the neutralization section to minimize the amount of additional effluents such as offensive materials and undesirable by-products. Releasing acid-containing vapors to the regular relief-gas system is impractical because of corrosion and odor problems as well as other environmental and safety concerns. The system is composed of the relief-gas scrubber, KOH mix tank, circulating pumps, and a KOH regeneration tank. All acid vents and relief valves are piped to this relief section. Gases pass up through the scrubber and are contacted by a circulating KOH solution to neutralize the HF acid. After the neutralization of the acid, the gases can be safely released into the refinery flare system.
The KOH is regenerated on a periodic basis in the KOH regeneration tank by using lime to form calcium fluoride (CaF2) and KOH. The CaF2 settles to the bottom of the tank and is directed to the neutralizing basin, where acidic water from acid sewers and small amounts of acid from the process drains are treated. Lime is used to convert any fluorides into calcium fluoride before any waste effluent is released into the refinery sewer system.
Distillation System
The distillation and recovery sections of HF Alkylation units have also seen considerable evolution. The modern isostripper recovers relatively high-purity isobutane as a sidecut that is recycled to the reactor. This recycle is virtually acid-free, thereby minimizing undesirable side reactions with the olefin feed prior to entry into the reactor. A small rectification section on top of the modern isostripper provides for more efficient propane rejection.