This is the addition of an alkyl group to any compound but in petroleum refining it is used for the reaction o low molecular weight olefins with an iso paraffin to form higher molecular iso paraffin. Although this reaction is simply the reverse of cracking, the belief that paraffin hydrocarbons are chemically inert delayed its discovery until about 1935.
The need for high octane aviation fuels during the World War II acted as a stimulus to the development of the alkylation process for production of isoparaffinic gasolines of high octane number.
Although alkylation can take place in high temperatures and pressure without catalysts, the only processes of commercial importance involve low temperature alkylation conducted in the presence of either sulphuric acid or hydrofluoric acid. The reactions occurring in both processes are complex and the product has rather a wide range of boiling. By proper choice of operating conditions, most of the product can be made to fall within the gasoline boiling range with motor octane numbers from 88-94 and research octane numbers from 94-99.
The chemical reaction.(Smith 2005).
Toa chemist, alkylation covers a broad range of reactions that stic k molecules together when one of them is a paraffin (alkane).But to a refiner alkylation is the reaction of propylene or butylenes with iso butane to form isoparaffin called an alkylate.
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Under the ideal conditions alkylate would consist of isoheptane and isooctane, two excel lent, high octane gasoline blending components. Propylene and butylenes are very reactive, however that other not so high octane components in the gasoline range form as by products. But not enough are formed to undermine the attractiveness of an alky plant.
The process involves the combination of the two chemicals that is the Isobutene and the propylene which results to Isoheptane as below:
Isobutane Propylene Isoheptane
C4H10 + C3H6 = C7H16
Isobutane Butylene Isooctane
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The above represents the Alkylation of propylene and butylenes.
Alkylation has a volumetric effect on refining operations that is the inverse of cracking because there is significant amount of shrinkage. With propylene as the feed, 1 bubble of propylene and 1.6 bubble of isobutene yield 1.8 bubble of product. As in cracking, the weight in equals the weight out. Nothing gets lost. Only the densities and volumes change. (Spivey 2000).
Propylene and butylenes are hyper enough that the chemical reaction could be made to take place by just subjecting the isobutene and olefins to high pressures.But the equipment would be very expensive to handle t his route to alkylation. Like a lot of other processes, catalysts have been developed to facilitate the process and simplify the hardware. Alkylation plants use either sulfuric cid or hydrofluoric acid to act as catalyst. In this case the catalyst is liquid, in contrast to the solids in cat cracking. The processes using both catalysts are basically the same. Those plants that use hydrofluoric acid are called HF plants; the others are called sulfuric plants. Both have safety concerns because hydrofluoric and sulfuric acids are seriously nasty items, corroding al but specially lined vessels and piping around them. Hydrofluoric has an additional concern. If it escapes, it floats in a cloud and can travel great distances, to the annoyance of refinery neighbor. Sulfuric acid will form droplets if it escapes and quickly settles down to the ground, though that is not much consolation to anyone working in the immediate vicinity. (Smith 2005).
Sulfuric plants work better for butylenes alkylation; HF plants are better for propylene. Sulfuric ac id seems to be slightly more popular, so thee main research will be concentrated on the sulphuric route. The alky plant has seven main parts that is the chiller reactor, the acid separator, the caustic wash, and three distilling columns.
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Function of the seven parts:-
Alkylation with sulfuric acid catalyst works best at temperatures in the neighborhood of about forty degrees Fahrenheit. The olefin feed,a propane /propylene and/or butane/ butylenes stream comes from the cat cracker, or coker, but not the hydrocracker, which makes no olefins. The strean is mixed with isobutene and sulfuric acid and pumped through a chiller. The pressure is high enough to keep the mixture in liquid form. Sometimes the chilling is done right in the reactor. (Spivey 2000).
The reaction time for the alkylation process is relatively long, so the mixture is pumped through a large reactor. The reactor hold so much total volume that by the time the contents turn over once, the residence time of any one molecule is quite long, about twenty to twenty five minutes. As the liquid passes through the reactors, it encounters mixers to assure that olefins come in good contact with the isobutene and the acid catalyst, promoting the reaction.
The mixture then moves to the acid settler the acid and hydrocarbon s separate like oil and water. The hydrocarbon is drawn off the top; the acid is drawn off the bottom. The acid is then recycled back to the feed side.
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The hydro carbon from the acid separator will have some of acid in it, so it is treated with caustic soda in a vessel. Caustic soda does to the hydro carbon what Alka-Seltzer does to your stomach when you have ingestion- it neutralizes the acid. What is left in the alky plant is a mixture of hydrocarbons and some salts from the caustic treating, ready to be separated.
Three standard fractionators separate the alkylate and saturated gases. Any un reacted isobutane is recycled to the feed.
During the alkylation process, a number of side reactions occur, some of which are more or less undesirable. Because there a lot of molecules forming and reacting, there are small amounts of propane, butane, and pentane formed, which are not too bad; but a small amount of tar forms, as well as a thick, brownish oil containing a mixture of complex hydrocarbons. The molecules are so heavy they usually build up in the ac id and exit the scene when the acid is sent back to the supplier for regeneration. (Smith 2005).
The alky plant manager has to watch a number of key variables to keep too man y side reactions from occurring that could cause the quality of the alkylate to deteriorate, as evidenced by such things as lower octane number, poor color, and high vapor pressure.
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Temperatures too low cause the sulfuric acid to get syrupy. That inhibits complete mixing and the olefins do not completely react. High temperatures cause compounds other than isoheptane and isooctane to occur, lowering the alkylate quality.
As the acid circulates through the process, it gets diluted with water that inevitably comes in with the olefins, and it also picks up tar. As the acid concentration goes from 99% down to about 89 %, it is drawn off and sent back to the acid supplier for refortifying.
Propylene and butylenes are very reactive, particularly in the presence of the ac id catalyst. They will react with each other as well as with isobutene. For that reason, excess isobutene is mixed in, as much as 10 times the amount needed for the reaction. That way the propylene and butylenes are 10 times as likely to react with isobutene as with each other.
Olefin space velocity
The length of time the fresh olefin feed resides in the reactor causes alkylate quality to vary.
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Smith R (2009) Chemical Process: Design and Integration 2 Sub edition Wiley Publishers.
Spivey J (2000) Catalysis: Are view of chemical literature: vol 15 Mac-Graw Hill publisher.
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