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Alkylation was first commercialized in 1938 where it was used to produce aviation fuel, then in the mid-1950's alkylation interests shifted towards using alkylate as a blending component in automotive motor fuel.(Kenneth A. Kobe, John J. Mcketta (1958))
Alkylation is a reaction that transfers an alkyl group from one molecule to an other, in petrochemical industries it is used to convert a mixture of light alkenes and iso-butane into a mixture of highly branched low-vapour-pressure high octane blending, low sulphur content alkane component that are blended with gasoline in order to increase its octane number producing a higher grade fuel. Alkylation can be carried out using a catalyst or without a catalyst at high pressure a temperature. Although only catalytic alkylation is of commercial importance today. (Kenneth A. Kobe, John J. Mcketta (1958))
Mechanism of alkylation:-
Thermal Alkylation: thermal alkylation can be described on the basis of a free-radical chain mechanism which is initiated by a small amount of alkane of alkene cracking in which a free radical is produced which then reacts with the alkane producing a free alkyl radical which adds to the alkene to produce a higher-molecular weight free radical which then reacts with the alkane producing the alkylation product plus a new alkyl radical which starts a new cycle. Thermal alkyation can be occurs under milder conditions ( lower temp., pressure) by adding compounds such as Chloroform, benzyl chloride which decomposes into radicals at relatively low temperatures(Kenneth A. Kobe, John J. Mcketta (1958))
Catalytic Alkylation: only occurs with isoalkanes containing a tertiary carbon atom with an alkene, this kind of reaction can be described on the bases of the conversion of the isoalkane to an active intermediate capable of adding to the alkene to yield a product which reacts with the isoalkane to yield the alkylation product and the active intermediate that stats a new cycle.
Alkylation is catalyzed by protonic acid (mixture of sulphuric acid and hydrogen flourid) or by Friedel-Crafts-type halide catalysts (Aluminium chloride promoted by hydrogen chloride), catalysts used for alkylation also catalyze alkenes polymerization, which result in a decrease yield and product quality, an excess of isoalkane is maintained in the reaction zone to limit polymerization. (Kenneth A. Kobe, John J. Mcketta (1958))
The importance of alkylation lays in its ability to produce a high-octane-number blending alkylate required for the manufacture of high quality fuels thus improving the quality of motor fuel making a cleaner burning fuel which reduces the amount of air pollutant produced after it combustion in automobile engines .
Prior to the commercialization of isoalkane alkylation process, equivalent blending materials were manufactured by polymerization of an alkene and then hydrogenation of the polymerized alkene. The advantage of alkylation process lies in the fact that the single process can cause a complete reaction conversion in a ratio approximately one mole of blending component per mole alkene. (Kenneth A. Kobe, John J. Mcketta (1958))
H2SO4 Alkylation process:-
Fig.1(Meyers, Robert A. (2004)
Catalyst used : sulphuric acid
Alkenes and isobutane are alkalized in the presence of sulphuric acid catalyst in the reactor section. The alkene feed is initially mixed with the recycled isobutane and cooled to 15.6 oC by exchanging heat with net effluent stream, then the reactants are fed to a contractor reactor ( a horizontal pressure vessel containing an inner circulation tube, a tube bundle to remove the heat of reaction and a mixing impeller) after being combined with the refrigerant recycle. Fig.2 (Meyers, Robert A. (2004)
Refrigeration section is then used to remove heat of reaction and light hydrocarbon and other impurities mainly SO2. (Meyers, Robert A. (2004))
The net products from the reactor are then treated in the effluent treating section where reaction by-products (esters) are removed because they will foul the equipment.
Finally products are separated, unreacted isobutane is recycled to the reactor section and remaining hydrocarbons undergo further separation to recover desired products(isooctane).Used acid is treated in the blow down section before being discharged. (Meyers, Robert A. (2004)
Process economics: Meyers, Robert A. (2004)
Table 1: estimated utility and chemicals per barrel of alkylate
Electric power, kW 15 Seam, lb 194
Cooling water, gal 1370 Acid, lb 13
Process water, gal 4
2.HF Alkylation process
Fig.3 (Meyers, Robert A. (2004)
Catalyst used : hydrofluoric acid
Process mechanism: Alkenes are charged along with recycled and makeup isobutane with the acidic catalyst to the reactor section, heat of reaction is removed by a heat exchanger, the acid is separated from the hydrocarbons in the settler and recycled back, hydrocarbons are pumped into the isostripper (distillation column). Product alkylate(isooctane) is recovered from the bottom and unreacted isobutane is recycled back, an acid regenerator is provided for start-up after turnarounds, KOH treaties are used to purify the products/by-products from the acid that might break through separation.(Meyers, Robert A. (2004))
Process economics: (Meyers, Robert A. (2004))
Table.2 Production cost
Total operating cost 1909
Hydrogen fluoride has definite advantages over sulphuric acid as an alkylation catalyst, which include: 1. Stability(enables recovery by distillation) where as sulphuric acid has to be discharged because the catalyst is spent when diluted, 2.selectivity over a wider range of temperatures which permits its use for a wider range of product quality and eliminates the need for refrigeration, 3.catalyst make-up is limited to mechanical losses and minor process losses(less than 0.25 lb of HF lost per barrel of product- 20-80lb of sulphuric acid lost per barrelof product), 4.higher isobutane solubility which reduces power requirement in mixing.
Although sulphuric acid is still more most widely used for commercial alkylation because of the hazards involved in handling hydrofluoric acid. (Kenneth A. Kobe, John J. Mcketta,1958)
Catalyst used: mainly aluminium chloride promoted by hydrogen chloride.
This Friedel-Crafts-type Catalyst is used to catalyze the alkylation reaction of isobutane with ethylene yielding a product containing 72% of
2,3-dimethylbutane which is superior to isooctane which is produced in plants that use hydrofluoric or sulphuric acid.
The isomerisation activity of the Friedel-Crafts-type Catalyst permits the use of lower-cost normal butane as an alkane instead of isobutane, but the cost of ethylene is a disadvantage of the process. ( Kenneth A. Kobe, John J. Mcketta,1958)
A catalyst life of more than two barrels of alkylate per lb of aluminium chloride with reasonable operating costs.
Although the Friedel-Crafts-type Catalyst have not found acceptance in the petroleum refining industry because of its economic disadvantage over sulphuric acid and hydrofluoric acid catalyst processes.( Kenneth A. Kobe, John J. Mcketta,1958)
Recent research and development
Research in the area of solid catalyst for alkylation has been on going for many years.
Some of the most important research cared out by different companies are illustrated below:
AlkyClean: based on a two year research, a prototype refinery demonstration unit was built that uses an environmentally friendly zeolite alkylation catalyst that is reliable and cost competitive alternative to liquid acid technologies. The new catalyst process produced a high quality alkylate without all the drawbacks of the acid technologies (acid soluble oils and spent acids) reducing maintenance and monitoring requirements while reducing environmental concerns.( Broekhoven, E.H, 1999)
AlkyleneTM: Alkylation process developed by UOP that uses a catalyst called HAL-100 (specific nature of the catalysts is not given), the process require extensive feed Pre-treatment before it can be charged into the reactor, since contaminants such as sulphur, oxygen, and nitrogen compounds, which are typically found in the alkene feed, can severely decrease the catalytic Activity and permanently deactivate the catalyst. (S.I. Hommeltoft, 2001)
fixed-bed alkylation (FBATM) technology: was developed by Haldor Topsøe, the developers used a supported liquid phase type catalyst, in which a liquid super acid is supported on a porous support material. The FBATM technology has proven to be stable toward variations in feed composition and flow. The catalyst system is capable of processing most typical alkylation feedstocks without pre-treatment. However, impurities in the feed meant that acid recovery capacity was needed and the choice of feed pre-treatment becomes an economic trade-off between the cost of the pre-treatment versus the cost of acid recovery. (S.I. Hommeltoft, 2001)
. kenneth A. kobe, john j. mcketta (1958). advances in petroleum chemistry and refining. new york: interscience publishers, inc. 336-383
.S.I. Hommeltoft / Applied Catalysis A: General 221 (2001) 421-428
. Meyers, Robert A. (2004). Handbook of petroleum refining processes. 3rd ed. New York, London : McGraw-Hill . 8-50.
. Broekhoven, E.H. van, Mas Cabré, F.R., Bogaard, P., Klaver, G., Vonhof, M., US Patent No.5.986.158 (1999).