Difference Between Catalytic Cracking And Alkylation |BEST|
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An alkylation unit (alky) is one of the conversion processes used in petroleum refineries. It is used to convert isobutane and low-molecular-weight alkenes (primarily a mixture of propene and butene) into alkylate, a high octane gasoline component. The process occurs in the presence of an acid such as sulfuric acid (H2SO4) or hydrofluoric acid (HF) as catalyst. Depending on the acid used, the unit is called a sulfuric acid alkylation unit (SAAU) or hydrofluoric acid alkylation unit (HFAU). In short, the alky produces a high-quality gasoline blending stock by combining two shorter hydrocarbon molecules into one longer chain gasoline-range molecule by mixing isobutane with a light olefin such as propylene or butylene from the refinery's fluid catalytic cracking unit (FCCU) in the presence of an acid catalyst.
Refineries examine whether it makes sense economically to install alkylation units. Alkylation units are complex, with substantial economy of scale. SAAU and HFAU have comparable capital investment costs. It is not surprising that the two processes are competitive on a capital cost basis, when one considers the basic process differences. The SAAU has a more expensive reactor section and requires refrigeration. However, equal costs are realized in the HF unit by the need for feed driers, product treating, regeneration equipment and more exotic metallurgy. In addition, most refiners will require a dedicated cooling system for an HF unit, to remove the risk of site-wide corrosion in the case of an HF leak. These capital cost estimates do not account for the additional safety and mitigation equipment now required in HF units. Due to the possible hazardous aerosol formation when the HF catalyst is released as a superheated liquid, expensive mitigation systems are now required in many locations throughout the world where HF is used as an alkylation catalyst.
In addition to a suitable quantity of feedstock, the price spread between the value of alkylate product and alternate feedstock disposition value must be large enough to justify the installation. Alternative outlets for refinery alkylation feedstocks include sales as LPG, blending of C4 streams directly into gasoline and feedstocks for chemical plants. Local market conditions vary widely between plants. Variation in the RVP specification for gasoline between countries and between seasons dramatically impacts the amount of butane streams that can be blended directly into gasoline. The transportation of specific types of LPG streams can be expensive so local disparities in economic conditions are often not fully mitigated by cross market movements of alkylation feedstocks.
The common source of the C3 alkenes for the alkylation is made available from the gas recovery unit processing the effluents of the Fluid catalytic cracking Unit. Isobutane is partly made available from the Catalytic reforming and from the Atmospheric distillation, although the proportion of the isobutane produced in a refinery is rarely sufficient to run the unit at full capacity and additional isobutane needs therefore to be brought to the refinery. The economics of the international and local market of gasolines dictates the spread that a buyer need to pay for isobutane compared to standard commercial butane.
Spent sulfuric acid is regenerated by thermal decomposition outside the battery limits of the sulfuric acid alkylation unit. This may be accomplished on the refinery site in sulfuric acid regeneration equipment operated by the refinery or in a commercial sulfuric acid regeneration plant which serves several refiners. The choice between these two options is site specific and usually depends on capital versus operating cost considerations and the proximity of the refinery to an existing commercial regeneration plant.As there is low risk from the sulfuric acid itself, the choice to regenerate on-site the acid or elsewhere is based on consideration of economic nature. Of course, even this relatively minor risk is eliminated with on-site sulfuric acid regeneration equipment.
Hello. Welcome to lesson eight. We will talk about more catalytic conversion processes. We'll talk about in this class about alkylation, about polymerization, catalytic reforming, and isomerization. All of these processes are catalytic processes to produce high octane number gasoline.Remember, for high performance, high power, we needed to produce high octane number gasoline. FCC obviously is the principal process in a refinery to produce high octane gasoline. Catalytic reforming, developed also during the Second World War, was really popular, very popular process.Now the feed stock for catalytic cracking comes from the light ends unit. You'll remember the naphtha fractionater in the light ends unit. The heaviest product from the light ends unit is the heavy naphtha. The reason it's heavy-- it has a lot of naphthenes, or cycloalkanes in its composition.So what happens in catalytic reforming, essentially converting these naphthenes or cycloalkanes into aromatics. Aromatics have very large octane numbers. Benzene, for example, has octane number of 100. So that it's highly, highly desirable high octane number component in the refinery blending scheme, if you will.So dehydrogenation of naphthenes using a precious metal catalyst like platinum is pretty straightforward if you do have a clean heavy naphtha. If you have sulphur, associated with clean, or with naphtha feed, you need to hydro treat it to remove, because platinum is very susceptible to poisoning by sulphur. So you need a pre hydro treatment before catalytic reforming.Now, up until 1990s, cat reforming was one of the most popular processes in the refinery as far as producing high octane number gasoline. But with the introduction of 1990 Clean Air Act amendments, the amount of gasoline or benzene and aromatics in gasoline were limited because of environmental issues or reasons of toxicity.So all of a sudden now, catalytic reforming that produces a high aromatic content wasn't so desirable. But the refiners could not give up catalytic reforming. Why? Because there is a byproduct from cat reforming that is very, very valuable for the refinery. It has become, of course, increasingly valuable in the recent times. And that is hydrogen.If you do dehydrogenate naphthenes, the byproduct is hydrogen, in addition to making aromatics. And hydrogen is needed in hydro treating processes and finishing processes that we will be talking about in the next lesson. So that is the cheapest source of hydrogen.Obviously, you can make hydrogen from natural gas by reforming natural gas. And that is the done in refineries as well to produce additional hydrogen. But the cheapest source of hydrogen in a refinery comes from catalytic reforming. So it's still used in US refineries to make gasoline that is a reformate and of course, the byproduct hydrogen.The second process we will talk about this alkylation. Alkylation is, in a sense, the opposite of cracking, where you have a larger molecule, you crack it into smaller molecules. In Alkylation, we do the opposite. We take the smaller molecules and combine them into larger molecules that would fall in the boiling range for gasoline.So the feed stocks for alkylation are three to four carbon atom alkanes, isoalkanes. Isobutane is the principal feed stock that comes from FCC. And also, olefins, three to four carbon atom olefins. That is propene and butene.So combining isobutane with propene or propene or butene will put you in the gasoline boiling range with seven eight-carbon atoms. And the resulting product will be an isoalkane with a high octane number. That would be essentially making up the alkylate.So alkylation is an alternative to catalytic reforming to make high octane gasoline without the aromatic. So that looks like really a nice alternative. But there is one problem. And the problem is for alkylation, you would need as catalyst highly concentrated acids, like highly concentrated sulfuric acid or highly concentrated hydrofluoric acid. These are, of course, not easy to our work with or transport or to have around, because of the risks involved with using this highly acidic materials.Another process that generates larger hydrocarbons from smaller fragments are called polymerization. The difference between alkylation and polymerization is that we use just olefins in polymerization. No isobutane here.So we use three to four carbon atom olefins-- typically again come from FCC process-- combine them using a milder acid as a catalyst this time-- so phosphoric acid. So the problems with handling phosphoric acids are not as severe as those with highly concentrated sulphuric or hydrogen fluoride used in alkylation. So polymerization produces branched olefins, which also have respectable octane numbers.The last process we will talk about is isomerization. That's actually adding branching to straight-run paraffins. This is essentially the light naphtha that comes from the light end unit, as opposed to heavy naphtha that is naphthenic like naphtha is paraffinic. But it has only straight chain paraffins with low octane numbers. So isomerization add branching to these straight chains to increase the octane number. So all these processes just make high octane gasoline, which is, of course, the most important fuel in US refineries.
Among the catalytic conversion processes developed just before and during the Second World War are included, in addition to catalytic cracking, polymerization processes that were introduced in the mid- to late 1930s, and alkylation and isomerization processes that were developed in the early 1940s. The principal impetus for developing these processes was to meet the demand for high-octane-number gasoline required by the high compression gasoline engines, including those used in the aircraft. Catalytic reforming and catalytic isomerization were developed in the 1950s to increase the high-octane-number gasoline yields from refineries. These processes are still important in current refineries that are directed to maximize gasoline yield from the crude oil feedstock. By-products from some of these processes, such as LPG and hydrogen, have gained significance because of the increasing demand in modern refineries for LPG recently used as automobile fuel and for hydrogen to supply the increasing demand for hydrotreating and hydrocracking processes. 2b1af7f3a8