Sulfuric acid, alkylation reactors using

Alkylation is an association reaction that is exothermic. Therefore, it has a favorable equilibrium only at low temperatures. The process is catalyzed by hquid acids of solid AICI3, and modem alkylation reactors use sulfuric acid or hquid HF as catalysts operating at 0°C in a refrigerated reactor that is stirred rapidly to dissolve and create bubbles of the hydrocarbons in the acid. 

Relatively low temperatures are required in alkylation reactors using sulfuric acid catalyst. They are necessary in order to slow down polymerization reactions and reduce the formation of undesirable acid soluble and hydrocarbon soluble by-products. Typically, most commercial units operate with reaction temperatures in the range of 35 F. to 65 F. Design temperatures are usually set at 50 F. For minimum acid make-up the reactor section should be operated as cold as possible. This means operating... 

Commercial reactors using sulfuric acid as the catalysts currently employ residence times in the 15- to 30-min range. Times as little as 2-5 min have been used in the laboratory to produce high quality alkylates (8,29). With HF as a catalyst, times as little as 5-10 s are used (30). Yet, information is still needed on how the kinetics of reactions varies as CPs and water build up in the acid phase. As the acidity of sulfuric acid decreases, the rates of formation of isoalkyl acid sulfates eventually surpass the rates of decomposition (or reaction) of these sulfates in which case, the acidity of the acid drops rapidly, and simultaneously the rate of alkylate production decreases precipitously. Such a highly undesired phenomenon is referred to as an acid runaway. A similar phenomenon can occur with HF. 

There are several ways to reduce acid consumption (and hence acid costs) significantly. These costs frequently account for about one-third of total operating costs of an alkylation unit using sulfuric acid as the catalyst. First, the refinery s choice of the feed acid has a fairly large effect on acid consumption. As a comparison, feed acids of between 98% and 99.5% acidity values will be considered, and the acids will be reduced to acidity values of 90%. In the first case, the acidity is decreased by 8% but for the second acid by 9.5%. Assuming that the acidity decreases per pass in the reactor are equal, then approximately 18-19% more alkylate are produced with the more concentrated feed acid. Several refineries currently use 99.5% feed acids, but others employ 98.5% feed acids. 

Recently, significant improvements have been made for processes using sulfuric acid. First, acid consumption has been reduced by as much as 50 /o in at least select refineries. Second, simultaneously the quality of alkylates has increased. Even further improvements of these processes seem likely, especially if improved reactors are built. 

Cooling costs to remove the heats of reaction are much lower than those units that use sulfuric acid as the catalyst. All commercial alkylation reactors using HF as the catalyst operate at about 35-40°C, or at temperatures that can be achieved using cooling water. If the HF-type plants were to use lower temperatures (and refrigeration), improved quality allgrlates would... 

Liquid/hquid reactions of industrial importance are fairly numerous. A hst of 26 classes of reactions with 61 references has been compiled by Doraiswamy and Sharma Heterogeneou.s Reactions, Wiley, 1984). They also indicate the kind of reactor normally used in each case. The reactions range from such prosaic examples as making soap with alkali, nitration of aromatics to make explosives, and alkylation of C4S with sulfuric acid to make improved gasoline, to some much less familiar operations. 

Fig. 11. Stratco Contactor reactor used in sulfuric acid-catalyzed alkylation (12).

The major discharges from sulfuric acid alkylation are the spent caustics from the neutralization of hydrocarbon streams leaving the alkylation reactor. These wastewaters contain dissolved and suspended solids, sulfides, oils, and other contaminants. Water drawn off from the overhead accumulators contains varying amounts of oil, sulfides, and other contaminants, but is not a major source of waste. Most refineries process the waste sulfuric acid stream from the reactor to recover clean acids, use it to neutralize other waste streams, or sell it. 

For purposes of plant design and for optimum operation consistent with feed stock availability, it is necessary to be able to predict accurately the octanes of the alkylate produced under varying operating conditions. Such a correlation developed from several hundred pilot plant and commercial plant tests is presented in Figures 7, 8, and 9. This correlation is applicable to sulfuric acid alkylation of isobutane with the indicated olefins, and was developed specifically for the impeller-type reaction system, although it also appears to be satisfactory for use with some other types of reactors. 

Sulfuric acid alkylation also is used. In addition to the type of acid catalyst used, the processes differ in the way of producing the emulsion, increasing the interfacial surface for the reaction. There also are important differences in the manner in which the heat of reaction is removed. Often, a refrigerated cascade reactor is used. In other designs, a portion of the reactor effluent is vaporized by pressure reduction to provide cooling for the reactor. 

Industrial fluid-fluid reactors may broadly be divided into gas-liquid and liquid-liquid reactors. Gas-liquid reactors typically may be used for the manufacture of pure products (such as sulfuric acid, nitric acid, nitrates, phosphates, adipic acid, and other chemicals) where all the gas and liquid react. They are also used in processes where gas-phase reactants are sparged into the reactor and the reaction takes place in the liquid phase (such as hydrogenation, halogenation, oxidation, nitration, alkylation, fermentation, oxidation of sludges, production of proteins, biochemical oxidations, and so on). Gas purification (in which relatively small amounts of impurities such as C02, CO, COS, S02, H2S, NO, and... 

Chemetics has developed a process for treating spent alkylation sulfuric acid with nitric acid to produce a sulfuric acid that can be used to acidulate phosphate rock, the major use for sulfuric acid. The organic contaminants are converted to carbon particles that are removed with the gypsum on filtration of the phosphoric acid. Special alloys are used in the fabrication of the acid reactor. Topsoe developed and, by the year 2005 had built, more than 45 Wet Sulfuric Acid (WSA) process units. This process is especially suited for... 

OATS [Olefinic Alkylation of Thiophenic Sulfur] A gasoline desulfurization process. Thiophenes and mercaptans are catalytically reacted with olefins to produce higher-boiling compounds that can more easily be removed by distillation prior to hydrodesulfurization. This minimizes hydrogen usage. The process uses a solid acid catalyst in a liquid-phase, fixed bed reactor. Developed by BPAmoco in 2000 and tested in Bavaria and Texas. First used commercially at the Bayernoil refinery, Neustadt, in 2001. The process won a European Environment Award in 2002. 

Although no specific analysis was made to Identify sec-butyl sulfate in the acid phase, there seems little doubt that this sulfate was the reaction product obtained in the n-butene runs. First, the n-butenes and acid reacted on essentially a 1 1 molar basis only a slight excess of acid was needed In all cases. Of Interest, formation of butyl sulfates has also been reported to occur In conventional alkylation reactors (13). Second, the reaction product was similar 1n many respects as compared to sec-propyl sulfate the product was unstable at higher temperatures and 1t reacted with isobutane in the presence of sulfuric aicd to form alkylate (9). Third, the reaction products from all three n-butenes alkylated in identical manners as will be discussed later (9). Fourth, McCauley (14) has shown that butyl fluorides can be formed and are quite stable at low temperatures such as used here he had contacted HF with n-butenes. 

The most significant impurities in the feed streams to a typical alkylation unit are ethylene, dlolefins, sulfur compounds and water. Corrosion Inhibitors and other chemicals used in upstream processing can also be present in some cases, and these can have harmful effects. The amount of each iaqiurlty that reaches the alkylation reactor varies considerably from refinery to refinery. If accurately determined and properly accounted for, these impurities can explain an appreciable percentage of the acid make-up reported by various operating units. The impurity data shown in Table I can be used to evaluate the merit of Improved upstream process control and/or more efficient feed pretreatment methods. 

Alkylation was first practiced for gasoline production about 60 yr ago. At that time, most of the alkylate was used as fuel for the airplanes used in World War II. Four quite distinct reactors were developed in which isobutane and olefins were introduced as liquids to the reactor. In the reactor, the hydrocarbon liquids are contacted with either liquid sulfuric acid or liquid hydrofluoric acid (HF), which acts as a catalyst. Dispersions of these two relatively immiscible liquids are formed. The alkylate product formed is a mixture of mainly C5-C16 isoparaffins. Alkylate products often have research octane numbers (RONs) varying from 93 to 98 (the motor octane numbers tend to be two to three units lower). 

In current processes that use either sulfuric acid or HF, isobutane in large excess and olefins are introduced as liquids into the reactor. After completion of the reactions, the liquid-liquid dispersions are separated by decanting. The alkylate product is separated by distillation or stripping from the unreacted isobutane, which is recirculated to the reactor. This entry reviews the chemistry, physicochemical phenomena, current processes, and finally suggests methods to improve significantly the alkylation process. 

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