Refinery alkylation

About 60 metres and 220 metric tons without internals a new Iso-stripper column for an alkylation unit. The significant decrease in column diameter is a signature feature within the skyline of every alkylation unit owed to the product composition after removal and recirculation of the surplus i-C4 only a small amount of lights ends (C2-C3) remain as an overhead product. Picture courtesy of M. Wilhelm 

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Refinery alkylation Liquid alkanes (e.g., isobutane) Gaseous alkenes (e.g., 1-butene) HF or H2SO4... 

The isobutylene in the C4 stream generally ends up in one of four places a refinery alkylation plant (covered further below), an MTBE plant, a polymerization process, or in gasoline. The first three are methods of removing isobutylene from the C4 stream the fourth is the default—the isobutylene just follows the other butanes and butylenes to the gasoline blending pool. 

The butanes are used as gasoline-blending components. Normal butane is sometimes an olefins plant feed. Isobutane is used in refinery alkylation plants with propylene or butylene to make alkylate, a high-octane gasolineblending component. 

Figure 10.18 Calibration describing the relationship between chemical shift and sulfuric acid concentration in spent acid from a refinery alkylation process.

Refinery alkylation processes utilize either sulfuric acid or hydrofluoric acid as reaction catalysts. The feedstock for both alkylation processes originates primarily from hydrocracking and catalytic cracking operations. Coker gas oils also serve as feedstock in some applications. The differences and similarities between sulfuric acid alkylation and hydrofluoric acid alkylation are shown in TABLE 2-5. Typical alkylation reactions are shown in FIGURE 2-9. A sulfuric acid alkylation unit is illustrated in FIGURE 2-10. 

For example, adding lighter propane to heavier isobutane in a refinery alkylation unit is a common practice to increase refrigerant circulation. But this will work only when the compressor driver has spare... 

So the competition will apparently continue, each refinery alkylation plant and catalyst selection being evaluated on an individual basis. 

Multiple supercritical isobutane regenerations of a partially deactivated USY solid acid catalyst also was tested utilizing a refinery alkylation feed blend Error Reference source not found,). The catalyst activity recovery was compared with the results of experiments that utilized a synthetic feed blend. 

Refinery alkylation Fiquid alkanes Gaseous alkenes HForH2S04... 

Alkylation fScheme 6.41 or refinery alkylation is the reaction of isobutane with short-chain olefins (C3—C5) in the presence of highly acidic catalysts. Alkylates are particularly suitable... 

Refinery alkylation, the reaction of isobutane (or isopentane) with light alkenes to produce high octane-number fuels, is a very important example of the technical use of branched light alkanes. This process is highlighted in Section 6.10. 

Scheme 6.10.1 Refinery alkylation is an acid-catalyzed reaction with very high relevance for producing high quality fuels.

The term refinery alkylation is applied to the reaction of low molecular weight olefins (propene, butenes, or pentenes) with isoparaffins to form higher molecular weight isoparaffins. The latter are very important hydrocarbon compounds for the production of high-quality fuel (Scheme 6.10.1). Currently, approximately 13-15% of the gasoline pool is produced by refinery alkylation. Refinery alkylation products are characterized by high research octane numbers (RONs) (93-97) and motor octane numbers (MONs) (90-95). 

Refinery alkylation takes place at high temperatures and pressures without catalysts. However, all industrially relevant processes proceed at low-temperatures in the presence of highly acidic catalysts. By appropriate choice of operating conditions, most of the alkylate can be made to fall within the gasoline boiling range with RONS of94-99 and MONs of 88-94. 

Figure 6.10.2 shows the simplified main reaction cycle in refinery alkylation. An olefin is added to the ferf-butyl cation to give the corresponding Cg carbocation. This Cg carbocation may isomerize via hydride and methyl shifis to form a more stable carbenium ion and subsequently undergoes, again, hydride transfer from isobutane. This latter step forms the saturated hydrocarbon and regenerates the tert-butyl cation to perpetuate the catalytic cyde. 

Scheme 6.10.2 Initial steps of the refinery alkylation reaction proton addition to 2-butene, isomerization via methyl shift or hydride transfer with isobutane to form a tert-butyl cation.

Figure 6.10.2 Main catalysis cycle of the refinery alkylation of 2-butene with isobutane.

In addition to the reactions described above, several side reactions occur in the refinery alkylation process that reduce the quality of alkylate and are highly undesirable. The most important of these are polymerization, disproportionation, cracking, and self-alkylation. They are described in more detail in Example 6.10.1 [see also Corma and Martinez (1993), and Albright (2003, 2009) for further information]. 

To run a refinery alkylation unit isobutane and light olefins are required as feedstock. However, the composition of the olefin stream varies significantly with the local refinery situation and this requires careful adjustment of the process conditions. The most commonly used olefins are butenes and propene but sometimes the use of pentenes is also considered. New gasoline specifications and the Chan Air Act (a United States federal law) amendments make it necessary to remove pentenes from the gasoline pool, because of their potential for atmospheric pollution. The main sources of olefins are catalytic cracking and coking processes. The isobutane feed for alkylation units is mainly obtained from hydrocrackers, catalytic crackers, and catalytic reformers. Additional amounts of isobutane are directly available from crude distillation and natural gas processing. Moreover, n-butane can be... 

Table 6.10.1 Influence of feedstock impurities on sulfuric acid consumption in a refinery alkylation unit (Corma and Martinez, 1993 and Albright, 2003).

Isobutane concentration is generally expressed in terms of the isobutane-to-olefin ratio (I/O). This ratio is the most important process variable to control in terms of refinery alkylation productivity, yield, and quality of alkylate, as well as the add... 

Isobutane is almost insoluble in the liquid acid catalysts used in refinery alkylation. Therefore, intense mixing is necessary to make sure that at least this low isobutene equilibrium concentration is maintained throughout the readion in the acid phase to allow the reaction to proceed properly. As the alkylation readion occurs at or near... 

A detailed comparison of the three sulfuric add based alkylation processes (Corma and Martinez, 1993) reveals that the time-tank process produces the best alkylate quality but has higher energy consumption related to isobutene recycling and refigeration compared to the other two. This is the reason why in the last 35 years no new time-tank refinery alkylation units have been installed. However, several refineries still operate older time-tank units. 

Refinery alkylation is a very important refinery process that produces high-quality fuel by the reaction of light alkenes and alkanes. 

Important undesired side reactions of refinery alkylation are polymerization, disproportionation, cracking, and self-alkylation. 

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