Hydrofluoric alkylation

Other acid plants are under serious consideration by refiners at this time. Plants as small as 15 tons/day have been quoted in the fourth quarter of 1976. Once the first small plant has been built and operated successfully, it is the writer s considered opinion that many of these plants will be built. This will definitely influence the ratio of sulfuric/hydrofluoric alkylation plants many small HF alkylation plants were built simply because sulfuric acid was unavailable. 

Other kinds of liquid-liquid equilibria are encountered in processes such as alkylation, where anhydrous hydrofluoric acid (HF) is partially soluble in hydrocarbons. 

A freshly made solution behaves as a strong monobasic acid. Neutralized solutions slowly become acidic because of hydrolysis to monofluorophosphoric acid and hydrofluoric acid. The anhydrous acid undergoes slow decomposition on distillation at atmospheric pressure, reacts with alcohols to give monofluorophosphoric acid esters, and is an alkylation (qv) and a polymerization catalyst. 

Fluorosulfuric acid [7789-21-17, HSO F, is a colodess-to-light yellow liquid that fumes strongly in moist air and has a sharp odor. It may be regarded as a mixed anhydride of sulfuric and hydrofluoric acids. Fluorosulfuric acid was first identified and characterized in 1892 (1). It is a strong acid and is employed as a catalyst and chemical reagent in a number of chemical processes, such as alkylation (qv), acylation, polymerization, sulfonation, isomerization, and production of organic fluorosulfates (see Friedel-CRAFTSreactions). 

The catalysts used in the industrial alkylation processes are strong Hquid acids, either sulfuric acid [7664-93-9] (H2SO or hydrofluoric acid [7664-39-3] (HE). Other strong acids have been shown to be capable of alkylation in the laboratory but have not been used commercially. Aluminum chloride [7446-70-0] (AlCl ) is suitable for the alkylation of isobutane with ethylene (12). Super acids, such as trifluoromethanesulfonic acid [1493-13-6] also produce alkylate (13). SoHd strong acid catalysts, such as Y-type zeoHte or BE -promoted acidic ion-exchange resin, have also been investigated (14—16). 

The reduction ia tetraethyl lead for gasoline production is expected to iacrease the demand for petroleum alkylate both ia the U.S. and abroad. Alkylate producers have a choice of either a hydrofluoric acid or sulfuric acid process. Both processes are widely used. However, concerns over the safety or potential regulation of hydrofluoric acid seem likely to convince more refiners to use the sulfuric acid process for future alkylate capacity. 

Alkylate. Alkylation means the chemical combination of isobutane with any one or a combination of propylene, butylenes, and amylenes to produce a mixture of highly branched paraffins that have high antiknock properties with good stabiUty. These reactions are cataly2ed by strong acids such as sulfuric or hydrofluoric acid and have been studied extensively (98—103). In the United States mostly butylenes and propylene are used as the olefins. 

Alkylations—for example, of olefins with aromatics or isoparaffins—are catalyzed by sulfuric acid, hydrofluoric acid, BF3 and AICI3. 

The principal use of the alkylation process is the production of high octane aviation and motor gasoline blending stocks by the chemical addition of C2, C3, C4, or C5 olefins or mixtures of these olefins to an iso-paraffin, usually isobutane. Alkylation of benzene with olefins to produce styrene, cumene, and detergent alkylate are petrochemical processes. The alkylation reaction can be promoted by concentrated sulfuric acid, hydrofluoric acid, aluminum chloride, or boron fluoride at low temperatures. Thermal alkylation is possible at high temperatures and very high pressures. 

A large oil refinery had a failure in their alkylation unit resulting in a significant release of hydrofluoric acid (HF). This incident and others in refineries during recent years prompted members of the hydrocarbon processors to turn to HAZOP to better understand their risks. 

Either concentrated sulfuric acid or anhydrous hydrofluoric acid is used as a catalyst for the alkylation reaction. These acid catalysts are capable of providing a proton, which reacts with the olefin to form a carbocation. For example, when propene is used with isohutane, a mixture of C5 isomers is produced. The following represents the reaction steps ... 

Both sulfuric acid and hydrofluoric acid catalyzed alkylations are low temperature processes. Table 3-13 gives the alkylation conditions for HF and H2SO4 processes. One drawback of using H2SO4 and HF in alkylation is the hazards associated with it. Many attempts have been tried to use solid catalysts such as zeolites, alumina and ion exchange resins. Also strong solid acids such as sulfated zirconia and SbFs/sulfonic acid resins were tried. Although they were active, nevertheless they lack stability. No process yet proved successful due to the fast deactivation of the catalyst. A new process which may have commercial possibility, uses... 

Commercial alkylation is the reaction of isobutane with C3 through Cg olefins in the presence of either sulfuric acid or hydrofluoric acid (see Example 10-1). Etherification is the reaction of a tertiary olefin with an alcohol or water in the presence of an acidic catalyst (see Example 10-2). 

Alkylation is one of the refining processes in which light olefin molecules are reacted with isobutane (in the presence of either sulfuric or hydrofluoric acid) to produce a desirable gasoline component called alkylate. 

In 1950 the Fischer-Tropsch synthesis was banned in Germany by the allied forces. Sinarol, a high paraffinic kerosene fraction sold by Shell, was used as a substitute. This ban coincided with the rapid development of the European petrochemical industry, and in due time Fischer-Tropsch synthesis applied to the production of paraffins became uneconomic anyway. After the war there was a steady worldwide increase in the demand for surfactants. In order to continually meet the demand for synthetic detergents, the industry was compelled to find a substitute for /z-paraffin. This was achieved by the oligomerization of the propene part of raffinate gases with phosphoric acid catalyst at 200°C and about 20 bars pressure to produce tetrapropene. Tetrapropene was inexpensive, comprising a defined C cut and an olefinic double bond. Instead of the Lewis acid, aluminum chloride, hydrofluoric acid could now be used as a considerably milder, more economical, and easier-to-handle alkylation catalyst [4],... 

The alkylation proceeds with aluminum chloride or hydrofluoric acid as catalyst, by which the importance of aluminum chloride diminishes. Today approximately 70% of all manufacturers use the HF process [4]. In addition, LAB is produced by the alkylation of secondary chloroparaffins (Wibarco in Germany) and by the alkylation of olefins (EniChem Augusta in Italy) over an aluminum chloride catalyst [12]. 

The LAB production process (process 1) is mainly developed and licensed by UOP. The N-paraffins are partially converted to internal /z-olefins by a catalytic dehydrogenation. The resulting mixture of /z-paraffins and n-olefins is selectively hydrogenated to reduce diolefins and then fed into an alkylation reactor, together with an excess benzene and with concentrated hydrofluoric acid (HF) which acts as the catalyst in a Friedel-Crafts reaction. In successive sections of the plant the HF, benzene, and unconverted /z-paraffins are recovered and recycled to the previous reaction stages. In the final stage of distillation, the LAB is separated from the heavy alkylates. 

Alkad A process for improving the safety of alkylation processes using hydrofluoric acid as the catalyst. A proprietary additive curtails the emission of the acid aerosol that forms in the event of a leak. Based on observation of G. Olah in the early 1990s that liquid polyhydrogen fluoride complexes (of amines such as pyridine) depress the vapor pressure of HF above alkylation mixtures. Developed by UOP and Texaco and operated at Texaco s refinery at El Dorado, TX, since 1994. A competing process is ReVAP, developed by Phillips and Mobil. 

Since the discovery of alkylation, the elucidation of its mechanism has attracted great interest. The early findings are associated with Schmerling (17-19), who successfully applied a carbenium ion mechanism with a set of consecutive and simultaneous reaction steps to describe the observed reaction kinetics. Later, most of the mechanistic information about sulfuric acid-catalyzed processes was provided by Albright. Much less information is available about hydrofluoric acid as catalyst. In the following, a consolidated view of the alkylation mechanism is presented. Similarities and dissimilarities between zeolites as representatives of solid acid alkylation catalysts and HF and H2S04 as liquid catalysts are highlighted. Experimental results are compared with quantum-chemical calculations of the individual reaction steps in various media. 

With hydrofluoric acid (23,50), and to a lesser degree also with zeolites (14,81, 87-89), a significant fraction of the product stems from self-alkylation, which is sometimes also termed hydrogen transfer. The importance of this mechanism depends on the acid, the alkene, and the reaction temperature. Self-alkylation... 

The same principles regarding the acidity can be applied to hydrofluoric acid-catalyzed alkylation, which is more sensitive towards water, so that the feed must be thoroughly dried before entering the reactor. Furthermore, the acid dilution by... 

Hydrofluoric acid selenic acid Alkyl phenols Fluorine chlorine bromine ... 

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