Butane isomerization unit

After the war the need for aviation alkylate declined rapidly, and most of the isomerization units closed down. During the motor gasoline octane race in the 1950 s, a number of butane isomerization units were placed on stream. Several pentane isomerization units were placed on stream in the 1960 s, and it is believed that only one or two plants today are being used to isomerize a C5/C6 straight run cut (41). 

A plant was operated by Cities Service for several years. Some years ago the entire refinery where the SARP unit was located was discontinued. As was the case with the pilot unit, the SARP unit was easy to operate, and in general a steadier operation was obtained. Upsets and poor operation of units not directly a part of the recovery section, such as the feed splitter, butane isomerization unit, and deisobutanizer were not as serious when SARP was in operation. The general feeling was that an octane increase was obtained, as well as a demonstrated two thirds to three fourths reduction in acid consumption. Sufficiently precise runs under stable conditions were not made in the pilot unit or in either of two commercial plants to be certain that an increase in octane was obtained. 

With the start of World War II, several processes were developed to the point where commercialization was possible. The first commercial butane isomerization unit went on stream in the fall of 1941. Use of the process increased rapidly until by the end of the war, four years later, total isomerization capacity was 40,000 bbl./day (12). Soon after the end of hostilities most of these units were shut down as the need for aviation gasoline dropped. 

The isomerate can be blended directly into gasoline or sent to an alkylation unit. N-butane from an alkylation unit can be recycled to a butane isomerization unit to achieve nearly total conversion of n-butane into isobutane or alkylate. 

Butamer [Butane isomerization] A process for converting n-butane into iso-butane conducted in the presence of hydrogen over a dual-functional catalyst containing a noble metal. Developed by UOP and licensed worldwide since 1959. In 1992, more than 55 units had been licensed. 

Butane isomerization is usually carried out to have a source of isobutane which is often reacted with C3-C5 olefins to produce alkylate, a high octane blending gasoline [13]. An additional use for isobutane was to feed dehydrogenation units to make isobutene for methyl tert-butyl ether (MTBE) production, but since the phaseout of MTBE as an oxygenate additive for gasoline, this process has decHned in importance. Zeolitic catalysts have not yet been used industriaUy for this transformation though they have been heavily studied (Table 12.1). 

The first commercial isomerization plant was a butane unit at Shell s Houston refinery. It began operation in November 1941. By the end of the war, a total of 43 isomerization units had been built and placed in operation—38 in the United States and the remainder in Canada, the Caribbean area, and Arabia. 

The 34 domestic commercial units employing these five butane-isomerization processes contributed substantially to the war effort. 

In the event of another major war, it is probable that all existing isomerization units would be reactivated and pushed to capacity. Although production of Grade 115/145 aviation fuel required by newer aircraft engines may place somewhat greater emphasis on aromatics, there would still be a demand for maximum alkylate production, and butane isomerization would again play an important role. Expansion of pentane and naphtha isomerization is somewhat less certain and would depend on future developments in aircraft fuels. 

We first review in Part 1 the basics of plantwide control. We illustrate its importance by highlighting the unique characteristics that arise when operating and controlling complex integrated processes. The steps of our design procedure are described. In Part 2, we examine how the control of individual unit operations fits within the context of a plantwide perspective. Reactors, heat exchangers, distillation columns, and other unit operations are discussed. Then, the application of the procedure is illustrated in Part 3 with four industrial process examples the Eastman plantwide control process, the butane isomerization process, the HDA process, and the vinyl acetate monomer process. 

The reactivity pattern displayed by platinum crystal surfaces for alkane isomerization reactions is completely different from that for aromatization. Studies revealed that maximum rates and selectivity (rate of desired reaction /total rate) for butane isomerization reactions are obtained on the flat crystal face with the square unit cell. Isomerization rates for this surface are four to seven times higher than those for the hexagonal surface. Isomerization rates are increased to only a small extent by surface irregularities (steps and kinks) on the platinum surfaces (Figure 7.39). 

The determination of fluorine in various liquid and gaseous hydrocarbons is vital at many points in the refining process primarily in any blend component that has been sourced fiom the hydrogen fluoride (HF) Alkylation Unit. Fluorinated compounds poison process catalysts therefore, it is essential that process feeds be as free of fluorine as possible. As an example, butane is used to produce methyl tertiary-butyl ether (MTBE). The butane must be fluorine free prior to butane isomerization to prevent the poisoning of the process catalyst, fri addition, any HF acid or its combustion products may be extremely destructive in any environment. Therefore, any finished hydrocarbon product or synthesized material that is utilized in the presence of sufficient heat (i.e., car engine), such as frel and lubricating oils, must be free of fluoride. 

One morning he was walking past the isomerization unit on a routine task to collect environmental information. Suddenly, without any type of warning, a pump on the unit blew a seal and a large cloud of butane vapor was emitted. 

The profitability of the DIP column was determined based on the value of separating 1C5 for direct blending to gasoline and nC to be used as feed for the Cs/Cg isomerization unit, less the utility and downstream isomerization unit opportunity costs incurred to do so. Lighter feed eomponents, such as n-butane, were assumed to always be fractionated into the DIP overhead, and components heavier than nCs were assumed to always be found in the DIP bottoms stream. Thus, only the disposition of iCs and nCs components were considered in the profitability calculation. Therefore, the objective function for optimizing the DIP tower is defined as... 

In units that isomerize n-pentane and n-hexane, the reactions are catalyzed either by Pt/alumina or Pt on zeolite. The zeolite catalysts require higher temperatures, but they are less sensitive to water. As with butane isomerization, the reactions are controlled by equilibrium, so lower reaction temperatures favor branched isomers. The high temperatures required by zeolite catalysts reduce the octane of the product relative to products made at lower temperatures with chlorided alumina catalysts. 

The feed to a butane (C4) isomerization unit should contain maximum amounts of n-butane and only small amounts of isobutane, pentanes, and heavier material. The feed is dried, combined with dry makeup hydrogen, and charged to the reactor section at 230 to 340° F (110 to 170°C) and 200 to 300 psig (1480 to 2170 kPa). H2 is not consumed by isomerization reactions, but it suppresses polymerization of the olefin intermediates that are formed during the reaction. A small amount of organic chloride promoter, which is added to maintain catalyst activity, converts completely to HCl in the reactors. 

TABLE V. Isomerization Energies for Butane and Pentane (units kcal/mole)... 

Muller et al. focused on polybead molecules in the united atom approximation as a test system these are chains formed by spherical methylene beads connected by rigid bonds of length 1.53 A. The angle between successive bonds of a chain is also fixed at 112°. The torsion angles around the chain backbone are restricted to three rotational isomeric states, the trans (t) and gauche states (g+ and g ). The three-fold torsional potential energy function introduced [142] in a study of butane was used to calculate the RIS correlation matrix. Second order interactions , reflected in the so-called pentane effect, which almost excludes the consecutive combination of g+g- states (and vice-versa) are taken into account. In analogy to the polyethylene molecule, a standard RIS-model [143] was used to account for the pentane effect. 

The isomerization of the butanes and of neopentane has been studied over various types of evaporated platinum films by Anderson and Baker (68) and Anderson and Avery (108,24). Table II gives some typical results. It is clear that the proportion of parent hydrocarbon reacting to isomeric rather than to hydrogenolytic product is considerably smaller for a hydrocarbon with an unbranched as opposed to a branched chain containing an isostructural unit indeed, neopentane was studied as the archetypal molecule of the latter class. 

Feed stock for the first sulfuric acid alkylation units consisted mainly of butylenes and isobutane obtained originally from thermal cracking and later from catalytic cracking processes. Isobutane was derived from refinery sources and from natural gasoline processing. Isomerization of normal butane to make isobutane was also quite prevalent. Later the olefinic part of the feed stock was expanded to include propylene and amylenes in some cases. When ethylene was required in large quantities for the production of ethylbenzene, propane and butanes were cracked, and later naphtha and gas oils were cracked. This was especially practiced in European countries where the cracking of propane has not been economic. 

The increase of 1 unit of the RON corresponds to about 900.000 US per year for a 300.000 tpa hydroisomerization unit (1). In Figure 7.1, several major refinery processes to improve RON are shown these include isomerization, reforming, addition of FCC-Naphtha, alkylation, addition of oxygenates or polygas or butanes. The effect of these options with respect to the new specifications is different for each particular process. Keeping in mind the Californian ban on MTBE and also the fact... 

The hydrocracking process often produces relatively large amounts of iso-butane that can be used in alkylation units to prepare alkylate for gasoline blending. Hydrocracking, depending on the catalyst, can also cause isomerization of the paraffinic products that benefit liquid fuels in terms of pour point control and smoke point. 

A crucial difference between solid and liquid acids is the ability of certain solids with strong Bronsted sites to isomerize -butane to i-butane via a bimolecular mechanism. A Cg carbocation is formed which isomerizes and undergoes p-fission. In this fission, the formation of two M0-C4 units is apparently preferred. Only if fission is preceded by extensive isomerization of the Cg carbocation can isotopic scrambling reach the randomization predicted by the binomial law. 

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