HF alkylation unit

Figure 11.4-2 shows process flows for an HF alkylation unit. The three sections are 1) reaction, 2). settling and 3) fractionation. In the reaction section isobutane feed is mixed with the olefin feed (usually propylene and butylene) in approximately a 10 or 15 to 1 ratio. In the presence of the HF acid catalyst the olefins react to form alkylate for gasoline blending. The exothermic reaction requires water cooling. The hydrocarbon/HF mixture goes to the settling... 

At the heart of the UOP HF alkylation unit is a vertical reactor-heat exchanger, shown in Fig. 14. The isobutane-alkene mixture enters the shell of the reactor through several nozzles, and HF enters at the bottom of the reactor. The reaction heat is removed by cooling water, which flows through cooling coils inside the reactor. After phase separation in the settler, the acid is recycled to the reactor. The hydrocarbon phase together with a slipstream of used acid and makeup isobutane is sent to the isostripper , where the alkylate product, n-butane, and isobutane are separated. The isobutane is recycled to the reactor. During normal... 

Hydrofluoric acid (HF) alkylation units have small acid remn units to purify the acid for reuse. HF units do not have a spent acid or spent caustic waste stream. Any leaks or spills that... 

The early experimental work showed that the alkylation reactions could be carried out thermally and by using hydrofluoric acid or aluminum chloride as catalysts. Successful processes were developed with the hydrofluoric acid catalyst, and the first commercial HF unit started up on Christmas Day 1942, having a capacity of 1950 BPD (3). Other commercial HF alkylation units followed quickly. The thermal and aluminum chloride-catalyzed alkylation processes had limited commercial acceptance. 

Typical Motor Alkylate from, HF Alkylation Unit Operating without Refrigeration and without Rerun Column... 

Fig. 1. F-l clear octanes produced at various reactor temperatures in an HF alkylation unit when charging butane-butylenes.

Low temperatures in the alkylation reaction zone favor the isomerization of butylene-1 to butylene-2 during the alkylation reaction. Where low temperatures are not used for economic reasons, however, as is the case with many HF alkylation units, this isomerization is sometimes carried out in a separate unit and the isomerized product then charged to the alkylation unit. Catalysts for this isomerization reaction are phosphoric acid and silica-alumina. Some polymer is usually made when isomerizing with these catalysts, but when the polymer production is limited to diisobutylene, it will be alkylated to produce the trimethylpentanes. Any polymer made from normal butylenes, however, is an inferior feed for alkylation units. When this type of polymer is made, it is usually removed by fractionation before charging the isomerized butylene feed to the alkylation unit. 

High isobutane recycle purity is not required on HF alkylation units as is required on many H2SO4 units because relatively high normal butane concentrations in the reaction zone do not appreciably affect the quality of the alkylate. Isobutane purities below 60% are usually avoided, however, since this purity definitely gives lower-quality alkylate and the cost of recycling the normal butane is considerable in heat requirements as well as fractionation equipment requirements. 

The reaction and acid storage sections of an HF alkylation unit are constructed of carbon steel, stress-relieved, and X-rayed. When properly constructed, these sections are essentially free of corrosion. Monel trim is used on all valves, pumps, and instruments. 

Stewart and McVey (1994) describe the design of a deinventory system for a hydrofluoric acid (HF) alkylation unit. The purpose of this deinventory system is to mitigate an accidental release by removing the hazardous materials rapidly from the leaking or damaged system to a safe location. 

NACE Task Group Report T-5A-6. Summary of Questionnaire Replies on Corrosion in HF Alkylation Units. Corrosion 15, 5(1959) p. 237t. 

The HF alkylation process is the dominant route for LAB production utilized by the plants built before 1995. The olefin feed to a conventional HF alkylation unit is typically derived from the Pacol process. Olefin feed is combined with an excess of benzene before the addition of the HF acid. Diluting the olefin in benzene followed by intense mixing helps minimize the formation of by-products, such as dialkylbenzene. The typical operating temperature for the HF alkylation reaction is 30 0° C. Operating pressure is sufficient... 

Figure 12.3 Effect of reactor temperature on research octane number (RON) of alkylate produced in an HF alkylation unit [8].

Subsequently, over several months, the demo unit reliably operated on a nearly continuous basis (except for holiday shut-down periods) while producing high quality product. The unit utilizes piped slipstreams of actual refinery feeds. To date, we have tested both predominantly C4 olefin and mixed C3/C4 olefin feed streams, and the demo has produced alkylate of comparable quality to that from Fortum s Porvoo Refinery HF alkylation unit. 

Crude methanol from the methanol synthesis will be distilled in order to remove the water content and hence improve the overall process economy. The distilled methanol is then converted to hydrocarbons in the fluid-bed reaction system. The hydrocarbon products are processed in a gas fractionation unit to produce C5 plus hydrocarbons, alkylation feed (C3/C4 fraction) and olefin recycle. The C3/C4 fraction is processed in the HF alkylation unit to produce alkylate. 

Block valves and drains have been installed in HF alkylation units in the water piping on the... 

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. 

Top impurity accumuiation. In some coliunns, overhead is totally condensed and then decanted to form an aqueous stream and an organic product. The organic product is then sent to a stripper to remove traces of the aqueous phase. The stripper overhead is then recycled to the column condenser inlet. A tsqiical example is a refinery HF alkylation unit depropanizer/HF stripper system. 

Depending on the feedstock used, there may be additional fractionation columns included within the HF alkylation unit. For example, when linear paraffins are fed to a dehydrogenation unit upstream of the HF alkylation unit, there are unreacted linear paraffins contained within the mixture of linear olefins. To fractionate and recover these linear paraffins, the benzene column bottom is sent to a paraffin column where the unreacted paraffins go overhead and returned to the dehydrogenation unit. The alkylated products in the paraffin column bottom are then charged to a rerun column where the LAB is recovered from the overhead system. A heavy alkylate by-product (HAB) is removed as bottom product. 

The LAB product is recovered by distillation. Typically, the distillation scheme is similar to the fractionation section of an HF alkylation unit. Benzene is recovered and recycled along with n-parafflns if n-parafflns are used as the feedstock. The HAB is separated in the third column... 

HF alkylation units require special operating experience because of the risks of corrosion to certain items of equipment and the safety aspect associated with the toxicity of HF. 

T8-20 Task Group NACE, A survey of plant practices and experience in HF alkylation units. Corrosion 94 NACE, Paper N° 511... 

Downstream units that process FCCU olefins are adversely affected by diolefins. A sudden increase in acid-soluble oils in an HF alkylation unit or a dramatic increase in catalyst consumption on a Dimersol polymerization unit is likely caused by an increase in diolefins in olefin feedstock. 

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