Hydrofluoric acid alkylation conversion and expansion
Much of the cost of switching from hydrofluoric acid to sulphuric acid alkylation can be avoided by using existing equipment.
SHANE PRESLEY and JASON NUNEZ
DuPont Clean Technologies
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At a time when refiners face concerns around the rise of electric vehicles and the eventual peak in global gasoline demand, alkylate as a gasoline blend component is more popular than ever. The unique properties of alkylate, including high octane, lack of olefins and aromatics, and extremely low sulphur, make it the only blending component that truly enhances all aspects of the gasoline pool, helping refiners meet tightening specifications. Due to these blend qualities, a transformation has occurred in the last few years whereby alkylate is sought out worldwide and is now sold and exported as a stand-alone product to serve those regions of the world where refining technology is not able to keep up with changing fuel specifications.
The commercially adopted alkylation processes in refineries utilise two main catalyst types: sulphuric acid and hydrofluoric acid (HF). Refiners that use HF in their alkylation units are burdened with safety and environmental concerns, leading to tremendous pressure from both regulatory agencies and community activists. This has led to a recent resurgence in the interest in conversion or replacement of HF alkylation units with alternative technologies.
As part of the 2017 Revamps edition of PTQ, an article titled ‘Advances in HF alkylation conversion and expansion’ was published, providing an overview of the DuPont ConvEx technology for converting HF alkylation units to sulphuric acid alkylation. This article explores the technology in greater detail and includes an in-depth case study utilising the novel reactor solution.
DuPont ConvEx HF conversion and expansion technology
DuPont Clean Technologies developed the ConvEx HF conversion and expansion technology for both gravity-flow and pumped-flow HF alkylation units to utilise sulphuric acid as catalyst for the alkylation reactions. The conversion options for the reaction section include: traditional Stratco Contactor reactors and a novel reactor design. The Stratco Contactor reactor option was discussed extensively in the 2017 article, so this article will focus primarily on the novel reactor approach.
The first conversion option using Stratco Contactor reactors is suitable for both gravity-flow and pumped-flow HF alkylation units and will match the performance of a grassroots Stratco alkylation unit. For the conversion of a gravity-flow HF alkylation unit, the acid coolers will be replaced by Contactor reactors, and the existing vertical acid settler will be retrofitted to perform as an acid settler for the converted sulphuric acid alkylation unit. For the conversion of a pumped-flow HF alkylation unit, the existing horizontal HF acid settler will be reused as a sulphuric acid settler.
The second conversion option using a novel reactor design is also suitable for both gravity-flow and pumped-flow HF alkylation units. For the conversion of a gravity-flow HF alkylation unit, the existing vertical acid settler will be retrofitted with proprietary internals for mixing and will perform the function of an alkylation reactor, acid settler, and compressor suction vessel. For the conversion of a pumped-flow HF alkylation unit, the existing horizontal acid settler will be retrofitted with proprietary internals for mixing and will perform the function of alkylation reactor, acid settler, and compressor suction vessel. In both bases, the reaction zone modifications are limited to vessel retrofits, new piping and new pumps. The novel reactor design incorporates innovations developed through extensive research while utilising proven design elements that are familiar to refinery operators. Due to the fact that no new reaction vessels are required, this conversion solution can be very economical, while still providing acid consumption and alkylate product properties similar to grassroots Stratco alkylation units.
Expansion through conversion
In both HF and sulphuric acid alkylation units, the ratio of isobutane to olefins in the reaction zone must be maintained adequately high to prevent unfavourable olefin-to-olefin reactions from occurring that can result in low quality alkylate and higher acid consumption. How this ratio is achieved in these processes is quite different, however. This difference is leveraged as part of the ConvEx technology to achieve a significant increase in throughput by converting an HF alkylation unit to one using sulphuric acid as the catalyst.
For sulphuric acid alkylation units, there are two sources of isobutane that circulate back to the reaction zone. The first source is the fractionation section, where net effluent is separated into its components (isobutane, normal butane and alkylate). The isobutane stream from the top of the deisobutaniser tower provides recycle isobutane to the reaction zone. This stream makes up about one half of the total isobutane required for the reaction zone. The second source is from the refrigeration section. In sulphuric acid alkylation units, isobutane is flashed in the reaction zone to provide cooling for the reactors, which typically operate at around 45-60°F (7-15°C). This flashed isobutane is then compressed, condensed and cooled in the refrigeration section before being routed back to the reaction zone as refrigerant recycle. The refrigerant recycle stream provides the remainder of the isobutane required in the reaction zone.
In HF alkylation units, the reaction zone is operated at much higher temperatures, so no refrigeration section is required. Since there is no refrigeration section providing refrigerant recycle back to the reaction zone, all the isobutane required in the reaction zone must come from the fractionation section. As a result, the fractionation equipment in HF alkylation units is significantly larger than in sulphuric acid alkylation units.
A comparison of isobutane flows in HF and sulphuric acid alkylation units is shown in Figure 1.
This difference between the isobutane recycle streams in HF and sulphuric acid alkylation units is the basis for unit expansion capabilities at the same time as conversion with minimal additional cost. The fractionation equipment and effluent piping in HF alkylation units is approximately twice the size of that of a sulphuric acid alkylation unit of similar size. As part of any HF conversion to sulphuric acid alkylation, a new refrigeration section is required to reduce reaction temperatures. This refrigeration section will provide additional isobutane (refrigerant recycle) flow to the reaction zone, unloading the fractionation equipment. Instead of operating the unit in a very unloaded fashion, it may be beneficial to utilise this additional capacity to expand the throughput of the unit.
DuPont has evaluated many HF alkylation units for conversion to sulphuric acid alkylation using the ConvEx technology. Table 1 shows the extent to which expansion was possible as part of this evaluation. In each of these examples, the units were expanded up to the limits of the fractionation equipment or other major unit constraints. Of the six examples shown in this table, three of these resulted in an expansion potential that more than doubled the capacity of the existing HF alkylation unit. Doubling unit capacity may not always be possible as part of a conversion of an HF alkylation unit, but in all the cases examined, considerable expansion was possible, providing an economic incentive for refiners to consider HF conversion.
The case study presented here utilises DuPont’s novel reactor design to retrofit an existing pumped-flow HF alkylation unit (see Figure 2) for conversion to a sulphuric acid alkylation unit.
This HF alkylation unit was originally designed to produce 10500 b/d of alkylate product using two HF reactors, acid settlers, and acid circulation pumps with a feed consisting of FCC butylene and propylene (see Figure 3).
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