Dec-2017

Advances in HF acid alkylation conversion and expansion

Recent technology developments offer economic conversion and expansion of HF acid alkylation units for sulphuric acid operation.

SHANE PRESLEY, RANDY PETERSON, DIWAKAR RANA and JASON NUNEZ
DuPont Clean Technologies

Article Summary

DuPont has introduced the ConvEx technology for converting from hydrofluoric (HF) acid to sulphuric acid alkylation while also achieving sizeable expansion. These new options have been tailored to minimise capex while maximising alkylate production, making HF conversion projects more attractive to refiners than in the past.

DuPont’s technical and operations experts have been consulting with HF alkylation refiners to better understand concerns and existing constraints. Based on this feedback, the primary goal of this effort is to provide low cost conversion solutions while increasing, perhaps more than doubling, the original design capacity of an existing HF alkylation unit.

Of course, simply converting an HF alkylation unit to a sulphuric acid alkylation unit and keeping the same capacity typically has poor economics. It was always assumed that no refiner would ever convert from HF to sulphuric acid unless it was mandated or if the risk of continuing to use HF was deemed to be too great. But if the capacity is also increased in conjunction with a conversion, the economics start looking much better for many refiners.

With a ban on HF being considered in some parts of the US and significant doubt around the long term viability of HF technology, refiners must consider whether additional investment in existing HF alkylation units is wise. With the introduction of the ConvEx technology, it is possible to keep the infrastructure of the existing alkylation unit along with much of the equipment as part of a conversion to sulphuric acid alkylation, and answer the call of increased alkylate demand by increasing capacity.

HF conversion and expansion solutions
DuPont has developed low cost ConvEx HF conversion and expansion solutions for both major HF alkylation technologies: gravity flow reaction section and pumped flow reaction section.

The first conversion option using Stratco Contactor reactors will match the performance of a grassroots Stratco alkylation unit. The second option includes a patent pending novel reactor, which utilises proven design elements and includes reaction research innovations. This option is intended to be a lower cost solution with a minor performance debit compared to the other option. These two HF conversion choices allow refiners to select the solution that works best with their unit configuration and best aligns with their operational and business needs.

Stratco Contactor reactor option     
The ConvEx HF technology solution utilising Stratco Contactor reactors is suitable for refiners with either a gravity flow or pumped flow HF alkylation unit. The case study featured in this article employs this option for the conversion of a gravity flow HF alkylation unit, an attractive conversion solution.

The gravity flow HF alkylation technology uses a single large vertical acid settler with sieve trays and multiple acid coolers. The HF catalysed alkylation reaction between olefins and isobutane occurs in the acid riser pipe; multiple large HF acid coolers use cooling water to remove the heat released by the alkylation reaction.

Figure 1 depicts such a conversion where Contactor reactors are installed in place of the existing HF acid coolers. The reactors can also be elevated on a deck for closer proximity to the settler. This arrangement can minimise unit downtime by allowing refiners to install the new reactors while the existing HF acid coolers remain in service.

Four reactors are shown in the schematic for illustration purposes, but the actual number of reactors will be determined based on the required plant capacity. The larger Stratco Model 74 Contactor reactors can be used to reduce equipment count, minimise plot space and reduce cost.

The existing HF acid settler is shown in Figure 2 and will be modified to function optimally in sulphuric acid emulsion service. The HF settler modifications include segmenting of the acid rich zone to allow for acid staging and adding two stages of coalescing media for the separation of acid and hydrocarbon. The lighter hydrocarbons in the combined settler effluent are flashed across a pressure control valve prior to entering the tube bundle inlets to cool and remove the heat of reaction. The tube bundle outlet is routed to a new refrigeration section before proceeding to fractionation. The flashed vapours are subsequently compressed, condensed and returned to the reactor feed, providing additional cooling and contributing a large amount of isobutane to the isobutane to olefin ratio. The recycle isobutane from fractionation and the refrigeration section ensures that conditions are optimum for the alkylation reaction. The converted acid settler can be segmented such that one large acid settler can function as multiple individual settlers.

Novel reactor option
DuPont recently developed a novel reactor (patent pending), which utilises proprietary mixing and separation equipment. This option can also be used to convert and expand either a gravity flow or pumped flow HF alkylation unit. The existing HF acid settler is converted to a sulphuric acid alkylation reactor utilising common refinery equipment with no moving parts within the converted acid settler. It is a lower cost, yet robust design option for HF conversion.

Conversion basics
Sulphuric acid alkylation reactions are typically optimised at a reaction temperature of 45°F (7°C) while most HF alkylation units operate at around 100°F (38°C). Although the heat generated by the exothermic alkylation reactions is similar regardless of the catalyst, the reaction heat released from an HF alkylation unit can be removed by cooling water, while sulphuric acid alkylation units require refrigeration to achieve the colder reactor temperatures. Both conversion options require the addition of a refrigeration section and new feed/effluent heat exchangers to achieve the desired 45°F (7°C) reactor temperature.

To protect the downstream fractionation section from corrosion and fouling, an acid coalescer and dry alumina treaters are installed. Existing HF recovery equipment in the HF alkylation unit may be repurposed for these applications. Removal of SO2 from the propane product is also required in the sulphuric acid alkylation unit; however, existing equipment in the propane product treating section can typically be reused for this purpose without significant modifications.

Finally, the entire fractionation section, which is over-sized for sulphuric acid alkylation, can typically be reused in its entirety with little or no changes.

As discussed above, the Stratco Contactor reactor and novel reactor design may be suitable options for refiners for the conversion and expansion of either a pumped flow HF alkylation unit or a gravity flow HF alkylation unit.