A flexible approach to refinery olefin alkylation

Safety issues concerning HF alkylation units, combined with incentives to process a wider range of olefin feedstock, has resulted in options for converting these units to sulphuric acid technology

Andrew R Tyas and Tony Parker, DuPont Clean Technologies

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Article Summary

Tighter fuel specifications and increasing emphasis on refinery safety are creating problems for refineries that utilise hydrofluoric acid (HF) alkylation. More alkylate is needed, but, with many units already fully debottlenecked, further capacity increases will only be possible at substantial capital cost.

With increasing pressure on gasoline RVP and olefin content, the requirement to alkylate C5 olefins (amylenes) is becoming a priority for many operators. While HF alkylation technology can cope well with small quantities of amylenes in the feed, high levels of iso-pentane production make the processing of substantial levels of C5 uneconomic if HF is used as the catalyst.

Oil companies are adopting sophisticated quantified risk analysis (QRA) techniques to understand the risk levels associated with their HF based operations. While all types of alkylation technology have excellent safety records, QRA studies have shown that in many cases were an HF unit is present, there is a much higher potential to cause a catastrophic offsite incident, impacting on many people, causing huge public outrage and having a negative impact on shareholder value.

The cost of the conversion from HF alkylation to H2SO4 alkylation is a fraction of that of a grassroots unit as it uses most of the existing equipment. It is also likely that the cost to convert to H2SO4 will be of the same order as the cost to install the modern HF mitigation systems. Because H2SO4 alkylation requires lower isobutane to olefin (I:O) ratios than HF alkylation, the capacity of the fractionation and recycle systems of the unit are substantially increased by the conversion, meaning the unit can be debottlenecked without requiring an expensive fractionation section revamp.

H2SO4 alkylation chemistry is also much better suited to processing amylenes, as isopentane production is less of an issue. Recent work has demonstrated that using new technology amylenes can be processed using H2SO4 without significant increases in acid consumption (an issue in the past). Also, the cyclic nature of petrochemical production may render it attractive to consider alkylation of propylene in some units. Processing schemes can be devised to process propylene on H2SO4 alkylation units to maximum advantage including HF units converted to use H2SO4 catalyst.

Overall, these developments allow an existing refiner to increase capacity, improve feedstock flexibility, and remove the risks associated with HF.
Readers will be familiar with the Euro IV (2005) specifications including the phase in of 10ppm sulphur material. For the refiner, these specifications will mean:
- Less sweetened FCC gasoline can be used
- Desulphurising FCC gasoline reduces octane or octane barrels to a greater or lesser extent
- Less reformate can be used in gasoline blends, especially the “super” grades (97–98 RON)
- Additional oxygenates could be used up to limits, but there may be future pressure to reduce/eliminate usage behind moves in the USA
- Reformate and FCC gasoline blend well together because they dilute each other. With less reformate to dilute the FCC gasoline olefins, less of the latter can be used unless other components are present.

Alkylate is recognised as the “perfect gasoline blending component”. One of the options available to meet these specifications is to debottleneck alkylation unit capacity. Additional feedstock can be obtained via FCC capacity increases or operational changes, purchased feed, or by additional feed components from the FCC (eg amylenes). While there has been no indication of further reductions in gasoline RVP in future European specifications, FCC C5 alkylation could provide some relief for refiners who are extremely tight on this specification. The traditional designs of alkylation unit use either hydrofluoric or sulphuric  acids as the catalyst. Over the past 10 years, issues with the safety and risks associated with large inventories of HF in refineries have been examined and understood. Solid bed catalysts are being developed and, once commercially proven, could provide alternatives to the traditional technologies.
HF issues
The owners and operators of HF alkylation units in most parts of the world are faced with public concern and regulatory attention regarding the safety of their units. While the industry’s record is good, the potential for incidents extending outside the confines of a facility has led environmental groups and regulatory bodies to attack the use of HF in refining and chemical plants.

For these reasons it is often very difficult to obtain permits to expand an existing HF unit or build a new HF alkylation unit. This trend is clearly seen by the fact that the majority of the alkylation units constructed worldwide during the 1990s use H2SO4 catalyst.

Refiners may come across regulatory blocks when contemplating debottlenecking projects on HF units. Alternatively they may be faced with bringing forward safety related modifications that have been scheduled for future shutdowns if they wish to debottleneck their units. In addition to these issues, many HF units may be fully debottlenecked already and could be operating at limiting reaction section parameters with less than ideal operating conditions such as low I:O ratio.

Refiners faced with pressures on gasoline RVP, have the problem of finding an outlet for a C4/C5 cut from their FCC gasoline. When C5 olefins are alkylated, hydrogen transfer reactions result in the production of isopentane. The reaction results in two moles of isobutane being consumed for every mole of amylene reacted. The rate of isopentane production is approximately three times greater when using HF than with H2SO4.

HF options
What options do refiners have in responding to regulatory pressure intended to assure the safety of their HF alkylation units? Fundamentally, there are three approaches that may be taken: mitigation, HF modifiers, or conversion to another catalyst such as sulphuric acid.

Currently, mitigation of the downwind impact of an HF release is the option most frequently employed. Mitigation systems usually include detection, isolation, water spray, and remote de-inventory facilities. The first goal of these systems is to detect an incipient acid release.
Once a leak is detected, the goal is to isolate reliably the major inventories of acid from the release, remove the acid from the leaking portion of the unit to a safe storage location and, finally, erect a wall of water between the leak and the community to absorb a substantial amount of the acid cloud on site. The typical cost of adding mitigation facilities to an existing HF alkylation unit is between US$20 million and $30 million. One Los Angeles refiner reportedly spent $50 million on their mitigation system.

To be effective, mitigation facilities must be fast-acting. The water curtain and water cannons must be in operation within seconds after the onset of the release. The API recommends designing for a 40:1 ratio of water to HF in order to mitigate 90 per cent of a release, but a risk-based quantification of a release must be made to determine the quantity of water.

Isolation and HF de-inventory operations must take place within minutes. All of these facilities must be tested at some regular frequency to ensure they are operable. Many existing refinery water mitigation facilities, while adequate to give substantial help in the case of a small HF leak, are inadequate to deal with a medium or large release of HF.

De-inventory systems usually involve large vessels with large acid movement devices. Plant isolation systems may require 10 to 80 remote-operated, fireproofed, testable valves and may also require additional pressure relief valves in HF service.
The reliability of mitigation systems is at present unknown. Also, this large capital expenditure provides no process benefit to the alkylation unit (no increase in capacity or product quality).

HF modifiers
Modification of the physical properties of the HF catalyst is a second avenue. Much research has focused on the development of chemical additives or diluents that reduce the volatility and aerosol formation properties of HF. Recent tests of two such additives have indicated substantial reductions in HF aerosol and vapour cloud formation. Figures quoted are in the range of 63–80 per cent reduction of airborne HF, due to the additive.

These additives are still in the developmental stage. Their effectiveness in reducing HF aerosol formation is commercially unproven at present. The capital and operating cost of a commercial installation are not known, but are likely to be significant. The developers of the HF modifiers foresee using the additive in conjunction with an effective mitigation system. When coupled with a water application system designed for a 40:1 water:HF ratio, reduction in the quantity of airborne HF is claimed to be in the range of 95–97 per cent, compared to an unmitigated release without additive.

This combination may be required to meet the ultimate rules set by the regulators. Should the additives be a success, they could command significant royalty charges, particular if they become commercially proven by a major release scenario.

The third option is to convert the alkylation unit to use a catalyst that has no aerosol forming tendencies. HF properties are much different from those of sulphuric acid catalyst, and the optimum reaction conditions are different. The major process differences in a sulphuric acid unit are lower reaction temperatures and pressures, higher mixing energies, and treatment of the reactor effluent prior to fractionation (verses post fractionation treatment with HF).

One of the major benefits of the conversion is a lower optimum isobutane to olefin ratio. Less fractionation capacity is required, meaning that the feed and product rates could be substantially increased without sacrificing alkylate quality.

The conversion discussed in a previous NPRA paper (AM-88-67) required replacement of most of the equipment in the unit except for the distillation towers. This included replacement of the reaction zone with proprietary Stratco Contactor reactors and an effluent refrigeration system. This form of conversion has octane, acid consumption, and capacity advantages over the one discussed later in this article. However, the downside of the previous form of conversion is that it is typically more capital intensive and requires more plot space.

Upgrading existing equipment
The proprietary Alkysafe process reuses both the reaction and distillation sections. It may also be possible to construct much of the acid blowdown section from existing equipment. Most of the new equipment (packaged refrigeration unit, effluent treating system, acid blowdown and tankage sections) can be constructed in advance of the conversion. With planning, the remaining modifications and tie-ins can be completed within a four-week FCC turnaround.
The short downtime and low capital equipment requirements make this process cost-competitive with the mitigation systems currently being installed on HF alkylation units.
The process flow of the converted unit will somewhat resemble the time tank units built between 1938 and 1958. The converted reaction zone will consist of acid settlers with external emulsion pumps and reaction chillers. Alkylate octane may be up to one number lower and the acid consumption will be approximately 10 per cent higher when compared with results from modern Stratco effluent refrigerated alkylation units with state-of-the-art reactors.
However, in many cases, the converted unit’s alkylate octane will be higher than the original HF unit especially if the feeds contain a high concentration of MTBE raffinate or amylenes.
The following is a description of the process differences and the resulting modifications, which must be made in order to convert an HF unit to an H2SO4 alkylation unit via the Alkysafe process.

Reaction zone
The sulphuric acid alkylation reaction is optimised by emulsifying the H2SO4 and hydrocarbon reaction mixture to maximise the surface area of the isobutane within the continuous acid phase. This reduces the side reactions and increases the desired alkylation reaction. Only a small amount of mixing is required in the reaction zone of an HF unit because isobutane is much more soluble in HF than in H2SO4 . Therefore, the conversion requires equipment to provide sufficient emulsification in the reaction zone.
Emulsion pumps and static mixers are added between the acid settlers and the reaction coolers. This provides the necessary turbulence to emulsify the H2SO4  and hydrocarbon mixture. The hydrocarbon feeds are injected into the suction of the emulsion pumps rather than directly into the reaction chillers as with the original HF unit. The emulsion will flow from the pumps through the reaction chillers and then to the acid settler.
Additional surface area may be required in the reaction chiller depending on the desired capacity. The trays and other internals of the acid settlers are removed or modified to minimise turbulence. Since the H2SO4  and hydrocarbon are highly emulsified, the hydrocarbon takes much longer to separate from the H2SO4  than from the HF although the much greater density difference between H2SO4  and hydrocarbon will help reduce the settling time.
Depending on the residence time in the system, additional settling volume to facilitate separation of the hydrocarbon and spent acid is required. However, some carryover of acid in the hydrocarbon effluent is not a problem since it will be recovered in the downstream acid wash. Most types of Monel material in potential contact with H2SO4  (>50 wt%) should be replaced with carbon steel or Alloy 20. Valves in frequent contact with H2SO4  should be constructed with Alloy 20 trim.

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