Most HF alkylation units are particularly well-suited for revamping to ExSact alkylation. A study commissioned by a refiner to evaluate the revamp of an HF alkylation unit revealed the following revamp opportunities.
Feed Zone: The feed zone prepares the feed streams for alkylation. The feed section of an HF unit typically includes a butane splitter, feed driers, and feed storage tanks and pumps. In the revamp design, these units are retained in their entirety under their current service. The revamped unit utilizes the same feed rates and conditions as the HF unit, allowing these units to continue their operation unchanged.
Reaction Zone: The reaction zone is where the feedstocks (isobutane and olefin) are contacted with the catalyst. The ExSact fixed-bed reactor design is unlike conventional HF mixer/settlers – so there is no easy way of retrofitting the HF reactors for use with ExSact. ExSact therefore requires two new fixed bed reactors and their associated recirculation pumps. However, the ExSact reactors require an effluent surge vessel to hold the liquid contents of a reactor while it is being regenerated. The HF settler tank is suitable for this service and is reused. The surge vessel is simply a holding tank and requires no particular internals. The settler can be used without modification for this purpose, which is not significantly different from its current service.
Separation Zone: The separation zone is a distillation train typically consisting of two or three columns. The distillation sections of an HF unit and an ExSact unit are identical and operate at the same isobutane-to-olefin ratio. Therefore the existing Isostripper (Main fractionator) and Depropanizer columns continue in their present service while unit capacity is maintained. Column operation and maintenance is simplified by eliminating the presence of HF in the columns. Product purity specifications are maintained since the column feed composition and flow is unchanged by the revamp. Smaller operations related to HF capture, such as the HF stripper, are idled.
Product Finishing Zone: No leaching of acid is possible from the ExSact catalyst. Therefore, product washing and neutralization steps, including the defluorinator and caustic (KOH) treaters are idled. The solid waste generated from the neutralization steps is also eliminated, reducing operating cost and complexity. Product coolers and storage tanks are retained in their current service.
Regeneration Zone: The equipment needed for regenerating the ExSact catalyst is different from that used to regenerate HF. However many pieces of equipment available in an HF plant can be modified for use in the new regeneration cycle. For example, the HF unit uses a fired heater as a re-boiler for the main fractionator in order to achieve temperatures high enough to decompose alkyl fluorides formed in the reactor. Such high temperatures are no longer required in the SAC process. This fired heater can be re-serviced for catalyst regeneration to raise the regeneration gas stream to 250°C and a new reboiler installed in place of the fired heater. The new re-boiler can be steam heated in order to reduce maintenance and safety concerns.
Cost Savings
The HF revamp option saves considerable capital over other alkylation options. Capital cost of the revamp is about 50% of a grassroots ExSact unit and about 20% of a new sulfuric acid facility (including acid regeneration). |
