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The refining industry is seeking more environmentally acceptable and economical methods of producing reformulated gasoline (RFG).
Alkylate, the cleanest gasoline-blending stream produced in a refinery, is a prime blend stock for RFG production.
Alkylation with solid acid catalysts has potential environmental and safety advantages over conventional liquid-acid alkylation. This is especially true for the HF alkylation process.
Industry and academia have been researching and developing solid-acid catalysts for more than 25 years. Several processes have reached the pilot-plant stage, but none has been commercialized.
Difficult technical challenges must be overcome in the next few years to achieve a commercially successful solid-acid alkylation process. Substantial innovations in catalysts, catalyst regeneration, reactor design, and product separation will be required for solid catalyst processes to replace the incumbent processes, which are being improved continually.
U.S. alkylate production is expected to grow rapidly through the year 2000, then level out. If an economically and environmentally superior solid-catalyst
alkylation process is not commercially available soon, the window of opportunity will close. As a result, new alkylation units will use sulfuric acid catalysts, and existing HF units will continue to operate or be retrofitted to use H[sub.2]SO[sub.4].
Alkylate constitutes about 13 vol % of the U.S. gasoline pool. It has a high octane number (typically 90-94 RON), low vapor pressure, and low aromatics and olefins contents. These properties make alkylate a valuable ingredient of the reformulated gasolines required by the 1990 Clean Air Act Amendments and the California Air Resources Board.
Alkylation units using HF and H[sub.2]SO[sub.4] catalysts have been operated commercially since the 1940s, but growing concerns about their safety and environmental hazards are restricting their use.(1)
At least four of the new solid-acid processes under development have reached the pilot-plant stage, but many technical barriers to commercialization remain. These barriers must be overcome in the next few years if a new solidcatalyst alkylation process is to be commercialized successfully.
A brief review of the commercial alkylation processes, including a discussion of the technical and commercial aspects of solid-catalyst alkylation, reveals the obstacles that must be overcome in order to successfully commercialize a solid-catalyst alkylation process. From these obstacles, the authors have devised a set of research and development (R&D) guidelines for achieving commercialization.
About 60% of worldwide alkylate capacity is based on HF. Of the 115 HF alkylation units operating worldwide, about 60 are in the U.S. Of the 300,000 tons/year of HF produced in North America, 5% (15,000 tons/year) is used as alkylation catalyst.(2)
The main disadvantage of HF is that a release of HF (a gas at ambient conditions) creates a ground-hugging toxic vapor cloud that can be carried by the wind and injure many people in the plant and surrounding communities.
Proposed legislation in California (Assembly Bill 3276) would ban large-scale storage of HF and force HF users to switch to a less-hazardous substance.(3 4)
Measures to improve the safety of HF include:
* Isolation valves, physical containment, and water sprays to mitigate the impact of an HF release.(2)
* Additives that reduce the volatility and cloud-forming tendency of HF by 60-90% (such additives have been developed and used by UOP and Texaco Inc., and by Mobil Corp. and Phillips Petroleum Co. (2 5-8)
* On site HF recovery and regeneration systems that reduce the hazards of HF transportation and handling, such as AlliedSignal Inc's Aquatech separation technology.(9)
Retrofitting existing HF units to use H[sub.2]SO[sub.4] is costly. The cost of converting the HF alkylation unit at Mobil's Torrance, Calif., refinery to use modified HF, for example, would cost an estimated $10 million, compared to $100 million to revamp the refinery to use H[sub.2]SO[sub.4].(7)
Refiners that currently use HF probably will be permitted to continue using HF, thus avoiding the need to shut down their alkylation units or convert them to another acid catalyst at great expense. No new HF alkylation units, however, are likely to be constructed.
Sulfuric acid is less hazardous than HF, but its consumption per unit of alkylate produced is greater. For a 10,000 b/d alkylate plant, the H[sub.2]SO[sub.4] traffic is typically 100 tons/day. The acid consumption increases further with the C[sub.3] and C[sub.5] olefin content of the feed.
Sulfuric acid alkylation processes usually operate at about 10 degrees C. and therefore require refrigeration. The HF processes operate at …