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ALKYLATION; ISOOCTANE PRODUCTION; AND MAINTENANCE, RELIABILITY, AND SAFETY
Publication date:1Q 2011
Alkylation; Isooctane Production; and Maintenance, Reliability, and Safety
Refinery alkylation processes offer a common and well-established method for the production of a high-quality gasoline blending component known as alkylate.
Alkylation comprises the reaction of isobutane with C3-C5 olefins in the presence of an acid catalyst to produce a high-octane blendstock product, characterized by low RVP, minimal sulfur, and no aromatics or olefins. In the US, alkylate is responsible for about 11-13% of the total gasoline pool, depending on regional demand and seasonal factors.
One of the primary drivers for increasing alkylate demand in the US and abroad is the increasing use of high-RVP ethanol in the gasoline pool, that forces producers to seek out lower-RVP blending components. The need to balance high volumes of ethanol blending with less-volatile fuel components is particularly emphasized during the spring and summer months, when more-stringent limitations on gasoline RVP levels are mandated. Back in 2008, the increased blending of ethanol into the gasoline pool led to an alkylate shortage in the US during the spring and summer months, which some analysts believe was partly to blame for high gasoline prices. Citigroup Global Markets analyst Doug Leggate claimed, "Supply of [alkylate] will set the price of summer gasoline—not [gasoline] inventory levels." With E10 gasoline currently in use, and E15 gasoline being introduced for consumption sometime in 2011, an alkylate supply problem could arise again; refiners and fuel blenders would be wise to monitor the situation closely to avoid significant supply shortages and to take advantage of positive pricing trends.
Alkylation can be carried out non-catalytically using high temperature and pressure, although refiners prefer low-temperature, acid-catalyzed processes to provide better alkylate yields. In the 1960s, approximately three times more alkylate was produced using sulfuric acid (H2SO4) as the catalyst than using hydrofluoric acid (HF). Since then, the trend in alkylation shifted to favor the use of HF, followed by a return to sulfuric acid. In North America, approximately the same amount of alkylate is produced from both types of units.
Alternatives to conventional liquid acid technologies (i.e. solid acid or ionic liquid catalysts) may become viable in certain situations in the near future. Solid acid catalysts, in particular, offer improved performance in terms of plant safety. Aside from safety considerations, solid acids will be of interest where liquid acid regeneration facilities are not economically feasible. Catalyst supply and toxic mitigation costs can also be decreased by implementing alternative systems, and it is estimated that the elimination of either HF or H2SO4 from a refinery alkylation unit in favor of an alternative, less-toxic catalyst may provide a potential savings of $250MM/y. In the coming years, commercialization of these technologies as well as further improvement of conventional operations will help to shape the role of alkylation.
Continued alkylation technology developments have focused on improving the contact between the liquid acid catalyst and olefin feed material to improve alkylate yields and quality. Integration of a refinery's alkylation unit with petrochemical sulfur plants is also covered and will allow for improved efficiency and environmental performance. Two case studies are presented that investigate revamping a HF alkylation plant to either solid acid or sulfuric acid service for improved safety. R&D work related to alkylation was mostly focused on alternative technologies, specifically ionic liquids and solid acid catalysts. Additionally in the alkylation section, new products and topics covered include:
In light of the MTBE phase-out in the US, the increased blending of low-RVP ethanol, and concerns surrounding the safety of acid-catalyzed alkylation processes, the production of isooctane via the dimerization and hydrogenation of isobutane has become an attractive option for a number of refiners to yield a high-octane, low-RVP gasoline blending component. Two primary factors have made the installation of isooctane production units particularly attractive: (1) refiners can take advantage of stranded MTBE production facilities and available feedstocks to produce high-quality isooctane (and/or isooctene) with minimal capital investment, and (2) isooctane production avoids the use of highly toxic acid catalysts that are needed for refinery alkylation processes and pose a significant threat to the health and safety of plant workers.
Outside of the US, the use of MTBE remains prevalent, and countries in the EU and other areas have focused on efforts to improve the storage of the gasoline additive to prevent groundwater contamination rather then banning its use. Additionally, gasoline demand makes up a larger portion of the total fuel usage in the US than in Europe so the need for high-quality blending components is not as urgent.
A number of case studies and revamp projects have been completed regarding the conversion of existing MTBE facilities into isooctene/isooctane production units. Furthermore, recent developments in the area of isooctane production processes have largely focused on the use of various oxygenates as catalysts modifiers to improve dimerization selectivity. Specifically, butanol (TBA), secondary butyl alcohol, MTBE, ETBE, and carboxylic acids have been mentioned in both commercial process and in R&D work. Many of the isooctane production processes discussed also include recycle configurations to limit makeup oxygenate requirements. Additionally, in the isooctane production section new products and topics covered include:
The refining industry—indeed, industry in general—has come to recognize reliability and safety as essential to profitability. Lost production, whether from equipment failure or human error, can cost a company significantly—in lost profit, in regulatory violations, in injuries or exposure to risk of personnel and the community. This has driven increasing attention not only to equipment and procedures that minimize failure and error, but to methodologies for identifying and assessing modes of failure, as well.
Reliability in refining involves the elimination of failure for equipment and systems. Although smaller auxiliary units will inevitably fail, avoiding large-scale upsets of major processing units and/or critical processing equipment should be a primary goal. Reliability will go hand-in-hand with refinery maintenance: reliability engineers will attempt to predict and mitigate failure while maintenance engineers look to restore operational failure quickly and efficiently. Furthermore, preventative maintenance is implemented to avoid failure and improve overall reliability of a plant.
Safety issues will also be a primary driver for the planning and execution of refinery maintenance programs. As such, another focus will be on safety issues in the refinery, as a whole, as well as safety concerns related to specific units. Reaching a clear and concise understanding of risk factors in refining and defining Process Safety Management (PSM) principles is the first step in providing a safe work environment for refinery personnel. Furthermore, a number of strategies focusing on inspection and maintenance activities, process control and automation, and best practices are presented with the goal of providing options for improved safety in refining.
Overall, improvements in both refinery reliability and safety on a plant-wide and unit-by-unit basis can help enhance production, efficiency, and profitability. Technology offerings from a wide range of companies can be utilized to help minimize the frequency and financial impact of process upsets. The integration of process control and automation systems with advanced plant-wide monitoring and asset management programs allows refiners to utilize real-time process data to evaluate, plan, and optimize operations. Furthermore, improvements in data and asset management can be coupled with expanded training programs and the implementation of industry best practices will help to mitigate many of the health and safety risks that refinery personnel are exposed to on a daily basis. Additionally, in the maintenance, reliability, and safety section, new products and topics covered include:
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