Application of Hazop, Lopa and Sil to an Alkylation Unit in a Refinery: a Case Study

Abstract

In a refinery, an alkylation unit is a process for producing gasoline range material (alkylate) from olefins such as propylene, isobutene, butylene, and pentene. In the alkylation process, light hydrocarbons from the Fluid Catalytic Cracking Unit (FCC) react in presence of a catalyst (hydrofluoric acid or sulphuric acid), to form a mixture of heavier hydrocarbons. After the reaction, the alkylate meets the specification as high octane gasoline. Important safety considerations in this process are related to the catalyst. For example, if hydrofluoric acid (HF) is used, its high vapour pressure, making it easily vaporized if a leak occurs in the units. HF can travel a significant distance as a dense vapour. Even though a small concentration of HF is used in the reaction, it is still enough to cause human and environmental problems, since it is a very corrosive and toxic inorganic acid. The objective of this study was to evaluate the hazards in the alkylation unit at a refinery located in Latin America, applying HAZOP, LOPA, and SIL methodologies. This study is based on the assumption that the process operates according to the conditions intended in its design, thus the risk and operability problems are unlikely to occur. The HAZOP (Hazard and Operability Study) analysis covered 13 sections of the alkylation process. Hazardous scenarios were categorized as: related to safety, environmental, and financial; each topic was also risk ranked to assess the strength of existing safeguards. The causes of hazardous scenarios were identified including human error, equipment failure, and external events. The severity of deviations was evaluated without any protection or safeguards. From all the deviations that were analysed, 24 scenarios were identified as significant risk and of high risk. These scenarios were then analysed by LOPA (Layer of Protection analysis) and SIL (Safety Integrity Level) methodologies, to identify the layers of protection that mitigate the hazardous scenarios found in the HAZOP analysis, as well as assigning a PFD (probability of failure on demand) value on each safeguard. Based on this, recommendations are provided, to mitigate the consequences or improve the operational capab