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Global demand for middle distillates, especially diesel, is increasing. For this reason, refiners are seeking new opportunities to increase diesel yields and profitability from existing refinery configurations with minimum investment or without additional capital costs. This trend is particularly relevant in countries like India, where the demand for diesel is greater than the demand for gasoline.
Most refineries employ either low-pressure or high-pressure diesel hydrotreating (DHT) units to reduce sulfur content, increase cetane number, etc., and meet Euro 4 specifications. Another important requirement is to adhere to ASTM D-86 T95, which is restricted to 360°C in the final diesel product. These hydrotreaters are usually designed to process certain types of feed, such as straight-run gasoil (SRGO), coker gasoil, fluid catalytic cracking (FCC) light-cycle oil, etc. Most DHT units process diesel feedstock with a T95 distillation temperature of approximately 365°C and achieve diesel product T95 of about 360°C. However, an opportunity exists to extend the endpoint of diesel feeds and increase the overall diesel yields by processing heavy diesel feedstock, such as light vacuum gasoil (LVGO) obtained from the refinery's vacuum distillation unit (VDU).
This work will explore the possibility of processing LVGO in a DHT unit by optimizing process conditions within the existing unit constraints, thus increasing the diesel yield and improving profitability. Two options are available to reduce the endpoint of distillates: mild hydrocracking, or isomerization of the heavy end of the diesel fraction. The mild hydrocracking option was studied by escalating the reactor temperature, thereby increasing the cracking of the heavy portion of diesel to meet distillation specifications.
Historical background. Hindustan Petroleum Corp. Ltd. (HPCL) is a publicsector refinery in India with two refinery units, in Mumbai and Visakhapatnam. In the Visakhapatnam refinery, a surplus of LVGO is processed in an FCCU, or it may be downgraded to fuel oil as cutter stock. Since the LVGO has higher distillation T95 in the range of410°C-420°C, blending LVGO in diesel will increase the feed T95 and result in higher T95 in product diesel while processing in the DHT unit. This problem was referred to the HPCL R&D Center recently established at Bangalore.
The process flow diagram of the DHT unit of the Visakhapatnam refinery is shown in FIG. 1. This unit processes a blend...