Content area
Full text
Solvent deasphalting (SDA) is emerging as a low-cost, bottom-of-the-barrel upgrading process to recover valuable oil from short residue. This oil can either be further processed to make lube-oil-based stocks or cracked in secondary processing units. To achieve a higher deasphalted oil (DAO) yield, these units were designed for high solvent-to-feed ratios (values of eight to 10 are not uncommon). This huge quantity of solvent is recovered by evaporation through multiple flashes, and then compressed and recycled back to the extractor. This process is highly energy intensive.
Most of the deasphalting units in operation were designed long ago, without using process integration and optimization tools. Moreover, a severe problem of choking and fouling (layer formation) in the asphalt-propane mix/asphalt rundown heat exchanger (E-6) exists, due to the processing of dirty streams on both sides and a huge A T at the heat exchanger entrance. The fouling of this heat exchanger not only restricts throughput of the propane deasphalting (PDA) unit, but also leads to a significant increase in the quantity of fuel in the furnace and cooling water in the asphalt cooler. Therefore, a modified process scheme to reduce energy consumption would benefit the refiner.
EXISTING PDA PROCESS
A rigorous study on one of Chennai Petroleum Corp. Ltd.'s (CPCL's) PDA units was conducted to reduce energy consumption and address fouling issues. The PDA unit consists of four sections: a feed and extraction section, a DAO recovery section, an asphalt recovery section and a propane recovery section.
Vacuum residue coming from the storage tank is mixed with propane to reduce its viscosity. This mixture is cooled-via a water cooler-to the specified temperature to maintain the extractor top temperature. The raffinate (DAO and propane mixture) phase from the extractor is heated with the high-pressure (HP) flash vapor, and fed to the low-pressure (LP) flash drum. The bottom part of the LP drum is routed to the HP flash after the heat exchange with the DAO and vapor coming from the asphalt recovery section flash drum. The bottom of the HP flash drum is routed to a separate section, consisting of a flash drum and stripper, to recover a minor portion of propane. Subsequently, it is routed to a stripper column to remove traces of propane from the DAO....