(28e) Preflash Drum When Processing Heavy Oils: Paradox or Reality ?

The first and logical thinking, when processing heavy oils, is not to use either a preflash drum nor a preflash tower, as the amount of light hydrocarbons (combustible gas, LPG and light naphtha) is very low. With this configuration it's necessary to prevent vaporization along the preheating train and before the atmospheric furnace-pass control valves. On the other hand, two aspects appear related to heavy oil processing. The first one is that desalting is more difficult, requiring higher temperatures. Desalted crude water content is higher due to the effect of higher water solubility, higher content of surfactants, and higher conductivity among other effects that lower desalting efficiency. The second one is that very high inlet atmospheric furnace temperature can be achieved due to greater heavy products yields and the use of pinch technology to reduce energy consumption. The result of the combination of high temperature and higher salty water content in the desalted crude is a necessity of a very high operating pressure to prevent vaporization and sometimes, a special metallurgy. In some cases it would be required even to change to a more expensive pressure class material. Thus, as a consequence, the better solution is to use a configuration with a preflash drum to allow the vaporization of the water and some amount of light hydrocarbons, reducing the vapor pressure at the furnace inlet, and eliminating any aqueous phase at the preheating trains after desalters. In this paper, a case study of a grassroot refinery design having a 16 °API gravity feedstock will be presented. Some different oil and water mixtures had those vapor pressures studied using process simulators and compared with PVT cell experimental results.