Effect of extended heavier hydrocarbon fraction (Cn+) composition on optimum surface separation pressure and temperature

Hydrocarbon fluids are made up of defined components which include N2, CO2, H2S, C1, C2, C3, iC4, iC5, and C6  and undefined components known as heavier fractions (Cn+) which include paraffinic, naftenic and aromatic compounds. The hydrocarbons are separated on the surface before they are sent to the market. There are several factors affecting the hydrocarbons surface separation condition which include; pressure, temperature, gas liquid flow rates, surging or slugging tendencies of the feed stream, presence of impurities such as paraffin and sands. This work is limited to the study of the effects of pressure and temperature. To obtain stabilized hydrocarbons phases optimum surface separation, pressure and temperature must be selected. Several empirical models have been developed to obtain optimum surface separation pressure and temperature. However, these models do not consider the full composition of the well stream, and the heavier fractions are most often treated as a single component. This paper presents the estimation of an optimum surface separation-pressure and temperature of crude oil  while including the complete composition of the well stream and extended composition of the heavier fraction. The optimum pressure was estimated through the fluid properties such as oil formation volume factor, gas oil ratio and API gravity.  Optimum pressure and temperature is the one that produces maximum liquid yield (by minimizing oil formation volume factor and gas oil ratio) of maximum quality (by maximizing API gravity).  The fluid properties were predicted by phase equilibrium calculations using Peng Robinson thermodyinamic Model. The optimum pressure was first estimated considering the heavier fraction as single component and second the heavier fraction was splitted in pseudo components, both including the full composition of the well stream. Ahmed splitting method was used to extend the heavier fraction compostion, Kesler and Lee’s correlation was apllied to assign critical properties of the pseudo components. The results indicate that it is possible to estimate more accurately the optimum separation pressure by extending a composition of heavier fraction and accurate values of fluid properties were obtained. Key words: C7+ fraction, splitting scheme, equation of state, Peng-Robison thermodynamic model, optimum separator pressure.