EOM (EQUATION ORIENTED MODEL) For Crude Distillation Unit

Crude distillation unit is at heart of any refinery and processes all the crude that refinery purchases. The complexity in the modelling of crude distillation unit is enormous. The first challenge is the characterization of feed stock. Crude oil is composed of thousands and probably millions of components, with the current analytical techniques – it is impossible to measure them all. Typical approach is to model composition with the boiling point based pseudo components, which has its own short comings such as accuracy of results depend upon the choice of the lumping scheme and correlations used for determining their physical properties. In the present paper, author provide a novel lumping scheme using molecular type and carbon number based surrogate components which provides consistent properties (such as paraffinic component will have lesser gravity and critical temperature compared to aromatics component with same carbon number). For VLLE (Vapour – Liquid – Liquid – Equilibrium) calculations, author adopt an approach presented by Kamath et. al. (2010)  to model Peng- Robinson equation of state (1976) calculations with API- Water calculations for handling the aqueous phase. A rigorous tray by tray distillation model is presented with rigorous (MESH) equations and correlation for pressure drop. Pump around in rigorous tower is modelled using the simplified UA based exchanger where a correlation is used to adopt UA values according to process conditions.  Pump around heat exchangers are linked with preheat exchangers with consideration of pressure drop. A simple correlation was developed to model furnace which heats up the crude to the flash zone temperature. The numbers of stages in crude distillation column were adjusted to match the observed temperatures and the column model was solved with product purity (distillation end point, cloud point and flash point). An entire EO (equation oriented) model - EOM was solved with IpOpt Solver (2012). The results were compared with published literature and industrial studies. It was found that EOM approach is internally consistent (because of molecular approach), fast (because of equation oriented nature compared to simulation based approach which requires a lot of recycle) and reliable. An illustrative example is also presented where EOM model was used in refinery planning and scheduling compared to the traditional approach – the results suggests a better crude purchase decision is obtained resulting in savings in purchase cost. To the best of author’s knowledge, this is the first molecular based, entirely heat integrated and fully equation oriented model presented for crude distillation unit.

Key words: Crude Distillation Unit, Equation Oriented, Molecular characterization, Heat Integration

References:

1. Ravindra S. Kamath, Lorenz T. Biegler, Ignacio E. GrossMann (2010); "An equation-oriented approach for handling thermodynamics based on cubic equation of state in process optimization", Computers and Chemical Engineering 34, 2085–2096
2. IpOpt version 3.0 downloaded from website(2012): https://projects.coin-or.org/Ipopt
3. Peng, D. Y., & Robinson, D. B. (1976). A new two-constant equation of state. Industrial and Engineering Chemistry Fundamentals, 15, 59–64.