Distillation Simulation with Cosmo-rs

COSMO-RS is a novel and efficient method for the a-priori prediction of thermodynamic properties of mixtures. It is based on unimolecular quantum chemical calculations that provide the necessary information for the evaluation of molecular interactions in liquids. Thus, the method provides an alternative to group contribution methods such us UNIFAC for the true prediction of activity coefficients and K-values. The starting point for our work was an existing computer program for performing multicomponent, multistage separation process calculations using Newton's method. The existing code is the one included in the software package ChemSepTM. The simulation model was combined with the COSMOtherm code that implements the COSMO-RS thermodynamic property prediction methods. Comparisons of column profiles predicted by COSMO-RS and other models (UNIQUAC and UNIFAC) have shown that sometimes COSMO-RS is in good agreement with these other models and sometimes it is not. On the other hand even conventional activity coefficient models don’t always agree with one another! The conclusion is that COSMO-RS could become an alternative to conventional thermodynamic property models for the preliminary design of distillation processes and for the design of any process for which no data of any kind exists. However, more work is needed in order to show that COSMO-RS is able to predict adequately the VLE behavior of multicomponent mixtures. It was also found that the combination of computational models is rather more time consuming than, for example, UNIFAC, but the times are not excessive and the approach does not require extreme computational times (as would be the case with other purely predictive methods based on molecular dynamics, for example). The Equilibrium Stage Model The starting point for our work was an existing computer program for performing multicomponent, multistage separation process calculations using Newton's method. The existing code is the one included in the software package ChemSepTM [4,5]. The form of the model used in this program is outlined below. The equations that model equilibrium stages are termed the MESH equations, MESH being an acronym referring to the different types of equations that form the mathematical model. The M equations are the Material balance equations, of which there are two types: The Total Material Balance 0 ) 1 ( ) 1 ( 1 1 = + − + − + + ≡ − + j L j j V j j j j j L r V r F L V M (1) and the Component Material Balances 0 ) 1 ( ) 1 ( , , , 1 , 1 1 , 1 = + − + − + + ≡ − − + + j i j L j j i j V j j i j j i j j i j ij x L r y V r z F x L y V M (2) where the j r are the ratios of sidestream flows to interstage flows. j L j L j j V j V j L S r V S r / ; / = = (3) The E equations are the Equilibrium relations 0 , , , , = − ≡ j i j i j i j i x K y E (4) The S equations are the Summation equations 0 1 ; 0 1 1 , 1 , = − ≡ = − ≡ ∑ ∑ = = c