Computer Aided Molecular Design Of Solvents For Separation Processes

It is thoroughly investigated in the paper how the solvents for a certain separation process (extraction or extractive distillation) are found by computer aided molecular design. A new method, the classified enumeration is represented to design the solvents, and a novel strategy, the synthetical evaluation is proposed to rate them. Furthermore, the classified enumeration is also successful to be applied to design the heating medium and the refrigerants.In the classified enumeration, all groups are divided into two parts: skeleton groups and function groups. The former are the groups that can constitute alkanes, cycloalkanes, and aromatic hydrocarbons; the latter are the reminder ones. With the former, we enumerate all molecules of alkanes, cycloalkanes and aromatic hydrocarbons. Then the function groups substitute the corresponding skeleton ones in these molecules to build other compounds. At last, the properties of each molecule are calculated by group contribution methods. As there are not more than three kinds of function groups in the molecule of solvent due to the industrial cost, a combinatorial explosion will be avoided.In addition, because the illogical combinations, such as CH3CO (OOCCH3), may emerge according to the octet rule only, it is necessary to modify the rule. Considering the difference of free valences, they are separated into two classes: unrestricted ones and restricted ones. The former are the free valences on the carbon atom in the group and the latter on the other atom. Because the unrestricted valences can link to any free ones while the restricted can only connect with the unrestricted ones, for the chain organic molecules, the sum of the unrestricted cannot be less than that of the restricted to guarantee the match of the free valences and eliminate the illogical combinations. As to the molecules containing rings, the ring can be looked on as one group so that the molecule is changed into a chain one, which can be dealt with as mentioned before.A synthetical evaluation is proposed to rate the solvents. In it all solvents are sorted into two classes: principal ones and secondary ones. The former are those that contain not more than one kind of function groups; the latter are the rest. Then a multiple-objective decision-making theory is introduced to estimate the separating capability of every solvent. During the process, a transform function is presented to eliminate the enormous difference among the factors to describe the separating capability.Finally, the classified enumeration performances well in solving the computer aided molecular design of the heating medium and the refrigerants, as indicate that it is general in computer aided molecular design based on group contribution methods.