(524g) Separation of Isomers

Authors: 
Enders, S., KIT
Zeiner, T., Graz University of Technology
Goetsch, T., TU Dortmund University
In chemical industry there are several reactions in which a linear product and a number of branched side products are simultaneously produced. One example is the hydroesterification of long-chain unsaturated esters of fatty acids where linear and branched diesters are produced. For application only the linear ones are of interest, so that there is a demand to separate these isomers. This separation is a challenging task as distillation is very elaborative because of small separation factors and extraction often does not meet the high purity demands. Possible unit operations for the separation of isomers are the solvent crystallization and the adsorption. Solvent crystallization is promising because linear and branched molecules show different solubilities in a solvent. The challenge is the appearance of a superposition of a liquid-liquid equilibrium (LLE) and a solid-liquid equilibrium (SLE) during the separation process. Since this superposition can affect the product properties it has to be understood in order to build up suitable crystallization processes. To design a suitable adsorption process, the adsorption isotherms must be estimated. But in both cases, it is challenging purchasing the pure isomers to analyze the solid-liquid-liquid equilibrium (SLLE) or the adsorption isotherm. Here a thermodynamic model can be used to overcome these limitations.

The basic idea of the developed methodology is to only perform experiments with linear molecules and use the gained results for the prediction of phase equilibria of systems containing linear and branched molecules. For this purpose, it is mandatory to use a thermodynamic model that considers the architecture of the branched molecules. The lattice cluster theory (LCT) was developed to consider the architecture of the molecules without any additional adjustable parameter. The LCT therefore allows for the prediction of thermodynamic properties of branched molecules based on pure component parameters of linear molecules. In the case of SLLE calculations, the investigated solution consists of an alcohol and an alkane. To consider the self-association of the alcohol the LCT is combined with the chemical association lattice model (CALM). To calculate the adsorption isotherms of alkanes, the LCT is combined with the density functional theory (DFT).

In both cases the developed methodology can be divided into two steps. First, experiments with linear molecules are required. Regarding the solvent crystallization, binary LLE of a linear alkane dissolved in an alcohol were estimated. For the adsorption, pure component isotherms of linear alkanes were used. These experimental data are then used to fit the required model parameters. In the framework of the LCT applied to incompressible systems for the SLLE calculation, there is only one adjustable parameter for a binary system, namely the interaction energy Δε. Additionally, there are two adjustable parameters to consider the self-association of the alcohol in case of LLE. The LCT parameter for the compressible version to calculate the adsorption isotherms of alkanes were fitted to their vapor-liquid equilibria and the external potential of the DFT is fitted to pure substances adsorption isotherm. In the second step the fitted parameters are used for the prediction of the LLE of binary systems including a branched alkane and an alcohol and to predict the adsorptions isotherms of the branched isomer. This methodology will be applied and discussed for the design of solvent crystallization and adsorption processes.