Equilibrium or kinetically controlled reactions are common in chemical and biochemical manufacturing. This type of reaction is usually characterized by low process yield or low selectivity towards the desired product, when parallel reactions occur. On-site removal of product(s) enhances the yield, suppresses undesired side reaction(s) and therefore leads to reduced processing times of batch processes. The integration of separation and reaction can save investing and operating resources leading to more sustainable process, however specific “knock-off” criteria with respect to the reaction phase, catalyst, residence time and operating temperature and pressure must be satisfied.

Combination of at least two unit operations, basing on different physical phenomena, is called a hybrid process since they jointly contribute to fulfil the process step. In general, two types of hybrid processes are distinguished: reaction-separation for example the combination of batch reaction and membrane-based separation, and separation-separation like coupling distillation with pervaporation. The design of hybrid processes need to take into account interdependency of combined processes and it is possible to identify reliable and feasible design alternatives through a model-based computer-aided technique, saving thereby valuable experimental resources.

In this work the propyl-propionate synthesis from propionic acid and n-propanol is investigated. The design algorithm consists of four mains steps. At step 1 of the methodology, concentration profiles published in [1] are used to determine a temperature dependent kinetic constants, this includes thermodynamic analysis and identifying the operating window of the reaction. The esterification reaction has a conversion of around 64% (equimolar). In step 2 the objective is defined in order to increase the process productivity and to simplify the complex downstream processing due to thermodynamic non-idealities of the reaction mixture. Pervaporation is selected as the membrane-based separation technique (step 3) and a hydrophilic membrane is applied to remove selectively the by-product water. In step 4 various process scenarios (batch reaction, membrane assisted batch reaction, etc.) are analysed with the ICAS-MoT modelling environment. The separation characteristics of the applied polymeric membrane (Sulzer Pervap 2201) for binary and quaternary mixtures were obtained from a previous work at a multipurpose PV/VP lab-scale plant at University of Dortmund [2]. The transmembrane flux is calculated with a semi-empirical approach. The operating window in terms of reactant ratio, temperature and ratio of mass of catalyst and mass of reaction medium is investigated experimentally. The experiments show good agreement with the simulations.

The results of this work show, that a reactor coupled with a pervaporation unit is a promising process configuration with respect to product yield and batch time by overcoming limitations of kinetically controlled reaction. By the application of the membrane reactor concept it is possible to increase the conversion up to 95% at a decreased operating time of 4 h. Moreover, this high conversion benefits in less complex downstream operation for the final purification of the ester.

Reference:

[1] Duarte, C., Buchaly, C., Kreis, P., Loureiro, J.M., Inz.Chem.Proc. (2006)

[2] Lauterbach, S., Kreis, P., Euromembrane 2006 Taormina, Italy (2006)