Benefits of Entrainer-based Reactive Distillation over Reactive Distillation for the Synthesis of Fatty Acid Esters
Advancing the chemical engineering fundamentals
Distillation, Absorption & Extraction - I (T2-10a)
Keywords: Reactive Distillation, Entrainer, Modelling, Fatty Acids
Benefits of Entrainer-based Reactive Distillation over Reactive Distillation for the Synthesis of Fatty Acid Esters
M.C. de Jong (a) A.C. Dimian (b) N.J.M Kuipers (a) A.B. de Haan (c)
(a) Separation Technology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands, email address: m.c.dejong@utwente.nl
(b) HIMS, Faculty of Science, University of Amsterdam, Nieuwe Achtergracht 166, 1018 VW Amsterdam, The Netherlands
(c) Process System Engineering, Technical University of Eindhoven, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
Keywords: Reactive Distillation, Entrainer, Modelling, Fatty Acids
Where in conventional Reactive Distillation (RD) a reaction column and a recovery column are necessary to reach high conversions, Entrainer-based Reactive Distillation (ERD) may take place in only one column. ERD not only uses separation to improve reaction, but also the introduction of an entrainer feed to make the separation feasible by increasing the relative volatility of one of the products. ERD appears to be advantageous for the synthesis of fatty acid esters. The entrainer increases the relative volatility of water (by-product) compared to the alcohol (reactant), such that during the reaction the water can be continuously removed by distillation by breaking the azeotrope. In this way the chemical equilibrium is shifted such that higher conversions can be obtained.
The ERD column consists of a reactive section with a distillation section placed on top, in where the entrainer is responsible for removing the water out of the reactive section. The top vapour is condensed and two phases are obtained through decantation: an aqueous phase and an entrainer rich phase which is refluxed back into the column. Of course adding an entrainer also has disadvantages: the most important one is that pollution of the product has to be prevented, therefore the entrainer should not enter the reactive section. The more entrainer is added the more critical this is.
Objective and Approach
The conceptual feasibility of ERD for the synthesis of fatty acid esters is already shown by Dimian et al. [1]. This work will concentrate on the gains that can be obtained using ERD with regarding to conventional RD.
The objective of this work was to compare the ERD process with a conventional RD process for the synthesis of fatty acid esters. An operating window was constructed which contains the necessary conditions to be able to produce fatty acid esters by ERD. Within this operation window both processes were compared and the advantage of ERD, which can be expressed in decreasing number of stages and the omission of a recovery column, was weighted against the constraints that are introduced together with the entrainer. The comparison was done by process models made in Aspen Plus© for the esterification of lauric acid with isopropanol using a heterogeneous catalyst.
Results
From the Aspen Plus© simulations can be seen that the entrainer in the distillation section indeed enhances the water removal from the reactive section without entering the reactive section itself and thus the equilibrium is favourably shifted. It also causes a higher alcohol concentration and therefore a higher reaction rate in the reactive section. The operating window in which this occurs is determined by the entrainer to feed ratio. A minimal value has to be reached to make the process work. When a certain value is exceeded (maximum) the entrainer completely ends up in the bottom of the distillation section, which results in a decreasing product purity. Within this operating window the number of stages in the reactive section necessary to achieve a certain conversion is significantly smaller for the reaction section for ERD than for the conventional RD column. The entrainer to feed ratio shows an optimum with respect to the capacity of removing water, however product purity is not influenced. It is desirable that the entrainer does not reach the reactive section. This can be prevented by introducing heat at the top stage of the reactive section. The more entrainer is added the more entrainer ends up in the bottom of the distillation section, such that more energy is needed to prevent the entrainer from entering the reactive section. Thus, the optimal entrainer to feed ratio is a compromise between water removal capacity/number of reactive stages and the amount of entrainer in the bottom of the distillation section which has to be prevented from entering the reactive section.
Conclusion
Within the operation window where ERD can be used, its use results in a significantly smaller reactive section compared to conventional RD. Therefore ERD could become an economically interesting alternative process for the production of fatty acid esters.
1. Dimian, A.C., F. Omota, and A. Bliek, Entrainer-enhanced reactive distillation. Chem. Eng. Process., 2004. 43: p. 411-420.
Presented Monday 17, 12:11 to 12:30, in session Distillation, Absorption & Extraction - I (T2-10a).
