ausblenden:
Schlagwörter:
-
Zusammenfassung:
The formation of allyl alcohol by means of the selective catalytic hydrogenation of acrolein as the lowest α,β-unsaturated aldehyd using molecular hydrogen is one of the, in a one-step procedure, most difficultly realisable reaction [1]. The optimisation of conversion and selectivity is a demanding and complex intension. Intensive research is devoted to develop new and more specific catalysts allowing to improve selectivity and yields with respect to valuable thermodynamically limited products. An attractive option consists in establishing local compositions in such a way that a desired selectivity is maximised. Membrane reactors can be such a promising and innovative development [2, 3].
In this theoretical study results should be presented for the hydrogenation of acrolein (AC) to allyl alcohol (AyOH) carried out in a fixed-bed- (FB), in a membrane reactor (MR) and in a membrane reactor cascade using reduced 1D reactor models. The aim of this contribution is to evaluate the potential of a reactant feeding at discrete reactor positions or by dosing in a distributed manner using one or several reactor stages in a broad range of operating conditions [4-7].
In preliminary experimental investigations of TU Darmstadt the heterogeneously catalysed gas phase reaction of the hydrogenation of acrolein to allyl alcohol and propionaldehyde on a Ag/SiO2-catalyst was determined in a conventional fixed-bed reactor. This experimental study forms the basis for the performed simulation and the evaluation of all specified reactor concepts in a broad concentration-, residence time- and temperature interval. The estimated experimental data of the fixed-bed reactor could be satisfactorily described by the derived kinetics and the simple mathematical reactor model. The reaction operation of the MR is characterised in contrast to the conventional co-feed-mode by a selective and purposeful dosing of molecular hydrogen in the reaction zone using a membrane (Fig.1).
The membrane reactor concept was distinguished by high yields of the desired product allyl alcohol and was superior to the conventional tubular reactor in many ranges of operating regions. The results show a significantly higher yield of allyl alcohol in the one stage membrane reactor (double yield) as well as in a cascade consisting of three membrane reactor stages (more than double yield) with an adjusted dosing profile compared to the conventional fixed-bed for the experimentally investigated data field (Fig. 2).
Obviously, the enhancement of the allyl alcohol yield is characterised by the concentration- and residence time effects in membrane rectors based on the separate reactant feeding. For the example of hydrogenation of allyl alcohol the concentration effect is not justified by reducing concentration of the dosed component (hydrogen). In fact the reason is the high initial concentration (beginning of reaction zone) of the feeded reactant (acrolein) at the reactor entrance. Thus, for the given reaction kinetics and/or constellation of exponents of acrolein in the derive kinetic expressions, leads to a significant increase of the allyl alcohol selectivity. Furthermore, the well known residence time effect contributes to intensification of conversion in membrane reactors. Based on the continuously reactant dosing using membranes a noteworthily higher residence time can be achieved. This means for the discussed application of acrolein, an obtainable doubling of conversion compared to the conventional tubular reactor. Both effects described above are more explicitly developed in a multi stage membrane reactor with an optimised dosing profile. For this reason an additional and remarkable increase of the allyl alcohol selectivity could be realised. An experimental validation of the membrane reactor concept in a one or multi stage configuration should be reasonable against the background of the performed theoretical study.
Literatur:
[1] P. Claus, Topics in Catalysis, 1998, 5, 51-62.
[2] J. G. Sanchez-Macano, T. T. Tsotsis, Catalytic Membranes and Membrane Reactors, Wiley-VCH, Weinheim, 2002.
[3] J. Coronas, J. Santamaría, Catal. Today, 1999, 51, 3-4, 377-389.
[4] Y. L. Lu, A. G. Dixon, W. R. Moder, Y. H. Ma, Catalysis Today, 1997, 35, 443-450
[5] C. Hamel, S. Thomas, K. Schädlich, A. Seidel-Morgenstern, Chem. Eng. Sci., 2003, 58, 4483-4492
[6] V. Diakov, A. Varma, Chem. Eng. Sci., 2003, 58, 801-807
[7] F. Klose, T. Wolff, S. Thomas, A. Seidel-Morgenstern, Applied Catalysis A: General, 2004, 257, 193-199
