OPTIMUM WATER/SYNGAS MOLAR RATIO IN THE FEED FOR MINIMIZING DEACTIVATION BY COKE IN THE SINGLE STEP SYNTHESIS OF DME
Advancing the chemical engineering fundamentals
Catalysis - I (T2-13a)
Keywords: DME, syngas, catalyst deactivation, coke
Considerable attention is being paid in the literature to the synthesis of dimethyl ether (DME) from hydrogen and CO in a single step for the following reasons: i) DME is considered to be one of the clean fuels with greater potential for use in the medium term; ii) this process allows for using alternative sources to oil (natural gas, coal and biomass) and has better perspectives for CO2 incorporation as reactant; and iii) the use of a single reaction step allows for shifting the reaction equilibrium towards DME production.
Unfortunately, deactivation by coke deposition is severe in this reaction, which gives way to a pronounced decrease in the yield of DME with time on stream. This paper approaches the viability of co-feeding water with the synthesis gas as an efficient strategy for minimizing deactivation by coke. However, this strategy of co-feeding water has the inconvenience of reducing the initial DME yield, due to the thermodynamic limitations of the reaction and to the adsorption of water in the acidic function [1]. Thus, another aspect of this study is to find the optimum water/syngas molar ratio in the reactor feed.
The bifunctional catalyst used is CuO-ZnO-Al2O3/g-Al2O3, prepared by mixing the dry metallic function (CuO-ZnO-Al2O3, for the methanol synthesis from H2+CO) and the acidic function (g-Al2O3, for the dehydration of methanol to DME) in an aqueous solution at a ratio of 2/1 by mass [2]. The reaction conditions selected are the following: 275 ºC (low enough to avoid CuO sintering), 30 bar, H2/CO molar ratio= 3/1, space time= 6.4 (g of catalyst) h-1 (mol of reactants)-1. Under these conditions, the catalyst undergoes severe deactivation by coke.
A water/syngas molar ratio of 0.20 (corresponding to a water flowrate of 0.005 ml/min) effectively reduces the deactivation by coke of the bifunctional catalyst (the DME yield hardly decreases with time on stream), without a significant decrease in the initial yield of DME. A higher water/syngas molar ratio (0.60) is not advisable, as the initial DME yield is severely affected by thermodynamic limitations and water adsorption on the Lewis sites of the g-Al2O3. Water/syngas molar ratios lower than 0.20 do not lead to an acceptable limitation of deactivation by coke.
References:
[1] Aguayo A. T., Ereña J., Sierra I., Olazar M., Bilbao J., Catal. Today, 106 (2005) 265.
[2] Ereña J., Garoña R., Arandes J. M., Aguayo A. T., Bilbao J., Catal. Today, 107-108 (2005) 467.
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Presented Thursday 20, 09:05 to 09:25, in session Catalysis - I (T2-13a).
