Kinetic Model for the Transformation of Bioethanol into Olefins over a HZSM-5 Zeolite Treated with Alkali

Abstract

A kinetic model for the transformation of bioethanol (aqueous ethanol into hydrocarbons) has been established for a catalyst with slow deactivation by coke (prepared from a HZSM-5 zeolite treated with alkali in order to moderate its acid strength and generate mesopores in the zeolite structure). The kinetic model allows quantifying the distribution of the lumps of products (ethylene, C3−C4 olefins, gasoline or C5−C10, and C1−C3 paraffins) and is particularly interesting for quantifying the production of C3−C4 olefin lump, which is the one of greater commercial interest. The experimental data have been obtained in an isothermal fixed-bed reactor in a wide range of operating conditions (temperature of 300−400 °C, space time and water content in the feed). The remarkable features of the kinetic model are (i) consideration of the autocatalytic formation of C3−C4 olefin lump following their reaction with ethylene, (ii) consideration of the combined involvement of ethylene and the light olefin lump in the formation of heavier hydrocarbons; and (i) quantification of the effect of water in the reaction medium, which attenuates the rate of each one of the steps in the kinetic scheme, except the dehydration of ethanol which is instantaneous under these conditions.

Deactivation kinetics of a HZSM‐5 zeolite catalyst treated with alkali for the transformation of bio‐ethanol into hydrocarbons

“…26 The reaction products were grouped into four lumps: (i) ethylene, which is really the reactant because ethanol dehydration is very fast under the conditions studied; (ii) light olefins (propylene and butenes), which commercially are the most interesting products; (iii) components in the C 4 -C 12 range (briefly denoted lump of gasoline), comprising C 4þ paraffins, C 5þ olefins and aromatics; (iv) light paraffins (methane, ethane and propane). The kinetic equations for the scheme in Figure 1 are:…”

“…Given the uncertainty concerning the dependence of deactivation on component concentration in the reaction medium, different expressions have been assayed for the kinetic equation of deactivation (Table 3) (26) which is dependent on water content in the reaction medium, X W (expressed by mass unit of organic components). Different expressions have been tested (exponential, potential, hyperbolic) to relate y d with X W but, for the sake of simplicity, the results shown below correspond only to an exponential expression of y d (X W ) function, which is that of best fit:…”

“…The operating conditions are controlled by a software specifically designed for this process called Adkir. 26 The data for the kinetic modeling have been obtained under the following operating conditions: temperature, between 300 and 400 C; pressure, 1 atm; space-time, up to 1.53 (g of catalyst) h (g of ethanol)…”

“…25 Subsequently, a reaction scheme and kinetic model that quantify the yields and product distribution at zero time on stream have been proposed. 26 It should be noted that this model also quantifies the attenuation of the rates of the reaction steps by the water content in the medium.…”

“…Kinetic scheme for bio-ethanol transformation. 26 negligible, which has been proven by reaction-regeneration cycles in which the catalyst maintains its performance.…”