Calculations of effectiveness factors and the criteria of mass transfer effect for high-temperature methanation (HTM) catalyst

International Journal of Low-Carbon Technologies, Aug 2015

Natural gas is an extremely important bridge fuel to a low-carbon energy economy for improving local air quality. Coal to synthetic natural gas (SNG) is an effective way to convert the high-carbon energy (coal) into the low-carbon energy with rich hydrogen (natural gas). For the modern coal to SNG industry, the high-temperature methanation (HTM) catalyst plays an important role, and the advanced evaluation process should necessitate the elimination of mass transfer effect. Some simple but effective model catalysts, such as slab and sphere, can be very helpful in defining the reaction conditions, and thus facilitating the evaluation process for real HTM catalysts. In this work, slab and sphere model catalysts were adopted to derive mass transfer and reaction-coupled equations, the numerical methods were used to solve the coupled equations for the concentration profiles in catalysts, and the effectiveness factors were accordingly calculated. By taking advantage of the Thiele module φ and the Weisz–Prater module Φ, the criteria for the elimination of mass transfer effect in the HTM catalyst evaluation process were successfully defined. This work also complements the Weisz–Prater criterion by incorporating ‘negative reaction orders’.

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Calculations of effectiveness factors and the criteria of mass transfer effect for high-temperature methanation (HTM) catalyst

International Journal of Low-Carbon Technologies Natural gas is an extremely important bridge fuel to a low-carbon energy economy for improving local air quality. Coal to synthetic natural gas (SNG) is an effective way to convert the high-carbon energy (coal) into the low-carbon energy with rich hydrogen (natural gas). For the modern coal to SNG industry, the high-temperature methanation (HTM) catalyst plays an important role, and the advanced evaluation process should necessitate the elimination of mass transfer effect. Some simple but effective model catalysts, such as slab and sphere, can be very helpful in defining the reaction conditions, and thus facilitating the evaluation process for real HTM catalysts. In this work, slab and sphere model catalysts were adopted to derive mass transfer and reaction-coupled equations, the numerical methods were used to solve the coupled equations for the concentration profiles in catalysts, and the effectiveness factors were accordingly calculated. By taking advantage of the Thiele module w and the Weisz - Prater module F, the criteria for the elimination of mass transfer effect in the HTM catalyst evaluation process were successfully defined. This work also complements the Weisz - Prater criterion by incorporating 'negative reaction orders'. *Corresponding author; zhaolijun@nicenergy; com 1 INTRODUCTION Natural gas, composed mainly of methane, is known as a quality clean-and-low-carbon energy. However, natural gas is very limited in many parts of the world [1]. With rapid urbanization and increasing living standard, the demand for natural gas is huge, and the short supply will get even worse in future without unconventional input. Coal reserves are abundant, and therefore coal to synthetic natural gas (SNG) has been seriously considered as one of the solutions for natural gas shortage. The H/C ratio in coal can be raised four to six times higher by converting into natural gas. The total thermal efficiency for coal to SNG can be as high as 62–65%, by comparison, coal to oil is 40–50% and coal to electricity is as low as 36–38% [2]. The high efficiency of coal to SNG process reduces relatively the CO2 emission, and the CO2 gets highly concentrated in the process for treatment or utilization. In addition, pollutants such as sulfur can be reclaimed as useful byproducts in the coal to SNG process. The high-temperature methanation (HTM) catalyst is crucial for the modern coal to SNG industry, and is of much superior hydrothermal stability at high temperatures to the conventional methanation catalysts [3, 4]. Usually, there are two evaluation processes applied. By comparison, the process for ‘activity change’ measurement has been regarded most favorable for the evaluation of the HTM catalyst [4]. To apply the process, the measurement conditions must be carefully defined. Only under the conditions of the elimination of mass transfer effect, can intrinsic reactions occur, and the measured reaction rates can be used to quantify the activities of catalyst. Mass transfer in porous catalysts has been of great interest for a very long time, and the first papers appeared in the late 1930s [5]. Due to the ubiquitous nature of mass transfer, the concentrations of reactants are always higher on the surfaces than in the interior of catalyst particles. The effectiveness factor h has been defined as ‘the ratio of the real reaction rate of the catalyst particle to the imaginary reaction rate when the whole particle is assumed to bathe in the surface reactant concentration’ [6, 7]. Numerous efforts have been directed to the approximate solution of model catalyst systems for the effectiveness factor h changing with complex reaction kinetics. In contrast, the Weisz – Prater criterion, which was introduced in 1954 [8], has been widely applied in reaction kinetics measurements since it dictates the conditions for the elimination of mass transfer effect. The Weisz – Prater criterion has been derived by analyzing a model catalyst with ‘zero and positive reaction orders’ and by using the general assumption for mass transfer elimination if jh j 0:05 [8]. However, the methanation reaction has been known to proceed via a negative order of 20.5 with respect to CO in high concentrations [9, 10], for which the Weisz– Prater criterion could not give definite answers for the mass transfer effect. Therefore, detailed investigation, including positive to negative reaction orders, should be made of the effectiveness factor h in relation with the Thiele module w and the Weisz – Prater module F, so that the effect of mass transfer can be evaluated and eliminated in the advanced evaluation process of ‘activity change’ measurement for the HTM catalyst. 2 CATALYST MODELS AND CALCULATION METHODS Catalyst models, mass transfer and reaction-coupled equations and the solving methods and the equation for effectiveness factor h calculations will be discussed successively in this section. 2.1 Slab and sphere (...truncated)


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Li-Jun Zhao, Qi Sun. Calculations of effectiveness factors and the criteria of mass transfer effect for high-temperature methanation (HTM) catalyst, International Journal of Low-Carbon Technologies, 2015, pp. 288-293, 10/3, DOI: 10.1093/ijlct/ctu005