Thermokinetic Modelling of the Gas Phase Partial Oxidation of Methane to Methanol in a CSTR.

H.E. Curry-Hyde and R.F.Howe (Editors), Natural Gas Conversion I1 0 1994 Elsevier Science B.V. All rights reserved.

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Thermokinetic Modelling of the Gas Phase Partial Oxidation of Methane to Methanol in a CSTR. B.G. Charlton", G.A. Fouldsb,B.F. Gray', J.F. Griffithsdand G.S. Walker"' "CSIRO Division of Coal and Energy Technology, PMB 7, Menai, New South Wales, 2234, Australia. bChemicalEngineering & Industrial Chemistry, School of Molecular Sciences, James Cook University, Townsville, Queensland 48 11, Australia. 'Department of Mathematics and Statistics, The University of Sydney, Sydney, New South Wales, Australia. dSchoolof Chemistry, University of Leeds, Leeds LS2 954T England.

1. INTRODUCTION In the direct conversion of methane to methanol by partial oxidation, to date, the most promising experimental results have been reported for the homogeneous gas phase reaction, but with considerable variation in methanol selectivities [ 11. In addition, multiple steady states [2] and oscillations [3] have also been observed under certain process conditions. A range in conversion and selectivity has also been reported for the reaction models that have been developed thus far for the homogeneous gas phase reaction [4-81. The aim of the present study is to obtain a workable quantitative numerical model. The only control parameters at the disposal of the experimentalist are the equivalence ratio of CH4/02,the surface to volume ratio, the total pressure, the ambient temperature, and the residence time in a CSTR. We have previously considered the effect of total pressure, equivalence ratio, and ambient temperature, on the discontinuity in maximum temperature attained, the hysteresis effect associated with this discontinuity,and the negative temperature coefficient of the heat generation rate at higher temperatures [4]. A comprehensive kinetic scheme containing 20 chemical species was developed. In a continuation of this work, we have extended the thermokinetic modelling studies to include the effects of overall heat transfer coefficient, oxygen concentrationin the feed, and residence time, on the discontinuity and its associated hysteresis, the methanol selectivity, and the occurrence of oscillations.

2. EXPERIMENTAL The present work is based on the kinetic scheme described earlier [4], and the modelling procedure is basically derived using the exact species and energy conservation equations. In the model, CSTR behaviour is assumed, and as a result, the transport properties of the numerous transient speciescan be ignored. As before, ambienttemperature(T,,,,) was chosen as the preferred or bifurcationparameter. X,the product of the overall heat transfer coefficient, I, and the surface to volume ratio, SN,was varied from 7500 to 3000 Wm-3K-'at a residence time of 20 seconds, while the other system parameters were maintained at 9.5% 0, in the feed and a pressure of 3.0 MPa. The effect of varying feed oxygen concentration was

'Acknowledgements: The authors wish to thank the Australian Research Council for the award of an Australian Senior Research Fellowship (B.F.G.), D. Chivers, K. Wong and B.H.P Co. Ltd. for financial assistance.

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calculated for 9.5,7.5,5.0 and 2.5% 0, in the feed (pressure = 3.0 MPa, residence time = 20 seconds, X = 4500 W ~ I - ~ K 'The ) . effect of varying the residence time was also investigated using residence times ranging from 5 to 35 seconds (feed oxygen concentration = 9.5%, pressure = 5.0 MPa). In each case the magnitude and position of the hysteresis loops were determined and the optimum methanol concentrations were calculated. 3. RESULTS AND DISCUSSION

Figure 1 illustrates the effect of X on the position and size of the hysteresis loop. As X is decreased, the magnitude of the hysteresis [AT = T (reaction temperature) - Twall] increases, with the discontinuity moving to lower Twdr.Methanol yield is not significantly affected by the decrease in X, which can be explained by the fact that although Twalldecreases considerably, the reaction mixture temperature remains relatively constant over the range of X. In addition, near the extinction point, in all cases showing hysteresis, the upper state is oscillatory and the extinction represents an extinction of oscillation. Methanol yields are highest at the lowest upper stable state before oscillation starts.

I5O

t

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/---

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Twall /'C

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Figure 1. Effect of X [I(SN)] on position and magnitude of hysteresis. (A; I ( S N ) = 5000 Wm"K-', B; I(SN) = 6000 W I ~ ' ~ K C; - ' , I(SN) = 6500 W I ~ - ~ KD; - ' I(SN) , = 7500 Wm"K-') Figure 2 depicts the data generated by the kinetic model for 2.5%,5.0%,7.5%and 9.5% O2 in the feed. It is clear that methanol selectivity passes through a maximum, which decreases with increasing oxygen concentration. In addition, hysteresis and oscillations are present using 9.5% 02,but are absent when the feed oxygen concentration is 2.5%,5.0% and 7.5% 0,. These results, agree well with experimental results obtained in the annular flow reactor

PI.

To study the effect of residence time, simulations were carried out at residence times ranging from 5 to 35 seconds, using 9.5%0, in the feed and a pressure of 5.0 MPa. As the residence time is increased, the magnitude of the hysteresis decreases, both in range of wall temperature and AT. More significantly, the maximum yield of methanol increases with increasing residence time, but at longer residence times (>20s) flattens out, and is essentially constant (27%) for further increases in residence time in the range considered. The inverse of this trend is manifested by the methane conversion, which initially decreases as the residence time is increased. but then levels off at 6.0%.

60 2.5% 0 2 5.0% 02

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I

I

I

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I

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Figure 2. Simulation of methanol selectivity vs Twn,, using various 0, concentrations

4.CONCLUSIONS Detailed numerical modelling of the direct partial oxidation of methane under fuel rich conditions at high pressures, including heat transfer effects, gives results which are in semi-quantitative agreement with experimental CSTR results. Observed phenomena such as hysteresis, bifurcation, sustained oscillation and homoclinic extinction, as well as parain metric dependencies of methanol selectivity on wall temperature, residence time, %02 feed, and heat transfer characteristics, are all accounted for in semi-quantitative fashion by the numerical approach.

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