Effect of reactor dimensions on the performance of an O2 pump integrated partial oxidation reformer—a modelling approach

doi:10.1016/j.ijhydene.2005.03.012

International Journal of Hydrogen Energy

Volume 31, Issue 1, January 2006, Pages 1-12

https://doi.org/10.1016/j.ijhydene.2005.03.012 Get rights and content

Abstract

This paper describes a two-dimensional model of an oxygen permeable membrane reactor for partial oxidation of methane (POM). The model covers all aspects of major chemical kinetics, heat and mass transfer phenomena in the tubular reactor with oxygen permeation in radial direction through an ion-conducting membrane. The temperature and concentrations of the major species in the membrane tube were calculated. Results showed that the overall performance of the reactor was strongly dependent on the reactor dimensions and operating conditions. A reactor with smaller diameter (D) and greater length-to-diameter ratio (L/D) gives better performance in terms of high methane (CH $_{4}$ ) conversion and high hydrogen (H $_{2}$ ) concentration. Under inlet gas temperature of 900 K and space velocity of 15,000 h $^{- 1}$ , the CH $_{4}$ conversion and dry H $_{2}$ concentration can reach 98% and 64%, respectively, in the reactor with diameter D around 10 mm and L/D $> 25$ .

Introduction

One of the popular techniques to produce hydrogen or syngas (mixture of H $_{2}$ and CO) is by partial oxidation of methane (POM) [1], [2]. In a conventional partial oxidation (POX) process, air is usually used as the oxidant. Thus, the product gas is diluted by high concentration of nitrogen in air of the supplied gas mixture causing low hydrogen concentration in the product gas and high energy loss through discharging the nitrogen in the exhaust. However, running a POX reactor with pure oxygen is always a costly exercise.

The above-mentioned drawbacks in POX reactors can be overcome by supplying only oxygen instead of air via an O $_{2}$ ion-conducting membrane (such as perovskite-type ceramic membrane) [3], [4], [5], [6], [7], [8], [9], [10]. Some studies showed that the permeation of oxygen through the membrane becomes significant at the membrane temperature of above 1000 K, which is very similar to the operating temperature of POX process. Hence, this membrane can be integrated into a POX reactor for oxygen separation. In a typical oxygen permeable membrane reactor, the tubular catalyst bed is packed in an O $_{2}$ ion-conducting membrane. One side of the membrane is exposed to gas in the catalyst bed, while the other side is exposed to air. The difference of oxygen partial pressures between the two adjacent sides due to oxidation reaction in the catalyst bed region causes oxygen to permeate from the airside to the catalyst bed through the membrane by ionic and electronic diffusion mechanism [5], [6].

There have been some experimental studies on POM in a membrane reactor packed with Ni catalyst, such as the studies of ${La}_{1 - x} A_{x} {Co}_{0.2} {Fe}_{0.8} O_{3 - δ}$ (with $x = 0.8$ and $0.6, A = Ba$ , Ca, or Sr) [10], ${Ba}_{0.5} {Sr}_{0.5} {Co}_{0.8} {Fe}_{0.2} O_{3 - δ}$ [11] and ${La}_{0.6} {Sr}_{0.4} {Co}_{0.2} {Fe}_{0.8} O_{3 - δ}$ [12] membrane reactors. The results showed that the methane conversion is larger than 96% under low CH $_{4}$ partial pressure in the range of operating temperature of about 1100–1200 K.

Modelling of POM in oxygen permeable membrane reactors has been of great interests by in recent years [13], [14]. The one-dimensional model is simple and easy to handle, but it is only preferred to simulate the conventional POX reactor when plug flow is assumed in the axial direction of the reactor. In an oxygen permeable membrane reactor, the transport of oxygen and products of oxidation reactions from the vicinity of the membrane towards the centre of the reactor has significant impact on the reforming performance. Therefore, two-dimensional model is preferred to account for this effect.

The objective of this paper is to study the performance of the POM in a tubular perovskite-type membrane reactor packed with Ni catalyst by two-dimensional modelling approach. The study is focused on the effect of reactor dimensions on the POX performance under different operating conditions so that the optimal reactor dimensions under prescribed operating conditions can be determined.

Section snippets

Chemical reaction scheme

In this study, the mechanism of indirect POX proposed by De Groote and Froment [1] is applied. Four major reactions are considered in this the study, including the exothermic complete combustion of a fraction of feed methane: ${CH}_{4} + 2 O_{2} \to {CO}_{2} + 2 H_{2} O, Δ H_{1 (298)} = - 802, 000 kJ / kmol$ followed by the endothermic STR reaction to CO: ${CH}_{4} + H_{2} O \leftrightarrow CO + 3 H_{2}, Δ H_{2 (298)} = 206, 000 kJ / kmol$ accompanied by water–gas shift (WGS): $CO + H_{2} O \leftrightarrow {CO}_{2} + H_{2}, Δ H_{3 (298)} = - 41, 000 kJ / kmol$ and some methane steam-reformed directly to CO $_{2}$ : ${CH}_{4} + 2 H_{2} O \leftrightarrow {CO}_{2} + 4 H_{2}, Δ H_{4 (298)} =$

Membrane reactor with oxygen permeation

Fig. 1 shows the schematic diagram of a typical membrane reactor used in this study. It is a membrane tube packed with Ni catalyst inside and surrounded by air and shell outside. The membrane is of $10 μ m$ perovskite-type ceramic layer coating on the surface of the support tube made of porous $α$ -alumina. The shell constitutes an air tunnel between the shell and the membrane tube. Air flows through the shell, while methane flows through the membrane tube packed with catalysts. One side of the

Simulation results and discussions

The model in this study consists of a set of six governing Eqs. (16–21) describing the mass and energy balance of air, gas, and solid phase of the reactor. These equations are solved simultaneously by a finite difference method with prescribed initial and boundary conditions (22–27) to determine the concentrations and thermal properties of gas at each increment of time and reactor coordinate. At each increment of time, the computation along z-axis is started from the membrane $(r = R_{c}$ ) to the

Conclusion

A two-dimensional model for oxygen permeable membrane reactor with partial oxygen of methane has been successfully developed. The model covers all aspects of major chemical kinetics; heat and mass transfer phenomena in the reactor. The model allows the determination of the gas temperature and concentrations of major species in the membrane tube (catalyst bed) and air region. The primary results are shown to match with those of the previous studies. The reactor dimensions and operating

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Effect of reactor dimensions on the performance of an O2 pump integrated partial oxidation reformer—a modelling approach

Abstract

Introduction

Section snippets

Chemical reaction scheme

Membrane reactor with oxygen permeation

Simulation results and discussions

Conclusion

Catal Today

J Membr Sci

J Membr Sci

Solid State Ion

Catal Today

Catal Today

Solid State Ion

J Power Sources

Catal Today

J Membr Sci

Chem Eng Sci

Effect of reactor dimensions on the performance of an O $_{2}$ pump integrated partial oxidation reformer—a modelling approach