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Methane Reforming
Demonstration Problem

Reference: Wang Shuyan et. al., Simulation of effect of catalytic particle clustering on methane steam reforming in a circulating fluidized bed reformer, Chemical Engineering Journal 139 (2008) 136–146.

Purpose and Overview

Steam reforming of methane (conversion of CH4 into syngas: CO and H2) is of interest for applications such as fuel cells, conversion of natural gas to liquid fuels, etc. Similar reactors are of interest for coalto-liquid (CTL) and biofuel synthesis applications.

A simple demonstration problem has been developed in SINDA/FLUINT, using both the Sinaps© nongeometric (sketchpad) GUI and the Thermal Desktop© with FloCAD© geometric (CAD‐based) GUI.

Since both sets of models are available for inspection and for use as a starting point or template, only brief descriptions are included in this document as general guidance. A basic understanding of SINDA/FLUINT modeling is assumed.

Reaction Kinetics

Two simultaneous reactions of five species are considered:

CH4 + H2O ↔ CO + 3H2 (reaction 1)

CO + H2O ↔ CO2 + H2 (reaction 2)

Reaction 2 is the water‐gas shift (WGS) reaction.

The referenced paper (Shuyan, et. al.) contains the reaction kinetics used in this problem, based on the presence of Haldor Topsoe Ni/Mg Al2O4 spinel calatytic particles. Because of the demonstrative nature of this problem, the details of the catalyst are neglected: the catalyst is assumed to operate at full activity.

Shuyan lists formulae for forward reaction rates for these reactions,1 using equilibrium constants as the basis for estimating reverse rates. Assuming full catalytic activity, the current reaction rate can be calculated as a function of temperature, pressure, and partial pressures. The temperature dependencies for both the reaction rate constants and the equilibrium constants are exponential functions of an Arrhenius form.

Notes on Fluid Properties

In SINDA/FLUINT, each species is assigned a letter identifier:

Water may be a condensable (two‐phase) fluid. However, in this case the temperatures will always be high enough such that the liquid phase will never occur.2 Therefore, either a perfect gas (8000 series fluid) or a real gas (“NEVERLIQ” 6000 series fluid) could have been used. Versions of such files are available (http://www.crtech.com/properties.html) that were built from the NIST program REFPROP, perhaps subsequently simplified to a perfect gas using the SINDA/FLUINT PR8000 utility.

Unfortunately, the NIST database does not extend to sufficiently high temperatures for CO, CH4, and H2.

For example, the upper temperature limit for CO is only 500K in the current version of REFPROP.

Therefore, properties for all five gases were generated using NASA’s free CEA chemical equilibrium program combined with a C&R utility for converting outputs to FPROP DATA format.3 While this interface is intended to prepare fluids representing equilibrium reacting mixtures with variable molecular weights, the ‘only’ mode in CEA can be used to produce properties for single constituents with constant molecular weights. Therefore, despite the resulting generalized 6000 series “real gas” fluid tables, it should be noted that the actual properties reflect CEA’s intrinsic assumption of perfect gases. Heat of formation (HFORM) and diffusion volume (DIFV) information were then added to these fluid files, noting that CEA uses the heat of formation as the basis of the enthalpy at 25°C. The heat of formation of water is therefore diminished by the heat of vaporization to reflect the all‐gas nature of this CEA‐derived fluid file. (For a full two‐phase water description, the uncorrected heat of formation should instead be applied, since a liquid state exists at standard temperature and pressure.)

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