Abstract
In this paper we discuss the use of an approximated form of the mass balance equation for a tubular reactor in which a gas phase reaction is carried out. By means of three examples it is shown that if density variations are not taken into account in a correct way, surprisingly high errors can occur. More specifically, if the mass balance is written down in a generally applied approximate form, i.e. using udc/dz instead of d(uc)/dz—u, c and z respectively being the gas velocity, concentration and length coordinate—even for density variations as low as 5%, errors of a magnitude higher than 5% can be introduced. This is particularly true for multiple reaction systems. The inexpediency of approximating d(uc) by udc is stressed: the approximation is in no way a simplification. Therefore, this commonly seen approximation should never be applied.
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