Comments on discrimination of three approaches to evaluate heat flux~ for wslkooled fixed bed chemical reactors

In a recent paper, Vortmcycr and Haidegger (1881) prcsented a comparison of thm models to evaluate heat iluxes fur wall-cooled fixed-bed chemical rtaEtvrs. We have the following comments about this mper. Vortmeya and Haidegger used the fast exothermic oxidation of etbane to test three (quasi-) homogeneous m&Is. They concluded that the wall ks c~tducri~~mcdel WBS the best thaw of kttcr agreement with the experjmental data and physical reality. We, however, feA tbrt under the rcaction conditions considered the homogeneous models are not applicable, and a beterogenevua model ghould have ken used instead. In a recent article (Wijngaarden and Westerterp. 1989) we aplained how improper dependenccs and relationa are introducbd when an improper mMe.1 is used. To prove thi we tested Vortmeym and Huidegger-s experimental data from their graphs 3(a)-&), with a criterion given by Mears (1971). The interparticle heat transfer rektThe results oC our calculations are presented in Tables 1 and 2. However, the Nu! relations are not given in the paper ol Vortmeyer and Hmdegger. In the VDI-W%rme~tla~ (VDI, 198X) Nu, i~umkrs arc given lor Fe above 500. We refix to the work al Kunii and Suzuki (1967), from which it can be found that Nu, for the given Rc, values varies between 4 and 12 This leads to 47 < ccP -C 141 W/(mZ XC). The measured temperatures were assumed to be. gas temperatures, In all experiments, particularly just before the hot spot where concentrations and tcntperatutes are relatively high, the criterion is not satisfied. Therefore, temperature differences between catalyst surface and surrounding gas cannot bc neglected.