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This design example is suggested from hydrocarbon oxidation processes such as benzene oxidation into maleic anhydride or the synthesis of phthalic anhydride from o-xylene. Such strongly exothermic processes are carried out in multitubular reactors, cooled by molten salt that is circulating around the tubes and that exchanges heat to an internal or external boiler. The length of the tubes is 3 m and their internal diameter 2.54 cm. One reactor may contain 2500 tubes in parallel and even up to 10,000 in the latest versions. In German processes the catalyst is V205 on promoted silica gel and the operating temperature range is 335 to 415°C. The particle diameter is 3 mm and the bulk density is 1300 kg/m3. The hydrocarbon is vaporized and mixed with air before entering the reactor. Its concentration is kept below 1 mole %, in order to stay under the explosion limit. The operating pressure is nearly atmospheric. The phthalic anhydride production from such a reactor with 2500 tubes is 1650 tons/yr. It follows that with this catalyst a typical mass flow velocity of the gas mixture is 4684 kg/m2 hr. With a mean f1uid density of 1.293 kg/m3 , this leads to a superficial f1uid velocity of 3600 m/hr. A typical heat of reaction is 307,000 kcal/kmol and the specific heat is 0.237 kcal/kg ºC (0.992 kJ/kg K). In this example the kinetic equation for the hydrocarbon conversion will be considered in first approximation to be pseudo first order, due to the large excess of oxygen: ra = k·pb0·p where pb0 = 0.208 atm represents the partial pressure of oxygen. Let k be given by: Ln k = 19.837 - 13,636 / T From Kunii and Smith's correlation it follows that at Re = 121,er = 0.67 kcal/m hr ºC and from Yagi and Kunii's equation: aw = 134 kcal/m2 hr ºC, so that Pehr = 5.25, whereas Pemr = 10. In all cases the feed inlet temperature equaled that of the salt bath.
GRADIENTE MÁXIMO Corrientes paralelas:
Suponer Tr0 Resolver Ec. Dif. Tr ,z=L TrL FIN
