Chemical Engineering Science 54 (1999) 4437}4448 Cyclic operation of the oxidative dehydrogenation of propane D. Creaser, B. Andersson, R. R. Hudgins*, P. L. Silveston Chemical Technology, Lulea  University of Technology, S-971 87 Lulea  , Sweden Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, N2L 3G1, Canada Department of Chemical Reaction Engineering, Chalmers University of Technology, S-412 96, Go ( teborg, Sweden Abstract The cyclic operation of the oxidative dehydrogenation of propane over a V}Mg}O catalyst by the alternate feeding of propane and oxygen gas mixtures was investigated. Generally, the response of the reaction products following a step-change in composition determined the cyclic conditions that resulted in an improved yield of propene. By alternating propane and oxygen with a 1 : 1 cycle split, time-average propene yields higher than steady state could be obtained. The optimal cycle period was about 60 s. This same period provided high propane conversion, as well as high propene selectivity. The cycle split could be varied by shortening the oxygen-feed half of the cycle with little e!ect on the time-average results. The oxygen half-cycle needed only to be long enough to reoxidize the catalyst. The propane feed scheme was varied by feeding increasing amounts of propane in the oxygen half-cycle. The change in feed scheme caused an increase in the time-average propane conversion that resulted in some further improvement in the propene yield despite some loss in propene selectivity.  1999 Elsevier Science Ltd. All rights reserved. Keywords: Propane oxidative dehydrogenation; V}Mg}O; Periodic operation 1. Introduction Oxidative dehydrogenation o!ers advantages of min- imal catalyst deactivation and energy savings compared to direct thermal dehydrogenation. However, the selec- tivity of oxidative dehydrogenation remains a serious problem that so far prevents its commercial adoption. It has been thought that the selectivity of partial oxidation reactions may be dependent on how the reactants } hy- drocarbons and oxygen } are contacted with the catalyst. If so, non-conventional reactor designs or operation could be used to tailor reactant and catalyst contact to minimize deep oxidation. To this end, there has been some recent studies in the use of catalytic membrane reactors for the oxidative dehydrogenation of ethane (Coronas et al., 1995; Ton- kovich et al., 1996) and propane (Pantazidis et al., 1995; Te H llez et al., 1996). By controlled feeding through a mem- brane, the oxygen could be better distributed to regulate the extent of the side reactions, thus leading to higher yields and selectivities. Working with propane, both Pan- tazidis et al. (1995) and Te H llez et al. (1996) have examined * Corresponding author. Tel.: 1-519-885-1211-2092; fax: 1-519-746- 4979. the use of di!erent inert membrane materials to distrib- ute oxygen to a "xed bed of V}Mg}O catalyst. Panta- zidis et al. claimed that the membrane feeding of oxygen resulted in a marked increase in propene yields, com- pared with conventional gas-phase co-feeding. Te H llez et al. (1996) on the other hand, did not report any notable improvement in yields by such a reactor con"guration. Instead they pointed out that by distributing the oxygen feed, one can extend the range of safe reactor operation by avoiding #ammable mixtures of the reactants and reducing the risk of hot spot formation. In another study the electrocatalytic oxidation of pro- pane on V}Mg}O catalysts was tested Wang et al. (1994). The V}Mg}O catalyst was deposited on a silver electrode to which oxygen ions could be electrochemically pumped. It was claimed that the electrochemically sup- plied oxygen was more selective for propene production than that supplied by the gas phase. In this work, a non-conventional mode of reactor operation is examined for oxidative dehydrogenation: cyclic or periodic operation. The reaction is carried out by cyclically feeding reactant mixtures of di!erent compositions. Transient responses to this type of forcing can also reveal additional information about the reaction mechanism and kinetics (Renken, 1990; Wang and Hof- mann, 1990). 0009-2509/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 0 9 - 2 5 0 9 ( 9 9 ) 0 0 1 3 5 - 9