Ind. Eng. Chem. Res. 2006, 45, 2661-2671
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The Oxidative Dehydrogenation of n-Butane in a Fixed-Bed Reactor and in an Inert Porous Membrane ReactorsMaximizing the Production of Butenes and Butadiene David Milne, David Glasser, Diane Hildebrandt,* and Brendon Hausberger Centre of Material and Process Synthesis, UniVersity of the Witwatersrand, Johannesburg, PriVate Bag 3, WITS 2050, South Africa
The oxidative dehydrogenation (ODH) of n-butane (butane) produces three isomers of butene (1-butene, trans2-butene, and cis-2-butene) which in turn are oxidized to form butadiene. Butane also is oxidized directly to butadiene. In this simulation study, the authors have analyzed the operating conditions required to produce the maximum amount of butenes, i.e., all three isomers, and butadiene in a fixed-bed reactor (FBR) and in an inert porous membrane reactor (IMR). The theoretical maximum yields of butenes and butadiene were found to be 0.119 and 0.800 carbon mass fractions, respectively. The reactor configuration in both instances was a large IMR operating at a low constant partial pressure of oxygen in the stream of reactants and products. It was found that 99.7% and 83% of the theoretical maximum yields of butenes and butadiene, respectively, can be achieved in an IMR with a constant oxygen partial pressure of 0.25 kPa. The corresponding residence times are 75 and 322 s. Candidate attainable regions have been identified for the system subspaces butanebutenes and butane-butadiene. Introduction Olefins and dienes are precursors for a wide range of useful chemicals. A very attractive route to make them is via the oxidative dehydrogenation of hydrocarbons, as these are readily available from crude oils and Fischer Tro¨psch synthesis. The problem with this route is to try to minimize the oxidation of these hydrocarbons to other products such as carbon monoxide, carbon dioxide, and water. However, such routes to olefins and dienes will only become practical when both the yield of product and the selectivity to the desired product are high. 1,3-butadiene is a high-volume and valuable intermediate organic chemical used in many industrial processes to produce rubber, resins, and plastics. It is involved in several different reactions including addition, oxidation, and substitution reactions, but its main use is for polymerization. Most 1,3-butadiene is used in synthetic elastomer production and in adiponitrile production, the raw material for nylon-6,6 production. The overall demand for butadiene is expected to increase because of the growth of specialty uses for it.1 Butadiene is usually produced by one of two processes (a) recovery from a mixed hydrocarbon stream and (b) by the oxidative dehydrogenation (ODH) of butenes.1 In this paper, we examine the ODH of n-butane to butenes and butadiene. Butane is a readily available feedstock and is produced from crude oils and Fischer Tro¨psch synthesis, and we believe that its conversion to butadiene offers potentially significant economic benefits. Another requirement is to achieve a high selectivity of butane to butadiene allied to high yields of butadiene. Once the kinetics of the reactions are known, it is important to optimize the reaction system to ensure that the economics of the process make it an attractive industrial option. In this paper, we examine the possible maximum yields and selectivities and then the ways of achieving them in practice. * To whom correspondence should be addressed. E-mail:
[email protected]. Tel.: +27 (11) 717 7557. Fax: +27 (11) 717 7557.
Figure 1. Reaction scheme for the oxidative dehydrogenation of butane to butenes and butadiene.
In an earlier paper,2 the authors studied the ODH of 1-butene to butadiene in a fixed-bed reactor (FBR) and in an inert porous membrane reactor (IMR). It was found that, in an IMR where the inlet oxygen partial pressure was maintained at a constant level along the length of the reactor, the maximum yield of butadiene increased as the oxygen partial pressure was reduced. This earlier paper acknowledged the work done on the ODH of butane by Te´llez et al.3-5 and Assabumrungrat et al.6 The catalyst used in the FBR and IMR reactors was a V/MgO catalyst containing 24% (by mass) of V2O5. The reaction network for the ODH of butane was postulated3,4 as shown in Figure 1. The three isomers, 1-butene, trans-2butene, and cis-2-butene, have been lumped together as C4H8 in reactions 7-9. The mathematical model created to describe and simulate the ODH of butane assumed isothermal conditions and atmospheric pressure. Maintaining atmospheric pressure in the reactor implied varying the size of the catalyst bed to attain the desired yields of butenes and butadiene. Matlab, Version 6, Release 13, was used for all the simulations. The kinetic rate expressions for the oxidation of butane, butenes, and butadiene were taken from Te´llez.3 These expressions have as variables the partial pressures of oxygen and the hydrocarbons, butane, butenes, and butadiene. In principle, one would like to analyze the system using the attainable region (AR) method, because this would give results for the optimum conditions and reactor structure to achieve a desired product. In this particular ODH study, the size of the problem is too large to be currently analyzed using this approach.
10.1021/ie050120l CCC: $33.50 © 2006 American Chemical Society Published on Web 03/09/2006
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Figure 2. FBR configuration.
However, when doing our analyses, some of the thinking behind this method is employed. Results An initial feed mixture of butane and oxygen was used, and the partial pressure of oxygen was varied over the range 0.25 to 85 kPa. The feed temperature and the reactor isothermal temperature was 773 K. As in our earlier paper,2 all hydrocarbon concentrations are expressed in terms of mass fractions of carbon. Three scenarios were considered. The first was feeding butane and oxygen, the latter at an initial specified partial pressure, to a stabilized (steady state) FBR and permitting the reaction to continue until either all the oxygen or all the butane was depleted. The effect of oxygen partial pressure in the feed stream upon the yields of butenes (Case 1) and butadiene (Case 2) was studied. In the second scenario, using a stabilized IMR, the partial pressure of oxygen was maintained at a constant specified level by the addition of fresh oxygen along the length of the IMR. Again, the effect of oxygen partial pressure in the feed stream upon the yields of butenes (Case 3), butadiene (Case 4), and butenes and butadiene combined (Case 5) was studied. In a third scenario, the authors have explored the effect upon the candidate attainable region of deploying two very large IMRs in series and by incorporating a policy of bypass and mixing. The effect of residence time upon yields of butenes and butadiene was examined. In all instances, the reaction was permitted to attain equilibrium, at which stage either the oxygen or the butane had been depleted. In effect, the stoichiometric ratio of oxygen in the feed was varied to simulate different reactant compositions. Despite there being a spectrum of seven products other than butane and oxygen in the product stream, this study has concentrated only on butenes and butadiene. The yields of carbon monoxide, carbon dioxide, and water were not considered. Scenario 1, Case 1: Depletion of Oxygen in a FBRs Production of Butenes. The reactor configuration for this scenario is shown in Figure 2. Using the given rate equations and the initial conditions, that is, of pure butane with the specified oxygen concentration (i.e., partial pressure), a total operating pressure of 1 atm, and an isothermal temperature of 773 K, one can integrate the differential equations to obtain the results shown in Figure 3, where all butane and butenes concentrations are expressed in mass fractions of carbon. In Figure 3, and in subsequent figures of concentration profiles, the various points on the profiles represent the concentrations of reactant and product were the reaction to be stopped at that point, i.e., after the concomitant residence time. At initial oxygen partial pressures of 85 and 86 kPa, the reaction proceeds until, at equilibrium, all the oxygen has been depleted. When this occurs, the residual butane and butenes concentrations for an oxygen partial pressure of 85 kPa are 0.075 and 0.017, respectively. The other components present on completion of the reaction, other than butane, butenes, and butadiene, are carbon monoxide, carbon dioxide, and water. All
Figure 3. Profiles of butane and butenes at various oxygen partial pressures in a FBR.
the oxygen has been utilized in the oxidation of butane, butenes, and butadiene. If the initial partial pressure of oxygen is increased to 87 kPa, at equilibrium, all the butane, butenes, and butadiene are oxidized and there is residual oxygen present on completion of the reaction. At this initial partial pressure of oxygen, the supply of butane is the limiting factor. At oxygen partial pressures
