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Models and Simulation of Liquid-Phase Membrane Reactors B. Park, V. S. Ravi-Kumar, and T. T. Tsotsis* Department of Chemical Engineering, University of Southern California, University Park, Los Angeles, California 90089-1211
A model of a liquid-phase membrane reactor is presented here. The model utilizes a reaction scheme and rate data applicable to the hydrocracking of California Hondo crude derived asphaltenes. Two reactor configurations have been investigated. In the first reactor configuration (configuration A) the shell-side reactor volume is kept constant by continuously pumping into the reactor pure solvent. In the second reactor configuration (configuration B) no additional solvent is added to the reactor shell side, whose volume, as a result, decreases as the solvent and solutes diffuse across the membrane from the shell side to the tube side. The model has been utilized to identify optimal reactor and process conditions and membrane characteristics, which maximize conversion and product yields. Introduction Membranes have been used in reactive applications to enhance the yield/ selectivity of gas-phase reactions (for a review see Sanchez and Tsotsis, 1996). Relatively fewer recent investigations have reported the use of membranes to enhance the yield of liquid-phase reactions. Most of these studies involve pervaporation applications, and the majority have been reviewed in a recent publication (Zhu et al., 1996). One potentially interesting liquid-phase application involves the use of high-temperature membranes in reactive processes involving petroleum and coal liquid macromolecules. Membrane reactors show promise in this area because of the presence of difficult separation problems (for example, separating unreacted coal and mineral matter from valuable liquid fuel products) together with selectivity limitations due to retrogressive reactions and coking. Membrane reactors, which combine reaction and separation in a single unit, thus creating a strong synergy, offer some potential advantages here. Preliminary experimental investigations, for example, of coal thermal dissolution in a high-temperature membrane reactor (Yang et al., 1997), have shown promising results. In this case the membrane acts as an ultrafiltration device, which completely separates the unreacted coal and mineral matter from the valuable liquid fuel products. Another potential application area is the hydrocracking/hydroprocessing of heavy petroleum liquids to useful products. A model compound that has been used to represent such liquids is asphaltenes, a solubility class of components, which contain a large fraction and the most refractory of the heteroatoms found in the original liquid. Their hydrocracking reaction to useful products (lumped together here as the “maltenes” fraction) is limited by the low reactivity of the inert asphaltene cores and retrogressive reactions to coke and gas. Kinetic evidence exists, furthermore (Blazek and Sebor, 1993; Koseoglu and Phillips, 1988; Ravi-Kumar, 1996; Soodhoo and Phillips, 1988a,b; Wiehe, 1992), that the maltenes themselves, through either condensation or * Author to whom correspondence is addressed. Phone: 213740-2225. Fax: 213-740-8053. E-mail:
[email protected].
hydrogen abstraction reactions, produce asphaltene-like molecules, thus creating conditions akin to a pseudoequilibrium limited reaction. This being the case, one expects the reaction to potentially shift to higher conversions, as a result of the preferential removal of maltenes. Inorganic membranes, with their narrow pore size distributions and high thermal stability, can be used to remove maltenes preferentially over asphaltenes and to completely exclude coke and the other products of retrogressive reactions; thus, they offer the potential for application in the development of efficient membrane reactors. As in the case of gas-phase reactions, however, in the choice of membranes one must strike a fine balance between permeance and permselectivity. During reactor operation high permeances generally imply significant reactant loss and diminished reactor yields. Membranes with low permeances and high permselectivities, on the other hand, limit reactant loss but only provide a limited shift in equilibrium conversion. In practice, furthermore, they are more vulnerable to plugging and further permeance decrease. The performance of such membrane reactors is expected to be a strong function of operating conditions and membrane characteristics. Preliminary experimental investigations with petroleum asphaltene hydrocracking have so far shown moderate improvements in reactor yield (Ravi-Kumar, 1996). To further improve the conversion and yield of such reactors, one must optimize their performance through the use of appropriate reactor design models. The development of such models and their validation by experimental data have been the focus of our investigations. Some preliminary results are presented here. These models describe the behavior of the reactor system utilized in our laboratory. They are also generic enough, however, to capture the basic behavior of other more complex reactor systems. Reactive separations for the processing of petroleum and coal liquids utilizing membranes have yet to become a commercial reality, and they probably will not be until many of the same problems that have so far plagued the commercialization of all other reactive separations involving high-temperature inorganic membranes are resolved. Such problems include the questionable op-
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Figure 1. Schematic of the membrane reactor system.
erational robustness of membrane materials and seals and the high cost of reactor systems and membranes. These problems notwithstanding, such processes remain of current interest. The diminishing petroleum resources are shifting the emphasis of hydroprocessing onto heavier crudes and resids. The efficient processing of the heavier components of these materials (i.e., asphaltenes) still remains a challenge. Even more challenging (and from an environmental standpoint more important) remains the separation of valuable products from unreacted reactants and the undesirable products of retrogressive reactions. High-temperature inorganic membrane based reactive separations appear to have an important role to play in this area. In what follows, the membrane reactor system used in the experimental investigations is first briefly described. Subsequently, the reactor models are presented and discussed. These models can be used to identify membrane and process conditions under which the desired product yields can be significantly enhanced. For the sake of conciseness only a limited discussion of the basic model features is presented here. Further, more detailed discussions can be found in a recently completed doctoral thesis (Ravi-Kumar, 1996).
The Reactor System The development of the reactor models that are presented here has been motivated by experiments involving asphaltene hydrocracking. The asphaltenes were isolated from a Hondo California resid (UNOCAL, Feed No. F4272) utilizing n-heptane and a procedure described elsewhere (Ravi-Kumar, 1996; Dolbear and Phan, 1988). The solvent used for the reactor experiments is tetralin. The membranes used are commercial multilayered, composite porous alumina tubes (ALCOA Membralox) of inner diameter 7, and outer diameter 10 mm, and 7 cm length. The first layer, inside the tube, has a fairly monodisperse pore structure with a diameter of 18 Å), there is significant asphaltene loss through the membrane and the membrane has a negative effect on the asphaltene conversion and maltene. For this case increasing pressure gradients lead to decreasing conversions and yields. For intermediate pore sizes (∼16 Å) the maltene yield shows a maximum at around 5 atm; at higher pressure gradients, the yield decreases due to the loss of asphaltenes through the membrane. The above points are made clearer by looking at Figures 5 and 6, which show several 2D cross sections through the 3D surfaces of Figure 4 along the pore diameter (Figure 5) and pressure (Figure 6) coordinates. Asphaltene conversion and maltene yields at a different temperature (300 °C) are shown as a function of pore diameter and pressure in Figure 7 for reactor configuration A. A similar asymptotic effect of pressure and the existence of an optimum pore size can be clearly observed. At this lower temperature, as expected (see Table 1) the ultimate asymptotic reactor conversions in
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Figure 5. Effect of pore size on asphaltene conversion and maltene weight fraction yield at 350 °C for different values of the pressure gradient. The dotted line is for the conventional reactor.
Figure 6. Effect of pressure gradient on asphaltene conversion and maltene weight fraction yield at 350 °C for different values of the pore size. The dotted line is for the conventional reactor.
the conventional reactor are lower than those attained at the higher temperature (Figure 4). The membrane reactor, on the other hand, shows significantly higher relative gains in conversions and yield over the corresponding conventional reactor values than those observed at a temperature of 350 °C. For example, up to 40% relative increase in the maltene weight fraction yield can be observed in Figure 7. For comparison with results observed with configuration A, Figure 8 shows the reactor behavior for configuration B. In this case we utilized the same membrane with configuration A but the shell side was filled with sufficient solution to prevent it from drying during the 200 h. The reactor volume was taken to be 20 times larger than that of configuration A. Absent from Figure 8 are the asymptotic effects of the pressure gradient on the asphaltene conversion and maltene yield shown in Figure 4. For small pore sizes (
