1. Field of Invention
The field of art to which the invention pertains is hydrocarbon, petrochemical, and chemical processing. In particular, this invention pertains to the catalysts used in certain heterogeneous catalytic processes in which the activity, selectivity characteristics, and life of the catalysts are improved by increasing the volume and diameter of the pores in the catalyst structure.
2. Description of the Prior Art
During the past 40 years there have been major advances in the catalytic conversion of one form of carbon containing molecule to another. This development has been particularly striking in the petroleum refining industry. In the early years petroleum fractions were, for example, reformed, cracked, or polymerized by thermal means under controlled conditions of temperature, pressure, and time. Today, the industry uses almost exclusively catalytic means to carry out the various transformations desired, the majority of the processes being of the heterogeneous catalytic type. In this type catalysis the reaction occurs on a solid inorganic surface in a series of steps as follows:
a. Mass transfer of the molecules from the flowing stream to the surface of the catalyst pellets.
b. Diffusion of the reactant molecules through the catalyst pores to the inner catalyst surface.
c. Reaction of the molecules on the inner surface.
d. Diffusion of the products of reaction from within the pellet, through the pores, and out to the outer surface of the pellet.
e. Mass transfer of the products of reaction from the outer pellet surface to the main flowing stream.
Items a and e, the mass transfers of feed and products between the flowing stream and the catalyst pellet have not been a serious problem, being usually controlled by proper engineering design of the reaction system, in particular the proper flowing velocity of the feed stream through the bed of catalyst pellets. In the past, most of the development work has been directed toward item c, the reaction on the inner surface. This work has been directed toward new catalyst formulations, increasing the active surface of the catalyst, and increasing the dispersion of metals which may be deposited on the surface to promote the catalytic reaction. It has been found, however, that in many reactions the type of internal surface is equally or more important than the amount of internal surface available. This has been the result of a greater understanding and appreciation of the role of items b and d, the diffusion of the feed and products within the catalyst pellet.
The most common type diffusion problem occurs when the reaction of the molecules at the catalyst surface is much more rapid than the rate of diffusion of the feed molecules through the pellet to the catalyst inner surface. Under these conditions the reaction occurs in the outer part of the catalyst pellet before the feed molecules have an opportunity to penetrate into the inner portion of the pellet. As a result the inner portion of the catalyst pellet is not being effectively used and the reaction rate is much less than one would theoretically expect. Under these conditions the reaction is commonly referred to as diffusion controlled.
There are two types of diffusion normally considered. One is Knudsen or molecular diffusion which occurs when the mean free path between intermolecular collisions is large compared to the pore diameter. The other is Bulk diffusion which occurs when the mean free path is small compared to the pore diameter. The Knudsen diffusion constant per unit cross-sectional area of the pore is given by: ##EQU1## where r is the pore radius in cm, v is the average molecular velocity in cm/sec, T is the temperature in .degree.K., and M is the molecular weight. The Knudsen diffusion rate is directly proportional to the pore radius. The Bulk diffusion coefficient in a mixture of similar mass and molecular diameter is given by: EQU D.sub.b =1/3vL
where v is the average molecular velocity and L is the mean free path in cm.
The combined Knudsen and Bulk diffusion coefficient in a given pore as recommended by A. Wheeler in "Catalysis" Vol 2, Reinhold, N.Y. 1955 is given by the formula: ##EQU2## The coefficient D.sub.c within a catalyst granule is simply the coefficient for a single pore times the number of pores per unit surface of the pellet, which reduces to: EQU D.sub.c =1/2.theta.D
where D.sub.c is the overall diffusion coefficient within a porous catalyst pellet, .theta. is the porosity, and D is the diffusion coefficient for a single pore as given above. It is apparent from the above relationships that the catalyst physical characteristics which affect diffusion within a catalyst pellet or granule are the catalyst porosity and the catalyst pore radius.
Another factor which must be considered in the formulation of a catalyst is the role of poisons on catalyst performance. Of particular concern are those conditions where the poison molecules are deposited after a few collisions with the catalyst surface. In this situation the outer pore mouths become severely poisoned while the inner surface of the catalyst remains essentially clean. As the outer pore mouths become smaller in diameter with the deposition of poisons, it is possible that diffusion through the poisoned pore mouth can become a slow process which can become rate determining. In this type catalytic process, proper formulation of the catalyst is required to minimize the deleterious effect of this poisoning of the catalyst.
There are a number of present day processes in which the diffusion problem discussed above is a factor in the performance of the catalyst. P. B. Weisz and R. D. Goodwin discussed the combustion of carbonaceous deposits within porous catalyst particles in the Journal of Catalysis 2, 397-404 [1963]. In the regeneration of 0.20 cm catalyst beads from the commercial TCC cracking process, it was demonstrated that at high temperatures the outer shell of the bead could be free of carbon whereas the center of the bead could be still black with carbon. Under these conditions the reaction rate of the burning of coke with air was greater than the rate of diffusion of the air through the pores. As a result the oxygen as it entered the catalyst particle would be consumed by reaction with coke before it could diffuse to the center of the particle.
An example of catalyst deactivation through pore mouth plugging during petroleum residuum desulfurization was given by F. M. Dautzenberg et al of Shell Research in The Netherlands in Chemical Reaction Engineering--Houston, ACS Symposium Series #65. Dautzenberg et al showed that in the processing of petroleum residua over conventional desulfurization catalysts there is a gradual deposition of the metals originally present in the oil, principally nickel and vanadium, on the catalyst. These metals gradually plugged the pores in the outer zone of the catalyst. This causes a slow loss in desulfurization activity over a longer period of time. Ultimately, the catalyst becomes totally inactive for desulfurization because the still active inner core of the catalyst has become completely inaccessible to the sulfur bearing molecules.
The effect of catalyst pore size on performance in the desulfurization of petroleum residua was also reported by K. L. Riley of the Exxon Baton Rouge Laboratories in the Preprints of the Division of Petroleum Chemistry, American Chemical Society, Vol 23 No 3 August 1978. This work showed that the reaction rate for the removal of nickel and vanadium from the petroleum residua was increased two to three fold for a two fold increase in the pore diameter of the catalyst. The desulfurization activity also increased as the pore size was increased, though the activity started to decrease at the high pore size level.
It is apparent from the above that the role of pore dimensions has been receiving increased attention in respect to their affect on catalyst performance. As a result, a number of methods have been developed to control catalyst porosity. In one of these techniques pore dimensions are controlled by varying precipitation or gelation conditions and aging of the gel with respect to concentration, temperature, and gelation agents. Typical data for commercial grades of alumina so prepared are given in Table 1.
TABLE 1 ______________________________________ Commercial Grades of Alumina Surface Pore Pore area volume diameter sq.m./gr cc/gr Angstrom ______________________________________ 350 0.43 49 245 0.60 98 200 0.77 154 ______________________________________
It is typical of conventional catalyst preparation techniques that the surface area and intrinsic activity of the catalyst decrease as the pore diameter is increased. As illustrated in the R. L. Riley reference above on the desulfurization of petroleum residua, when the pore diameter is low, the catalyst loses its high intrinsic activity very rapidly as a result of pore mouth plugging due to both carbon and metal deposition in the outer shell of the catalyst. As the pore diameter is increased, this effect is decreased and improved activity is realized. At very high pore diameters, the pore mouth plugging problem may be greatly minimized, but the effect of the low surface area and low intrinsic activity is now realized and a loss in desulfurization activity starts to be observed.
Another technique of increasing pore volume and decreasing pore diffusion problems has been to add to the hydrous catalyst precipitate before drying a dry inert type material. This apparently has the effect of increasing the amount of large pores in the catalyst. Burbidge et al in U.S. Pat. No. 3,162,607, for example, added water insoluble organic fibers to an alumina gel and after drying the organic fibers were removed by calcination at an elevated temperature. This preparation technique gives an anticipated result in that it appears quite reasonable that the removal of the fiber by calcination, solvent extraction, or some other technique would leave voids in the space previously occupied by the fiber and result in increased porosity. Improved desulfurization performance was illustrated for catalysts using the alumina prepared by this technique to increase the amount of large pores.
It is apparent from the above that catalyst pore size plays an important role where diffusion limitations and/or pore plugging is encountered. Also, a number of techniques have been developed to increase catalyst pore size and decrease pore diffusion problems. In the practice of this invention catalyst of varying pore size can be prepared by incorporating in the catalyst structure a special additive material, which technique is more effective than other known methods in increasing pore size with a minimum loss in surface area and intrinsic catalyst activity, and without the necessity to remove the additive by any special extraction technique.