Energy & Fuels 2002, 16, 1599-1600
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Communications Improved Methods for Conversion of Our Fossil Resources to Fuels Theodore B. Simpson U.S. Dept of Energy, ME-222 Germantown Building, 1000 Independence Avenue SW, Washington, D.C. 20585-1290 Received June 24, 2002 In the last thirty years, only the so-called Direct Coal Liquefaction1 or the Indirect Coal Liquefaction2 processes have been significantly studied. While the two approaches provided transportation fuel products that fully met currently proposed specifications, excessive cost and the risk in the commercializing of new technology discouraged domestic application. Future improvements to the Indirect process will likely be limited to catalyst improvement or process engineering developments. These are not expected to be significant improvements. Hence, Direct Liquefaction and related approaches are the areas of interest here. The following eight potential improvements have been identified by this author or by the other authors cited. (1) U.S. Patent 6,054,0433describes a novel form of direct liquefaction that claims higher yields of volatile transportation-range fuels with a lower capital cost and lower hydrogen requirement. Since the cost of hydrogen is presently approximately half of the cost of product, this is a significant advance. It has been demonstrated commercially that liquids, optionally containing catalyst, can be sprayed onto hot fluidized or moving beds of particulates at a selected temperature without causing the bed to bog down. Here an inert particulate with or without a catalyst on its surface is used. The fine droplets of spray deposit and spread on the surface of the particulate and react with the reducing gas used also as the fluidizing means until they convert to lighter hydrocarbons and are stripped from the particulate surface by the upflowing reducing gas. The efficient heat transfer capability of the fluidized or moving bed almost instantaneously brings the feed to bed temperature and thereafter spreads the large heat of reaction of the reduction reaction so that it can be removed from the bed by conventional means. Clearly, staged reactors can be used to give a degree of plug flow reaction of the feed. As a further improvement, when the feed droplet has reacted so as to become volatile, the product evaporates off at a temperature below its boiling point as a result of the fluidization gases stripping it off and carrying it overhead as it is formed. Accomplishing this has always been an objective of the designer of a coal or resid liquefaction reactor, for under the severe conditions of coal (1) Comolli, A. G.; et al. DOE Report DE-92148-TOP-09, 2000, January. (2) Zhang, Y. Q.; Davis, B. H. Catalysis 2000, 15, 138-189. (3) Simpson, T. B. U.S. Patent 6,054,043, 2000.
liquefaction the volatiles held in the reaction bed tend to retrogress to products that are unlikely to be liquefied again. In this type of process, unreactive organic (nearly pure carbon) catalyst, if any, and particulate can be recovered for reuse. In the process when the feed is coal, a prior step is a vigorously agitated, short-contact-time reaction vessel sparged with a reducing gas such as hydrogen and fed with some form of dry coal. If the feed is a petroleum resid, this first step is not necessary. (2) Reference 2 points out the need for such features as result from (item 1) above, although these needs are not pointed out by its author. The cause is that conventional direct liquefaction is extremely mass transfer limited. The test results given show this in that when the hydrogen is diluted (by the steam from the water added) the conversions decrease unless the water added improves the activity of an added catalyst. Also, he finds that even addition of solvent vs no solvent reduces conversion, presumably due to adding resistance to hydrogen transfer to the coal. (3) Reference 5 provides another significant guideline to providing the ideal conditions for direct liquefaction. It shows that the conditions required for virtually instant conversion of particulate coal feed into a liquid reaction medium are the following: (a) adequate presence of active hydrogen, (b) near instant heat-up to a temperature, and (c) agitation high enough to provide the desired kinetics. If, on the other hand, these requirements are not met, the coal remains particulatesin fact, it grows somewhat in size. Reference 6 supports these findings and adds that high hydrogen pressure and presoaking the coal in the solvent increase reaction rates. A conclusion is that reaction conditions and contacting must be ideal in order to achieve optimum yields. Item (1) above is believed to provide an optimum means of providing such ideal reaction conditions. (4) Clearly, in the slurry, bubble-column-type reactor, vaporization of the volatile products of liquefaction that are formed is limited to the gas bubble-to-liquid slurry contact area. Here, too, the reducing gas partial pressure reduces the partial pressure of the volatiles that is required to allow them to escape, and the rate of escape is a function of this surface area. By comparison, this
10.1021/ef0201432 CCC: $22.00 © 2002 American Chemical Society Published on Web 10/11/2002
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surface area is huge in the case of the reactor design described in item (1) above. (5) Plug flow of reactants is a desirable attribute. Without plug flow the reactor is to some degree fully mixed, and as a result the product stream contains some concentration of fresh feed. In this respect, the slurry, bubble-column reactor is not a good choice. The apparatus of item (1) above has somewhat the same problem. This leads to making the reactor coke-bottle-shaped 7sa design which provides fluidization in the constricted top section and a moving bed in the lower section. In this way the sprayed feed is successfully deposited on the fluidized particulate, which converts to a moving bed and to slug flow in the lower section where the diameter is greater. Solids are recycled to the top from either the bottom of the reactor or from the particulate regenerator to complete the cycle. It may be desirable to provide horizontal mixers in the lower bed. (6) It is not known whether bench or PDU tests have been made of feeding a dry or nearly dry coal feed to a first stage of direct liquefaction. Developing a means to do so should not be a particularly difficult objective. Doing so would eliminate the cost of the recycle loop. It would also increase the overall degree of plug flow. It would encourage consideration of an overall process design in which the most unreactive portion of the exit stream is separated and used for gasification rather than being recycled in hopes of achieving its ultimate extinction. (7) Oil agglomeration de-ashing is one of the most effective ways of reducing ash in the feed coal. The agglomeration procedure also offers one of the most promising opportunities to add catalysts to the feed coal.3 This opportunity occurs when the oil for agglomeration is added to the ground-coal/water slurry to cause the agglomeration. This oil that causes agglomeration has been found to form a continuous thin film on the surface (4) Song, C.; et al. Proc. 14th Ann. Int. Pitt Coal Conf., 1997, Paper No. 8-4, Sept. 23-27. (5) Guin, S.; Tarrer, A.; et al. Ind. Eng. Chem., Process Des. Dev. 1976, 15 (4), 490-494. (6) Shaw, J. M.; et al. ACS 205th National Meeting, Division of Petroleum Chemistry, Symposium on Coal and Oil, Denver, 1993; pp 346-350. (7) Simpson, T. B. DOE Invention Disclosure, 2002, June 1.
Communications
of the individual ground-coal particles and to cause the agglomeration. If the catalyst is first dissolved in the oil, it should deposit on the coal as part of this continuous film. This offers the promise of being a procedure that is superior to the widely used incipient wetting technique, wherein a water solution of the catalyst is used to wet the surface of the coal. In this case the wet coal is then dried, and the catalyst-water solution doubtless draws up into beads as it dries and finally becomes particles of catalyst on the surface of the coal that are small but nowhere near the near uni-molecular thickness that should result in the proposed agglomeration. (8) The United States has large reserves of oil shale. Strenuous efforts have been made to develop processes for the economical production of a crude oil from oil shale, but without success because of the cost of the overall processing needed to produce specification product in an environmentally acceptable manner. One of the processes that was studied by Occidental Petroleum involved underground retorting to drive off and produce the product oil. Utilizing existing know-how, Occidental used explosives to break the selected underground volume to be produced into a rubble much like a reactor containing crushed ore. They then excavated to permit the introduction of combustion air and to provide a shaft up through which the combustion gas and the produced oil vapors could flow to the surface for clean-up and condensation of the product. Here the proposed approach8 is to use similar underground preparations and instead to gasify the shale to obtain a syn gas that would readily flow to the surface for Fischer Tropsch process conversion to clean transportation fuel or for other syn gas conversion options. In conclusion, each of the above process procedures or combinations of them is believed to offer significant process advantages and/or cost savings. This author recommends their further study in order to confirm their promise in hopes of improving the energy security of the United States. EF0201432 (8) McCarthy, H. E.; et al. AIChE Symp. Ser. 1976, 72 (155), 14-23.
