Catalyst from a fluid catalyst cracking (FCC) unit is regenerated because it is contaminated with coke ("coked up") in an operating fluid bed catalytic cracker. An FCC regenerator operates at very high temperatures due to the high heat release of burning coke. The capacity of many FCC units is limited by the regenerator operating at a temperature which approaches the limit beyond which the regenerator may not be operable because of its metallurgy. Hot regenerated catalyst (regen catalyst) is conventionally cooled in a catalyst cooler ("catcooler") by generating steam. The catcooler may be either internal or external. In this invention we cool the regen catalyst in an external catalyst cooler (ECC) which also functions as a dehydrogenation reactor to which alkanes are fed.
More specifically, this invention relates to coupling the dehydrogenation of a lower C.sub.2 -C.sub.6 alkane, preferably propane (C.sub.3p) and butane (C.sub.4b), and most preferably propane, with the operation of a FCC cracker and its regenerator, in the specific instance where the temperature of operation of the regenerator permits using the FCC catalyst as an effective propane dehydrogenation agent.
As will hereinafter be described and substantiated, the dehydrogenation reaction occurs in the ECC due to thermal catalytic cracking which is partly a pyrolytic thermal reaction, referred to herein as thermal dehydrogenation, and partly a dehydrogenation catalytic effect of the FCC catalyst. Since it is the FCC catalyst which is responsible for the dehydrogenation, we refer to it as "dehydrogenation catalyst" or "ECC catalyst" when it is in the ECC, just as we refer to the catalyst being regenerated as "regen catalyst", though it is only being regenerated. The thermal dehydrogenation of normally liquid hydrocarbons at a temperature in the range from 538.degree. C. to 750.degree. C. (1000-1382.degree. F.) by pyrolysis in the presence of steam, is disclosed in U.S. Pat. Nos. 3,835,029 and 4,172,816, inter alia, but there is no suggestion that such a reaction may be used as the basis for direct heat exchange, to cool regen catalyst in an ECC for a FCC unit.
At the present time, there exists a profusion of schemes for dehydrogenating propane, eventually, for the most part, converting it to gasoline or other products far more valuable than propane. For example, U.S Pat. No. 4,293,722 to Ward et al, teaches one such process. These schemes are unequivocally based on the catalytic effect of particular catalysts and rely on reactions which occur at substantially lower temperatures than those used in our process.
Still more specifically, this invention relates to an improvement in cooling regen catalyst, which improvement involves operating a fluid bed of a conventional FCC catalyst in an ECC under conditions such that the catalyst is cooled while it performs its alkane dehydrogenating function, and having done so, may be returned from the ECC to the regenerator. Conditions of operation of the bed of catalyst in the ECC (ECC bed) is closely tied to the amount of coke deposited during operation of the bed (cracker bed) of FCC catalyst in the cracker, which in turn dictates the amount of heat which will be generated in the bed (regen bed) of catalyst being regenerated in the regenerator. The particular conditions of operation of each of the three fluid beds will be decreed by the mass and energy balances required to accomplish what we have discovered may be done to improve the economics of operation of the FCC section of an oil refinery.
The FCC process converts petroleum feedstocks in the gas oil boiling range to lighter products such as gasoline. Though a wide variety of catalysts may be used in the cracker, most preferred is a zeolite cracking catalyst with a proclivity to be deactivated when coked up. This requires that much coke be removed from the catalyst when it is to be regenerated. As a result, regenerators are designed to be "hot-operated" and under pressure, that is, operated at a pressure in the range from about 25 psig to 40 psig, and as high a temperature as is practical from a materials standpoint. The temperature within a regenerator typically ranges from about 538.degree. C. to about 815.degree. C. (1000.degree.-1500.degree. F.) and the ECC operates in our process, in the same general range of pressure and temperature.
As stated, heat generated in a conventional regenerator is typically removed by internal coils (regen coils) functioning as an internal catcooler, or, by an ECC (external catcooler) in which hot catalyst is contacted with cooling fluid in heat exchanger tubes. Prolonged operation of a regenerator at a temperature at which the catalyst's efficiency is not deleteriously affected bestows upon the operation of any catcooler, a criticality which demands near-absolute reliability of operation. Because such reliability has been so well established by tubes carrying cooling fluid, a heat exchanger is a logical choice, and has been for many years. Because of the relatively high temperature, in the range from about 621.degree. C. to about 732.degree. C. (1150.degree.-1350.degree. F.), at which a large volume of regenerated catalyst must be returned to the cracker of an operating refinery, it will be evident that the drop in catalyst temperature due to cooling it, cannot be large, but the amount of heat to be removed is very large. This makes the generation of steam a logical choice. Since steam generation by indirect heat exchange with boiler feed water carried in tubes, is such a `perfect fit`, the practical onus of cooling regen catalyst by direct heat exchange which is necessarily inflexibly tied to the operation of an unrelated dehydrogenation reaction carried out in yet another (third) fluid bed, if ever given even cursory consideration, must understandably have been viewed with a lack of enthusiasm.
Since the dehydrogenation reaction is endothermic, by providing a fluid bed ECC we have provided a catalyst cooling chamber for removing enough heat from the system to compensate for the large amount generated by the regeneration of the catalyst.
Prior art FCC regenerators with catcoolers are disclosed in U.S. Pat. Nos. 2,377,935; 2,386,491; 2,662,050; 2,492,948; and 4,374,750 inter alia. All these prior art catcoolers remove heat by indirect heat exchange, typically a shell and tube exchanger. None removes heat by direct heat exchange, for example, by continuously diluting hot regenerated catalyst with cold catalyst, or by blowing cold air through the hot catalyst; more particularly, none removes heat by functioning as a reactor which supplies heat to an endothermic reaction.
U.S. Pat. No. 4,422,925 discloses the step-wise introduction of ethane, propane, butane, recycle naphtha, naphtha feed, raffinate naphtha, and fractionator bottoms recycle in the riser reactors of a FCC unit. In the riser reactors, the lower alkanes are contacted, in a transport zone, with hot regenerated catalyst which would dehydrogenate the alkanes, progressively decreasing the temperature of the suspension of catalyst and hydrocarbons as they progress upwards through the risers. The mixture of catalyst and reaction products is then contacted with a hydrocarbon feedstock suitable for catalytic cracking, such as virgin naphtha, virgin gas oil, light cycle gas oil, or heavy gas oil. (see col 2, lines 29-33). Clearly, the control of the catalyst temperature in the risers as well as the benefits of dehydrogenation occurring in the risers were both lost when the products from the risers were mixed with the products of the main cracker. Most important is that operation of the reference FCC unit as a combination dehydrogenator and cat cracker failed to provide control of the olefins generated, which control is essential if recovery and subsequent utilization of the olefins is a goal of the process.
The concept of cooling hot regenerated catalyst by using an endothermic reaction, specifically the catalytic dehydrogenation of butane, was disclosed more than four decades ago in U.S. Pat. No. 2,397,352 to C. E. Hemminger. Though unrelated to operation of a FCC unit, regeneration of the catalyst was required before it was returned to the dehydrogenation reactor. He provided a catalyst (chromic oxide supported on alumina or magnesia) heating chamber for supplying heat to the dehydrogenation reaction to compensate for that lost in the dehydrogenation reaction, and to preheat, at least in part, the butane to raise its temperature to reaction temperatures.
Since the disclosure of this old process, the use of large pore zeolites for cracking catalysts was discovered, as was the effectiveness of certain large and intermediate pore zeolites for the conversion of alkanes to olefins. Our process achieves at least 50%, and preferably 70% conversion of propane, and for the first time, makes the process practical in a refinery environment.
Though conceptually feasible, the '352 system required pressurizing catalyst powder which was to be recirculated. The result was that the catalyst did not recirculate, and over the years, improving upon the concept had been neglected. Particularly because the generation of steam is generally desirable in a typical refinery, and this could be done both reliably and economically with a conventional indirect heat exchanger, the teaching of the '352 was never related to cooling FCC catalyst. The application of the concept of cooling FCC catalyst in an ECC was more fortuitous than by design. It happened that the temperature for dehydrogenation of lower alkanes matched the temperature at which a large pore zeolite cracker catalyst is to be returned to the regenerator. Moreover, we found that the recirculation problem evaporated when the alkane was prevaporized and mixed with the catalyst to be recirculated before the catalyst was introduced into the dehydrogenation chamber.
Though other dehydrogenation processes are available, we have found that the "third bed" catalytic dehydrogenation of propane, tied to the operation of a FCC cracker and regenerator, provides numerous advantages. In addition to the desirable upgrading of ethane (C.sub.2p), propane (C.sub.3p) and butane to ethylene (C.sub.2 =), propylene (C.sub.3 =) and butylenes (C4=), no prior art process teaches the steps of our process which: (1)affords such flexibility in the design of the ECC `third bed` for optimum weight hourly space velocity (WHSV), and control of the dehydrogenation temperature so as to get maximum conversion; (2)affords flexibility in controlling the temperature at which the FCC regenerator operates so as to facilitate processing heavier than conventionally used feedstock in the FCC; (3) does not increase the throughput to the FCC reactor and main column system; (4) uses the FCC regenerator to burn coke made during alkane dehydrogenation; (5) eliminates internal regen coils for steam regeneration; and, (6) eliminates the ECC air compressor used to provide fluidization air to a conventional ECC catcooler, because hot propane gas provides the fluidization; and (7) minimizes undesirable thermal cracking in the FCC reactor by reducing the temperature of catalyst fed to the FCC reactor, as more fully described in a particular embodiment in which spent catalyst from the ECC is directly fed to the FCC reactor.