The present invention relates to the hydrotreatment of hydrocarbons. It particularly relates to a method whereby hydrocarbon feed stocks can be hydrotreated in a more economical and facile manner while maintaining a high product quality and quantity. It specifically relates to a method for recovering hydrogen gas from a hydrotreatment process effluent stream for recycle to the process reaction zone or for use in other hydrogen-consuming reactions with a purity which is improved over that obtainable with conventional processes.
It is wel known in the prior art that high quality gasoline boiling range products, such as aromatic hydrocarbons, e.g., benzene, toluene, and xylene, may be produced by the catalytic reforming of naphtha-containing feedstocks, utilizing a platinum-containing catalyst, in the presence of hydrogen to convert at least a portion of the feedstock into aromatic hydrocarbons. One of the predominant reactions in catalytic reforming involves a dehydrogenation of naphthenic hydrocarbons. The dehydrogenation function provides a net excess of hydrogen from the process which is available for other uses, such as hydrodesulfurization reactions, and the like. A considerable portion of the produced hydrogen, however, is required for recycle purposes in order that a proper partial pressure of hydrogen may be maintained over the catalyst in the catalytic reforming zone.
However, the catalytic reforming reaction also involves a hydrocracking function which segments hydrocarbons into relatively low molecular weight hydrocarbons, e.g., normally gaseous hydrocarbons, such as methane, ethane, propane, butane, etc., and in particular, C.sub.2 + hydrocarbons which then become contaminants in the gaseous hydrogen which is separated from the effluent of the reaction zone. These contaminants have the effect of lowering the hydrogen purity to such an extent that frequently external purification techniques must be used by those skilled in the art before the net hydrogen from the reformer can be used in other chemical reactions requiring relatively high purity hydrogen. Low hydrogen purity also has a significant effect in the reforming reaction by way of requiring considerable quantities of such low purity hydrogen in order to maintain the hydrogen partial pressure in the reaction zone at the proper level, as previously mentioned.
As those skilled in the art are familiar, the reforming reaction must have a hydrogen atmosphere in order for the various desired reactions to take place. This means that the separated hydrogen gas referred to above must, to a considerable extent, be returned to the catalytic reforming zone. Due to the large pressure drop through a conventional catalytic reforming system, typically comprising a plurality of catalytic reactors and separation vessels, the separated gas for recycle purposes must be compressed to at least the pressure of the reaction zone before it can be returned and properly used. Heretofore, the size of the hydrogen gas compressed has been a significant cost factor in constructing and operating catalytic reforming units for the production of gasoline boiling range products, such as benzene, toluene, and xylene. In other words, the large horsepower requirement for the recycle compressor is a substantial capital investment item and a substantial operating cost item for any catalytic reforming unit.
Moreover, due to current federal environmental regulations, there has been a trend in the catalytic reforming technology towards operating catalytic reforming processes with high severities. With high severity reforming operations, the problem presented by hydrogen purity is decreased by the increased hydrocracking function which accompanies high severity reforming conditions. This increased hydrocracking activity generates significant increases in the concentration of low molecular weight hydrocarbons. These high concentrations of low molecular weight hydrocarbons cause the hydrogen recycle gas purity to decrease and consequently increase the quantities of recycle gas necessary to maintain the hydrogen partial pressure in the reaction zone at the desired level. Increased quantities of the recycle gas must therefore be compressed. Accordingly, with high severity reforming operations, the problem of low hydrogen gas purity is rendered even more acute. It would be desirable, therefore, to provide a method for hydrotreating hydrocarbon feedstocks whereby relatively high purity hydrogen may be produced, not only for recycle purposes but also for other uses outside the hydrotreating reaction.
Heretofore, several attempts have been made to develop a method for purifying the make hydrogen present in a hydrotreating process effluent stream. Typically, this result has been attempted by removing from the hydrotreating process effluent stream a relatively impure hydrogen-containing gaseous stream, compressing this gaseous stream, admixing it with a liquid hydrocarbon stream to absorb therefrom some of the gaseous hydrocarbons, and then removing therefrom in a second separation an enriched hydrogen-containing gaseous stream. For example, in U.S. Pat. No. 3,431,195, a catalytic zone effluent stream is separated in a low pressure gas-liquid separation into an impure hydrogen-containing gaseous stream and a normally liquid hydrocarbon stream. The hydrogen-containing gaseous stream is then compressed and admixed with the liquid hydrocarbon stream from the low pressure separation in order to remove therefrom some of the gaseous hydrocarbons. This admixture is then subjected to a high pressure separation, producing an enriched hydrogen-containing gas for recycle to the catalytic reforming zone. Similarly, in U.S. Pat. No. 3,706,655, an impure hydrogen-containing gaseous stream is removed from the reforming zone effluent stream in a low pressure separation, compressed, admixed with the reformer hydrocarbon feedstock, and then subjected to a high pressure separation wherein a gaseous stream of increased hydrogen content is produced.
In a variation of the above described prior art processes, U.S. Pat. No. 3,520,799 describes a hydrogen purification process wherein a portion of the hydrogen-containing gaseous stream recovered in the high pressure separation is passed into an absorber column wherein it is countercurrently contacted with a C.sub.6 + bottoms material from the reforming system stabilizer column whereby further gaseous hydrocarbons are removed from the hydrogen-containing gas stream. The resultant higher purity hydrogen stream is then cooled, and subjected to an additional gas-liquid separation to produce a net hydrogen product for use in other refinery units. However, while this method produces a net hydrogen product of increased purity, the hydrogen gas recycled to the catalytic reforming zone is not subjected to these additional purification steps. Consequently, the hydrogen-containing gas stream recycled to the catalytic reforming zone contains substantial quantities of gaseous hydrocarbons which increase operating costs, particularly in high severity reforming operations. Moreover, in order to obtain the net hydrogen product of improved purity, the method of U.S. Pat. No. 3,520,799 requires subjecting the catalytic reforming zone effluent stream to a complex series of purification steps involving three gas-liquid separations, a gas-liquid absorption, and a fractionation, necessitating a substantial equipment capital investment.
In another variation of the method for purifying make hydrogen described in U.S. Pat. No. 3,431,195 and No. 3,706,655, U.S. Pat. No. 3,882,014 describes a hydrogen enrichment method wherein instead of a high pressure gas-liquid separation, the relatively impure hydrogen gas-containing gaseous stream recovered from the low pressure gas-liquid separator is countercurrently contacted in a contacting-condensation column with a descending stream of cooled fractionation zone liquid bottoms material. This contact produces partial condensation and selective absorption of the upwardly flowing gaseous stream, with a concomitant hydrogen enrichment. However, while substitution of this contacting-condensation step for the high pressure gas-liquid separation of the aforementioned patents produces a hydrogen recycle gas with increased purity, this method does not effect sufficient reduction in the low molecular weight hydrocarbons present in the hydrogen-containing gas stream to allow satisfactory use in a high severity reforming operation.
In view of the current necessity for reforming operations to operate at higher and higher severities, the complexity of the above prior art processes and/or the low purity of the hydrogen recycle gas obtained therefrom renders their use undesirable. Accordingly, there is a great need in the art for a method for hydrotreating hydrocarbon feedstocks whereby make hydrogen can be purified in an economical and facile manner, and which achieves a significant reduction in the concentration of low molecular weight hydrocarbons present in the hydrogen recycle gas.