Environmental concerns, especially with sulfur oxides and nitrogen oxides emissions, have led petroleum refiners to depend more heavily than in the past on hydrodesulfurization and hydrocracking processes. Availability of by-product hydrogen from naphtha reforming no doubt has also cooperated to foster this dependence. Other factors too, have come into play to make hydroprocessing of increasing importance. Among these factors is that high quality crude oils for lube and fuels refineries are expected to progressively become more scarce. Also, refineries that include a fluid catalytic cracking (FCC) plant generate large volumes of dealkylated, aromatic refractory effluents, commonly known as FCC Cycle Oils. Decrease in demand for the fuel oil products into which these FCC Cycle Oils were previously incorporated has to the practice of working them off by incorporation with a hydrocracker feedstock. The hydrocracking process, unlike catalytic cracking, is able to effectively upgrade these otherwise refractory materials.
Hydrocracking is an established petroleum refining process. The hydrocracking feedstock is invariably hydrotreated before being passed to the hydrocracker in order to remove sulfur and nitrogen compounds as well as metals and, in addition, to saturate olefins and to effect a partial saturation of aromatics. The sulfur, nitrogen and oxygen compounds may be removed as inorganic sulfur, nitrogen and water prior to hydrocracking although interstage separation may be omitted, as in the Unicracking-JHC process. Although the presence of large quantities of ammonia may result in a suppression of cracking activity in the subsequent hydrocracking step, this may be offset by an increase in the severity of the hydrocracking operation.
In the hydrotreater, a number of different hydrogenation reactions take place including olefin and aromatic ring saturation but the severity of the operation is limited so as to minimize cracking. The hydrotreated feed is then passed to the hydrocracker in which various cracking and hydrogenation reactions occur.
In the hydrocracker, the cracking reactions provide olefins for hydrogenation while hydrogenation in turn provides heat for cracking since the hydrogenation reactions are exothermic while the cracking reactions are endothermic; the reaction generally proceeds with generation of excessive heat because the amount of heat released by the exothermic hydrogenation reactions usually is much greater than the amount of heat consumed by the endothermic cracking reactions. This surplus of heat causes the reactor temperature to increase and accelerate the reaction rate, but control is provided by the use of hydrogen quench.
Conventional hydrocracking catalysts combine an acidic function and a hydrogenation function. The acidic function in the catalyst is provided by a porous solid carrier such as alumina, silica-alumina, or by a composite of a crystalline zeolite such as faujasite, Zeolite X, Zeolite Y or mordenite with an amorphous carrier such as silica-alumina. The use of a porous solid with a relatively large pore size in excess of 7A is generally required because the bulky, polycyclic aromatic compounds which constitute a large portion of the typical feedstock require pore sizes of this magnitude in order to gain access to the internal pore structure of the catalyst where the bulk of the cracking reactions take place.
The hydrogenation function in the hydrocracking catalyst is provided by a transition metal or combination of metals. Noble metals of Group VIIIA of the Periodic Table, especially platinum or palladium may be used, but generally, base metals of Groups IVA, VIA and VIIIA are preferred because of their lower cost and relatively greater resistance to the effects of poisoning by contaminants (the Periodic Table used in this specification is the table approved by IUPAC as shown, for example, in the chart of the Fisher Scientific Company, Catalog No. 5-702-10). The preferred base metals for use as hydrogenation components are chromium, molybdenum, tungsten, cobalt and nickel; and, combinations of metals such as nickel-molybdenum, cobalt-molybdenum, cobalt-nickel, nickel-tungsten, cobalt-nickel-molybdenum and nickel-tungsten-titanium have been shown to be very effective and useful.
One characteristic of the conventional hydrocracking catalysts is that they tend to be naphtha directing, that is, they tend to favor the production of naphthas, typically boiling below about 165.degree. C. (about 330.degree. F.) rather than middle distillates such as jet fuel and diesel fuel, typically boiling about 165.degree. C. (about 330.degree. F.), usually in the range of 165.degree. to 345.degree. C. (about 330.degree. to 650.degree. F.). However, the yield of middle distillates may be relatively increased by operating under appropriate conditions. For example, U.S. Pat. No. 4,435,275 to Derr et al. describes a process for producing low sulfur distillates by operating the hydrotreating-hydrocracking process without interstage separation and at relatively low pressures, typically below about 7000 kPa (about 1000 psig). The middle distillate product from this process is an excellent low sulfur fuel oil but it is generally unsatisfactory for use as a jet fuel because of its high aromatic content; this high aromatic content also makes it unsuitable for use as a diesel fuel on its own but it may be used as a blending component for diesel fuels if other base stocks of higher cetane number are available. Conversion is maintained at a relatively low level in order to obtain extended catalyst life between successive regenerations under the low hydrogen pressures used. Relatively small quantities of naphtha are produced but the naphtha which is obtained is an excellent reformer feed because of its high cycloparaffin content, itself a consequence of operating under relatively low hydrogen pressure so that complete saturation of aromatics is avoided.
The use of highly siliceous zeolites as the acidic component of the hydrocracking catalyst will also favor the production of distillates at the expense of naphtha, as described in U.S. Patent Application Ser. No. 744,897 now abandoned, filed 17 June 1985 and its counterpart EU 98,040 to La Pierre et al.
In conventional hydrocracking processes for producing middle distillates, especially jet fuels, from aromatic refinery streams such as catalytic cracking cycle oils, it has generally been necessary to employ high pressure hydrotreating typically about 2000 psig to saturate the aromatics present in the feed so as to promote cracking and to ensure that a predominantly paraffinic/-naphthenic product is obtained. The hydrocracked bottoms fraction is usually recycled to extinction even though it is highly paraffinic (because of the aromatic-selective character of the catalyst) and could form the basis for a paraffinic lube stock of higher value than the distillate produced by cracking it. Thus, the conventional fuels hydrocracker operating with a cycle oil feed not only is demanding in terms of operating requirements (high hydrogen pressure) but also degrades a potentially useful and valuable product.
A significant departure in hydrocracking is described in U.S. Patent Application Ser. No. 379,421 now abandoned and its counterpart EU 94,827. The catalyst used in the process is Zeolite Beta, a zeolite found to have a combination of unique and highly useful properties. Zeolite Beta, in contrast to conventional hydrocracking catalysts, has the ability to attack paraffins in the feed in preference to the aromatics. The effect of this is to reduce the paraffin content of the unconverted fraction in the effluent from the hydrocracker so that it has a relatively low pour point. By contrast, conventional hydrocracking catalysts such as the large pore size amorphous materials and crystalline aluminosilicates previously mentioned, are aromatic selective and tend to remove the aromatics from the hydrocracking feed in preference to the paraffins. This results in a net concentration of high molecular weight, waxy paraffins in the unconverted fraction so that the higher boiling fractions from the hydrocracker retain a relatively high pour point (because of the high concentration of waxy paraffins) although the viscosity may be reduced (because of the hydrocracking of the aromatics present in the feed). The high pour point in the unconverted fraction has generally meant that the middle distillates from conventional hydrocracking processes are pour point limited rather than end point limited. The specification for products such as light fuel oil (LFO), jet fuel an diesel fuel generally specify a minimum initial boiling point (IBP) for safety reasons but end point limitations usually arise from the necessity of ensuring adequate product fluidity rather than from any actual need for an end point limitation in itself. In addition, the pour point requirements which are imposed effectively impose an end point limitation of about 345.degree. C. (about 650.degree. F.) with conventional processing techniques because inclusion of higher boiling fractions including significant quantities of paraffins will raise the pour point above the limit set by the specification. When Zeolite Beta is used as the hydrocracking catalyst, however, the lower pour point of the unconverted fraction enables the end point for the middle distillates to be extended so that the volume of the distillate pool can be increased. Thus, the use of Zeolite Beta as the acidic component of the hydrocracking catalyst effectively increases the yield of the more valuable components by reason of its paraffin selective catalytic properties.
Another characteristic of Zeolite Beta is that it affects removal of waxy paraffinic components from the feed by isomerization as well as by conventional cracking reactions. The waxy paraffinic components, comprising straight chain end paraffins and slightly branched chain paraffins, especially the monomethyl paraffins, are isomerized by Zeolite Beta to form iso-paraffins which form excellent lubricant bases because the iso-paraffins possess the high viscosity index characteristic of paraffins without the high pour point values which are characteristic of the more waxy paraffins. A process employing this property of Zeolite Beta for dewaxing feeds to produce low pour point distillates and gas oil is described in U.S. Pat. No. 4,419,220.
Catalytic hydrodesulfurization is a well known process. Representative of prior art catalysts used for hydrodesulfurization are those alumina containing catalysts that include as hydrogenation component nickel and molybdenum or cobalt and molybdenum, the hydrogenation components being in the forms of metal or metal compounds. Phosphorus also is often present. Silica may be present in various modifications of such catalysts. An outstanding distinction between hydrocracking and hydrodesulfurization catalysts is that the former includes a strongly acidic component to enhance hydrocarbon cracking, while the latter catalyst is only mildly acidic to limit hydrocarbon cracking. U.S. Pat. No. 3,546,105 to Jaffe is incorporated herein by reference for background purposes, as are all of the other patents cited in the previous paragraphs.
The catalysts described in the previous paragraph are known to be effective for catalytic hydrodesulfurization of heavy hydrocarbon feedstocks, particularly feedstocks such as vacuum gas oils which may have an appreciable nitrogen content, that do not contain appreciable amounts of heavy residual materials. Although such catalysts also promote denitrogenation, a very important application is desulfurization of feedstocks for use as low sulfur heavy fuel to conform with air pollution requirements.
We have now found that a hydroconversion process for hydrocracking or hydrodesulfurizing a heavy oil feed contaminated with nitrogen can be improved by a simple physical modification of the catalyst bed, and without a need for changing the catalyst composition.
It is an object of this invention to provide a novel fixed bed of hydrocracking or hydrodesulfurization catalyst particles wherein the catalyst particles in either the upstream portion (top) or the downstream portion (bottom) of said catalyst bed is of larger particle size than the remainder of said bed, whereby imparting increased catalytic activity to said bed.
It is a further object of this invention to provide an improved hydrocracking or hydrodesulfurization process which utilizes the above-described bed of catalyst.
It is a still further object of this invention to provide an improved Moderate Pressure Hydrocracking Process (MPHC) wherein a fixed bed of hydrocracking catalyst comprising Zeolite Beta is used, and wherein nonuniform particle-sized catalyst is disposed in the fixed bed in the manner described above.
These and other objects will become evident to one skilled in the art on reading this entire specification and amended claims.