Ind. Eng. Chem. Res. 1990,29, 1995-1998
1995
Effect of Metal on Zeolite Catalysts for Extinction Hydrocracking Tsoung Y.Yan Mobil Research and Development Corporation, P.O. Box 1025, Princeton, New Jersey 08543-1025
The slow diffusivity of large molecules into the micropores results in shape selectivity in the conversion of mixed feeds. The metals deposit on the zeolite, as the hydrogenation components further reduce this diffusivity through pore filling and pore mouth blocking, leading to ineffective catalysts for extinction hydrocracking. By using active metals at low loadings, these adverse effects can be minimized. T o demonstrate this principle, experimental catalysts were compared. Unlike NiW /REX (REX = rare earth exchanged X-type zeolite), the experimental catalysts Pt and P d on REX a t 0.5 wt % levels were effective for the extinction hydrocracking of heavy gas oil blends. There was no heavy-end buildup in the recycle feed. The catalysts were active, low in aging rate, and high in selectivity for naphthas.
Introduction Zeolite catalysts are widely used in petroleum-refining and petrochemical industries. Their applications in hydroprocessing have been reviewed by Maxwell (1987). In the process, the gas oil and residue are converted to valuable distillates, kerosene, and, particularly, naphtha, which is an excellent feed for reforming to high-octane gasoline. For the maximum production of gasoline and kerosene, a hydrocracker can employ single or two or more stages of reactors, depending on the nature of the feedstocks. The first stage is for hydrotreating the feed, which is converted in the second stage where the main hydrocracking reactions take place. To achieve a high yield of the desirable products, i.e., naphthas and jet fuels, the conversion per pass in the second stage is typically controlled at about 60%. The unconverted feed from the bottom of the distillation tower is recycled to the second-stage reactor for complete conversion of the feed. This mode of operation is called extinction hydrocracking. The general principles and preparation of hydrocracking catalysts have been described (Ward, 1983). The second-stage hydrocracking catalysts are dual-functional and consist of hydrogenation and acidic cracking components. Because of their high activity, resistance to nitrogen compound poisoning, and low coke forming tendency, zeolites have become more favored as cracking component hydrocracking catalysts. The pore diameters of zeolites range between 4 and 10 8, and are close to the molecular sizes of hydrocarbons involved in hydrocracking. When the pore diameter of the zeolite approaches the critical diameters of the feed and product, molecular diffusion of the feed and product becomes hindered. Depending on the relative diameters of the molecules and the zeolite pores, the diffusivities of the molecules can span some 10 orders of magnitude (More and Katzer, 1972),leading to shape-selective catalysis. The implications of shape selectivity associated with zeolite catalysts in the extinction hydrocracking process have been demonstrated (Maxwell, 1987; Yan, 1983a,b). In the process, large refractory compounds build up in the recycle feed due to their lower reaction rate constants. In order to maintain the conversion per pass at the desired level, the operation severity has to be increased by raising the reactor temperature. However, the increased severity for converting the recycle feed becomes too severe for the fresh feed, leading to overcracking and product yield shifts to increased gas-make and reduced heavy naphtha. To overcome this problem, a zeolite/amorphous dual-catalyst system was developed that effectively hydrocracked feeds of wide boiling ranges to extinction (Yan, 1983a,b). Sim0888-5885/90/2629-1995$02.50/0
ilarly, a composite catalyst consisting of both zeolite and amorphous components in a particle has been developed to hydrocrack feeds of wide boiling ranges to extinction (Yan, 1989). It has been shown that the two components can operate complementarily in a single catalyst. In this study, the effect of metal components on the shape selectivity of the zeolite-based hydrocracking catalyst is examined. On the basis of these results, experimental, noble-metal hydrocracking catalysts with superior performance in extinction hydrocracking were prepared.
Approaches to This Study The second-stage hydrocracking consists of hyrogenation and cracking components. For optimum performance, the activities of these two components have to be balanced (Yan, 1983a,b). The zeolite base cracking component is so active that either its activity has to be tempered or the activity of the hydrogenation component has to be boosted. A catalyst with a hydrogenation activity that is too low often leads to excessive coking and rapid catalyst aging. To achieve this balance, the content of the hydrogenation components is generally higher for the zeolite cracking catalyst. A typical hydrocracking catalyst can contain 6 wt % Ni and 19 w t 5% W (Galbreath and Van Driesen, 1971). The NiW/REX system (REX = rare earth exchanged X-type zeolite) tested previously (Yan, 1983a) contains 4 and 10 wt % nickel and tungsten, respectively. Much of the nickel and tungsten would be deposited inside the pore, but some, particularly tungsten, would be expected to deposit at the outer surface, leading to pore mouth plugging. Since the pore diameter of X or Y zeolite is close to the critical diameters of the molecules in the hydrocracker feed, addition of metal oxide hydrogenation components would increase the shape selectivity of the zeolite and impair its effectiveness for extinction hydrocracking as observed by Maxwell (1987) and Yan (1983a,b). The diffusivity of HZSM-5 is dramatically reduced by deposition of MgO inside the pore or coke at the outer surface (Olson and Haag, 1984). The diffusivity of o-xylene in HZSM-5 was reduced by 2 orders of magnitude when about 8% of the pore was filled with MgO. Similarly, 4 % of coke deposition, mostly at the outer surface, reduced the o-xylene diffusivity in HZSM-5 by nearly 2 orders of magnitude. It is apparent that one of the approaches to improve the effectiveness of zeolite-based catalysts for extinction hydrocracking is to reduce the physical quantity of the hydrogenation component while the necessary hydrogenation activity is maintained. High-activity hydrogenation components, such as Pd and Pt, can be used in less quantity 0 1990 American Chemical Society
1996 Ind. Eng. Chem. Res., Vol. 29, No. 10, 1990
and are preferred. With this principle as the basis, we have studied the effect of hydrogenation components on zeolite-based catalysts for extinction hydrocracking. The study led to noble metals on zeolite catalysts, which are active, stable, and selective for extinction hydrocracking. Experimental Section Catalyst Preparation. The X-type zeolite in sodium form was exchanged with a mixture of rare earth chloride to a sodium level of 0.6%. This rare-earth exchanged X was calcined to 1000 "F (538 " C ) in air and then impregnated with aqueous solutions of the metal salts to the desired metal levels. The Pt/REX catalysts with 2.5, 1.0, 0.5, and 0.25 wt 70 Pt contents were prepared by impregnation using H2PtC&. Similarly, Pd/REX catalysts with 1.0 and 0.24 wt % Pd were prepared by impregnation using PdC1,. The Ni/REX catalyst with 4.4 wt % Ni was prepared by impregnation using Ni(N0J2. The Ni/Si02A1203 catalyst was prepared by impregnating amorphous silica alumina cracking catalyst with Ni(NO& to a Ni level of 4 wt %. NiW/REX was prepared by impregnating REX using ammonium paratungstate and nickel nitrate solutions in steps with calcination between the steps. The final composition was 4 and 10 wt % Ni and W, respectively. This catalyst has been studied previously (Yan, 1983a,b). All the catalysts were formed into 1/8-in.pellets without binder and calcined. The catalysts were tested in the form of pellets without crushing. Charge Stock Preparation. The raw feed consisted of 18.9 w t 70light coker gas oil, 12.2 wt % heavy coker gas oil, 18.4 wt % light catalytic cracker gas oil, and 50.5 wt % heavy catalytic cracker gas oil and furfural extract from lube oil production. The properties were specific gravity, 0.9402 (19.0" API); nitrogen, 710 ppm; sulfur, 1100 ppm; hydrogen, 10.69 wt %; and 95% boiling point, 832 O F (444 O C ) . This raw feed was hydrotreated over a commercial hydrogenation catalyst to 1 ppm nitrogen content. This was the fresh feed used in this study. Additional important properties of this fresh feed were specific gravity, 0.8665 (31.8O API); sulfur, 43 ppm; hydrogen, 12.62 wt %; and 95% boiling point, 800 "F (427 " C ) . Operation Procedure. The reaction system and operation procedure have been reported previously (Yan, 1989). The reaction system was set up to simulate the second-stage hydrocracker with recycle. The effluent from the reactor was separated in a high-pressure separator. The liquid product from this separator was charged to a continuous distillation column. This column was maintained to give a cut point of 380 OF (193 "C). The overhead, which was naphtha or lighter hydrocarbons, was drawn as the product, while the bottom product was the unconverted feed. This unconverted feed was returned to the feed line as the recycle feed to mix with the fresh feed for further reaction. The flow rate of the total feed, Le., fresh and recycle feeds, was fixed so that a constant total liquid hourly space velocity (LHSV) was maintained. The reactor temperature was adjusted to achieve 60% conversion per pass of total feed. The reactor temperature required is an indication of the catalyst activity. The activities of the fresh catalysts reported were the temperatures required at 10-14 days on-stream time after the catalyst activities had been stabilized. Analytical Section. The gaseous products were analyzed for light hydrocarbon distribution by gas chromatography and for hydrogen by the use of a mass spectrometer. The liquid products were distilled to obtain the
M o l Lcadim,
a:!
Figure 1. Reactor temperature required versus metal loading.
product distribution and for an inspection of the properties. The product yields and distribution were calculated from a material balance. The catalyst selectivitiesfor components or fractions are defined as follows:
si = lOOY,/C where Si is the selectivity for component i, %; Yi is the yield of component i, vol %; and C is the conversion of feedstock to products boiling below 380 O F , vol %. In addition to the conventional tests, both the fresh and recycle feeds were characterized by using a chromatograph equipped with silicone gum rubber columns. The chromatogram is a good simulation of boiling range distribution. The chromatogram also reveals the nature of the product. The sharp peaks and the shoulders represent n-paraffins and isoparaffins, respectively, while the broad background represents aromatics and naphthenic compounds. By integrating the areas in the chromatograms, the relative concentrations of the compound types can be estimated. Results and Discussion Metal Requirement for Adequate Hydrogenation. To balance the REX acidic cracking base for an effective hydrocracking catalyst, the precious metal requirement is low, about 0.5 wt 70. The effect of Pt and Pd loading on the hyrocracking activity in terms of the reactor temperature required for 60% conversion per pass is shown in Figure 1. The catalyst activity increases as the metal loading increases and levels off at a metal loading of about 0.5 wt % . This is a very low metal loading in comparison with NiW/REX, which contained 4 and 10 wt % Ni and W, respectively. The hydrogenation activities of Pt and Pd/REX with metal loadings greater than 0.5 wt % were adequate, since the recycle feeds were found to be highly saturated. Similarly, the Ni/REX and NiW/REX catalysts were found to have adequate hydrogenation activities. It is realized that the metal loading was never optimized for the overall long-term performance of the catalysts. Pore Filling by Metals. Because of its low loading, the pore filling by metals is low for precious metals and will impair the diffusivity of the catalyst significantly. If the metal is assumed to be all within the zeolite pores, the
Ind. Eng. Chem. Res,, Vol. 29, NO. 10, 1990 1997 Table I. Pore Filling by Metals
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catalyst NiW/REX P t / R E X
Ni/REX
4
~~~
composition, wt % Ni W Pt volume, cm3/100 g cat. NiO
wo3
PtO REX pore filled," vol %
4 10
4.4
2
0.5
0.68 1.76 20.60 11.8
0.75 0.06 24.62 0.2
23.66 3.2
Io 2
Reclcle Feed Fm NlWRM
"If all the metal oxides were assumed to be inside the pore, in reality, most of the tungsten oxide was believed to be outside the pore.
pore filling by impregnated 0.5 wt % Pt is estimated to be 0.2% of the zeolite pore volume (Table I). As a result, the Pt metal is not expected to cause significant additional diffusional resistance. A similar estimation on pore filling for NiW/REX was made (Table I). With 11.8% pore filling, the diffusivity of zeolite can be significantly impaired, leading to an ineffective catalyst for extinction hydrocracking, as reported previously (Yan, 1983a,b). Since the metals were deposited on the zeolite by impregnation in this study, in addition to pore filling, some of the metal would be located on the intercrystalline external surfaces. This is particularly true for tungsten oxide when the size of its precursor, tungstate ion, is considered. It is believed that most of the tungsten oxide was outside of the pore. Such deposits can effectively block the pore mouth, leading to reduced diffusivity. This type of pore blockage increases with the amount of metal deposited. With a pore filling of 3.290, the Ni/REX is expected to crack heavy ends much better than NiW/REX, which was proven to be true, as will be discussed in the following section. As the catalyst on-steam time increases, the coke deposition in the pore increases, leading to additional diffusional barrier and lower effectiveness for extinction hydrocracking. In addition to the mechanism of pore filling, the coke can deposit on the intercrystalline surface, leading to pore blockage and lower diffusivity. The compound effect of high metal oxide loading and coking on NiW/REX impairs the diffusion of large molecules in and out of the pores. Besides these physical considerations, it will be interesting to directly measure and compare the diffusivities of hydrocarbons of various sizes in the fresh and aged NiW/REX and Pt/REX catalysts to verify this postulate. Fortunately, this postulate is fully supported by the catalyst performances in extinction hydrocracking tests, to be discussed in the next section. Effect of Metals on Zeolite-Based Hydrocracking Catalyst for Extinction Cracking. The faujasite-based catalyst, with lower quantity and less bulky metals than the hydrogenation component, is rather effective for extinction hydrocracking. In contrast to NiW/REX, Pt/ REX, Pd/REX, and Ni/REX are effective for extinction hydrocracking, as judged by comparing the changes in properties of the recycle feeds and product yields and increase in the reactor temperature required as the onstream time increases. Heavy-End Buildup in the Recycle Feed. The typical chromatograms of the recycle feeds from hydrocracking using NiW/REX and Pt/REX catalysts are compared in Figure 2. By comparison with the fresh feed (Yan, 1989),it is evident that there is no heavy-end buildup
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10
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16
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Figure 2. Chromatograms of recycle feeds.
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c 0
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Figure 3. Content at 343 "C in the recycle feed versus on-stream time.
in the recycle feed from the Pt/REX catalyst. In fact, the chromatograms of recycle feeds from the Pt/REX catalyst and the amorphous catalysts are similar, indicating that the composite catalyst is effective in cracking heavy ends. In contrast to this, NiW/REX was incapable of cracking large compounds effectively, leading to buildup of heavy ends. The changes in fractions of heavy ends, 343+ " C (650+ OF) cut, in the recycle feeds from NiW/REX, Ni/REX, and Pt/REX as a function of on-stream time are shown in Figure 3. For NiW/REX, the 650+ OF fraction in the recycle feed decreased first and then gradually builds up, indicating that the catalyst is not effective in cracking the heavy ends. On the other hand, the 650+ OF fraction from both Ni/REX and Pt/REX decreased rapidly and leveled off at low levels. Pt/REX was particularly effective in cracking the heavy ends. Product Yield. The product yields from Pt/REX are plotted against on-stream time in Figure 4. For comparison, those from NiW/REX reported previously are also plotted. The product yield pattern remained rather constant throughout the 25 days of operation. The dry gases (Cl, Co, and C,) were low at 1.0-1.2 wt YO, and the naphtha yields were high at 11 and 90 vol 90 for light and heavy naphthas, respectively. This is a dramatic improvement over the NiW/REX catalyst. For the NiW/REX catalyst, the heavy naphtha yield dropped dramatically from 83 to 64 vol 90,while the C,s' increased from 12 to 30 vol 90 in 14 days of operation. This constancy in the product yield pattern, as well as the high naphtha selectivity, is consistent with the observation that
1998 Ind. Eng. Chem. Res., Vol. 29, No. 10, 1990
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Figure 5. Reactor temperature increase versus on-stream time.
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selectivity makes the zeolitic hydrocracking catalysts ineffective for extinction hydrocracking. Depending on the levels of loading, the metal deposited on the zeolite as the hydrogenation component further reduces the diffusivity of the catalyst through pore filling and pore mouth blocking. To minimize these effects, the active precious metals (such as Pt or Pd) are particularly preferred as the hydrogenation component. Unlike NiW/REX, Pt and P d on REX catalysts at metal loadings of 0.5 w t % were found to be effective for extinction hydrocracking of heavy gas oils. There was no heavy ends buildup in the recycle feed. The quality of the recycle feed was better than that of the fresh feed and was maintained throughout the extended testing period. The catalysts were active, low in aging rate, and high in selectivity for naphthas. Registry No. Ni, 7440-02-0; W, 7440-33-7; Pt, 7440-06-4; Pd, 7440-05-3.
Literature Cited Galbreath, R. B.; Van Driesen, R. P. Hydrocracking of Residual Petroleum Stocks. Proc. 8th World Pet. Congress 1971, 4, 129-137. Maxwell, I. E. Zeolite Catalysis in Hydroprocessing Technology. Catalysis Today 1987,I, 385-413. More, R. M.; Katzer, J. R. Counterdiffusion of Liquid Hydrocarbons in Type Y Zeolite: Effect of Molecular Size, Molecular Type, and Direction of Diffusion. AIChE J . 1972,18 (No.4), 816-824. Olson, D. H.; Haag, W. 0. Structure-Selectivity Relationship in Xylene Isomerization and Selective Toluene Disproportionation. ACS Symp. Ser. 1984,248,275-307. Ward, J. W. Design and Preparation of Hydrocracking Catalysts; In Preparation of Catalysts; Poncelet, G., Grange, P., Jacobs, P. A., Eds.; Elsevier Science: Amsterdam, 1983; Vol. 111. Yan, T. Y. Zeolite-Based Catalyst for Hydrocracking. Ind. Eng. Chem. Process Des. Dev. 1983a,22, 154-160. Yan, T. Y. Stabilization of Zeolite Hydrocracking Catalyst. Chem. Eng. Commun. 1983b,21,123-133. Yan, T. Y. Modified Zeolite-Based Catalyst for Effective Extinction Hydrocracking. Znd. Eng. Chem. Res. 1989,28, 1463-1466.
Receiued for reuiew December 27, 1989 Accepted June 29, 1990
