US3523887A - Catalytic hydrocracking with a mixture of crystalline aluminosilicate amorphous base cracking compounds promoted with one or more hydrogenation components - Google Patents

Description

United States Patent Office 3 523 887 CATALYTIC I-IYDRO CRACKING WITH A MIX- TURE OF CRYSTALLINE ALUMINOSILICATE AMORPHOUS BASE CRACKING COMPOUNDS. PROMOTED WITH ONE OR MORE HYDRO- GIENATION COMPONENTS Frgncls V. Htanioln IjlrlldoPlalg W. Snyder, Jr., Pitman, N.J.,

ss1gn0rs o o i i 0 t New York rp ra mm a corporauon of N0 Drawing. Fillet: C1116 1968, Ser. No. 766,958 11 13/02 US. Cl. 208-111 g Claims ABSTRACT OF THE DISCLOSURE A catalytic hydrocracking operation is described employing a catalyst mixture comprising a crystalline alumino-sihcate cracking base in admixture with an amorphous cracking base and promoted with one or more hydrogenating components wherein the activity and selectlvity of the catalyst is under substantial surveillance and the activity of the catalyst is maintained by an adustment of temperature and selectivity of the damaged mixed base catalyst is restored to substantially its original condition by ammonia treatment of the catalyst.

BACKGROUND OF THE INVENTION The present invention relates to catalytic hydrocracking and is concerned with the cracking of feed stocks containing aromatic and paraffin hydrocarbons. Hydrocracking, also known as destructive hydrogenation or hydrogenolysis, involves the cracking of hydrocarbonaceous material in the presence of hydrogen and a Suitable catalytic composite under conditions to effect a change in the molecular structure of the charge preferably to lower boiling products including gasoline and jet fuels. Hydrocracking is known as cracking under hydrogenating conditions such that the lower boiling products of the conversion reaction are substantially more saturated. In these hydrocracking operations, a selective hydrocracking is of particular importance when processing hydrocarbons and mixtures of hydrocarbons having boiling points at temperatures above that of the gasoline and middle distillate boiling range products desired. During hydrocracking under selective conditions, it is known that the catalysts become poisoned by basic nitrogen compounds and carbonaceous materials deposited on the catalyst from the feed stock. This poisoning effect is evidenced by decreased conversion of the feed stock under a given set of conditions or a change in product selectivity at the given set of operating conditions or both. It is known, however, that the poisoning effect of nitrogen compounds can to some extent be overcome by operating at higher temperatures. However, the use of higher temperatures often lead to increased deactivation-of the catalysts by coke as well as decreased catalyst life, and thus the refiner is caught When trying to maintain product selectivity over an extended operating period before effecting regeneration of the catalyst. The deactivation problem is part-icularly aggravated when using high boiling feed stocks containing aromatic constituents boiling above 600 F. It is also known that heavy polycyclic aromatic hydrocarbons become absorbed on the active cracking centers of hydrocracking catalyst and thereafter may not be effectively hydrogenated or cracked. Eventually the poly- 3,523,887. Patented Aug. 11, 1970 cyclics are converted to carbonaceous residues such as condensation products. In virtually all hydrocracking P cesses however, controlled and/or selective hydrocracking is highly desirable for the purpose of maintainmg high yields of desired liquid product and to assure a more effective catalytic action over an extended period of catalyst life. Selective hydrocracking is of particular importance when processing hydrocarbons and mixtures of hydrocarbons boiling above middle distillate boiling range materials and hydrocarbon fractions having an initial boiling point above about 550 F. and more usually above about 650 F.

It is known in the prior art that optimized hydrocracking conditions for converting parafiinic hydrocarbons differ considerably from the optimum conditions for converting aromatic hydrocarbons. Thus the prior art has used different types of catalyst varying in activity for hydrocracking different types of hydrocarbon constituents. For example, hydrocracking catalysts based on amorphous cracking bases have been found to display greater efficiency for the conversion of high molecular weight polycyclic aromatic hydrocarbons than those based on crystalline 'alumino-silicates but are considerably less efficient for converting paraffinic hydrocarbons. On the other hand, hydrocracking catalysts based upon certain crystalline aluminosilicates or zeolitic molecular sieve cracking bases have been found very efficient for the conversion of paraffinic hydrocarbons but are considerably less efiicient for converting the condensed ring polycyclic aromatic hydrocarbons. Thus in the hydrocracking of mixed feed stocks, the prior art has found that improved results can be obtained by contacting the feed stock with both types of catalyst rather than either catalyst alone. Thus in cases where separate catalyst beds were employed, it was found advantageous to locate the amorphous catalyst ahead of the crystalline catalyst so that polycyclic aromatics would be converted to a considerable extent before coming in contact with a crystalline aluminosilicate based hydrocracking catalyst. It has also been proposed in the prior art to employ a mlxture of the amorphous based and crystalline aluminosilicate based hydrocracking catalyst in a common reactlon zone as a mixed catalyst phase.

An object of the present invention is to improve a hydrocracking operation in maintaining product selectivity and catalyst life when using a hydrocrackmg catalyst comprising a mixture of crystalline aluminos hcate cracking base and amorphous silica-alumina cracking base promoted with one or more hydrogenating components.

BRIEF SUMMARY OF THE INVENTION The present invention relates to the hydrocracking of high boiling hydrocarbons to produce therefrom lower boiling hydrocarbons comprising gasoline arid/or et fuel products with a mixed phase hydrocracking catalyst having a particular selectivity for effecting the conversion of paraffins and polycyclic aromatic constituents found in the hydrocarbon charge. In a more particular aspect, a feature of the present invention resides in adding a catalyst reaction modifier in sufiicient quantity and at an onstream time during the hydrocracking operation which is commensurate with restoring the mixed base catalyst cracking components selectively after damage thereto and in balance to optimize desired product yields. More particularly, it may be said that the essence of the present invention is concerned with adding ammonia or a nitrogen compound yielding ammonia upon hydrogenolysis to a mixed base hydrocracking catalyst comprising in combination a crystalline aluminosilicate and an amorphous silicious cracking component after the selectivity of the catalyst has been damaged, with the ammonia yielding compound being added in amounts which will substantially restore the mixed base catalyst selectivity to its original condition for providing a desired product distribution even at a reduced cracking activity level.

DESCRIPTION OF SPECIFIC EMBODIMENTS Cracking operations carried out in the presence of hydrogen at relatively high temperatures and pressures do not impose undesirable limitations on the type of charge stock which may be used in the process. In fact, hydrocracking operations are most suitable for converting charged materials not suitable for conversion in other well known hydrocarbon refining processes. Thus gas oil feeds, both virgin and synthetic, heavy residual stocks, cycle stocks, coker gas oils and other charge stocks considered substantially less satisfactory or desirable can be converted in hydrocracking operations. However, in order to maintain catalyst activity and to avoid undesired deposition of carbonaceous material on the catalyst including condensation products thereof, it is generally necessary to employ relatively high hydrogen pressures to minimize the deactivating effects of these high boiling charge stocks on the catalyst.

In accordance with the present invention, it has been found that a superior hydrocracking catalyst composite is found in a mixture of two cracking components comprising a crystalline aluminosilicate component having cracking activity in admixture with an amorphous silicous or metal oxide cracking component with the cracking mixture thus formed containing one or more hydrogenating components. The crystalline aluminosilicate component is preferably of low sodium content which is less than about 4 percent by weight and may be base exchanged with a solution of ions selected from the group consisting of rare earth, hydrogen, hydrogen precursor, calcium, magnesium, zinc and magnesia ions and mixtures thereof with one another to reduce the sodium content to a value below about 4 percent. It is preferred that the sodium content of the final hydrocracking composite be less than about 1 percent. The amorphous base cracking component used herein may comprise substantially any of the well known silica containing cracking bases. Suitable cracking bases include, for example, co-precipitated or cogelled mixtures of two or more reducible oxides such as silica-alumina, silica-magnesia, silica-zirconia, aluminaboria, silica-titania, silica-zirconia-titania, silica-aluminaboria, and the like.

Hydrocracking operations contemplated in the process of the present invention are generally carried out at a temperature in the range of from about 450 F. to about 1000 F. and preferably from 550 F. to about 950 F. The hydrogen partial pressure in such an operation is maintained at a high order of magnitude generally within the range of from about 700 to about 3000 pounds per square inch gauge and preferably above about 1000 or 1500 pounds per square inch gauge. The liquid hourly space velocity of fresh feed, i.e. the liquid volume of hydrocarbons per hour per volume of catalyst, is maintained in the range of from about 0.1 to about 10 and preferably from about 0.25 to about 5. In general, the mole ratio of hydrogen to hydrocarbon charge employed is between about 2 to about 80 and preferably from about to about 40.

The hydrocracking selectivities of the catalyst described herein were evaluated by comparing their product distributions at fixed conversion levels. Conversion is defined and used herein as 100 minus the volume percent of charge remaining in the fresh feed boiling range. Products considered in this evaluation are dry gas (C -C C materials, light naphtha (C 180 F.), heavy naphtha (180- 390 F.), fuel oil (390-650 F.), and cycle stock (650 F. l). An overall measure of selectivity may be limited to the total yield of 0 -650" F. material at any given conversion level since this range includes products boiling within the jet fuel and product boiling range. As indicated hereinbefore, the present invention is concerned with restoring and maintaining the selectivity of a damaged hydrocracking catalyst comprising a mixture of crystalline and amorphous cracking components by the controlled addition of ammonia or ammonia forming compounds of the hydrocracking feed stock.

During a hydrocracking operation there are three basic types of evidence which indicate that a hydrocracking catalyst has been or is being damaged. These are (1) rapid loss of catalyst activity while onstream (i.e., aging rate), (2) rapid loss of selectivity while onstream (i.e., a shift toward lower average molecular Weight in products boiling below the end point of desirable products), and (3) a rapid shift toward higher average molecular weight in products boiling above the end point of desirable products. The latter two types of evidence effects are accelerated and therefore most frequently observed during hydrocracking of hydrocarbon charge stocks having end boiling points above about 800 F. in such a way that products boiling above the end point of desired products (i.e., about 400 F. if gasoline is desired, and about 550 F. if a jet fuel is desired) are recycled to extinction.

The above three effects are not necessarily concurrent, and some may occur without others. For example, when crystalline aluminosilicate-base catalysts rapidly lose selectivity during hydrocracking in which all products boiling above gasoline are recycled, there is usually an accompanying build-up of the higher molecular weight portion of the recycle liquid boiling above gasoline; but there is often little concurrent loss in activity. The critical deficiency of the crystalline-base catalysts in such a case is inability to hydrocrack polycyclic hydrocarbons having three or more condensed rings well enough to prevent their accumulation in the recycle liquid. As time goes on, therefore, these hydrocarbons occupy an ever increasing proportion of the total feed to the catalyst. Although it is unable to hydrocrack these polycyclics fast enough to prevent their build-up, the catalyst is still very active for hydrocracking less refractory cyclic and acyclic hydrocarbons. The consequence is increasing secondary conversion of gasoline range naphtha to dry gas and butanes, and lowering of average molecular weight of the naphtha (i.e., decreasing selectivity).

On the other hand, the results with amorphous-base catalysts are usually different from those with crystalline base catalysts. Even when amorphous-base catalysts age rapidly, the concurrent loss in selectivity is usually not very rapid; and there is no build-up of higher molecular weight products in the recycle liquid. That is, these catalysts convert those polycyclic hydrocarbons which are refractory as far as crystalline-base catalysts are concerned well enough to prevent their accumulation in the recycle liquid, and they minimize the selectivity loss at tendant upon that build-up. The deficiency of these catalysts is primarily high aging rate which leads to short cycle life, shortened total catalyst life, and frequently to poor selectivity during the latter (i.e., high temperature) portion of each cycle.

Type of catalysts involved in the invention Hydrocracking catalysts exhibiting the best performance features of both crystalline-base and amorphous-base catalysts can be made, it has been found, by incorporating both a crystalline and an amorphous base in the same catalyst. The crystalline component provides low aging rate and, thus, sustains activity; the amorphous component prevents accumulation of condensed-ring polycyclic hydrocarbons in the recycle liquid and attendant loss of selectivity. It is the restoration of selectivity to such mixed crystalline-amorphous base catalysts after they have been damaged that this invention is particularly concerned. Non-limiting examples of such catalysts are described and patented in US. Pat. 3,304,254. The crystalline aluminosilicate cracking base may be combined with the amorphous silica cracking base in substantially any amount in the range of from about 5 to about 95% by weight. In some applications it may be found advantageous to maintain a higher concentration of the amorphous cracking base so that the crystalline aluminosilicate cracking component is less than 50% by weight of the mixed cracking base.

THE INVENTION The present invention is illustrated by the data presented in the table. The crystalline aluminosilicate portion of the catalyst of this example was nickel-tungstensulfide on rare earth exchanged crystalline aluminosilicate, the amorphous component was nickel-tungsten-sulfide on silica-alumina, and there were approximately equal amounts of these two components in the finished catalyst.

Example 1 (Run 1) The above identified catalyst mixture, catalyst sample A, was used to hydrocrack a feed stock boiling in the range of from about 450 F. to about 800 F. and made by pretreating a mixture of coker gas oils and catalytic cracking cycle stocks over a catalyst comprising a hydrogenation component like nickel-tungsten-sulfide deposited on silica-alumina so as to produce a catalyst having a low activity cracking component at 2000 p.s.i.g., 1.2 LHSV, 7000 s.c.f. of hydrogen circulation per barrel of feed, no liquid recycle and a temperature producing about 1 p.p.m. of organically combined nitrogen in the product. This product was then passed over a relatively fresh sample of the mixed crystalline-amorphous based catalyst at 1625 p.s.i.g., 0.54 LHSV based on fresh feed, 0.90 LHSV based on total feed, 5500 s.c.f. of recycle hydrogen gas circulation per barrel of total feed, and 60 vol. percent conversion per pass to products boiling below 380 F. based on fresh feed. It required about 620 F. to meet these conditions and the product distribution is that of Run 1 (labeled fresh catalyst condition) in the table.

Example 2 (Run 2) A separate batch of the mixed crystalline-amorphous based nickel-tungsten-sulfide catalyst was used to hydrocrack a different feed stock made by pretreating a mixture of coker gas oils, catalytic cracking stocks, and secondary furfural extract under the same pretreating requirements of Example 1 and over a batch of the pretreating catalyst of Example 1 which had been operated at much higher temperatures than that of Example 1. That is, the feed stock boiling in the range of about 450 F. to about 930 F. of this example was made under more severe temperature conditions than that of Example 1; and it was made by pretreating a stock containing a petroleumderived component (secondary furfural extract) more deleterious to the performance of both the crystalline and the amorphous components of the mixed crystallineamorphous based hydrocracking catalyst of this example than any of the petroleum-derived components of the feed to the pretreating step of Example 1, and having an end point of about 930 F. as compared to about 800 F. in Example 1. In addition, the mixed crystalline-amorphous based catalyst had been subjected to accelerated aging (by operation at double the space velocity used in Example 1 and therefore at a higher temperature) relative to that of Example 1. The cumulative effect of the greater severities under which the mixed crystalline-amorphous based catalyst was used in this example relative to the severities of Example 1 was to damage the catalyst severely. In fact, the damage was so severe that, when the feed stock of Example 1 was subsequently passed over the catalyst under the same conditions as those of Example 1, catalyst activity was lower by 35 F. and product selectivity was much poorer. This is shown by comparison of Run 2 with Run 1 of the table; dry gas yield, for example, is up from 2.5 to 3.9 wt. percent, butane yield is up from 16.8 to 24.4 vol. percent, C 180 F. light naphtha yield is up from 30.8 to 38.0 vol. percent, l-380 F. heavy naphtha yield is down from 74.2 to 60.9 vol. percent, and the amount of material boiling above 650 F. in the recycle liquid was up from 40 to 65 wt. percent.

Example 3 (Run 3) The comparision of the results of this example with those of Examples 1 and 2 is a nonlimiting illustration of the heart of this invention. To the feed stock being processed over the mixed crystalline amorphous hydrocracking catalyst at the end of Example 2 was added enough pyridine to produce ppm. of amomnia by hydrogenolysis in the reactor. The result was a great improvement in catalyst selectivity accompanied by a further loss in catalyst activity of 50 F. This is shown by comparison of Run 3 with Run 2 in the table. In fact, the selectivity of the catalyst was so improved by adding the ammonia (in effect) to the reaction mixture that the subsequent selectivity was as good as would be expected if the catalyst had never been damaged but had just aged up to the 705 F. temperature requirement of that experiment. This great improvement in the selectivity of hydrocracking with the damaged catalyst when ammonia (or a precusor thereof) was added to the reaction mixture exemplifies this invention; the concurrent loss in catalyst activity was, of course, expected on the basis of prior art.

DISTINCTION OVER THE PRIOR ART The improvement in selectivity and in quality of the recycle liquid (i.e., decreased concentration of heavy ends) when the ammonia was added to the system is what distinguishes this invention over the prior art. It was previously known to add ammonia or its precursors to hydrocracking reactors in order to increase the temperature requirement for a given conversion; the increased temperature may, for example, be desired in order to induce increased octane number of the naphtha being produced. However, ammonia adidtion under these circumstances either produces no significant change in product selectivity or (if the resultant change in temperature is very great) a decrease in selectivity. These effects of ammonia addition which are known in the prior art are illustrated by comparison of the following two examples.

Example 4 (Run 4) Another preparation of the same catalyst, Catalyst B, as used in Examples 13 was used much in the same way as in Example 1 except for some insignificant differences in the conditions of hydrocracking in the step following the pretreating step. These differences are 1600 instead of 1625 p.s.i.g., and 6000 instead of 5500 s.c.f. of recycle hydrogen gas circulation per barrel of total feed. The hydrocracking results are shown in Run 4 of the table. Temperature requirement to achieve 60 vol. percent conversion per pass to products boiling below 380 F. was 620 F., just as it was in Example 1. The minor differences between Examples 1 and 4 in product distribution probably reflected that the experiments were done with different preparations of the same catalyst.

Example 5 (Run 5) To the feed stock being processed over the catalyst in Example 4 was added enough pyridine to produce 100 ppm. of ammonia by hydrogenolysis in the reactor. The result was a loss in catalyst activity of 70 F accompanied by a slight loss in selectivity. This is shown by comparing Run 4 with Run 5 in the table. This slight loss in selectivity upon the ammonia addition to the undamaged catalyst of this example is just what is expected on the basis of prior art.

Example 6 (Run 6) The feed stock processed over the catalyst of Example 4 was compared in an operation identified as Run 6 involving normal catalsyt aging until the temperature of the hydrocracking operation reached 690 F. The product distribution is shown in Run 6. It will be observed, when comparing Runs 5 and 6, that the departures in product distribution are not significant, thus further attesting to the fact that the catalyst was not damaged and had retained its selectivity.

The results obtained in Runs 4, 5 and 6, for comparative purposes, are in dramatic contrast to that obtained in Runs 1, 2 and 3 and, thus, identify the completely unexpected results forming the basis of the present invention.

The characteristics of the catalyst which lead to improved selectivity upon ammonia addition in this invention are two: 1) it is a mixed crystalline-amorphous based catalyst, and (2) it had been damage. Without both of these characteristics, the catalyst will react to ammonia addition as catalysts of the prior art dothat is with :1 decreases in selectivity if selectivity changes.

Without limiting this invention by theroy, the following explanation of the increase in selectivity when ammonia was added during hydrocracking over the damaged mixedbase catalyst in Example 3 above is offered. The results of Example 2 all suggest that the amorphous-base component of the catalyst had been damaged (i.e., deactivated) much more than the crystalline-base component during the high temperature processing of the pretreated high end-point (930 F.) feed stock containing the secondary furfural extract. This deactivation was probably associated with deposition on the catalyst of carbonaceous deposits (coke) which would interfere with adsorption of ammonia on it. When ammonia was added to the system, therefore, it most likely adsorbed preferentially on the least damaged (i.e., least coked) component of the catalyst (believed to be the crystalline-base component in this case). It is, of course, well known that adsorption of ammonia on hydrocracking catalysts deactivates them and raises the temperature required to produce a given desired conversion. Therefore, the preferential adsorption of the ammonia on the least damaged (i.e., coked) component (the crystalline-base component in this case) of the catalyst raised the operating temperature required to obtain the desired conversion over this component to a level which was high enough to restore hydrocracking activity to the more damaged component (the amorphous component in this case). With the amorphorus component functioning again because of the temperature increase induced by the deactivation associated with the preferential ammonia adsorption on the crystalline component, the accumulation of high boiling components in the recycle liquid was once again repressed. Both components of the catalyst were again functioning, each doing its own jobs; the activities of the amorphous and crystalline components have been brought into balance again and the result is the return to high. selectivity shown in Example 3.

If by some circumstance the catalyst were operated in such a way that the crystalline-base component was damaged more than the amorphous-base component (the opposite case to what we believe actually happened in Example 2) addition of ammonia will tend to bring the ac tivities of the two components back into balance in this case too. That is, the ammonia will always preferentially adsorb on the least damaged of the two catalyst components, further deactivate that component, and raise the temperature at which that component must be operated to 8 get the desired conversion. This increase in temperature will always tend to restore hydrocracking activity to the more damaged 0f the two components and bring the activities of the two components back into balance in this each component can serve its proper function.

Sources of Ammonia Ammonia itself can be used in the process of this invention, and any nitrogenous organic compound which yields ammonia by hydrogenolysis under hydrocracking conditions can be used. Most nitrogenous organic compounds fall in this category. Non-limiting examples are methylamine, ethylamine, butylamine, pyridine, piperidine and quinoline.

In the method of the invention, it is important to maintain a close surveillance of the activity and selectivity of the mixed base catalyst herein identified so that incremental operating temperature limits to maintain a given conversion level can be held to a desired low level at the same time catalyst selectivity is monitored to maintain product selectivity. Surveillance of the catalyst activity and selectivity can be achieved by several different methods, some of which are slower than others. Two methods of surveillance for selectivity which provide a rapid indication of the catalyst condition are (1) light adsorption spectroscopy and (2) gas-liquid chromatography. Either of these methods may be employed to monitor the total product stream or the recycle liquid stream of the hydrocracking operation. Thus, the method of this invention encompasses adjusting operating temperatures of hydrocracking so that to maintain a desired conversion level and the amount of adjustment will be dependent upon several factors including type of hydrocarbon charge, product distribution desired, conversion level desired and operating economics. The selectivity of the mixed base hydrocracking catalyst herein identified and comprising a mixture of crystalline aluminosilicate and amorphous silica-alumina is monitored to identify a damaged hydrocracking catalyst as evidenced by an undesired change in product distribution. One method of identifying undesired changes in product distribution can be related to changes in C yields and/or dry gas yields.

In the data of the table herein presented, the C product yield is conveniently relied upon to identify an undesired change in product distribution due to catalyst damage. These data show as previously discussed that treating an undamaged catalyst with ammonia had little or no effect on product distribution as evidenced by the yields obtained in Run 5 as compared with those obtained in Run 4 even though the operating temperature was raised F. to maintain a given desired conversion level. However, in Runs 1, 2 and 3, the selectivity of a damaged catalyst as represented by the yields obtained in Run 2 was substantially completely restored by treating the catalyst with ammonia of the order of about 100 ppm. to obtain the data represented for Run 3. That is, after treatment of the catalyst with ammonia, the product selectivity of Run 3 compares favorably with that obtained with the fresh catalyst of Run 1 even though the operating temperature to maintain a given conversion level was raised from 620 F. to 705 F., a change of F. Thus, applicants have unexpectedly found that the selectivity of a mixed base hydrocracking catalyst is not significantly affected by the addition of ammonia unless the catalyst has experienced damage as represented by a change in product distribution even though the hydrocracking temperature is raised to maintain a given conversion level.

Having thus provided a general description of the essence of the present invention and presented specific examples in support thereof, it is to be understood that no undue restrictions are to be imposed by reasons thereof except as identified by the following claims.

Catalyst sample Pressure p.s.i.g Recycle hydrogen gas,

s.c.f./b

Catalyst condition c.

Fresh Fresh Damaged P.p.m. NHa (as pyridine) in charge 0 Temp. required for 60% convJpass, F... ields:

Dry gas, wt.

percent C vol. percent. 0 vol. percent. O 180 F., vol.

percent 180-380 F., vol.

percent Material boiling above 650 F. in the recycle,

We claim:

1. In a process for efiecting the conversion of hydrocarbons by hydrocracking employing a mixed base hydrocracking catalyst comprising crystalline aluminosilicate in admixture with amorphous materials having cracking activity at a temperature in the range of 450 to 1,000 F. and a pressure in the range of 700 to 3,000 p.s.i.g., the improvement for producing desired jet fuel and gasoline products while limiting the production of C and lower boiling gaseous components which comprises: (a) initiating hydrocracking of a hydrocarbon charge with the mixed base hydrocracking catalyst at a temperature and pressure which will provide a desired product selectivity and conversion level for the fresh mixed base hydrocracking catalyst, (b) raising the hydrocracking temperature as required to maintain a desired conversion level as the catalyst ages, (0) monitoring the hydrocracked product to identify significant changes in product yield selectivity attributable to damage to the catalyst during the hydrocracking operation and '(d) contacting the damaged catalyst with an amount of ammonia which will restore substantially the catalyst product yield selectivity for the desired conversion level.

2. The process of claim 1 wherein the crystalline aluminosilicate hydrocracking component comprises from about 5 to about percent by Weight of the cracking base.

3. The process of claim 1 wherein selectivity of the catalyst is monitored as a'function of C products and/or dry gas yields and the catalyst selectivity is restored after identification of damage thereto by treatment with an appropriate amount of a compound yielding ammonia.

4. The process of claim 1 wherein product selectivity is continuously monitored by light adsorption spectroscopy and a change in 0, product yield greater than 20 volume percent is indicative of a damaged catalyst requiring treatment with ammonia in such amounts as to restore the catalyst selectivity commensurate with its selectivity before damage.

5. The process of claim 1 wherein product selectivity is continuously monitored by gas liquid chromatography to identify changes in product distribution indicative of catalyst damage.

References Cited US. Cl. X.R.