Catalytic dewaxing process with high temperature sorbent bed

Abstract

 A conventional catalytic dewaxing process is improved by high temperature, l77°C⁺ (350°F⁺) contact of feed with a sorbent before dewaxing. Preferred sorbents are high surface area materials, e.g., alumina. The sorbent bed helps the conventional dewaxing catalyst to regain most of its original activity after regeneration.

Description

[0001] This invention relates to improvements in processes for the catalytic hydrodewaxing of hydrocarbon chargestocks.

[0002] Particularly effective catalysts for catalytic dewaxing include ZSM-5 and related zeolites as described in U.S. Reissue Patent No. 28,398. Drastic reductions in pour point are achieved by selective conversion of the wax with hydrogen in the presence of ZSM-5.

[0003] Catalytic hydrodewaxing of middle distillates and higher boiling lubricating oil stocks has been successfully developed to a stage of commercial operation. The process effectively dewaxes most of the distillate feeds available. However, some feeds, of similar boiling point, are hard to dewax, and higher initial reaction temperatures are needed. Much of the difference may be due to compounds in the chargestock which are poisons to the catalyst. Nitrogen compounds are suspected of causing higher reaction temperatures.

[0004] It is known to remove nitrogen compounds prior to catalytic dewaxing. Hydrotreating and then hydrodewaxing is described in U.S. Patent No. 4,257,872. The hydrotreating catalysts are conventional.

[0005] It is also known to pretreat the feed with a zeolite molecular sieve maintained under sorption conditions. It was postulated that the feed contains minute amounts of catalytically deleterious impurities which were sorbed by the catalyst and served as catalyst poisons. Although the precise nature or composition of the catalyst poisons was not known, it was speculated that basic nitrogen compounds, and oxygen and sulfur compounds, may be involved. The substitution of a clay or other sorbent for the zeolite was also suggested as producing some increased activity, but of much lesser magnitude, than is achieved by the zeolite sorbent, although clay was thought to remove a greater fraction of the nitrogen compounds. Sorption conditions included 2 to 177°C (35° - 350°F). Patents disclosing such a process include U.S. 4,357,232; 4,358,362; and 4,358,363.

[0006] It has been found, however, that even upon removal of nitrogen compounds from the waxy hydrocarbon stock by solvent stripping or by use of ion exchange resins prior to catalytic dewaxing, the dewaxing catalyst ages in the same manner and, in fact, ages faster than catalysts dewaxing an untreated feed. Thus, it appears that the observed catalyst aging is more complex than any straight forward relationship to nitrogen content. Such is also shown in U.S. 4,357,232; 4,358,362; and 4,358,363 where, upon pretreating the feed to remove nitrogen compounds, catalyst behavior is not necessarily altered in a manner consistent with the removal of nitrogen compounds. Even after successive air regenerations of a dewaxing catalyst, catalyst deactivation is irreversible and that the aging rate of the catalyst increases over the fresh catalyst and further increases after each successive air regeneration.

[0007] Accordingly, there is a need for a dewaxing process which retains catalyst activity, even after multiple regenerations.

[0008] Accordingly, the present invention provides a process for preparing a high Quality lube base stock oil from a waxy chargestock by contacting the chargestock and hydrogen gas with a zeolite dewaxing catalyst comprising a zeolite having a constraint index of 1-12 to produce dewaxed lube base stock and deactivated dewaxing catalyst characterized by contacting the chargestock with a sorbent at temperatures greater than 177°C (350°F) before dewaxing.

Figure 1 is a plot illustrating aging of regenerated catalysts without a guard bed.

Figure 2 is a plot illustrating aging of regenerated and fresh catalysts without a guard bed.

Figure 3 is a plot illustrating aging of regenerated catalysts with a guard bed.

[0009] The feed may be any waxy hydrocarbon oil that has a pour point which is undesirably high. Petroleum distillates such as atmospheric tower gas oils, kerosenes, jet fuels, vacuum gas oils, whole crudes, reduced crudes, and propane deashalted residual oils are contemplated as suitable feeds. Also contemplated are oils derived from tar sands, shale, and coal. A particular embodiment of this invention is applicable to lube oil stocks in which the process of the present invention is used to prepare low pour point lube oil stocks with superior oxidation resistance.

[0010] Lubricating oils are based on petroleum fractions boiling above 232°C (450°F). The molecular weight is high. These fractions include almost all conceivable structures. This complexity and its consequences are referred to in "Petroleum Refinery Engineering," by W. L. Nelson, McGraw Hill Book Company, Inc., New York, New York, 1958 (4th Ed.)

[0011] The first step of the process of this invention requires that the feed be treated by contact with a sorbent to remove some of the deleterious impurity. These conditions may cover a fairly wide range of time, temperature and pressure, and may be conducted in the presence of hydrogen. The conditions, both broad and preferred, for this step of the process are indicated in Table 1.

[0012] The catalytically deleterious impurities, or poisons, will be referred to herein as "contaminants" regardless of whether these occur naturally associated with the feed or are acquired by the feed from some known or unknown source during transportation, processing, etc.

Table 1

Sorption Conditions
	Broad	Preferred
Temperature, °F	350+	400-800
°C	177+	204-427
Pressure, psig	0-3000	25-1500
kPa × 10⁻³	0.1-21	0.3-10
LHSV, hr⁻¹	0.1-100	0.2-20

It is preferred to pretreat feed in a fixed bed of 1.6 - 6.4 mm diameter extrudate or pellets. Other modes of contact may be employed such as slurrying the feed oil with a powdered sorbent followed by centrifugation and recycle of sorbent. The precise conditions for the sorption step may be determined by experiment.

[0013] The sorbents used as the guard bed include alumina, clay, bauxite, spent catalysts, etc. A sorbent having a surface area of at least 50 m²/g and at least l0% of the pores being in the range of 30-l00°A is useful in this invention. Gamma-alumina, eta-­alumina, or mixtures thereof are effective sorbents. Additionally, the sorbent may consist of molecular sieve type zeolites with or without exchange capacities having pores with an effective diameter of at least 5 Angstroms. Another class of sorbents that could be used also include a family of crystalline microporous aluminophosphates (U.S. Patent 4,385,994) and silicoaluminophosphate (U.S. Patent 4,440,871). These sorbents can be used alone or in combination with other refractory inorganic oxides.

[0014] Illustrative of zeolites with pores of 5 Angstroms are zeolite A in the calcium salt form, chabazite and erionite, which sorb normal paraffins but exclude all other molecules of larger critical diameter. Other zeolites which may be used which have larger pore diameters include zeolite X, zeolite Y, offretite and mordenite. The last group of zeolites sorbs molecules having critical diameters up to about 13 Angstroms, and all of them sorb cyclohexane freely.

[0015] In addition to these zeolites, any zeolites useful as dewaxing catalysts also may be used as sorbents. In fact, the zeolite sorbent and dewaxing catalysts may have the same crystal structure.

[0016] The pretreated feed is separated from the sorbent and passed to the catalytic dewaxing step.

[0017] In one embodiment the feed is contacted with a dewaxing catalyst under sorption conditions after which a pretreated feed is recovered and passed to storage. The sorbent is now treated, for

[0018] example with steam at elevated temperature, to remove the sorbed deleterious impurity, and the stored treated hydrocarbon is passed over the regenerated sorbent now maintained at dewaxing conditions. In general, however, it is more effective to employ at least one separate bed of molecular sieve zeolite as sorbent.

[0019] The catalytic dewaxing process is illustrated in U.S. Reissue 28,398 and U.S. 3,956,102 and 4,137,148. The dewaxing step may be conducted with or without hydrogen, although use of hydrogen is preferred. It is contemplated to conduct the dewaxing step at the dewaxing conditions shown in Table II.

Table II

	Broad	Preferred
	Without Hydrogen
Temperature, °F	400-1000	500-800
°C	204-538	260-427
LHSV, hr⁻¹	0.3-20	0.5-10
Pressure, psig	0-3000	25-1500
kPa × 10⁻³	0.1-21	0.3-10

	With Hydrogen
Temperature, °F	400-1000	500-800
°C	204-538	260-427
LHSV, hr⁻¹	0.1-10	0.5-4.0
H₂/HC mol ratio	1-20	2-1
Pressure, psig	0-3000	200-1500
kPa × 10⁻³	0.1-21	0.3-10

[0020] Useful dewaxing catalysts include zeolites having a constraint index of 1-12. Preferred are ZSM-5, ZSM-11, ZSM-12, ZSM-23, ZSM-35 ZSM-38, ZSM-48, and other similar materials.

[0021] ZSM-5 is described in US 3,702,866 and Re. 29,948.

[0022] ZSM-11 is described in U.S. 3,709,979.

[0023] ZSM-12 is described in U.S. 3,832,449.

[0024] ZSM-23 is described in U.S. 4,076,842.

[0025] ZSM-35 is described in U.S. 4,016,245.

[0026] ZSM-38 is described in U.S. 4,046,859.

[0027] ZSM-48 is described in U.S. 4,397,827.

[0028] When synthesized in the alkali metal form, the zeolite is conveniently converted to the hydrogen form, generally by intermediate formation of the ammonium form as a result of ammonium ion exchange and calcination of the ammonium form to yield the hydrogen form. In addition to the hydrogen form, other forms of the zeolite wherein the original alkali metal has been reduced to less than about 1.5 percent by weight may be used. Thus, the original alkali metal of the zeolite may be replaced by ion exchange with other suitable metal cations of Groups I through VIII of the Periodic Table, including, by way of example, nickel, copper, zinc, palladium, calcium or rare earth metals.

[0029] In practicing the invention, it may be useful to incorporate the above-described crystalline zeolite with a conventional matrix comprising another material resistant to the temperature and other conditions employed in the process.

[0030] Useful matrix materials include both synthetic and naturally occurring substances, as well as inorganic materials such as clay, silica and/or metal oxides.

[0031] Alumina, silica-alumina, silica-magnesia, silica-zirconia, silica-thoria, silica-beryllia, and silica-titania, as well as ternary compositions, such as silica-alumina-thoria, silica-alumina-zirconia, silica-alumina-magnesia and silica-magnesia-zirconia may be used. The matrix may be in the form of a cogel. The relative proportions of zeolite component and inorganic oxide gel matrix, on an anhydrous basis, may vary widely with the zeolite content ranging from 1 to 99 percent by weight and more usually in the range of 5 to 80 percent by weight of the dry composite.

Example 1 (Prior Art)

[0032] Bright stock raffinate having the properties set forth in Table 3 was dewaxed at 1 LHSV, 2900 kpa (400 psig), 450 n.l.l⁻¹ H₂ (2500 SCFB) over catalyst comprising 65% ZSM-5/35% alumina. Properties of the dewaxing catalyst are shown in Table 4. The catalyst was used for seven processing cycles with hydrogen reactivation in between the first four cycles (120 days) and oxygen regeneration, with air, for all subsequent cycles. In all cases air regeneration was carried out in situ circulating gas containing less than 1% 0₂ during initial coke burn. Final coke burn was accomplished by a stream containing 50% air at 510° (950°F).

[0033] The experiments were carrier out in stainless steel micro-units. The reactor was 5.3 mm (5/8˝) ID with a 3.2 mm (1/8˝) axial thermowell containing 15 cc (8.6 grams) of the as received extrudate catalyst diluted with 15 cc of 0.84 to 1.41 mm (14/20 mesh) vycor chips.

TABLE 3

Bright Stock Properties
Specific gravity	0.90

Viscosity
KV @ 100°C	29.71
KV @ 300°F (149°C)	9.31
Aniline point	251

Elemental analysis, wt.%
Carbon	85.53
Hydrogen	13.16
Sulfur	1.31
Nitrogen (ppm)	130
Basic Ni (ppm)	100
Ash, wt.%	0.01 to 0.02
Furfural (ppm)	8.0
Oil Content, wt%	80.62
Refractive index (70°C)	1.4872
COR, wt.%	.7
Paraffins	18
Naphthenes	34
Aromatics	48

Distillation:
	Wt.%	°F	°C
	IBP	800	427
	5	912	489
	10	944	507
	20	980	527
	30	998	537
	40	1014	546
	50	1030	554
	55	1045	563
	80	1100	593

TABLE 4

Catalyst Properties
Alpha activity	101
Crystallinity, %	55

Density, g/cc
Real	2.49
Particle	.80
Packed	0.58
Surface area, m²/g	350
Pore volume, cc/g	.714
Average pore diameter, A	82

Pore volume in pores of:
0-100 A	51
100-150 A	13
140-200 A	6
200-300 A	5
300+ A	25
Ni, wt.%	1.1
Na, wt.%	0.02

[0034] The aging curves in Fig. 1 summarize the reults of this study. Aging rates of 2.8°C/day (5°F/day), 4.7°C/day (8.5°F/day), and 5.6°C/day (l0°F/day) following first, second and third air regenerations were obtained. In addition, start of cycle temperature increased 2.8-5.6°C (5-l0°F) following every air regeneration. Upon completion of the dewaxing run following the third air regenerations, the catalyst was air regenerated and submitted for alpha and acid sites density mearsurements. The alpha values were evaluated after H₂S treatment of the fresh and spent catalysts. The reported alpha values were 120 and 140 for fresh and spent catalysts, respectively. Acid site densities of the fresh catalyst and the air regenerated spent catalyst are essentially the same at 0.202 and 0.221 MEQ N/g CAT, respectively.

Example 2 (Prior Art)

[0035] Bright stock of Example 1 was dewaxed in a micro-unit similar to the one used in Example 1. The reactor had a 5.3 mm (5/8˝) ID and contained 15 cc of the as received extrudate catalyst mixed with 10 cc of fine sand. In this example, the axial thermocouple was removed to maximize feed distribution throughout the catalyst bed. The catalyst was used for seven processing cycles with hydrogen reactivation in between the first four cycles and air regeneration for all subsequent cycles.

[0036] The aging curves for fresh and air regenerated catalyst are shown in Fig. 2. Aging rate increased from 1.3°C/day (2.3°F/day) for fresh catalyst to 2.3°C/day (4.1°F/day) and 3.1°C/day (5.5°F/day) for first and second air regeneration.

[0037] To partially answer the question of possible catalyst deactivation as a resut of air regeneration or steam dealumination, alpha value and acid site density of the spent and fresh catalysts were measured. Upon completion of the last cycle (7th cycle), the catalyst was air regenerated in a procedure similar to that used for the second air regeneration and submitted for analysis. Alpha values were evaluated after H₂S treatment of the fresh and spent catalysts. The reported alpha values were 125 and 115 for fresh and spent catalysts, respectively. Acid site densities of the fresh catalyst and the air regenerated spent catalyst are essentially the same at 0.23 and 0.24 MEQ N/g CAT, respectively.

[0038] These data are consistent with build-up of "materials" in the zeolite or the pore mouth or catalyst surface modification.

Example 3 (Invention)

[0039] The experiments were carried out in stainless steel two-stage reactor. The first reactor 5.3 mm (5/8˝) I.D. contained 30 cc of American Cynamid alumina (surface area 200m²/g) mixed with 20cc sand. The second reactor 5.3 mm (5/8˝) I.D. contained 15 cc of the as-received 101 alpha dewaxing catalyst of Example 1 mixed with 15 cc of the same sand. The experiments consisted of seven processing cycles with air regeneration in between the first four cycles. Standard hydrogen activation before the fifth cycle and air regeneration before the last two cycles. Hydrogen activation was done at 482°C (900°F) for 24 hours at 2200 kPa (305 psig) in once-through hydrogen circulation 450 n.l.l⁻¹ (2500 SCF/B), based on 1 LHSV. A fresh alumina guard bed was used with each new cycle (the alumina was dried by purging with hydrogen at 149°C (300°F).

[0040] Fresh and air regenerated catalyts were presulfided prior to catalytic evaluation. The guard bed, which was changed at the end of each cycle, was bypassed during high temperature purging, oxygen regeneration and presulfiding. Bypassing the guard bed kept desorbed materials from the dewaxing catalyst. Hydrogen and feed were passed over the alumina guard bed at 0.5 LHSV 288°C (550°F) and 2900 kPa (400 psig), and the product was cascaded over the dewaxing catalyst. Dewaxing conditions over the catalyst were 0.9 LHSV, 2900 kPa (400 psig) and 450 n.l.l⁻¹ (2500 SCF H₂/B). The guard bed temperature was held constant at 288°C (550°F), while the dewaxing reactor temperature was set to maintain specification pour point on the product.

[0041] Over four processing cycles with air regenerations in between cycles, the observed catalyst aging rates were 1.6°C/day, 1.4°C/day, 1.2°C/day, 1.4°C/day, (2.8°F/day, 2.5°F/day, 2.2°F/day and 2.6°F/day) for first, second, third and fourth cycle respectively. Start-of-cycle temperature as well as aging rate (aging rate accurate + 0.3°C (+ 0.5°F/day) are constant from one cycle to another, see Figure 3.

[0042] In Examples 1 and 2, with no alumina guard bed, catalyst aged as the number of cycles increased, regardless of catalyst reactivation mode. The use of a guard bed eliminated or drastically reduced irreversible catalyst aging. The guard bed did not change product yields or quality, and demonstrated the stability of the ZSM-5 catalyst. Based on these results, in the absence of guard bed, feed contaminants, or the interaction of the contaminants with the dewaxing catalyst can affect catalyst performance by restricting or slowing down the diffusion of branched and cycloakyl hydrocarbon into the zeolite.

[0043] Following the completion of the fourth cycle, the effectiveness of hydrogen rejuvenation in the presence of guard bed was studied. Compared to the fresh catalyst, hydrogen reactivation increased start of run conditions from 288 to 293°C (550° to 560°F). Aging rate increased from 1.6°C/day (2.8°F/day) for fresh catalyst to 2.4°C/day (4.3°F/day) following hydrogen reactivation. Standard hydrogen activation may restore activity but does not remove all the hydrocarbon deposits. These remaining hydrocarbon deposits slow down the diffusion of branched and cycloalkyl hydrocarbons. Two successive air regenerations (cycle 6 and 7) were carried out following the hydrogen reactivation cycle (cycle 5).

[0044] Cycle No. 6 lasted 42 days. The catalyst aged at 1.6°C/day (2.9°F/day). The catalyst was then air regenerated. Then bright stock was processed over this seventh cycle for seven days. During this short cycle the catalyst aging rate was 1.6°C/day (2.9°F/day).

[0045] The guard bed stabilizes the cycle to cycle dewaxing performance of dewaxing catalysts.

Claims

1. A process for preparing a high quality lube base stock oil from a waxy chargestock by contacting the chargestock and hydrogen gas with a zeolite dewaxing catalyst comprising a zeolite having a constraint index of 1-12 to produce dewaxed lube base stock and deactivated dewaxing catalyst characterized by contacting the chargestock with a sorbent at temperatures greater than l77°C (350°F) before dewaxing.

2. The process of claim 1 further characterized in that the sorbent is selected from the group of alumina, clay, bauxite, zeolites, and microporous crystalline solids containing phosphates.

3. The process of claim 1 further characterized in that the sorbent has a surface area of more than 50m²/g and pores and at least 10% of the pores are in the range of 30-100°A.

4. The process of claim 1 further characterized in that the sorbent is gamma alumina, eta-alumina or mixtures thereof.

5. The process of any preceding claim further characterized in that the chargestock contacts sorbent at a temperature of 204 to 427°C (400°F to 800°F); a pressure of 0.1 to 21000 kPa (0 to 3000 psig) at a LHSV at O.1-100.

6. The process of any preceding claim further characterized in that the sorbent is regenerated and is isolated from the dewaxing reactor during sorbent regeneration.

Drawing