EP0181066A2

EP0181066A2 - Process for dewaxing heavy distillates and residual liquids

Info

Publication number: EP0181066A2
Application number: EP85306342A
Authority: EP
Inventors: Philip Varghese
Original assignee: Mobil Oil Corp
Current assignee: ExxonMobil Oil Corp
Priority date: 1984-10-29
Filing date: 1985-09-06
Publication date: 1986-05-14
Also published as: JPS61108693A; AU4685985A; CA1253107A; ZA856718B; AU586980B2; EP0181066A3

Abstract

Process for dewaxing heavy distillate and residual feed. Easily converted waxes are partially removed in a first catalytic dewaxing step, and the resulting product is separated to remove the light ends. The heavy liquid fraction, which contains difficult to convert waxes, such as poly-branched or mid-methyl branched paraffin waxes, is then introduced into a second dewaxing stage, where the liquid stream is dewaxed to a specification pour point. The process may be implemented via double pass dewaxing in existing units or by passing through two consecutive dewaxing units. Improvements are due to a decoupling of temperature requirements in each reaction step and removal of cracked products which inhibit dewaxing and accelerate aging in the second dewaxing stage.

Description

The dewaxing of hydrocarbons to liquids of lower pour point is a process of great commercial significance. The use of shape-selective catalysts such as ZSM-5 to selectively convert those paraffins that contribute the most to high pour points has many advantages over other methods.
Catalytic dewaxing of hydrocarbon oils to reduce the temperature at which precipitation of waxy hydrocarbons occurs is a known process and is described, for example, in the Oil and Gas Journal, January 6, 1975, pages 69-73. U. S. Patent No. Re 28,398 describes a process for catalytic dewaxing with a catalyst comprising ZSM-5 and a hydrogenation/dehydrogenation component. A process for hydrodewaxing a gas oil with'ZSM-5 is described in U. S. Patent No. 3,956,102. A mordenite catalyst containing a Group VI or Group VIII metal may be used to dewax a distillate from a waxy crude, as described in U. S. Patent No. 4,100,056. U. S. Patent No. 3,755,138 describes a process for mild solvent dewaxing to remove high quality wax from a lube stock, which is then catalytically dewaxed to specification pour point
Catalytic dewaxing processes may be followed by other processing steps such as hydrodesulfurization and de- nitrogenation in order to improve the qualities of the product U. S. Patent No. 3,668,113 describes a catalytic dewaxing process employing a mordenite dewaxing catalyst which is followed by a catalytic hydrodesulfurization step over an alumina-based catalyst U. S. Patent No. 4,400,265 describes a catalytic dewaxing/hydrodewaxing process using ZSM-5 wherein gas oil is catalytically dewaxed followed by hydrodesulfurization in a cascade system.
In catalytic dewaxing processes using shape-selective catalysts, such as ZSM-5, the waxy components, particularly the n-paraffins, are cracked by the zeolite into lighter products, such as paraffins, olefins and aromatics, some of which remain in the lube oil boiling range. Some lighter products are produced in the naphtha boiling range (boiling at less than 204°C (4000F)). Olefinic fragments are unstable to oxidation so the dewaxed oil may be subsequently hydrogenated over catalysts to saturate the olefins and improve the oxididation stability of the oil. The hydrogenation catalysts generally used are mild hydrogenation catalysts, such as a CoMo/Al₂O₃ type. The color of the oil may also be improved in this hydrofinishing.
U. S. Patent No. 4,428,819 to Shu et aI discloses a process for hydrofinishing a catalytically dewaxed oil in which the residual wax content of the dewaxed oil is isomerized over a hydroisomerization catalyst
Typically, heavier lube fractions (boiling above 316°C (600°F)) contain waxy components comprising normal paraffins, branched paraffins and cyclo paraffins. When a shape-selective catalyst, such as HZSM-5, is used to dewax these feeds, the normal paraffins crack much faster than the branched paraffins and cycloparaffins.
Recent experience with ZSM-5 based catalytic dewaxing has shown that the dewaxing of such heavier lube fractions pose significantly greater problems than that experienced with lighter feeds.
Heavier feeds cause catalysts to display a more rapid loss of activity. This loss of activity results in higher catalyst aging rates, so the reactor temperature must increase more rapidly.
Some of the reasons why heavier feeds'are harder to process have now been discovered. Specifically, the same degree of pour point reduction requires the conversion of substantially greater proportions of branched paraffins and other shape selectively hindered species for the heavier feeds. Thus, waxes from heavy neutral or bright stock raffinates, for example, contain smaller proportions of n-paraffins while light neutral derived waxes are largely n-paraffins. Secondly, it is now found that the branched paraffins may be classified into different groups with unique reactivity characteristics. Because of shape-selective cen- siderations, n-paraffins or end-methyl branched paraffins are significantly easier to convert than other waxes. Thirdly, mid-methyl branched paraffins and larger sterically hindered high molecular weight waxes, such as poly-branched paraffin waxes, are harder to convert than n-parrafin or end-methyl branched paraffins. In fact, the conversion of the harder' to convert waxes is inhibited by the presence of large quantities of the easier to convert waxes and the lower molecular weight analogs (primary products) derived from the molecular cracking of the easier to convert waxes. These primary products appear to be able to interact with remaining high molecular weight materials to cause rapid catalyst aging. Thus, small amounts of easy to convert light hydrocarbons, such as paraffins and olefins derived from primary conversion of n-paraffins and end-methyt branched paraffins, substantially inhibit the conversion of the less easily converted poly-branched and mid-methyl branched paraffin waxes.
It was discovered that a substantially improved dewaxing process for heavy feeds was possible by separating the easy and difficult conversion stages, and by removing the relatively light, cracked products intermediate easy and difficult conversion stage.

Fig. ₁ is a block flow diagram of an embodiment of the invention showing a first stage dewaxing unit, a separation unit, and a second stage dewaxing unit;
Fig. 2 shows a block flow diagram of a preferred embodiment comprising at least one reactor in each dewaxing stage;
Fig. 3 is a block flow diagram of a second embodiment of the invention showing a single stage dewaxing unit, a separation unit, tankage and an intermittent recycle stream to the single dewaxing unit;
Fig. ₄ is a block flow diagram of a third embodiment of the invention showing a single dewaxing unit, a separation unit and a continuous recycle stream to the single dewaxing unit;
Fig. ₅ is a plot of pour point versus days on stream for 3 feeds with varying amounts of 204°C' (400°F) hydrocarbons; and
Fig. 6 is a plot of pour point versus days on stream comparing one stage dewaxing to two-stage dewaxing for a single overall space velocity.

The present process is applicable to feedstocks, including lube stocks, when a low wax content is desired in the final product. This process is especially useful for feeds with pour points higher than 21 °C (70°F). The feeds may be whole crudes or fractions, and may have been subjected to other refinery processes.
A feedstock 2, as shown in Fig. 1, comprising high pour point waxy feed, passes through a preheater (not shown) and contacts a dewaxing catalyst contained in a first stage dewaxing unit 4. First stage dewaxing unit 4 operates at a temperature of 204 to 427°C (400° to 800°F), and pressure of 1,500 to 7,000 kPa (200 to 1000 psig), and liquid hourly space velocity (LHSV) between 0.5 and 3 hr.1 . The feedstock 2 comprises n-paraffin waxes, end methyl branched waxes, poly-branched waxes and mid-methyl branched waxes. In the first stage dewaxing unit 4, the easily converted waxes, such as the n-paraffin waxes and end-methyl branched waxes, are cracked to lighter products, such as C-, gases and light paraffinic and olefinic fragments, some of which remain in the lube oil boiling range, but most of which are in the naphtha range. An effluent stream 6 from the dewaxing unit 4 discharges into separator 8. Separator 8 separates stream 6 into a vapor stream 10 and a liquid stream 12. The separation may be accomplished by lowering the pressure and flashing the effluent stream 6 or by distilling the effluent stream 6 or by allowing vapor liquid separation to occur at an elevated pressure and temperature. The separator removes those materials boiling below 204°C (400°F), and preferably those boiling below 371 °C (700°F). The composition of the liquid stream 12 and vapor stream 10 can be adjusted by adjusting the temperature and pressure in separator 8.
The vapor stream 10 may be sent to downstream processing, such as distillation, while the liquid stream 12 passes into the second stage dewaxing unit 14 which may operate within the same ranges of temperature and pressure specified for the first dewaxing unit.
The relative operating conditions in the second stage dewaxing unit 14 are preferably more severe than those of the first stage dewaxing unit 4, to obtain a product stream 16 that meets pour point specifications by cracking the difficult to convert waxes, such as the poly-branched waxes and mid-methyl branched waxes, in liquid stream 12. Preferably, the ratio of LHSV in the first stage dewaxing unit 4 relative to dewaxing unit 14 will be 5:1 to about o.5:1.The second stage dewaxing unit 14 then produces the product stream 16 which is passed to downstream processing, such as hydrofinishing into final product.
This promotes removal of the primary reaction products of the cracking of n-paraffin waxes and end-methyl branched waxes from the feed to the second stage de- waxer.The primary products inhibit the cracking of remaining uncracked stock. It is also theorized that the primary products react with the remaining uncracked stock because the primary reaction products are often olefins which can cyclize and/or alkylate to heavier components in the stock. The primary reaction products, such as light hydrocarbons, especially naphtha boiling range products, may inhibit the reaction of the heavier uncracked stock because they are more rapidly absorbed into catalyst volume, thus in effect accelerating the measured rate of catalyst aging for dewaxing to the desired product.
The catalysts employed in the first and second stage dewaxing units 4, 14 may be the same type or different. Preferably, they possess shape-selective paraffin cracking ability. Catalysts that have shape-selective qualities include crystalline zeolite catalysts and crystalline silica alumina phosphate (SAPO) catalysts. These materials may be bound in a variety of matrices, such as silica alumina or silica and alumina alone. The catalysts should contain a hydrogenation/dehydrogenation component hereafter. The preferred hydrogenation components are the noble metals of Group VIII, especially platinum and palladium, but other noble metals, such as irridium, ruthenium or rhodium, may also be used. Combinations of noble metals with non-noble metals, such as nickel, rhenium, tungsten, chromium and molybdenum are of interest.
Combinations of Group VIB and Group VIII are also of interest. Base metal hydrogenation components may also be used, especially nickel, cobalt, molybdenum, tungsten, copper or zinc. Up to 15% metal may be added, though usually much less noble metal promoter is needed.
The metal may be incorporated into the catalyst by any suitable method such as impregnation or exchange onto the zeolite. The metal may be incorporated in the form of a cationic, anionic or a neutral complex,, such as Pt(NH,) 9-and cationic complexes of this type will be found convenient for exchanging metals onto a zeolite. Anionic complexes are also useful for impregnating metals into the zeolites.
A portion of zeolites useful herein are termed medium pore zeolites and are characterized by an effective pore size of generally less than about 7 angstroms, and/or pore windows in a crystal formed by 10-membered rings. The medium pore zeolites include ZSM-5, ZSM-11, ZSM-23, ZSM-35, ZSM-38, ZSM-48 and TMA Offretite.
Another class of zeolites important to the present invention are large pore zeolites. These have a pore size sufficiently large to admit the vast majority of components normally found in a feedstock, generally in excess of 7.5 angstroms and/or formed by 12-membered rings. The large pore zeolites are represented by ZSM-4, ZSM-12, ZSM-20, Zeolite Beta, Mordenite, TEA Mordenite, Dealuminized Y, and Rare Earth Y. Additionally, the large pore component may include a low sodium Ultrastable Y molecular sieve (USY).
ZSM-4 is described in U. S. 3,923,639.
ZSM-5 is described in U. S. 3,702,886.
ZSM-₁₁ is described in U. S. 3,709,976.
ZSM-12 is described in U. S. 3,832,449.
ZSM-20 is described in U. S. 3,972,983.
ZSM-23 is described in U. S. 4,076,842.
ZSM-35 is described in U. S. 4,016,245.
ZSM-38 is described in U. S. 4,046,859.
ZSM-48 is described in U. S. 4,397,827.
Zeolite Beta is described in U. S. 3,308,069 and Re. 28,341.
USY is described in U. S. 3,293,192 and 3,449,070.
The first and second stage dewaxing units 4, 14 respectively, may be part of the same reactor, may be in separate vessels, or may each consist of a plurality of vessels. A preferred embodiment, as shown in Fig. 2, comprises a series of two or more reactors 40, 42, 44, 46 and 48 for each stage. As shown in Fig. 2, the first stage dewaxing unit 4 comprises two reactors 40, 42, and the second stage dewaxing unit 14 comprises two other reactors 44, 46. However, the units 4, 14 could operate with only one reactor per unit.The reactors 40, 42, 44 and 46 contain dewaxing catalysts and are operated in series. While the reactors 40, 42, 44 and 46 are in operation, the catalyst in the remaining reactor 48 could be reactivated/ regenerated.A solid line, shown in Fig. 2, represents the path of hydrocarbons through the reactors 40, 42, 44 and 46. A feedstock passes into the first reactor 40 to produce an outlet stream 50 which passes into the second reactor ₄2. The outlet stream 52 from the second reactor 42 becomes an effluent stream 6 which passes into the separation unit 8 to form a vapor stream 10 and a liquid stream 12. The liquid stream 12 passes into the third reactor 44 to produce an outlet stream 54 which passes into the fourth reactor 46 to produce an outlet stream 56 which forms the product stream 16.
As shown by Fig. 2, headers 24, 26 and 28 are provided so that the reactors may be rotated, thus allowing for on-line reactivation/regeneration of catalysts in any one of the reactors. The headers 24, 26 and 28 allow flashing or distillation between any two or the reactors 40, 42, 44, 46 and 48. The feedstock 2 feeds the feed header 24 which is attached to each of the reactors 40, 42, 44, 46 and 48. The outlets 50, 52, 54, 56 and 58 from each reactor 40, 42, 44, 46 and 48, respectively, can feed either the product header 28 or the separation unit header 26. The effluent 6 from the first stage dewaxing unit passes into the separation header 26 and subsequently into the separation unit 8. The product stream 16 passes into the product header 28 and subsequently to downstream processing. This arrangement will have an added benefit because catalysts that age far enough to be unsuitable for use in the more severe second stage dewaxing unit 14 could be switched to duty in the first stage dewaxing unit 4, allowing another reactor with its catalyst charge to be freed for rotational reactivation/regeneration. Appropriate valving (not shown) would be provided to direct flow to the correct headers and units.
Fig. 3 shows an alternative embodiment of the invention, in which a feedstock 2 passes into a dewaxing unit 30 under the first set of operating conditions outlined above. The effluent 6 then passes to a separation unit to form a vapor stream 10 and a liquid stream 12.The liquid stream 12 is then stored in tankage 18, such as any suitable tankage storage area located on a plant site. Then, after all of the feedstock 2 has been run through the dewaxing unit 30, alternately to catalytic dewaxing of the feedstock 2, an effluent 20 comprising the hydrocarbons from the liquid stream 12 is catalytically dewaxed in the dewaxing unit 30, which operates at the second set of operating conditions outlined above. The effluent 6 would then form a product stream 16 which passes to downstream processing.
An alternate embodiment of the invention is shown in Fig. ₄, in which a feedstock 2 and a recycle stream 22 are catalytically dewaxed in a dewaxing unit 30 to produce an effluent 6. The dewaxing unit 30 temperature is 204 to 426°C (400°F to 800°F). pressure is 1,500 to 7,000 kPa (200 to 1000 psig), LHSV is 0.25 to 5 hrl, based on feedstock 2, and the recycle ratio of recycle stream 22 to feedstock 2 is 0.5 to 20.The dilution of fresh feed by once processed and partially dewaxed stock ensures that the level of light hydrocarbons, developed as primary products, present in the reactor will be substantially reduced.The effluent 6 passes to a separation unit 8 to form the liquid stream 12 and the vapor stream 10. The liquid stream 12 is then separated into the product stream 16 and the recycle stream 22 is combined with the feedstock 2 and recycled to the dewaxing unit 30. It should be understood that in all of the above embodiments catalytic dewaxing may occur in the presence or absence of added hydrogen.
By dewaxing hydrocarbons using dewaxing units under two sets of conditions, dewaxing may be accomplished by an easy conversion step and a relatively more difficult conversion step. By separating the easy and difficult conversion steps one can control the temperature in the two steps, allowing significantly lower overall aging rates and thus, higher capacity factors. Separating a vapor stream from a dewaxing unit effluent prior to a second pass over dewaxing catalyst removes components which inhibit further dewaxing and accelerate catalyst aging.

Examoles

Laboratory tests, described below, were conducted on a bright stock comprising a furfural extracted, propane deasphalted vacuum resid having the following properties:
The tests were conducted at a constant space velocity, pressure and temperature and record the change in pour point versus the number of days on stream. Such comparisons allow both an estimate of dewaxing ability and aging rates to be determined simultaneously. Example 1, shown in Fig. 5, is a plot of pour point versus days on stream and compares feed run by itself over catalyst against feed run, with 3% and 6% added hydrocarbons boiling below 204°C (400 °F). These were mixtures of paraffins boiling in the naphtha range, i.e., liquids boiling below 204°C.These tests were run at a temperature of 354°C (670°F), a pressure of 2,900 kPa (400 psig) and a space velocity of 1 hr-^1. As explicitly shown in Fig. 5, the light hydrocarbons caused an inhibition of catalyst activity and acceleration of the aging rate. These results demonstrate that small proportions of easily cracked and diffusionally favored light hydrocarbons can have significant inhibitive and deactivating effects on the catalyst relative to the main process objective, which is the conversion of waxes in heavier hydrocarbons to produce a product with a desired pour point specification.
Example 2, shown in Fig. 6, compares the pour point versus days on stream at constant space velocity and temperature of a feed dewaxed in a single dewaxing stage as opposed to a feed dewaxed at the same overall space velocity, temperature and pressure in two. stages with separation and removal of a vapor stream in between the two stages. The test was run at 299°C (570°F), pressure of 2,900 kPa (400 psig) and 1.0 LHSV for the single stage unit and 2.0 LHSV per stage for the two-stage units. Both the one-stage and two-stage units contain the same amount of catalyst. The catalyst was a Ni-ZSM-5. The separation was a laboratory fractionation, to remove 204°C (400°F) and lighter material from the liquid feed to the second stage. This shows lower pour points and extended catalyst life when two stages and a separation stage are used.

Claims

1. A method for dewaxing a relatively heavy wax containing hydrocarbon feedstock (2) by catalytically dewaxing the feedstock at conventional catalytic dewaxing conditions in a first stage reactor (4) to produce a first stage effluent stream (6) comprising a relatively light fraction comprising naphtha boiling range and lighter hydrocarbons and a relatively heavy, partially dewaxed feed characterized by separating the effluent stream (6) into a relatively heavy partially dewaxed feed (12) with a reduced naphtha content and a light fraction (10) and catalytically dewaxing the partially dewaxed feed (12) at conventional conditions in a second stage catalytic dewaxing reactor (14) to produce a dewaxed product stream (16).

2. The method of Claim 1 wherein the feedstock comprises at least one of n-paraffin waxes and end-methyl branched waxes.

3. The method of Claim 1 or 2 wherein the partially dewaxed feed comprises at least one of poly-branched waxes and mid-methyl branched waxes.

4. The method of any of Claims 1 to 3 wherein the partially dewaxed liquid (12) comprises 371°C (700°F^-) hydrocarbons.

5. The method of any of Claims 1 to 4 wherein the LHSV in the first reactor (4) is 0.5 to 3 hr-' and the LHSV in the second reactor is 0.1 to 6 (14).

6. The method of any of Claims 1 to 5 wherein the first stage reactor (4) catalyst comprises at least one of ZSM-4, ZSM-5, ZSM-11, ZSM-12, ZSM-20, ZSM-23, ZSM-35, ZSM-38, ZSM-₄8, TMA Offretite, Mordenite, TEA Mordenite, Dealuminized Y, Rare Earth Y, Ultrastable Y and Zeolite Beta.

7. The method of any of Claims 1 to 6 wherein the catalyst in the second reactor (14) comprises at least one zeolite of ZSM-₄, ZSM-5, ZSM-11, ZSM-12, ZSM-20, ZSM-23, ZSM-35, ZSM-38, ZSM-48, TMA Offretite, Mordenite, TEA Mordenite, Dealuminized Y, Rare Earth Y, Ultrastable Y and Zeolite Beta.

8. The method of any of Claims 1 to 7 wherein the partially dewaxed liquid (12) is accumulated in tankage (18), prior to catalytic dewaxing in the second stage (30) and wherein the same reactor (30) and catalyst are used for both first and second stage dewaxing.

9. The method of any of Claims 1 to 8 wherein separation of the light fraction from the partially dewaxed feed is accomplished in a vapor liquid separator intermediate the two dewaxing reactors.

10. The method of any of Claims 1 to 9 wherein separation of the light fraction from the partially dewaxed feed is accomplished in a fractionator intermediate the two dewaxing reactors.

11. The method of any of Claims 1 to 10 wherein the partially dewaxed feed (12) to the second stage contains less than 6 wt.% naphtha and lighter boiling range materials.

12. The method.of any of Claims 1 to 11 wherein the feed (2) contains a mixture of n-paraffin, end-methyl branched mid-methyl branched and poly branched waxes and wherein a majority of the conversion of n-paraffin and end methyl branched waxes occur in the first stage (4) and a majority of the conversion of mid-methyi and poly-branched waxes occurs in the second stage (14).