Catalytic dewaxing of gas oils

Abstract

Improved catalytic dewaxing and desulfurization of high pour point, high sulfur gas oils to lower their pour point to about 10.degree.F or lower and their sulfur content by contacting a high pour point gas oil first with a ZSM-5 type zeolite hydrodewaxing catalyst which may contain a hydrogenation/dehydrogenation component, in the presence or absence of added hydrogen under low pressure conditions, followed by conventional hydrodesulfurization processing of the dewaxed intermediate.

Description

This invention relates to petroleum processing. It more particularly refers to upgrading certain gas oil fractions to low pour point fuels.

Liquid hydrocarbons in the 400.degree. to 850.degree.F boiling range include kerosine, Diesel fuel and No. 2, or home heating fuel. Petroleum fractions boiling in this range often have a significant long chain paraffin content which accounts for their rather high pour point. It is not unusual for fractions of this type to have pour points in the 50.degree. to 100.degree.F range. One significant specification for some No. 2 fuel oil is a pour point of 0.degree.F or lower. Thus reduction in pour point is accomplished by dewaxing or removing the long chain normal paraffins which seem to be principally responsible for this property.

This dewaxing has traditionally been accomplished by a solvent dewaxing technique according to which the gas oil is thoroughly mixed with a suitable solvent, often methyl ethyl ketone, into which the wax dissolves. In the past, the wax was recovered from the solvent, by evaporation of the ketone, purified and sold for seals, candles and other uses. More recently, the petroleum industry has seen a marked decline in this market and has therefore sought more profitable and less costly outlets for this wax constituent.

It has therefore been proposed to use shape selective zeolitic catalysts to crack, preferably hydrocrack, these paraffinic components and convert them into smaller, suitably paraffinic fragments which may be collected as light gas, or larger but not waxy paraffinic materials which remain in the boiling range of the gas oil being treated and therefore enhance the yield of the dewaxing operation. The smaller cracked fragments may be used as fuel, sold as such or resolved and disposed of by components.

Most recently it has been proposed to utilize the new ZSM-5 type of zeolite as a cracking or hydrocracking dewaxing catalyst because it converts not only the normal paraffins but the slightly branched paraffins as well thereby more efficiently lowering the pour point of the product with better yields than have heretofore been achieved. (See U.S. Pat. No. 3,700,585).

ZSM-5 type of zeolite is a group of small pored zeolites, the most noteworthy members of which are ZSM-5, ZSM-8 and ZSM-11. U.S. Pat. Nos. 3,702,886 directed to ZSM-5 zeolite and 3,709,979 directed to ZSM-11 zeolite are in point and are incorporated herein by reference. Abandoned U.S. Patent application Ser. No. 865,418, filed Oct. 10, 1969 is directed to ZSM-8 and is also in point and is incorporated herein by reference. The ZSM-5 family of zeolites can be used to catalyze cracking of normal or slightly branched paraffins in the absence of added hydrogen or to catalyze hydrocracking of such paraffins in the presence of added hydrogen. Typical cracking conditions include a space velocity of about 0.25 to 200 LHSV, a temperature of about 400.degree.F to 1100.degree.F and a pressure from subatmospheric to several hundred atmospheres. Typical hydrocracking conditions include a space velocity of about 0.1 to 10 LHSV, a hydrogen to hydrocarbon mole ratio of about 1 to 20 to 1, a temperature of about 650.degree.F to 1000.degree.F and a pressure of about 100 to 3000 psig (see U.S. Patent 3,700,585).

For most petroleum processing, it is conventional to pretreat the charge stock in some manner calculated to remove sulfur and/or metals and/or nitrogen from the charge stock prior to converting and upgrading such charge stock. Thus catalytic hydrodesulfurization is a unit process which is widely used in a refinery. It precedes reforming and other conversion processes because sulfur, nitrogen and/or metals present in the charge stock often have detrimental effects upon hydrocarbon conversion catalysts. Metals, sulfur and nitrogen tend to concentrate in the heavier petroleum fractions. Therefore conversion processes utilizing such heavier petroleum fractions would normally be considered prime users of the conventional desulfurization followed by catalytic conversion processing sequence.

Thus in the dewaxing of sulfur containing gas oils boiling in the 400.degree. to 900.degree.F range, or any part thereof, one of ordinary skill in the art would conventionally think of subjecting such gas oils to catalytic hydrodesulfurization prior to dewaxing. Indeed this may be a desirable modus operandi for dewaxing of gas oils with a "conventional" shape selective catalyst of the erionite type. It has been most unexpectedly found however that operating according to this sequence with ZSM-5 type zeolite dewaxing catalyst leads to a significantly decreased activity as compared to what one would expect (see Examples 1 and 2 infra). In fact the evidence would seem to lead one to the conclusion that the more sulfur and/or nitrogen there is in the gas oil charge stock, the greater is the diminution of ZSM-5 type zeolite catalyst life. Yet the product finally produced as a result of this dewaxing operation must have a reduced sulfur and/or nitrogen content or preferably substantially no sulfur or nitrogen content at all. This is because this product goes predominantly into the jet fuel, Diesel fuel and home heating market, where nitrogen and sulfur emissions must be closely controlled.

It is therefore an important object of this invention to provide an efficient gas oil dewaxing, desulfurization and denitrogenation process.

It is another object of this invention to provide such a process capable of taking advantage of the use of ZSM-5 type of zeolite dewaxing catalyst.

It is a further object of this invention to provide such a process for producing high yields of No. 2 fuel oil having a pour point of about 0.degree.F.

Other and additional objects of this invention will become apparent from a consideration of this entire specification including the claims hereof.

In accord with and fulfilling these objects, one important aspect of this invention resides in a processing sequence consisting essentially of catalytically dewaxing a sulfur and/or nitrogen containing gas oil having a boiling range in the range of about 400.degree.F to 900.degree.F and thence subjecting at least the liquid product resulting from such dewaxing to desulfurization and/or denitrogenation. In a preferred aspect of this invention the gas oils to which this process is directed have a total sulfur and nitrogen content of at least 0.1 wt. percent and 10 ppm, respectively, and a pour point of at least +20.degree.F. It is preferred to use a ZSM-5 type of zeolite catalyst for dewaxing and to operate the dewaxing portion of this process at about 300.degree. to 1000.degree.F, 0 to 2000 psig, 0.1 to 10 LHSV and a hydrogen to hydrocarbon ratio of about 0 to 25 to 1. Where the dewaxing portion of the process is of the hydrodewaxing type, the ZSM-5 type zeolite may have incorporated therein a hydrogen transfer functional component such as nickel, palladium, or platinum in a proportion of about 0.5 to 5 weight percent based on the total weight of dewaxing catalyst. The dewaxing catalyst may be used in a fixed or fluidized bed configuration as desired. In a fixed bed operation, the catalyst particles should be sized between one thirty-second and one-eighth inch. In a fluidized bed the catalyst particles should be about 80 to 400 mesh. The catalyst may be of the matrix type with alumina, silica, silica-alumina or other similar known matrix materials. In this matrix embodiment, the ZSM-5 type zeolite should constitute about 5 to 95 weight percent of the total catalyst matrix.

The product of catalytic dewaxing may be resolved into a liquid and gas portion by cooling to a prescribed temperature at an appropriate pressure to obtain whatever liquid-gas demarcation is desired. The liquid may then be subjected to desulfurization and denitrogenation. It is preferred however to subject the entire dewaxed product, without intermediate product resolution, to desulfurization and denitrogenation. By not resolving the intermediate product there is obtained a great saving in energy, particularly heat, because this resolution requires cooling and the subsequent desulfurization and denitrogenation requires reheating. Desulfurization and denitrogenation are usually accomplished simultaneously, one treatment serving to liberate both sulfur and nitrogen from the feed stock. This treatment can be carried out in any manner desired, e.g. conventionally at about 550.degree. to 800.degree. F, 200 to 1000 psig, 0.5 to 4.0 LHSV, 2 to 10 moles of added hydrogen per mole of feed stock, in effective contact with a cobalt-molybdenum-alumina-catalyst. This procedure converts sulfur and nitrogen values in the feed stock to hydrogen sulfide and ammonia which are later recovered from the gaseous portion of the final product.

While operating in the manner and sequence set forth results in excellent dewaxing and pour point reduction to acceptable levels with liquid yields of over about 80 percent and dewaxing catalyst life of about 20 to 400 days between regenerations to a total life of about 6 to 60 months, reversing the sequence of operations, that is desulfurizing and denitrogenating under the same conditions as set forth above and directly dewaxing the intermediate product thus formed to the same final product specifications, permitted operation of the dewaxing catalyst for about 1 to 24 hours between regenerations.

This invention will be illustrated by the following Examples which are not to be considered as limiting on the scope or applicability thereof. In these Examples parts and percentages are by weight unless expressly stated to be on some other basis.

EXAMPLE 1 (PRIOR ART)

A gas oil having a boiling range of 500.degree. to 800.degree. F, a pour point of +50.degree. F, a sulfur content of less than 3 wt. % and a nitrogen content of less than 3000 ppm was contacted with ZSM-5 zeolite at 700.degree. F, 400 psig, 5.0 hydrogen to hydrocarbon mole ratio and 3.0 LHSV. The conversion to 330.degree.F + liquid was 84 percent with the pourpoint reduced to 0.degree.F. Time between catalyst regenerations was 20 days.

EXAMPLE 2 (PRIOR ART)

A gas oil having a boiling range of 500.degree. to 800.degree. F, a pour point of + 50.degree. F, a sulfur content of 2.5 percent and a nitrogen content of 0.02 percent was contacted with a cobalt-molybdenum-alumina catalyst at 700.degree. F, 400 psig, 5.0 hydrogen to hydrocarbon ratio and 1.0 LHSV. The product thus formed was directly, and without intermediate product resolution, contacted with ZSM-5 zeolite at 700.degree. F, 400 psig, 5.0 hydrogen to hydrocarbon mole ratio and 3.0 LHSV. The yield of 330.degree.F + liquid was 96 percent. However, its pour point was reduced only from +50.degree.F to +25.degree.F, indicating that the ZSM-5 dewaxing catalyst was substantially deactivated.

EXAMPLE 3

Example 2 was exactly repeated except that the sequence of processing steps was reversed. The yield of 330.degree.F+ liquid was 84 percent and the pour point was reduced from +50.degree.F to 0.degree.F. Time between ZSM-5 catalyst regenerations was 20 days.

Claims

What is claimed is:

1. In the process of reducing the pour point of a sulfur and nitrogen containing gas oil boiling in the range of 400.degree. to 900.degree.F by treating such with a ZSM-5 type of zeolite at about 300.degree. to 1000.degree.F; the improvement, whereby reducing the pour point of said gas oil and purifying such to reduce the sulfur and nitrogen content thereof, which comprises subjecting said gas oil first to said zeolite treatment and then subjecting the whole, unresolved zeolite treated, reduced pour point intermediate to desulfurization and denitrogenation.

2. The improved process claimed in claim 1 wherein said hydrodesulfurization is carried out at about 550.degree. to 800.degree. F, 200 to 1000 psig hydrogen partial pressure, 0.5 to 4.0 LHSV and in effective contact with a cobaltmolybdenum-alumina catalyst.

3. The improved process claimed in claim 1 wherein said gas oil has a minimum of 0.1 wt. percent total sulfur and 10 ppm nitrogen before processing.

4. The improved process claimed in claim 1 wherein said zeolite treating is carried out at a hydrogen partial pressure of about 0 to 2000 psig and a space velocity of about 0.1 to 10 LHSV.