Production Of Low Pour Fuel Oils

Abstract

Low pour fuel oils are obtained by first converting waxy pour residua into the corresponding viscous pour residua by deep vacuum fractionation, with or without auxiliary visbreaking or thermal or catalytic cracking, and then blending with sufficient low pour cutter stock to produce fuel oils having commercially acceptable viscosities and pour points.

Description

This invention relates to the production of fuel oils of commercially acceptable viscosities and pour points from waxy crudes such as Libyan crudes. The invention includes methods whereby residual fractions obtained from such crudes by vacuum fractionation, with or without visbreaking, catalytic cracking or other auxiliary treatment, are converted by blending with low pour point cutters into fuel oils having acceptable viscosities and unexpectedly low pour points.

Waxy crudes can be defined as those whose atmospheric reduced crudes have high pour points and which cannot be converted into fuel oils meeting a prescribed pour point specification (50.degree.F. or lower for low pour point fuel oils) without using so much cutter oil as to reduce the viscosity of the resulting fuel oils to an unacceptably low level. Viscous pour residua are those whose pour point is primarily due to their viscosity whereas, on the other hand, waxy pour residua are those whose pour points are primarily due to their wax content. For a given viscosity residuum, a waxy pour residuum will have a higher pour point than a viscous pour residuum because the wax present therein will cause it to solidify at a higher temperature.

U. S. Pat. No. 3,303,128 describes a procedure for converting waxy crudes into fuel oils that meet commercial requirements in Great Britain, where relatively high pour points are tolerated. This patent shows that a maximum pour point of 70.degree.F. and a viscosity range of 40 to 90 cs at 75.degree.C. can be obtained by vacuum distillation of waxy crudes under conditions to obtain a final vacuum residue boiling above 550.degree.C. (1,022.degree.F.) at a pressure corrected to 760 mm. Hg but below the temperature at which cracking of the feedstock commences. The patent shows that when this is done the heat-modified residues can be blended with a typical catalytically cracked gas-oil cutter stock, having a pour point of 20.degree.F., in proportions such that the resulting fuel oil will meet British specifications.

In the United States, most fuel oils must have viscosities of about 50 to 240 Saybolt Universal Seconds (SUS) at 210.degree.F. and a pour point of 50.degree.F. or less. This combination is considerably more difficult to obtain than the British requirements mentioned above. For example, Table 2 of the patent shows that with a Nigerian crude residue a fuel oil having a 40.degree.F. pour point is obtainable only after vacuum distillation to a TBP cut point of 595.degree.C. (1,103.degree.F.) followed by cutting the residue with 52 percent by weight of gas oil and that this pour point could not be reached at all with a Libyan crude, even when distillation temperatures as high as 620.degree.C. were used.

By our invention fuel oils meeting the viscosity and pour point requirements conventionally required in the United States can be obtained from waxy crudes by blending their viscous pour residua with certain diluents hereinafter defined as low pour cutter stocks. We have discovered that blends containing these low pour cutters exhibit two unexpected and hIghly important properties.

The first of these is that their pour points are much lower (usually more than 50.degree.F. lower) than would be expected frOm calculations based on linear blending of component pours. This is illustrated by the following table wherein 1,020.degree.F.+ Libyan residua are blended with the +20.degree.F. cutters used in U.S. Pat. No. 3,303,128 and with low pour cutters having a pour point of -10.degree.F. ##SPC1##

The second advantage possessed by our low pour cutters is that they are effective with a much wider range of viscous pour residua than could be used with the cutters previously employed. Thus, Libyan virgin crudes need be distilled only to 950.degree.F. or higher residua, and by the use of visbreaking the depth of fractionation required to obtain a residuum that will blend with a low pour cutter to meet fuel oil requirements can be still further reduced. To obtain a suitable fuel oil from less waxy crudes, such as Mata crude, the virgin residuum need be cut only to a 850.degree.F. or higher IBP. This advantage will be further described and illustrated with reference to the accompanying drawings.

We have found that the above and other advantages are obtained by using cutters with pour points between 0.degree.F. and -85.degree.F. or lower. Thus any cutter with a pour point of 0.degree.F. or lower can be used to obtain low pour fuel oils from waxy crudes, and these are the materials herein designated as low pour cutter stocks. It is an important advantage of the invention that they may be obtained from the same stocks used in producing high pour cutters, having pour points on the order of +20.degree.F. or higher, by reducing the end point of the distillation in which they are produced. To illustrate this, light gas oils obtained from the total liquid product from once-through visbreaking of Libyan reduced crude were tested for pour points. The 350.degree.-550.degree.F. fraction had a -30.degree.F. pour point and the 350.degree.-650.degree.f. had a +5.degree.F. pour point. Thus a 35.degree.F. lower pour point cutter was obtained by reducing the cutter end point from 650.degree.F. to 550.degree.F. They can also be obtained by other procedures such as thermal cracking, catalytic cracking, visbreaking and the like. Representative cutter stocks obtained by these procedures are further described in Example 1.

The amount of cutter to be mixed with viscous pour residua will depend largely on the fuel oil specifications that must be met. Since cutter stocks are normally worth more than the fuel oils into which they are blended, it is desirable to add only enough cutter to meet maximum viscosity specifications as well as the 50.degree.F. maximum pour point mentioned above. Within these limits the exact amount will vary with the particular viscous pour residua being treated and the type of cutter stock used, but will usually be within the range of about 15 percent to 45 percent by weight and, in most cases, within the preferred range of 20 percent to 40 percent, based on the weight of the mixture of residuum and cutter. It is understood that in the preferred practice of the invention, a waxy crude is first pretreated by vacuum distillation, with or without an auxiliary treatment such as visbreaking thermal cracking or both, or catalytic cracking, to convert its waxy pour residuum into viscous pour residuum after which this viscous pour residuum is blended with an amount of a low pour cutter sufficient to produce a fuel oil having a maximum pour point of 50.degree.F. and a viscosity within the range of about 50 to 240 SUS at 210.degree.F. The preferred new fuel oils of the invention are those produced by this procedure.

In addition to the advantages outlined above, our invention also makes available a source of low sulfur fuel oils of great commercial importance. This is because waxy crudes normally have low sulfur contents. This is shown in the following tabulation where 30 weight percent of cutter was added to various residua from waxy crudes and the sulfur content of the resulting blends was determined.

No. Waxy Resid Cutter Fuel Oil Blend Crude Type Type Sulfur, Wt. % 1 Libyan 1020.degree.F. + 350.degree.-650.degree.F. 0.72 Virgin VisbrOken 2 Libyan 850.degree.F.+ do. 0.60 Visbroken 3 Mata 850.degree.F.+ 350.degree.-650.degree.F. 1.36 4 Nigerian 1020.degree.F.+ 350.degree.-650.degree.F. 0.43 Virgin Visbroken 5 1020.degree.F.+ do. Orito 1.00 Virgin

Our invention also includes certain processing procedures in which our low pour point blending is applied on a commercial scale. The simplest of these is vacuum distillation by the procedure shown diagrammatically in FIG. 1 of the drawings, wherein a waxy crude is first charged to atmospheric fractionating tower 1 for the separation of volata e volatile A portion of a 350.degree.-550.degree.F. fraction, taken off through line 2, is bypassed through line 3 for use as a low pour cutter, as has been described above. The residue leaving through line 4 is fed into a vacuum tower 5, where an overhead waxy gas oil portion is separated. The viscous pour residuum leaving through line 6 is converted into low pour fuel oil in accordance with the present invention. It will be understood that the temperature employed in tower 5 will vary from one feedstock to another, the controlling factor being the point where the residuum goes from a predominatly waxy pour to a predominantly viscous pour.

FIG. 2 shows diagrammatically a process in which visbreaking is combined with the procedure shown in FIG. 1. The tower 1, line 2 and bypass line 3 are the same as in FIG. 1 but the residue leaving the tower 1 through line 14 is passed through a visbreaking furnace 15 where it is heated to incipient cracking. It is then discharged into an atmospheric flash tower 16, from which the partially cracked overhead product is returned through line 17 to tower 1 for fractionation with the waxy crude feedstock. The visbroken residuum leaves flash tower 16 through line 18 and is fed into a vacuum tower 19, similar in operation to tower 5 of FIG. 1, where an overhead waxy gas oil portion is separated The residuum, which for example may be the 850.degree.F.+ residuum of a Libyan waxy crude, leaves tower 19 through line 20 and now has a viscous pour. It therefore is converted into a fuel oil meeting the desired specifications by blending with the requisite quantity of the low pour cutter obtained from line 3.

In some instances, particularly in foreign countries it is often desired to maximize middle distillate (furnace oil or diesel fuel) production from a crude. Two procedures for accomplishing this objective are shown in FIGS. 3 and 4 of the drawings.

In the process shown diagrammatically in FIG. 3 the atmospheric reduced waxy crude, such as a Libyan crude, is visbroken in a furnace 21 and charged to atmospheric column 22 operating at atmospheric pressure, where gases, gasoline and a furnace oil or diesel fuel fraction are separated. The stripped residue passes through line 23 to a vacuum distillation column 24, the overhead fraction of which is passed through line 25 to a thermal cracking furnace, 26. In this furnace it is heated to a cracking temperature, after which it is recycled through line 27 to atmospheric after which it is recycled through line 27 to atmospheric column 22 in admixture with further quantities of visbroken feed. The residuum from vacuum column 24, which now has a viscous pour, is withdrawn through line 28 for blending with a low pour cutter such as a fraction of the diesel or furnace oil boiling in the 350.degree.-550.degree.F. range.

In a modification of this procedure the atmospheric reduced waxy crude is not visbroken. The overhead from vacuum tower 24 would then be a waxy gas oil which would be recycled to extinction through a visbreaker furnace. To maximize middle distillates production, the residuum in line 28, produced from the vacuum tower, would be a high IBP material (1,000.degree.F. IBP or greater), the quantity of which would be correspondingly reduced. By cutting a deep vacuum residuum in this manner the available visbreaker recycle would be increased, giving rise to greater production of middle distillates from recycle thermal cracking heater 26.

In FIG. 4, a procedure somewhat similar to that of FIG. 3 is shown in which the thermal cracker on the recycle stream is replaced by a catalytic cracker. In this process the atmospheric reduced waxy crude is not visbroken; it is introduced through feed line 31 into a vacuum distilling column 32. A distillate from this column, having a 550.degree.-650.degree.F. IBP, is withdrawn through line 33 and introduced into fluid catalytic cracking unit 34. In order to maximize middle distillate production, instead of gasoline, this cracker is operated at reduced severity. Thus, instead of obtaining about 50 volume percent per pass of gas oil conversion to gasoline and lighter products, the per pass conversion is reduced to about 30 volume percent or lower.

The products leaving catalytic cracker 34 through line 35 are separated in the usual manner in atmospheric column 36, the bottoms being withdrawn through line 37 and recycled to extinction by admixture with the catalytic cracker feed. The desired furnace oil or diesel fuel fraction is withdrawn through line 38 as a side stream from column 36.

The column 32 is operated at temperatures such that the residuum leaving through line 40 will have a viscous pour. Thus in the case of a reduced Libyan crude the residue in this line will have an IBP of at least 950.degree.F. and preferably about 1,020.degree.F. The material in this line can therefore be converted into a low pour fuel oil by blending with suitable quantities of low pour cutter, such as a suitable fraction obtained through line 41.

Typical results obtainable by the processes of FIGS. 3 and 4 are described in Examples 4 and 5, respectively.

The invention will be further described and illustrated by the following specific examples. It should be understood, however, that although these examples may describe some of the more specific features of our invention they are given primarily for purposes of illustration and the invention in its broader aspects is not limited thereto.

Example 1

Low pour point cutter stocks suitable for use in practicing the invention are obtainable from virgin and cracked light gas oils:

CUTTER NO. 1

This is a 350.degree.-550.degree.F. fraction derived from an Amna virgin crude and has the following properties:

API Gravity 45.2 Viscosity, SUS at 100.degree.F. 32.1 Pour Point, .degree.F. -20 Sulfur, Wt. % 0.091

The following were obtained from Libyan crudes. The thermal cracked light gas oils were from visbreaking runs charging Libyan reduced crude.

Cutter NO. 2

Thermal cracked 350.degree.-650.degree.F. gas oil: API Gnavuty 35.5 VaScosity, SUS at 100.degree.F. 33.4 Pour Point, .degree.F. -5 Sulfur, Wt. % 0.35

CUTTER NO. 3

Thermal cracked 350.degree.-650.degree.F. gas oil: API Gravity 35.7 Viscosity, SUS at 100.degree.F. 32.2 Pour Point, .degree.F. -10 Sulfur, Wt. % 0.36

CUTTER NO. 4

Thermal cracked 350.degree.-650.degree.F. gas oil: API Gravity 32.9 Viscosity, SUS at 100.degree.F. 33.3 Pour Point, .degree.F. 31 20 Sulfur, Wt. % 0.25

CUTTER NO. 5

Thermal cracked 350.degree.-650.degree.F. gas oil: API Gravity 32.4 Viscosity, SUS at 100.degree.F. 33.6 Pour Po nt, .degree.F. -35

CUTTER NO. 6

This was a catalytically cracked 350.degree.-550.degree.F. gas oil fraction. It has a pour point of -85.degree.F.

CUTTER NO. 7

This was a catalytically cracked 350.degree.-650.degree.F. gas oil fraction having a pour point of -10.degree.F.

The use of these cutters will be illustrated in subsequent examples. It will be understood that, if desired, they can be used in admixture with each other, or in admixture with other gas oils, to obtain cutter compositions having any desired pour point of 0.degree.F. or lower. Such mixtures are sometimes more compatible with certain residua, and particularly with some visbroken residua, than are single gas oil fractions.

EXAMPLE 2

Typical fuel oils derived from Libyan crude are shown in the following table. ##SPC2##

The importance of converting the residua from waxy pour to viscous pour is evident from the above results.

EXAMPLE 3

Visbreaking is a process in which a petroleum feedstock such as a topped crude is heated and thermally cracked slightly in a visbreaker furnace. It is described, for example, on page 154 of the September 1969 issue of "Hydrocarbon Processing." We have found that visbreaking cracks or alters the wax in the vacuum residuum of a waxy crude and thus converts its pour characteristics from waxy to viscous.

Fuel oils obtained by cutting visbroken Libyan residua with low pour point cutters are shown in Table 3. ##SPC3##

EXAMPLE 4

Visbreaking With Recycle Thermal Cracking

The process shown in FIG. 3 of the drawings was operated with a Libyan reduced waxy crude charge using the following conditions:

Fresh Feed.

650.degree.F.+Reduced Crude Recycle 650.degree.-1020.degree.F. Gas Oil Recycle Fresh Feed Volume Ratio 2.9 Per Pass F. F. Conversion, Volume Percent (350.degree.F.) 7 Yields: Vol. % Wt. % Dry Gas -- 6.7 Butanes 3.7 2.3 Pentanes 2.9 2.0 115.degree.-350.degree.F. Naphtha 11.4 9.0 350.degree.-650.degree.F. Gas Oil 47.6 43.3 * 1020.degree.F.+Residuum 33.5 36.7 * If the atmospheric reduced crude is not visbroken this yield of residuum increases to bout 42.5 column percent.

EXAMPLE 5

Mild Catalytic Cracking at 920.degree.F. Reactor Temperature

The process of FIG. 4 of the drawings was operated with the following material and results;

Fresh Feed 650.degree.-1020.degree.F. Gas Oil Recycle 650.degree.F.+Gas Oil Recycle: Fresh Feed Volumn Ratio 2.0 Per Pass F.F. Conversion Volume Percent (350.degree.F.) 20 Yields: Vol. % Wt. % Dry Gas -- 4.2 Butanes 7.8 5.1 Pentanes 4.8 3.4 115.degree.14 350.degree.F. Naphtha 18.7 15.5 350.degree.-650.degree.F. Gas Oil 34.6 32.9 1020.degree.F.+Residuum 33.0 35b4

Claims

We claim:

1. A method of producing a low pour fuel oil from a waxy crude which comprises the steps of producing a viscous pour residuum from a waxy crude by subjecting a waxy pour atmospheric residuum obtained from the waxy crude to visbreaking, separating the resulting product under atmospheric pressure into distillates and a residuum, subjecting the residuum so obtained to a deep vacuum distillation to produce a viscous pour vacuum residuum having an initial boiling point of at least 1,020.degree.F. and mixing the viscous pour residuum so obtained with from about 15-45 percent based on the weight of the mixture of a low pour cutter, having a pour point not higher than 0.degree.F., sufficient to produce a fuel oil having a pour point not higher than 50.degree.F. and a viscosity within the range of about 50 to 240 SUS at 210.degree.F.

2. A method according to claim 1 in which at least a portion of the distillate from the vacuum distillation is thermally cracked and the thermally cracked product so obtained is combined with the product of the visbreaking.

3. A method according to claim 1 in which the viscous pour residuum is produced by vacuum distillation of a waxy crude feedstock at a temperature sufficiently high to produce a viscous pour residuum and the low pour cutter is produced by cracking an overhead fraction from the said vacuum distillation.

4. The method according to claim 2 in which the cracking is fluidized catalytic cracking.

5. A method according to claim 1 in which the cutter is a 350.degree.-550.degree.F. gas oil fraction.