This invention relates to fuel oil compositions and to a method of treating them. More especially it relates to improving the compatibility of two or more different types of fuel oils on blending them, and to improving the stability of the resulting blends, and in particular to a method of inhibiting the precipitation of asphaltenes from a blend of a residual fuel oil with a different fuel.

It is known to prepare residual fuels, i.e., fuels i containing residuum, by blending residues, for example the tar from steam cracking (pyrolysis fuel) or thermally cracked (visbroken) residues, and diluents, e.g., flashed distillates or gas oils. The residues contain asphal¬ tenes, carbenes, and resins which are soluble, or colloidally soluble, in the residue but which are likely to precipitate out from the blend either very soon after blending or after long term storage. This precipitation has in the past been largely avoided by blending residual fuels with distillate from the same or similar crudes but, as the need arises to process component fractions more severely and to blend residual fuels with distillate from very different crudes, e.g., one having a low aromatic or naphthenic content (and hence less likely to maintain the asphaltenes in solution), it has become increasingly necessary to treat the blend or its com¬ ponents to prevent or reduce asphaltene precipitation. Precipitation is likely to occur when the fuel

blend is required to have a low sulphur content, when the distillate is derived from a low sulphur paraffinic crude and the residual component is relatively low - up to 15% - such blended, intermediate, fuels being very suscep¬ tible to asphaltene precipitation.

The addition of alkylaryl sulphonic aids to fuel oil compositions to inhibit precipitation has been proposed, for example, in U.S. Patent No. 4,182,613. In this patent, it is noted that although the sulphonic acids are readily oil soluble heating and agitation may be neces¬ sary to an extent sufficient to overcome viscosity effects, it being suggested to heat to 90 ^* C or higher with stirring until the additive is dissolved. If incompatibility on blending the components of the fuel oil is expected, the additive may be incorporated into one of the fractions and it is stated, though without giving reasons, that mixing with the residuum fraction is particularly effective.

It has now unexpectedly been found that if, before blending, the residuum is treated with the additive at a temperature in excess of 200 ^* C for a prolonged period, the inhibition of sedimentation may be greatly enhanced. This enhancement is both in relation to pretreat ent of the residuum at lower temperatures for the time needed to effect solution and in relation to treatment at the same high temperature and for the same prolonged period carried out on the blend.

Accordingly, the present invention provides a method of inhibiting the formation of asphaltene sediment in a blend of fuel oils one of which is a residuum which method comprises adding an effective stabilizing amount of an alkylaryl sulphonic acid having from 10 to 70 carbon atoms to the residuum and maintaining the residuum containing the sulphonic acid at a temperature of at least 200 ^* C for a time sufficient to inhibit sediment formation in the eventual blend, and subsequently blending the residuum with the other component or components of the blend.

The sulphonic acid advantageously contains from 26 to 46 carbon atoms, and the alkyl substituent has or substituents have a total content of 18 to 40, preferably from 22 to 28, carbon atoms, and may be of straight or branched chain structure. Mixtures of two or more sulphonic acids may be used. Especially preferred are those in which there are two alkyl groups meta to each other on a benzene ring, one alkyl group having from 1 to 14 carbon atoms, the other having from 14 to 36 carbon atoms. Particularly useful acids have a molecular weight in the range of from 300 to 750, advantageously from 450 to 700. Mixtures of sulphonic acids with straight and branched chain alkyl groups may be used. As examples of specific acids there may be mentioned the sulphonic acids of tetradecyl benzene, hexadecyl benzene, icosyl benzene.

tetracosyl benzene, octacosyl benzene and dotricosyl benzene.

Suitable alkylaromatics may be obtained by numerous techniques and subsequently sulphonated by any one of several reagents. For example, benzene, toluene, or naphthalene may be alkylated with an olefinic fraction or a chlorinated alkane using a Friedel-Crafts catalyst. The olefin may be an oligomer of ethylene or a 1-alkene. Alternatively, appropriate natural petroleum products may be used. Sulphonation may be carried out using oleum, concentrated sulphuric acid, sulphur trioxide or chloro- sulphonic acid. The procedures for obtaining the alklyaryl sulphonic acids are well known and will not be detailed here.

The residual fuel oils in which the additive may be incorporated accordingly to the method of the invention are oils containing residua, for example straight residuum, vacuum residuum, steam cracking, and, especially, thermal cracking residuum. The residuum will generally have an initial boiling point of at least 315 ^* C, and advantageously about 345"C at atmospheric pressure.

The treated residua may be blended with a variety of diluents, more especially vacuum, flash or middle distillate e.g., 15θ'C to 345 ^* C, oils, particularly heavy gas oils e.g., 260 ^* C to 345 ^* C oils.

As indicated above, in the method of the present

invention, the sulphonic acid is introduced into the residuum at an elevated temperature. The temperature is at least 200'C, and is preferably in the range of from 200 ^* C to 350 ^* C. The time of treatment at elevated temperature will be at least sufficient to inhibit formation of sediment in the eventual blend, and is generally at least 2 minutes, and is advantageously for 10 to 60 minutes, preferably for 15 to 45 minutes, and most preferably about 30 minutes. Generally, for a given treatment, a higher temperature will require a shorter time.

The propensity for a fuel to form sediment, and the ability of an additive to inhibit sediment formation, are measured by the SHF (Sediment by Hot Filtration) Test, as described in "Industrial and Engineering Chemistry", iα, 678 to 680 (1938). It is a matter of simple routine experiment to ascertain for a given additive at a given treat rate in a given residuum to be blended with a given distillate whether the time/tempera¬ ture regime of the treatment has reduced the sediment formed by the eventual blend in the SHF test, either to an acceptable level, or to that regarded as economically appropriate. Accordingly, it is not appropriate to lay down here precise treatment regimes, except to say that at higher temperatures the effect of increasing treatment times is greater.

The treat rate of the sulphonic acid is

advantageously in the range of from 0.01 to 2%, preferably from 0.1 to 1.0%, and most preferably from 0.2 to 0.8%, based on the total weight of the eventual blend. Thus, for example, if a blend of 10% residuum, 10% gas oil is to be made, a treat rate based on the residuum of 10 times the ranges given above will be appropriate.

In the U.S. patent mentioned above, it is pointed out that the proportion of additive required to prevent sediment formation depends on the sediment formed by the i untreated fuel as measured by the SHF Test, and that an addition range of 50 to 250% by weight additive based on the SHF Test sediment result was appropriate, with a treat rate of 100 to 150% being preferred.

The method of the present invention, however, achieves reduction in sediment comparable with that in the above-mentioned U.S. Patent at a treat rate of 30 to 60%.

In addition to measurement of SHF, other measures of the effectiveness of an additive are available. One of these is based on the fact that asphaltenes are soluble in aromatic, but not in aliphatic, solvents. Hence, a comparison of the aromaticity, calculated from density and average boiling point, of a given residual oil as measured by its BMCI with the aromaticity required to retain the asphaltenes present in that oil in solution, as measured by TEF gives a value for "solubility

reserve". BMCI is the Bureau of Mines Correlation Index as described in the paper:

Smith, H. R., "Correlation Index to Aid in Inter¬ preting Analyses", Bureau of Mines Technical Paper 610, U.S. Dept. of the Interior, U.S. Govt. Printing Office, Washington, D.C. 1940.

TEF is the "Toluene Equivalence Filtered" test, as described in the report of the ASTM Marine Fuels Symposium, Miami, Dec. 8, 1983. i

Any treatment that reduces TEF without correspond¬ ingly reducing the BMCI of a product will enable a blend having a higher proportion of gas oil to be formed. It has been found that the treatment method of the invention is effective in reduction of TEF and improving solubility reserve, and more so than the treatment of the residual oil at lower temperatures.

The following Examples illustrate the invention:

Example 1

Various residual fuel oil compositions were prepared containing a residuum from thermal cracking and various diluents in several proportions. In the procedure according to the invention, the residuum was treated with a C ₂ straight chain alkylbenzene sulphonic acid at 200 to 250 ^* C for 30 minutes before blending. In the com¬ parison procedure, the residuum was blended with the diluent and the blend treated with the same sulphonic acid in the same proportions at 200'C for 30 minutes.

In Table 1 below, the residuum is designated VBT; two different types are used. Type A is a visbroken tar from a vacuum residual feed, TEF 81; BMCI 85.1; HFS 0.07%; Type D is a visbroken tar, TEF 69; BMCI 75; HFS 0.06%; Diluent B is paraffinic gas oil from atmospheric distillation, BMCI 28; Diluent C is a low sulphur straight-run heavy fuel oil, BMCI 45; TEF < 23; HFS 0.03%.

Table I

The stabilizer is a 90% solution of the acid; i.e., the active constituent is present in a proportion of 0.45%, based on the total weight of fuel oil.

Example 2 The effect of treatment time on the SHF results is illustrated in the following Table 2.

Residuum is treated with 0.5% alkylbenzene sulphonic acid for various times at 200 ^* C, then blended to form compositions comprising 25 and 20% gas oil by weight. An untreated blend containing 25% gas oil had an SHF of 1.7%; that containing 20% had an SHF of 0.9%

Table 2

Example 3

A further measure of the effectiveness of the treatment according to the invention is the "solvency reserve".

Table 3 below shows the reduction in TEF by addition of a poly(n-butene)benzene.sulphonic acid, molecular

weight 675, to a VBT of TEF 81 and BMCI 85.1 fuel oil; treat rate 0.3% at 200'c for 30 minutes. The TEF of the fuel oil blended with a straight run gas oil (BMCI 28) is also reduced.

Table 3

VBT 81

VBT + Additive 67

VBT 80%, GO 20% 88

VBT 80%, GO 20% + Additive 73 The solubility reserve is also found to increase with increasing treat rate.

Sxarøple 4 As has been indicated above, additive treatment methods should improve not only compatibility of blends made immediately after treatment, but also blends made a prolonged time after treatment. In this example, one sample of a visbroken tar (TEF = 81) is treated with an alkylbenzene sulphonic acid, 0.5%, at 200 ^* C for 30 minutes, and a second sample is untreated. Both samples are stored for 1 month at 50 ^* C, and then formed into a 75%:25% blend with a paraffinic gas oil.

Table 4 below shows the improvement in HFS values.

Table 4

HFS, * Blended, Untreated VBT 1.7

Blended, Treated VBT 0.05

Example 5 The effect of temperature on the reduction in TEF achieved by the poly-n-butene benzene sulphonic acid used in Example 3 is shown in Table 5 below. 0.3% by weight of the acid was used at 50 ^* C and 200 ^* C, for 30 minutes in each case, to treat a VBT (AM413, TEF 81; BMCI 85.1) and the resulting TEF's measured.

Table 5

TEE AM 413, no additive 81

AM 413, 50 ^* C treatment 75

AM 413, 200"C treatment 67

Example 6 To ascertain the improvement in solubility reserve, samples of a VBT (TEF 81; BMCI 85.1) were left untreated, or treated with 0.5% by weight of Additive A (90% solution of a C2 branched chain alkylbenzene sulphonic acid), or with 0.5% by weight of Additive B (60% solution of the acid used in Example 5), in each case for 30 minutes at 200 ^* C. The results are shown in Table 6 below.

Table §

BMCI TΣL Reserve

VBT 85.1 81 4.1

VBT + A 85.2 73 12.2

VBT + B 85.7 67 18.7