EP2076583A1

EP2076583A1 - Stabilizing additive for fuel oil

Info

Publication number: EP2076583A1
Application number: EP07821841A
Authority: EP
Inventors: Andrea Zanotti; Milena Mantarro; Marco Buccolini
Original assignee: Chimec SpA
Current assignee: Chimec SpA
Priority date: 2006-10-26
Filing date: 2007-10-25
Publication date: 2009-07-08
Also published as: WO2008049887A1; ITRM20060581A1

Abstract

The present invention relates to additives for the stabilization of fuel oil, compositions containing them and use processes thereof, in particular in the stabilization of fuel oil exiting Visbreaking (VSB) plants.

Description

STABILIZING ADDITIVE FOR FUEL OIL

DESCRIPTION

Field of the invention

The present invention relates to additives for the stabilization of fuel oil, compositions containing them and use processes thereof, in particular in the stabilization of fuel oil exiting from Visbreaking (VSB) plants .

State of the prior art Visbreaking (VSB) is a thermal conversion process applied to petroleum distillation residues, otherwise intended for bitumen or fuel oil (products which are well-known to be of low commercial value) , to partially convert them into lighter products of higher value. The conversion provides in part distilled products, such as gas, gasolines, kerosene, gas oil, and in part residue (hereinafter also referred to as tar or TAR) that can still constitute the basis for use as fuel oil. The conversion process is conducted in such a manner as to maximize the yield of distilled products, while reducing the overall yield of tar oil. It should be noted that even a conversion increase equal to 1% of the feedstock is to be considered extremely satisfactory in terms of economic yield. The VSB plant comprises the following parts: a heat exchanger bank through which the feedstock enters the VSB plant for pre-heating; a furnace in which the actual thermal cracking takes place; a fractionating column, from the top of which the distillates are extracted and from the bottom of which the residue (TAR) leaves. This residue passes through heat exchangers, transferring part of its heat to the feedstock, and is then stored in tanks. A soaker and a preflash column can be provided between the furnace and the fractionating column. The VSB plant operative conditions may be briefly described as follows: a feedstock, consisting generally of a primary or vacuum distillation residue, is fed into the furnace, which operates at temperatures ranging from 420⁰C to 500⁰C and at pressures of between 3 bar and 20 bar. The feedstock treated in this manner passes to the fractionating column, which operates using known procedures; from it there may be obtained, for example in the following proportions: light distillates (gas and gasolines, 3-10%) , medium distillates (kerosene and gas oil, 15-20%) and a residue (TAR, 65-75%) ; the percentages are by weight on the total products leaving the VSB plant .

The TAR, present in a greater proportion (aliquot) than the other components, must conform to specifications set by the market, for example those indicated in Table 1, if it is to be used as fuel oil.

TABLE 1

Determination Method Specification

Water and sediments ASTM D 1796 Max 0.5 %v

Asphaltenes IP 14 3* * * Max 6

%w

Heat of combustion ASTM D 240 Min 9.850 Kcal/Kg

Ash ASTM D 482 Max 0.03

%w

Density at 15°C ASTM D 1298 Max 990 Kg/me

Distillation at 300⁰C ASTM D 86 Max 60* %v

Distillation at 350°C ASTM D 86 <85^ %v

HFT (Hot Filtration Test) ASTM D 4870 Max 0.30

%w

Inflammability, Pensky ASTM D 93 Min 65 Martens °C

Upper pour point ⁰C ASTM D 97 Max +40

Conradson carbon residue ASTM D 189 Max 13

%w

Nickel AAS** Max 60 ppm

Sodium AAS** * * Max 100 ppm

Vanadium AAS** * * Max 90 ppm

Viscosity at 50 ⁰C ASTM D 445 Max 400*

CSt

Viscosity at 50 ⁰C ASTM D 455 Max 52.6* 0E**

Sulphur ISO14596 Max 1.0*

%w

* Required by Italian Law

** Viscosity E at 5O⁰C greater than 12 and ASTM color diluted min 4.0;

***IP: The Institute of Petroleum (recognized standard method); ****SAA: Spectrophotometry by Atomic Absorption

Usually the plant is operated so that the TAR obtained, except for slight viscosity corrections, already satisfies the required specifications for fuel oil, and as such "TAR" and "fuel oil" shall hereinafter be considered essentially as synonyms for the purposes of the present invention.

Annexed is the method ASTM D4870 for HFT determination

The preceding patents EP-B-0321424 and EP-B-0818524 (Chimec Spa) , describe methods for decreasing the amount of TAR produced, Visbreaking feedstock amounts being equal. I.e., increasing the yield in light distillates (high-value products) while decreasing the amount of (low-value) residue produced. To do this, VSB furnace temperature, and anyhow running severity in general, have to be raised. Beside the teaching in the patent, this presently constitutes a general trend, as leading to the production of greater amounts of valuable products and therefore to a high economic yield of the plant. However, managing the VSB plant at the utmost possible severity entails some drawbacks due to the formation of more carbonaceous residues and fouling compounds (due to polymerization and dehydrogenation phenomena, consequent to the increase in process temperatures) . Moreover, it has been tested that in some situations the tar produced under high severity has the drawback of not being stable over time, something prejudicing its sale as fuel oil meeting commercial specifications. Fuel oil leaving the VSB plants is stored in tanks, where it is maintained at a temperature between 70 and 110⁰C, until the time of its pumping into tankers or its arrival in the burners. This time can be some weeks. During these manipulations, the oil can come into contact with air and is subjected to heat, conditions which, combined with the variable time, can foster the formation of insoluble compounds in the oil.

In fact, it has been noted that, over time, i.e. during storage in the tanks, the fuel oil shows ageing phenomena, highlighted by a progressive increase in the HFT value (indicating the presence of sediments), until the fuel oil, even within a very short time, of the order of a few days, lies outside specification. The aged TAR no longer conforms to specifications set by the markets; its viscosity increases so as has its HFT value. This results operationally, on use (combustion) in the occurrence of the following drawbacks : - Abundant formation of carbon in the preheaters, due to the coking of heavy carbon containing materials

Abundant formation of soot produced by the incomplete combustion of precipitated insoluble heavy compounds - Sediments, paraffins and substances similar to gum found on the lines of the combustion system.

Current solutions adopted to counteract TAR instability all have drawbacks. For example, the exit temperature of the VSB furnace can be lowered, but this reduces the production of (valuable) light products. Alternatively, the TAR can be mixed with suitable distillates, for example gas oil, so as to counteract the separation of the insoluble compounds present in the oil, but in this manner a low value-added product is diluted with other products of a higher added value. In both cases, therefore, the solutions adopted have too high a cost. The problem of effective and at the same time economically advantageous stabilization of TAR or fuel oil consequently remains unsolved to this day.

Therefore, scope of the present application is to offer novel advantageous solutions to the above-mentioned problems .

Summary of the Invention

The present invention is based on the unexpected finding that compounds belonging to the class of stable free radicals are useful as additives able to stabilize fuel oil during storage and counteract the formation of insoluble compounds therein.

Therefore, object of the present invention is a stabilizing composition, soluble in fuel oil, containing at least one compound belonging to the class of stable free radicals, preferably selected from nitroxides, dinitroxides, trinitroxides or mixtures thereof, and a solvent, or a mixture of solvents, able to stably maintain said compound in solution, in the temperature range between -10⁰C and +50⁰C. Preferably, the fuel oil is the residue or tar of a Visbreaking (VSB) thermal conversion process.

A second object of the invention is a method for stabilizing fuel oils, such as those at the basis of the residue (tar) of the Visbreaking (VSB) thermal conversion process, comprising a step in which the composition of the invention is added to the oil in an amount ranging from 1 to 2000 ppm with respect to the fuel oil, for example from 100 to 600 ppm with respect to the fuel oil.

A third object of the invention are the stabilized fuel oils obtained in accordance with the method of the invention .

Further objects are thermal conversion process and plant for the production of petroleum distillation products, such as plants for conducting Visbreaking processes, comprising a station for adding to the distillation residue (tar) stabilizing compositions according to the invention on the line between the column and the storage tank.

The compositions of the invention afford to the distillation residue (tar) , and therefore to the fuel oil obtainable therefrom, surprising characteristics of storage stability, low viscosity, low solid-fraction content as demonstrated in detail in the experimental section of the application. These advantageous properties increase the intrinsic and commercial value of the product . Detailed description

For the purposes of the present invention, for stabilizing additive, compound or composition it is meant an additive, compound or composition able to influence at least one of the parameters usually considered as indicative of the stability of the residue or tar of a petroleum thermal conversion process. Such parameters are the HFT (Hot Filtration Test) , indicating the presence of sediments. The parameter is determined by carrying out the ASTM D4870 method described in detail in the examples, and must not have a value higher than 0.30% w/w. Higher values denote levels outside commercial specifications for the solid fraction contained in oil and the oil viscosity. A second parameter is the PV, indicating the stability of asphaltenes placed under flocculation condition. This parameter is calculated according to the SHELL method. Under standard running conditions of a Visbreaking process, the parameter value is 1.1. Lower values reflect a decreased stability of the residue . The stabilizing compositions of the invention comprise as essential component one or more stabilizing compounds able to counteract the increase in HFT value during storage. These pertain to the class of stable free radicals, and in particular to the class of nitroxides, dinitroxides, trinitroxides, preferably nitroxides or dinitroxides or mixtures thereof containing a free radical in their molecule. The nitroxides of the invention are compounds having the following general formula (I) :

R R I i

R-C-N-C-- R'

Formula (I)

where the substituents R, R' , and R" represent a group selected from H; (C1-C15) alkyl, e.g. methyl, ethyl, n-propyl, iso-propyl, n-butyl, sec-butyl, ter-butyl, pentyl, hexyl; (C3-C15) cycloalkyl, e.g. cyclobutyl, cyclopentyl, cyclohexyl; (C5-C15) aryl, e.g. phenyl, benzyl, napthyl, or where two symmetrical substituents

[R] or [R'] or [R"] together form a cyclic core, all substituted or unsubstituted with heteroatoms like 0, N, S, P or halogens. Alternatively, the compound belonging to the class of stable free radicals is a dinitroxide or a trinitroxide, or mixtures of all preceding compounds.

Examples of compounds useful in the present invention are : 4-hydroxy-l-oxyl-2, 2, 6, 6-tetramethylpiperidine (4OH TEMPO) ;

4-ethoxy-l-oxyl-2, 2, 6, 6-tetramethylpiperidine;

4-propoxy-l -oxyl-2, 2, 6, 6-tetramethylpiperidine;

4-acetamido-l-oxyl-2, 2, 6, 6-tetramethylpiperidine; 1-oxyl-2, 2, 6, 6-tetramethylpiperidine (TEMPO) ; l-oxyl-2, 2, 6, 6-tetramethylpiperidin-4-one (0X0 TEMPO) ; l-oxyl-2, 2, 6, 6-tetramethylpiperidin-4-yl acetate; l-oxyl-2, 2, 6, 6-tetramethylpiperidin-4-yl-2 ethylhexanoate; l-oxyl-2, 2, 6, 6-tetramethylpiperidin-4-yl stearate; 1 -oxyl-2, 2, 6, 6-tetramethylpiperidin-4-yl benzoate; l-oxyl-2, 2, 6, 6-tetramethylpiperidin-4-yl 4-t-butyl- benzoate;

N- (l-oxyl-2, 2, 6, 6-tetramethylpiperidin-4-yl) caprolactam;

N- (l-oxyl-2, 2, 6, 6-tetramethylpiperidin-4-yl) dodecylsuccinimide;

4,4' -ethylenebis (l-oxyl-2, 2, 6, βtetramethyl piperazin-3-one) ;

2-oxyl-l, 1, 3, 3-tetramethyl-2-isobenzazole; l-oxyl-2, 2, 5, 5-tetramethylpyrrolidine; bis (l-oxyl-2, 2, 6, 6-tetramethylpiperidin-4-yl) sebacate; bis (l-oxyl-2, 2, 6, 6-tetramethylpiperidin-4-yl) succinate; bis (l-oxyl-2, 2, 6, 6-tetramethylpiperidin-4-yl) adipate; bis (l-oxyl-2, 2, 6, 6-tetramethylpiperidin-4-yl) , n- butylmalonate; bis (l-oxyl-2, 2, 6, 6-tetramethylpiperidin-4-yl) phthalate; bis (l-oxyl-2, 2, 6, 6-tetramethylpiperidin-4-yl) isophthalate; bis (l-oxyl-2, 2, 6, 6-tetramethylpiperidin-4-yl) terephthalate; bis (l-oxyl-2, 2, 6, 6-tetramethylpiperidin-4-yl) hexahydroterephthalate;

N, N' -bis (l-oxyl-2, 2, 6, 6-tetramethylpiperidin-4-yl) adipamide;

N, N-bis- (1, 1, 3, 3-tetramethylbutyI) nitroxide;

2,4, 6-tris- [N-butyl-N- (l-oxyl-2, 2, 6, 6- tetramethylpiperidin-4-yl) ] -s-triazine.

Preferred nitroxides or dinitroxides are:

4-hydroxy-2, 2, 6, 6 tetramethyl-piperidin-N-oxyl (4OH

TEMPO); bis (l-oxyl-2, 2, 6, 6-tetramethylpiperidin - 4 yl) sebacate; 2,2,6,6 tetramethyl-piperidin-N-oxyl (TEMPO); l-oxyl-2, 2, 6, 6 tetramethyl-piperidin-4-one (OXO TEMPO);

4-acetamide-2, 2,6,6 tetramethyl-piperidin-N-oxyl .

Essential condition of the free radical compound is that it be soluble in the residue (tar) or in the fuel oil at the various operating temperatures to which the residue or the fuel oil are subjected during preparation or storage. The compound should be soluble, preferably in the temperature range between 50⁰C and 300⁰C and in particular between 70⁰C and 110⁰C, storage temperature of the fuel oil.

The compounds of the invention are known to a person skilled in the art (see WO-A-00/36052) and all commercially accessible from Sigma Aldrich Corporation or A. H. Marks and Company Limited (UK) .

The compositions subject-matter of the invention further comprise a solvent or a mixture of solvents suitable for solubilizing and stably maintaining in solution the free radical compound within a wide temperature range, i.e. of at least between -10⁰C and +50⁰C. Examples of useful solvents are water, alcohols, glycols, glycol-ethers, aromatic solvents and mixtures thereof, such as: water, 2-butoxy-ethanol, iso-octyl alcohol, iso-butyl alcohol, isopropyl alcohol, methyl alcohol, toluene, ortho- meta- and para-xylene, ethylbenzene or mixtures thereof. The compound belonging to the class of stable free radicals is present in the composition in any percentage amount suitable for reaching the desired stabilizing effect, e.g. of from 0.1% to 50% w/w of the composition.

The stabilizing method according to the invention provides the stabilizing composition to be added to the tar or fuel oils in an amount ranging from 1 to 2000 ppm with respect to the product to be stabilized. Advantageous results were obtained by using amounts ranging from 50 to 600 ppm. Preferably, composition addition is on the tar exiting the bottom of the VSB column on the line leading to the storage tank. The stabilized product thus obtained, be it tar or fuel oil, maintains the characteristic specifications set by the market and by provisions of the law as indicated in Table 1, with extremely reasonable additional costs. In actual fact, its combustion causes scarce formation of coke in the preheaters, scarce formation of soot and decreased gum-like sediments of paraffin nature on the lines of the combustion system.

The use of the stabilizing compositions of the invention has a positive impact on the running of the entire Visbreaking process. In fact, another relevant advantage afforded by the invention is the option of increasing the conversion, by increasing the process severity, and therefore of improving the yield of high- value products, without anyhow losing in terms of quality of produced tar. In fact, the increase in conversion attainable, e.g., through a temperature increase, would be negatively balanced by a downgrading of the produced tar, if the latter were not stabilized with the compositions of the invention. In other words, the advantage of a higher yield of valuable products would be nullified by the fact that the tar would not reach the envisaged specifications and therefore would not have the commercial value usually foreseen.

For this reason, the implementation of the invention involves a modification of the known plants for conducting classic Visbreaking processes, introducing a station provided with suitable inletting devices, like a suitable detached section off the tar line, equipped with safety valves, a line dedicated to the product, an addition pump and a storage tank, allowing addition of the stabilizing composition directly to the tar exiting the VSB column on the line between the latter and the tar storage tank. Thanks to the invention, the Visbreaking method itself is advantageously modified with respect to its usual implementation, by providing the additional step of adding the stabilizing composition. Hereinafter the invention is described in all experimental details in the following examples, which are by way of illustration and not for limitative purposes. TESTING Example 1: HFT test determination

The HFT method envisages a hot filtration (100⁰C, held by means of a steam-heated thermostatic jacket) on GF/A Grade fiberglass filters, of an aliquot of the tar

(about 10 grams) to be analyzed. Filtration is performed under vacuum and the sediments left on the filter are washed with a mixture of solvents (85% n-heptane and 15% toluene) . The deposit is weighed and compared to the weight of filtered tar: the result is expressed as percentage of sediments left on the filter.

Example 2 :

The tar samples, obtained by VSB and collected

(withdrawn) after raising the furnace exit temperature (FET) of +3°C and +6°C with respect to temperatures normally held, were stabilized with a composition A formulated as follows:

20% 4OH-TEMPO and 40% 2-butoxy-ethanol and 40% iso- octyl alcohol. A standard procedure envisaged by method ASTM D 4870 A and a more severe non-standard procedure, in which ageing at a 70⁰C temperature is protracted for

96 h, were followed to age the TAR.

1) VSB TAR at FET +3°C

The tar sample collected with a +3°C raise of the FET was stabilized, attaining an HFT containment below the 0.3% even under severe ageing conditions.

The blank used as reference is the same sample without stabilizing additive.

The samples, with and without additive, are placed in an oven or a thermostatic bath for 24 h at 100⁰C to simulate ageing (STD ASTM D 4870A procedure) .

The tar, a VSB residue, used in the tests (collected after having raised the FET of +3°C) has a PV value equal to 1.06 and a HFT value tel quel of less than 0.01%w/w. Under normal running conditions the PV value of the VSB tar is equal to 1.1. Such a PV value demonstrates that FET raising caused an increase in severity, thereby producing a less stable tar (lower PV) .

The sample was stabilized by addition of composition A at different dosages, to test its effectiveness towards the blank. Sample ageing was performed according to the standard method, therefore at a 100⁰C temperature for 24 h.

The results are reported in Table 1

Table 1

Product HFT ( %w/w)

Blank 0.18

Product A (300 ppm) 0.04 (-78 %)

As shown by the data in Table 1, the tar sample has a clear tendency to ageing, reaching an HFT value of about 0.2 w/w. Use of the additive in a 300 ppm ratio contained 78% of the ageing of the sample, bringing it back to an HFT value of below 0.05% w/w.

Then, wishing to test the occurrence of this behavior under more severe ageing conditions, an aliquot of stabilized sample was aged at 70⁰C for 96 h. The results are reported in Table 2.

Table 2

According to the non-standard procedure used, the sample has an ageing tendency 47% greater than that detected by the STD procedure and is found to have an HFT outside the required specification (0.3% w/w). Product A, at the optimal dosage of 300 ppm, counteracts ageing for a 27% bringing the sample back inside the specification.

2) VSB TAR at FET +6°C

The sample, collected after having raised the furnace exit temperature (FET) of 6°C, has a PV value equal to 1.01 and an HFT tel quel of <0.01%w/w. Under normal running conditions, the PV value of the VSB TAR is equal to 1 . 1 .

The sample was additioned with Product A, at different dosages, to test its effectiveness towards the blank .

Sample ageing was performed according to the standard method, therefore at a 100⁰C temperature for 24h. The results are reported in Table 3.

Table 3

Product HFT ( %w/w)

Blank 0 46

Product A (300 ppm) 0 38 (-17 -o )

Product A (500 ppm) 0 29 (-37 -o )

The VSB tar sample collected under severe running conditions exhibits a behavior decidedly more critical than the sample collected at + 3°C. The HFT value is 61% higher, falling outside the specification by 35%. Product A, at the optimal dosage of 500 ppm, brings the HFT value back inside the required specification. The sample processed according to the NON STD procedure at +96h exhibited a critical HFT value equal to 0.66%w/w (Table 4); Product A, at a 500 ppm dosage, contained said ageing by as much as 27%, though the specification value was not exceeded. The results are reported in Table 4. Table 4

Product HFT ( %w/w)

Blank 0.66

Product A (500 ppm) 0.48 (- 27 %)

Example 3: Plant test

Composition A was also tested in a plant during a 3- month industrial trial. In order to adequately illustrate the trial there has to be recalled the following concept, namely that the VSB plant is operated with the aim of obtaining maximum transformation into medium and light distillates (high value-added products) . This aim is achieved by increasing the Furnace Exit Temperature

(FET) , but this entails the drawback of obtaining an unstable TAR which falls easily outside specification. Adding the additive allows the FET to be raised while at the same time obtaining a stable TAR.

In Table 6 it is reported the pattern of FET (raised of about 8 ⁰C during the test) during the period 01 February - 04 May 2006. Composition A was added, with an average dosage of about 100 ppm, to TAR produced by the plant, during times in which the furnace exit temperature had gradually been raised. The samples, collected by pouring, were subjected to analysis upon ageing in an oven for 96 h and the results are reported in Table 5 below.

Table column "HFT96h (additive-treated) " indicates the value (%w/w) of sediment content in the sample collected after the addition of additive.

Table column "HFT24h (BLANK) indicates the value (%w/w) of sediment content on the sample collected before the addition of additive.

As it is highlighted from data reported in Table 5, the composition A used in the plant demonstrates an average efficiency of 28%. Table 5

In Table 6 there have been gathered the data of HFT (without ageing) performed on samples collected in storage tanks, throughout the test. During the time preceding the plant test, though FET had not been raised compared to the normal running of the plant, as many as 17 collections outside specification were recorded over a 2-month time, with very high HFT values, in the neighborhood of 0.5 - 0.6 %. The results are reported in Table 6.

Table 6

As shown in the data reported in Table, throughout the test period, by raising the FET up to a maximum of 8°C (initial FET = 480⁰C) and then increasing the system severity, HFT values on additive-treated samples always remain inside the required specification. During the first month of testing (February) , the average HFT of fuel oil produced was 0.226 % (FET monthly average: 482.2⁰C); during the second month (March), average FET was raised to 485°C, but average HFT did not increase; in fact, thanks to the effect of product A, it decreased to 0.198 %.

For the third month of testing, average FET was again raised to 487.8°C but average HFT value was contained to 0.185 %.

Claims

1. A stabilizing composition, soluble in fuel oil, characterized in that it contains at least one compound belonging to the class of stable free radicals and a solvent, or a mixture of solvents, able to stably maintain said compound in solution, in the temperature range between -10⁰C and +50⁰C.

2. The composition according to claim 1, characterized in that the fuel oil is the residue (tar) of a Visbreaking (VSB) -type thermoconversion process.

3. The composition according to any one of the claims 1 or 2, characterized in that the compound belonging to the class of free radicals is selected from nitroxides, dinitroxides, trinitroxides or mixtures thereof .

4. The composition according to claim 3, characterized in that the compound is a nitroxide of formula (I ) :

R R

R'-C-N -C— R'

R" O ° R" Formula (I) where R, R' , and R" represent a group selected from H, (C1-C15) alkyl, (C3-C15) cycloalkyl, (C5-C15) aryl, substituted or unsubstituted with heteroatoms selected from O, N, S, P, halogens, or where the two R groups form a cyclic structure of mixtures of such compounds.

5. The composition according to claim 3, characterized in that the compound belonging to the class of free radicals is selected from: 4-hydroxy-2, 2, 6, 6 tetramethyl-piperidin-N-oxyl (4OH TEMPO); bis(l-oxyl- 2, 2, 6, 6-tetramethylpiperidin - 4 yl) sebacicate; 2,2,6,6 tetramethyl-piperidin-N-oxyl (TEMPO); l-oxyl-2, 2, 6, 6 tetramethyl-piperidin-4-one (OXO TEMPO) ; 4-acetamido- 2,2,6,6 tetramethyl-piperidin-N-oxyl .

6. The composition according to any one of the claims 1 or 5, characterized in that the solvent is selected from water, alcohols, glycols, glycol- ethers, aromatic solvents or mixtures thereof.

7. The composition according to any one of the claims 1 or 6, characterized in that the compound belonging to the class of stable free radicals is present in a percentage amount of from 0.1% to 50% w/w of the composition .

8. A method for stabilizing fuel oils, comprising a step in which the composition according to any one of the claims 1 to 7 is added to the oil in an amount ranging from 1 to 2000 ppm with respect to the fuel oil.

9. The method according to claim 8, wherein the composition is added to the oil in an amount ranging from 50 to 600 ppm with respect to the fuel oil.

10. The method according to claims 8 or 9, wherein the fuel oil is the residue (tar) of the Visbreaking (VSB) thermal conversion process.

11. A stabilized fuel oil obtainable through the method according to any one of the claims 8 to 10.

12. A Visbreaking thermal conversion process comprising a step in which the distillation residue (tar) is added with the composition according to any one of the claims 1 to 7 in an amount ranging from 1 to 2000 ppm with respect to the amount of residue.

13. The process according to claim 12, characterized in that the step of adding the stabilizing composition occurs on the residue coming from the outlet of the column on the line to the storage tank.

14. A thermal conversion plant for the production of petroleum distillates, characterized in that it comprises a station for adding to the distillation residue (tar) the composition according to any one of the claims 1 to 7 on the line between the column and the storage tank.

15. The plant according to claim 14, characterized in that it is a Visbreaking (VSB) plant.