DE69421523T2 - OIL ADDITIVES AND COMPOSITIONS - Google Patents

Description

This invention relates to oil compositions, primarily fuel oil compositions, and more particularly to the control of foaming in such compositions.

In the processing and transportation of hydrocarbon oils, foaming often occurs when the oil is transferred from one vessel to another. Foaming can interfere with oil pumping and can be so severe that a reduction in pumping speed is required to collapse the foam and prevent oil spillage. It is desirable to control foaming to allow higher oil transfer rates.

The problem of foaming is particularly important in the distribution of oils such as liquid petroleum oils, particularly fuel and lubricating oils. Such oils are typically passed through a distribution network, which involves pumping through pipes or a series of storage tanks.

Fuel oils derived from plant or animal material, also known as biofuels, are considered to be less harmful to the environment when burned and are obtained from a renewable resource. Certain biofuels can be used as full substitutes for fuel oils such as diesel fuel oils. Similarly, certain biofuels can be used as partial substitutes for such fuel oils, blended into the oils in appropriate proportions.

Biofuels as such have lower foaming tendencies than typical fuel oils such as diesel fuel oil. Unfortunately, it has now been found that blends of biofuel with diesel fuel oil have much higher foaming tendencies than the fuel oil as such. This finding is surprising since it was thought that minor components of fuel oils which are of a surface active nature are responsible for stabilizing foam and are different from the components of these biofuels. Accordingly, it was not expected that the addition of an amount of biofuel to a fuel oil would increase the foaming tendency of the resulting blend.

It has now surprisingly been found that foaming in a mixture of petroleum-based fuel oil and biofuel can be successfully controlled in the sense that the initial foam height upon agitation is reduced by an antifoam additive conventionally used to control foaming in petroleum distillate fuel oils (hereinafter referred to as petroleum fuel antifoam agent).

Such additives are themselves surface active in nature and are believed to somehow interfere with the foam stabilizing tendency of other components of the fuel mixture.

US-A-2 862 885 describes the use of silicon-containing antifoam agents to prevent foaming of petroleum substitutes.

FR-A-2 579 481 describes the use of siloxane-oxyalkylene block copolymers as antifoam agents for hydrocarbon fuel oils.

GB-A-658 494 describes antifoam compositions containing a water-immiscible organic liquid.

The invention therefore provides, in its first aspect, a fuel oil composition comprising a major proportion of a mixture of petroleum-based middle distillate fuel oil and biofuel and a minor proportion of petroleum fuel antifoam, wherein the antifoam is a silicon-containing composition and the fuel oil has a sulfur concentration of 0.05% or less by weight based on the weight of the fuel.

The petroleum fuel antifoams used in this first aspect of the invention are silicon-containing compositions.

It has further been found that foam formation in the mixture of the first aspect can be successfully controlled in the sense that foam collapse is accelerated when the mixture contains at most 65% by weight of biofuel.

The invention therefore provides, according to a preferred embodiment of the first aspect, a fuel oil composition comprising a major proportion of a mixture of petroleum-based fuel oil and biofuel and a minor proportion of antifoaming agent for petroleum fuel, the fuel oil mixture containing at most 65% by weight of biofuel and at least 35% by weight of petroleum-based fuel oil.

The invention also provides in a second aspect a concentrate comprising a petroleum fuel antifoam agent mixed with a mixture of petroleum-based fuel oil and biofuel, and in a third aspect the use of a petroleum-based antifoam agent to control foaming in a mixture of petroleum-based fuel oil and biofuel.

The invention is described in more detail below.

Petroleum-based fuel oil (according to all aspects of the invention)

The petroleum-based fuel oil is suitably a middle distillate fuel oil, i.e. a fuel oil obtained in the refining of crude oil as a fraction between the lighter kerosene and jet fuel fraction and the heavier fuel oil fraction. Such distillate fuel oils generally boil within the range of about 100°C to about 500°C (ASTM D-86), e.g. 150°C to about 400°C, for example those having a relatively high final boiling point of above 360°C, such as 380°C. The fuel oil may comprise atmospheric distillate or vacuum distillate or cracked gas oil or a mixture of any proportions of directly distilled and thermally and/or catalytically cracked distillates. The most common petroleum distillate fuels are kerosene, jet fuels, diesel fuels, heating oils and heavy fuel oils. The heating oil may be a direct atmospheric distillate or it may contain small amounts, e.g. up to 35 wt%, of vacuum gas oil or cracked gas oils, or both. Heating oils may be produced from a mixture of virgin distillate, e.g. gas oil, naphtha, etc., and cracked distillates, e.g. material from the catalytic cycle.

A representative specification for a diesel fuel includes a minimum flash point of 38ºC and a 90% distillation point between 282 and 380ºC (see ASTM designations D-396 and D-975).

The fuel oil has a sulfur concentration of 0.05 wt% or less, and preferably 0.01 wt% or less. The prior art describes processes for reducing the sulfur concentration of hydrocarbon middle distillate fuels, such processes including solvent extraction, sulfuric acid treatment, and hydrodesulfurization.

Biofuel (according to all aspects of the invention)

The biofuel may be one or more oils derived from animal or vegetable material or both that can be used as fuel.

Oils obtained from animal or vegetable material are mainly metabolites comprising triglycerides of monocarboxylic acids, e.g. acids containing mainly 10 to 25 carbon atoms and having the form

in which R is an aliphatic radical having predominantly 10 to 25 carbon atoms, which may be saturated or unsaturated. Preferably, R is an aliphatic radical having 10 to 25 carbon atoms. In general, such oils contain glycerides of a variety of acids, the number and type of which vary with the source of the oil, and may additionally contain phosphoglycerides. Such oils can be obtained by methods known in the art.

Examples of derivatives of such oils are alkyl esters such as methyl esters of fatty acids from vegetable or animal oils. Such esters can be produced by transesterification.

References within this description to oils derived from animal or vegetable material therefore include references to oils obtained from the animal or vegetable material or both, as well as derivatives thereof.

Examples of oils derived from animal or vegetable material are rapeseed oil, coriander oil, soybean oil, cottonseed oil, sunflower oil, castor oil, olive oil, peanut oil, corn oil, almond oil, palm kernel oil, coconut oil, mustard seed oil, beef tallow and fish oils. Other examples include oils derived from wheat, jute, sesame, shea nut, arachis oil and linseed oil and can be derived from these by methods known in the art. Rapeseed oil, which is a mixture of fatty acids partially esterified with glycerin, is preferred because it is available in large quantities and is easily obtained by pressing rapeseed. Rapeseed oil typically contains the esters of, in addition to about 11 to 19% saturated C₁₆ to C₁₈ acids, about 23 to 32% monounsaturated, 40 to 50% diunsaturated and 4 to 12% triunsaturated C₁₆ to C₂₈ acids, mainly oleic, linoleic, linolenic and erucic acids.

The following are suitable as lower alkyl esters of fatty acids, for example as commercially available mixtures: the ethyl, propyl, butyl and especially methyl esters of fatty acids with 12 to 22 carbon atoms, for example of lauric acid, myristic acid, palmitic acid, palmitoleic acid, stearic acid, oleic acid, elaidic acid, petroselinic acid, ricinoleic acid, eleostearic acid, linoleic acid, linolenic acid, eicosanoic acid, gadoleic acid, docosanoic acid or erucic acid, which have an iodine number of 50 to 150, in particular 90 to 125. Mixtures with particularly advantageous properties are those which are mainly, i.e. at least 50% by weight, methyl esters of fatty acids with 16 to 22 carbon atoms and 1, 2 or 3 double bonds.

The preferred lower alkyl esters of fatty acids are the methyl esters of oleic acid, linoleic acid, linolenic acid and erucic acid.

Commercially available mixtures of the type mentioned are obtained, for example, by splitting and esterifying animal and vegetable fats and oils by transesterification with lower aliphatic alcohols. To produce lower alkyl esters of fatty acids, it is advantageous to start with fats and oils with a high iodine number, such as sunflower oil, rapeseed oil, coriander oil, castor oil, soybean oil, cottonseed oil, peanut oil or beef tallow. Lower alkyl esters of fatty acids based on a new type of rapeseed oil are preferred.

Although many of the above oils can be used as biofuels, vegetable oils or derivatives are preferred, with particularly preferred biofuels being alkyl ester derivatives of rapeseed oil, cottonseed oil, soybean oil, sunflower oil, olive oil or palm oil or the respective oils, with rapeseed oil methyl ester being particularly preferred.

The mixture (according to all aspects of the invention)

The proportion of biofuel in the mixture can be in the range from 0.1% to 99.9% by weight, preferably 0.1% to 90% by weight and in particular 0.5% to 85% by weight. The proportion of biofuels is advantageously at most 65% and in particular at most 55% by weight.

It is within the scope of the invention to use as a mixture two or more petroleum-based fuel oils or in particular two or more biofuels mixed with one or more of the other fuel type.

The mixture may contain additives other than the petroleum fuel antifoam agent. Thus, one or more co-additives such as low temperature flow improvers, stabilizers, dispersants, antioxidants, corrosion inhibitors, cetane improvers (ignition accelerators), emission reducers, odor improvers, lubricity agents, antistatic additives or demulsifiers may be present in the mixture. These co-additives may be used simultaneously with the petroleum fuel antifoam agent. oil fuel to the mixture, for example the antifoam agent and the coadditives may comprise a single additive. Alternatively, one or more coadditives may be added to the mixture independently of the antifoam agent or to the petroleum-based fuel oil or biofuel prior to mixing.

The petroleum fuel antifoam agent (according to all aspects of the invention)

The antifoam agent is advantageously insoluble in the fuel being treated, but is dispersible therein to form a stable dispersion, if necessary with the aid of a suitable dispersant or solvent with or without the use of mechanical dispersing aids.

As an antifoaming agent, a silicon-containing composition can be used as indicated above. Such a composition advantageously comprises a siloxane polymer, for example a block copolymer containing siloxane blocks and oxyalkylene blocks. Preferred siloxane polymers are those with the general formula (i)

in which R represents a hydrocarbon group, n represents a number in the range of 1 to 3 and m represents a number 2. The hydrocarbon group may be a relatively simple hydrocarbon group having 1 to 30 carbon atoms or may be a polymeric group. The groups represented by R may be the same or different in any given siloxane group or within the siloxane polymer and the value of n may be the same or different in the different siloxane groups in the siloxane polymer.

The preferred polymers are block copolymers comprising at least two blocks, one block comprising siloxane groups represented by the general formula (i) and the second block comprising oxyalkylene groups having the general formula (ii)

(R¹-O) (ii)

includes.

The siloxane block and the oxyalkylene block can be connected to one another via a divalent hydrocarbon group, this can be R in the general formula (i). Thus, each siloxane block contains at least one group with the general formula (i), where at least one group represented by R is a divalent hydrocarbon group. The siloxane block has a ratio of hydrocarbon groups to silicon atoms of 1:1 to 3:1.

The hydrocarbon groups represented by R in the general formula (i) may be alkenyl groups, for example vinyl and allyl, cycloalkenyl groups, for example cyclohexenyl, alkyl groups, for example methyl, ethyl, isopropyl, octyl and dodecyl, aryl groups, for example phenyl and naphthyl, aralkyl groups, for example benzyl and phenylethyl, alkaryl groups, for example styryl, tolyl and n-hexylphenyl, or cycloalkyl groups, for example cyclohexyl.

The divalent hydrocarbon groups represented by R in the general formula (i) may be alkylene groups such as methylene, ethylene, propylene, butylene, 2,2-dimethyl-1,3-propylene and decylene, arylene groups such as phenylene and pp'-diphenylene, or alkarylene groups such as phenylethylene. Preferably, the divalent hydrocarbon group is an alkylene group containing two to four carbon atoms in sequence.

These divalent hydrocarbon groups are linked via a silicon-carbon bond to a silicon atom of the siloxane blocks and bound to an oxygen atom of the oxyalkylene block via a carbon-oxygen bond.

The siloxane block in the copolymers may contain siloxane groups represented by the general formula (i) with either the same hydrocarbon groups bonded to the silicon atoms (e.g. the dimethylsiloxy, diphenylsiloxy and diethylsiloxy groups) or different hydrocarbon groups bonded to the silicon atoms (e.g. the methylphenylsiloxy, phenylethylmethylsiloxy and ethylvinylsiloxy groups).

The siloxane block in the copolymers may contain one or more types of siloxane groups represented by the general formula (i), provided that at least one group has at least one divalent hydrocarbon substituent. For illustration purposes only, ethylenemethylsiloxy groups may be present in the siloxane block or the siloxane block may contain more than one type of siloxane group, e.g., the block may contain both ethylenemethylsiloxy groups and diphenylsiloxy groups or the block may contain ethylenemethylsiloxane groups, diphenylsiloxy groups and diethylsiloxy groups.

The siloxane block may contain trifunctional siloxane groups (e.g., monomethylsiloxane groups, CH₃SiO1.5), bifunctional siloxane groups (e.g., dimethylsiloxane groups, (CH₃)₂SiO, monofunctional siloxane groups (e.g., trimethylsiloxane groups, (CH₃)₃SiO0.5) or combinations of these types of siloxane groups with the same or different substituents. Due to this functionality of the siloxane groups, the siloxane block may be predominantly linear or cyclic or cross-linked or may have combinations of these structures.

The siloxane block may contain organic endblocking groups or organic chain end groups in addition to the monofunctional siloxane chain end groups included in the general formula (i). Organic endblocking groups may be hydroxyl groups, aryloxy groups such as phenoxy, alk oxy groups such as methoxy, ethoxy, propoxy and butoxy and acyloxy groups such as acetoxy.

The siloxane blocks in the copolymers contain at least two siloxane groups represented by the general formula (i) (such that m is a number 2). Preferably, the siloxane blocks contain a total of 5 to 20 siloxane groups represented by the general formula (i) where m is a number in the range of 5 to 20. That part of the average molecular weight of the copolymer which is attributable to the siloxane blocks can be as high as 50,000, but is preferably 220 to 20,000. If that part of the average molecular weight of the copolymer which is attributable to the siloxane blocks exceeds 50,000 or if the siloxane blocks contain a total of more than 20 siloxane groups represented by the general formula (i), the copolymers are usually not useful, e.g. For example, they may be too viscous for convenient use in the additives of the invention.

The oxyalkylene blocks in the copolymers each contain at least two oxyalkylene groups represented by the general formula (ii) where R¹ is an alkylene group. Preferably at least 60% by weight of such groups represented by the general group (ii) are oxyethylene or oxypropylene groups.

Other oxyalkylene groups represented by the general formula (ii) which may also be present in the oxyalkylene block in amounts preferably not exceeding 40% by weight are oxy-1,4-butylene, oxy-1,5-amylene, oxy-2,2-dimethyl-1,3-propylene or oxy-1,10-decylene groups.

The oxyalkylene blocks in the copolymers may contain oxyethylene or oxypropylene groups alone or together with one or more of the various types of oxyalkylene groups represented by the general formula (ii). The oxyalkylene blocks may contain only oxyethylene groups, or only oxypropylene groups, or both oxyethylene and oxypropylene groups, or other combinations of the various types of oxyalkylene len groups represented by the general formula (ii).

The oxyalkylene blocks in the copolymers may contain organic endblocking or chain end groups. Such endblocking groups may be hydroxy groups, aryloxy groups such as phenoxy, alkoxy groups such as methoxy, ethoxy, propoxy and butoxy, and alkenyloxy groups such as vinyloxy and allyloxy. A single group may serve as an endblocking group for more than one oxyalkylene block, for example, the glyceroxy group may serve as an endblocking group for three oxyalkylene chains.

The oxyalkylene blocks in the copolymers contain at least two oxyalkylene groups represented by the general formula (ii). Preferably each block contains from 4 to 30 such groups. That portion of the average molecular weight of the copolymer attributable to the oxyalkylene blocks may vary from 176 (for C₂H₄O)₄) to 200,000, but is preferably from 176 to 15,000. However, provided that each oxyalkylene block contains at least two oxyalkylene groups represented by the general formula (ii), the number of oxyalkylene groups and that portion of the average molecular weight of the copolymer attributable to the oxyalkylene blocks is not critical, provided that the resulting copolymer is not rendered physically incompatible with oily liquids. However, those copolymers in which the portion of the average molecular weight attributable to the oxyalkylene blocks exceeds 200,000 or which contain more than 50 oxyalkylene groups per block have been found to be less useful, e.g. they are too viscous for convenient use in the additives of the invention.

The copolymers may contain siloxane blocks and oxyalkylene blocks in any relative amounts. To have desirable properties, the copolymer should contain 5 parts by weight to 95 parts by weight of siloxane blocks and 5 to 95 parts by weight of oxyalkylene blocks per 100 parts by weight of copolymer. The copolymers contain 5 parts by weight to 50 parts by weight of the siloxane blocks and 50 parts by weight to 95 parts by weight of the oxyalkylene blocks per 100 parts by weight of copolymer.

The copolymers may contain more than one of each of the blocks and the blocks may be arranged in various configurations such as linear, cyclic or branched configurations. The most preferred copolymers have the general formula (iii)

in which p is a number ≥ 2 and preferably a number in the range from 4 to 30, c is a number in the range from 0 to 2, m is a number 2, R² represents a monovalent hydrocarbon radical having 1 to 12 carbon atoms, preferably a linear aliphatic radical, for example a methyl or ethyl group, R³ is a divalent hydrocarbon radical having 1 to 12 carbon atoms, preferably an alkylene group having at least two carbon atoms, for example ethylene, 1,3-propylene or 1,4-butylene, R⁴ represents the same or different divalent hydrocarbon radicals having 2 to 10 carbon atoms, such as ethylene, 1,3-propylene or 1,6-hexylene, and R⁵ is a divalent hydrocarbon radical having 2 to 10 carbon atoms, such as ethylene, 1,3-propylene or 1,6-hexylene. represents a monovalent hydrocarbon group having 1 to 12 carbon atoms or a terminal group such as hydroxyl or hydrogen. It is preferred that R⁴ represents different hydrocarbon radicals and most preferably represents a mixture of at least one each of ethylene and 1,3-propylene radicals.

The block copolymers described above can be prepared by an addition reaction between siloxanes containing silicon-bonded hydrogen atoms and oxyalkylene polymers containing end-blocking alkenyl groups in the presence of a platinum catalyst. These copolymers can also be prepared by a metathesis reaction between siloxanes containing silicon-bonded chloroorgano groups and an alkali metal salt of hydroxy-end-blocked oxyalkylene polymer.

According to all aspects of the invention, the concentration of the antifoam agent in the fuel oil composition may, for example, be in the range of 0.1 to 10,000 ppm by weight, preferably 0.5 to 5,000 ppm by weight and most preferably 1 to 100 ppm by weight (active ingredient) per weight of fuel oil. Particularly advantageous concentrations are in the range of 1 to 20 ppm.

concentrate

Concentrates are convenient as a means of introducing the antifoam agent into the bulk of the fuel mixture. The introduction can be carried out by methods known in the art. The concentrates can also contain coadditives as required and as previously described and preferably contain from 3 to 75% by weight, more preferably from 3 to 60% by weight, most preferably from 10 to 50% by weight of additives, preferably dissolved in oil. Examples of carrier liquids are organic solvents including hydrocarbon solvents, for example petroleum fractions such as naphtha, kerosene, diesel and fuel oil, aromatic hydrocarbons such as aromatic fractions, e.g. those sold under the trade name "SOLVESSO", and paraffinic hydrocarbons such as hexane and pentane and isoparaffins. The carrier liquid must of course be selected for its compatibility with the additives and the fuel mixture.

According to the second aspect of the invention, the fuel mixture that makes up the mass into which the concentrate is introduced is suitable as the carrier liquid.

The antifoam agent may be introduced into the mass of the fuel mixture by other methods such as those known in the art. Coadditives may be introduced into the mass of the fuel mixture simultaneously with the antifoam agent or at another time.

The invention is now illustrated as follows, by way of example only.

Fuels used in the examples

Two petroleum-based fuel oils and a biofuel having the characteristics shown in Table 1 were used in the following examples of the invention. Both fuel oils were diesel fuels, while the biofuel was rapeseed methyl ester. Table 1

IBP = initial boiling point, FBP = final boiling point

Examples of antifoams useful in the present invention (i) Silicon-containing compositions Antifoam A

Proprietary block copolymer comprising siloxane blocks and oxyalkylene blocks and belonging to the general class previously described. Antifoam A is commercially available for the treatment of middle distillate fuel oils.

Example according to the invention

The fuel oil compositions defined in Table 5 were prepared by a conventional laboratory mixing technique using a WARING blender. The foaming tendency of each biofuel and petroleum-based fuel oil blend was measured after the addition of Antifoam A using a test procedure in which 100 mL of test fuel oil was agitated by hand in a previously cleaned and dried 4 oz (118.3 cc) bottle and the bottle was then placed in its normal position on a stationary flat surface. The subsequent time (in seconds (s)) during which the foam generated by shaking collapsed sufficiently to reveal a clear liquid region at the surface was recorded as a measure of foaming tendency, with longer foam collapse times indicating greater foam stability.

The silicon-containing antifoam agent significantly reduced foam collapse time in all blends containing fuel oil A and 60% or less by weight biofuel and in all blends containing fuel oil B and 70% or less by weight biofuel. These results indicate that the silicon-containing antifoam agent is generally effective in fuel oil blends containing about 65% or less by weight biofuel.

The foaming tendency of each mixture in Table 5 was also measured as the initial foam height obtained upon agitation. In Table 5, the foam height of each antifoam containing fuel oil composition (in Klam mern) as a percentage of the foam height of the corresponding untreated mixture.

The foam height of the mixture shows that the initial foam height is generally reduced with the addition of petroleum fuel antifoam. Table 5

Claims (23)

1. A fuel oil composition comprising a major proportion of a mixture of petroleum-based middle distillate fuel oil and biofuel and a minor proportion of a petroleum fuel antifoam, wherein the antifoam is a silicon-containing composition and the fuel oil has a sulfur concentration of 0.05% or less by weight based on the weight of the fuel. 2. Composition according to claim 1, wherein the fuel oil mixture contains at most 65% by weight of biofuel and at least 35% by weight of petroleum-based fuel oil. 3. A composition according to claim 2, wherein the mixture contains 0.1% to 65% by weight of biofuel. 4. A composition according to claim 3, wherein the mixture contains 0.5 % to 55% by weight of biofuel. 5. A composition according to any one of claims 1 to 4, wherein the fuel oil has a sulfur concentration of 0.01 wt% or less, based on the weight of the fuel. 6. A composition according to any one of claims 1 to 5, wherein the middle distillate fuel oil is heating oil or diesel fuel. 7. The composition of claim 6, wherein the fuel is diesel fuel. 8. A composition according to any one of claims 1 to 7, wherein the middle distillate fuel oil has been subjected to hydrodesulfurization. 9. A composition according to any one of claims 1 to 8, wherein the silicon-containing composition comprises a siloxane polymer. 10. The composition of claim 9, wherein the siloxane polymer is a block copolymer comprising siloxane blocks and oxyalkylene blocks. 11. A composition according to claim 9 or claim 10, wherein the siloxane polymer has the general formula (i) wherein R represents a monovalent or divalent hydrocarbon group, n represents a number in the range of 1 to 3 and m represents a number ≥ 2 and the groups represented by R may be the same or different in any given siloxane group within the siloxane polymer and the value of n may be the same or different in the different siloxane groups in the siloxane polymer. 12. Composition according to one of claims 1 to 11, in which the biofuel is one or more alkyl esters of fatty acids of vegetable or animal oils. 13. A composition according to claim 12, wherein the alkyl esters are methyl, ethyl, propyl and butyl esters of fatty acids having 12 to 22 carbon atoms. 14. A composition according to claim 12 or 13, wherein the alkyl esters are methyl esters of oleic acid, linoleic acid, linolenic acid and erucic acid. 15. Composition according to one of claims 12 to 14, in which the biofuel is a mixture containing at least 50 wt% methyl esters of fatty acids having 16 to 22 carbon atoms and 1, 2 or 3 double bonds. 16. A composition according to any one of claims 12 to 15, wherein the biofuel is a rapeseed methyl ester. 17. A composition according to any one of claims 1 to 16, which contains 0.1 to 10,000 ppm by weight of antifoam agent. 18. Composition according to one of the preceding claims, wherein the mixture contains one or more coadditives. 19. Composition according to claim 18, in which the mixture contains as coadditives one or more low-temperature flow improvers, stabilizers, dispersants, antioxidants, corrosion inhibitors, cetane improvers (ignition accelerators), emission reducers, deodorants, lubricants, antistatic additives or demulsifiers. 20. A concentrate comprising antifoam for petroleum fuel, wherein the antifoam is a silicon-containing composition blended with a mixture of petroleum-based middle distillate fuel oil having a sulfur concentration of 0.05% or less by weight based on the weight of the fuel, and biofuel. 21. A concentrate according to claim 20, further comprising one or more coadditives. 22. A concentrate according to claim 21, which contains one or more coadditives as defined in claim 19. 23. Use of a petroleum fuel antifoam agent, wherein the antifoam agent is a silicon-containing composition, for controlling foaming in a mixture of petroleum-based middle distillate fuel oil having a sulfur concentration of 0.05% or less by weight, based on the weight of the fuel, and biofuel.