Methods for Making the Same

The present invention relates to fuel compositions comprising heavy hydrocarbon oil streams, their use and methods for preparing, storing and transporting them.

Hydrocarbons are organic compounds which contain predominantly carbon and hydrogen. Carbonaceous fossil fuels, such as coal, oil and gas, have derived from organic matter over millions of years, and contain hydrocarbons of varying complexity. They may also contain small quantities of substances other than carbon and hydrogen, such as mainly sulphur, nitrogen, oxygen and trace elements such as vanadium and nickel . Energy can be released from these fuels by combustion, whereby the carbon and hydrogen present in the fuel react with oxygen to form carbon oxides, water and other materials.

Mixtures such as crude oil can be processed to purify or separate the hydrocarbons contained within them, to break down large hydrocarbon molecules into smaller ones or to cause larger molecules to be formed. Fossil fuels are usually processed prior to use, leading to a range of hydrocarbon- containing streams for use as fuels and in other applications. Since there is only a finite supply of fossil fuels, it is desirable to use all components produced from them where possible.

Heavy hydrocarbon oil streams are one type of hydrocarbon- containing stream obtained when processing crude oil. Their physical characteristics, including high viscosity, high density, low flowability and low volatility in a combustion environment, mean they are both difficult to combust and present handling difficulties. For example, in a combustion environment, it is difficult to atomise heavy hydrocarbon oil streams into droplets which are small enough to permit combustion with a high level of carbon burnout. Consequently, heavy hydrocarbon oil streams find relatively little industrial applicability and have in the past not been considered suitable for use as fuels without modification or moderation, usually by adding lighter and more valuable hydrocarbons .

Attempts have been made to utilize heavy hydrocarbon oil streams as fuels by using them to form oil-in-water emulsions. For example, US 6,530,965 (Warchol) discloses oil-in-water emulsions wherein the droplet sizes permit combustion of the emulsion. US2003/0131526 (Warchol et al.) discloses the use of a diluent with oil-in-water emulsions to help reduce the viscosity of heavy oil residuum, rendering it suitable for use as a combustible fuel. Fuel emulsions containing super-heavy oils and water are also disclosed in US 5,551,956 (Mariyama et al.), which discusses the use of various interfacially active materials to stabilize the emulsions.

The high levels of metals such as vanadium and the like present in heavy hydrocarbon oils can result in corrosion of the equipment in which the oil is combusted, and additives may be used to counter these effects. For example, CA 2243054 (Rivas) discloses water-in-oil-in-water emulsions wherein the inner water phase contains additives suspended therein to combat the adverse effects of vanadium. Additives are introduced into the fuel without leading to excessive use of interfacially active materials to stabilise the emulsion. Such water-in-oil-in-water emulsions are also disclosed in US 5,505,877 (Krivohlavek) , wherein additives in the innermost water phase are found to improve storage stability.

Nevertheless, the efficient handling and combustion of heavy hydrocarbon oils or residues still presents problems and there is still a need for compositions and methods which allow the efficient manufacture, handling and use of heavy hydrocarbon oil streams as fuels.

In particular, there is a need to be able to store and transport dispersions or emulsions of heavy hydrocarbon oil streams in water without degradation of the dispersion or emulsion due to creaming, sedimentation, agglomeration and/or separation in storage tanks and in equipment such as pumps and pipes. In part, degradation of the dispersion arises due to the difference in density between the oil phase and the continuous water phase within which the oil is dispersed. Where there is a large difference in these densities, the tendency to instability will be increased.

The high density of heavy hydrocarbon oil phases can be attributed to residues arising from high density crude oil sources and hydrocarbon residues that result from a high level of processing, particularly in thermal cracking or solvent precipitation processes. These residues may also have very high viscosities and need significantly elevated temperatures to keep them sufficiently mobile to be successfully processed into an emulsion. Some increase in density of the continuous water phase can be achieved, for example by dissolving inorganic solutes, which helps to reduce the density difference between the water and oil phases. However, this can only lead to a marginal reduction in the density difference while placing an additional inorganic load on the device used to combust the emulsion fuel.

Therefore there is a need for compositions and methods which result in a smaller density difference between the oil phase and the continuous water phase.

Summary of the Invention

In a first aspect, the present invention provides a fuel composition comprising a continuous water phase with heavy hydrocarbon oil droplets dispersed or emulsified in said phase, wherein a gas is dissolved and/or entrained within the heavy hydrocarbon oil droplets. Such compositions are suitable for use as fuels and can be readily stored and transported. Preferably, the heavy hydrocarbon oil droplets in the produced fuel comprise gaseous bubbles entrained within the droplets. Thus, preferred fuel compositions of the invention may be described as gas-in-oil-in-water emulsions.

The invention also provides a method of preparing a fuel composition according to the invention, comprising: (i) mixing a heavy hydrocarbon oil with a gas to form a heavy hydrocarbon oil phase containing a dissolved and/or entrained gas at an elevated temperature and pressure; followed by (ii) dispersing the resultant heavy hydrocarbon oil phase in water at an elevated temperature and pressure to form an immiscible oil- in-water dispersion or emulsion fuel composition; followed by (iii) cooling and de-pressurisation to bring the fuel composition to ambient pressure and a temperature suitable for storage, preferably a temperature in the range of about I ⁰ C to about 99°C, more preferably in the range of about 20 ⁰ C to about 75 ⁰ C.

The invention further provides the use of the compositions according to the invention as fuels .

The xnvention also provides a method for reducing the density of a hydrocarbon oil phase comprising mixing a heavy hydrocarbon oil phase with a gas to form a hydrocarbon oil phase comprising gaseous bubbles entrained within the droplets of the hydrocarbon oil phase.

The invention also provides a method of increasing the combustibility of a hydrocarbon oil phase comprising dissolving and/or entraining a gas within the hydrocarbon oil phase .

The invention also provides a method of improving the stability of an oil-in-water dispersion or emulsion comprising entraining a gas in the oil phase to form a gas-in-oil in water dispersion or emulsion.

Brief Description of the Figure

Figure 1 illustrates in schematic form some of the aspects and preferred features of the invention showing a series of steps which can be used to prepare the compositions of the present invention.

Detailed Description

The compositions of the invention are referred to herein as "gas-in-oil-in-water" dispersions or emulsions.

As used herein, a "gas-in-oil-in-water emulsion" is an emulsion or dispersion of oil droplets dispersed in a continuous water phase where the oil droplets contain gaseous bubbles entrained within them. The term "continuous water phase" as used herein refers to the aqueous phase which contains the heavy hydrocarbon oil droplets .

By "heavy hydrocarbon oil" is meant a substance that comprises a hydrocarbon oil, hydrocarbon oil mixture or hydrocarbon oil fraction with one or more of the following properties:

(i) having a density greater than 1.01 kg/1, and preferably greater than 1.04kg/l, at 20 ⁰ C;

(ii) having a kinematic viscosity measured according to

ASTM D445 of greater than 30OcSt at 100 ⁰ C preferably greater than 55OcSt at 90 ⁰ C or a kinematic viscosity measured according to ASTM D2170 of greater than 65 cSt at 135°C;

(iii) having a ring and ball softening point measured according to ASTM D36 of greater than 80 ⁰ C;

(iv) having a penetration index at 25°C measured according to ASTM D5-06el of less than 20mm, preferably less than 12mm;

(v) being a solvent precipitated asphaltenic fraction or having an asphaltene content of at least 6%, preferably between 6% and 75% weight/weight;

(vi) being a residue remaining after processing crude oil hydrocarbon mixtures by atmospheric distillation, vacuum distillation, thermal cracking, visbreaking, hydrodesulphurisation, catalytic cracking, hydrocracking or a combination of these processes including in combination with solvent precipitation processes .

Preferably, the heavy hydrocarbon oil phase is characterised by having a density according to (i) . More preferably, the heavy hydrocarbon oil phase is characterised by having a density according to (i) and one or more property selected from a kinematic viscosity according to (ii) , a ring and ball softening point according to (iii) , and a penetration index according to (iv) .

Asphaltenes are contained in the heavy fractions derived from carbonaceous sources such as petroleum or oil shale.

Asphaltenes are insoluble in low boiling point paraffin solvents such as heptane, but are generally soluble in benzene, toluene and other aromatic solvents. It is not normally practical to purify these complex fractions into individual compounds or narrower fractions. Asphaltenes are typically characterised by having a molecular mass of greater than 1,000 gmol ^"1 .

In addition to the hydrocarbon constituents, the "heavy hydrocarbon oil" may contain non-hydrocarbon constituents such as clay fines, catalyst fines, water (including formation water), inorganic contaminants, additives (including demulsification chemicals) and other minor constituents.

The heavy hydrocarbon oil phase used in the composition of the invention may become sufficiently viscous at certain temperatures and pressures, particularly at temperatures close to ambient temperatures, as to be essentially solid. Thus, the compositions of the invention may also be described herein as an oil-in-water dispersions.

By "oil-in-water dispersion" is meant a dispersion of highly viscous, essentially solid oil particles in a continuous water phase. Whether a composition of the invention may more properly be described as a dispersion or an emulsion depends on the viscosity of the heavy hydrocarbon oil phase, which is a function of the temperature and pressure. As such, the distinction between an oil-in-water emulsion and a dispersion of the invention is not necessarily clear cut. Therefore, unless it is clear from the context, oil-in-water emulsions and oil-in-water dispersions as used herein should be considered as being synonymous, and likewise unless it is clear from the context, oil droplets and oil particles should be considered as being synonymous.

By "gas" is meant any substance which is gaseous at standard atmospheric pressure and at all temperatures of between about 0 ⁰ C and about 100 ⁰ C, and which can be entrained or dissolved in the heavy hydrocarbon phase at the elevated temperatures and pressures experienced in the manufacturing process. Preferably, the substance is gaseous at ambient pressure and temperatures of -15°C or above. Single gases or mixtures of gases may be used. Suitable gases include carbon dioxide, methane, ethane, ethylene, nitrogen, oxides of nitrogen, oxygen, air, sulphur dioxide, dimethyl ether or mixtures thereof.

Typically, the gas is selected to minimise costs. However, other factors also influence the suitable choice of gas. For example, where the fuel composition is to be combusted in equipment with flue-gas clean-up facilities, gases that are otherwise more polluting may be used, such as sulphur dioxide or oxides of nitrogen. Alternatively, where the fuel composition is to be combusted in a diesel engine, a gas with a high cetane number such as dimethyl ether may be used.

As the gas is entrained or dissolved in the oil phase under pressure, it then effervesces and expands on cooling and de-pressurisation to form stable pockets or bubbles of gas inside the oil droplets . At ambient fuel storage pressure the bubbles of gas have an average diameter less than about 1 μm, preferably less than about 0.1 μm and most preferably less about 0.05 μm. The oil droplets containing the entrained bubbles of gas following cooling and de-pressurisation will have an average density similar to that of the continuous phase, and hence the composition will be suitable for storage and transport.

By "dissolved" is meant that the gas forms a homogeneous solution in the oil phase at the elevated temperatures and pressures experienced in the manufacturing process.

By "entrained" is meant that the gas is present in the oil phase as discrete bubbles or pockets, thus forming a gas in oil dispersion at the elevated temperatures and pressures experienced in the manufacturing process. The size of any bubbles of gas will depend at least in part on the temperature and pressure of the fuel composition and the means of mixing. Under certain conditions, it is possible that the oil phase could comprise both dissolved and entrained gas. Where some of the gas is entrained, the manufacturing process results in the size of these bubbles or pockets being sufficiently small and dispersed such that, on cooling and de-pressurisation, the bubble size distribution meets the criterion of having an average diameter less than about 1 μm, preferably less than about 0.1 μm and most preferably less about 0.05 μm.

It is preferable that at least some of the gas should be entrained and not completely dissolved in the heavy hydrocarbon oil phase prior to the cooling and depressurisation step, as small gas bubbles can act as centres for gas evolution and speed the process of gas effervescence during the cooling and de-pressurisation manufacturing stage.

By "effervesces" or "effervescence" is meant any gas dissolved in the oil phase comes out of solution to form gaseous bubbles .

By including gas within the heavy hydrocarbon oil droplets, the difference in density between the hydrocarbon phase and the water continuous phase can be more readily matched to address problems of storage and transport stability. The applicant has recognised that differences in density can lead to creaming or sedimentation and coalescing of the hydrocarbon oil droplets present in dispersions or emulsions of the present invention. With heavy hydrocarbons where the density is greater than that of the continuous water phase, sedimentation of the heavy hydrocarbon droplets can occur over time when the heavy hydrocarbon-in-water dispersion or emulsion is stored. Sedimentation can be prevented by stirring the emulsion or dispersion, or by recirculating the emulsion or dispersion stored in a storage tank using a suitable pump. However, continuous and heavy use of recirculation techniques can also increase the tendency for the emulsion to become unstable with the dispersed droplets coalescing and causing emulsion or dispersion de- stabilisation.

When there is a large difference in the densities of the oil and water phases, the oil phase must have small droplet sizes to reduce storage instability. In the present invention, by- entraining a gas in the heavy hydrocarbon oil phase, the density of the heavy hydrocarbon oil droplets can be adjusted to match that of the continuous water phase. This has the potential advantage of allowing oil droplet sizes to be larger and hence reducing the level of chemical stabilisers that may be needed. A lower processing temperature may also be used when forming the oil-in-water emulsion.

Furthermore, the viscosity of compositions according to the present invention which contain heavy oils is lower than the viscosity of the oils themselves, thereby addressing handling problems associated with heavy oils. The presence of the gas lowers the viscosity of the heavy hydrocarbon oil phase, allowing reduced temperatures for the emulsion forming process of dispersing the oil phase into the continuous water phase. The level of reduction in temperature will depend on the quantity and type of gas and may be greater than 5 ⁰ C, but will typically be no greater than 25°C. Where a temperature of about 180 ⁰ C would be required to generate a conventional fit for purpose emulsion, by choosing a suitable quantity of a suitable gas the temperature of manufacture to generate a fit for purpose emulsion according to the present invention may be reduced to as low as about 16O ⁰ C.

The density of the continuous water phase may be increased by the addition of suitable components to the water. Suitable components include solids or solutes such as ammonium nitrate. Preferably these solids or solutes promote combustion and do not add significantly to the undesirable species in the flue gas such as nitrogen oxides, particulates or ash. Where the solids or solutes add to the particulate or ash content, they preferably reduce its harmful effect such as reduced corrosivity, reduced reflectivity or stickiness. One such class of compounds is the soluble salts of magnesium such as magnesium nitrate.

Ideally the densities of the oil and water phases in the compositions of the invention should be near identical over the range of temperatures that are likely to be experienced by the composition during manufacture, storage, transport and use. However, as the density-temperature relationship for water is non-linear, whereas the density-temperature relationship for the heavy hydrocarbon is typically near- linear, this is not always possible. Theoretical calculations under conditions of both laminar and turbulent flow in pumps or pipes, particularly at bends or restrictions, indicate the density difference should preferably be less than about 0.02 kg/1. Empirical evidence from experience with a wide range of dispersions of hydrocarbons-in-water indicate that for systems where the continuous water phase is approximately 30% by weight, it is desirable to reduce the density difference to less than about 0.05 kg/1, preferably less than about 0.02 kg/1. Emulsions in which the density difference is outside of this range might be expected to show significant creaming or sedimentation over a relatively short period of time.

A further benefit of entraining a gas in the heavy hydrocarbon oil phase is the increase in combustibility of the fuel composition of the present invention. Without wishing to be bound by theory, it is postulated that the inclusion of gas in the heavy hydrocarbon oil droplets makes the heavy hydrocarbon oil droplets more amenable to combustion because, under suitable conditions of temperature and pressure, typically elevated temperature and near ambient pressure, present in the combustion equipment, any dissolved gas effervesces and any gaseous bubbles expand rapidly causing the heavy hydrocarbon droplets to fracture into smaller droplets. This greatly improves the extent to which, and the ease with which, the fuel can be combusted, aiding complete combustion of the carbon components in the heavy hydrocarbon oil phase. Breaking the heavy hydrocarbon oil into smaller droplets allows the reaction with oxygen and other species present in the combustion environment to occur more easily and more efficiently, and generally at an even temperature that is lower than usually required for the combustion of oil droplets atomised in the combustion environment by conventional atomisation methods. Furthermore, the use of a readily combustible gas such as methane or ethane dispersed in the heavy hydrocarbon phase also enhances combustion.

One benefit of using the compositions of the invention as fuels is therefore that the proportion of excess oxygen in the combustion effluent gas stream may be kept to very low levels, promoting overall system efficiency while avoiding the formation of high levels of carbon monoxide, particulates and smoke. Furthermore, more even burning, particularly in intimate contact with a high water concentration, results in fewer regions of elevated temperature and hence results in lower production of thermal nitrogen oxides .

Thus, the present invention allows heavy hydrocarbon oils to be used effectively and efficiently as fuels.

The compositions of the invention may additionally comprise one or more interfacially active materials (e.g. emulsifiers and/or surfactants) to help stabilise the dispersions or emulsions and/or to help control the size of the heavy hydrocarbon oil droplets. Preferably, an interfacially active material is present at the interface of the continuous water phase and the heavy hydrocarbon oil droplets .

Suitable interfacially active materials are known in the art. For the stabilisation of the oil-in-water emulsion or dispersion all classes of interfacially active material known to stabilise such emulsions or dispersions can be used. Such materials are generally water soluble. Thus, water soluble non-ionic, cationic and anionic interfacially active materials or mixtures thereof can be used. Cationic interfacially active materials, such as quaternary fatty alkyl trimethylenediamine ethoxylates and N-tallow alkyl-1,3- diaminopropane, are particularly good at stabilising heavy hydrocarbon emulsions or dispersions.

The heavy hydrocarbon stream used in preparing the fuel is sufficiently viscous as to be essentially solid at ambient temperatures, such that when the hydrocarbon oil is emulsified/dispersed in the continuous water phase and subsequently cooled to ambient temperature during manufacture, any gaseous bubbles in the hydrocarbon phase are effectively frozen and hence kinetically stabilised.

Other more volatile and/or more combustible hydrocarbon components may be added to the oil phase in order to assist combustion. In addition to the beneficial effect on combustion, these additional hydrocarbon components may also contribute to reducing the water phase / hydrocarbon phase density differential and to reducing the hydrocarbon phase viscosity. Suitable volatile and/or combustible hydrocarbons are those with one or more of the following properties: very low or, preferably, zero asphaltene content; low molecular weight (preferably below 1,000); high hydrogen to carbon ratios such as found in compounds with a high paraffinic or naphthenic content although lower molecular weight high-aromatic content compounds would also be suitable; or hydrocarbons having a boiling point below 650 ⁰ C, preferably below 500 ⁰ C.

Suitable materials include vacuum gasoil, de-asphalted oil or light distillate hydrocarbons such as gasoil, kerosene and naphtha. Slurry from catalytic cracking processing containing some catalyst fines may also be utilised.

This provides a greater proportion of material that is volatile in a combustion environment to aid rapid combustion and to help ensure complete carbon burnout .

Any of the known combustion improving additives may be present in the composition in the oil and/or in the continuous water phase. These include, for example, ammonium nitrate. These additives assist in obtaining better and more rapid combustion of the fuels. Combustion improvers that dissolve in a non-oil phase are generally cheaper than oil soluble combustion improvers, and hence additives soluble in the water phase are preferred. They can also be soluble in the heavy hydrocarbon phase and be based on metal containing additives which reduce the ignition temperature of soot particles. For example, iron based catalysts that promote the complete oxidation of the heavy hydrocarbon components, or combination additives containing both iron and magnesium which inhibit the high temperature vanadic corrosion of combustion equipment, may be used. Compounds containing manganese, platinum and cerium can also be used.

Where the fuel is to be combusted in a diesel engine, the addition of compounds that have a high cetane number and low auto-ignition temperatures (e.g. below 280 ⁰ C), such as di- methyl ether, will aid combustion to give complete combustion and low NOx and low particulate emissions.

The volumetric median size of the heavy hydrocarbon oil droplets is preferably below about 75 μm, more preferably below about 20 μm, and still more preferably from about 5 to about 15 μm, as measured using a Malvern Instruments Mastersizer Micro, Mastersizer S or Mastersizer 2000. 90% by volume of the heavy hydrocarbon oil droplets is preferably below about 100 μm, more preferably below about 60 μm, and still more preferably below about 25 μm. The volumetric mean should preferably be less than about 70 μm, more preferably below about 20 μm, and still more preferably from about 5 to about 15 μm.

The bubbles of gas inside the oil droplets suitably have an average diameter less than about 1 μm, preferably less than about 0.1 μm and most preferably less about 0.05 μm. The proportion of gas volume to oil volume in the oil droplets should be adjusted during manufacture such that. the composite droplets have an average density similar to that of the continuous phase.

The invention also provides a method of preparing a fuel composition according to the invention comprising: (i) mixing a heavy hydrocarbon oil with a gas to form a heavy hydrocarbon oil phase containing a dissolved and/or entrained gas at an elevated temperature and pressure, followed by; (ii) dispersing the resultant heavy hydrocarbon oil phase in water at an elevated temperature and pressure to form an immiscible oil-in-water dispersion or emulsion fuel composition, followed by,- (iii) cooling and de-pressurisation to bring the fuel composition to ambient pressure and a temperature suitable for storage, preferably a temperature in the range of about 1°C to about 99 ⁰ C, more preferably in the range of about 2O ⁰ C to about 75 ⁰ C.

In one embodiment, in step (i) the gas and heavy hydrocarbon oil phase are pressurised and intimately mixed, allowing the gas to dissolve and/or disperse finely in the oil. Alternatively, the gas and heavy hydrocarbon oil may be intimately mixed at low pressure, preferably between ambient pressure and about 10 bara (1 MPa), before pressurising the system to the operating pressure required to dissolve a substantial proportion of the gas into the hydrocarbon phase. The operating pressure in step (i) will be sufficiently high to cause the gas to dissolve in the oil to a level that is greater than the amount that will be dissolved in the oil following the cooling and de-pressurisation step (step (iii)). Typically, the pressure in step (i) is about 100 bara (10 MPa) or less, preferably about 50 bara (5 MPa) or less.

Step (i) is carried out at an elevated temperature such that the viscosity of the hydrocarbon oil phase is sufficiently low that, on mixing, gas dispersion and dissolution can occur. The temperature used will depend both on the operating pressure and the nature of the hydrocarbon oil phase. Typically the temperature is above about 100 ⁰ C, preferably about 110 ⁰ C to about 300 ⁰ C, more preferably about 110 ⁰ C to about 250 ⁰ C, even more preferably about 120 ⁰ C to about 200 ⁰ C, and most preferably from about 130 ⁰ C to about 180 ⁰ C

Step (ii) is carried out at a temperature such that the viscosity of the gas-in-oil phase is suitable for the manufacture of an emulsion with the necessary particle size distribution. Typically, the viscosity of the gas-in-oil phase is from about 80 to about 700 cSt, preferably from about 150 to about 500 cSt, even more preferably from about 200 to about 400 cSt. Typically, the temperature in step (ii) will be from about 120 ⁰ C to about 200 ⁰ C, preferably from about

13O ⁰ C to about 18O ⁰ C. Typically, the temperature of step (i) will be at or above the temperature of step (ii) , preferably at the temperature of step (ii) . Preferably, the pressure at the end of the process of dispersing the oil droplets in the continuous water phase (step (ii)), prior to cooling to below about 100 ⁰ C if storage and/or transport of the fuel is required, is between about 2 bara (0.2 MPa) and about 100 bara (10 MPa) . This ensures that the continuous water phase is maintained in the liquid phase. Typically, the pressure of step (i) will be at or above the pressure of step (ii) , preferably at the pressure of step (ii) . The water phase may require heating to a temperature similar to, or preferably lower than, the oil phase temperature. The difference in temperature of the water continuous phase and the oil phase in step (ii) should be no more than about 120 ⁰ C, and preferably no more than about 50 ⁰ C, to ensure that the thermal shock of bringing the low viscosity cool water into contact with the relatively higher viscosity and hotter gas- containing heavy hydrocarbon is minimised, and the gas- containing heavy hydrocarbon continues to pass through the mixing device and associated equipment without blocking the equipment. The pressure in step (ii) will be higher than is required to keep the continuous water phase liquid at the temperature of step (ii) .

An interfacially active material may be added during step (ii) , or may already be present in the water used in step (ii) . The presence of an interfacially active material at this stage is normally necessary in order to prevent the emulsion from destabilising. The process conditions, temperature, water content and interfacially active material are chosen so that the hydrocarbon oil droplets have a suitable size profile.

Preferably, the ratio by mass of heavy hydrocarbon gas-in-oil droplets to continuous water phase in the compositions of the invention is from about 60:40 to about 80:20, more preferably from about 68:32 to about 80:20. More preferably, the continuous aqueous phase comprises less than about 30% by mass of the total composition.

The quality of the water used to form the continuous water phase should not inhibit formation of a suitably stable emulsion or dispersion of hydrocarbon in water. For example, it should have a pH consistent with the type of interfacially active material used to stabilise the emulsion or dispersion. In addition, the water should not contain solid materials at levels which would require economically excessive amounts of interfacially active material. Any constituent that can increase the corrosive qualities of the stored and transported fuel emulsion to an unacceptable level for the equipment

(tanks, pipes, pumps, injectors etc.) through which it passes should be avoided. Any constituent that has a negative impact on the post-combustion pollution load should preferably not be included unless such pollution load can be adequately managed.

Waste water streams may be used. Oily water streams such as may be found in an oil processing facility are suitable, as are streams that contain dissolved gases such as hydrogen sulphide provided suitable safety measures are taken. In an upstream oil situation, formation water from a hydrocarbon reservoir may be suitable for some or all of the continuous water phase depending on the level of inorganic material present. Excessive inorganic material may lead to osmotic thickening and ultimately to emulsion inversion.

The quantity and mixture of gases dissolved into the pressurised hot heavy hydrocarbon oil are sufficient that on evolution during the de-pressurisation stage they result in an expansion of the oil droplet volume to a point where the droplet average density is compatible with the continuous phase of the emulsion. Where the source heavy hydrocarbon oil specific gravity is 1.1 at the storage temperature, this will require a volume expansion of 10%. Where the specific gravity of the heavy hydrocarbon oil is greater than this, a proportionally greater volume expansion will be required.

For example, a 0.1 kg/litre modification to the oil-phase density requires the quantity of gas to be dissolved per kilogram of oil to be 4 to 5 millimole of gas in excess of the quantity of gas that remains dissolved in the hydrocarbon after completion of the cooling and de-pressurisation stage. If the gas used were pure carbon dioxide this would require the addition of between 180 and 220 milligrams of carbon dioxide per kilogram of hydrocarbon phase. Where the gas being used is a 50/50 (by volume) mixture of carbon dioxide and methane, this would require the addition of between 130 and 140 milligrams of gas per kilogram of hydrocarbon phase.

The amount of gas present in the oil phase will depend on the intended use of the compositions, and the conditions to which they will be exposed during and prior to use. For example, if the compositions are to be stored and/or transported before use, sufficient gas should be used to ensure that the compositions are stable under the storage/transport conditions. For example, it is preferable if the compositions of the invention can be stored and transported at a temperature in the range of about 20 ⁰ C to about 75 ⁰ C.

In some cases it may be necessary to cool and de-pressurise the composition from a temperature of greater than 100 ⁰ C and a pressure greater than one atmosphere to ambient temperature and pressure in order to maintain the continuous water phase as a liquid, and/or to stabilize the hydrocarbon oil droplets.

The compositions of the invention may be stored prior to use. Where the compositions are stored for combusting at a later time or date, and the continuous water phase is substantially comprised of water and has an atmospheric boiling point of approximately 100 ⁰ C, storage temperatures above 0 ⁰ C and below 100 ⁰ C, for example in the range of about 20 ⁰ C to about 75 ⁰ C, and storage at ambient pressure are preferred. Preferably, suitable interfacially active materials are also present, and are added during the manufacturing stage in order to help ensure that the composition remains stable with respect to agglomeration and coalescence over the period of storage.

As discussed above, the compositions of the invention are useful as fuels. The choice of optional components is determined in part by how the compositions are intended to be used. For example, if they are used immediately or soon after preparation, and/or in the vicinity of their preparation, it is less important to stabilize the compositions with respect to time, storage or transportation requirements. Conversely, significant storage or transport of the compositions prior to their use as fuels may bring stability considerations to the forefront, and the use of interfacially active materials and additional density altering substances may be more important.

The compositions of the invention can be transported to a liquid fuel burner equipped with a suitable burner tip that maintains the pressure in the feed line above the pressure required to keep the water in the liquid phase. At the burner tip, the release of pressure to near atmospheric pressure, the heat content of the hot fuel and the effect of the heat flux from the hot boiler flame environment causes most of the water in the fuel to vaporise instantaneously. The dissolved gasses inside the oil droplets also effervesce, and any entrained gaseous bubbles rapidly expand, causing the oil droplets to be further broken up into smaller droplets such that complete burnout can take place within the residence time of the hot combustion zone environment .

Where compositions of the invention have been cooled for storage and/or transport, it may be necessary to heat them, under pressure, prior to combustion to ensure sufficient heat is available as the fuel leaves the burner tip to vaporise a significant proportion, typically more than about 60% and preferably more than about 80% by weight, of the water contained in the fuel in the region close to the burner tip.

Pressure can be reduced prior to or during combustion, thereby enhancing the vaporisation of dissolved gas and expansion of entrained gas bubbles and hence the combustion process. It is also possible to expose the composition to higher temperatures whilst maintaining high pressures, thereby preparing the fuel for combustion, prior to reducing the pressure and combusting the composition.

Figure 1 illustrates schematically the steps that may be used in the preparation of fuel compositions according to the invention.

The gas 12 and heavy hydrocarbon oil 11 are pressurised and fed to "System Device 1" 10 where they are intimately mixed and the gas allowed to dissolve and/or disperse in the oil. Alternatively, the gas and the heavy hydrocarbon oil are intimately mixed at low pressure, preferably between ambient pressure and about 10 bara (1 MPa), before pressurising the system to the operating pressure required to partially dissolve the gas in the hydrocarbon phase. The device can incorporate mixing means sufficient to ensure that the gas is completely dissolved. Alternatively and preferably, when mixing the gas and hydrocarbon phase together at the system operating pressure, the process may be taken to the point where small gas bubbles remain in the hydrocarbon phase, provided that over about 80%, preferably over about 90%, of the bubbles are sufficiently small that the bubbles of gas inside the oil droplets after cooling and de-pressurisation will have an average diameter less than about 1 μm, preferably less than about 0.1 μm and most preferably less about 0.05 μm.

The gas-containing heavy hydrocarbon oil phase 21 and the water phase 24, formed from water 22 and interfacially active chemicals 23, are mixed in "System Device 2" 20. The system is maintained at a suitable pressure to ensure that the continuous aqueous phase is maintained in a liquid state when the mixture is fed into "System Device 2" 20. For example, where the operating temperature is 160 ⁰ C, the system pressure must be greater than 6.2bara (0.62 MPa); for 190 °C, the system pressure must be greater than 12. δbara (1.26 MPa); or for 260 ⁰ C, the system pressure must be greater than 46.6bara (4.66 MPa) .

The system pressure in "System Device 1" 10 and "System Device 2" 20 is elevated to a level required to ensure that the gas either dissolves into the heavy hydrocarbon oil or is entrained such that the gas bubbles are below the maximum size distribution required. Preferably, the system pressure is in the range 6bara to lOObara (0.6 to 10 MPa), preferably in the range lObara to 60bara (1 to 6 MPa) . The quantity of gas dissolved and/or dispersed into the oil is determined to be sufficient to meet the requirement for gas expansion and evolution during the cooling and de-pressurisation step.

After mixing and formation of the required level of gas bubbles, the heavy hydrocarbon oil containing the dissolved gas 21, or the dissolved and entrained gas, is fed to "System Device 2" 20 at the same pressure where it is dispersed into a continuous phase 24 comprising water 22 together with interfacially active chemicals 23. Mixing to form the composition of the invention may be effected by any suitable device known in the art. Suitable systems include pressure- shock systems, static mixer systems, rotator-stator mills, colloid mills, in-line blenders and homogenisers .

After mixing in "System Device 2" 20, the composition is then cooled and depressurised 30. The rate of cooling and de- pressurisation is controlled so that the dissolved gas may effervesce and the gas bubbles may expand before the oil droplets become so viscous that the pressure in the gas bubbles is substantially greater than ambient. This may be done in a single step or as a series of steps.

As one example, the composition produced from "System Device 2 " 20 is cooled to a temperature that is close to the softening point of the heavy hydrocarbon oil, preferably more than 5 ⁰ C above the softening point, more preferably more than 2 °C above the softening point, and the pressure is reduced to the lowest pressure required to maintain the continuous water phase in the liquid state. This allows rapid effervescence and expansion of the gas bubbles while the hydrocarbon phase is still moderately soft.

In some circumstances, the temperature may be reduced in a series of steps, with the final expansion stage taking place at a temperature below the hydrocarbon phase softening point.

As another example, the pressure is first reduced to the lowest pressure required to maintain the continuous phase in a liquid state. Sufficient storage may be provided for the cooled material at this intermediate stage to allow an adequate residence time for the evolution of the dissolved gases. Subsequently, the temperature is further reduced to just below the boiling point of the continuous phase of the emulsion - below 100 ⁰ C in the case of pure water. The emulsion is then de-pressurised to ambient pressure. The emulsion is further cooled to a normal storage temperature in the range of I ⁰ C to 99 ⁰ C, preferably in the range of 20 ⁰ C to 75 ⁰ C, to complete the process of solidifying the oil droplets and entraining the evolved gas within the oil droplets of the emulsion 31.