Desulphurisation process

Abstract

The present invention provides the use of a solid agent comprising a reductant for reducing the amount of reactive sulphur in an oil and / or reducing the dielectric dissipation factor of an oil, wherein either: (i) the oil is contacted with (a) the solid agent comprising the reductant, and (b) a liquid agent suitable for dispersing said solid agent; (ii) the oil is contacted with a solid substrate having the reductant on its surface; or (iii) the oil is an ester and is contacted with a particulate adsorbent support having the reductant on its surface.

Description

[0001] DESULPHURISATION PROCESS

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a desulphurisation process and is based on the finding that certain forms of reductant can be used to reduce the amount of sulphur in an insulating oil and remove reactive sulphur in a very efficient way. This is of particular use in the field of transformer oil regeneration.

[0004] More specifically, the present invention provides a process for reducing total sulphur content and removing S8, S4, S5, S6, S7, mercaptans, monosulphides and / or disulphides, oxygenated sulphur compounds, typically sulphoxides, sulphones and sulphur oxy acids from mineral insulating liquids and synthetic ester insulating liquids for the use in power transformers. The present invention also provides desulphurised insulating oils obtained or obtainable by the process of the invention, as well as compositions and equipment for use in the desulphurisation process of the invention and preparation methods for the compositions.

[0005] BACKGROUND TO THE INVENTION

[0006] The presence of sulphur compounds, specifically S8, monosulphides, disulphides, oxygenated sulphur compounds, typically sulphoxides, sulphones and sulphur oxy acids in oils can cause corrosion of metallic components made of metals such as silver and copper. Thus, power transformers typically feature components made of silver and / or copper, and the corrosiveness of insulating liquids, such as mineral oils and esters to such components is an issue for the power industry because it can lead to transformer breakdowns. On top of this, known procedures for regenerating used mineral insulating oils include ones featuring the use of adsorbents based on aluminium oxide and aluminium silicate in online systems. During the adsorbent reactivation process using high-temperature combustion technology, S8, S2, S4S5, S6, S7, hydrogen sulphide, mercaptans can form as side products. Such regeneration methods may therefore contribute to corrosion by S8, S2, S4, S5, S6, S7, hydrogen sulphide and mercaptans.

[0007] Methods which desulphurise such oils are known. For instance, W02018190741 describes a process wherein mineral oil is circulated through a stainless-steel column filled with a synthetic adsorbent made by depositing silver ions and ammonium ions on annealed silicon dioxide. This removes sulphur and also removes amine derivatives of tolyltriazole from the oil. However, if it was possible to devise a desulphurization process which offered improvements in terms of efficiency and / or the extent of desulphurisation and overall lower cost of method, then this would be of obvious benefit.

[0008] SUMMARY OF THE INVENTION

[0009] The present invention provides the use of a solid agent comprising a reductant for reducing the amount of reactive sulphur in an oil and / or reducing the dielectric dissipation factor of an oil, wherein either

[0010] (i) the oil is contacted with (a) the solid agent comprising the reductant, and (b) a liquid agent suitable for dispersing said solid agent;

[0011] (ii) the oil is contacted with a solid substrate having the reductant on its surface; or

[0012] (iii) the oil is an ester and is contacted with a particulate adsorbent support having the reductant on its surface.

[0013] The present invention also provides the use of a solid agent comprising a reductant for reducing the amount of reactive sulphur in an oil (especially an insulating oil, such as a mineral insulating oil or an ester insulating oil) and / or reducing the dielectric dissipation factor, oil acidity and particles, while increasing interfacial tension, wherein either

[0014] (i) the oil is contacted with (a) the solid agent comprising the reductant, and (b) a liquid agent suitable for dispersing said solid agent;

[0015] (ii) the oil is contacted with a solid substrate having the reductant on its surface; or

[0016] (iii) the oil is an ester and is contacted with a particulate adsorbent support having the reductant on its surface.

[0017] The forms for the reductant identified in options (i), (ii) and (iii) above have been found to be highly effective in reducing the content of active sulphur in oils and / or reducing the dielectric dissipation factor of an oil and / or reducing oil acidity and / or reducing particle levels, and / or increasing interfacial tension, and are particularly useful for regenerating transformer oils which can contain various different types of compounds containing active sulphur and can lose their insulating properties over time during use (active sulphur is also referred to in places herein as reactive sulphur). The form of the reductant in option (iii) has been found to be surprisingly effective in situations where the oil is an ester. In a first embodiment, the present inventor has devised a new method of removing S8from oils such as mineral insulating oils and esters (especially mineral oils), which employs a selective liquid-liquid-soiid process. The present invention therefore provides a means to mitigate power transformer failures induced by the presence of S8.

[0018] The process is a selective desulphurisation procedure that utilises a liquid-liquidsolid interphase chemical reaction to effectively and selectively remove S8. This approach has surprisingly been found to enable the selective removal of S8in a highly efficient way. The present invention solves the problem of providing a desulphurisation process which offers improvements in terms of efficiency and also the extent of desulphurisation.

[0019] The process of this first embodiment of the present invention effectively and highly selectively removes S8, thereby preventing corrosion reactions that the S8would otherwise cause. The process can be used to provide post-treated oils meeting the physical, chemical and electrical criteria that oils for use in power transformers must meet. It therefore enables the regeneration of corrosive mineral insulating oils and their re-use as insulating oils for power transformers. Further, a single dispersion reagent as defined herein in accordance with the present invention may be used to carry out this regeneration multiple times. The ability to recycle the dispersion reagent this way has obvious benefits in terms of efficiency, cost, waste reduction and environmental profile.

[0020] Additionally, corrosive mineral insulating oils can be made non-corrosive very quickly - e.g. within 60 to 90 minutes. The method can also be used to remove aging products from mineral insulating oils, further improving their regeneration. Another benefit is that low metal-to-oil ratios are used in the present method, rendering the entire process very cost-efficient. Relatedly, readily available and low-cost reagents can be used as the starting materials, thereby further increasing the cost-efficiency of the method.

[0021] In a second embodiment, the present inventor has devised a new method of reducing total sulphur content, more specifically total reactive sulphur content and removing sulphur, more specifically S8,S7, S6, S5, S4disulphides and / or sulphoxides, sulphones and sulphur oxy acids from oils such as mineral insulating oils and synthetic ester insulating liquids, which may employ the use of silver-plated coils made of iron, steel, aluminium, or copper, having silver plating layer of 0.005 to 0.05 mm Fig. 6 (copper, iron or aluminium silver- plated wire, diameter 8-10 mm, length 5-15 m, silver plating layer of 0.005 to 0.05 mm) In another embodiment silver plated-coils are wrapped around heating cylinders made of copper, or iron, or steel with silver-plating.

[0022] Depending on the need to vary surface of available silver to the mass of oil and the concentration of total reactive sulphur content in the oil, silver-plated coils are placed in batch reactor, with or without silver-plated heating cylinders (Fig. 6A). Silver-plated coils are placed in the batch reactor in two different possible arrangements: one is silver plated coil alone (Fig. 6), and another is silver-plated coil wrapped around silver-plated heating element in the shape of cylinder, (Fig. 6B).

[0023] One to four silver plated coils are fixed at the lid of the batch reactor, with silver- plated coils wrapped around the heating cylinder or alone, positioned 40 cm from the top of the reactor and immersed in the oil (Fig. 7, 7A, 7B, 7C). Ratio of silver-plated surface to oil mass in total in batch reactor is specifically in the range from 0.0025 to 0.01 m2 / kg.

[0024] One to three silver-plated coils (Fig. 8A) are inserted in tubular reactor made of stainless steel, length 80 -130 cm, diameter 28 - 40 cm (Fig. 8 and 8B) connected to the preheater chamber with working fluid volume of 700-900 cm3, made of iron steel (Fig. 9), or to a batch reactor to enable continuous flow of the oil between tubular reactor and preheater or continuous flow between tubular or batch reactora.

[0025] Silver-plated coils with or without heating cylinder are heated at specified temperatures in two different ranges, depending on the flash point of the treated oil, concentration and type of sulphur reactive species in the oil as well as desired reaction time. Lower temperature range is from 100°C to 140 °C, usually suitable to mineral oils and higher range of temperature is from 160°C to 200°C, usually suitable to synthetic ester insulating liquids.

[0026] Silver-plated coils are immersed in the oil in batch reactor and in tubular reactor.

[0027] Batch reactor filled with oil is heated to temperatures of 100°C-120°C.

[0028] Oil in tubular reactor is heated by the silver-plated coils and in preheater which is connected to the tubular reactor.

[0029] The second embodiment of the present invention therefore provides a means to mitigate power transformer failures induced by the presence of reactive sulphur, namely S8,S7, S6, S4disulphides and oxygenated sulphur compounds, typically sulphoxides, sulphones and sulphur oxy acids. The process is a desulphurisation procedure that reduces total sulphur content and total reactive sulphur content and utilises a solid-state reaction of silver and sulphur compounds such as S8disulphides and oxygenated sulphur compounds, typically sulphoxides, sulphones and sulphur oxy acids. This approach has surprisingly been found to enable the selective removal of S8, S7, S6, S4disulphides and sulphoxides, sulphones and sulphur oxy acids in a highly efficient way. The second embodiment of the present invention solves the problem of providing a desulphurisation process which offers improvements in terms of efficiency and also the extent of desulphurisation in both mineral and synthetic ester insulating liquids.

[0030] The process of the second embodiment of the present invention effectively reduces total reactive sulphur content and removes S8S7, S6, S4and sulphoxides, sulphones and sulphur oxy acids, thereby preventing corrosion reactions that sulphur compounds, S8S7, S6, S4disulphides and oxygenated sulphur compounds, typically sulphoxides would otherwise cause. Further, a single agent herein in accordance with the second embodiment of the present invention may be used to carry out this treatment multiple times (after removal of deposited sulphides by mechanical treatment / sanding). The absence of chemicals and waste materials has obvious benefits in terms of efficiency, cost and environmental profile.

[0031] Additionally, corrosive mineral insulating oils can be converted to non-corrosive very quickly - e.g. within 30 to 360 minutes, but longer periods can be applied if necessary (one to three days). Another benefit is that low metal-to-oil ratios are used in the present method, rendering the entire process very cost-efficient. Relatedly, readily available materials for the multiple use are further increasing the cost-efficiency of the method.

[0032] In a third embodiment, the present inventor has devised a new method of reducing the amount of reactive sulphur in the form of mercaptans, disulphides, sulphoxides, sulphones, sulphur oxy acids in an ester oil and / or reducing the dielectric dissipation factor of an ester oil, the method comprising contacting the ester oil with a particulate adsorbent support having the reductant on its surface.

[0033] This third embodiment is particularly useful for removing elemental sulphur and disulphides that may be present in the ester oil in wide range of concentrations and in the same time obtain lower dielectric dissipation factor of the oil after treatment, as a important oil property for further use of oil in power transformers. Below is described an illustrative method for preparing a particulate adsorbent support having the reductant on its surface, wherein the adsorbent is based on silicon dioxide and incorporates silver and ammonium ions. This illustrative method has three stages:

[0034] 1 - annealing the adsorbent support consisted of silicon dioxide

[0035] 2 - deposition of silver ions on the adsorbent support by silver nitrate aqueous solution for removal of sulphur compounds corrosive to silver and

[0036] 3 - deposition of ammonium hydroxide aqueous solution on the adsorbent support for neutralization of acidic by-products after removal of the sulphur compounds on the adsorbent.

[0037] Stages 2 and 3 are key steps, because deposition of the specified components produces an adsorbent highly efficient and selective for the removal of corrosive moieties.

[0038] As an illustration of how the particulate adsorbent support having the reductant on its surface may be used to treat ester oil in a method of the invention, it may be used in a method comprising ester oil circulation under pressure with flow of 1000 to 2000 lit. / h, through a stainless-steel column filled with a bed of the particulate adsorbent support having the reductant on its surface, in the ratio of 2 to 10 mas. % relative to the mass of oil, at working temperatures from 80°C to 90°C to enable conditions of effective diffusion of reactive sulphur species on the adsorbent active sites because of high viscosity of the ester oil. In case of removal of high concentrations of reactive sulphur compounds from the oil (up to 50 mg / kg S8, up to 200 mg / kg dibenzyl disulphide, sulphones, sulphoxides and sulphur oxy acids), treatment may be conducted in a narrower temperature range from 85°C to 90°C, possibly in a two-stage process, i.e. with two portions of fresh adsorbent with maximum amount of adsorbent relative to oil of up to 10 mas.%. High efficiency results from the bonding of the silver ions to the adsorbent support, namely to acidic sites of -Si-O- H bonds, which react with the reactive sulphur compounds and molecules thereby efficiently removing them from the oil, while ammonium hydroxide is efficient for neutralization of acidic by-products of chemisorption of the sulphur compounds on the adsorbent. The adsorbent can be used in a packed bed in a stainless steel column / cartridge (Fig.19) which can be placed in an oil treatment plant and / or connected to a transformer, enabling continuous oil circulation between the column / cartridge and the power transformer (on-line, while transformer is working).

[0039] Such methods enable removal of reactive corrosive sulphur compounds, especially elemental sulphur in high concentrations up to 50 mg / kg and disulphides in concentrations up to 200 mg / kg, as determined by testing the oil according to methods: IEC 62697 part 1 (quantification of dibenzyl disulphide), IEC TR 62697part 3 (quantification of eight atom molecule of sulphur - S8) and ASTM D 1275- 15 (silver strip corrosion test).

[0040] Common feature of all three embodiments is the removal of elemental sulphur (S8) from insulating oils, in particular synthetic ester oils, obtaining oils non-corrosive to silver, by applying processes and methods that include heterogeneous chemical reactions in boundary liquid-solid interphase.

[0041] BRIEF DESCRIPTION OF THE FIGURES

[0042] Figure 1 is a chromatogram displaying the S8content of a mineral insulating oil before, during and after treatment by a method of the invention (see Example 1).

[0043] Figure 2 is a chromatogram displaying the S8content of a mineral insulating oil before and after treatment by a method of the present invention (see Example 1).

[0044] Figure 3 is a chromatogram displaying the S8content of a mineral insulating oil before and after treatment by a method not according to the invention (see Example 1A).

[0045] Figure 4A shows a silver plate that has been in contact with corrosive oil before treatment, containing a typical amount of S8, according to the DIN 51353 oil corrosion test.

[0046] Figure 4B shows a silver plate that has been in contact with non-corrosive oil after treatment, containing a non-detectable amount of S8, according to DIN 51353 oil corrosion test.

[0047] Figure 5 A shows a copper plate that has been in contact with non-corrosive oil after treatment, in accordance with the IEC 62535 oil corrosion test.

[0048] Figure 5B shows a silver plate that has been in contact with non-corrosive oil after treatment, in accordance with the DIN 51353 oil corrosion test.

[0049] Figure 6 is a drawing of a silver-plated coil for batch reactor

[0050] Figure 6A is a drawing of a silver-plated heating cylinder

[0051] Figure 6B is a drawing of a silver-plated coil wrapped around heating cylinder for application in a batch reactor

[0052] Figure 7 is a drawing of batch reactor with mounted silver-plated coils, front view Figure 7 A is a drawing of batch reactor with mounted silver-plated coils, side view Figure 7B is a drawing of batch reactor with mounted silver-plated coils wrapped around silver-plated heating cylinder, side view

[0053] Figure 7C is a drawing of batch reactor with mounted silver-plated coils wrapped around silver-plated heating cylinder, front view

[0054] Figure 8 is a drawing of silver-plated coil for tubular reactor,

[0055] Figure 8A is a drawing of tubular reactor

[0056] Figure 8B is a drawing of tubular reactor with silver plated coils

[0057] Figure 9 is a drawing of modular reactor system containing preheater, tubular reactor and batch reactor with mounted silver-plated coils with continuous flow between preheater and tubular reactor and continuous flow between tubular reactor and batch reactor

[0058] Figure 10 is a chromatogram displaying the S8content of a synthetic ester insulating liquid containing 7.6 ppm S8before treatment and chromatograms of S8, during and after treatment by a method of the invention (see Example 7).

[0059] Figure 11A is displaying the results of silver strip corrosion test ASTM D 1275-15 of synthetic ester oil before treatment (see Example 8).

[0060] Figure 1 IB is displaying the results of silver strip corrosion test ASTM D 1275-15 of synthetic ester oil after treatment (see Example 8).

[0061] Figure 12A is displaying the results of silver strip corrosion test ASTM D 1275-15 of synthetic ester oil before treatment - an additional experiment containing oxidized sulphur compound and low total sulphur content (see Example 8).

[0062] Figure 12B is displaying the results of silver strip corrosion test ASTM D 1275-15 of synthetic ester oil after treatment- an additional experiment containing oxidized sulphur compound and low total sulphur content (see Example 8).

[0063] Figure 13 is a chromatogram displaying the S8content of a synthetic ester insulating liquid containing 7.6 ppm S8and lower oil to silver surface ratio, before, during and after treatment by a method of the present invention (see Example 9).

[0064] Figure 14A is displaying the results of silver strip corrosion test ASTM D 1275-15 synthetic ester oil before treatment (see Example 9). Figure 14B is displaying the results of silver strip corrosion test ASTM D 1275-15 of synthetic ester oil after treatment (see Example 9).

[0065] Figure 15 is a chromatogram displaying S8content of a synthetic ester before (1.51 mg / kg S8), during and after treatment by a method of the present invention (see Example 10).

[0066] Figure 16 is a chromatogram displaying the S8content of synthetic ester oil before (0.9 mg / kg S8), during and after treatment by a method of the present invention (see Example 10).

[0067] Figure 17 is a chromatogram of mineral oil containing 7.6 ppm of S8before, during and after treatment (see Example 11).

[0068] Figure 18A is displaying the results of silver strip corrosion test DIN 51353 of mineral oil before treatment.

[0069] Figure 18B is displaying the results of silver strip corrosion test DIN 51353 mineral oil after treatment.

[0070] Figure 19 is a drawing of a stainless-steel column with ratio of length (L) to diameter (D) from 2.3 to 3 containing the particulate adsorbent support (having the reductant on its surface) for use in the third embodiment of the invention.

[0071] DETAILED DESCRIPTION OF FIRST EMBODIMENT

[0072] The present invention provides the use of a solid agent comprising a reductant for reducing the amount of active sulphur in an oil and / or reducing the dielectric dissipation factor of an oil and oil acidity, while in the same time increasing interfacial tension, wherein the oil is contacted with (a) the solid agent comprising the reductant, and (b) a liquid agent suitable for dispersing said solid agent. Preferred aspects of this use correspond to those outlined below in connection with the method of the first embodiment.

[0073] The present invention provides (in a first embodiment) a method for reducing the amount of S8in an oil which contains S8, the method comprising contacting the oil with (a) a solid agent comprising a reductant, and (b) a liquid agent suitable for dispersing said solid agent.

[0074] The reductant preferably comprises one or more metals. Preferably said one or metals are selected from copper, iron, zinc, aluminium, nickel, and tin. More preferably the reductant comprises copper. Most preferably the reductant is copper. The solid agent preferably comprises (and most preferably consists of) solid carrier particles having the reductant on the surface thereof. This does not require all of the reductant to be found exclusively on the surface of the solid carrier particles, however it does require at least some of the reductant to be on the surface thereof. Preferably the majority (over 50 %) of the reductant is on the surface of the solid carrier particles, and more preferably substantially all of the reductant is present on the surface of the solid carrier particles.

[0075] Preferably the solid carrier particles are in the form of granules or a powder. The size of the solid carrier particles is not particularly limited and can be varied as appropriate in order to tailor the rate of reaction, with smaller particle sizes (e.g. using powder rather than granules) enabling increased rates of reaction (this applies to both the desulphurisation reaction and also - if / when appropriate - the reduction of copper salt to copper during preparation of the dispersion of the invention as described herein). The solid carrier particles preferably include at least some particles having a particle size of less than or equal to 4.00 mm, more preferably less than or equal to 2.00 mm. Preferably the majority (>50 wt%) of the solid carrier particles, more preferably >80 wt%, and more preferably still >90 wt% of the solid carrier particles have a particle size of less than or equal to 4.00 mm, more preferably less than or equal to 2.00 mm. Typically, all of the solid carrier particles have a particle size of less than or equal to 4.00 mm, more preferably less than or equal to 2.00 mm. Particles meeting these size requirements can be obtained using standard designation sieves and are also readily available commercially. There is no particular lower limit as regards the size of the solid carrier particles besides the natural physical limits - the presence of relatively small particles is not problematic. Typically, though, the proportion of the solid carrier particles having a particle size of ≤88 μm is <50 wt%, preferably <20 wt%, and more preferably <10 wt%. Again, standard designation sieves can be used to identify such particles.

[0076] The solid carrier particles preferably comprise iron and / or zinc. In this embodiment the particles may also further comprise one or more salts of iron and / or zinc, preferably one or more salts selected from sulphates, nitrates, chlorides, and bromides. Typically, the solid carrier particles are iron and / or zinc particles, which may optionally feature a proportion of oxidised iron and / or zinc (such as sulphates, nitrates, chlorides, and bromides). (The oxidised iron and / or zinc is generally present as a consequence of how the solid agent is preferably made - as described below, it may for instance be made by adding iron and / or zinc particles to a copper salt solution, such that the iron and / or zinc reduce the copper, leading to the formation of elemental copper on the surface of the particles, along with a proportion of correspondingly oxidised iron and / or zinc.)

[0077] The liquid agent is preferably a polymer, more preferably a polyether, and more preferably still a polyethylene glycol. The number average molecular weight (Mn) of the polymer is not particularly limited though in practice will be limited to some extent by the requirement for the agent to be a liquid under the conditions in which it is used. Suitable polymers are readily available commercially. The polymer preferably has a number average molecular weight (Mn) of at least 300 g / mol, more preferably at least 350 g / mol. The polymer preferably has an Mn of no greater than 800 g / mol, more preferably no greater than 600 g / mol, more preferably still no greater than 550 g / mol, more preferably still no greater than 500 g / mol. A typical range is 300 to 600 g / mol, with 350 to 550 g / mol being more preferred, and 350 to 500 g / mol more preferred still. Mn may be measured by gel permeation chromatography however generally it is unnecessary to measure Mn because suitable agents are readily available commercially. For instance, a particularly suitable agent is the commercially available polymer PEG-400 (polyethylene glycol having an Mn of 400).

[0078] The nature of the solid agent and the reductant comprised within it, and the amounts of these components, can be varied in order to control the rate at which desulphurisation proceeds.

[0079] Preferably the amount of the solid agent relative to the amount of the oil is from 0.05 to 10 wt.%.

[0080] Preferably the amount of reductant relative to the amount of the oil is from 0.01 to 2 wt %, more preferably 0.02 to 1.5 wt %.

[0081] Preferably the reductant accounts for 0.5 to 25 wt% of the solid agent.

[0082] Preferably the amount of liquid agent relative to the amount of the oil is from 1.5 to 40 wt%.

[0083] In one embodiment, the method comprises a step of contacting the oil with a dispersion wherein the solid agent is dispersed within the liquid agent. Preferably said dispersion further comprises water. However, it is not essential to form a dispersion prior to contacting the oil with components (a) and (b). Thus, in an alternative embodiment components (a) and (b) may be contacted with the oil separately. For instance, component (b) may be added to the oil first, followed separately by component (a). The amount of dispersion relative to the total amount of oil plus dispersion is preferably from 1.5 to 40 wt.%.

[0084] The dispersion is preferably a dispersion of the invention as described below.

[0085] In the method of the present invention, the agents (a) and (b) as described above are the only essential components. The inclusion of further components is not excluded, provided that such further components do not interfere with the desulphurisation reaction. Also, in some instances the inclusion of further components can be advantageous as it may help facilitate the desulphurisation reaction - e.g. including polar liquids may help. Thus, in a preferred embodiment the method of the invention comprises contacting the oil with (a) a solid agent comprising a reductant, (b) a liquid agent suitable for dispersing said solid agent, and (c) one or more polar liquids, such as one or more polar liquids selected from water, alcohols and polyols. Most preferably the method of the invention further comprises contacting the oil with water, optionally in combination with one or more alcohols and / or polyols. Preferred alcohols are C1-3alcohols such as methanol, ethanol and propanol, with methanol being most preferred. Preferred polyols are C1-3diols and C1-3triols, with glycerol being most preferred. Generally, it is preferable not to include the optional alcohol and / or polyol components (i.e. preferably the method of the invention comprises contacting the oil with (a) a solid agent comprising a reductant, (b) a liquid agent suitable for dispersing said solid agent, and (c) water). However, including an alcohol and / or a polyol may be advantageous if heavier liquid agents are used, for example if the liquid agent is a polymer (such as a polyethylene glycol) with a relatively high Mn - e.g. an Mn of >400, such as ≥500, or ≥550.

[0086] In the method of the invention the oil, solid agent and liquid agent are preferably subjected to stirring and / or ultrasound treatment.

[0087] In the method of the invention the oil, solid agent and liquid agent are preferably heated to a temperature of 50 to 120 °C, more preferably 60 to 105 °C, more preferably still 70 to 99 °C. In this regard, the reaction mixture is preferably heated to such temperatures for a period of at least 20 minutes, more preferably at least 30 minutes. The reaction mixture is preferably heated to such temperatures for a period of up to 4 hours, more preferably up to 3 hours, and more preferably still up to 2 hours. Typically, the reaction mixture is heated to such temperatures (e.g. 50 to 120 °C) for 20 minutes to 3 hours, and more typically for 30 minutes to 2 hours. The oil which contains S8may be a mineral oil or an ester oil. Esters may be synthetic or natural.

[0088] The oil which contains S8may be a mineral oil or a synthetic oil (e.g. an ester oil). Preferably it is a mineral oil.

[0089] The oil which contains S8is preferably an insulating oil for use in a transformer, and more preferably a mineral insulating oil for use in a transformer. Typically, it is a used insulating transformer oil, and more typically it is a used mineral insulating transformer oil.

[0090] The oil which contains S8preferably contains S8in an amount of at least 2.0 mg / kg oil based on the total amount of oil, more preferably at least 5.0 mg / kg, and yet more preferably at least 10.0 mg / kg.

[0091] In the method of the invention, the content of S8is preferably reduced to a level of less than 5.0 mg / kg oil based on the total amount of oil, preferably less than 2.0 mg / kg, more preferably less than 1.0 mg / kg, yet more preferably less than 0.2 mg / kg.

[0092] The content of S8in the oil is preferably measured by IEC TR 62697-3 / 2018.

[0093] Once the method of the invention has been carried out, the oil (which consequently has a reduced level of S8in it) can be separated from the other components, e.g. by sedimentation. Said other components should be present in the form of a dispersion. The dispersion may just be composed of the components (a) and (b) (plus some copper sulphide) though may also contain further components if other agents were additionally used (e.g. water, optionally combined with alcohol and / or polyol).

[0094] Thus, the present invention also provides a process comprising:

[0095] (a) reducing the amount of S8in an oil which contains S8by a method of the invention as defined above; and

[0096] (b) subsequently separating the oil from the other components, wherein said other components are present in the form of a dispersion comprising the solid agent and the liquid agent.

[0097] The separated oil with a reduced content of S8may be used in applications wherein it will come into contact with components that may be susceptible to attack by S8- e.g. it may be used as an insulating oil in a transformer featuring such components. Meanwhile, the separated dispersion can be re-used to desulphurise other oils. Thus, the present invention also provides a process comprising: (a) reducing the amount of S8in an oil which contains S8by a method of the invention as defined above;

[0098] (b) subsequently separating the oil from the other components, wherein said other components are present in the form of a dispersion comprising the solid agent and the liquid agent; and

[0099] (c) using the thus obtained dispersion to reduce the amount of S8in an oil which contains S8by a method which is independently as defined above.

[0100] Certain dispersions (containing the solid agent and the liquid agent as defined above) which are suitable for use in accordance with the method of the present invention are believed to be novel. Thus, the present invention also provides a dispersion comprising water and elemental copper, wherein said water and copper are dispersed within a liquid polymer. The preferred aspects of the liquid agent described above (component (b) in the method of the present invention) apply correspondingly, but independently, to said liquid polymer.

[0101] The dispersion of the invention preferably comprises solid carrier particles having the copper on the surface thereof. Thus, the present invention provides a dispersion comprising water and solid carrier particles having the copper on the surface thereof, wherein said water and particles are dispersed within a liquid polymer. The solid carrier particles are preferably defined in the same way as described above in connection with the method of the present invention.

[0102] The amount of water in the dispersion is preferably 1 to 50 wt%, more preferably 2 to 40 wt%.

[0103] The amount of liquid polymer in the dispersion is preferably 40 to 98 wt%, more preferably 45 to 97 wt%.

[0104] The amount of copper in the dispersion is preferably 0.02 to 4.0 wt%, more preferably 0.05 to 3.0 wt%.

[0105] When the dispersion of the invention comprises solid carrier particles having the copper on the surface thereof, the amount of the solid carrier particles in the dispersion (not including the copper on the surface thereof) is preferably 0.1 to 12.0 wt%, more preferably 0.2 to 10.0 wt%. For the avoidance of doubt, these preferred aspects of the dispersion of the invention apply also to the dispersion for use in the method of the present invention as defined above.

[0106] The present invention also provides a process of preparing a dispersion of the invention as defined above, the process comprising the following steps:

[0107] (i) preparing an aqueous solution of a copper-containing agent or a suspension of a copper-containing agent, and

[0108] (ii) dispersing said solution or suspension in the liquid polymer; wherein if the copper-containing agent in step (i) features copper in oxidised form (i.e. if the copper is present in the form of copper cations - e.g. if it is obtained by adding a copper salt), then said process further includes a step of reducing the copper in the copper- containing agent in between steps (i) and (ii). In this regard, the copper in the copper- containing agent is reduced so as to form elemental copper.

[0109] Preferably step (i) is preparing an aqueous solution of a copper salt, such as copper sulphate. In this regard, the aqueous solution of the copper salt is preferably prepared using:

[0110] (a) H2O;

[0111] (b) a H2O / alcohol mixture, wherein preferably the alcohol is a C1-3alcohol such as methanol, ethanol or propanol, and more preferably methanol; or

[0112] (c) a H2O / polyol mixture, wherein preferably the polyol is a C1-3diol or C1-3triol, and more preferably glycerol.

[0113] In a preferred aspect of the above process, the copper-containing agent in step (i) features copper in oxidised form (such that the process further includes a step of reducing the copper in the copper-containing agent in between steps (i) and (ii)), and the reduction reaction involves introducing to the aqueous solution or suspension one or more reducing agents selected from transition metals and post-transition metals, and more preferably from elemental iron, elemental zinc, elemental aluminium, elemental nickel, and elemental tin. In a further preferred aspect, the reducing agent(s) is preferably in the form of granules or a powder. Also, the preferred aspects of the solid carrier particles which may be used in accordance with the method of the invention as defined above apply correspondingly to the reducing agent(s) in this context. Typically, particles of a metallic reducing agent are used to reduce the copper in the copper-containing agent (e.g. copper sulphate), such that elemental copper forms on the surface of said particles. The method of the present invention produces oils with beneficial properties which make them particularly suited to use as insulating oils in transformers. It is also believed to impart to the oil a ‘fingerprint’ which renders it different from previous known oils (even ones with low S8contents). One aspect of this is that the method is believed to selectively desulphurize oils while causing little or no change to other components such as the aromatic, paraffin, and naphthenic components. Thus, the present invention also provides an oil obtained or obtainable by a method as defined above.

[0114] Such oils preferably have a total S8content of ≤0.5 mg / kg (more preferably ≤0.2 mg / kg),

[0115] Such oils preferably have a content of mercaptans and disulphides of ≤2 mg / kg.

[0116] Such oils preferably have a dielectric dissipation factor at 90 °C of <0.005.

[0117] Such oils preferably have an acid number of <0.02 mgKOH / g.

[0118] Such oils preferably have an oil-water interfacial tension of ≥35 mN / m.

[0119] Such oils preferably have an FT-IR spectrum within a wavelength range of 700 to

[0120] 1300 cm-1which is substantially the same as that of the oil prior to the desulphurisation method.

[0121] Such oils preferably have particles of size ≥4 μm in an amount of ≤320 particles / ml (typically the number of such particles is in the range of >160 to ≤320 particles / ml).

[0122] Such oils preferably have particles of size ≥6μm in an amount of ≤320 particles / ml (typically the number of such particles is in the range of >20 to ≤40 particles / ml).

[0123] Such oils preferably have particles of size ≥4μm in an amount of ≤320 particles / ml, and particles of size ≥6μm in an amount of ≤40 particles / ml.

[0124] Such oils preferably have: a sum of mercatpans, sulphides and disulphides of ≤2 mg / kg an S8content of ≤ 0.5 mg / kg a dielectric dissipation factor at 90 °C of < 0.005, an acid number of <0.02 mgKOH / g, an oil-water interfacial tension of ≥35 mN / m, an FT-IR spectrum within a wavelength range of 700 to 1300 cm-1which is substantially the same as that of the oil prior to the desulphurisation method, and / or particles of size ≥4μm in an amount of ≤320 particles / ml, and particles of size ≥6μm in an amount of ≤40 particles / ml.

[0125] In line with the above preferred aspects of the invention, such oils preferably have ≤15 / 12 ISO code (ref: ISO 4406, standard for coding particle number and size). The first number, 15, is the code for a particle size of 4 μm to be >160 particles / ml and up to and including 320 particles / ml. The second number, 12, is the code for a particle size of 6 μm to be >20 particles / ml and up to and including 40 particles / ml.

[0126] Particle size and count may be measured in accordance with IEC 60970:2007.

[0127] The above preferred features of the oil obtained or obtainable by the method are particularly preferred in cases where the oil is a mineral oil. In cases where the oil obtained or obtainable by the method is an ester oil, the preferred properties may sometimes differ.

[0128] Such ester oils preferably have a total sulphur content of ≤0.5 mg / kg (more preferably ≤0.2 mg / kg),

[0129] Such ester oils preferably have a total S8content of ≤0.5 mg / kg (more preferably ≤0.2 mg / kg),

[0130] Such ester oils preferably have a content of mercaptans and disulphides of ≤0.2 mg / kg.

[0131] Such ester oils preferably have a dielectric dissipation factor at 90 °C of < 0.020.

[0132] Such ester oils preferably have an acid number of <0.03 mg KOH / g.

[0133] Such ester oils preferably have an oil-water interfacial tension of ≥35 mN / m.

[0134] Such ester oils preferably have particles of size ≥4μm in an amount of ≤320 particles / ml (typically the number of such particles is in the range of >160 to ≤320 particles / ml).

[0135] Such ester oils preferably have particles of size ≥6μm in an amount of ≤320 particles / ml (typically the number of such particles is in the range of >20 to ≤40 particles / ml).

[0136] In line with the above preferred aspects of the invention, such ester oils preferably have ≤15 / 12 ISO code (ref: ISO 4406, standard for coding particle number and size). The first number, 15, is the code for a particle size of 4 μm to be > 160 particles / ml and up to and including 320 particles / ml. The second number, 12, is the code for a particle size of 6 μm to be >20 particles / ml and up to and including 40 particles / ml.

[0137] Particle size and count may be measured in accordance with IEC 60970:2007.

[0138] DETAILED DESCRIPTION OF SECOND EMBODIMENT

[0139] The present invention provides the use of a solid agent comprising a reductant for reducing the amount of active sulphur in oil, wherein the oil is contacted with a solid substrate having the reductant on its surface. Preferred aspects of this use correspond to those outlined below in connection with the method of the second embodiment.

[0140] The present invention provides (in a second embodiment) a method for reducing the amount of S8, S7, S6, S5, S4, disulphides and / or sulphoxides, sulphones and sulphur oxy acids in an oil which contains S8, S7, S6, S5, S4, disulphides and / or sulphoxides, sulphones and sulphur oxy acids the method comprising contacting the oil with a solid substrate having a reductant on its surface.

[0141] The substrate is preferably made of metal. Preferably the substrate comprises aluminium, copper, zinc, brass and iron steel. More preferably the substrate is aluminium, copper, zinc, brass and iron steel. The most preferred materials to use are aluminium and copper. Thus, preferably the substrate is an aluminium or copper substrate.

[0142] The reductant preferably comprises one or more metals. Preferably said one or metals are selected from silver, zinc, aluminium, nickel and tin. More preferably the reductant comprises silver. Most preferably the reductant is silver.

[0143] The reductant is preferably a different material to the substrate. So if (e.g.) the substrate is aluminium then the reductant is preferably not aluminium.

[0144] The substrate should have structural integrity such that it may be inserted / immersed into the oil in order to provide the desired treatment. The shape of the substrate is not particularly limited. A coil shape may conveniently be used but in principle any shape offering a similar or higher ratio of surface area to volume may suitably be employed.

[0145] Preferably the solid substrate having reductant on its surface is a reductant-plated substrate, more preferably a reductant-plated metal substrate (e.g. a reductant-plated metal coil), and wherein typically:

[0146] - the reductant is one or more selected from silver, zinc, aluminium, nickel and tin, and preferably is silver; and - the metal coil comprises, or is, one or more selected from aluminium, copper, zinc, brass, iron and steel, and preferably selected from aluminium and copper.

[0147] The size and surface area of the substrate (e.g. a metal coil), and in particular the surface area of reductant thereon (which is to come into contact with the oil) may be varied by a skilled practitioner depending factors such as on the desired level of performance and the cost of preparing the substrate having reductant on its surface. The ratio of the surface area of reductant to mass of oil is preferably from 0.00001 to 0.01 m2 / kg, more preferably 0.00005 to 0.005 m2 / kg, more preferably still 0.0001 to 0.001 m2 / kg, and yet more preferably 0.00015 to 0.00080 m2 / kg.

[0148] When the substrate is a coil, said coil preferably has a diameter of 2-20 mm, more preferably 4-15 mm, more preferably still 6-12 mm, most preferably 8-10 mm.

[0149] When the substrate is a coil, said coil preferably has a length of 2-30 m, more preferably 3-25 mm, more preferably still 4-20 mm, most preferably 5-15 m.

[0150] When the substrate having reductant on its surface is a reductant-plated substrate (more preferably a reductant-plated metal substrate, e.g. a reductant-plated metal coil such as a silver-plated metal coil), the plating layer preferably has a thickness of 0.0005 to 0.2 mm, more preferably 0.001 to 0.1 mm, yet more preferably 0.003 to 0.08 mm, and most preferably 0.005 to 0.05 mm.

[0151] In one preferred aspect, the solid substrate comprises (and more preferably is) a reductant-plated metal coil wrapped around a heater.

[0152] In a preferred aspect of this second embodiment of the invention, the oil is heated to a temperature of 100°C to 220 °C.

[0153] If the oil is a mineral oil, the oil is preferably heated to a temperature from 100°C to 160 °C, more preferably 100°C to 140 °C.

[0154] If the oil is an ester, the oil is preferably heated to a temperature from 160°C to 180 more preferably 180°C to 200 °C.

[0155] Preferably, the oil is heated for a period of at least 30 minutes, more preferably 60 minutes and most preferably 120 minutes. Long heating periods can be used if desired for ≤72 hrs, more typically ≤48 hrs, yet more typically ≤20 hrs, and most typically not longer than 10 hrs. The second embodiment of the present invention provides a method for reducing the amount of total sulphur content and removal of elemental sulphur (in different forms, such as: S8, S7, S6, S4), disulphides, sulphoxides, sulphones and sulphur oxy acids in an oil which contains different forms of elemental sulphur, disulphides, sulphoxides, sulphones and sulphur oxy acids. In insulating oils which are used in power transformers, operating conditions, i.e. related temperature and pressure may lead to the predominant presence of such species, and S8in particular. In a preferred aspect the method comprises contacting the oil with heated silver-plated coils. More preferably the heated silver-plated coils attract reactive sulphur species, namely elemental sulphur (S8, S7, S6, S4but predominantly S8), disulphides and / or sulphoxides, sulphones and sulphur oxy acids to react and deposit silver sulphides on its surface.

[0156] In another preferred aspect the reductant comprises one or more metals, preferably said one or metals are selected from zinc, aluminium, copper, silver. More preferably the reductant comprises silver. Most preferably the reductant is silver.

[0157] In another preferred aspect the solid substrate comprises (and most preferably consists of) a solid substrate in the shape of a coil made from aluminium, copper, zinc, brass, iron steel, preferably aluminium, or copper.

[0158] In another preferred aspect the substrate is in the form of coil (Fig.6). The coil preferably includes wire with diameter of 0.8 -2 cm, length of 10 m size, or less than or equal to 7 m, more preferably less than or equal to 3 m.

[0159] In another preferred aspect the substrate having reductant on its surface is a silver- plated coil having diameter between 100 and 150 mm, preferably 130 mm and length between 300 and 1000 mm, preferably 450 mm and more preferably 700 mm each, with surface area ratio to the oil mass in the range from 0.00015 to 0.0008 m2 / kg oil (Fig. 6).

[0160] The silver-plated coils may be wrapped around heating cylinders made from aluminium or copper, or iron steel (Fig. 6B) which may have silver plating on its surface.

[0161] In aspects of this second embodiment of the invention which feature a heater, the heater may be a copper and / or silver-plated heating cylinder having surface area from 0.001 to 0.010 m2 / kg oil, preferably 0.007 m2 / kg oil (Fig. 6A).

[0162] The present invention provides a batch reactor, a tubular reactor, or a reactor system comprising a batch reactor and a tubular reactor, wherein said batch reactor, said tubular reactor and at least one of the reactors in said reactor system comprises at least one solid substrate having reductant on its surface as defined above, wherein the solid substrate having reductant on its surface is either located within that reactor or is moveable so as to be mounted and de-mounted therein. This aspect of invention is important for the efficient cleaning of the silver-plated coil.

[0163] In a preferred aspect the coils are positioned in batch reactor, preferably wrapped around cylinder heating element mounted, e.g. in the upper zone of the batch reactor (Fig. 8).

[0164] In another preferred aspect silver-plated coils with and without silver-plated heating cylinders are placed in batch reactor (Figures 8, 8A and 8B, 8C).

[0165] In another preferred aspect a batch reactor (height 170 cm, diameter 90 cm), is made of stainless steel 316 (Fig. 7) is equipped with stirrer and has capacity to be filled with 600- 900 cm3of insulating liquid (Figure 8C).

[0166] In another preferred aspect the number of coils with heating copper or silver-plated cylinders immersed in the batch reactor is 2 to 4, more preferably 4, while in a preferred aspect of the tubular reactor the number of silver-plated coils is one to three, more preferably 2.

[0167] In another preferred aspect one to three silver-plated coils are used in tubular reactor. In this regard the silver-plated coils may have diameter between 100 and 150 mm, preferably 130 mm and length between 300 and 1000 mm, preferably 450 mm and more preferably 700 mm each, with surface area ratio to the oil mass in the range from 0.00015 to 0.0008 m2 / kg oil (Figure 8).

[0168] In another preferred aspect a tubular reactor is made of stainless steel 316 (length 1500 mm and diameter 280 mm) (Figure 8A).

[0169] In another preferred aspect silver-plated soils are placed in the middle of tubular reactor cross section (Figure 8B).

[0170] In another preferred aspect tubular reactor and batch reactor with silver-plated coils composes modular reactor system (Figure 9). Suitable tubular reactor and preheater are connected in the loop for circular flow and if necessary tubular reactor is connected to the batch reactor in the loop if the required reaction time is higher than 120 minutes for additional treatment. Otherwise, tubular reactor is disconnected and / or connected to a preheater chamber in the loop for certain number of turns and time and then again connected to the batch reactor.

[0171] In another preferred aspect the number of silver-plated coils, total available silver- plated surface and the number of oil circulations in the loop and back treatments can be varied in order to control the rate at which desulphurisation proceeds. Preferably the total number of silver-plated coils in both batch and tubular reactor is 6.

[0172] Preferably the total silver-plated surface is 0.005 to 0.0110 m2.

[0173] Surface of coils and heating cylinders is determined using equations for the roller surface, length and diameter measurement using meter and vernier.

[0174] In another preferred aspect of the method of this second embodiment of the invention the oil is circulated in the loop between preheater chamber and tubular reactor with embedded silver-plated coils and heated from ambient temperature to the temperatures in the following range: 100°C to 220 °C, more preferably from 100°C to 140 °C in the case of mineral oil, while more preferably from 160°C to 180 °C, more preferably still 180°C to 200 °C in the case of ester oil. In this regard, the oil is heated for a period of at least 30 minutes, more preferably 60 minutes and most preferably 120 minutes. Following the step above, the oil is preferably subjected to stirring in batch reactor treatment which contains four silver- plated coils and / or silver-plated coils wrapped around silver-plated heating cylinder, using the same temperatures as stated above. As additional option oil is circulated between tubular and batch reactor for a period of time.

[0175] If the concentration of S8, disulphides and sulphoxides, sulphones and sulphur oxy acids is up to 5 mg / kg oil the reaction time is preferably in the range from 30 minutes to 360 minutes, at least 30 minutes, more preferably 60 minutes and most preferably 120 minutes.

[0176] If the concentration if S8, disulphides and sulphoxides, sulphones and sulphur oxy acids is above 5 mgS / kg oil, the reaction time is preferably above 360 minutes, up to 2000 minutes.

[0177] The oil which contains sulphur compounds, S8S7, S6, S4disulphides and sulphoxides, sulphones and sulphur oxy acids may be a mineral oil or an ester oil (e.g. a synthetic ester oil).

[0178] The oil which contains S8S7, S6. S4disulphides and sulphoxides, sulphones and sulphur oxy acids is preferably insulating oil for use in a transformer. Typically, it is a used insulating transformer oil, and more typically it is a used mineral insulating transformer oil and most preferably it is a used synthetics ester insulating oil.

[0179] The synthetic ester insulating oil which contains sulphur preferably contains total sulphur in an amount of at least 1 mg / kg oil based on the total amount of oil, more preferably at least 5.0 mg / kg oil, and yet more preferably at least 10.0 mg / kg oil.

[0180] The mineral insulating oil which contains sulphur preferably contains total sulphur in an amount of at least 10 mg / kg, more preferably at least 100 mg / kg, and yet more preferably at least 800 mg / kg.

[0181] In the method of the invention, the content of total sulphur in synthetic ester oil is preferably reduced to a level of less than 5.0 mg / kg based on the total amount of oil, preferably less than 2.0 mgS / kg, more preferably ≤ 1.0 mg / kg.

[0182] The content of total sulphur in the oil is preferably measured by ASTM D 5453.

[0183] The content of sulphoxides and sulphones is measured using HPLC, LC MS / MS, or GC / MS.

[0184] Sum of mercaptans, disulphides and elemental sulphur is measured using automatic potentiometric titration using zinc granules (Zn), according to CIGRE A2.32 TF 3 method.

[0185] The oil which contains sulphur preferably contains S8in an amount of at least 0.2 mg / kg oil based on the total amount of oil, more preferably at least 5.0 mg / kg, and yet more preferably at least 10.0 mg / kg.

[0186] In the method of the invention, the content of S8is preferably reduced to a level of less than 5.0 mg / kg oil, preferably less than 1.0 mg / kg, yet more preferably less than 0.2 mg / kg.

[0187] The content of S8in the oil is preferably measured by IEC TR 62697-3 / 2018.

[0188] Once the method of the invention has been carried out, the oil (which consequently has a reduced level of S8, S7, S6, S5, S4, disulphides and / or sulphoxides, sulphones and sulphur oxy acids) can be separated from the substrate having reductant on its surface. And the substrate having reductant on its surface may be re-used (optionally after having been treated to remove sulphide deposits).

[0189] Thus, the present invention also provides a process comprising: (a) reducing the amount of S8, S7, S6, S5, S4, disulphides and / or sulphoxides, sulphones and sulphur oxy acids in an oil by a method as defined above; and

[0190] (b) subsequently separating the oil from the solid substrate having reductant on its surface.

[0191] The present invention also provides a process comprising:

[0192] (a) reducing the amount of S8, S7, S6, S5, S4, disulphides and / or sulphoxides sulphones and sulphur oxy acids in an oil by a method as defined above;

[0193] (b) subsequently separating the oil from the solid substrate having a reductant on its surface; and

[0194] (c) using the thus obtained solid substrate having a reductant on its surface to reduce the amount of S8, S7, S6, S5, S4, disulphides and / or sulphoxides, sulphones and sulphur oxy acids in an oil which contains S8, S7, S6, S5, S4, disulphides and / or sulphoxides, sulphones and sulphur oxy acids by a method which is independently as defined above.

[0195] The method of the present invention produces oils with beneficial properties which make them particularly suited to use as insulating oils in transformers. It is also believed to impart to the oil a ‘fingerprint’ which renders it different from previous known oils (even ones with low S8contents). One aspect of this is that the method is believed to selectively desulphurize oils while causing little or no change to other components such as the ester, aromatic, paraffin, and naphthenic components. Thus, the present invention also provides an oil obtained or obtainable by a method as defined above.

[0196] When the oil is a synthetic ester oil, it preferably has a total sulphur content ≤ 0.5 mg / kg (more preferably ≤0.2 mg / kg)

[0197] Such oils preferably have S8content of ≤ 0.5 mg / kg (more preferably ≤0.2 mg / kg).

[0198] Such oils preferably have disulphide content ≤ 10 mg / kg, more preferably ≤ 5 mg / kg.

[0199] Such oils preferably have a dielectric dissipation factor at 90 °C of <0.020, more preferably ≤ 0.010.

[0200] Such oils preferably have an acid number of ≤0.03 mg KOH / g.

[0201] Such oils preferably have an oil-water interfacial tension of ≥30 mN / m.

[0202] Such oils preferably have particles of size ≥4μm in an amount of ≤320 particles / ml (typically the number of such particles is in the range of >160 to ≤320 particles / ml). Such oils preferably have particles of size ≥6μm in an amount of ≤320 particles / ml (typically the number of such particles is in the range of >20 to ≤40 particles / ml).

[0203] Such oils preferably have particles of size ≥4μm in an amount of ≤320 particles / ml, and particles of size ≥6μm in an amount of ≤40 particles / ml.

[0204] In line with the above preferred aspects of the invention, such oils preferably have ≤15 / 12 ISO code (ref: ISO 4406, standard for coding particle number and size). The first number, 15, is the code for a particle size of 4 μm to be >160 particles / ml and up to and including 320 particles / ml. The second number, 12, is the code for a particle size of 6 μm to be >20 particles / ml and up to and including 40 particles / ml.

[0205] Particle size and count may be measured in accordance with IEC 60970:2007.

[0206] The above preferred features of the oil obtained or obtainable by the method are particularly preferred in cases where the oil is a mineral oil. In cases where the oil obtained or obtainable by the method is an ester oil, the preferred properties may sometimes differ.

[0207] Such ester oils preferably have a total sulphur content of ≤0.5 mg / kg (more preferably ≤0.2 mg / kg),

[0208] Such ester oils preferably have a total S8content of ≤0.5 mg / kg (more preferably ≤0.2 mg / kg),

[0209] Such ester oils preferably have a content of mercaptans and disulphides of ≤0.2 mg / kg.

[0210] Such ester oils preferably have a dielectric dissipation factor at 90 °C of ≤0.020.

[0211] Such ester oils preferably have an acid number of ≤0.03 mgKOH / g.

[0212] Such ester oils preferably have an oil-water interfacial tension of ≥35 mN / m.

[0213] Such ester oils preferably have particles of size ≥4 μm in an amount of ≤320 particles / ml (typically the number of such particles is in the range of >160 to ≤320 particles / ml).

[0214] Such ester oils preferably have particles of size ≥6μm in an amount of <320 particles / ml (typically the number of such particles is in the range of >20 to ≤40 particles / ml).

[0215] In line with the above preferred aspects of the invention, such ester oils preferably have ≤15 / 12 ISO code (ref: ISO 4406, standard for coding particle number and size). The first number, 15, is the code for a particle size of 4 μm to be >160 particles / ml and up to and including 320 particles / ml. The second number, 12, is the code for a particle size of 6 μm to be >20 particles / ml and up to and including 40 particles / ml.

[0216] Particle size and count may be measured in accordance with IEC 60970:2007.

[0217] DETAILED DESCRIPTION OF THIRD EMBODIMENT

[0218] The present invention provides the use of a solid agent comprising a reductant for reducing the amount of reactive sulphur in the form of mercaptans, disulphides, oxygenated sulphur compounds, such as sulphoxides, sulphones and sulphur oxy acids in oil and / or reducing the dielectric dissipation factor and oil acidity, wherein the oil is an ester and is contacted with a particulate adsorbent support having the reductant on its surface. Preferred aspects of this use correspond to those outlined below in connection with the method of the third embodiment.

[0219] The present invention provides (in a third embodiment) a method for reducing the amount of reactive sulphur in an ester oil and / or reducing the dielectric dissipation factor of an ester oil, the method comprising contacting the oil with a particulate adsorbent support having the reductant on its surface.

[0220] Preferably the particulate adsorbent support comprises silicon dioxide and / or calcium oxide. Typically, the particulate adsorbent support is based on silicon dioxide. Preferably the particulate adsorbent support has a silicon dioxide content of ≥60 wt%, ≥80 wt%, ≥90 wt%, or ≥94 wt%. In each case the upper limit for the silicon dioxide content is 100 wt%, and preferably 99 wt%. More preferably the particulate adsorbent support has a silicon dioxide content of 90 to 100 wt%, typically 94 to 99 wt%, and a calcium oxide content of 0 to 10 wt%, typically 1 to 6 wt%.

[0221] The reductant typically comprises one or more transition metal ions, and preferably comprises silver ions. Most preferably the reductant is silver ions. Typically, the silver ions are deposited on the particulate adsorbent support by contacting said support with aqueous silver nitrate solution. The reductant (typically silver ions) is preferably present in an amount of ≥1 wt%, ≥2 wt%, or ≥3 wt%, based on the weight of the particulate adsorbent support. The reductant (typically silver ions) is preferably present in an amount of ≤20 wt%, ≤10 wt%, or ≤6 wt%, based on the weight of the particulate adsorbent support. The reductant (typically silver ions) is preferably present in an amount of 1 to 20 wt%, 2 to 10 wt%, or 3 to 6 wt%, based on the weight of the particulate adsorbent support. The particulate adsorbent support preferably also has an alkaline agent on its surface. This can help neutralize acidic by-products. A suitable agent is one containing ammonium ions, such as ammonium hydroxide. Thus, the particulate adsorbent support preferably (also) has ammonium ions on its surface, and preferably has ammonium hydroxide on its surface. Typically, the alkaline agent is deposited on the particulate adsorbent support by contacting said support with aqueous ammonium hydroxide. The alkaline agent (typically ammonium hydroxide) is preferably present in an amount of ≥1 wt%, ≥2 wt%, or ≥3 wt%, based on the weight of the particulate adsorbent support. The alkaline agent (typically ammonium hydroxide) is preferably present in an amount of ≤30 wt%, ≤20 wt%, or ≤10 wt%, based on the weight of the particulate adsorbent support. The alkaline agent (typically ammonium hydroxide) is preferably present in an amount of 1 to 30 wt%, 2 to 20 wt%, or 3 to 10 wt%, based on the weight of the particulate adsorbent support.

[0222] In a preferred embodiment the particulate adsorbent support comprises silicon dioxide and / or calcium oxide and the reductant comprise of silver ions, and even more preferably the particulate adsorbent support has ammonium ions on its surface.

[0223] In a further preferred embodiment the particulate adsorbent support has a silicon dioxide content of 90 to 100 wt. % (typically 94 to 99 wt. %) and a calcium oxide content of 0 to 10 wt. % (typically 1 to 6 wt. %); the reductant comprises silver ions which are present in an amount of 1 to 20 wt. % (typically 3 to 6 wt. %), based on the weight of the particulate adsorbent support; and the particulate adsorbent support has ammonium hydroxide on its surface, which his present in an amount of 1 to 30 wt. % (typically 3 to 10 wt%), based on the weight of the particulate adsorbent support.

[0224] Preferably the particulate adsorbent support has a particle size of ≥50 μm, ≥100 μm, or ≥200 μm. The particle size is preferably ≤5000 μm, ≤2000 μm, or ≤1200 μm. The particle size is typically 50 to 5000 μm, 100 to 2000 μm, or 200 to 1200 μm.

[0225] Preferably the particulate adsorbent support having the reductant on its surface is located within a container, typically a column-shaped container / cartridge (preferably a stainless-steel column) and the ester oil is passed through it, e.g. via a percolation procedure.

[0226] In a preferred aspect of this third embodiment of the invention, the oil is heated to a temperature of 40°C to 120 °C. More preferably it is heated to a temperature of from 50°C to 100 °C, more preferably from 75°C to 90 °C. In a preferred aspect the particulate adsorbent support having the reductant on its surface may be prepared by a technological procedure consisting of three stages: a first stage of annealing a silicon dioxide particulate adsorbent support; a second stage of depositing silver ions; and a third stage of depositing ammonium hydroxide. An illustrative example is set out below.

[0227] • In the first stage (annealing of adsorbent), an adsorbent based on silicon dioxide, 94-98%, particle size 200 to 1200 μm, is annealed at 150°C, for 18-24 h at atmospheric pressure, in order to remove adsorbed moisture.

[0228] • In the second stage (activation of adsorbent by Silver ion deposition) the annealed silicon dioxide is treated with 4-10 wt. % of silver nitrate relative to the mass of the adsorbent, by applying an aqueous silver nitrate solution to the adsorbent, which is followed by gradual evaporation of water from the adsorbent in the following way:

[0229] - heating at temperature of 30°C and pressure from 40 - 45 mBar during 2 hours

[0230] - heating at temperature of 40°C and pressure from 70-75 mBar during 2 hours

[0231] - heating at temperature of 50°C and pressure from 120 - 130 mBar during 2 hours, followed by annealing at temperature of 120°C to 130°C at atmospheric pressure, during 18 to to 24 hours.

[0232] • In the third stage (addition of ammonium hydroxide for neutralization of acidic byproducts) silicon dioxide adsorbent incorporated silver ions of chemical composition: Si (30 - 40%, 20 - 33 at. %), O (60 - 65 %, 74 - 80 at.%), Ag (3 - 6 %, 0.50 - 1.15 at.%), Ca (0.25 - 0.80 %, 0.10 - 0.40 at.%) is treated with aqueous solution of ammonium hydroxide, 5 to 10 wt. % relative to the mass of adsorbent, depending on the concentration of corrosive compounds to be removed and acidic by-products to be neutralized. Afterwards gradual evaporation of water is performed at atmospheric pressure over 5 hours in temperature program from 30 °C to 120 °C, with temperature increase of 0.3 °C per minute. In the second step additional annealing of the adsorbent is performed at temperature of 125 °C to 130 °C during 18 to 24 hours.

[0233] When the particulate adsorbent support having the reductant on its surface is used in accordance with the invention (e.g. in a percolation process), the ester oil may be contacted therewith under conditions that could be determined by a skilled person. For instance, the ester oil may be circulated under pressure, at a temperature 80°C to 85°C, with flow from 1000 to 2000 lit. / h, through a stainless-steel column having a ratio of length to diameter of 2.3 to 3, filled with the particulate adsorbent support having the reductant on its surface, thus performing chemisorption of active sulphur. Afterwards the oil may be subjected to vacuum filtration at said temperature through sintered glass filter porosity 4-10 micrometer followed by additional vacuum application (1 mBar) for a period of one to 4 hours.

[0234] Use of the particulate adsorbent support having the reductant on its surface enables surprisingly efficient removal of mercaptans, disulphides, oxygenated sulphur compounds, such as sulphoxides, sulphones and sulphur oxy acids, removal of high concentrations of compounds containing reactive sulphur - e.g. up to 50 mg / kg of elemental sulphur (S8), and up to 200 mg / kg of dibenzyl disulphide and in reducing total sulphur content. It is also surprisingly effective at reducing the dielectric dissipation factor of the ester oil, as well as Aluminium and Iron content in the oil. Dielectric dissipation factor may be measured by IEC 60247:2008 (a Baur Oil Tester DPA 75C may be used). Concentration of aluminium, iron, copper and silver in the oil is measured according to ASTM D 7151 or ASTM D 2622.

[0235] The content of total sulphur in the oil is preferably measured by ASTM D 5453.

[0236] The content of sulphoxides and sulphones can be measured using HPLC, LC MS / MS, or GC / MS, preferably GC / MS.

[0237] Sum of mercaptans, disulphides and elemental sulphur is measured using automatic potentiometric titration according to CIGRE A2.32. TF3 method.

[0238] Once the method of this third embodiment of the invention has been carried out, the ester oil (which consequently has a reduced amount of active sulphur and / or a reduced dielectric dissipation factor and acid content) can be separated from the particulate adsorbent support having the reductant on its surface. And the particulate adsorbent support having the reductant on its surface may be used multiple times and re-used (optionally after having been treated to remove sulphide deposits).

[0239] Thus, the present invention also provides a process comprising:

[0240] (a) reducing the amount of active sulphur in an ester oil and / or reducing the dielectric dissipation factor of an ester oil by a method as defined above; and

[0241] (b) subsequently separating the ester oil from the particulate adsorbent support having the reductant on its surface.

[0242] The present invention also provides a process comprising: (a) reducing the amount of active sulphur in an ester oil and / or reducing the dielectric dissipation factor of an ester oil by a method as defined above;

[0243] (b) subsequently separating the ester oil from the particulate adsorbent support having the reductant on its surface; and

[0244] (c) using the thus obtained particulate adsorbent support having the reductant on its surface to reduce the amount of active sulphur in an ester oil and / or reduce the dielectric dissipation factor of an ester oil, by a method which is independently as defined above.

[0245] The method of the third embodiment of the present invention produces ester oils with beneficial properties which make them particularly suited to use as insulating oils in transformers. It is also believed to impart to the oil a ‘fingerprint’ which renders it different from previous known ester oils (even ones with low active sulphur contents). Thus, the present invention also provides an ester oil obtained or obtainable by a method of the third embodiment of the present invention as defined above.

[0246] Such ester oils preferably have a total sulphur content of ≤0.5 mg / kg (more preferably ≤0.2 mg / kg),

[0247] Such ester oils preferably have a total S8content of ≤0.5 mg / kg (more preferably ≤0.2 mg / kg),

[0248] Such ester oils preferably have a content of mercaptans, disulphides, sulphoxides and sulphones ≤0.2 mg / kg.

[0249] Such ester oils preferably have a dielectric dissipation factor at 90 °C of ≤0.020.

[0250] Such ester oils preferably have an acid number of ≤0.03 mgKOH / g.

[0251] Such ester oils preferably have an oil-water interfacial tension of ≥35 mN / m.

[0252] Such ester oils preferably have particles of size ≥4μm in an amount of ≤320 particles / ml (typically the number of such particles is in the range of >160 to ≤320 particles / ml).

[0253] Such ester oils preferably have particles of size ≥6μm in an amount of <320 particles / ml (typically the number of such particles is in the range of >20 to ≤40 particles / ml).

[0254] In line with the above preferred aspects of the invention, such ester oils preferably have ≤15 / 12 ISO code (ref: ISO 4406, standard for coding particle number and size). The first number, 15, is the code for a particle size of 4 μm to be >160 particles / ml and up to and including 320 particles / ml. The second number, 12, is the code for a particle size of 6 μm to be >20 particles / ml and up to and including 40 particles / ml.

[0255] Particle size and count may be measured in accordance with IEC 60970:2007.

[0256] DEFINITIONS

[0257] Unless indicated otherwise, unqualified references herein to “copper”, along with references herein to “elemental copper”, refer to copper with an oxidation state of zero, and the same considerations apply to other elements mentioned herein. (An instance wherein a contrary meaning is indicated is where reference is made herein to a “copper-containing agent” for use in the process of preparing the dispersion of the invention - as indicated above, in this specific context the copper optionally may be in oxidised form.)

[0258] Unless indicated otherwise, unqualified references herein to “sulphur”, along with references herein to “elemental sulphur”, refer to sulphur with an oxidation state of zero, and the same considerations apply to other elements mentioned herein.

[0259] Unless indicated otherwise, amounts given in terms of % refer to weight %.

[0260] Unless indicated otherwise, the term “reactive sulphur” or “active sulphur” may be defined sulphur which reacts with silver in the DIN 51353 test and silver and copper in ASTM D 1275 - 15.

[0261] EXAMPLES

[0262] Examples 1 to 6 relate to the first embodiment of the invention set out above, and Examples 7 and 9 to 11 relate to the second embodiment of the invention set out above. Example 8 relates to the third embodiment of the invention set out above.

[0263] Example 1: Treatment of corrosive oil with copper as the solid reductant and PEG as the liquid dispersing agent

[0264] Step (1): preparation of dispersion containing copper particles

[0265] 0.6 g copper (II) sulphate pentahydrate was added to a glass vial, then 5 g water was added. The mixture was shaken for 3 minutes to dissolve the copper salt in the water. Then, 0.6 g zinc granules with a diameter ranging from 0.3-1.5mm was added to the vial, and the mixture was shaken for a further 3 minutes at room temperature. Once the blue colour of the solution disappeared and it became colourless or darkly transparent, this indicated the reduction reaction was complete. Next, the mixture was added to a glass beaker containing 180g pure PEG-400 (Fisher Chemical, UK; CAS: 25322-68-3). The resulting mixture was heated and then stirred for 15 minutes at 40 °C to obtain the dispersion, which contained about 0.08 wt.% copper particles.

[0266] Step (2): corrosive oil preparation

[0267] To simulate a corrosive mineral insulating oil, 0.009 g S8was added to 600g waste oil from a power transformer.

[0268] Step (3): desulphurisation of corrosive oil

[0269] The corrosive oil obtained from step (2) was heated to 95°C and added to the dispersion obtained from step (1). The temperature was controlled within the range 92-97°C using a magnetic stirrer (LLG labware) set to a stirring speed of approximately 600 RPM. The reaction duration was 120 minutes. The amount of copper in the reaction mixture, i.e. the amount of copper relative to the total combined weight of the dispersion and oil, was about 0.019 wt.%.

[0270] Samples were collected at specific intervals and measured using a GC chromatogram (Agilent Technologies 7890B with an ECD detector) following the guidelines of the IEC TR 62697-3 / 2018 standard. The results are set out in Table 1A below and are also depicted in Figures 1 and 2.

[0271] Table 1A: change in oil S8content over time

[0272] As indicated in Table 1 A and Figures 1 and 2, the S8had been almost completely removed after 90 minutes. The S8content was reduced still further after an additional 30 minutes, at which stage S8was not detected.

[0273] Step (4): work up Following completion of step (3), stirring and heating of the liquid-liquid-solid interphase oil-and-dispersion mixture was ceased. This allowed the oil to naturally separate and form a top layer, while the dispersion, including solids, settled at the bottom of the vessel. After 20 minutes, the oil was separated from the dispersion via decantation, without undergoing any cooling steps, and a desulphurised oil was obtained. The obtained oil was analysed to assess the characteristics of the oil before and after treatment with the dispersion. The results are set out in Table IB below and in Figures 1, 2, 4A and 4B.

[0274] Table 1B: change in oil characteristics before and after treatment

[0275] As indicated in Table IB and Figures 1 and 2, the treatment completely removed the S8from the oil. Furthermore, as indicated in Table IB and Figures 4A and 4B, the oil was corrosive before treatment (Fig. 4A) whereas the oil was non-corrosive after treatment (Fig. 4B). This illustrates the ability of the present invention to render non-corrosive mineral insulating oils for power transformers that were formerly corrosive.

[0276] The data presented in Table IB were obtained through additional steps of regenerating, drying, and degassing the oil, which are standard procedures for treated oils with reagents. The various characteristics were measured as indicated in the following table.

[0277] Example 1A: Treatment of corrosive oil with copper sulphate solution and PEG as a liquid dispersing agent (Comparative Example)

[0278] To illustrate the treatment of corrosive oils using only copper (II) sulphate pentahydrate (without reduction by zinc or any other reducing agent), another experiment was conducted using the same conditions and equipment as in Example 1 with the exception that no zinc was added in step (1). The results are set out in Table 1C below and also depicted in Figure 3.

[0279] Table 1C: change in oil S8content over time

[0280] As indicated in Table 1C and Figure 3, a dispersion having only Cu2SO4·5H2O as its active ingredient is much less effective at removing S8from mineral insulating oils.

[0281] Example 2: Treatment of corrosive oil with a higher concentration of solid reductant A further experiment was conducted using the same equipment as Example 1. Steps (including reagents and amounts) and conditions were the same as in Example 1 apart from the following:

[0282] Step (1) 32.4 g Cu2SO4·5H2O, 32.4 g zinc granules, and 150 g water were used which was dispersed in 180g of pure PEG-400, and the dispersion contained about

[0283] 2.4 wt.% copper particles.

[0284] Step (2) A new oil (Nynas 4000x) in which was added 0.009g of S8was used instead of a waste oil.

[0285] Step (3) The temperature range was between 90 - 105°C. The amount of copper in the reaction mixture, i.e. the amount of copper relative to the total combined weight of the dispersion and oil, was about 0.81 wt.%.

[0286] The results are set out in Table 2 below and in Figures 5A and 5B.

[0287] Table 2: change in oil S8content over time

[0288] The data in Table 2 demonstrate that a greater rate of S8content reduction can be achieved using a higher concentration of copper relative to the total amount of dispersion and oil. After 60 minutes, the amount of S8in the oil was reduced to non-detectable levels.

[0289] Figures 5A and 5B show that the post-treated oil was not corrosive to copper or silver.

[0290] Example 3: batch treatment

[0291] Industrial scale batch treatment of corrosive oil containing a typical amount of S8was simulated on a laboratory scale using the same equipment as in Example 1.

[0292] Step (1): preparation of dispersion containing copperparticles as a solid reductant

[0293] 3.6 g copper (II) sulphate pentahydrate was added to a glass vial, then 15 g water was added. The mixture was shaken for 6 minutes to dissolve the copper salt in the water. Then, 3.6 g zinc granules with a diameter ranging from 0.3-1.5mm was added to the vial, and the mixture was shaken for a further 6 minutes at room temperature. Next, the mixture was added to a glass beaker containing 180g pure PEG-400 (Fisher Chemical, UK; CAS: 25322-68-3). The resultant mixture was heated and stirred for 15 minutes at 40 °C to obtain the dispersion containing about 0.46wt.% copper particles.

[0294] Step (2): corrosive oil preparation

[0295] 15 batches of oil containing approximately 15.4 mg S8 / kg oil were prepared in the same way as in step (2) in Example 1.

[0296] Step (3): desulphurisation of corrosive oil

[0297] The corrosive oil obtained from step (2) was preheated to 95 °C and added to the first dispersion obtained from step (1). The temperature was controlled within the range of 92- 97 °C using a magnetic stirrer (LLG labware) set to a stirring speed of approximately 600 RPM. The amount of copper in the reaction mixture, i.e. the amount of copper relative to the total combined weight of the dispersion and oil, was about 0.11 wt.%.

[0298] Samples were collected at the end of 90 minutes: after the 90-minute period and a subsequent sedimentation process of 20 minutes, similar to Example 1, the oil was separated from the dispersion by decantation. A fresh batch of corrosive oil was then added to the dispersion and the new batch underwent the same process for the next 90 minutes. This process was repeated four times until five batches had been treated.

[0299] After the treatment of the fifth batch, the dispersion was replenished with copper particles in accordance with step (1) by adding 3.6 g copper (II) sulphate pentahydrate, 3.6 g zinc granules and 15 g water to the (five times used) dispersion. The process was then repeated on a further five batches, thus bringing the total number of treated batches to 10.

[0300] After the treatment of the tenth batch, the dispersion was replenished with copper particles (again in accordance with step (1)) by adding 3.6 g copper (II) sulphate pentahydrate, 3.6 g zinc granules and 15 g water to the dispersion. The process was then repeated on a further five batches, thus bringing the total number of treated batches to 15.

[0301] Hence, a total of 9kg oil (600g per batch for 15 batches) was processed using only 180g PEG. This clearly demonstrates the improved efficiency and environmental benefits of the present invention. The range of PEG content in relation to the oil varied from 30 wt.% in the first batch to 2 wt.% in the last batch.

[0302] The results are set out in Table 3.

[0303] Table 3: post-treatment S8content in each batch of oil

[0304] These results show that the dispersion for use in accordance with the present invention can be re-used multiple times and still provide effective desulphurisation, particularly when appropriate replenishing steps are taken. The possibility of recycling the dispersion has self- evident benefits in terms of environmental impact and efficiency / costs.

[0305] Example 4: Treatment on larger scale

[0306] The following Example was conducted to investigate the efficacy of the invention on a greater-than-laboratory scale.

[0307] 31.4 g copper (II) sulphate pentahydrate was added to a glass beaker, then 400 g water was added. The mixture was stirred for 6 minutes to dissolve the copper salt in the water. Then, 31.4g iron metal powder was added to the beaker, and the mixture was stirred at room temperature for a further 10 minutes (until the reduction reaction was complete). Then, 7 kg pure PEG-400 was placed into a 100 L stainless steel vessel and mixed with 26 kg corrosive oil containing approximately 12.7 mg S8 / kg oil that hadbeen preheated to 75°C. Then, the copper solution was added to the vessel. The temperature was controlled within the range of 75-78 °C and the mixture stirred with an overhead stirrer set to a stirring speed of approximately 750 RPM. The reaction duration was 120 minutes. The amount of copper in the reaction mixture, i.e. the amount of copper relative to the total combined weight of the dispersion and oil, was about 0.023 wt.%.

[0308] Samples were collected at specific intervals and measured using a GC chromatogram (Agilent Technologies 7890B with an ECD detector) following the guidelines of the IEC TR 62697-3 / 2018 standard. The results are set out in Table 4 below.

[0309] Table 4: change in oil S8content over time

[0310] As indicated in Table 4, the S8had been completely removed (to a non-detectable level) after only 90 minutes. This confirmed the ability of the present method to remove corrosiveness of mineral insulating oils on an industrial scale.

[0311] Step (4): work up

[0312] After 120 minutes, stirring and heating of the liquid-liquid-solid interphase oil-and- dispersion mixture was ceased. This allowed the oil to naturally separate and form a top layer, while the dispersion, including solids, settled at the bottom of the vessel. After 20 minutes, the oil was drained from a side outlet of the vessel, without undergoing any cooling steps, and a desulphurised oil was obtained.

[0313] Example 5: Treatment of corrosive oil with copper as the solid reductant and PEG as the liquid dispersing agent without water Step (1): preparation of dispersion containing copper particles

[0314] 0.6 g copper (II) sulphate pentahydrate was added to a glass vial, then 5 g of pure PEG-400 was added. The mixture was shaken for 3 minutes to obtain dispersion of copper salt. Then, 0.6 g zinc granules with a diameter ranging from 0.3-1.5mm powder was added to the vial, and the mixture was shaken for a further 10 minutes at room temperature. Next, the mixture was added to a glass beaker containing 175g pure PEG-400 (Fisher Chemical, UK; CAS: 25322-68-3). The resulting mixture was heated and then stirred for 15 minutes at 40 °C to obtain the dispersion, which contained about 0.08 wt.% copper particles.

[0315] Step (2): corrosive oil preparation

[0316] A corrosive oil was prepared at same way in step (2) as in Example 1.

[0317] Step (3): desulphurisation of corrosive oil

[0318] The corrosive oil obtained from step (2) was heated to 95 °C and added to the dispersion obtained from step (1). The temperature was controlled within the range 92-97 °C using a magnetic stirrer (LLG labware) set to a stirring speed of approximately 600 RPM. The reaction duration was 120 minutes. The amount of copper in the reaction mixture, i.e. the amount of copper relative to the total combined weight of the dispersion and oil, was about 0.019 wt.%.

[0319] Samples were collected at specific intervals and measured using a GC chromatogram (Agilent Technologies 7890B with an ECD detector) following the guidelines of the IEC

[0320] TR 62697-3 / 2018 standard. The results are set out in Table 5

[0321] Table 5: change in oil S8content over time

[0322] As indicated in Table 5 a dispersion without added water is effective at removing S8from mineral insulating oils. However, a comparison with the data in Table 1 A above shows that including water in the dispersion leads to even more efficient desulphurisation.

[0323] Step (4): work up

[0324] Following completion of step (3), stirring and heating of the liquid-liquid-solid interphase oil-and-dispersion mixture was ceased. This allowed the oil to naturally separate and form a top layer, while the dispersion, including solids, settled at the bottom of the vessel. After 20 minutes, the oil was separated from the dispersion via decantation, without undergoing any cooling steps, and a desulphurised oil was obtained.

[0325] Example 6: Treatment in a larger scale in the absence of water

[0326] The following Example was conducted to investigate the efficacy of the invention on a greater-than-laboratory scale in the absence of water.

[0327] 31.4 g copper (II) sulphate pentahydrate was added to a glass beaker, then 400 g of pure PEG-400 was added. The mixture was stirred for 6 minutes to obtain a dispersion of the copper salt in the PEG. Then, 31.4g iron metal powder was added to the beaker, and the mixture was stirred at room temperature for a further 10 minutes.

[0328] Then, 6.6 kg pure PEG-400 was placed into a 100 L stainless steel vessel and mixed with 26 kg corrosive oil containing approximately 12.0 mg S8 / kg oil that had been preheated to 75 °C. Then, the copper dispersion was added to the vessel. The temperature was controlled within the range of 75-78 °C and the mixture stared with an overhead stirrer set to a stirring speed of approximately 750 RPM. The reaction duration was 120 minutes. The amount of copper in the reaction mixture, i.e. the amount of copper relative to the total combined weight of the dispersion and oil, was about 0.023 wt.%.

[0329] Sample was collected at the end of treatment and measured using a GC chromatogram (Agilent Technologies 7890B with an BCD detector) following the guidelines of the IEC TR 62697-3 / 2018 standard. The results are set out in Table 6 below.

[0330] Table 6: change in oil S8content over time

[0331] As indicated in Table 6 desulphurisation occurs. A comparison with the results when water is also added (see Table 4 above) again reveals the surprising benefits of including water.

[0332] Step (4): work up

[0333] After 120 minutes, stirring and heating of the liquid-liquid-solid interphase oil-and- dispersion mixture was ceased. This allowed the oil to naturally separate and form a top layer, while the dispersion, including solids, settled at the bottom of the vessel. After 20 minutes, the oil was drained from a side outlet of the vessel.

[0334] Example 7: Treatment of synthetic ester corrosive oil with high S8content (second embodiment)

[0335] First treatment

[0336] Step (1) desulphurisation of high content of S8 in corrosive synthetic ester oil

[0337] To simulate a corrosive synthetic ester insulating oil, 0,0016 g of S8was added to 213 g of synthetic ester insulating oil.

[0338] Step (2): desulphurisation of corrosive synthetic ester oil

[0339] The oil obtained in step (1) was heated with silver plate was heated to 200°C with silver plated coil. Ratio of oil volume to the surface of silver was 0.01 cm2 / g oil. Prior treatment oil was purged with Argon for 10 minutes.

[0340] The temperature was controlled within the range 200±2°C. The reaction duration was 30, 60, 120, 150, 210 and 270 minutes.

[0341] Samples were collected at specific intervals and measured using a GC chromatograph (Agilent Technologies 7890B with an EC detector) following the guidelines of the IEC TR 62697-3 / 2018 standard. The results are set out in Table 7 below and shown in Figure 10.

[0342] Table 7: change in oil S8content over time

[0343] As indicated in Table 7 and Figure 10 the S8had been almost completely removed after 210 minutes. The S8content was reduced still further after an additional 60 minutes, at which stage S8was detected in minute concentration, between limit of detection and limit of quantification of the method.

[0344] Example 8: Treatment of synthetic ester containing S8(third embodiment) To simulate a corrosive synthetic ester insulating oil, 0,0011 g of S8was added to 213 g of synthetic ester insulating oil. desulphurisation of corrosive synthetic ester oil using silver reductant incorporated on adsorbent surface

[0345] The oil obtained in step (1) was treated with silver embedded adsorbent at 80°C during 10 cycles in quantity of 2 %wt. of adsorbent to the oil weight. The temperature was controlled within the range ±2°C.

[0346] Samples were collected at specific intervals and measured using a GC chromatograph (Agilent Technologies 7890B with an EC detector) following the guidelines of the IEC TR 62697-3 / 2018 standard. The results are set out in Table 8 below and shown in Figure 11.

[0347] Table 8: change in oil S8content over time

[0348] Table 9; change in synthetic ester oil properties before and. after treatment

[0349] Additional experiment containing oxygenated sulphur compound and low total sulphur content Desulphurisation of corrosive synthetic ester oil containing 1.9 ppm of total sulphur and detected sulphonyl benzazepine was performed using silver embedded adsorbent at 80°C during 10 cycles with mass 2 wt. % of adsorbent to the oil weight. The temperature was controlled within the range ±2°C.

[0350] Sample collected after 10 cycles was measured to determine sulphonyl benzazepine content using a GC chromatogram MS detector. The results are set out in Table 10 below and depicted in Figure 12.

[0351] Table 8: change in sulphur content in the oil after 10 treatment cycles

[0352] Example 9: Treatment of synthetic ester with S8content (second embodiment)

[0353] To simulate a corrosive synthetic ester insulating oil, 0,0016 g of S8 was added to 213 g of synthetic ester insulating oil. desulphurisation of high content of S8 in corrosive synthetic ester oil with lower available silver surface

[0354] The corrosive oil obtained from step (1) was heated to 200°C with silver plated coil. Ratio of oil volume to the surface of silver was 0.01 cm2 / g oil and 0.007 cm2 / g oil. Prior treatment oil was purged with Argon for 10 minutes. The temperature was controlled within the range 200±2°C. The reaction duration was 30, 60, 120, 150 and 210 minutes.

[0355] Samples were collected at specific intervals and measured using a GC chromatogram (Agilent Technologies 7890B with an ECD detector) following the guidelines of the IEC TR 62697-3 / 2018 standard. The results are set out in Table 11 below. Table 11: change in oil S8content over time

[0356] As indicated in Table 11 and Figure 13, the treatment removed the S8from the oil after 210 minutes to the low values on the limit of quantification of the method. Furthermore, as indicated in (Fig. 14A) at the oil was corrosive before treatment whereas the oil was non- corrosive after treatment (Fig. 14B). This illustrates the ability of the present invention to render non-corrosive insulating oils for power transformers that were formerly corrosive.

[0357] Example 10: Treatment of synthetic ester oil with a lower S8content (second embodiment)

[0358] A further experiment was conducted using the same equipment as in Example 9. Steps and conditions were the same as in Example 9 apart from the following:

[0359] Step (1): corrosive oil preparation

[0360] To simulate a corrosive synthetic ester insulating oil, 0,0003 g of S8was added to 213 g of synthetic ester insulating oil.

[0361] Samples were collected at specific intervals and measured using a GC chromatogram (Agilent Technologies 7890B with an ECD detector) following the guidelines of the IEC TR 62697-3 / 2018 standard. The results are set out in the table below and depicted at Figure 15.

[0362] Table 12: change in oil S8content over time Additional experiment with low concentration of S8in synthetic ester oil

[0363] Step (1): corrosive oil preparation

[0364] To simulate a corrosive synthetic ester insulating oil, 0.0002 g of S8was added to 213 g of synthetic ester insulating oil.

[0365] Samples were collected at specific intervals and measured using a GC chromatogram (Agilent Technologies 7890B with an ECD detector) following the guidelines of the IEC TR 62697-3 / 2018 standard. The results are set out in the table below and shown in Figure 16.

[0366] The data in Table 13 demonstrate that shorter contact time is needed to completely convert S8in 60 minutes.

[0367] Table 13: change in oil S8content over time n.d. - not detected

[0368] Example 11: Treatment of mineral oil with higher S8 content (second embodiment)

[0369] Step (1): corrosive oil preparation

[0370] To simulate a corrosive synthetic oil, 0,0003 g of S8was added to 213 g of synthetic ester insulating oil.

[0371] Step (2): desulphurisation of corrosive mineral oil

[0372] The corrosive oil obtained from step (1) was heated to 200°C for 30 min., 60 min. and 120 min. with silver plated coil. Ratio of oil volume to the surface of silver was 0.014 cm2 / g oil and 0.007 cnr / g oil. Prior treatment oil was purged with Argon for 10 minutes. The temperature was controlled within the range 200±2°C. The reaction duration was 30, 60, 120 minutes.

[0373] Oil samples were collected at specific intervals and measured using a GC chromatograph (Agilent Technologies 7890B with an EC detector) following the guidelines of the IEC TR 62697-3 / 2018 standard. The results are set out in Table 14 below and are also depicted in Figure 17.

[0374] Tablel4A: change in oil S8content in mineral oil over time

[0375] As indicated in Table 14A and Figure 17, the S8had been almost completely removed after 120 minutes. The S8content was reduced still further after an additional 30 minutes, at which stage S8was bellow limit of quantification.

[0376] Step (3): work up

[0377] Following completion of step (3), the oil was analysed to assess the properties of the mineral oil before and after treatment.

[0378] The data presented in table 14 B were obtained through additional steps of regenerating, drying, and degassing the oil, which are standard procedures for treated oils. The various characteristics were measured as indicated in the following table.

[0379] Table 14 B: change in mineral oil properties before and after treatment The numbered clauses [1] to

[0025] below correspond to claims of the earlier applications from which the present case claims priority, and define aspects of the invention. These numbered clauses are not the claims of this application. The claims of this application appear further below in a separate section which is titled “CLAIMS”.

[0380] [1] A method for reducing the amount of S8 in an oil which contains S8, the method comprising contacting the oil with (a) a solid agent comprising a reductant, and (b) a liquid agent suitable for dispersing said solid agent.

[0381] [2] The method of claim 1, wherein the reductant comprises one or more metals selected from copper, iron, zinc, aluminium, nickel, and tin, and preferably the reductant is copper.

[0382] [3] The method of claim 1 or 2, wherein the solid agent comprises solid carrier particles having the reductant on the surface thereof.

[0383] [4] The method of claim 3, wherein the solid carrier particles are in the form of granules or powder.

[0384] [5] The method of claim 3 or 4, wherein the solid carrier particles comprise iron and / or zinc, and optionally further comprise one or more salts of iron and / or zinc, preferably one or more salts selected from sulphates, nitrates, chlorides and bromides.

[0385] [6] The method of any preceding claim, wherein the liquid agent is a polymer, preferably a polyether, more preferably polyethylene glycol.

[0386] [7] The method of any preceding claim, wherein the amount of the solid agent relative to the amount of the oil is from 0.05 to 10 wt.%.

[0387] [8] The method of any preceding claim, which comprises a step of contacting the oil with a dispersion wherein the solid agent is dispersed within the liquid agent, and wherein said dispersion preferably further comprises water.

[0388] [9] The method of claim 8, wherein the amount of dispersion relative to the total amount of oil plus dispersion is from 1.5 to 40 wt.%.

[0389]

[0010] The method of any preceding claim, wherein the oil, solid agent and liquid agent are subjected to stirring and / or ultrasound treatment.

[0011] The method of any preceding claim, wherein the oil, solid agent and liquid agent are heated to a temperature of 50 to 120°C, preferably 60 to 105 °C, more preferably 70 to 99 °C.

[0390]

[0012] The method of any preceding claim, wherein the content of S8 as measured by IEC TR 62697-3 / 2018 is reduced to a level of less than 2.0 mg / kg oil based on the total amount of oil, preferably less than 1.0 mg / kg, more preferably less than 0.2 mg / kg.

[0391]

[0013] A process comprising:

[0392] (a) reducing the amount of S8 in an oil by a method as defined in any one of claims 1 to 12; and

[0393] (b) subsequently separating the oil from the other components, wherein said other components are present in the form of a dispersion comprising the solid agent and the liquid agent.

[0394]

[0014] A process comprising:

[0395] (a) reducing the amount of S8 in an oil by a method as defined in any one of claims 1 to 12;

[0396] (b) subsequently separating the oil from the other components, wherein said other components are present in the form of a dispersion comprising the solid agent and the liquid agent; and

[0397] (c) using the thus obtained dispersion to reduce the amount of S8 in an oil which contains S8 by a method which is independently as defined in any one of claims 1 to 12.

[0398]

[0015] A dispersion comprising water and elemental copper, wherein said water and copper are dispersed within a liquid polymer, wherein said liquid polymer is preferably a polyether, and more preferably polyethylene glycol.

[0399]

[0016] A dispersion according to claim 15, comprising solid carrier particles having the copper on the surface thereof.

[0400]

[0017] A dispersion according to claim 16, wherein the solid carrier particles comprise iron and / or zinc, and optionally further comprise one or more salts of iron and / or zinc, preferably one or more salts selected from sulphates, nitrates, chlorides, and bromides.

[0018] A dispersion according to any one of claims 15 to 17, wherein the liquid polyether is polyethylene glycol having a number average molecular weight of from 300 to 600 g / mol.

[0401]

[0019] A process of preparing a dispersion as defined in any one of claims 15 to 18, the process comprising the following steps:

[0402] (i) preparing an aqueous solution of a copper-containing agent or a suspension of a copper-containing agent, and

[0403] (ii) dispersing said solution or suspension in the liquid polymer; wherein if the copper-containing agent in step (i) features copper in oxidised form, then said process further includes a step of reducing the copper in the copper- containing agent in between steps (i) and (ii).

[0404]

[0020] The process of claim 19, wherein step (i) is preparing an aqueous solution of a copper salt, preferably copper sulphate.

[0405]

[0021] The process of claim 20, wherein the aqueous solution of the copper salt is prepared using:

[0406] (a) H2O;

[0407] (b) a H2O / alcohol mixture, wherein preferably the alcohol is a C1-3alcohol such as methanol, ethanol or propanol, and more preferably methanol; or

[0408] (c) a H2O / polyol mixture, wherein preferably the polyol is a C1-3diol or C1- 3triol, and more preferably glycerol.

[0409]

[0022] The process of any one of claims 19 to 21, wherein the liquid polymer is a polyether, and preferably is a polyethylene glycol.

[0410]

[0023] The process of any one of claims 19 to 22, wherein the copper-containing agent in step (i) features copper in oxidised form, such that the process further includes a step of reducing the copper in the copper-containing agent in between steps (i) and (ii), and wherein the reduction reaction involves introducing to the aqueous solution or suspension one or more reducing agents selected from transition metals and posttransition metals, and more preferably from elemental iron, elemental zinc, elemental aluminium, elemental nickel, and elemental tin; and wherein the reducing agent(s) is preferably in the form of granules or a powder.

[0024] An oil obtained or obtainable by a method as defined in claim 13.

[0411]

[0025] An oil according to claim 24, wherein the oil: has a sum of mercatpans, sulphides, disulphides of ≤2 mg / kg has S8 content of ≤0.5 mg / kg has a dielectric dissipation factor at 90 °C of <0.005, has an acid number of <0.02 mgKOH / g, has an oil-water interfacial tension of ≥35 mN / m, has an FT-IR spectrum within a wavelength range of 700 to 1300 cm-1 which is substantially the same as that of the oil prior to the desulphurisation method, and / or has particles of size ≥4μm in an amount of ≤320 particles / ml, and particles of size ≥6μm in an amount of ≤40 particles / ml.

Claims

CLAIMS1. Use of a solid agent comprising a reductant for reducing the amount of active sulphur in an oil and / or reducing the dielectric dissipation factor of an oil, wherein either:(i) the oil is contacted with (a) the solid agent comprising the reductant, and (b) a liquid agent suitable for dispersing said solid agent;(ii) the oil is contacted with a solid substrate having the reductant on its surface; or(iii) the oil is an ester and is contacted with a particulate adsorbent support having the reductant on its surface.

2. The use of claim 1, wherein the active sulphur is provided by S8, and the oil is contacted with (a) a solid agent comprising a reductant, and (b) a liquid agent suitable for dispersing said solid agent.

3. A method for reducing the amount of S8in an oil which contains Ss, the method comprising contacting the oil with (a) a solid agent comprising a reductant, and (b) a liquid agent suitable for dispersing said solid agent.

4. The use of claim 2 or the method of claim 3, wherein the reductant comprises one or more metals selected from copper, iron, zinc, aluminium, nickel, and tin, and preferably the reductant is copper.

5. The use of claim 2 or the method of claim 3 or 4, wherein the solid agent comprises solid carrier particles having the reductant on the surface thereof, and wherein:- preferably the solid carrier particles are in the form of granules or powder; and- more preferably the solid carrier particles comprise iron and / or zinc, and optionally further comprise one or more salts of iron and / or zinc, preferably one or more salts selected from sulphates, nitrates, chlorides and bromides.

6. The use of claim 2 or the method of any one of claims 3 to 5, wherein the liquid agent is a polymer, preferably a polyether, more preferably polyethylene glycol.

7. The use of claim 2 or the method of any one of claims 3 to 6, wherein the amount of the solid agent relative to the amount of the oil is from 0.05 to 10 wt.%, and the amount of reductant relative to the amount of the oil is preferably from 0.01 to 2 wt%.

8. The use of claim 2 or the method of any one of claims 3 to 7, which comprises a step of contacting the oil with a dispersion wherein the solid agent is dispersed within the liquid agent, and wherein said dispersion preferably further comprises water.

9. The use of claim 1, wherein the active sulphur is provided by S8, S7, S6, S5, S4, disulphides and / or sulphoxides, and the oil is contacted with a solid substrate having a reductant on its surface.

10. A method for reducing the amount of S8, S7, S6, S5, S4, disulphides and / or sulphoxides in an oil which contains S8, S7, S6, S5, S4, disulphides and / or sulphoxides, the method comprising contacting the oil with a solid substrate having a reductant on its surface.

11. The use of claim 9 or the method of claim 10, wherein the solid substrate is a reductant- plated metal coil, and wherein:- the reductant is one or more selected from silver, zinc, aluminium, nickel and tin, and preferably is silver; and- the metal coil comprises, or is, one or more selected from aluminium, copper, zinc, brass, iron steel, and preferably selected from aluminium and copper.

12. The use of claim 9 or the method of claim 10 or 11, wherein the ratio of the surface area of reductant to mass of oil is from 0.00015 to 0.00080 m2 / kg.

13. The use of claim 9 or the method of any one of claims 10 to 12, wherein the solid substrate comprises a reductant-plated metal coil wrapped around a heater.

14. The use of claim 9 or the method of any one of claims 10 to 13, the oil is a synthetic ester.

15. The use of claim 9 or the method of any one of claims 10 to 14, wherein the oil is contacted with said solid substrate, or more than one such solid substrates, in a batch reactor and / or a tubular reactor.

16. The use of claim 1, wherein the oil is an ester and is contacted with a particulate adsorbent support having the reductant on its surface.

17. A method for reducing the amount of active sulphur in an ester oil and / or reducing the dielectric dissipation factor of an ester oil, the method comprising contacting the oil with a particulate adsorbent support having the reductant on its surface.

18. The use of claim 16 or the method of claim 17, wherein the particulate adsorbent support comprises silicon dioxide and / or calcium oxide and the reductant comprises silver ions, and wherein the particulate adsorbent support also has ammonium ions on its surface.

19. The use of claim 16 or the method of claim 17 or 18, wherein the particulate adsorbent support has a particle size of 200 to 1200 μm.

20. A process comprising:(a) reducing the amount of S8in an oil by a method as defined in any one of claims 3 to 9; and(b) subsequently separating the oil from the other components, wherein said other components are present in the form of a dispersion comprising the solid agent and the liquid agent.

21. A process comprising:(a) reducing the amount of S8in an oil by a method as defined in any one of claims 3 to 9;(b) subsequently separating the oil from the other components, wherein said other components are present in the form of a dispersion comprising the solid agent and the liquid agent; and(c) using the thus obtained dispersion to reduce the amount of S8in an oil which contains S8by a method which is independently as defined in any one of claims 3 to 9.

22. A dispersion comprising water and elemental copper, wherein said water and copper are dispersed within a liquid polymer, wherein said liquid polymer is preferably a polyether, and more preferably polyethylene glycol.

23. A dispersion according to claim 22, wherein(a) the dispersion comprises solid carrier particles having the copper on the surface thereof, and preferably the solid carrier particles comprise iron and / or zinc, and optionally further comprise one or more salts of iron and / or zinc, preferably one or more salts selected from sulphates, nitrates, chlorides, and bromides; and / or(b) the liquid polyether is polyethylene glycol having a number average molecular weight of from 300 to 600 g / mol.

24. A process of preparing a dispersion as defined in claim 22 or 23, the process comprising the following steps:(i) preparing an aqueous solution of a copper-containing agent or a suspension of a copper-containing agent, and(ii) dispersing said solution or suspension in the liquid polymer; wherein if the copper-containing agent in step (i) features copper in oxidised form, then said process further includes a step of reducing the copper in the copper-containing agent in between steps (i) and (ii).

25. The process of claim 24, wherein step (i) is preparing an aqueous solution of a copper salt, preferably copper sulphate; and wherein preferably: the aqueous solution of the copper salt is prepared using:(a) H2O;(b) a H2O / alcohol mixture, wherein preferably the alcohol is a C1-3alcohol such as methanol, ethanol or propanol, and more preferably methanol; or(c) a H2O / polyol mixture, wherein preferably the polyol is a C1-3diol or C1-3triol, and more preferably glycerol;• the liquid polymer is a polyether, and preferably is a polyethylene glycol; and / or• the copper-containing agent in step (i) features copper in oxidised form, such that the process further includes a step of reducing the copper in the copper- containing agent in between steps (i) and (ii), and wherein the reduction reaction involves introducing to the aqueous solution or suspension one or more reducing agents selected from transition metals and post-transition metals, and more preferably from elemental iron, elemental zinc, elemental aluminium, elemental nickel, and elemental tin; and wherein the reducing agent(s) is preferably in the form of granules or a powder.

26. A process comprising:(a) reducing the amount of S8, S7, S6, S5, S4, disulphides and / or sulphoxides in an oil by a method as defined in any one of claims 10 to 15; and(b) subsequently separating the oil from the solid substrate having a reductant on its surface.

27. A process comprising:(a) reducing the amount of S8, S7, S6, S5, S4, disulphides and / or sulphoxides in an oil by a method as defined in any one of claims 10 to 15;(b) subsequently separating the oil from the solid substrate having a reductant on its surface; and(c) using the thus obtained solid substrate having a reductant on its surface to reduce the amount of S8, S7, S6, S5, S4, disulphides and / or sulphoxides in an oil which contains S8, S7, S6, S5, S4, disulphides and / or sulphoxides, by a method which is independently as defined in any one of claims 10 to 15.

28. A batch reactor, a tubular reactor, or a reactor system comprising a batch reactor and a tubular reactor, wherein said batch reactor, said tubular reactor and at least one of the reactors in said reactor system comprises at least one solid substrate having reductant on its surface as defined in any one of claims 10 to 15, wherein the solid substrate having reductant on its surface is either located within that reactor or is moveable so as to place it therein.

29. A process comprising:(a) reducing the amount of active sulphur in an ester oil and / or reducing the dielectric dissipation factor of an ester oil by a method as defined in any one of claims 17 to 19; and(b) subsequently separating the ester oil from the particulate adsorbent support having the reductant on its surface.

30. A process comprising:(a) reducing the amount of active sulphur in an ester oil and / or reducing the dielectric dissipation factor of an ester oil by a method as defined in any one of claims 17 to 19;(b) subsequently separating the ester oil from the particulate adsorbent support having the reductant on its surface; and(c) using the thus obtained particulate adsorbent support having the reductant on its surface to reduce the amount of active sulphur in an ester oil and / or reduce the dielectric dissipation factor of an ester oil, by a method which is independently as defined in any one of claims 17 to 19.

31. An oil obtained or obtainable by a method as defined in claim 16, 26 or 29.

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