Desulphurisation process

Abstract

A method for reducing the amount of S8 in an oil containing S8, comprising contacting the oil with (a) a solid agent comprising a reductant, and (b) a liquid agent suitable for dispersing said solid a

Description

FIELD OF THE INVENTION 5 The present invention relates to a desulphurisation process. More specifically, the present invention provides a process for removing elemental sulphur in the form of Ss from oils such as mineral insulating oils used in power transformers. The present invention also provides desulphurised oils obtained or obtainable by the process of the invention, as well as compositions for use in the desulphurisation process of the invention and preparation 10 methods therefor. BACKGROUND TO THE INVENTION The presence of Sx in oils can be problematic as Sx can be corrosive towards 15 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 mineral insulating oils 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 20 oxide and aluminium silicate in online systems. During the adsorbent reactivation process using high-temperature combustion technology, Sx can form as a side product. Such regeneration methods may therefore contribute to corrosion by Sx. Methods which desulphurise such oils are known. For instance, WO2018190741 describes a process wherein the oil is circulated through a stainless steel column filled with 25 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. 30 SUMMARY OF THE INVENTION xt CM CO CM CM The present inventor has devised a new method of removing Ss from oils such as mineral insulating oils, which employs a selective liquid-liquid-solid process. The present invention therefore provides a means to mitigate power transformer failures induced by the presence of Ss. 5 The process is a selective desulphurisation procedure that utilises a liquid-liquid- solid interphase chemical reaction to effectively and selectively remove Ss. This approach has surprisingly been found to enable the selective removal of Ss in 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. 10 The process of the present invention effectively and highly selectively removes Ss, thereby preventing corrosion reactions that the Ss would 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. 15 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. Additionally, corrosive mineral insulating oils can be made non-corrosive very 20 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. 25 BRIEF DESCRIPTION OF THE FIGURES Figure 1 is a chromatogram displaying the Ss content of a mineral insulating oil before, during and after treatment by a method of the invention (see Example 1). 30 Figure 2 is a chromatogram displaying the Ss content of a mineral insulating oil before and after treatment by a method of the present invention (see Example 1). Figure 3 is a chromatogram displaying the Ss content of a mineral insulating oil before and after treatment by a method not according to the invention (see Example 1 A). Figure 4 shows the results of DIN 51353 oil corrosion tests on silver plates (see Example 1). Figure 4A shows a plate that has been contacted with a pre-treated oil containing a typical amount of Sg. Figure 4B shows a plate that has been contacted with a post-treated oil containing a non-detectable amount of Ss. 5 Figure 5 shows the results of oil corrosion tests on metal plates using a treated mineral insulating oil (described in Example 2). Figure 5A shows a copper plate that has been contacted with a post-treated oil in accordance with the IEC 62535 oil corrosion test. Figure 5B shows a silver plate that has been contacted with a post-treated oil in accordance with the DIN 51353 oil corrosion test. 0 DETAILED DESCRIPTION The present invention provides a method for reducing the amount of Sg in an oil which contains Ss, wherein the oil is an insulating oil for use in a transformer, the method 15 comprising contacting the oil with (a) a solid agent comprising copper (a reductant), and (b) a liquid agent suitable for dispersing said solid agent, wherein the liquid agent is polyethylene glycol, the solid agent is dispersed within the liquid agent, and the dispersion further comprises water. The solid agent may further comprise one or more other metals (reductants). 20 Preferably said one or more other metals are selected from iron, zinc, aluminium, nickel, and tin. The solid agent preferably comprises (and most preferably consists of) solid carrier particles having the copper on the surface thereof. This does not require all of the copper to be found exclusively on the surface of the solid carrier particles, however it does require 25 at least some of the copper to be on the surface thereof. Preferably the majority (over 50 %) of the copper is on the surface of the solid carrier particles, and more preferably substantially all of the copper is present on the surface of the solid carrier particles. 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 30 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 5 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 10 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 pm is <50 wt%, preferably <20 wt%, and more preferably <10 wt%. Again, standard designation sieves can be used to identify such particles. The solid carrier particles preferably comprise iron and / or zinc. In this 15 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) on the surface thereof. (The oxidised iron and / or zinc is generally present as a 20 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.) 25 The liquid agent is a polyethylene glycol. The number average molecular weight (Mn) of the polyethylene glycol 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 polyethylene glycols are readily available commercially. The polyethylene glycol preferably has a number average molecular weight (Mn) of at least 30 300 g / mol, more preferably at least 350 g / mol. The polyethylene glycol 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). 5 The nature of the solid agent and the copper comprised within it, and the amounts of these components, can be varied in order to control the rate at which desulphurisation proceeds. Preferably the amount of the solid agent relative to the amount of the oil is from 0.05 to 10 wt.%. 10 Preferably the amount of copper comprised in the solid agent relative to the amount of the oil is from 0.01 to 2 wt %, more preferably 0.02 to 1.5 wt %. Preferably the copper comprised in the solid agent accounts for 0.5 to 25 wt% of the solid agent. Preferably the amount of liquid agent relative to the amount of the oil is from 1.5 to 15 40 wt%. The method comprises a step of contacting the oil with a dispersion wherein the solid agent is dispersed within the liquid agent, and the 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 20 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.%. The dispersion is preferably a dispersion of the invention as described below. 25 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, 30 in a preferred embodiment the method of the invention comprises contacting the oil with (a) a solid agent comprising copper, (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, wherein the liquid agent is polyethylene glycol, the solid agent is dispersed within the liquid agent, and the dispersion further comprises water. Thus, 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 C1-3 alcohols such as methanol, ethanol and propanol, with methanol being most preferred. Preferred 5 polyols are Ci-.diols and Ci-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 10 agents are used, for example if the liquid agent (a polyethylene glycol) has a relatively high Mn - e.g. an Mn of >400, such as >500, or >550. In the method of the invention the oil, solid agent and liquid agent are preferably subjected to stirring and / or ultrasound treatment. In the method of the invention the oil, solid agent and liquid agent are preferably 15 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 20 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 Ss may be a mineral oil or a synthetic oil. Preferably it is a mineral oil. The oil which contains Sx is an insulating oil for use in a transformer, and 25 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. The oil which contains Sx preferably contains Sx in 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 30 preferably at least 10.0 mg / kg. In the method of the invention, the content of Ss is 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. The content of Ss in the oil is preferably measured by IEC TR 62697-3 / 2018. Once the method of the invention has been carried out, the oil (which consequently has a reduced level of Ss in 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 5 dispersion may just be composed of the components (a) and (b) and water (plus some copper sulfide) though may also contain further components if other agents were additionally used (e.g. alcohol and / or polyol). Thus, the present invention also provides a process comprising: (a) reducing the amount of Ss in an oil which contains Ss by a method of the 10 invention as defined above; 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. The separated oil with a reduced content of Ss may be used in applications wherein 15 it will come into contact with components that may be susceptible to attack by Ss - 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 Ss in an oil which contains Ss by a method of the 20 invention as defined above; (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 Ss in an oil 25 which contains Ss by a method which is independently as defined above. Also described herein, for possible use in accordance with the invention, is a dispersion comprising water and elemental copper, wherein said water and copper are dispersed within a polyethylene glycol liquid polymer. The preferred aspects of the liquid agent (polyethylene glycol) described above (component (b) in the method of the present 30 invention) apply correspondingly, but independently, to said polyethylene glycol liquid polymer. This dispersion preferably comprises solid carrier particles having the copper on the surface thereof. Thus, described herein is a dispersion comprising water and solid carrier particles having the copper on the surface thereof, wherein said water and particles are dispersed within a polyethylene glycol 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. 5 The amount of water in the dispersion is preferably 1 to 50 wt%, more preferably 2 to 40 wt%. The amount of liquid polymer in the dispersion is preferably 40 to 98 wt%, more preferably 45 to 97 wt%. The amount of copper in the dispersion is preferably 0.02 to 4.0 wt%, more 10 preferably 0.05 to 3.0 wt%. When this dispersion 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%. 15 For the avoidance of doubt, these preferred aspects of this dispersion apply also to the dispersion for use in the method of the present invention as defined above. Also described is a process of preparing this dispersion as defined above, the process comprising the following steps: (i) preparing an aqueous solution of a copper-containing agent or a suspension 20 of a copper-containing agent, and (ii) dispersing said solution or suspension in the polyethylene glycol 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 25 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. 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 30 using: (a) H2O; (b) a H2O / alcohol mixture, wherein preferably the alcohol is a Ci-salcohol such as methanol, ethanol or propanol, and more preferably methanol; or (c) a H2O / polyol mixture, wherein preferably the polyol is a Ci-sdiol or Ci-striol, and more preferably glycerol. 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 5 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 10 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. 15 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 Sg contents). 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 20 aromatic, paraffin, and naphthenic components. Possible features of an oil obtained or obtainable by a method as defined above are described below. Such oils preferably have a total Ss content of <0.5 mg / kg (more preferably <0.2 mg / kg), Such oils preferably have a content of mercaptans and disulfides of <2 mg / kg. 25 Such oils preferably have a dielectric dissipation factor at 90 °C of <0.005. Such oils preferably have an acid number of <0.02 mgKOH / g. Such oils preferably have an oil-water interfacial tension of >35 mN / m. Such oils preferably have 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 30 method. Such oils preferably have particles of size >4pm 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 >6pm in an amount of <320 particles / ml (typically the number of such particles is in the range of >20 to <40 particles / ml). Such oils preferably have particles of size >4pm in an amount of <320 particles / ml, and particles of size >6pm in an amount of <40 particles / ml. 5 Such oils preferably have: a sum of mercatpans, sulfides and disulfides of <2 mg / kg an Sg content of <0.5 mg / kg a dielectric dissipation factor at 90 °C of <0.005, an acid number of <0.02 mgKOH / g, 10 - an oil-water interfacial tension of >35 mN / m, an FT-IR spectrum within a wavelength range of 700 to 1300 cm' which is substantially the same as that of the oil prior to the desulphurisation method, and / or particles of size >4pm in an amount of <320 particles / ml, and particles of size >6pm in an amount of <40 particles / ml. 15 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 pm 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 pm to be >20 particles / ml and up to and including 40 particles / ml. 20 Particle size and count may be measured in accordance with IEC 60970:2007. DEFINITIONS 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 25 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 described herein - as indicated above, in this specific context the copper optionally may be in oxidised form.) Unless indicated otherwise, amounts given in terms of % refer to weight %. 30 EXAMPLES Example 1: Treatment of corrosive oil with a solid agent comprising copper and PEG as the liquid dispersing agent Step (1): preparation of dispersion containing copper particles 5 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 10 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. 15 Step (2): corrosive oil preparation To simulate a corrosive mineral insulating oil, 0.009 g Sg was added to 600g waste oil from a power transformer. 20 Step (3): desulphurisation of corrosive oil 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 25 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.%. 30 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. Table 1A: change in oil Ss content over time Reaction time, min Ss content, mg / kg oil 0 15.4 10 11.79 30 4.67 60 1.78 90 0.52 120 0 (n.d.) n.d. - not detected 5 As indicated in Table 1A and Figures 1 and 2, the Ss had been almost completely removed after 90 minutes. The Ss content was reduced still further after an additional 30 minutes, at which stage Ss was not detected. xt C\l Step (4): work up CO 10 Following completion of step (3), stirring and heating of the liquid-liquid-solid interphase CM 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 15 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. Table IB: change in oil characteristics pre- and post-treatment Characteristic Pre-treatment Post-treatment Ss content, mg / kg oil 15.4 0 (n.d.) Dielectric dissipation factor (tg 6), %o 11.4 2.9 Acid number (Nb), mgKOH / g 0.05 0.00 Oil-water interfacial tension (o), mN / m 25 38 Specific electrical resistance p, GQm 22 127.9 Content of DBPC (2,6-di-t-butylparacresol), % 0.25 0.21 Corrosion test on silver plate according to DIN 51353 Corrosive (see Fig. 4A) Non-corrosive (see Fig. 4B) Content of copper, mg / kg oil / <5 Content of zinc, mg / kg oil / <5 Content of iron, mg / kg oil / <20* n.d. - not detected * since the Zn reagent used in step (1) was not 100 % pure, Fe content was checked As indicated in Table IB and Figures 1 and 2, the treatment completely removed the Ss 5 from the oil. Furthermore, as indicated in Table IB and Figures 4A and 4B, the pretreatment oil was corrosive (Fig. 4A) whereas the post-treatment oil was non-corrosive (Fig. 4B). This illustrates the ability of the present invention to render non-corrosive mineral insulating oils for power transformers that were formerly corrosive. 10 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. Characteristic Measurement Content of the Ss, mg / kg oil IEC TR 62697-3 / 2018, Agilent 7890B with ECD detector Dielectric dissipation factor (tg 8), %0 IEC 60247:2008, Baur Oil Tester DPA 75C Acid number (Nb), mgKOH / g IEC 62021-2:2007-05, colorimetric titration Oil-water interfacial tension (g), mN / m ASTMD971-99a (2004), KrussKl 1 Specific electrical resistance p GQm IEC 60247:2008, Baur Oil Tester DPA 75C Content of the additive (DBPC), % IEC 60666:2011, FT-IR Nicolet iS 10 Corrosion test on silver plate according to DIN 51353 DIN 51353, Furnace Memert UF55 Content of the copper, mg / kg ICP-MS Content of the zinc, mg / kg ICP-MS Content of the iron, mg / kg ICP-MS xt CM CO CM CM Example 1A: Treatment of corrosive oil with copper sulphate solution and PEG as a liquid dispersing agent (Comparative Example) 5 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 IC below and also depicted in Figure 3. Table IC: change in oil Ss content over time Reaction time, min Ss content, mg / kg oil 0 17.9 90 14.7 As indicated in Table IC and Figure 3, a dispersion having only CuzSO^SFEO as its active ingredient is much less effective at removing Ss from mineral insulating oils. Example 2: Treatment of corrosive oil with a higher concentration of copper in the solid agent 5 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 I apart from the following: Step (1) 32.4 g Cu2SO4*5H2O, 32.4 g zinc granules, and 150 g water were used 10 which was dispersed in 180g of pure PEG-400, and the dispersion contained about 2.4 wt.% copper particles. Step (2) A new oil (Nynas 4000x) in which was added 0.009g of Ss was used instead of a waste oil. 15 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.%. 20 The results are set out in Table 2 below and in Figures 5A and 5B. Table 2: change in oil Ss content over time Reaction time, min Ss content, mg / kg oil 0 14.5 60 0 (n.d.) 90 0 (n.d.) n.d. - not detected 25 The data in Table 2 demonstrate that a greater rate of Ss content 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 Sg in the oil was reduced to non-detectable levels. Figures 5A and 5B show that the post-treated oil was not corrosive to copper or silver. Example 3: batch treatment Industrial scale batch treatment of corrosive oil containing a typical amount of Ss was 5 simulated on a laboratory scale using the same equipment as in Example 1. Step (1): preparation of dispersion containing copper particles 3.6 g copper (II) sulphate pentahydrate was added to a glass vial, then 15 g water was 10 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 15 obtain the dispersion containing about 0.46wt.% copper particles. CM Step (2): corrosive oil preparation 15 batches of oil containing approximately 15.4 mg Ss / kg oil were prepared in the same 20 way as in step (2) in Example 1. Step (3): desulphurisation of corrosive oil The corrosive oil obtained from step (2) was preheated to 95 °C and added to the first 25 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.%. 30 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. After the treatment of the fifth batch, the dispersion was replenished with copper particles 5 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. After the treatment of the tenth batch, the dispersion was replenished with copper particles 10 (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. Hence, a total of 9kg oil (600g per batch for 15 batches) was processed using only 180g 15 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. The results are set out in Table 3. 20 Table 3: post-treatment Ss content in each batch of oil Batch Ss content, mg / kg oil 1 1.85 2 0.68 3 0 (n.d.) 4 0.77 5 0 (n.d.) 6 0 (n.d.) 7 0 (n.d.) 8 0.77 9 0 (n.d.) 10 0 (n.d.) 11 0 (n.d.) 12 0 (n.d.) 13 0 (n.d.) 14 0 (n.d.) 15 0 (n.d.) n.d. - not detected 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 5 appropriate replenishing steps are taken. The possibility of recycling the dispersion has self-evident benefits in terms of environmental impact and efficiency / costs. Example 4: Treatment on larger scale ^r CM 10 The following Example was conducted to investigate the efficacy of the invention on a 00 greater-than-laboratory scale. C\J 31.4g 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. 15 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 Ss / kg oil that had been preheated to 75 20 °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.%. 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. 5 Table 4: change in oil Ss content over time Reaction time, min Ss content, mg / kg oil 0 12.7 10 6.7 30 4.6 60 1.3 90 0 (n.d.) 120 0 (n.d.) n.d. - not detected As indicated in Table 4, the Ss had been completely removed (to a non-detectable level) CM after only 90 minutes. This confirmed the ability of the present method to remove 00 10 corrosiveness of mineral insulating oils on an industrial scale. CM Step (4): workup CM After 120 minutes, stirring and heating of the liquid-liquid-solid interphase oil-and- 15 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. 20 Example 5: Treatment of corrosive oil with a solid agent comprising copper and PEG as the liquid dispersing agent without water (Reference Example) Step (1): preparation of dispersion containing copper particles 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, 5 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. Step (2): corrosive oil preparation 0 A corrosive oil was prepared at same way in step (2) as in Example 1. Step (3): desulphurisation of corrosive oil xt CM CO CM CM 15 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 20 0.019 wt.%. 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 5 25 Table 5: change in oil Ss content over time Reaction time, min Ss content, mg / kg oil 0 15.4 120 1.08 As indicated in Table 5 a dispersion without added water is effective at removing Sx from mineral insulating oils. However, a comparison with the data in Table 1A above shows that 30 including water in the dispersion leads to even more efficient desulphurisation. Step (4): work up Following completion of step (3), stirring and heating of the liquid-liquid-solid interphase 5 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. 10 Example 6: Treatment in a larger scale in the absent of water (Reference Example) The following Example was conducted to investigate the efficacy of the invention on a greater-than-laboratory scale in the absence of water. 15 31.4g 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. 20 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 Sx / 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 stirred with an overhead stirrer set to a stirring speed of approximately 750 RPM. The reaction duration was 120 minutes. 25 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.%. Sample was collected at the end of treatment and measured using a GC chromatogram (Agilent Technologies 7890B with an ECD detector) following the guidelines of the IEC 30 TR 62697-3 / 2018 standard. The results are set out in Table 5 below. Table 6: change in oil &content over time Reaction time, min Ss content, mg / kg oil 0 12.0 120 6.77 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. 5 Step (4): work up 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 10 minutes, the oil was drained from a side outlet of the vessel. xt CM CO CM CM

Claims

1. A method for reducing the amount of Ss in an oil which contains Ss, wherein the oil is an insulating oil for use in a transformer, the method comprising contacting the oil5 with (a) a solid agent comprising copper, and (b) a liquid agent suitable for dispersing said solid agent, wherein the liquid agent is polyethylene glycol, the solid agent is dispersed within the liquid agent, and the dispersion further comprises water.10 2. The method of claim 1, wherein the solid agent comprises solid carrier particleshaving the copper on the surface thereof.

3. The method of claim 2, wherein the solid carrier particles are in the form of granules or powder.

154. The method of claim 2 or 3, 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.20 5. The method of any preceding claim, wherein the amount of the solid agent relative tothe amount of the oil is from 0.05 to 10 wt.%.

6. The method of claim 5, wherein the amount of dispersion relative to the total amount of oil plus dispersion is from 1.5 to 40 wt.%.

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

8. The method of any preceding claim, wherein the oil, solid agent and liquid agent are 30 heated to a temperature of 50 to 120°C, preferably 60 to 105 °C, more preferably 70 to 99 °C.

9. The method of any preceding claim, wherein the content of Sx 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.5 10. A process comprising:(a) reducing the amount of Ss in an oil by a method as defined in any one of claims 1 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 agent10 and the 1 i qui d agent.

11. A process comprising:(a) reducing the amount of Sx in an oil by a method as defined in any one of claims 1 to 9;15 (b) subsequently separating the oil from the other components, wherein said othercomponents 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 Ss in an oil which contains Ss by a method which is independently as defined in any one of claims20 1 to 9, wherein the oil is an insulating oil for use in a transformer.

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