Method of removing sulphur from crude methanol

Abstract

The present invention relates to a method of removing sulphur from crude methanol the method comprising the steps in temporal order of: distilling the crude methanol to remove at least a portion of one or more sulphurous compounds from the crude methanol and provide a methanol-rich stream, extracting at least a further portion of one or more sulphurous compounds from the methanol-rich stream thereby providing a second methanol-rich stream, contacting the second methanol-rich stream with an oxidant, whereby at least a part of the sulphur compounds in the stream is oxidized, and contacting the second methanol-rich stream with an acidifying agent, whereby ammonia present in the stream reacts to form ammonium sulphate in an acidified second methanol-rich stream.

Description

METHOD OF REMOVING SULPHUR FROM CRUDE METHANOLTECHNICAL FIELD

[0001] The present invention relates to a method of removing sulphur from crude methanol. The present invention further relates to a system for removing sulphur from crude methanol. Still further the present invention relates to a use of the system for carrying out a method of removing sulphur from crude methanol.BACKGROUND

[0002] In a pulp mill methanol is mainly formed during sulphate cooking by alkali-catalysed elimination of methanol from 4-O-methylglucuronic acid residues in hemicellulose. Methanol is also formed by oxidation reactions in phenolic lignin during Ch-delignification. Methanol yield is around 6- 15 kg / ADT depending on the wood type and pulping process. Cooking also produces organic sulphur compounds, as the sulphide- and hydrogen sulphide ions react with lignin. Methanol and other volatile compounds formed during cooking end up in black liquor, a by-product of the cooking process, which comprises an aqueous solution of lignin residues, hemicellulose and inorganic chemicals as well as organic and inorganic solids. Due to methanol’s high volatility, it vaporises with water and other volatiles and condenses in the evaporation plant’s foul liquor.

[0003] Pulp mill produces foul liquor as a result of black liquor evaporation. This foul liquor is stripped, in order to generate better quality stripped liquor having a total reduced sulphur (TRS) content of < 5 mg / L As a result of the stripping, methanol from the foul liquor ends up in stripper off-gases, which are taken to the methanol liquefaction plant. In the methanol liquefaction plant, methanol concentration is increased to 60 - 80%.

[0004] In addition to methanol, foul liquor contains volatile sulphur compounds such as hydrogen sulphide (H2S), methyl mercaptan (MM), dimethyl sulphide (DMS) and dimethyl disulphide (DMDS). A portion of the volatile sulphur compounds are found in the methanol, which is called crude methanol, after stripping while a larger portion is found in concentrated non-condensable gases after stripping. The foul liquor still further comprises less-volatile sulphur compounds, which remain with methanol when treated in the stripper. The amount of less-volatile sulphur compounds in the foul liquor is a fraction of that of volatile sulphur compounds. The recovered methanol concentration is fairly high (>74 w-%, by weight of crude methanol), but as mentioned, it contains sulphur compounds normally over 5 w-%, by weight of crude methanol. Water, ethanol, acetone, ammonia (NH3) and turpentine compounds are the other main impurities. Methods have been developed to remove nitrogen and sulphur and provide purified methanol. Using methods of the prior art involving acidification of a liquor followed by oxidation of the liquor, however, results in the consumption of large amounts of oxidant and in the formation of polymerised terpene compounds, sticky stuff, which foul, e.g., distillation columns resulting in plant shutdowns for regular periodic cleaning.SUMMARY

[0005] The invention is defined by the independent claims. Some specific embodiments are defined in the dependent claims.

[0006] According to a first aspect of the present invention there is provide a method removing sulphur from crude methanol, the method comprising the steps in temporal order of distilling the crude methanol to remove at least a portion of one or more sulphurous compounds from the crude methanol and provide a methanol-rich stream, extracting at least a further portion of one or more sulphurous compounds from the methanol-rich stream thereby providing a second methanol-rich stream, contacting the second methanol-rich stream with an oxidant, whereby at least a part of the sulphur compounds in the stream is oxidized, and contacting the oxidised second methanol-rich stream with an acidifying agent, whereby ammonia present in the stream reacts to form ammonium sulphate in a third methanolrich stream.

[0007] According to a second aspect of the present invention there is provided a system for removing sulphur from crude methanol. The system comprises a distillation column, an extraction unit, an oxidation unit, and an acidification unit. The distillation column has an inlet for receiving crude methanol, an outlet for a distillate, and an outlet for a methanol-rich stream. The extraction unit has an inlet for a methanol-rich stream, an outlet for an extracted second methanol-rich stream, an inlet for extraction media, and an outlet for spent extraction media. The oxidation unit has a first inlet for receiving a second methanol-rich stream, a second inlet for receiving an oxidant, and an outlet for feeding the contents of the oxidation unit to the acidification unit. The acidification unit has an inlet for receiving a feed of an oxidised second methanol-rich stream from the outlet of the oxidation unit, a second inlet for receiving an acidifying agent, and an outlet for an acidified third methanol-rich stream essentially free of polymerised terpenes.

[0008] According to a third aspect of the present invention there is provided a use of a system according to the second aspect for carrying out a method according to the first aspect.

[0009] Considerable benefits are gained with the aid of the present invention. A simple, easy to apply inexpensive treatment, industrially applicable under moderate conditions provides methanol with a reduced sulphur content that is essentially free of polymerised terpenes, which foul mill equipment, such as distillation columns. Extraction of sulphur compounds prior to oxidation reduces the charge of oxidant that is required.BRIEF DESCRIPTION OF THE DRAWINGS

[0010] FIGURE 1 is a flow chart illustrating removing sulphur from crude methanol in accordance with some embodiments.

[0011] FIGURE 2 is a flow chart illustrating removing sulphur from crude methanol in accordance with some embodiments.

[0012] FIGURE 3 is a flow chart illustrating removing sulphur from crude methanol in accordance with some embodiments.

[0013] FIGURE 4 is a flow chart illustrating removing sulphur from crude methanol in accordance with some embodiments.EMBODIMENTS

[0014] For the purposes of embodiments of the present invention, crude methanol may be taken to mean any alkaline liquor from a pulp mill comprising methanol for example liquified stripper off gases (gases from foul liquor stripping), liquefied methanol, crude methanol from a methanol plant, foul liquor, or any type of liquor or condensate, such as foul condensate or condensate formed from stripper off gases etc. Indeed, embodiments of the method are not limited to liquors from pulp mills but include any alkaline liquid comprising methanol and one or more components selected from the group consisting of sulphurous organic compounds, ammonia, acetone, alcohols such as ethanol, extractives such as turpentine and other non-structural components of lignocellulosic material.

[0015] In embodiments, streams are described as being essentially free of polymerised terpenes. For the purposes of embodiments of the present invention, “essentially free” means that any presence of polymerised terpenes is so low that fouling of equipment is at such a minimal level that no maintenance beyond that which is normal is required.DETAILED DESCRIPTION

[0016] The present invention relates to a method removing sulphur from crude methanol, the method comprising the steps in temporal order of distilling the crude methanol to remove at least a portion of one or more sulphurous compounds from the crude methanol and provide a methanol-rich stream, extracting at least a further portion of one or more sulphurous compounds from the methanolrich stream thereby providing a second methanol-rich stream, contacting the second methanol-rich stream with an oxidant, whereby at least a part of the sulphur compounds in the stream is oxidized, and contacting the second methanol-rich stream with an acidifying agent, whereby ammonia present in the stream reacts to form ammonium sulphate in an acidified second methanol-rich stream.

[0017] FIGURE 1 illustrates a method and system of removing sulphur from crude methanol in accordance with at least some embodiments of the present invention.

[0018] Crude methanol 10 is optionally diluted with water and fed into a decanting unit 200. Turpentine 20 is decanted from the decanting unit and directed to further processing. The crude methanol with reduced turpentine content 300 is then fed from the decanting unit into a distillation column 100 having an outlet for sulphurous compounds 30 and an outlet for a first methanol-rich stream 310. The first methanol-rich stream 310 is fed from the distillation column 100 into an extraction unit 201 whereit is contacted with an extracting agent 210. Spent extracting agent 220 is recovered and regenerated and a second methanol-rich stream 320 is fed from the extraction unit 201 into an oxidation unit 101 where it is contacted with an oxidant 40 providing an oxidised stream 330 which is fed from the oxidation unit 101 into an acidification unit 103. The oxidised stream 330 is then contacted with an acidifying agent 60 and an acidified second methanol-rich stream 340 is led from the acidification unit 103 to further processing. The acidified second methanol-rich stream 340 is essentially free of polymerised terpenes.

[0019] FIGURE 2 is a flow diagram illustrating a method and system of removing sulphur from crude methanol in accordance with at least some embodiments of the present invention.

[0020] Crude methanol 10 is optionally diluted with water and fed into a decanting unit 200. Turpentine 20 is decanted from the decanting unit 200 to provide a crude methanol with reduced turpentine content 300. The crude methanol with reduced turpentine content 300 is then fed from the decanting unit into and distilled in a distillation column 100 having an outlet for sulphurous compounds 30 and an outlet for a methanol-rich stream 310. The methanol-rich stream 310 is fed from the distillation column 100 into an extraction unit 201 where it is contacted with an extracting agent 210. Spent extracting agent 220 is recovered and regenerated and a second methanol-rich stream 320 is fed from the extraction unit 201 into an oxidation unit 101 where it is contacted with an oxidant 40. One stream 50 of oxidation products is led to further processes, and a second oxidised stream 350 is led from the oxidation unit 101 to an acidification unit 103 where it is contacted with an acidifying agent 60, where by ammonia present in the second oxidised stream 350 reacts to form ammonium sulphate. An acidified second methanol-rich stream 360 is then led from the acidification unit 103 to a second distillation column 105, where the acidified second methanol-rich stream 340 is distilled to provide a distilled methanol rich stream 370 and stream comprising ammonium sulphate 70.

[0021] FIGURE 3 is a flow diagram illustrating a method and system of removing sulphur from crude methanol in accordance with at least some embodiments of the present invention.

[0022] Crude methanol 10 is optionally diluted with water and fed into a decanting unit 200. Turpentine 20 is decanted from the decanting unit 200 to provide a crude methanol with reduced turpentine content 300. The crude methanol with reduced turpentine content 300 is then fed from the decanting unit into and distilled in a distillation column 100 having an outlet for sulphurous compounds 30 and an outlet for a methanol-rich stream 310. The methanol-rich stream 310 is fed from the distillation column 100 into an extraction unit 201 where it is contacted with an extracting agent 210. Spent extracting agent 220 is recovered and regenerated and a second methanol-rich stream 320 is fed from the extraction unit 201 into an oxidation and acidification unit 104 where it is first contacted with an oxidant 40, whereby oxidation products are formed, and second contacted with an acidifying agent 60, whereby ammonium sulphate is formed, thereby providing an acidified methanol rich stream 340 comprising ammonium sulphate and oxidation products.

[0023] FIGURE 4 is a flow diagram illustrating a method and system of removing sulphur from crude methanol in accordance with at least some embodiments of the present invention.

[0024] Crude methanol 10 is optionally diluted with water and fed into and distilled in a distillation column 100 having an outlet for sulphurous compounds 30 and an outlet for a methanol-rich stream 380. The methanol-rich stream 380 is fed from the distillation column 100 into an extraction unit 201 where it is contacted with an extracting agent 210. Spent extracting agent 220 is recovered and regenerated and a second methanol-rich stream 390 is fed from the extraction unit 201 into an oxidation unit 101, where it is contacted with an oxidant 40. An oxidised stream 400 is led from the oxidation unit 101 to an acidification unit 103 where it is contacted with an acidifying agent 60, whereby an acidified methanolrich stream 410 is formed and directed to further processing. The acidified methanol-rich stream 410 is essentially free of polymerised terpenes.

[0025] FIGURE 5 is a flow diagram illustrating a method and system of removing sulphur from crude methanol in accordance with at least some embodiments of the present invention.

[0026] Crude methanol 10 is optionally diluted with water and fed into a decanting unit 200. Turpentine 20 is decanted from the decanting unit 200 to provide a crude methanol with reduced turpentine content 300. The crude methanol with reduced turpentine content 300 is then fed from the decanting unit into and distilled in a distillation column 100 having an outlet for sulphurous compounds 30 and an outlet for a methanol-rich stream 310. The methanol-rich stream 310 is fed from the distillation column 100 into an extraction unit 201 where it is contacted with an extracting agent 210. Spent extracting agent 220 is recovered and regenerated and a second methanol-rich stream 320 is fed from the extraction unit 201 into an oxidation unit 101 where it is contacted with an oxidant 40. One stream 50 of oxidation products is led to further processes, and an oxidised stream 350 is led from the oxidation unit 101 to an acidification unit 103 where it is contacted with an acidifying agent 60, where by ammonia present in the second oxidised stream 350 reacts to form ammonium sulphate. An acidified second methanol-rich stream 360 is then led from the acidification unit to a mechanical separation unit 106 where oil 80 and optionally turpentine are separated from the stream 360 to provide a mechanically treated methanol stream 420 which is directed to second distillation column 105, where the acidified second methanol-rich stream 340 is distilled to provide a distilled methanol rich stream 430 and stream comprising ammonium sulphate 70.

[0027] As described above, the present technology relates to a method of removing sulphur from crude methanol. The method comprises the steps in temporal order of distilling the crude methanol to remove at least a portion of one or more sulphurous compounds from the crude methanol and provide a methanol-rich stream, extracting at least a further portion of one or more sulphurous compounds from the methanol-rich stream thereby providing a second methanol-rich stream, contacting the second methanol-rich stream with an oxidant, whereby at least a part of the sulphur compounds in the stream is oxidized, and contacting the oxidised second methanol-rich stream with an acidifying agent, whereby ammonia present in the stream reacts to form ammonium sulphate in an acidified second methanol-richstream. The acidified second methanol-rich stream is essentially free of polymerised terpenes. It has surprisingly been found that by distilling the crude methanol and extracting sulphur compounds before carrying out an oxidation step the amount of oxidant needed to oxidise sulphurous compounds is reduced by almost half. Oxidising the second methanol-rich stream before an acidification step provides an additional benefit that terpenes remaining in the second methanol-rich stream are far less likely to polymerise forming a sticky stuff comprising polymerised terpenes. It has been found that the amount of polymerised terpenes in a liquor in which an acidification step is carried out before an oxidation step is at such a level that fouling of mill equipment, for example distillation columns, demands regular and periodic cleaning.

[0028] As mentioned above the crude methanol may contain hydrogen sulphide and sulphurous organic compounds. Such compounds include methyl mercaptan (MM) and dimethyl sulphide (DMS). In the distilling step the sulphurous compound content of the crude methanol, for example in an embodiment at least a portion of the H2S is removed from the crude methanol. In another embodiment at least a portion of the MM is removed. In a further embodiment at least a portion of DMS is removed. In one embodiment at least a portion of DMDS is removed. Removal of at least one or more sulphurous compounds from the crude methanol has been found to reduce the oxidant charge required in the oxidation step by up to 50%, typically by 20 to 50 %. In an embodiment distilling removes at least a portion of so called light sulphurous compounds such as hydrogen sulphide, MM, and DMS, and at least a portion of DMDS is also removed. After the distilling, the first methanol-rich stream is subjected to extraction during which at least a portion of heavier sulphur compounds, such as DMDS, are removed thereby providing a second methanol-rich stream. The second methanol-rich stream is provided by contacting the first methanol-rich stream with an extracting agent. The extracting agent, befouled with said at least a portion of heavier sulphur compounds, is regenerated in a steam-stripping process. The second methanol-rich stream is then contacted with an oxidant in the oxidation step. In the oxidation step remaining sulphurous compounds or at least a portion thereof are oxidised for example methyl mercaptan (MM) is oxidised to dimethyl disulphide (DMDS), which is in turn oxidised to dimethyl sulphoxide, hydrogen sulphide is oxidised to sulphur dioxide, dimethyl disulphide is oxidised to dimethyl sulphoxide and dimethyl sulphoxide is in turn oxidised to dimethyl sulphone. Another possible oxidation product of MM and DMDS is methane sulfonic acid. It is speculated that the generation of sulphate ions in the oxidation phase reduces the amount of acidifying agent needed in the acidification phase for the protonation of ammonia (NH3). In the acidification stage ammonia reacts to form ammonium sulphate. Optionally, water is added to maintain the concentration of ammonium sulphate below the concentration at which it precipitates.

[0029] In an embodiment extracting is carried out by contacting the first methanol-rich stream with an extracting agent. In an embodiment the extracting agent is a paraffinic oil. In one embodiment the extracting agent comprises a non-polar organic solvent having a boiling point higher than 100 °C, preferably higher 150 °C, 200 °C, 250 °C or 300 °C. In an embodiment the extracting agent comprisesa solvent comprising at least 60 wt.%, preferably at least 70, 75, 80, 85, or 90 wt % alkanes and or cycloalkanes having a carbon number of 14 or higher. In a further embodiment the extracting agent comprises less than 2.5 wt% polycyclic aromatic hydrocarbons. In one embodiment the extracting agent has a viscosity of not more than 100 cSt, preferably not more than 75cSt, 50cSt or 25 cSt. In an embodiment the extracting agent comprises a mixture of hydrocarbons, e.g. mineral oil, white oil, and paraffin oil. In a further embodiment the extracting agent has a density at 25 °C in the range of 0.80 to 1.20 g / cm3, preferably 0.82 to 0.91 g / cm3, particularly 0.83 to 0.90 g / cm3, suitably 0.87 to 0.88 g / cm3.

[0030] In an embodiment ammonium sulphate is removed from the methanol-rich stream, whereby ammonium sulphate is recovered and a liquid comprising methanol is recovered. Recovered ammonium sulphate has a plethora of uses, for example the ammonium sulphate can be used as a fertilizer, a food additive, a flame retardant, an ingredient in vaccines, a cleaning additive, a pH buffer, a binding material in wall board, a dough conditioner in bread products. There are many other uses of ammonium sulphate for which the recovered ammonium sulphate would be suitable. The recovered liquid comprising methanol can be used in a number of applications. In a pulp mill the recovered liquid comprising methanol can be used as fuel in a lime kiln, as an auxiliary fuel in burners or in a recovery boiler. Using the recovered liquid comprising methanol as a fuel in a recovery boiler helps reduce sulphur and NOx emissions of the recovery boiler, which becomes a more and more desirable aim as environmental legislation becomes more and more stringent. The recovered liquid comprising methanol is also suitable for use, e.g., as a fuel outside the confines of a pulp mill. Optionally, the recovered liquid comprising methanol can be processed further, for example distilled, to remove alcohols such as ethanol, and acetone from the methanol and provide a purified methanol stream which is suitable for converting into various industrially-useful chemicals as well as being ideal for fuel.

[0031] In one embodiment the crude methanol starting material is diluted with water. In an embodiment the water: crude methanol ratio is in the range of 1.0 - 1.5:1, by weight of the crude methanol, preferably 1.3: 1. In a further embodiment turpentine is decanted from the crude methanol before the distilling step. The further step of decanting turpentine from the crude methanol provides the additional benefit of removing at least a portion of the terpenes and at least a portion of heavier sulphur compounds from the liquor. As a natural consequence, reactions between sulphur compounds and oxidants are reduced whereby the amount of oxidant added to the liquor for the oxidation of sulphurous compounds may be reduced. The step of diluting the liquor with water before distilling facilitates and improves the decantation of turpentine in an embodiment.

[0032] Acidification of the second methanol-rich stream lowers its pH. In one embodiment the pH of the oxidised liquor is adjusted to a pH in the range of 5.0 - 6.0. Reducing the pH in the acidification step to a pH in the range of 5.0 - 6.0, suitably 5.1, 5.2, 5.3, 5.4, 5.5, 5.6, 5.7, 5.8 or 5.9 increases the yield of ammonium sulphate, whereby the amount of sulphate ions in solution is increased. Traditionally, pH has been reduced to a much lower level of about 2.0. Due to the distillation, the extraction and the oxidation prior to the acidification, a large portion of the sulphurous compounds areno longer present in the methanol-rich stream and thus the amount of sulphurous compounds to be acidified are no longer present and less acid is required.

[0033] In a further embodiment, the acidified second methanol-rich stream is mechanically treated by filtering and / or coalescing. Filtering removes oil droplets and coalescing in a coalescer collects the small droplets into larger droplets that are then separated from the acidified second methanol-rich stream by gravity. Removing the oil drops at this stage facilitates downstream further processing. Thus, in an embodiment the method comprises the further step of filtering and / or coalescing the acidified second methanol-rich stream before further processing.

[0034] In one embodiment, the oxidant is selected from the group consisting of peroxide, typically hydrogen peroxide, oxygen, ozone, sodium hypochlorite, and a mixture thereof. In a preferred embodiment the oxidant is in liquid form, particularly suitable is hydrogen peroxide in liquid form. Oxidants in liquid form, such as hydrogen peroxide in liquid form are easier to handle than gaseous oxidants. In one embodiment the oxidant is added in a dose in the range of 4 to 8 % by weight of the crude methanol, typically 4.5 to 7.5 %, for example about 5 %, 6 %, or about 7 %. The charge of oxidant depends on the sulphur content of the feed to the oxidation unit. The amount of oxidant required is proportional to the amount of sulphur. In other words, if the crude methanol comes from a source with a high sulphur content, even after a distillation step the sulphur content may be higher than in crude methanol from a low sulphur source, e.g. sulphur in crude methanol from a eucalyptus pulp mill is higher than sulphur in crude methanol from a birch pulp mill. The charge of oxidant will be adjusted accordingly.

[0035] In a further embodiment the acidifying agent is selected from the group consisting of sulphuric acid, hydrochloric acid, nitric acid, phosphoric acid, ethanoic acid, methanoic acid, carbon dioxide, sulphur dioxide, waste liquid from carbon dioxide production, acid bleach plant waste liquor, sodium sulphate bleaching filtrates such as A-stage bleaching, and a mixture thereof. In an embodiment, sulphuric acid is favoured due to its wide availability in pulp mills and can be produced from concentrated non-condensable gases (CNCG). The readily available sulphuric acid or that which can be produced from CNCGs has the further advantage of having fewer impurities than some of the other mentioned acidifying agents, i.e., filtrates.

[0036] In a still further embodiment the method is carried out at a temperature in the range of 35 °C - 60 °C, preferably 40 °C to 55 °C, suitably at a temperature of 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53 or 54 °C. Typically oxidation and acidification take place at a temperature in the range of 50 °C to 60 °C. The methanol, forming the most valuable part of the liquor has a boiling point of 64 °C. It is preferred that the methanol remains in the liquid form during treatment of the liquor and that none is lost in the gaseous form. Distilling in this temperature range allows for the removal of sulphurous compounds having boiling points around this temperature, e.g. DMS has a boiling point in the range of 35 to 41 °C.

[0037] In an embodiment the method is carried out at ambient pressure, meaning that in an embodiment the pressure is not adjusted, in other words no pressure adjustment step is carried out.

[0038] Embodiments may be carried out in one or more vessels. In one embodiment the oxidising step and the acidifying step are carried out in the same vessel. Carrying out both oxidation and acidification in the same vessel improves the economy of the process in terms of both equipment costs and in terms of energy, for example, heat / vapour generated in the exothermic oxidation and acidification steps can be used to strip away the methanol in the same vessel. In a further embodiment the oxidising step and the acidifying step are carried out in different vessels. Carrying out the oxidation and acidification steps in different vessels provides greater control over the process since both steps are exothermic.

[0039] In embodiments in which the oxidising step and acidifying step are carried out in different vessels, it is necessary to transfer the oxidised second methanol-rich stream from a first vessel to a second vessel. Thus, in an embodiment the oxidised second methanol-rich stream is transferred from a first vessel to a second vessel.

[0040] As described above, in one embodiment the methanol-rich stream is directed to further processing. In one embodiment further processing comprises distilling the acidified third methanol-rich stream to provide a methanol-rich distillate and a residue comprising ammonium sulphate and oxidation reaction products. Typically, the distillation is carried out at a temperature in the range of 50 to 70 °C, whereby the temperature at the condenser is typically in the range of 64 to 68 °C, most usually in the range of 65 to 67 °C. In an embodiment the ammonium sulphate is separated from the oxidation reaction products. In a further embodiment the ammonium sulphate is recovered for further processing.

[0041] In addition to a method, the present technology also relates to a system of treating crude methanol 10. In an embodiment the system comprises a distillation column 100, an extraction unit 201, an oxidation unit 101 and an acidification unit 103, wherein the distillation column 100 has an inlet for receiving crude methanol 10, an outlet for a distillate 30, and an outlet for a methanol-rich stream 310, 380. The extraction unit 201 has an inlet for a methanol-rich stream 310,380, an outlet for an extracted methanol-rich stream 320,390, an inlet for extraction media 210, and an outlet for spent extraction media 220. The oxidation unit 101 has a first inlet for receiving an extracted methanol-rich stream 320,390, a second inlet for receiving an oxidant 40, and an outlet for feeding the contents of the oxidation unit 330, 350, 400 to the acidification unit 103. The acidification unit 103 has an inlet for receiving a feed of a methanol-rich stream 330, 350, 400 from the outlet of the oxidation unit 101 , a second inlet for receiving an acidifying agent 60, and an outlet for an acidified methanol-rich stream 340, 360, 410. The acidified methanol-rich stream is essentially free of polymerised terpenes. As is familiar to those of skill in the art, the oxidation unit 101, the acidification unit 103 and the extraction unit 201 may all comprise an agitator for mixing the contents of the said unit. Agitators may be selected from those known in the art for example suitable agitators include propellers, blades, anchors, turbines, paddles and helical agitators.

[0042] In a further embodiment, the system further comprises a first decanting unit 200 for receiving crude methanol 10, optionally diluted with water, said decanting unit being for decanting turpentine 20 from the crude methanol 10 before feeding a crude methanol stream with reduced turpentine content 300 from an outlet of the decanting unit 200 to the inlet of the distillation column 100.

[0043] In one embodiment, the oxidation unit 101 comprises a second outlet for removing oxidation products 50 from the oxidation unit 101.

[0044] Further embodiments according to the present technology describe a system in which oxidation and acidification can be carried out in a single vessel. Thus, in an embodiment, the system of treating crude methanol 10 comprises a decanter 200, a distillation column 100, an extraction unit 201, and an oxidation and acidification unit 104. The decanter 200 has a first inlet configured to receive the crude methanol 10, a first outlet for decanting turpentine 20, and a second outlet configured to feed a crude methanol with reduced turpentine content 300 to distillation column 100. The distillation column 100 has an inlet for receiving crude methanol with a reduced turpentine content 300, an outlet for a distillate 30, and an outlet for a methanol-rich stream 310, 380. The extraction unit 201 has an inlet for a methanol-rich stream 310, 380 an outlet for an extracted methanol-rich stream 320, 390, an inlet for extraction media 210, and an outlet for spent extraction media 220. The oxidation and acidification unit 104 has a first inlet for receiving a feed of an extracted methanol-rich stream 320, 390 from the extraction unit 201 , a second inlet for receiving an oxidant 40 a third inlet for receiving an acidifying agent 60 and an outlet for delivering an acidified methanol-rich stream 410 to further processing.

[0045] In a further embodiment the oxidation and acidification unit 104 is configured to receive the oxidant 40 via the second inlet before it receives the acidifying agent 60 through the third inlet. In a further embodiment the oxidation and acidification unit 104 is configured so that the third inlet is closed when the second inlet is open to receive oxidant 40. In a still further embodiment the oxidation and acidification unit 104 is configured so that the second inlet is closed when the third inlet is open to receive acidifying agent 60. As is familiar to those of skill in the art, the oxidation unit and acidification unit 104 may comprise one or more agitators for mixing the contents of the said unit. Agitators may be selected from those known in the art, for example suitable agitators include propellers, blades, anchors, turbines, paddles, and helical agitators.

[0046] Also envisaged are embodiments in which any of the above-described embodiments of systems are used to carry out any of the above-described embodiments of

[0047] The following examples illustrate at least some embodiments of the present technology.EXAMPLESExample 1A crude methanol oxidation treatment process was simulated in laboratory experiments. Water was added to crude methanol in the ratio of 1.3 (1.3 water: 1 crude methanol by weight) and the formed turpentine layer was decanted from the top of the water phase. Hydrogen peroxide was added to the decanted methanol solution. Oxidation was performed for 1 hour at 50 °C with 9.8 w / w% H2O2 dosage by weight of crude methanol. The oxidized methanol solution was acidified with H2SO4 to adjust the pH to approximately 2.2. The acidified methanol solution was evaporated until temperature 95 °C was reached. The distillation residue left in the flask was filtered. As on outcome the evaporation flask was clear, and no solids was observed in the filter paper or gravimetrically measured.Example 2To find out the effectiveness of distillation as a pretreatment, a distillation step was added prior to oxidation and lower oxygen charge was used. Water was added to crude methanol in the ratio of 1.3 (1.3 water: 1 crude methanol by weight) and the formed turpentine layer was decanted from the top of the water phase. The decanted stream was distilled at approximately 40°C to remove light sulphuric compounds. The distilled methanol stream was directed to oxidation process, where the methanol-rich stream was oxidized with hydrogen peroxide for one hour at 50°C with 5.1 w / w% H2O2 dosage by weight of predistilled methanol-rich stream. The oxidated stream was acidified with H2SO4 to adjust the pH to approximately to 2.2. The acidified methanol solution was evaporated until temperature 95 °C was reached. The distillation residue left in the flask was filtered. As an outcome, very little solids were observed in the filter paper.Example 3An extraction prior to oxidation eliminates solids formation and decreases the concentration of sulphuric compounds, and consecutively, decreases the consumption of chemicals. The procedure described in Example 2 was repeated with additional extraction step after predistillation and prior to oxidation. The distillation was performed at approx. 40°C to distill light sulphuric compounds from the methanol-rich stream. The distilled stream was treated with extraction step, where paraffinic oil was utilized to extract sulphuric compounds from the methanol-rich stream, which was further directed to oxidation. The methanol-rich stream was oxidized with hydrogen peroxide for one hour at 50°C with 5.8 w / w% H2O2 dosage by weight of predistilled methanol-rich stream. The oxidated stream was acidified with H2SO4 and evaporated. As an outcome in comparison to Example 1 no solids were observed in the filter paper, residual peroxide was higher, and the consumption was lowered as there was less sulphur in the feed due to extraction.Table 1: Treatment of crude methanol with and without distillation and extraction.INDUSTRIAL APPLICABILITY

[0048] Embodiments of the method find various uses in industry for example in pulp mills the embodiments may be applied to foul condensate, crude methanol or SOGs to oxidise sulphurous compounds, to remove turpentine and ammonia, e.g. by forming ammonium sulphate from unreacted ammonia. Embodiments allow the charge of oxidant to be reduced significantly while the formation of polymerised terpenes is greatly reduced or even eliminated. The oxidised compounds and ammonium sulphate are easily removable from the treated liquor for further processing, providing a methanol stream comprising further alcohols and acetone and being essentially free of polymerised terpenes which are known to foul columns and reactors.REFERENCE NUMBERS

Claims

CLAIMS1. A method of removing sulphur from crude methanol (10), the method comprising the steps in temporal order of:• distilling the crude methanol (10) to remove at least a portion of one or more sulphurous compounds (30) from the crude methanol (10) and provide a methanolrich stream (310, 380),• extracting at least a further portion of one or more sulphurous compounds from the methanol- rich stream (310, 380) thereby providing a second methanol-rich stream (320, 390)• contacting the second methanol-rich stream (320,390) with an oxidant (40), whereby at least a part of the sulphur compounds in the stream is oxidized, and• contacting the second methanol-rich stream (330, 350, 400) with an acidifying agent (70), whereby ammonia present in the stream reacts to form ammonium sulphate in an acidified second methanol-rich stream (340, 360, 410).

2. The method according to claim 1, comprising the further step of directing the second acidified methanol-rich (340, 360, 410) stream to further processing.

3. The method according to claim 1 or 2, comprising the further step of diluting the crude methanol (10) with water before distilling.

4. The method according to claim 1 to 3, comprising the further step of decanting turpentine (20) from the crude methanol (10) before distilling the crude methanol (10).

5. The method according to any of the preceding claims, comprising adjusting the pH of the oxidised residue to a pH in the range of 5.0 - 6.0, suitably 5.1, 5.2, 5.3, 5.4, 5.5, 5.6, 5.7, 5.8 or 5.9.

6. The method according to any of the preceding claims, wherein the oxidant (40) is selected from the group consisting of hydrogen peroxide, peroxide, oxygen, ozone, sodium hypochlorite, and a mixture thereof.

7. The method according to any of the preceding claims, wherein the acidifying agent (70) is selected from the group consisting of sulphuric acid, hydrochloric acid, nitric acid, phosphoric acid, ethanoic acid, methanoic acid, carbon dioxide, sulphur dioxide, waste liquid from chlorine dioxide production, acid bleach plant waste liquor, sodium sulphate bleaching filtrates such as A- stage bleaching filtrate, and a mixture thereof.

8. The method according to any of the preceding claims carried out at a temperature in the range of 35 °C - 60 °C, preferably 40 °C to 55 °C, suitably at a temperature of 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53 or 54 °C.

9. The method according to any of the preceding claims carried out at ambient pressure.

10. The method according to any of the preceding claims, wherein the oxidising step and the acidifying step are carried out in the same vessel (104).

11. The method according to any of claims 1 to 9, wherein the oxidising step and the acidifying step are carried out in different vessels (101, 103).

12. The method according to claim 11, comprising the step of transferring the second methanol-rich stream after the oxidising from a first vessel (101) to a second vessel (103).

13. The method according to any of the preceding claims comprising a mechanical separation step such as filtering and / or coalescing the acidified second methanol-rich stream (340, 360, 410) before further processing.

14. The method according to any of the preceding claims, wherein further processing comprises distilling the acidified second methanol-rich stream to provide a methanol-rich distillate (370, 430) and a residue comprising ammonium sulphate (70) and oxidation reaction products (50).

15. A system of removing sulphur from crude methanol (10), characterized in that the system comprises:• a distillation column (100), an extraction unit (201), an oxidation unit (101) and an acidification unit (103), wherein o The distillation column (100) has■ an inlet for receiving crude methanol (10),■ an outlet for a distillate (30), and■ an outlet for a methanol-rich stream (310, 380); o The extraction unit (201) has■ an inlet for a methanol-rich stream (310, 380),■ an outlet for an extracted methanol-rich stream (320, 390),■ an inlet for extraction media (210), and■ an outlet for spent extraction media (220); o The oxidation unit (101) hasa first inlet for receiving an extracted methanol-rich stream (320, 390), a second inlet for receiving an oxidant (40), and■ an outlet for feeding the contents of the oxidation unit (350, 330, 400,) to the acidification unit (102), o The acidification unit (102) has■ an inlet for receiving a feed of a methanol-rich stream (350, 330, 400) from the outlet of the oxidation unit,■ a second inlet for receiving an acidifying agent (60), and■ an outlet for an acidified methanol-rich stream essentially free of polymerised terpenes (340, 360, 410).

16. The system according to claim 15 further comprising a first decanting unit (200) for receiving crude methanol (10), optionally diluted with water, said decanting unit being for decanting turpentine (20) from the crude methanol (10) before feeding a crude methanol stream with reduced turpentine content (300) from an outlet of the decanting unit to the inlet of the distillation column (100).

17. The system according to any of claims 14 to 16, characterized in that the oxidation unit (101) comprises a second outlet for removing oxidation products (50) from the oxidation unit (101).

18. A system of treating crude methanol characterized in that the system comprises a decanter (200), a distillation column (100), an extraction unit (201), and an oxidation and acidification unit (104), wherein• the decanter (200) has■ a first inlet configured to receive the crude methanol (10),■ a first outlet for decanting turpentine (20), and■ a second outlet configured to feed a crude methanol with reduced turpentine content (300) to distillation column 100;• the distillation column (100) has■ an inlet for receiving crude methanol with a reduced turpentine content (300),■ an outlet for a distillate (30), and■ an outlet for a methanol-rich stream (310);• the extraction unit (201) has■ an inlet for a methanol-rich stream (310),■ an outlet for an extracted methanol-rich stream (320320),■ an inlet for extraction media (210210), and■ an outlet for spent extraction media (220220);• the oxidation and acidification unit (104) has■ a first inlet for receiving a feed of an extracted methanol-rich stream (320) from the extraction unit (201),■ a second inlet for receiving an oxidant (40),■ a third inlet for receiving an acidifying agent (60), and■ an outlet for delivering an acidified stream (340) to further processing.

19. The system according to claim 18 characterized in that the oxidation and acidification unit ( 104) is configured to receive the oxidant (40) via the second inlet before it receives the acidifying agent (60) through the third inlet.

20. The system according to claim 18 or 19 characterized in that the oxidation and acidification unit (104) is configured so that the third inlet is closed when the second inlet is open to receive oxidant.

21. The system according to any of claims 18 to 20 characterized in that the oxidation and acidification unit (104) is configured so that the second inlet is closed when the third inlet is open to receive acidifying agent 60.

22. The system according to any of claims 15 to 21 further comprising a mechanical separation unit (106) such as a filtering / coalescing unit for receiving an acidified methanol stream (340, 360, 410) from the outlet of the acidification unit (103, 104)23. Use of a system according to any of claims 15 to 22 for carrying out a method according to any of claims 1 to 14.

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