Patent Description:
This section illustrates useful background information without admission of any technique described herein being representative of the state of the art.

Methanol is formed during sulphate cooking when lignin is hydrolyzed in an alkali. In addition to methanol, the cooking produces also organic sulphurous compounds, such as methyl mercaptan, dimethyl sulfide and dimethyl disulfide as the sulfide and hydrogen sulfide ions react with lignin.

During cooking, large amount of methanol ends up to the black liquor, and since methanol is quite volatile, it vaporizes together with water to the evaporator foul condensate to be recovered as raw methanol. Raw methanol is recovered also from other points of pulping process, like from the condensates of digester off gases. The foul condensate may be treated at foul condensate stripper by stripping methanol from the condensate, and afterwards the methanol can be liquefied from the stripper off gases as raw methanol in the methanol column which decreases the water content of the raw methanol.

Foul condensate contains also volatile sulphurous compounds like H<NUM>S, methyl mercaptan, dimethyl sulfide that have a low boiling point, but also other less volatile sulfurous compounds. These will end up to raw methanol when treated in the stripper. In case methanol is recovered during softwood pulping while digesting spruce or pine, raw methanol contains turpentine and ammonia together with the above mentioned sulphurous compounds.

Because raw methanol obtained in chemical pulping of softwood contains a complex mixture of impurities, removal of the impurities is needed before the methanol can be used. The solubility of softwood turpentine present in the raw methanol decreases after acidification and dilution, and distinct phases can be separated by decantation. A heavier bottom phase can be collected after the acidification, while methanol and turpentine remain in the upper phase(s). Some turpentine may remain dissolved or entrained in the raw methanol. Turpentine can be removed from the raw methanol in the extraction column.

Some processes to purify the methanol from the sulphurous and other compounds have been described. For example <CIT> describes a process where methanol is treated with an acid to precipitate salts, and volatile sulphurous compounds like H<NUM>S, methyl mercaptan, and dimethyl sulfide are removed as a lighter fraction, while methanol remains in the heavier bottom fraction. Furthermore, the remaining sulphurous compounds are treated with an oxidizing agent to make them more volatile and easier to separate by distillation.

Another process for methanol purification is presented at <CIT>, where ammonium is acidified by sulphuric acid and the formed salt is kept soluble below the precipitation point by diluting with water. After acidification the methanol is evaporated and treated as described in <CIT>.

<CIT>describes a process where raw methanol is first acidified to vaporize H<NUM>S, methyl mercaptan, and dimethyl sulfide. After this the raw methanol is washed with non-polar organic solvent to purify the methanol further. Finally water, acetone, and ethanol in the washed methanol is reduced by distillation.

A problem in purification of raw methanol stream from softwood pulping is that the precipitate formed in the acidification step of raw methanol also gets attached and grows on the equipment surfaces exposed to the acidified raw methanol. When such a precipitate is formed during the process conditions, it needs to be removed and thereby causes non-desired blockages and process interruptions.

It is an object of the present invention to provide a method for preventing precipitation when purifying raw methanol obtained as a by-product in the Kraft pulping of softwood.

According to a first example aspect there is provided a heavy oil obtainable from a raw methanol side stream from chemical hardwood pulping, wherein the heavy oil is manufactured by a method comprising:.

According to a second example aspect there is provided a method for manufacturing heavy oil comprising:.

In an embodiment of the method of the second aspect, the method is for manufacturing the heavy oil of the first aspect. The method is illustrated in <FIG>. According to a third example aspect there is provided a method of purifying raw methanol obtained in chemical pulping, the method comprising:.

In an embodiment of the third aspect the raw methanol is obtained from chemical pulping of softwood. The method of purifying raw methanol is illustrated in <FIG>.

In another aspect is provided a use of the heavy oil in purification of raw methanol obtained in chemical pulping.

When softwood or hardwood is processed in chemical pulping, the resulting raw methanol, or raw methanol stream, can be processed by acidifying the raw methanol. Optionally, the raw methanol stream can also be diluted with water. The chemical composition of the raw methanol obtained from a chemical pulping process varies depending on the type of wood used in pulping, as well as the processing parameters. Therefore raw methanol stream components obtained in softwood pulping may have different characteristics, and behave in a different manner, in the acidification step, and the optional dilution step, when compared to the raw methanol stream components obtained in hardwood pulping.

It has now been surprisingly found that components dissolved in the raw methanol obtained from softwood pulping tend to form after acidification a solid precipitate, and that this precipitation can be prevented by using a heavy oil obtained with the present method from a raw methanol stream from hardwood pulping.

Without being bound to any theory, it is assumed that the components found in the recovered heavy oil form during the hardwood pulping process, and some chemical conversion may additionally occur during the acidification and/or heating step. During the black liquor evaporation step of the pulping process the heavy oil components follow methanol, and during the purification process they are separated due to acidification into a distinct phase, and be collected.

Examples of heavy oils include hardwood oil, birch oil, eucalyptus oil, mixtures of said oils in any proportions, and components that can be separated from said oils and mixtures. Also examples of heavy oils include oils or compounds which have been synthesized during the acidification or heating (distillation and evaporation), and sulfurous compounds produced during cooking process.

The term heavy oil composition means a composition which comprises any amount of heavy oil or its component.

As used herein, the term "comprising" includes the broader meanings of "including", "containing", and "comprehending", as well as the narrower expressions "consisting of" and "consisting only of".

The birch oil, which is an example of heavy oil, may contain aliphatic or cyclic sulphurous compounds with <NUM> to <NUM> sulphur atoms, and have a molecular weight in the range <NUM>-<NUM>.

In an embodiment the raw methanol stream is a side-stream from chemical pulping of softwood.

A raw methanol stream means a stream of raw methanol, such as a stream which is produced in a continuous process.

In an embodiment mixing the raw methanol with an acid forms a salt solution. The salts, such as ammonium salts, can be removed by methods known in the art, such as by settling.

In an embodiment the heavy oil has a density of about <NUM>/m<NUM> at <NUM>.

When the heavy oil is collected in the process, it has a temperature of <NUM>-<NUM> upon collection.

In an embodiment the raw methanol has a density of about <NUM>/m<NUM> at <NUM>.

In an embodiment the heavy oil is enriched in organic sulphurous compounds compared to the raw methanol side stream.

Also examples of heavy oils include oils or compounds that are synthesized from the heavy oil, such as birch oil, during the acidification, heating, distillation and/or evaporation.

The amount of terpenes is increased in softwood cooking, whereas their amount in hardwood cooking is low. Typically in chemical pulping both hardwood and softwood campaigns are run, and consequently the streams, such as methanol streams, produced in these campaigns may be mixed. Thus, when analyzing e.g. the composition of hardwood methanol stream or heavy oil, the analysis results may reveal components that have formed in a previous campaign with softwood. However, the decrease in the amounts of typical softwood turpentine components in the heavy oil or raw methanol from hardwood cooking indicates that the amount of such a component is much higher in the softwood turpentine. For example the amount of terpenes is lower in heavy oil.

In particular the amount of sesquiterpenes is increased when heavy oil is manufactured according to the present process.

The GC/MS analysis can be carried out as described for the recent heavy oil from hardwood raw methanol (Niemelä <NUM>). For the MS identifications, a commercial MS database can be used, together with an extensive collection of spectra from previous studies and published sources, including NIST Chemistry WebBook (https://webbook. Additional MS data searches from literature can be conducted to assist in identification of certain compounds. For comparison, a number of MS datafiles from several previous methanol analyses can be checked. Results from a chemical analysis of heavy oil and a comparison with softwood turpentine are shown in Table <NUM>.

As the skilled person understands, the present heavy oil composition is impractical to be characterized by its exact chemical composition because it may contain hundreds of different compounds, some of which can be volatile or prone to degradation. Instead, the present disclosure characterizes the heavy oil by referring to its manufacturing method. Additional chemical and physical parameters are also used to characterize the heavy oil and differentiate it e.g. from softwood turpentine and raw methanol streams. For example the presence or absence, or a change in concentration, of a certain component of the heavy oil, compared to raw methanol stream or softwood turpentine, serves as a chemical characteristic which is an intrinsic feature of the present heavy oil. The skilled person can easily determine the presence or absence of such a feature in the heavy oil, or in another substance.

In an embodiment the heavy oil is reduced in at least one of α-pinene β-pinene, nitrogen compounds, and sulphurous and nitrogenous compounds, compared to the raw methanol.

In an embodiment the heavy oil contains at least <NUM>% sulphurous compounds analyzed by GC/MS, preferably more than <NUM>% sulphurous compounds analyzed by GC/MS.

In an embodiment the heavy oil contains less than <NUM>% terpenes and at least <NUM>% sulphurous compounds analyzed by GC/MS, preferably <NUM>-<NUM>% terpenes and <NUM>-<NUM>% sulphurous compounds analyzed by GC/MS.

In an embodiment the heavy oil contains <NUM>-<NUM>%, preferably <NUM>-<NUM>%, most preferably about <NUM>% β-pinene analyzed by GC/MS.

In an embodiment the heavy oil comprises oils obtainable from raw methanol obtained in hardwood pulping.

In an embodiment the heavy oil is manufactured from a mixture of raw methanol from hardwood pulping and from softwood pulping. Preferably the mixture predominately contains raw methanol from hardwood pulping. However, the raw methanol from softwood pulping in said mixture does not prevent manufacturing of the heavy oil, because the present process is able to separate the heavy oil from such mixtures.

In an embodiment, when the heavy oil is used in the raw methanol purification, the heavy oil is added in amount selected from the range <NUM>-<NUM> mass-%, preferably <NUM>-<NUM> mass-%, more preferably <NUM>-<NUM> mass-%, and most preferably <NUM>-<NUM> mass, based on the total mass of the methanol mixture.

<FIG> shows an embodiment of a system for producing heavy oil. The relevant elements of the system <NUM> are described below in detail with a reference to the corresponding method for producing heavy oil as described in <FIG>.

The system <NUM> has an acidification unit <NUM>, to which raw methanol stream(s) from hardwood pulping <NUM> is fed. Acid stream <NUM> is fed to the acidification unit <NUM> to acidify the raw methanol in the acidification unit <NUM>. The acidification of the raw methanol stream(s) <NUM> is also shown in <FIG> as the acidification step <NUM> including an optional addition of water.

Volatiles <NUM> are optionally removed from the acidification unit <NUM>. Optionally, turpentine <NUM> is also removed and conducted to a turpentine recovery unit <NUM>.

Heavy oil <NUM> can optionally be recovered to a heavy oil recovery unit <NUM> from the acidification unit, which can also be used to separate liquid phases after acidification. As the acidification of the raw methanol stream in the acidification unit <NUM> results into separation of an aqueous salt solution, salts such as ammonium salts can be removed. Further, phase separation <NUM> occurs in the acidification unit <NUM> as shown in <FIG>, and heavy oil can be recovered <NUM> as the bottom phase after settling of the liquid phases. A lighter methanol containing phase forms above the liquid bottom phase as an acidified raw methanol stream <NUM>.

The acidified raw methanol stream <NUM> is fed into a distillation unit <NUM> to separate volatiles <NUM> by distillation <NUM>. Optionally, a salt solution comprising ammonium sulfate <NUM> formed during distillation is removed, as shown in <FIG> as the step <NUM>.

From the distillation unit <NUM> the raw methanol distillate <NUM> is fed into a buffer tank <NUM> to which water <NUM> is optionally fed. Upon addition of water <NUM>, the raw methanol distillate <NUM> is diluted as also shown in <FIG> as the step <NUM>. Dilution <NUM> results into phase separation of liquid phases (step <NUM> in <FIG>), from which heavy oil <NUM> can be recovered into the heavy oil recovery unit <NUM>, and methanol purification <NUM> can be continued by feeding the raw methanol and water feed <NUM> to an extraction column <NUM>.

The optional addition of water is advantageous to prevent ammonium sulphate from precipitation as the methanol reduces its solubility. The chemical composition of the raw methanol may vary in industrial processes, and the optional addition of water can be used to optimize the process for cases when the concentration of ammonium sulfate rises above its solubility level.

From the extraction column <NUM> a feed <NUM> is optionally fed to an optional solvent stripping unit <NUM> to which steam <NUM> can be fed. Gaseous feed <NUM> is conducted to rich gas system, and a condensate <NUM> containing water is removed. Feed <NUM> containing aliphatic paraffinic oil can be returned to the extraction column.

From the extraction column a feed <NUM> containing methanol, ethanol, acetone and water is conducted to a series of distillation columns <NUM>, <NUM>, <NUM>, in which acetone and volatile impurities <NUM> are initially removed from methanol, ethanol and water feed <NUM> in the distillation column <NUM>, then water <NUM> is separated from methanol and ethanol feed in the distillation column <NUM>, and a methanol feed <NUM> is separated from less volatile impurities such as propanol, butanol and ethanol feed <NUM> in the distillation column <NUM>.

<FIG> shows a flow diagram of the heavy oil manufacturing. Briefly, raw methanol stream from hardwood pulping is provided <NUM>, and acidified <NUM> with an optional addition of water. When adding the acid <NUM> there may be an evaporation of methyl mercaptane (MM) and H<NUM>S. In the existing process also dimethyl sulfide (DMS) is removed together with MM and H<NUM>S in a distillation column ("Sulphur column"). Salt is removed as a salt solution <NUM>, and distinct liquid phases separate <NUM>. A heavy bottom phase containing heavy oil is recovered <NUM>, and the acidified raw methanol stream is distilled <NUM> and the resulting distillate is optionally diluted <NUM> with water, which results into separation of liquid phases <NUM>. A heavy bottom phase containing heavy oil is recovered <NUM> and a lighter methanol containing phase is taken to methanol purification <NUM>. The methanol purification step <NUM> comprises collectively the steps carried out downstream of the buffer tank <NUM> in <FIG>.

In an embodiment the acidification step <NUM> is carried out by using a strong acid in aqueous solution, preferably <NUM>-<NUM> wt-% H<NUM>SO<NUM>. Optionally, water is added. Addition of water is advantageous in particular when using strong acid, such as <NUM> wt-% H<NUM>SO<NUM>. The acidification results into formation of ammonium salts.

In an embodiment in the method for manufacturing heavy oil, as shown in <FIG>, the raw methanol side stream contains less than <NUM>% terpenes and less than <NUM>% sulphurous compounds analyzed by GC/MS, preferably less than <NUM>% terpenes and less than <NUM>% sulphurous compounds analyzed by GC/MS.

In an embodiment the method for making heavy oil, as shown in <FIG>, further comprises distilling the middle phase, optionally diluting the resulting distillate with water to separate phases, and recovering a heavy bottom phase containing heavy oil <NUM>. Thus, recovery of heavy oil can be further enhanced by collecting the heavy oil after the distillation step <NUM>.

In a further embodiment heavy oil is recovered only after distillation, i.e. the first recovery step before distillation <NUM> is omitted, and only the recovery point <NUM> is employed.

Precipitate formation in raw methanol from softwood pulping is accelerated at low pH below <NUM> indicating it being an acid catalyzed reaction. Precipitate formation is further accelerated by a temperature higher than <NUM>. Acidification of the raw methanol stream is also used to vaporize volatile sulphurous compounds and to keep NH<NUM> in solution.

Precipitate formation in raw methanol can be prevented by adding the present heavy oil.

In an alternative or additional embodiment the heavy oil is added to the raw methanol from softwood pulping before the methanol purification process is started, such as during or after the methanol recovery from the foul condensate stripper (not shown in <FIG>).

In another embodiment the heavy oil is mixed together with the raw methanol after the acid has been mixed but before the distillation, as shown with the dashed lines in <FIG>. In case the raw methanol is diluted with water, heavy oil can be added before or after water is added. In a preferred embodiment the heavy oil is added after turpentine separation, to ensure that the heavy oil is not dissolved in the turpentine.

The acidification step can be followed by heating to evaporate volatiles, such as H<NUM>S, mercaptane and dimethyl sulfide, from the methanol-containing phase which is subsequently recovered as a distillated form the distillation column(s).

In a preferred embodiment the raw methanol purification method comprises distillation and evaporation by using a multi-stage distillation and evaporation process comprising a series of distillation columns. In such a process heavy oil can be added to the methanol containing feed before a distillation or evaporation step is started. The heavy oil can thus be added at least once before the multi-stage distillation process is started, it can be added before each individual distillation step is started in the series of distillation columns, or it can be added before a certain distillation step is started.

The precipitations are mainly formed when the methanol is evaporated/distilled from the salt solution. When the methanol content becomes low during evaporation/distillation, the temperature increases and the solubility of precipitating compounds decreases. In a preferred embodiment the heavy oil is added or fed just before this step, i.e. before temperature begins to increase as a result of decreased methanol content.

When heavy oil is added before a distillation or evaporation step is started, its dosage can be selected from the range <NUM>-<NUM>/day.

In a preferable embodiment the first distillation column is configured to separate water with dissolved (NH<NUM>)<NUM>SO<NUM>.

In a preferable embodiment the second distillation column is configured to separate compounds that are more volatile than methanol, such as acetone.

In a preferable embodiment the third distillation column is configured to separate water.

In a preferable embodiment the fourth distillation column is configured to separate compounds that are less volatile than methanol, such as ethanol.

As shown in the examples below, when heavy oil was added to the raw methanol stream obtained from softwood pulping, the resulting raw methanol mixture remains clear, and during distillation no precipitation formation was observed. Thus, distillation could be carried out directly to the raw methanol mixture without further steps to remove a precipitate. Typically, when using prior art purification processes for purifying raw methanol by distillation, precipitate is seen in the distillation columns and in the raw methanol liquor, and the precipitate has to be removed before distillation.

<FIG> shows in a flow diagram an overview of the present method for purifying raw methanol by using the heavy oil. In step <NUM> raw methanol is provided, and heavy oil is added <NUM> before the distillation step <NUM>. After obtaining a methanol mixture with the added heavy oil, the methanol mixture is distilled in step <NUM>, and methanol is recovered in step <NUM>.

In a preferred embodiment the addition of heavy oil is controlled such that no significant phase separation occurs when it is mixed with the acidified raw methanol. This has an advantage of enhancing the capability of heavy oil to prevent precipitation.

In <FIG> an optional acidification step <NUM> is shown as a dashed box, and when this step is employed, heavy oil is added to the acidified raw methanol before or after the acidification step <NUM>, as shown with the arrows having dashed lines. In this embodiment either or both of the addition points can be employed. Preferably heavy oil is added after acidification.

With the present process involving use of heavy oil, the methanol purification process is simplified as the raw methanol mixture containing heavy oil can directly be distilled.

In an embodiment the raw methanol stream <NUM> mainly comprises methanol, wood extractives, sulphurous compounds and optionally nitrogenous compounds. The amounts of these components may vary depending on the type of wood used in pulping, as well as the process parameters of the pulping. Typically the amount of water, wood extractives, sulfurous compounds and nitrogenous compounds are in the range <NUM>-<NUM> mass-% in raw methanol from softwood pulping.

The methanol content of the raw methanol from softwood pulping is in the range <NUM>-<NUM> mass-%.

In an embodiment the wood extractives present in the raw methanol comprise terpenes, preferably monoterpenes and sesquiterpenes.

The raw methanol stream may also contain sulphurous compounds and nitrogenous compounds. Sulphur compounds and sulfurous compounds refer to organic compounds containing at least one sulphurous atom as a heteroatom but no nitrogen. Nitrogenous compounds and nitrogen compounds refer to organic compounds containing at least one nitrogen atom as a heteroatom but no sulphur. Sulphur and nitrogen compounds refers to organic compounds that contain at least one sulphur and at least one nitrogen atom as heteroatoms.

In an embodiment the raw methanol is obtained from chemical pulping of softwood, hardwood or a mixture thereof. In a preferred embodiment the raw methanol comprises raw methanol from chemical pulping of softwood.

In another embodiment the heavy oil does not contain nitrogenous compounds, such as pyrrole and amines, that are typically present in raw methanol obtained in pulping of softwood.

In an embodiment the methanol mixture is a one-phase solution, a dispersion or a two-phase mixture. In a two-phase system the phases can be dispersed or not dispersed, and raw methanol is mainly in one phase, but it may contain some dispersion (not necessary visible). However, as the raw methanol is acidified or/and water is added, then the solubility of for example terpenes are reduced and they form more clear dispersion. In a preferred embodiment the methanol mixture is a one-phase solution. The solubility of terpenes is reduced by adding dilution water. In an embodiment only strong acid is added, and the acid will be consumed by forming a white ammonium salt precipitation.

Preferably no precipitate is seen in the methanol mixture after addition of heavy oil.

In an embodiment the raw methanol is acidified before distilling or evaporating to a pH value below <NUM>, preferably acidified to a pH value below <NUM>. In another embodiment the methanol mixture is immediately distilled after acidification, i.e. no precipitation needs to be removed and no decanting or settling of phases is needed before the methanol mixture can be distilled.

In an embodiment light turpentine phase is removed before distillation.

In an embodiment the heavy oil is added to the raw methanol before distillation or evaporation.

In an embodiment the heavy oil is added to the raw methanol after acidification.

In an embodiment ammonium sulphate is removed as a solution. As salts such as ammonium salts remain dissolved during evaporation and/or distillation, they can be easily removed as a bottom product.

In an embodiment the heavy oil is added in amount selected from the range <NUM>-<NUM> mass-%, preferably <NUM>-<NUM> mass-%, more preferably <NUM>-<NUM> mass-%, and most preferably <NUM>-<NUM> mass-%, based on the total mass of the methanol mixture. In case a plurality of distillation columns are used in a series, said amount can be added before the first, at least one, some, or each of the distillation columns.

In an embodiment of the method the methanol is recovered by distillation by using a plurality of distillation columns in a series.

In an embodiment the raw methanol stream <NUM> is obtained from chemical pulping of softwood or hardwood, or a mixture thereof. Raw methanol obtained from softwood pulping is particularly advantageous in the present invention.

In an embodiment the raw methanol is acidified before distilling or evaporating. Preferably the acidified raw methanol is distilled or evaporated without a precipitate removal step, and/or a decanting step.

In an embodiment in softwood raw methanol purification a turpentine fraction is removed after dilution water is added. In an embodiment the turpentine content is about <NUM>% in the raw methanol from softwood. Without dilution water the turpentine remains dissolved and then follow the methanol to the extraction column.

In an embodiment the methanol mixture obtained after addition of heavy oil is clear, or essentially clear. Clarity of the methanol mixture can be verified e.g. simply by visual inspection.

As is clear to the skilled person, the method illustrated in <FIG> and <FIG> can be modified to further control or improve certain steps of the method or products obtained with it. For example products obtained with the method can be taken to further purification, separation, derivatization or to at least one chemical conversion step to separate components present in the product, or to chemically convert them into further products.

The following experiments were performed to compare the performance of the present raw methanol purification method with a method conducted according to prior art.

The analysis results of the heavy oil are summarized in Table <NUM>. As the results show, the present heavy oil is characterized by an unusually high amount of sulfurous compounds. Also the amount of sesquiterpenes is high, and clearly different compared to e.g. softwood turpentine.

In a <NUM> glass cylinder a <NUM> of acidified raw methanol from softwood pulping and containing ammonium sulphate in water was boiled in the atm pressure to simulate the conditions at the first distillation column after methanol acidification. During the methanol water boiling it was noticed that a solid precipitate was formed, similarly as described in previous publications in connection with methanol purification processes. Solid precipitate was not dissolvable to the common solvents tested, like acetone, sulfuric acid, hydrochloric acid, paraffinic oil, or NaOH.

This test was started as in the Comparative example <NUM> using raw methanol sample from softwood pulping. Before heating and boiling, <NUM> of heavy oil (birch oil) was added. During boiling and methanol evaporation no precipitation was noticed. The heavy oil thus prevented precipitation.

Additionally, it was found that the heavy oil is able to dissolve the solid precipitation formed in the Comparative example <NUM>. A seriously contaminated distillation column that was used to purify raw methanol from a softwood campaign could be cleaned with raw methanol containing the heavy oil: after a couple of days with hardwood raw methanol most of the liquid exposed parts in the column were nearly clean. The addition of heavy oil thus effectively prevented precipitation and even dissolved a previously formed precipitation.

Claim 1:
Heavy oil obtainable from a raw methanol side stream from chemical hardwood pulping, wherein the heavy oil is manufactured by a method comprising:
an acidification step comprising mixing the raw methanol side stream with an acidic aqueous solution, and heating to vaporize low boiling point sulphurous compounds, to provide acidified raw methanol;
a separation step comprising separating from the acidified raw methanol a liquid top phase, a liquid aqueous middle phase, and a liquid bottom phase; and
collecting the liquid bottom phase as the heavy oil having a density of about <NUM>/m<NUM> at <NUM>.