Patent Description:
In a kraft production process of cellulose, known as pulping, cellulose and black liquor are formed during the cooking stage. Methanol is also formed during the cooking stage. The methanol will dissolve into the black liquor and be separated together with the black liquor from the cellulose. The black liquor is then concentrated during an evaporation stage before finally being burned in a soda-recovery boiler. During the evaporation stage condensate streams are formed, which mainly consist of water. Parts of these condensate streams also comprise other compounds or substances, such as, methanol, Total Reduced Sulphur (TRS), etc. The methanol and the TRS may be enriched in a separate condensate stream to form a so-called foul condensate. The foul condensate may then be purified in a so-called stripper or stripper column. A stripper conventionally uses water vapour to remove substances or pollutants from a liquid stream, where the pollutants are removed together with the stripping gases. In this case, the stripping gases, also commonly referred to as stripper off gas (SOG), will, besides water vapour and other pollutants, also contain a considerable amount of methanol.

The SOG, having a water content of <NUM>-<NUM>%, can be incinerated or upgraded in a distillation column, where the methanol is concentrated and separated from the major part of the water content of the SOG. The SOG as well as the upgraded methanol is highly polluted by primarily three (<NUM>) different substances or pollutants: nitrogenous compounds, mainly in the form of ammonium; sulphurous compounds, mainly in the form of TRS; and turpentine and/or other non-water-soluble hydrocarbons. It should be noted that turpentine is primarily present when using so-called softwood, such as, spruce or pine, during the pulping process, while using so called hardwood, such as, e.g. eucalyptus, more oil-related substances will be present instead, albeit in a lesser degree.

The SOG could also be condensed to form a methanol-containing condensate, which can be further treated and thereby further upgraded to a higher quality. Depending on the applications for usage of the methanol, there is a need to be able to provide an improved purification of methanol from foul condensate of a pulping process.

<FIG> depicts an example of a system <NUM> for obtaining methanol from foul condensate <NUM> of a pulping process according to prior art. The foul condensate <NUM> is feed into a primary stripper <NUM>. In the primary stripper <NUM>, water vapour <NUM> is used to remove volatile pollutants in the foul condensate. The methanol-containing water vapour being driven off may be referred to as stripping gases or stripper off gas (SOG). The remaining part of the foul condensate in the primary stripper <NUM> can be removed as a purified condensate <NUM> from the primary stripper <NUM>. The SOGs are then transferred from the primary stripper <NUM> to a primary reflux condenser <NUM>. In some cases, the primary reflux condenser <NUM> may be considered to form part of the primary stripper <NUM>. The primary reflux condenser <NUM> will cool the SOGs. The major part of the SOGs will be condensed in the primary reflux condenser <NUM> and sent back to the primary stripper <NUM> in a so-called reflux stream, e.g. via a reflux tank <NUM>, but the remaining part of SOGs, which is enriched with methanol, will be transferred into a distillation column <NUM>. In the distillation column <NUM>, water vapour <NUM> is normally provided to the bottom of the distillation column <NUM> to drive the distilling process. The distilled methanol from the distillation column <NUM> is then condensed via a distiller reflux condenser 108A and a distiller final condenser 108B. The distiller reflux condenser 108A is arranged to return the distiller reflux condensate to the distillation column <NUM> while a vapour stream is directed to the distiller final condenser 108B such that a methanol rich condensate from the distiller final condenser 108B is collected in a methanol tank <NUM> from which the methanol <NUM> can be extracted. It should also be noted that non-condensable gases <NUM>, i.e. the non-condensable parts coming out of the distiller final condenser 108B, can be transferred away from the system <NUM>. The major part of the water is found in the stream <NUM> coming out of the bottom the distillation column <NUM>.

In order to further improve the process disclosed in <FIG> acid may be added in the process. Methods for purification of methanol from foul condensate are for example disclosed in <CIT> which discloses the acidifying of the SOGs from a foul condensate before entering a second stripper. However, even though the method in <CIT> discloses a method for producing methanol there is a desire for a product having higher purity. Also the publication "<NPL>) discloses the use of acidifying in order to improve the purification of methanol from foul condensate. An essentially identical process is disclosed also in <CIT>. However, the method disclosed in the publication by<CIT> also needs to be improved in order to provide a more pure methanol product.

An object of the invention is to improve the purification of methanol from foul condensate of a pulping process.

According to a first aspect of embodiments herein as defined in the present claim <NUM>, the objective is achieved by a system for obtaining methanol condensate from foul condensate of a pulping process. The system comprises a primary stripper arranged to strip the foul condensate from pollutants. The system also comprises at least a primary reflux condenser, which may be integrated with the primary stripper or be a separate unit, which partly condenses the vapour from the primary stripper and the condensate is returned to the primary stripper as so-called reflux condensate. The remaining vapour, often referred to as stripping gases or stripper off gases (SOGs), can be condensed in a SOG condenser, whereby a first methanol-containing condensate is obtained from the polluted SOGs of the primary stripper. If desired or needed, one or more additional SOG condensers can be used to obtain the methanol-containing condensate from the polluted SOGs. The SOGs from the primary stripper and the primary reflux condenser or condensers will be referred to as a first stripper off gas (SOG) stream. For example, the primary reflux condenser could be arranged to partly condense the polluted stream of SOGs from the primary stripper such that a vapour stream of highly volatile substances including methanol is withdrawn from the upper part of the reflux condenser and guided further to the SOG condenser while a liquid fraction comprising less volatile substances is withdrawn from the lower part of the reflux condenser to provide a reflux stream to the stripper. In the SOG condenser, the major portion of the stream including methanol is condensed to be used as a first methanol-containing condensate in the further steps of the methanol producing process while a smaller fraction of highly volatile substances such as non-condensable gases can be transferred away from the system or possibly treated and purified from pollutants. Also, the system comprises at least one first acidification unit arranged to acidify the first methanol-containing condensate in one or more stages with an acid, preferably sulfuric acid. The system further comprises a secondary stripper arranged to strip the acidified first methanol-containing condensate in order to remove pollutants together with the SOGs from the secondary stripper and to obtain a second methanol-containing condensate as the remaining condensate in the second stripper. Furthermore, the system comprises a distillation column arranged to distil the second methanol-containing condensate from the second stripper in a distillation column so as to obtain purified methanol.

According to a second aspect of embodiments herein as defined in the present claim <NUM>, the objective is achieved by a method performed in a system for obtaining methanol from foul condensate of a pulping process. The method comprises the step of stripping the foul condensate from pollutants in a primary stripper and primary reflux condenser, whereby a first methanol-containing condensate is obtained by condensing the polluted stripper off gases (SOGs) from the primary stripper passing through the primary reflux condenser by the use of one or several SOG condensers. Also, the method comprises the step of acidifying the first methanol-containing condensate from the SOG condenser in one or more stages. The acidification is performed by adding acid to the first methanol containing condensate in a first acidification unit before the first methanol condensate is guided to a secondary stripper. The method further comprises the step of stripping the acidified first methanol-containing condensate from pollutants in the secondary stripper in order to remove the pollutants together with a second stream of stripper off gases (SOG) and to obtain a second methanol-containing condensate as the remaining condensate from the second stripper. Furthermore, the method comprises the step of distilling the second methanol-containing condensate from the second stripper in a distillation column so as to obtain purified methanol.

By performing a second stripping of the first acidified methanol-containing condensate, a large part of the pollutants, such as, e.g. organic nitrogenous compounds, reduced sulphurous compounds, turpentine, etc. will be stripped off. The acidic condition will contribute to make the pollutants more volatile. For example, pollutants such as hydrogen sulphide and methyl mercaptan are much more volatile at an acidic condition. Other non-polar components such as turpentine and organic sulphides, which often have a high boiling point, will also have an increased volatility at an increased ionic strength. By adding the acid to the first methanol-containing condensate before the condensate is guided to the secondary stripper the concentration of acid and pH-level may be controlled to a desired level and the desired reactions may start in the acidification unit before the condensate is introduced into the secondary stripper. The second stripping will lead to a minor amount of methanol vaporizing which will mix with volatile pollutants so as to form a SOG stream of polluted by-products from the second stripper and a major amount of further purified methanol will remain in the second methanol-containing condensate. Hence, the remaining second methanol-containing condensate will comprise much less pollutants than what previously has been achieved. This highly purified second methanol-containing condensate can then be distilled to remove the water phase comprising the ammonium sulphate and thus obtain an increasingly purified methanol. Hence, the purification of methanol from foul condensate of a pulping process is improved.

Further possible features and benefits of this solution will become apparent from the detailed description below.

The figures herein are schematic and simplified for clarity, and they merely show details which are essential to the understanding of the embodiments presented herein, while other details have been left out.

It should also be noted that the embodiments of the system <NUM>, <NUM>, <NUM>, <NUM> shown below in <FIG> describe non-limiting examples and that the differing features or parts of the embodiments of the system <NUM>, <NUM>, <NUM>, <NUM> may be considered as independent embodiments or may be considered in any combination with each other, thus forming further combined embodiments which have not been illustrated further for the sake of clarity.

<FIG> depicts embodiments of a system <NUM> for obtaining methanol from foul condensate <NUM> of a pulping process. Here, it should be noted that the system <NUM> may be similar or identical to the system <NUM> according to the prior art shown in <FIG>, except in that it comprises further features and parts. These further features and parts of the embodiments are indicated in <FIG> by the dashed lines. The similar features or parts of the system <NUM> according to the prior art shown in <FIG> and the embodiments of system <NUM> are indicated by being denoted with the same reference numerals.

For example, the system <NUM> comprises a primary stripper <NUM>. The primary stripper <NUM> being arranged to strip the foul condensate <NUM> from pollutants, whereby a first methanol-containing condensate is obtained by condensing a first stream of stripper off gases (SOGs) from the primary stripper <NUM> in a primary SOG condenser <NUM>. The primary stripper <NUM> usually comprises a primary reflux condenser <NUM>, or heat exchanger, arranged to partly condense the polluted SOGs from the primary stripper <NUM> so as to provide a reflux stream to the primary stripper <NUM>. The first stream of SOGs may thus stream via the primary reflux condenser <NUM>, if such a condenser is present in the system, before being guided to the primary SOG condenser to obtain the first methanol-containing condensate. One or more additional primary SOG condensers <NUM> can be used to condense the first stream of SOGs so as to obtain the first methanol-containing condensate from the polluted SOGs. Hence, it is the condensate derived from the first stream of SOGs produced in the primary stripper <NUM> and condensed in the SOG condenser <NUM> which will be the first methanol containing condensate used as basis for production of a further methanol enriched and purified product in the following steps.

The system <NUM> also comprises a first acidification unit <NUM>. The first acidification unit <NUM> is arranged to acidify the first methanol-containing condensate. This can be performed by adding acid <NUM>, such as, e.g. sulfuric acid, H<NUM>SO<NUM>, to the first methanol-containing condensate obtained from the at least one primary SOG condenser <NUM>. By adding acid, preferably sulfuric acid, to the first methanol-containing condensate, the nitrogenous compounds, mainly in the form of ammonium, in the first methanol-containing condensate will be bound forming ammonium sulphate. This means that later upon vaporizing in the distillation column <NUM>, the methanol will be transferred to the vapour phase while the ammonium sulphate will be retained in the remaining liquid phase stream <NUM>. However, it should be noted that other nitrogenous compounds, primarily organic, and the sulphurous compounds may not be retained in the liquid phase in the same way. Although, by adding acid, the level of ions in the first methanol-containing condensate will increase, which in turn will increase the volatility of the different pollutants, such as, e.g. organic nitrogenous compounds, reduced sulphurous compounds, turpentine, etc., dissolved in the acidified first condensate. The turpentine and other hydrocarbon on the other hand may be separated from the first methanol-containing condensate by decanting, as described further below.

Furthermore, the system <NUM> further comprises a secondary stripper <NUM>. The secondary stripper <NUM> is arranged to strip the acidified first methanol-containing condensate from pollutants and obtain a second methanol-containing condensate. In other words, the secondary stripper <NUM> is arranged to strip the acidified first methanol-containing condensate in order to remove the pollutants together with a second stream of SOGs <NUM> from the secondary stripper <NUM>. This is performed in the secondary stripper <NUM>, e.g. by water vapour <NUM> being introduced at the bottom of the secondary stripper <NUM>. Due to the increased acidity and level of ions in the first methanol-containing condensate, the increased volatility of some of the pollutants, such as organic nitrogenous compounds, reduced sulphurous compounds, turpentine etc., will enable an increased part of the pollutants in the first methanol-containing condensate to be removed with the second stream of SOGs <NUM>.

The second methanol-containing condensate is obtained as the remaining condensate in the secondary stripper <NUM>. The second methanol-containing condensate is then transferred to the distillation column <NUM>. According to the embodiments herein, the distillation column <NUM> is arranged to distil the second methanol-containing condensate from the secondary stripper <NUM> so as to obtain a methanol. The distilled methanol from the distillation column <NUM> can then be condensed via the distiller reflux condenser 108A and the distiller final condenser 108B. The methanol from the distiller final condenser 108B can then be collected in the methanol tank <NUM> from which the methanol <NUM> may be extracted.

Hence, in the secondary stripper <NUM>, it is the condensate from the secondary stripper which forms a main flow which will be basis for production of a further methanol enriched product in the following steps while there is a minor flow of pollutants, by-products and some methanol removed as the second stream of SOG <NUM>. This is in contrast to the primary stripper <NUM> wherein it is the first stream of SOG which is condensed in the primary SOG condenser <NUM> and used as the product flow containing the main portion of methanol while a liquid fraction of purified condensate <NUM> is removed from the process.

It should be noted that the second methanol-containing condensate obtained in the secondary stripper <NUM> will comprise substantially less amounts of pollutants, e.g. organic nitrogenous compounds, reduced sulphurous compounds, turpentine etc., than the first methanol-containing condensate. This means that distilled methanol obtained from the distillation column <NUM> in the system <NUM> will have a significantly higher degree of purification than, for example, the distilled methanol of the system <NUM> in <FIG> according to the prior art. Hence, the system <NUM> will provide an improved purification of methanol from foul condensate of a pulping process.

<FIG> depicts embodiments of a system <NUM> for obtaining methanol from foul condensate <NUM> of a pulping process. Here, it should be noted that the embodiments of the system <NUM> may be similar or identical to the system <NUM> according to the embodiments described with reference to <FIG>, except in that it comprises further features and parts. These further features or parts of the embodiments are indicated in <FIG> by the dashed lines. The similar features or parts of the embodiments of system <NUM> and the embodiments of system <NUM> are indicated by being denoted with the same reference numerals.

In some embodiments, the system <NUM> may comprise a first adding unit <NUM>. The first adding unit <NUM> may be arranged to add sulphite ions to the acidified first methanol-containing condensate. The sulphite ions may, for example, be provided in the form of sodium bisulphite, NaHSO<NUM>. This may be performed so as to facilitate the separation of turpentine, or other hydrocarbons forming their own phase, from the acidified first methanol-containing condensate. This advantageously also creates a buffer in the first acidified methanol-containing condensate which simplifies and eases the control of the pH-level when, for example, providing amounts of acid <NUM> into the first methanol-containing condensate by the first acidification unit <NUM> and/or second acidification unit <NUM> (see <FIG> below). This is because during the acidification of the first methanol-containing condensate, there is a risk that the pH-level reaches low levels, such as, e.g. < pH <NUM>, i.e. below pH <NUM>, and, at those low pH-levels, there is a risk of polymerization of the turpentine in the first methanol-containing condensate. This polymerization may create a sticky phase which may cause severe clogging.

To further avoid or reduce the risk of polymerization, the system <NUM> can comprise a first decanter <NUM>. The first decanter <NUM> may be arranged to decant the first methanol-containing condensate. This may be performed so as to separate turpentine <NUM>, or other hydrocarbons forming their own phase, from the first methanol-containing condensate. Hence, the at least one first decanter <NUM> will also allow some of the turpentine <NUM> to be extracted from the first methanol-containing condensate. Furthermore, since the decanting of the first methanol-containing condensate by the first decanter <NUM> is improved by high levels of ions, performing at least some of the acidification in the first acidification unit <NUM> and adding of sulphite ions in the first adding unit <NUM> before the decantation may be advantageous in order to separate more of the turpentine from the first methanol-containing condensate.

Further, according to some embodiments, the system <NUM> can also comprise a second acidification unit <NUM>. The second acidification unit <NUM> can be arranged to acidify the decanted first methanol-containing condensate from the first decanter <NUM>. This means that the acidification of the first methanol-containing condensate may be performed in one or more stages before and/or after the decantation in the first decanter <NUM>. In this case, a first stage may be adding a first amount of acid <NUM> in the first acidification unit <NUM> before the decantation in the first decanter <NUM>. Here, the first amount of acid <NUM> may be selected in order to avoid or reduce polymerization of the turpentine <NUM> in the first methanol-containing condensate in view of the pH-level. After that, a second stage may be adding a second amount of acid <NUM> in the second acidification unit <NUM> after the decantation in the first decanter <NUM>. Since some of the turpentine <NUM> has been extracted and lowered the risk of polymerization, the second amount of acid <NUM> may be chosen taken this into consideration.

From the above it should also be noted that, according to some embodiments, the decanting of the first methanol-containing condensate may also be performed before the acidification and/or the adding of sulphite ions. In other words, in some embodiments, the decanting may be performed in one or more stages before, in between, and/or after the one or more stages of acidification and/or adding of sulphite ions in the system <NUM>.

<FIG> depicts embodiments of a system <NUM> for obtaining methanol from foul condensate <NUM> of a pulping process. Here, it should be noted that the embodiments of the system <NUM> may be similar or identical to the system <NUM>, <NUM> according to the embodiments described with reference to <FIG>, except in that it comprises further features and parts. These further features or parts of the embodiments are indicated in <FIG> by the dashed lines. The similar features or parts of the embodiments of system <NUM> and the embodiments of system <NUM>, <NUM> are indicated by being denoted with the same reference numerals.

In some embodiments, the system <NUM> may comprise an oxidation adding unit <NUM>. The oxidation adding unit <NUM> may be arranged to add an oxidation agent in order to oxidize the second methanol-containing condensate from the secondary stripper <NUM> prior to distilling the second methanol-containing condensate in the distillation column <NUM>. By adding an oxidant, i.e. a chemical compound such as hydrogen peroxide (H<NUM>O<NUM>) or chlorine dioxide (ClO<NUM>) that will cause oxidation of other compounds, the oxidation adding unit <NUM> may advantageously increase the degree of purification of the second methanol-containing condensate in reference to the sulphurous compounds in the second methanol-containing condensate. This is because the oxidation in the oxidation adding unit <NUM> will cause oxidation of the sulphurous compounds in the second methanol-containing condensate. This means that the sulphurous compounds in the second methanol-containing condensate will become less volatile, and also odourless. This, in turn, means that the oxidized sulphurous compounds in the second methanol-containing condensate to a large extent will remain in the water phase <NUM>, in the distillation column <NUM> instead of being transferred away with the distilled methanol out of the distillation column <NUM>.

In some embodiments, the system <NUM> may also comprise a second decanter <NUM>. The second decanter <NUM> may be arranged to decant the second methanol-containing condensate from the secondary stripper <NUM> prior to distilling the second methanol-containing condensate in the distillation column <NUM>. This may be performed so as to separate some of the remaining hydrocarbons <NUM>, such as turpentine, forming their own phase, from the second methanol-containing condensate if needed.

Furthermore, according to some embodiments, the system <NUM> may also comprise a third acidification unit <NUM>. The third acidification unit <NUM> may be arranged to acidify the second methanol-containing condensate from the secondary stripper <NUM> prior to distilling the second methanol-containing condensate in the distillation column <NUM>. Since the second methanol-containing condensate from the secondary stripper <NUM>, or the decanted second methanol-containing condensate from the second decanter <NUM>, may comprise relatively low amounts of turpentine, or other hydrocarbons forming their own phase, it may be advantageous to perform the final acidification before the distilling, e.g. in this third acidification unit <NUM>. This is because an excess of acid may here be added in order to ensure that substantially all of the ammonium is retained in the water phase during the distillation in the distillation column <NUM> instead of being transferred away with the distilled methanol out of the distillation column <NUM>.

Although the oxidation, decanting and acidification of the second methanol-containing condensate from the secondary stripper <NUM> may be performed in no particular order, it should also be noted that it may also be particularly advantageous to perform the oxidation in the oxidation unit <NUM> prior to the acidification in the third acidification unit <NUM>. This is because the oxidation in the oxidation unit <NUM> will prevent or reduce the risk of polymerization of the turpentine in the second methanol-containing condensate, when acidifying the second methanol-containing condensate down to low pH-levels (e.g. <pH <NUM>) in the third acidification unit <NUM>. Also, the oxidation will further help to reduce the pH-level of the second methanol-containing condensate even further (e.g. <pH <NUM>).

In some embodiments, the system <NUM> may further comprise a third decanter <NUM>. The third decanter <NUM> may be arranged to decant the remaining condensate from the distillation column <NUM>. This may be performed so as to separate some of the remaining turpentine, or other hydrocarbons forming their own phase, from the water phase of second methanol-containing condensate remaining after the distillation in the distillation column <NUM>. Hence, the third decanter <NUM> will also allow some of the hydrocarbons <NUM>, such as turpentine, forming their own phase to be extracted from the from the water phase of the condensate from the distillation column <NUM>, while the remaining water phase <NUM> may be transferred away to, for example, a waste water treatment facility. A waste water treatment facility is typically found at a pulp processing mill. This is also advantageous as said water phase comprise ammonium sulphate, which contributes as nutrients for microbes.

In some embodiments, the system <NUM> may further comprise a second adding unit <NUM>. The second adding unit <NUM> may be arranged to add turpentine to first methanol-containing condensate when the amount of turpentine in the first methanol-containing condensate is below a determined threshold level. This may advantageous in order to further extract sulphurous components in the first methanol-containing condensate, when the turpentine is insufficient for a satisfying extraction. The turpentine may be added to the first methanol-containing condensate at different stages.

<FIG> depicts embodiments of a system <NUM> for obtaining methanol from foul condensate <NUM> of a pulping process. Here, it should be noted that the embodiments of the system <NUM> may be similar or identical to the system <NUM>, <NUM>, <NUM> according to the embodiments described with reference to <FIG>, except in that it comprises further features and parts. These further features or parts of the embodiments are indicated in <FIG> by the dashed lines. The similar features or parts of the embodiments of system <NUM> and the embodiments of system <NUM>, <NUM>, <NUM> are indicated by being denoted with the same reference numerals.

In some embodiments, the system <NUM> may also comprise at least one secondary SOG condenser <NUM>. The at least one secondary SOG condenser <NUM> is arranged to condense the SOGs from the secondary stripper <NUM> comprising the pollutants into a polluted condensate. The SOGs from the secondary stripper <NUM> may also be referred to as a second stripper off gas stream. In some cases, the polluted condensate may, for example, be transferred away for destruction, e.g. to be burned together with the black liquor after the evaporation stage in a soda recovery boiler. However, this means that the methanol remaining in the polluted condensate will be lost.

An alternative way of handling the polluted condensate according to some embodiments herein is to transfer the polluted condensate to an extraction unit <NUM> that may be comprised in the system <NUM>. The extraction unit <NUM> may be arranged to extract pollutants in the polluted condensate by mixing the polluted condensate with turpentine <NUM>. Advantageously, the turpentine <NUM> used may here be turpentine extracted within the system <NUM>, such as, e.g. turpentine <NUM> extracted from the first decanter <NUM>, turpentine <NUM> extracted from the second decanter <NUM>, and/or turpentine <NUM> extracted from the third decanter <NUM>. However, preferably, the turpentine <NUM> extracted from the first decanter <NUM> should be used as it is most likely to be the large and less polluted extracted turpentine. In this case, the system <NUM> may further comprise a fourth decanter <NUM>. The fourth decanter <NUM> may be arranged to decant the remaining polluted condensate and turpentine mixture, so as to separate turpentine <NUM> from the polluted condensate and turpentine mixture. This will be advantageously as the turpentine, or any other non-polar extraction medium for that matter, will extract other non-polar components, such as, e.g. organic sulphides, organic nitrogen compounds, etc., from the methanol <NUM> in the remaining polluted condensate and turpentine mixture, thereby reducing the pollutants in the remaining methanol <NUM>. This enables the methanol <NUM> in the polluted condensate from the secondary SOG condenser <NUM> to be extracted and used. For example, the methanol <NUM> may be added back into the system <NUM>, for example, to the first methanol-containing condensate. Also, as shown in <FIG>, the non-condensable gases <NUM> of the secondary SOG condenser <NUM> transferred away from the system <NUM>.

Furthermore, in some embodiments, the system <NUM> may comprise a cooling arrangement <NUM>. The cooling arrangement <NUM> may be arranged to cool the acidified first methanol-containing condensate at the upper-most equilibrium stage of the secondary stripper <NUM>. In controlling and optimizing the operation of the secondary stripper <NUM>, the supply of water vapour, i.e. the water vapour or steam feed, within the secondary stripper <NUM> may be controlled, i.e. decreased, maintained or increased. A high water vapour feed through the secondary stripper <NUM> will lead to a high vapour flow out of the top of the secondary stripper <NUM>. If the latter is too high, then this may cause too much of the methanol to be transferred away together with the SOGs of the secondary stripper <NUM>. Hence, by cooling the acidified first methanol-containing condensate in the upper-most equilibrium stage of the secondary stripper <NUM>, the vapour flow may at least partly be condensed and thus remain in the secondary stripper <NUM>, which ensures that not too much methanol is transferred away together with the SOGs to, for example, the secondary SOG condenser <NUM>. Therefore, it should be noted that the pollutant removal efficiency of the secondary stripper <NUM> is strongly depended on the ratio of flows of vapour and the second methanol-containing condensate in the secondary stripper.

According to one example, the cooling arrangement <NUM> may be arranged to extract some of the condensed second methanol-containing condensate from a lower equilibrium stage of the secondary stripper <NUM>, cool the extracted second methanol-containing condensate e.g. in a separate cooler, and then return the cooled-off second methanol-containing condensate to the secondary stripper <NUM> again at the upper equilibrium stage. However, it should also be noted that other ways of cooling the first methanol-containing condensate in an upper equilibrium stage may also be envisioned, such as, for example, extending pipes into the secondary stripper <NUM> with cooling liquid inside, having a cooling coil at the upper-most equilibrium stage of the secondary stripper <NUM>, or by sub-cooling the condensate feed of the secondary stripper <NUM>.

In the systems <NUM>, <NUM>, <NUM>, <NUM> in <FIG>, there is disclosed the use of a separate primary reflux condenser <NUM>. However, such a reflux condenser could form part of the primary stripper <NUM>. Furthermore, it is disclosed a single primary SOG condenser but there could be several such condensers present instead of only one primary SOG condenser.

Example of embodiments of a method performed in a system <NUM>, <NUM>, <NUM>, <NUM>, for obtaining methanol from foul condensate <NUM> of a pulping process will now be described with reference to the flowchart depicted in <FIG> is an illustrated example of actions or operations which may be performed in the embodiments of the system <NUM>, <NUM>, <NUM>, <NUM>.

Action <NUM>. The system <NUM>, <NUM>, <NUM>, <NUM> strips the foul condensate <NUM> from pollutants in a primary stripper <NUM>, whereby a first methanol-containing vapour is obtained from the primary stripper <NUM>.

Action <NUM>. The system <NUM>, <NUM>, <NUM>, <NUM> condenses the polluted stripper off gases (SOGs) of the primary stripper <NUM> to obtain the first methanol-containing condensate.

Action <NUM>. As an option, in some embodiments, the system <NUM>, <NUM>, <NUM>, <NUM>, may add turpentine to the first methanol-containing condensate when the amount of turpentine in the first methanol-containing condensate is below a determined threshold level.

Action <NUM>. The system <NUM>, <NUM>, <NUM>, <NUM> acidifies the first methanol-containing condensate. The acidification may for example be performed by adding acid <NUM> to the first methanol containing condensate in a first acidification unit <NUM>.

Action <NUM>. After the acidification of the first condensate in Action <NUM>, the system <NUM>, <NUM>, <NUM>, <NUM> may add sulphite ions to the acidified first methanol-containing condensate.

Action <NUM>. After the adding of the sulphite ions in Action <NUM>, the system <NUM>, <NUM>, <NUM>, <NUM> may decant the first methanol-containing condensate.

Action <NUM>. After the decanting of the first condensate in Action <NUM>, the system <NUM>, <NUM>, <NUM>, <NUM> may acidify the decanted first methanol-containing condensate.

Action <NUM>. The system <NUM>, <NUM>, <NUM>, <NUM> strips the acidified first methanol-containing condensate from pollutants in a secondary stripper <NUM> to obtain a second methanol-containing condensate.

Action 608A. After the stripping in Action <NUM>, the system <NUM>, <NUM>, <NUM>, <NUM> may condense the second stream of SOGs comprising the pollutants from the secondary stripper <NUM> into a polluted condensate.

Action 608B. After the condensing in Action 608A, the system <NUM>, <NUM>, <NUM>, <NUM> may extract at least part of the pollutants in the polluted condensate by mixing the polluted condensate with turpentine.

Action 608C. After the extraction in Action 608B, the system <NUM>, <NUM>, <NUM>, <NUM> may decant the remaining polluted condensate and turpentine mixture, so as to separate turpentine from the polluted condensate and turpentine mixture. Here, it should be noted that the remaining methanol may be added back into the system <NUM>, <NUM>, <NUM>, <NUM>, for example, to the first methanol-containing condensate in Action <NUM>.

Action <NUM>. In the stripping in Action <NUM>, the system <NUM>, <NUM>, <NUM>, <NUM> may also cool the acidified first methanol-containing condensate in an upper-most equilibrium stage of the secondary stripper <NUM>.

Action <NUM>. After the stripping in Action <NUM> or cooling in Action <NUM>, the system <NUM>, <NUM>, <NUM>, <NUM> may oxidize the second methanol-containing condensate.

Action <NUM>. After the oxidation in Action <NUM>, the system <NUM>, <NUM>, <NUM>, <NUM> may decant the second condensate.

Action <NUM>. After the decantation in Action <NUM>, the system <NUM>, <NUM>, <NUM>, <NUM> may acidify the second methanol-containing condensate.

Action <NUM>. The system <NUM>, <NUM>, <NUM>, <NUM> distils the second methanol-containing condensate from the secondary stripper <NUM> in a distillation column <NUM> so as to obtain methanol.

Action <NUM>. After the distilling in Action <NUM>, the system <NUM>, <NUM>, <NUM>, <NUM> may decant the remaining methanol-containing condensate.

Action <NUM>. Finally, the system <NUM>, <NUM>, <NUM>, <NUM> may obtain the methanol from the distillation column <NUM>.

Here, it should be noted that the acidification in Action <NUM> and <NUM> may, according to some embodiments, be performed in one or more stages after the stripping in Action <NUM>, after the decantation in Action <NUM>, or after the stripping in Action <NUM>, or in any combination thereof. It should also be noted that the decantation in Action <NUM> may be performed in one or more stages after the stripping in Action <NUM>, or after or in between one of the acidification stages described in Action <NUM> and <NUM>, or in any combination thereof.

The terminology used in the detailed description of the particular embodiments illustrated in the accompanying drawings is not intended to be limiting of the described system or method therein. As used herein, the term "and/or" comprises any and all combinations of one or more of the associated listed items. Further, as used herein, the common abbreviation "e.g.", which derives from the Latin phrase "exempli gratia," may be used to introduce or specify a general example or examples of a previously mentioned item, and is not intended to be limiting of such item. If used herein, the common abbreviation "i.e.", which derives from the Latin phrase "id est," may be used to specify a particular item from a more general recitation. The common abbreviation "etc.", which derives from the Latin expression "et cetera" meaning "and other things" or "and so on" may have been used herein to indicate that further features, similar to the ones that have just been enumerated, exist.

As used herein, the singular forms "a", "an" and "the" are intended to comprise also the plural forms as well, unless expressly stated otherwise. It will be further understood that the terms "includes," "comprises," "including" and/or "comprising," when used in this specification, specify the presence of stated features, actions, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, actions, integers, steps, operations, elements, components, and/or groups thereof.

Claim 1:
A system (<NUM>; <NUM>; <NUM>; <NUM>) for obtaining methanol from foul condensate (<NUM>) of a pulping process, the system (<NUM>; <NUM>; <NUM>; <NUM>) comprising:
a primary stripper (<NUM>) arranged to strip the foul condensate (<NUM>) from pollutants, whereby a first methanol-containing condensate is obtained from the polluted stripper off gases (SOGs) of the primary stripper (<NUM>) by condensing the polluted gases in a primary SOG condenser (<NUM>),
wherein the system (<NUM>; <NUM>; <NUM>; <NUM>) is characterized by further comprising:
a first acidification unit (<NUM>) arranged to acidify the first methanol-containing condensate,
a secondary stripper (<NUM>) arranged to strip the acidified first methanol-containing condensate in order to remove the pollutants together with the SOGs (<NUM>) to obtain a second methanol-containing condensate as the remaining condensate in the secondary stripper (<NUM>), and
a distillation column (<NUM>) arranged to distil the second methanol-containing condensate from the secondary stripper (<NUM>) so as to obtain the methanol.