WO2013191640A1

WO2013191640A1 - Method of obtaining a fraction of cellulose fibers with increased solvent accessibility

Info

Publication number: WO2013191640A1
Application number: PCT/SE2013/050740
Authority: WO
Inventors: Roland Agnemo; Anna Svedberg
Original assignee: Domsjoe Fabriker AB
Current assignee: Domsjoe Fabriker AB
Priority date: 2012-06-21
Filing date: 2013-06-20
Publication date: 2013-12-27
Anticipated expiration: 2014-12-21

Description

METHOD OF OBTAINING A FRACTION OF CELLULOSE FIBERS WITH INCREASED SOLVENT ACCESSIBILITY

Technical field

The present invention relates to the field of production of regenerated cellulose and cellulose derivatives from dissolving pulps. Background

Dissolving pulp is a cellulose pulp with a high cellulose content (above 90 % w/w) and low contents of lignin, hemicellulose and resin. These features make dissolving pulp suitable as a raw material for the production of regenerated cellulose. Dissolving pulp is mainly produced by the acid sulfite process or the prehydrolysis kraft processes. The acid sulfite process is the most common and benefits of this technique include high recovery rates of the inorganic cooking chemicals and a totally chlorine free bleaching. One disadvantage with the method is that it results in pulps with a broad molecular weight distribution of cellulose (Christofferson 2005, Sixta et al. 2004).

Dissolving pulp is mainly used for production of regenerated cellulose and as a raw material in the production of different cellulose derivatives such as carboxymethyl cellulose (CMC), methyl cellulose (MC), hydroxypropyl cellulose (HPC) and hydroxyethyl cellulose (HEC). In the production of regenerated cellulose, the cellulose is dissolved in a solvent to form a cellulose dope which is processed to regenerate the cellulose fibers in different forms. There are a number of different methods of producing regenerated cellulose known in the art, including the viscose process, the Lyocell process, the cellulose acetate process and the cellulose carbamate process. The viscose process has been used industrially for many years for the production of rayon fibers, used in the textile industry. One advantage with the method is that it is possible to use wood as a starting material whereas several other methods need lignin-free cellulose as starting material. The viscose process can be divided into the following steps:

-Mercerization

-Pre-ripening

-Xanthation

-Dissolving

-Ripening and

-Regeneration

In the Mercerization step the cellulose is typically soaked in 17-20% (w/w) NaOH solution at room temperature for a few hours such that the cellulose is converted into alkali-cellulose. During this process the cellulose fibers are activated by the swelling of the fibers leading to an increased accessibility of the cellulose fibers to the chemicals in the solvent (e.g. carbon disulfide) such that the reactivity of the cellulose fibers with the chemicals is increased. In addition to the increased accessibility, the breakage of intra- and inter molecular hydrogen bonds in and between the cellulose chains caused by the alkaline conditions also contributes to the increased reactivity of the swelled fibers. In the pre-ripening step, the alkali-cellulose will be degraded into desired degree of polymerization through depolymerization reaction with the oxygen in the air. The reaction can be catalyzed by cobalt or manganese. In the xanthation step, the pre-riped alkali-cellulose is mixed with carbon disulfide (CS2) resulting in a cellulose xanthate derivativewhich is dissolved in a dilute sodium hydroxide solution to from a cellulose xanthate solution (viscose solution). This is followed by a ripening step where some of the alkali xanthate groups are removed from the cellulose. In the regeneration process the cellulose xanthate is applied to a coagulation bath. Typically, this involves pumping of the cellulose xanthate through spinnerets, into an acid bath where cellulose is regenerated in the form of long filaments. In this process the CS2 will be released which enables recycling of the chemical. Another industrially established process for production of regenerated cellulose is the Lyocell process. In this process the cellulose pulp is mixed with a water solution of N-methyl morpholine N-oxide (NMMO) in a heated vessel. NMMO is believed to be a direct solvent of cellulose which means it can dissolve cellulose without derivatization. Other methods for dissolving cellulose and for production of solutions comprising dissolved cellulose using amine oxides are known in the art, see e.g. US4196282 and US2179181 . In contrast to the viscose process which is an alkaline process, the Lyocell process can be performed at neutral pH in a water/NMMO solution.

Other methods of preparing regenerated cellulose or cellulose derivatives include the cellulose acetate process wherein cellulose is reacted with acetic acid and acetic anhydride in the presence of sulfuric acid. The cellulose is thereafter partially hydrolyzed to remove sulfate and some of the formed acetate groups to give the produced cellulose acetate product the desired properties. Thereafter the cellulose acetate can be dissolved in acetone and cellulose acetate fibers can be regenerated by extrusion through spinnerets and evaporation of the acetone.

Regenerated cellulose can also be produced by the cellulose carbamate process, see e.g. US6590095. This process includes reacting cellulose with urea at elevated temperatures to form cellulose carbamate. The formed cellulose carbamate can be dissolved in a sodium hydroxide solution such that a spinnable cellulose carbamate dope is formed.

Summary of the present disclosure

Preparation of regenerated cellulose usually involves reacting the cellulose to form cellulose derivatives such as for example cellulose xanthate, cellulose acetate and cellulose carbamate. To get a high yield in the reactions it is important that the reactivity of the cellulose and accessibility of the fibers to the solvent is high. This is also true for production of other cellulose derivatives such as cellulose sulfate, cellulose phosphate, cellulose nitrate, ethyl hydroxyethyl cellulose, carboxymethyl cellulose, methyl cellulose, hydroxypropyl cellulose and hydroxyethyl cellulose. In some processes such as the Lyocell process, the cellulose is dissolved without derivatization.

Nevertheless a high accessibility of the fibers to the solvent is important to get a high solubilization of the cellulose. In fact, some types of dissolving cellulose have been shown to be less suitable for the Lyocell process due to poor solubilization of the fibers in the NMMO/water solution. The present inventors have realized that this, at least partly is due to poor accessibility of the fibers to the solvent.

In strongly alkaline processes where the cellulose is swelled at a pH above 9.0 the activation of the fibers and the swelling of the fibers are usually highly effective and thus the accessibility of the solvent to the fibers are high. In contrast, in neutral or acidic processes, such as the Lyocell process and the cellulose acetate process, the swelling of the fibers are often not as effective and the accessibility of solvent and chemicals to the fibers are not as high. Thus, the present inventors have realized that there is a need to improve the accessibility of the fibers to the solvent in processes where the fibers are swelled at a pH below 9.0.

By fractionation of a dissolving pulp in a hydrocyclone into an over flow fraction and an under flow fraction, the present inventors have surprisingly discovered that the swellability in water of the fibers present in the over flow fraction is significantly higher than for the fibers present in the under flow fraction or in the unfractionated dissolving pulp. This shows that the

accessibility of the cellulose fibers in the over flow fraction, for some reason, is higher than for the fibers in the under flow fraction or in the unfractionated cellulose pulp. In line with this discovery, the present inventors have further demonstrated that the solubility of the fibers in the over flow fraction in a cellulose acetate process is significantly higher than for the under flow fraction or the unfractionated dissolving pulp, most likely due to the increased accessibility of the fibers in the over flow pulp fraction. The present inventors have thus realized that a fraction of cellulose fibers with increased accessibility of the fibers to the solvent can be generated by fractionation of a cellulose pulp into an over flow fraction and an under flow fraction using a hydrocyclone. The present inventors have further realized that the over flow fraction, due to the increased accessibility of the fibers to the solvent, is particularly suitable as a starting material in processes for production of cellulose products where the fibers are swelled at a pH below 9.0.

Thus one aspect of the present invention relates to a method for producing at least one cellulose product comprising the following steps:

a) providing a cellulose pulp, preferably by a method comprising pulping lignocellulosic biomass by a pulping process such that a cellulose pulp is obtained

b) fractionating the cellulose pulp such that an over flow pulp fraction and an under flow fraction is obtained

c) swelling at least part of the cellulose fibers present in the over flow fraction by soaking at least part of the over flow fraction in a first liquid having a pH of 9.0 or lower such that a first set of swelled cellulose fibers is obtained

d) producing at least one first cellulose product from the first set of

swelled cellulose fibers obtained in step c)

wherein the at least one first cellulose product is a cellulose derivative and/or a cellulose dope.

Definitions:

Dissolving pulp: A cellulose pulp having a dry weight cellulose content of at least 90 % (w/w)

Cellulose dope: A liquid solution of dissolved cellulose fibers.

Swellability: Ability to swell in a solvent. A pulp sample of cellulose fibers with a high swellability in water will have a larger increase in volume after incubation in water than a pulp sample of cellulose fibers with a lower swellability in water.

Over flow fraction: A fraction of the pulp which exits the hydrocyclone at or near the top of the hydrocyclone ( i.e. in the over flow).

Under flow fraction: A fraction of the pulp which exits the hydrocyclone at or near the bottom of the hydrocyclone (i.e. in the underflow). Detailed description

Methods for production of cellulose derivatives and cellulose dope usually involve swelling of cellulose fibers prior to dissolving and/or derivatization of the cellulose fibers. The swelling step is important for increasing the accessibility of chemicals and/or the solvent to the cellulose fibers and a sufficient swelling of the fibers are therefore highly beneficial for the reactivity in the derivatization reaction and for the dissolving of the cellulose fibers to a cellulose dope. In some of these methods the cellulose fibers needs to be swelled at an acidic or close to neutral pH or at least at a pH below 9.0. A drawback with these kinds of methods is that the swelling of the fibers are less effective compared to methods where the fibers are swelled in a more alkaline liquid, such as in liquids having a pH above 9.0. Therefore, some types and qualities of dissolving cellulose is presently not used in industrial processes for production of regenerated cellulose or cellulose derivatives by processes which requires swelling of fibers at pH below 9.0. Some of these methods are important industrial process, which are highly suitable for production of different high quality regenerated cellulose products and/or cellulose derivatives. It is therefore desired that also such types of dissolving pulps could be used as starting material in these kinds of methods and that the swelling of the fibers are as efficient as possible.

The present invention relates to a method for isolation of a fraction of a dissolving pulp with an increased swellability in liquids having a pH below 9.0. Thereby a fraction of cellulose fibers, which is more suitable for production of regenerated cellulose and/or cellulose derivatives, by methods including swelling of fibers at a pH below 9.0, can be obtained. This enables use of dissolving pulps which previously have been regarded as unsuitable or less suitable for these kinds of methods. The present invention also enables a more efficient swelling of the fibers at pH below 9.0.

Therefore, one aspect of the invention relates to a method for producing at least one cellulose product comprising the following steps:

b) fractionating the cellulose pulp using a hydrocyclone such that an over flow fraction and an under flow fraction is obtained

d) producing at least one first cellulose product from the first set of

swelled cellulose fibers obtained in step c)

A hydrocyclone (often referred to in the shortened form cyclone) is a device to classify, separate or sort particles in a liquid suspension based on the ratio of their centripetal force to fluid resistance. A hydrocyclone will normally have a cylindrical section at the top where liquid is being fed tangentially, and a conical base. The angle, and hence length of the conical section, plays a role in determining operating characteristics. A hydrocyclone has two exits on the axis: ususally one smaller on the bottom (underflow exit) and a larger at the top (overflow exit). In the present disclosure, the term "over flow fraction" should be interpreted as a pulp fraction that exits the hydrocyclone through an exit located at or near the top of the hydrocyclone and the term "under flow fraction" should be interpreted as a fraction of the pulp which exits the hydrocyclone at or near the bottom of the hydrocyclone. In the present disclosure the term hydrocyclone and cyclone is used interchangeably.

When cellulose pulp are fractionated in a hydrocyclone, fibers with a lower density ends up in the over flow fraction and fibers with higher density in the under flow fraction. The present inventors have also demonstrated that it is possible to perform the fractionation such that the fiber length of the fibers present in the over flow can be larger than in the under flow faction. Therefore in some embodiments the over flow fraction can also be referred to as the "long fiber fraction" and the under flow faction can be referred to as the "short fiber fraction".

It has been described that regenerated cellulose such as e.g. viscose can be produced from a paper grade pulp, see US4210747A. Nevertheless, in industrial processes used at present, regenerated cellulose, cellulose dopes and cellulose derivatives are usually produced from dissolving pulps which are fairly pure cellulose pulps with low levels of hemicellulose and lignin and wherein the dry weight proportion of cellulose is above 90 % (w/w). Therefore it is preferred that the cellulose product is prepared from a dissolving pulp. Depending on the pulping process, a dissolving pulp can be obtained directly after the digesters in the pulping step (i.e. step a) in the present invention) or hemicellulose might need to be degraded and/or removed in a step subsequent to the pulping step to obtain the dissolving pulp. For example if the pulping process in step a) is a pre-hydrolysis kraft pulping process hemicellulose have been removed in a pre-hydrolysis step, prior to the pulping step, and thus a dissolving pulp (i.e. a cellulose pulp having a dry weight cellulose content of at least 90 % (w/w)) can be obtained already at step a). If the pulping process is an acid sulfite pulping process, the dry weight cellulose content can some times be below 90 % (w/w) directly after the pulping step and thus hemicellulose can be degraded in a step

subsequent to step a) such that a dissolving pulp is obtained prior to step c). In industrial processes for production of dissolving pulp using the acid sulfite pulping process, hemicellulose is usually degraded subsequently of the pulping step in a bleaching step. Besides of the pre-hydrolysis kraft pulping process and the acid sulfite pulping process, it has also been described that a paper pulp can be upgraded to dissolving pulp by extracting and/or degrading hemicellulose from the paper pulp, see e.g. US6057438.

Therefore in a preferred embodiment the cellulose pulp provided in step a) and/or the over flow fraction swelled in step c) is a dissolving pulp. In one embodiment the cellulose pulp provided in step a) is a dissolving pulp having a dry weight cellulose content of at least 90 % (w/w). In one embodiment the cellulose pulp provided in step a) has a dry weight cellulose content below 90 % (w/w) and the method is further comprising a step:

a2) removing or degrading hemicellulose at a step prior to step c) such that the cellulose content in the over flow fraction is at least 90 % (w/w) in step c). In one embodiment the removing or degradation of hemicellulose in step a2) is performed prior to step b). In another embodiment the removing or degradation of hemicellulose in step a2) is performed subsequently of step b). In one embodiment the over flow fraction swelled in step c) and/or the pulp provided in step a) is a sulfite dissolving pulp. In one embodiment the pulping process in step a) is an acid sulfite pulping process. In one embodiment step a2) includes a bleaching step, such as hydrogen peroxide bleaching step, a chlorine dioxide bleaching step or a chlorine bleaching step. In another embodiment the cellulose pulp provided in step a) is a pre-hydrolysis kraft dissolving pulp. In one embodiment the pulping process in step a) is a pre- hydrolysis kraft pulping process. In one embodiment the over flow fraction swelled in step c) is a dissolving pulp upgraded from a paper pulp.

In strong alkaline solutions, such as pH above 9.0 and in particular at pH above 10 or at a pH above 1 1 , the swellability of the fibers are high and thus the fibers with lower swellability in water will still be sufficiently swelled and activated in this kind of processes. Thus the inventors have realized that the under flow fraction is suitable for use in alkaline processes such as for example a viscose process or a cellulose carbamate process. By using the more easily swelled over flow fraction in the more challenging swelling at pH below 9.0 and the under flow fraction at processes where the demand of high swellability is less, the cellulose fibers are more efficiently used.

Thus, in a preferred embodiment the method further comprising the step of: e) swelling at least part of the cellulose fibers present in the under flow fraction by soaking at least part of the under flow fraction in a second liquid having a pH above 9.0, preferably above 10 such as above 1 1 , such that a second set of swelled cellulose fibers is obtained

f) producing at least one second cellulose product from the second set of swelled cellulose fibers obtained in step e).

In a preferred embodiment the under flow fraction swelled in step e) is a dissolving pulp. In one embodiment the dry weight cellulose content of the under flow fraction swelled in step e) is at least 90 % (w/w).

In one embodiment the first cellulose product is a cellulose dope having a pH of 9.0 or lower and in one embodiment the second cellulose product is a cellulose dope having a pH of above 9.0. In one embodiment the first cellulose product is selected from Lyocell dope, cellulose acetate dope, cellulose sulfate, cellulose phosphate and cellulose nitrate. In one

embodiment the second cellulose product is selected from a viscose solution, solid cellulose carbamate, cellulose carbamate dope, Ethyl hydroxyethyl cellulose, carboxymethyl cellulose, methyl cellulose, hydroxypropyl cellulose, hydroxyethyl cellulose, ethyl cellulose, propyl cellulose and ethyl - hydroxymethyl cellulose.

In a preferred embodiment, the pH of the first set of swelled fibers is not raised above 9.0 at any time in between step c) and step d). In one

embodiment the pH of the second set of swelled fibers is not decreased below 9.0 at any time in between step e) and step f). In one embodiment at least 60 % (w/w) of the fibers present in the over flow fraction obtained in step b), preferably at least 75 % (w/w), preferably at least 90 % (w/w) is used in step c). In one embodiment at least 60 % (w/w) of the fibers present in the under flow fraction obtained in step b), preferably at least 75 % (w/w), preferably at least 90 % (w/w), preferably 95 % (w/w) is not used in step c). In one embodiment at least 60 % (w/w) of the fibers present in the under flow fraction obtained in step b), preferably at least 75 % (w/w), preferably at least 90 % (w/w), preferably at least 95 % (w/w) is used in step e). In one embodiment at least 60 % (w/w) of the first set of swelled cellulose fibers obtained in step c), preferably at least 75 % (w/w), preferably at least 90 % (w/w) is used in step d). In one embodiment at least 60 % (w/w) of the second set of swelled cellulose fibers obtained in step e) preferably at least 75 % (w/w), preferably at least 90 % (w/w), preferably at least 95 % (w/w) is used in step f).

In one embodiment less than 40 % (w/w), preferably less than 30 % (w/w), preferably less than 20 % (w/w) of the fibers in the over flow fraction have a fiber length between 0.2-1 mm and more than 60 % (w/w) preferably more than 70 % (w/w) preferably more than 80 % (w/w) of the of fibers in the over flow fraction have a fiber length between 1 -5 mm. In one embodiment less than 40 % (w/w), preferably less than 30 % (w/w), preferably less than 20 % (w/w) of the fibers in the under flow fraction have a fiber length between 1 -5 mm and more than 60 % (w/w), preferably more than 70 % (w/w), preferably more than 80 % (w/w) of the fibers in the under flow fraction have a fiber length between 0.2-1 mm.

In some cases, for example in the production of a Lyocell dope, the swelling of the fibers and the actual dissolving of the cellulose fibers in the

water/NMMO solution occur simultaneously in a common step. In the Lyocell process the cellulose pulp is mixed with a water solution of NMMO in a heated vessel and incubated until the cellulose fibers are dissolved by the water/NMMO solvent. During this incubation, the fibers are initially swelled before they are dissolved, otherwise the fibers are not accessible to the solvent and can not be dissolved. However, in this method the swelling is not performed in a separate step but is rather performed in the same vessel, in parallel with the actual dissolving of the fibers. The same is also true for example in the production of cellulose phosphate, cellulose nitrate and cellulose sulfate. In the production of these cellulose derivatives, the cellulose pulp is incubated with phosphoric acid, nitric acid and sulfuric acid

respectively. During this incubation the fibers are first swelled and then derivatized. Again the swelling and the reaction is not performed in separate steps but rather in the same vessel in parallel. In other processes, such as for example the viscose process the swelling is performed at a step which is separate from the derivatization and/or dissolving step. In the viscose process the cellulose fibers are swelled by soaking of the fibers in 17-20% sodium hydroxide in the mercerization step. The formed alkali-cellulose is derivatized by reaction with carbon disulfide to form the cellulose xanthate derivative in the xanthation step and finally the viscose solution is formed by dissolving of the cellulose xanthate derivative in sodium hydroxide of a concentration of about 5-10 % (w/w) in the dissolving step.

Therefore, in one embodiment the swelling in step c) is performed

simultaneously with the production of the first cellulose product in step d) and in one embodiment the swelling in step e) is performed simultaneously with the production of the second cellulose product in step f). In another embodiment the swelling in step c) is performed prior to the production of the first cellulose product in step d) in a step separate from step d) and in one embodiment the swelling in step e) is performed prior to the production of the second cellulose product in step f) in a step separate from step f). Preferably the first product can be selected from Lyocell dope, cellulose acetated dope, cellulose sulfate, cellulose phosphate and cellulose nitrate. As described above, in the production of a Lyocell dope the fibers are preferably swelled in a mixture of water and an amine oxide such as NMMO. In the production of a cellulose acetate dope, the fibers are preferable swelled in a mixture of acetic acid and sulfuric acid. In the production of cellulose sulfate the fibers are preferable swelled in sulfuric acid. In the production of cellulose nitrate the fibers are preferable swelled in nitric acid. In the production of cellulose phosphate, the fibers are preferable swelled in phosphoric acid. Thus in one embodiment the first liquid having a pH of 9.0 or lower comprises acetic acid, sulphuric acid, nitric acid, phosphoric acid or at least one amine oxide. In one embodiment the amine oxide is selected from NMMO, trimethylamine, triethylamine, tripropylamine, monomethyldiethylamine, dimethylmonoethylamine, monomethyldipropylamine, Ν,Ν-dimethyl-, N,N- diethyl- or Ν,Ν-dipropylcyclohexylamine, N,N-diemethylmethylcyclohexamine and pyridine, In a preferred embodiment the amine oxide is NMMO. In another embodiment the first liquid having a pH of 9.0 or lower is acetic acid, sulphuric acid, nitric acid, phosphoric acid or a mixture of acetic acid and sulfuric acid. In one embodiment the first liquid having a pH of 9.0 or lower is a water solution comprising at least one amine oxide such as NMMO. In one embodiment the NMMO is NMMO-monohydrate. Suitable amine oxide concentration for swelling cellulose fibers is well known to the skilled person. For example the concentration of NMMO in the first liquid can be about 75 % (w/w ), such as between 60 % (w/w) and 85 % (w/w).

In one embodiment the production of the first cellulose product in step d) comprises dissolving at least part of the swelled cellulose fibers obtained in step c) with at least one amine oxide, such as for example NMMO, trimethylamine, triethylamine, tripropylamine, monomethyldiethylamine, dimethylmonoethylamine, monomethyldipropylamine, Ν,Ν-dimethyl-, N,N- diethyl- or N,N-dipropylcyclohexylamine, N,N-diemethylmethylcyclohexamine or pyridine. In a preferred embodiment the amine oxide is NMMO and in one embodiment the first cellulose product produced in step d) is Lyocell dope. In another preferred embodiment the first liquid having a pH of 9.0 or lower comprises acetic acid and optionally sulfuric acid. In one embodiment the production of the first cellulose product produced in step d) comprises dissolving at least part of the swelled cellulose fibers obtained in step c) by acetylation of the cellulose. In one embodiment the first cellulose product produced in step d) comprises reacting the swelled cellulose fibers obtained in step c) with acetic anhydride. In one embodiment the first cellulose product produced in step d) is cellulose acetate dope. In another embodiment the first cellulose product produced in step d) is selected from cellulose sulfate, cellulose phosphate and cellulose nitrate.

In one embodiment the second liquid having a pH of above 9 comprises sodium hydroxide. In one embodiment the second liquid having a pH of above 9 is sodium hydroxide. In a preferred embodiment the second liquid having a pH of above 9 comprises 15-25 % (w/w), such as 17-20% (w/w) sodium hydroxide. In one embodiment alkali-cellulose is formed in step e). In one embodiment the production of the second cellulose product in step f) comprises mixing at least part of the alkali-cellulose formed in step e) with carbon disulfide such that cellulose xanthate is formed. In one embodiment at least part of the cellulose xanthate is dissolved in a sodium hydroxide solution such that a viscose solution is formed. In one embodiment at least part of the cellulose xanthate is dissolved in a solution comprising 5-10 % (w/w) sodium hydroxide such that a viscose solution is formed. In one embodiment the second cellulose product produced in step f) is a cellulose xanthate derivative such as a viscose solution.

In another embodiment the second cellulose product produced in step f) is produced by a cellulose carbamate process and in one embodiment the second cellulose product produced in step f) is cellulose carbamate. In one embodiment the second cellulose product produced in step f) is a cellulose carbamate dope. Pulp from the cooking step usually contains some unwanted solid material. If the starting material is wood chips, some of the chips may not have been fiberized properly, and some of the fibrous material may not be completely in the form of individual fibers. Furthermore, contaminants such as bark, sand or gravels might enter the cooker together with the wood chips. Therefore, this unwanted material is usually removed by a screening step prior to the production of cellulose dopes, cellulose derivatives or other cellulose products. In the present invention it is desirable that this screening step is performed prior to the fractionation in step b). However it is possible, even though less desirable to perform the screening subsequently of the

fractionation in step b). Thus in one embodiment the method further comprising a step

a3) screening the cellulose pulp provided in step a) wherein the screening preferably is performed prior to step b). In a less preferred embodiment the pulp screened in step a3) is the over flow fraction and/or the under flow fraction obtained in step b).

The over flow fraction and the under flow fraction can preferably be dried prior to step c). Often the production of the cellulose dope or the cellulose derivative is performed at another site or in another plant than the production of the cellulose pulp. Therefore drying of the pulp is desired for transportation and storage of the pulps. Thus in one preferred embodiment the method further comprises the step

b2) drying the over flow fraction obtained in step b) prior to step c) and in another preferred embodiment the method comprises the step

b3) drying the under flow fraction obtained in step b) prior to step e).

In one embodiment the method further comprises a step

g) producing regenerated cellulose form the cellulose dope produced in step d) and/or f).

In one embodiment the regenerated cellulose produced in step g) is selected from cellulose fibers such as ryons and cellulose films, such as cellofan. The present inventors have demonstrated that in a cellulose acetate process, the mean reactivity with acetic anhydride of the fibers in the over flow fraction is higher than the mean reactivity with acetic anhydride of the fibers in the under flow fraction and the fibers in the unfractionated pulp, see table 2. The present inventors have further demonstrate that the swellability in water is higher (se table 3) and that the water retention value (WRV) is higher for the over flow fraction compared to the reference and the under flow fraction. This shows that the accessibility of the cellulose fibers in the over flow fraction is higher than for the fibers present in the reference and the under flow fraction. Thus, in one embodiment the mean reactivity with acetic anhydride, of the fibers in the over flow fraction is higher than the mean reactivity with acetic anhydride of the fibers in the under flow fraction and of the fibers in the unfractionated pulp provided in step a). In one embodiment the mean reactivity with acetic anhydride is measured as amount of produced and dissolved cellulose acetate in a cellulose acetate process. In one embodiment the mean swellability in a water solution, such as water, for the cellulose fibers in the over flow fraction is higher than the mean swellability in water for the cellulose fibers in the under flow fraction and in the unfractionated cellulose pulp provided in step a). In one embodiment the water retention value for the cellulose fibers in the over flow fraction is higher than the mean water retention value for the cellulose fibers in the under flow fraction and in the unfractionated cellulose pulp provided in step a). The mechanisms behind the increased accessibility of the fibers in the overflow fraction, demonstrated in the present disclosure, are not known in detail. It is however evident that fibers with increased WRV and increased reactivity with acetic anhydride and increased swellability in water ends up in the over flow fraction. Without being bound by theory it is possible that fibers of these characteristics have a lower density than other fibers and therefore end up in the over flow fraction. Interestingly, in the experiments performed by the present inventors, the average length of the fibers in the over flow fraction have also, in many cases, been longer than the fibers in the under flow fraction. It is therefore possible that longer fiber lengths also, to some extent, can be associated with increased accessibility.

In a preferred embodiment the lignocellulosic biomass is wood such as wood chips. In one embodiment the lignocellulosic biomass is softwood such as pine, spruce, cedar, fir, larch, douglas-fir, hemlock, cypress, redwood, or yew. In another embodiment the wood is hardwood such as birch, beech, eucalyptus, acacia, oak, ash, elm, aspen, poplar or maple. In one preferred embodiment the lignocellulosic biomass is wood chips from spruce, pine, birch, beech, acacia and/or eucalyptus. In one preferred embodiment the lignocellulosic biomass is wood chips from spruce or pine. In one preferred embodiment the lignocellulosic biomass is wood chips from spruce,

The present invention relates to an industrial process for production of cellulose products. Therefore in step a) an industrially relevant amount of pulp is produced, in step b) an industrially relevant amount of pulp is fractionated, in step c) an industrially relevant amount of the over flow fraction is swelled and in step d) and industrially relevant amount of first cellulose product is produced. Thus in one embodiment at least 100 kg pulp, preferably at least 1 ton, is produced in step a). In one embodiment at least 100 kg pulp/h, such as at least 500 kg/h is produced in step a). In one embodiment at least 100 kg of pulp is fractionated in step b). In one embodiment at least 100 kg of pulp is swelled in step c). In one embodiment, at least 100 kg of the first cellulose product is produced in step d).

Examples

Example 1 "Fractionation of dissolving cellulose":

A sample of hydrogen peroxide bleached sulfite dissolving pulp was taken out at a position in an acid sulfite process prior to drying of the pulp. The cellulose concentration in the wet sample wasl .5 % (w/w) and the sample was further diluted with water to a concentration of 0.2 % (w/w). The sample was fractionated with a hydrocyclone at room temperature such that an over flow fraction and a under flow fraction were obtained. The pH at the fractionation was about 6, the ingoing pressure was 4.5 bar and the outgoing pressure was 0.2 bar. The over flow fraction, the under flow fraction and the unfractionated reference pulp was dewatered and dried for further experiments. Fiber

length Proportion Average fiber length

(mm) % (w/w) (mm)

Reference pulp 0.2-1 48 1 .33

1 -5 52

Over flow

fraction 0.2-1 16 1 .90

1 -5 84

Under flow

fraction 0.2-1 84 0.70

1 -5 16

Table 1 :

Fiber lengths and proportion of long fibers (1 -5 mm) and short fibers

(0.2-1) in the over flow fraction, the under flow fraction and in the

unfractionated reference pulp.

Example 2 "Acetylation of cellulose":

1 g each of the dried over flow fraction, under flow fraction and reference pulp was torn and added to a vessel containing 50 ml Glacial acetic acid (>98 %). The mixture was stirred for 3 h in room temperature. The pulps were thereafter dewatered by vacuum filtration in a Buchner funnel. The dewatered pulps were then activated in a solutions consisting of 0.6 ml sulfuric acid and 100 ml glacial acetic acid for 2h at room temperature. The pulps where thereafter acetylated by addition of 5 ml acetic anhydride followed by 30 minutes incubation at 35 ⁰ C. The acetylation process was terminated by addition of 2 ml of a solution consisting of 60 %(w/w) acetic acid, 38 % (w/w) water and 2 % (w/w) ammonium acetate. In the processes for production of cellulose acetate the over flow fraction gave rise to a clear solution whereas both the reference pulp and the under flow fraction gave rise to cloudy slightly yellow solutions. This demonstrate that significantly more cellulose acetate is produced and dissolved when the over flow fraction is used as a starting material compared to if the under flow fraction or the reference pulp is used. Thus the reactivity with acetic anhydride is higher in the over flow fraction than in the under flow fraction or the reference pulp. The turbidity, which is a measure of cloudiness or haziness of a fluid caused by individual particles, for the different pulps is shown in table 2. Sample Turbidity (NTU)

Over flow fraction 1 12

Under flow fraction 203

Reference 166

Table 2: Turbidity in NTU for the over flow fraction, the under

flow fraction and the reference pulp.

Example 3 "swellability in water"

1 g each of the dried over flow fraction, under flow fraction and reference pulp was soaked in water and defibrillated followed by shaping of the pulps into cylindrical forms and drying of the pulps at 40°C. The dried cylindrical pulp samples had a volume of 6 ml. In the swelling experiment 25 ml of water was added to the top of each of the dried pulp samples and the volume was measured after 5 and 30 minutes. As shown in table 3, the swellability in water is higher in the over flow fraction than in the under flow fraction and the reference. This demonstrates that the accessibility of the fibers in the over flow fraction is higher than for the fibers in the under flow fraction or in the reference pulp.

Table 3: Swellability in water for the over flow fraction, the under flow fraction and the reference pulp, The volume incensement was measured after 5 minutes and 30 minutes.

The experiments described above were repeated in three different types of hydrocyclones using different parameters in the separation, and in all cases the over flow fraction showed a higher reactivity with acetic anhydride a higher WRV and a higher swellability in water compared to the reference or the under flow fraction.

REFERENCES

Christoffersson (2005) "Dissolving pulp : Multivariate Characterisation and Analysis of Reactivity and Spectroscopic Properties" Ph.D thesis Umea University. Sixta, et al "Evaluation of new organosolv dissolving pulps. Part I: Preparation, analytical characterization and viscose processability". (2004) Cellulose 11 (1 ): 73-83

Claims

1 . Method for producing at least one cellulose product comprising the following steps:

d) producing at least one first cellulose product from the first set of

swelled cellulose fibers obtained in step c)

wherein the at least one first cellulose product is a cellulose derivative and/or a cellulose dope

2. Method according to claim 1 wherein the cellulose pulp provided in step a) and/or the over flow fraction swelled in step c) is dissolving pulp

3. Method according to any of the previous claims wherein the cellulose pulp provided in step a) has a cellulose content below 90 % (w/w) and wherein the method further comprising the step:

a2) removing or degrading hemicellulose at a step prior to step c) such that the cellulose content in the over flow fraction is at least 90 % (w/w) in step c)

4. Method according to any of the previous claims wherein the pulping process in step a) is an acid sulfite pulping process

5. Method according to any of the claims 1 -3 wherein the pulping process ^' step a) is a pre-hydrolysis kraft pulping process

6. Method according to any of the previous claims further comprising the step of :

e) swelling at least part of the cellulose fibers present in the under flow pulp fraction by soaking at least part of the under flow fraction in a second liquid having a pH above 9.0 such that a second set of swelled cellulose fibers is obtained

f) producing at least one second cellulose product from the second set of swelled cellulose fibers obtained in step e)

7. Method according to any of the previous claims wherein the first cellulose product is a cellulose dope having a pH of 9.0 or lower

8. Method according to claims 6-7 wherein the second cellulose product is a cellulose dope having a pH of above 9.0

9. Method according to any of the previous claims wherein the first cellulose product is selected from Lyocell dope, cellulose acetated dope, cellulose sulfate, cellulose phosphate and cellulose nitrate

10. Method according to claims 6-9 wherein the second cellulose product is selected from a viscose solution, solid cellulose carbamate, cellulose carbamate dope, Ethyl hydroxyethyl cellulose, carboxymethyl cellulose, methyl cellulose, hydroxypropyl cellulose, hydroxyethyl cellulose, ethyl cellulose, propyl cellulose and ethyl -hydroxymethyl cellulose

1 1 . Method according to any of the previous claims wherein less than 40 % (w/w), preferably less than 30 % (w/w), preferably less than 20 % (w/w) of the fibers in the over flow pulp fraction have a fiber length between 0.2-1 mm and wherein more than 60 % (w/w) preferably more than 70 % (w/w) preferably more than 80 % (w/w) of the of fibers in the over flow fraction have a fiber length between 1 -5 mm

12. Method according to any of the previous claims wherein less than 40 % (w/w), preferably less than 30 % (w/w), preferably less than 20 % (w/w) of the fibers in the under flow fraction have a fiber length between 1 -5 mm and wherein more than 60 % (w/w), preferably more than 70 % (w/w), preferably more than 80 % (w/w) of the fibers in the under flow fraction have a fiber length between 0.2-1 mm

13. Method according to any of the previous claims wherein the first liquid having a pH of 9.0 or lower comprises water, acetic acid, sulphuric acid, nitric acid, phosphoric acid and/or at least one amine oxide such as NMMO

14. Method according to any of the previous claims wherein the mean reactivity with acetic anhydride of the fibers in the over flow fraction is higher than the mean reactivity with acetic anhydride of the fibers in the under flow fraction and the fibers in the unfractionated pulp provided in step a)

15. Method according to any of the previous claims wherein the mean swellability in a water solution such as water for the cellulose fibers in the over flow fraction is higher than the mean swellability in water for the cellulose fibers in the under flow fraction and in the unfractionated cellulose pulp obtained or provided in step a)