WO1995000698A1

WO1995000698A1 - Hemicellulose recovery during kraft pulping

Info

Publication number: WO1995000698A1
Application number: PCT/US1994/004798
Authority: WO
Inventors: J. Robert Prough; Bruno S. Marcoccia; R. Fred Chasse; Richard O. Laakso
Original assignee: Kamyr, Inc.
Priority date: 1993-06-25
Filing date: 1994-04-29
Publication date: 1995-01-05

Abstract

During the production of kraft pulp, by kraft cooking, one or more hemicellulose-rich extractions are treated to separate and remove a significant portion of the hemicellulose. This may be accomplished by treating the extraction with an oxidant, or by acidification to precipitate lignin out of the solution. Treatment may also include ultrafiltration at a temperature greater than 100 °C utilizing inorganic membranes, and reconstitution such as cross-linking to promote re-polymerization, or agglomeration to produce coagulated hemicellulose. After separation of the hemicellulose it is redeposited on the kraft pulp, for example in an E bleach stage, downstream of any oxygen or ozone stages. There is also a method of bleaching kraft pulp according to the sequence OODEDD, OODEPDD, or OOZEP, where the E stage includes a saccharide redeposition treatment. The kraft pulp product has increased yield and bonding compared to conventional kraft pulp.

Description

HEMICELLULOSE RECOVERY DURING .KR.AFT PULPING

BACKGROUND .AND SUMMARY OF THE INVENTION

In conventional continuous kraft pulping operations, extractions from the pulping operation, which contain significant quantities of hemicellulose, Hgnin, and like extractives from wood, are usually directed to recovery and burned as one of the components of the high solids concentration black liquor. Hemicellulose is a polymer of five different sugars, glucose, mannose, galactose, xylose and arabinose; that is, it is a saccharide.

It has recently been recognized that it is desirable to maintain the dissolved organic materials (DOM) concentration low throughout kraft cooking (see copending application Serial No. 08/056,211 filed May 4, 1993), and therefore in ensuing pulping operations it is highly likely that liquor extracted after the impregnation stage or early in the co- current cooking stage which is relatively rich in dissolved hemicellulose, will have to be dealt with. It is undesirable to merely burn the hemicellulose component of this extraction, but rather it is desirable to reintroduce the hemicellulose into the fiberline at a location that will permit redeposition of the hemicellulose onto the fiber, and thus increase the yield and bonding of the resulting kraft pulp.

It has been recognized according to the invention that one particularly desirable location for reintroduction of the hemicellulose into the fiberline is in the bleach plant, particularly downstream of any oxygen deligm^'fi cation or ozone bleaching stages (since oxygen and ozone attack hemicellulose). One particularly desirable location is in an alkaline extraction (E) stage since the hemicellulose rich extraction stream is alkaline.

It is desirable to separate the hemicellulose from other components of the stream containing dissolved hemicellulose, including colored compounds (e.g. dissolved hgnin), and one particularly desirable manner of separating the hemicellulose. is by ultrafiltration at high temperature (greater than 100° C, utilizing inorganic membranes), which may be followed by re-polymerization of fractionated hemicellulose, or agglomeration to coagulate the hemicellulose, prior to introduction into the fiber- line.

According to one aspect of the present invention, a method of producing kraft pulp is provided comprising the following steps: (a) Kraft cooking fibrous cellulosic material to produce at least one dissolved hemicellulose- rich liquid extraction, and kraft pulp, (b) Treating the hemicellulose-rich extraction to separate and remove a significant portion of the hemicellulose therefrom. And (c) redepositing the removed hemicellulose on the kraft pulp.

Step (a) is typically practiced so as to maintain the dissolved hemicellulose concentration during cooking at about 15 g/1 or less (preferably less than 10 g l). The concentration of the dissolved hemicellulose in the extraction in conventional MCC® digesters is on the order of 40 - 60% of the total organics, or typically about 40 - 60 g/l, in the bottom circulation, and about 30 - 60 g/l in the trim circulation. Where step (a) is practiced in a continuous digester, the hemicellulose-rich extraction is preferably taken from one or more of the bottom circulation line, trim circulation, and top circulation line.

The method of the invention typically has as a further step bleaching the kraft pulp in a plurality of bleaching stages, including at least one oxygen delignifi cation or ozone stage, in which case step (c) is practiced downstream of all oxygen delignifi cation or ozone stages. The bleaching stages typically include at least one E stage, and step (c) is preferably practiced in the E stage. Typical bleaching sequences are: OODEDD, OODEPDD and OOZEP; in the first of these sequences, peroxide typically is added in the E stage.

Step (b) is typically practiced to remove colored compounds from the hemicellulose-rich stream, such as by acidification to lower the pH below 9 and thereby precipitate hgnin out of solution, or by treating the extraction liquid with an oxidant, such as hydrogen peroxide or ozone, which bleaches out the color. Step (b) is particularly desirably practiced by filtration to produce a product stream and a filtrate, and by reconstituting the hemicellulose in the product stream. The filtration step is preferably either reverse osmosis or ultrafiltration. Ultrafiltration, typically at a temperature greater than 100° C utilizing inorganic membranes, is preferred. Reconstituting may be practiced by adding a cross---h-iking agent to the product stream to promote re-polymerization, or by adding a surface active reagent and otherwise practicing agglomeration to promote coagulated hemicellulose production.

The invention also contemplates kraft pulp produced by the method set forth above. The kraft pulp so produced, in addition to resulting in increased yield of the digestion process, has increased bonding compared to pulps produced without redeposition of the hemicellulose.

The invention also contemplates apparatus for producing kraft pulp. The apparatus typically comprises: Means for kraft cooking comminuted cellulosic fibrous material to produce kraft pulp. Means for withdrawing a hemicellulose-rich liquid stream from the cooking means. Means for treating the hemicellulose-rich stream to separate and remove a significant portion of the hemicellulose therefrom. And means for redepositing the hemicellulose removed by the treating means on the kraft pulp.

The redepositing means preferably comprises an E bleach stage and a conduit extending from the treating means of the E bleach stage. The means for treating the hemicellulose-rich stream preferably includes an inorganic membrane ultrafiltration unit, and downstream of the ultrafiltration unit means for re-polymerization fractionated hemicellulose or means for agglomerating hemicellulose. The means for treating also may include means for adding an oxidant to effect oxidation of colored compounds, or means for acidifying the stream to precipitate hgnin out. The kraft cooking means typically comprises a Kamyr, Inc. continuous digester having a top circulation Line, bottom circulation line, or trim circulation, and the means for withdrawing the hemicellulose-rich stream comprises one or more of the top circulation line, bottom circulation line, and trim circulation.

According to yet another aspect of the present invention, a method of bleaching of kraft pulp utilizing an E stage which includes a saccharide redeposition treatment is provided.

Yet another aspect of the present invention contemplates a method of bleaching kraft pulp according to the sequence OODEDD, OODEPDD, or OOZEP, where the E stage includes a saccharide redeposition treatment.

It is a primary object of the present invention to insure effective utilization of hemicellulose separated during kraft cooking of pulp, allowing pulp yield and bonding of the resulting pulp to be increased. This and other objects of the invention will become clear from an inspection of the invention, and from the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGURE 1 is a schematic view of basic exemplary apparatus according to the present invention for practicing an exemplary method according to the present invention;

FIGURE 2 is a schematic view of one exemplary hemicellulose-rich extraction treatment apparatus according to the invention;

FIGURE 3 is a schematic view of an alternative exemplary hemicellulose-rich extraction stream treatment apparatus according to the invention; and

FIGURES 4 and 5 are schematic representations of two additional exemplary components of apparatus according to the present invention. DETAILED DESCRIPTION OF THE DRAWINGS

FIGURE 1 schematically illustrates a chip bin, steaming vessel, and like conventional conventional equipment for handling wood chips or like comminuted cellulose fibrous material, shown schematically at 10, for enitraining the wood chips in hquid and feeding it to a high pressure feeder 11 such as that sold by Kamyr, Inc. of Glens Falls, New York. Feeder 11 feeds the chips slurry to the top of a conventional Kamyr, Inc. continuous digester 12, either directly or through an impregnation vessel.

FIGURE 1 schematically illustrates a single vessel hydraulic digester 12, having a return or top circulation line 13 to the high pressure feeder 11 and a trim circulation Line 14. A two vessel hydraulic digester could alternatively be provided, in which case the line 13 is a bottom circulation (BC) line. Other conventional digester systems could be utilized. The digester 12 illustrated in FIGURE 1 includes an extraction from the top circulation line 130, an extraction from the trim circulation line 140, and a conventional line extraction 15. Lines 130, 140 and 15 may, according to the present invention, optionally be connected to a line 19 which leads to a treatment means 20.

The hquid extractions from the bottom circulation and trim circulation in a conventional two vessel hydraulic system have hemicellulose concentrations on the order of about 30 - 60 g/l, and it is desirable to practice kraft cooking in the digester 12 (in fact in the entire cooking process) so that the DOM concentration is at 100 g/l or less throughout substantially the entire kraft cook, and so that the dissolved hemicellulose concentration is about 15 g/l or less (preferably about 10 g/l or less) throughout substantially the entire kraft cook, as disclosed in copending application Serial No. 08/056,211 filed May 4, 1993.

The apparatus 20 comprises means for treating the hemicellulose- rich stream in line 19 to separate and remove a significant portion of the hemicellulose therefrom. The details of the separation means options will be described below. The apparatus of FIGURE 1 also includes a kraft pulp discharge (blow) line 22 from the continuous digester 12, which is operatively connected to a plurality of bleaching stages 24. A particular exemplary sequence illustrated in FIGURE 1 is first and second oxygen dehgnification stages 25, 26, an ozone stage 27, an alkaline extraction stage 28, and a peroxide stage 29, resulting in bleached pulp being produced and discharged in line 30. According to the present invention there also is provided means for redepositing the hemicellulose removed by the treatment means 20 on the kraft pulp. In the exemplary embodiment this means includes the discharge line 32 from the apparatus 20 which has separated hemicellulose (other products being shown as schematically discharged at 33 in FIGURE 1), the conduit 32 leading to the alkaline extraction stage 28 wherein the removed hemicellulose is redeposited on the kraft pulp, increasing the yield. The pulp in line 30, produced according to the invention, has increased bonding compared to conventional kraft pulp.

FIGURE 2 illustrates an apparatus 35 that may be utilized downstream of the apparatus 20 in FIGURE 1. The apparatus 35 illustrated in FIGURE 2 is for oxidant bleaching of the hemicellulose-rich stream. Ozone, hydrogen peroxide, or like oxidant is added via line 36 in order to bleach colored compounds, such as dissolved hgnin, which would otherwise counteract the bleaching and dehgnification done in the stages of the bleach plant 24 upstream of the connection of the line 32 thereto.

FIGURE 3 illustrates another alternative or additional mechanism for treating the hemicellulose extraction stream in line 19. FIGURE 3 illustrates a filtration stage 38, including a filtrate line 39 and a product line 40. The product line 40 leads to a re-polymerization stage 42 into which a conventional cross-linking agent is introduced via line 43 to promote re-polymerization of fractionated hemicellulose.

The filtration stage 38 may comprise a reverse osmosis unit, but preferably comprises an ultrafiltration unit. Most desirable is ultrafiltration at high temperature (that is greater than 100° C. Conventional ultrafiltration is limited to temperatures less than 100° C which requires the filtrate to be cooled, but it is undesirable to cool the streams according to the invention. Ultrafiltration can be performed at high temperatures according to the present invention by using inorganic membrane technology. One particularly desirable inorganic membrane that may be utilized is the ceramic membrane filtration product sold under the trademark "Membralox"® by U.S. Filter Corporation of Warrendale, Pennsylvania. Not only does utilization of the ceramic membranes allow treatment at high temperature, the ceramic ultrafiltration units are easier to clean than conventional membranes. The micron-size pores in conventional ultrafiltration membranes are prone to fouling and plugging, and are difficult to clean. Ceramic membranes, unlike conventional organic membranes, can be acid cleaned or pyrolyzed to burn off contaminants, and are more structurally stable than conventional membranes, being more rigid, easier to handle, and having structural rigidity.

FIGURE 4 illustrates another exemplary treatment for the hemicellulose-rich extraction utilized according to the invention, as well as an alternative particular bleaching sequence for the pulp in line 22. FIGURE 4 utilizes an ultrafiltration unit 38, but instead of a re- polymerization unit 42 an agglomeration means 45 is provided. A surface active reagent may be added in line 46 to the agglomeration means 45 in order to facilitate coagulation of the hemicellulose before it is reintroduced in line 32 to the E stage 28. FIGURE 4 illustrates a different bleaching sequence than illustrated in FIGURE 1, this sequence utilizing two oxygen dehgnification stages 25, 26, as in FIGURE 1, then a first chlorine dioxide stage 48, followed by the alkaline extraction stage 28, and then by other chlorine dioxide stages 49, 50. In the FIGURE 4 embodiment, the E stage preferably comprises Ep stage, peroxide being added.

In the embodiment of FIGURE 5, a filtration unit 38 is replaced by an acidification stage 52 in which the pH of the hemicellulose-rich stream is reduced to below 9 so that the dissolved hgnin precipitates out, as illustrated at 53 in FIGURE 5. The Hgnin depleted, hemicellulose- rich stream in line 54 may be passed directly to the E stage 28, or through a re-polymerization or agglomeration means 42, 45 if desired. FIGURE 5 also illustrates another bleaching sequence, similar to that of FIGURE 4 except induding the P stage 29 between the E stage 28 and the D-^ stage 49.

The E stage according to the present invention, illustrated in each of FIGURES 1, 4, and 5, may be denoted as an "E_gR" stage, "SR" standing for saccharide redeposition treatment. This is a unique bleaching stage, per se.

It will thus be seen that according to the present invention an advantageous method and apparatus are provided for producing kraft pulp, which utilizes hemicellulose extracted during digestion in an effective manner, including to increase yield and to produce a new kraft pulp, having increased bonding. While the invention has been herein shown and described in what is presently conceived to be the most practical and preferred embodiment thereof, it will be apparent to those of ordinary skill in the art that many modifications may be made thereof within the scope of the invention. For example, a batch digester could be utilized for cooking the pulp. Therefore, the invention's scope is to be accorded the broadest interpretation of the appended claims so as to encompass all equivalent methods, apparatus, and products.

Claims

WHAT IS CLAIMED IS:

1. A method of producing kraft pulp, comprising the steps of:

(a) kraft cooking fibrous cellulosic material to produce at least one dissolved hemicellulose-rich hquid extraction, and kraft pulp;

(b) treating the hemicellulose-rich extraction to separate and remove a significant portion of the hemicellulose therefrom; and

(c) redepositing the removed hemicellulose on the kraft pulp.

2. A method as recited in claim 1 wherein step (a) is practiced so as to maintain the dissolved hemicellulose concentration during cooking at about 15 g/l or less, and wherein the hemicellulose-rich extraction has about 30 - 60 g/l dissolved hemicellulose.

3. A method as recited in clai 1 comprising the further step of bleaching the kraft pulp in a plurality of bleaching stages m uding an alkaline stage, and wherein step (c) is practiced in the alkaline stage.

4. A method as recited in claim 3 wherein the alkaline stage is an alkaline extraction stage E.

5. A method as rerited in daim 1 comprising the further step of bleaching the kraft pulp in a plurality of bleaching stages mduding at least one oxygen dehgnification or ozone stage, and wherein step (c) is practiced downstream of all oxygen dehgnification or ozone stages.

6. A method as recited in claim 5 wherein the bleaching stages indude at least one alkaline stage, and wherein step (c) is practiced in the alkaline stage.

7. A method as recited in claim 6 wherein the alkaline stage is an alkaline extraction stage E.

8. A method as redted in daim 1 wherein step (b) is practiced to remove colored compounds from the hemicellulose-rich stream.

9. A method as recited in claim 8 wherein step (b) is further practiced by treating the hemicellulose-rich extraction with an oxidant.

10. A method as redted in daim 8 wherein step (b) is further practiced by addification to lower the pH below 9 and thereby precipitate hgnin out of solution.

11. A method as redted in claim 8 wherein step (b) is further practiced by filtration to produce a product stream and a filtrate, and by reconstituting the hemicellulose in the product stream.

12. A method as redted in daim 11 wherein said filtration step is selected from the group consisting essentially of ultrafiltration and reverse osmosis.

13. A method as redted in daim 11 wherein said filtration step is practiced by ultrafiltration at a temperature greater than 100° C utihzing inorganic membranes.

14. A method as redted in claim 13 wherein said reconstituting step is practiced by a method selected from the group consisting essentially of cross-linking to promote re-polymerization, and agglomeration to promote coagulated hemicellulose production.

15. A method as recited in claim 11 wherein said reconstituting step is practiced by a method selected from the group consisting essentially of cross-linking to promote re-polymerization, and agglomeration to promote coagulated hemicellulose production.

16. A method as redted in claim 1 wherein step (a) is practiced in a continuous digester having a top circulation line, a bottom circulation line, or a trim drculation, and wherein the hemicellulose-rich extraction is taken from one or more of the top circulation line, bottom circulation line, and trim circulation.

17. A method as redted in daim 1 wherein step (a) is practiced in a batch digester having a top circulation line, a bottom circulation line, or a trim drculation, and wherein the hemicellulose-rich extraction is taken from one or more of the top circulation, bottom drculation, and trim circulation.

18. A method as redted in claim 1 comprising the fiirther step of subjecting the kraft pulp to a bleaching sequence selected from the group consisting essentially of OODEDD, OODEPDD and OOZEP.

19. A method as recited in claim 18 wherein step (c) is practiced in the alkaline stage.

20. A method as recited in claim 19 wherein the alkaline stage is an alkaline extraction E stage.

21. A method as redted in claim 1 wherein the extraction from step (a) is taken from a continuous digester system in an impregnation stage or early in a co-current cooking stage.

22. Apparatus for producing kraft pulp, comprising: means for kraft cooking comminuted cellulosic fibrous material to produce kraft pulp; means for withdrawing a hemiceUulose-rich hquid stream from said cooking means; means for treating the hemicellulose-rich stream to separate and remove a significant portion of the hemicellulose therefrom; and means for redepositing the hemicellulose removed by said treating means on the kraft pulp.

23. Apparatus as redted in daim 22 wherein said redepositing means comprises an E bleach stage and a conduit extending from said treating means to said E bleach stage.

24. Apparatus as redted in daim 22 wherein said means for treating the hemicellulose-rich stream includes a ceramic membrane ultrafiltration unit.

25. Apparatus as redted in claim 24 wherein said treating means further comprises, downstream of said ultrafiltration unit, means for re- polymerizing fractionated hemicellulose, or means for agglomerating hemicellulose.

26. Apparatus as recited in daim 22 wherein said means for treating the hemicellulose-rich stream indudes means for adding an oxidant to effect oxidation of colored compounds, or means for acidifying the stream to predpitate hgnin therefrom.

27. Apparatus as redted in daim 25 wherein said means for treating the hemicellulose-rich stream indudes filtration means.

28. Apparatus as redted in claim 22 wherein said kraft cooking means comprises a continuous digester having a top circulation line, bottom circulation line, or trim circulation; and wherein said means for withdrawing a hemicellulose-rich stream comprises one or more of said top circulation line, bottom circulation line, and trim circulation.

29. A method of bleaching kraft pulp according to the sequence OODEDD, OODEPDD, or OOZEP, where the E stage indudes a saccharide redeposition treatment.

30. A method as redted in daim 29 wherein the bleaching sequence is OODEDD, and wherein peroxide is added during the E stage.

31. A method of bleaching kraft pulp utihzing an E stage which includes a saccharide redeposition treatment.

32. Kraft pulp produced by the method of kraft cooking fibrous cellulosic material to produce at least one hemicellulose-rich extraction and kraft pulp; treating the hemicellulose-rich extraction to separate and remove a significant portion of the hemicellulose therefrom; and redepositing the removed hemicellulose on the kraft pulp to thereby increase the bonding of the kraft pulp produced.