EP0287960A2 - A process for the production of a hemicellulose hydrolysate and special pulp - Google Patents

Abstract

The invention relates to a process for the pro­duction of a hemicellulose hydrolysate and special pulp through two steps, the first step comprising the prehydrolysis of the material and the second step the dissolving of the lignin contained in the prehydro­lyzed material. According to the process the lignin dissolving is carried out by means of a neutral sul­phite cooking with anthraquinone or a derivative thereof as a catalyst, the pH of the cooking liquor being initially at least 10.

Description

• The invention relates to a process for the pro­duction of a hemicellulose hydrolysate and special pulp from a material containing lignocellulose through two steps, the first step comprising the hydrolysis of hemicelluloses into simple sugars and the second step the dissolving of lignin for liberating cellulose fibres.
• Traditionally, there are two processes for the production of special pulps having a high content of alpha cellulose, such as dissolving pulp: the far­advanced acidic bisulphite cooking and the prehydroly­sis-sulphate cooking. The former was developed at the beginning of the 20th century and the latter in the 1930's, see e.g. Rydholm, S.E., Pulping Processes, p. 649 to 672, Interscience Publishers, New York, 1968. The basic idea in both processes is to remove as much hemicellulose as possible from cellulose fibres in connection with the delignification so as to obtain a high content of alpha cellulose. This is essential because the various uses of dissolving pulp, for in­stance, do not tolerate short-chained hemicellulose molecules with indefinite structure. In the sulphite process, the removal of hemicellulose takes place during the cooking simultaneously with the dissolving of lignin. The cooking conditions are highly acidic and the temperature varies from 140 to 150°C, whereby the hydrolysis is strong. The result, however, is always a compromise with delignification, and no high content of alpha cellulose is obtained. Another draw­back is the decrease in the degree of polymerization of cellulose and the yield losses, which also limit the hydrolysis possibilities. Various improvements have been suggested in traditional sulphite cooking, the use of additional chemicals, for instance. Such additional chemicals, used in addition to the basic chemicals of sulphite cooking, include sulphide, white liquor, and anthraquinone, see e.g. Finnish Patent Specification 67 104 and U.S. Patent Specification 4 213 821. These sulphite cooking variations do not, however, imply hydrolytic conditions.
• A separate prehydrolysis step is interesting in the view of the fact that it enables the adjustment of the hydrolysis of hemicelluloses as desired by varying the hydrolysis conditions. In the prehydrolysis-sul­phate process the delignification is not carried out until in a separate second cooking step. The prehydro­lysis is carried out either as a water prehydrolysis or in the presence of a catalyst. Organic acids libe­rated from wood in the water prehydrolysis perform a major part of the process, whereas small amounts of mineral acid or sulphur dioxide, in some cases even sulphite waste liquor, are added to the digester in "assisted" prehydrolysis. It has previously been ne­cessary to effect the lignin dissolving step after the prehydrolysis as sulphate cooking which has several drawbacks. The prehydrolysis-sulphate process has e.g. the following drawbacks:
      - The yield is low because of the strong alka­line reaction conditions which cause splitting of cel­lulose. Thus the wood consumption per one ton of cel­lulose is high.
      - The content of residual lignin is rather high because the step for the removal of residual lignin in the sulphate cooking process is extremely non-select­ive. Thus there is a great need of bleaching for com­plete removal of lignin, and the consumption of chemi­cals is high; further, at least five bleaching steps are required.
      - Industrial realization of sulphate cooking is complicated, and the cost of investment very high.
• Previously the use of sulphite cooking has not been possible, because it is not possible to dissolve from wood material lignin deactivated in the pre­hydrolysis by means of traditional sulphite cooking processes. It has been regarded as impossible to use a sulphite cooking step (cf. Rydholm above) even though it would have advantages over sulphate cooking.
• It has now been found out unexpectedly that ex­cellent results can be obtained by effecting the lignin dissolving after the prehydrolysis by an alka­line neutral sulphite cooking with anthraquinone or a derivative thereof as a catalyst. Such a cooking is known per se from the prior art (see e.g. U.S. Patent Specification 4 213 821); on the contrary, a com­bination of prehydrolysis and such a cooking has not been set forth previously.
• The invention relates to a process for the pro­duction of hemicellulose hydrolysate and special pulp from a material containing lignocellulose through two steps, the first step comprising the prehydrolysis of the material and the second step dissolving of the lignin contained in the prehydrolyzed material. The process is characterized in that the dissolving of lignin is carried out by means of neutral sulphite cooking with anthraquinone or a derivative thereof as a catalyst, the pH of the cooking liquor being in­itially at least 10.
• Suitable prehydrolyzing agents include e.g. water, mineral acid, sulphur dioxide, sulphite cooking acid, and sulphite waste liquor. Preferred prehydro­lyzing agents include sulphur oxide, sulphuric acid, and water.
• A suitable prehydrolyzing temperature is 100 to 180°C, preferably 155 to 170°C, and a suitable hydro­lyzing time is 10 to 200 minutes, preferably 90 to 170 minutes.
• The material containing lignocellulose prefer­ably consists of softwood or hardwood.
• The cooking step is suitably carried out with a cooking liquor comprising 100 to 400 g of sodium sul­phite/kg of dry wood; 10 to 100 g of sodium carbon­ate/one kg of dry wood; sodium hydroxide for rising the pH of the cooking liquor to a value between 10 and 13; and 0.01 to 0.2%, calculated on dry wood, of anthraquinone or a derivative thereof.
• The cooking temperature preferably ranges from 160 to 180°C, and the cooking time is suitably 100 to 200 minutes after the temperature has risen 0.1 to 2°C/min from a temperature varying between room tem­perature and 100°C.
• It is typical of the prehydrolysis-neutral sul­phite-anthraquinone process (PH-NS-AQ process) that delignification to a low content of residual lignin is easy to carry out while the yield of cellulose fibre, however, remains on an exceptionally high level. Thus it is possible to use strong prehydrolysis conditions (e.g. strong acids, such as H₂SO₄), whereby the hydro­lysis of hemicelluloses into simple sugars is effi­cient; on the other hand, the alpha cellulose content representing the content of residual hemicellulose in cellulose fibre is high and the content of residual pentosan is low. Due to these properties the process is particularly suitable for the production of high­quality dissolving pulp, for instance, whereby mono­saccharides are obtained simultaneously.
• As to the new process, it was found out that the use of the so called neutral sulphite anthraqui­ none cooking process effects a partial ionization of the lignin inactivated in the prehydrolysis, the in­itial pH being at least 10, e.g. 11 to 12, and that anthraquinone as an additive in the cooking catalyzes the breaking of nucleophilic beta aryl ether bonds, which at the end results in the liberation of fibres, i.e. a successful cooking. It was further found out that sulphite ions in neutral sulphite cooking react simultaneously and participate in the decomposing of the structure of lignin and above all sulphonate the lignin material and fragments which thus become more hydrophilic and dissolve more easily in the cooking liquor, thus contributing to the formation of a successful cooking and to the continuation thereof to a very low content of residual lignin. In short, the prehydrolysis-neutral sulphite anthraquinone process according to the invention not only gives a result as successful as that of the sulphate process but also provides all the advantages typical of sulphite cooking.
• The increased yield of the process according to the invention is due to the fact that there does not occur splitting of cellulose to any greater degree during the neutral sulphite cooking step. In sulphate cooking, on the contrary, the high alkalinity causes alkaline hydrolysis, and the peeling-off reaction in particular results irrevocably in a yield loss. The process according to the invention enables the recov­ery of nearly all of the high molecular weight cellu­lose material originally contained in the wood ma­terial.
• In the process-chemical sense, another advan­tage is that pulp which has undergone neutral sulphite anthraquinone cooking is easy to bleach, i.e. the re­sidual lignin remaining in the fibre after the cooking is easy to remove. This is due to the fact that the delignification resembles sulphite cooking; the con­densation of the structure of lignin is insignificant; and the sulphonation makes lignin more hydrophilic. Contrary to this, the residual lignin in sulphate cooking is strongly condensated and the content there­of is on a higher level. The removal of this kind of residual lignin in bleaching requires five to six bleaching steps and plenty of expensive chlorine di­oxide. The bleaching of pulp obtained by means of the process according to the invention can be carried out by three steps only and the demand of chemicals, too, is lower.
• The process according to the invention has the following advantages:
      - The yield of the special pulp to be produced in connection with the production of sugars is in­creased, which improves the production economy.
      - The process after the prehydrolysis is sim­plified, which decreases the cost of investment.
      - The easier delignification in the cooking step decreases the need of bleaching, thus improving the production economy and reducing the emission of chlorinated compounds from the bleaching.
      - The oxygen or peroxide step after the cook­ing is extremely efficient as compared with that of the prehydrolysis-sulphate process, whereby the re­covery and economy are improved.
      - Small-scale production is economically more interesting because it is possible to operate in con­nection with an existing sodium-based sulphite pulp mill without any appreciable additional investments.
• The following examples are illustrative of the invention.
• The following abbreviations are used in the examples:
Steps of the bleaching processes
• O = Oxygen step
  D = Chlorine dioxide step
  E = Alkali extraction
  P = Peroxide step
  H = Hypochlorite step
  C = Chlorination
  
Standards
• SCAN = Scandinavian standard
  TAPPI = U.S. standard
  
Example 1 Production of a birch hydrolysate and special pulp by means of the PH-NS-AQ process from birch chips
• Chips and a prehydrolyzing liquor were metered into a chip basket positioned in a 20-litre forced circulation digester. The cover of the digester was closed and the prehydrolysis was carried out according to the temperature program by heating the digester circulation indirectly by means of steam. After the hydrolysis time had passed, the hydrolysate was re­moved from the digester and recovered. The prehydro­lyzed chip material contained in the digester was washed in the digester for 5 minutes with warm water, the cover was opened, and the chips were passed into a centrifuge in which excess water was removed. The cen­trifugalized material was weighed and a dry substance sample was taken for determining the hydrolysis loss.
• The prehydrolyzed chip material was returned to the digester, cooking liquor and anthraquinone were added, the cover was closed, and the cooking was carried out according to the temperature program. At the end of the cooking the cooking liquor was removed rapidly and the digester was filled with cold water, whereafter water was allowed to flow for 10 hours for washing the cooked chip material. After the wash the pulp was disintegrated by means of a wet disintegrator for one minute and assorted with a flat screen plate of 0.35 mm. Shives were recovered and weighed dry for determining the shive content. The accepted fraction was passed into the centrifuge for dewatering, homo­genized, and weighed. Laboratory analyses were carried out on this pulp and the pulp was further used in bleaching tests.
Prehydrolyzing step
• Wood amount, g of abs. dry chips      2000
  Prehydrolyzing agent      SO₂
  Amount of prehydrolyzing agent, % on dry wood      0.25
  Liquor ratio      6:1
  Temperature rising time, min      40
  Prehydrolysis temperature, °C      155
  Prehydrolysis time, min      170
  Prehydrolysis loss, % on wood      26.6
  
Cooking step
• Na₂SO₃, % on wood as NaOH      22
  Na₂CO₃, % on wood as NaOH      5
  Anthraquinone, % on wood      0.1
  Liquor ratio      4.5:1
  pH of the cooking liquor      11.3
  Rising of the temperature °C/min      1
  Cooking temperature, °C      175
  Cooking time, min      170
  Yield, % on wood      39.3
  Kappa number      17.2
  Shive content, % on wood      0.1
  Properties of O-D-E-D bleached pulp
  Final yield, % of wood      36.7
  ISO brightness      87.1
  Alpha cellulose %      94.2
  Viscosity, SCAN dm³/kg      764
  
Example 2 Production of a birch hydrolysate and special pulp by the PH-NS-AQ process from birch chips
• The test was carried out as disloced in Example 1.
Prehydrolyzing step
• Wood amount, g of abs. dry chips      2500
  Prehydrolyzing agent      SO₂
  Amount of prehydrolyzing agent,
  % on dry wood (SO₂)      0.25
  Liquor ratio      3.5:1
  Temperature rising time, min      40
  Prehydrolysis temperature, °C      155
  Prehydrolysis time, min      170
  
Cooking step
• Na₂SO₃, % on wood as NaOH      20
  Na₂CO₃, % on wood as NaOH      6
  Anthraquinone, % on wood      0.1
  Liquor ratio      4.5:1
  pH of the cooking liquor      11.3
  Rising of the temperature °C/min      1
  Cooking temperature, °C      175
  Cooking time, min      170
  Yield, % on wood      46.7
  Kappa number      48.1
  Shive content, % on wood      1.35
  Properties of O-P-H bleached pulp
  Final yield, % on wood      39.7
  ISO brightness      87.1
  Alpha cellulose %      91.7
  Viscosity, SCAN dm³/kg      530
  
Example 3 Production of a birch hydrolysate and special pulp by the PH-NS-AQ process from birch chips
• The test was carried out as disclosed in Example 1.
Prehydrolyzing step
• Wood amount, g of abs. dry chips      2500
  Prehydrolyzing agent      H₂SO₄
  Amount of prehydrolyzing agent, % on dry wood      1.0
  Liquor ratio      3.5:1
  Temperature rising time, min      40
  Prehydrolysis temperature, °C      155
  Prehydrolysis time, min      90
  Prehydrolysis loss, % on wood      25.4
  
Cooking step
• Na₂SO₃, % on wood as NaOH      22
  Na₂CO₃, % on wood as NaOH      5
  Anthraquinone, % on wood      0.1
  Liquor ratio      4.5:1
  pH of the cooking liquor      11.3
  Rising of the temperature °C/min      1
  Cooking temperature, °C      175
  Cooking time, min      170
  Yield, % on wood      37.0
  Kappa number      24.9
  Shive content, % on wood      0.6
  Properties of C-E-D bleached pulp
  Final yield, % on wood      34.2
  ISO brightness      90.0
  Alpha cellulose % 94.6
  Viscosity, SCAN dm³/kg 730
  Properties of O-P-D bleached pulp
  Final yield, % on wood      34.7
  ISO brightness      84.4
  Alpha cellulose %      94.5
  Viscosity, SCAN dm³/kg 720
  
Example 4 Production of a pine hydrolysate and special pulp by the PH-NS-AQ process from pine chips
• The test was carried out as disclosed in Example 1.
Prehydrolyzing step
• Wood amount, g of abs. dry wood      2000
  Prehydrolyzing agent      H₂0
  Liquor ratio      6:1
  Temperature rising time, min      45
  Prehydrolysis temperature, °C      170
  Prehydrolysis time, min      15
  Prehydrolysis loss, % on wood      13.2
  
Cooking step
• Na₂SO₃, % on wood as NaOH      22
  Na₂CO₃, % on wood as NaOH      5
  Anthraquinone, % on wood      0.2
  Liquor ratio      4.5:1
  pH of the cooking liquor      11.3
  Rising of the temperature °C/min 1
  Cooking temperature, °C 175
  Cooking time, min      170
  Yield, % on wood      40.3
  Kappa number      16.5
  Shive content, % on wood      0.4
  Properties of O-D-E-D bleached pulp
  Final yield, % on wood      37.2
  ISO brightness      84.2
  Viscosity, SCAN dm³/kg      890
  
Reference example
• It was studied how lignin dissolves in cooking processes generally in use as compared with the cook­ing step of the process according to the invention when the chips are prehydrolyzed according to the prior art. Sulphate cooking and various modifications of sulphite cooking are processes in general use.
• In the tests the prehydrolysis/cooking was car­ried out as follows:
Test 1
• Sulphur dioxide water prehydrolysis, normal
  Normal acidic Ca bisulphite cooking step
  Kappa number 150
  
Test 2
• Sulphur dioxide water prehydrolysis, normal
  Normal acidic Ca bisulphite cooking step
  Kappa number 126
  
Test 3
• Water prehydrolysis, weak
  Normal acidic Ca bisulphite cooking step
  Kappa number 118
  
Test 4
• Sulphur dioxide water prehydrolysis, weak
  Neutralizing lime milk treatment
  Acidic Ca bisulphite cooking step with an ex­tremely high bound SO₂
  Kappa number 106
  
Test 5
• Sulphur dioxide prehydrolysis
  Cooking step 1: ammonium neutral sulphite cooking
  Cooking step 2: sulphur dioxide water acidic sulphite cooking
  Kappa number 141
  
Test 6
• Sulphur dioxide water prehydrolysis, normal
  Neutral sulphite-anthraquinone cooking step
  Kappa number 48
  
Test 7
• Sulphur dioxide water-prehydrolysis, normal
  Sulphate cooking step, normal
  Kappa number 14
• Lignin concentrations measured from the digest­er during the cooking step by means of a cooking liquor analyzer as a function of the cooking time re­duced to the same scale appear from the attached figure 1. The curves thus illustrate the dissolving of lignin as measured as an increase in the lignin con­tent of the cooking liquor. The results show that the cooking step after the prehydrolysis in Tests 1 to 4 does not dissolve lignin efficiently even though at­tempts have been made to improve these sulphite pro­cesses as much as possible. The dissolving obtained in Test 5 was better because the prehydrolysis is excep­tional and not technically reasonable. The content of residual lignin in Test 5 (the kappa number exceeding 100) is, however, technically impossible, the reason­able level being the kappa number of about 50 (= about 10% of lignin in cooked pulp). In Tests 6 and 7, lignin starts to dissolve rapidly in the relative cooking time of 100, the subsequent step being the main delignification of a successful cooking which is completed by a slow residual delignification towards the end of the cooking. In this way, the kappa level of 40 in Test 6 and the kappa level of 15 in Test 7 were achieved. Accordingly, it is obvious that an efficient removal of lignin from prehydrolyzed chip material takes place in the cooking step of the pro­cess according to the invention such as disclosed in Test 6; thus, it can replace the sulphate cooking used in Test 7.
• The tests carried out show that normal tech­nical prehydrolysis conditions inactivate lignin to such an extent that no cooking modification within an acidic or neutral cooking pH range is able to dissolve lignin even though the chip material would be neutral­ized between the prehydrolysis and the cooking. The sulphite cooking step used in the process according to the invention is operative only when the cooking con­ditions and the cooking catalyst are chosen appro­priately.

Claims (5)

1. A process for the production of a hemicellu­lose hydrolysate and special pulp from a material con­taining lignocellulose through two steps, the first step comprising the prehydrolysis of the material and the second step the dissolving of the lignin con­tained in the prehydrolyzed material, charac­terized in that the dissolving of lignin is carried out by means of a neutral sulphite cooking with anthraquinone or a derivative thereof as a cata­lyst, the pH of the cooking liquor being initially at least 10.

2. A process according to claim 1, char­acterized in that the material containing lignocellulose is hardwood.

3. A process according to claim 1, char­acterized in that the material containing lignocellulose is softwood.

4. A process according to any of the preceding claims, characterized in that the prehy­drolysis is carried out by means of water, sulphur di­oxide or sulphuric acid at a temperature of 155 to 170°C for 90 to 170 minutes.

5. A process according to any of the preceding claims, characterized in that the cooking is carried out by means of a cooking liquor comprising 100 to 400 g of sodium sulphite/kg of dry wood; 10 to 100 g of sodium carbonate/kg of dry wood; sodium hy­droxide for rising the pH of the cooking liquor to a value varying from 10 to 13; and 0.01 to 0.2% (cal­culated on dry wood) of anthraquinone or a derivative thereof, at a temperature of 160 to 180°C for 100 to 200 minutes after the temperature is raised 0.1 to 2°C/min from a temperature varying between room tem­perature and 100°C.

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