EP0584675A1 - Wood pulping with acetic acid and formic acid - Google Patents

Abstract

In a process for producing pulps from lignocelluloses, the production is carried out by heating with aqueous acetic acid under pressure and addition of formic acid, a pulp with a very low residual lignin content being obtained, which can be bleached with ozone and peracetic acid to give high degrees of whiteness, and acetic acid and formic acid being recovered by distillation, so that therefore no effluents arise.

Description

The invention relates to a method for pulp production and bleached pulp and chemical pulp obtainable by this process. This procedure can also be used to obtain lignin and hemicelluloses.

Conventional processes for pulp production such as the sulfite and sulfate processes lead to sulfur-containing waste liquors, the combustion of which contains SO₂-containing waste gases. The high residual lignin contents of the pulps of 4 to 5% by weight require large amounts of bleaching chemicals, which lead to chlorinated organic compounds in waste water. A further disadvantage of these processes is that plants with a minimum size of 1000 tp cellulose are required because of the chemical recovery by waste liquor combustion.

US Pat. No. 3,553,076 describes the wood pulping with aqueous acetic acid under pressure at 150 to 205 ° C., pulps with residual lignin contents of 2 to 3% by weight (corresponding to Kappa numbers of 12 to 20) being obtained. According to DE-A-34 35 132, wood can be digested even at normal pressure if catalytic amounts of hydrochloric acid are added to the acetic acid (acetosolve process). However, the residual lignin content of the cellulose does not decrease as a result, and chloride ions are highly corrosive in the presence of acetic acid.

Other mineral acids such as sulfuric acid, phosphoric acid, perchloric acid, MgCl₂ or nitric acid have been investigated as catalysts in wood digestion with acetic acid, which, however, consistently gave pulps with higher residual lignin contents and lead to problems in the recovery of the mineral acids.

Formic acid has also been proposed as a means of digesting wood. So in one Two-stage process Wood chips treated with formic or acetic acid in the first stage and hydrogen peroxide added in the second stage and heated to 70 to 100 ° C. However, the amounts of hydrogen peroxide required for this are too high with regard to economical process management (Poppius et al., Paper and Timber 73 (2) 154-158 (1991).

The object of the present invention is to provide a method for the production of cellulose, whereby cellulose with a significantly lower residual lignin is obtained.

This object is achieved by a process in which lignocelluloses are heated with aqueous acetic acid under pressure and with the addition of formic acid.

Wood or annual plants can be used as starting lignocelluloses. The digestion temperature is preferably between 130 and 190 ° C. The concentration of acetic acid in the digestion medium is preferably between 50 and 95% by weight, that of formic acid from 5 to 40% by weight and that of water below 50% by weight. The weight ratio of the lignocellulose to the digestion solution is preferably 1: 1 to 1:12.

According to another embodiment, the method can also be used to obtain lignin and hemicelluloses from lignocelluloses. The process can be carried out continuously and batchwise, with the comminuted lignocellulose being introduced into a pressure cooker in the case of a continuous process is extracted in countercurrent from the digestion solution and on the other hand continuously leaves the digester in extracted form. For example, 2 to 20 digestion vessels can be connected in series.

According to further preferred embodiments, the pulping of the pulp and the washing process of the pulp are included in the methods according to the invention. The lignocelluloses can be pre-extracted with a solvent to remove the ingredients, and acetic anhydride and bleaching agents can be added to the digestion solution. According to a further preferred embodiment, the lignocelluloses are impregnated with formic acid, acetic acid, acetic anhydride or their vapors before being introduced into the digestion vessel. The impregnation can also be carried out with a solvent or its vapors, which forms an aceotrope with water.

The lignins and hemicelluloses, which are also obtained with a high degree of purity, can be used, for example, for glue production.

The process according to the invention has the advantage over the conventional processes for the production of cellulose that it does not use any inorganic pulping chemicals and therefore no SO₂-containing waste gases or heavy metal-containing waste water are produced. The pulps can be bleached with ozone in acetic acid and / or hydrogen peroxide, whereby neither chlorine nor waste water containing heavy metals is formed. Formic and acetic acid are recovered by distillation, so that the lignin and the hemicelluloses do not need to be burned for chemical recovery. Another advantage is that the digestion temperature of about 10 ° C is lower than in the conventional methods, which significantly reduces energy costs.

The pulp obtained according to the invention has a significantly lower residual lignin content and improved properties. From Table 1 it can be seen that the addition of 10% by weight of formic acid under otherwise similar pulping conditions results in a reduction in the Kappa number from 15.6 to 3.6, corresponding to a lignin content from 2.5 to 0.5, while the Yield drops only slightly.

The same applies to poplar and miscanthus pulp (Tab. 1). The whiteness of the three pulps increases accordingly by 8 to 15%. Low kappa numbers and higher degrees of whiteness mean less use of the expensive bleaching chemicals, which are important for the economics of the process.

Tab. 1 compares the pulp properties after digestion (2h, 180 ° C, poplar and miscanthus 170 ° C) with 85% acetic acid and with 85% acetic acid and 10% formic acid. As can be seen from this table, the strength properties of the pulps obtained with the addition of formic acid also increase significantly. This applies in particular to the tear resistance, which is generally lower in the case of acidic digestion processes than in the case of alkaline ones, for example the sulfate process. Since sulfate pulps are now generally regarded as the standard for paper production, the increase in tear resistance is of particular importance in the process according to the invention. Table 1 Acetic acid 85% Acetic acid 85% + formic acid 10% Spruce poplar Misc. Spruce poplar Misc. Kappa number 15.6 9.2 13.3 3.6 3.1 3.2 Yield (%) 49.0 50.1 48.5 46.8 50.3 48.2 Whiteness (% ISO) 20.3 20.0 25.9 28.0 34.7 33.8 GVZ (ml / g) 1050.0 1005.0 1022.0 1179.0 849.5 1012.0 DPW 3035.0 2850.0 2910.0 3490.0 2430.0 2870.0 Rip-off * (cN) 59.1 31.1 51.5 77.1 42.7 90.4 Bursting area * (m²) 62.9 31.4 24.6 70.9 38.0 43.9 Tearing length * (km) 10.3 7.5 5.2 11.4 7.3 8.1 R-10 (%) 90.1 85.8 88.5 93.6 89.4 91.0 Mannose (%) - 2.3 - 0.5 1.2 0.2 Xylose (%) - 5.4 - 1.5 2.4 3.3 Glucose (%) - 92.3 - 94.2 93.5 96.2 * Paper strength at 30 ° SR freeness

In this table, GVZ mean the intrinsic viscosity according to Staudinger, DPW the degree of polymerization and R-10 the remaining pulp, which is insoluble in 10% NaOH.

The increase in the R-10 values in connection with the low xylose and mannose contents, also evident from table 1, means lower hemicellulose contents of the cellulose obtained with formic acid addition and thus its suitability as starting materials (chemical cellulose) for the production of cellulose derivatives. The process according to the invention offers advantages in particular in the production of cellulose acetate, because at the pre-swelling of the pulp in acetic acid before acetylation and an acetic acid recovery step are eliminated.

The optimal formic acid concentration depends on the digestion temperature, the digestion time, the type of wood and the water content of the digestion medium. As can be seen from Table 2, at 190 ° C with 20% formic acid, the lignin condensation already prevails after 1 h, which is why in Table 1 a two-hour digestion with 10% formic acid was chosen at 180 and 170%. The acetic acid concentration in Tab. 2 is 85%.

Formic acid increases the acidity of the digestion medium and thus the lignin breakdown, while the lignin condensation increases more slowly. Compared to mineral acids as catalysts, the selectivity of formic acid in lignin degradation appears to be increased. In addition, formic acid increases the solubility of the lignin in the exclusion medium.

The chlorine-free bleaching of the cellulose obtained by the process according to the invention is fundamentally simplified compared to that of conventional cellulose. Today, conventional bleaching usually uses five bleaching stages required with oxygen, peroxide, ozone, sodium hydroxide solution and possibly chlorine dioxide, two to three bleaching stages with small amounts of ozone in acetic acid and / or peracetic acid are sufficient for the bleaching of the process according to the invention.

In the following examples, the percentages relate to the weight.

example 1

Spruce wood chips (20 x 35 x 5-6 mm) with a moisture content of 8% were poured with 6 times the amount by weight of 85% acetic acid, which contained 10% formic acid, and heated in a rotary autoclave at 180 ° C for 2 hours (heating time 40 min). Then part of the cooking liquor was evaporated to below 100 ° C., the fibrous material was pressed off on a suction filter and washed with 85% acetic acid. The filter cake was opened with a laboratory mixer in a large beaker under 85% acetic acid and suction filtered again. The pulp obtained was splinter-free and had the properties given in Table 1.

For comparison, spruce wood chips with 85% acetic acid, which did not contain formic acid, were digested and worked up under otherwise identical conditions. The properties of the pulp obtained under these conditions are also shown in Table 1.

The spruce pulp obtained with the addition of formic acid (Table 1) was washed on a suction filter with glacial acetic acid, pressed to a consistency of 35%, fluffed up in a coffee grinder for 30 seconds and then in a rotary flask on a rotary evaporator with a 3% ozone-oxygen mixture fumigated. The pulp was then washed on a suction filter first with water and then with a 0.2% peracetic acid solution in water, pressed to 15% consistency, heated at 80 ° C for 1 h and finally washed on the suction filter with water. The bleached spruce pulp has the properties shown in Table 3.

In a second approach, spruce pulp was bleached at 15% consistency only with peracetic acid, first in acetic acid at 0.7% at 80 ° C, 90 min, and then in water at 1.3% at 80 ° C, 120 min. The results can also be found in Tab. 3.

Example 2

Wood chips (80 x 20 x 5 mm) of a six-year-old poplar (Populus nigra from the clone "Rapp") with a moisture content of 10% were poured with six times the amount of 85% acetic acid, which contained 10% formic acid, and put in a rotary autoclave for two hours heated to 170 ° C. The pulp was worked up, defibred and washed as described in Example 1 for spruce pulp. The pulp properties are shown in Tab. 1.

The pulp was bleached in two stages with peracetic acid, first with 0.7% in 6.6 parts of glacial acetic acid for 90 minutes at 80 ° C, and then with 1.3% peracetic acid in 6.6 parts of water for 120 minutes at 80 ° C . The properties of the bleached pulp are shown in Tab. 3.

Example 3

Stems of Miscanthus sinensis "Giganteus" chopped to a length of 2.5 cm and having a moisture content of 18% were poured with ten times the amount of 85% acetic acid, which contained 10% formic acid, and heated in a rotary autoclave at 170 ° C. for 2 hours (Heating up time 40 min). Refurbishment, defibration and the pulp was washed as described in Example 1 for spruce pulp. The pulp was splinter-free. Its properties are shown in Table 1 and are compared to those of cellulose obtained under the same conditions but with the exclusion of acetic acid.

The pulp was bleached in two stages using peracetic acid, as described for poplar cellulose in Example 2. The properties of the bleached pulp can be found in Tab. 3.

Example 4

Spruce woodchips of the type specified in Example 1 were poured over with six times the amount of 85% acetic acid, which contained 5, 10, 15 or 20% formic acid in four batches, and were each heated to 190 ° C. in a rotary autoclave for 1 hour. The resulting pulps were worked up, defibred and washed in the same manner as in Example 1. After that, the pulps were splinter-free. Their residual lignin contents, degrees of whiteness and yields are shown in Table 2. Table 3 Properties of the pulp bleached with ozone (Z) and peracetic acid (Pa) at 20 SR freeness cellulose Bleach Amount (%) Whiteness (% ISO) Tearing length (km) Bursting area (m²) Rip-off (cN) Spruce Z / Pa 0.6 / 1.3 84.3 9,315 52.5 76.7 Father 0.7 / 1.3 72.1 9.113 50.3 79.1 poplar Father 0.7 / 1.3 83.4 6.68 28.8 46.0 Miscanthus Father 0.7 / 1.3 83.0 6,933 35.6 89.6

Example 5 A. Information

Cooker: 2.5 x 10 m = 49 m³
Digestion solution: acetic acid / water / formic acid (75:15:10)
Temperature: 160-180 ° C
Time: 1 - 2 h
Fleet ratio: 1: 5
One cooker is sufficient for the test phase (batch process, 25 t of pulp / d), while 6 to 12 cookers are connected in series for production (semi-continuous process, maximum 300 t of pulp / d). Only by connecting several cookers in series is it possible to extract the wood chips according to the countercurrent principle with optimal use of the digestion solution. The wood chips are heated by pumping the digestion solution heated externally in heat exchangers.

B. bleaching with hydrogen peroxide

The first bleaching stage is carried out with 1 to 2% hydrogen peroxide in the cooker after the digestion has ended and the extract has been displaced by fresh digestion solution, 1 to 2 hours at 70 to 90 ° C. A uniform distribution of the H₂O₂ takes place by pumping around the bleaching solution, the composition of which, apart from the H₂O₂, does not differ from the digestion solution. The active agent is peracetic acid, the formation of which is catalyzed by the formic acid present.

C. Sorting

Sorting consists of post-defibrating (separation), rough sorting and cleaning. For the first two steps, a slot sorter equipped with stirring arms, slot width about 0.4 mm, in the manner of a pipe centrifuge, and a hydrocyclone for cleaning are proposed. The diffuser (thickener) must be very effective in order to get from a consistency of about 1%, which is necessary for the sorting, to at least 8%, from which a consistency of about 40% for the ozone bleaching is achieved in a screw press got to.

In the sorting process, the cellulose washing takes place at the same time. A separate wash, as in the conventional processes, is not necessary since no inorganic pulping chemicals have to be washed out and the pulp leaving the cooker hardly contains any lignin. The effectiveness of the sorting can be improved by several pipe centrifuges or cyclones connected in series.

D. Ozone bleaching

Ozone bleaching takes place in a rotary drum at 20 to 50 ° C and a consistency of about 40%, the residence time of the pulp should be at least 10 minutes, ozone amount about 0.5%, calculated on pulp. Because of the good solubility of the ozone in acetic acid, it is not necessary to fluff up the pulp.

Due to the risk of explosion of acetic acid vapors with oxygen-ozone mixtures, an explosion-proof version of the rotating drum is required. The exhaust gases should be kept in a circuit or a closed system. Ozone discharged with the pulp decomposes within a certain time. A check in the distillation column appears to be absolutely necessary, especially with high ozone doses (0.5%). Possibly the acetic acid / butyl acetate mixture to be distilled would have to be degassed in vacuo or the excess ozone would have to be chemically removed (test with KJ solution).

E. Solvent exchange

After ozone bleaching, the pulp still contains about 60% digestion solution, which is displaced in an extraction tower (2.0 × 10 m) with butyl acetate. Because the pulp swells more in acetic acid than in butyl acetate, there should be no clogging problems in the tower.

The amount of butyl acetate that leaves the tower at the top together with the digestion solution should be about 60% of the dry weight of the pulp if the wood moisture content of the wood chips is 10%, because then in the subsequent distillation 20% water, based on the pulp weight, than Aceotropically with butyl acetate leaves the distillation column in the upper part. Under these conditions, the butyl acetate would completely leave the solution mixture as an aceotrope, while 2.5% water remained in the digestion solution, which flows back undistilled into the storage tank. Small amounts of extract substances, furfural etc. remain in the digestion solution and do not interfere with the digestion. A separation of the Formic acid from acetic acid by distillation is not required. Changes in the composition of the digestion solution (see under A.) must be compensated for by adding the component present in the deficit.

F. Exchange the butyl acetate for water

The butyl acetate is exchanged for water with steam in a desolventizer. The consistency of the pulp is brought to about 40% by a screw press upstream of the desolventizer. Since the enthalpy of vaporization of the butyl acetate is only about 1/5 of that of the water, the pulp leaves the desolvent coating with 12% moisture and is then pressed in a press into sheets of 1 m².

G. Evaporation of the waste liquor

The waste liquor leaving the stove contains 17% dissolved lignin and hemicelluloses. They are concentrated to a 50% thick liquor in a six-stage tube evaporator with a pressure gradient, using the heat of condensation of the evaporated digestion solution.

H. distillation

The distillation column only serves to separate the water introduced with the chips from the digestion solution as an aceotrope with butyl acetate. After the water has been distilled off, the digestion solution flows undistilled into the storage tank. The capacity of the column depends on the wood chip moisture. If this is 10%, there are 200 kg of water per ton of pulp (together with approx 600 kg of butyl acetate).

Since the digestion solution contains 15% water, 1.333 m³ of digestion solution + 0.6 m³ of butyl acetate would be required per ton of pulp. With a wood chips moisture of 20%, the amount doubles. It should therefore be considered whether the wood chips can be pre-dried, which would also be advantageous for the storage capacity of the wood chips. Drying the wood chips has no influence on the wood digestion using the Formacell process.

I. Spray drying the thick liquor

Laboratory and pilot plant trials have resulted in a powder containing about 1% acetic acid.

K. General comments

All parts of the system that come into contact with the hot digestion solution must use steels that are corrosion-resistant to acetic acid / formic acid / water mixtures. This applies in particular to the cooker, the distillation column and the spray dryer.

Pulp grades

Tab. 4 compares the properties of unbleached spruce pulps obtained by the process according to the invention (Formacell process) with those of sulfate and acetosolv pulps. Table 4 Formacell sulfate Acetosolv Kappa number 3.6 30.6 15.6 Whiteness (% ISO) 28.0 24.8 20.3 GVZ (ml / g) 1179.5 902.2 1059.0 DP 3490 2470 3035 R-10 (%) 93.6 88.3 90.1 Yield (%) 46.8 47.4 49.0

What is striking is the very low kappa number of the Formacell pulp with only a slightly lower yield than the conventional sulfate pulp, which requires a significantly lower need for bleaching chemicals. Due to their high R10 value, Formacell pulps are also suitable for the production of cellulose derivatives.

Tab. 4 also shows the improvements of the Formacell process compared to the earlier Acetosolv process, which are mainly due to a significant improvement in delignification and an increase in whiteness.

Fig. 1 shows a comparison of the strengths (tear length and tear strength) of Formacell and sulfate spruce pulp depending on the degree of grinding.

While the tear length of Formacell spruce pulp is above the values of the sulfate pulp at all grades, the tear strength of the Formacell pulp is lower. Overall, the strength potential of the Formacell pulp is roughly comparable to that of the sulfate pulp.

Even more favorable values are obtained with Miscanthus pulps, as can be seen from Table 5. Here not only lower kappa numbers and higher degrees of whiteness are obtained, but also significantly higher tear resistance than with the conventional soda process. In recent investigations, even higher tear resistance, which comes close to that of spruce sulfate pulps, could be obtained. Since the Formacell process, in contrast to the soda process, does not produce lye containing sodium silicate, it is particularly suitable for the digestion of annual plants. Tab. 5 Properties of Miscanthus pulps obtained by three different processes Formacell soda Acetosolv Kappa number 3.2 27.2 13.2 Whiteness (% ISO) 33.8 26.6 25.9 GVZ (ml / g) 1012 1010 1022.5 DP 2870 2870 2910 R-10 91.0 - 88.5 Yield (%) 48.2 54.6 48.5 Penetr. (cN) 90.4 63.2 51.5 Bursting Fl. (m²) 43.9 41.2 24.6 Tearing length (km) 8.1 7.08 5.2

Claims (14)

Process for the production of cellulose, lignin and hemicelluloses from lignocelluloses by heating to 130 to 190 ° C under pressure in a digestion medium, characterized in that the digestion medium contains 50 to 95% by weight of acetic acid, 5 to 40% by weight of formic acid and up to 50 Contains% water. A method according to claim 1, characterized in that the lignocellulose is wood or an annual plant. Method according to one of the preceding claims, characterized in that the moisture content of the lignocellulose can be reduced by pretreatment at elevated temperature or by solvent vapors. Method according to one of the preceding claims, characterized in that the weight ratio of the lignocellulose to the digestion solution is 1: 1 to 1:12. Method according to one of the preceding claims, characterized in that the comminuted lignocellulose is continuously introduced into a pressure cooker, in which it is extracted from the digestion solution in countercurrent and continuously leaves the cooker on the other side in extracted form. A method according to claim 5, characterized in that a continuous extraction is achieved in that 2 to 20 digestion vessels are connected in series and the digestion solution extracts the lignocellulose in countercurrent. Method according to one of the preceding claims, characterized in that the pulping of the pulp is included. Method according to one of the preceding claims, characterized in that the washing process of the pulp is included in the continuous extraction. Method according to one of the preceding claims, characterized in that the lignocellulose is pre-extracted with a solvent to remove the ingredients. Method according to one of the preceding claims, characterized in that acetic anhydride is added to the digestion solution. Method according to one of the preceding claims, characterized in that a bleaching agent is added to the digestion solution. Method according to one of the preceding claims, characterized in that the lignocellulose is impregnated with formic acid, acetic acid, acetic anhydride or their vapors before being introduced into the digestion vessel. Method according to one of the preceding claims, characterized in that the lignocellulose is impregnated with a solvent or its vapors which forms an aceotrope with water. Method according to one of the preceding claims, characterized in that the bleaching of the pulp is included.