US3664919A

US3664919A - Vapor phase polysulphide liquid pulping of lignocellulosic materials

Info

Publication number: US3664919A
Application number: US883475A
Authority: US
Inventors: David W Clayton; Asahi Sakai
Original assignee: Pulp Paper Res Inst
Current assignee: Pulp and Paper Research Institute of Canada
Priority date: 1969-12-09
Filing date: 1969-12-09
Publication date: 1972-05-23
Anticipated expiration: 1989-05-23
Also published as: CA873779A

Abstract

Impregnating lignocellulosic material, previously saturated with water, in a first stage with an alkaline polysulphide cooking liquor, removing excess liquor, and then cooking the impregnated material in a second stage by rapidly heating with steam.

Description

United States Patent Clayton et al.

[54] VAPOR PHASE POLYSULPHIDE LIQUID PULPING OF LIGNOCELLULOSIC MATERIALS [72] Inventors: David W. Clayton, Hudson, Quebec; Asahi Sakai, St.-Laurent, Quebec, both of Canada Pulp and Paper Research Institute of Canada, Pointe Claire, Quebec, Canada [73] Assignee:

[ Notice: The portion of the term of this patent subsequent to May 9, 1989, has been disclaimed.

[22] Filed: Dec. 9, 1969 21 Appl. No.: 883,475

[52] US. Cl ..162/19, 23/48, 23/49, 23/134, 162/68, 162/82 [51] Int. Cl ..D21c 3/26 [58] Field of Search ..162/19, 68, 82, DIG. 2, 84; 23/134, 49, 48

[451 *May 23, 1972 FOREIGN PATENTS OR APPLICATIONS 7,501 1963 Japan Primary ExaminerS. Leon Bashore Assistant ExaminerArthur L. Corbin AttorneyStevens, Davis, Miller & Mosher [57] ABSTRACT lmpregnating lignocellulosic material, previously saturated with water, in a first stage with an alkaline polysulphide cooking liquor, removing excess liquor, and then cooking the impregnated material in a second stage by rapidly heating with steam.

9 Claims, 2 Drawing Figures Patented May 23, 1972 2 Sheets-Sheet l KAPPA NUMBER INVENTORS DAVID w. CLAYTON,

ASAHl SAKAI M, wmwh ATTORNEYS Patented May 23, 1972 2 Sheets-Sheet 2 AT KAPPA No.30

30V 0 5; AJDQ 52.0%

KP-A

POLYSULPHIDE CONSUMED. ON WOOD INVENTORS 8 ha u N N R 0 0 W in A MA LA C wm A W8 VA A D VAPOR PHASE POLYSULPHIDE LIQUID PULPING OF LIGNOCELLULOSIC MATERIALS The present invention relates to the production of pulp in improved yield from lignocellulosic materials such as wood, straw and grasses by a vapor phase polysulphide process. In particular the present invention relates to the production of cellulosic pulp in increased yield by a modification of the conventional liquid polysulphide process in which process the amount of polysulphide and alkali required is substantially reduced.

In the conventional kraft or sulphate process for the production of cellulosic pulp, in particular wood pulp, the subdivided wood or other lignocellulosic material is cooked in an aqueous solution of sodium hydroxide and sodium sulphide at a temperature of the order of 170 C. for the time required to produce a pulp in the required yield. The resulting pulp contains lignin and carbohydrate in a ratio which is detennined by the specific conditions of the pulping process i.e. the temperature cycle, the time, the liquor-to-wood ratio and the ratio of chemical to wood. In general the ratio of carbohydrate to lignin in the pulp from a given material at a given yield is a quantity which varies within very narrow limits being virtually fixed by the nature of the process.

In the conventional single stage polysulphide process the aforesaid cooking is effected in the presence of polysulphide ion suitably provided by adding sodium polysulphide or elemental sulphur to the cooking liquor and the pulping operation is carried out under appropriately modified conditions which are well known to those skilled in the art. The polysulphide pulps which are obtained have a higher ratio of carbohydrate to lignin than the pulps obtained in the conventional kraft or sulphate process at the same yield, due to the stabilization of the wood polysaccharides against alkaline degradation by the polysulphide ion. However, while the product pulps of the aforesaid single stage polysulphide process compare favorably with the pulps obtained in the conventional kraft or sulphate process and in particular pulps produced by the polysulphide process are usually darker in color than kraft pulps with the same Kappa number, and have easier beatability, higher strength in burst and tensile, slightly lower strength in double fold and tear and have similar bleachability, the conventional single stage polysulphide process is subject to several problems inter alia based upon the tendency of the polysulphide ion to decompose in the presence of hydroxyl ions to form hydrosulphide and thiosulphate ions. The rate of decomposition is increased appreciably when the temperature exceeds 100 C. and in a conventional single stage polysulphide process most of the polysulphide is destroyed before the maximum cooking temperature is reached. Thus, due to the tendency of the polysulphide ion to decompose in the presence of the hydroxyl ion it is found that for a given Kappa number the percentage of polysulphide ion consumed with increase in yield of the pulp rises steeply, with the result that the sulphur loading on the recovery system of the mill is excessive and extremely difficult and expensive to handle. As a result the single stage conventional polysulphide process has not met with substantial commercial success.

In an attempt to overcome the aforesaid problem of decomposition of the polysulphide ion and its attendant increase in the amount of alkali consumed in the single stage polysulphide process, it is proposed in Japanese Pat. No. 7,501 (1963) to effect the process in two stages, involving the treatment with polysulphide in the absence of alkali at a temperature of about 130 C. over a period of an hour and to stabilize the polysaccharides against degradation in the lignocellulosic material and subsequently after removal of the excess polysulphide liquor, which liquor, as it contains residual polysulphide ion, can be reused in the process, cooking of the stabilized lignocellulosic material to 170 C. for about an hour with sodium hydroxide or kraft liquor with a 15 percent sulphidity to obtain the required pulp. Whilst the two stage process of the Japanese patent gives yield increases of 5 to 7 percent based on wood when compared with kraft pulps of the same lignin content, the process of the Japanese patent still has the disadvantage that a substantial quantity of the polysulphide is destroyed in the stabilization stage without it reacting with the polysaccharides in the lignocellulosic material. As such, the percentage of polysulphide consumed for a given increase in yield of the pulp at a constant Kappa number is still high and the recovery system of a conventional kraft pulp mill would find it expensive and difficult to handle the sulphur loading.

In a further improvement of the polysulphide process as set forth in our co-pending U.S. application Ser. No. 665,710 filed Sept. 6, 1967, the production of pulp from lignocellulosic material by the polysulphide process is effected in three stages in which the decomposition of the polysulphide during the impregnation of the polysaccharides in the lignocellulosic material is substantially reduced and at the same time an increase in pulp yield is obtained at essentially the same Kappa number over that obtained by the conventional kraft or single stage polysulphide process. Thus, in accordance with the invention of U.S. application Ser. No. 665,710 it was found that when the lignocellulosic material in subdivided form is subjected to impregnation in a separate stage from the stabilization stage, and in the impregnation stage the temperature is maintained sufficiently low so that essentially no polysulphide ion decomposes and the excess of polysulphide liquor containing no added sodium hydroxide is removed from the material between impregnation and stabilization stages, it is possible to effect both the impregnation and stabilization stages under conditions which are optimum for both impregnation and stabilization and thus avoid the problem which previously occurred, particularly in the process of the Japanese patent, namely that the conditions for stabilization are directly opposed to the conditions for desirable impregnation. In particular in US. patent application Ser. No. 665,710 there is provided a process for the production of cellulosic pulp for lignocellulosic material which comprises impregnating said material in subdivided form e.g. as chips, with a polysulphide liquor containing no added sodium hydroxide at a temperature below that at which substantial decomposition of the polysulphide occurs, removing excess polysulphide liquor from the impregnated materials, stabilizing the impregnated materials against alkaline degradation by increasing the temperature of said materials and subsequently delignifying said stabilized materials by cooking said materials in a cooking liquor containing sodium hydroxide. In practice, in the process of the aforesaid U.S. application Ser. No. 665,710 the impregnation is effected at a temperature not exceeding 1 10 C. and usually in the range of to C., the stabilization is effected at a temperature not less than C. usually under superatmospheric pressure and the stabilized material is cooked at a temperature in the range to C. Thus by subdividing the first stage into indivudial stages for impregnation and stabilization it is possible to obtain optimum conditions for stabilization without the necessity of protecting the polysulphide against decomposition because the excess of polysulphide is withdrawn and the impregnation is also carried out under conditions in which the least loss of polysulphide occurs. This process, which is effected in three stages in which the decomposition of the polysulphide during impregnation of the polysaccharides in the lignocellulosic material is substantially reduced, produces an increase in pulp yield over that obtained by the conventional Kraft, single stage polysulphide process and the two stage polysulphide process of the Japanese patent when the pulp yields are compared at substantially the same Kappa number.

In the three stage polysulphide process of U.S. application Ser. No. 665,710 the polysulphide solution in the impregnation stage contains no added sodium hydroxide and the polysulphide solution has a pH below 12.5 which avoids substantial decomposition of the polysulphide ions into thiosulphate and hydrosulphide ions, the rate of which decomposition becomes appreciable above a pH of about 12.5. In a further improvement of the three stage process of U.S. application Ser. No. 665,710 it was found that it was also desirable to maintain the pH in the impregnation stage above about 1 1.5 to avoid conversions of the polysulphide ions in the impregnation liquor into sulphur and hydrosulphide ions and this is effected according to the process as disclosed in our co-pending U.S. application Ser. No. 864,740 by the addition of ammonium hydroxide to the impregnation stage, and in the subsequent stabilization stage after removal of the impregnation liquor, the impregnated lignocellulosic material has its temperature raised above 130 C., suitably by steaming, with addition if necessary of further ammonia to provide optimum stabilization conditions. This is followed by the delignification stage which involves the addition of sodium hydroxide or kraft liquor in a similar manner as in the process of U.S. application Ser. No. 665,710

According to the invention of our co-pending U.S. application Serial No. 864,740 there is provided in a process for production of a cellulosic pulp from lignocellulosic material which comprises substantially completely impregnating said material in subdivided form with a polysulphide liquor containing no added sodium hydroxide and at a pH below 12.5 and at a temperature below that at which substantial decomposition of the polysulphide occurs, removing the excess of polysulphide liquor from the impregnated material, stabilizing the impregnated material against alkaline degradation by increasing the temperature of said material and subsequently delignifying the stabilized material by cooking said material in a cooking liquor containing sodium hydroxide, the improvement which comprises effecting impregnation of the material for a polysulphide liquor containing ammonium hydroxide in an amount sufiicient to maintain a pH of at least H5 and effecting stabilization by heating the impregnated material in the presence of ammonia gas under superatmospheric pressure. In this process it has been found that a still further increase in pulp yield is obtained at essentially the same Kappa number over that obtained by the conventional Kraft, single stage polysulphide process, two stage polvsulphide process of the Japanese patent and three stage polysulphide process of U.S. application Ser. No. 665,710 for a particular polysulphide consumption and corresponding alkali consumption.

In the aforesaid process of U.S. applications Ser. Nos. 665,710 and 864,740 the impregnation of the chips with the liquor is effected on either dry chips or water-saturated chips, the use of water-saturated chips being preferred as the impregnation of wet chips can be effected substantially at atmospheric pressure whereas with dry chips to obtain uniform impregnation within a reasonable period of time one has to use pressure which may be inconvenient.

It has now been found according to the present invention that there is an alternative process to the aforesaid three stage processes of US. applications Ser. Nos. 665,710 and 864,740 and in particular when moisture saturated chips are impregnated with a conventional polysulphide liquor such as that used in the single stage polysulphide process at a temperature below 95 C. and after removal of the excess of liquor the impregnated chips are subjected to cooking at elevated temperatures in the vapor phase at an elevated temperature preferably in the range 170 to 185 C., that the pulp yield obtained at constant Kappa number for a particular polysulphide consumption, particularly below 6 percent on wood, is superior to that obtained in the single stage polysulphide process, below about 4 percent polysulphide consumption on wood is superior to that obtained in the three stage process of U.S. application Ser. No. 665,710 and below about 2.5 percent on wood is substantially superior to that obtained by the process of U.S. application Ser. No. 864,740. According to the present invention therefore there is provided in a process for the production of cellulosic pulp from lignocellulosic material which comprises heating said material in subdivided form with an alkaline cooking liquor containing polysulphide ion, the improvement which comprises in a first stage impregnating said subdivided material previously saturated with water, with said cooking liquor at a temperature below that at which substantial decomposition of the polysulphide ions occurs, and removing excess liquor from said impregnated material and in a second stage cooking said impregnated material at elevated temperature in the vapor phase.

Thus, in the aforesaid three stage polysulphide processes of U.S. application Ser. Nos. 665,710 and 864,740 the processes inherently involve protection of the polysulphide in the impregnation stage, complete and uniform impregnation into the wood, promotion of the stabilization reaction on the polysaccharides in the wood, suppression of degradation of the polysaccharides during the stabilization stage, promotion of delignification, avoidance of hydrolysis of the polysaccharides. However, in the aforesaid three stage processes, in the final cooking stage and possibly also in the stabilization stage the wood chips are heated with hot alkali, e.g. sodium hydroxide containing polvsulphide ions, and it is believed that in such processes the hemicelluloses in the wood are readily dissolved by the alkaline solution at such high temperature and are therefore lost, even though the reducing end groups of the hemicelluloses are stabilized against the alkaline peeling reaction by the presence of the polysulphide ion. It is thus believed that by operating with conditions which avoid the heating of the wood chips with a large volume of strong alkali solution, particularly heating more than twice with such solution, the dissolution of the hemicelluloses is reduced to a minimum and thus the process of the present invention, in addition to the above advantages to be obtained in the aforesaid three stage process, has the additional advantage of preventing dissolution of wood hemicelluloses and therefore increasing the total pulp yield for a particular polysulphide consumption. However, it is also found that an application of a low liquor-to-wood ratio in the stabilization stage also helps to reduce the leaching out of the impregnated polysulphide from the wood chips and thus helps to maintain the highest possible concentration of polysulphide in the chips at a given polysulphide charge. A low liquor-to-wood ratio is of course inherent in a vapor phase cooking of the impregnated chips.

Thus the process of the present invention is essentially a two stage process, the first stage of which is an impregnation stage in which the wood chips are impregnated with the polysulphide cooking liquor, which is essentially an aqueous solution of sodium hydroxide e.g. kraft liquor containing polysulphide ions, suitably obtained by the addition of sodium polysulphide to the liquor. The impregnation stage desirably effected at a temperature below 95 C. and suitably in the range of 70 to C., the maximum temperature of C. being a lower temperature than that C.) specified as maximum for the three stage polysulphide pulping process, because the decomposition of the polysulphide ion in the presence of an excess of sodium hydroxide is much more rapid than for a solution of polysulphide alone, since the latter can be heated to 110 C. without undue decomposition.

To achieve the advantages of the present invention it is essential that the wood chips be fully saturated in order to obtain a uniform impregnation of the chips and to obtain a sufficient charge of polysulphide in the chips. The saturated moisture content of the chips may be obtained by merely soaking the chips in warm water for a sufficient time. The impregnation of the wet chips with the polysulphide liquor allows the impregnation to be effected substantially at atmospheric pressure as opposed to superatmospheric pressure with dry chips. As used herein the term water saturated as applied to the lignocellulosic material refers to subdivided material, e.g. chips in which at least the centers thereof are saturated with water. For softwoods the water content is usually in the range 180 percent to 200 percent by weight and for hardwoods the water content is in excess of percent by weight. The saturated wood chips generally have a moisture content in excess of about percent by weight. The impregnation of the wet chips is suitably effected for a period of the order of 9s an hour to an hour as it is found that in this range of time the wet chips can be impregnated with of the quantity of the chemical which can be introduced after 4 days. Further after A an hour The cooked chips were then disintegrated and the pulp was screened on a vibrating flat screen with 0.010 inch slits. The .weights of the screen rejects and the screened d pregnated charge containing polvsulphide sul to the percentages, based on ovendry wood 1.

lncooksS-192, 196,193,250, 251,197,171,172,173 and 174 the impregnated chips were then steamed for 15 minutes at a temperature of 100 C. and were subsequently steamed at a temperature of 175 C. for 90 minutes at 115 psig. In cooks 8-106, 107, 108 and 109, the impregnated chips were raised directlv to a temperature of 175 C. over 60 minutes and were 10 maintained at that temperature for 90 minutes by steaming at a pressure of 1 15 psig.

etermined and these were summed up to provide the total yield. The conditions and results are given in Table 1 followmg.

the unimpregnated area in the wet chips is found to have disappeared in cross section thereof and accordingly where a sufficient amount of chemicals can be introduced within a short time by means of a high concentration the size of the impregnation vessel can be reduced because the throughput is higher.

The concentration of the polysulphide sulphur in the liquor is suitably in the range up to 50 grams per liter, for above about 50 grams per liter no substantial advantage is gained.

The liquor-to-wood ratio in the impregnation stage is suitably within the range 2.5 to 1 to 4 to l, as above the ratio of 4 to 1 an infinite effect is approached, where a two fold increase in the ratio leads only to a 10 percent increase in impregnation under the same conditions. A high liquor volume places a great load on the circulation system and inherently exposes a large quantity of polysulphides to the risk of decomposition in a given time. Accordingly to obtain efficient impregnation it is preferable to keep the liquor-to-wood ratio as low as possible and to increase the polysulphide concentration. As aforesaid a desirable impregnation is below 6 percent by weight preferably below 4 percent by weight and more preferably below 2 percent by weight on ovendry wood, which gives optimum increases in yield over the aforesaid three stage polysulphide processes and over the single conventional polysulphide process.

Subsequent to the impregnation of the wood chips, which from a practical point of view must be as complete and uniform as possible in order to produce a useful pulp, the excess of liquor is removed from contact with the chips and may be recycled for further use in impregnation of fresh chips. The impregnated chips are then subjected to a vapor phase cooking procedure in which the temperature of the chips is raised, suitably by steaming, to a temperature in the range 170 to 185 C. and preferablv in the range 170 C. to 175C.

In a particular embodiment of the invention, low pressure steaming after impregnation of the chips and before the vapor phase cooking of the chips is desirably effected, as this operation ensures that the stabilization reaction is complete before delignification takes place. The vapor phase cooking of the chips is suitably effected at a cooking pressure in the range 115 to 150 psig, suitably for a period in the range to 90 minutes.

The present invention will be further illustrated by way of the following examples:

EXAMPLE 1 Black spruce guillotine chips with a size of 1 inch by (grain direction) by l/6 inch thick were prepared from black spruce logs. The chips were air-dried at atmospheric pressure to a moisture content of 10 percent based on dry wood and were separated into aliquots of 326 grams (ovendry weight). Each aliquot was saturated with water in a pressure vessel by two steam purges at 30 psig for 2 minutes each, followed by forced impregnation with water at 70 C. under a pressure of nitrogen gas at 100 psig for 1 hour. Following this the chips were sealed in a polyethylene bag and were stored in a cold room at 3 to 5 C. for from 2 to 5 weeks. Before the chips were subjected to cooking, any free water was drained away by keeping the wet chips in a wire mesh basket (60 mesh) for 1 hour. At the point when the surface of the chips was starting to dry the moisture content of the chips was l79fl percent (based on dry wood), which approached the water saturation point for spruce wood (190 to 200 percent). In a series of cooks according to the present invention a number of these aliquots were subjected in an impregnation stage to heating with 1,305 ml. of an aqueous kraft liquor containing sodium tetrasulphide at a liquor-to-wood ratio of 4 to l. The impregnated chips were in each case heated to 90 C. over a period of 15 minutes in a 2.5 liter bomb and were maintained at this temperature at atmospheric pressure for 60 minutes. The effective alkali, sulphidity, sodium hydroxide content, sodium monosulphide, and sodium tetrasulphide content of the liquor are set for each cook in Table l. The excess of polysulphide liquor was then removed, leaving an im- The concentration of the polysulphide in the fresh and withdrawn liquors was determined by the acidimetric method proposed by K. Johnsen, (Norsk Skogind 20, No. 3:91 95, Mar., 1966) and improved by P. Ahlgren, (Svensk Papperstidn. 21 No. 152730-733, Nov. 15, 1967). From the above results graphs were plotted as shown in the accompanying drawings in which:

FIG. 1 shows plots of Kappa number against total pulp vield at different polysulphide consumptions. The figures in parenthesis by each curve are polysulphide sulphur consumptions, expressed as percentages based on ovendry wood.

FIG. 2 shows plots of total pulp yield against polysulphide consumption at Kappa number 30 for the process of the present invention identified as II-P, as well as for the process of U.S. application Ser. No. 864,740 identified as III-Q, U.S. application Ser. No. 665,710 shown as III-E and a conventional single stage polysulphide process shown as l-C.

In FIG. 2 the plots identified as III-Q, III-E and I-C are taken.

directly from FIG. 2 in U.S. application Ser. No. 864,740, which graphs are based upon the results therein and the results in U.S. application Ser. No. 665,710.

It will be seen from the graphs that below a polysulphide consumption of about 2.5 percent by weight, the two-stage 1. In a process for the production of pulp from a lignocellulosic material which comprises heating said material in chip form with an alkaline cooking liquor containing polysulphide ion the improvement which comprises in a first stage impregnating said lignocellulosic material in chip form previously saturated with water with said cooking liquor at a temperature below 95 C. and removing the excess of liquor from said impregnated material, and in a second stage cooking said impregnated material with said impregnating polysulphide liquor by rapidly heating to an elevated temperature with steam.

2. A process as claimed in claim 1 in which the polysulphide consumption on wood is up to 6 percent.

3. A process as claimed in claim 1 on which the polysulphide consumption on wood is less than 4 percent.

4. A process as claimed in claim 1 in which the polysulphide consumption on wood is less than 2.5 percent.

5. A process as claimed in claim 1 in which the impregnated material is cooked in the vapor phase by steaming, under superatmospheric pressure.

6. A process as claimed in claim 1 in which the impregnation temperature is in the range 70 to 90 C.

7. A process as claimed in claim 1 in which the cooking temvapor phase polysulphide pulping process (II-P) of the present 2 5 perature 1S m the range 175 to 185 invention gives higher pulp yields, at the same Kappa number- (FIG. 2) for the same polysulphide consumption, than the processes III-Q, III-E and LC; for a polysulphide consumption of up to about 4 percent by weight, it is better than processes Ill-E and LC, and better than process I-C for a polysulphide consumption below about 6 percent by weight.

We claim:

8. A process as claimed in claim 7 in which the cooking pressure is from 1 15 to psig.

9. A process as claimed in claim 1 in which the impregnated chips are steamed at a temperature of about 100 C. to effect stabilization and the temperature subsequently raised by steaming to be in the range to C. to effect delignification.

Claims

2. A process as claimed in claim 1 in which the polysulphide consumption on wood is up to 6 percent.
3. A process as claimed in claim 1 on which the polysulphide consumption on wood is less than 4 percent.
4. A process as claimed in claim 1 in which the polysulphide consumption on wood is less than 2.5 percent.
5. A process as claimed in claim 1 in which the impregnated material is cooked in the vapor phase by steaming, under superatmospheric pressure.
6. A process as claimed in claim 1 in which the impregnation temperature is in the range 70* to 90* C.
7. A process as claimed in claim 1 in which the cooking temperature is in the range 175* to 185* C.
8. A process as claimed in claim 7 in which the cooking pressure is from 115 to 150 psig.
9. A process as claimed in claim 1 in which the impregnated chips are steamed at a temperature of about 100* C. to effect stabilization and the temperature subsequently raised by steaming to be in the range 175* to 185* C. to effect delignification.