US2583994A - Wood pulp digestion - Google Patents

Description

i plicated Jan. 29,. i952 OFFICE WOOD PULP DIGESTION Ben ll. Briggs, Shelton, Wash., assignor to Rayonier Incorporated, Shelton, Wash, a corporation of Delaware No Drawing. Application June 21,, M46, Serial No, 618,360

C 2 Claims. I i

This invention relates to wood pulp digestion and has for its object the provision of an improved method of digesting wood for the production of cellulose fiber. More particularly, the invention contemplates an improved alkaline process for wood digestion capable of producing a wide variety of chemical wood pulps ranging from those somewhat superior to the products of conventional alkaline digestion to those of the highest quality characterized by high alpha cellulose content and low pentosan content and suitable for the manufacture of high grade papers, regenerated cellulose and cellulose derivatives.

In the manufacture of chemical wood pulp it is often highly desirable to produce a product of low pentosan content and high alpha cellulose content. One outstanding chemical character istic of the unbleached pulp made by an alkaline digestion process, such as the "soda process, the "sulfate process, and modifications of these two processes, is a relatively high pentosan content, which persists through to the bleached products unless some special procedure, such as extraction in cold caustic, is resorted to. Moreover, the alpha cellulose content of alkaline-digested pulps is relatively low, and hot alkaline treatments, such as are efiective in improving the alpha cellulose content of sulflte pulps, have but negligible effect on the alpha cellulose content of alkaline pulps.

Various proposals have been made for lowering the pcntosan content as well as increasing the alpha cellulose content of alkaline-digested wood pulp. Such proposals have generally involved a pretreatment step in which the wood chips, prior to alkaline digestion, are cooked in an acid solution such as an acid sulfite, sulfurous acid or other mineral acid solution. Such acid pretreatments do effect an increase in the alpha cellulose content and a decrease in the pentosan content of a subsequently alkaline-digested pulp. Pretreatment by cooking in water, either with or without lowconcentrations of various chemical reagents, has also been proposed. Such pretreatment steps require either special equipment, such as acid-resistant cooking vessels, or unusually long treating periods. Not only does special equipment add to the cost of producing digested pulp, but acid reagents are unavoidably carried over from the pretreatment stage to the alkaline digestion stage and represent a substantial loss of the acidic reagents. Furthermore. substantial amounts of the alkaline reagents used in the alkaline digestion stage are lost in neutralizing these acidic reagents carried over into the digestion stage. Thus, the advantages of such pretreatments are frequently outweighed by substantial increases in operating costs and in non-productive tie-up of equipment by prolonged treating periods.

The method of the present invention is characterized by elimination of chemical reagents and special equipment in the pretreating stage and additionally by a marked reduction of the required pretreating time for effectively increasing the alpha cellulose content and decreasing the pentosan content of subsequently alkalinedigested pulp. In its broad aspect, the method of the invention comprises subjecting wood chips while substantially free of added liquid and chemical reagent to the action of steam under pressure, and subsequently digesting the steamtreated chips with an alkaline cooking liquor in accordance with conventional alkaline-digestion practice. The pretreatment of the wood chips with steam is carried out at a temperature of from 140 to 200 C. The steam is preferably saturated, and the pressure within the digester, corresponding to the temperature range or 140 to 200 C. under practical digester conditions, will range from about to 235, pounds absolute per square inch. Depending upon the steam temperature used, the subsequently alkaline-digested pulps range from products (with mild steaming conditions) differing only slightly in alpha cellulose and pentosan contents from conventional alkaline pulps to products (with severe steaming conditions) of the highest chemical purity as evidenced by very high alpha cellulose and low pentosan contents. The duration of the steaming treatment may vary from 20 to minutes, depending upon the steam temperature and the desired type or quality of digested pulp. Preferably, the wood chips are substantially freed of air or deaerated prior to the steaming treatment, and such condensate as forms during steaming is continuously drained from the chips to avoid immersion of the chips in condensate.

In its simpler and broader aspect, the inven-- tion involves the preparation of chemical wood pulp from wood chips by the following sequence of steps:

1. Steam pretreatment at a temperature 01' from to 200 C. in the absence of any free liquid.

2. Alkaline cook under conditions essentially normal for a one-stage alkaline cook.

The invention may be practiced in any conventional equipment suitable for wood pulp digestion with alkaline liquors. The steam pretreatment produces some rather weak acids which generally are not unduly corrosive toward mild steel cooking vessels. However, more corrosionresistant materials such as stainless steel may be used with advantage in the construction or lining of digesters used in practicing the invention.

In practicing the invention, the wood chips are charged into the digester, and the digester is steamed directly, preferably with saturated steam, in such a way as to effect as complete air removal as possible through the relief line. When air in the digester has been essentially displaced by steam, the relief line is throttled and the digester temperature is raised by continued direct steaming to a temperature between 140 and 200 C., and preferably between 160 and 180 C., in a total time of roughly 20 to 90 minutes.

The pressure corresponding to the prevailing steaming temperature will vary somewhat depending upon the resinous nature of the wood chips and also upon the amount of air not vented during the early stages of the presteaming operation. If reasonable care is exercised toward good air relief, the pressures corresponding to the aforementioned steaming temperatures (140 to 200 C.) will be from about 55 to 235 pounds absolute per square inch.

I have found that the presence of air during the steaming treatment adversely afiects the quality and yield of pulp. Hence, air should be removed from the chips as completely as possible before the maximum presteaming temperature is reached. When air is swept out of the digester by the initial direct steaming, as hereinabove described, the amount of air left in the digester will depend upon a number of factors, among the more important of which are the time allowed for displacement of air with steam, variations in the rate of steam input, and the relative location of steam inlet and relief vent. The allowable time for air displacement, that is the time required for the contents of the digester to reach a temperature of about 100 C., will usually be dictated in practice by digester capacity available at a given mill. This time will in general be not much less than minutes nor much more than 60 minutes. Sudden surges in the rate of steam input should be avoided in order to obtain the most efiective air relief. The usual stationary digester is equipped for direct steaming at the bottom and for air and steam relief at the top. This arrangement is admirably adapted for carrying out the deaerating and steaming steps of the invention by direct st?aming. When the steam inlet of the digester is at the top, air must be vented at the bottom.

Instead of relying upon slow, steady steaming from one end of the digester and relief from the other end to effect air removal, other procedures may be used to effect substantially complete air removal prior to the presteaming treatment. Thus, for example, substantially complete air removal may be obtained by evacuating the digester after it is charged with wood chips but before admitting the steam. Another procedure comprises packing the digester with chips, filling the digester with water and then evacuating to boil out the trapped air. After the evacuation has been continued for a sufilcient length of time to effect the desired air removal, the water is drained from the digester while admitting steam under pressure in such manner as to force the water out of the digester without introducing extraneous air. Still another procedure comprises filling the digester with water, after it has been charged with chips, and then heating the water to the boiling point by steaming while maintaining the relief line open at the top. After fifteen minutes or more of boiling, the relief valve is closed and a drain line to a storage tank is opened, and continued steaming displaces the water, which is saved for reuse in a subsequent cook.

The presence of free liquid should be avoid-ed as far as practicable during the steaming treatment. For example, if water has been added to the digester packed with chips in order to facilitate the removal of occluded air, the water should be thoroughly drained from the digester. During the steaming treatment condensate will form inasmuch as the steam used is preferably at or near the saturation point. The condensate formed will not entirely drain from the chip mass and collect in the digester bottom since the chips as charged to the digester will usually imbibe considerable Water during steaming. Any condensate which does drain from the chip mass during the steaming treatment is drained off continuously. The reason for these precautions is that I have found that most effective results are obtained when the steaming operation is carried out in such manner as to permit the greatest penetration of steam into the chips. Free liquid in contact with the wood chips tends to impede such penetration.

The duration of the pretreatment by steaming will be determined by the temperature used and type of product desired, and in general may vary from 20 to minutes. A higher temperature for longer time favors the production of higher alpha pulps of lower pentosan content, lower yield, and, when made into paper, of lower Mullen and higher tear values. By controlling the severity of the steaming treatment from low temperature and mild steaming conditions to high temperature and severe steaming conditions, pulps may be obtained ranging from those suitable for the manufacture of high strength papers to those suitable, after bleaching, for conversion to rag content or other high grade papers or to regenerated cellulose or cellulose derivatives.

At the conclusion of the steaming treatment, an alkaline cooking liquor is pumped into the digester containing the presteamed chips, preferably for steam economy without first relieving the steam pressure. The conditions prevailing during the alkaline digestion are comparable to those used in normal singlestage alkaline pulping operations. The alkaline cooking liquor may be a "kraft or sulfate liquor containing principally sodium sulfide (NazS) and sodium hydroxide (NaOH) with lesser amounts of sodium carbonate (NazCOs) and sodium sulfate (NazSOh, or a soda liquor in which the principal alkaline ingredient is sodium hydroxide with lesser amount of sodium carbonate, or a modification or combination of those liquors with or without additional agents such as sodium sulfite (NazSOz) etc. Generally, the ratio of total NazO to bone dry wood in such strongly alkaline liquors is between 0.14 and 0.30. The digester is brought up to a temperature of normally to C. in roughly 1 to 2 hours and held at this temperature for 1 to 5 hours, and the cook is blown and washed. Usually the liquid charged to the digester includes a certain amount of organic solids derived from earlier cooks, it being common practice to recycle some spent or black" liquor solids for reasons 'of steam economy. Where a kraft" or soda liquor is used in an indirectly heated digester, the following conditions are suitable:

a. The liquid to wood ratio (i. e. the ratio of the weight of liquor charged plus the weight of water in the wood chips to the weight of the bone dry chips) may be varied from about 3 to '7. In

' bone dry chips) may be varied from 0.16 to 0.24

and should preferably be about 0.20.

c. The sulfidity (i. e. the ratio in percent of the weight of sodium sulfide to the combined weight of sodium sulfide and sodium hydroxide, all expressed as NazO) may range from to about 60%.

d. The maximum digestion temperature may be varied between about 160 and 180 C., but is usually around 170 C.

e. The time required to bring the digester charged with chips and alkaline liquor up to this maximum temperature is not critical, but prelerably should be at least about 1.5 hrs.

i. The digestion time at the desired maximum temperature, depending on the type of pulp desired and other chosen conditions, may be varied from 1 to 4. hours, but will usually be around 2.5 hrs.

For reasons of steam economy, some black liquor solids from a previous digestion and also any condensate drained from the presteaming operation may be included in the alkaline cooking liquor.

At the conclusion of the alkaline digestion, the digester is blown and the stock is washed by any conventional means. The black liquor is advantageously passed through a recovery system in which the alkaline compounds contained therein are largely converted to sodium hydroxide in the case of soda liquor or to sodium hydroxide and sodium sulfide in the case of kraft liquor.

The following examples illustrate practices of the invention:

Example 1 A digester charged with western hemlock chips was steamed from the bottom with the top relief valve open and condensate was bled off from the bottom as formed. The rate of steaming was such that steam was observed coming from the relief line about 8 minutes after starting the operation. The relief valve was then closed and steaming was continued at such a rate as to give an approximately linear temperature rise to 1'75 C. in 22 minutes. The observed gauge pressure to maintain this temperature was about 118 lbs/sq. in.

After steaming for 45 minutes at 175 C., the steam inlet and condensate bleed-off valves were closed and a kraft cooking liquor containing all of the condensate from the steaming operation was pumped into the digester.

The following conditions prevailed in the alkaline digestion stage:

Liquid: wood ratio (approx) Active alkali: wood ratio 0.20 Sulfidity 40% Maximum temperature 170 C.

Time to maximum temperature 1.5 hrs. Time at maximum temperature 2.5 hrs.

The effectiveness of the steaming operation of the invention is illustrated in the following comparison of the properties of the pulp produced in 8 Example 1 and the properties of a kraft pulp produced under substantially the same alkaline digestion conditions without the presteaming treatment:

Pulp produced Ki-alt" by invention pulp Alpha cellulose, per cent 96. 8 93. 6 Hot l0 Per Cent KOH $01., Per Cent.. 2. 2 5. 8 Peulosans, per cent (J. 9 6. 0 Cupremmonium visc., sec" l0. 5 27 TAPPI KMnOl No 13. 5 14.6 Mullen at 400 C. S. Freeness l50 205 'lear at 400 C. S. Frceness 2.8 2.7

In all the examples of this specification, the pentosan content was determined by a modification of the hydrobromic acid distillation-barbituric acid precipitation method proposed by Jayme (Die Naturwissenschaften, 28 No. 52,822, 1940). According to this modified procedure a weighed sample of pulp in 27% hydrobromic acid is boiled in a distilling flask. Distilled water is added constantly to the pulp mass at such rate as to maintain a substantially constant volume. The condensate containing furfural derived from the pentosans is acidified with hydrochloric acid, and barbituric acid is then added to precipitate furfural as furfurylidenemalonylcarbamide. Although this procedure gives lower pentosan values than the older hydrochloric acid-phloroglucinol method, the analytical method described herein is preferred because it is believed to give a more accurate measure of the pe-ntosans as they exist in wood pulps. The TAPPI (Technical Association of the Pulp and Paper Industry) permanganate (KMnO4) number is a measure of the bleach demand. Briefly, it is the number of milliliters of 0.1 N potassium permanganate solution that is reduced by one gram of oven dry pulp in 5 minutes under certain specified conditions, closely paralleling TAPPI method T214 m-S'I.

Pulp Mullen or bursting strength increases to a maximum as the pulp undergoes mechanical action in a beater or similar device. Tear, in general. decreases during beating. As a pulp is beaten its water retaining properties are enhanced and it is said to lose freeness. Freeness is a measure of the rate at which a pulp stock may be partially dewatered, and decreases as beating progresses. Since Mullen and tear both change with freeness, it is more meaningful to say that a pulp has a certain Mullen and a certain tear at some givenfreeness, which in the examples of this application is set at 400 as determined with a Canadian Standard (C. S.) freeness tester in accordance with TAPPI T-227 m-43. The Mullen strength and tear of the examples were determined according to TAPPI T-220 m-42. Mullens are reportedas pounds per square inch bursting pressure multiplied by and divided by the pound basis weight (480 sheets, 24" x 36"). Tears are reported in grams (Elmendorf tear tester reading) multiplied by 16 divided by the product of the number of sheets tested and the basis weight.

Example 2 Western hemlock chips were treated under essentially the same conditions as in Example 1, except that a lower maximum temperature of C. was maintained during the steaming operation. The resulting alkaline-digested pulp had the following characteristics:

Western hemlock chips were treated by steaming and alkaline digestion under essentially the same conditions as in Example 1, except that a lower maximum steaming temperature of 140 C. was maintained. The resulting pulp had the following characteristics:

Alpha cellulose per cent 93.8 Hot 10% KOH Sol do 5.3 Pentosans -do 5.2 Cuprammonium visc. sec 20.8 TAPPI KMnOi N 13.3

Mullen at 400 C. S. freeness 202 Tear at 4000. S. freeness 2.7

Examples 1, 2 and 3 show that the steaming pretreatment in accordance with the invention increases the alpha 'cellulose content and decreases the pentosan content of the subsequently alkaline-digested pulp as compared with a' pulp produced by the same alkaline digestion but without steam pretreatment. The examples further show that decreasing steaming temperatures, other conditions remaining essentially unchanged, efiect progressively less increase in alpha cellulose content and progressively less decrease in pentosan content over normal alkalinedigested pulp. Lower steam treating temperatures also have less effect in altering the solution viscosity, and Mullenand tear strengths.

Example. 4

In order to demonstrate the effect of variations in the length of the, steam treating period, western hemlock chips were treated under essentially the same conditions as those prevailing in Example 1, except that the treating period at 175 C. was only 30 minutes as against 45 minutes in Example 1. The results of these two tests are compared in the following table:

Steaming Period 30 Min. 45 Min.

Alpba cellulose, per cent 96. 2 96. 8 Hot 10 Per Cent KOH 801., Per Cent 2. 7 2. 2 Pentosans, Per Cent 1.3 0.9 Cuprammonium visc., sec 11.4 10.5 TAPPI KMnO; No 11.4 13.5

In general, the longer the steaming period (over the range of 20 to 90 minutes), at any one temperature, the higher is the alpha cellulose content and the lower is the pentosan content of the subsequently alkaline-digested pulp.

Example gester was steamed from the bottom with the top relief open until a temperature of about C. was attained to eifect substantial air removal. About 10 minutes was taken to bring the digester to this temperature in each run where the top relief was kept open to permit air displacement. No other steps were taken to effect more complete air removal, the amount of air removed in the manner described being suflicient to show the advantages 'of air removal in the steaming pretreatment. The alkaline digestion in each run of each pair was essentially the same as that described in Example 1. As shown in the following tabulation of results, air removal in the steaming operation has a pronounced eifect on the yield and bleach demand (permanganate number) of the digested pulp, although it has no significant effect upon the alpha cellulose content of the pulp under the conditions of the comparative runs:

Run 1 2 3 Steaming 'lemp., degrees C Steaming period, hours 0. 50 0.38 Changes Effected by Air Remov Increase in yield, per cent 3 5 4 Decrease in bleach demand. per cent 18 Example 6 i added to the alkaline liquor before it was charged to the digester. The digested pulp had the following characteristics:

Alpha cellulose. per cent 97.1 Hot 10% KOH sol do 2.3 Pentosans do 0.8 Cuprammonium visc. sec 9.6

TAPPI KM1'104 NO 13.0

The results indicate that the re-use of black liquor in the alkaline stage following steam pretreatment as practiced here has no more than slight effects on the analytical characteristics of the resulting pulp.

The runs in the foregoin examples were made on western hemlock chips. However, the invention is in no way limited to the treatment, of this or any other specific wood species. Thus, the invention has been practiced with marked success with other species of wood including western red cedar, Douglas fir, various southern pines, and black gum. While in these examples, the presteaming step has been followed by a cook in kraft liquor, other types of alkaline cooking liquors may be used. Thus, the presteaming step of the invention may be advantageously used in conjunction with any alkaline cooking liquor that satisfactorily pulps wood chips in a one-stage cooking process.

It will be seen from the foregoing examples that by changing conditions in the steaming operation th characteristics of the subsequently alkalinedigested pulp can b materially altered. Thus, by controlling the steaming conditions, pulp products can be obtained having only slightly higher alpha cellulose content and only slightly lower pentosancontent than the usual pulps obtained by similar alkaline digestion alone. Steaming conditions may be so chosen, however,

as to produce other pulps having extremely high alpha cellulose and low pentosan contents. Accordingly, the method of the present invention may be used for the preparation of pulps havin a wide range of characteristics and suitable for widely different purposes. For example, mild conditions in the presteaming step (e. g. 140 C. for 45 minutes) leads to a pulp product having an alpha cellulose content slightly higher than that of conventional alkaline-digested pulps. Such pulps either before or after suitable bleaching possess paper-making characteristics similar to those of the best grades of kraft." A paper pulp of distinctly better tear than and almost as good Mullen strength as the highest grade of kraft can be made by using more severe presteaming conditions. And by still further increasing the severity of the presteaming treatment, a pulp product suitable for use in photographic, rag content, and other high grade papers can be made.

When making pulps for dissolving purposes, a relatively mild presteaming treatment yields a product which after proper bleaching is suitable for use in the manufacture of viscose cellophane or rayon where the highest tenacity is not required. By a more severe condition of presteaming, pulps of higher alpha cellulose and lower pentosan contents are obtained, which after suitable bleaching are suitable for the preparation of high tenacity yarns and tire cord by the viscose process. Such pulps may also be converted to yarn by the cuprammonium process, and are suitable for nitration and acetylation.

The method of wood pulp digestion of the in vention offers advantages in quality of product and economy oi operation over the methods of the prior art. For example, careful analysis and study of pulp products produced in accordance with the invention show that they possess unusually uniform and good cellulose chain length distribution. Uniformity of cellulose chain length is highly desirable in pulps which are to be dissolved and particularly for pulps to be used in the manufacture of high tenacity yarns by the viscose process. The economic advantages of the present invention may be summarized as follows:

1. The pretreatment with steam and the alkaline digestion may be carried out as successive stages in the same digester.

2. The steam pretreatment and alkaline digestion may be carried out in digesters presently in use in plants using any alkaline cooking liquor. The acids developed during the steaming operation are so weak that they are only slightly corrosive toward the mild steel commonly used for the construction of such digesters.

3. The steaming operation requires no provision for makeup, storage or handling oi'treating solutions. Only steam is required for the pretreatment stage.

4. The steaming operation-presents no problem of efliuent disposal, the condensate produced during the steaming operation being used with advantage in making up the cooking liquor for the subsequent alkaline digestion.

5. ,The steaming operationis characterized by high thermal efliciency for the reason that a major portion of the heat put into the digester during this operation is ultimately used in the alkaline digestion.

6. The steaming pretreatment offers savings 7 in cost of materials. The cost of supplying steam for this operation is negligible. No costs are ining operations of the prior art.

7. The steaming operation is of much shorter duration than any. known pretreatment with water, acid solutions, or the like. As a result, less digester capacity is required for a given pulp output.

I claim: a

1. In the production of pulp of high alpha cellulose and low pentosan content from wood chips, the improved method which comprises subjecting wood chips to a treatment which consists essentially of subjectingthem to the direct action of steam at a temperature-of from 160 C. to 200 C. for a period of from 20 to minutes. said treatment continuing after acid re-' sulting therefrom has formed in the chips, and promptly digesting the steam-treated chips with an alkaline cooking liquor in which the alkali to wood ratio (weight of sodium hydroxide plus sodium sulfide, if any, expressed as NazO per unit weight of bone dry chips) is from 0.14 to 0.30 at a temperature of from C. to 180 C. for from 1 to 4 hours.

2. In the production of pulp of high alpha cellulose and low pentosan content from wood chips,

the improved method which comprises subjecting wood chips to a treatment which consists essentialiy of subjecting them to the direct action of steam at a temperature of from 160 C. to 180 C. for a period of from 20 to 90 minutes, said steaming being continued after acid resulting therefrom has formed in the chips, draining from the wood chips such condensate as forms, digesting the steam-treated chips with an alkaline cooking liquor in which the ratio of the weight of active alkali expressed as NazO to the weight of bone dry wood is from 0.16 to 0.24 at a temperature of from 160 C. to 180 C., maintaining the digestion at this temperature for from 1 to 4 hours, and then blowing the digester.

- BEN T. BRIGGS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 109,742 Keen Nov. 29, 1870 150,111 Voelter Apr. 21, 1874v 480,334 Maste Aug. 9, 1892 1,004,473 Saylor Sept. 26, 1911 1,138,907 Thickens May 11, 1915' 1,739,683 Lucas Dec. 17, 1929 1,771,598 Wells July 29,, 1930 1,782,869 Roza, Nov. 25, 1930 1,857,432 Codwise May 10, 1932 1,857,985 Taube May 10, 1932 2,161,763 Jones June 6,1939 2,269,985 Olsen Jan. 13, 1942 FOREIGN PATENTS Number Country Date 62,064 Norway Apr. 28, 1939 OTHER REFERENCES Paper Trade Journal." Dec. 18, 1941, pages 37,42, and 44; Jan. 18. 1945, page 47.

Wood Chemistry, by Wise (-1944,), pages 768, 769.