CA1153852A - Process for acid sulfite digestion of wood - Google Patents
Process for acid sulfite digestion of woodInfo
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- CA1153852A CA1153852A CA000355337A CA355337A CA1153852A CA 1153852 A CA1153852 A CA 1153852A CA 000355337 A CA000355337 A CA 000355337A CA 355337 A CA355337 A CA 355337A CA 1153852 A CA1153852 A CA 1153852A
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- wood
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-
- D—TEXTILES; PAPER
- D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
- D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
- D21C3/00—Pulping cellulose-containing materials
- D21C3/04—Pulping cellulose-containing materials with acids, acid salts or acid anhydrides
- D21C3/06—Pulping cellulose-containing materials with acids, acid salts or acid anhydrides sulfur dioxide; sulfurous acid; bisulfites sulfites
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Abstract
P. C. Leithem 2x PROCESS FOR ACID SULFITE
DIGESTION OF WOOD
Abstract of the Disclosure:
The production of chemical pulp by the acid sulfite digestion process is improved by increasing the ratio by weight of combined SO2 to wood to a range of from 4 to 12 (based on one part of combined SO2 to 100 parts of dry wood) and by increasing the minimum average rate of heating to the substantially maximum cooking temperature to 40°C per hour.
DIGESTION OF WOOD
Abstract of the Disclosure:
The production of chemical pulp by the acid sulfite digestion process is improved by increasing the ratio by weight of combined SO2 to wood to a range of from 4 to 12 (based on one part of combined SO2 to 100 parts of dry wood) and by increasing the minimum average rate of heating to the substantially maximum cooking temperature to 40°C per hour.
Description
, - 2 - ~. C~ Leithe~ 2X
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This invention relates to a process for the acid sulfite digestion of wood.
The acid sulfite digestion process (sometimes referred to as acid bisulfite) has been and continues to be widely used for the production of -high quality pulps. The acid sulfite process possesses the versatility of being capable of use for producing a variety of pulp grades, ranging from paper grades containing relatively large amounts of hemicellulose to high purity dissolving grade chemical pulps having very small amounts of hemi-eellulose.
Increasing the acidity of the cooking acid in such processes increases hydrolysis of both the celluloses and hemicelluloses. Thus, for paper grade eelluloses where it is desirable to preserve hemieelluloses, less aeidity produees higher yield pulps with more hemicellulose. Conversely, dissolving grade pulps require higher aeidity to reduce hemicellulose content.
One of the key components of the eoo~ing acid in acid sulfite processes is so-called "combined S02," generally defined as the amount of S2 bound as neutral sulfite. Combined S02 has a basic character. It has been widely assumed for many years in the pulping industry that low levels of eombined S02 are neeessary for dissolving pulp grades to develop the acidity neeessary during cooking to reduce hemieellulose content.
The aeid sulfite digestion proeess has always been eharaeterized by the long ti~e period required to reach maximum cooking temperature.
Normally, aeid sulfite digestion also requires eritieal control of the time-temperature sehedule, partieularly the time to reach the eritieal temperature tllO to 125C), a temperature below which full penetration of the wood ehips with the cooking aeid must oeeur. Quite frequently heating is interrupted or retarded or about an hour at about 110 - 125C to eomplete penetration and prevent a so-ealled burnt eook from oecurring. Total t~me to maximum temperature for an aeid sulfite mill digester typically ranges from 3 to 5 hours for soluble base liquors and longer for insoluble base liquor. Attempts to shorten this time period have resulted in poor delignification and excessive carbohydrate degradation.
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P. C. Leithem 2X
It is a primary object of the present invention to provide a process for producing chemical pulp which pro-cess has increased flexibility by allowing a much en-larged range of acceptable cooking parameters.
It is an additional object of the present invention to provide a process for lowering the production costs for producing chemical cellulosic pulp.
It is yet another object of this invention to pro-vide a process for producing acid sulfite pulp which pro-duces a spent sulfite liquor effluent which is less cor-rosive than prior effluents and which is more amenable to recovery processes.
It is still an additional object of this invention to provide a process for producing chemical pulps of lower lignin content which are more easily bleached than prior chemical pulps.
The present invention involves the discovery that the proportion of combines SO2 used in the digestion process can be varied as a function of the rate of heating. An increase in the pxoportion of combines S02 used in the digestion process combined with an increase in the heating rate allows a considerable shortening of the total digest-ion time. The discovery of this relationship i~ the acid sulfite process between the amount of combined SO2 and cooking rate has, insofar as is know, never before been recognized. The process of the invention uses amounts of combined S02 normally used for high hemicellulose content paper pulps and previously believed incapable of producing low hemicellulose dissolving pulps.
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1 ~3~iZ
P. C. Leithem 2X
- 3a -Specifically, the process of the invention com-prises the digestion of wood to produce chemical pulp by the acid sulfite digestion process in which the ratio by weight of total SO2 to combined So2 is at least 4 to 1 by heating in a closed vessel wood chips in an acid sulfite cooking liquor having a concentration of free SO2 no greater than 16% to a maximum cooking temperature no greater than 180C and at a maximum pressure no greater than 170 psig for a period of time sufficient to defiber the wood, the ratio by weight of one part of combined S02 to 100 parts of dry wood being from 4 to 12 and ~he minimum average rate of hea~ing to a temperature which is the substan-tially maximum cooking temperature being 40C per hour.
- 3a -" 1~S3l~52 _4_ P. C. Leithem 2X
The success of the rapid heating in the process of the inve~tion contradicts well accepted theory that a sufficient period of time must be allowed for base penetration or impregnation to avoid so-called burnt cooks.
Hence, acid sulfite pulping technology specifies rather slow raees of heating to ensure penetration or cooking liquor to the center of the chips, for example, 10-30C or in some instances as high as 35C per hour. The rates of heating for the present process are greater than 40 C per hour, frequently greater than 50C per hour and may be greater than 100 C per hour with properly designed digesting equipment.
"Substantially maxi~um cooking temperature" as used herein is the temperature of the digester at the end of the rapid rise to cooking tempera-ture. This temperature may subsequently be slightly exceeded prior to termination of digestion. The l'average rate of heating" as used herein is the quotient of the difference between the starting temperature of the digester and the substantially maximum temperature divided b~ the time from start of heating the digester to substantially maximum temperature.
The invention will be better understood by reference to the accompany-ing drawing in which:
FIG. 1 is a graph of three different time-temperature cooking curves comparing a typical curve of the invention with prior art mill and laboratory digestion curves, and FIG. 2 is a graph of polarization curves of spent sulfite liquor of the invention as compared to prior art spent sulfite liquors to show the difference in corrosive behavior.
A principal advar,tage of the invention is an increase in productivity resulting from an approximately 10-25% reduction in digestion time. In conventional acid s-~lfite digestion processes, time to maximum temperature normally ranges from 3 to 5 hours in a com~ercial pulping operation. In the present invention, this time is reduced to 2 hours or less when using a hot ~e.g. 60-85C) liquor as is customary in a commercial operation. In the laboratory, or i~ other instances where the starting cooking liquor is at ambient temperatures, this time is still reduced to 2 1/2 hours or less in l~.S~
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accordance with the invention. In many cases, this heating time will be less than 1 1/2 hours and may range to as low as one hour or even less.
Moreover, as will be brought out more fully below, pulp quality and yields may be improved. In addition, spent sulfite liquor effluent from the digestion process has physical and chemical characteristics which are less corrosive and are more compatible with effluent recovery systems.
Technically, the process of the invention is classified in the acid sulfite range because of the relatively high ratio of total 52 to combined 52 and the resulting low pH of the cooking liquor. The ratio of total S02 to combined S02 on a weight basis ranges from about 4:1 to 12:1 in the process of the invention and the p~ of the digester liquor ranges from about 1 to 2. ~owever, the actual levels of combined SO2 used in the present process and hence the combined S02 to wood charged are typical of bisulfite pulping. In the preferred practice of the invention, the ratio by weight of one part of com~inea SO2 to 100 parts of dry wood will range from ~.5-to 9.0 and, even more preferably, will be a minimum of 5. Conventional acid sulfite pulping at these relatively high combined levels is extremely slow and pulp hemicellulose levels are distinctly higher than those achieved via combined levels in accordance with the present invention. Although the invention is useful in multi-stage or continuous processes, the process of the invention will preferably be a single stage digestion process. The prior art teaches that faster heating rates may be used by chip impregnation prior to digestion.
Such pre-impregnation is not necessary to obtain the advantages of the invention. The invention is particularly suitable for soluble base digestion processes using sodium or ammonium base cooking liquors.
As will be shown below, the process may be used to produce pulp of unique analytical properties which contradict results expected from either conventional acid sulfite or conventional bisulfite pulping data. In addition, the use of rapid heat input in a single stage process to achieve cooking temperature without specifically allowing for a base impregnation period is unique to both types of pulping technology. Pulp yields in accordance with the invention are about 45~ which is typical of conventional aqueous sulfite - 6 - P. C. Leithem 2X
processes. (Yields are identified in the Examples as percentages of "screened yields" which are defined below.) The process of the present invention is carried out in dilute aqueous solution at conventional acid sulfite digestion pressures. It does not involve large increases in S02 concentration. If the concentration of 52 in the cooking liquor were to be increased substantially over the dilute concentration normally used in acid sulfite processes, digestion pressures would have to be correspondingly increased. This results from the higher vapor pressures of unco~bined S02, particularly as cookinq temperatures are approached, and the consequent necessity of increased pressures to prevent the loss of S02. Moreover, the invention does not require the presence in the cooking liquor of lower alcohols or other organic solvents. The present invention involves an alteration of the amount of one specific chemical present in the cooking liquor in relatively small quantities, combined S02.
Free S02, and thus total cooking chemicals, may also be increased to keep the process in the acid sulfite range. ~owever, the free 52 concentration of the cooking li~uor introduced into the digester should not be greater than 16~ (16 grams of SO2 per 100 ml of liquor), will usually be from about 4 to 12~ and even more preferably from 5 to 10%. As the cook is heated to maximum temperature and pressure, the solubility of free SO2 goes down~ Thus the amount of free S02 at maximum pressure will normally not exceed 19~
after pressure is relieved. The maximum pressure should be about 170 psig (1170 KPa(g)) and normally this maximum will be from 70 to 150 psig (482-1034 KPatg)), pressures which are conventional for acid sulfite digestion.
The liquor to wood ratio will normally range from 3:1 to about 6:1 based on liters of cooking acid added to kilograms of oven dried wood.
Using too little liquor outside this range would present problems in covering chips to assure adequate cookingi too much liquor would be im-practical for commercial operation. Typical mill liquor to wood ratios range from 4-5: It should be noted that additional moisture entering the digester with the chips is typically not included in this calculation, but would result in relatively small additional dilution. If the liquor - 7 - P. C. Leithem 2X
~5;~8'~
to wood ratio is varied in the present invention, a corresponding variation shou~ be maae in the concentration o~ c~m~ined SO~ in the cookin~ ~cid so that the combined S02 to wood range is between 4 and 12. For example, a cooking acid containing 1.2 g/dl combined S02 will result in a combined S02 to wood ratio of 6 to 1 ~g/100 kg O.D. wood) when cooking liquor is used at a S:1 liquor:wood ratio. ~owever, to achieve the same combined S02 to wood ratio, a cooking liquor of 1.5 g/dl combined S02 must be used at a 4:1 liquor:wood ratio.
By acid sulfite cooking we mean, as per Rydholm, that the ratio of total 52 to combined SO2 would be at least 4:1, such that the pH of the cooking acid (at room temperature) would fall in the general range of 1-2.
(Reference to "Rydholm" herein is to the text Pulping Processes, S. A.
Rydholm, Interscience Publishers, 1965.) This ratio of total S02 to combined S2 in the cooking acid at room tem~erature may be as great as 12:1 but will generally be lower in accordance with the free S02 concentration of the cooking acid which will typically range from 4 to 12 g/dl free S02. By free S02 we mean the portion of the S02 that is the sum of the actual free S02 plus one-half of the S02 combined as bisulfites, determined by titration according to TAPPI Standard Method T604. (This is the definition of free S02, set forth in TAPPI definition T 1201 OS-72.) The total S02 content of the cooking liquor refers to the grams of S02 per 100 ml of solution also as determined by titration according to TAPPI Standard Method T604. The total 52 concentration of a cooking liquor is the sum of the free plus the combined S02 concentrations. The actual ratio of total S02 to combined S02 will in all cases be greater than 4:1 but will vary to reflect the free S02 concentration of the cooking liquor which will not exceed 16 g/dl free S02.
For example, if a cooking liquor containing 8.0 g/dl free S02 and 2.0 g/dl combined S02 or 10.0 g/dl total S02 is chaxged, the ratio of total S02 to combined S02 would be 5.0:1 0. If the cooking liquor contained 8.0 g~dl free S02 and 1.2 g/dl combined S02 or 9.2 g/dl total S02, the ratio of total SO2 to combined SO2 would be 7.7:1Ø
~15~85~
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As previously indicated, the process of the invention uses pressures which are essentially conventional for acid sulfite digestions processes. At the maximum pressure of 170 psig, about 10~ free SO2 would remain when the digester temperature reached 120C, even if free 52 had been present in greater quantity in the cooking acid (at lower temperature).
For temperatures greater than 120C, even less S02 would be present, i.e.
more would be lost in relieving digester pressure at 170 psig. Less than 10%
free S02 would remain in the digester liquor at 120C at lower, more practical maximum digester pressures, e.g. 70 to 150 psig or even more pre-greatly ferably 90-120 psig. Com~ined S02 does not contribute/to vapor pressure and hence its concentration would not be limited by a pressure limitation. The relationship between pressure, free S02 and temperature in an acid sulfite digestion process is more fully set forth in an article entitled "Chemical Equilibria in ~eated Sulfite Solutions" by O.VO Ingruber, in Pulp and Paper Magazine of Canada, 66:T215 to T228, April 1965.
In the following examples, data is given for various properties of both unbleached pulp and spent sulfite liquor. The definition of this property data is set forth below.
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9 l~L5385~ P. c. Leithem 2X
.
K number ~also called permanganate number, see page 1112 Rydholm) is a measure of lignin remaining in unbleached pulp. Ihe determination is based on the fact that lignin is much more rapidly oxidized by a O.lN
; (normal) solution of potassium permanganate than the cellulose and hemi-cellulose ?resent in unbleached pulp. Specifically, K number corresponds to the number of milliliters of O.lN potassium permanganate consumed by 1 g of dry unbleached pulp under standard conditions.
I.V. is cuene intrinsic viscosity (see page 1118 Rydholm) and refers to the intrinsic viscosity of a 0.5% solution of unbleached pulp in cuene (cupriethylenediamine hydroxide). The intrinsic viscosity of this solùtion is related to the average degree of polymerization of the carbo-hydrate polymers (cellulose and hemicellulose) in unbleached pulp.
Dissolving grade pulps are usually digested to a specified I.V.
Lignin is deter~ined by hydrolyzing and dissolving the cellulose and hemicellulose in hot 72% sulfuric acid. A portion of the lignin (insoluble llgnin) remains as an insoluble residue that is collected and dried. ~nother portion goes into solueion and is termed soluble lignin; it is measured in solution by a spectroscopic method. These lignins are expressed as a percentage of the dry unbleached pulp. A higher portion of soluble lignin is considered desirable.
Slo and Slg (see Rydholm page 1116 and 1117) refer to the solu-bility of unbleached pulp in 10 and 18% solutions of caustic under a standard set of conditions. Slg is a reflection of the amount of hemicellulose, whereas Slo is a reflection of the amount of degraded cellulose plus hemi-cellulose present in the unbleached pulp. Lignin is removed from the pulp by a chlorite oxidation method before alkali solubility determinations are made (see ~ydholm page 1118).
Xylan and mannan content are a reflection of the two major types of hemicellulose present in chemical wood pulp. Xylan is a major constituent of one type, whereas mannan is a ~aior constituent of the other. Xylan and mannan _ 9, _ - lo ~ 3~5~ P. c. Leithem 2X
were measured by hydrolyzing pul? to its monomeric compcnents with an acid soluticn, and measuring ~he iiberated xylose and mannose by a paper chroma-tographic technique.
Tailings refers to that por~ion of wood chips that are not digested to the point where the wood fibers separate. Tailings are removed from the liberated fibers by a screening method. Screened yield is the percent of dry unbleached pulp (based on dry wood) recovered after digestion and screening to remove tailings. The percent dry weight of tailings removed is also based on the dry weight of wood digested.
Brightness refers to the amount of reflected light coming from a sheet of pulp compared to the amount of reflected light from a standard white plate. Measurements are made in an Elrepho photoelectric reflection photometer No. 50-38-00 under standard conditions. ~igher unbleached pulp brightness implies improved delignification during digestion.
The following example illustrate the practice of the invention.
Unless otherwise indicated, all parts ahd percentages are by weight.
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Western hemlock wood chips (65.8 kg~ containing 48.6% dry wood were placed in a 0.2 m3 capacity stainless steel laboratory pulping digester.
Chips were of typical commercial size having the following average dimensions:
length, 19.8 mm; width, 19.0 mm; thickness, 3.7 mm. After placing on the lid, 160 Q of ammonium base cooking liquor having a composition falling in the range of the invention (7.25 g/dl free S02, and 1.22 g/dl combined S02) was pumped into the digester. The combined S2 to wood was 6.1 kg of combinet S02 to 100 kg of oven dried wood. The s~stem was then heated by circulating the liquor out of the top of the digester, through a steam-heated heat exchanger, and back into the digester bottom. The average heating rate to the maximum cooking temperature was 64Clhr (1.07C/min), bringing the system from 20C
to 148~ in 2.0 hr. The digester pressure was not allowed to exceed 758 kPa(g). -After holding the d1gester at 148 C for 1 hr and 55 min, the pressure was lowered to 551 kPa(g) and the digester contents blown to a tank at atmospheric pressure. Total digestion time was 3 hrs and 55 min.
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The unbleached pulp and spent sulfite pulping liquor (SSL) wese separated with a centrifuge. The pulp was then washed with water and passed through a screen with 0.2 ~ slots to remove tailings and dewatered to 31.7% drv weight, and weighed. ~aterial not passing the screen (tailings) were collected, dried and weighed.
~ portion of the SSL was stripped of free S02 by counter current contact with steam in a packed colu~n under 200 kPa(g) of pressure.
Stripped liquor was evaporated to 50% solids at atmospheric pressure in an indirect-contact steam-heated evaporator. Viscosity of the evaporated liquor was measured on a Brookfield viscometer.
Examole 2 A second digestion typical of conventional acid sulfite pulping was performed in a similar manner except the cooking liquor contained 7.17 g/dl free S2 and 0.65 g/dl combined SO2, and that the digester was heated from 20C to 142C in 3 hr and 30 min,- an average heating rate of 34.9C/hr (0.58C/min). The combined S02 to wood ratio was 3.3 (k~ of S02 to lO0 kg of oven dried wood). The digester was held at 142C for 50 min before blowing. Total digestion time was 4 hr and 20 min. The maximum cooking temperature in Example l was 6 higher than in this Example 2. It is desirable, although not necessary as demonstrated in further examples below, that such a higher temperature be used to help shorten total cooking time.
Properties of the unbleached pulp and SSL from Examples l and 2 are set forth in Table I.
385~
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Conventional Invention Acid Sulfite ~nbleached Pulp Exa~ple 1 Examole 2 Screened yield, ~ 46.5 47.2 Tailings, % 0-5 0-5 I.V., dl/g ll.0 10.8 K number 8.8 11.6 K no./I.V. 0.80 . 1.07 10.5 11.8 slo ~ ~
9.0 10.2 S18, %
Xylan, ~ 2.0 1.8 ~annan, % 4.6 5.5 Xylan plus mannan, % 6.6 7.3 Lignin, soluble, ~ 1.50 1.40 Lignin, insoluble, % 0.09 0.42 Total lignin ~%) 1.59 1.82 Brightness, % 60.2 53.0 Spent Liquor Unstripped SSL pH 3.14 1.87 Stripped SSL pH 4.54 3~11 Stripped and evaporated 27 60 SSL vlscosity at 90C, nPa.s : ~, 3~35Z
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It can be seen from the table that the process of the invention, compared with conventional acid sulfite pulping, can give pulp of about the same yield, tailings and I.V., but with tne advantages of 25 min shorter digestion time, and i~proved pulp lignin and hemlcellulose contents and higher brightness. Further re, as more fully discussed below, the waste liquor from the invention has a higher pH, making it less corrosive to process equipment. Also, evaporating the invention SSL produces a lower viscosity heavy liquor that would be more econo~ic to burn in a recovery boiler.
The cooking curves of Examples 1 and 2 are shown in FIG. 1 together with a representative curve o~ an acid sulfite mill digestion process. The mill curve is shown because time-temperature curves in the mill tppically differ in certain respects from comparable laboratory runs of the type shown in Examples 1 and 2. As shown in FIG. 1, the maximum cooking temperature is reached in two hours in Example 1 and in 3 1/2 hours in the prior art process of Example 2. In the mill curve the maximum cooking temperature (140C) is reached after 3 hours. Xowe~er, the temperature frequently rises a few degrees (e.g. 1 to 10C) in mill digestion operations after achieving "maximum cooking temperature" and for this reason, the term "substantially the maximum cooking temperature" is herein used to define the end of the rapid temperature rise, namely 140C at three hours in the mill curve of FIG. 1. In practice, the temperature may rise a few degrees above 140C in a mill operation after the "substantially maximum temperature" is reached at the end of the three hour period in FIG. 1. ~ote also that the mill curve starts with a 60C cooking liquor and ~hat the temperature tails off at the end of the digestion cycle, as is typical of mill operations.
The corrosivity of the unstripped SSL of Examples 1 and 2 to 317L
stainless steel was evaluated by potentiodynamic polarization using a Princeton Applied Research ~odel 331-1 corrosion measurement system. The temperature was 65 C. Traditionally, 317L stainless steel (SS) is one of the materials used in commercial equipment made to handle SSL.
1 1~5~5'~:
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Anodic polarization curves of the SSL are shown in FIG. 2. In this technique, an increasing potential is applied between a S cm metal anode (317L SS) and an inert saturated colomel electrode tscE), or cathode, and the resultant currents measured.
As the anodic polarization increases, passivation occurs; i.e., the current flowing from the anode goes through maximum, then decreases to the passive current density. This indicates that the stainless steel has the ability to self passivate. The wider the passive potential range, the more likely that anodic passivation will occur, and that the stainless steel will resist corrosion. It can be seen from FIG. 2 that SSL from the inven-tion has a wider passive potential range than for SSL from the conventional acid sulfite process. The anodic curve of the invention starts at a lower potential than the conventional process because of a higher pH.
Examples 3-7 Two digestion runs were carried out as in Example 1 but using - sodium base rather than ammonium base cooking liquor. ~emlock wood chips were used as furnish. Table II compares the results of these digestion runs with two similar runs carried out in accordance with conventional acid sulfite digestion processes and one run in accordance with conventional bisulfite digestion processes. Examples 5 and 6 at 3.1 combined SO2 to wood ratio is a typical conventional acid sulfite ratio for dissolving grade pulp. By comparison, Example 7 is typical of bisulfite pulping suitable for paper end use. Note that tha combined SO2 to wood in Example 7 is much higher t8.7) than Examples 5 and 6. Note also that the maximum tempera-ture used is over twenty degrees higher for the bisulfite cook than for the conventional acid sulfite cooks. In addition, total cooking time is long, approaching six hours for Example 7. Higher maximum temperature and longer total cooking times are well known to be required in bisulfite pulping to compensate for the slowing down of the delignification rate which is caused by the higher levels of combined S02 to wood.
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The cooking conditions and the resulting unbleached pulp ant spent liquor properties for these five exa~ples (except ~here not measured) are set forth in Table II.
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TABLE II
Conventional Process Invention ~id Sulrite Bisul~ite E~ample ~o. 3 4 5 6 7 Cooking Conditions Combined S02 (g/dl) 1.20 1.22 0.66 0.66 1.9 Free S02 (g/dl) 6.93 7.09 7.05 7.08 1.9 Total sO2/Com~ined S2 6:1 6:1 12:1 12:1 2:1 Combined SOg:Wood (kg/100 kg O.D.) 6.0 6.1 . 3.3 3.3 8.7 ~aximum Temperature (C)148 148 142 142 165 Time to (hrs:min) 2:00 2:00 3:30 3.30 3:00 Time at (hrs:~in) 1:55 1:55 0:45 0:50 2:45 Average Xeating Rate to (Clhr) 64 64 3'~.9 34.9 48.3 Total Cooking Time (hrs:~in) 3:55 3:5; 4:15 4:20 5:45 Pul~ Pro~erties Screened Yield (%) 44.0 44.7 45.7 44.5 53.6 Tailings (%) 0.7 0.6 0.4 0.7 0.6 I.V. (dl/g) 10.5 10.9 11.2 10.0 11.6 K ~umber 8.4 8.8 10.7 10.2 19.8 K ~o./I.V. Ratio 0.80 0.81 0.96 1.02 1.?1 Total Lignin (7.) 2.5 1.~ ~.4 2.0 --Lignin Soluble (%) 1.4 1.6 1.3 0.9 Lignin Insoluble (%)1.1 0.3 1.1 1.1 --S10 (Z) 11.2 11.3 12.0 l2.0 __ S18 (%) 9.6 9.7 10.9 10.2 ~-Brightness, % 56.8 54.2 50.3 49.8 __ Spent Liquor Combined--- Combined---Unstripped pa 3.00 1.75 --Stripped pa 4.35 3 05 ~
Vanillin Yield 0.042 0.038 --(gm/gm SSL solids) ll.S31~5~
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Table II shows that Exa~ples 3 and 4 of the invention produced at shorter times pulp properties which were at least equivalent to the conventional acid sulfite pulps of Examples S and 6. Note also that Examples 3 and 4 had lower K ~o./I.V. ratios. "l~ nu~ber" is ~ measure of the lignin content of the pulp, the lower the K ~o., the less lignin in the pulp. I.V. is a measure of degradation of the pulp, the higher the I.V., ehe less degradation. Thus the lower the ratio of K ~o./I.V., the better the quality of the pulp within a given I.V. range. After almost six hours of cooking the bisulite pulp of Example 7 is characterized by poorer delignification than acid sulfite Examples 5 and ~. Table II also shows hi8her pH's and improved vanillin yields from the spent sulfite liquors of the examples of the invention.
Examples 8-12 A further series of laboratory pulping runs were made to compare the effects of rapid heating schedules on conventional acid sulfite and conventional bisulfite combined/wood ratios. All parameters of the process were those of conventional runs except for the heating schedule. All examples used sodium base cooking liquor and hemlock wood chips. The cooking conditions and the results of each of these runs are set forth in Table III.
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T~BLE III
Acid Sulfite _ Bisulfite Fast Fast Process Conventional Heating Conventional Heatin~
Example ~o. 8 9 10 11 12 Cooking Conditions Combined S02 (g/dl) 0.65 0.64 0.65 1.90 1.91 Free SO2 (g/dl) 7.2 6.9 7.2 1.9 1.9 Total S02/Combined S02 12/1 12/1 12/1 2/1 2/1 Co~bined S02/~ood (kg/100 kg O.D.) 3-3 3.2 3.3 10.9 11.2 Maximum Temperature (C) 142 142 142 165 165 Tim~ to (hrs:min) 3:30 2:00 1:00 3:00 1:00 Time at (hrs:min) ~:30 1:00 2:00 2:45 2:45 Average Heating Rate to (C/hr) 34.5 61.0 122.0 48.3 145.0 Total Cooking Ti~e (hrs:min) ~ 4:00 3:00 3:00 5:45 3:45 Maximum Pressure ~Pa(g)) 758 758 758 1240 1240 Cooking Liquor pH 1.5 1.3 1.5 3.1 4.0 Pulp Properties Screened Yield (%) 49.3 45.6 -- 48.9 51.7 Tailings (%) 0.8 3.9 11.6 0.3 2.3 K No./I.V. Ratio 1.40 3.05 2.70 1.76 2.32 I.V. (dl/g) 10.5 7.9 8.6 10.3 11.7 K Number 14.7 24.0 23.2 18.1 27.2 Total Lignin (Z) 3.5 5.8 5.6 -- --Lignin, Soluble (%) 1.2 0.4 0.8 -~ --Lignin, Insoluble (%) 2.3 5.4 4.8 -- --Brightness (Z) 40.8 35.8 33.0 -- --Xylan (%) 1.6 1.4 1.6 -- 2.6 Mannan (%) 4.8 3.8 3.8 -- 10.5 Total Xylan plus Mannan (%) 6.4 5.2 5.4 -- 13.1 llS385Z
- 19 - P. C. Leithem 2X
It will be seen by ~xamination of Table III, in which all exa~ples are outside the scope of the invention, that the acid sulfite pulp5 produced with fast heat (Examples 9 and 10) e~hibit higher tailings, higher ~ numiers, higher K ~o./I.V. ratios as com~ared to the corresponding conventional process. Furthermore, Examples 9 and 10 show evidence of lignin condensa-tion (compare insoluble lignin contents) and carbohydrate degradation as evidenced by lower I.V.'s. These data substantiate the well known properties of a burnt cook in conventional acid sulfite pulping. The success of the corresponding cooks using the process of the invention under the same rates of heaeing indicates that it is the overall quantity of co~bined S02/wood which limits the rate of heating in conventional acid sulfite pul?ing.
Exa~ples 11 and 12 in Table III also shows the effect of fast heating rise on bisulfite cooking. The ~ ~o./I.V. ratio and tailings increase as in fast rise conventional acid sulfite pulping. Also as in conventional acid bisulfite pulping, the use of a fast rise to maximu~
temperature to reduce total cooking time is not plausible with bisulfite pulping because pulp quality is impaired as evidenced by the 50% jump in K nu~ber. Table III supports the conclusion that the success of the process of the invention is the result of coupling a fast temperature rise with an appropriate cooking acid combined ratio and substantial a unes of free S02.
Examples 13-15 These e~amples illustrate the preparation of an acid sulfite pulp to a target I.V. of 11 from slash pine furnish with an ammonium base cooking liquor. Examples 13 and 14 are in accordance with the invention. Example 15 is a comparable acid sulfite digestion process using a conventional combined S2 to wood ratio and heating rate. The maximum cooking pressure in all examples was 758 RPa gauge. The digestion conditions and pulp and spent liquor properties are set forth in Table IV.
1.~53~52 - 20 - P.C. Leithem 2'f.
TABLE IV
Process Invention Conventional Example ~o. 13 14 15 Cooking Condi~ions Combined S02 (g/dl) 1.51 1.19 0.85 Free S02 (g/dl) 6.95 6.93 6.99 Co~bined S02:Wood (kg/100 kg O.D.) 5.6 4.4 3.2 ~aximum Temperature (C) 145 145 140 Time to (hrs:min) 2:00 2:15 4:00 Time at (hrs:min) 2:45 2:05 1:29 Average ~eating Rate to (C/hr) 62.5 55.6 30.0 Total Cooking Time (hrs:min) 4:45 4:20 5:29 Pulp Properties Screened Yield (%) 45.8 45.9 46.3 Tailings (%) 1.6 1.8 1.9 I.V. (dl/g) 11.1 11.1 10.8 K Number 8.0 9.5 10.1 K No./I.V. Ratio ' 0.72 0.86 0.94 Total Lignin (Z) 2.1 2.1 3.3 Lignin Soluble (%) 1.4 1.4 1.0 Lignin Insoluble (~) 0.7 0.7 2.3 Slo (~) 10.3 10.1 10.9 S18 (%) ~ 8.6 8.6 9.2 Brightness (%) 67.2 64.4 61.5 Spent Liquor Unstripped pH 2.55 2.3 1.9 Stripped pH 4.1 3.9 2.7 - 20 ~
3~35~
- 21 - P. C. Leithem 2X
It will be seen fro~ Table IV that a pulp of improved K nu~er, K ~o./I.V. ratio, lignin content and brightness was produced in accordance with the invention at a significant reduction in digestion time. It should also be noted from the S18 results that the hemicellulose level is une~pectedly low for the total cookin~ times and combined S02/wood levels used. At these cooking times and co~iined levels, it would normally be expected to have higher hemicellulose levels. This is further shown in the following Examples 16 and 17 and Table V. Finally, the p~ levels of the spent sulfite liquor are significantly higher for Examples 13 and l4 of the invention.
Exam~les 16 and 17.
Two pulping runs were mRde to compare fast heating in accordance with the invention with conventional heating, both runs using a high combined level within the scope of the invention. Both samples used sodium base cooking liquors and hemlock wood chips. Maximum pressure was 758 kilopascals gauge in both exa~ples. The results are set forth in Table V.
~5'~85Z
- 22 - P. C. Leithem 2X
Process Invention onventional Exa~ple No. 16 17 _ooking Conditions Combined S02 (g/dl) 1.19 1.16 Free S02 (g/dl) 7.18 7.24 Total sO2/Co~bined S2 7/1 7/1 Combined SO2/Wood ~ (kg/ 100 kg O.D.) 6.0 5.8 Maximum Temperature (C) 142 142 Time to (~I:M) 1:00 3:30 Time at (H:~) 2:00 0:30 Average Heating Rate to ~-(C/hr) 122.0 34.8 Total Cooking Time (H:~) 3:00 4:00 Cooking Liquor p~ 1.5 - 1.4 Unbleached Stock Screened Yield (%j 45.8 53.3 Tailings (%) 2.8 0.6 K Wo¦I.V. Ratio 1.42 1.70 I.V. (dl/g) 12.7 14.9 K Number 17.9 25.2 Total Lignin (~) 2.6 Lignin, Soluble (%) 1.6 --Lignin, Insoluble (%) 1. 0 --Brightness (%) 44 . 5 40 . 8 Slo (%) 11.5 13.9 Slg (Z) 10.3 12.9 Xylan (%) 2.1 2.3 ~annan (%) 4.6 6.1 Xylan plus Mannan (Z) 6.7 8 . 4 ~;~
1 ~L5 3 ~ 5 2 - 23 - P. C. Leithem 2X
Table Y shows that at a high combined S2 level, the process of the invention has produced a 12.7 I.V. pulp and a 17.9 K number while Example 17, at the same high combined level but at conventional heating rate, produced a higher I.V. and K number pulp in an hour longer total cooking time. Note also a lower hemicellulose content in Example 17 as evidenced by lower S18 and total x~lan and mannan content.
Examples 18-l9.
Two digestion runs were made at the same max$mum cooking tempera-ture and to the same approximate I.V. range. The I.V. range used in this example is typical of paper grade pulps. The first of these runs, Example 18, was in accordance with conventional acid sulfite digestion practice with respect to combined S02 to wood ratio, heating rate and cooking time~
Example l9 was within the scope of the invention. Both examples used slash pine furnish, an ammonium base cooking liquor and a maximum cooking pressure of 758 KPa gauge. The cooking conditions and pulp properties are set forth in Table VI.
_ _ _ ~1S3~Z
- 24 - P. C. Leithem 2X
T~BLE VI
Process Conventional Invention Example ~o. 18 19 Cookino Conditions Combined S02 (g/dl) 0.65 1.70 Free S02 (g/dl) 7.30 7.09 Combined S02:Wood (Rg/100 Rg O.D.) 3.3 8.5 ~aximum Temperature (C) 142 142 Time to (hrs:min) 3:30 1:00 Time at (hrs:min) 0:30 2:30 Average Heating Rate to (C/hr) 34.9 122.0 Total Cooking Time (hrs:min) 4:00 3:30 Pulp Pro~erties Screened Yield (Z) 45.6 48.5 Tailings (%) 3.0 3.4 I.V. (dl/g) 11.9 12.6 - K Number 19.5 15.7 K No./I.V. Ratio 1.64 1.25 Total Lignin (%) 3.5 4.0 Lignin Soluble (%) 0.2 2.9 Lignin Insoluble (%) 3.3 1.1 Xylan Plus ~annan (%) 8.0 9.0 Slo (%) 11.1 10.8 S18 (%) 9.1 g,o Brightness (%) 48.5 57.6 t - 24 -25 ~ 5~3~ ~ 2 p. C. ~eithem 2X
It will be seen that the process of the invention, Example 19, arrived at approximately the s~me I.V. range as conventional Example 18 with less cooking time and produced a pulp of slightly better quality as evidenced by K number and K No./I.V. ratio. Table VI also indicates that the maximum temperature for the process o~ the invention need not be any ¦ higher than that used in conventional acid sulfite cooking, whereas bisulfite conditions which use high combined S02 to wood ratios require a higher cooking temperature to effectively defiber the wood. See Example 7 above which shows a conventional bisulfite process using a combined SO2 to wood ratio of 8.7 which is comparable to Example 19 but which requires a maximum cooking temperature of 165C.
Examples 20-21.
Examples 20 and 21 compare the use of cooking liquor at ambient temperature with a substantially ide~tical digestion run starting with a hot cooking liquor to simulate a mill run. Ammonium base cooking liquors having a combined SO2 level within the scope of the invention were pumped from a holding vessel into a laboratory digester containing hemlock chips.
In Example 20, the cooking acid was pumped to the digester without first heating the liquor, whereas in Example 21 the liquor was first heated to 96C. The maximum cooking pressure in both examples was 758 KPa(g). The same heating curve was used in both examples, however 35 minutes was eliminated from the first portion of the cooking curve of Example 20 by starting with hot cooking liquor. This time saving is reflected in the 35 minute shorter total cooking time of Example 21 compared with Example 20. The results are set forth in Table VII.
~ - 25 ~
111 ~3~3S~
- 26 - P. C. Leithem 2X
TABLE ~II
Example No. 20 21 Cooking Conditions Combined S2 (g/dl) 1.20 1.19 Free S02 (g/dl) 7.02 6.93 Combined S02:Wood (Kg/100 Kg O.D.) 6.3 6.3 Temperature at Start of Heating (C) 20 84 Maximum Temperature (C) 142 142 Time to (hrs:min) 2:00 1:25 Time at (hrs:min) 1:30 1:30 Average Heating Rate to (C/hr~ 61.0 40.9 Average Heating Rate to 110C (C/hr) 83.1 62.4 Total Cooking Time (hrs:min) 3:30 2:55 Pul~ Proverties Screened Yield (%) . 47.4 48.9 Tailings ~%) 1.5 1.6 I.V. (dl/g) 14.0 14.0 K Number 15.6 21.6 K No./I.V. Ratio 1.11 1.54 i - .
3~2 - 27 - P. C. ~eithem 2X
Examples 20 and 21 and Table VII show that the invention is equally applicable to the use of both ambient temperature and preheated cooking liquor, the latter being commonly used in co~mercial practice.
Since the heating curves are not linear, the heating rate is influenced by the starting temperature of the cooki~g liquor.
Thus, the process of the invention greatly enlarges the raDge of cooking parameters acceptable in acid sulfite digestion processes.
This flexibility permits increased productivity by allowing an overall reduction in digester cooking time, typically 10-25~ or even more. The process produces spent sulfite liquor whose physical and chemical charac-teristics are improved and which is more compatible with effluent recovery systems. The process also makes possible the production of improved quality pulp with lower bleach chemical requirements because of reduced lignin content.
3~r~æ
This invention relates to a process for the acid sulfite digestion of wood.
The acid sulfite digestion process (sometimes referred to as acid bisulfite) has been and continues to be widely used for the production of -high quality pulps. The acid sulfite process possesses the versatility of being capable of use for producing a variety of pulp grades, ranging from paper grades containing relatively large amounts of hemicellulose to high purity dissolving grade chemical pulps having very small amounts of hemi-eellulose.
Increasing the acidity of the cooking acid in such processes increases hydrolysis of both the celluloses and hemicelluloses. Thus, for paper grade eelluloses where it is desirable to preserve hemieelluloses, less aeidity produees higher yield pulps with more hemicellulose. Conversely, dissolving grade pulps require higher aeidity to reduce hemicellulose content.
One of the key components of the eoo~ing acid in acid sulfite processes is so-called "combined S02," generally defined as the amount of S2 bound as neutral sulfite. Combined S02 has a basic character. It has been widely assumed for many years in the pulping industry that low levels of eombined S02 are neeessary for dissolving pulp grades to develop the acidity neeessary during cooking to reduce hemieellulose content.
The aeid sulfite digestion proeess has always been eharaeterized by the long ti~e period required to reach maximum cooking temperature.
Normally, aeid sulfite digestion also requires eritieal control of the time-temperature sehedule, partieularly the time to reach the eritieal temperature tllO to 125C), a temperature below which full penetration of the wood ehips with the cooking aeid must oeeur. Quite frequently heating is interrupted or retarded or about an hour at about 110 - 125C to eomplete penetration and prevent a so-ealled burnt eook from oecurring. Total t~me to maximum temperature for an aeid sulfite mill digester typically ranges from 3 to 5 hours for soluble base liquors and longer for insoluble base liquor. Attempts to shorten this time period have resulted in poor delignification and excessive carbohydrate degradation.
11~3~35~:
P. C. Leithem 2X
It is a primary object of the present invention to provide a process for producing chemical pulp which pro-cess has increased flexibility by allowing a much en-larged range of acceptable cooking parameters.
It is an additional object of the present invention to provide a process for lowering the production costs for producing chemical cellulosic pulp.
It is yet another object of this invention to pro-vide a process for producing acid sulfite pulp which pro-duces a spent sulfite liquor effluent which is less cor-rosive than prior effluents and which is more amenable to recovery processes.
It is still an additional object of this invention to provide a process for producing chemical pulps of lower lignin content which are more easily bleached than prior chemical pulps.
The present invention involves the discovery that the proportion of combines SO2 used in the digestion process can be varied as a function of the rate of heating. An increase in the pxoportion of combines S02 used in the digestion process combined with an increase in the heating rate allows a considerable shortening of the total digest-ion time. The discovery of this relationship i~ the acid sulfite process between the amount of combined SO2 and cooking rate has, insofar as is know, never before been recognized. The process of the invention uses amounts of combined S02 normally used for high hemicellulose content paper pulps and previously believed incapable of producing low hemicellulose dissolving pulps.
:
1 ~3~iZ
P. C. Leithem 2X
- 3a -Specifically, the process of the invention com-prises the digestion of wood to produce chemical pulp by the acid sulfite digestion process in which the ratio by weight of total SO2 to combined So2 is at least 4 to 1 by heating in a closed vessel wood chips in an acid sulfite cooking liquor having a concentration of free SO2 no greater than 16% to a maximum cooking temperature no greater than 180C and at a maximum pressure no greater than 170 psig for a period of time sufficient to defiber the wood, the ratio by weight of one part of combined S02 to 100 parts of dry wood being from 4 to 12 and ~he minimum average rate of hea~ing to a temperature which is the substan-tially maximum cooking temperature being 40C per hour.
- 3a -" 1~S3l~52 _4_ P. C. Leithem 2X
The success of the rapid heating in the process of the inve~tion contradicts well accepted theory that a sufficient period of time must be allowed for base penetration or impregnation to avoid so-called burnt cooks.
Hence, acid sulfite pulping technology specifies rather slow raees of heating to ensure penetration or cooking liquor to the center of the chips, for example, 10-30C or in some instances as high as 35C per hour. The rates of heating for the present process are greater than 40 C per hour, frequently greater than 50C per hour and may be greater than 100 C per hour with properly designed digesting equipment.
"Substantially maxi~um cooking temperature" as used herein is the temperature of the digester at the end of the rapid rise to cooking tempera-ture. This temperature may subsequently be slightly exceeded prior to termination of digestion. The l'average rate of heating" as used herein is the quotient of the difference between the starting temperature of the digester and the substantially maximum temperature divided b~ the time from start of heating the digester to substantially maximum temperature.
The invention will be better understood by reference to the accompany-ing drawing in which:
FIG. 1 is a graph of three different time-temperature cooking curves comparing a typical curve of the invention with prior art mill and laboratory digestion curves, and FIG. 2 is a graph of polarization curves of spent sulfite liquor of the invention as compared to prior art spent sulfite liquors to show the difference in corrosive behavior.
A principal advar,tage of the invention is an increase in productivity resulting from an approximately 10-25% reduction in digestion time. In conventional acid s-~lfite digestion processes, time to maximum temperature normally ranges from 3 to 5 hours in a com~ercial pulping operation. In the present invention, this time is reduced to 2 hours or less when using a hot ~e.g. 60-85C) liquor as is customary in a commercial operation. In the laboratory, or i~ other instances where the starting cooking liquor is at ambient temperatures, this time is still reduced to 2 1/2 hours or less in l~.S~
- 5 - P. C. Leithem 2X
accordance with the invention. In many cases, this heating time will be less than 1 1/2 hours and may range to as low as one hour or even less.
Moreover, as will be brought out more fully below, pulp quality and yields may be improved. In addition, spent sulfite liquor effluent from the digestion process has physical and chemical characteristics which are less corrosive and are more compatible with effluent recovery systems.
Technically, the process of the invention is classified in the acid sulfite range because of the relatively high ratio of total 52 to combined 52 and the resulting low pH of the cooking liquor. The ratio of total S02 to combined S02 on a weight basis ranges from about 4:1 to 12:1 in the process of the invention and the p~ of the digester liquor ranges from about 1 to 2. ~owever, the actual levels of combined SO2 used in the present process and hence the combined S02 to wood charged are typical of bisulfite pulping. In the preferred practice of the invention, the ratio by weight of one part of com~inea SO2 to 100 parts of dry wood will range from ~.5-to 9.0 and, even more preferably, will be a minimum of 5. Conventional acid sulfite pulping at these relatively high combined levels is extremely slow and pulp hemicellulose levels are distinctly higher than those achieved via combined levels in accordance with the present invention. Although the invention is useful in multi-stage or continuous processes, the process of the invention will preferably be a single stage digestion process. The prior art teaches that faster heating rates may be used by chip impregnation prior to digestion.
Such pre-impregnation is not necessary to obtain the advantages of the invention. The invention is particularly suitable for soluble base digestion processes using sodium or ammonium base cooking liquors.
As will be shown below, the process may be used to produce pulp of unique analytical properties which contradict results expected from either conventional acid sulfite or conventional bisulfite pulping data. In addition, the use of rapid heat input in a single stage process to achieve cooking temperature without specifically allowing for a base impregnation period is unique to both types of pulping technology. Pulp yields in accordance with the invention are about 45~ which is typical of conventional aqueous sulfite - 6 - P. C. Leithem 2X
processes. (Yields are identified in the Examples as percentages of "screened yields" which are defined below.) The process of the present invention is carried out in dilute aqueous solution at conventional acid sulfite digestion pressures. It does not involve large increases in S02 concentration. If the concentration of 52 in the cooking liquor were to be increased substantially over the dilute concentration normally used in acid sulfite processes, digestion pressures would have to be correspondingly increased. This results from the higher vapor pressures of unco~bined S02, particularly as cookinq temperatures are approached, and the consequent necessity of increased pressures to prevent the loss of S02. Moreover, the invention does not require the presence in the cooking liquor of lower alcohols or other organic solvents. The present invention involves an alteration of the amount of one specific chemical present in the cooking liquor in relatively small quantities, combined S02.
Free S02, and thus total cooking chemicals, may also be increased to keep the process in the acid sulfite range. ~owever, the free 52 concentration of the cooking li~uor introduced into the digester should not be greater than 16~ (16 grams of SO2 per 100 ml of liquor), will usually be from about 4 to 12~ and even more preferably from 5 to 10%. As the cook is heated to maximum temperature and pressure, the solubility of free SO2 goes down~ Thus the amount of free S02 at maximum pressure will normally not exceed 19~
after pressure is relieved. The maximum pressure should be about 170 psig (1170 KPa(g)) and normally this maximum will be from 70 to 150 psig (482-1034 KPatg)), pressures which are conventional for acid sulfite digestion.
The liquor to wood ratio will normally range from 3:1 to about 6:1 based on liters of cooking acid added to kilograms of oven dried wood.
Using too little liquor outside this range would present problems in covering chips to assure adequate cookingi too much liquor would be im-practical for commercial operation. Typical mill liquor to wood ratios range from 4-5: It should be noted that additional moisture entering the digester with the chips is typically not included in this calculation, but would result in relatively small additional dilution. If the liquor - 7 - P. C. Leithem 2X
~5;~8'~
to wood ratio is varied in the present invention, a corresponding variation shou~ be maae in the concentration o~ c~m~ined SO~ in the cookin~ ~cid so that the combined S02 to wood range is between 4 and 12. For example, a cooking acid containing 1.2 g/dl combined S02 will result in a combined S02 to wood ratio of 6 to 1 ~g/100 kg O.D. wood) when cooking liquor is used at a S:1 liquor:wood ratio. ~owever, to achieve the same combined S02 to wood ratio, a cooking liquor of 1.5 g/dl combined S02 must be used at a 4:1 liquor:wood ratio.
By acid sulfite cooking we mean, as per Rydholm, that the ratio of total 52 to combined SO2 would be at least 4:1, such that the pH of the cooking acid (at room temperature) would fall in the general range of 1-2.
(Reference to "Rydholm" herein is to the text Pulping Processes, S. A.
Rydholm, Interscience Publishers, 1965.) This ratio of total S02 to combined S2 in the cooking acid at room tem~erature may be as great as 12:1 but will generally be lower in accordance with the free S02 concentration of the cooking acid which will typically range from 4 to 12 g/dl free S02. By free S02 we mean the portion of the S02 that is the sum of the actual free S02 plus one-half of the S02 combined as bisulfites, determined by titration according to TAPPI Standard Method T604. (This is the definition of free S02, set forth in TAPPI definition T 1201 OS-72.) The total S02 content of the cooking liquor refers to the grams of S02 per 100 ml of solution also as determined by titration according to TAPPI Standard Method T604. The total 52 concentration of a cooking liquor is the sum of the free plus the combined S02 concentrations. The actual ratio of total S02 to combined S02 will in all cases be greater than 4:1 but will vary to reflect the free S02 concentration of the cooking liquor which will not exceed 16 g/dl free S02.
For example, if a cooking liquor containing 8.0 g/dl free S02 and 2.0 g/dl combined S02 or 10.0 g/dl total S02 is chaxged, the ratio of total S02 to combined S02 would be 5.0:1 0. If the cooking liquor contained 8.0 g~dl free S02 and 1.2 g/dl combined S02 or 9.2 g/dl total S02, the ratio of total SO2 to combined SO2 would be 7.7:1Ø
~15~85~
- 8 - P. C. Leithem 2X
As previously indicated, the process of the invention uses pressures which are essentially conventional for acid sulfite digestions processes. At the maximum pressure of 170 psig, about 10~ free SO2 would remain when the digester temperature reached 120C, even if free 52 had been present in greater quantity in the cooking acid (at lower temperature).
For temperatures greater than 120C, even less S02 would be present, i.e.
more would be lost in relieving digester pressure at 170 psig. Less than 10%
free S02 would remain in the digester liquor at 120C at lower, more practical maximum digester pressures, e.g. 70 to 150 psig or even more pre-greatly ferably 90-120 psig. Com~ined S02 does not contribute/to vapor pressure and hence its concentration would not be limited by a pressure limitation. The relationship between pressure, free S02 and temperature in an acid sulfite digestion process is more fully set forth in an article entitled "Chemical Equilibria in ~eated Sulfite Solutions" by O.VO Ingruber, in Pulp and Paper Magazine of Canada, 66:T215 to T228, April 1965.
In the following examples, data is given for various properties of both unbleached pulp and spent sulfite liquor. The definition of this property data is set forth below.
.~
9 l~L5385~ P. c. Leithem 2X
.
K number ~also called permanganate number, see page 1112 Rydholm) is a measure of lignin remaining in unbleached pulp. Ihe determination is based on the fact that lignin is much more rapidly oxidized by a O.lN
; (normal) solution of potassium permanganate than the cellulose and hemi-cellulose ?resent in unbleached pulp. Specifically, K number corresponds to the number of milliliters of O.lN potassium permanganate consumed by 1 g of dry unbleached pulp under standard conditions.
I.V. is cuene intrinsic viscosity (see page 1118 Rydholm) and refers to the intrinsic viscosity of a 0.5% solution of unbleached pulp in cuene (cupriethylenediamine hydroxide). The intrinsic viscosity of this solùtion is related to the average degree of polymerization of the carbo-hydrate polymers (cellulose and hemicellulose) in unbleached pulp.
Dissolving grade pulps are usually digested to a specified I.V.
Lignin is deter~ined by hydrolyzing and dissolving the cellulose and hemicellulose in hot 72% sulfuric acid. A portion of the lignin (insoluble llgnin) remains as an insoluble residue that is collected and dried. ~nother portion goes into solueion and is termed soluble lignin; it is measured in solution by a spectroscopic method. These lignins are expressed as a percentage of the dry unbleached pulp. A higher portion of soluble lignin is considered desirable.
Slo and Slg (see Rydholm page 1116 and 1117) refer to the solu-bility of unbleached pulp in 10 and 18% solutions of caustic under a standard set of conditions. Slg is a reflection of the amount of hemicellulose, whereas Slo is a reflection of the amount of degraded cellulose plus hemi-cellulose present in the unbleached pulp. Lignin is removed from the pulp by a chlorite oxidation method before alkali solubility determinations are made (see ~ydholm page 1118).
Xylan and mannan content are a reflection of the two major types of hemicellulose present in chemical wood pulp. Xylan is a major constituent of one type, whereas mannan is a ~aior constituent of the other. Xylan and mannan _ 9, _ - lo ~ 3~5~ P. c. Leithem 2X
were measured by hydrolyzing pul? to its monomeric compcnents with an acid soluticn, and measuring ~he iiberated xylose and mannose by a paper chroma-tographic technique.
Tailings refers to that por~ion of wood chips that are not digested to the point where the wood fibers separate. Tailings are removed from the liberated fibers by a screening method. Screened yield is the percent of dry unbleached pulp (based on dry wood) recovered after digestion and screening to remove tailings. The percent dry weight of tailings removed is also based on the dry weight of wood digested.
Brightness refers to the amount of reflected light coming from a sheet of pulp compared to the amount of reflected light from a standard white plate. Measurements are made in an Elrepho photoelectric reflection photometer No. 50-38-00 under standard conditions. ~igher unbleached pulp brightness implies improved delignification during digestion.
The following example illustrate the practice of the invention.
Unless otherwise indicated, all parts ahd percentages are by weight.
~xam~
Western hemlock wood chips (65.8 kg~ containing 48.6% dry wood were placed in a 0.2 m3 capacity stainless steel laboratory pulping digester.
Chips were of typical commercial size having the following average dimensions:
length, 19.8 mm; width, 19.0 mm; thickness, 3.7 mm. After placing on the lid, 160 Q of ammonium base cooking liquor having a composition falling in the range of the invention (7.25 g/dl free S02, and 1.22 g/dl combined S02) was pumped into the digester. The combined S2 to wood was 6.1 kg of combinet S02 to 100 kg of oven dried wood. The s~stem was then heated by circulating the liquor out of the top of the digester, through a steam-heated heat exchanger, and back into the digester bottom. The average heating rate to the maximum cooking temperature was 64Clhr (1.07C/min), bringing the system from 20C
to 148~ in 2.0 hr. The digester pressure was not allowed to exceed 758 kPa(g). -After holding the d1gester at 148 C for 1 hr and 55 min, the pressure was lowered to 551 kPa(g) and the digester contents blown to a tank at atmospheric pressure. Total digestion time was 3 hrs and 55 min.
.~
~ ~3~35~:
~ P. C. Leithem 2X
The unbleached pulp and spent sulfite pulping liquor (SSL) wese separated with a centrifuge. The pulp was then washed with water and passed through a screen with 0.2 ~ slots to remove tailings and dewatered to 31.7% drv weight, and weighed. ~aterial not passing the screen (tailings) were collected, dried and weighed.
~ portion of the SSL was stripped of free S02 by counter current contact with steam in a packed colu~n under 200 kPa(g) of pressure.
Stripped liquor was evaporated to 50% solids at atmospheric pressure in an indirect-contact steam-heated evaporator. Viscosity of the evaporated liquor was measured on a Brookfield viscometer.
Examole 2 A second digestion typical of conventional acid sulfite pulping was performed in a similar manner except the cooking liquor contained 7.17 g/dl free S2 and 0.65 g/dl combined SO2, and that the digester was heated from 20C to 142C in 3 hr and 30 min,- an average heating rate of 34.9C/hr (0.58C/min). The combined S02 to wood ratio was 3.3 (k~ of S02 to lO0 kg of oven dried wood). The digester was held at 142C for 50 min before blowing. Total digestion time was 4 hr and 20 min. The maximum cooking temperature in Example l was 6 higher than in this Example 2. It is desirable, although not necessary as demonstrated in further examples below, that such a higher temperature be used to help shorten total cooking time.
Properties of the unbleached pulp and SSL from Examples l and 2 are set forth in Table I.
385~
- 12 - P. C. Leithe~ 2X
Conventional Invention Acid Sulfite ~nbleached Pulp Exa~ple 1 Examole 2 Screened yield, ~ 46.5 47.2 Tailings, % 0-5 0-5 I.V., dl/g ll.0 10.8 K number 8.8 11.6 K no./I.V. 0.80 . 1.07 10.5 11.8 slo ~ ~
9.0 10.2 S18, %
Xylan, ~ 2.0 1.8 ~annan, % 4.6 5.5 Xylan plus mannan, % 6.6 7.3 Lignin, soluble, ~ 1.50 1.40 Lignin, insoluble, % 0.09 0.42 Total lignin ~%) 1.59 1.82 Brightness, % 60.2 53.0 Spent Liquor Unstripped SSL pH 3.14 1.87 Stripped SSL pH 4.54 3~11 Stripped and evaporated 27 60 SSL vlscosity at 90C, nPa.s : ~, 3~35Z
- 13 - P. C. Leithem 2X
It can be seen from the table that the process of the invention, compared with conventional acid sulfite pulping, can give pulp of about the same yield, tailings and I.V., but with tne advantages of 25 min shorter digestion time, and i~proved pulp lignin and hemlcellulose contents and higher brightness. Further re, as more fully discussed below, the waste liquor from the invention has a higher pH, making it less corrosive to process equipment. Also, evaporating the invention SSL produces a lower viscosity heavy liquor that would be more econo~ic to burn in a recovery boiler.
The cooking curves of Examples 1 and 2 are shown in FIG. 1 together with a representative curve o~ an acid sulfite mill digestion process. The mill curve is shown because time-temperature curves in the mill tppically differ in certain respects from comparable laboratory runs of the type shown in Examples 1 and 2. As shown in FIG. 1, the maximum cooking temperature is reached in two hours in Example 1 and in 3 1/2 hours in the prior art process of Example 2. In the mill curve the maximum cooking temperature (140C) is reached after 3 hours. Xowe~er, the temperature frequently rises a few degrees (e.g. 1 to 10C) in mill digestion operations after achieving "maximum cooking temperature" and for this reason, the term "substantially the maximum cooking temperature" is herein used to define the end of the rapid temperature rise, namely 140C at three hours in the mill curve of FIG. 1. In practice, the temperature may rise a few degrees above 140C in a mill operation after the "substantially maximum temperature" is reached at the end of the three hour period in FIG. 1. ~ote also that the mill curve starts with a 60C cooking liquor and ~hat the temperature tails off at the end of the digestion cycle, as is typical of mill operations.
The corrosivity of the unstripped SSL of Examples 1 and 2 to 317L
stainless steel was evaluated by potentiodynamic polarization using a Princeton Applied Research ~odel 331-1 corrosion measurement system. The temperature was 65 C. Traditionally, 317L stainless steel (SS) is one of the materials used in commercial equipment made to handle SSL.
1 1~5~5'~:
- 14 - P. C. Leithem 2X
Anodic polarization curves of the SSL are shown in FIG. 2. In this technique, an increasing potential is applied between a S cm metal anode (317L SS) and an inert saturated colomel electrode tscE), or cathode, and the resultant currents measured.
As the anodic polarization increases, passivation occurs; i.e., the current flowing from the anode goes through maximum, then decreases to the passive current density. This indicates that the stainless steel has the ability to self passivate. The wider the passive potential range, the more likely that anodic passivation will occur, and that the stainless steel will resist corrosion. It can be seen from FIG. 2 that SSL from the inven-tion has a wider passive potential range than for SSL from the conventional acid sulfite process. The anodic curve of the invention starts at a lower potential than the conventional process because of a higher pH.
Examples 3-7 Two digestion runs were carried out as in Example 1 but using - sodium base rather than ammonium base cooking liquor. ~emlock wood chips were used as furnish. Table II compares the results of these digestion runs with two similar runs carried out in accordance with conventional acid sulfite digestion processes and one run in accordance with conventional bisulfite digestion processes. Examples 5 and 6 at 3.1 combined SO2 to wood ratio is a typical conventional acid sulfite ratio for dissolving grade pulp. By comparison, Example 7 is typical of bisulfite pulping suitable for paper end use. Note that tha combined SO2 to wood in Example 7 is much higher t8.7) than Examples 5 and 6. Note also that the maximum tempera-ture used is over twenty degrees higher for the bisulfite cook than for the conventional acid sulfite cooks. In addition, total cooking time is long, approaching six hours for Example 7. Higher maximum temperature and longer total cooking times are well known to be required in bisulfite pulping to compensate for the slowing down of the delignification rate which is caused by the higher levels of combined S02 to wood.
- 15 - P. C. Leithem ZX
The cooking conditions and the resulting unbleached pulp ant spent liquor properties for these five exa~ples (except ~here not measured) are set forth in Table II.
~ - 15 -~S3~
- 16 - P. C. Leithem ~X
TABLE II
Conventional Process Invention ~id Sulrite Bisul~ite E~ample ~o. 3 4 5 6 7 Cooking Conditions Combined S02 (g/dl) 1.20 1.22 0.66 0.66 1.9 Free S02 (g/dl) 6.93 7.09 7.05 7.08 1.9 Total sO2/Com~ined S2 6:1 6:1 12:1 12:1 2:1 Combined SOg:Wood (kg/100 kg O.D.) 6.0 6.1 . 3.3 3.3 8.7 ~aximum Temperature (C)148 148 142 142 165 Time to (hrs:min) 2:00 2:00 3:30 3.30 3:00 Time at (hrs:~in) 1:55 1:55 0:45 0:50 2:45 Average Xeating Rate to (Clhr) 64 64 3'~.9 34.9 48.3 Total Cooking Time (hrs:~in) 3:55 3:5; 4:15 4:20 5:45 Pul~ Pro~erties Screened Yield (%) 44.0 44.7 45.7 44.5 53.6 Tailings (%) 0.7 0.6 0.4 0.7 0.6 I.V. (dl/g) 10.5 10.9 11.2 10.0 11.6 K ~umber 8.4 8.8 10.7 10.2 19.8 K ~o./I.V. Ratio 0.80 0.81 0.96 1.02 1.?1 Total Lignin (7.) 2.5 1.~ ~.4 2.0 --Lignin Soluble (%) 1.4 1.6 1.3 0.9 Lignin Insoluble (%)1.1 0.3 1.1 1.1 --S10 (Z) 11.2 11.3 12.0 l2.0 __ S18 (%) 9.6 9.7 10.9 10.2 ~-Brightness, % 56.8 54.2 50.3 49.8 __ Spent Liquor Combined--- Combined---Unstripped pa 3.00 1.75 --Stripped pa 4.35 3 05 ~
Vanillin Yield 0.042 0.038 --(gm/gm SSL solids) ll.S31~5~
- 17 - P. C. Leithem ~X
Table II shows that Exa~ples 3 and 4 of the invention produced at shorter times pulp properties which were at least equivalent to the conventional acid sulfite pulps of Examples S and 6. Note also that Examples 3 and 4 had lower K ~o./I.V. ratios. "l~ nu~ber" is ~ measure of the lignin content of the pulp, the lower the K ~o., the less lignin in the pulp. I.V. is a measure of degradation of the pulp, the higher the I.V., ehe less degradation. Thus the lower the ratio of K ~o./I.V., the better the quality of the pulp within a given I.V. range. After almost six hours of cooking the bisulite pulp of Example 7 is characterized by poorer delignification than acid sulfite Examples 5 and ~. Table II also shows hi8her pH's and improved vanillin yields from the spent sulfite liquors of the examples of the invention.
Examples 8-12 A further series of laboratory pulping runs were made to compare the effects of rapid heating schedules on conventional acid sulfite and conventional bisulfite combined/wood ratios. All parameters of the process were those of conventional runs except for the heating schedule. All examples used sodium base cooking liquor and hemlock wood chips. The cooking conditions and the results of each of these runs are set forth in Table III.
- 18 - P. C. Leithem 2X
T~BLE III
Acid Sulfite _ Bisulfite Fast Fast Process Conventional Heating Conventional Heatin~
Example ~o. 8 9 10 11 12 Cooking Conditions Combined S02 (g/dl) 0.65 0.64 0.65 1.90 1.91 Free SO2 (g/dl) 7.2 6.9 7.2 1.9 1.9 Total S02/Combined S02 12/1 12/1 12/1 2/1 2/1 Co~bined S02/~ood (kg/100 kg O.D.) 3-3 3.2 3.3 10.9 11.2 Maximum Temperature (C) 142 142 142 165 165 Tim~ to (hrs:min) 3:30 2:00 1:00 3:00 1:00 Time at (hrs:min) ~:30 1:00 2:00 2:45 2:45 Average Heating Rate to (C/hr) 34.5 61.0 122.0 48.3 145.0 Total Cooking Ti~e (hrs:min) ~ 4:00 3:00 3:00 5:45 3:45 Maximum Pressure ~Pa(g)) 758 758 758 1240 1240 Cooking Liquor pH 1.5 1.3 1.5 3.1 4.0 Pulp Properties Screened Yield (%) 49.3 45.6 -- 48.9 51.7 Tailings (%) 0.8 3.9 11.6 0.3 2.3 K No./I.V. Ratio 1.40 3.05 2.70 1.76 2.32 I.V. (dl/g) 10.5 7.9 8.6 10.3 11.7 K Number 14.7 24.0 23.2 18.1 27.2 Total Lignin (Z) 3.5 5.8 5.6 -- --Lignin, Soluble (%) 1.2 0.4 0.8 -~ --Lignin, Insoluble (%) 2.3 5.4 4.8 -- --Brightness (Z) 40.8 35.8 33.0 -- --Xylan (%) 1.6 1.4 1.6 -- 2.6 Mannan (%) 4.8 3.8 3.8 -- 10.5 Total Xylan plus Mannan (%) 6.4 5.2 5.4 -- 13.1 llS385Z
- 19 - P. C. Leithem 2X
It will be seen by ~xamination of Table III, in which all exa~ples are outside the scope of the invention, that the acid sulfite pulp5 produced with fast heat (Examples 9 and 10) e~hibit higher tailings, higher ~ numiers, higher K ~o./I.V. ratios as com~ared to the corresponding conventional process. Furthermore, Examples 9 and 10 show evidence of lignin condensa-tion (compare insoluble lignin contents) and carbohydrate degradation as evidenced by lower I.V.'s. These data substantiate the well known properties of a burnt cook in conventional acid sulfite pulping. The success of the corresponding cooks using the process of the invention under the same rates of heaeing indicates that it is the overall quantity of co~bined S02/wood which limits the rate of heating in conventional acid sulfite pul?ing.
Exa~ples 11 and 12 in Table III also shows the effect of fast heating rise on bisulfite cooking. The ~ ~o./I.V. ratio and tailings increase as in fast rise conventional acid sulfite pulping. Also as in conventional acid bisulfite pulping, the use of a fast rise to maximu~
temperature to reduce total cooking time is not plausible with bisulfite pulping because pulp quality is impaired as evidenced by the 50% jump in K nu~ber. Table III supports the conclusion that the success of the process of the invention is the result of coupling a fast temperature rise with an appropriate cooking acid combined ratio and substantial a unes of free S02.
Examples 13-15 These e~amples illustrate the preparation of an acid sulfite pulp to a target I.V. of 11 from slash pine furnish with an ammonium base cooking liquor. Examples 13 and 14 are in accordance with the invention. Example 15 is a comparable acid sulfite digestion process using a conventional combined S2 to wood ratio and heating rate. The maximum cooking pressure in all examples was 758 RPa gauge. The digestion conditions and pulp and spent liquor properties are set forth in Table IV.
1.~53~52 - 20 - P.C. Leithem 2'f.
TABLE IV
Process Invention Conventional Example ~o. 13 14 15 Cooking Condi~ions Combined S02 (g/dl) 1.51 1.19 0.85 Free S02 (g/dl) 6.95 6.93 6.99 Co~bined S02:Wood (kg/100 kg O.D.) 5.6 4.4 3.2 ~aximum Temperature (C) 145 145 140 Time to (hrs:min) 2:00 2:15 4:00 Time at (hrs:min) 2:45 2:05 1:29 Average ~eating Rate to (C/hr) 62.5 55.6 30.0 Total Cooking Time (hrs:min) 4:45 4:20 5:29 Pulp Properties Screened Yield (%) 45.8 45.9 46.3 Tailings (%) 1.6 1.8 1.9 I.V. (dl/g) 11.1 11.1 10.8 K Number 8.0 9.5 10.1 K No./I.V. Ratio ' 0.72 0.86 0.94 Total Lignin (Z) 2.1 2.1 3.3 Lignin Soluble (%) 1.4 1.4 1.0 Lignin Insoluble (~) 0.7 0.7 2.3 Slo (~) 10.3 10.1 10.9 S18 (%) ~ 8.6 8.6 9.2 Brightness (%) 67.2 64.4 61.5 Spent Liquor Unstripped pH 2.55 2.3 1.9 Stripped pH 4.1 3.9 2.7 - 20 ~
3~35~
- 21 - P. C. Leithem 2X
It will be seen fro~ Table IV that a pulp of improved K nu~er, K ~o./I.V. ratio, lignin content and brightness was produced in accordance with the invention at a significant reduction in digestion time. It should also be noted from the S18 results that the hemicellulose level is une~pectedly low for the total cookin~ times and combined S02/wood levels used. At these cooking times and co~iined levels, it would normally be expected to have higher hemicellulose levels. This is further shown in the following Examples 16 and 17 and Table V. Finally, the p~ levels of the spent sulfite liquor are significantly higher for Examples 13 and l4 of the invention.
Exam~les 16 and 17.
Two pulping runs were mRde to compare fast heating in accordance with the invention with conventional heating, both runs using a high combined level within the scope of the invention. Both samples used sodium base cooking liquors and hemlock wood chips. Maximum pressure was 758 kilopascals gauge in both exa~ples. The results are set forth in Table V.
~5'~85Z
- 22 - P. C. Leithem 2X
Process Invention onventional Exa~ple No. 16 17 _ooking Conditions Combined S02 (g/dl) 1.19 1.16 Free S02 (g/dl) 7.18 7.24 Total sO2/Co~bined S2 7/1 7/1 Combined SO2/Wood ~ (kg/ 100 kg O.D.) 6.0 5.8 Maximum Temperature (C) 142 142 Time to (~I:M) 1:00 3:30 Time at (H:~) 2:00 0:30 Average Heating Rate to ~-(C/hr) 122.0 34.8 Total Cooking Time (H:~) 3:00 4:00 Cooking Liquor p~ 1.5 - 1.4 Unbleached Stock Screened Yield (%j 45.8 53.3 Tailings (%) 2.8 0.6 K Wo¦I.V. Ratio 1.42 1.70 I.V. (dl/g) 12.7 14.9 K Number 17.9 25.2 Total Lignin (~) 2.6 Lignin, Soluble (%) 1.6 --Lignin, Insoluble (%) 1. 0 --Brightness (%) 44 . 5 40 . 8 Slo (%) 11.5 13.9 Slg (Z) 10.3 12.9 Xylan (%) 2.1 2.3 ~annan (%) 4.6 6.1 Xylan plus Mannan (Z) 6.7 8 . 4 ~;~
1 ~L5 3 ~ 5 2 - 23 - P. C. Leithem 2X
Table Y shows that at a high combined S2 level, the process of the invention has produced a 12.7 I.V. pulp and a 17.9 K number while Example 17, at the same high combined level but at conventional heating rate, produced a higher I.V. and K number pulp in an hour longer total cooking time. Note also a lower hemicellulose content in Example 17 as evidenced by lower S18 and total x~lan and mannan content.
Examples 18-l9.
Two digestion runs were made at the same max$mum cooking tempera-ture and to the same approximate I.V. range. The I.V. range used in this example is typical of paper grade pulps. The first of these runs, Example 18, was in accordance with conventional acid sulfite digestion practice with respect to combined S02 to wood ratio, heating rate and cooking time~
Example l9 was within the scope of the invention. Both examples used slash pine furnish, an ammonium base cooking liquor and a maximum cooking pressure of 758 KPa gauge. The cooking conditions and pulp properties are set forth in Table VI.
_ _ _ ~1S3~Z
- 24 - P. C. Leithem 2X
T~BLE VI
Process Conventional Invention Example ~o. 18 19 Cookino Conditions Combined S02 (g/dl) 0.65 1.70 Free S02 (g/dl) 7.30 7.09 Combined S02:Wood (Rg/100 Rg O.D.) 3.3 8.5 ~aximum Temperature (C) 142 142 Time to (hrs:min) 3:30 1:00 Time at (hrs:min) 0:30 2:30 Average Heating Rate to (C/hr) 34.9 122.0 Total Cooking Time (hrs:min) 4:00 3:30 Pulp Pro~erties Screened Yield (Z) 45.6 48.5 Tailings (%) 3.0 3.4 I.V. (dl/g) 11.9 12.6 - K Number 19.5 15.7 K No./I.V. Ratio 1.64 1.25 Total Lignin (%) 3.5 4.0 Lignin Soluble (%) 0.2 2.9 Lignin Insoluble (%) 3.3 1.1 Xylan Plus ~annan (%) 8.0 9.0 Slo (%) 11.1 10.8 S18 (%) 9.1 g,o Brightness (%) 48.5 57.6 t - 24 -25 ~ 5~3~ ~ 2 p. C. ~eithem 2X
It will be seen that the process of the invention, Example 19, arrived at approximately the s~me I.V. range as conventional Example 18 with less cooking time and produced a pulp of slightly better quality as evidenced by K number and K No./I.V. ratio. Table VI also indicates that the maximum temperature for the process o~ the invention need not be any ¦ higher than that used in conventional acid sulfite cooking, whereas bisulfite conditions which use high combined S02 to wood ratios require a higher cooking temperature to effectively defiber the wood. See Example 7 above which shows a conventional bisulfite process using a combined SO2 to wood ratio of 8.7 which is comparable to Example 19 but which requires a maximum cooking temperature of 165C.
Examples 20-21.
Examples 20 and 21 compare the use of cooking liquor at ambient temperature with a substantially ide~tical digestion run starting with a hot cooking liquor to simulate a mill run. Ammonium base cooking liquors having a combined SO2 level within the scope of the invention were pumped from a holding vessel into a laboratory digester containing hemlock chips.
In Example 20, the cooking acid was pumped to the digester without first heating the liquor, whereas in Example 21 the liquor was first heated to 96C. The maximum cooking pressure in both examples was 758 KPa(g). The same heating curve was used in both examples, however 35 minutes was eliminated from the first portion of the cooking curve of Example 20 by starting with hot cooking liquor. This time saving is reflected in the 35 minute shorter total cooking time of Example 21 compared with Example 20. The results are set forth in Table VII.
~ - 25 ~
111 ~3~3S~
- 26 - P. C. Leithem 2X
TABLE ~II
Example No. 20 21 Cooking Conditions Combined S2 (g/dl) 1.20 1.19 Free S02 (g/dl) 7.02 6.93 Combined S02:Wood (Kg/100 Kg O.D.) 6.3 6.3 Temperature at Start of Heating (C) 20 84 Maximum Temperature (C) 142 142 Time to (hrs:min) 2:00 1:25 Time at (hrs:min) 1:30 1:30 Average Heating Rate to (C/hr~ 61.0 40.9 Average Heating Rate to 110C (C/hr) 83.1 62.4 Total Cooking Time (hrs:min) 3:30 2:55 Pul~ Proverties Screened Yield (%) . 47.4 48.9 Tailings ~%) 1.5 1.6 I.V. (dl/g) 14.0 14.0 K Number 15.6 21.6 K No./I.V. Ratio 1.11 1.54 i - .
3~2 - 27 - P. C. ~eithem 2X
Examples 20 and 21 and Table VII show that the invention is equally applicable to the use of both ambient temperature and preheated cooking liquor, the latter being commonly used in co~mercial practice.
Since the heating curves are not linear, the heating rate is influenced by the starting temperature of the cooki~g liquor.
Thus, the process of the invention greatly enlarges the raDge of cooking parameters acceptable in acid sulfite digestion processes.
This flexibility permits increased productivity by allowing an overall reduction in digester cooking time, typically 10-25~ or even more. The process produces spent sulfite liquor whose physical and chemical charac-teristics are improved and which is more compatible with effluent recovery systems. The process also makes possible the production of improved quality pulp with lower bleach chemical requirements because of reduced lignin content.
Claims (13)
I CLAIM:
1. In a process of digesting wood to produce chemical pulp by the acid sulfite digestion process in which the ratio by weight of total SO2 to com-bined SO2 is at least 4 to 1 comprising heating in a closed vessel wood chips in an acid sulfite cook-ing liquor having a concentration of free SO2 no greater than 16% to a maximum cooking temperature no greater than 180°C and at a maximum pressure no greater than 170 psig for a period of time sufficient to de-fiber the wood, the improvement in which the ratio by weight of one part of combined SO2 to 100 parts of dry wood is from 4 to 12 and the minimum average rate of heating to a temperature which is the substantially maximum cooking temperature is 40°C per hour.
2. The process of Claim 1 in which the temperature is raised to substantially the maximum temperature from an ambient temperature cooking liquor in a time of less than 2 1/2 hours.
3. The process of Claim 1 in which the temperature is raised to substantially the maximum temperature from a heated cooking liquor in a time of less than 2 hours.
4. The process of Claims 2 and 3 in which the time is less than 1 1/2 hours.
5. The process of Claim 1 in which the ratio of combined SO2 to wood is 4.5 to 9.
P. C. Leithem 2X
- 28a -
P. C. Leithem 2X
- 28a -
6. The process of Claim 1 in which the ratio of combined SO2 to wood is at least 5.
7. The process of Claim 1 in which the average minimum rate of heating is 50°C per hours.
8. The process of Claim 1 in which the initial pH of the cooking liquor ranges from about 1 to 2.
9. The process of Claim 1 in which the ratio by weight of total SO2 to combined SO2 ranges from 4 to 12.
10. The process of Claim 1 in which the process is a single stage digestion process.
- 28a -- 29 - P. C. Leithem 2X
- 28a -- 29 - P. C. Leithem 2X
11. The process of Claim 1 in which the process is a soluble base digestion process.
12. The process of Claim 1 in which the concentration of free SO2 ranges from 4 to 12% and does not exceed 10% at cooking temperatures above 120°C.
13. The process of Claim 1 in which the maximum pressure is 150 psig.
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US5452579A | 1979-07-03 | 1979-07-03 | |
| US54,525 | 1979-07-03 | ||
| US131,813 | 1980-03-19 | ||
| US06/131,813 US4295929A (en) | 1979-07-03 | 1980-03-19 | Process for acid sulfite digestion of wood |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1153852A true CA1153852A (en) | 1983-09-20 |
Family
ID=26733125
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000355337A Expired CA1153852A (en) | 1979-07-03 | 1980-07-03 | Process for acid sulfite digestion of wood |
Country Status (2)
| Country | Link |
|---|---|
| CA (1) | CA1153852A (en) |
| DE (1) | DE3024420A1 (en) |
-
1980
- 1980-06-28 DE DE19803024420 patent/DE3024420A1/en active Granted
- 1980-07-03 CA CA000355337A patent/CA1153852A/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| DE3024420A1 (en) | 1981-01-08 |
| DE3024420C2 (en) | 1992-07-09 |
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