US3841962A - Hydrogen sulfide pretreatment of lignocellulosic materials in alkaline pulping processes - Google Patents

Abstract

DIRECTION OF THE SUPPORT TUBE (8) AND WHICH WHILE THE REACTOR IS IN OPERATION IS SUPPORTED ON THE LOCKING PLATE (5,6), AND WHILE THE REACTOR IS SHUT DOWN IS SUPPORTED ON THE SLEEVE TUBE (3) BY MEANS OF A PAWL DEVICE (14), THE LENGTH OF TRAVEL OF THE AUXILIARY TUBE (12) BEING SUCH THAT WHEN THE PAWL DEVICE IS IN OPERATION THE SUPPORT TUBE (8) IS AT LEAST PARTLY RELIEVE OF THE WEIGHT OF THE MODERATOR COLUMN.

1. IN AN ALKALINE PULPING PROCESS IN WHICH LIGNOCELLULOSIC MATERIAL IS PRE-TREATED WITH HYDROGEN SULFIDE, IN WHICH THE PRE-TREATED MATERIAL IS DIGESTED UNDER HEAT AND PRESSURE IN AN ALKALINE DIGESTING LIQUOR COMPRISING A SODIUM BASED DIGESTING AGENT TO FORM THE PULP, IN WHICH THE DIGESTED PULP IS SEPARATED FROM THE DIGESTING LIQUOR AFTER DIGESTION, AND IN WHICH THE DIGESTING LIQUOR IS PROCESSED FOR RECOVERY OF THE ALKALINE DIGESTING AGENT, THE IMPROVEMENT WHICH COMPRISES: PROVIDING FROM THE DIGESTING LIQUOR PROCESSED FOR RECOVERY OF THE ALKALINE DIGESTING AGENT, A HIGH SULFIDITY LIQUOR CONTAINING AT LEAST ONE MEMBER SELECTED FROM THE GROUP CONSISTING OF SODIUM SULFIDE AND SODIUM HYDROSULFIDE AND HAVING A SULFIDITY OF ABOUT 75% TO 200% AND A WEIGHT RATIO OF SULFUR TO SULFUR PLUS SODIUM OF FROM ABOUT 0.35-0.58; CONTACTING THE LIGNOCELLULOSIC MATERIAL WITH SAID HIGH SUFIDITY LIQUOR IN A PRE-TREATMENT VESSEL PRIOR TO ALKALINE DIGESTION; INTRODUCING CARBON DIOXIDE GAS INTO SAID HIGH SULFIDITY LIQUOR IN SAID VESSEL, IN AN AMOUNT IN EXCESS OF STOICHIOMETRIC TO GENERATE HYDROGEN SULFIDE FROM SAID HIGH SULFIDITY LIQUOR; MAINTAINING THE TEMPERATURE IN SAID PRE-TREATMENT VESSEL WITHIN THE RANGE OF ABOUT 120 TO 165*C. FOR ABOUT 20 TO 200 MINUTES AND UNDER A PARTIAL PRESSURE OF HYDROGEN SULFIDE GAS OF ABOUT 10 TO 80 P.S.I. TO EFFECT REACTION BETWEEN HYDROGEN SULFIDE AND THE LIGNOCELLULOSIC MATERIAL IN SAID PRE-TREATMENT VESSEL TO INCREASE PULP YIELD OVER THAT OBTAINED BY AN ALKALINE PULPING PROCESS IN WHICH THE LIGNOCELLULOSIC MATERIAL IS DIGESTED IN SAID ALKALINE DIGESTING LIQUOR; SEPARATING PRE-TREATED LIGNOCELLULOSIC MATERIAL FROM THE ACCORDING TO WHICH THE SUPPORT TUBES PASS THROUGH THE BOTTOM OF THE REACTOR VESSEL AND ARE PROVIDED AT THE OUTER END WITH AT LEAST ONE PLUG WHICH ACTS AS A RADIATION SHIELD AND A COOLING-GAS SEAL, IN WHICH THE SUPPORT TUBE (8) IS FIXED IN A SLEEVE TUBE (3) BY MEANS OF A LOCKING PLATE (5, 6) SUCH THAT BETWEEN THE SUPPORT TUBE (8) AND THE SLEEVE TUBE (3) THERE IS AN ANNULAR GAP (11) CONTAINING AN AUXILIARY TUBE (12) WHICH IS MOVABLE IN THE AXIAL

Description

Oct. 15, 1974 Filed- May 21, 1973 A. R. PROCTER ET L HYDROGEN SULFIDE PRETREATNENT F LIGNOCELLULOS MATERIALS IN ALKALINE PULPING PROCESSES 3 Sheets-Sheet 2 f 5 THE COMPOSITION OF THE SULFIDE, HYDROSULFIDE, H28 SYSTEM AND THE CARBONATE BICARBONATE SYSTEM As A FUNCTION OFPH.

s E 0.6- v 3 E 0.4 0.2 X

o 2 4 6 8 I0 I2 I4 p THE EFFECT OF FEED LIQUOR CHEMICAL COMPOSITION A? -To THE PRETREATMENT VESSEL 0N H28 PARTIAL PRESSURE AT EQUILIBRIUM IN A CONTINUOUS SYSTEM 06 FEED LIQUOR %s APPLIED ON WOOD I I 8 Lu L z 0.5 2 2 k D K lg E fly Y Q (I) H 8 PARTIAL 'PRESSURE AT [40C Oct. 15, 1974 Filed May 21. 1973 A. R. PROCTER E AL HYDROGEN SULFIDE PRETREATMENT 0F LIGNOCELLULOSIC MATERIALS IN ALKALINE PULPING PROCESSES 3 Sheets-Sheet J 4 THE EFFECT OF H25 PARTIAL PRESSURE 0N PULP YIELD INCREASE PRETREATMENT CONDITIONS, OF /4oc,5o MIN.

S/S+Na WT. FRACTION IN FEED THE EFFECT OF FEED LIQUOR CHEMICAL COMPOSITION TO THE PRETREATMENT VESSEL ON CO AT EQUILIBRIUM C OMS UMPTI ON 00 CONSUMPTION, 0N WOOD United States Patent rm. (:1. D218 3/00, 11/12 US. Cl. 162-30 Claims ABSTRACT OF THE DISCLOSURE A pretreatment of lignocellulosic material, such as wood chips, prior to digestion in an alkaline pulping process, such as the kraft pulping process and the alkaline sulfite pulping process, is performed with hydrogen sulfide which is generated in the pretreatment by mixing a sulfide which is compatible with the pulping process liquors and carbon dioxide gas. The pretreated lignocellulosic materials are separated from the excess pretreatment chemicals and then pulped in the usual manner in a kraft or alkaline sulfite pulping process.

This is a continuation-in-part of application Ser. No. 276,853, filed July 31, 1972, now abandoned.

This invention relates to improved hydrogen sulfide pretreatment in alkaline pulping processes, and particularly the kraft pulping and alkaline sulfite pulping processes.

The present process is an improvement over the process of US. Pat. 3,520,773, to Vinje and Worster. A process for increasing the pulp yield by pretreating lignocellulosic material with hydrogen sulfide is described in this patent. According to the patent, a significant increase in pulp yield (6 to 7% wood) is obtainable if lignocellulosic material, such as wood chips, sawdust and the like, is pretreated with hydrogen sulfide under heat and pressure in the presence of an alkaline bufier which is compatible with the pulping liquors and sufficient to maintain the pH of the pretreatment medium slightly alkaline. The pretreated lignocellulosic material is pulped by the regular kraft, soda or alkaline sulfite process under conventional conditions.

In the prior Vinje and Worster process, the hydrogen sulfide makeup requirement generally ranges from 1.0 to 1.5% on dry wood, particularly When applied to softwoods. This makeup hydrogen sulfide indicates sulfur losses that cannot be tolerated under existing and anticipated future pollution abatement regulations, unless the process is combined with another process that regenerates pure hydrogen sulfide from mill chemicals. Processes that are presently available for hydrogen sulfide regeneration are expensive and complex.

The present process has the following advantages over the process of Pat. 3,520,773:

(1) No external production of pure H 8 is required, eliminating expensive, awkward gas handling systems.

(2) The overall process is simpler and completely self contained, requiring no external H S recovery system in the mill.

(3) H 8 gas is confined within a single vessel giving a safer process operation.

(4) The pretreatment process can be applied to a oneline mill without complex sodium and sulfur balance problems.

3,841,962 Patented Oct. 15, 1974 The present process is an improvement in alkaline pulping processes wherein lignocellulosic material is digested under heat and pressure in an alkaline digesting liquor. The improvement comprises pretreating the lignocellulosic material before it is contacted with the alkaline digesting liquor with hydrogen sulfide generated in the pretreatment by mixing a high sulfidity liquor which is compatible with the liquors of the alkaline pulping process and an excess of carbon dioxide. The excess of the carbon dioxide acts as a buffer. The pretreatment is conducted under conditions of temperature and time and a stable partial pressure of hydrogen sulfide that effect reaction of the hydrogen sulfide with the lignocellulosic material and provide higher pulp yields than obtained by conventional alkaline pulping process alone, calculated on the dry weight of the lignocellulosic material. The lignocellulosic material is preferably wood chips, but it may consist of annual plants, sawdust and the like. The hydrogen sulfidecarbon dioxide mixture is preferably sufficient to maintain the pH of the pretreatment liquor in the neutral range.

The above outlined process is improved when the high sulfidity feed liquor solution contains sodium sulfide and/ or sodium hydrosulfide, together with sodium carbonate, sodium bicarbonate and sodium hydroxide.

The improvement according to this invention consists of generating the required amount of hydrogen Sulfide in a pretreatment vessel by mixing a suitable high sulfidity liquor with an excess of carbon dioxide. Sodium sulfide is preferred, but other sulfides such as potassium sulfide or ammonium sulfide can be used. It has been found advan tageous to allow part of the generated hydrogen sulfide gas to react with the lignocellulosic material and another part thereof to be recycled together with unused carbon dioxide gas. The makeup flow of carbon dioxide and the selected high sulfidity liquor in a continuous system is regulated so as to provide a stable partial pressure of hydrogen sulfide sufficient for a 5 to 6% pulp yield increase calculated on the dry lignocellulosic material. An advantage of this invention is that the pretreatment process can be applied to any kraft or alkaline sulfite pulping mill without upsetting the sodium and sulfur material balances of that mill.

A significant increase in pulp yield is obtained if the lignocellulosic material, such as wood chips, is pretreated in a suitable vessel with a hydrogen sulfide-carbon dioxide gas mixture under heat and pressure in the presence of a high sulfidity liquor which is compatible with the pulping liquors of the kraft pulping process or of the alkaline sulfite pulping process. The pressure of the H S/CO gas mixture in the vessel is kept at a level sufficient to maintain the pH of the pretreatment liquor in the neutral range, that is, approximately 6 to 7.5. The pretreated lignocellulosic material is then removed from the pretreatment vessel and pulped by the regular kraft or alkaline sulfite pulping process under conventional conditions.

Examples of this invention are illustrated in the accompanying drawings, in which FIG. 1 diagrammatically illustrates the process for a kraft pulp mill,

FIGS. 2 to 5 are graphs illustrating various aspects of this invention.

In the process illustrated in FIG. 1, the sulfide containing liquor used is kraft green liquor, which is essentially a mixture of sodium carbonate and sodium sulfide in a 3 to 1 to 2.5 to 1 mole ratio, this being the equivalent to a 25% to 28.6% sulfidity green liquor. The pretreatment takes place in a pretreatment vessel 10. It is important to maintain the sulfidity of the makeup pretreatment liquor directed into the vessel at 11 at a very high level, at least above and it can be as high as 200%. If the liquor sulfidity is not at a high level, the efilciency of the in situ hydrogen sulfide generation is impaired. High sulfidity makeup pretreatment liquor can be prepared by evaporating green liquor in a crystallizer/evaporator 14, which causes precipitation of sodium carbonate which is discharged at so as to give a maximum level of about 95% sulfidity in the evaporated green liquor which is directed to the pretreatment vessel. This high sulfidity liquor may also be prepared by selective dissolution of sodium sulfide in the smelt in the dissolving tank 17 by using water or spent pretreatment liquor, since sodium carbonate is less soluble than sodium sulfide. The concentrated high sulfidity liquor may be diluted with water or spent pretreatment liquor prior to charging the pretreatment vessel. It has been found, however, that the best results are obtained when the required makeup liquor for the pretreatment stage is prepared by first precarbonating the green liquor with a carbon dioxide containing gas to reduce the pH from about 13.5 to about 12, although good results are attained with the pH within 10 and 13. At this pH level the green liquor contains a maximum of sodium hydrosulfide and a minimum of sodium bicarbonate (see FIG. 3. This is important since maximum amounts of sodium hydrosulfide are required to generate maximum partial pressures of hydrogen sulfide in the pretreatment vessel, and minimum quantities of sodium bicarbonate are required to avoid increases in lime demand in the recausticizing part of the mill recovery system. The precarbonated liquor is then subjected to an evaporation/crystallization step whereby essentially all the sodium carbonate is removed by crystallization. The concentrated supernatent liquor, which contains sodium hydrosulfide, sodium sulfide and residual sodium carbonate and bicarbonate is then used as a feed liquor to the pretreatment vessel.

The hydrogen sulfide-carbon dioxide gas mixture is directed into vessel 10. This gas is preferably recycled from a previous pretreatment in a continuous system, together with makeup carbon dioxide from a gas purifier 21. The gas pressure in vessel 10 preferably ranges from 120 to 250 psi. Reaction of the makeup sodium sulfide and carbon dioxide results in the generation of more hydrogen sulfide by the following reactions:

From these equations it can be seen that the hydrogen sulfide concentration is controlled by the chemical ratio [NaSH]/[NaHCO This ratio is in turn controlled by the sulfur to sodium ratio in the feed liquor to the pretreatment vessel. The relationship between generated hydrogn sulfide partial pressure (or overpressure) and the feed chemical composition expressed as a ratio of weight of sulfur upon total weight of sulfur plus sodium, is shown in FIG. 3. To sustain eifective pretreatment conditions, sufiicient to maximize the kraft pulp yield increase with Western hemlock wood chips, it has been found that an H S partial pressure of at least 23 psi. measured at 140 C. is required (FIG. 4). However, partial pressures ranging from 10 to about 80 p.s.i. can be used with variations in the temperature. Typical pretreatment conditions are 140 C. for 50 minutes, with a total pressure of about 180 p.s.i.g. in the pretreatment vessel. Here again, the temperature can vary from 120 to 165 C. and the time from 200 down to minutes. A study of FIG. 4 confirms the lower yield increases that are obtained in the presence of low H 8 partial pressures. These conditions arise when lignocellulosic material is treated with NaSH or Na s alone, as specified in prior art, where no gas recovery or recycling is specified so as to maintain a stable partial pressure of H 8 gas.

Typical pretreatment conditions used with this system involve a sulfur consumption by the lignocellulosic material of about 1.7% as sulfur on wood. This sulfur is organically bound to the liguocellulosic material. Depending on the efficiency of the washing in zone 24 of vessel 10 after the pretreatment, additional sulfur in the form of sodium hydrosulfide in the entrained pretreatment liquor may be carried over to the kraft stage with the pretreated chips which are directed at 26 to the kraft digester 28. To maintain an overall sulfur balance in a one-line kraft mill, it is important that this total sulfur carry-over not exceed about 2.5%, which is the normal amount of available active sulfur (as sodium sulfide) in the green or white liquor of a kraft mill.

From the above equations and FIG. 3, it is obvious that a liquor with a high i S+Na ratio should be used for makeup and that sodium bicarbonate must not be allowed to build up to a high level during pretreatment since this would shift the chemical equilibrium resulting in a lower hydrogen sulfide partial pressure. To minimize the sulfur carryover to the kraft stage, the pretreated chips are washed with water or degassed sulfur lean spent pretreatment liquor in zone 24. Spent pretreatment liquor is removed from the pretreatment vessel and flashed in tank 30 to recover dissolved hydrogen sulfide and carbon dioxide gases. The spent pretreatment liquor is directed by 31 to tank 17 to dissolve the smelt which comes from the kraft recovery furnace 33. Alternatively, the spent pretreatment liquor can be used as a wash liquor in the pretreatment vessel.

The crystallized sodium carbonate can be dissolved in tank 35 in weak wash, a dilute kraft mill solution consisting of sodium hydroxide-sodium sulfide, to form a low sulfidity green liquor. This solution is then recausticized at 37 in the normal manner using lime from kiln 38-, to produce a low sulfidity white liquor which is directed by 39 into the kraft mill digester 28. In the following pulping stage, the sulfur associated in both organic and inorganic form with the pretreated chips is rapidly reacted with sodium hydroxide to form a regular kraft cooking liquor of sodium hydroxide and sodium sulfide.

The carbon dioxide from lime kiln 38 can be directed to the precarbonator 42 and then to gas purifier 21, whence it is directed into pretreatment vessel 10 or into the gas flashing system 30. Nitrogen and other inert gases have to be removed from the lime kiln gas stream before it enters the pretreatment vessel. As an alternative, external pure carbon dioxide can be used to provide the gas makeup requirements.

As an alternative, at least a portion of the spent pretreatment liquor can be recycled back into pretreatment vessel 10, as indicated at 41.

Since the hydrogen sulfide pretreatment system of this invention can be readily incorporated into an existing kraft mill with no additional external chemical makeup requirements over those normally employed, as illus trated in FIG. 1, there is no net change in the sulfur and sodium balances of the kraft m-ill. It should be mentioned that there should be an excess of carbon dioxide in the pretreatment vessel, and this excess acts as a buffer. With continued application of thehigh sulfidity green liquor and recirculation of recovered hydrogen sulfide and carbon dioxide in a continuous system, the partial hydrogen sulfide pressure builds up to a level sufiicient for carbohydrate stabilization.

If an external supply of hydrogen sulfide is to be avoided, using a high sulfidity liquor such as green liquor and a hydrogen sulfide-carbon dioxide gas mixture, then the pretreatment process should be operated within fairly narrow limits of temperature and pressure to obtain maximum yield increases. It has been found that the maximum operating temperature is about C. for Western hemlock wood chips, above this temperature pulping via acid hydrolysis will occur, and this leads to weaker pulps,

However, with other species the temperature can be up to about 165 C. and as low as about 120 C. As the temperature decreases to below about 130 C., a hydrogen sulfide pressure of 80 p.s.i. or greater is required to obtain maximum yield increases with this species. An equilibrium mixture of approximately 4 to 7 parts carbon dioxide and about 1 part hydrogen sulfide results in continuous pretreatment of lignocellulosic material in the manner described herein. For a hydrogen sulcfide pressure 20 p.s.i. at 140 C., the total operating pressure should be about 180 p.s.i., including steam pressure. The preferred operating conditions for Western hemlock wood chips, are from about 135 to about 145 C at a total pressure of from about 120 to about 250 p.s.i. for about 20 to about 200 minutes. The sulfur application to the pretreatment in the form of sodium, potassium, or ammonium sulfide is between about 2.5 and about 5.0 as percent sulfur on wood. About 2.5% as sulfur on wood is derived from the smelt and the balance is derived from the hydrogen sulfide in the recycled gas which can build up to a level of between about 1 and 2% as sulfur on wood, from which partial pressures in excess of 30 p.s.i. at 140 C. can be obtained. The carbon dioxide makeup requirement depends on the sulfur application in the pretreatment stage and the hydrogen sulfide consumption, as organic sulfur on the lignocellulosic material, during pretreatment. The makeup carbon dioxide is normally added to an extent of about 2 to 4% on wood, to maintain a total pressure of about 160 to 185 p.s.i. in the pretreatment vessel.

Both the pretreatment and the subsequent pulping stages can be operated as a 'batch or a continuous system.

Of the various alternative methods of routing the spent pretreatment liquor, as indicated in FIG. 1, the best choice will depend on the optimum operating conditions of the pretreatment in any particular mill. This will depend pri-marily on wood species and its response to the process in terms of sulfur uptake and yield increase, and also the sulfur available in any particular green liquor system, and the lime capacity of the mill. Generally, if the sulfur application requirement for the pretreatment from the green liquor is less than, or equal to that normally available in the green liquor system, then the spent pretreatment liquor may be added to the dissolving tank 35, white liquor, or black liquor, depending on factors such as the lime capacity and etc. of the mill. If the sulfur application is greater than that normally available in the green liquor, then some recycling of the spent pretreatment liquor must be made, either to the smelt tank, or to the green liquor ahead of the crystallizer evaporator, or to the high sulfidity green liquor stream, or directly to the pretreatment vessel, or to the wash zone of the pretreatment vessel after degassing.

Summing up, the present process for pretreating lignocellulosic material with hydrogen sulfide generated in the pretreatment by mixing a compatible high sulfidity liquor and an excess of carbon dioxide gas is conducted generally under the following conditions:

Temperature 120 to 165 C. Time (at temp.) 20 to 200 minutes. Partial pressure of H 8 10 to p.s.i. Total pressure 120 to 250 p.s.i. Sulfidity of liquor 75 to 200%. Weight ratio of sulfur to sulfur plus sodium in liquor 0.35 to 0.58.

The present invention is further illustrated by the following examples, which should not be construed to limit the operating limits for the process. In every case, commercial Western hemlock wood chips from different wood lots have been used, however the process is equally applicable to other wood species and plant material. The pretreated chips were subsequently digested by kraft liquors.

EXAMPLE 1 A series of pretreatments followed by kraft cooks were carried out in a 2.5 cu. ft. stationary digester with forced liquor circulation, under conditions as detailed in Table I below. In this series of experiments the pretreatment conditions were established with H S and NaSH as active chemicals, together with inactive N gas.

TABLE I Trial A B C D E Pretreatment conditions:

Time to temperature, min. None 30 25 25 2 5 Time at temperature, min None 110 110 110 Temperature, O None 145 145 145 140 His partial pressure at 140 C., p.s.i None 25 50 75 60 Total pressure, p.s.i None 150 150 150 150 NazO application, percent on wood None 2 2 2 1 Percent sulfur uptake by chips None 1. 25 1. 62 1. 98 1. 90 Results of subsequent kraft pulping:

Screened yield, percent on wood 48. 3 48. 70 48.30 50. 60 Percent 136180135 0. 26 0. 40 0. 30 0. 20 0. 50 Permanganate number p. o. 25. 3 25. 0 22.0 19. 8 26. 9 Screened yield at 22 p. No 42. 8 47. 3 48. 7 48. 8 49. 1 Screened yield increase at These results show that a pulp yield increase of at least 5% can be expected using pretreatment conditions of about 145 C. for minutes with a hydrogen sulfide partial pressure of about 40 p.s.i. only.

EXAMPLE 2 In a second series of experiments using a different chip lot, the active chemicals were provided by a simulated high sulfidity green liquor, essentially an aqueous solution of sodium sulfide, together with carbon dioxide gas and a portion of H 5 gas, to simulate H 8 gas recovery and recycling in a continuous system. The conditions and results are shown in Table II.

TABLE II Trial A B C D E F Pretreatment conditions;

HzS applied, as percent S on wood.... None None 1.91 3.22 3. 88 5.54 N as apphed, as percent S on wood... None None 2.96 3. 12 3. 09 3. 09 NaiCO; applied, as percent NazCO on wood None None 0. 44 0. 42 0. 43 0.44 Time to temperature, min. None None 31 30 30 35 Time at temperature, min. None None 103 104 104 104 Temperature, C None None 145 145 145 145 HiS partial pressure at C None None 29 35 51 52 C02 applied, percent on wood as 002.. None None 10. 0 8.0 8. O 6. 0 Results of subsequent kraft pulping;

Screened yield, percent on wood 43. 4 44.4 47. 4 48. 2 48. 48. 3 Percent rejects 0.3 0. 5 0 0. 2 0. 2 0. 1 Permanganate number (p. No 21.5 24.1 21.8 22.1 22.4 20.2 Screened yield at 22 p. No-.. 43. 6 43. 4 47.5 48.2 48. 0 48.9 Screened yield increase at 22 p. No 4.0 4. 7 5. 1 5. 4

These experiments show that with an H 5 partial pressure of 40-50 p.s.-i., about a 5% yield increase is possible under these pretreatment conditions. In this instance the pretreatments were carried out under conditions closely simulating those anticipated under continuous operation, with high sulfidity liquor and a CO /H S gas phase.

EXAMPLE 3 Having established that good yield increase are possible withlow H S partial pressures in the presence of high sulfidity liquors and CO gas, a continuous process simulation was carried out. In this simulation, wood chips were pretreated with high sulfidity green liquor and CO; gas only, after which the gas was recovered by flashing and compressing into a storage tank. The spent pretreatment liquor was drained from the chips into a storage tank, and the chips were washed briefly with a small amount of water and CO: to simulate the operation of the washing zone. The wash liquor was then drained from the chips and added to the stored pretreatment liquor. This entire liquor sample was then flashed to recover the last traces of free H S or CD which were added to the gas storage tank. The spent liquor was removed from the system. The chips were then pulped in the usual manner with a low sulfidity kraft liquor.

In successive pretreatments fresh high sulfidity green liquor was used; together with the recovered gas from the previous pretreatment and makeup CO: to 185 p.s.i.g. at 140 C. At no time was any external H 8 added to the system. Six sucessive pretreatments were carried out using the same recycled gas throughout. Pretreatment conditions were 140 C. for 110 minutes in each case. The results were shown in Table 111 using a third Western hemlock chip lot.

ratio in the feed liquor, less chemical as percent S on wood is required to generate the same partial pressure (or overpressure) of hydrogen sulfide. For Western hemlock chips, the relation between hydrogen sulfide partial pressure under similar pretreatment conditions and screened pulp yield increase, as shown in FIG. 4. These results were obtained using a batch laboratory digester with forced liquor circulation. At a hydrogen sulfide partial pressure of 25 p.s.i., a 5.5% yield increase is attained. With a feed liquor containing sodium sulfide only S m: about 0.35,

a 6% application as sulfur on wood is required. However, with a feed liquor containing only sodium hydrosulfide =about 0.58,

only a 2.8% application of sulfur is required, FIG. 3. Feed liquors containing a proportion of both sodium sul- TABLE III A B o D E F H Pretreatment conditions;

Percent onwoodin greenliquormakeup.. None None 4.8 4.8 4.8 3.4 3.4 4.0 Percent-Supplied from gas storage None None 0 0.85 1.01 1.32 1.12 1.02 Totals applied, percent on wood None None 4.8 5.6 5.8 4.7 4.5 5.0 CO: makeup, percent on wood.. None None 12.0 5.0 3.3 3.3 3.1 4.3 00: applied, percent on wood.. None None 12.0 15.6 15.8 14.2 13.6 14.4 Sulfur recovery, percent 8 on wood in spent liquor- None None 1.52 1.48 1.88 1.03 1.39 1.57 Perc'entSon wood ingas storage-. None None 0.85 1.01 1.32 1.12 1.02 1.33 Totalsrecovered None None 2.37 2. 49 3.20 2.15 2. 41 2.09 Percent S carried over to kratt None None 2. 4 3.1 2.6 2.5 2. 1 2.1 Total pressure (140 C p s 1.. None None 159 179 162 174 167 172 Pulping re ts;

P.No' 22.3 21.4 24.8 25.1 25.1 22.9 26.3 24.2 Total eld 44.9 44.0 49.7 50.6 49.7 50.0 50.0 49.3 Screened yield 43.9 44.2 48.2 50.4 40.6 49.3 50.0 49.2 31 yield at 22 1%.... 44.7 44.8 48.9 49.7 48.9 49.7 48.0 48.6 Screened yield at 22 p. N0 43.7 4 .0 47.4 49.5 48.7 49.0 49.0 48.5 Screened Yield increase at 22 p. No 3.5 5.6 4.9 5.1 5. 1 4.6

These results clearly demonstrate that a hydrogen sulfide and sodium hydrosulfide would normally be obfide partial pressure can be built up and sustained in a tamed with a green liquor precarbonation process. Any continuous system, without any external H 8 addition, excess sodium carbonate or bicarbonate in the feed liquor sufiicient to give excellent pulp yield increases over reguwould decrease the lat kraft cooks. In addition, the sulfur carryover to the kraft stage can be kept at or below 3% on wood. Close insepection of these results shows that the amount of H 8 S recovered is controlled by the sulfur application from t high sulfidity green liquor.

EXAMPLE 4 ratio. High In this case the process simulations were carried out on a 15 ton per day continuous pilot plant, consisting of a pretreatment vessel and digester of similar design S and operating principle to that commonly used in full size S+N commercial installations. Western hemlock wood chips were used as feed material; feed chemicals for the pretreatment vessel were prepared from sodium hydrosulvalues for the feed liquor are prepared by precarbonating fide and sodium hydroxide solutions; carbon dioxide for regular green liquor from a pH about 13.5 to a pH of the pretreatment was supplied from a storage tank; regular about 12, whereby higher concentrations of NaSH are mill cooking liquor (white liquor) was used in the dlgester. generated, see FIG. 2, prior to the crystallization/evap- Feed liquors of different chemical composition were oration step. Impure carbon dioxide can be used for prepared and feed continuously to the pretreatment ves- 4 the precarbonation process. This could be obtained directsel, in which the pressure was maintained at 180 psig. ly from the lime kiln or recovery boiler of a kraft mill.

TABLE IV The efleet of feed liquor composition and application on the generated H23 partial pressure and CO1 consumption in a continuous chip pretreatment system with continuous gas recycling C02 consump- Feed hquor Generated tion at equi- Feed liquor composiapplications H28 partial librium as tion as weight ratio of as percent pressure at percent on 8/8 Na S on wood equilibrium wood 0.58 2. 9 28. 2 1. 8 0.58 2. 9 28. 2 2. 6 0.58 2. 4 21. 2 1. 7 0.58 5. 45. 8 3. 0 0 .58 2. 6 2. 0 0.41 2. 5 13.0 3. 2 0.41 3. 5 18. 0 3. 7 0.58 5. 0 50. 2 0.41 5. 8 18. 6 6. 8 0.47 5. 0 21. 1 0.58 3. 1 2.0 0.50 3. 7 26. 5 5. 4 0.45 3. 7 21. 3 3. 6 0.58 2. 9 27. 4 2. 0 0.43 5. 6 26. 1 8. 7 0.50 3. 0 23. 1 2. 4 0.41 3. 0 l4. 9 3. 8 0.46 5.6 31.9 5.4 0.54 5.6 50.0 2.3 0.41 3. 7 19. 0 4. 6 0.54 3.7 39.7 2. 6 0.33 l. 8 9.4 5.1 0.42 1.8 13. 8 2. 3 0.50 4. 3 29. 4 4. 8 0.44 3. 0 22. 2 4. 6 0.38 3. 0 15. 2 8. 0 0.36 5.4 21. 0 11. l 0.37 4. 6 18. 6 12. 2 0.36 3. 7 13. 5 8. 9

TABLE V Trial A B C D E Pretreatment conditions:

Time at temperature, min.. None 50 50 50 50 Temperature, C None 140 145 150 155 HaS partial pressure at 140 p.s.i None 17. 4 12. 9 14. 5 12. 8 Total pressure, p.s.i None 180 180 180 180 Results after subsequent krait pulping:

Screened yield, percent on wood 43.9 48.0 48.0 Percent rejects 0. 7 0. 5 0. 3 2 Permenganate number (p.

No. 23. 9 21. 3 21. 5 20. 3 22. 4 Screened yield at 22 p. No.. 43. 3 48. 2 48. 2 46. 5 40. 8 Screened yield increase at 22 p. No 4.9 9 3 These results show that increasing the temperature to 150 C. or above reduces the yield increase with this wood species. The preferred temperature conditions for the pretreatment is between 140 and 145 C.

EXAMPLE 6 A series of chip pretreatments followed by kraft cooks were carried out as in Example 2, but in this instance, the pretreatment time was varied. The results shown in Table VI show that with Western hemlock chips about 50 minutes is the minimum time requirement for the most effective pretreatment.

TABLE VI Trial A B C D E F Pretreatment conditions:

Time at temperature, min None Temperature, C None 138 138 138 138 138 as partial pressure at 140 p.s.i. None 20. 2 17. 5 16.4 23. 8 18. 6 Total pressure, p.s.i None 156 156 172 177 Results after subsequent krait pulping:

Screened yield, percent on wood 42.6 47.3 47.1 48. 0 47.0 47.1 Percent rejects 0.8 0.7 0. 8 0. 5 0.2 0. Permanganate number (p. No.) 22.6 26.2 25.0 26.2 23.3 24.0 Screened yield at 22 p. N o 42.4 45.9 46. 1 46. 6 46.6 46.4 Screened yield increase at 22 p. No 3. 5 3. 7 4. 2 4. 2 4. 0

Consumption of carbon dioxide while operating the 'What is claimed is:

continuous pilot plant under conditions described above, is shown in FIG. 5 and Table IV. It is evident that by the presence of sodium hydrosulfide in the pretreatment feed liquor (higher ratios), less pure makeup carbon dioxide to the pretreatment vessel is required.

in a 2.5 cu. ft. stationary digester with forced liquor circulation under conditions as detailed in Table V.

1. In an alkaline pulping process in which lignocellulosic material is pre-treated with hydrogen sulfide, in which the pro-treated material is digested under heat and pressure in an alkaline digesting liquor comprising a sodium based digesting agent to form a pulp, in which the digested pulp is separated from the digesting liquor after digestion, and in which the digesting liquor is processed for recovery of the alkaline digesting agent, the improvement which comprises:

providing from the digesting liquor processed for recovery of the alkaline digesting agent, a high sulfidity liquor containing at least one member selected from the group consisting of sodium sulfide and sodium hydrosulfide and having a sulfidity of about 75% to 200% and a weight ratio of sulfur to sulfur plus sodium of from about 0.35-0.58;

contacting the lignocellulosic material with said high sulfidity liquor in a pre-treatment vessel proir to alkaline digestion;

reaction between hydrogen sulfide and the lignocel lulosic material in said pre-treatment vessel to increase pulp yield over that obtained by an alkaline pulping process in which the iignocellulosic material is digested in said alkaline digesting liquor;

separating pro-treated lignocellulosic material from the pre-treatrnent liquor present in said pre-treatment vessel;

separating hydrogen sulfide gas from the separated pre-treatment liquor; and

recycling at least a portion of said hydrogen sulfide gas separated from said pre-treatment liquor to said pretreatment vesselfor treatment of additional lignocellulosic material introduced thereto, whereby the need for adding make-up hydrogen sulfide for said pretreatment from an external source is eliminated.

2. A process according to claim 1 wherein the lignm cellulosic material comprises wood chips.

3. The process according to claim 1 wherein the pH of the liquor in the pre-treatment vessel is maintained in the neutral range.

4. The process according to claim 1 wherein the total pressure in the pre-treatment vessel ranges from about 120 to 250 p.s.i.

5. The process according to claim 1 wherein the pretreatment liquor separated from said lignocellulosic material is introduced into the digesting liquor recovery system of the alkaline pulping process.

6. The process according to claim 1 wherein the processing of the alkaline digesting liquor includes the formation of a smelt and wherein the pre-treatment liquor separated from the lignocellulosic material is used for dissolution of said smelt.

7. The process according to claim 1 in which the alkaline pulping process is the kraft pulping process.

8. A method as in claim 1 in which the high sulfidity feed liquor solution contains sodium carbonate, sodium bicarbonate and sodium hydroxide.

9. A method as in claim 7 in which processing of the kraft digesting liquor includes formation of a green liquor and in which said high sulfidity liquor is obtained from said green liquor.

10. A method according to claim 1-wherein hydrogen sulfide gas is separated from the. separated pre-tratmen liquor in the presence of carbon dioxide gas.

References Cited 4 UNITED STATES PATENTS S. LEON BASHORE, Primary Examiner A. L. CORBIN, Assistant Examiner US. Cl. X.R.

Claims (1)

1. IN AN ALKALINE PULPING PROCESS IN WHICH LIGNOCELLULOSIC MATERIAL IS PRE-TREATED WITH HYDROGEN SULFIDE, IN WHICH THE PRE-TREATED MATERIAL IS DIGESTED UNDER HEAT AND PRESSURE IN AN ALKALINE DIGESTING LIQUOR COMPRISING A SODIUM BASED DIGESTING AGENT TO FORM THE PULP, IN WHICH THE DIGESTED PULP IS SEPARATED FROM THE DIGESTING LIQUOR AFTER DIGESTION, AND IN WHICH THE DIGESTING LIQUOR IS PROCESSED FOR RECOVERY OF THE ALKALINE DIGESTING AGENT, THE IMPROVEMENT WHICH COMPRISES: PROVIDING FROM THE DIGESTING LIQUOR PROCESSED FOR RECOVERY OF THE ALKALINE DIGESTING AGENT, A HIGH SULFIDITY LIQUOR CONTAINING AT LEAST ONE MEMBER SELECTED FROM THE GROUP CONSISTING OF SODIUM SULFIDE AND SODIUM HYDROSULFIDE AND HAVING A SULFIDITY OF ABOUT 75% TO 200% AND A WEIGHT RATIO OF SULFUR TO SULFUR PLUS SODIUM OF FROM ABOUT 0.35-0.58; CONTACTING THE LIGNOCELLULOSIC MATERIAL WITH SAID HIGH SUFIDITY LIQUOR IN A PRE-TREATMENT VESSEL PRIOR TO ALKALINE DIGESTION; INTRODUCING CARBON DIOXIDE GAS INTO SAID HIGH SULFIDITY LIQUOR IN SAID VESSEL, IN AN AMOUNT IN EXCESS OF STOICHIOMETRIC TO GENERATE HYDROGEN SULFIDE FROM SAID HIGH SULFIDITY LIQUOR; MAINTAINING THE TEMPERATURE IN SAID PRE-TREATMENT VESSEL WITHIN THE RANGE OF ABOUT 120 TO 165*C. FOR ABOUT 20 TO 200 MINUTES AND UNDER A PARTIAL PRESSURE OF HYDROGEN SULFIDE GAS OF ABOUT 10 TO 80 P.S.I. TO EFFECT REACTION BETWEEN HYDROGEN SULFIDE AND THE LIGNOCELLULOSIC MATERIAL IN SAID PRE-TREATMENT VESSEL TO INCREASE PULP YIELD OVER THAT OBTAINED BY AN ALKALINE PULPING PROCESS IN WHICH THE LIGNOCELLULOSIC MATERIAL IS DIGESTED IN SAID ALKALINE DIGESTING LIQUOR; SEPARATING PRE-TREATED LIGNOCELLULOSIC MATERIAL FROM THE ACCORDING TO WHICH THE SUPPORT TUBES PASS THROUGH THE BOTTOM OF THE REACTOR VESSEL AND ARE PROVIDED AT THE OUTER END WITH AT LEAST ONE PLUG WHICH ACTS AS A RADIATION SHIELD AND A COOLING-GAS SEAL, IN WHICH THE SUPPORT TUBE (8) IS FIXED IN A SLEEVE TUBE (3) BY MEANS OF A LOCKING PLATE (5, 6) SUCH THAT BETWEEN THE SUPPORT TUBE (8) AND THE SLEEVE TUBE (3) THERE IS AN ANNULAR GAP (11) CONTAINING AN AUXILIARY TUBE (12) WHICH IS MOVABLE IN THE AXIAL

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