US3145134A

US3145134A - Wood pulping process

Info

Publication number: US3145134A
Application number: US199748A
Authority: US
Inventors: Julius D Robertson; George M Dickinson
Original assignee: Sonoco Products Co
Current assignee: Sonoco Products Co
Priority date: 1962-06-04
Filing date: 1962-06-04
Publication date: 1964-08-18
Anticipated expiration: 1981-08-18

Description

18, 1964 J. D. ROBERTSON ETAL 3,145,134

WOOD PULPING PROCESS Filed June 4, 1962 INVENTORS: Jun u 5 D. wzoaEa rsoN ZMGEQRGE M. New MsoN United States Patent 3,145,134 Wiflrlllf) lllllLPlNG PRGCESS Julius ll). Robertson and George M. Dickinson, Huntsville, S.., assignors to Sonoco Products Company, Harte ville, SAL, a corporation of South Carolina Filed June 4, 1962, Ser. No. 199,743 8 Claims. (Cl. 162-4 3) This invention relates to a wood pulping process and more particularly to a process for recovering solubilized wood components in a concentrated form from wood chips in a neutral sulfite semi-chemical pulping process.

In the pulping of wood chips using a sodium based sulfite process, commonly known as a neutral sulfite semichcmical process, the wood chips are cooked in a digester in the presence of a cooking liquor to convert certain Wood components such as the lignins into a solubilized form. At the completion of the digestion step, these solubilized wood components are then removed and recovered by means of a leaching or washing operation.

This washing operation is generally carried out by means of suitable apparatus such as vacuum washers in which the solubilized wood components are washed from the digested wood chips using a substantial quantity of dilution water so as to effectively carry out this leaching or washing step. Not only is it generally necessary to wash the chips in separate apparatus after digestion, but as a result of the large amount of water used in the washing operation, the liquor obtained has a low solids content. Therefore, before the liquor can be effectively utilized, as is generally accomplished by an extraction or burning operation, it must be concentrated by evaporation in special apparatus such as evaporators or the like.

The vacuum washers employed to perform a pulp washing operation and the evaporators used to perform the liquor evaporating operation are generally large and expensive. Furthermore, the power required to operate such equipment, particularly the evaporators where the steam consumption is substantial, adds considerably to the cost of producing concentrated black liquor from pulped wood-chips.

Another factor which contributes to the cost of rcovering these wood components during a pulping operation is the inefficiency of the pulp washing apparatus presently used in semi-chemical pulping. It is generally accepted that the formation of the potentially soluble wood components such as lignosulfonates and acetates occurs very early in the pulp digesting process and that approximately one hour of cooking or digestion in the presence of cook.- ing liquor is sufficient to form these salts. However, the cooking process is generally extended for considerably more than one hour in the process presently used for the purpose of hydrolytically solubilizing the solid lignosulfonates and acetates and other soluble material and diffusing the resulting soluble material from the wood chips into solution. It is believed that the present day employment of black liquor for said hydrolytic solubilization and diffusion inhibits the dilfusion of the soluble wood solids because of the salting out effect of the inorganic salts present in said black liquor, and accordingly requires additional processing time for complete diffusion.

Accordingly, a primary object of this invention is to provide a new and novel method for simultaneously performing the digestion, delignification and liquor concentration operations in a wood pulping process.

Another object of this invention is to provide a new and novel method for the recovery of solubilized wood components in a concentrated form from wood chips subjected to a pulping process.

Still another object of this invention is to provide a new and novel method for the recovery of black liquor in concentrated form from wood chips pulped by a neutral white 3,145,134 Patented Aug. 18, 1964 semichemical process which permits the degree of liquor concentration to be readily determined and which enables the removal of substantially all of the available solubilized wood components in the chips.

A further object of this invention is to provide a new and novel method for washing pulped wood chips and for evaporating the black liquor obtained thereby which is highly efiicient so as to require a minimum of power consumption and which produces a black liquor in a sufficiently concentrated form so that it can be utilized without further concentration.

A still further object of this invention is to provide a new and novel method for obtaining concentrated black liquor from wood chips pulped by a neutral sulfite semichemical pulping processwhich eliminates the need for expensive, bulky, inefiicient and difficult to maintain washing and evaporating apparatus and which materially reduces the time required for obtaining concentrated black liquor in comparison with present day practices.

This invention further contemplates the provision of a new and novel method for simultaneously performing delignification, leaching, and liquor evaporation in a neutral sulfite semi-chemical process for pulping wood chips, which eliminates the need for external dilution water, which produces considerable savings in labor and maintenance, and which permits the recovery of substantially all of the available solubilized wood components from the wood chips in a highly efllcient manner.

Other objects and advantages of the invention will become apparent from the following description taken in connection with the accompanying drawings.

In general, the objects of the invention and related objccts are accomplished by cooking a quantity of wood chips in a digester at a predetermined pressure and temperature in the presence of a cooking liquor by a conventional sulfite pulping process. When the reaction between wood components and pulping chemicals which constitutes the initial phase of the digestion process is completed, free black liquor produced is conducted into a pressure tank communicating with the digester and in pressure equilibrium therewith. A head of black liquor in the tank functions as a pressure seal between the tank and digester. Vacuum-compression means are provided for reducing the pressure on the surface of the liquor in the pressure tank so that the volatile components in the liquor are vaporized to concentrate the liquor, vaporization being limited to the liquor surface as a result of the pressure seal. The vapor is conducted out of the tank and is compressed by the vacuum-compression means. The compressed vapor is then conducted in heat exchange relationship with the liquor in the pressure tank to condense the vapor and form a condensate. This relatively clear condensate is then introduced into the digester whereby the condensate flows through the chips in the digester performing the final phase of the digestion by hydrolytically solubilizing solid lignosulfonates and other wood components and concomitantly removing by leaching these solubilized wood components from the chips. This condensate containing the solubilized wood components then flows from the digester into the pressure tank in a closed system so that the cycle is continuous.

The novel features which are believed to be characteristic of the invention are set forth with particularity in the appended claims. The invention, however, both as to its organization and method of operation may be best understood by reference to the following description taken in conjunction with the accompanying drawing in which:

FIGURE 1 is a diagrammatic View of the preferred form of the apparatus for practicing the novel method of the invention.

Referring now to the drawing, there is shown one form of apparatus suitable for carrying out the novel method arsena s of the invention. The method of the invention will be described hereinafter with reference to that pulping process known as a neutral sulfite semi-chemical process in which hardwood chips are cooked in a digester in the presence of a sodium based sulfite cooking liquor. As is well known, the active ingredients of this cooking liquor are primarily sodium sulfite and sodium bicarbonate. It should be understood however that the method of this invention lends itself to any similar pulping process in which it is desired to separate and recover solubilized wood components in a concentrated form.

According to present day practices, the wood chips are washed and the liquor resulting from this sodium based sulfite pulping process containing dissolved wood solids principally sodium lignosulfonate, sodium acetate, sodium formate, sodium bicarbonate, waxes, tannins, resins, sugars, etc., is recovered and preferably concentrated for subsequent use. The best practicable concentration of this liquor for further use is in the range of about 40% to 45% solids content although concentrations ranging from 25% to 60% solids content may be used. This concentrated liquor can then be burned for its fuel value and its smelt recovered or further processed for the recovery of certain constituents such as acids and the like.

In the novel method of the invention using the apparatus of FIGURE 1, the liquor is recovered in a concentrated form without the need for subsequent concentration such as by evaporation nor does the novel process of the invention require that the chip washing operation be performed in separate apparatus as presently necessary for removing the solubilized wood solids.

In the apparatus of FlGURE l, a digester 6, commonly referred to as an upright circulating digester, is provided which is in the form of a large tank having a side wall 7, a tapered bottom wall 8, a top wall 9 having an open end through which a load or charge of wood chips to be pulped can be introduced into the digester. The open end of the digester is closed by means of a lid 1t and suitable clamping means for clamping the lid in the closed position are provided.

Communicating with the interior of the digester 6 and extending through the digester side wall 7 is a steam conduit 11 having a valve 12 therein. A pressure relief valve 13 and a cooking liquor inlet conduit 14 having a valve 15 are also provided which communicate with the interior of the digester 6.

The bottom wall 8 of the digester 6 is provided with an outlet 15 to which is connected a conduit 17 having a valve 18 therein. The conduit 17 is arranged to conduct the pulp from the digester 6, after the solubilized solids are removed from the wood chips in accordance with the invention, to a paper machine or the like (not shown) for manufacture into paper in the well known manner.

Also connected to the bottom wall 8 of the digester 6 is a conduit 19 communicating with

conduits

21, 22 having located therein valves 23, 2 respectively. Conduit 21 communicates with a conduit 26 having a valve 27 therein and with a conduit 28 extending downwardly into a suitably supported pressure vessel or tank designated generally by the numeral 29. The pressure tank 29 is arranged to be filled to a predetermined level with black liquor obtained from the wood chips pulped in the digester 6. Conduit 26 communicates with a suitable black liquor storage tank or a burner supply or the like (not shown).

It will be noted that the conduit 28 extends through the top wall 31 ,of the tank 29 and throughout substantiallythe entire height of the tank so that the conduit outlet 32 is adjacent the bottom wall 33 of the tank. As a result of this positioning of the conduit outlet 32, the filling of the tank 29 with liquor 34 from the digester 6 to a predetermined level as indicated by the liquor upper surface 36, there is a substantial height or head of liquor between the conduit outlet 32 and the liquor upper surface. The upper surface 36 of the liquor 34 introduced into the pressure tank 29 thus defines with the top wall 31 of the tank 29 a space referred to hereinafter as a vapor zone and designated generally by the numeral 37.

The vapor zone 37 in the tank 29 communicates by means of a conduit 533 connected to the tank top wall 31 with the suction or inlet side of suitable vacuum compression means such as a centrifugal compressor 39. While any suitable type of vacuum compression means may be employed, such as a pump or blower, a centrifugal compressor of any well known construction is preferably employed. The outlet or pressure side of the centrifugal compressor 339 communicates by means of a conduit it? with a coil 41 positioned within the liquor pressure tank 29.

As can be seen, the coil 41 is arranged in spaced relationship with the top wall 31 of the tank so as to be completely immersed in the liquor 34. This fluid and/ or vapor flowing in the coil 4-1 is maintained in heat exchange relationship with the liquor 34. The lower discharge end of the coil communicates by means of a conduit 42 with the conduit 22 and a valve 43 is positioned in the conduit 42.

The conduits 22, 42; communicate with a conduit 44 communicating in turn with the inlet of a pump 45, the outlet of which communicates by means of conduit 46 with condensate heating means such as a heat exchanger 47. A condensate bleed conduit 43 having a valve 49 is connected to conduit as for removing a portion of the fluid pumped by pump 45, if desired.

The heat exchanger 47 may be of any conventional construction such as the tube and shell type in which the fluid flowing in its tubes is maintained in heat exchange relationship with steam or the like surrounding the tubes. In order to supply steam to the heat exchanger 47, a steam supply conduit 51 is provided connected to the heat exchanger as shown and a steam condensate outlet conduit 52 is provided as shown for the removal of the steam condensate from the heat exchanger.

The heat exchanger 47 is arranged to discharge fluid circulated therein into a conduit 53 connected to the heat exchanger outlet. Conduit 53 is connected in turn with the top wall 9 of the digester 6 so as to communicate with the interior of the digester.

In carrying out the novel method of the invention with the apparatus of FIGURE 1, hardwood chips are introduced into the digester 6 through the top opening and the lid 10 is clamped in the closed position as shown. Valves 12, 18 on the digester are opened and all the other valves shown in the drawing,

valves

13, 15, 23, 24, 27, 43 and 49 are closed. Steam is then introduced in the top of the digester through the steam conduit 11 from a suitable source (not shown) and valve 18 permits the escape of air and volatile material as well as the drainage of condensate accumulated in the digester through the conduit 17. This steaming is continued until the escape of a steady flow of steam from the conduit 17 and/or the attainment of a temperature of approximately C. in the digester which indicates that the flow of steam has traversed the column of wood chips and completely filled the digester.

Valves 12, 18 are then closed and valve 15 in the cooking liquor supply conduit 14 is opened. A predetermined amount of neutral sulfite semi-chemical cooking liquor, consisting primarily of sodium sulfite and sodium bicarbonate in aqueous solution, is then introduced into the digester through the supply conduit 14. When the introduction of cooking liquor into the digester 6 has been completed, valve 15 is then closed.

Valve 24 is now opened and steam is supplied to the heat exchanger 47 through the steam conduit 51. The pump 45 is started and the cooking liquor in the digester is pumped from the bottom of the digester, through conduit 19 and is conducted through

conduits

22, 44,

and 46 in to the heat exchanger 47 wherein the liquor is heated. The heated liquor then flows out of the heat exchanger 4-7 through conduit 53 and is introduced back into the digester 6 at the top.

It should be understood that although heat is introduced into the digester by heating the liquor as it circulates through the heat exchanger 47, this heat may also be introduced into the digester, if desired by the introduction of saturated steam into the charge of chips and liquor in the digester. This heating and circulation of the liquor is continued until a pressure of about 100-110 p.s.i.g. is attained in the digester 6.

At this stage in the digestion process, the digester contains a mixture of volatile gases such as carbon dioxide and steam and as a result the temperature of the digester charge is significantly less than the gauge pressure would indicate for saturated steam. In order to elevate the temperature to a value more closely approaching the theoretical value, the valve 13 at the top of the digester is now opened and these volatile gases are permitted to escape. This valve 13 is then closed and the digestion is continued for about one hour at a pressure of approximately 1001l0 p.s.i.g. and at a temperature of about 170 C. At the end of this period of time, the reaction between the sodium sulfite in the cooking liquor and the lignin in the wood chips during which solid lignosulfonates are formed and which constitutes the first phase of the digestion process is substantially complete.

At this time, valve 24 is closed and valves 23 and 43 are opened. The liquor from the digester 6 then drains down through conduits 1h, 21 and 2% through the conduit outlet 32 into the pressure tank 2% to fill the tank with liquor to approximately the level indicated at 36 so that the coil 41 is immersed in the liquor as shown. The initial discharge of liquor from the digester 6 into the presure tank 29 results in a slight pressure drop in the digester but a pressure equilibrium is quickly established between the tank and digester.

Communication between the interior of the digester 6 and the liquor 34 in the tank 29 is continuously main tained by means of the

conduits

19, 21 and 255 as the opening 32 of conduit 28 remains open to permit liquor to pass from the digester into the tank 29, but the pressure equilibrium between the tank and digester is also maintained as a result of the interconnection of the tank 29 and digester 6 in a closed system as previously described.

The compressor 39 is now started so that the pressure in the vapor Zone 37 above the liquor surface 36 is reduced and evaporation of the volatile constituents in the liquor 34 is initiated. The head of liquor 34 in tank 29 functions as a pressure seal between the tank 29 and digester 6 so that liquor evaporation is confined to the surface of the liquor in the tank 29 and does not extend to the interior of the digester 6. The relatively clear vapor is drawn from the tank 29 through the conduit 38 as indicated by the arrows and after being compressed in the compressor 39 is discharged in a compressed state from the compressor outlet through the conduit id into the coil 41. As a result of this compression of the vapor, its temperature is raised.

As the compressed vapor circulates through the coil 41 in the direction of the arrows 1, heat is transferred from the compressed vapor to the liquor 34 and the vapor condenses in the coil 41 to form a relatively clear condensate which is mostly water. This condensate is drawn through conduits 42, 44 by means of the pump 45 and flows through the heat exchanger 4'7 wherein heat is added to the condensate by the steam from conduit 51.

The heated condensate then flows through conduit 53 and is introduced into the top of the digester 6 where it diffuses downward over the chips by gravity, hydrolytically solubilizing solid lignosulfonates and other wood components in the second or final stage of the digestion process, concomitantly leaching out these and other mechanically held solubilized wood components that are present in the chips and further removing wood com ponents as they are solubilized by hydrolysis. The heating of the condensate in the heat exchanger 4'7 is carried out at a rate which insures that heat loss by radiation and the like is compensated for and the temperature and pressure within the digester is maintained at a predetermined level so that the digestion process in the digester 6 is carried out in the shortest possible time and in the most efficient manner.

As this condensate flows through the digester charge carrying with it solids from the charge, the chips are washed and hydrolytic solubilization of solid lignosulfonates and other wood components is eifected in a highly eiiicient manner due to the relatively clear condensate which is used. This condensate then flows back into the tank 20 through conduits 119, 21 and 23 in a closed system constantly adding solids to the liquor 34%.

Thus, simultaneously with the removal of the wood solids from the chips in the digester 6 and the transfer of these solids from the digester to the tank 29, the volatile constituents of the liquor 34 which are mixed with the solids are continuously removed by vaporization as the compressor 39 removes the vapor from the vapor zone 37. The liquor 34 is thereby concentrated to the desired degree as the condensate, which is brought in contact with the wood chips, replaces the solids removed from the chips with the chips holding this condensate mechanically for subsequent removal. The liquor 34 in the tank 29 thus contains a high concentration of the wood solids which permits the liquor to be used without further evaporation.

If it is desired to increase the degree of concentration of the liquor 34 in the tank 29 above that concentration which can be obtained in the manner described above, a portion of the condensate flowing in the system can be removed through the outlet conduit 48 by opening the valve &9. By removal of a portion of this condensate, there is less of an excess of water over that which the chips can absorb and therefore less water present to dilute the liquor 34 in the tank 29.

The condensate is circulated through the system as described above until the digestion of the chips has proceeded to the desired degree and sufficient solubilized wood components have been removed from the charge in the digester. It has been determined that approximately four hours of processing at a pressure of approximately p.s.i.g. will complete the cook with substantially all of the solubilized wood components contained in the tank 29 and much of the water or condensate held in the chips.

Valves 23 and 43 are now closed, valve 18 is opened and the pulp is blown out of the digester through the conduit 17 for utilization in a paper making machine or the like. Valve 27 is then opened and the concentrated liquor is blown from the pressure tank 29 to storage or to a burner supply according to the desired end use.

The following is an illustrative but non-limiting example of the process of the invention.

Example Hardwood chips equivalent to about 2500 pounds of bone dry wood substance are introduced into the digester 6 and steamed until there is a steady escape of steam through the conduit 1'7 and/ or a temperature of approximately 100 C. in the digester. Approximately 27.62 cubic feet of neutral sulfite semi-chemical cooking liquor containing about 179.53 pounds of sodium sulfite and about 89.77 pounds of sodium bicarbonate both expressed as equivalents of sodium carbonate are introduced into the digester. The cooking liquor is circulated and heated until a pressure of approximately 100110 p.s.i.g. is reached. The volatile gases accumulated in the digester are then blown through valve 13 and the digestion is con- 'tinued thereafter for about one hour at a pressure of approximately 100ll0 p.s.i.g. and at a temperature of approximately 170 C. The liquor is then drained from the digester 6 into the pressure tank 29 and the compressor 39 is operated to compress vapors from the tank into condensate and condensate is circulated continuously through the digester for a period of approximately two to three hours. During this circulation of condensate or Washing of the chips, the temperature and pressure in the digester is maintained at the levels referred to above. About 2,000 pounds of bone dry cooked chips comparatively free of soluble material remain in the digester 6 and are removed and the black liquor 34 in the tank 29 has a concentration of about 40% and can be blown from the tank 29 for recovery.

It can be seen that with the novel process of the invention a simple and inexpensive system has been provided by means of which wood chips pulped by a sulfite pulping process can be simultaneously subjected to digestion and delignification while at the same time the solubilized wood components can be removed from the chips and can be concentrated to a level which permits utilization of the solids or black liquor without further treatment and without the introduction of any water other than that already present in the system. An outstanding feature of the process of the invention is the simultaneous performance of the final stage of the digestion process wherein the solid lignosulfonates and other soluble wood components are hydrolytically solubilized together with the Washing or leaching of the chips with relatively clear condensate and concentration of the removed solubilized solids without further evaporation. In addition, the use of relative- 1y clear condensate for the hydrolytic solubilization enhances the efficiency of this step and eliminates the salting out eflfect on the wood solids produced by the organic chemicals in black liquor used in present day processes. The novel method of the invention practiced in this apparatus enables the elimination of separate pulp washing apparatus such as vacuum washers or the like and the elimination of evaporators for concentrating the black liquor resulting from the pulping process such as are required in present day pulping processes.

In addition to permitting the use of simple and compact apparatus, the novel method of the invention is carried out in a highly efficient manner and with an absolute minimum of power consumption so that not only is pulp obtained for paper making and the like from which has been removed substantially all of the available wood components but the resulting black liquor can be obtained at the desired concentration which is readily controlled in accordance with the unique features of the subject invention. The maintenance on the apparatus employed to practice the subject invention is relatively low and the initial cost of this apparatus is substantially smaller than presently available apparatus used for the same purpose. Furthermore, in present day practices, the amount of water which must be added to such a pulping process for the proper washing of the solubilized wood components from the chips is eliminated with the resulting drop in the needed power consumption for evaporation.

While there has been described what at present is considered to be the preferred embodiment of the invention, it will be understood by those skilled in the art that van ious changes and modifications may be made therein without departing from the invention and, therefore, it is the aim of the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

Having thus described the invention, what is claimed is:

1. A pulping process comprising the steps of, reacting wood chips in the presence of a cooking liquor in a digester by a sulfite pulping process to form solid lignosulfonates and partially complete the digestion of said wood chips, conducting the liquor from said digester to a reservoir in pressure equilibrium therewith whereby the liquor {L3 in said reservoir is isolated from the chips in said digester, evaporating the volatile components of the liquor in said reservoir to form a vapor, condensing said vapor to form a condensate, introducing said condensate into said digester in intimate contact with said wood chips to complete said digestion by hydrolytically solubilizing said solid lignosulfonates and other components in said wood chips while simultaneously removing by leaching said solubilized wood components from said wood chips to form a liquor and adding said liquor to the liquor in said reservoir in a closed system to thereby transfer the solubilized material from said wood chips to said reservoir.

2. A process in accordance with claim 1 including the step of removing a portion of said condensate from said system to further concentrate the liquor in said reservoir.

3. The pulping process comprising the steps of, reacting wood chips in the presence of a cooking liquor in a digester by a sulfite pulping process to form solid lignosulfonates and partially complete the digestion of said wood chips, conducting the liquor from said digester to a vessel in pressure equilibrium therewith, isolating the liquor in said vessel from the chips in said digester, evaporating the volatile components of the liquor in said vessel to form a vapor, condensing said vapor to form a condensate, introducing said condensate into said digester in intimate contact with said wood chips to complete said digestion by hydrolytically solubilizing said solid lignosulfonates and other components in said Wood chips while simultaneously removing by leaching said solubilized wood components from said wood chips to form liquor and adding said formed liquor to the liquor in said vessel in a closed system to thereby transfer the solubilized material from said wood chips to said vessel.

4. A pulping process comprising the steps of, reacting Wood chips in the presence of a cooking liquor in a digester by a sulfite pulping process to form solid lignosulfonates and partially complete the digestion of said wood chips, conducting the liquor from said digester to a tank in pressure equilibrium therewi h, isolating the surface of the liquor in said tank from the interior of said digester, reducing the pressure on the surface of the liquor in said tank to vaporize the volatile components in said liquor, removing said vapor from said tank, compressing said vapor, conducting said compressed vapor in heat exchange relationship With the liquor in said tank to heat said liquor and to condense said compressed vapor, introducing said condensate into said digester in intimate contact with said wood chips to complete said digestion by hydrolytically solubilizing said solid lignosulfonates and other components in said wood chips while simultaneously removing by leaching said solubilized wood components from said wood chips to form liquor and adding said formed liquor to the liquor in said tank in a closed system to thereby transfer the solubilized material from said wood chips to said tank.

5. The pulping process comprising the steps of, reacting wood chips in the presence or" a cooking liquor in a digester by a sulfite pulping process to form solid lignosulfonates and partially complete the digestion of said Wood chips, conducting said liquor from said digester to a tank in pressure equilibrium therewith to form a head of liquor in said tank, utilizing said head of liquor in said tank to form a pressure seal between the interior of said digester and the upper surface of the liquor in said tank,

reducing the pressure on the surface of the liquor in said tank to vaporize the volatile components in said liquor, removing said vapor from said tank, compressing said vapor, conducting said compressed vapor in heat exchange relationship with the liquor in said tank to heat said liquor and to condense said compressed vapor, introducing said condensate into said digester in intimate contact with said wood chips to complete said digestion by hydrolytically solubilizing said solid lignosulfonates and other components in said wood chips while simultaneously removing by leaching said solubilized wood components from said 9 wood chips to form a liquor and adding said formed liquor to the liquor in said tank in a closed system to thereby transfer the solubilized material from said wood chips to said tank.

6. A pulping process comprising the steps of, reacting wood chips in the presence of a cooking liquor in a digester by a sulfite pulping process to form solid lignosulfonates and partially complete the digestion of said wood chips, conducting the liquor from said digester to a tank through a conduit having an outlet positioned a suitable distance below the surface of the liquor in said tank to define a head of liquor, forming a pressure seal with said head of liquor to isolate the upper surface of the liquor in said tank from the interior of said digester, reducing the pressure on the surface of the liquor in said tank to vaporize the volatile components in said liquor, removing said vapor from said tank, compressing said vapor, conducting said compressed vapor in heat exchange relationship with the liquor in said tank to heat said liquor and to condense said compressed vapor, introducing said condensate into said digester in intimate contact with said wood chips to complete said digestion by hydrolytically solubilizing said solid lignosulfonates and other components in said wood chips while simultaneously removing by leaching said solubilzed wood components from said wood chips to form a liquor and adding said formed liquor to the liquor in said tank in a closed system to thereby transfer the solubilized material from said wood chips to said tank.

7. A pulping process comprising the steps of, reacting wood chips in the presence of a cooking liquor in a digester by a sulfiite pulping process to form solid lignosulfonates and partially complete the digestion of said wood chips, conducting the liquor from said digester to a tank through a conduit having an outlet positioned a suitable distance below the surface of the liquor in said tank to define a head of liquor, forming a pressure seal with said head of liquor to isolate the upper surface of the liquor in said tank from the interior of said digester, providing vacuum-compression means having a suction side in communication with said liquor upper surface, reducing the pressure on said liquor surface with said vacuum-compression means to vaporize the volatile components in said liquor, removing said vapor from said vessel, compressing said vapor in said vacuum-compression means, conducting said compressed vapor from said vacuum-compression means in heat exchange relationship with the liquor in said tank to heat said liquor and to condense said compression vapor, introducing said condensate into said digester in intimate contact with said wood chips to complete said digestion by hydrolytically solubilizing said solid lignosulfonates and other components in said wood chips while simultaneously removing by leaching said solubilized wood components from said chips to form a liquor and adding said formed liquor to the liquor in said tank in a closed system to thereby transfer the solubilized material from said wood chips to said tank.

8. A pulping process comprising the steps of, reacting wood chips in the presence of a cooking liquor in a digester by a sulfite pulping process, recirculating said cooking liquor through a heat exchanger to heat said cooking liquor during said reacting step to form solid lignosulfonates and partially complete the digestion of said wood chips, conducting the liquor from said digester to a tank having a bottom wall through a conduit having an outlet adjacent said tank bottom wall thereby defining a head of liquor extending from the upper surface of said liquor to said conduit outlet, forming a pressure seal with said head of liquor to isolate the upper surface of the liquor in said tank from the interior of said digester, providing vacuum-compression means having a suction side and a pressure side with said suction side in communication with said liquor surface, subjecting the surface of the liquor in said tank to a reduced pressure at the suction side of said vacuum-compression means to vaporize the volatile components in said liquor, removing the vapor from said vessel with said vacuum-compression means, compressing said vapor in said vacuum-compression means, conducting said compressed vapor from the pressure side of said vacuum-compression means through a coil immersed in the liquor in said tank in heat exchange relationship therewith to heat said liquor and to condense said compressed vapor, pumping the condensate from said coil through said heat exchanger, heating said condensate to a predetermined temperature in said heat exchanger, introducing said heated condensate into said digester in intimate contact with said wood chips to complete said digestion by hydrolytically solubilizing said solid lignosulfonates and other components in said Wood chips while simultaneously removing by leaching said solubilized wood components from said wood chips to form a liquor and adding said formed liquor to the liquor in said tank in a closed system to thereby transfer the solubilized material from said wood chips to said tank.

References Cited in the file of this patent UNITED STATES PATENTS 1,669,234 Dunbar May 8, 1928 1,793,264 Tschuwikowsky Feb. 17, 1931 1,852,011 Hatch Apr. 5, 1932 2,041,597 Dunbar a May 19, 1936 2,329,727 Rosenblad Sept. 21, 1943

Claims

1. A PULPING PROCESS COMPRISING THE STEPS OF REACTING WOOD CHIPS IN THE PRESENCE OF A COOLING LIQUOR IN A DIGESTER BY A SULFITE PULPING PROCESS TO FORM SOLID LIGNOSULFONATES AND PARTIALLY COMPLETE THE DIGESTION OF SAID WOOD CHIPS, CONDUCTING THE LIQUOR FROM SAID DIGESTER TO A RESERVOIR IN PRESSURE EQUILIBRIUM THEREWITH WHEREBY THE LIQUOR IN SAID RESERVOIR IS ISOLATED FROM THE CHIPS IN SAID DIGESTER, EVAPORTING THE VOLATILE COMPONENTS OF THE LIUQOR IN SAID RESERVOIR TO FORM A VAPOR, CONDENSING SAID VAPOR TO FORM A CONDENSATE, INTRODUCING SAID CONDENSATE INTO SAID DIGESTER IN INTIMATE CONTACT WITH SAID WOOD CHIPS TO COMPLETE SAID DIGESTION BY HYDROLYTICALLY SOLUBILIZING SAID