JPH0244957B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for cooking cellulose derivative materials such as wood chips in a batch process.
More particularly, the present invention relates to a method of steaming cellulose derivative materials, which includes a step of recovering and efficiently utilizing the heat used during the steaming process.

[Background of the invention]

In a typical batch process for cooking wood chips, a digester is filled with chips and then the digester is charged with cooking chemicals. In the soda method, this agent essentially contains a caustic soda solution, and in the Kraft method, it further contains a solution containing a sulfur compound. The digester is then sealed and the steam raises the temperature of the digester to the cooking temperature.
At the end of this steaming process, the blow valve of the digester is opened and the contents of the digester are discharged into the blow tank. Most of the thermal energy acquired by the digester contents during the processing of this pulp is released through the blow tank together with the exhaust steam. However, these recovery devices were not truly efficient. To save on energy costs, some pulp producers have chosen to equip a continuous cooking process. Such processes are usually characterized by a much more efficient use of heat than is obtainable in conventional batch processes. However, the cost of equipment required for a continuous process is considerably greater than the cost of equipment required for a batch process.

Prior art known to the applicant illustrates various attempts to recover heat within the spent liquor produced during the pulp manufacturing process. An example of such prior art is US Pat. No. 1,690,032, which discloses a sulfate process for cooking pulp. In the process described in this patent, the hot liquor collected in the blow pit is flushed out of the digester together with the wood chips, packing the chips and increasing the temperature of the chips. However, in this process, as in the conventional batch soda process and Kraft process mentioned above, when the contents of the digester are discharged into the blow pit, considerable energy is released from the blow pit together with the exhaust steam. .

U.S. Pat. No. 2,195,378 discloses a process for chemically producing pulp, the white liquor of which is heated by a heating coil by pumping spent liquor through the coil. Heated in the accumulator. There are many practical problems with using a pump in this manner to remove spent liquor from the digester mixture. Moreover, it is difficult to recover in a practical manner the heat contained in the spent liquor when it is discharged from the heating coil described in this patent.

[Summary of the invention]

It is therefore an object of the present invention to provide a method for cooking cellulosic materials, which makes it possible to efficiently recover and utilize thermal energy in a practical manner.

Since the usual source of energy for carrying out the steaming process is steam, the objective is to provide a steaming process which ultimately results in a reduction in the amount of steam used.

It is another object of the present invention to ensure that heat is recovered from the spent liquor present in the digester after the final cooking process in a manner that promotes efficient recovery and utilization even when relatively low temperature levels are involved. Another object of the present invention is to provide a method for steaming cellulose derivative materials.

That is, in the steaming method according to the present invention, wood chips made of a cellulose derivative material and liquor made of a cooking agent are loaded into a digester, and then the loaded material is subjected to a steaming treatment to produce delignified pulp. After the steaming treatment of the charge, the resulting delignified pulp remains in the digester and under pressure,
By pumping a displacement liquid into the digester from below, the hot spent liquor after steaming is discharged from the top of the upper part of the digester and into the accumulator, thereby displacing the spent liquor with the digester. The spent liquor thus displaced and transferred into the accumulator is pumped into a digester loaded with wood chips for the next cooking process. Therefore, the present invention is characterized in that the heat contained in the used liquor is transferred to the wood chips to preheat them.

According to the intent of the present invention, the spent liquor in the digester after the steaming process of the cellulosic material is removed by introducing a displacement liquid to displace the spent liquor in the digester. , is discharged from the digester.
The pressure within the digester is maintained during this displacement to prevent the liquor from escaping rapidly.

The used liquor collected in this way is
It is used to supply heat to the subsequent digester charge.

It is further contemplated in embodiments of the invention to separate and collect the liquid discharged from the digester at two temperature levels. A liquid at a relatively low temperature level is prepared by immersing the cellulosic material therein.
It can be used for initial preheating of the charge of cellulosic material in the digester. The relatively high temperature level liquid can be used to further heat the cellulosic material which has undergone displacement of the relatively low temperature liquid in the digester with this relatively high temperature liquid.

In carrying out such separate collection, the relatively high temperature spent liquor discharged by the displacement liquid is transferred to a high temperature accumulator, as described below, and the discharged relatively low temperature liquid is then transferred to a low temperature accumulator. In this way, the preheating liquid in two temperature ranges can be separated and collected. After the relatively high-temperature used liquor discharged by the above-mentioned displacement liquid is transferred to a high-temperature accumulator, this liquid is used for preheating before supplying the liquor consisting of the above-mentioned cooking agent. The liquid from the high-temperature accumulator, whose temperature has thus decreased, can also be transferred to the low-temperature accumulator. In addition, when preheating the wood chips to be steamed, first, a comparatively low temperature liquid is filled from the low temperature accumulator into the digester loaded with the wood chips and preheated, and then a It is also possible to replace the used liquor at temperature by pumping it into the same digester, thereby providing further preheating of the wood chips.

Other features and advantages of the present invention include more efficient use of cooking chemicals in the cooking process and reduced usage of such chemicals, higher yields and improvements in pulp due to relatively greater selectivity in the delignification reaction. This gives rise to the possibility of using the digester as a cleaning device during the process, and also to control scale build-up within the heat exchanger.
Furthermore, existing equipment for carrying out the batch cooking process can be readily converted to carry out the method of the invention.

[Specific description of the invention]

These features and advantages of the invention will become more apparent following the following description with reference to the accompanying drawings.

As mentioned above, in the chemical cooking of pulp by batch process, it is not necessary to charge the cellulosic material, i.e., wood chips, into the digester and then introduce the reaction liquid (liquor) containing the reactive agent into the digester. , which is done in the normal way. In the case of the soda process, the reaction liquid (cooking agent) is white liquor.
It is essentially an aqueous solution of caustic soda. . In craft law,
This white liquor contains sulfur compounds. Steaming is
With the contents of the digester at a certain elevated temperature and pressure, the internal temperature of the digester is
It is typically in the range of about 165° to 177°C (330° to 350°).

According to the present invention, at the end of the steaming period, the replacement liquid is pumped into the bottom of the digester while maintaining the pressure inside the digester. As this displacement liquid fills the digester from the bottom, it gradually displaces the hot spent liquor (or black liquor) produced by the boiling process, causing the digester to displace it. liquid is replaced. This hot spent liquor is discharged from the digester at the temperature of the digester at the end of the cooking period and collected in the black liquor accumulator.

In an embodiment of the present invention, it is preferable to use a washed filtrate obtained by washing the delignified pulp obtained in the steaming process before the steaming process as the replacement liquid. . Such a filtrate (by passing through a heated pulp)
It has a temperature higher than normal room temperature. In practicing the invention, such wash filtrate can be considered to have a temperature in the range of, for example, 60° to 74°C (140° to 165°C).

The temperature of the liquid leaving the digester when the volume of liquid leaving the digester approaches the liquid holding capacity of the digester, i.e. the volume of the digester minus the volume occupied by the solids in the digester, is: Such liquid will become essential to the filtrate used in the displacement step. This liquid, however, will have a somewhat higher temperature than the original filtrate. This is because it is heated by the delignified pulp held in the digester.

Relatively high temperature spent liquor removed from the digester may be collected in a high temperature accumulator, and relatively low temperature liquid subsequently exiting the digester may be collected in a low temperature accumulator. . Each of these liquids can be utilized in subsequent batch processing of the cellulosic material in providing a portion of the heat required to obtain suitable cooking temperatures. When such preheating is performed, the liquid from the hot accumulator, whose temperature has been reduced by heat exchange, can be collected in the cold accumulator, as described above.

Apparatus that can be used in carrying out the invention is schematically illustrated in FIG. It should be understood that this figure is a schematic representation and many instrumentation devices such as gauges, pressure relief holes, pumps and valves required in the actual apparatus have been omitted from illustration for the sake of brevity.

Referring to FIG. 1, reference numeral 10 is a digester of the type typically used in the chemical steaming treatment of wood chips. Although not shown in detail, such digesters have a conventional removable cover, which is removed when loading the digester with wood chips.

A valve-controlled conduit 14 having a conductor 14 and a valve 16 connects to the top of the digester and provides a passageway for transferring liquid from the digester to the black liquor reservoir 12. During pulp production,
The spent black liquor collected in the black liquor storage tank 12 is converted to white liquor using conventional methods to provide the active agent for the wood chip cooking process. The details of this conversion method are not necessary for understanding the present invention and are therefore omitted from this description.

Denoted at 20 is a low-temperature accumulator containing relatively low-temperature liquid. A valve-controlled conduit, including conduit 22 and valve 24, provides a liquid flow path from the top of the digester to cryogenic accumulator 20. The liquid contained within the cryoaccumulator 20 is returned to the digester 10 by actuating the pump 26, which removes liquid from the cryoaccumulator 20 and directs it through conduit 28 and valve 30 to the digester 10. Inject into the bottom of 10.

As previously mentioned, when practicing the present invention, after the chips have been cooked, the spent liquor in the digester is drained from the digester by means of a displacement liquid, which liquid preferably comprises:
This is a filtrate collected from a washing device provided for washing the delignified pulp after steaming treatment after being taken out from the digester. A first washer filtrate reservoir for this purpose is indicated by 32 and this double filtrate is supplied via conduit 34. Filtrate is removed from reservoir 32 using pump 36, conduit 38 and valve 40 to digester 10.
is sent into the

Hot spent liquor extruded from the digester 10 is collected in a hot accumulator 42 . This liquor reaches the hot accumulator 42 via valve 44 and conduit 46. The liquid in this high temperature accumulator 42 is introduced to the bottom of the digester 10 via a pump 48 and a conduit and valve 52 connected to the bottom of the digester 10 . According to one embodiment of the invention, a portion of the liquor in the high temperature accumulator 42 can be used to preheat the liquor comprising the cooking agent introduced into the digester for cooking purposes. . That is, what is shown at 54 is a heat exchanger for this purpose. Hot spent liquor from the hot accumulator 42 is delivered to the heat exchanger 54 via pump 56 and conduit 58. As the hot spent liquor from the hot accumulator 42 passes through the exchanger, it transfers most of its heat to the liquor from conduit 63 before being transferred to the cold accumulator 20 through conduit 60.

Shown at 62 is a storage container for hot white liquor. Liquor entering exchanger 54 through conduit 63 reaches container 62 via conduit 64 .
Hot liquor is conveyed from container 62 to the bottom of digester 10 by pump 66 via conduit 68 and valve 70 .

If desired, provision may be provided for heating the contents of container 62 above the temperature of the liquor introduced into the container by conduit 64. This can be done by using a variety of conventional heating devices. As illustrated in the drawings, the heating utilizes a steam-based heat exchanger 82 with steam supplied by conduit 83 to transfer the contents of the container 62 via a pump 84 to the exchanger 82. This can be done by circulating it through.

A heating device can additionally be provided to heat the contents of the digester to the final cooking temperature. Such heating devices can take various forms. In the apparatus illustrated in FIG. 1, heating is provided by a steam heat exchanger 86 to which steam is supplied via conduit 87. Liquid from digester 10 enters heat exchanger 86 via conduit 88 and pump 90. Exchanger 86
Liquid exiting the digester enters the digester 10 near the top and bottom of the digester by conduits 92, 94.

Hereinafter, the steaming method according to the present invention will be explained in relation to each step when it is used for batch steaming of wood chips. In this description, representative temperature and volume relationships will be shown. However, it should be understood that these numerical values, processing time, and processing speed will change as appropriate depending on the scale of the equipment and the object to be processed. To begin this explanation, first consider that a high-temperature accumulator contains a liquid, or hot black liquor, at a temperature of approximately 163°C (325°C). The low-temperature accumulator contains liquid at approximately 98.9°C (210°C).
How to obtain these conditions will become clear later in this description.

First, the digester is filled with the wood chips to be treated, which have a temperature of approximately room temperature or 15.5°C (60°C).

After closing the top of the digester, liquid from the cryogenic accumulator 20 is pumped into the digester 10, as shown in FIG. Dip the chips into the liquid. By bringing the liquid into direct contact with the chip, the temperature of the chip increases. As an example, using a digester with an internal volume of 170 m ³ (6000 ft ³ ), when it is filled with chips, approximately 11.3 m ³ (400 ft ³ ) will be occupied by the wood material. Dew. Therefore, when the digester is filled with chips, 159 m ³ (5600 ft ³ ) of liquid holding capacity remains in the digester.

Normally slightly more than this liquid holding capacity will be introduced into the digester when filling it with liquid from the cryogenic accumulator. For example, 1.5 to 1.7 times this capacity, the chip is actually filled with cold liquid. This cryogenic liquid treatment lowers the temperature of the digester contents to approximately 93°C (200°C).
〓) has the effect of increasing

Excess cryogenic liquid leaving the digester is directed to black liquor storage tank 12.

After this initial treatment with the cryogenic liquid to raise the temperature of the chips, the relatively hot spent liquor from the hot accumulator 42 is fed into the bottom of the digester, as shown in FIG. . During such pumping, valve adjustments are made to connect the top of the digester to a cold accumulator 20 by a tube 22 as shown in FIG. The hot liquid introduced into the bottom of the digester displaces the cold liquid previously introduced into the digester and returns this cold liquid to the cold accumulator. Pumping is carried out against a back pressure valve provided in the pipe 22, so that a certain pressure condition is maintained within the digester to prevent a rush of hot liquor.

In this and other substitution steps performed during this process, the operation of replacing the liquid in the digester with another liquid is substantially quantitative, in other words, the replacement liquid is replaced with another liquid in the digester. It has been found that most of the liquid that leaves the digester during the introduction up to the liquid holding capacity of , is the liquid that initially entered the digester. That is, by introducing an amount of replacement liquid equal to the liquid holding capacity of the digester and then introducing more replacement liquid, most of the liquid discharged from the digester becomes replacement liquid. This can be demonstrated by taking temperature measurements of the liquid exiting the digester, as described below.

The pump is stopped when a quantity of hot liquor equal to the liquid holding capacity of the digester has been introduced. The amount of hot liquor pumped also depends on the amount present in the hot accumulator, and the appropriate amount pumped is determined by the balance of the system. Direct contact between the chips and the hot liquor in the digester causes the temperature of the chips to be around the temperature of the hot liquor, e.g.
The temperature rises to 300〓).

Referring to FIG. 4, at the end of this stage of the process, when the chips are already at a high temperature level, liquor from the hot white liquor storage container 62 is pumped to the bottom of the digester using pump 66. entered. By making appropriate valving, hot liquor dislodged from the digester is returned to the hot accumulator 42, as shown in FIG. In a typical pulping process, 50 to 70% of the liquid holding capacity of the digester is introduced at this time. This is because this is the range of liquor normally required to introduce the chemicals necessary for cooking into the digester. Pumping is performed against a back pressure valve in line 46 so that a pressure condition is maintained within the digester to prevent a rush of liquor.

In practice, the latter part of the hot black liquor replenishment and the liquor replenishment can be carried out simultaneously in order to save the cooking time and to adjust the concentration of the liquor.

As can be seen in FIG.
As mentioned above, it is possible to preheat the liquor (white liquor) which is sent through this exchanger and into the storage container 62. If it is desired to raise the temperature of the liquor in storage vessel 62 above that obtained by passing the liquor through exchanger 54, steam-based heat exchanger 82 may be activated to pump the liquor. 84 can be used to circulate through the exchanger.

In the method according to the invention, the temperature of the liquor contained in the storage container 62 is preferably in the range of 150 DEG to 163 DEG C. (300 DEG to 325 DEG C.).

The digester is now filled with an appropriate amount of liquor and the temperature of the contents inside the digester is between 150° and
When the temperature of the digester contents is in the range of 163 degrees Celsius (300 degrees to 325 degrees Celsius), a steam-based heat exchanger 86 is used to circulate the contents of the digester through the exchanger using a pump 90. By increasing the temperature to normal steaming temperature, i.e. 165° to 177°
The temperature can be raised to 330° to 350°C.

At the end of the cooking period, the hot black liquor, ie the hot spent liquor in the present digester, is pumped from the filtrate reservoir 32 to the bottom of the digester, as illustrated in FIG. It can then be discharged from the digester. The first liquor to leave the digester is
For example, having a temperature of about 165°C (330°C), this hot spent liquor is directed into a high temperature acumulator. At this time, the pressure within the digester is maintained to prevent a rush of liquor.

As previously explained, displacing the liquid in the digester by pumping a displacement liquid (filtrate from the filtrate reservoir 32 in the example above) into the bottom of the digester substantially reduces the amount of It is carried out in a regular manner. This is verified by measuring the temperature of the liquid leaving the digester. Therefore, as the filtrate from the filtrate reservoir 32 was pumped into the bottom of the digester, temperature measurements of the liquid exiting the digester were taken. 6th
The figure diagrammatically illustrates the results obtained from such measurements. As can be seen in Figure 6, this figure illustrates the relationship of the temperature of the liquid leaving the digester to the volume of liquid introduced into the digester expressed as a percentage of the liquid holding capacity of the digester, and the pump The temperature of the discharged liquid remains approximately constant and is slightly lower than the cooking temperature used until the volume of the displacement liquid introduced in the digester is approximately equal to the liquid holding capacity of the digester. When the volume of displacement liquid introduced into the digester becomes equal to the liquid holding capacity of the digester, the temperature of the displaced liquid drops very rapidly. In practicing the invention, the filtrate pumped into the digester will preferably be in an amount equal to about 120% of the liquid holding capacity of the digester. An amount of liquid displaced while being pumped into the digester, approximately equal to the liquid holding capacity of the digester, is directed to the high temperature accumulator. The remaining liquid sent from the digester is directed to a cold accumulator, the average temperature of which is approximately midway between the temperature of the filtrate used and the temperature of the hot liquor initially displaced.

After separating and recovering each of the hot and cold liquids as described above, the cooked pulp fibers within the digester are removed from the digester by any of a number of methods. For example, referring to Figure 1, pressurized steam is introduced into the top of the digester, increasing the pressure within the digester (with little temperature increase), opening the blow-off tube 96, and discharging the contents of the digester normally. transferred to a blow tank. Alternatively, the contents of the digester may be removed from the digester using a wash fluid, such as filtrate from reservoir 32, introduced via conduit 98. Regardless of the method used, after the pulp fibers are removed from the digester, they are washed and the water used for washing is collected in a filtrate reservoir 32 as a first wash filtrate. Since such washing liquid contacts the hot pulp (and hot water is used for washing), the filtrate in the filtrate reservoir has a temperature above room temperature.

The liquid fed to the high temperature accumulator in the embodiment of the invention described above is the hot spent liquor discharged from the digester at the end of the boiling process. The liquid supplied to the low temperature acumulator is the liquid from the high temperature acumulator 42 that has passed through the heat exchanger 54 and the filtrate used in the final stage of discharging the liquor from the digester. These origins and the temperature changes due to heat transfer to the chips determine the temperature of the liquid present in both accumulators.

A brief summary of some of the advantages of the process outlined above and variations of this process are as follows.

The steaming process according to the invention can be accomplished with significantly less steam than is required to perform the heating operations required by conventional methods.
This is because the hot spent liquor removed from the digester is effectively used as the primary heat source for the subsequent steaming process.

Another advantage achieved by the present invention is the reduction in liquor (white liquor) required for steaming. Residual active agent present in the spent liquor and wash filtrate is recycled into the system and treated with wood acid for neutralization before delignification by the newly introduced white liquor. and other initial reactions of steaming. As a result, the cooking agents in the white liquor are not needed to carry out these initial reactions. Therefore, the method of the present invention is also highly effective in reducing the amount of chemicals used.

A further associated feature and advantage is the relatively high selectivity in the delignification that occurs during the cooking process. When cooking occurs, a high concentration of active alkali is present, which produces a relatively rapid delignification action that must be terminated before the useful cellulose is destroyed. The agents introduced into the cooking liquor to improve its characteristics are also stored for this reintroduction into the apparatus.

Yet another advantage of the present invention is that the pulp can be thoroughly washed in the digester as a result of replacing the spent liquor after cooking with the washing filtrate. In practice, the digester may be used as the washing vessel during the first washing of the pulp being produced. In some new installations, this feature makes it possible to save money on cleaning equipment. In some existing equipment, other cleaning processes are possible and several advantages arise from this.

Using conventional methods, the calcium present in the wood dissolves in the liquor during the early stages of cooking when the heat exchange equipment used to heat the bagged material is under high load. Acts to increase calcium ion concentration. If the hydroxide ion concentration is high at this time, calcium carbonate begins to precipitate, creating scale within the strainer and heat exchanger. In preparation for introducing the white liquor according to the invention and before the hydroxide ion concentration increases, calcium ions are created when the black liquor contacts the chips. When the white liquor is charged into the digester, hydroxide ions immediately increase rapidly and calcium carbonate precipitates within the chip columns. At this time, there is no load on the strainer and heat exchanger, and calcium carbonate precipitation in the strainer and heat exchanger is minimal. Thus, the present invention provides control of scale build-up, which is a problem in conventional procedures.

Another advantage of the process outlined above is the use of pressurized hot liquor to displace other liquids in the digester. That is, it is possible to cause pressure impregnation of the chips with the pressurized liquid, which is advantageous in further improving the efficiency of the steaming process.

A final point to be noted is that existing equipment for conventional steaming processes can be easily modified into an apparatus for carrying out the invention. This makes the invention attractive to pulp producers who are using conventional equipment and wish to reduce their energy requirements.

In the above embodiments, the use of a single digester has been described. Obviously, the operating principle of the invention can be applied to a cooking process using one or more digesters.

Although particular embodiments of the invention have been described, many modifications and changes are possible without departing from the invention. Accordingly, such modifications and changes as would be obvious to those skilled in the art are intended to be included in the present invention.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram illustrating in simplified form a digester and related equipment that can be used in carrying out one embodiment of the present invention, and FIGS. Simplified schematic diagram illustrating the main equipment used in the steps, No. 6
The diagram shows the temperature of liquor discharged from a digester versus the percentage of the total liquid holding capacity of that digester.
2 is a graph illustrating a representative relationship that exists between the amount of discharged liquid shown as 10...Digester, 12...Black liquor storage tank, 1
4... Conduit, 16... Valve, 20... Low temperature accumulator, 22... Conduit, 24... Valve, 26... Pump, 28... Conduit, 30... Valve, 32... Washing filtrate storage tank, 34 ... Conduit, 36 ... Pump, 3
8... Conduit, 40... Valve, 42... High-temperature accumulator, 44... Valve, 46... Conduit, 48... Pump, 50... Conduit, 52... Valve, 54... Heat exchanger, 56... ...Pump, 58... Conduit, 60...
Conduit, 62... Hot white liquid storage container, 63... Conduit,
64... conduit, 66... pump, 68... conduit,
70... valve, 82... heat exchanger, 83... conduit,
84... pump, 86... heat exchanger, 87... conduit, 88... conduit, 90... pump, 92,94
... conduit, 96 ... blow-off pipe, 98 ... conduit.

Claims (1)

[Claims] 1. To obtain delignified pulp by loading wood chips made of a cellulose derivative material and a liquor made of a cooking agent into a digester, and then subjecting the loaded material to a cooking process. The method of steaming comprises, after the steaming treatment of the charge, displacing liquid into the digester while the obtained delignified pulp remains in the digester and under pressure. By pumping from below, the hot spent liquor after the steaming process is discharged from the top of the top of the digester and into a high temperature accumulator, thereby substantially completely filling the spent liquor with the displacing liquid. By pumping the spent liquor thus displaced and transferred into a high-temperature accumulator into a digester loaded with wood chips for the next steaming process, the spent liquor is replaced. A steaming method characterized in that the heat contained in the wood chips is transferred to preheat them. 2. The method according to claim 1, wherein the displacing liquid is a washing filtrate used for washing the delignified pulp obtained after the steaming treatment. 3. Transferring the high-temperature spent liquor discharged by the displacement liquid to a high-temperature accumulator;
The relatively low-temperature liquid discharged from the upper part of the digester is transferred to a low-temperature accumulator, and in this way, the liquids in two temperature ranges are separated and collected as preheating liquids. Method described. 4. After transferring the hot spent liquor discharged by said displacing liquid to a high temperature accumulator, this liquid is used to preheat the liquor consisting of the cooking agent from the white liquor conduit, and by this preheating 4. A method as claimed in claim 3, in which liquid from the high temperature accumulator is transferred to the low temperature accumulator. 5. When preheating the wood chips to be steamed, first fill the digester loaded with the wood chips with relatively low temperature liquid from the low temperature accumulator to preheat it, and then heat the high temperature liquid stored in the high temperature accumulator. 4. A method as claimed in claim 3, characterized in that the preheating of the wood chips is carried out by replacing the spent liquor of the wood by pumping it into the same digester.