FI79724C - Process for boiling cellulose material - Google Patents

Description

1 79724

This invention relates to the cooking of a cellulosic material, such as wood chips, in a batch type process.

More particularly, the invention relates to a process for cooking a cellulosic material which is characterized by an efficient and practical way of recovering the heat used in the process.

In a conventional batch type method for cooking wood chips, the cooking pot is filled with chips and a cooking chemical is then placed in the cooking pot, which in the soda process consists mainly of sodium hydroxide and in the sulphate-cellulose method consists of a solution further containing a sulfur compound. The boiler is then closed and the temperature of the boiler is raised to the cooking temperature by means of steam. At the end of cooking, the boiler blow-off valve is opened and the contents of the boiler are removed to the pulp pit. Much of the heat accumulated in the contents of the cooking pot during pulp treatment is discharged through the pulp pit with 20 exhaust gases. In order to recover this energy, efforts have been made to conduct these vapors through various heat recovery systems. However, these recovery systems are not fully efficient. In order to save energy costs, some cellulose manufacturers have decided to install a continuation-25 cooking method. Such a method is usually characterized by more efficient utilization of heat than is achieved in a conventional batch process. However, the cost of the equipment required in the continuous process is substantially higher than the equipment required in the batch-30 type process.

A prior-art patents, of which I am aware, presents a variety of other efforts to recover the heat formed in the cooking process used keittoliuokses-O. An example of such a prior art patent 35 is U.S. Patent 1,697,032, which discloses a sulfite process for cooking pulp. According to the method disclosed in this patent 2,79724, the hot solution recovered from the pulp pit is allowed to flow downwards through the cooking device together with the wood chips, whereby a tighter packaging of the chips and a slight heating of the chips is achieved. However, in this process, as in conventional batch-type soda and sulphate pulp processes, as described above, a considerable amount of energy is removed from the pulp pit with the exhaust fumes.

U.S. Patent 2,195,378 discloses a process for chemically producing cellulose-10 pulp in which white liquor is heated in an accumulator tank by means of a heating coil which is heated by pumping the spent cooking solution through the coil. In practice, there are difficulties in separating the spent cooking solution from the cooked mixture in a boiler using a pump, as shown. In addition, it is difficult to recover thermal energy from a practically used cooking solution at the temperature prevailing according to the patent for heating coil removal.

The object of the invention is thus generally to provide a method for batch-type cooking of a cellulosic material, which method enables the efficient recovery of thermal energy in a practical manner.

The presence of water vapor in the cooking process of a conventional energy source results in the above as a result of providing a batch-type cooking method with a reduced need for water vapor.

Another object of the invention is to provide a method for cooking a cellulosic material, wherein heat is recovered from the used cooking solution in the cooking pot after final cooking in a manner that promotes efficient recovery even at relatively low temperature levels.

According to the invention, the spent cooking solution 35 is removed from the cooking vessel after cooking the cellulosic material, leading to a displacing solution 3,79724, which removes the used cooking solution and replacing it in the cooking pot. The boiler is maintained under pressure during this replacement to prevent the solution from flaring. The spent cooking solution recovered in this way is then used to supply heat to the next cooking charge.

Another feature of a particular embodiment of the invention is the recovery of cooking solutions removed from the cooking vessel at two temperature levels. The lower temperature cooking solution can be used to preheat a 10 batch of cellulosic material in a cooking pot by immersing the cellulosic material in a lower temperature cooking solution. The higher temperature cooking solution can be used to further heat the cellulosic material by removing the lower temperature solution from the cooking vessel using the higher temperature solution.

Other features and advantages of the invention include more efficient use of cooking chemicals in the cooking process and reduced consumption of cooking chemicals; higher yields and improved pulp quality due to better selectivity of the lignin removal reaction; the ability to use the boiler as a means to perform the washing treatment in the process; and descaling control of the heat exchanger. In addition, current equipment for performing batch cooking processes can be easily modified to perform the method of the invention.

These and other objects and advantages of the invention will become more apparent from the following description with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram showing in simplified form 30 cooking pots and apparatus connected to a cooking pot which can be used to practice the invention; Figures 2-5 are simplified schematic representations illustrating the main apparatus used in the various stages of the cooking process, and Figure 6 shows graphically the typical correspondence between the amount of cooking solution 4,79724 removed from the digester, expressed as a percentage of the total liquid volume of the digesters.

As mentioned above, in a chemical process for the chemical cooking of cellulose, in a batch-type process, the batch is fed to the cooking vessel with a cellulosic material, for example wood chips, and then fed to the cooking vessel a reactive solution containing the reactive chemical. In the case of the soda process, the reactive solution, known as white liquor, is mainly an aqueous solution of sodium hydroxide.

10 In the sulphate process, the white liquor contains a sulfur compound.

The extraction takes place in the contents of the boiler at elevated temperatures and pressures, the temperature in the boiler being typically in the range of 165-177 ° C.

According to the present invention, at the end of the cooking step 15 and maintaining the pressure in the boiler, a displacing solution is pumped to the bottom of the boiler. When this displacing solution fills from the bottom of the cooking pot, it continuously removes and replaces the hot, used cooking solution or black liquor formed during cooking. This hot black liquor, as observed, leaves the digester substantially at the end of the cooking stage at the prevailing temperature and can be recovered at a high temperature in the black liquor accumulator tank.

In a particular embodiment of the invention, it is recommended to use as a displacement solution the filtrate obtained from the washing step of the pulp or lignin-purified fibers, which is obtained at the initial stage of cooking the chips. The temperature of this filtrate is above the usual room temperature (due to its passage through the warm fibers) and in the practice of the invention it can be assumed that the temperature of this filtrate is in the range of 60-74 ° C.

As the volume of solution displaced from the boiler approaches the volume limit of the boiler liquid, i.e. the volume of the boiler minus the volume of solids in the boiler 35, the temperature of the solution leaving the boiler decreases and the temperature of this solution becomes substantially the same as the filtrate used in the displacement step 5,79724. However, the temperature of this solution is slightly higher than that of the original filtrate due to the heating effect of the pulp fibers in the cooking pot.

The relatively high temperature spent cooking-5 solution is removed from the boiler and can be recovered in a high temperature accumulator tank, and the lower temperature solution, which then exits the boiler, can be recovered in a low temperature accumulator tank. These solutions, in turn, can be used to supply some of the heat required to achieve the proper cooking temperature in subsequent batches of cellulosic material. When performing this heating step, the solution leaving the high temperature accumulator tank, the temperature of which decreases due to the heat exchange, can be recovered in the low temperature accumulator tank.

Figure 1 schematically shows a device that can be used in the application of the invention. It should be noted that the representation is schematic, in which several devices such as gauges, pressure relief pipes, pumps and valves, which are characteristic of the actual installation, have been removed from the drawing for simplicity.

Referring to Figure 1 of the drawing, there is shown a cooking pot 10 of the type commonly used in the chemical extraction of wood chips. Although not specifically shown, such a cooking pot is provided with a conventional removable lid which is removed when the wood chips are charged into the cooking pot.

Attached to the top of the digester and providing 30 for the flow of liquid from the digester to the black liquor storage vessel is a valve controlled tube including tube 14 and valve 16. The spent black liquor recovered from Chemical cabbage. The details of this procedure have been omitted from this disclosure because they are unnecessary to an understanding of the invention.

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Section 20 shows a low temperature accumulator tank. The valve-controlled tube comprising the tube 22 and the valve 24 forms a passageway for the flow of liquid from the top of the cooking boiler to the low temperature accumulator tank. The solution in the low temperature accumulator tank can be returned to the digester by using a pump 26 which removes the solution from the accumulator hopper and pumps it through line 28 and valve 30 to the bottom of the digester.

As indicated above, in the practice of the invention, the spent cooking solution in the cooking vessel, after cooking the chips, is removed from the cooking vessel by means of a displacing solution and this is preferably filtrate from the first washing machine used to wash cellulose 15 after removal from the cooking vessel. The tank of eodet obtained from the first washing device is shown at 32 and is fed to it from the filtrate via a pipe 34. The filtrate can be removed from the storage tank 32 and pumped to the bottom of the digester using a pump 36 through a pipe 38 and a valve 20 40.

The spent hot cooking solution removed from the digester is recovered in a high temperature accumulator tank, as shown in step 42. The cooking solution is transferred to the accumulator tank through the valve 44 and the pipe 46. The solution in the high temperature accumulator tank can be fed to the bottom of the cooker using a pump 48 through a pipe 50 and a valve 52 connected to the bottom of the cooker.

According to one embodiment of the invention, part of the cooking solution in the high temperature accumulator tank can be used to preheat a white liquor containing the active chemical and fed to the cooking pot for cooking. For this purpose, a heat exchanger is shown in section 54. The hot cooking slurry from the high temperature accumulator tank 35 is pumped into the heat exchanger using pump 56 and tube 58. This broth, of which 7,79724 of heat is transferred to the white liquor, passes through tube exchanger through tube exchanger to low temperature accumulator.

At 62, a hot white liquor storage-5 tank is shown. The white liquor entering the heat exchanger through and through the heat exchanger 63 passes through the tube 64 to the storage tank 62. The hot white liquor is transferred from the tank to the bottom of the digester 10 by a pump 66 which pumps the digestion solution through the tube 68 and valve 70.

If desired, means may be used to heat the contents of the vessel 62 above the temperature of the cooking solution introduced therethrough through the tube 64. This can be done using a variety of conventional heating systems. As shown in the drawing, heating is performed using a steam-heated heat exchanger 82 to which steam is passed through a tube 83 and circulating the contents of the vessel 62 through the heat exchanger by a pump 84.

An additional heating means is used to heat the contents of the cooking pot to the final cooking temperature. Again, this heating means 20 may be different. In the specified embodiment shown, the heating is performed by means of a steam-operated heat exchanger 86, to which steam is passed through a pipe 87. The solution leaving the digester enters the heat exchanger through line 88 and pump 90. The solution leaving the heat exchanger is led to the digester through pipes 92, 94 near its top and bottom.

The method according to the invention is presented below applied to batch-type cooking of wood chips. This review presents typical temperatures and volume-30 ratios, although it should be noted that these values vary from installation to installation. Initially, this review assumes that the high temperature accumulator tank contains a solution, i. hot black liquor at a temperature of about 163 ° C. The low temperature accumulator tank contains the solution at a temperature of about 35.98 ° C. How these conditions have been achieved will become apparent at a later stage in this presentation.

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The cooking pot is initially filled with wood chips and their temperature can be approximately room temperature, about 15.5 ° C.

After closing the boiler lid, as shown in Figure 2 of the drawings, a solution is pumped from the low temperature 5 storage tank 20 to the boiler to fill it completely (and remove air from the boiler) and immerse the wood chips in the liquid. When the liquid is in direct contact with the chips, the temperature of the chips rises.

Assuming, by way of example, that a cooking space with an internal volume of 170 m is used and when it is filled with chips, 3 the wood material needs a volume of about 11.3 m.

3 In this case, the liquid volume of the cooking appliance remains at 159 m when it is filled with chips.

When filling with liquid, the low-temperature accumulator-15 is usually fed to the cooking boiler slightly more than its liquid-absorbing capacity, for example 1.5 to 1.7 times its tensile capacity, so that in fact the chips are rinsed with a low-temperature solution. This treatment with a low temperature liquid raises the temperature of the contents of the boiler-20 to about 93 ° C.

Excess low temperature liquid leaving the digester is led to the black liquor storage tank 12.

After this treatment with the low temperature solution, to raise the temperature of the chips as described above and according to Fig. 3, the cooking solution obtained from the high temperature accumulator tank 42 is pumped to the bottom of the cooking pot. By adjusting the valves during this pumping, the top of the cooking pot is connected to the low temperature accumulator tank through a pipe 22, as shown in Fig. 3 30. The high temperature cooking solution fed to the bottom of the digester displaces the low temperature solution previously fed to the digester and this low temperature solution returns to the low temperature accumulator tank. The pumping is carried out via a back-pressure valve 35 in the pipe 22, whereby a pressure is maintained in the cooking boiler to prevent the flash of the high-temperature cooking solution 9 79724.

It has been found that in this and other possible displacement steps in the process, the displacement of the liquid in the boiler by the second liquid is substantially quantitative so that during the supply of the displacing liquid until the draft of the boiler liquid is reached, the main liquid leaving the boiler is When a displacement liquid has been fed in an amount equal to the traction capacity of the 10 boiler liquids, and when more displacement liquid is fed, the liquid leaving the boiler becomes mainly a displacement liquid, which can be demonstrated by performing temperature measurements in the liquid leaving the boiler, as shown below.

15 When the high temperature cooking solution has been fed in an amount corresponding to the capacity of the boiler liquid, pumping can be stopped. The amount of high temperature cooking solution can also be affected by the amount in the high temperature accumulator tank and how much can be pumped to balance the system. Due to the direct contact of the high temperature cooking solution with the chips in the cooking pot, the temperature of the chips rises close to the temperature of the high temperature cooking solution, for example to a temperature in the range of 143-149 ° C.

According to Figure 4, at the end of this step of the process, when the temperature of the chips has risen, the white liquor can be pumped from the hot white liquor storage vessel 62 to the bottom of the cooking pot by means of a pump 66. By appropriately adjusting the valve 30, the hot cooking solution is returned from the cooking pot to the high temperature accumulator tank 42, as shown in Figure 4. In a typical cellulose process, 50-70% of the boiler liquid draw volume is fed this time, as this is the amount of white liquor normally required to feed the reactive chemical required for extraction into the digester. The pumping is performed against a back pressure valve in the pipe 10 79724 46, whereby the pressure is maintained in the cooking pot to prevent the cooking solution from igniting.

In practice, in order to shorten the cooking time and adjust the concentration of the white liquor 5, the filling of the rest of the hot black liquor and the filling of the white liquor can be done simultaneously.

As can be seen with reference to Figure 1, hot black liquor from the high temperature accumulator tank can be pumped through the heat exchanger 54 to preheat the white liquor 10, which is pumped through the heat exchanger and passed to a storage vessel 52. through the heat exchanger 54, a steam-operated heat exchanger 82 can be used by circulating the white liquor through the heat exchanger by means of a pump 84.

In the process of the invention, the temperature of the white liquor in the storage vessel 62 may be in the range of 150-163 ° C.

When the boiler is now filled with a suitable amount of 20 white liquors and the boiler contents temperature is in the range of 150-163 ° C, the boiler contents temperature can be raised to a conventional cooking temperature, e.g. 165-177 ° C, using a steam-operated heat exchanger 86 and circulating the boiler contents. by means of pump 90.

At the end of the cooking step, the hot black liquor or spent cooking solution in the cooking pot is then removed from the cooking pot by pumping the filtrate stored at the bottom of the cooking pot from the filtration tank 32, as shown in Fig. 5. The cooking solution initially leaving the digester is fed to a high temperature accumulator tank at a temperature of about 165 ° C. The boiler is pressurized to prevent the cooking solution from igniting.

As previously mentioned, the displacement of liquid in the boiler 35 by pumping the displacing liquid to the bottom of the boiler is substantially quantitative and this can be seen by measuring the temperatures of the liquid η 79724 leaving the boiler. Thus, temperature measurements are made of the liquid leaving the digester by pumping the stored filtrate from the filtrate storage tank 32 to the bottom of the digester. Figure 6 shows the 5 obtained from these measurements and the results graphically. As can be seen from Figure 6, which shows the temperature of the solution leaving the boiler as a percentage of the volume of solution fed to the boiler as a percentage of boiler liquid uptake, up to the volume of pumped displacement remains essentially constant and only slightly below the cooking temperature used. When the volume of the displacement solution fed to the cooking pot reaches the liquid-absorbing capacity of the cooking pot, the temperature of the displacement solution drops relatively sharply. In the practice of the invention, the amount of filtrate pumped into the digester may be approximately 120% of the digestibility of the digester. The liquid, which is displaced by pumping into the cooking boiler a volume of liquid approximately equal to the liquid-absorbing capacity of the cooking pot-20, is led to a high-temperature accumulator tank. The remainder of the solution removed from the digester is passed to a low temperature accumulator having an average temperature of approximately half the temperatures of the filtrate used and initially of the displaced hot broth.

Once the high and low temperature solutions have been recovered, as described above, the extracted cellulose fibers in the digester can be removed therefrom in a number of ways. For example, and with reference to Figure 1, pressurized water vapor can be fed to the top of the boiler to increase the pressure in the boiler, (thereby an insignificant increase in temperature) and the exhaust pipe 96 is opened, transferring the contents of the boiler to a conventional ground well. Alternatively, the contents of the cooking pot 35 may be removed from the cooking pot using a rinsing liquid such as filtrate from a tank 32 fed through a pipe 98.

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Whatever procedure is used, once the cellulosic fibers have been removed from the digester, they are then washed in which the used water forms the first wash filtrate which is recovered in the filtrate storage tank 32. Since this wash-5 liquid contains warm cellulose (and warm water is used in the wash), the filtrate above.

In the illustrated embodiment of the invention, the solution fed to the high temperature accumulator tank is spent cooking solution displaced from the cooking device at the end of cooking. The solution fed to the low temperature accumulator tank is hot black liquor passing through a heat exchanger 54 and the filtrate used in the final step of displacing the cooking liquor from the digester. These sources and the cooling from the heat transfer to the chips cause the solution temperatures in the accumulator tanks.

Briefly, some of the advantages of the process and its variations presented can be used in batch-type lignin removal to use significantly less water vapor to perform the required heating than is required in a conventional process. This is due to the use of the hot, spent cooking solution removed from the digester as the main heat source in subsequent extraction charges.

Another advantage of the invention is the reduction in the amount of white liquor required to extract the batch of material. The residual amount of active chemical in the cooking liquor used and the amount of active chemical in the wash filtrate is returned to the system and this residual chemical is available to react with wood acids to neutralize these acids and neutralize other initial cooking reactions before lignin removal using new white liquor. As a result, cooking chemicals in white liquor are not required to perform these initial reactions. A slightly similar feature and advantage is the greater selectivity in lignin removal that occurs in the cooking pot during cooking. During cooking, the active alkali content is i3 79724, which results in a relatively rapid removal of lignin, which can be stopped before the useful cellulose decomposes. Special chemicals that are added to the broth to improve its properties can also be saved by returning them to the system.

Yet another advantage of the invention is the achievement of a significant washing of the cellulose due to the displacement of the cooking solution used in the cooking pot in the first washing of the filtered cellulose. In a new installation, this feature allows the cost of 10 wash installations to be reduced. In existing installations, a second washing step is permitted with its benefits.

When using conventional methods, the calcium contained in the wood tends to dissolve, causing an increase in the concentration of calcium ions in the cooking solution at the initial stage of cooking, whereby the load on the heat exchanger used to heat the charge is high. With a high hydroxyl ion content at this stage, calcium carbonate begins to precipitate, forming scale in filters and heat exchangers. In the present invention, calcium ions are formed upon contact of the chips with the black liquor before the white liquor is charged and before the hydroxyl ion content is high. When the white liquor is fed to the digester, the concentration of hydroxyl ions increases immediately and strongly and calcium carbonate precipitates in the 25 chip towers. At present, there is no load on the filters and heat exchangers and the deposition of calcium carbonate on the filters and heat exchangers is minimized. Thus, the invention allows the control of scale, which has been a difficulty in conventional methods.

Another advantage of the presented method is based on the use of a hot cooking solution under pressure to displace the second solution in the cooking pot. In this case, pressure saturation of the chips occurs due to the effect of the pressurized cooking solution.

Finally, it should be noted that a conventional plant can be relatively easily converted into a plant useful for the application of the invention. The use of the invention therefore makes sense for cellulose manufacturers who have used conventional equipment and who want to reduce energy requirements.

The invention has been presented in connection with a single cooking pot. Obviously, the operating principles of the invention can be applied to extraction processes using one or more cooking pots.

Although a particular embodiment of the invention has been described above, modifications and variations are possible without departing from the invention. The invention thus encompasses those modifications and variations that will be apparent to those skilled in the art.

Claims (5)

A process for batch boiling, wherein a digester is charged with cellulose material and boiling liquid and the batch is boiled at elevated temperature and overpressure to obtain in the digester a delignified pulp column and hot liquor, characterized in that after boiling and maintaining the cellulose material. the delignified pulp column substantially in undamaged state in the digester, the hot liquor is quantitatively suppressed by pumping into the digester a first volume portion of low temperature liquid having a volume approximately equal to the boiler's liquid capacity, leading to the hot liquor exceeding the boiler's elevation. temperature collector container substantially under overpressure, that liquid remaining in the digester is quantitatively displaced from the top end of the digester by pumping a second volume portion of said low temperature liquid, and while maintaining the hot exhaust underneath the overpressure, heat is transferred n cellulose material kit 20 to maintain and utilize the free heat of the hot liquor to preheat the second cellulosic material kit. 2. A batch cooking process according to claim 1, characterized in that the first volume portion of the low temperature liquid used to quantitatively displace the hot liquor is heated with the free heat in the cellulose material batch and is quantitatively displaced by pumping an additional volume of said liquid in the cellulosic material set to collect the free heat in the first volume portion of the liquid, and that the first volume portion of the liquid is transferred to another cellulosic material set to preheat this second cellulosic material set. A batch boiling method according to claim 2, characterized in that the first volume portion of the low temperature liquid is pumped to said second boiler for filling and pressurizing it and that hot exhaust is then pumped to the lower end of said second boiler for to displace the first liquid from the upper end of this second boiler. 4. A batch cooking process according to claim 1, characterized in that the first liquid, displaced from the second digester, is transferred from the digester system to a black liquor storage container, and that hot, fresh boiling liquid displaces the hot liquor which is transferred to the still liquor. another cellulose pulp kit for heating the same. 10 5. A process for batch cooking, characterized in that the low temperature liquid consists of filtrate obtained from pulp washing.