FI103815B - Power boilers with white liquor added to the washing circulation - Google Patents

Abstract

In the continuous kraft pulping of comminuted cellulosic fibrous material (e.g. wood chips) to produce paper pulp, viscosity for a given Kappa Number is significantly increased (e.g. at least about 20%) by adding kraft white liquor to the wash recirculation loop (38-40) in a continuous digester (19). Optimum results are obtained when kraft white liquor is also added to a recirculation loop (28-30) at a central portion of the digester, so that the majority of the digester has countercurrent flow between white liquor and cellulosic material. About 5-20% of the total white liquor necessary to effect kraft cooking is introduced in the wash circulation, and about 10-20% is introduced into the central portion recirculation. The temperature in the wash zone is maintained at about 140-175 DEG C. <IMAGE>

Description

103815

KRAFTKOKO, WHICH IS ADDED TO THE WASHING CIRCLE OF KRAFTKOKNING MED VITLUT TILLSATT TILL TVÄTTCIRKULATIONEN

The continuous process for producing kraft pulp, from which pulp 5 is produced from chopped cellulosic fibrous material (e.g. wood chips), has been greatly improved in recent years. In conventional continuous cooking systems, white liquor is added only to the return circuit between the digester and other parts of the system and / or to 10 absorption vessels. However, around 1983, it was found that the whiteness of the pulp is improved and the bleaching chemical is preserved if kraft white liquor is added to the process at several points, e.g. (and most of all) the liquor circulation in the middle of the continuous standing stove. The black liquor 15 is then removed by means of strainers between the upper and middle parts of the digester, whereupon a portion of the kraft white liquor flows upstream of the cellulosic material.

Although the improved method described above - marketed by 20 Kamyr, Inc. of Glens Falls, New York and Kamyr AB of Karlstad, Sweden under the trademark "MCC" - has resulted in a huge improvement in many commercial establishments where it has been applied since 1985, There are 25 more ways to improve kraft soup.

The continuous kraft cooking process has been greatly improved by the present invention, according to which a portion of the kraft white liquor is added to the washing cycle of the lower part of the digester so that the kraft white liquor flows into the fiber. upstream of the digester. The temperature of the wash zone is typically maintained at about 140-165 ° C.

Not only does the invention provide an improvement in pulp viscosity at a given kappa number (at least about 20%, 35 normally at least about 30%), but the digester is also easier to use (chip and lye levels are easier to control and the process progresses more smoothly). Although the desired result according to the invention is achieved only by adding 2,103,815 kraft liquor to the washing cycle, the most effective effect is also achieved by adding kraft liquor to the middle part of the digester (and other conventional objects in the process).

One embodiment of the invention is a continuous kraft cooking process wherein the chopped cellulosic fibrous material is cooked in a vertical digester having a top, a middle and a bottom. The process comprises the steps of: a) feeding the pulverized cellulose fiber material mixed with kraft white liquor into the top of a 10 digester; b) removing the black liquor with at least one strainer between the top and bottom of the digester; c) extracting the liquid from the first part of the digester and circulating it in the first cycle; d) adding the washing liquid to the bottom of the digester; e) removing fluid from the bottom of the digester 15 and circulating it in a second cycle, i.e. a washing cycle; f) adding the kraft white liquor to the second cycle, whereby a portion of the lye added to the circulation is upstream of the fiber and the amount of white liquor added is such that the viscosity and strength property of the pulp produced is improved compared to the pulp produced by the without step f); and g) removing the kraft pulp from the bottom of the digester. Step b) is preferably disposed between the top and center of the digester and step c) is centered on the digester, and the system further comprises step h) of adding kraft white liquor to the first cycle. The amount of kraft white liquor added to the second cycle is at least about 5%, preferably about 10-20% (e.g., 15%) of the total amount of white liquor needed for kraft cooking. In step: h) the amount of kraft white liquor to be added is at least about 10%, preferably 10-20% of the total amount of white liquor needed for kraft cooking.

Another embodiment of the invention is a kraft cooking apparatus for continuously cooking the ground cellulosic fibrous material. The apparatus includes a vertical cooker having a top, a middle and a bottom; means for supplying to the upper part of the digester 3 103815 pulverized cellulosic fibrous material mixed with white liquor; means for removing black liquor from at least one screen between the top and center of the digester; in the central part of the digester, means for extracting and circulating the liquid in the first cycle; means for adding washing liquid to the bottom of the digester; near the bottom of the digester, means for removing and circulating fluid in a second cycle, i.e., a washing cycle; means for removing kraft pulp from the bottom of the digester; and means for adding kraft white liquor 10 to said second cycle. Preferably, the apparatus also includes means for feeding white liquor to the first cycle. Each cycle includes an exhaust duct, a pump, a heater and a return duct. The white liquor is fed to the supply duct by the supply elements into the outlet before 15 pumps and the heater.

Another embodiment of the invention is another continuous kraft cooking process for cooking kraft pulp from chopped cellulosic fibrous material. This process comprises the following steps: a) feeding the pulverized cellulose fiber material mixed with the kraft white liquor into the top of the digester; b) removing the black liquor from at least one strainer between the top and center of the digester; c) extracting the liquid from the central part of the digester and circulating it for the first 25 cycles; d) adding Krafval white liquor to the first cycle; e) adding the washing liquid to the bottom of the digester; f) removing liquid from the bottom of the digester and circulating it in a second cycle, i.e. a washing cycle; (g) removing the kraft pulp from the bottom of the digester; and h) adding 30 kraft white liquors to the second cycle will increase the viscosity of the pulp produced by at least about 20% compared to the pulp produced by only steps a) to g).

It is a primary object of the invention to improve conventional kraft cooking methods.

. The invention will now be explained in more detail with reference to the accompanying drawings in which 4 103815

Figure 1 is a schematic side view of an exemplary kraft cooking apparatus according to the invention; Figure 2 is a graph showing viscosity versus kappa number when comparing the invention to prior art kraft cooking processes; and

Figure 3 graphically shows, as in Figure 2, a viscosity / -10 kappa ratio as a function of kappa number.

Figure 1 shows an exemplary kraft cooking apparatus according to the invention. The equipment includes e.g. the following standard equipment: a chips container 10 to which the chips or other 15 correspondingly comminuted cellulosic fiber material is fed from line 11. From the chips container 10, the fiber material is fed to the horizontal steaming vessel 12 and the vertical channel 13, where the mixture of fibrous material and kraft white liquor is fed to the impregnation vessel 16, if an impregnation vessel is connected to the system, and further along the pipeline 17 to the upper portion 18 of the continuous digester 19, the liquid and fibrous material separator 20 being preferably disposed an arrangement like that. The digester 19 also includes a central portion 21 and a lower portion 22.

Between the upper part 18 and the central part 21 of the digester 19, there are screen / screen screens 23 and a channel 24 for extracting the black liquor from the digester 19. The black liquor is typically passed through the channel 24 to the expansion vessel set 25 as is conventionally done. The central portion 21 of the digester 19 also has discharge outlets 35 that are connected via the outlet passage to the pump 28 and the heater 29 through which the discharged liquid is returned to the digester at the return passage 30, typically just above the screens 27. The bottom 22 of the digester 5 103815 is removed in the kraft liner along the channel 33 (a scraper or similar conventional device may be mounted on the bottom of the digester if necessary) and the washing liquor is supplied from the source 34 via the feed channel 35.

Near the bottom of the digester 19 is a washing screen 37 (row or rows) through which the fluid is discharged into the duct by pump 38 and further passed through the heater 39 to a return duct, from where it is fed back to the digester 10 at position 40 just above screen 37. From point 40 to the screens 23, the liquid flows upstream of the fiber as described by arrows 42, while above the screens 23, the liquid flows in a direction parallel to the fiber.

15

What has been described above and furthermore, the main channel 44 from which the kraft white liquor is fed through the channel 45 into the digester

of the central portion 21, is typical of TM

Kamynn MCC technology. The apparatus 20 according to the invention further comprises a channel 46 through which the main channel 44 communicates with a second return cycle, i.e. a wash cycle consisting of parts 37-40, to an outlet channel.

According to the invention, the amount of kraft white liquor added to the channel 25 46 is such that the viscosity and strength properties of the pulp produced are significantly better than that of the pulp produced without the addition of kraft white liquor to the channel 46. The amount of kraft white liquor and typically about 10 to 20% (preferably about 15%).

As is known per se, kraft white liquor is preferably added to channel 45 in addition to channel 46; the amount added to channel 45 is at least about 10% and preferably 35 is about 10-20%.

The apparatus shown in Figure 1 also has a return cycle arranged in the central portion of the infusion vessel 16 including a 6 103815 strainer 48, an outlet duct 49, a pump 50, and a return duct 51. Kraft white liquor is also preferably added from source 14 as indicated by 52, 53, 54; return channels, from the digester to the suction vessel 16 and from the suction vessel 16 to the high pressure feeder 15. In conventional continuous kraft cooking processes, most of the kraft white liquor used is normally fed through points or channels 52-54.

Figures 2 and 3 graphically illustrate some of the improvements achieved by the present invention over conventional ones. In Figure 2, the viscosity of three different major processes is plotted as a function of kappa number. The first process, described by line 55, is a conventional parent kraft soup. Another process,

, TM

described by line 56 is the Kamyr MCC system and process. Points 57 illustrate the results obtained with the invention. In the processes for which the results are shown in Figure 2, all parameters were substantially the same except for the addition of white liquor and, in accordance with the invention, the temperature of the wash zone increased by about 140 ° C.

175 ° C when used in conventional kraft soup and Kamyrin TM

The MCC process is 125-130 ° C. The conditions that exist in the processes for which the results are shown in Figures 2 and 3 are explained in the following table: 7 103815 TABLE 1

Soup Conventional MCC ™ Invention

Pre-steaming time, min 5 55 temperature, ° C 110 110 110

Absorption, liquid / wood = 4 white liquids for feed, 16.2 (90%) 11.7 (65%) 10.8 (60%)% NaOH from wood (% of total white liquor) time, min 30 30 30 temperature, ° C 120 120 120

Parallel flow phase white liquor to black liquor circulation, 1.8 (10%) 3.6 (20%) 2.7 (15%)% NaOH wood (% of total white liquor) highest temperature, ° C 170 170 165 total time, min 90 45 45

Intermediate white liquor,% NaOH wood - 2.7 (15%) 1.8 (10%) (% of total white liquor) highest temperature, ° C - 170 165 time, min - 60 60

Washing step white liquor,% NaOH wood - - 2.7 (15%) (% of total white liquor) highest temperature, ° C 135 135 165. time, min 180 180 180

In all soups, the total addition of effective alkali (white liquor) was 18% NaOH

wood.

8 103815

In a conventional kraft process experiment with direct 55 results, white liquor was added only in steps 53 and 54. In the experiments described by direct 56, white liquor was also added to channel 45. Now, in the 5 experiments of the present invention with 57 points, about 15% the kraft white liquor needed for kraft cooking was added to channel 46, about 10% to channel 45, and the rest in steps 53 and 54.

In the process of the invention, white liquor is normally added in step 53 to about 50-60% of the total amount of white liquor and about 10-20% in step 54 (about 10-20% to channel 45 and 5-20% to channel 46).

Figure 3 graphically depicts the viscosity / kappa ratio as a function of the kappa number from the same test results as Figure 2. Line 60 graphically represents the result of a conventional kraft soup, line 61 the results of the MCC ™ process and system and points 62.

As shown in Figures 2 and 3, the viscosity obtained according to the invention is at least 30% better than the best

TM

The viscosity achieved by the prior art system (MCC) is £ 2 and can be expected to be at least 20% better under almost any condition.

According to the invention, not only the viscosity of the pulp produced is increased but also the pulp strength properties (tear, tensile and puncture strength) are improved. In addition, the entire kettle is surprisingly easier to use, which is why it is advisable to apply the method according to the invention alone in practice.

Advantageous results are obtained according to the invention whether or not the system includes a suction cup 16, and most advantageous results are obtained according to the invention, 103815 whether or not white liquor is added to the channel 45. However, optimum results are obtained when the suction vessel 16 is connected to the system and white liquor is also added to the channel 45.

5

Thus, it will be appreciated from the above that a novel improved method and apparatus for cooking kraft pulp from continuous cellulosic material has been developed in accordance with the invention. Although the invention has been described and explained above in what is currently considered to be the most practical and preferred embodiment of the invention, the same principles can be applied to single-boiler hydraulic boilers and either single or double boiler steam / slurry boilers, and that the invention may be practiced in many ways without departing from the scope of the invention, which is to be interpreted as broadly as possible in accordance with the appended claims, so as to cover all such methods and structures.

20 '' i ««

Claims (11)

10 103815 A process for the continuous production of kraft pulp from a finely divided cellulosic fibrous material 5 using a vertical digester (19) having an upper portion (18), a lower portion (22) and a central portion (21), comprising at least the following steps: the upper part; (B) removing the black liquor from at least one of the screens (23) between the top and center of the digester; (c) removing and circulating fluid in the central portion of the digester in a first pass (28, 29), wherein the fluid circulated in the digester passes upwardly in the digester upstream of the flow of material 15; (d) adding the washing liquid (34) from the bottom of the digester; (e) removing the kraft pulp from the bottom of the digester; (f) removing and circulating liquid near the bottom of the digester in a second washing cycle (38, 39); (g) adding white liquor to the first cycle; characterized in that, after step (f), after step (f), kraft white liquor (14) is added to the second cycle, whereby the liquid to be recycled in the digester passes upwardly in the digester upstream of the flow of the material; and wherein the amount of white liquor to be added to the white liquor in the second cycle is sufficient to increase the viscosity and lumen properties of the pulp to be produced compared to the result of the same method using only the same material, same number and other parameters. Process according to claim 1, characterized in that step (h) is carried out by adding about 5 to 20% of the total amount of kraft white liquor used for making kraft pulp during step (h). A process according to claim 1, characterized in that steps (a) to (f) are carried out by adding most of the kraft white liquor in step (a) and adding at least 5% of the total amount of U 103815 kraft liquor used in kraft cooking to step (h) and in step (h) is about 140-175 ° C. Method according to claim 1, characterized in that it further uses an impregnation vessel (16) and a flow channel (17) arranged between the impregnation vessel and the digester, and that step (a) is carried out by adding kraft white liquor (14) to the flow channel and impregnation vessel. and that most of the kraft white liquor is added to the flow passage and the impregnation vessel, about 5-20% of the kraft white liquor used for kraft cooking is added in step (h), and about 10-20% of the used kraft white liquor is added in step (g). 15 Method according to Claim 1, characterized in that the liquid in both the first and second cycles is pumped (28, 38) and heated (29, 39) before being returned to the digester, and that steps (g) and (h) are performed by feeding the kraft white liquor 45, 46) to the return circuit prior to pumping and heating the recycled liquid. Method according to claim 1, characterized in that step (7) is carried out in such a way that the viscosity of the produced kraft pulp * 'increases by at least 20% with respect to the pulp produced using only the aforementioned steps (a) - (g). ). The method according to claim 6, characterized in that. this includes the step of maintaining the wash zone temperature at about 140-175 ° C. Method according to claim 1, characterized in that step (h) is carried out in such a way that the viscosity of the prepared kraft pulp is increased by at least 30% by a certain kappa number compared to pulp produced using 12103815 only from steps (a) to (g) by maintaining the temperature in the washing step at about 140-175 ° C and adding about 15% kraft white liquor in the flow passage (46), about 10% in the flow passage (45) and the rest in the flow passages (53 and 54). 5 An apparatus for continuously producing kraft pulp from a finely divided cellulosic fibrous material, the apparatus comprising at least: a vertical cooking vessel (19) having an upper portion (18), a lower portion 10 (22), and a central portion (21); means (17) for feeding the finely divided cellulosic fibrous material carried with the white liquor to the top of the digester; means (23, 24) for separating the black liquor from at least one screen (23) between the upper and the middle of the digester; means (27, 28) in the central part of the digester 15 for removing and circulating fluid in the first cycle; means (34, 35) for adding washing liquid to the lower part of the digester; means (33) for removing kraft pulp from the bottom of the digester; means (37, 38) near the bottom of the digester for removing and circulating fluid in a second wash cycle; and 20 means (14, 44, 45) for supplying white liquor to said first circulation, characterized by means (14, 44, 46) for supplying white liquor to said second circulation. Apparatus according to claim 9, characterized in that each circulation comprises an outlet channel, a pump (28, 38), a heater (39, 29) and a return channel (30, 40), and said means for supplying white liquor to each cycle comprises at least means (45, 46) for supplying liquor to said outlet duct prior to the pump and heater. 30th Device according to Claim 9, characterized in that it comprises at least an impregnation vessel (16) having an upper and a lower part and a flow channel (17) for connecting the lower part of the impregnation vessel to the upper part of the digester and (48-51) in the central portion of the impregnation vessel for removing and circulating the liquid in the first cycle and means (14, 52) for supplying white liquor to the third circuit.