FI80085B - FLERSTEGSKOKNINGSFOERFARANDE FOER CELLULOSA MED EN DISKONTINUERLIG KOKARE. - Google Patents

Description

1 80085 MULTI-STAGE 3ELLULOSE COOKING METHOD BREAKER WITH COOKER

The present invention relates to the chemical cooking of wood chips to produce a pulp suitable for papermaking, and to the successive use of different cooking solutions in the same cooker for predetermined cooking times to increase the efficiency of the process and to save energy.

Different types of methods for cooking wood chips are highly known in papermaking. Cooking usually takes place in a cooker specially designed for this purpose, which is filled with wood chips, which are usually compacted in it. Hot solutions of sodium hydroxide alone or of sodium hydroxide and sodium sulfide are then charged to the digester. The temperature of the digester can be controlled by introducing steam and after the chips have been kept in contact with the digestion solution for a predetermined period of time, the digester blow valve can be opened to discharge the contents into the pulp pit.

There is a considerable amount of heat loss in the batch cooking process and although several systems have been proposed to minimize this heat loss, none of them are particularly effective. A few papermakers have switched to continuous cooking methods to improve cooking efficiency, but the equipment costs of a continuous cooking system are very high.

Recently, an improved type of dosing method has been developed to eliminate these difficulties. In this new method, the wood chips are boiled in a digester and the hot black liquor formed is removed by replacing the filtrate from the washing section. This filtrate is added to the bottom of the digester and pushes the hot spent solution through the chip column without substantially mixing the two liquids. The replaced hot black liquor is then directed to a pressurized collector. The digester is emptied by adding 80085 to the top of the digester, forcing the pulp out through the blow valve into the pulp pit. After the mass is blown from the digester, it is filled evenly with chips.

S Hot black liquor is pumped from the collector to the bottom of the digester, where it heats the chips. At this point, the excess black liquor is used over the capacity of the digester so that the excess black liquor is removed from the top of the digester and transferred to the dilute black liquor tank. The fresh white-10 head is then used to transfer the black liquor from the bottom of the digester and the resulting spent solution is then transferred to the dilute black liquor storage space. The contents of the digester are then heated with steam to the desired temperatures and the required cooking times are stored. When the contents of the kettle have reached the cooking temperature, the steam supply is stopped.

After boiling, the hot solution is removed as in the initial step and the cycle starts again. This type of process is disclosed in Fagerlund U.S. Application No. 434,758, filed October 18, 1982, and assigned to the owner of the present application.

The present invention provides a multi-stage method of cooking wood chips using a single digester, in which the wood chips are introduced into the digester and soaked in warm black liquor to remove air for the most part from the digester and the chips. After a suitable soaking time, the warm black liquor is replaced from the digester by a mixture of the first hot black liquor and the hot white liquor with a relatively high relative amount of white liquor. The temperature of the contents of the digester is raised to the cooking temperature by circulating the contents, usually through a heat exchanger, through which steam is added. After a suitable cooking period, which is the longest in the multi-stage process, the digester solution is replaced by a mixture of another hot black liquor and a hot white liquor. This mixture has a lower relative amount of 35 hot white liquors than the first n 3,80085 solution. The temperature of the digester is again raised to the cooking temperature, and when the chips have reached a predetermined degree of cooking, the solution is replaced in the digester with a liquid filtrate derived from the washing of the pulp. Finally, the contents of the digester si-5 are emptied by applying gas pressure to the inside of the digester.

More generally, the present invention relates to a multi-stage cooking process for wood chips, in which the chips are sequentially cooked in a digester using soups, C2, ........

10 sets. The cooking is performed with a liquid with a relatively high amount of white liquor, and with a relatively long cooking time T ^. Cooking C2 is performed with a solution L2 with a lower relative amount of white liquor than the liquid L2, and its cooking time is shorter than T1. The subsequent soups up to cooking C are carried out successively with lower relative amounts of white liquor and successively with shorter times. Only two steps can be used, but three steps are preferred. If necessary or desired, more than three 20 steps can be used.

The following conditions can be used in a three-stage softwood chip cooking process using a sulfate cellulose process. The total white liquor can typically be about 25% of the digester's liquid capacity minus the amount of chips in the digester. The first cooking is performed with an amount of white liquor of 50 to 75% of the total white liquor over a period of 25 to 40 minutes, the second cooking is performed with an amount of white liquor of 10 to 30% of the total amount over a period of 10 to 20 minutes, and the third cooking is performed. with an amount of white liquor that is 5 to 20% of the total amount of white liquor, and this takes 5 to 15 minutes.

The only figure in the drawing schematically shows an installation for carrying out the multi-stage wood chip cooking process according to the present invention.

μ 80085

In the accompanying drawing, reference numeral 10 is used to denote a generally conventional kettle with a removable lid 11. The contents of the digester can be heated to the digester by pumping it through a pump 12 5 and a valve 13 through a heat exchanger 14 with a steam inlet pipe 15 and a steam condensate outlet pipe 16.

The warm solution collector 17 stores the black liquor at a relatively low temperature. This warm black liquor, which is at a substantially lower temperature than that required for cooking, is initially pumped by means of a pump 18 through a pipe 19 controlled by a valve 20 to the bottom of the digester 10 via an inlet valve 21.

The system also includes a first hot black liquor collector 22 that contains liquor for the first cooking step. This relatively high temperature black liquor is pumped through pump 23 and then through valve 24, valve 21 and to the bottom of the digester. The hot white liquor collector 25 serves as a storage vessel for fresh hot white liquor which is pumped from the collector 20 by a pump 26 through a flow controller 27 and then connected by a pump 23 to the liquor pumped from the first hot black liquor collector 22. The flow controller 27 can be used to adjust the relative proportions of the hot white liquor and the hot black liquor in the first cooking step.

The hot white liquor, after being passed through the inlet pipe 28, is preheated by a heat exchanger 29 before it enters the hot white liquor collector 25. The heat exchange is performed by removing part of the hot black liquor 30 from the collector 22 through the pump 30 and the pipe 31. This hot black liquor is also used as a heat exchange fluid for another. - for the heat exchanger 32, which is explained below.

The hot black liquor passing through the two heat exchangers 29 and 32 is removed as hot but not hot liquid 35 through line 33, after which it is supplied to the hot liquor collector 17. Periodically the warm black liquor is removed from the collector 17 via line 66 and transported to the black liquor evaporator. .

5 80085

The second hot liquor collector 34 is used to store black liquor for the second soup. Pump 35 conveys a stream of hot black liquor at a lower temperature than the hot black liquor in collector 22 to the hot white liquor from collector 25. The hot white liquor is supplied by the pump 36 through the flow regulator 37, where it is connected to the discharge volume of the pump 35, the combined discharge volume then passes through the valve 38 and to the bottom of the digester through the valve 21. The relative proportion of hot white liquor in this cooking is lower than that used in the first cooking, and the cooking-15 temperature is lower. The steam is optionally fed to a second hot liquor collector 34 via a steam pipe 39.

A third hot liquor collector 40, which contains hot black liquor for a third cooking, is provided with a pump 41 for discharging its contents. A pump 42 connected to the hot white liquor 20 collector 25 supplies a metered amount of hot white liquor through a pipe 43 and a flow controller 44 which mixes with the hot black liquor which is pumped through the pump 41. The combined flow passes through valve 45 and through valve 21 to the bottom of the digester 10. The combined flow of the third broth has a lower white liquor concentration than the previous broths and is at a lower temperature.

The pulp washing filtrate recovered from the second part of the papermaking mill (not shown) is fed to a collector 46 via a pipe 30 47. The pump 48 is arranged to supply the filtrate through the valve 49 and to the bottom of the digester through the valve 21. A portion of the filtrate in the collector 46 may be pumped by the pump 50 through a tube 51 and then through a heat exchanger 32 where it passes in a heat exchange relationship 35 with 6,80085 hot black liquors from the first hot liquor collector 22. This preheated filtrate is then directed to a hot filtrate collector 52. Pump 53 delivers the heated filtrate through valve * 49 to digester 10.

Finally, an outlet valve 54 is provided to drain the contents of the digester 10. For this purpose, air or other liquid is fed through the inlet pipe 55 at the end of the cooking, after which the valve 54 is opened and the contents of the digester are transferred to the pulp pit or other container 10 through the outlet pipe 56.

The various cooking liquors are then returned to the collectors at the end of the individual cooking steps. Tube 57 and valve 58 are used to return dilute cooking liquor to the hot liquor collector 17. Tube 59 and valve 15 are used to supply hot black liquor to the first hot liquor collector 22. Similarly, tube 61 and valve 62 return liquor from the boiler 10 to the second hot liquor collector 34. Material is recycled back to the third hot liquor collector 40 by means of pipe 63 20 and valve 64.

The process of the present invention can be used in a modified sulfate or soda cellulose process. The multi-stage system preferably consists of three stages, as shown in the drawing, but two or more stages than three may be used. Multi-stage cooking removes non-cellulosic material from wood chips in a manner that improves pulp yield, improves pulp quality as measured by average molecular size, and improves pulp whiteness. In addition, this new process allows for a more complete removal of lignin from the pulps produced for bleaching further processing and thus requires a milder bleaching treatment using smaller amounts of bleaching chemicals. Another advantage of this invention is that most

II

7 80085 of pulp washing to remove spent cooking chemical and dissolved organic matter is performed in a digester.

The time, temperature and concentration conditions of the active cooking chemical can be adjusted within reasonable limits 5 between the cooking steps of the wood chips used to optimize the desired pulp properties. This feature provides greater flexibility in cellulose cooking.

The following explanation is provided to illustrate the entire process cycle.

10 The empty digester 10 is filled with wood chips by removing the lid 11. These chips can be compacted to increase the number of loaded chips, and to provide a more uniform chip tightness. It is preferred that chips that are too thick (more than 6 mm) be removed from the chip storage.

15 When the digester 10 is closed, the warm black liquor is pumped from the collector 17 by means of a pump 18 to the valve 21 of the pipe 19 on the bottom of the digester, which is substantially filled with chips. The kettle is completely filled with this lye and filled with a small excess. The excess leaves the digester 20 through a separator grid (not shown) located in the cover of the digester 10. Excess lye is returned to the hot liquor collector 17 through line 57. This initial soaking with warm lye at a temperature well below cooking temperatures removes 25 air from most of the digester and chips, heats the chips, and neutralizes a few organic acids associated with the wood chips. Excess dilute black liquor that develops in the cooking and washing system is periodically removed to the black liquor evaporator through an outlet pipe 66 30.

The hot black liquor of the first hot black liquor collector 22 and the hot white liquor of the hot white liquor collector 25 are pumped together by means of pumps 23 and 26, respectively.

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β 80085 through valve 2 4 to the bottom of the digester, which is now filled with warm black liquor. The replaced warm liquor exits the digester through a separation screen located in the digester cover and is returned to the warm black liquor 5 collector 17 via a pipe 57.

The liquor of the first hot black liquor collector 22 is not only used to fill the digester 10, but is also used to preheat fresh white liquor in the heat exchanger 29 and also to preheat the first 10 portions of the wash filtrate in the heat exchanger 32. The black liquor exiting the two heat exchangers passes through the hot liquor

The hot white liquor coming through the pipe 28 is also heated by a heat exchanger 29 before it enters the hot val-15 collector collector 25.

The temperature of the contents of the digester 10 filled with the mixture of hot black liquor from the first hot liquor collector 22 and the hot white liquor from the collector 25 is raised to the desired cooking temperature by circulating the contents of the digester 20 through valve 13 and heat exchanger 14 by pump 12. Forced recirculation of digester liquor is preferred to ensure uniform temperature and chemical distribution in digester 10. In the sulfate-cellulose cooking process for a three-step cycle, the first cooking can take place with a liquor containing 50-75% of the total white liquor used and a cooking time of 25-40 minutes. No soup in this multi-step process requires more than 50 minutes of cooking.

At the end of the desired first stage cooking time, 30 hot black liquors are pumped from the second hot liquor collector 34 and the hot white liquor from the collector 25 to one of the pumps 35 and the like. 35 using the bottom of the cooker 10. The proportioning of the relative amounts is performed by the flow controller 37. This second cooking liquor contains a smaller relative amount of 9,80085 white liquors and uses a shorter cooking time than the first cooking. Typically, the second cooking is performed with white liquor, which makes up 10 to 30% of the total white liquor, within 10 to 20 minutes.

5 The kettle is brought to the cooking temperature by circulating the lye through the heat exchanger 14.

Alternatively, steam may be added to the second hot liquor collector 34 through the steam pipe 39.

At the end of the desired second stage cooking time, the hot black liquor from the third hot black liquor collector 40 and the hot white liquor from the pump 42 and flow controller 44 are combined and pumped together through valve 45 to the bottom of the digester 10. The replaced black liquor exits the digester 10 and passes through the put-15 ken 61 and valve 62 to another hot blackhead collector 34.

The temperature of the contents of the digester filled with hot black liquor and hot white liquor is raised to the desired cooking temperature by circulating it through the heat exchanger 14.

Alternatively, the heat exchanger can be eliminated and steam can be sprayed directly into the circulation tube, whereby the contents of the digester are removed from the top and pumped to the bottom by means of a pump 12. Alternatively, steam may be added to the third black liquor collector 40.

Under typical conditions of the third cooking cycle, the proportion of white liquor is 5 to 20% of its total amount and the cooking time is 5 to 15 minutes.

At the end of the desired third stage cooking time, the filtrate from the pulp wash is pumped to the bottom of the digester. The replaced black liquor exits the top of the digester and passes to a third hot black liquor collector 40 through line 63 and valve 64.

10 80085

The first portion of the filtrate is pumped from the collector 46 by a pump 50 to a heat exchange ratio with the hot black liquor circulating through the heat exchanger 32. Preheating the first portion of the filtrate reduces the total amount of steam mi-5 required in the cooking system, allows the wash water added to the liquor system to be used efficiently with countercurrent flow, and maintains a low concentration of black liquor in the final cooking steps.

After replacing the hot black liquor from the third stage boiler with a scrubbing filtrate, compressed air is passed through line 55 to the top of the digester and the contents, pulp and scrubbing filtrate are forced out of the digester bottom through valve 54 into a suitable storage vessel or pulp pit via line 56. The removal of chips by means of an air curtain is described in more detail in co-pending U.S. Application Ho 402,636, filed July 28, 1382.

Typical cooking conditions for a three-stage softwood pulp cooking process using a sulphate process are shown in the following table: li n 80085% of total max. Mass final white cooking boiler final lye liquor cooking liquor heating capacitor step return% (min.) Oq I 69 50 171 100 25 II 22 30 170 40 12 III 9 25 168 25 4

The present invention allows the use of relatively short total cycle times and thus improves mass production rates. Typical times for different operations in the three-step process of the present invention are:

Action time_min.

wood chip filling 15 hot lye filling 20 first hot lye filling 15 10 first cooking stage 35: second hot lye filling 15 second cooking stage 15 • | third hot liquor filling 15 third cooking stage 10 15 hot liquor replacement 20 blowing 15 spare 5 full digester cycle time 180

Although the drawing shows a three-step process and this is the preferred embodiment, the invention is more general than this. In essence, the invention comprises a multi-stage cooking process for wood chips, in which the chips are successively cooked in a digester for soups C ^, ........

series. Cooking is performed with a lye with a relatively high relative amount of white liquor and a relatively long cooking time T 1. The cooking is carried out in a lye with a lower relative amount of white liquor than in lye L 1, and it takes less than ΤΊ. Subsequent soups up to cooking C are performed successively with lower relative amounts of white liquor and successively with shorter times.

The total amount of white liquor used in the digester is determined by (1) the degree of defibering or lignin removal desired, (2) the amount of chips filled on an oven-dry basis, and (3) the concentration of active cooking chemicals, sodium hydroxide and sodium sulfide in the white liquor.

For example, it has been found that an active cooking chemical, expressed as sodium oxide, is required to provide 15% of the lignin in the oven to provide a properly lignin-free pulp. A 170,000 liter digester containing 27,216 kilograms of oven-dried wood requires 15% 15,272,200 kilograms or 4,082 kilograms of active alkali. The amount of val alkali found by the test to contain 2,72 kg of active alkali per 28,3169 liters and required for the batch is then calculated to 42,475 liters.

In the example above, the entire amount of white liquor is added to the batch during initial filling in a conventional batch cooking system. In the new multi-stage process, assuming the same use of total white liquor, in a three-stage system the application could be as follows: 2 5 stage i_lS2lS2_i ^ Yt0sta I 28,317 liters 66.7% II 8,495 "20.0% III _5i66 3" _13_, 3_% total 42,475 liters 100.0% It will be appreciated that various modifications may be made to the embodiments shown without departing from the present invention. framework of the invention.

Claims (8)

1. Wood chip cooking method of batch type comprising the steps of - introducing wood chips into a digester, 5. soaking the chips in said digester with a hot black liquor at a temperature significantly below that required for boiling to remove air, - displacing the hot black liquor from the digester with a preheated cooking liquor, 10 - increase the temperature of the cooking liquor in the digester to the boiling temperature, - when the chips have reached a predetermined degree of boiling, displace the liquor in said digester with a liquid-filtered extract from pulp washing and recover some of the heat of the the offset, hot black liquor having a temperature near the boiling temperature by heat exchange with the white liquor used as a cooking liquor, and - discharging the contents of said cooker by applying gas pressure to the interior of said cooker, characterized in that the cooking of the chips the cooker is carried out in a series of cooks C1, C2> ... at - each of the said liquors is a mixture of hot white liquor and a hot black liquor, and - the first boil C 2 is carried out with a liquor L 2 with a proportionately high proportional amount of the total white liquor required to complete the the second boiling is carried out with a lye having a proportional amount of the white liquor less than and for a time T2 shorter than »and - the subsequent boiling to boil Cn is carried out with successively less proportional quantities of white liquor and gradually shorter times. ie 80085 2. A method according to claim 1, characterized in that the cooking of the chips is carried out in three boils Cp C2 and C2. 3. A method according to claim 1 or 2, characterized in that each of the cooks Cp C2, ... Cn is carried out in less than 60 minutes. Method according to any of claims 1 to 3, characterized in that the white liquor used in the first boiling C1 is in the range of 50 to 75% of the total amount of white liquor required to complete the desired total boiling, in the second one. the boil C2 is in the range of 10 to 30%, and in the third boil C3 is in the range of 5 to 20%. 5. A process according to any one of claims 1 to 4, characterized in that the first cooked for a period of 25 to 40 minutes, the second cooked C2 for a period of 10 to 20 minutes and the third cooked. C3 stir for 5 to 15 minutes. 6. A process according to any one of claims 1 to 5, characterized in that the hot white liquor is preheated by heat exchange with the hot black liquor from the cooker C, | . Process according to any of claims 1 to 6, characterized in that the liquid filtrate used to displace the liquor in the digester is preheated by heat exchange with the hot black liquor from the boiler C Method according to any one of claims 1 to 7, characterized in that the raising of the boiling liquor Lp L2 ... temperature in the digester to a boiling temperature is carried out by circulating the contents of said digester to heat exchange exchange with a meadow. t