FI70441C - FOERFARANDE FOER AOTERVINNING AV SEKUNDAERVAERME VID SATSVIS KOKNING AV SELLULOSA. - Google Patents

Description

70441

The present invention relates to a method for recovering the secondary heat of a pulp batch soup when the cooking solution at the end of the cooking cycle is fed from the digester to a separate pressurized surge tank.

Secondary heat recovery methods are already known, which are based on the recovery of heat from the expansion steam of the buffer tank or from the material streams taken directly from the digester. The disadvantage of these methods is the periodicity of the material and heat flows to the recovery devices, which makes it difficult to control the process and that the secondary heat is only available as hot water, which is often generated in excess of the process needs. In addition, in known methods, the periodically released odorous gases have to be equalized by means of an intermediate storage tank or by introducing leaking air into the system when no odorous gas is released from the process before being fed to the continuous treatment.

Finnish patent publication 18 429 discloses that in batch pulp cooking, the cooking solution is transferred to a separate tank at virtually the same pressure and temperature after cooking and that the heat content of the pulp remaining in the digester is transferred as reduced pressure steam to one or more steam accumulators. As the above publication does not show how the heat content of the cooking solution must be utilized in practice and how it is possible to carry out the digester butt, this method has not been applied in practice.

The importance of heat economy is also growing in the pulp industry and attempts have long been made to improve it, especially in the pulp batch cooking process. The present invention relates to the process mentioned at the beginning, which is characterized by a good temperature economy of 70,441 and a pressurized cold butcher of the digester, preferably in the range of 90-105 ° C, and in which the pulp can be prewashed in the digester.

The method according to the invention is characterized mainly in that the cooking solution or the steam released from the cooking solution is fed in a controlled flow from a surge tank for use in a cooking process or auxiliary processes or to a heat recovery unit for use outside its heat content and for the surge tank pressure energy to be used.

In the preferred embodiment of the invention, the hot cooking solution is fed from the surge tank in a controlled flow to a lower pressure expansion cyclone, which may be several. The expansion steam separated in the expansion cyclones can be used in the early stages of the cooking process as well as in separate auxiliary processes instead of fresh steam. Excess expansion steam is led to surface condensers, i.e. a heat recovery unit where hot water is produced. Depending on the expansion steam stream, the released odorous gases and methanol vapor can be fed directly to the odorous gas treatment through a known multi-stage surface cooling process due to the flow controllability.

The free cooking solution contained in the digester is sucked out through a strainer at the bottom of the digester. After emptying the cooking liquor, the secondary heat contained in the broth remaining in the chips in the digester is recovered by a down-gasification technique known per se. The pressure in the kettle is reduced to approximately atmospheric pressure. The released secondary steam is led to the heat recovery unit together with or instead of the steam stream from the last expansion cycle. A solution is introduced into the expansion cyclone from the surge tank, adjusted so that the combined steam flow remains substantially unchanged.

The kettle is then filled with the filtrate from the scrubber and emptied using the steam pressure of the surge tank. The use of a leach filtrate provides one pre-slurry step.

The wash filtrate is fed to the digester at a relatively low temperature so that the mixture temperature of the filtrate and the pulp becomes 105 ° C or lower. The contents of the digester can thus be buffered as a cold buffer at a temperature of 90-105 ° C.

It is advantageous to feed the leach filtrate through the bottom sieve of the digester in order to obtain a liquid effect and its even distribution. During the butt, it may also be advantageous to feed a controlled filtrate stream through the bottom screen to the lower cone to reduce the flow resistance of the pulp and thus facilitate emptying of the digester.

When disturbances occur in a pulp mill, e.g. in a cooking plant, which momentarily reduce steam consumption, according to the invention it is possible to direct fresh steam from steam boilers to a surge tank and thus prevent steam blowing out in recovering minor balance disturbances.

The method according to the invention achieves numerous advantages in terms of plant steam consumption, process, pulp quality, pulp washing efficiency and environmental protection, as can be seen from the following presentation.

Factory steam consumption benefits

Theoretically, 100% of the latent heat of cooking is available as adjustable secondary steam. Compared to a conventional heat recovery system, greater freedom of choice is achieved in the utilization of secondary heat. In addition, the secondary heat is recovered at a higher energy level than before and the variations in the product steam flow to the digester can be reduced.

process Benefits

In general, the best features of a continuous and batch cooking process are combined.

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The secondary heat flow from the digester can be connected to various auxiliary processes with greater certainty, as they can no longer be directly affected by disturbances in the main process.

Better pulp quality

The method allows the digester to be pushed at a low temperature of 90-105 ° C. This so-called the cold butt reduces the shattering damage to the fiber wall that occurs in a conventional butt when the liquid inside the fiber evaporates vigorously in a rapid pressure drop. Thus, the strength properties of the pulp are improved.

The variation in the number of pieces is reduced, because it is even more economical to use a high liquid / wood ratio in cooking.

In the event of malfunctions in the pulp processing units (laundry, bleaching mo), this method opens up the possibility of allowing the finished soup to wait in the digester without reducing the number of pieces. The degassing of the digester to near atmospheric pressure and the supply of the leach filtrate lower the temperature of the pulp and at the same time the rate of cooking reactions to a level that no longer affects the number of pieces of pulp.

Better washing performance

The free cooking solution, in which the concentration of cooking chemicals and wood-dissolved ingredients is high, is removed from the digester and replaced with a washing filtrate in which the concentration of said ingredients is lower. The mixing of the filtrate and the pulp takes place partly in the digester, the buffer tube and the buffer tank. Thus, the mass flow leading to the washing is more dilute with respect to the above-mentioned ingredients and, as a result, the washing result is better. In addition, the diffusion of chemicals from the cooking solution absorbed into the fibers to the more dilute wash filtrate takes place in the buffer tank.

In the filter washer, the temperature of the wash water can be further raised without a greater risk of the liquid boiling in the suction foot of the first filter: the temperature of the basic broth coming as a filter is lower than before.

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Better environmental protection

The kitchen's odor gases are in a closed system at all times and can be routed to a continuous controlled flow.

The process according to the invention will be described in more detail below with reference to the accompanying drawings, in which Figure 1 shows a temperature / time diagram of sulphate pulp cooking with conventional heat recovery, Figure 2 shows a corresponding process in the process according to the invention and Figure 3 schematically shows a plant for carrying out the process.

The conventional method consists of the following steps: 1 Filling the A - B chips 2 B - C steaming 3 C - D feeding the cooking solution 4 D - E heating with low pressure steam 5 E - F heating with high pressure steam 6 F - G cooking time 7 G - H down-gasification 8 H - I buffer A - A 300 minutes including 40 min Interval.

Figure 2 shows how the new method can be implemented in a plant of the type shown in Figure 3. The plant consists of several cooking pots, which are usually about 8-12 pieces, of which only one boiler marked with the number 1 is shown. The circulating heating is arranged by means of a heat exchanger 2 provided with a circulating pump 3. External steam is supplied to the heat exchanger 2 via a line 4.

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The circulating cooking solution is returned to the boiler 1 via a line 5 with a valve 6. The line 5 communicates via a line 7 with a valve 8 to a surge tank 9, from which a line 10 with a valve 11 leads back to the boiler 1. From a boiler 1 a line 12 leads to a buffer tank shown. Line 10 communicates with line 14 for the supply of fresh cooking solution. This line is provided with a valve 15. A line 16 provided with a valve 17 is connected to the circulation line via a heat exchanger 2 and to a screen at the bottom of the boiler. The bottom of the surge tank 9 is connected via a line 18 partly via a line 19 to the first depressurization cyclone 20 and partly via a line 21 to the depressurization cyclone 22. The bottom of the depressurization cyclone 20 is connected via a line 23 to the depressurization cyclone 22. from the top of boiler 1. Lines 24, 25 are also connected to other boilers not shown. The steam outlet of the depressurization cyclone 22 is connected via a line 27 to a line 28 coming from the top of the boiler 1 and provided with a valve 29. The line 28 leads to a heat recovery unit 30 from which odorous gases escape via line 31. A line 32 leads from the liquid outlet of the depressurization cyclone 22 to the black liquor tank. From the top of the surge tank 9, a line 34 with a valve 26 leads to the top of the boiler. The top of the boiler is connected to line 28 by valve 35. Line 36 with valve 37 is connected to line 16. Line 19 is provided with valve 38. Line 23 is provided with valve 39. Line 21 is provided with valve 40.

The plant operates as follows: 1 A - B chip filling 2 B - C steaming 3 C - D cooking solution supply 7 70441 4 D - E heating with low pressure steam and steam from the depressurization cyclone 5 E - F heating with high pressure steam 6 F - G cooking time 7 G - H cooking solution emptying 8 H - I down-gasification to the recovery system 9 I - J down-gasification to the heat recovery unit 10 J - K washing solution supply 11 K - L pressure increase for boiler butt 12 L - M cold butt 13 M - N final gasification A - B Chip filling is started by opening the boiler valve and a chip chipper valve as well as starting the chip feed. Fresh steam or secondary steam from line 24 is passed to a wood chipper. When the correct chip level is reached in the boiler, the valves are closed.

B - C During steaming, the chips are preheated in the boiler by supplying expansion steam from the depressurization cyclone 20 through the valve 33. At the same time, air is removed from the boiler through the valve 37 of the line 36. Preheating with steam from the depressurization cyclone 20 is continued until the desired temperature or pressure is reached in the boiler, after which the valve 33 in the line between the boiler and the cyclone is closed. The temperature control closes the evacuation valve 37 when the steam flow reaches the control point in the line 36.

C - D The cooking solution is fed to the bottom strainer of the boiler. To increase the feed rate, the cooking solution can also be fed through a central screen. The supply of black liquor to the cooking solution can advantageously take place directly via a line 10 with a flow regulator from the cooking solution surge tank. This measure saves fresh steam.

When a sufficient amount of cooking solution has been supplied via line 14 and 8 70441 via line 10, the supply valves 11 and 15 are closed. Valve 17 is closed.

D - E The broth circulation pump 3 is started. Indirect heating of the cooking solution with fresh steam is started by temperature control, which senses the removal temperature of the cooking solution from the calorizer 2. Valve 6 is open and valve 8 is closed.

At the same time, steam is supplied directly to the boiler from line 24 through valve 33. The supply is stopped automatically when the boiler pressure has risen close to the pressure of the depressurized cyclone.

The inert gases that accompany the expansion gas from line 24 to the boiler are removed from the boiler through a turpentine gasification line, the valve 35 of which is kept slightly open during heating.

E - F The final heating to the cooking temperature takes place indirectly in the calorizer by means of fresh steam.

F - G Soup.

G - H The cooking medium is emptied of free cooking solution (part of the cooking solution is bound to the chips) during the actual cooking time by means of a cooking solution circulation pump. The valve 6 of the cooking solution circulation line is closed and the valve 8 of the line leading from the heat exchanger 2 to the cooking solution expansion tank and the valve 17 on the suction side of the pump 3 are opened. Drainage is automatically interrupted when a level switch in the boiler at a suitable height closes the valve 8 of the line leading to the equalization tank and stops the circulation pump.

H-I The boiler pressure is reduced by opening the valve 33 in the line 25 leading from the boiler to the line 24. The steam valve 33 is controlled by a pressure control which first closes the valve 38 704 located in the pipeline leading from the surge tank to the cyclone. This is done to compensate for the load peak caused by the steam supplied from the boiler.

I - J When the boiler pressure has dropped to the same level as the pressure of the cyclone 20, the valve 33 of the line 25 leading from the boiler closes automatically and the cyclone 20 is again supplied with liquid from the surge tank.

The final pressure reduction of the boiler takes place by degassing by means of a valve 29 to the heat recovery unit for hot water production. The change in the degassing load from the boiler is measured by a flow meter in line 28. The variation of this steam flow is compensated primarily by changing the liquid flow to the depressurization cyclone 22 by valve 40, which produces so much steam that the sum of steam flows in lines 27 and 28 is substantially constant. The steam flow to the heat recovery unit for hot water production is measured with a flow meter.

If the throttling of the valve 40 is not sufficient, the valve 29 is throttled as a secondary to provide a constant total flow to the unit 10. If time so requires, final gasification can take place simultaneously through valves 33 and 29. In this case, the valve 33 is controlled by the pressure in the line 24. Valve 29 is controlled by the desired sum flow of lines 27 and 28.

J - K During degassing, a washing solution (filtrate from the washing machine) is fed to the boiler through valve 15. Valve 17 is closed. The washing solution is pumped in through a bottom sieve, and the flow is stopped by a level regulator placed at a suitable level. The valves 35 and 29 in the line for degassing to the heat recovery unit are also closed.

K - L The boiler is pressurized by opening the valve 26 in the cooking solution surge tank.

10 704 41 L - M The soup is pushed from the kettle into the buffer tank. When the entire boiler has been buffered, the valves 13 and 26 of the buffer pipe and the line coming from the surge tank are closed automatically.

M - N The boiler is degassed once more to the heat recovery plant by valve 29. When the boiler is at atmospheric pressure, the boiler lid opens and the boiler is ready for the next cooking cycle.

The depressurization cyclone 20 acts as an integrated part of the cooking system. It enables the recovery of down-gasification vapors and compensates for momentary final gasification peaks and steam demand peaks in the cooking compartment.

The energy flows leaving the basic system are measured by three flow meters located in lines 28, 27 and 23. The data from the flow measurement are fed to a control system which primarily controls the valve 40 and secondarily controls the gasification valves 29 and 35.

Claims (3)

11 70441 A method for emptying a cellulose batch digester from pulp, wherein the cooking liquid at the end of the cooking cycle is passed to a pressurized surge tank and degassed until a substantially atmospheric pressure or slightly higher pressure prevails therein, characterized in that during said degassing the filter is preferably to prewash the pulp at a relatively low temperature, preferably such that the temperature of the mixture of washing liquid and pulp becomes 105 ° C or lower, after which steam is passed from the surge tank to the digester to blow out its pulp content at 90-105 ° C, which means so-called cold blowing. A method according to claim 1, characterized in that at the end of the cooking cycle, the free liquid of the chain is sucked out through a strainer at the bottom of the digester. A method according to claim 1 or 2, characterized in that after the free liquid has been sucked out of the digester, the washing liquid is introduced into the digester through a sieve at the bottom of the digester to distribute the washing liquid evenly to the bottom of the digester.