JPS6221917B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for secondary heat recovery in batch cooking of cellulose in a digester, in which during the final step of the cooking cycle the heat treatment solution is placed in a balance tank under pressure. be guided.

Secondary heat recovery methods have long been known in the pulp industry. These methods are based on recovering heat from the expanding steam or material stream from the blowing tank, which originates directly from the digester. Disadvantages of these methods include the discontinuity of material and heat flow to the heat recovery system, which means that the process is difficult to control.
More obviously, secondary heat is obtained in the form of hot water, usually in excess of that normally used in the plant (process). And because of the discontinuous nature of the material and heat flows in the process, post-treatment requires a storage tank for the malodorous gases released in the system. If the emission of dirty gases is controlled by supplying air to the system, the emission of dirty gases can be carried out continuously in the case described above.

Finnish Patent No. 18429 discloses a cellulose cooking method, in which the heat treatment solution is heated under approximately the same pressure and temperature as that prevailing in the digester after cooking. The steam remaining in the pulp remaining in the digester is also directed to one or more steam storage units. There is no disclosure of how the heat capacity of the heat treatment solution is actually used in this method, and this method has not been put to practical use.

Thermal economy is becoming increasingly important in the pulp industry, and numerous efforts have been made to improve it, particularly in the kraft pulp industry. The object of the invention is also to provide a process of the type mentioned at the outset, which is characterized by good thermal economy and which allows the pulp to be blown at relatively low temperatures, preferably in the range from 90°C to 105°C. This allows a so-called "cold blowing" process to be carried out, allowing the pulp to be pre-cleaned in the digester.

The method of the invention provides that the heat treatment liquid or the vapor expanded from this heat treatment solution is balanced under flow control for utilization in a cooking process or an auxiliary process or in a heat recovery collector allowing external use of the heat capacity. led from the tank.

From this balance tank the hot heat treatment solution is directed to one or more flash cyclones. These are at a lower pressure than the balance tank for the heat treatment solution. The steam generated in the cyclone is partially available to the cooking plant and other auxiliary processes at the beginning of the next cooking cycle.
In this way, active steam is saved compared to conventional systems. The remainder of the expanded steam is directed to a surface condenser or heat recovery collector where hot water is produced. Flow control through well-known multi-stage condensation processes allows the malodorous gases and methanol gas released with the expanded steam to be directed to a contaminated gas cleaning system.

After the heating solution has been discharged, the secondary heat in the solution remaining in the chip is recovered by relief gas treatment, which is known per se. The pressure within the digester is then reduced to a value somewhat above atmospheric pressure. The generated steam stream is directed to a heat recovery concentrator together with or instead of the steam stream from the flash cyclone. The cycle is supplied with solution from a balance tank. The continuing steam flow is then controlled so that the combined steam flow remains approximately constant. The solution free in the digester is sucked out through a sieve or strainer at the bottom of the digester.

The digester is then filled with filtrate from the pulp washer and emptied by blowing with the aid of expanded steam from the balance tank.

The filtrate or wash liquor is preferably fed to the digester at a relatively low temperature. As the temperature,
Temperature in the mixture of cleaning solution and pulp to 105℃
The temperature below is preferred. The contents of the digester are then desirably blown, or "cold blown", preferably at a temperature of 90 to 105°C.

In order to uniformly distribute the cleaning liquid at the bottom of the digester, it is advantageous to feed the cleaning liquid to the digester via a filtration device located at the bottom of the digester.

Additionally, supplying a controlled flow of wash liquid through a filtration device at the bottom of the digester is useful during blowing into the conical bottom of the digester to reduce friction in the flow through this part of the digester. is considered preferred, which facilitates draining the digester.

If there is a disturbance in a pulp plant, e.g. a digester, in which the steam consumption decreases periodically,
According to the invention, there is the possibility of introducing external high-pressure steam, ie steam from the boiler room, into the balance tank, so that this steam can be saved instead of being blown into the free atmosphere.

This new process has various advantages in terms of plant steam consumption, process, pulp quality, cleaning efficiency and environmental protection, as will be clear from the description below.

Benefits regarding plant steam consumption: Theoretically, 100% of the latent heat of the digester contents is
Available in the form of controlled secondary steam flow.
Compared to known heat recovery devices, greater freedom is provided in the selection regarding the use of secondary heat. Furthermore, secondary heat is recovered at a higher enthalpy level than before.

Fluctuations in the main steam flow to the cooking plant can be reduced.

Process Advantages In general, it can be said that the best features of continuous and discontinuous batch cooking processes are combined.

Since the heat flow is not affected by disturbances in the main process, the secondary heat flow from the cooking plant can be supplied to different auxiliary processes more safely.

Improved pulp quality The method allows blowing of the digester at low temperatures (90 to 105°C). This method, so-called "cold blowing", reduces the risk of fiber wall damage due to rapid expansion of liquid condensed within the fibers compared to known blowing. In this way, pulp strength was improved.

In cooking, using a hot water/wood relationship is more economical than before, so the fluctuations in the Kappa-Number are reduced.

If there are blockages in the downstream lines (washers, bleach plants), the method of the invention provides the opportunity to park the finished cooking charge in the digester without reducing the number of cutters. The temperature of the pulp is lowered by treating the digested product with a relief gas to a pressure not far from atmospheric pressure and by supplying the filtrate from the washer. means to lower the level below the level at which it is received.

Improving Cleaning Efficiency The free heating solution with a high concentration of heat treatment chemicals extracted from the wood is removed from the digester and replaced with a cleaning liquid with a low concentration of said substances. The mixing of the washing liquid and the pulp takes place partly in the digester and partly in the blowing pipe and partly in the blowing tank. In this blowing tank, diffusion of chemicals from the concentrated heat treatment solution absorbed into the fibers to the more dilute cleaning solution takes place. This dilution results in a cleaning effect.

The temperature of the cleaning water in the filter washer can be increased without significant danger to the heat treatment in the suction leg. In addition, the temperature of the filter cleaning liquid is lowered than in the past.

Improved environmental protection Channeling foul-smelling gases from cooking plants
It can be treated as a constant controlled flow.

Hereinafter, the method of the present invention will be explained in detail with reference to the drawings.

In the conventional method, the following steps are performed.

1 A-B Chip filling 2 B-C Steam injection 3 C-D Filling of heat treatment solution 4 D-E Heating with low-pressure steam 5 E-F Heating with high-pressure steam 6 F-G Cooking 7 G-H Top injection 8 H-I Low Blowing A-A 300 minutes including 40 minutes down time FIG. 2 discloses a process diagram for the novel method carried out in the plant disclosed in FIG. The plant usually has a digester consisting of 8 to 12 units;
One digester, designated with reference numeral 1, is shown. Circulatory heating is performed by a heat exchanger 2 equipped with a circulation pump 3. External steam is supplied via line 4. The circulating heated solution is returned to the digester 1 via line 5 with valve 6 . Conduit 5
is connected to a balance tank 9 via a line 7 with a valve 8. The tank 9 has a valve 11
It is connected to the digester 1 via a conduit 10 having a .

A line 12 with a valve 13 leads from the digester 1 to a blow tank (not shown). Conduit 1
0 is connected to line 14 for supplying fresh heat treatment solution. This line has a valve 15. Line 16 with valve 17 connects heat exchanger 2
and is connected to a circulation line leading to a sieve or filtration device at the bottom of the digester. The bottom of the balance tank 9 is connected to the first flash cyclone 2 via a conduit 18 and then via a conduit 19.
0 and a flash cyclone 22 via a conduit 21.

The bottom of the flash cyclone 20 is connected to the pipe line 23.
It is connected to the flash cyclone 22 via. The steam outlet of the flash cyclone 20 is connected via a line 24 to a steam pipe 25 from the top of the digester 1 . Pipe line 24, 2
5 is also connected to other digesters not shown. The steam outlet of the flash cyclone 22 is connected via a line 27 to a line 28 which extends from the top of the digester 1 and is provided with a valve 29. Line 28 is connected to a heat recovery collection device 30 from which the malodorous gases are discharged via line 31. A line 32 leads from the solution outlet of the flash cyclone 22 to the black solution tank. The top of the balance tank is connected to a line 34 having a valve 26.
It is connected to the top of the digester 1 via.
The top of the digester 1 is connected via a valve 37 to a line 28 . A line 36 with a valve 37 is connected to line 16 . Conduit 19 is equipped with a valve 38 . Conduit 23 is provided with a valve 39 and conduit 21 is provided with a valve 40.

The plant operates as follows.

1 A-B Chip filling 2 B-C Steam injection 3 C-D Filling with heat treatment solution 4 D-E Heating with low-pressure steam and steam from flash cyclone 5 E-F Heating with high-pressure steam 6 F-G Cooking 7 G-H Discharge of heat treatment solution 8 H-I Top injection to recovery system 9 I-J Relief gas treatment to heat recovery collective unit 10 J-K Supply of cleaning liquid 11 K-L Digester injection pressure Increment 12 LM Cold Blow 13 MN Final Relief Gas Treatment A-A 300 minutes including 35 minutes down time A-B Chip filling is initiated by opening the digester valve and starting the chip feed. Ru.
Activated steam and steam from line 24 are supplied to the chip filling apparatus. The valve is closed when the chip level in the digester reaches a predetermined level.

B-C During steam blowing, the chips are preheated in the digester by adding expanded steam from flash cyclone 20 via valve 33. At the same time, air in the digester is forced out via a valve 37 in line 36. Preheating with steam from the flash cyclone 20 continues until the desired temperature or pressure is achieved in the digester, after which the valve in the line between the digester and the cyclone is closed. Then, when the steam flow reaches the detection point of the control system in the conduit 36, the exhaust valve 37 is closed by the temperature control means.

CD The heat-treated solution is fed to the digester via a bottom filtration device. To increase the feed rate, it is also possible to feed the heat treatment solution via an intermediate filtration device. The black solution charge can be added, preferably via line 10, from a balance tank for the heat treatment solution, with controlled flow rates.
This mode of operation results in a saving of active steam. When a sufficient amount of heat treatment solution has been supplied through the pipes 14 and 10, the valves 11,1
5 and 17 are closed.

DE The heat treatment solution circulation pump 3 is started. While detecting the outlet temperature of the heat treatment solution from the heat exchanger 2 and controlling the temperature, indirect heating of the heat treatment solution using active steam is started. Valve 6 is opened and valve 8 is closed.

At the same time, active steam is supplied from line 24 via valve 33 to the digester. This supply is then automatically shut off when the digester pressure rises to a pressure close to the flash cyclone pressure.

Inert gas with expanded steam from line 24 to the digester is discharged from the digester via the turpentine degassing line. The valve 35 of this pipeline is
It is held somewhat open during the heating period.

The remaining heating period up to the EF boiling temperature is carried out indirectly in a heat exchanger with activated steam.

F-G Cooking G-H The free heat-treated solution (a portion of the heat-treated solution is bound into the chips) is discharged from the digester during the cooking period in question by the heat-treated solution circulation pump. carried out with the help of
The valve 6 in the heat treatment solution circulation line is closed, and the valve in the line from the heat exchanger 2 to the heat treatment solution balance tank and the valve on the suction side of the pump 3 are opened. Discharge is then automatically stopped when, at the preferred level in the digester, the level indicator is actuated to shut off the valve 8 in the line leading to the balance tank as well as the circulation pump.

H-I The pressure inside the digester is reduced by opening the valve 33 provided in the conduit 25 leading from the digester to the conduit 24. Steam valve 33 is controlled by a pressure control device. The controller first shuts off the valve 38 in the liquid supply pipe from the balance tank to the cyclone 20 to compensate for load peaks caused by the steam supply from the digester.

I-J When the pressure in the digester drops to a value close to that of the cyclone 20, the valve 33 in the line 25 from the digester is automatically shut off, and the cyclone 20 is also supplied with water from the balance tank. Liquid is supplied again.

Pressure reduction in the digester is continued by degassing via a valve 29 leading to a heat recovery collection device for hot water production. The degassing load change from the digester is measured by a flow meter in line 28. Changes in this flow are
Primarily, this is compensated for by varying the liquid flow to flash cyclone 22 via valve 40. A large amount of steam is generated from here, such that the amount of steam flow in the pipes 27 and 28 is approximately constant. Steam flow to the heat recovery collection device for hot water production is measured by flow meters.

If the throttling of valve 40 does not appear to be sufficient, valve 29 is then throttled to stabilize the flow rate to concentrator 10. If required for time reasons, it is also possible to carry out the degassing process simultaneously via valves 33 and 29. Valve 33 is then controlled by the pressure in line 24. Valve 29 connects lines 27 and 2
It is controlled by the desired flow rate within 8.

During the J-K degassing period, cleaning liquid (filtrate from the washer) is fed to the digester via valve 15. Also, valve 17 is shut off. The cleaning liquid is pumped through the bottom filtration device.
This flow is then controlled by a level control device provided at a suitable level. Valves 35 and 2 installed at the same height of the degassing treatment pipeline leading to the heat recovery collection device
9 is blocked.

The digester is placed under pressure by opening the valve 26 to the K-L heat treatment solution balance tank.

The contents of the LM digester are blown into the blowing tank. When all contents have been blown out, valves 13 and 26 in the lines from the blowing pipe and the balance tank are automatically shut off.

The digester is once again degassed by M--N valve 29 to the heat recovery collection unit. Atmospheric pressure in the digester opens the cap and the digester is ready for a new cooking cycle.

The flash cyclone 20 operates as an integral part of the cooking system. It then allows return of degassed steam to compensate for temporary degassing and heat demands in the cooking plant.

The energy flow from the basic system to the outside is measured by three flowmeters installed in the lines 23, 27, 28. Data from the flow measurements are fed into a control system, which first acts on valve 40 and then on degassing valves 29 and 35.

[Brief explanation of the drawing]

FIG. 1 is a temperature/time diagram for batch-type kraft pulp cooking with heat recovery, which is known in the art. FIG. 2 is a similar diagram in the method according to the invention. FIG. 3 schematically shows a plant for carrying out the method according to the invention. 1... Digestion device, 2... Heat exchanger, 3... Circulation pump, 4, 5, 7, 10, 12, 14, 16, 1
8, 19, 21, 23, 24, 25, 27, 2
8, 31, 32, 34, 36...pipe line, 6, 8, 1
1, 13, 15, 17, 26, 29, 33, 3
7, 38, 39, 40... Valve, 9... Balance tank, 20... First flash cyclone, 22...
Flash cyclone, 30...Heat recovery collection device.

Claims (1)

[Scope of Claims] 1. A secondary heat recovery method in batch-type cooking of cellulose in a digester, comprising:
During the final stage of the cooking cycle, the washing liquid, preferably superfluous from the filter washing, is fed to the digester for pre-washing of the pulp at a relatively low temperature, in which case it is led under pressure to the balance tank, and the desired The temperature is 105°C or lower in the mixture of cleaning liquid and pulp, into which the steam flow means so-called cold blowing.
A method for secondary heat recovery in batch-type cellulose cooking, characterized in that it is introduced from a balance tank into the digester to blow out the pulp content at a temperature of 90-105°C. 2. The method of claim 1, wherein after the completion of the heat treatment cycle, the free liquor in the digester is directed to be discharged therefrom through the bottom of the digester.