NO157109B - PROCEDURE FOR BATTLE COOKING OF CELLULOSE IN COOK. - Google Patents

Description

It is known to recover heat by boiling cellulose pulp, where the cooking liquid in the final phase of the cooking cycle is fed to a pressurized leveling tank, cf. Norwegian patent no. 44 111 and no. 57 679. From Finnish patent no. 18 429 it is known in the production of cellulose pulp to transfer the cooking liquid after the end of cooking, under essentially the same pressure and temperature as prevailed in the cooker, to a separate vessel, and to over-

lead the steam from the remaining mass in the boiler to one or more steam accumulators.

From US patent 1 776 761, a method is known for blowing out pulp from a boiler, whereby part of the cooking liquid is transferred to a holding vessel outside the boiler. When boiling is finished, this liquid is returned to the digester to facilitate the transport of pulp out of the digester.

US patent 1 836 862 describes how the last remaining cooking liquid is displaced from the pulp fibers using water at a temperature of 100°C. During this operation, the boiler is kept under pressure using compressed air. At the start of washing, the mass and remaining cooking liquid have a temperature of 170°C.

When all the cooking liquid has been displaced, the pulp and water have a temperature of 100°C.

The present invention relates to a method for batch cooking of cellulose in a boiler, whereby the heat content of the cooking liquid is used in the cooking process, and where the cooking liquid in the final stage of the cooking cycle is fed to a pressurized leveling cistern, characterized in that washing liquid, preferably filtrate from filter washing, is supplied to the boiler for pre-washing of the pulp at a relatively low temperature, preferably so that the temperature in the mixture of washing liquid and pulp becomes 105°C or lower, after which a stream of steam is led from the leveling cistern to the boiler in order to blow out its pulp content from there at a temperature of 90-105°C, which means so-called cold blasting. It will be seen that the method involves a combination of pre-washing the mass at a relatively low temperature and blowing out the mass using a stream of steam from the cistern.

From the leveling cistern, the hot cooking liquid is led to one or more relaxation cyclones. These are under lower pressure than the leveling tank for cooking liquid. The steam produced in the cyclones can partly be used in the cooker at the start of the next cooking cycle as well as in separate auxiliary processes. This saves fresh steam compared to a conventional system. The rest of the de-stressed steam is led to surface condensers, i.e. a heat recovery unit in which hot water is produced. The odor gases and methanol vapors released with the de-stressed steam can, because the flow is regulated via a well-known multi-stage condenser process, be led to the odor gas purification system.

After emptying the cooking liquid, the secondary heat in the liquid remaining in the tile is taken care of with the help of cr, in cg known degassing technique. The pressure in the boiler is thereby lowered to slightly above atmospheric pressure. The produced steam stream is fed to a heat recovery unit together with or instead of a steam stream from a relaxation cyclone to which liquid is fed from the leveling cistern, the latter steam stream being regulated so that the combined steam stream is kept essentially constant. The free liquid in the boiler is extracted through a strainer in the boiler bottom.

The boiler is then filled with filtrate from the bulk laundry and blown empty using steam released from the leveling cistern. This achieves a pre-washing of the mass.

The filtrate, i.e. the washing liquid, is suitably supplied to the boiler at a relatively low temperature, i.e. preferably so that the temperature in the mixture of washing liquid and pulp is 105°C or lower. The boiler contents can then be blown at a temperature of preferably 90-105°C, which involves cold blowing.

It is advantageous that washing liquid is supplied to the boiler through a strainer arranged in the bottom of the boiler with the aim of distributing washing liquid evenly in the boiler bottom.

It may also be appropriate to supply a regulated flow of washing liquid through a strainer arranged in the bottom of the boiler during the blowing into the conical part of the bottom of the boiler with the aim of reducing the friction in the flow through this part of the boiler and thereby facilitating the emptying of the boiler.

In such cases where disturbances occur in a pulp factory, for example at a boiler, so that steam consumption is occasionally reduced, according to the invention there is a possibility to direct external high-pressure steam, i.e. steam coming from the steam boiler, to the equalization cistern in order to utilize this steam instead to blow it into the air.

The new method shows a number of advantages with regard to the factory's steam consumption, the process, the pulp quality, the washing effect and environmental protection, as can be seen from the table below.

Advantages regarding the factory's steam consumption

Theoretically, 100% of the boiler's latent heat is available

in the form of a controlled secondary steam flow. Compared with a conventional heat recovery system, a greater freedom of choice has been achieved in the utilization of the secondary heat. In addition, the secondary heat is maintained at a higher energy level than previously.

The variations in the primary steam flow to the cooker can be reduced.

Process benefits

In general, it can be said that the best features of the continuous and batch cooking process are combined.

The secondary heat flow from the cooker can be connected to various auxiliary processes with greater certainty than before, as the flows are not affected by disturbances in the main process.

Improved pulp quality

The method allows blowing the boiler at a low temperature (90-105°C). This so-called cold blowing means that the risk of breakage of the fiber wall due to violent expansion of liquid bound in the fiber is reduced compared to what is the case with conventional blowing. This improves the mass's strength properties.

The Kappa number's variation decreases as it is more economical than previously to use a high water/wood ratio for cooking.

If problems arise during operation during the further treatment of the pulp (laundry, bleaching), this method provides an opportunity to leave a finished boil waiting in the digester without causing a reduction in the kappa number. The degassing of the boiler to close to atmospheric pressure as well as the supply of filtrate from the laundry lowers the temperature of the mass, which causes the reaction rate of the boiling process to be reduced to below a level that does not affect the kappa number of the mass.

Improved washing effect

The free cooking liquid with a high concentration of cooking chemicals and components dissolved from the wood is removed from the cooker and replaced with a washing liquid with a lower concentration of the above-mentioned substances. Remixing of washing liquid and pulp takes place partly in the boiler, partly in the blowing line and the blowing tank. In the blowing tank, a diffusion of chemicals also takes place from the concentrated cooking liquid absorbed in the fiber to the weaker washing liquid. This achieves a dilution and thereby a washing effect.

It is possible to raise the temperature of the washing water in the filter washing without there being any greater risk of boiling in the suction line: the temperature of the base liquid (base water tank) to the filters is lower than before.

Improved environmental protection

The odorous gases from the cooker can be directed to treatment as a steady, controlled stream.

The method according to the invention will be described in more detail below with reference to the three figures in the drawing, where

fig. 1 shows a temperature/time diagram for a batch process

sulphate pulp boiler with conventional heat recovery; fig. 2 shows the same for the method according to the invention;

fig. 3 schematically shows a plant for carrying out the method according to the invention.

The conventional method consists of the following steps:

A-A 300 minutes incl. 40 min. intermediate time.

In fig. 2 shows how the new method can be carried out in a plant of the type shown in fig. 3. The plant consists of a number of boilers, usually 8-12, of which only one boiler, with the reference number 1, is shown. Circulation heating is arranged using the heat exchanger 2, which is equipped with a circulation pump 3. External steam is supplied to the heat exchanger 2 through line 4. The circulating cooking liquid is returned to the boiler 1 through a line 5 with a valve 6. The line 5 is via a line 7, which is provided with a valve 8, in connection with an equalization cistern 9, from which a line 10, which is provided with a valve 11, leads back to the boiler 1. From the boiler 1 leads a line 12, which is provided with a valve 13, from the boiler to the blowing cistern, which is not shown. Line-10 is in connection with a line 14 for the supply of new cooking liquid. This line is provided with a valve 15. A line 16, which is provided with a valve 17, is connected to the circulation line through the heat exchanger 2 and to the strainer in the boiler bottom. The bottom of the leveling cistern 9 is connected through a line 18 partly via a line 19 in connection with a first relaxation cyclone 20 and partly via a line 21 in connection with a relaxation cyclone 22. The bottom of the relaxation cyclone 20 is connected via a line 23 to the relaxation cyclone 22. The relaxation cyclone 20 steam outlet is via a line 24 connected to a steam line 25, starting from the top of the boiler 1. The lines 24, 25 are also connected to other boilers that are not shown. The steam outlet of the relaxation cyclone 22 is connected via a line 27 to a line 28, which comes from the top of the boiler 1 and is equipped with a valve 29. The line 28 leads to the heat recovery unit 30, from which odorous gases are withdrawn through a line 31. From the liquid drain of the relaxation cyclone 22, a line 32 leads to a black liquor cistern. From the top of the leveling cistern 9 leads a line 34 with a valve 26 to the cooker top. The cooker top is connected via a valve 35 to a line 28. Line 36, which is supplied nt with a valve 37, is connected to line 16. Line 19 is provided with a valve 38. Line 23 is provided with a valve 39. Line 21 is provided with a valve 40.

The facility works as follows:

A-A 300 minutes incl. 35 min. intermediate time.

A-B The chip filling is started by opening the boiler valve and the chip filler's valve and starting the chip supply. Fresh steam or secondary steam from line 24 is supplied to the chip packer. When the correct chip level in the boiler is achieved, the valves are closed.

B-C During the steam treatment, the chip is preheated in the boiler by supplying de-stressing steam from the de-stressing cyclone 20 via valve 33. At the same time, the air in the de-stressing boiler is expelled through a valve 37 in line 36. Pre-heating with steam from the de-stressing cyclone 20 is continued to the desired temperature or the desired pressure in the boiler is reached, after which the valve 33 in the line between the boiler and cyclone is closed. A temperature regulation closes the evacuation valve 37 when the steam flow reaches the regulation's measurement point in line 36.

C-D Boiling liquid is supplied through the bottom strainer in the boiler. To increase the feed rate, cooking liquid can also be fed through a middle strainer. Supply of black liquor to the cooking liquid can advantageously take place directly through a line 10 with flow regulation from the leveling tank for cooking liquid. This precaution involves a saving of fresh steam.

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

D-E Circulation pump 3 for cooking liquid is started. The indirect heating of the cooking liquid with fresh steam is started with a temperature control which knows the outlet temperature of the cooking liquid from the caloriser 2. Valve 6 is open and valve 8

closed.

At the same time, the boiler is supplied with steam directly from line 24 through valve 33. The supply is automatically interrupted when the boiler pressure has risen to a pressure close to that in the relaxation cyclone.

The inert gases, which follow the relaxation steam from line 24 to the boiler, are vented out of the boiler via the turpentine degassing line, whose valve 35 is kept slightly open during heating.

E-F The rest of the heating to boiling temperature takes place indirectly

in the caloriser using fresh steam.

F-G Cooking.

G-H Emptying the boiler of free cooking liquid (part of the cooking liquid is bound in the tile) is carried out during the relevant cooking time using the circulation pump for cooking liquid. The valve 6 in the circulation line for cooking liquid is closed, and the valve in the line from the heat exchanger 2 to the leveling tank for cooking liquid and the valve 17 on the suction side of the pump 3 are opened. Emptying is automatically interrupted when a level indicator at an appropriate height in the boiler closes the valve 8 in the line to the leveling cistern and stops the circulation pump.

H-I Boiler pressure is lowered by opening valve 33 in line 25 from the boiler to line 24. Steam valve 33 is controlled by a pressure regulator, which primarily closes valve 38 in the pipeline for liquid supply from the equalization cistern to cyclone 20. This is to compensate for the load peak caused by the steam supply from the cooker. I-J When the boiler pressure has been lowered to the level of the pressure in the cyclone 20, the valve 33 in line 25 from the boiler is automatically closed, and the cyclone 20 is again fed with liquid from the equalization cistern.

The remaining pressure reduction in the boiler takes place through degassing with valve 29 to the heat recovery unit for hot water production. The load change of the degassing from the boiler is measured with a flow meter in line 28. The variation in this steam flow is compensated primarily by

to change the liquid flow to the relaxation cyclone 22 with valve 40, from which so much steam is thereby produced that the sum of the steam flows in the lines 27 and 28 becomes essentially constant. The steam flow to the heat recovery unit for hot water production is measured with a flow meter.

If throttling of valve 40 does not extend tii, is used

secondary valve 29 for obtaining a constant result flow to aggregate 10. If required for temporal reasons, the blowdown can take place simultaneously through valves 33 and 29. In this way, valve 33 is controlled by the pressure in line 24. Valve 29 is controlled by the desired result flow in the lines 27 and 28.

J-K During the time during which the degassing is in progress, washing liquid (filtrate from laundry) is supplied to the boiler through valve 15. Valve 17 is closed. The washing liquid is pumped in through the bottom strainer, and the flow is controlled by a level control placed at the appropriate level. Also valve 35 and 29 in the line for

degassing until the heat recovery unit is closed.

K-L The boiler is pressurized by opening valve 26

to the leveling tank for cooking liquid.

L-M The boiled is blown out of the boiler into the blowing tank. When the entire boiler has been blown away, the valves 13 and 26 in the blowing line and the line from the leveling cistern are closed automatically. M-N The boiler) is once again degassed to the heat recovery unit with valve 29. At atmospheric pressure in the boiler, the boiler lid is opened, and the boiler is ready for a new cooking cycle.

Relaxation cyclone 20 works as an integral part of the cooking system. It enables the return of degassing steam and compensates for the momentarily occurring degassing and heat requirements in the cooker.

Outgoing energy flows from the basic system are measured by three flow meters placed in lines 28, 27 and 23. Data from the flow measurement is fed to a control system which primarily controls valve 40 and secondarily controls the degassing valves 29 and 35.

Claims (4)

1. Method for batch cooking of cellulose in a boiler, whereby the heat content of the cooking liquid is used in the cooking process, and where the cooking liquid in the final stage of the cooking cycle is fed to a leveling cistern under pressure, characterized in that washing liquid, preferably filtrate from filter washing, is supplied to the boiler for pre-washing the mass at relatively low temperature, preferably so that the temperature in the mixture of washing liquid and pulp is 105°C or lower, after which a stream of steam is led from the leveling cistern to the boiler in order to release its pulp content from there at a temperature of 90 - 105°C, which involves so-called cold blowing. 2. Method according to claim 1, characterized in that the boiler's free liquid is caused to leave through the bottom of the boiler after the completion of the cooking cycle. 3. Method according to claim 1, characterized in that, after the completion of the cooking cycle, the cooking liquid is displaced from the cooker by means of washing liquid. 4. Method according to claim 3, characterized in that the cooking liquid is displaced from the cooker by means of filtrate introduced into the lower part of the cooker, so that the cooking liquid departs from the upper part of the cooker.