Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a method for secondary
heat recovery in batchwise digesting of cellulose in digesters,
the cooking liquor being conducted, during the final stage of
the digesting cycle, to an equalizing tank standing under
pressure.
Methods for secondary heat recovery in the pulp in-
dustry are known. These are based on the recovery of heat from
the expansion steam from the blowing tank, or from material
flows, which originate directly from the digester. These methods
are disadvantageous in that the material and heat flows to the
heat recovery system are discontinuous, which means, that it
will be difficult to control the process. Further the secon-
dary heat is recovered as warm water, which is normally avail-
able in excess o~f the normal needs of the plant(process). Asthe material and heat flows according to said methods aredis-
continuous, there is needed an accumulating tank for the odor-
ous foul gases liberated in the system so that subsequent treat-
- ments may be effected. It is possible to eliminate the dis-
continuity in the liberating of the foul gases on such occa-
sions, when foul gases are not liberated by feeding air to the
system.
In the Finnish Patent Specification No. 18,429 a
cellulose digesting method is disclosed, according to which cook-
ing liquor is conducted, after the digestion is terminated, un-
der substantially the same pressure and temperature as that,
which was prevailing in the digester, to a separate tank, and
according to which any steam left in the pulp remaining in the
digester is transferred to one or several steam accumulators.
As there is no disclosure of how to utilize the heat content
of the cooking liquor practically in the method suggested,
this has not been applied practically.
j 35 The heat economy is growing in importance in the pulp
industry, and many efforts have been made to improve the same,
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especially in the kraft pulping industry. The pres~nt inven-
tion provides a method of the type mentioned at the introduction,
which is characterized by a good heat economy, and which permi-ts
blowing of the pulp at a relatively low temperature, prefer-
ably within the range 90 - 105C, which means so called "cold
blow", and in which the pulp may be prewashed within the digester.
According to the present invention there is provided
in the batchwise digesting of cellulose in a cooker from which
a cellulose mass is to be discharged, the method which comprises
feeding cooking liquor and cellulose to said cooker to digest
the cellulose, feeding cooking liquor, during a final stage of
said digesting, from the cooker to an equalizing tank maintained
under pressure, thereafter degassing the cooker to substantially
atmospheric pressure, feeding washing liquor, during said de-
gassing, to the cooker and there prewashing the cellulose
mass at a ]ow temperature, and conducting a steam flow from the
equalizing tank to the cooker to discharge its content oE cel-
lulose mass at a temperature of 90 to 105C, thereby providing
a cold blow.
Thus according to the invention the cooking liquor
or steam, expanded from cooking liquor is conducted, under flow
control, from the equalizing tank for utilizing in the digest-
ing process, in auxiliary processes or in heat recovery ag-
gregates for external use of its heat content.
From the equalizing tank the hot cooking liquor is
conducted to one or several flash cyclones. These stand under
lower pressure than the equalizing tank for cooking liquor.
The steam generated in the
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1 cyclones cc~n be utilized partly in the digesting plant at the beginnin~
2 of the next digesting cycle and in separate auxili~ry processes. In
3 this way live steam is saved compared with a conventional system. The
~ rest of the expanded steam is conducted to surface condensers, i.e.
a heat recovery a,ggregate, in which hot water iB produced. The odorous
6 foul gases and the methanol gases llberated with the expanded steam can
7 be conducted to a foul gases purification system thanks to the flow
8 control by a well known multi step condenser process.
9 After the discharge of the cooking liquor the secondary heat
in the liquor remaining in the chips is recovered by relief gassing,
1 known per se. ~he pressure in the digester is lowered to a value some-
12 what above atmosperic pressure. The steam flow generated is conducted
l3 to a heat recovery aggregate together with or instead of a steam flow,
14 coming from a flash cyclone, to which liquor is fed from the equilizing
tank, which last steam flow is controlled, in order that the combined
16 steam flow is kept substantially constant. ~he free liquor in the
17 digester is suctioned out through a sieve in the digester bottom.
18 The digester is then filled with filtrate from the pulp washer
19 and i6 blown empty by the aid of steam, expanded from the equilizing
tank. Thus there is provided a prewash of the pulp.
21 The filtrate, i.e. the washing liquid is fed, preferably, to
22 the digester at a relatively low temperature, i.e. preferably at such
23 a temperature, that the temperature in the mixture of washing liquid
24 and pulp will be 105C or lower. The content of the digester can be
blown preferably at a temperature of 90 - 105C, which means "cold
26 blow7l.
27 It is advantageous to feed washing liquid to the digester
28 through a sieve in the bottom of the digester in order to distribute
29 washing liquid evenly in the bottom of the digester.
It may also be suitable to feed a controlled flow of washing
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1 liquid through a sieve in the bottom of the digeter, durlng the blowing
2 into the bottom cone of the digester in order to reduce the friction in
3 the flow through this part of the digester, whereby the discharge of
4 the digester is facilitated.
In such cases, when there are disturbances in a pulp plant,
6 for instance at a digester, so that the steam consumption is periodi-
7 cally reduced, there is, according to the invention, a possibility to
8 conduct external high pressure steam, i.e. steam from the boiling house,
9 to the equilizing tank, in order to save this steam instead of blowing
it into the free air.
11 The new method has many advantages, considering the steam
12 consumption of the plant, the process, the quality of the pulp, the
13 washing efficiency and the environmental protection, which will be
t4 obvious from the following list.
Advantages relating to the steam consumption of the plant
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16 Theoretically 10~o of the latent heat of the digester content
17 is available in the form of a controlled secondary steam flow. Compared
t8 to a conventional heat recove7y system there is thus provided a greater
19 freedom in the choice of utilization of the secondary heat. ~urther,
the se~ondary heat is recGvered at a higher enthalpy level than before.
21 The variations in the primary steam flow to the digesting
22 plant can be reduced.
23 Process advantages
24 Generally it can be said, that the best features of the con-
2S tinuous and the discontinuous batchwise digesting processes have been
26 combined.
Z7 'rhe secondary heat flow from the digesting plant can be fed,
28 more safely, to different auxiliary processes, as the flows are not
29 influenced by disturbances in the main process.
Improved pulp quality
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~ The method permits blowing of the digester at a low tempera-~
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1 ture (90 - 105 C). This socalled "cold blow" means, that the risk for
2 a ruptur~ in the fibre wall by violent expansion of liquid, bound in
3 the fibre, i8 reduced compared to the risk in conventional blow. In
4 this way the strength of the pul~ is improved.
~he variation of the Kappa-Number is reduced, as it is more
6 economical than previously to use a high water/wood-relationship in
7 the digesting
8 If there are disturbances in the subsequent process line
9 (washer, bleaching plant) this method gives an opportunity to let a
completed digester charge wait in the digester without reducing the
11 Kappa-Number. Relief gassing of the digester to a pressure not too far
l2 from atmospheric pressure and feed of filtrate from the washer lowers
13 the temperature of the pulp, which means, that the reaction velocity of
14 the digesting process is reduced below a level, where the Kappa-Number
is influenced.
16 Improved washing efficiency
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17 m e free cooking liquor with a high concentration of cooking
t8 chemicals and components extracted from the wood are removed from the
19 digester and are rèplaced by a washing liquid with a lower concentration
of said substances. Mixing of washing liquid and pulp is carried out
21 partly in the digester and partly in the blowing pipe and in the blowing
~2 tank. In the blowing tank there iB also a diffusion of chemicals from
23 concentrated cooking liquor, absorbed in the fibre to the more diluted
24 washing liquid. Hereby a dilution os obtained, and thus a washing effect.
It is possible to raise the temperature of the washing water
26 in the filtre washer without any greater risk for cooking in the suction
27 leg. The temperature of the washing liquid to ghe filtre is lower than
28 previously.
29 Improved environmental protection
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The foul gases from the digesting plant can be conducted for
treatment as a constant, controlled flow.
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1 The method according to the invention shall now be described
2 more in detail, reference being made to the accompanying figures, of
3 which
4 fi~ure 1 shows a temperature/time-diagram for a batchwise kra~t pulp
di~esting with conventionnl heat recovery;
6 figure 2 shows a corresponding diagram for the method according to the
7 invention;
B fi~ure 3 shows, schematically, a plant for carTying out the method
9 according 'LO the invention.
In the conventional method there are the following steps:
11 1. A - ~ chip filling
12 2. B - C steam blowing
l3 3. ~ - D filling of cooking liquor
14 4. D - E heating with low pressure steam
5. E - F heating with high pressure steam
16 6, ~ - G digesting
17 7. G - H tbp blowing
18 8. H - I bottom blowing
19 A - A 300 minutes incl. of 40 minutes intermed. time.
In figure 2 there is disclosed a process diagr~m for the new
21 method, carried out in plant disclosed in figure 3. m is plant comprises
22 a number of digesters, normally 8 - 12 units, of which only one digester
23 with reference 1 is 6hown. Circulation heating is provided by a heat
24 exchanger 2, which i9 provided with a circulation pump 3. External steam
i8 fed to heat exchanger 2 through a line 4. The circulating cooking
26 liquor is returned to digester 1 via a line 5 with a valve 6. The line 5
27 i8 connected, via a line 7, provided with a valve 8, to an equalixing
28 tank 9, connected via a line 10, provided with a valve 11, to digester 1.
29 ~rom digester 1 there is drawn a line 12, provided with a valve 13, to
a blowing tank, which is not shown. Line 10 is connected to a line 14
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1 for ~eeding`of new cooking liquor This line is provided with a valve 1
2 A line 16, provided with a valve 17, is connected to the circulating
- 3 line, with heat exchanger 2, to a sieve in the di~eOEter bcttom. The
~ bottom of the equalizing tank 9 is connected through a line 18, partly,
via a line 19 to a first flash cyclone 20 and partly, via a line 21 to
6 a flash cyclone 22.
7 m e bottom of flash cyclone 20 is connected, via a line 23 to
8 flash cyclone 22. The steam outlet of flash cyclone 20 is connected,
9 via a line 24 to a steam pipe 25, coming from the top of digester 1.
The lines 24, 25 are also conneoted to the other digesters, which are
11 not shown. The steam outlet of flash cyclone 22 is connected via a line
12 27, to a line 28, coming from the top of digester 1, and provided with
13 a valve 29. Line 28 i9 connected to a heat recovery aggregate 30, from
14 where odorous foul gases are discharged through a line 31. From the
liquid outlet of flash cyclone 22 a line 32 goes to a black liquor tank.
16 m e top of equalizing tank 9 is connected, via a line 34, with a valve
17 26 to the top of digester 1. m e top of digester 1 is connected, via a
18 valve 37 to line 28. A line 36, provided with valve 37 is connected to
19 li~e 1G. Line 19 is provided with a valve 38 Line 23 is provided with
a valve 3~. Line 21 is provided with a valve 40.
21 me plant operates in the following way:
22 1. A - B chip filling
23 2. B - C steam blowing
24 3. C - D filling of cooking liquor
4. D - E heating with low pressure steam and stea~ from
26 flash cyclone
27 5. E - F heating with high pressure steam
28 6. F - G digesting
29 7. G - ~ discharge of cooking liquor
8. ~ - I top blowing to recovery system
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1 9 I - J relief gassing to heat recovery a6greg~te
2 10. J - K feed of washing liquid
3 11. K - L increa~e of pressure for blowing of dlgester
4 12. L - M cold blow
S 13. M - N final relief gassing
6 A - A 300 minutes incl. of 35 intermed. time.
7 A - B The cip filling is started by opening the digester valve and
8 starting the cip feed. Live steam and secondary steam from the
line 24 are fed to the chip filler. When the correct chip
level in the digester has been achieved, the valves are shut.
11 ~ - C During the steam blowing the chips are preheated in the diges-
l2 ter by adding expanded steam from flash cyclone 20 through
l3 valve 3~. At the same time air in the digester is forced out
14 through valve 37 in line 36. The preheating with steam from
flash cyclone 20 is continued until the desired temperature
16 or pressure has been achieved in the digester, whereafter valve
17 35 in the line between the digester and the cyclone is shut.
t8 ~y a temperature control means the evacuating valve 37 is
19 shut, when the steam flow reaches the sensing point, of the
control system, in iine 36.
21 C - D Cooking liquor is fed to the digester through the bottom
22 sieve. In order to increaee the feeding rate cooking liquor
23 can also be fed through a middle sieve. The black liquor charge
24 can be added, preferably through line 10, with flow control,
from the equalizing tank for cooking liquor. ~y this mode of
26 operation live steam is saved.
27 When a sufficient amount of cooking liquor has been fed
28 through line 14 and line 10, valves 11, 15 and 17 are shut.
29 D - E Circulating pump 3 for cooking liquor is started. The indirect
heating of cooking liquor with live steam is started with a
temperature control, sensing the outlet temperature of the
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1 cbokin~ liquor from heat exch~nger 2. Valve 6 is open and
- 2 valve 8 is shut
3 Simultaneously live steam is ~ed to the digester from line 24
4 throu~h valve 33. The feed is interrupted automatically when
the digester pressure has risen to a pressure close to that
6 in the flash cyclone.
7 The inert gases, accompanying the expanded stea~ from line 24
8 to the digester, are vented out of the digester via the
9 turpentine degassing line, the valve of which 35 is kept open
to some degree during the heating period.
11 E - ~ The rest of the heating period to boiling temperature is
12 carried out indirectly in heat exchanger 2 by live steam.
13 ~ - G Digesting
14 G - H The discharge of free cooking liquor (part of the cooking
liquor is bound in the chips) from the digester is carried
16 out during the digesting period in question by aid of the
17 circulating pump for cooking liquor. Valve 6 in the circulating
t8 line for cooking liquor is shut, and the valve in the line
19 from heat exchanger 2 to the equalizing tank for cooking liquor
and valve 17 on the suction side of pump 3 are opened. The
21 discharge is automatically interrupted when a level indicator
22 at a suitable level in the digester acts to shut valve 8 in
23 the line to the equalizing tank and shuts the circulating
24 pump.
25 H - I The pressure in the digester is lowered by opening valve 33
26 in line 25 from the digester to line 24. The steam valve 33
27 is controlled by a pressure controller, which shuts primarily
28 valve 38 in the pipe fo~r liquid feed from the equalizing tank
29 to cyclone 20, to compensate the load peak, caused by steam
feed from the digester.
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1 I - J When the pressure of the digester has been lowered down to a
2 value close to that in cyclone 20, valve 33 in line 25 from t
3 the digester will be automatically shut, and cyclone 20 i3
~ again fed with liquid from the equalizing tank.
The rest of the pressure lowering in the digester i8 carried
6 out by degassing through valve 29 to the heat recovery aggre-
7 gate for production of hot water. The load change of the
8 degassing from the digester is measured by a flow meter in
9 line 28. Variations in this flow are compensated primarily by
changing the liquid flow to flash cyclone 22 by valve 40,
11 from which thus so much steam is produced, that the sum of
l2 the steam flows in lines 27 and 28 will be substantially
l3 constant. The steam flow to the heat recovery aggregate for
14 hot water production is measured by a flow meter.
If throttling of vlave 40 would not be enough, valve 29 is
16 ~ secondarily throttled in order to achieve a constant flow sum
17 to aggregate 10. If it is necessary for time reasons, the
18 degassing can be carried out simultaneously through valves 33
19 and 29. Then valve 33 is controlled by the pressure in line 24.
Valve 29 is controlled by the desired flow sum in lines 27
21 and 28.
22 J - K During the degassing period, washing liquid (filtrate from the
23 washer) is fed to the digester through valve 15. Valve 17 is
24 shut. The washing liquid is pumped in through the bottom sieve
and the flow is controlled by a level controller, provided at
26 a suitable level. Even valves 35 and 29 in the line for de-
27 ga~sing to the heat recovery aggregate are shut.
28 K - L The digester is put under pressure by opening valve 26 to the
29 equalizing tank for cooking liquor.
L - M The content of the digester is blown to the blowing tank.
en the total content has been blown out, valves 13 and 26
in the blowing pipe and the line from the equalizing tank are
shut automatica]ly.
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t M - N The digester is degassed once more to the heat recovery
2 aggre~ate by vlave 29. With atmospheric pressure in the
3 digester the cap is opened and the digester is ready for a
4 new digesting cycle.
Flash cyclone 20 operates as an integrated part of the
6 digesting system. It makes possible a return of degassing
_-- 7 vapors and compensates the mementary degassings and heat
8 demands in the digestin~ plant.
9 The outgoing energy flow~ from the basic system are measured
by three flow meters, provided in lines 23, 27 and 28. Data
11 from the flow measurements are fed to a control system,
12 primarily acting upon valve 40 and secondarily acting upon
13 degassing valves 29 and 35.
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