FR2553797A1 - Method of recovery of secondary heat during the batchwise digestion of cellulose. - Google Patents

Abstract

DURING THE LAST STAGE OF THE DIGESTION CYCLE, THE EXTRACTION LIQUOR IS DELIVERED FROM THE DIGESTION APPARATUS 1 TO A PRESSURE BALANCER 9. THE EXTRACTION LIQUOR OR THE VAPOR EMITTING THEREFROM IS DELIVERED, IN A CONTROLLED FLOW, FROM THE TRAY 9 TO A ZONE OF USE DURING THE PROCESS OR AN APPENDIX PROCESS OR TO A HEAT RECOVERY SET 30. THE LIQUOR MAY PARTICULARLY BE TAKEN FROM BAC 9 TO A FIRST CYCLONE 20. THE LIQUID LEAVING THE CYCLONE IS THEN SUPPLIED TO A SECOND CYCLONE 22 PROVIDED WITH A VAPOR OUTLET AND A LIQUID OUTLET.

Description

1 2553797

  METHOD FOR RECOVERING SECONDARY HEAT DURING

  DIGESTION OF CELLULOSIS BY LOADS

  The present invention relates to a process for recovering secondary heat during the digestion of cellulose by charges in digestion apparatus, of the type in which the extraction liquor is brought to a pressure balancing tank during from the final stage of

digestion cycle.

  The pulp and paper pulp industry has long known secondary heat recovery processes These processes recover heat from the expansion vapor coming from the blowing tank or from material flows coming directly from the digestion These processes have the drawback that the flows to the heat recovery system are discontinuous. It is therefore difficult to control the process. It can also be observed that the secondary heat is recovered in the form of hot water, which normally exceeds the requirements of the installation for carrying out the process. Since the flows of materials and heat are discontinuous in the above processes. above, a tank for the accumulation of fetid gases released into the system is required so that subsequent treatment can be carried out. The discontinuity in the release of fetid gases can be eliminated in such cases, when the gases are not

  not released when the system is supplied with air.

  Finnish Patent No. 18,429 describes a cellulose digestion process in which, after the end of digestion, the extraction liquor is brought into a separate tank, at a pressure and a temperature which are substantially the same as those which exist in the digestion apparatus The vapor contained in the pulp which remains in the digestion apparatus is transferred to one or more vapor accumulators Since this document does not

  not describe how the thermal content of the extraction liquor can be used according to the suggested method, there has been no practical application.

  Pulp and pulp industry

  attaches increasing importance to energy saving and numerous efforts have been exerted to save heat, in particular in the pulp industry for the production of kraft. The present invention aims to provide a process of the type mentioned above which ensures a satisfactory heat saving and which makes it possible to blow the pulp at a relatively low temperature, advantageously between 90 and 105 ° C. (so-called "cold" blow); the invention also aims to make it possible to prewash the pulp in the digestion apparatus.

  According to the invention, washing liquor, advantageously filtrate from a washing by filter, is brought to the digestion apparatus in order to prewash the mass of cellulose at relatively low temperature, advantageously at such a temperature. that the temperature of the mixture of washing liquid and cellulose mass is at most equal to 105 ° C. and that a digestion stream is brought in to blow out its cellulose mass content at a temperature ranging from 90 at 105 ° C, i.e. carry out a

cold blowing.

  The hot extraction liquor, often designated by the technical term "cooking liquor", is brought from the balancing tank to a flash cyclone or more These 25 cyclones are under a lower pressure than the balancing tank The steam generated in the cyclones can be used partly in the digestion apparatus at the start of the following digestion cycle and in separate auxiliary processes. Thus, an economy is achieved of live steam, in comparison with a conventional system. The rest of the expanded steam can be brought to surface condensers, i.e. a heat regulator assembly, in which hot water is produced. Fetid gases and methanol-containing gases released at the same time as the expanded vapor can be brought to a foul gas purification system by a well fractionated condensation process

  known thanks to the flow control.

  The digestion apparatus is then filled with the filtrate coming from the pulp washer, then it is emptied by blowing with the aid of steam which relaxes from the balancing tank. The pulp is then prewashed. The liquor free in the digestion apparatus is sucked through a filter in the bottom of the apparatus

digestion.

  The filtrate, that is to say the washing liquid, is advantageously brought to the digestion apparatus at a relatively low temperature, advantageously at a temperature such that the temperature of the mixture of washing liquid and pulp is 105 ° C. or less. The contents of the apparatus can advantageously be blown

  digestion at a temperature of 90 to 105 ° C, that is to say perform a cold blowing.

  It is advantageous to supply the washing liquid to the digestion apparatus through a filter provided at the bottom of the digestion apparatus, so as to distribute the washing liquid regularly at the bottom of the digestion apparatus. It may also be desirable to bring a

  controlled flow of washing liquid through a filter 25 at the bottom of the digestion apparatus during blowing The filter can be placed in the bottom cone of the digestion apparatus This reduces friction in the flow through this part of the digestion apparatus and it facilitates the flow out of the digestion apparatus.

  In such cases, when incidents occur in a pulp mill, for example on a digestion apparatus and result in a temporary reduction in steam consumption, the invention provides the possibility of supplying steam to high pressure of external origin, for example from the boiler, to the balancing tank so as to save this vapor at

  instead of discharging it to the atmosphere.

  We will now list some of the advantages

  of the process according to the invention, concerning the consumption of steam of the installation, the process, the quality of the pulp, the washing efficiency and the protection of the environment.

  Advantages relating to the steam consumption of the installation In principle, the latent heat of the contents of the digestion apparatus is entirely available in the form of a controlled secondary steam flow. By comparison with a conventional heat recovery system, greater freedom is therefore obtained in the choice of the use of secondary heat. In addition, secondary heat is recovered at a higher enthalpy level than in the past. Variations in the vapor flow rate can be reduced. primary water brought to

the digestion facility.

Advantages of the process

  We can say that we combine the favorable characteristics of continuous and discontinuous processes (by loads) of digestion We can bring the flow of secondary heat from the digestion plant 25 safer, to different auxiliary processes since flows are not influenced by disturbances in the main process.

  Improving the quality of the pulp The process allows the blowing of the digestion apparatus at a low temperature {9 D at

  C) Such a "cold blowing" reduces the risk of rupture of the wall of the fibers due to the violent expansion of the liquid retained by the fiber, in comparison with conventional blowing. The resistance of the pulp is thus improved.

  We reduce the variations of the Kappa number, because it

  it is more economical than before to use a high water / wood ratio during digestion.

  If incidents occur on the line of

  manufacturing during the subsequent phases (washing, bleaching), the process according to the invention gives the possibility of leaving a charge of a complete digestion apparatus in the apparatus, without reduction of the Kappa number Per escape of contained gas in the digestion apparatus at a pressure which is not too far from atmospheric pressure and feeding of filtrate from the washer, the temperature of the pulp is reduced, which results in a reduction in the reaction rate during the digestion process below the level for which the number 15 Kappa is influenced.

  Improvement of 19% Washing efficiency The free extraction liquor with a high concentration of chemical reagents and components extracted from wood is removed from the digestion apparatus, then replaced by a washing liquid o these substances are at low concentration The washing liquid and the pulp are mixed partly in the digestion apparatus and partly in the blowing line and the blowing tank In this latter, there is also dif25 fusion of chemicals coming from liquor

  concentrated extraction, absorbed in the fiber, to the more diluted washing liquid. This gives a dilution, therefore a washing.

  The temperature of the washing water can be increased in the filter washer without the risk of extraction in the suction branch. The temperature of the washing liquid towards the filter is lower than before.

  Improvement in Environmental Protection Fetid or foul-smelling gases can be brought from the digestion station to a

  treatment in the form of constant and controlled flow.

  The invention will be better understood on reading

  the following description of a process and an installation which constitute a particular embodiment thereof, given by way of nonlimiting example The description refers to the accompanying drawings, in

  which: FIG. 1 is a diagram showing the variation of the temperature as a function of time during digestion by loads of pulp for Kraft paper, with conventional heat recovery; Figure 2 is a corresponding diagram, when implementing a method according to the invention; Figure 3 schematically shows an installation for implementing a method according to the invention. In FIG. 1, which shows the conventional process, the successive segments designate the following stages: 1 A-B: filling in chips and fragments; 2 B-C: steam blowing; 3 C-O: filling with extraction liquid; 4 D-E: low pressure steam heating; 25 5 E-F: high pressure steam heating; 6 F-G: digestion;

  7 G-H: head blowing 8 H-I: bottom blowing.

  The interval A-A is 300 minutes, including 30 40 minutes of duration between two cycles.

  Figure 2 shows a diagram corresponding to the new process, capable of being implemented in the installation shown in Figure 3 This installation comprises several digestion apparatuses, the name of which typically ranges from 8 to 12 A single digestion apparatus 1 is shown The heat is supplied by circulation in a circuit which includes a heat exchanger 2 and a circulation pump 3 The external steam is injected into the heat exchanger 2 via the line 4 The extraction liquor in circulation 5 is brought back to the digestion apparatus 1 by a pipe provided with a valve 6 The pipe 5 is connected, by a pipe 7 provided with a valve 8, to a balancing tank 9 connected by a pipe 10 provided with a valve 11 to the digestion device 1 A pipe 12 provided with a valve 10 13 goes from the digestion device 1 to a blowing tank not shown The pipe 10 is connected to a pipe 14 for supplying liquor d fresh extraction This pipe is t fitted with a valve 15 A pipe 16 fitted with a valve 17 connects the circulation pipe, side 15 heat exchanger 2, to a filter provided at the bottom of

  the digestion apparatus The bottom of the balancing tank 9 is provided with a line 18 which is connected, on the one hand, to a first flash cyclone 20 by a pipe 19 and, on the other hand, to a second cyclone flash 22 by a con20 duite 21.

  The bottom of the flash cyclone 20 is connected to the flash cyclone 22 by a pipe 23 The steam outlet of the cyclone 20 is connected, by a pipe 24, to a steam pipe 25 coming from the upper end of the digestion 1 The pipes 24 and 25 are also connected to the other digestion devices (not shown) The steam outlet of the cyclone 22 is connected by a pipe 27 to a pipe 28 coming from the upper part of the digestion device 1 and provided 30 of a valve 29 The pipe 28 is connected to a heat recovery unit 30 from which the foul or smelly gases escape through a pipe 31 A pipe 32 goes from the outlet of the cyclone 22 to a tank of black liquor The upper part of the balancing tank 9 35 is connected by a pipe 34 to a valve 25 leading to the upper part of the digestion apparatus This upper part is connected by a valve 35 to the pipe 28 A pipe 36 provided with a valve 37 is connected to the pipe 16 The pipes 1 9, 23 and 21 are provided with

respective valves 38, 39 and 40.

  The stages in the operation of the installation are as follows: 1 A-B: filling with chips and fragments; 2 B-C steam blowing; 3 C-O: filling with extraction liquor; 4 D-E: heating with low pressure steam and steam from the flash cyclone; E-F: high pressure steam heating; 6 F-G: digestion; 7 G-H: evacuation of the extraction liquor; 8 H-I: head blowing in the recovery system; 9 I-3: gas escape towards the heat recovery unit; 20 10 3-K: washing liquid supply; 11.K-L: pressure increase to blow the digestion apparatus; 12.L-M: cold blowing;

13.M-N: final gas exhaust.

  The time interval A-A is 300 minutes, including

  including 35 minutes of duration between two cycles.

  We will now describe the successive stages of

in detail.

  A B: The filling with chips and fragments is started by opening the valve of the digestion apparatus and by starting the feeding with chips. Live steam and secondary steam coming from line 24 are admitted into the filling apparatus.

  When the proper level of chips in the digestion apparatus is reached, the valves are closed.

  BC: During steam blowing, the chips are preheated in the digestion apparatus by adding expanded steam from the flash cyclone 20 via the valve 33. At the same time the air in the apparatus digestion is expelled by the valve 37 in the line 36 The steam from the cyclone 20 is continued to preheat until the desired temperature or pressure is reached in the digestion apparatus 20 The valve 33 in the line between the digestion apparatus and the cyclone is then

  closed The discharge valve 37 is closed by temperature control means when the steam flow rate in line 36 reaches the threshold of the control system.

  CD: The extraction liquor is injected into the digestion apparatus through the bottom filter To increase the feed rate, it is also possible to introduce extraction liquor through a median filter The charge can be added of black liquor, advantageously via line 10, with flow control, from the balancing tank of the extraction liquor This saves the

live steam.

  When a sufficient quantity of extraction liquor has been introduced via lines 14 and 10,

the valves il, 15 and 17 are closed.

  DE The pump 3 for circulation of the extraction liquor is started. The indirect heating of the extraction liquor is started by live steam by carrying out a temperature control by means of a detection of the temperature at which the extraction liquor exits heat exchanger 2 Valve 6 is open while

valve 8 is closed.

  At the same time, live steam is supplied to the digestion apparatus via line 24 through the valve 33. The supply is automatically interrupted when the pressure in the aeration valve reaches a value close to that which prevails.

in the lightning storm.

  E F: During this period, additional heating up to the boiling temperature is carried out indirectly with live steam in

  the heat exchanger 2. 20 F G: Digestion.

  GH: The free extraction liquor (part of the extraction liquor being retained in the chips and fragments) is evacuated from the digestion apparatus during the digestion phase in question, using the extraction liquor circulation pump The valve 6 in the extraction liquor circulation line is closed, while the valve in the line from the heat exchanger 2 to the balancing tank is opened and the valve 17 c 8 pump suction head 3 Evacuation is automatically interrupted when the level indicator placed at a suitable level in the digestion device intervenes, closes valve 8 in the line towards the balancing tank and stops the circulation pump. Il H I: We lower the pressure in the digestion apparatus by opening the valve 33 of the line 25

  which goes from the digestion apparatus to the pipe 24.

  The steam valve 33 is controlled by a Jon tress control device, which closes the valve 38 in the liquid supply line of the cyclone 20 from the balancing tank, to compensate for the load peak caused by

  the supply of steam from the digestion apparatus.

  IJ: When the pressure in the digestion apparatus is lowered to a value close to that prevailing in cyclone 20, the valve 33 in the line 25 which leaves from the digestion apparatus I) closes automatically and the cyclone is again supplied with liquid from

of the balancing tank.

  J K: -We introduce cu washing liquid (filtrate from the washer) in the digestion apparatus through 20 the valve 15 The valve 17 is closed We pump the

  washing liquid through the bottom filter and the flow is controlled using a flow controller placed at an appropriate level Valves 35 and 29 in the gas exhaust line to the recovery assembly heat are closed.

  K L: The digestion apparatus is put under pressure by opening the valve 26 towards the balancing tank

for the extraction liquor.

  LM: The contents of the digestion apparatus are blown towards the blowing tank When all the contents have been blown, the valves 13 and 26 in the blowing line and the line which leaves from the balancing tank are closed automatically. 35 MN: The digestion apparatus is degassed to the heat recovery unit using the valve 29 When the atmospheric pressure prevails in the digestion apparatus, the lid is opened The digestion apparatus is then ready for a new digestion cycle. The cyclone flash 20 functions as an integral part of the digestion system This makes possible a return of degassing vapor and accounts for the momentary gas escapes and the

  thermal calls in the digestion plant.

  The energy flows leaving the basic system are measured by three flowmeters arranged on the lines 23, 27 and 28 The data from the flow measurements are introduced into a control system which acts first on the valve 40, then, in the alternative, on the valves

gas exhaust 29 and 35.

Claims (3)

REVEN? IC TT ', G   1 Process for recovering secondary heat during the digestion by loads of cellulose in digestion apparatuses, the extraction liquor 5 being brought to a balancing tank under pressure during the final stage of the digestion cycle, characterized in that washing liquor, advantageously filtrate from a washing by filter, is brought into the digestion apparatus for prewashing the mass of cellulose at relatively low temperature, advantageously at a temperature such as the temperature of the mixture of washing liquid and cellulose mass is at most equal to 105 ° C., and in that a digestion stream is brought in to blow out its cellulose mass content at a temperature ranging from 90 to 105 ° C, i.e. hunting by cold blowing.   2 Method according to claim 1, characterized in that, after the end of the digestion cycle, the free liquid is aspirated from the digestion apparatus through a   filter provided at the bottom of the digestion apparatus.   3 Method according to claim 1, characterized   in that, after the end of the digestion cycle, the extraction liquor is moved out of the management apparatus using the washing liquid. J Fo / FG 4.2 1985