NO119622B - - Google Patents

Description

Process and plant for continuous boiling

preferably by alkaline digestion of

cellulose-containing fibrous substances.

The present invention relates to a method for continuously

equal boiling, preferably by alkaline digestion of cellulose-containing fibers

substances, and more specifically aims to achieve a greater continuity in cooking

the digestion and absorption thanks to regulated conditions for the cooking and digestion so that the individual fibers that are finally released in the process

are set for more accurate tolerances with regard to the total chemical treat-

ling and/or the resulting entanglement.

In a typical chemical digestion process such as the power process, an

active absorption media such as sodium hydroxide and sodium sulphide are used

fid for processing the pulp, which usually consists of chips. The dissolved organic components in the used absorption liquids are burned for steam evolution

ling, and the inorganic leaching chemicals are recovered and reused.

Although the "mechanics" of such a chemical entrapment process is fairly easy to understand, it has so far been difficult to achieve an advantageous continuous entrapment process due to the extremes in the process conditions observed during the processing of chips and similar starting material for the production of cellulose. Although the present invention is not limited to only the power process, and can easily be adapted to any mainly chemical digestion process (as opposed to a mainly mechanical process), in the following the invention will primarily be explained in connection with the power process.

The present invention results in improved continuity in the overall process by subjecting the tile to such a "temperature-time-pressure-chemical concentration ratio" that a predetermined variation in the speed of the chemical reaction is produced, during which the speed of the chemical reaction on the chemicals in the individual fibers actually decrease from the outer periphery to the inner periphery of a chip, fiber bundle or particle in the process. Until now, it has been argued that the only practical way to achieve uniform cooking conditions had to involve a uniform initial saturation or impregnation of the tile with cooking liquid under relatively moderate conditions,

in order to avoid rather than induce a reaction increase in the rate of the chemical reaction from the outside to the inside, or vice versa, of the tile. One therefore leaned towards the notion that a uniform rate of the chemical reaction throughout the entire tile pieces was preferable. However, experts in this field strongly disagree both with regard to several theories and to practical proposals, and although it may very well be desirable to induce simultaneous and essentially identical boiling conditions throughout the entire piece of tile in order to achieve uniform boiling, it will be clear that when really strive for a continuous boiling and/or absorption in the true meaning of the word, you start by distancing yourself from this fundamental notion, although this starting point will, according to the opinion of many experts, entail difficulties.

However, this invention is not only based on the exact opposite of such a way of thinking, but intends to prescribe a practically feasible method with this starting point in mind. The invention aims, among other things, to provide a method in which an increased chemical reaction rate is produced from the outside to the inside of the chip pieces in order to release the individual peripheral fibers from the chip pieces at high speed, under regulated conditions for rapid boiling, in such a way that ( 1) the fibers that are first exposed to the most active chemical reaction at the periphery of the tile are removed very quickly from the area of active chemical reaction, and (2) the next inner fiber layer will then become the outer peripheral fiber layer of the tile and will then pass through it same treatment (1). Essentially, this proposal involves the use of treatment conditions which have hitherto been considered too harsh and which would inevitably lead to overcooking of the first chemically attacked fibers at the periphery of the tile; but in practice, the boiling according to the invention is carried out during a certain movement or stirring of the tile so that the first chemically attacked fibers will first weaken in their bonds to the pieces of tile and thus more easily be torn apart under relatively mild conditions (in contrast to mechanical treatment), and the thus freed fibers are removed fairly quickly from the reaction area by a liquid stream which transports the free fibers to a sieve in the form of a narrowed drain from the boiler. This narrowed drainage passage is dimensioned in such a way that it only allows the passage of free fibres, i.e. the narrowing forms a drainage opening with an insignificantly greater width than the size of the free fibres. In this way, the free fibers are brought away from the area of the greatest chemical activity, and passed through the sieve (the narrowing), after which they are freed from a significant part of the free liquid by mechanical separation, so that any subsequent boiling conditions to which the fibers are exposed, easily can be controlled and regulated, and the fibres, which in this condition mainly have the same size and chemical content, will be cooked evenly. In contrast, they become pieces of tiles etc. which has not yet been reduced to free fiber size or condition, circulated rapidly through the area of greatest chemical activity, so that the peripheral fibers thereof are repeatedly subjected to short-term chemical attack which weakens the bonds to the chip pieces and then the fibers are allowed to release for removal of these from the area of chemical influence, or at least away from the area of greatest or more drastic chemical influence, while the piece of tile etc. which at all times produces new, firmly bound fibers until the area of maximum chemical action is kept continuously in this area or circulated through it in such a way that a more or less continuous sequence of maximum chemical actions takes place on the periphery of the tile followed by rather rapid release of the thus affected fibers on the surface of the tile and uncovering of new fibers. In this continuous process, the conditions are regulated to such an extent that each fiber, which in its original state was bound to a fiber bundle or the outside of a wood chip, is exposed to approximately the same average amount of chemical influence, but during a relatively shorter period of time than what is normally involved with continuous processes according to previous proposals, and in a much shorter time than with the known batch processes.

The method according to the invention is characterized by the combination of the following features: (a) continuous circulation of a liquid flow at a pre-determined pressure and boiling temperature through a circuit consisting essentially of a boiling line which feeds over a discharge zone into

a mechanical separation zone and then back to the cooking zone; (b) as per se known introduction of water and cooking chemicals forming a cooking liquid,

and lignin-cellulose fiber particles in the digester to continuously maintain a thin suspension with a solids to liquid ratio of between

1:100 and 1:8; (c) continuously maintaining in the cooking zone a chemical boil

and entrapment where the relationship between concentration, pressure and temperature is sufficient to effect continuous chemical entrapment with resultant weakening of the bonds of the outer fibers to the particles at substantially greater strength and speed than for the inner fibers in such particles; (d) continuously subjecting the suspension to gentle stirring which is sufficient to quickly detach from such particles individual outer fibers whose bonds have been weakened, thereby exposing new fibers and exposing them to the conditions in the cooking zone, thereby reducing the residence time of the fibers in the cooking liquid to a minimum to prevent these from breaking down in the cooking liquid; (e) continuous to put

the said particles in motion in the suspension in the immediate vicinity of narrowed drain openings in the discharge zone, which drain opening(s) have a "size" corresponding to the size of the liberated fibers, in countercurrent to the fiber-carrying liquid flow entering these openings, to continuously clean the openings; (f) continuous circulation of the liquid stream which transports said freed fibers through narrowed drain openings and into the mechanical separation zone, where a part - between 10 and 95% - of the free liquid is mechanically separated from the liberated fibers and recycled to the digester; and (g) during the aforementioned steps (a-f) of the process continuously maintaining a sufficiently large pressure immediately adjacent to the surface of the particles and fibers to prevent mechanical bursting of these upon relief of internal pressure.

From the foregoing, it appears with complete clarity that an important purpose of the present invention is to provide an improved continuous boiling

and chemical containment process and system.

Other objects, features and advantages of the present invention will be apparent from the following detailed description with reference to the drawings. Fig. 1 schematically shows a cooking or soaking system for carrying out the invention; Fig. 2 and 3 schematically show two embodiments of the system according to fig. 1 included cooking container (boiler); Fig. 4 is a partial section on an enlarged scale, taken in the area of the aforementioned narrowed drain from the cooking container, where the drain opening had a size corresponding to the free fibers. In fig. 1, 2 and 3, this strainer or constriction is denoted by R, while R. in reality denotes a propeller in fig. 4. These propellers are also schematically shown in the other figures. 1 fig. 1, the entire system is designated as one with the reference number 10. The system comprises a boiling vessel or boiler 11 with narrowed drains R (which will be described in detail later), a mechanical separation zone or device 1Z which is only shown schematically, but which in practice can be formed by a normal overpressure press. With regard to the constructive design of the devices included in the system, attention is drawn to the state of the art, e.g. as described in U.S. patents Nos. 2,466,290, 2,789,052 and 3,096,734, which show various devices that can be used in practice in connection with the present invention. Many of the devices included in the system are also common merchandise and are therefore only shown heavily schematized.

In order to follow the cellulose-containing components through that in fig. 1 shown system, it will be seen that the wood chip C enters the top of the boiler 11 via a normal pressure feeding device 21, from where the chip falls down to the liquid level Li-l which is indicated by dashed lines inside the boiler 11. The liquid in the boiler is continuously circulated and as soon as the chip pieces have absorbed sufficient moisture to sink into the circulating liquid streams inside the digester 11, the previously mentioned incipient chemical action occurs and the peripheral fibers are rapidly and continuously removed from the chip pieces solely by means of a mild (weak) stirring motion exerted on the pieces of tile moving around in the fluid streams by the rotating action of the propellers. With reference to fig. 4, it will be seen that the propellers R^ have the form of a kind of paddle wheel 36 mounted on a rotating shaft 30, mounted in a pair of bearings 31,32. The shaft 30 is arranged concentrically in a sleeve-shaped housing 33 for the removal of liquid-transporting free fibres. The housing 33 is connected to a drainage line 34 which is connected to a main line 35a (Fig. 1) or 35b (Fig. 2) or 35c (Fig. 3) which leads to a mechanical separator 12 in the form of an overpressure press. In the example, the rotation shaft 30 is made in one with the actual vane wheel or propeller 36, which is designed with a number of wings (blades) or vanes 36a, 36b, 36c that protrude from a flat disk-like bottom 36x. The paddle wheel (propeller) 36 can, instead of being made in one piece with the shaft 30, of course be manufactured separately and rigidly connected to it. The paddle wheel 36 serves to exert centrifugal forces on the chip pieces, fiber bundles, etc. solids dispersed in the liquid in the digester as they attempt to approach the constricted drain openings 37, which are formed by an annular passage of minimal thickness between the outer circumferential underside of the impeller disk-shaped bottom 37 and an annular rib 38 (see Fig. 4 ), mounted on the

the container wall 39. The annular rib 38 can of course be mounted on

an intermediate layer plate or otherwise made adjustable for regulation of the opening 37. The annular rib 38 is ground in such a way that the formation of dead zones is avoided, as the vane wheel which moves in relation to the rib 38 continuously ensures a self-cleaning of the opening 37. The wings (the blades ) 3^a, 36b, 36c etc. also serve to circulate solid particles in countercurrent to the liquid flow in through the opening 37 and thus lead these particles or unreduced chip pieces away from the drain slot 37 to continuously clean this also in this way. In addition, the paddle wheels (propellers) can also be used as the only source to achieve circulation of the liquid in the boiler 11. Under relatively high pressure conditions in the boiler 11, 11b, lic, there will in each case be a tendency for a relatively significant flow of liquid under each of these vane wheels (propellers) 36 in the narrowed drainage zones (empty zones) and out of the lines 34 to the main line 35a, 35b, 35c. This rather significant flow of liquid will quickly transport with it free fibers from the interior of the boiler and thus bring these quickly away from the area of maximum chemical activity, while these rotating bodies 36 at the drain constrictions 37 at the same time serve to stir the pieces of wood chips in the liquid and circulation of the liquid, so that they simultaneously perform several important functions that are important in the present process and apparatus. It will thus be understood that the wood chip pieces, particles or fiber bundles in the system according to fig. 1 will remain in the boiler 11 until they are reduced to

"free-fiber" size during this continuous circulation process in the area of maximum chemical activity and shortly after they are reduced to the so-called "free-fiber" size, the resulting free fibers themselves will find their way through the associated openings 37 in each drain and enter the main line 35a and from there to the separator (overpressure press) 12 for pulp/liquid. In the separator 12, a screw conveyor (conveyor screw) is arranged which works inside a screening device, so that the mass is pressed and the free liquid is pressed out of the mass and through the sieve (screening device), so that the liquid is again brought into the circular process as indicated by 40 in fig . 1. The "pressed" mass which is then freed from approx. 10 to 95% of the free liquid is extracted by the screw press 12 through a line 41, while it is still under pressure, and fed into a separator container 42 which is also under pressure. The container 42 has an air line 43 which leads to a main air line 44, which is also connected to a line leading from the top of the boiler 11 for the discharge of air, volatile substances such as turpentine etc. periodically must be released to create the conditions for smooth, continuous operation of the plant. Between the air lines 45, 43 and the main blow-off 45, a valve 46 can be connected which can be regulated so that it is avoided

a total pressure loss in the digester 11 or from the digester 11 all the way down to the container 42, as it is desirable to maintain a significant excess pressure throughout the treatment course for the cellulosic material. The cellulose material that enters the separator container 42 is in the form of a relatively concentrated mass, i.e. it has a relatively high consistency, but it also has

a relatively high temperature and if immediately exposed to free atmosphere would result in a tendency to burst the fibers due to the sudden evaporation of the water in the fibers. In practice, the facility for carrying out the method according to the invention at the various stations will be equipped with appropriate devices to keep the pressure on the immediate periphery of the fibres, pieces of wood chips etc. sufficiently large to exclude mechanical bursting of these at all times when the internal pressure is released. This obviously means that the pressure in the boiler 11 is kept sufficiently high to exclude the formation of a vacuum or negative pressure below the saturation pressure for the liquid in the particles at the propeller blades or vanes

36a, 36b, 36c etc. This is carried out exclusively by maintaining a sufficiently large total pressure in the boiler to compensate event. local minimum pressure areas that may arise during the use of the stirring devices etc. , and this presents no difficulties whatsoever because the ratio between dry matter and liquid is quite low in the boiler; approximately 1:100, in some cases up to 1:8. The liquid suspension in the boiler is thus rather thin and can thus be exposed to the influence of the weak stirring devices and/or suitable rapidly rotating circulation devices, forming noticeable areas of negative pressure in the boiler, where the temperature will also be significantly above normal outside temperature; actually above the water's boiling point in most cases, so that if the mass in the boiler was exposed to the external atmosphere, the water inside the fibers would evaporate away and cause the fibers to burst mechanically. It will be understood that at all times an overpressure is maintained in the liquid suspension in the digester and in the liquid flow which transports the free fibers in the lines 34 and the main line 35a down to the overpressure press 12 and through the cellulose fiber extraction line 41 to the separator container 42, where the mass, which has a relatively high consistency, is first exposed to cooling washing water via the line 49. The cooling washing water has only the purpose of reducing the temperature throughout the fiber body to a temperature below the liquid's evaporation temperature (under the pressure conditions at the outlet of the valve 50), so that the wood pulp fibers can be pulled without risk of bursting out of the bottom of the container and via a control valve 50 into a line 51 which leads to washing devices for the pulp. The control valve 50 is affected by a liquid level regulator LLC illustrated purely schematically, which regulator senses

the liquid level in the container and ensures that it is kept constant under the desired operating conditions. In this way, a conventional pressure regulation is maintained in the container 42 and adjustable draining of liquid from the same, whereby the mass is fed to the washing devices in the line 51 at a temperature which is lowered by the washing water from the line 49 in order to exclude the possibility of mechanical bursting of the fibers.

The foregoing mainly describes the "advance process" of the cellulosic components through the system 10. It has previously been indicated that this "advance process" does not take place until fibers are exposed in the digester, so that previously only a circulation of the chip etc. can take place. in the boiler, while later at the same time a continuous "forward process" of the liberated cellulose fibers takes place in the manner just described.

The stirrers R (agitators) are concentrically mounted in the drain openings from the boiler. The stirring devices themselves in the form of propellers (paddle wheels) are common merchandise (e.g. Morden's "SLUSHMAKER").

As regards the main liquid circuit, it has already been mentioned that a substantial liquid flow will pass behind the agitators and out through the outlets 34 to the main line 35a for pulp/liquid (mainly transporting free fibers) through the mechanical separation device 12, in which a substantial part of the liquid is pressed out of the mass which with increased consistency is led via the line 41 to the container 42, the separated liquid being drawn away through the circulation line 40 to the main circulation line 60 which in turn feeds a circulation pump 61.. A normal pH value regulation device 62, which is only visualized purely schematically, senses the pH value in the suction line 60 of the circulation pump 61 and controls a valve 63 for adjustable supply of fresh liquid from a supply device 64 to the suction side of the circulation pump via the main line 60. It will be understood that there is a consumption of chemicals of the one or the other variety used in the fresh liquid during the boiling and soaking operation nene, and this consumption can e.g.

on the basis of the weight of completely dry fibers, be about 14 to 18 percent by weight of the active alkalis used (i.e., the sodium oxide equivalent in the case of sodium hydroxide alkali). The chemicals that are thus consumed during the process must therefore be returned to the system via the circulation pump 61 and it is advantageous to first dilute the chemicals, in the form of fresh liquid from the supply source 64, with the circulating liquid in the main line 60

so that local zones with too high a concentration of chemicals do not occur in the boiler.

Liquid circulation continues from the outlet of the circulation pump

61 through a control valve 65, a conventional heat source 66, which is shown in the drawing in the form of a conventional heat exchanger in which steam is fed controllably into the so-called "outer" phase of the exchanger 66 and condensate is removed in a known manner, so that direct vapor contact heating is not used, as the liquid passes through the "internal" phase of the heat exchanger 66. Such heating is not absolutely required, but is only discussed to show that heat can be returned to the system according to the invention without the need for supply of water, if this is desirable. The line 68 from the circulation pump 61 leads through the valve 65, the heat exchanger 66 and into the boiler 11 to complete the circuit for the liquid, which circuit is adiabatic for practical reasons, as it is of course desirable to keep the heat in the liquid back in the boiler, within practical limits, than to draw the liquid out of the kettle and cool it, then have to heat the liquid up again. The fresh liquid from the supply source 64 will of course have a cooling effect, and some addition of heat will therefore be required, e.g. via the heat exchanger 66 to compensate for the fresh liquid, and there will also inevitably be a certain heat loss in the circulation system regardless of how effectively the lines 40,60,68 etc. are insulated. The main purpose of the various heating devices is to heat the wood chips to the desired boiling conditions. Other heat recovery systems can also be used.

It will further be seen that an additional control is provided on the basis of conventional equipment, in this case by means of a consistency sensor device 69 of usual construction, inserted into the main line 35a for pulp/liquid from the digester 11, which device emits a signal (indicated with the dashed line 69a) to the control valve 65 to regulate the degree of consistency in the circulating liquid in such a way that the total amount of circulating liquid maintains a predetermined consistency in the main line 35a, which consistency may vary to adapt to the current needs, but preferably stays within the previously mentioned narrow consistency range as suggested for the cooker itself. It will be realized that a significant amount of liquid passes through the main line 35a in the return flow and transports free fibers with it, so that even lower consistencies will occur than the previously stated limits for the boiler. The consistency is adapted in each case to the relevant need, so that the operator can effectively keep the narrowed drain openings R so set that an unobstructed passage of the freed fibers out of the boiler is achieved at the desired speed. To carry this out in any given system, a certain amount of fluid is required as a carrier and this amount of fluid can be easily determined in any given situation by the operator, and as soon as it is determined it can be used in the previously described control system 69, 69a, 65.

The boiler 11 will also have a liquid level regulator, indicated schematically with the box LLC, which senses the level L-l in the boiler 11 through the sensor line 71, regulation of the liquid level in the boiler being achieved by means of a normal regulation line 72 with a liquid level control valve 73 for extraction of liquid from the circulation system via a liquid recovery line 74, which exits from line 40 from the mechanical separation zone 12. It will thus be understood that a continuous increase in the total liquid or water content in the boiler cannot be allowed, and a certain amount of used liquid must is extracted through the line 74 via the control valve 73 continuously during operation to compensate for the addition of fresh liquid from the supply source 64, and thus maintain a constant liquid level in the boiler. The liquid recovery system, to which the line 74 leads, is of a completely ordinary structure and therefore does not need to be described in more detail. It will be seen, however, that a return line 75 from the mass-flushing apparatus (which is also of known design) is shown, which return line 75 can return a certain amount of used liquid to the liquid recovery line 74 for the same purpose. In this way, the entire fluid system should be satisfactorily explained.

With regard to the heat balance of the system, it has already been explained that a significantly larger amount of liquid with a significant heat content will be used in the boiler 11, so that the rather significant volume of liquid used in the boiler 11 for the purpose of achieving the desired thinness must necessarily flow through the circuit 35a, 40, 60, 68 and back to the digester 11 in a manner which must necessarily involve certain heat losses, despite seeking to keep such heat losses within reasonable limits to maintain essentially adiabatic conditions with respect to the truly recycled liquid. Heat loss will occur in the mass that is removed from the mechanical separation zone 12

via wire 41; in the fluid supplied to the recovery system via line 74, and in the new additive fluid via control valve 63 (the latter of which is technically not a heat loss but rather an insufficient heat input since the added material is not hot enough); and the additional heat to the liquid system can assume one of two forms, possibly. both of these in combination. One of these forms of heat supply is the so-called dry- . heating system 66 where fresh steam is not allowed to come into contact with the liquid itself in the heat exchanger 66, and the other form of heat supply is indicated schematically by the line 75 in which fresh steam is fed from a steam main line 76 through a control valve 77 and directly into the boiler lii above - agreement with temperature regulation signals via the line 78a to a

signal receiver 78 which controls the valve 77 via the signal line 78b; all these parts represent a normal temperature control system and are therefore only shown schematically.

Although any "set" operating conditions will cause variations in the total operation, which the operator of the plant is familiar with, in the following it shall; a typical "set" operating condition is described: As the wood chips are fed through the pressure feed device 21 directly into the digester 11, there will be a nominal "unknown" with respect to the "time-temperature-chemical reaction" relationship while the chips absorb enough moisture to enter in the circuit, but this so-called "unknown" will in reality turn out to be a predetermined nominal number of minutes; close to five minutes on average. The wood chips used are of the usual size and grade for e.g. the power process. The trapped air and non-condensable gases that enter together with the wood chips will evaporate away very soon on contact with the hot liquid in the boiler and this evaporation will produce a pressure head via the back pressure on the vent valve 46, so that a pressure of 9.84 kg/ cm on the boiler, on the separator container 42 and on the parts of the system located between these. When using a dry fibre/liquid ratio of 1:20 in the cooker, set the temperature regulator to 171°C. The incoming raw steam through the valve 77 is set to the aforementioned temperature. The consistency sensor 69 is set to control the valve 65 to keep the consistency inside the cooker within the previously mentioned limits, in connection with a predetermined liquid level regulation through the regulator LLC. The alkali consumption for this particular system is calculated on the basis of 16% of the dry fiber weight, and the sulphidity in the cooking liquid in the boiler 11 is kept at 25% in this particular example, which means substantial control of the access to fresh liquid via the pH value regulator 62 The fresh liquid in the supply source 64 contains a concentration of sodium sulphide and alkali such that not only is the predetermined sulphidity maintained in the digester 11, but also the range of maximum chemical activity in the digester 11 is maintained so that the effective alkali concentration therein is 30 grams/litre. These concentrations in boiler n are maintained via the pH-value regulator 62, and the amount of fresh liquid that is needed through the control valve 63 requires a measurable amount of additional heating in the relevant liquid line 68, so that the steam on the heat exchanger 66 is set manually to this heat addition, while the temperature regulator 78 will automatically maintain the exact temperature conditions in the cooker.

The rotation of the propellers at the narrowed drains will of course cause the total circulation of the liquid and the chips in the digester 11, while the peripheral fibers on the chip pieces are exposed to the maximum chemical influence, have their bonds weakened and are released from the chip pieces finally after a minimal residence time in the digester to flow out of the drains and into the main line 35a. The speed in the mechanical separation zone 12 is controlled so that some further "compensating" or "equalizing" boiling can take place in this zone which operates under approximately the same pressure as the boiler, and under approximately the same temperature to reduce the heat losses in the overall circulation system - tem.

It will be understood that the chemical boiling of the free fibers is slowed down how

rise by the addition of washing water via line 49 into the separator container and by the subsequent mass washing in the mass - rinsing devices, so that the actual elapsed cooking time is the time in the boiler 11, in the main line 35a and in the separator 12, with an insignificant additional cooking time in the separator container during the boiling slowdown as a result of the cooling effect from the wash water. The average time the wood chip pieces maintain their "particle state" (i.e. before they are reduced to free fiber size and can pass the constricted drains) during the continuous circular process in the digester in the area of maximum chemical influence, is calculated on the basis of 15 minutes in present system and the residence time for the free fibers to travel from the drains R to the container 42 is calculated on the basis of a residence time of approximately 5 minutes, resulting in a total cooking time of only 20 minutes in the example described.

It will be understood that from a theoretical point of view the peripheral fibers of a particular wood chip will undergo only a nominal reaction to chemical

one in the area of the maximum chemical influence in the boiler 11, under conditions which only involve the theoretical exposure of the outer of the peripheral fibers to the area of the maximum chemical influence with the internal (internal) binders, i.e. the lignins, protect the back of the fiber on a nominal way, so that theoretically at the rate of chemical boiling used here, in reality the outer layer of fibers becomes rapidly boiled

ere than the interior, but it is finally the boiling of the inner side of the fiber that results in the binding, which weakens the bond (lignins) and enables the weak stirring to release the fiber. Once the fiber is detached or released, it temporarily remains in the area of maximum chemical action in the digester under conditions of maximum effective chemical "attack". It-

ne time period is preferably a time period which is reduced by the circulation rate of the liquid inside the boiler 11, so that there is a net effect of the theoretical circulation rate which enables "washing away" of the free

fibers through the narrowed drain openings 37, in an average time of approximately one minute residence time per fiber. The foregoing is admittedly a mainly theoretical method of calculation, but it is included in the hope of clarifying the invention. The basic principle is that the so-called "lignin'-

filled" fibers will remain glued to the chipboard and thus partially protected against maximum chemical attack until the entanglement of this particular single fiber has been carried to such an extent that it breaks free from the chipboard. Up to this point, only a part of the uncovered side of the fiber and even a much more nominal part of it immediately within the fiber under direct chemical "maximum attack". As soon as the fiber is detached, however, it will find itself surrounded by maximum chemical activity intended to produce the desired increase in chemical action from the exterior to the interior of the tile, can be a stronger chemical influence than what is otherwise used in batch cooking

ning or other slow-acting boiling and entanglement methods, so that it is required that the circulation speed is such that the residence time for the free fibers is reduced to a minimal time, and that the free fibers are quickly allowed to pass out through the drains R and into the separator 12. The transport of the free fibers in the cooking liquid through the main line 35a can of course be carried out in a few seconds, so that the preceding cooking can be neglected, but in the physical separation zone 12 there will again be a measurable residence time during which the mass will at least initially be exposed for chemical action, temperature and pressure which are essentially identical to those in the region of maximum chemical activity in the digester.

As previously mentioned, the mechanical separation zone includes an overpressure press or similar device which relatively quickly presses a significant part of the liquid out of it. the fiber mass, which starts an immediate but controlled slowing down of the maximum chemical impact, whereby it will immediately begin to reduce the relative chemical concentration on dry fibers, even if pressure and temperature do not change significantly. This reduction in the chemical concentration/dry fiber ratio occurs under regula-

conditions, in fact very easily adjustable conditions, as the working method of overpressure presses or similar separators for pulp/liquid is relatively easily adjustable, and the ratio between the chemicals present and dry fiber mass can be varied under controlled conditions over a predetermined period of time, such as e.g. 5 minutes as suggested, so that the exact degree of actual chemical boiling that takes place in the mechanical separation

zone can be assumed to be determined on this basis, provided that a substantially constant temperature and pressure is maintained on the individual fibers during the regulated slowing down of the chemical boiling. In the

mechanical separation zone, there will be a nominal number of fibers which are presumably "overcooked" (i.e. have boiled too long) as well as a nominal number of fibers which, presumably due to accident, have been released and pulled out too early from the cooker, so that they have boiled too little, but the overwhelming majority of the fibers in the mass in the mechanical separation zone 12 will be within a relatively limited area of so-called "cooked conditions" which can be easily measured by the average lignin content. Under regulated conditions, the average lignin content in the mechanical separation zone in this particular example will be around 18% (plus or minus approx. 1%) for around 90% of the fibres. The average residence time for a released fiber in the area of maximum chemical activity can be relatively easily regulated by using a high-speed liquid cycle process and the narrowed drain openings for removing such free fibers in the manner just described. It is now desired to reduce this average 18% lignin content in the aforementioned 90% of the fibers in the mechanical separation zone 12, and it will be understood that the speed of the liquid overpressure press or a similar device can be easily regulated; e.g. if it is desired to reduce this lignin content constantly to only 2% on average, the press is regulated so that the residence time in it is approx. 5 minutes, and if a reduction of e.g. 5%, the residence time in the mechanical separation zone is regulated so that it becomes correspondingly longer. An equalizing effect takes place in connection with the mechanical separation whereby the fibers with maximum lignin content will tend to be more susceptible to chemical influence than fibers with minimal lignin content (more or less as a logical consequence of the law of mass action in the chemistry), as many of the "variables" that could alter the propensity of maximum lignin concentrations to undergo maximum chemical attack have been eliminated. There are thus mainly no local high pressure or high temperature zones in these mechanical separation zones; the real ratio between chemical content and dry fiber content can be regulated almost instantaneously throughout the entire mechanical separation zone or process, and the previously mentioned "constriction drainage system" from the digester entails an introduction of almost only free fibers into the separation zone 12 with essentially a uniform "physical " size, fibrillation, matting etc., so that the conditions for chemical equalization of the lignin contents are as favorable as possible.

With reference to fig. 2, it is assumed that approximately the same operating conditions are maintained with respect to time, temperature, concentrations, pressure etc. as for the boiler lii fig. 1, as the boiler in fig. 2 broadly has the same constructive design as 11, with the exception that the narrowed drains with the circulation propellers R, in fig. 2 is placed near the bottom of the boiler 11a, in such a way that no local dead zones can form, but directly in the path of a substantial flow of the circulating liquid which flows directly past the drains R.R^ and out of the bottom of the boiler through a suction line 80 into a circulation pump 81 which brings the liquid back to the boiler 1a via a short return line circuit 82. The return liquid is brought into the boiler 1a at a predetermined submerged level indicated by arrow 82a, in order to give the system a additional control factor ("variable"). The four narrowed drains R, R^ will again extract a substantial liquid stream containing free fibers which is fed into the main line 35b for mechanical separation at 12, as shown in fig. 1, but the circulation system 80,81,82 is intended to enable an additional regulation of the real residence time for the free fibers inside the boiler bed.

It must be noted that the mode of operation of the boiler 11 or 11a, or 11b

in fig. 3, does not exclusively consist in washing a significant amount of wood chips with a relatively small amount of liquid so that loose fibers will be washed off and their residence time in the digester is accurately calculated on the basis of the residence time of the circulating liquid passing through the digester, as an important feature in the invention, the use of such significant amounts of liquid means that a mainly continuous liquid phase is maintained with a phase of wood chips dispersed therein, and the liquid dispersion has the desired fluidity. When this is the case, there will always be a risk of a certain (small) number of free fibers entering the circulation of the liquid instead of being removed through the extraction system, which is to some extent unavoidable. On the other hand, by selectively regulating the circulation system and the speed of liquid circulation through the boiler via one or more streams, it is possible to effect a sufficiently rapid movement of a significant amount of the liquid in the boiler out through the narrowed drain openings R, so that the part of the free fibers will have a very short average residence time in the boiler 11, 11a, 11b, as already explained, in the area of maximum chemical activity. An important feature of the invention is not only the production (instead of the proposed "destruction") of the reaction rise from the outside to the inside of a particular piece of tile, but also the reduction of the residence time of the released fibers in this area for maximum chemical influence. This is simply carried out at regulated flow rates of the circulating liquid streams.

From fig. 3, it will appear that the arrangement here is somewhat different, although the boiler itself is constructed in roughly the same way as the boiler in fig. 2 with the narrowed drain openings with the agitators placed near the bottom of the digester 11b. A main drain line 80b leads to the suction side of a circulation pump 81b which is connected on the pressure side to a short circulation line 82b, which in the same way as with the device in fig. 2 leads back to the boiler and opens below the liquid surface in it. The fundamental liquid circulation via the mechanical separation zone 12 and the liquid system 40, 60, 68, etc. in fig. 1 is also used in the arrangement in fig. 3, but the devices in fig. 2 and 3 involve further regulation of the effective flow rate of the liquid through the boiler lia or 11b.

A further feature of the device in fig. 3 consists of a pre-wetting

of the chip C. The wood chip C, which is fed into the system at 90, is pre-moistened with white liquor which is fed in at 91, after which this pre-moistened mixture is pressed and fed through an overpressure press 92 into the circulation pump 81b. The overpressure press 92 is in the embodiment equipped with a conventional screw conveyor 92a, which is driven by a motor 92b and is enclosed near the discharge end by a cylindrical screen 93 of a known type, which serves to collect excess white liquor which is circulated back through the line 91 to be re-added to the tile. This system brings certain advantages in that the chip is actually fed into the short circuit 80b, 81b, 82b at approximately the point where the particles begin their movement through the circuit after they have just released their outer layer (cover) of peripheral fibers in the area near the narrowed drainage openings R, R. The particles (pieces of wood chips) which have just "formed" new, uncovered peripheral fibers are thus mixed with damp chip pieces which also form fibers which are in effect the newly uncovered peripheral fibers which can be compared to those on the particles located themselves in the circular process.

As a method considered, it will be understood that the system 10 with or without the alternative designs in fig. 2 and 3 involves a continuous boiling and chemical sintering method, which comprises (1) continuously circulating a sintering liquid stream at a predetermined pressure and boiling temperature through a circuit essentially consisting of a boiling zone which is continued above a draining zone by a mechanical separation zone and then back to said cooking zone; (2) introducing cooking liquid (water and sintering chemicals) and lignin-cellulose fiber particles (wood chip pieces) into the digester to maintain a thin liquid suspension with a solids to liquid ratio of between 1:100 and 1:8; (3) in the cooking zone at all times to maintain a chemical boiling and entanglement where the relationship between concentration, temperature and pressure is sufficient to continuously cause chemical entanglement with the following weakening of the "bonding" of the peripheral fibers to the particles (wood chip pieces) with a significant greater strength than that of the internal fibers in such particles; (4) continuously subjecting the suspension to gentle agitation sufficient to continuously loosen such individual peripheral fibers whose "bond" is thus weakened, thereby exposing new peripheral particulate fibers to conditions (3) in the cooking zone; (5) continuous circulation of said liquid flow transporting said free fibers (4) through one or more narrowed drains, the opening of which has a size corresponding to the size of said free fibers, in said emptying zone and into the mechanical separation zone where a substantial part , from 10 to 95% of the free liquid is mechanically separated from the free fibers and circulated back to the cooking zone in the circuit at (1); (6) continuously setting the said particles (wood chip pieces) in motion in the suspension in the immediate vicinity of said constricted drain openings to set the particles in motion countercurrently to the fiber-carrying fluid flow entering the constricted drain opening to thereby continuously clean the drain opening (e); and during said steps (1) - (6) continuously maintaining a sufficiently large pressure on the immediate periphery of said particles and fibers to prevent their bursting upon release of internal pressure. The agitation is kept so weak that the fibers are not exposed to mechanical damage. The mechanically separated free fibers and liquid contained therein are still held under said predetermined pressure and subjected to continued chemical cooking to reduce the lignin content to below the average content of free fibers leaving the cooking zone, after which the fibers are quenched to reduce the internal fiber temperature to below 100° C to allow rapid reduction of pressure to normal atmospheric pressure.

In a typical system, process step (3) involves temperatures between .100 and 200°C, and excess pressure between 1.41 and 17.6 kg/cm . In the kraft process, the alkali concentrations should lie at from 5 to 25% of the dry fiber weight on the basis of 100% sodium hydroxide concentration, and at 10 to 50% sulphidity in the digester itself. In other processes, pressures and temperatures can conveniently be kept within the limits drawn here, but other chemicals such as combined alkali-SO^ are used in relatively high concentrations, or only alkali can be used in the concentrations mentioned here.

The combination of temperature, pressure and chemical concentration should generally be such that a reaction gradient is produced from the outside inwards on the individual chip and/or particle. In most cases, this will involve a reaction ratio between the aforementioned variables that effectively exceeds the rate rates for the boiling and digestion by at least 25%

on the basis of chemical influence and preferably by at least several hundred percent, so that the residence time for the free fibers in general in the area of maximum chemical influence is reduced to a fraction of the time that otherwise comes into consideration in previously proposed rate processes. In this respect, the invention involves chemical boiling or digestion at such a strength and

speed that the weakening of the bond of the peripheral fibers is significantly accelerated and occurs much faster than the weakening of the bond of the internal fibers. This can of course be difficult to illustrate purely numerically, although it will be easily understood by professionals in the field, as it essentially represents an "exaggeration" of a chemical boiling result that professionals in the field have tried to avoid for years and it is connected to the idea of circulating excess amounts of liquid (from the previous point of view) through a narrowed opening zone with the intention of preventing the extraction of unfree fibers but withdrawing the free fibers very quickly from the area of maximum chemical action. These "impeded" or retained free fibers are of course recycled and in the process according to the invention this recycling takes place very quickly and is due to the chemical influence which is so great that the particles will very quickly deposit a new peripheral coating or layer of free fibres, and this newly deposited peripheral coatings of free fibers must then be removed from the area of maximum chemical action at the same rate employed in the release and removal of the first circumferential layer of fibers on any given chip or particle fed into the system.

The invention also includes an apparatus system in the form of a combination of means, previously described in detail, but referred to generally as a continuous boiling-chemical digestion system, comprising (1) means for continuously circulating a digestion fluid stream by overpressure and boiling temperature through a circuit consisting mainly of a boiling zone which above a discharge zone is continued by a mechanical separation zone and then back to said boiling zone; (2) means for continuously introducing into the digester digester liquid and lignin cellulose fiber particles to continuously maintain a thin suspension in said digester with a solids to liquid ratio of from 1:100 to 1:8; (3) means for continuously maintaining in the cooking zone a ratio of concentration, temperature and pressure for the chemical boiling and engorgement which is sufficient to effect chemical engorgement with the consequent weakening of the bonds of the peripheral fibers to said particles at a significantly greater strength than for internal fibers in such particles; (4) means for continuously putting the suspension in a gentle stirring motion sufficient to continuously detach from such particles individual peripheral fibers whose bonds have been weakened in this way and thereby expose new peripheral fibers, which are thereby subjected to the same treatment (3); (5) means forming narrowed drainage openings in the emptying zone whose size essentially corresponds to the free fiber size, which drainage openings are formed between relatively movable surfaces to divide the liquid flow into a first and a second liquid flow; (6) means for continuously circulating the first liquid stream transporting free fibers (4) through said narrowed drain openings and into the mechanical separation zone where a substantial part, between 10 and 95%, of the free liquid present is mechanically separated from the free fibers and is circulated back to the digester under essentially adiabatic conditions; (7) means for continuously circulating particles that have not yet been reduced to free fibers away from said constricted drain in the second fluid stream, and thereunder continuously cleaning the drain openings, and directly back into the circuit (1) also under essentially adiabatic conditions; and (8) means operably associated with previously mentioned means (l)-(7) for continuously maintaining a pressure-temperature relationship at the immediate periphery of said particles and fibers in the aforementioned means (l)-(8) such that the minimum pressure at any time and at any place in the system is above the saturation pressure of the liquid in the particles and in the fibers, thereby preventing mechanical bursting of the fibers by release of internal pressure.

Claims (7)

1. Procedure for continuous boiling, preferably by alkaline digestion of cellulose-containing fiber substances, characterized by the combination of the following features: a) continuously circulating a sintering liquid stream at a predetermined pressure and boiling temperature through a circuit consisting essentially of a boiling zone which, over a draining zone, feeds into a mechanical separation zone and then back to the boiling zone; b) as is known per se introducing water and cooking chemicals forming a cooking liquid, and lignin-cellulose fiber particles into the digester to continuously maintain a thin liquid suspension with a solids to liquid ratio of between 1:100 and 1:8; c) continuously maintain in the cooking zone a chemical boiling and entanglement where the relationship between concentration, pressure and temperature is sufficient to cause continuous chemical entanglement with the resulting weakening of the bonds of the outer fibers to the particles at significantly greater strength and speed than for the inner fibers in such particles; d) continuously subjecting the suspension to gentle stirring which is sufficient to quickly detach from such particles individual outer fibers whose bonds have been weakened, thereby uncovering new fibers and exposing them to the conditions in the cooking zone, thereby reducing the residence time of the fibers in the cooking liquid to a minimum to prevent these being broken down by the cooking liquid; e) continuously setting said particles in motion in the suspension in the immediate vicinity of constricted drain openings in the discharge zone, which drain opening(s) have a "size" corresponding to the size of the released fibers, in countercurrent to the fiber- conveying fluid flow entering said apertures to continuously clean said apertures; f) continuous circulation of the liquid flow which transports said freed fibers through the narrowed drain openings and into the mechanical separation zone, where a part - between 10 and 95% - of the free liquid is mechanically separated from the freed fibers and recycled to the digester; g) during the aforementioned steps (a-f) in the process to continuously maintain a sufficiently large pressure immediately adjacent to the surface of the particles and fibers to prevent mechanical bursting of these upon relief of internal pressure. 2. Method according to claim 1, characterized in that an overpressure is maintained while the containment liquid circulates from the cooking zone, through the emptying zone to the mechanical separation zone and back to the cooking zone, and that a relationship between pressure and temperature is maintained immediately up to the surface of the fiber particles in the cooking , emptying and the mechanical separation zone which is such that a minimum pressure above the liquid's saturation pressure is maintained at any time and in any place in these zones. 3. Method according to one of the claims 1 -2, characterized in that the continuous binding reduces the lignin content in the individual outer fibers to a value of between 15 and 20%. 4. Method according to one of claims 1 - 3, characterized in that the particles which have not been reduced to free fibers are continuously exposed to centrifugal forces in the dispersion essentially radially away from said narrowed drain opening(s) in the emptying zone. 5. Method according to claims 1-4, characterized in that the mechanically separated free fibers with retention liquid therein, after they have left the separation zone, are subjected to continuous pressure and chemical boiling in order to bring the lignin content below the average content of the free fibers leaving the cooking zone , and that the mechanically separated fibers with the retention liquid are then cooled rapidly to lower the internal fiber temperature below 100°C, so that the pressure can be quickly lowered to normal atmospheric pressure. 6. Device at a plant for continuous boiling and chemical digestion of cellulose-containing fibrous substances using the method according to claims 1 or 4, characterized in that said narrowed drain opening (37) is formed between relatively rotating surfaces at a drain opening of the cooking container (11) . 7. Device according to claim 6, characterized in that one of said drain opening-limiting surfaces is formed by the back of a propeller-like disc (36) rotating inside the container (11), the other (stationary) surface being formed by an annular rib etc. (38) around the cooking container's drain opening (33), whereby said narrowed drain opening takes the form of an annular slot which opens into the container at the circumference of said propeller-like disc.