WO1988006201A1 - Process for production of cellulose pulp and/or delignification of secondary fibers - Google Patents

Abstract

In a process for digesting plant and wood fibers or for delignification, optinally with preceding deinking, of fibers from chemical or mechanical pulps, optionally with succeeding bleaching, the fibrous raw material is digested or delignified in an alkaline pumpable slurry in a pressurized tube system and conducted therethrough by the use of pulp pumps. On its way through the pressurized tube system the pulp is repeatedly dewatered and after each dewatering, save the last one, is rediluted with pressed out process liquor recycled from a downstream dewatering step and/or from a downstream pumping stage of the process. While being conducted through the pressurized tube system the pulp is subjected to stepwise increasing pressure, the pressure increase being caused by the pulp pumps. An apparatus for use in carrying out the process comprises an inner perforated tube (83) surrounded by an outer tube (85) which at its ends is sealed against the inner tube (83). The outer tube (85) which is capable of enduring high pressures is provided with one or more tubes (86) for receiving process liquor pressed out from the pulp through the perforations of the inner tube (83).

Description

Process for production of cellulose pulp and/or delignification of secondary fibers

Field of the invention

The present invention relates to continuous production of cellulose pulp by means of a combined chemical and mechanical method for digesting plant and wood fibers to cellulose pulp to be used as raw material for paper, board, fiber boards and fiber-containing products. The present process is also well suited for deinking, delignification and optionally bleaching of secondary fiber and can also include fiber fractionation.

The prior art

Cellulose pulp has for several decades been produced by means of the well known sulphite process or the well known sulphate process. Both processes today require very large plants to enable them to be carried out economically, and it is very expensive to build new cellulose plants for the production of cellulose by means of the sulphite process or the sulphate process. Nevertheless, in the recent years some new large cellulose mills have been built in the developed countries, and after starting up they have suffered great economic losses.

Currently some similar mills are being built in developing countries and it can in advance very probably be stated that these mills will impose big economic losses and burdens on these countries in the years to come.

The reason is the fact that the conventional technology of today for continuous production of pulp has become far too expensive compared with the prices which the cellulose markets have been able to offer in the last decades. This situation is now particularly unsatisfactory and untenable for developing countries which need to build their own cellulose and paper industry.

If new continuous mills for the production of cellulose shall be able to run at a profit, a thorough revision and renewal of the cellulose process must be made with a view to obtaining a substantial reduction of investment and capital costs per ton of annual production capacity. Today's technology for continuous production of pulp is mainly based on the sulphate process which in recent years has experienced a steadily increasing widespread use at the cost of the sulphite process. This fact is due thereto that the sulphate process may be used for most fiber raw materials which are of interest, in contrast to e.g. the sulphite process. Moreover, for the sulphate process of today an established system is at disposal for the recovery of cooking chemicals and for the combustion of organic compounds in the so called "black liquor". Accordingly, in recent years the sulphate process has been considered to be the process for the production of cellulose which among the cellulose processes is the less detrimental one to the environments.

However, several serious disadvantages are still connected with the process, and these may be summarized as follows:

1. The sulphate process has developed into a far too capital demanding process and requires extremely high production capacities as a consequence of the so called "economy of scale". Such large plants put correspondingly strong demands to capacity of transportation and infrastructure, however, plants having a capacity of above 200000-300000 tons per annum have in the recent years suffered economic losses and have been insufficiently profitable.

2. In the sulphate process sulphur compounds are used, and this is the cause of bad smell and contamination of the air. Moreover, in connection with the bleaching chlorine-containing compounds are as a rule used which yield toxic and harmful emissions to the environments.

3. A sulphate mill is a complicated system composed of a large number of strongly different process and machinery elements, and the mill is divided into separate departments for respectively cooking, screening, washing and bleaching, each department having separate heat, water, chemicals and effluent systems.

Thus, the sulphate mill of today is usually characterized by upright high pressurized digesters into which the fibers are fed at the top and at a high consistency which cannot be pumped, and later the fibers move vertically down through the digester in accordance with the "free fall" principle, and subsequently the fibers are diluted to a pumpable or "blowable" consistency at the bottom of the digester.

4. Requirements of the sulphate process to volume and ground area are substantial because the process proceeds relatively slowly. This is due thereto that the fiber pulp on its way is subjected to relatively little movement which again causes the reaction to proceed more slowly than what the case would have been had the fiber pulp been subjected to more movement and turbulence ("washing machine effect") .

5. The temperature and pressure used during the digestion of the pulp are usually determined by the fixed relationship between pressure and temperature of saturated steam which traditionally is used as heat source in the digester.

6. Upright high diffusers which usually are connected with complicated heat recovery systems are used for "blowing" the pulp. after the digesting.

7. Repeated dilution and thickening of the pulp in connection with several different pulp consistencies during the process, by. internal transportation, screening, washing and bleaching are performed by means of heavy and complicated equipment which often is located in various stocks in high, heavy and expensive factory buildings.

8. Large amounts of water and effluent chemicals must be handled in large, area demanding and expensive recovery and/or purification plants, 9. The pulp mill comprises a very large number of different machines, operations and functions which all represent possible sources of failure and which demand corresponding surveillance, maintenance and a comprehensive and expensive inventory of several spare parts.

Object of the invention

It is an object of the invention to provide a process for the production of cellulose pulp and/or delignification of secondary fibers possibly with bleaching, including possible deinking or other kinds of washing, and the process is to be free of the very substantial disadvantages which have been mentioned above in connection with the sulphate process which is representative of the prior art which is the most interesting one and the one which comes closest to the process according to the invention. The present process involves, compared with the conventional cellulose pulp manufacturing process of today, a substantial simplification of the process operation and of the process equipment with reduced demand for capital, area, surveillance and maintenance. By means of the present process an extensive recirculation and reuse of chemicals and heat energy in combination with the so called "washing machine" effect are obtained, and the process proceeds far more rapidly from raw material to finished cellulose pulp than the processes which currently are conventionally used. The extensive recirculation and reuse of chemicals and heat energy are to secure optimum process economy for the present process and, moreover, shall reduce the water consumption whereby effluents to natural water sources possibly may be eliminated. It is also an object of the invention that with regard to the equipment for carrying out the process and with regard to the process steps per se it shall be able to divide these into modules which by means of simple internal modifications are to present possibilities for combinations for the treatment of all fiber raw materials of interest. Further, it is an object of the invention that such modules shall lend them selves to being mounted into frames for standard transportation containers whereby simple and inexpensive assembly, transportation, starting up and possibly moving of the plant from place to place according to demand are ensured.

Summary of the invention

The invention relates to a process for digesting plant and wood fibers or for delignification,optionally with preceding deinking, of fibers from chemical or mechanical pulps, possibly with subsequent bleaching, in order to obtain a pulp which is suitable as raw material for paper, board, fiber boards and other products containing plant and/ or wood fibers, wherein the fiber-containing raw material is introduced into a digestion zone and is digested or delignified therein in alkaline slurry at elevated temperature and pressure using an alkaline cooking chemical in combination with oxygen and, optionally, minor amounts of other additions, like e.g. anthraquinone, and removing the cooking chemicals from the digestion process or delignification process in the form of a black liquor which is deposited or is subjected to recovery of chemicals chracterized in that the pulp raw material in the form of a pumpable slurry during the digestion or the delignification and the optionally preceding deinking and optionally subsequent bleaching is transported through a closed, continuous and pressurized tube system by the use of pulp pumps which are simultaneously used as mixing aggregates for the slurry and the chemicals, the pulp while being transported through the tube system being subjected to repeated dewaterings by pressing out liquid and prior to each dewatering, apart from the last dewatering of the process, the pulp is diluted with liquor which has been pressed out and has been circulated back from a downstream dewatering step of the process and/or from a downstream pumping stage, the pulp while being transported through the tube system being subjected to stepwise increasing pressure in one or more pumping steps and prior to the last dewatering step being diluted and optionally cooled with fresh water and/or bleach liquor supplied under pressure, and after the last dewatering in the pressurized digestion or delignification system the pulp is washed and optionally bleached in a continuation of the pressurized system or in the non-pressurized state after cold or hot blowing.

Brief description of the drawings

In the drawings Fig. 1 shows a flowsheet for an embodiment of the process according to the invention, including four dewatering steps and with stepwise building up of the pressure from the first dewatering step to the fourth dewatering step, Fig. 2 shows schematically the relative pressure increase according to the flowsheet of Fig. 1, from the start of the digestion process to the termination thereof, from left to right in the Figure,

Fig. 3 shows a Sankey diagram for the process according to the flowsheet shown in Fig. 1, for the supplied and recirculated amounts of liquids or chemical solutions, and Fig. 4-24 will be more detailed discussed below in connection with the description of the process.

Detailed description of the process and the drawings

An embodiment of the present process will now be further disclosed with reference to the drawings. According to Fig. 1 and 2 raw material 1 and fresh water 2 are supplied to a pulper 3, and from the pulper the raw material and fresh water is by means of a pump P₁ and a cleaning device 4 transferred to a pump P₂ in which the pressure exerted upon the slurry, which has a consistency of about 4%, is increased to the pressure level p₂. From the pump P₂ the slurry is pushed through a dewatering device 5 which will be further described below and in which the slurry is dewatered until it reaches a consistency of from about 10 to 30%. In the tube between the dewatering device and a pump P₃ the dewatered pulp is diluted with cooking liquor which has been recycled from pressure increasing stages and/or pressure dewatering stages further downstreams in the process. Moreover, also fresh solutions of chemicals and/or further heat 6 can be supplied to the slurry in the tube section between the dewatering device and the pump P₃. The liquid 7 pressed out in the dewatering device 5 is mainly transferred to a heat exchanger 8 in which the pressed out liquid (waste liquor or waste water) is passed in heat exchange relationship to fresh water supplied or possibly to waste water 9 from a bleach plant . Part of the liquor which has been pressed out from the dewatering device 5 can also be supplied to the pulper 3 in order to form the raw material slurry therein which has a consistency of about 4%. Before the slurry enters the pump P₂ and thence into the dewatering device 5 it is provided with a used solution 12 of cooking chemicals which has been recycled from the digesting tube immediately after the pump P₃ which is responsible for the next following pressure increase stage. From the pumping stage p₃ (Fig.2) the pulp is transferred to a new pumping stage p₄ in which the pressure exerted upon the slurry is further increased and with recirculation of cooking chemicals 13 to the slurry between the two pumping stages p₃ and p₃. The used cooking liquor 14 which is recycled to the tube section between the pumping stage p₃ and the pumping stage p₄ is taken from the digesting tube immediately after the next following pressure increasing pumping stage p₅. From the pump P₄ the slurry is then, while still at a pulp consistency of about 4%, transferred to the next pressure increasing pumping stage p₅, and from this pumping stage the slurry is transported further and again with supply of recirculated used cooking liquor 15 to the pressure pump P₆ from which the slurry is conducted further through the next dewatering device 16 in which the slurry is again dewatered until a pulp consistency of from about 10 to 30%. From the dewatering device 16 the pulp is transported again with dilution in order to form a pulp slurry having a pulp consistency of about 4% by the use of recycled liquid 17, to a next pressure increasing pump P₇ and from this pump into a next dewatering device 18 in which the slurry is again dewatered until a mass consistency of from about 10 to 30%. From the dewatering device 18 the pulp is transported to the last shown pressure increasing pump P₈, again with dilution of the slurry until pulp consistency of about 4% by the addition of liquid 19 and fresh water or optionally waste water 9 from a bleaching plant to which the pulp can be transferred after having been removed from the digestion section shown in Fig. 1. This liquid ( 9 ) which is supplied prior to the pressure increasing pump P₈ is preferably relatively cool in order to be able to cool the pulp to a such low temperature that when the pulp finally leaves the pressurized tube system it will have temperatures below 100ºC at atmospheric pressure, i.e. it will not expand, or, more specifically, it will be "cold blown" when it leaves the pressurized tube system. From the pressure increasing pump P₈ the slurry arrives at the last shown dewatering device 20 in which it is again dewatered to a pulp consistency of from 10 to 30%. The dewatered slurry having a pulp consistency of 10 to 30% is then removed from the system through a closure cone 21 at the end of the tube. This closure cone is of known construction and yields a back pressure which is controlled by a bellow filled with air and the controllable pressure of which is exerted against trie base of the conical closure means.

The amount of pressurized water injected into the system is controlled by means of the control valve R₂, and the effluent from the system is controlled by means of the control valve R₁. For the control the valves are interconnected so that the volume of liquid pressed into the system will approximately correspond to the effluent.

The flowsheet according to Fig. 1 shows upgrading of secondary fibers or the production of unbleached cellulose of annual plants. The main consistency during the conveyance through the tube system is, as mentioned, about 4%, and the consistency after dewatering is about 10-30%. On the flowsheet of Fig. 1 three reaction modules (M₁, M₂ and M₃) for delignification has been shown which are succeeded by two washing stages (at P₇ and P₈). A bleach section can be connected to the system shown in Fig. 1, and for the bleaching the same method may be used as has been described above in connection with the upgrading or the digestion, however, obviously using chemicals of a type and in an amount sufficient to obtain the desired bleaching.

The present process can be used for all kinds of vegetable fibers, e.g. of bagasse, kenaaf, rice straw, rubber trees, waste from the production of palm oil, bamboo and also for hardwood (leave trees) and softwood (needle trees). Similarly, the process is well suited for delignification and optional bleaching of secondary fibers. Moreover, when using printed secondary papers the layout of the system causes deinking of the raw material without the use of extra mechanical additional equipment. The present process offers a wide spectrum of reaction possibilities which cover production of pulp having a quality which varies within the range of from mechanical pulp (MP) through thermo-mechanical pulp (TMP) and chemical-thermomechanical pulp (C-TMP) to pure chemical pulps (CP).

As stated above, the flowsheet shown in Fig. 1 pertains to the digestion or cooking of one year old plants, like straw, bagasse or kenaaf or for the upgrading or secondary fibers. For digestion or cooking of sawdust, hardwood or needlewood the process must be supplied with mechanical processing in refiners. After the refiners pressure screens can be used which separate the coarse chips from the accept and return the coarse chip fraction sorted out for renewed chemical and mechanical treatment. This opens for possibilities for a number of alternative programs which can be adapted to all fiber qualities of interest.

As stated under the Object of the Invention it is also an object of the invention that the equipment for carrying out the process and the process steps per se shall lend themselves to being divided into modules. In the enclosed Fig. 4 the main principles of an example of the present process have been shown somewhat simplified. According to Fig. 4, after the slurrying 3 both impregnation, delignification and defibration 23 take place within a closed pressurized tube system indicated by 22, followed by washing 24, optionally bleaching 25 and renewed washing 26 before the finished fibers leave the process system through the outlet means 21.

Fig. 5 shows how the flowsheet for annual plants may look, the flowsheet having been built up into three modules 27, 28 and 29 which each comprises a pulp pump and tube system, whereas an example of the flowsheet for the production of pulp from sawdust, hardwood or needlewood has been shown in Fig. 6. Compared with the flowsheet of Fig. 5 the flowsheet of Fig. 6 has been supplied with refiners 30, 31 and 32 for mechanical processing, and after the refiners pressure screens 33, 34 and 35 are arranged which separate the coarse chips from the accept and return the separated coarse chip fraction 36, 37 and 38 to renewed chemical and mechanical treatment.

It is rather natural to use the three modules 27, 28 and 29 shown in Fig. 5 and 6 for impregnation, delignification and defibration respectively, and these three operations constitute the total digestion.

After this impregnation, delignification and defibration 23 washing 24 is carried out, according to Fig.4, still within the closed pressurized system 22. This "cooking"- section is terminated with pressurized dewatering which is preferably carried out in a pressure-plug thickener shown in Fig. 1. The pressure-thickening device is further described below.

The pulp from the digestion can be transferred for washing into a module, and the conveyance of the pulp to a washing module IV (50) has been shown in Fig. 7. In this module three pressurized dewatering presses are connected in series and in the drawing these have been indicated as three pressurized screw presses, however, they could just as well have been replaced by three pressure-plug thickeners with preceding pressure pumps. It appears from Fig. 7 that fresh water or waste water from bleaching 39 is injected prior to the entrance of the last one of the pressurized dewatering presses connected in series and that the liquid 40 which has been pressed out is recycled to before the entrance to the second pressurized dewatering press. This is done in order to dilute the slurry, after pressurized dewatering, to a pulp consistency which is suitable for the further conveyance of the pulp, e.g. about 4%.

According to Fig. 4 the washed pulp, still in a pressurized system, is conveyed to a bleaching plant, and in Fig. 8 it has been shown how this conveyance can take place and how the bleach plant according to the embodiment shown has also been built up of three modules 51, 52 and 53 which each includes a tube system with pressure pump. From the last bleach plant module VII, 53, the bleached pulp, as shown in Fig. 4, is transferred for washing, similarly with three pressurized dewatering presses or with three pressure- plug thickeners (module VIII, 54 ). Prior to each pressurized dewatering the pulp slurry is diluted with liquid. Fresh water 55, possibly from scrubber or heat exchanger, is being supplied prior to the last pressurized dewatering device whereas liquid 56 which has been pressed out in the last pressurized dewatering device is recirculated to the pressurized tube system in front of the inlet to the other pressurized dewatering device. The liquid 57 pressed out therein can if desired by recirculated to the pressurized tube system and/or be introduced into the bleached pulp prior to the inlet to the first pressurized dewatering device 58.

From the last pressurized dewatering device the pulp 59 is transferred for storage or paper production.

The pulp can upon impregnation, delignification and defibration also be washed and, if desired, bleached in a subsequent non-pressurized system. A such unpressurized washing has been schematically shown in Fig. 9. The pulp from the digesting section 45 is then first "blown" 46 (if desired through a heat exchanger 47) before it enters into a module IX, 48,which is a washing plant module and wherein three dewatering presses are used in the same manner as for the pressurized washing, however, the three dewatering presses are, of course, not pressurized. After this pressureless washing the pulp may be transferred for bleaching in conventional manner, i.e. non-pressurized bleaching, or it can be transferred for storage or paper production.

For a plant producing unbleached or bleached cellulose pulp at a capacity of about 30000 tons per annum, i.e.

100 tons per day or 4 tons per hour, which can be built up of the system herein described and give profitable operation because investment costs for plant area and equipment are relatively modest, figures and dimensions referring to this production capacity have been stated below. A factory of this production capacity is also of a size which is favourable for new factories in developing countries.

As previously mentioned, in connection with the digesting section shown in Fig. 5 and Fig. 6 it is a natural choice to use module I ,27 , as impregnating module and the modules

II and III ,28 and 29, respectively for delignification and defibration with or without use of mechanical defibration in addition carried out "in-line" with pressurized refiners.

As examples of working parameters for a such factory the following can be stated:

1⁾Calculated as dry NaOH based on bone dry fibers, bagasse 2⁾ Modified operation for the substance dissolving part

³⁾ Higher values for sawdust, hardwood and needlewood

⁴⁾ Finely chopped chips 5⁾ Reduced production capacity compared with annual plants 6⁾ Calculated as NH₄OH based on bone dry fibers 7⁾ Calculated as O₂ on bone dry fibers 8⁾ OCC= old corrugated cardboard 9⁾ ONP= old newsprint paper It appears from the lower part of the table that the use of refiners and pressurized screens is dependent upon the raw material to be be prosessed. However, the modules have been designed in agreement with a standard configuration with room reserved for alternative uses of machinery.

The termination of the digesting section can be made in two different ways, i.e. a) using the traditional blowing tank, preferably in combination with a scrubber plant for the recovery of heat from released steam, followed by a pressure-less dewatering device with recirculation of pressed out cooking liquor for recovery and transfer to pulper. b) With a pressurized dewatering device the system can be substantially simplified and also yield a more closed system and a more optimum heat economy.

Washing of the pulp is usually desirable as part of the cellulose pulp production, subsequent to the first delignification and defibration of the raw material and after the bleaching. By means of the present process the washing can be carried out in a particular washing module which comprises 1, 2 or 3 washing steps after the first dewatering, see module IV in Fig. 7 , 50, or Fig. 9 , 48.

With regard to bleaching of the pulp, both the bleaching plant and the associated washing plant can work at atmospheric pressure if the temperature of the pulp line does not exceed 100ºC. In that case the present process can, after the washing plant in the cellulose mill (i.e. after module IV), be carried out by the use of non-pressurized dewatering presses in the washing section of the bleach plant. This consideration also pertains if module IV is pressurized ( 50 in Fig. 7) because "cold blowing" from the pressurized module will here take place due to the cooling down caused by the addition of injected colder washing water in the form of fresh water or as waste water from a subsequent bleach plant. However, the present process makes it possible to obtain maximum heat recovery and economy and simultaneously a significant reduction of the total water consumption. Thereby also the effluent conditions become correspondingly improved. Optimum heat recovery is obtained if the cold blowing of the pulp from the pressurized system is moved to the end of the washing section of the bleaching plant. However, this requires that pressurized equipment also is being used in the bleaching plant and in the associated washing section. Accordingly, two pressure alternatives exist.

However, there are several reasons why pressurized equipment also ought to be used for the bleaching plant and its washing section. When this is done, the bleaching plant and its washing section can be built up of identical modules which are the same as the ones used for the cellulose pulp roduction and for its associated washing section.

The additional costs due to the machinery to be used in the pressurized system are relatively modest compared with the costs for building a non-pressurized system. This circumstance must also be considered against the background of the fact that the demand for capital for a total pressurized system for carrying out the present process is modest compared with the demand for capital for conventional cellulose processes.

Accordigly, the use of pressurized elements throughout the present system involves that maximum standardization of machinery elements which have to be used can be realized. This standardization involves in its turn significant advantages and savings with respect to surveillance, maintenance and spare parts.

Accordingly, against the background of that which has been stated above a pressurized bleaching plant with associated washing section can look like depicted in Fig. 8. However, it is to be remarked that pressure-plug thickeners are preferable, both in module IV ,50, and in module VIII, 54.

In Table II below some working parameters for operating a such bleaching plant as has been shown in Fig.8 are disclosed.

If the bleaching is carried out without chlorine, i.e. only with NaOH or NH₄OH, O₂ and/or O₃, the waste water from the bleach plant can be used as washing water in the washing section of the cellulose mill, and from the washing section the waste water is recirculated to the beginning of the cellulose pulp process.

The present process can be carried out utilizing a high degree of self-adjustment. A cooking plant and bleach plant which work in pressure stages provided by pulp pumps in series will themselves control the pressure difference by means of recirculation of the chemicals countercurrent to the movement of the fiber pulp through the plant.

A such interconnection of cooking plant and bleach plant also results in only one place for the discharge of used chemicals from the plant. This discharge may be located further forward, almost at the beginning of the process, and the "black liquor" which is withdrawn can, as mentioned, be conveyed in heat exchange relationship 8 to fresh water and/or waste water from the bleach plant, as shown in Fig. 1 and Fig. 3.

According to the Sankey diagram shown in Fig. 3, the amount of cooking liquid used which is discharged from the system will be approximately equal to the amount of (fresh) water which is added at the end of the plant as washing water for the pulp subsequent to the bleaching.

If significant amounts of chlorine are used in the bleach plant, the return flow of waste water and chemicals from the bleach plant and its associated washing section must be interrupted prior to the cellulose mill and branched from the system. This branching presupposes new consumption of water in the washing section of the cellulose mill. A such branching off will then, due to use of chlorine in the bleach plant, necessarily have to cause increased consumption of water. Nevertheless, the present process can be carried out using substantially lower amounts of water than what is the case in conventional cellulose mills. Moreover, when using moderate additions of chlorine in the bleach plant the chemicals liquid can be further returned to the cooking section and used as part of the digesting liquor.

Moreover, it appears from Fig. 1 that the addition of chemicals and/or the addition of heat 6 is made immediately in front of the pulp pumps, whereby several advantages are obtained, as follows: a) The process pressure is increased in order to be able to use process temperatures above 100ºC. b) The pressure increase is carried out stepwise across the pulp pumps which are arranged in series, with increase to the next pressure stage for each passing through a new pulp pump. c) Increased pressure at each stage also enables recirculation and use of the counter-current principle between fiber flow and chemicals throughout the plant. d) Raw material and chemicals are intensely mixed, a procedure which is steadily repeated. Due to the agitation obtained and the recirculation of the process chemicals the fibers are "washed" more or less free of lignin through influence of the chemicals and through agitation.

The modules shown in Fig. 5, 6, 7, 8 and 9 have been shown with reaction tubes 60 arranged in succession. For each module there have as an example been shown four reaction tube parts which together constitute a "package". This tube package together with a pulp pump, optionally also with refiner and screen can be assembled within the frame of a standard transport container, and in the same manner for all succeeding modules. A such assembly within a standard transport container has been schematically indicated in Fig. 10.

It also appears from the above tables that the retention period in each reaction tube package of each module is about 10 minutes. An example of dimensioning of these reaction tubes is as follows:

Tube diameter 1 m Tube volume 15 m³

Total tube length 18 m

Length of each tube section 4,5 m

A suitable tube quality is ST23-33 with a wall thick ness of 13 mm. A such tube quality will endure an operating pressure of 20 bar and an operating temperature of 175ºC. If desired, tubes of quality ST23-43 having a wall thickness of 12 mm can be used.

The valves in the tubes for circulation of liquids and return of liquids and shown in Fig. 1, 2 and 3 are pressure operated automatic valves which open at a preset pressure. Because the pressure increase when carrying out the present process takes place in steps or stages, these valves will automatically adjust the back flow through the process and thereby the counter flow between fiber flow and process chemicals and with respect to amounts determined by the amount of liquid (water) which is pressed into the last step of the process through the valve R₂ in Fig. 1, 2 and 3. The principle of the self-adjusting pressure distribution when carrying out the process appears clearly from Fig. 1 and 2. The amount of fresh water added (through R₂) will also in agreement with this embodiment determine the amount of black liquor discharged from the systems (through R₁) and thereby also the concentration of solids in the black liquor.

The pressure controlled automatic valves between the various pressure increasing stages shown in Fig. 2 will besides providing for automatic control of the back-flow through the process and thereby the counter-flow between fiber flow and process chemicals also control the back pressure and thereby the pressure drop across the pressurized dewatering devices, i.e. the pressure controlled valves will provide for control of the degree of dewatering or the amount of liquid pressed out of the pulp slurry.

Some examples of chemical process conditions for the present process when the present process comprises cooking, washing, bleaching and washing, have been given in the below Table III.

It appears from Table III that when cooking a temperature varying from 90 to 150ºC is used, dependent upon the fiber material fed in, and as cooking chemical sodium hydroxide or ammonia, oxygen and, optionally auxiliary cooking adjuvants, like anthraquinone (AQ), are used. The total cooking period varies from 10 to 90 minutes dependent upon the fiber material supplied, however, a satisfactory delignification with optional deinking of secondary fiber will take place in the course of from 2 to 40 minutes.

The bleaching is carried out in alkaline environment using chlorine, O₂, peroxide (oxygen bleaching) or O₃, and a temperature during the bleaching is maintained between 70 and 120ºC. The washing between the cooking and the bleaching can be carried out at a temperature of from 70 to 140ºC, and the washing time can vary from 0.5 to 10 minutes. Subsequent to the bleaching a finishing washing is carried out. It appears from Table III that all the time fiber flow and liquid flow are conveyed in opposite directions to each other. Because the entire process takes place in a pressurized closed system also ammonia will be well suited as cooking chemical.

For the pressurized system in which the present process is carried out it has been stated that the dewatering is performed as a pressure dewatering. It has further been stated that this dewatering can be performed in a so called "pressure-plug thickener" or in a pressurized screw press.

In Fig. 11 the principle of a such pressure-plug thickening device with preceding pressure pump P has been shown. According to Fig. 11 the pressure p_I minus the resistance of friction between the sliding pulp plug and the tube wall is higher than the pressure p_III. The pulp plug which is within the central perforated tube will then be pressed out of the system past the conical discharge control means to the right of the drawing, while pressed out liquid at the pressure p_II will be circulated back to one or more places upstreams in the process as indicated by the direction of the arrow at the entrance to the circulation tube shown.

As the first and later the intermittent dewatering device within the system the device shown in Fig. 12 can be used. This corresponds to the dewatering device which has been shown in Fig. 11, however, in Fig. 12 it has been shown that for the further conveyance of the pulp subsequent to the dewatering in the pressure-plug thickener the pulp is in a tube mixer 70 diluted with process liquor which is returned to the pressurized tube system through one or two tube legs 71 and 72 in order to redilute the pulp which now has a pressure of p₃, to a low pulp consistency, e.g. about 4%.

These "pressure-plug thickeners" are without movable parts and have been dimensioned to tolerate operating pressures up to about 16 bar, in the same manner as the pumps which can consist of standard centrifugal pumps for pulp and designed for low consistency or intermediate consistency pulp suspensions, i.e. consistencies within the range of from 6 to 15%. When using high pressure back flushing, as disclosed below, it can be required to dimension the jacket around the perforated tube and the pertaining tube system for much higher pressures.

The pumps are to be certified for pressures up to 15-16 bar in order to endure the pressure which is created when several pumps are connected in series within the system. The pressure head for one pump ought to be within the range of from 10 to 20 m or higher. Further the number of revolutions of the pump wheels and the diameter of the pump wheels can advantageously be so dimensioned that the peripheral velocity preferably should not be lower than 20-30 m/seconds in order to obtain the desired mixing and dispersing effect between fiber material, process liquid, the chemicals used and the gases which are added to the pressurized tube system at several places along its length.

However, the strength requirements to the inner perforated tube in the pressure-plug thickeners are not so substantial as to the outer surrounding tube which is formed like a mantel around the perforated tube and which, thereby, also functions as protection around the inner tube. Accordingly, the outer tube must comply with all safety regulations which pertain to the working pressures used, i.e. also when, if desired, performing high pressure back flushing at pressures of up to 30 and 50 bar and higher. For example, the inner tube can consist of acid resistant steel having a wall thickness of 1-5 mm.

For the last dewatering which is performed when carrying out the present process also a perssurized screw press can be used. The pressure-plug dewatering devices or thickening devices, abbreviated PPT, have been invented for carrying out the present process and consist of a means which with respect to its design represents a substantial simplification compared with the thickening apparatuses currently used within the pulp industry. When pressing out liquid using PPT relatively high fiber concentration of up to 10-30% is obtained. The PPT is compact, requires little space and in addition offers the advantage that it can work at temperatures above 100ºC and at pressures substantially higher than 1 bar. A natural pressure limit for the PPT is 16 bar which is a conventional classification pressure in connection with dimensioning of materials for pumps and tubes of pressurized process systems with respect to strength. This circumstance opens for new possibilities and degrees of freedom with respect to selection of temperature and pressure for carrying out the present process and, moreover, enables a complete and integrated utilization of the countercurrent principle.

The pressure-plug thickener comprises an internal thin-walled perforated tube, and outside and around this tube another non-perforated tube has been arranged as a jacket, and in the space formed between the inner perforated tube and the outer tube water or liquid is received which due to the pressure differential between the inner perforated tube and said space is pressed through the perforations of the inner tube.

The plug of thickened pulp which is successively formed in the inner tube when the pulp suspension is dewatered and the pulp consistency increases will under suitable conditions be axially pushed through the perforated inner tube in the direction of movement of the pulp suspension, and this pushing will be caused by the pump pressure exerted against one end of the perforated tube. This pressure will be higher than the pressure in the surrounding tube.

Because the pulp plug is pushed towards the outlet end of the pressure-plug thickener in this manner, new volume exposing a fresh perforated inner tube face is liberated which is utilized for dewatering and thickening of the fresh fiber-containing suspension flowing into the thickener. In this manner a renewed dewatering takes place in association with and behind the pulp plug which already has been formed, and this pulp plug will then be continuously extended and renewed at the same pace as the pulp plug slides out of the perforated tube.

The wall of the inner perforated tube is selected to be so thin as possible in order to avoid that the perforations in the tube wall will become obstructed and blocked by fibers from the pulp suspension. Accordingly, the wall thickness of the inner tube has been calculated and dimensioned with modest safety margin and only to endure the stresses caused in the material as a consequence of the pressure differential between the inner and outer tube chamber and which usually does not exceed 2 or 3 bar. However, as mentioned, the outer tube must be dimensioned in order to comply with all safety requirements and regulations with respect to pressurized tube or steam systems to operate under expected elevated working pressure.

When several pressure-plug thickeners are connected in series, dewatering and thickening can be performed in several stages, with intermittent dilutions, with pressures in the system which can reach 20 bar in the last stage. Accordingly, the outer tube jacket of the PPT has been dimensioned in order to endure a such elevated expected working pressure.

The movement of the pulp plugs through the inner per forated tube is determined by the outlet means in the last step of the process. The outlet means can consist of a pressure-loaded, resilient cone which presses against the pumping pressure within the system and balances this or it can consist of a sluice chamber, in principle in the form of a piece of tubing between two valves which open and close alternately in order to sluice out portions of the pulp plug formed within the perforated tube in the last pressure dewatering step of the pressurized system.

When a pressure cone is used at the outlet of the pressurized system, the pulp plug will under suitable conditions leave the pressurized system continuously and with approximately uniform movement whereas use of a sluice chamber will cause the pulp plug to move forward and out of the pressurized system with rythmic movements and advances determined by the pace at which the valves of the sluice chamber are opened and closed.

In order to facilitate the movement of the pulp plug through and out of the inner perforated dewatering tube it can be necessary at intervals to send pressure impulses into the inner dewatering tube in a direction opposite to the pressure drop between the inner and the outer tube chamber, in order to thereby cause a brief return of pressed out liquid to the inner perforated tube. This small amount of liquid will force itself in through the perforations and temporarily reduce the fiber concentration in the outer plug layers against the perforated tube wall, whereby "lubrication" is obtained between the outer layers of the pulp plug, in the radial direction, and the inner face of the perforated tube. Subsequent to this impulse injection of liquid with consequential impulse movement of the plug forward within the perforated tube, the pressure drop ratio will when the impulse injection ceases return to the normal pressure drop ratio for dewatering and thickening.

The requirements to the obtainment of a safe sliding of the pulp plug through the pressure-plug thickeners and out of these in the form of a coherent plug are the provision of a friction between the inner tube wall of the perforated tube and the fiber plug which is less than the internal shear forces between the internal fiber layers within the pulp plug, considered in axial direction.

Whether this friction-reducing "lubrication" by pulsatory and brief back pressing of small amounts of pressed out liquid is required in order to ensure the actual displacement of a coherent and unitary pulp plug through the pressure-plug thickeners will be dependent upon several operating parameters, among which may be mentioned pulp pressure, pressure differential between the inner and outer tube chamber in the pressure- plug thickeners, the consistency of the pulp suspension, the type of fibers and the degree of milling of the fibers, the wall thickness of the perforated tube and the perforation pattern of the tube and, finally, the surface condition of the inner tube face.

Some embodiments of such pressure-plug thickeners and the use thereof in pressurized tube systems as well as some examples of degree of perforation and pattern of perforation for the inner perforated tube will now be described with reference to Figure 13.

The fundamental design of the pressure-plug thickener (PPT) has been shown in Fig. 13. It appears that the pulp suspension is conveyed into a pump 81 which pumps the pulp further into tubes 82, 83 and 84.

Around the tube 83 a tube 85 has been placed from which an outlet tube 86 leads.

Between the tubes 82, 83 and 85 seals 87 are present, and between the tubes 83, 85 and 84 a seal 88 is present, and these seals have been so constructed that the tube 83 will form an inner pressure chamber 89 which has been sealed in the lateral direction against an outer pressure chamber 90. The arrows show the direction of flow of the pulp suspension and the water respectively.

The construction is such that the PPT can easily be opened, whereby the inner perforated tube 83 easily can be replaced by another tube having new perforation or another perforation pattern.

The PPT functions as follows: At point A the pulp suspension usually has a fiber concentration of 3-6% and a pressure p 1. (So called MC pulp concentrations ("Medium consistency") can also be used, i.e. a fiber concentration of 6-15%, however, this presupposes use of special so called MC-pumps).

By means of the pump 81 the pressure is increased to p 2 at the point B at which point by the way the suspension has the same fiber concentration as at point A. When it moves through the tube 83 from point B to point C the pulp suspension will flow past the perforations in the tube 83.

Because the pressure p 2 within the tube 83 is higher than the pressure p 4 in the outer and surrounding pressure chamber 90, some of the liquid in the suspension will be pressed out through the perforations in the tube wall, as indicated by the arrows. The fibers of the pulp suspension will largely remain in the tube 83, whereby the fiber concentration in the tube 83 will increase correspondingly. Accordingly, the fiber concentration of the pulp suspension will gradually increase while the suspension moves from B to C. The fiber concentration at the inlet to the tube 84 will normally vary between 10 and 30% and is dependent upon the existing conditions, like pressure differential (p 2 - p 4), type of fibers, degree of digestion, degree of milling of the fiber suspension, the perforation pattern, the percentage open area in the perforated surface compared with the total tube surface, and the diameter of the perforated tube in relation to said parameters and the pressure drop (p 2 - p 3).

The liquid pressed out is conveyed from the tube 85 out through the tube 86.

Several PPT's can advantageously be connected to a continuous pressurized tube system, and the pressure will then increase stepwise within the system as a result of the pumping pressure from the pumps connected in series between the separate pressure-plug thickeners (PPT). For a pressure increase of about 2 bar for each pump and PPT unit, and with 8 units connected in series in a system, mentioned as an example, the entire system must be constructed for enduring a maximum pressure of about 16 bar which, moreover, is a conventional classification limit for pumps and tubes for pressurized systems.

However, as mentioned the tube 83 can be made of relatively thin tube material, preferably of a thin acid resistant steel sheet having a thickness of 0.5-2.0 mm in which the desired perforation pattern is made, e.g. by burning with the aid of a numerically controlled laser machine, whereupon the perforated sheet is polished on the inside before it is bent around and welded into a tube. The pumps employed can be conventional centrifugal pulp pumps.

With a suitable discharge means at the end of the pressurized system, for example disclosed below in connection with Fig. 15, there can be obtained a point of balance for the pressurized movement of the pulp suspension through the pressurized tube system at which the pumping pressure ( p 2) at the entrance to the last pressure-plug thickener of the pressurized system will counterbalance the pressure exerted against the discharge means and the frictional force between the pulp plug and the inner tube wall and the discharge means (in Fig. 15 shown in the form of a discharge cone). For the formation of a coherent movable fiber plug in the pressure-plug thickener the inner diameter of the perforated tube similarly plays an important part because the axial resultant force of the pumping pressure across the entire tube cross-section increases with the square of the radius of the tube whereas the internal frictional force between the pulp plug and the inner tube surface only increases linearly with the tube diameter for otherwise equal pressure conditions and equal tube length. Accordingly, this type of plug movement can more easily be obtained when the tube diameter is increased provided the pulp plug having a sufficiently high fiber concentration.

It is important that the pressure-plug thickeners are so constructed that the pulp plugs formed therein do not encounter hindrances in the form of tapered tube cross-sections when the pulp move through and out of the perforated tube. Fig. 14 shows how pressure-plug thickeners can be connected in series in a closed and pressurized system.

The thickened pulp suspension at D which is a representative of a fiber plug having a fiber concentration of 10- 30%, is pressed into a diluting chamber 91 in which it is provided with diluting liquid through an inlet tube 92. By means of a pump 93 the diluted suspension is pumped into a pressure-plug thickener 94 in which the pulp is again dewatered as described above in connection with Fig. 13. The thickened pulp is pushed by the pumping pressure past point G and into a new diluting chamber 97 which similarly to the diluting chamber 91 can have approximately the same main dimensions and the same main design as the pressure-plug thickener but obviously without the inner perforated tube.

When thickened pulp has been mixed with diluting liquid in the chamber 97, the diluted suspension continues past the point H and is transferred into a pump 98 whereupon the thickening operation is repeated in PPT 99.

Fig. 15 schematically shows the pulp thickener (PPT) arranged as the last stage of a cellulose process or washing process. In Fig. 15 a stop element 101 has been shown at the outlet end of the tube from the pulp thickener, and this stop element 101 preferably is in the shape of a conical device which is pressed against the tube end by means of an adjustable pressure member, preferably an air bellow 102, which exerts pressure against the stop element and thereby against the pulp flow within the tube system. The pressure p4 will vary in response to the formation of the plug in the perforated tube, as follows: When the pulp plug which is initially formed at L increases in length in the tube in the direction towards K, the fiber layers will cover an increasingly larger part of the inner surface of the perforated tube, whereby the liquid flow is delayed and the pressure p4 increases. Maximum pumping pressure occurs when the fiber plug covers the entire part of the perforated tube from L to K and the liquid flow through the perforations has approximately ceased. By controlling the pressure of the air bellow 102 a point of balance can be obtained at which the pressure exerted by the bellow against the cone balances the pumping pressure p 4 minus the frictional forces against the inner tube wall and the surface of the cone against the movement of the pulp. During operation this balance will be automatically maintained because the length of the pulp plug will be approximately constant. When an extension of the length of the pulp plug covers the perforations and causes increase of the inlet pressure, the pressure increase will cause a larger discharge of thickened pulp across the cone, whereby a larger perforated area will become uncovered so that the pressure will drop and the balance becomes reestablished. Fig. 16 shows another embodiment of a last thickening step of a pressurized pulp suspension system. According to Fig. 16, the discharge is controlled by means of a discharge sluice which is constituted by two ball or calotte valves 105 and 106 respectively arranged at a distance from one another in the same tube and being alternatingly opened and closed. The flow opening 107 of the valve must be at least equal to the cross-sectional area of the tube 108 in order to ensure an unhindered passage of pulp. This type of discharge sluice can be used when the thickened fiber suspension, i.e. the pulp plug, to be discharged from the pressurized system to atmospheric pressure has a temperature higher than 100ºC and develops internal steam when the valve 106 is open against atmospheric pressure while the valve 105 is closed. The pulp will then be pushed or blown out of the sluice and the valve 106. The entire sluice volume will thereby be blown approximately empty of fibers and will at the same time become filled with steam at atmospheric pressure. When the valve 106 is closed and the valve 105 is opened in order to let out the next portion of the pulp plug in the tube 108, the steam or air which is present within the sluice will become compressed to pressure corresponding to the pressure of the pressurized system and thereby give room for a new portion of pulp plug to be sluiced out from the system. Under certain conditions of production the valves 105 and 106 may advantageously be designed as flap valves with the flaps having a particularly sharp and cutting design which better can divide or cut the pulp plug. Fig. 17 shows the same sluice means as is shown in Fig. 16, however, provided with an inlet tube 110 and a valve 111 which are used when the content within a sluice 112 does not move out of the sluice by itself. Further, two valves 113 and 114 respectively have been shown, and when the valve 113 is closed and the valve 114 is open, pressurized air supplied because the valve 111 is opened will blow the content in the sluice out of the system through the valve 114. Then the valves 111 and 114 are closed and the valve 113 is opened, and the next portion of the pulp plug will be let in, as described above in connection with Fig. 16.

Fig. 18 shows an additonal means which facilitates the movement of the pulp plug within and out of the thickener. A pump 115 and various parts 116, 117, 118 and 119 for constructing the thickener are as disclosed above in connection with Fig. 13.

The additional means consists of a three-way valve 120 arranged in an outlet conduit 119 and 122 and having supply from a tube 121 which conveys liquid or gas at a higher pressure than the pumping pressure in the pulp tubes. A flexible tube connection 123, e.g. in the form of a rubber collar, arranged as a connecting link between a divided pulp tube of the pressurized system can accomodate and absorb the short pressure pulsations occurring in the same tube. An alternative means for absorbing pressure pulsations is a standard membrane expansion tank 124 shown in Fig. 18C. The tank is dimensioned in agreement with the pressure pulsations which are expected to occur in the system. The additional means is operated as follows: According to Fig. 18A dewatering respectively thickening of the fiber pulp takes place in the same manner as disclosed above in connection with Fig. 13. In order to facilitate the movement of the pulp plug within and out of the thickener reversions of short duration of the pressure conditions in the pulp thickener are made at regular intervals.

This appears from Fig. 18B which shows how the three-way valve 120 upon a brief turning by 90 in the clock direction closes the outlet 122 and lets in water, mixture of chemicals or gas (e.g. oxygen) at higher pressure in the opposite direction through the outlet tube 119 and into the outer pressure chamber 116, whereupon previously pressed out liquid from the pulp suspension in the tube 117 is pressed back through the perforations and into the tube 117.

The amount of liquid which in the case shown in Fig.18B is pressed back through the perforations and into the tube 117, can be adjusted by means of the size of the pressure or the length of the time during which the external pressure is allowed to be effective, or both. The amounts of liquid pressed back which here will function as lubricant between the pulp plug and the internal surface of the tube 117 are so small that they are of little or no importance to the fiber concentration per se of the pulp plug which for several reasons is wanted to be as high as possible.

After an injeciton of short duration of "lubricant" liquid into the tube 117, as shown in Fig. 18B, with consequential movement of the pulp plug, the three-way valve 120 is turned back to the same position as the position shown in Fig. 18A, whereupon pulp thickening and formation of new plug continue, and the same cycle as disclosed above is repeated.

Fig. 19 shows a PPT having the same function as disclosed for the PPT shown in Fig. 18A, 18B and 18C, however, instead of a three-way valve and a separate tube system for injecting medium back into the inner perforated tube in the form of a pressure pulse, the liquid which has been pressed out from the pressure-plug thickener is here used for the same purpose.

Thus, Fig. 19A shows that in an outlet tube 125 a cylinder 126 has been arranged which has a piston 127 which is actuated by means of an actuator, e.g. an air bellow 128. When the position of the piston is as shown in Fig. 19A, dewatering and thickening of the pulp suspension will take place as previously disclosed.

Fig. 19B shows that a piston movement of short duration to an upper position as shown in the Figure causes both that the outlet flow from the tube 125 is interrupted and, moreover, that the pressure conditions within the pressure-plug thickener are changed due thereto that a volume of liquid Δ V is forced back into the pressure-plug thickener with corresponding thinning and "lubricating" effect as disclosed above in connection with Fig. 18. The brief pressure pulse is accomodated by an elastic tube connection 123 or a pressuretank 124, as shown in Fig. 18A and B, or by similar means.

Fig. 19C and D show the way of working for a corresponding installation, however, where a cylinder, a piston 131 and a movable element 130 constitute a branching from an outlet tube 125. Fig. 19C shows the position of a valve 129 and of a piston 131 during dewatering and thickening of the pulp in the perforated tube in the pressure-plug thickener with which the tube 125 is connected.

Fig. 19D shows a closure of short duration of the valve 129, whereupon the piston 131 by the movable element 130 is moved towards the outlet tube 125. Thereby, reversed pressure conditions in the pressure-plug thickener and a movement of liquid in through the perforations of the perforated tube are caused, as disclosed above.

Fig. 19E and F show the principle of a means for a pressure-plug thickener where the reverse pulsating pressure is provided by means of an outer pressing body 132 which is pressed against an elastic tube connection or adapter 133 inserted between an outlet tube 134 from the pressure-plug thickener and an outlet tube 135 from the adapter 133 after a valve 136 has been regulated so that it closes the outlet tube 135. The adapter 133 can consist of rubber or another elastic material which can endure the relevant pressures and temperatures. This type of means in order to provide the reversed pressure pulse is of particular interest when washing cellulose fibers, and in those cases where the pulp suspension has a relatively low temperature of below 100ºC.

Fig. 20A, B, C and D show various types of perforations for the inner tube 117 of pressure-plug thickeners and of a fiber sorting means (the latter is described below).

Fig. 20D shows a tube with relatively large openings which on its outside has been covered by a fine mesh of non-corroding metal wires. Fig. 20D, I, shows wire qauze or screen and openings viewed from outside the tube, and Fig. 20D, II, shows wire gauze and openings viewed from the inside of the tube.

Fig. 20E, I, shows in section through the perforated tube wall with the applied fine wire gauze how a fiber suspension is thickened against the wire gauze during the dewatering, and Fig. E, II, which shows a section in the same way as I, shows how the fiber accumulation formed which constitutes a part of the pulp plug which is being formed within the tube, is pressed back in direction towards the inner of the tube by means of the pulsating back pressing of pressed out liquid. The liquid which has been pressed back through the wire gauze and into the tube will follow the way of least resistance and become distributed as a liquid layer between the pulp plug and the internal surface of the tube wall. Thereby the friction between pulp plug and tube wall is reduced, and the pulp plug will be pushed forward in the tube by the pumping pressure in the direction indicated by the heavily drawn arrow. Instead of round holes also coarse slits may be taken outin the tube wall below the wire gauze. Embodiments of interest of the perforations for the tube of the pressure-plug thickener have been shown by way of examples in Fig. 20A, B, C and D, and measures and dimensions of interest for such perforations appear from the below Table IV.

The TPS process can also include fractionating of fibers, i.e. long and short fibers of the pulp suspension are separated and conveyed out of the pressure system through two separate outlets. The separation is carried out in an apparatus built up of the same elements which have been disclosed above in connection with the pressure-plug thickeners (PPT). This separation or fiber fractionation has been schematically shown in Fig. 21A and B which shows how a pump 141 transports the diluted pulp suspension from a preceding lower pressure stage through an elastic tube connection 142 or through some other previously disclosed pressure-pulse equalizing means and into a perforated tube 143. However, the separate perforation of the tube 143 are substantially larger than the perforations disclosed above for the pressure- plug thickeners (PPT). Outside and about the perforated tube

143 a tube 144 has been arranged, and the space which is thereby formed between the inner tube 143 and the outer tube

144 is in open connection with a cylinder 146 whose piston is moved by an actuator which is air-controlled and which in the drawing has been shown as an air bellow 145. Reciprocating movements of the piston cause a pulsating pressure with alternating pressure drop both ways across the perforations of the inner tube 143. This pressure pulsation causes the fiber suspension, when it moves from the pump 141 towards an outlet 159, to be pressed respectively sucked several times in and out of the perforations. Accordingly, these perforations of the inner tube 143 will cause a screening effect which in its turn causes the long fibers of the pulp suspension to preferentially remain within the perforated tube 143 whereas the shorter fibers will more easily be let through the perforations. Accordingly, the short fibers will become preferentially collected in the space between the inner perforated tube 143 and the outer tube 144 whereafter they are conveyed out of the system through pressure-plug thickener 150. This sorting of fibers or fractionation process which takes place between the inner perforated tube 143 and the space surrounding it occurs without the pulp becoming dewatered or thickened, in contrast to what takes place within the pressure-plug thickeners. This means that the pulp consistency in the inner tube 143 and in the space between the tube 143 and the tube 144 remains approximately the same.

After this sorting of fibers the two fractions of short fibers and long fibers respectively are conveyed through separate suspension or pulp runs out of the pressurized system and into a pressure-plug thickener 150 and a pressure-plug thickener 158 respectively and provided with associated outlet closure means 151 and 159 respectively. The functioning of the two pressure-plug thickeners 150 and 158 is as disclosed in connection with the pressure-plug thickeners shown in the other Figures. However, the two pressure-plug thickeners 150 and 158 shown must have differently dimensioned perforations because the perforation for the long fiber fraction, i.e. for the pressure-plug thickener 158, must be so dimensioned that it is larger than the perforation for the pressure-plug thickener 150 intended for the short fiber fraction.

The outlet means 151 and 159 respectively can consist of a pressure-loaded cone, as shown, or it can consist of a sluice means with two valves as disclosed above. Moreover, the outlet means can under favourable circumstances consist of a single flow- or pressure-controlled valve.

The valves shown, 147 and 152 (R₄ and R₃) respectively, automatically control the desired ratio of flow between the two fiber fractions. Thus, reduction of the amount of suspension through R₄ will cause a stronger concentration of long fibers in this fraction, whereby the short fibers mainly will be contained in the fraction which is conveyed through R₃ and which has the larger volume. Similarly, a relatively large proportion of fibers through R₄ will cause a larger number of short fibers to be conveyed out of the system along with the long fiber fraction.

A cylinder piston 154 has been shown which moves in step with a closure valve 156, and similarly a piston 162 work with opposite movements, as shown in Fig. 22A and 22B.

Two pressure equalizers 148 and 155 have been shown which will dampen and partly eliminate the pressure pulsations in the tubes 143, 149 and 157.

The liquid pressure which is used for back-flushing through the perforations in the pressure-plug thickeners 158 and 150 when the pistons 154 and 162 work against closed valves 156 and 161 respectively, is secured by the use of overflow valves which are connected in parallel with the valves 156 and 161 in conventional manner and not shown in the Figure.

The pressure variations in the tube 144 caused by a piston 146 is of a far smaller magnitude than the pressures which occur in the tubes 158 and 150 because the larger perforation in the tube 143 yields less resistance to flow and thereby more rapid pressure equalizations when the pressure drop alternates in both directions by movement of the piston 146. Due to the relatively modest pressure drop across the more coarsly perforated tube 143 safety means in the form of overflow valves are here not required.

The liquid pressed out from the pressure-plug thickeners 158 and 150 is conveyed back through a tube 160 counter- currently to the fiber flow through the process, as disclosed above.

In Fig. 21A and B two actuators 153 and 163 are shown which move at a such frequency that they always work in opposite directions, i.e. not with parallel movements. Further, the actuator 145 can move indepently upon the frequency of movement of the actuators 153 and 163. Moreover, the actuator 145 has a continuous and uninterrupted reciprocating movement whereas the actuators 153 and 163 only have two pulsating movements of short duration against closed outlet valve in order to ensure a brief back-flushing of pressed out liquid back in through the perforations in the tubes 157 and 149 respectively, whereby the movement of the pulp plug out of the pressure system is supported, as disclosed above. If the movements of the actuators 145, 153 and 163 are to be synchronized, this must take place as shown in Fig. 22A and 22B and e.g. thereby that the actuator 145 can be set into several reciprocating movements between each time the actuators 153 and 163 come into function.

The dimensioning of the perforations in the tube 143 used for the fiber fractionation must be made in accordance with the type or the types of fibers contained in the pulp suspension. The same relates to the dimensioning of the perforations of the pressure-plug thickeners used for the discharge of the long fiber fraction and the short fiber fraction respectively. In Table IV below examples of design and dimensions of the perforations have been stated.

_P

Fig. 22 shows how several pressure-plug thickeners (PPT) have been connected in series and work in a synchronized manner and in response to the sluice means for discharge of pulp from the system. Fig. 22A shows the normal dewatering and thickening situation using three interconnected PPTs 168, 169 and 170 respctively with water for dilution flowing into diluting vessles 171 and 172 and with the pressed out liquid flowing out of outlets 179, 180 and 181 respectively. When a valve 173 is closed, there are no axial movements of the fiber plugs which are continuously formed within the inner perforated tubes of the pressure-plug thickeners.

Fig. 22B shows the same system as has been shown in Fig. 22A, however, with the pressure drop having been reversed in the pressure-plug thickeners after closing a valve 174 and opening the valve 173. The pulp plug in the pressure-plug thickener 170 is thereby allowed to get into an outlet sluice between the valves 173 and 174, as described in details above in connection with Fig. 16 and 17. Further, in Fig. 22A and B injection means 176, 177 and 178 respectively have been shown which concurrently push pressed out liquid back into the pressure-plug thickeners 168, 169 and 170 respectively, and the pulp plugs therein will simultaneously become "lubricated", and as a consequence of the pumping pressure which the pulp pumps together exert on the pulp plugs, the pulp plugs will be concurrently pushed forward towards the outlet sluice until the outlet sluice has become filled with the pulp plug due to compressing of the remaining steam or air from the last emptying of the sluice. The pressure pulsations which are caused by the injection means 176, 177 and 178 will be caught by dampers 182, 183 and 184 respectively as disclosed above. Then the valve 173 is closed and the valve 174 and, optionally, a valve 175 (the latter for blowing the sluice chamber) are opened concurrently with opening of the injection members 176, 177 and 178 for renewed outlet of pressed out liquid through the outlets 179, 180 and 181 respectively. Thereupon the same pressure pulsation cyclus is repeated. As pressure pulsation inducers other means as disclosed above in connection with Fig. 18 and 19 can be used in the same manner.

A flow sheet which is a more detailed flow sheet but which otherwise corresponds to the flow sheet shown in Fig. 1 has been shown in Fig. 23. In particular, the three reaction modules, M₁, M₂ and M₃, have been shown in more details in Fig. 23, and it appears from the Figure that in front of each pump there is a tube mixer 70 for mixing chemicals liquid and pulp slurry and after each pump a means constructed in the same manner is arranged, however, the means then functions as distributor for chemicals. It has again been shown how the pressed out liquid 7 from the first dewatering device to the left in Fig. 23 is conveyed for heat exchange with fresh water 9 supplied or with possible waste water from a bleach plant before it leaves the system. The water consumption is controlled by means of the control valves R₂ and R₁.

In order to further illustrate the combination of the countercurrent principle and the addition of chemicals for the carrying out of the present process Fig. 24 shows the principles for the process operation. Again the drawing showing these principles is based on the main steps: delignification (and optionally deinking), washing, bleaching and renewed washing, and the number of circulation and return circuits is dependent upon the number of pumps and thickeners which are used. Similarly, the number of places for addition of chemicals will vary with respect to the different raw materials and will be dependent upon the fiber pulps to be produced. It appears from Fig. 24 that due to the counter-current principle used in carrying out the present process, which can be termed "The Tube Pulping System" or abbreviated the "TPS process" production of cellulose can be integrated within the system so that the washing water which is injected into the system at the end of the process will firstly be converted into bleach liquid and then to cooking liquid by the addition of heat and chemicals on its way towards the beginning of the process. The same liquid will on its way to the beginning of the process also take up dissolved matter from the raw material and carry this out of the system when the cooking liquid leaves the system at the beginning of the process, which appears from the Sankey diagram shown in Fig. 3.

During the progress of the process the pumps effect recirculation and reuse of cooking liquid and chemicals and, moreover, mixing and dispersing of added fresh amounts of chemicals and gas to the cooking liquid, stirring and thereby "washing machine effect" within the system and, moreover, internal heat recovery.

In Fig.24 A denote washing zone, B bleaching zone, C washing zone and D the zone for impregnation, delignification and optionally deinking. Added washing water 190 is changed into bleach liquor by addition of bleach chemicals 191 before the process liquid is changed into cooking and/or deinking liquid by the addition of chemicals and heat 192 as previously disclosed.

It has previously been mentioned that by means of the present process the consumption of water is strongly reduced compared with the consumption of water for conventional cellulose digesting processes. This has been rendered possible thereby that the same pulp concentration is maintained throughout the process and because the process (the TPS process) is carried out in a continuous,closed and pressurized tube system wherein impregnation, cooking and optionally bleaching of the pulp are carried out, and because pressurized dewatering devices are used "in line" for dewatering of the pulp and recirculation and reuse of chemicals and heat, whereby the machinery needed for carrying out the process can be correspondingly simplified, and production of cellulose without effluence to natural water can be made possible.

An example of a calculation in order to determine the water consumption when producing pulp by means of the TPS process at a yield of about 50% has been shown below.

It is assumed that fiber raw material having a solids content of 33% is supplied to the TPS process and that the amount of raw material is 2 kg, calculated as bone dry substance. At the beginning of the TPS process black liquor is pressed out in the first pressurized dewatering device, and the black liquor will contain about 1 kg of dissolved solids. At the end of the TPS process, i.e. immediately before the last pressurized dewatering device, water (washing water) is injected, and the amount of water is assumed to be X liters per kg of cellulose fibers produced. The cellulose fibers which are produced are assumed to have the same solids content as the raw material, i.e. 33%, and from the 2 kg of raw material initially introduced 1 kg of fibers and 1 kg of dissolved solids will remain. The solids will be dissolved in the injected liquid, and after the last pressurized dewatering device 1 kg of fibers will be removed from the system, whereas the liquid injected with the solids dissolved therein will be conveyed countercurrently to the fiber flow and towards the beginning of the process. The theoretically least amount of water which must be used in order to obtain the said conditions for the cellulose produced, the returned liquid and the black liquor discharged can simplified be calculated as follows: If the black liquor is assumed to contain 15% of the dissolved solids and will thereby be ready for evaporation, the least theoretical amount of liquid (X liters) will be:

produced or 6.7 tons of water for each ton of cellulose fibers produced.

This very low amount of liquid supplied for each ton of produced cellulose is significantly lower than the conventional water consumption when producing cellulose by means of the conventional cellulose manufacturing processes.

The circumstance that the present TPS process is carried out in a continuous and closed pressurized system makes it well suited for utilizing cooking liquids, e.g. NH₄OH, which easily emits volatile gases. Because the waste liquor also leaves the process at one place only, the degasing of the liquid can take place in a controlled manner and simultaneously be combined with recovery of chemicals and heat.

The recovery of chemicals from liquids, e.g. NH₄OH, which easily emit gases, can be made by adding a basic agent which will enhance the driving off of the volatile gases. When using NH₄OH as cooking liquor CaO or CaOH in pulverulent form or in the form of a slurry can be used as agent for enhancing the driving off of NH₃. When the waste liquor is then leaving the pressurized system, NH₃ gas is liberated which is conveyed away from the remaining process to become absorbed in fresh water in a scrubber plant whereby fresh cooking chemical, i.e. NH₄OH, is formed and at the same time the heat content of the NH₄ gas is recovered.

The countercurrent principle utilized for carrying out the present process enables deinking of secondary fiber from various print qualities to be carried out within the present TPS without having to resort to extra mechanical additional equipment. The deinking which is supported by particular chemicals added at this stage of the process takes place in the pulp prior to or at the same time as the pulp arrives for delignification and, if desired, to be subsequently bleached. A suitable chemical in order to facilitate the deinking is a non-ionic surface active nonylphenol alkylene oxide adduct.

By means of the present process also so called "explosion pulp" can be produced by causing the pulp to leave the pressurized system without the preceding cooling of the liquid-fiber mixture to a temperature below 100ºC before discharging it to atmospheric pressure. In this context the uniqueness of the present process is that it also enables "explosions" of pulps from for example relativel elevated pressures of from 15 to 20 bar (dependent upon the number of pressure pumps in the system) and at relatively low temperature in the pressurized system, i.e. from 110 to 150ºC, because pressures and temperatures within the TPS used can be selected independently of one another, i.e. in contrast to conventional cellulose production where pressure and temperature are determined by the conventionally used saturated steam. The advantage of low temperatures during the present process is that the cellulose will then not so easily become deteriorated and damaged by the preceding treatment with chemicals, in contrast to what is the case at the extremely elevated temperatures corresponding to temperatures of saturated steam at such elevated pressures as from 15 to 20 bar.

In Fig. 1, 2 and 3 the first pressure increasing means of the TPS is the pump P₂. However, the TPS can be supplied with extra pressure pumps connected in series if the desired temperature in the system should make a corresponding pressure increase necessary in order to prevent development of steam within the system.

Moreover, it is a general characteristic of the present TPS process and the apparatus used for the carrying out of the present process that they in a very flexible manner can be adapted to the different raw materials to be processed and to the fiber pulps which are to be produced.

When bleaching with O₂ has been stated, of course, this shall also cover bleaching by the addition of chemicals which will release O₂ in the process, e.g. peroxide.

The back flushing mentioned above of liquid back into the perforated tube of the pressure-plug thickeners has also a further effect apart from providing "lubrication" between the fiber plug and the inner surface of the perforated tube. By applying the back flushing the perforations in the perforated tube are kept free of fibers and other solid matter due to the high back flushing pressure used and the resulting high speed of the flushed black liquid. The back flushing pressure is controlled by adjustment of a conventional overflow valve, as disclosed above but which has not been shown in the drawings. The back flushing pressures used will vary according to the pertaining production conditions and will vary within the range of from 10 to 60 bar absolute pressure, preferably within the range of from 3 to 40 bar above the existing pressure in the inner tube of the pressure-plug thickener. The pressure increase caused by each pump in series, i.e. the pressure increase of each pumping stage, may be within the range from 0,5 to 10 bar, preferably not above about 5 bar.

A technical scale plant for carrying out the present process may have a capacity of about 100 tons per day, and a suitable length for the pressure-plug thickeners used in carrying out the present process will correspond to a perforated tube length of from 2 to 5 m with the perforated tube having a diameter of from 100 to 400 mm.

Claims

C l a i m s

1. A process for digesting plant and wood fibers or for delignification, optionally with preceding deinking of fibers from chemical or mechanical pulps, optionally with succeeding bleaching, for the production of a pulp suited as raw material for paper, carboard, fiber boards and other products which contain plant and/or wood fibers, wherein the fibrous raw material is introduced into a digestion zone and is digested or delignified therein in alkaline slurry at elevated temperature and pressure using an alkaline cooking chemical in combination with oxygen and, if desired, minor amounts of other adjuvants, like e.g. anthraquinone, and wherein the cooking chemicals are removed from the digestion or delignification process in the form of a black liquor which is deposited or transferred for recovery of chemicals, characterized in that the pulp -raw material in the form of a pumpable slurry during the digestion, the delignification and the optionally preceding deinking and the optionally succeeding bleaching is conveyed through a closed continuous and pressurized tube system using pulp pumps which are simultaneously utilized as mixing- aggregates for the slurry and the chemicals, the pulp while being conveyed through the tube system being repeatingly subjected to dewatering and pressing out of liquid and prior to each dewatering, apart from the last dewatering of the process, is diluted with pressed out liquid from the process and recycled from a downstream dewatering step of the process and/or from a downstream pumping stage of the process, and wherein the pulp while being conveyed through the tube system is subjected to stepwise increasing pressures in one or more pumping steps and is diluted immediately prior to the last dewatering step and optionally cooled with fresh water and/or bleach liquor supplied under pressure, and after the last dewatering carried out in the pressurized digestion or delignification system the pulp is washed and, if desired, bleached in a continuation of the pressurized system or in non-pressurized state upon cold or hot blowing.

2. Process as claimed in claim 1, characterized in that the raw material is digested and bleached using identical apparatus assembly for the digestion and the bleaching.

3. Process as claimed in claim 1 or 2, characterized in that the fiber raw material is conveyed through the process by maintaining essentially the same pulp concentration only disrupted by increased pulp concentration of short duration immediately after each pressurized dewatering step.

4. Process as claimed in anyone of claims 1 to 3, characterized in that the same basic main chemical is used throughout the process.

5» Process as claimed in claim 4, characterized in that sodium hydroxide or ammonium hydroxide is used as the basic main chemical.

6. Process as claimed in anyone of claims 1 to 5, characterized in that it is carried out using pressures which are controlled to be higher than the pressure of saturated steam at the working temperature used.

7. Process as claimed in claim 6, characterized in that the finished fiber pulp is removed from the pressurized tube system under "explosion" conditions.

8. Process as claimed in anyone of claims 1 to 7, characterized in that the addition of chemicals is made at various stages of the process and at various pressure steps.

9. Process as claimed in anyone of claims 1 to 8, characterized in that ammonium hydroxide is used as alkaline main chemical and that to the black liquor which is pressed out from the fiber pulp in the first pressurized dewatering of the process calcium based alkali is added before the black liquor is removed from the pressurized system, whereby gaseous ammonia is expelled from the black liquor upon subjecting the black liquor to pressure release , and collect ing the ammonia driven off by absorbing it in water and introducing the ammoniacal solution obtained in this manner as fresh cooking liquor into the digestion or delignification process.

10. Process as claimed in anyone of claims 1 to 9, characterized in that the pulp slurry is dewatered by pressing it into a thin-walled perforated tube surrounded by a thick-walled outer tube dimensioned according to the pressures used in the process, and letting the pressure differential between the inner perforated tube and the pressure in the space between the inner perforated tube and the outer tube jacket cause liquid from the pulp slurry to become pressed radially out of the inner tube through the perforations therein, while retaining substantially all fiber material in the inner tube, whereby the pulp is thickened into a pulp plug.

11. Process as claimed in anyone of claims 1 to 10, characterized in that the last pressurized dewatering step is provided with a discharge means for discharging the thickened pulp slurry from the pressurized system, the discharge means consisting of a conical stop device which has been placed in the end of the outlet tube from the pressurized system and which presses against the inner pressure of the system and balances the inner pressure by being actuated by the pressure of a pressurized air bellow or a similar pressure exerting means.

12. process as claimed in anyone of claims 1 to 11, characterized in that the thickened fiber plug formed in the pressurized dewatering devices is conveyed through the dewatering devices by means of the pumping pressure created by the immediately preceding pulp pump , the forward movement of the fiber plug within the pressurized dewatering device being assisted by intermittently reversing the pressure differential across the perforated tube wall in order to intermittently press liquid back through the perforations and into the inner tube of the dewatering device in order to create a lubricating effect between the outer periphery of fiber plug and the inner surface of the perforated tube.

13. Process as claimed in claim 12, characterized in that there are used more than one pressurized dewatering device in series and that the intermittent reversing of the pressure differential across the perforated tube wall in the different pressure dewatering devices is carried out in a synchronized manner and in step with the discharge of the fiber pulp from the fiber pulp producing system.

14. Process as claimed in anyone of claims 1 to 10 and 12 to 13, characterized in that the last pressurized dewatering step is provided with a discharge means in the form of a discharge sluice formed between two alternatingly operating valves arranged at a distance from one another in the outlet tube for thickened pulp from the pressurized system.

15. Process as claimed in claim 14, characterized in that the discharge sluice is connected with a pressurized system, preferably a pressurized air system, which by opening of the inlet valve to the sluice will cause the discharge sluice to be essentially flushed clean upon discharge.

16. Process as claimed in anyone of claims 12 to 15, characterized in that pressurized dewatering devices are used which are provided with an outlet tube for liquid pressed out through the perforations in the inner tube of the pressurized dewatering devices, the outlet tube being provided with a three-way valve which at short intermittent intervals is operated to close off the outlet flow of pressed out liquid and to simultaneously in the same outlet tube open for an incoming flow of liquid or gas at elevated pressure which causes flushing back through the perforations of the inner tube of previously pressed out liquid, there being used an elastic tubular connection in the pulp conveying tube shortly before the entrance to the pressurized dewatering device or devices in order to elastically catch the pressure pulses caused by the back flushing of liquid through the perforations of the inner tube, alternatively using a membrane pressure tank connected to the pulp conveying tube into the pressurized dewatering devices and immediately in front of these for catching the pressure pulses caused by the back flushing of liquid through the perforations of the inner tube.

17. Process as claimed in claim 16, characterized in that the back pressure pulse is provided by the use of a piston cylinder arranged in the outlet tube or tubes from the pressurized dewatering device or dewatering devices for pressed out liquid from the pulp slurry, the piston being operated pneumatically or hydraulically, or by means of a piston cylinder arranged in a branched off tube from said outlet tube which is provided with a closure valve for the flow of pressed out liquid through the outlet tube, and with the valve in closure position operating the piston to cause the back pressure of short duration for flushing pressed out liquid back through the perforations of the inner tube.

18. Process as claimed in claim 16, characterized in that the outlet tube from the pressurized dewatering device or devices for pressed out liquid from the pulp slurry is provided with an elastic section against which a pressure exerting device is applied in order to press the elastic tubular section together with the outlet valve from the outlet tube closed, in order to create the back pressure for flushing pressed out liquid from the pulp slurry back through the perforations of the inner tube of the dewatering device or devices.

19. Process as claimed in anyone of claims 1 to 18, characterized in that prior to be discharged from the pressurized pulp producing process the pulp slurry is passed through a tube having relatively coarse perforations and being surrounded by an outer tube forming a jacket around the inner tube, and applying pulses of liquor in and out of the perforations to thereby cause the shorter fibers to flow out through the perforations and into the space formed between the inner tube and the outer tube jacket, and branching off from the outer tube jacket the slurry of shorter fibers and passing the slurry through a final pressurized dewatering device before discharging the pulp from the process, while passing the slurry of long fibers from the inner perforated tube of the fiber fractionating means into a final pressurized dewatering device and dewatering the pulp slurry before discharging the pulp from the process.

20. Process as claimed in claim 19, characterized in that the perforations of the inner tube of the fiber fractionating means are coarser than the perforations in the inner tube of the pressurized dewatering device for the long fiber pulp slurry, which are coarser than the perforations of the inner tube of the pressurized dewatering device for the slurry of shorter fibers.

21. Apparatus for use in carrying out the process according to anyone of claims 1 to 20, characterized in that it is a pressurized dewatering device for providing a thickened pulp plug under pressure and comprising an inner perforated tube surrounded by an outer tube which at its ends are sealed against the inner tube and consists of a material capable of enduring higher pressures than the inner perforated tube, the outer tube being provided with one or more outlet (3) tubes for receiving liquid pressed out from pulp slurry in the inner perforated tube and for intermittently pressing back pressed out liquid or other flushing media into the space formed between the inner perforated tube and the outer surrounding tube and from there back into the inner tube through the perforations therein.

22. Apparatus as claimed in claim 21, characterized in that the perforations consist of holes through the inner tube, the holes being covered by a wire gauze or screen with mesh openings which are substantially smaller than the perforations through the tube wall.