NO870562L - PROCEDURE FOR PREPARING PLANTS OF PLANT AND / OR FIBER AND SUITABLE AS MATERIAL FOR OTHER PAPER, PAPER OR FIBER PLATES. - Google Patents

Description

Process for continuous production of

cellulose pulp and/or delignification of return fibre

The technical area

The present invention relates to continuous production

of cellulose pulp using a combined chemical and mechanical method for breaking down plant and wood fibers into cellulose pulp as a raw material for paper, cardboard, fiberboard and other fiber-containing products. The present method is also suitable for deinking, delignification and possible bleaching of return fibre.

State of the art

Cellulose pulp has for decades been produced by the well-known sulphite process or the well-known sulphate process. Both processes today require very large facilities in order to be carried out economically and it is very expensive to build new cellulose factories for the production of cellulose by sulphite or sulphate pro-

the session. Nevertheless, in recent years, a number of new large cellulose factories have been built in the industrialized part of the world and which, after commissioning, have caused large financial losses.

Currently, some similar factories are under construction in

winding countries, and it can be said in advance with great probability that these factories will cause these countries great economic losses and burdens in the years to come.

The reason for this is the fact that today's usual technology for continuous mass production has become far too expensive compared to the prices that the cellulose markets have been able to offer in recent decades. This situation is now particularly unsatisfactory and unsustainable for developing countries that need to build up their own cellulose and paper industries.

If new, continuous factories for the production of cellulose are to be made profitable, a thorough revision and renewal of the cellulose production process must be carried out with the aim of achieving a significant reduction of the investment and capital costs per annual tonne production capacity. Today's technology for continuous pulp production is mainly based on the sulphate process, which in recent years has become increasingly widespread at the expense of the sulphite process. This situation is due to the fact that the sulphate process is applicable to most relevant fiber raw materials, in contrast to, for example, the sulphite process. In addition, the current sulphate process is also an established one

system for the recovery of cooking chemicals and the combustion of organic compounds in the so-called "black liquor" available. The sulphate process has therefore in recent years been regarded as the most environmentally friendly process for the production of cellulose.

However, there are still several serious disadvantages with the process, and these can be listed as follows: 1. The sulphate process has developed to be far too capital-intensive and requires extremely large production capacities as a result of the so-called "economy of scale" . Such large facilities make correspondingly large demands on transport capacity and infrastructure, but even so, facilities over 200,000-250,000 annual tonnes have in recent years produced large financial losses and had insufficient profitability. 2. For the sulphate process, sulfur compounds are used, and this causes unpleasant odors and pollution of the air. In addition, chlorine-containing compounds are usually used in connection with bleaching, which give off toxic and harmful emissions to the environment. 3. A sulphate factory is a complicated system which is composed of a large number of very different process and machine elements and is divided into separate departments for cooking, silage, laundry and bleaching respectively, each with separate heating, water, chemical and sewage systems .

Today's sulphate factory is usually thus characterized by standing high pressure digesters where the fibers are fed in at the top with a high, non-pumpable consistency, to later pass vertically through the digester according to the "free fall" principle, to then be diluted to a pumpable or " blowable" consistency at the bottom of the cooker.

4. The sulphate process's need for volume and ground area are large because the process proceeds

respectively slowly. This is because the fiber mass is exposed to relatively little movement along the way, and this in turn has the consequence that the reactions take place more slowly than would have been the case with more movement and turbulence ("washing machine effect").

5. The temperatures and pressures used during pulp boiling are usually determined by the fixed ratio between pressure and temperature in saturated steam which is traditionally used as a heat source in the boiler. 6. Upright, tall diffusers, which are usually associated with complicated heat recovery systems, are used to "blow" the mass after cooking. 7. Repeated dilution and thickening of the pulp in connection with many different pulp consistencies during the process, by internal transport, screening, washing and bleaching, is carried out with the help of heavy and complicated equipment which is often placed on different floors in high, heavy and expensive factory buildings. 8. Large quantities of water and effluent chemicals must be dealt with in large, space-consuming and expensive recycling and/or purification plants. 9. The mass factory comprises a very large number of different machines, operations and functions, all of which represent possible sources of error and which require corresponding supervision, maintenance as well as a rich and expensive stock of many spare parts.

Purpose of the invention

The invention aims to provide a method for the production of cellulose pulp and/or delignification of return fiber with possible bleaching,

including any deinking, as the

the procedure should not be beset with the very large disadvantages mentioned above in connection with the sulphate process as representative of the most interesting and close state of the art. With the present method, compared to today's usual cellulose pulp production processes, a strong simplification of the process management and of the process equipment is to be obtained, which reduces the need for capital, space, monitoring and maintenance. With the present method, an extensive recycling and reuse of chemicals and heat energy is to be achieved in combination with the so-called "washing machine"-

effect, whereby the process proceeds much faster from raw material to finished cellulose pulp than the currently commonly used processes. The extensive recycling and re-use of chemicals and heat energy will ensure optimal operating economics for the present method, and also reduce water consumption, whereby discharges to water may possibly be eliminated. The aim of the invention is that the equipment for carrying out the method and the method steps themselves should be able to be divided into modules

which with simple internal modifications should provide combination possibilities for the treatment of all applicable fiber raw materials. Furthermore, the aim is that such modules should be able to be fitted into frames for standard transport containers, thereby ensuring simple and cheap assembly, transport, start-up and possibly moving the plant from place to place as required.

Summary of the invention

The invention relates to a continuous method for digesting and possibly bleaching plant and wood fibres

and/or for delignification and possibly deinking of return fiber while obtaining a pulp that is suitable as raw material for paper, cardboard, fiber boards and other products containing plant and/or wood fibers, where the fiber-containing raw material is introduced into a digestion zone and digested in this in alkaline slurry at elevated temperature and pressure using an alkaline cooking chemical in combination with oxygen and possibly smaller amounts of other additives, such as anthraquinone, where the cooking chemicals are removed from the digestion process in the form of a black liquor that goes to disposal or chemical recycling, and the method is characterized by the fact that the pulp raw material in the form of a pumpable slurry during digestion and eventual bleaching is transported through a closed, continuous and pressurized pipe system using pulp pumps which are also used as a mixing unit for the slurry and the chemicals, as the pulp is transported through the pipe system out tes for repeated dewatering by squeezing out liquid and before each dewatering, except for the last dewatering, is dewatered with squeezed liquid returned from a down-

stream dewatering step in the process and/or from a downstream nnmno f ri nn r\ rr n ri Q +■ m a e; c? n m d T~| o rlctrt f ranennrforoc monn About

the pipe system is exposed to gradually increasing pressure in one or more pump stages and before the last dewatering stage it is drained and possibly cooled with fresh water and/or bleaching effluent supplied under pressure and that the mass after the last dewatering in the digester pressure system is washed and possibly bleached in a continuation of the pressure system or in depressurization condition after cold or hot blasting..

Brief description of the drawings

The drawings show three different forms for an embodiment as an example of the present method. Specifically, is on

Fig. 1 shows a flowchart for the digesting part of the process, with four dewatering stages and with a gradual build-up of the pressure from the first dewatering stage to the fourth dewatering stage, Fig. 2 schematically shows the relative pressure increase according to the flowchart shown in Fig. 1, from the beginning of the digestion process to its conclusion from left to right in the Figure, and Fig. 3 shows a Sankey diagram for the process according to the flow chart shown in Fig. 1, for the supplied and recycled amounts of liquids or chemical solutions. Fig. 4-16 will be discussed below in connection with the description of the process.

Detailed description of the process and the drawings

An embodiment of the present method will now

be described in more detail below, with reference to the drawings.

According to Fig.l and 2, raw material and fresh water are fed to a dissolver and from this via a pump P-j and purification device fed to a pump P^in which the pressure on the slurry which has a consistency of approx. 4%, is increased to the pressure level p^. From the pump P^, the slurry is pushed through a dewatering device

which will be described in more detail below, in which the slurry is dewatered to a consistency of approx. 20%. The dewatered mass is in the pipe between the dewatering device and a pump<p>_ diluted with digestion liquid that has been returned from

later pressure boosting step and/or pressure dewatering step further downstream in the process. In addition, fresh chemical-potassium solutions and/or additional heat can also be added to the slurry in the pipe section between the dewatering device and the pump P ^ . The liquid squeezed out in the dewatering device is essentially fed to a heat exchanger in which the squeezed liquid (sewage or waste water) is fed in heat exchange conditions to introduced fresh water or possibly waste water from a bleaching plant. Part of the lye that has been pressed out of the dewatering device,

can also be added to the solvent to form the raw material slurry in the solvent with a consistency of approx. 4%. Before the slurry enters the pump P^ and from there into the dewatering device

it is supplied with used cooking chemical solution returned from the digester pipe immediately after the pump P ^ which accounts for the next closest pressure increase step. From the pump stage p^, the mass is then fed to a new pump stage where the pressure on the slurry is further increased and as recycled cooking chemicals are returned to the slurry between the two pump stages p^and p^, with the returned used cooking liquor to the pipe section between the pump stage p^and the pump stage p^ is taken from the slurry pipe immediately after the next consecutive pressure-increasing pump stage p,.. From the pump P^, the slurry is then transferred with a mass consistency of approx. 4% to the next pressure-increasing pump stage p^, and from this pump stage the slurry is carried on, again under the supply of recycled

used caustic soda, to the pressure pump P^from which the slurry is passed through the next dewatering device in which the slurry is again dewatered to a mass consistency of approx. 20%. From the dewatering device, the mass is conveyed further, again under dilution to form a mass slurry with a mass consistency of approx. 4% with returned spent cooking liquor, to a next pressure increase pump P^ and from there into a next dewatering device in which the slurry is again dewatered to a mass consistency of approx. 20%. From the dewatering device, the slurry is fed on to the last shown pressure-increasing pump Pg, again while diluting the slurry to a mass consistency of approx. 4% when supplying liquid, which in this case is fresh water or possibly waste water from a bleaching plant

the mass can be transferred after it has been removed from the digestion section shown in Fig.l. This liquid, which is supplied before the pressure-increasing pump Pg, is preferably relatively cool in order to be able to cool the mass to such a low temperature that when it finally leaves

the pressurized pipe system, at atmospheric pressure it will have a temperature below 100°C, i.e. that it will not expand, or more precisely that it will be "cold blown" when it leaves the pressurized pipe system. From the pressure-increasing pump P g, the slurry enters the last shown dewatering device, in which it is again dewatered to a mass consistency of approx. 20%.

The dewatered slurry with a pulp consistency of approx. 20% is then taken out of the system via the shut-off cone at the end of the pipe, as this shut-off cone is of known construction and provides a counter-pressure which is controlled by an air-filled bellows whose adjustable pressure is exerted against the cone-shaped rod device's base surface. The flowchart according to Fig. 1 shows the upgrading of return fiber or the production of unbleached cellulose from one-year crops. As mentioned, the main consistency during transport through the pipe system is approx. 4% and the consistency after dewatering approx. 20%. The flowchart shows 3 reaction modules (Mn,1^ and M.J for delignification and 2 steps for washing (at

P_ and P0). A bleaching section can

is connected to the system shown in Fig. 1, and for the bleaching can

the same procedure is used as described above in connection with the upgrading or digestion, but of course with the use of chemicals of a type and in a sufficient quantity to achieve the desired bleaching.

The present method is applicable to all kinds of vegetable fibres, e.g. bagasse, kenaaf, rice straw, rubber trees, waste from palm oil production, bamboo and for hardwoods (hardwoods) and softwoods (conifers). Likewise, the method is suitable for delignification and possible bleaching of return fibre. When using printed recycled paper qualities, the system's design also results in deinking (de-inking) of the raw material without the use of additional mechanical equipment. The present process offers a wide spectrum of reaction possibilities, covering the production of pulp with a quality that varies in the range from mechanical pulp (MP) via thermo-mechanical pulp (TMP) and chemical-thermo-mechanical pulp (C-TMP) to pure chemical masses (CP).

As mentioned, the shown flowchart according to Fig. 1 applies to digesting or cooking annual crops, such as straw, bagasse or kenaaf.. For digesting or cooking sawdust, hardwood or softwood, the process must be supplemented with mechanical processing in irafiners. After the refiners can an

yendes pressure sieves that separate coarse

the chip from the acceptance and returns the sorted coarse chip

fraction to renewed chemical and mechanical support. This provides opportunities for a number of alternative arrangements that can be adapted to all relevant fiber types.

As mentioned under the purpose of the invention, the invention also aims to ensure that the equipment for carrying out the method and the method steps themselves can be divided into modules. The attached Fig. 4 shows the main principles for

an example of

the present method, and on the attached Fig. 5

is shown how the flow chart for one-year increments can

out as this is built orp in three modules, each of which includes a pulp pump and pipe system, while Fig. 6 shows an example of the flow chart for the production of cellulose from sawdust, hardwood or softwood. In relation to the flow chart shown in Fig. 5, the flow chart shown in Fig. 6 is supplemented with refiners for mechanical processing, and after . the refiners are equipped with pressure screens that separate the coarse chips from

accepted and returns the sorted coarse chip fraction to renewed chemical and mechanical digestion.

It is natural to use the three modules in Fig. 5

.and 6 for respectively impregnation, delignification and

defibration which constitutes the total support.

After this impregnation, delignification and defibration, washing according to Fig. 4 follows, still in the closed, pressurized system. This "Kokery" part ends with pressurized dewatering, preferably carried out in a pressure-plug dewatering device as shown in Fig. 1. Pressure-plug dewatering

the device will be described in more detail.

The mass from the digestion can be transferred to washing in

a module, and the transfer of the mass to a washing module IV

is shown in Fig. 7. I

this module includes 3 pressurized dewatering presses connected in series, but these 3 pressurized dewatering presses could just as easily be replaced by 3 pressure-plug dewatering devices with a front pressure pump. It appears from Fig. 7 that fresh water or wastewater from the bleaching plant is introduced before entering the last of the series-connected pressurized dewatering presses and that the liquid that has been pressed out in this is returned to before entering the second pressurized dewatering press. This is done in order, after pressurized dewatering, to thin the slurry to a mass consistency that is suitable for the further transport of the mass, for example approx. 4%.

According to Fig. 4, the washed pulp is still transferred

in a pressurized system, to a bleaching plant, and on Fig. 8 it is shown how this transfer can take place and how, according to the embodiment shown, the bleaching plant is also built up of wooden modules, each of which includes a pipe system with a pressure pump. From the last bleaching module VII

the bleached mass is transferred, as shown in Fig. 4, to washing, likewise with 3 pressurized dewatering presses or with 3 pressure-plug dewatering devices (module VIII). Before each pressure dewatering, the pulp slurry is fed with liquid, fresh water possibly from a scrubber or heat exchanger being supplied before the last pressure dewatering device, while liquid squeezed out in the last pressure dewatering device is returned to the pressure pipe system in front of the inlet to the second pressure dewatering device. The liquid squeezed out in this can optionally be returned to the pressure pipe system and introduced into the bleached mass before the inlet to the first pressure dewatering device.

From the last pressure dewatering device, the pulp is transferred to storage or paper manufacturing.

After impregnation, delignification and defibration, the pulp can also be washed

possibly bleached in a subsequent non-pressurized system. Such

pressureless washing is shown schematically in Fig. 9. The mass from the cooker is then first "blown" before it enters a module IV which is a laundry module and in which, in the same way as for the pressurized washing, three dewatering presses are used, but these are of course not pressurized. After this pressureless washing, the pulp can of course be transferred to bleaching in the usual way, i.e. not pressurized bleaching, or it can be transferred for storage or paper manufacture.

For a factory for the production of unbleached or bleached cellulose with a capacity of approx. 30,000 annual tonnes, i.e. 100 daily tonnes or 4 tonnes per hour, which can be built up by the system described here and provide profitable operation because the investment costs in plant space and equipment are relatively modest, numbers and dimensions that refer to this production capacity have been indicated below. A factory with this production capacity has a favorable size for new factories in developing countries.

As previously mentioned, in connection with the cooking part as shown in Fig. 5 and Fig. 6, it is natural to use module I as impregnation part and modules II and III for delignification and defibration, with or without the use of mechanical defibration additionally carried out "in-line " with pressurized refiners. As examples of working parameters for such a factory, the following can be stated:

It appears from the lower part of the table that the use of refiners and pressure sieves depends on the raw material to be processed. However, the modules are built according to a standard form that has allocated space for alternative machine applications.

The termination of the digestion part can be carried out in two different ways, i.e. a) with a traditional blowing tank,

preferably in combination with a scrubber system for the recovery of heat from released steam, followed by a pressureless dewatering device with the return of squeezed cooking liquid for recovery and solvent.

b) With a pressurized dewatering device, the system can be significantly simplified, which also results in a more closed system and

optimized heat economy.

Washing the pulp is often desirable as part of cellulose production, after the first delignification and defibration of the raw material, and after bleaching. In the present method, the washing can be carried out in a special washing module which is made up of 1, 2 or 3 washing stages after the first dewatering, see module IV in Fig. 7 or 9.

As for bleaching the pulp, the bleacher can do it as well

like the associated laundry, work without pressure if the temperature in the pulp string does not exceed 100°C. In that case, the present method can be carried out after the laundry in the cellulose factory (ie after module IV) by using pressureless dewatering presses in the washing part of the bleaching plant. This condition also applies if module IV is pressurized, as here a "cold blow" will take place out of the pressurized module due to the cooling resulting from the addition of injected, colder washing water in the form of fresh water or as

wastewater from the bleaching plant.

However, the present method enables

maximum heat recovery and economization through a significant reduction in total water consumption. Thereby, the emission conditions are also correspondingly improved. Optimum heat recovery is achieved if the cold blowing of the mass out of the pressure system is moved to the end of the washing part of the bleaching plant. However, this assumes that pressurized equipment is also used in the bleaching plant and the associated laundry. There are therefore 2 print options.

However, there are several reasons why pressurized equipment should also be used for the bleacher and its laundry. In this way, the bleaching plant with its laundry can be built up from the same, identical modules that are used for cellulose production and its associated laundry.

The additional mechanical costs for a pressurized system are relatively modest compared to the installation price for an unpressurized system. This situation must also be seen against the background of the fact that the capital requirement for an overall pressurized system for carrying out the present method is modest compared to the capital requirement for conventional cellulose processes.

The use of pressurized elements throughout the present system consequently means that a maximum standardization of the machine elements that must be used can be carried out. This standardization in turn entails significant advantages and savings in terms of operational supervision, maintenance and spare parts.

Based on the above, a pressurized bleaching plant with associated laundry will therefore look as shown in Fig. 8. However, it should be noted that pressure plug dewaterers are preferable.

Below, in Table II, some working parameters for the operation of such a bleaching plant as shown in Fig. 8 are reproduced.

If the bleaching is carried out without chlorine, i.e. only with NaOH or NH^OH, 0^ and/or "0^, the wastewater from the bleaching plant can be used as wash water in the cellulose factory's laundry, and from the laundry the wastewater is returned to the beginning of the cellulose process.

The present method can be carried out with a

high degree of self-regulation. A coking plant and bleaching plant that works in pressure stages provided by pulp pumps connected in series will itself regulate the pressure difference through recycling of the chemicals in countercurrent to the movement of the fiber mass through the plant.

Such a connection of the cookery and the bleaching plant also provides

only one place for discharge of used chemicals from the plant. This discharge can be placed furthest forward, almost at the beginning of the process, and this "black liquor" which is taken out can, as mentioned, be fed in heat exchange conditions with fresh water and/or waste water from the bleaching plant, as shown on Flg. 1, 2 and 3.

According to the Sankey diagram shown in Fig. 3 will

the amount of cooking liquid used that is discharged from the system,

be approximately equal to the amount of (fresh) water that is added at the end of the plant as washing water for the pulp after bleaching.

If significant amounts of chlorine are used in the bleaching plant,

will return flow off

wastewater and chemicals from the bleaching plant and its laundry had to be broken before the pulp mill and led out of the system.

This division requires new water consumption in the cellulose factory's laundry. Thus, such a division due to the use of chlorine in the bleaching plant will necessarily lead to increased water consumption. Even so, the present method will be able to be carried out with a significantly lower water consumption than is the case in ordinary cellulose factories.

In case of moderate additions of chlorine in the bleaching plant, the chemical liquid can still be returned to the boiler section and used as part of the digestion liquid.

It also appears from Fig. 1 that the addition of chemicals and/or the addition of heat is carried out immediately before the mass pumps, whereby several advantages are achieved, as follows: a) The process pressure is increased in order to be able to use process temperatures above 100°C. b) The pressure increase takes place step by step over the mass pumps which are arranged in series, with an increase to the next pressure step for each

review of new mass pump.

c) Increased pressure at each stage also enables recirculation and application of the counter-flow principle between fiber flow and

chemical potash pumping throughout the plant.

d) Raw materials and chemicals get an intense mixture that is constantly repeated. Due to the achieved agitation and

recirculation of the fibres, these are "washed" more or less clean of lignin by chemical action and agitation.

The modules shown in Fig. 5, 6, 7, 8 and 9 are shown with reaction tubes arranged one behind the other. For each module there are, for example, 4 reaction pipe parts which together make up a "package".

This pipe package together with a mass pump, possibly also with

refiners. and strainer, can be built into the frame of a standard transport container, and this in the same way for all modules that follow one another. Such an installation in a standard transport container is perspectively indicated in Fig. 10.

It also appears from the above tables that the residence time in each reaction tube package in each module is approx. 10 minutes. A suitable dimensioning for these reaction tubes is as follows:

A suitable pipe quality is ST23-33 with a pipe thickness of 13 mm in stainless steel. Such a pipe quality will withstand a working pressure of 20 bar and a working temperature of 175°C. Optionally, pipes of quality ST23-43 with a material thickness of 12 mm in acid-proof steel can be used.

The valves shown in Fig. 1, 2 and 3 are pressure-controlled automatic valves that open at a pre-set pressure. Due to the fact that the pressure increase during the execution of the present method takes place in steps, these valves will automatically regulate the backflow in the process and thus the counterflow between fiber flow and process chemicals, quantitatively determined by the amount of liquid (water) that is pressed into the last step of the process, see Fig. 5, 6, 7 8 and 9. The principle of the self-regulated pressure distribution in the implementation of the present method can be seen clearly from Fig. 1 and 2.

The amount of fresh water added will also determine the amount of black liquor discharged from the system and thus also the concentration of solids in the black liquor.

The pressure-controlled automatic valves between the different pressure increase stages shown in Fig. 2 will, in addition to ensuring automatic regulation of the backflow in the process and thus the counterflow between fiber flow and process chemicals, also regulate the back pressure and thereby the pressure drop across the pressurized dewatering devices, i.e. they will ensure a control of the degree of dewatering or the amount of liquid squeezed out of the pulp slurry.

Some examples of chemical process conditions for the present method when this includes boiling (possibly deinking), washing, bleaching and washing are given in Table III below.

It appears from the table that during cooking a temperature varying from 90 to 150°C is used depending on the fiber material fed, and as cooking chemicals sodium hydroxide or ammonia, oxygen and any cooking aid additives, such as anthraquinone (AQ), are used. The total cooking time varies from 8 to 60 minutes depending on the fiber material fed, but a satisfactory delignification

with possible deinking of return fiber will take place within 2-40 minutes.

The bleaching is carried out in an alkaline environment with chlorine or peroxide (oxygen bleaching), and the temperature during the bleaching is kept between 70 and 120°C. The washing between boiling and bleaching can be done at a temperature of 70-140°C, and the washing time can vary from 0.5 to 15 minutes. After the bleaching is done

then a final wash. It appears from the table that fiber flow and liquid flow are constantly carried in opposite directions in relation to each other, and as the entire process takes place in a pressurized closed system, ammonia will also be suitable as a cooking chemical.

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

Fig. 11 shows the principle of such a pressure-plug dewatering device with a front-mounted pressure pump. According to Fig. 11, the pressure p (minus the frictional resistance between the sliding mass plug and the pipe wall) is greater than the pressure Pjjj-Thus, the mass plug located inside the central perforated pipe will be pushed out of the system past the cone-shaped discharge control device arranged on the right of the drawing while extruded liquid with the pressure p will be fed back to one or more places upstream in the process as indicated by the direction of the arrow at the entrance to the circulation pipe shown.

As the first and later intermittent dewatering device within the system, the device shown in Fig. 12 can be used. This corresponds to the dewatering device shown in Fig. 11, but it is shown in Fig. 12 that for the further transport of the mass after the dewatering in the pressure-plug dewatering device, the mass is diluted with process fluid which is returned to the pressure pipe system via the two pipe legs shown, to again dilute the mass, which now has a pressure of p, to a low mass consistency, e.g. about. 4%.

These "pressure-plug dewaterers" have no moving parts and are designed for operating pressures up to approx. 16 bar, in the same way as the pumps which can be standard centrifugal pulp pumps and designed for low-consistency or medium-consistency pulp suspensions, i.e. in the range 3-15%. The pumps must be certified for pressures up to 15-16 bar in order to withstand the pressure that occurs when several pumps are connected in series within the system. The pressure head for one pump should be in the range of 10-20 m or more. Furthermore, the speed and diameter of the impellers will advantageously be dimensioned in such a way that the peripheral speed will not fall below 25-30 m/second to achieve

the desired mixing and dispersing effect between fiber material, process fluid and the chemicals and gases used which are added to the pressure pipe system in several places along the way.

However, the strength requirements for the inner, perforated tube in the pressure plug dewaterers are not as great as for the outer surrounding tube which is shaped like a shell around the perforated tube and which thus also functions as protection around it. The outer pipe must therefore comply with all safety regulations that apply to the working pressures used. The inner tube can, for example, consist of acid-resistant material with a wall thickness of 1-2 mm. The perforation can advantageously be given a special design to avoid a resealing of the holes, and the holes can open approx. 60° in the direction of movement of the liquid flow, see Fig. 13.

For the last dewatering that is carried out during the execution of

the present method, a pressurized screw press can also be used. The principle of such a screw press is shown in Fig. 14. The mass suspension enters via the preset pump, with a pressure p^, and the extruded liquid is returned to the process with a pressure p2 which is lower than the pressure p^.

To the right of the Figure, the atmospheric pressure is denoted by p^.

Furthermore, it should be noted that for the first dewatering

which is carried out during the execution of the present method, a non-pressurized press can be used which is in principle arranged as shown in Fig. 14, but with a simpler mechanical design as p2 will be approximately the external atmospheric pressure Pr

The dissolution of the thickened mass takes place all the time

with returned liquid, countercurrent to the fiber flow, as can be seen from the Sankey diagram shown in Fig. 3.

A flowchart that is a more detailed flowchart, but

which otherwise corresponds to the flow diagram shown in Fig. 1, is shown in Fig. 15. In particular, the three reaction modules, M^, M.^°9M3, are shown in more detail in Fig. 15, and it appears from this that in front of each pump is a pipe mixer for mixing chemical-liquid and mass slurry and after each pump a device of the same design arranged, but the device then acts as a chemical distributor. It is again shown how the extruded liquid from the first dewatering device on the left in Fig. 15 is returned for heat exchange with incoming fresh water or possibly wastewater from a bleaching plant. The fresh water or possibly waste water from bleaching which has been led in heat exchange conditions to the black liquor squeezed out in the first dewatering device on the left in Fig. 15, is via the valve R ? introduced under pressure into the pipe mixer in front of the pump Pg (for washing stage 2), and liquid that has been squeezed out in the last dewatering device after the pump PQo is returned, among other things, to the pipe mixing device (washing stage 1) which is arranged in front of the pump V^.

To further illustrate the present method's combination of counterflow principle and chemical addition, Fig. 16 shows a schematic diagram of the process. Again, the principle drawing is based on the main steps of delignification (and possibly deinking), washing, bleaching and washing, and the number of circulation and return loops (which depends on the number of pumps and dewaterers used) as well as the number of addition points for chemicals will be different for those for different raw materials and depending on the fiber masses to be produced. It is clear from the principle drawing shown in Fig. 16 that due to the counter-flow principle used in the execution of the present method which can be given the name "The Tube Pulping System" or abbreviated the TPS process, the 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, on its journey towards the beginning of the process it is first converted into bleaching liquid and later into cooking liquid by the addition of heat and chemicals along the way. Along the way, the same liquid will also take with it dissolved substance from the raw material and bring this out of the system when the cooking liquid leaves the system at the beginning of the process, as can be seen from the Sankey diagram shown in Fig. 3.

Along the way, the pumps effect recycling and reuse

of cooking liquid and chemicals and also mixing and dispersing the added fresh amounts of chemicals and gas in the cooking liquid, stirring and thus "washing machine effect" within the system and also internal heat recovery.

It has previously been mentioned that with the present method the water consumption will be greatly reduced in relation to the water consumption that takes place in normal cellulose boiling processes. This is made possible by the fact that the same mass concentration is used throughout the process, and because the process (TPS process) is carried out in a continuous,

closed and pressurized pipe system in which both impregnation, boiling and possibly bleaching of the pulp is carried out, that pressurized dewatering devices are used "in-line" for dewatering the pulp and

recycling and reuse of chemicals and heat and that the same concentration is used throughout the process, whereby mechanical equipment is simplified accordingly, a cellulose production without discharge to water can be made possible.

Below is a calculated example of the water consumption when producing pulp in the TPS process with a yield percentage of around 50.

It is assumed that for the TPS process raw

material with a dry matter content of 33% and that the amount of raw material is 2 kg dry. At the beginning of the TPS process, black liquor is squeezed out in the first pressure dewatering device, and the black liquor will contain approx. 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 provisionally set to X liters per kg of cellulose produced. The cellulose that is taken out is assumed to have the same dry matter content as the raw material, namely 33%, and from the 2 kg of raw material that was originally introduced, 1 kg of fiber and 1 kg of dissolved solids will remain. The solid substance will be dissolved in the injected liquid, and after the last pressure dewatering device, 1 kg of fiber will be removed from the system, while the injected liquid with the solid substance dissolved in it will be pushed out in the last pressure dewatering device and carried countercurrently to the fiber flow towards the beginning of the process. The theoretical minimum amount of water that must be used to obtain the aforementioned conditions for the produced cellulose, the returned liquid and the extracted black liquor can be calculated simply as follows: If the black liquor is assumed to contain 15% dissolved solids and is thus ready for evaporation, it will least theoretical

be:

X , 15 v _ 100 , _ ,

"lOO-" * 15 = ' water/kg fiber

produced, or 6.7 tonnes of water/tonne of cellulose produced.

This very low added liquid value per ton

produced cellulose is significantly lower than

the usual water consumption in the production of cellulose in the usual cellulose production processes.

The fact that the present TPS process is carried out

in a continuous and closed pressure system makes it suitable for the use of cooking liquids, e.g. NH^OH, which readily gives off volatile gases. As the effluent also leaves the process in only one place, the degassing of the liquid can take place in a controlled manner and at the same time be combined with chemical and heat recovery.

The chemical recovery of liquid which easily emits gases, for example NH^OH, can be carried out by adding a base stripping agent to the effluent before it leaves the pressure system.

When using NH^OH as the cooking liquid, CaO or CaOH, in powder form or as a slurry, can be used. When the effluent then leaves the pressure system, NH3~ gas is released, which is led away from the rest of the process to then be absorbed in fresh water in a washing plant, whereby new cooking chemicals, i.e. NH^OH, are formed, while the heat of the NH3 gas is recovered.

In the present method, the counterflow principle is used optimally between chemical flow and fiber flow. The fresh water that is added as washing water at the end of the process moves forward in the direction of movement of the mass and adds heat and chemicals along the way. The amount of liquid thus carries out washing, possibly bleaching and new washing, then digestion, delignification, possibly deinking,

and impregnation of the raw material, before it leaves the pressure system

at the beginning of the process. The liquid now has the character of "black liquor" and then goes to discharge or recycling, preferably after heat exchange with added fresh

fluid for the process.

This embodiment of impregnation, possibly deinking, delignification, bleaching and washing is made possible due to the mechanical structure of the TPS process as well as the fact that the process uses the same basic chemicals and the same mass consistency for all steps throughout the process (except for the intermittent and short-term thickenings and associated dilutions of the mass).

The used counter-flow principle enables a deinking or de-inking of return fiber from different printing paper qualities within the TPS system without the insertion of additional mechanical equipment. The deinking, which is supported by added special chemicals at this stage of the process, takes place in the pulp before it reaches delignification and possible bleaching. A suitable chemical to promote the deinking is a nonionic surfactant nonylphenol alkylene oxide adduct.

With the present method, so-called "explosive masses" can also be produced by causing the mass to leave the pressure system without prior cooling of the liquid-fibre mixture to below 100°C before it comes out at atmospheric pressure. The peculiarity of the present method in this context is that it also enables "explosions" of masses from, for example, relatively high pressures of 15-20 bar (depending on the number of pressure pumps in the system) with relatively low temperatures in the pressure system, i.e. 110-150° C, because pressure and temperature within the TPS system used here can be chosen independently of each other, i.e. in contrast to what the relationship is in conventional cellulose production where pressure and temperature follow the conventionally used saturated steam. The advantages of low temperatures during the process are that the cellulose will not be so easily degraded and damaged by the previous chemical treatment, in contrast to what is the case at extremely high temperatures corresponding to temperatures for saturated steam at pressures as high as 15-20 bar.

In Fig. 1, 2 and 3 is the first pressure increasing device

in the TPS system, pump P2-This can, however, be supplemented with additional pressure pumps connected in series if the desired temperature in the system should make a corresponding increase in pressure necessary to prevent vapor development within the system.

It is also a general characteristic of the present TPS process and the apparatus system used for its execution that they can be very flexibly adapted to the different raw materials to be processed and to the fiber masses to be produced.

When bleaching with 02 is indicated, this should of course also cover bleaching with the addition of chemicals that will emit O2, e.g. peroxides.

Claims (10)

1. Procedure for digesting and possibly bleaching plant and wood fibers and/or for delignification and possibly deinking of return fiber while obtaining a pulp that is suitable as raw material for paper, cardboard, fiberboard and other products containing plant and/or wood fibers , where the fiber-containing raw material is introduced into a digestion zone and in this is digested in an alkaline slurry at elevated temperature and pressure using an alkaline cooking chemical in combination with oxygen and possibly smaller amounts of other additives, such as anthraquinone, and where the cooking chemicals are removed from the digestion process in the form of a black liquor that goes to disposal or chemical recycling, characterized by the mass raw material in the form of a pumpable slurry during digestion and possible bleaching being transported through a closed, continuous and pressurized pipe system using pulp pumps which are also used as a mixing unit for the slurry and kj the chemicals, as the mass while being transported through the pipe system is subjected to repeated dewatering by squeezing out liquid and before each dewatering, except for the last dewatering in the process, it is washed with squeezed liquid returned from a downstream dewatering stage in the process and/or from a downstream pumping stage, and as the mass while it is being transported through the pipe system, it is exposed to gradually increasing pressure in one or more pump stages and before the last dewatering stage it is washed out and possibly cooled with fresh water and/or bleaching liquor supplied under pressure, and that after the last dewatering in the digester pressure system the mass is washed and possibly bleached in a continuation of the pressure system or in a depressurized state after cold or hot blowing. 2. Method according to claim 1, characterized in that the raw material is digested and bleached using an identical apparatus assembly for digestion and bleaching. 3. Method according to claim 1 or 2, characterized in that the fiber raw material is carried through the entire process with maintenance of essentially the same mass concentration only interrupted by a short-term increased mass concentration immediately after each pressure-dewatering step. 4. Method according to claims 1-3, characterized in that the same main basic chemical is used throughout the process. 5. Method according to claim 4, characterized in that sodium hydroxide or ammonium hydroxide is used as the main basic chemical. 6. Method according to claims 1-5, characterized in that it is carried out using mutually independent pressure and temperature settings. 7. Method according to claim 6, characterized in that it is carried out using pressure which, at the working temperature used, is significantly above the pressure that would have occurred for saturated water vapor at the same temperature. 8. Method according to claim 7, characterized in that the finished fiber mass is removed from the pressurized pipe system under "explosion" conditions. 9. Method according to claims 1-8, characterized in that the chemical addition is carried out at different stages of the process and at different pressure levels. 10. Method according to claims 1-9, characterized in that ammonium hydroxide is used as the main alkaline chemical and that calcium-based alkali is added to the black liquor after it has been squeezed out, but before it is removed from the pressure system, so that ammonia gas is driven off from the black liquor when it is subjected to depressurization .