EP1797235A1

EP1797235A1 - Improved alkaline process and system for producing pulp

Info

Publication number: EP1797235A1
Application number: EP05795252A
Authority: EP
Inventors: Lasse Hernesniemi; Antti Tuominen; Tuomo NYKÄNEN; Rami Lampinen
Original assignee: Metso Paper Oy
Current assignee: Valmet Technologies Oy
Priority date: 2004-10-04
Filing date: 2005-10-04
Publication date: 2007-06-20
Also published as: CN101068977A; FI120547B; FI20041288A0; EP1797235A4; CA2581923A1; WO2006037860A1; FI20041288A; CN101068977B; US20070256801A1

Abstract

A process and an apparatus for producing pulp from lignocellulosic material by an alkaline cooking process, in which process preheated chips are treated in an impregnation stage with impregnation liquor, which is taken from the impregnation stage, and concentrated to obtain both a high inorganic and organic dry solid content. The temperature is higher than in the heating of the chips and in the removal of air, and sufficient to enable the finishing of the reactions in the impregnation stage before the cooking stage. The majority of the alkali is added to the impregnation stage. In the cooking stage alkali is added to enable the delignification to be performed to a desired degree, and at the end of the cook black liquor is added to regulate the liquid-to-wood ratio. The process can be applied both to a continuous cook and to a batch cook.

Description

Improved alkaline process and system for producing pulp.

Field of the invention The invention relates to an alkaline process for preparing pulp according to the preamble of claim 1 as well as a system for carrying out the process.

Background of the invention

The objective of chemical pulping is to remove lignin so that the fibres can be separated with minor mechanical work. Among the chemical pulping methods, alkaline cooking processes and especially kraft or sulfate cooking are dominant in the production of chemical cellulose pulp because they provide pulp fibers which are stronger than those obtained from other commercial pulping process. The lignocellulosic material, typically cut into wood chips, is treated in either batch or continuous digesters with cooking liquor.

The active components of an aqueous solution of sulfate cooking liquor are hydroxide (OH) and hydrogen sulfide ions (HS). The delignification takes place mainly by action of OH ions, but also the HS content in decisively significant in two ways: the hydrogen sulfide ions protect the carbohydrates of wood material whereby carbohydrate yield is improved, and on the other hand, they accelerate delignification reactions. Like the reactions of OH ions, also the rate of these reactions is increased with increased temperature.

In practice, it is not possible to cook high-quality cellulose using merely hydroxide, but catalytically acting anthraquinone (AQ) is additionally to be used. AQ may also be used in an ordinary kraft cooking, generally resulting in better yield and/or faster cooking. The yield can also be improved by a polysulfide treatment which has not shown to be easy to carry out in practice in industrial processes. The use of anthraquinone is accociated with problems, as well: The use of AQ involves additional costs, and on the other hand, it cannot be used in all cellulose applications because of its toxicity. Pores inside fresh wood chips are partly filled with liquid and partly with a gas mixture consisting mainly of air. The ratio is, among others, determined by the density, moisture and dry content of wood. The air should be removed from the chips before they can be perfectly impregnated with cooking liquor. This is usually done by treating the chips with steam, and with respect of the present invention, for example, by using a process and a apparatus according to Finnish patent application FI 20021208.

There are two major stages where the transport of chemicals into the chips can occur: (1) the penetration in the impregation stage where the chips are moistened with chemical- containing liquid before delignification reactions begin, and (2) in the continuous movement of chemicals to the reaction sites during the cooking stage. The penetration of the steamed chips with the impregnation liquor takes place fast. After that, reactive ions must diffuse into the chips. The rate of diffusion is most significantly influenced by differences in the concentration of chemicals in the liquid outside the chips and in the penetrated liquid inside the chips. In addition, the so-called Donnan effect is to be taken into account which means that there must be a certain external ion concentration (generally given as Na concentration) in order to make it generally possible for the ions to be transported into chips.

In cooking there is a critical balance between the rate of ion transportation, wood porosity, chip dimensions and the rate of chemical reaction. For instance raising the temperature increases the rate of transportation, but also the rate of reaction increases. Chip dimensions are of major importance in this context. The longer, wider and especially the thicker the chips are, the longer is the transportation distance to the central parts of the chips. If the transportation distance is too long and the rate of transportation too slow, the chemicals may be completely consumed before the cooking liquor can reach the chip centers, resulting in non-uniform cooking.

The higher the cooking temperature, the shorter time is required time to reach a certain delignification degree. A commonly used measure for the time and temperature required for cooking reactions is the so-called Vroom H-factor expressing time integrale of the reaction rate as a function of time. In order to reach a certain delignification degree, a certain chip raw material can be understood to require, under constant conditions, an approximately constant H factor value. Thus, e.g. a higher cooking temperature provides a required H factor value in a shorter time.

A low cooking temperature is an objective strived for, besides for the reasons of energy economy, also because of the yield and uniformity of the cook, but on the other hand, it means slower cooking reactions and thus a longer reaction time to obtain a certain H factor and a certain delignification degree. It leads naturally to larger digester volume, which, besides increasing investment costs, may at higher production levels cause problems in runnability of the process and thus in pulp quality.

The delignification reactions are generally divided to take place in three different steps: extraction delignification (1), bulk delignification (2) and residual delignification (3). In practice, the majority of the delignification reactions take place in step (2), and the reactions of step 3 are attempted to be avoided, because in that step cooking selectivity is essentially poorer than in step 2. The delignification occuring during the step 1 is not selective, either, due to the fact that in this step many various reactions with chemical compounds of wood material take place. Extraction delignification can be said to be based on the extraction of lignin bound to various carbohydrates, such as hemicellulose, from wood material. This, however, requires reactions with carbohydrates and thus a decrease in carbohydrate yield.

In existing continuous processes, typically referred to as the Kamyr type, chips are heated with steam and air is removed from the chips to facilitate later liquor impregnation. In continuous cooking, the chip impregnation stage typically takes place, in various modifications in a retention time of 10 to 60 minutes at a temperature of 100 to 145 ⁰C. Since penetration rates increase with increased pressure, impregnation stages typically operate at operating pressures of about 5 to 10 bars at the aforesaid temperatures. Subsequent to impregnation, the chips are heated directly in a vapour phase and/or in several liquor heating circuits to a cooking temperature, and then typically cooked for at least 90 to 150 minutes in a concurrent and/or in a countercurrent cooking zone at temperatures below 165 ⁰C, in some overstressed digesters the maximum temperature may be even higher than that. Practical experience shows that the process becomes limited by chemical diffusion at cooking times of below about 90 to 150 minutes and temperature above 165 °C.

Typical cooking temperatures for softwood material are 145 - 165 °C, whereas for hardwood lower temperatures of 135 - 150 °C can be used, cf. e.g. international patent application WO 98/35091. Thus, a cooking time of about 90 - 150 minutes is required. In addition, subsequent to the concurrent cooking zone, there is usually a concurrent or countercurrent zone of 60 to 240 minutes at temperatures of 130 - 160 °C. Contemporary continuous cooking processes as e.g. ITC, EMCC and Lo-solids cooking typically retain the cooking temperature through almost all of the aforesaid cooking zones, i.e. utilizing nearly the whole volume of the pressure vessel for cooking. These modern digesters have thus a total cooking zone of about 180 - 360 minutes. For the countercurrent zone and/or for cooling the blow pulp, washing filtrate is pumped into the bottom of the vessel. A blow temperature is typically 85 - 95 °C.

Several methods have been described to increase the HS ion concentration especially hi the impregnation stage. Both in batch and continuous processes, these methods are based on the utilization of the black liquor from the cooking stage in the impregnation, e.g. US patents 5,053, 108 and 5,236,553. In practice, this means, however, an increase in the relative HS ion content compared to the OH ion content since significantly more OH ions than HS ions are consumed by the delignification reactions. However, a high HS-to-OH ratio does not necessarily mean that the absolute HS content would be very high compared e.g. to white liquor. Thus, the content difference being a driving force for diffusion, the efficiency of the impregnation is not necessarily efficient as to the HS ions.

The concentrations of the cooking liquor (dissolved dry matter, OH and HS ions) in a certain cooking stage are determined by the required amount of alkali, i.e. alkali consumption, the moisture of the chips and the dilution introduced with chemicals. The HS^' ion content in white liquor, being essential for the rate of the delignification reactions, is in fact high, but it cannot be added to exceed the amount of consumption in the cooking stage, resulting in spent liquor still containing a large amount of alkali, thus impacting the recovery of chemicals. In addition, a too high OH concentration affects negatively the yield of the cooking. On the other hand, the cooking liquor is considerably diluted by chip water. For this reason, it would be advantageous that the chips are as dry as possibly when entering the process. This is, however, not reasonable with respect to the quality nor the operability of the process, and it is not possible, either, to dry chips energy efficiently under control.

Patent application WO 03/062524 describes a method in which black liquor taken from cooking is evaporated before it is returned to the beginning of the cooking stage. From the end of the cooking stage the black liquor is intended to be fed to impregnation. The aim of this is to obtain a higher dry content which should improve cooking yield. The arrangement of this kind would also increase the absolute concentration of HS ions in the cooking stage. However, in several studies, it has been found that HS ions should be present already at the beginning of the whole extraction delignification and the bulk delignification to fully utilize the yield advantage and the increased rate of the cooking reactions obtained by them.

Swedish patent SE 521678 describes a method to increase sulfide concentration in impregnation in which method black liquour is taken from two different stages of the cooking and from which the latter can be, introduced to the impregnation through a expansion tank. The independent claim states that the cooking temperature is in the range of 150 - 180 °C, and in connection with sulfide treatment the temperature should be at least 10 °C lower than the temperature of the cooking stage. The method enables to increase the HS concentration to some extent in the impregnation and in the initial stage of the cooking, but the black liquor from the cooking stage cannot have a very high HS concentration without adding a relative great amount of alkali to the cooking stage since the majority of the consumption takes place in the impregnation stage. This would, in turn, lead to essentially decreased yield.

Disclosure of the invention

An objective of the present invention is to provide a process and a system which enable the use a temperature as high as possible in the impregnation of an alkaline cooking process so that carbohydrate yield after both the impregnation and the cooking stage can be maximized.

A process according to the invention is characterized by what is disclosed in the characterizing part of claim 1. A system according to the invention is characterized by features disclosed in the characterizing part of claim 44.

The process according to the invention provides impregnation conditions enabling high concentrations of dissolved organic and inorganic dry matter of the impregnation liquor compared to those in the prior art. In the cooking process according to the invention, the extraction delignification is carried out under optimal conditions as far as possible so that the extraction delignification would occur as little as possible or not at all in connection of the bulk delignification.

The system according to the invention enables to carry out the alkaline cooking according to the invention so that the liquid circulations of the impregnation stage and the cooking stages are separated from each other, and the impregnation liquor circulating in the impregnation stage can be concentrated.

When proceeding according to the invention, carbohydrate yield after all delignification stages is maximized. In addition, due to the extraction delignification to be carried out in the impregnation stage and due to the efficient sulfide and/or AQ treatment, the bulk delignification to be carried out in the actual cooking stage can be carried out rapidly. Advantages achieved by the invention are i.a.: - possibility to use a lower cooking temperature whereby the energy economy of the cooking process is improved and production costs are reduced

- possibility to increase the production capacity of a process apparatus of a certain size

- increased cooking yield results in lower consumption of raw material and cooking chemicals - a lower cooking temperature enables the use of a low enthalpy steam enabling higher production of electricity by means of a counterpressure turbine

- evaporation of the impregnation liquor reduces correspondingly the evaporation required in an evaporation plant enabling technically and economically improved evapo¬ ration arrangements.

The process according to the invention can be carried out in a continuous cooking process in a one- or two-digester cooking process, in a steam/liquid phase digester or in a hydraulic digester. Impregnation is performed either in an upper part of the digester or before the cooking stage or in a separate impregnation vessel. The main principles of the process according to the invention are applicable also to a batch cook, especially a displacement batch cook. The raw material can in principle be any lignocellulosic material. Different raw materials such as softwood, hardwood, bagasse and the like can be utilized in this process. Likewise, various end products such as bleached or unbleached pulp and a high- yield kraft pulp which optionally requires mechanical deliberation, can be produced by this process.

In an embodiment according to the invention, the black liquor from the cooking stage is not introduced to the impregnation in order to increase the level of the HS and of dry matter level which is a significant difference compared to prior art embodiments which utilize the HS/OH ion ratio of the black liquor generated in the cooking stage. Liquid is transported to the cooking from the impregnation stage only inside the chips and in an amount carried during the transport of the chips.. When the liquid-to-wood ratios of the impregnation stage and the cooking stage are separated from each other, the possibility to regulate the process in different stages is improved. In addition, the liquids differ from each other as to their compositions and they contain reaction products from corresponding stages and cooking chemicals required for a stage. By the conditions in the impregnation, such conditions of the extraction delignification are maintained inside the chips which enable the reactions of the lignocellulosic material and the HS ions and/or the AQ reactions to occur as efficiently as possibly, i.e. a high HS ion concentration and/or AQ concentration in free liquid and a temperature as high as possible.

Detailed description

In this description by impregnation liquor (IL) is meant an alkaline process liquor used in the impregnation stage. By cooking liquor (CL) is meant a liquor to be used in the bulk delignification, which liquor is removed from the digester or blown off together with pulp after the end of the cooking.

The terms white liquor (WL), black liquor (BL) and green liquor have their common meanings used in sulfate processes.

By fresh alkali is meant an alkaline solution from the preparing system of chemicals, which in sulfate processes is white liquor or white liquor and green liquor. In soda processes the fresh alkali is mainly NaOH. In soda processes the fresh alkali corresponds to white liquor but it does not contain substantially sulfide. Fresh alkali does not substantially contain dissolved organic dry matter.

Essential in the concept according to the invention is that free water is remover from the impregnation in stem form, e.g. by evaporation or by expansion, whereby high concentration differences required for diffusion maintain high. In one special embodiment the removal of water is effected partly or wholly in liquid form, e.g. by a membrane film. The function of cations of the inorganic dry solid matter present in the impregnation liquor is to counteract the aforesaid Donnan effect, and the function of the organic matter is to buffer the impregnation liquor against yield losses of the chip material. By retaining in the impregnation carbohydrates dissolved in the hydrolyse occuring at the beginning of the ^• impregnation, improved yield is obtained and additionally the carbohydrates have a catalyzing effect on the cooking. The level of the: inorganic and organic dry solid matter dissolved in the impregnation liquor is between 15 - 50%, preferably 20 - 35%.

This enables the use of a higher impregnation temperature without negative effects on yield, on the conformity of the cooking or on the paper technical properties of the pulp.

Due to the high temperature, hydrogen sulfide reacts with lignin and is attached chemically to wood matrix. The sorption of hydrogen sulfide and the reactions with lignin are the primary objectives of the high impregnation temperature. This leads to a higher rate of the bulk delignification and to an increase in carbohydrate yield. If the alkali added to the process does not contain sulfide, or it is desired to make the sulfate process more effective, anthraquinone (AQ) or a corresponding additive is added. The aforementioned, with respect to hydrogen sulfide advantageous conditions can preferably be utilized in the use of AQ.

The impregnation takes place by means of an alkaline impregnation liquor circulating in the impregnation stage to which fresh alkali is fed as an alkali addition and wherein the impregnation liquor is concentrated by removing water, e.g. by evaporating or by flashing, from it. The process according to the invention is especially preferably sulfate cooking and the fresh alkali consists preferably essentially or entirely of white liquor. In an embodiment alkali addition consists merely of fresh alkali or essentially merely of fresh alkali. In a preferred embodiment the alkali addition consists of fresh alkali and black liquor which is fed up to 1.0 ffiVodt. The cooking process may also be soda cooking, whereby the fresh alkali consists of sodium hydroxide as dry or as a solution. In an embodiment, also anthraquinone is added to the impregnation stage and/or the cooking stage. In an embodiment the fresh alkali addition fed to the impregnation liquor may consist of green liquor or green liquor and another fresh alkali, particularly white liquor.

In the impregnation, the temperature is maintained as high as possible also in order to allow necessary degradations of the carbohydrates rapidly consume the hydroxide of the white liquor which will increase the HS-to-OH ratio. In addition, the removal of water increases the hydrogen sulfide concentration to a high level. The upper limit of the temperature is determined by the uniformity of the impregnation, the consumption of alkali and balances of the alkali as well as the starting of the bulk delignification. Thus, the temperature of the extraction delignification in the impregnation stage is 135 ± 55 ⁰C, preferably 135 ± 20 °C, strongly depending on the raw material. In one embodiment the temperature is 135 ± 35 °C, preferably 135 ± 10 °C It is also essential to carry out the extraction delignification stage as close to an end as possible so that this stage would not be carried out simultaneously with the bulk delignification or that extraction delignification would further occur as little as possible during bulk delignification. The target yield by the impregnation is 80 ± 15 %, preferably 80 ± 10%, depending on the raw material to be used. In this the total yield after all delignification steps is maximized. The impregnation may comprise several steps and consists preferably of one or two steps. In the first step, a rise of the ratio of HS ions to OH ions is achieved towards the end of the step since the hydroxide is rapidly consumed at the beginning of the impregnation. In this way by a certain alkali amount it is possible to provide more reactive HS ions inside the chips. The objective of the second step is to rapidly increase the alkali concentration before the bulk delignification which promotes uniform impregnation of the effective alkali to a chip, resulting in uniform cooking. Essential in the impregnation is a high dynamic liquid- to-wood ratio, i.e. between the chips and the liquid phase there is a two to sevenfold, preferably a three to fivefold flow rate difference. In a preferred embodiment of the process according to the invention the extraction delignification is carried out as completely as possibly during the impregnation, particularly in the first step thereof. In a continuous process, the first step of the impregnation is carried out in a concurrent flow, and the second step of the impregnation may be concurrent or countercurrent. In a preferred embodiment the impregnation stage is merely concurrent. Yet in another embodiment the impregnation stage is merely countercurrent. At the beginning of the impregnation steps the amount of the effective alkali does not exceed 1.5 mol/1 NaOH, is preferably 0.5 - 1.5 mol/1 NaOH, particularly 0.8 - 1.2 mol/1 NaOH, preferably neither during the impregnation steps. In an embodiment, a rapid rise of the alkali concentration in the second step of the impregnation may be carried out at the beginning of the cooking stage before the bulk delignification begins, when the temperature of the cooking stage is low, preferably 140 ± 10 -⁰C. The amount of the alkali required for the bulk delignification is small compared to the total consumption, particularly when using hardwood. In a preferred embodiment, due to the effective impregnation and hydrogen sulfide treatment, after the temperature rise in the cooking stage the process proceeds directly to bulk delignification which bulk delignification is rapid. In a preferred embodiment the impregnation comprises a first step and a second step, whereby in the second step the OH concentration and the HS concentration are essentially higher than the corresponding concentrations at the end of the first step. Yet in another embodiment the HS concentration in the second step of the impregnation is essentially higher than at the end of the first step. Yet in another embodiment the OH concentration in the second step is essentially higher than at the end of the first step. The alkaline cooking process according to the present invention for preparing pulp from lignocellulosic raw material comprises the impregnation of the raw material and the cooking of the impregnated raw material with cooking liquor. In an embodiment the raw material is steamed before its impregnation. In an embodiment of the process the concentration of the impregnation liquour is effected by heat energy from the cooking stage. In an embodiment the concentration of the impregnation liquor is effected partly by means of heat energy brought from the cooking stage, whereby additionally other energy is used. In an embodiment other energy than secondary energy from the cooking stage is used, e.g. low pressure steam or electric energy, e.g. by means of a compressor. In an embodiment water is evaporated from the impregnation liquor by heating the impregnation liquor first indirectly by steam and by evaporating, and after that, the impregnation liquor is heated indirectly by means of the black liquor from the cooking stage to an impregnation temperature.

In an embodiment of the process according to the invention no energy is introduced to the process from the outside after the impregnation stage to elevate the temperature.

In an embodiment of the process according to the invention the majority of the alkali to be added to the raw material is added preferably to the steps of the impregnation.

The cooking stage of the -process according to the invention may preferably comprise both a concurrent and a countercurrent zone, whose alkali concentrations and temperatures can be regulated.

The cooking reactions of the process according to the invention can be terminated in the digester e.g. by displacing the cooking liquor or by diluting using filtrate from a wash plant, whereby the blow is carried out at a temperature below 100 °C. In a further embodiment the pulp is blown out essentially at a temperature higher than 100 ⁰C, optionally into a blow tank provided with heat recovery. In a further embodiment the pulp is blown out through a mechanical defibering stage into a blow tank. According to laboratory test results, the arrangement according to the invention provides improved pulp yield due to the effective HS treatment. Likewise, the rate of the delignification increases significantly: according to the test results the H factor demand is even halved. Bleachability of the pulp is good due to the effective HS treatment.

Example 1

Pulp was cooked under prior art conditions from eucalyptus chips in a laboratory, whereby the temperature in the first step of the impregnation was 110 ⁰C (duration 45 min) and in the second step 12O⁰C (15 min). The cooking temperature was kept at 152 °C for 120 minutes to obtain cellulose pulp having kappa number 17, which expresses the degree of delignification of the cellulose pulp. The liquors used were typical as to their HS and dry solid contents. The H factor expressing the ratio of the temperature to the cooking time was about 460. Pulp yield from the chips was 53.7% on the wood amount used.

Example 2

Using the same material, by circulating and by concentrating the impregnation liquor during several cookings, such conditions were created which correspond to those according to the invention: the durations of the impregnation as in Example 1, the impregnation temperature of 130 ⁰C and the dissolved dry solid content and the HS concentration were approximately double compared to those, of the cooking of example 1. Nearly the same degree of delignification (kappa number 18) wasiobtained at a temperature of 149 ⁰C in a time of 100 minutes which means a H factor of 230 which is half of that of example 1. Cellulose pulp yield was 54.5% on the wood amount used.

Brief description of the drawing

Figure 1 shows a preferred embodiment of the invention schematically. Figure 2 shows a preferred embodiment of the invention with a separate impregnation vessel.

Figure 3 shows a preferred embodiment of the invention where impregnation and cooking are performed in the same vessel.

Detailed description of the process arrangement The arrangement according to the invention can be carried out e.g. according to fig. 1, wherein in chip treatment (1) the chips are treated with steam, which may be fresh or generated by flashing the liquor, to heat the chips and to remove air (gas mixture) from the chips. After that, a chips-liquor mixture with impregnation liquor is formed which mixture is introduced to impregnation (2). The impregnation (2) comprises one, two or more steps having different chemical and/or temperature profiles. The first step is carried out preferably in a concurrent flow, and the second step, depending on the case, in a concurrent or countercurrent flow. The duration of the first step of the impregnation (2) is 15 - 120 minutes, preferably 15 - 45 minutes, and that of the second step of the impregnation is 5 — 60 minutes, preferably 10 - 45 minutes.

Liquor (BL) can be removed at the beginning of the impregnation steps (2), between the impregnation steps or before the cooking stage (3) e.g. by means of a screen zone or a screen/screw arrangement. To this removed liquor (EL) fresh alkali (white liquor) can be added, it can be heated directly or indirectly (8) and be introduced to expansion and/or evaporation (9). After the expansion/evaporation the cooled liquor (IL) can be introduced into a level tank (10) for stabilizing flow variations. Separation of extractives and soap can also be carried out in the tank. Too much alkali (white liquor) should not be added to the liquors (BL) of the impregnation stage to avoid additional yield losses. The hydroxide peak caused by the white liquor addition is diluted by a high dynamic liquid-to-wood ratio, i.e. by circulating free liquid. -This dynamic liquid-to-wood ratio may be 2 - 10 m³/Odt wood, preferably 3 - 6 m³/Odt. In an embodiment of the process according to the invention the black liquor (e.g. from the cooking stage) may be added to the impregnation liquor, e.g. reject from fiber separation, 0 - 1.0 m³/Odt wood, preferably below 0.5 m³/Odt.

The removal of water (here evaporation) contained in the liquors (IL) of the impregnation stage (2) may be carried out e.g. in an expansion tank and/or in an evaporation unit (9), in one or more steps. Subsequent to that, the concentrated liquor flow may be heated to a desired temperature in one or more heat exchangers (11) wherein the heating flow may be e.g. steam, black liquor from the expansion screens of the digester or another secondary energy generated in the process or a combination thereof. This liquor (EL) to be circulated is returned to the impregnation stage at one or more points. From the impregnation stage (2) some or no free impregnation liquor is transferred to the cooking stage (3) of the cooking process, to the bulk delignification stage of the cooking. To the cooking stage (3), cooking liquor CL may be introduced which may consist of black liquor circulated from the end of the cooking and fresh alkali, here white liquor, optionally added thereto. The amount of the circulated black liquor depends on a desired dynamic liquid-to-wood ratio in the cooking stage (3). The dynamic liquid-to-wood ratio in the cooking stage (3) may be e.g. 2 - 6 mVODt (oven dry), preferably 3 - 4 m³/0Dt. If the liquid in the liquid to wood ratio is considered to be free water only, the corresponding values are 1-5 m³/0Dt, preferably 2-3 mVODt. In the cooking stage (3), the temperature is maintained at 150 ±20 ⁰C for a sufficient time to obtain the required H factor. The black liquor remaining at the end of the cooking stage (3) is first cooled by one or more heat exchangers (11), wherein the impregnation liquor is intended to be heated, and after that by one or more heat exchangers using cold water, if required. The cooled black liquor is introduced to recovery (5-7) of cooking chemicals. After the cooking stage (3) the pulp is cooled and transferred to washing (4).

The recovery (5 - 7) of cooking chemicals consists of evaporation units wherein black liquor (BL) is evaporated to raise its dry solid content to a level of 65 - 85 %. In practice, in the embodiment according -to the invention, a part of the evaporation work is carried out preferably in a water removal unit (9) in connection with the impregnation (2), whereby a number of the evaporation units (5) needed for the concentration of the black liquor can be correspondingly smaller. Since the dry solid level of the feed liquor (BL) from the cooking stage (3) is higher than that in a cooking plant, where the evaporation is not carried out in connection with the impregnation (2), so-called feed liquor strengthening is not necessarily required in the evaporation plant (5), resulting in an increase of the capacity of the evaporation plant (5) and in improved energy economy. In spite of that, the water removal unit (9), e.g. an evaporation unit, can be placed at the recovery (5 - 7), e.g. in a process modification to the embodiment according to the invention. In this case, however, the flow to be evaporated (IL) is still separated from other evaporation units (5) which evaporate black liquor (BL). As a last step, a superconcentrator (6) may be used before the black liquor is burnt in a soda recovery boiler (7). In this connection, a melt is formed which is dissolved to green liquor to obtain white liquor by causticizing (not shown in the figure).

The process according to the invention can be carried out in a continuous process or batch cooking process, especially in a displacement batch cook. A preferred embodiment of the invention in a batch cook comprises a step before the impregnation stage in which chips are treated with steam to heat the chips and to remove gases. In the batch cook a separate impregnation vessel may be used from which a chips-liquor mixture is filled to digesters. The process may, in principle, be performed in present apparatuses, by changing i.a. liquid circulations so that the liquid circulations of the impregnation stage and the cooking stage are essentially separated from each other. The impregnation (2) may be carried out e.g. in a separate impregnation vessel or in the same vessel as the cooking stage (3), however prior to the bulk delignification.

In the system according to figure 1 for carrying out the process according to the invention separated liquid circulations are provided for the impregnation liquor (IL) and the cooking liquor (CL) and they are essentially separated from each other.

In figure 2 is shown one preferred embodiment of the invention with a separate impregnation vessel. By letter P before a reference number in meant a pump. The chipped fiber material is fed through a chip bin to a chip treatment vessel (20), where air is removed from the chips. The said vessel (20) can be e.g. according to a heating, steaming and storage apparatus shown in the patent application FI 20040637. After this the fiber material is slurried by means of a liquid flow (50) which in this case is impregnation liquor and the slurried fiber material (51) is fed to a chip feeder (21). In the embodiment of figure 2, the chip feeder is a feeder with which material in slurry form can by transported from a first pressure system to a system having a second, higher pressure.

In the embodiment of figure 2 the impregnation is with two steps: before screens (Sl, S2) the impregnation is concurrent and after the screens countercurrent. The slurried fiber material is fed by means of the chip feeder (21) along line 52 to a continuous impregnation vessel (22). Impregnation liquor used for the transportation of the chips is separated from the chips in the upper part of the impregnation vessel by means of equipment known in the art. Transportation liquid (53) is returned by means of the chip feeder apparatus (21) to the evaporation circulation where impregnation liquor is concentrated. The impregnation liquor (54) leaving the chip feeder in pumped (P3) to a pressure screen (23) in order to remove fiber material, whereby the reject flow (55) obtained is guided back to the slurrying of chips. The pressure screen (23) can alternatively be replaced with a tube screen known in the art. Fresh alkali (here white Ii quor WL) is fed to the accept flow (56) from line 57. The impregnation liquor is heated in a heat exchanger (24) whereto low pressure steam (57) is fed.

The heated liquor flow is directed to a water removal unit 25 along line 84. In the water removal unit 25 the impregnation liquor is concentrated by removing water from it. Part of the steam (58) exiting the water removal unit 25 is directed along lines 59, 60 and 61 to be used in chip heating and pre-steaming, and part of it is removed wholly from the impregnation circulation and cooking circulation and is directed to the condenser.

Depending on the amount of chip water and on the concentration of the fresh alkali (57) added, the impregnation liquor may be concentrated further at the second water removal unit 26 and the steam generated is directed to condenser. Water removal units 25 and 26 can be e.g. expansion tanks and/or evaporation units. The impregnation liquor concentrated at water removal unit 25 is directed along line 85 to a water removal unit 26.

The concentrated impregnation liquor (62) is fed to a level tank (27). In the embodiment according to figure 2, soap is removed from the impregnation liquor in the level tank, after which impregnation liquor (63) is pumped (P5) to an impregnation vessel and if necessary, along line 50 for slurrying of the chips. The surface level of the level tank is regulated by the magnitude of flow 63. The impregnation liquor (63) fed to the impregnation vessel is heated in a heat exchanger (28) utilizing the heat of the black liquor (64) coming from the digester.

The heated liquor flow (65) is pumped (P4) through a heat exchanger (29) to the bottom of the impregnation vessel (22). Before the heat exchanger (29) and the pump (P4) the impregnation liquor is combined with a liquor flow from the upper part of the digester, which liquor has been used to transport the impregnated fiber material from the impregnation vessel to the digester (30). In the said heat exchanger (29) low pressure steam (66) directed thereto is used.

The concentration of the chip-liquor-mixture, here moving from the impregnation vessel to the digester, is regulated with pump P4. Impregnation liquor is removed from the impregnation vessel though screen S2 to line 68 and through a central tube screen along line 69 which joins line 68. Impregnation liquor that has passed through a second impregnation step, is pumped (PT) along line 68 to the impregnation liquor-chip-flow 52 going to the impregnation vessel, whereby this impregnation liquor is returned to the water removal of the impregnation liquor. From the first step of the impregnation, the impregnation liquor which is diluted by the water in chips, is removed through screen Sl and through the central tube screen and through line TO to line Tl wherefrom it is pumped to return circulation 53 and thereby returned to the water removal of the impregnation liquor. Part of the flow Tl is directed directly to a heat exchanger 24 preceding the water removal unit 25 by means of line T2. After the heat exchanger (28) of line 65, starting from the level tank (27) the impregnation liquor is directed through line 65' to the chip- impregnation liquor-flow (52) going to the first step of the impregnation vessel. Steam generated at the first water removal unit 25 can be directed to chip steaming in the chip treatment vessel (20) and/or to the chip bin preceding it (not in the figure). The impregnation liquor which is used in impregnation and is circulating in the impregnation, is concentrated when part of the steam generated in the water removal unit 25 and the steam (14) generated in the water removal unit 26 in its entirety, is removed from the whole system to the condenser, i.e. part of the steam is not returned to the chips. Steam and air (83) from the chip treatment vessel (20) is removed to the condenser.

The impregnated fiber material is removed from the bottom of the impregnation vessel (22) and is transferred to a continuous digester (30) along line T3. The embodiment according to figure 2 is a steam-liquid phase-digester. Impregnation liquor used for transportation of chips is removed from the top of the digester by means of equipment known in the prior art and the said impregnation liquor is returned along line T4 by means of pump P4, through a heat exchanger 29 to the bottom of the impregnation vessel (22). The digester (30) is heated with low pressure steam or with middle pressure steam (81) which preferably originate from the power plant of the mill. The dynamic liquid to wood- ratio in the upper part of the digester is regulated by pumping with pump PI l black liquor sucked from the central tube screen along line 75 to the upper part of the digester. Fresh alkali (here white liquor WL) is added to line 75 in order to regulate alkali . Black liquor removed by central tube screens is added through line 16 to the black liquor flow leaving the digester through expansion screens S3 and S4.

Black liquor removed from the digester (30) through screens (S3, S4) and through central tube screens is used to heat at the heat exchanger (28) the impregnation liquor from the level tank to the impregnation vessel, after which the black liquor is directed to a pressure screen 31. The accept stream of the pressure screen is directed to the chemical recovery and the reject (77) to washing filtrate (78) or to the stream (50) for slurrying the chips. From the screen S5 located in the lower part of the digester (30) black liquor is removed through stream 79 and is circulated to screen S5. White liquor (80) is added to this stream (79). Washing filtrate (78) is fed to the bottom of the digester (30) with pump P13. In case when the step below screen S4 is countercurrent, only a small amount of black liquor is removed to line 64' through the screen S5. When the step is concurrent, the black liquor is remover along line 64'. Stream 79 can be heated by means of heat exchanger 31, whereto low pressure steam (82) is fed.

In the system according to figure 2 for carrying out the process according to the invention separated liquid circulations are provided for the impregnation liquor (TL) and the cooking liquor (CL) and they are essentially separated from each other.

Figure 3 shows a preferred embodiment of the invention where impregnation and cooking are performed in the same vessel. Chips are fed to the chip bin (100) wherefrom the heated chips are fed by means of a low pressure feeder (101) to a pre-steaming vessel (102) to remove air from the chips. Chips are fed from the pre-steaming vessel (102) through a chip ^' chute (103) to a chip feeder (104) by means of which the chips are fed to a digester (105) with a higher pressure. The chip feeder (104) can be e.g. a high pressure feeder according to prior art. Impregnation and cooking of the chips is performed in the digester (105). In the figure letter P before a reference number refers to a pump. From the pre-steaming vessel (102) steam and air (108) can be removed and directed to a condenser.

The slurried fiber material is fed by means of the chip feeding apparatus (104) along line 150 to the digester (105). Impregnation liquor used for the transportation of the chips is separated from the chips in the upper part of the digester by means of equipment known in the art. Transportation liquid (151) is returned by means of the chip feeder apparatus (104) to water removal where impregnation liquor is concentrated. The impregnation liquor (152) exiting from the chip feeder (104) is pumped (P24) to a pressure screen (99) in order to remove fiber material, whereby the reject flow (182) obtained is guided back to the chip chute (103). The pressure screen (99) can alternatively be replaced e.g. with a tube screen known in the prior art. The accept flow (153) is heated in a heat exchanger (106) whereto fresh steam (154) is fed.

The heated impregnation liquor (155) is directed to the first water removal unit 107, where the impregnation liquor is concentrated by removing water from it. Part of the steam leaving from the water removal unit 107 is directed along line 156 to be used in chip pre- steaming (102) and the rest (157) is directed to a heat exchanger (109) to heat the impregnation liquor. The impregnation liquor can be concentrated further at the second water removal unit (108) wherefrom the leaving steam (158, 159) is directed to a condenser and to chip bin (100) for heating and steaming the chips. The water removal units (107, 108) can be e.g. expansion tanks and/or evaporation units.

The concentrated impregnation liquor (160) is directed to a level tank (110). From the line (160) impregnation liquor is returned to the water removal through line 161. In the line 161 fresh alkali (here white liquor WL) is added from line 162 to the impregnation liquor and the mixture is pumped with pump P25 to the heat exchanger 109 wherefrom it is directed^' to line 153.

In the level tank (110) soap is removed from the impregnation liquor which soap is removed along line 163 by means of pump P22. After soap separation the impregnation liquor (164) is pumped with a pump (P27) to impregnation. Impregnation liquor 164 which is fed to the impregnation steps is heated in a heat exchanger (111) utilizing the heat of the black liquor (165) coming from the digester.

In the embodiment according to figure 3, the impregnation is in two steps: before the first screen (S21) impregnation us concurrent and between the first (S21) and the second screen (S22) concurrent or countercurrent. Impregnation liquor is removed from the first step of the impregnation through screen S21 to water removal (stream 178). The concentrated impregnation liquor is returned to the first step of impregnation to the upper part of the digester (164) and to the second step of the impregnation along line 166 either to heating circulation 167 and/or 168. In case the second step of the impregnation is concurrent, line

168 and pump P29 are used. Lines 167 and 168 include heat exchangers (not shown in the figure) working with fresh steam in order to heat the impregnation liquor.

The cooking step subsequent to the second screen S22 is concurrent. Black liquor from the end of the cooking can be circulated from screen S23 by means of a pump P214 along line

169 to the beginning of the cooking, below the screen S22. The step between the screens S23 and S24 can be concurrent or countercurrent. In the countercurrent step liquor is circulated along line 170 which is equipped with a heat exchanger, to which line fresh alkali (here white liquor) is added (line 174) before pump (P210). Black liquor is removed from the digester along line 165. In case when the step between the screens S23 and S24 is concurrent, cooking liquor is removed along line_170'. The heat of the black liquor removed from the digester is utilized for heating of the white liquor in heat exchanger 111, after which the black liquor is directed to, via cooling and fiber separation (112, 113) to the chemical recovery along line 172. In the cooling the pressure of the black liquor is decreases in an expansion tank 114 and simultaneously odorous gases of the black liquor can be removed which gases are not removed in the connection of the water removal (107, 108) because of high pH. Final cooling is effected after the expansion tank 114 in heat exchanger 115, whereto fresh steam (179) is brought. In the fiber separation of the black liquor coming from the digester, accept (172) of the first pressure screen (112) is directed to the recovery of the cooking chemicals and reject (173) to the second pressure screen (113), the reject (174) of which is directed to the washing filtrate (line 175) or to the hue 182 leading to the chip chute (103). In the system according to figure 3 for carrying out the process according to the invention, separated liquid circulations are provided for the impregnation liquor (IL) and the cooking liquor (CL) and they are essentially separated from each other,

Claims

1. An alkaline cooking process for producing pulp from lignocellulosic material, said process comprising impregnation of the raw material and cooking of the impregnated raw material, characterized in that the impregnation is performed with an alkaline impregnation liquor circulating in the impregnation stage, to which liquor fresh alkali is introduced as an alkali addition and which impregnation liquor circulating in the impregnation stage is concentrated by removing water from it.

2. The process according to claim 1, characterized in that the cooking process is a sulfate cook, and the fresh alkali consists essentially or entirely of white liquor.

3. The process according to claim 1 or 2, characterized in that the alkali addition is merely fresh alkali or essentially merely of fresh alkali.

4. The process according to claim 1 to 3, characterized in that the alkali addition consists of fresh alkali and of black liquor which is introduced at the most 1.0 m³/Odt.

5. The process according to any of claims 1 to 4, characterized in that the level of the inorganic and organic dry solid content dissolved in the impregnation liquor is between 15 to 50 %.

6. The process according to any of claims 1 to 5, characterized in that the energy required for the evaporation to be carried out to concentrate the impregnation liquor is taken partly or entirely from the cooking stage.

7. The process according to any of claims 1 to 6, characterized in that water is evaporated from the impregnation liquor by heating the impregnation liquor first indirectly with steam^' and by evaporating impregnation liquor.

8. The process according to claim 1 to 7, characterized in that after the evaporation the impregnation liquor is heated by means of the black liquor from the cooking stage.

9. The process according to any of claims 1 to 8, characterized in that after the evaporation the impregnation liquor is introduced into a level tank for stabilizing flow variations.

10. The process according to claim 9, characterized in that the separation of extractives and soap is carried out in said tank.

11. The process according to any of claims 1 to 10, characterized in that the impregnation temperature is about 135 ± about 55 °C, preferably about 135 ± about 35 ⁰C.

12. A cooking process according to any of claims 1, 3 or 5 to 11, characterized in that the cooking process is a soda cook.

13. The process according to any of claims 1 to 12, characterized in that the impregnation comprises a first step and a second step.

14. The process according to claim 13, characterized hi that in the second step the OH concentration and the HS- concentration are essentially higher than the corresponding concentrations at the end of the first step.

15. The process according to claim 13, characterized in that in the second step the HS concentration is essentially higher than at the end of the first step.

16. The process according to claim 13, characterized in that in the second step the OH concentration is essentially higher than at the end of the first step.

17. The process according to any of claims 13 to 16, characterized in that the second step of the impregnation is carried out at the beginning of the cooking stage.

18. The process according to any of claims 1 to 17, characterized in that the impregnation comprises several steps.

19. The process according to any of claims 13 to 18, characterized in that the duration of the first impregnation step is 15 - 120 minutes.

20. The process according to any of claims 13 to 19, characterized in that the duration of the second impregnation step is 5 - 60 minutes.

21. The process according to any of claims 1 to 20, characterized in that the impregnation stage is merely concurrent.

22. The process according to any of claims 1 to 20, characterized in that the first step of the impregnation stage is concurrent and the second step is countercurrent.

23. The process according to any of claims 1 to 20, characterized in that the impregnation stage is merely countercurrent.

24. The process according to any of claims 1 to 23, characterized in that in the impregnation stage the effective alkali of the impregnation liquor does not exceed 1.5 mol/1 NaOH.

25. The process according to any of claims 1 to 24, characterized in that after the impregnation stage no energy is introduced to the process from the outside to elevate the temperature.

26. The process according to any of claims 1 to 25, characterized in that the majority of the alkali to be added to the raw material is added to the steps of the impregnation.

27. The process according to any of claims 1 to 26, characterized in that the cooking stage comprises both a concurrent and a countercurrent zone, whose alkali concentrations and temperatures can be regulated.

28. The process according to any of claims 1 to 27, characterized in that the cooking temperature is 150 ± about 20 ⁰C.

29. The process according to any of claims 1 to 28, characterized in that the dynamic liquid-to-wood ratio of the cooking stage is about 2 - 6 m³/Odt wood.

30. The process according to any of claims 1 to 29, characterized in that anthraquinone is added to the impregnation stage and/or to the cooking stage.

31. The process according to any of claims 1 to 30, characterized in that the cooking reactions are terminated in the digester by displacing the cooking liquor or by diluting using filtrate from a wash plant, whereby the blow is carried out at a temperature below 100 ⁰C.

32. The process according to any of claims 1 to 30, characterized in that the pulp is blown out at a temperature essentially higher than 100 °C, optionally to a blow tank provided with heat recovery.

33. The process according to claim 31 or 32, characterized in that the pulp is blown out through a mechanically defibering stage to the blow tank.

34. The process according to any of claims 1 to 33, characterized in that the cooking process is continuous.

35. The process according to claim 34, characterized in that the continuous cooking process is with one or two vessels, either a steam/liquid phase digester or a hydraulic digester.

36. The process according to claim 35, characterized in that middle pressure or low pressure steam is introduced to the steam/liquid phase digester for heating thereof.

37. The process according to claim 35, characterized in that the impregnation takes place in a separate impregnation vessel or in upper part of the digester before the cooking stage.

38. The process according to any of claims 1 to 33, characterized in that the impregnation and cooking stages are carried out in a batch cook.

39. The process according to claim 38, characterized in that the process is a displacement batch cook.

40. The process according to claim 38 or 39, characterized in that the impregnation stages are carried out in a separate impregnation vessel outside the digester.

41. The process according to any of claims 1 to 40, characterized in that the chips are heated with steam to remove gases prior to slurrying the chips with the impregnation liquor.

42. The process according to any of claims 1, 3 to 11, 13 or 16 to 41, characterized in that the fresh alkali addition introduced to the impregnation liquor consists of green liquor or green liquor and other fresh alkali, especially of white liquor.

43. The process according to any of claims 1 to 42, characterized in that water is removed in the form of steam.

44. A system for carrying out the process according to any of the claims 1 to 43, comprising means for impregnating and cooking the raw material, means for circulating the impregnation liquor, means for introducing alkali addition, characterized in that the apparatus comprises means outside the impregnation apparatus for concentrating the impregnation liquor by removing water in the form of steam.

45. A system according to claim 44, characterized in that the apparatus includes means for removing from the cooking process the steam or part of the steam generated in the said removal of water.