Abstract:
An alkaline batch process for the production of pulp from wood chips, wherein the preheated chips are subjected to an extended impregnation step outside the digester for at least 60 min, preferably longer, at a temperature not exceeding the impregnation liquor boiling point at atmos-pheric conditions, and a rapid heating and cooking period in the digester of less than about 90 min, preferably shorter, followed by cooling to below reaction tempera-ture. Fresh alkali is added both during impregnation and the heating/cooking period.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to the field of alkaline pulping.  
         BACKGROUND OF THE INVENTION  
         [0002]    Among the chemical pulping methods, alkaline cooking processes and especially kraft cooking are dominant in the production of cellulose or chemical pulp because alkaline cooking provides pulp fibers which are stronger than those from any other commercial pulping process. The lignocellulosic material, typically chopped into wood chips, is treated in either batch or continuous digesters.  
           [0003]    In chemical pulping processes as alkaline cooking, the chemical reactions with wood components are heterogenous-phase border reactions. To ensure uniform reactions, it is vital that all fibers in the wood get their proper share of chemicals and energy. Non-uniformity occurs when these criteria are not fulfilled and this may result in a larger amount of unfiberized wood (rejects) in the final pulp, discolored pulp, lower final yield and impaired bleachability and paper properties. The objective of chemical pulping is to remove lignin so that the fibers can separate. Ideally, each fiber should receive the same amount of chemical treatment for the same time at the same reaction site. This means that chemicals and energy must be transported uniformly to reaction sites throughout each chip. There are two major stages where this can happen; (1) the impregnation phase where chips are saturated with chemical-containing liquid before delignification reactions begin and (2) the continuous movement of chemicals to the reaction sites during the cooking phase. 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 centers of the chips  
           [0004]    Pores inside fresh wood chips are also partly filled with liquid and partly with air. The ratio is, among others, determined by the moisture content or dry content of wood. The air should be removed from the chips before they can be fully impregnated by cooking liquor. This is usually done by pre-steaming the chips.  
           [0005]    As cooking proceeds, reactive ions must diffuse into the chips. If the transportation distance is too long and rate of transportation too slow, the chemicals may be completely consumed before they can reach the chip centers, resulting in non-uniform cooking. Thus, in cooking there is a critical balance between the rate of ion transportation, wood porosity, chip dimensions and the rate of chemical reaction. E.g. raising the temperature increases the rate of transportation, but the rate of reaction increases still faster. On an average, long and thick chips will not delignify as fast as short and thin chips when the same cook parameters are used. In conventional batch cooks, long and thick chips generate more screening rejects than short and thin chips and shorter cooks and higher cooking temperatures aggravate the effect. The higher the cooking temperature, the shorter is the required time to a certain delignification degree and the higher is the rate difference between delignification reaction and ion transportation. Thus it seems that cooking uniformity would require that chips should be small and thin enough and that the size variation in the chip furnish should be as narrow as possible. However, in practice many other important parameters as e.g. chipping parameters, chipping capacity, fiber cutting, fiber length, paper properties, the bulk density of the chip column, the permeability of the chip column, produced saw dust and pin chips amounts and plugging of digester screens has also to be taken into account. Saw dust is material passing through a 3 mm hole screen; pin chips is material that passes through 7 mm hole screens but is retained on a 3 mm hole screen.  
           [0006]    It has also been proposed that the rate of heating influence the result. A shorter heating period requires a smaller chip size to ensure sufficient uniformity, and it has been demonstrated that shredding of the chips will reduce the screening when using very rapid cooks (cycle-to-cycle) at high temperatures, and almost no heating period. Such conditions, i.e. fast heat-up and high temperatures prevail in certain types of continuous digesters as saw dust and pin chips digesters. In the Kamyr type of continuous digester as well as in most batch digesters, slow cooking, i.e. low rate of reaction, long heating period and lower maximum temperature, allow a more uniform cooking also of ordinary mill size chips. As both shredding and chipping to smaller size will affect fiber length, pulp quality and efficiency of operation, it is desirable to provide conditions that allow the normal industrial chip size.  
           [0007]    For processing ordinary mill size chips, it has been proposed that cooking non-uniformity can be reduced and perhaps eliminated by proper chip manufacturing and screening, proper impregnation and sufficiently low cooking temperature. Thus low rate of chemical reactions and long cooking stage in combination with impregnation and uniform chips.  
           [0008]    In a conventional alkaline batch cooking process, wood chips are fed to a digester and cooking liquor is added. The chips can be steam treated before, during or after chip filling to pre-heat the chips and remove air. The pressure is about atmospheric after liquor addition. When chips and liquor have been added, the cook is immediately started by introduction of heat either indirectly or directly by steam. Impregnation occurs during heating. The cook itself consists of a heating period and an “at pressure” period. Typical heat-up times and at pressure times are 60 to 150 minutes and 60 to 120 minutes, respectively. Typical sum of heat-up and at pressure times is about 150 minutes. At the conclusion of the cook lo when the delignification has proceeded to the desired reaction degree, a blow valve in the digester is opened and the contents of the digester are discharged into a blow tank, as the hot liquor in the digester flashing into steam and forces the cooked pulp out of the digester. The cooked material is cooled and defiberized.  
           [0009]    The difficulty with conventional batch cooking is the non-uniform and poor quality of the pulp, high energy consumption and environmental concerns. Therefore, one of the most important objectives has been the attempt to improve the efficiency of the cooking process and to improve the properties of the resulting pulp and the uniformity, especially in relation to above mentioned conventional process. Continuous flow processes and displacement batch processes have therefore been developed.  
           [0010]    Displacement batch pulping processes were developed in the 80&#39;s, originally for the sake of energy economy. Following a batch cook, the black liquor was recovered, divided into fractions according to temperature and chemical content, stored and introduced into a digester charged with fresh lignocellulosic material in order to transfer the heat of the completed cook to a subsequent cook. The total duration of the black liquor impregnation stage in batch displacement processes is typically below 30 min at temperatures below 100° C. Heating is carried out by displacement with a black liquor having a higher temperature than the impregnation liquor. Following these initial stages, white liquor is introduced, and a main cooking stage follows. Typically, the total duration of the hot liquor fill stage, temperature adjustment and the cooking stage is in the range 95-120 min. Practical experience shows that the process becomes chemical transportation rate limited at total heating and cooking times below about 95 min.  
           [0011]    Several variations of the batch displacement cooking process have been developed to optimize utilization of cooking chemicals and to handle variations in raw material, where it may be advantageous to adjust the composition and/or sequence of pretreatment liquors. In addition to energy economy, adjustment possibility, pulp quality and flexibility are advantages when displacement batch cooking processes are used. On the other hand, these processes require a significant amount of tanks and piping to handle the various liquors involved. As the trend is towards closed cirquits and lower emissions from plants, accumulation problems may also occur.  
           [0012]    In contemporary continuous processes, typically referred to as the Kamyr type, energy savings are achieved by pre-heating the chips with steam obtained from flashing the hot black liquor. In the pre-steaming of the chips, chips are preheated and air is removed from the chips to facilitate later liquor impregnation. In continuous cooking, the chip impregnation zone typically involves 30-60 min or shorter chip retention at a temperature of 115-130° C. and a high pressure to enhance the pre-impregnation of the chips and the ion transportation into the chips. Since penetration rates increase with increased pressure, impregnation stages operate at pressures that greatly exceed the liquor saturation pressure at the specified temperature, i.e. typically greater than 10 bars operating pressure for impregnation temperatures of 115-130° C. Subsequent to impregnation, the chips are heated directly in a vapor phase and/or in several liquor heating circuits to full cooking temperature, and then typically cooked for at least 1.5-2.5 hours in a concurrent cooking zone at temperatures below 165° C. Practical experience suggests that the process becomes chemical transportation rate limited at cooking times below about 1.5-2.5 hours and temperatures above 165° C. Therefore, typical cooking temperatures are between 150-165° C. but even cooking temperatures of 140-150° C. can also occur, see for example international patent application WO 98/35091. Thus, a minimum of 1.5-2.5 hours of cooking is required. In addition, subsequent to the concurrent cooking zone, a countercurrent zone, typically referred to as the hi-heat zone, usually follows for 2-4 hours at temperatures of 130-160° C. Contemporary continuous cooking as e.g. ITC, EMCC and Lo-solids cooking retains the cooking temperature, typically 150-160° C., all through the countercurrent zone, i.e. enlarging the cooking zone to the counter-current zone. These modern digesters have thus a total cooking zone of about 240-360 minutes. For the countercurrent zone, washing filtrate is pumped into the bottom of the vessel. The vessel bottom is also a blow dilution and cooling zone. Discharge temperature is typically 85-90° C.  
           [0013]    The continuous processes offer, compared to conventional batch digesters: more space efficiency, less installed power, lower volumes of inlet streams and outlet stream, steady-state operation vs. batch fill and discharge cycles, energy efficiency, lower environmental impact and a first stage of brownstock washing.  
           [0014]    However, it is found that while the typical continuous processes have the aforesaid advantages the pulp obtained has a number of properties, e.g. strength and uniformity of the pulp, which are inferior to e.g. pulp produced under well-controlled laboratory conditions. The continuous process still lacks sufficient impregnation and this has to be compensated by lower reaction temperatures and long retention times in the digester. This leads to expensive, large-size and huge digesters designed for high pressures and temperatures. The scale of contemporary digesters, typically of the type Kamyr, at higher production levels also causes mass-transfer problems in liquid circulations and displacement, which further is compensated by longer retention times and lower cooking temperatures.  
           [0015]    Impregnation theoretically requires small chips, but modern continuous digesters are based on the principle of maintaining sufficient liquid circulation and a good displacement efficiency. This calls for chip properties that are in conflict with some of the basic requirements for ensuring uniform delignification. Thus, a large chip size must be used, which leads to inferior impregnation and further longer retention times in cooking and expensive technology. Thus, the pulp maker has been trapped by his own technology. It is stated, that the 30-60 min retention time at 115-130° C. in impregnation zones of continuous mill digesters could never provide a completely uniform distribution of cooking chemical for all chips (mill chips) before the start of bulk delignification.  
           [0016]    In Swedish patent application 9602016-9, it is suggested that the way the chips are treated before continuous kraft cooking is disadvantageous for the strength of the pulp. It is proposed that the pre-impregnation at 110-130° C. in e.g. Kamyr continuous digesters is unfavourable and the chips should instead be cold impregnated. During impregnation it would only be necessary to have enough alkali present to neutralize possible by-products, in order to prevent formation of acidic regions that can damage the fiber properties. The alkali required during cooking can consequently be added after the pretreatment, and/or during cooking, i.e. a high alkali level is not required during impregnation. A pulping process is disclosed which comprises so-called “cold impregnation” as its main feature. A temperature of about 80-110° C. is specified, the time period being unlimited. However, an optimum of 2-3 hours is suggested. Pressure may be used in order to compress gas bubbles and cause sinking of the chips. The theory behind the cold impregnation stage is, that acid-generating processes within the chips shall be suppressed until the chips are filled with alkali sufficient to neutralize any acid released when reaction commences at higher temperatures, and the impregnation step is defined as resulting in “an alkali concentration sufficient to neutralize all acid produced”.  
           [0017]    The proposed process preferably uses a conventional continuous digester like MCC, EMCC or Lo-solids digester. Thus, the retention times in the cooking stage are in the order of several hours, typically around 2-5 hours. In addition, the figures of the application show a residence time in the impregnation stage approximately of the same order as in the cooking stage. However, it is found that while the proposed continuous processes have pulp strength advantages, the cooking stage still has a long retention time and low reaction temperature. This requires huge and expensive digesters designed for, from a technical point of view, high pressures and temperatures.  
           [0018]    In U.S. Pat. No. 3,215,588, and in a paper titled “Rapid alkaline cooking”, Pulp and Paper Magazine of Canada, No 7, pages T-275-T-283, a process is disclosed wherein an extended impregnation stage is utilized followed by a rapid cooking stage. Chip impregnation takes place at a pressure in excess of 10 bar, using cooking liquor. Subsequently, the chips are fed into a continuous digester having a steam zone where the chips are rapidly heated to 170-185° C. and thus cooked before entering a liquid zone where gradual cooling takes place prior to discharge. The paper teaches that the total cycle including impregnation is in the range of 30 to 45 minutes, mostly using impregnation temperatures of 130-150° C. The process results in pulp relatively low in lignin, having good bleachability according to the standards of the time, and the process is rapid due to the pressurized impregnation stage and the short heating stage. However, the screen rejects content is high and the strength reduced in comparison with pulp produced by conventional methods of the period. These disadvantages are addressed in U.S. Pat. No. 3,644,918, which introduces water saturation of the chips prior to impregnation. By using water-saturated chips, according to U.S. Pat. No. 3,644,918, it is possible to obtain complete and uniform impregnation at atmospheric pressure at e.g. 90° C. within a period of the order of 60 minutes. The whole amount of cooking liquor is added in the impregnation stage. Screen rejects are negligible, the yield is higher and the pulp shows better properties than according to U.S. Pat. No. 3,215,588. However, the authors have found that this process also lacks efficiency, since the water saturation of the chips increases the evaporation demand in the recovery cycle, the reject levels are high and the screened yield is low.  
           [0019]    Thus, development of both batch and continuous cooking technology has been characterized by improvements in various fields, e.g. energy efficiency. However, very little attention has been paid to important issues as how to really utilize both “the front-end” and “back-end” of the cooking process to simplify it, retain flexibility and also to improve the pulp quality. The failure to consider these issues has to a great extent been responsible for the development of larger and larger equipment as well as lowering the flexibility of the process and causing lower pulp quality. The development of rapid cooking has failed to recognize the conditions, which are required to economically produce high-quality pulp.  
         SUMMARY OF THE INVENTION  
         [0020]    According to the present invention, an improved, alkaline batch cooking process is provided, wherein the raw chip material is preheated and air purged, and impregnated with a liquor at a temperature no higher than the boiling point at atmospheric pressure of the impregnation liquor, at retention times of more than 60 minutes. Liquors including fresh cooking liquor are added to result in a concentration in the range from about 0.5 to 2.2 mol/l as OH −  ions; preferably said concentration is about 0.5 to 1.5 mol/l as OH − /l ions; more preferably said concentration is about 0.75 to 1.5 mol as OH − /l ions. A liquid-to-wood ratio in the range of 3 to 10 m 3 /t odw (m 3  per ton oven dry wood) is to be maintained during the impregnation step; preferably said ratio is in the range of 3 to 6 m 3 /t odw.  
           [0021]    The impregnated material is subsequently transferred to a batch digester and heated to a temperature T 2  of at least about 150° C., preferably in not more than 40 min (t 2 ), after which follows a cooking stage at a time t 3  with a maximum temperature T 3  of no more than 185° C., and a liquid-to wood ratio of at least 2.5 m 3 /t odw during a substantial part of the heating and cooking steps. The total of t 2  and t 3  shall not exceed 90 min. Fresh cooking liquor is added during the heating step, the cooking step or both. After the cooking step, the delignified material is cooled to a temperature where significant cooking reactions no longer occur.  
           [0022]    In practice, a temperature decrease to about 140° C. is sufficient to end the cooking step. Preferably, the time t 1  for impregnation is above 120 min, and the temperature T 1  in the range from 70° C. to the boiling point at atmospheric pressure of the impregnation liquor. Preferably, the total of t 2  and t 3  is less than 80 min. More preferably, the total of t 2  and t 3  is less than 70 min. Even more preferably, the total of t 2  and t 3  is in the range 10-60 min. Further, the liquid-to wood ratio during a substantial part of the heating and cooking steps is preferably at least 3 m 3 /t odw; more preferably, it is at least 3.5 m 3 /t odw.  
           [0023]    According to the invention, batch digesters over 10 m 3  are used. The heating and cooking time total in minutes may be expressed in relation to the digester volume V in m 3  as follows:  
             t   2   +t   3 ≦(0.09  V+ 63) min when  V≧ 100 m 3 ,  
             t   2   +t   3 ≦70 min when  V  is between 10 and 100 m 3 .  
           Preferably,  t   2   +t   3 ≦(0.09  V+ 53) min when  V≧ 100 m 3 , and  
             t   2   +t   3 ≦60 min when  V  is between 10 and 100 m 3 .  
           More preferably,  t   2   +t   3 ≦(0.09  V+ 43) min when  V≧ 100 m 3 , and  
             t   2   +t   3 ≦50 min when  V  is between 10 and 100 m 3 .  
           Even more preferably,  t   2   +t   3 ≦(0.09  V+ 33) min when  V≧ 100 m 3 , and  
           10 min≦t 2   +t   3 ≦40 min when  V  is between 10 and 100 m 3 .  
           [0024]    The heating time of a batch reactor is heavily dependent on size. A batch reactor of industrially significant size cannot be considered as a whole with regard to temperature, but each region within the reactor should ideally have the same temperature history. In a liquid displacement reactor equipped for bottom-to-top displacements, the material at the bottom reaches hot liquor displacement temperature long before the material at the top, but is correspondingly cooled earlier. Thus, the various parts of the digesters experience the same temperatures during approximately the same periods, but at a given point of time, the temperature may be different in various regions of the batch reactor.  
           [0025]    According to the present invention, the heating step during period t 2  is preferably carried out by means of liquid displacement, whereby the amount of heat delivered to each region of chips is essentially uniform throughout the digester. The cooking step during period t 3  is preferably carried out by liquor exchange as e.g. liquid circulation or displacement.  
           [0026]    According to the present invention, the cooling step is preferably carried out by means of liquid displacement.  
           [0027]    The average dry-solid of the material entering the impregnation stage is preferably over 40%; more preferably said dry-solid is over 45%.  
           [0028]    Preferably, impregnation takes place at low pressure, for the present purposes defined as up to 5 bar. Thus, low pressure equipment may be used, which saves investment costs. Use of pressure may be required to ensure sinking of the chips in the liquid phase. If high pressure equipment is installed, it may be utilized as expedient.  
           [0029]    As raw material for the process according to the invention, industrial wood chips are used. These commonly have an average length above 10 mm, typically 15-35 mm, and an average thickness above 2 mm, typically 3-7 mm.  
           [0030]    According to one aspect of the present invention, a digesting system is provided for carrying out the process of the invention. The digesting system comprises at least one impregnation vessel, batch digesters in fluid communication with the impregnation vessel; transfer lines between the impregnation vessel and the bottom of each digester for transporting the impregnated material to the digester; a separator, comprising a withdrawal space, disposed in connection with each digester for separating a transport liquid from the impregnated material; first return lines attached to each separator to conduct the transport liquid from the separator back to the transfer lines; second return lines connected to the first return lines and to the impregnation vessel for transferring a portion of the transport liquid to an inlet of the impregnation vessel; and a supply line connected to an inlet of the impregnation vessel. Due to the relation between the time periods for impregnation and cooking, the volume V of the impregnation vessel is larger than 1.5 times the volume of each digester, preferably larger than 3 times the volume of each digester, more preferably larger than 5 times the volume of each digester.  
           [0031]    The above process has given excellent results as shown in the examples, and it is a significant process simplification. The differences compared to prior art batch processes are the unique combination of:  
           [0032]    low temperature during impregnation, which enables low-pressure and -temperature equipment in impregnation, and long retention times, i.e. over 60 min, in a separate stage outside the digester  
           [0033]    alkali concentration of 0.5-2.2 mol OH − /l of added liquor in impregnation  
           [0034]    a short heating and cooking stage making possible a lower volume of cooking equipment  
           [0035]    liquor-to-wood ratio over 2.5 m 3 /t odw and fresh alkali addition in heating or cooking stage or both  
           [0036]    different pressures in the impregnation and cooking stages, which enables simpler equipment for low-pressure impregnation and a lower volume of high-pressure equipment, i.e. batch digesters.  
           [0037]    This results in process simplification and great flexibility. The batch cycle is significantly shortened compared to prior art processes. Experimental results show that the process gives high yield, improved bleachability and at least equal quality compared to prior art methods.  
           [0038]    The specific effective alkali concentrations, temperatures and times used in a process according to the invention are dependent on the type of wood and the purpose of the product. Hardwood cooking generally requires lower maximum cooking temperatures than softwood cooking. Pulp for unbleached products also normally require lower cooking temperatures than for bleached products. The impregnation time depends mainly on the type of chips and raw material. Material hard to impregnate, and consequently requiring longer times, may consist of long and thick chips, or have a large proportion of low-porosity material. The type of equipment and the space available are other factors.  
         DISCLOSURE OF THE INVENTION 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0039]    [0039]FIG. 1 shows schematically the time-temperature profile of prior art conventional batch cooking,  
         [0040]    [0040]FIG. 2 shows schematically the time-temperature profile of prior art displacement batch cooking,  
         [0041]    [0041]FIG. 3 illustrates schematically the time-temperature profile of prior art continuous cooking of the Kamyr type,  
         [0042]    [0042]FIG. 4 illustrates schematically the time-temperature profile of an embodiment of the invention,  
         [0043]    [0043]FIG. 5 is a schematic representation of the tank farm required combined with a flowchart showing an embodiment of the invention and the various liquor streams occurring during the production cycle,  
         [0044]    [0044]FIG. 6 is a representation corresponding to FIG. 5 showing an alternative embodiment of the invention,  
         [0045]    [0045]FIG. 7 is a representation corresponding to FIG. 5 showing a further alternative embodiment of the invention,  
         [0046]    [0046]FIG. 8 shows the brightness achieved versus consumption of active chlorine in pulps prepared using the conditions set forth in Table 1,  
         [0047]    [0047]FIG. 9 shows the brightness as a function of viscosity of pulps prepared according to the examples in Table 1,  
         [0048]    [0048]FIG. 10 shows the active chlorine consumption against bleached yields in the examples according to Table 1, and  
         [0049]    [0049]FIG. 11 shows the reject percentage as a function of impregnation time in pulp cooked to two kappa numbers according to the invention. 
     
    
     DETAILED DESCRIPTION  
       [0050]    FIGS.  1  to  4  show the temperature profiles of prior art pulping methods compared to that of the present invention. Referring now to FIG. 1, which shows the temperature curve against time in a conventional batch cook, region  1  of the curve represents the heat-up phase, region  2  illustrates cooking at about the maximum temperature and region  3  illustrates the discharge and cooling of the conventional batch digester.  
         [0051]    Typically, the duration of region  1  is 60 to 150 minutes, and that of region  2  60 to 120 minutes. The sum of region  1  and  2  is typically about 150 minutes.  
         [0052]    In FIG. 2 showing the corresponding curve of a prior art displacement batch process, region  5  represents the impregnation phase, region  6  the hot liquor fill phase, i.e. hot black liquor treatment and hot white liquor charge, region  7  represents the temperature adjustment phase, usually carried out by circulating the digester content and heating, region  8  illustrates the cooking phase at cooking temperature. Region  9  represents the displacement with cool wash liquid and region  10  represents the cold discharge.  
         [0053]    Typically, the duration of region  5  is typically about 30 minutes, but it can be 10 to 40 minutes depending on digester size, at a temperature below 100° C. Region  6  is typically about 30 minutes (can be 15 to 40 minutes depending on digester size). Regions  7  and  8  are typically 65 to 100 minutes. Thus, regions  6 ,  7  and  8  together typically represent 95 to 130 minutes. Region  9  is typically 45 minutes (can be 20 minutes to 60 minutes depending on, among other factors, digester size). The stages of regions  5 - 9  occur in the same batch equipment.  
         [0054]    In FIG. 3, illustrative of a prior art continuous process, region  11  represents the impregnation phase, region  12  represents heating and region  13  represents a cooking phase, which can occur in both concurrent and countercurrent modes. Region  14  represents displacement and cooling of the cooked material before discharge from the digester. Region  11  is typically 30 to 60 minutes or shorter at a temperature of 115-130° C. Regions  12  and  13  are over 90 minutes, typically 240 to 360 minutes.  
         [0055]    [0055]FIG. 4 shows the advantageous temperature profile of the present invention. Region  20  represents the impregnation phase, which as can be seen is substantially extended relative to processes presently in use. Region  21  represents the heating-up phase. Region  22  represents the short reaction time and region  23  the cooling of the cooked material before discharge from the digester. Between regions  20  and  21 , feeding of the pre-impregnated material to the batch digester takes place.  
         [0056]    [0056]FIG. 5 is a schematic representation showing the various tanks used in an embodiment of the invention, and a flowchart of the process together with the liquor streams occurring and their relation to the tanks. Wood chips are pre-steamed (point  1 ) and charged into the impregnation vessel. The chips can preferably be pre-heated to a temperature of 95-110° C., the retention time at that temperature being preferably 5-40 min. The transfer method and equipment between the presteaming phase and the impregnation vessel depends on the counter-pressure in the impregnation stage. The residence time of the impregnation stage (point  2 ) is at least 60 minutes. It can be significantly longer, depending on the available size of equipment. Longer times or impregnation times of more than about 24 h may be used for example when combining the impregnation stage with chip storage between the chipping unit and the cooking plant The impregnation time rarely exceeds 120 hours in the same equipment The impregnation equipment may be a down-flow vertical vessel or a horizontal conveyer type vessel with at least one inflow and at least one outflow point for the material, known to the person skilled in the art. Installed continuous digester vessels can be used e.g. when upgrading an existing plant. When using longer retention times, the impregnation device can be considered to be of the chip silo vessel type. Several vessels can be used in series or in parallel. According to the invention, the impregnation vessels are preferably dimensioned for a low pressure, i.e. pressure in the area from about atmospheric to 5 bar. Atmospheric conditions can be used. High-pressure equipment (over 5 bar design pressure) can be used when for example upgrading a plant to a method according to the invention. Liquor A is added to the stage. The liquor contains fresh alkali (point WLimp) and spent liquor from tank  4 . The amount may be, for example, 30 per cent or more of the total fresh alkali to be added calculated as total titrable alkali (TTA) per charged unit of wood, but additional fresh alkali is invariably added in the cooking stage. Spent liquor (from tank  4 ) is added as needed, recycled from e.g. a subsequent liquor-separation stage. The effective alkali concentration of the added liquors is 0.5-2.2 mol OH − /l; preferably, in the range 0.5-1.5 mol OH − /l; more preferably, in the range 0.75-1.5 mol OH − /l.  
         [0057]    The impregnation liquor A is a mixture of fresh alkali and spent liquor. The fresh alkali and spent liquor can be added together at one addition point, or in sequences during the impregnation. Spent liquor can be added first, and then fresh alkali is added and some spent liquor withdrawn. Fresh alkali can also be added first and then spent liquor. Parts of spent liquor and fresh alkali can also be added first, and then fresh alkali is added together with some withdrawal of spent liquor. The fresh alkali used can be both caustisized liquor, normally referred to as white liquor, and uncaustisized liquor, normally referred to as green liquor, or also derivates of the above mentioned liquors, e.g. a mother liquor from crystallization of sodium carbonate from green liquor. The temperature of liquor A may require adjustment to hold the preferable temperature between 70° C. and its atmospheric boiling point.  
         [0058]    Impregnated material is transferred from the impregnation reactor to the batch digester via a transfer system (points  3  and  4 ), which may be one of various combinations of discharge systems in the outlet part of the impregnation vessel and feeding technology known to the person skilled in the art. The system is supplied with liquor A 1  as required e.g. for dilution. Transfer systems to be used are for example pumps, chamber feeders (e.g. of the high pressure (HP) feeder type), screws, scrapers and injectors etc., and combinations therof, known to the person skilled in the art. Preferably, the digester is charged hydraulically by e.g. pumping from the bottom. Other methods, e.g. charging from the digester top after liquid separation, may also be used. Surplus liquor is removed at A 1  from for example the digester screen girdle, and is conducted to tank  4 .  
         [0059]    Following chip charge, hot black liquor B from tank  1  and hot white liquor C from tank  3  are charged in a hot liquor fill stage (point  5 ), initially displacing liquor A 2  to tank  4  and then, as the temperature rises above boiling point, D to tank  2 . The temperature is adjusted by means of circulation-based direct or indirect steam heating or direct steam heating of the digester (point  6 . 1 ). In accordance with the invention, at the end of the cooking stage (point  6 . 2 ), the effective alkali concentration of the cooking liquor can be 0.05-0.7 mol OH − /l, preferably in the range 0.1-0.5 mol OH − /l. Cooking is completed, and the batch is cooled by displacing the cooking liquor with cooler liquor (point  7 ), e.g. wash filtrate E from tank  5 , possibly containing also liquor from tank  4 . Displaced liquor is divided according to temperature and chemical content into fractions B 1  and D 1 , to tanks  1  and  2  respectively. When the temperature has decreased below about 100° C., the digester is discharged (point  8 ), preferably by pumping using additional filtrate F from tank  5  as required.  
         [0060]    In accordance with the flow balance, flow G, filtrate from the wash plant, may be used to dilute the white liquor, which is conducted to tank  3  while being heated by black liquor from tank  2 .  
         [0061]    [0061]FIG. 6 is analogous to FIG. 5, but no circulation heating is used in the digester, which consequently requires no heating circuit piping. Instead, the digester heating takes place using hot liquor displacement (point  5 ), whereby the temperatures of black liquor B and white liquor C are preferably adjusted by heat exchange before introduction into the digester. Liquors D and B displaced during the hot displacement stages are conducted to tanks  2  and  1 , respectively, depending on temperature and/or chemical content. According to this embodiment of the present invention, also cooking is carried out by displacement (point  6 ).  
         [0062]    [0062]FIG. 7 shows an embodiment where heating occurs by direct or indirect steam heating to the digester circulation or direct steam heating of the digester (point  6 . 1 ). Other differences are pressurized blow of the digester content at the end of cooking (point  8 ).  
         [0063]    Table 1 shows the results of, on the one hand, comparative cooking experiments (1-4) using various typical conditions for prior art continuous and batch cooking, and on the other hand experiments (5-11) using conditions according to the present invention.  
                                                                                                                         Example                1   2   3   4   5   6   7               Co king   Prior-art   Prior-art   Prior-art   Prior-art   Invention   Invention   Invention           Continuous   Continuous   Continuous   Batch   lab   lab   lab           mill   mill   lab   lab       Impregnation       Temperature, ° C.           90   80   95   95   95       EA mol OH − /l           3.05   0.3   1.25   1.25   1.25       Time, min           60   30   4320   60   60       Pressure, bar           0   5   0.5   0.5   0.5       Liquor-to-wood ratio m 3 /ft           4   5   4.6       Heating and C oking       EA of added liquor, mol OH − /l                   0.54   0.69   0.62       Liquor-to-wood ratio, m 3 /t           1.6   5   3.5   3.5   3.5       Heat-up, min           7   50   15   15   15       Heat-up + cooking time, min           29   91   25   25   25       Max cooking temperature, ° C.           175   160   169   176   169       End-of-cook residual EA, mol OH − /l               0.23   0.28   0.22   0.24       Unbleached pulp       Kappa Number   15.2   15.4   19.8   20.3   16.4   17.8   26.9       Brightness, ISO %   31.1   32.1   41.7   41.8   47.2   44.8   39.7       Total Yieid, %   nd   nd   54.2   55.4   54.3   55.3   57.2       Total Reject, %   nd   nd   2.66   0.97   0.12   0.88   1.2       Screened yield, %   nd   nd   51.5   54.4   54.2   54.8   56.3       Oxygen delignification       Time, min   60   20/60   nd   60   30/120   30/120   30/120       Temp, ° C.   100   90/100   nd   100   90/110   90/110   90/110       NaOH charge, kg/odt   15   10   nd   18   25   25   35       Oxygen pressure, Mpa   0.5   0.8/0.5   nd   0.6   0.8/0.5   0.8/0.5   0.8/0.5       Residual Ph   12   9.7   nd   11.8   11.7   11.4   11.8       Kappa Number   9.2   9.7   nd   12   8.6   8.5   11.3       Kappa reduction, %   39   37   nd   41   48   52   58       Viscosity, dm 3 /kg   895   908   nd   1065   1014   896   979       Brightness, % ISO   54.7   50.4   nd   59.6   72.0   71.4   68.0       ECF Bleaching       Stages used   D(EOP)(DnD)   D(EOP)DnD   nd   D(EOP)DnD   D(EOP)DD   D(EOP)DD   D(EOP)DD       Tot. conc. Act Cl, kg/odt   42.8   41.3   nd   38   13.8   17.5   18.7       Tot. conc. Act. Cl mult   0.46   0.43   nd   0.31   0.17   0.22   0.17       Brightness, % ISO   90.5   91   nd   92.0   92.0   91.7   91.5       Viscosity, dm 3 /kg   838   779   nd   800   922   818   871       Bleached yield, %               52.0   52.0   52.3   52.6       PFI beating results       Brightness, % ISO   nd   91.0   nd   90.5   92.0   91.7   91.5       SR 30       Tensile index, Nm/g   nd   82.8   nd   83.2   91.9   92   92.7       Tear index, mNm 2 /g   nd   10.8   nd   9.0   11.1   10.6   10.7                        Examples                    8   9   10   11                       Co king   Invention   Invention   Invention   Invention               lab   lab   lab   lab           Impregnation           Temperature, ° C.   95   95   95   95           EA mol OH − /l   1.25   1.25   1.25   1.25           Time, min   180   4320   60   60           Pressure, bar   0.5   0.5   10   10           Liquor-to-wood ratio m 34.6     4.6   4.6   46   /ft           Heating and C oking           EA of added liquor, mol OH − /l   0.64   0.41   0.62   0.64           Liquor-to-wood ratio, m 3 /t   3.5   3.5   3.5   3.5           Heat-up, min   15   15   15   15           Heat-up + cooking time, min   25   25   25   27           Max cooking temperature, ° C.   168   160   174   168           End-of-cook residual EA, mol OH − /l   0.27   0.31   0.24   0.29           Unbleached pulp           Kappa Number   30.6   32.5   19.7   27.4           Brightness, ISO %   42.6   42.0   42   40           Total Yieid, %   57.6   57.5   54.5   56.5           Total Reject, %   1.16   1.23   0.96   1.4           Screened yield, %   56.4   56.3   53.5   55.1           Oxygen delignification           Time, min   nd   nd   nd   nd           Temp, ° C.   nd   nd   nd   nd           NaOH charge, kg/odt   nd   nd   nd   nd           Oxygen pressure, Mpa   nd   nd   nd   nd           Residual Ph   nd   nd   nd   nd           Kappa Number   nd   nd   nd   nd           Kappa reduction, %   nd   nd   nd   nd           Viscosity, dm 3 /kg   nd   nd   nd   nd           Brightness, % ISO   nd   nd   nd   nd           ECF Bleaching           Stages used   nd   nd   nd   nd           Tot. conc. Act Cl, kg/odt   nd   nd   nd   nd           Tot. conc. Act. Cl mult   nd   nd   nd   nd           Brightness, % ISO   nd   nd   nd   nd           Viscosity, dm 3 /kg   nd   nd   nd   nd           Bleached yield, %   nd   nd   nd   nd           PFI beating results           Brightness, % ISO   nd   nd   nd   nd           SR 30           Tensile index, Nm/g   nd   nd   nd   nd           Tear index, mNm 2 /g   nd   nd   nd   nd                                                                                                                      
 
       EXAMPLES 1 and 2  
       [0064]    Mill-scale production according to prior-art “Kamyr” continuous cooking of industrial eucalyptus chips to typical kappa numbers of eucalyptus cooking. Sampled pulps were oxygen delignified and ECF bleached in the laboratory. Bleaching chemicals demand for a given pulp brightness was determined and the pulp strength measured by beating and testing.  
       EXAMPLE 3  
       [0065]    Production of eucalyptus pulp according to prior-art process disclosed in U.S. Pat. No. 3,664,918 (vapor phase pulping of water saturated lignocellulosic materials) and example 1 of U.S. Pat. No. 3,664,918.  
         [0066]    The industrial eucalyptus chips (5.5 kg oven dry weight) were first submerged in water overnight at 2 bar overpressure and room temperature. The excess water was separated. The water saturation resulted in chips of 44.6% dry solids. The water-submerged chips were metered into a chip basked positioned in a jacketed displacement digester with liquor circulation. The chips were impregnated with white liquor (liquor (EA charge of 33.7% NaOH calculated on wood, EA 122 g NaOH/l and sulfidity 30%) at a liquor-to-wood ratio of 4 m 3  per ton of dry wood at 90° C., 60 minutes and atmospheric pressure. After impregnation of the chips and removal of excess liquor, the impregnated chips were then subjected to steaming and the temperature of the chips was initially raised to 100° C. for 20 min and subsequently treated at 175° C. for a total of 36 min, including heating-time of 7 minutes. After cooking the digester content was cooled with water. After the cook the pulp was wet disintegrated and screened. Kappa number, yield, reject, brightness were determined on the cooked pulp.  
       EXAMPLE 4  
       [0067]    Production of normal eucalyptus pulp.  
         [0068]    Example 4 show laboratory simulation data of a process simulated according to prior-art displacement batch cooking of industrial Eucalyptus.  
         [0069]    4.5 kg eucalyptus chips (oven dry basis) were metered into a chip basket positioned in a 26-liter jacketed displacement digester with liquor circulation. The same chip raw material as shown in Example 3 were used. The chips were pre-steamed for 10 minutes at 100° C. Then impregnation liquor fill at 80° C. was conducted with an impregnation liquor containing 0.29 mol OH − /l of EA. After 30 minutes impregnation, hot black liquor treatment occurred for 20 minutes with a HBL containing 0.205 mol OH − /l of EA and a temperature of 148° C. Then hot white liquor (105 g NaOH/l as EA, sulfidity 40%) at a charge of 11.6% as NaOH (EA) was added for 10 minutes. The digester content was then heated for 20 minutes to the cooking temperature of 160° C. The time at cooking temperature was 41 minutes. After the cook the pulp was wet disintegrated and screened. Kappa number, yield, reject, brightness were determined on the cooked pulp. The cooked pulp was then oxygen delignified and ECF bleached in the laboratory. Bleaching chemicals demand for a given pulp brightness was determined and the pulp strength measured by beating and testing.  
       EXAMPLE 5  
       [0070]    Production of eucalyptus kraft pulp in accordance with an embodiment of the present invention.  
         [0071]    5.5 kg of oven dry eucalyptus chips was metered into a chip basket in a jacketed displacement digester with liquor circulation. The same chip raw material was used as in Example 3 and 4. The chips were first pre-steamed at 100° C. for 30 minutes. Impregnation occurred for 3 days at a temperature of 95° C. and a small overpressure of 0.5 bar. The alkalinity of the added liquor was 1.25 mol OH − /l and the liquor-to-wood ratio was 4.6 dm 3  per kg of dry wood. The added impregnation liquor contained white liquor at a sulfidity of 40% and spent liquor drained from previous impregnations using the same process. After impregnation of the chips and removal of excess liquor, pre-heated cooking liquor at various alkali concentrations (EA) was added for 5 minutes and the liquor-to-wood ratio was simultaneously adjusted to 3.5 m 3  per ton of dry wood. The digester content was heated to the cooking temperature in about 10 minutes and the temperature was kept at temperature for 10 minutes. After cooking, the digester content was cooled and the liquor was drained. After the cook the pulp was wet disintegrated and screened. Kappa number, yield, reject, brightness were determined on the cooked pulp. The cooked pulp was then oxygen delignified and ECF bleached in the laboratory. Bleaching chemicals demand for a given pulp brightness was determined and the pulp strength measured by beating and testing.  
       EXAMPLE 6  
       [0072]    The experiment was carried out as disclosed in Example 5, but the impregnation time was 60 min and the cooking conditions were adjusted to give about the same kappa number as in Example 5.  
       EXAMPLE 7  
       [0073]    The experiment was carried out as disclosed in Example 6, but the cooking conditions were adjusted to give a higher cooking kappa number.  
       EXAMPLE 8  
       [0074]    The experiment was carried out as disclosed in Example 7, but the impregnation time was adjusted to 180 minutes and the cooking conditions were adjusted to give slightly higher kappa number than in Example 7.  
       EXAMPLE 9  
       [0075]    The experiment was carried out as disclosed in Example 8, but the impregnation time was adjusted to 3 days and the cooking conditions were adjusted to give slightly higher kappa number compared to Example 8.  
       EXAMPLE 10  
       [0076]    The experiment was carried out as disclosed in Example 6, but the impregnation pressure was adjusted to 10 bar and the cooking conditions were adjusted to give slightly higher kappa number compared to Example 6.  
       EXAMPLE 11  
       [0077]    The experiment was carried out as disclosed in Example 8, but the impregnation pressure was adjusted to 10 bar and the cooking conditions were adjusted to give slightly lower kappa number compared to Example 8.  
         [0078]    Table 1 shows the cooking characteristics of Eucalyptus hardwood chips, unbleached pulp results and the subsequent oxygen delignification, ECF bleaching and PFI beating results. All oxygen delignifications, ECF bleachings, PFI beatings and tests were performed in the laboratory.  
         [0079]    The effect of impregnation time is shown in FIG. 11. The reject percentage is shown as a function of impregnation time as pulp is cooked according to the invention to kappa numbers  20  and  25  using a total heat-up and cooking time of 25 min. It is seen, that a satisfactory level is reached when 1 hour impregnation residence time is used; a further half per cent decrease is achieved by extending impregnation with a further hour. The improvement due to extension to even three days is marginal. However, table 1 shows that the bleaching chemical consumption is significantly lower and bleached pulp viscosity is higher when using 3 days impregnation.  
         [0080]    Based on the results in Table 1 and FIGS.  8  to  11 , the present invention offers the following surprising benefits over a state-of-the-art cooking process:  
         [0081]    remarkably shorter residence time in heating and cooking can be used compared to over 1.5 hours in prior-art Kamyr-type continuous digesters and in prior-art displacement batch digesters  
         [0082]    the required cooking volume is considerable reduced  
         [0083]    the unbleached and oxygen bleached pulp is brighter pulp at same kappa number  
         [0084]    lower or equal rejects amounts at same or higher kappa number. In a process according to the invention and according to the methods described, the reject level depends on the impregnation time and kappa number target (see FIG. 10 showing reject levels of pulps at kappa numbers  20  and  25  and impregnation times of 0-3 days using a retention time of 25 minutes in heating and cooking)  
         [0085]    the reject level is independent on impregnation pressure in the range 0.5 bar to 10 bar for pre-steamed chips implementing that low-pressure impregnation equipment can be used in impregnation  
         [0086]    higher unbleached screened yield  
         [0087]    higher kappa number reduction in oxygen delignification. Example 7 used more NaOH in oxygen delignification, but the additional cost for this is minor since oxidized white liquor from the recovery cycle, i.e. low-cost NaOH, is usually used in oxygen delignification.  
         [0088]    considerable lower active chlorine chemical consumption in ECF bleaching by about 50-65%  
         [0089]    bleached pulps gives a pulp of higher viscosity, see example 5,  
         [0090]    higher bleached yield at lower bleaching costs.  
         [0091]    higher tensile strength although the brightness of the pulps are higher  
         [0092]    The following examples make clear some advantages of the present invention over prior art kraft batch cooking when cooking industrial softwood chips.  
       EXAMPLES 12 AND 13  
       [0093]    Production of a normal “reference” softwood kraft pulp by using prior-art displacement kraft batch technology.  
         [0094]    4.2 kg Scandinavian softwood chips (oven dry basis) were metered into a chip basket positioned in a 26-liter jacketed displacement digester with liquor circulation. Industrial chips were used consisting of 10% over-thick chips (fraction retained on a 8 mm wide bar) and 90% of so-called accept chips (chip fractions retained between 8 mm wide bars and 13 mm holes). The lid of the digester was closed. The impregnation liquor (IL) was pumped into the digester. The amount of the IL was 4.5 l/kg o.d. wood and EA 0.3 mol/l. The conditions in the impregnation step were total time 20 min, temperature 90° C. and pressure 3 bar. After the impregnation stage followed immediately the hot black liquor stage and hot white liquor stage. The hot black liquor and hot white liquor displaced the IL. The amount of hot black liquor was 4.0 l per kg o.d. wood and EA 0.45 mol/l. The conditions in the hot black liquor and hot white liquor stage were: Total time 30 min, temperature 5° C. below cooking temperature and pressure 7.0 bar. Then temperature adjustment and cooking by circulation followed. The hot white liquor was also split charged, so that 70% was charged at the hot black liquor fill and 30% after 15 min at cooking temperature. The cooking time was varied by having different cooking temperatures. At the target H-factor, displacement liquor was pumped into the digester cooling the pulp. The conditions in the final displacement were: Temperature 80° C., time 50 min and total amount of liquor 7.0 l/kg o.d wood. After the cook, the pulp was wet disintegrated and screened. Kappa number, yield, reject, brightness and viscosity were analyzed on the pulp.  
         [0095]    Cooking characteristics are shown in table 2.  
                                           TABLE 2                           Results of Example 12 and 13                EXAMPLE 12   EXAMPLE 13                        Impregnation liquor, min   20   20       Impregnation liquor, EA mol OH − /l   0.3   0.3       Hot black liquor + hot white liquor   30   30       addition time, min       Hot black Liquor, EA mol OH − /l   0.45   0.45       Hot White Liquor Charge, EA %   16.9   16.6       NaOH       Temperature adjustment + cooking   80   60       time, min       Cooking temperature   169   172       H-factor   1050   1000       Total heating and cooking time, min   110   90       End of cook residual, EA mol OH − /l   0.45   0.5       Kappa Number   31.5   31.1       Total Yield, % on wood   47.7   47.6       Coarse reject, % on wood   1.03   1.98       Fine reject, % on wood   0.42   0.66       Screened yield, % on wood   46.3   45.0       Viscosity, dm 3 /kg   1154   1094       Brightness, % ISO   32   30                  
 
       EXAMPLE 14  
       [0096]    Production of softwood kraft pulp by using displacement kraft batch technology according to an embodiment of the invention.  
         [0097]    4.2 kg Scandinavian softwood chips (oven dry basis) were metered into a chip basket positioned in a 26-liter jacketed displacement digester with liquor circulation. The same chips as in examples 12 and 13 were used. The lid of the digester was closed. The chips were steamed for 20 min at 100° C. Impregnation liquor (IL) was pumped into the digester and circulation put on for 20 minutes. After 20 minutes of circulation was the circulation stopped and the chips were impregnated for a total time of 180 minutes. The temperature of the impregnation stage was 95° C. and the effective alkali concentration of added impregnation liquors was 1.3 mol OH − /l. The overpressure was 0.3 bar in the top of the digester. After the impregnation stage, a hot black liquor and hot white liquor fill stage followed immediately. The hot black liquor and hot white liquor displaced the spent impregnation liquor. The amount of hot black liquor was 4.2 l/kg o.d. wood and EA 0.45 mol OH − /l. The conditions in the hot black liquor and hot white liquor fill stage were: Total time 30 min, temperature 5° C. below maximum cooking temperature and pressure 7.0 bar. Then lo temperature adjustment and cooking by circulation followed. The total heating and cooking time was 70 min. At the target H-factor displacement liquor was pumped into the digester, cooling the pulp. The conditions in the final displacement were: Temperature 80° C., time 50 min and total amount of liquor 6.7 l/kg o.d. wood. After the cook, the pulp was wet disintegrated and screened. Kappa number, yield, reject, brightness and viscosity were determined on the pulp.  
                                       TABLE 3                           Results of Example 14                EXAMPLE 14                            Impregnation liquor, min   180           Impregnation liquor, BA mol OH − /l   1.3           Hot black liquor + hot white   30           liquor addition time, min           Hot black Liquor, EA mol OH − /l   0.45           Hot White Liquor Charge, EA % NaOH   6.4           Temperature adjustment +   40           cooking time, min           Cooking temperature   174           H-factor   808           Total heating and cooking time, min   70           End of cook residual, EA mol/l   0.45           Kappa Number   30.6           Total Yield, % on wood   48.1           Coarse reject, % on wood   0.76           Fine reject, % on wood   0.22           Screened yield, % on wood   47.1           Viscosity, dm 3 /kg   1138           Brightness, % ISO   33                      
 
         [0098]    Based on the results shown in tables 2 and 3, the present invention offers the following surprising benefits over a state-of-the-art cooking process:  
         [0099]    By using over 60 minutes pre-impregnation in a vessel outside of the displacement kraft batch digester according to method presented in example 14,  
         [0100]    the heat-up and cooking time can be reduced by over 50% compared to a prior art process at the same kappa number and reject level (sum of coarse and fine reject). Decrease of total cycle time by at least 40 min, which for a reference installation with total cycle time of 220 min means a production increase of at least 18%. A lower number of batch digesters or lower total batch digester volume can be used to reach a given production level.  
         [0101]    lower reject level although 40 min shorter heating and cooking time  
         [0102]    the screened yield is significantly higher.  
         [0103]    the unbleached brightness is slightly higher.  
         [0104]    The following examples make clear some advantage of the present invention over prior art kraft batch cooking in terms of the equipment required.  
       EXAMPLE 15  
       [0105]    Production of 1800 air dry tons softwood kraft pulp per day by using prior-art displacement kraft batch technology  
         [0106]    The total batch digester volume required is about 4000 m 3  using 10 times 400 m 3  digesters.  
       EXAMPLES 16-18  
       [0107]    Production of 1800 air dry tons softwood kraft pulp per day by using a displacement kraft batch cooking process according to the invention.  
         [0108]    Examples 16 to 18 show that the same production can be made with a total batch digester volume of 2400 to 2800 m 3  using a lower number of digesters. The examples show, that the volume ratios between the individual batch digesters and the impregnation vessel are about 2 to 6.8.  
                                                           TABLE 4                           Results of Example 15-18                EXAMPLE   EXAMPLE   EXAMPLE   EXAMPLE           15   16   17   18                        Production,   1800   1800   1800   1800       adt/d       Digester size,   400   350   350   300       m 3         Number of   10   8   8   8       batch digesters       Total volume   4000   2800   2800   2400       of batch       digesters, m 3         Impregnation   60   120   180       retention       time, min       Impregnation   700   1370   2050       vessel, m 3         Ratio between   2   3, 9   6, 8       impregnation       vessel and       batch digester       volume                  
 
         [0109]    Based on the results shown in table 4, the present invention offers the following surprising benefits over a state-of-the-art cooking process:  
         [0110]    A lower number of batch digesters can be used which also means less pressure vessels, piping, instruments etc.  
         [0111]    A lower total batch digester volume can be used to reach a given production level.  
         [0112]    In order to lower the batch digester volume and the number of batch digesters, an impregnation vessel is used which according to the invention can be designed for much lower pressure and temperature conditions. In addition, the building requirements are much lower with a method according to the invention as the digester is preferably filled with chips hydraulically from the bottom. The chip silo and capping valve above the batch digesters can also be eliminated. The ratio between the impregnation vessel digester volume is important in order to obtain sufficient pre-impregnation.