FIELD OF TECHNOLOGY

The invention relates to the production of cellulose from wood chips and it especially concerns pre-treatment of the chips wherein these are impregnated with a chemical solution.

BACKGROUND OF THE INVENTION

In a chemical or chemi-mechanical process for cellulose production the chips are cooked in a delignifying cooking liquor at a high temperature, normally between 160 and 180°C, and under an overpressure. Before the cooking temperature can be reached the chip pieces must be nearly fully impregnated by the cooking liquor; otherwise so-called "burning" will take place in the chips. For this reason, the impregnation must be performed at a temperature of less than 135 - 145°C.

In presently used cooking processes the impregnation step takes no less than one-fourth or even more than one-half of the total cooking time, depending on the quality of the cooking liquor. For example, in a sulphate process increasing the temperature to 170°C will typically take about 120 minutes, while the cooking step proper will only take about 100 minutes.

Diffusion is accelerated by raising the temperature and pressure in a range of 100 - 135°C (the speed is increased 2

- 3 times in this way) . However, at raised temperatures carbohydrate components and hemicelluloses in particular will begin dissolving. At about 140°C the speed of chemical reac¬ tions exceeds the rate of diffusion and accelerates quickly manyhundredfold as the cooking temperature is reached. For delignification, the hemi discharge reactions are unneces¬ sary, besides which they reduce the yield. In addition, they consume alkali in particular in the cooking liquor.

In processes today the chips are washed with water before the actual impregnation. Then air is removed from lumens in the chips, for example, by low-pressure steaming. 20 - 30 per cent of the air is removed in this way. As the steaming continues, steam in the lumens is condensed and water starts filling the lumens.

In patent publication FI-80083 (corresponds, for example, with the publication WO-89/07170) a so-called vacuum penetration method is proposed for use in impregnation. In this method, air is first removed from the chips under a negative pressure and the chips are then promptly surrounded with cooking liquor. The liquor hereby penetrates into the lumens, from which the chemicals are further diffused into the fibre walls. In this way impregnation is speeded up, because the diffusion distance will be short even into fibre walls located in the centre of the thickest chip piece.

Publication WO-92/13990 describes a control method used in vacuum penetration, especially in mechanical defibration.

DESCRIPTION OF THE INVENTION General description of the invention

A cooking method as defined in claim l has now been in¬ vented. Some advantageous applications of the method are presented in the other claims.

In the method the surface properties of the chips are ensured before the treatment with cooking liquor, which is performed by using a vacuum penetration method. Hereby the diffusion of chemicals into the cell walls takes place very quickly and uniformly all over. This results in improved cellulose yield and quality. Detailed description of the invention

The temperature and surface moisture of chips arriving for the production vary considerably, depending i.a. on the weather and on storing methods. Sometime the chips may be frozen, and in the warm season their temperature may be high due to biochemical activity.

In the method according to the invention the chips are heated or cooled with air when required before the vacuum penetration. The chips may be at no more than the boiling temperature of the penetration liquor in the vacuum used. The treatment with air eliminates such undeterminable effects on the filling level (liquor/dry matter) of the cell tissue which are caused by variations in the temperature, especially in the surface moisture and air content of the chips to be supplied into the production. In addition, heating promotes

penetration, because penetration is made more effective by the reduced viscosity of the penetration liquor. A filling level which is as high as possible intensifies the perfor¬ mance of the cooking step proper and the control of cooking and promotes a constantly optimum uniform pulp quality. A uniform filling level also promotes a constantly optimum low, but still sufficient, consumption of chemicals, and it inten¬ sifies the dissolution and bleaching of lignin.

The above-mentioned treatment of chips with air is preferably done in an intermediate storage container equipped with air blowing or suction equipment when required.

After the air-treatment, the chips are penetrated with cooking liquor using the vacuum penetration method. In this air is first removed from the chips in a vacuum (for example, 0.1 - 0.5 bar), whereupon cooking liquor is conducted in a swift flow to the chips. This is done because the water or chemical solution will quickly indurate the ends of the cell walls which were cut off during the chipping and have opened up in the vacuum process, whereafter hardly any penetration will happen. For this reason, the chips must not either be wetted before the vacuum process, for example by washing. The above-mentioned liquor penetration must be done in a minimum time of about 5 minutes. This is done much quicker in prac¬ tice, for example, in no less than about 2 minutes, preferably in no less than about 0.5 minute. The temperature of the penetration liquor must not be higher than its boiling point in the vacuum used. In vacuum penetration the liquid is made to fill the lumens practically completely, excepting an occurrence of a few resin and air pore inclusions. Neither the water content of the chips nor the air quantity has any affect on the filling level achieved.

It was further noticed in performed tests that penetration is equally complete with all kinds of tree (spruce, pine, birch) . It was also noticed that the type, concentration or specific weight of the cooking liquor (alkaline sulphate, neutral Na sulphite, formic acid) will not affect the filling level of the lumen space.

Nor was the filling level affected by the thickness (2 - 8 mm) or the length (6 - 35 mm) of the chip piece.

Penetration is preferably performed in a penetration space located before the digester and containing suitable means and automation to produce a vacuum and to conduct the cooking liquor into the vessel.

The chemicals content of the liquor to be used in the penetration can be controlled in accordance with the water content of the arriving chips. In this way the chemicals dosage per dry matter is kept constant despite any variations in the water content of the chips.

If additives are used in the cooking liquor (such as polysulphide and catalysts) they are preferably added already to the liquor which is used in the penetration. Additives may be used, for example, to promote the removal of incrust matter or to reduce unnecessary dissolution of hemi cel¬ lulose.

After the penetration the chip lumens are full of cooking liquor, having as high and homogenous a chemicals con- centration and temperature as possible. They also contain an optimum dose of chemicals in relation to the water content of the chips.

The chips are usually washed before cooking. The purpose of washing is first of all to remove sand and metal im- purities. In the process according to the invention this can be done only after the vacuum penetration. It is preferably done by applying conventional equipment and procedures in¬ tended for this purpose. It is preferred to circulate as washing liquor the cooking liquor already used in the vacuum penetration, preferably to increase even further the temperature of the chips.

Before the temperature can be increased to the level required by delignification reactions, the cooking liquor must be allowed to diffuse into the cell walls. In the method according to the invention diffusion is completed very quick¬ ly, because the chemicals content is as high as possible and the diffusion distance as short as possible (that is, the fibre wall thickness, which for example with spruce is 0.002 - 0.008 mm) .

In the method according to the invention diffusion takes place so quickly that it can be performed even at a low temperature without making the total process time any longer. When the temperature is below the liquor boiling point (for example, 90 - 95°C) , there is neither any need of pres¬ surizing production equipment for the diffusion.

Impregnation is preferably done as a separate step, because when applied, for example, to a sulphate process such temperature and pressure conditions and equipment arran- gements are used for the said impregnation which differ very much from the cooking conditions. Conventional principles and equipment can be applied to the connection and automation of functions.

In batch cooking, diffusion may be performed simply and advantageously, for example, in two or more intermediate vessels before the chips are moved into the digester. In continuous cooking the most flexible solution is to arrange the diffusion step as a continuous one.

Changes in cooking modes can be allowed for by controlling the through time. For example, in a plant having a separate chip impregnation vessel, the chips may be moved from the pre-treatment and vacuum penetration steps to a continuous diffusion state and from there further by a liquor at cooking temperature and pressure to the cooking space. Thus, in the early stage of cooking a major part of that part of the digester which is now used for impregnation can be used as a cooking space and thus add to the capacity and/or improve the pulp quality level by controlling the cooking temperature and/or the chemicals content. If there is no need to utilize the advantages offered by diffusion at a low temperature, the diffusion step may also be preferably performed at a raised temperature and under a pressure. For example, in the production of dissolving pulp the chemicals for dissolving hemis are placed in the penetration liquor and diffusion can be speeded up by raising the temperature, however, not above 120°C. Even then, of course, the shorter diffusion time is useful, because the time for harmful temperatures (about 120 - about 140°C) to exert an influence will be correspondingly shorter.

Pressurized diffusion treatments of the impregnated chips may be combined with the step where the digester temperature is increased.

In a test process (sulphate cellulose) vacuum-impregnated chips were used and the digester temperature was raised by using the highest rate of temperature rise (11.5°C/min) . Compared with normal reference cooking this process resulted in considerably reduced kappa and sliver values and in less consumption of alkali. After the diffusion step the cooking liquor is allowed to affect the chips at a temperature and a pressure required by the delignification reactions for the desired time.

Using conventional methods the chip temperature and pres¬ sure in the digester are preferably raised in, for example, a sulphate process by slightly pressurized (1 - 3 bar) so- called buffer black liquors obtained from previous cookings. The air surrounding the chips is removed at the same time. The temperature is then raised by using calorizer or steam heating or the black liquor is displaced by hot pressurized black liquor and/or white liquor.

The heat required in the penetration and diffusion steps is preferably obtained by moving heat obtained from the cooking in counterflow in relation to the direction in which the chips are moving in the process. The manner of utilization depends on the type of digester house and on heat economy arrangements.

Since the chips are impregnated as completely and uniformly as possible with cooking chemicals, unusually high cooking temperatures may also be used without any harmful effects to speed up the production.

The described diffusion of cooking chemicals under the effect of a suitable chemicals dosage and suitable conditions eliminates any harmful effects caused by lignin condensation products for the cooking and bleaching ability of the pulp. Cooking may be prolonged over the usual time and such chlorine-free bleaching chemicals can be used for removing any residual lignin, the use of which has been restricted by the costs of bleaching the residual lignin. An additional

advantage is the possibility of eliminating bleaching ef¬ fluents together with cooking waste liquors.

The separate performance of chip impregnation adds to the capacity of the digester house and/or improves yield and quality factors.

Figure 1 is a schematic view of an embodiment of the invention in a batch cooking process. The chips are conducted from storage 1 into an intermediate storage container 2. The container is equipped with air circulating equipment 3 for setting the desired chip temperature and surface moisture.

The treated chips are brought in batches along line 4 into penetration vessel 5. When the batch is in the vessel, valve 6 is closed and the desired vacuum is quickly created in the vessel using pump 7 and vacuum accumulator 8, whereby a powerful and sudden air flow from the chip lumens opens the so-called bordered pits which connect the fibres. When the desired vacuum is obtained, liquor is quickly conducted into the bottom part of the vessel from container 9 by using pump 10. While the vacuum is still effective and the penetrating liquor is filling the fibre lumens, capillary forces inten¬ sify a very quick filling of the capillary fibre lumen net¬ work. The automation closes valve 14 in the vacuum line, whereby the liquor pressure within the vessel will rise quickly to 1 - 3 atm. The described process should preferably take place at less than 30 s. Valve 12 in the exhaust line is then opened and the chips are moved into digester 13.