SE0200189A1 - Systems and procedures for controlling an operation of 16 - Google Patents

Abstract

The invention relates to a process for boiling chemical cellulose pulp in which the starting material, preferably in the form of chips, undergoes a successive temperature increase to boiling temperature while the chips are first impregnated with an impregnating liquid followed by boiling in a preferably alkaline cooking liquid. or at subsequent boiling, an improved delignification process is obtained where the pulp acquires improved properties with respect mainly to strength / viscosity relative to the current degree of delignification but also increased yield. The process can be used in both continuous and batch cooking of cellulose pulp. (Pig. 2) 0228se.doc '-.- = a: .aztfa fi iz / iz: -.,;:' - on gas

Description

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The present invention relates to a process for the production of cellulosic pulp according to the preamble of claim 1.

Teknikens StåndpunktKoktekniken has undergone significant development over several decades.

During the 60s and 70s, systems were used in which essentially all cooking liquid, including white liquor, was already charged during the pre-treatment. The boiling in continuous cooking spoon then in the same cooking liquid down through the boiler to then be subtracted. Relatively high alkali levels were established at the beginning of the cooking, so that sufficient alkalinity is maintained throughout the cooking. The high alkali levels at the beginning of the cooking proved to be detrimental to the pulp quality, so several different variants have been proposed for these systems. These variants include, among others, MCC (Modified Continuous Cooking), ICT (ISO THERMAL COOKING) with the same cooking temperature throughout the cooker and EMCC (Extended Modified Cooking).

Later in the 1980s, the black liquor impregnation was developed for both batch and continuous cooking, where spent cooking liquid from the boiler was returned to form part of the impregnation liquid. Here, the white liquor could be invested in the end of impregnation, or the beginning of the cooking. The spent cooking liquid, i.e. the black liquor, which was withdrawn from the boil could then have a relatively high residual alkali, whereupon the stripped black liquor was returned to the impregnation where the bulk of the residual alkali was consumed before deduction of spent impregnation liquid was sent to recycling (evaporation & boiler). With this technique, a lower alkali content could be established at the beginning of the cooking.

Several different solutions are known where black liquor is used as an impregnation liquid in an impregnation zone before boiling. US 5,080,755 shows a system with black liquor in the input. US 5,053,108 discloses a variant in which black liquor withdrawn from the digester is recirculated to the high-pressure layer to form a larger part of the treatment liquid in transfer circulation to the digester. EP 477,059 shows a further developed variant where the wood chips impregnated with black liquor are raised to O228se.doc -a ::: Ü -išiåå -2 /: 125- * boiling temperature before the main addition of the white liquor. These showed that quantities with different proposals for processes were studied in order to improve the pulp quality and the yield while maintaining a high degree of delignification in the pulp washed after cooking.

Another cooking technique that began to be marketed by Andritz-Ahlstrom in the 1990s, and which goes by the name LO-SOLIDS, means that man continuously removes cooking liquid with a high content of organic material and replaces it with new or treated cooking liquid that has a lower content of organic organic material ( DOM / Dissolved Organic material). These systems are punished, however, as the cooking process often results in lower yields, caused by the fact that the extracted organic material such as lignin also loses carbohydrates, mainly hemicellulose.

Other ways to improve the cooking process, both in terms of yield and pulp quality, use additives of polysulfide, AQ or released Xylan.

The yield is very important when cooking as an increase of one percentage point means that a production plant of normal size of 1,500 tonnes / day gives increased production of 15 tonnes, which with a pulp price of 700 'USD / ADT, gives an increased income of 10,500 USD per day .

Object and purpose of the invention During the development of the cooking technique, mainly for continuous cookers, there are today two dominant techniques. Partly the so-called LO-SOLIDS technology part and the COMPACT COOKING developed by Kvaerner Pulping AB.

In COMPACT COOKING, very high liquid-wood conditions are established in the first phases of the coke with a very large amount of black liquor present in the cooking liquid.

It has now surprisingly been found that, in direct contrast to the LO-SOLIDS principles, a very favorable delignification is established during cooking if black liquor is present in the cooking liquid. Thus, as in LO-SOLIDS way, boiling liquid with a high content of dissolved organic material should not be deducted and this liquid replaced with cooking liquid with a lower content of dissolved organic material. 0228561100 z - z åšiååß /: uš-. L i * The problem instead is that on the contrary you want to increase the proportion of released organic material but still retain other added liquids, white liquor etc., in quantity and content. In particular, you want to enrich the cooking liquid on the organic material that affects delignification favorably and also to some extent allows an increased yield.

The current type of desired additive to the cooking liquid is already present in the pulp mill, but then in the evaporation before recycling. Hitherto, no one has realized that a partially evaporated black liquor has a positive effect on the boil, and that the partially evaporated black liquor must be returned to the boil from the evaporation.

The main object of the invention is to increase the selectivity of the coquette by accelerating the delignification. This results in the same degree of delignification (kappa number reduction) obtaining improved pulp quality (viscosity / mass strength) and higher yield, alternatively that with the same mass strength and yield a higher degree of delignification is obtained.

The invention also allows a positive increase of OH 'and HS' ions during cooking, which in a manner known per se gives better selectivity and bleachability on and of the cellulose pulp produced.

Another object is that with the improved delignification effect under coke soy, this can be switched to at least one of the following advantages: - Less continuous boilers, ie. cheaper systems for given production volume (shorter residence time); - Increased production capacity in both batch and continuous cooking, while maintaining pulp quality.o Reduced need for cooking chemicals (alkali), which results in lower production costs. The invention can be applied to both steam phase boilers and hydraulic top cookers, with inverted top cookers as a down-feed top separator and types without a top separator and can be used in the production of cellulose pulp O228se.doc tu: .éicíaäq /: lz-š. Q å .. 1 according to both sulfite and the sulfate method. Similarly, hardwood, softwood, annuals (type bagass etc.) etc. can also be used. constitute the cellulosic raw material.

The invention can also be applied in batch cooking, so-called batch cooking, where the chips are fed to a vessel where then a sequence treatment with different impregnation liquids and cooking liquids is performed on the chip stationary in the vessel.

The invention can also be applied in continuous boilers, where the addition of pre-evaporated black liquor is supplied to cooking liquids which flow in cocurrent or countercurrent with the chips in the cooker, in the beginning of the cooker, in the middle or in the final phases of the cooker.

List of drawings Figure 1, shows in principle how the invention is applied in a cooking system.

Detailed Description of Preferred Embodiments Figure 1 schematically shows a cooking system with associated recycling of cooking chemicals. The chips are first handled in a chip handling system 1 (Chip) where the chips can preferably be sprayed with steam for expelling air and first heating of the chips. Here, a first addition of process liquid takes place so that the chips are mixed in an fl ice-liquid mixture.

In a subsequent impregnation step 2 (BL-lmp), black liquor BL1 is added which has been removed from a subsequent cooking step. The black liquor can be added with the chips at the beginning of the impregnation step and accompany chips in so-called co-current treatment, after which the black liquor is removed from the impregnation vessel. This black liquor BL2 normally has a relatively low alkali content of about 5-20 g / l, and after a pressure relief in a cyclone / pressure relief vessel FL, the recovery is sent. The pressure relief normally takes place to a pressure level not exceeding 0.5 bar overpressure, and usually essentially at atmospheric pressure. The boiling process takes place at elevated pressure, normally 5-25 bar overpressure in the boiler and previous black liquor impregnation can be either pressurized or non-pressurized.

After the black liquor impregnation, the treated chips are transferred to the boiler (Cook) where cooking liquid Cliq is added. The chips are boiled in the cooking step at a boiling temperature which is in the range 150i20 ° C, after which the cellulose mass 0228se.doc = __ =, §fd'a = 5 ./- 1Z :. : transferred to further delignification and bleaching (Bleach), preferably via an intermediate wash (not shown). The recovery conventionally consists of a number of evaporation steps 5a-5e where the substantially pressure-relieved black liquor (BL2 after passing FL) undergoes evaporation in several steps from an initial dry matter level TS ice black liquor (BL2) of 17-20%, to a level above 70-80%. The evaporation line 5a-5e consists of a number of evaporation steps, so-called effects, where the black liquor flows through in succession, while heating steam is conducted in countercurrent to the black liquor flow. Normally, the freshest and warmest steam is used in the evaporation step which treats the black liquor with the highest dry content, ie. the last step seen in the flow of black liquor. Evaporated volatile substances are also obtained from each evaporation step, which are disposed of in special ash cradle handling systems (not shown) or turpentine recovery systems or alternatively led to other steps to be mixed in with the heating medium (steam).

The last step in the evaporation can also include superconcentrators 6, the interior is burned in a recovery boiler 7. In the recovery boiler a melt is formed which is taken out in the bottom (indicated in figure 1) and which after a slurry to green liquor 20 (not shown) is sent to a causticizing plant. regenerated.

Of course, the primary evaporation step, unlike that shown in Figure 1, may be a specially designed evaporation step which only evaporates the amount of black liquor to be returned to the impregnation or boiling step. Such a step can thus be placed next to the boiler and does not necessarily have to be arranged in connection with other evaporation steps before the recovery boiler. In accordance with the invention, a portion of the partially evaporated is taken out; black liquor PV_BL from e.g. the first step 5a and is led back to the cooking step.'z - š 30 As shown schematically in the fi guren, the pre-evaporated black liquor PV__BL v can be fed to a boiler circulation where cooking liquid is drawn from the boiler, normally via strainers in a continuous boiler wall, and returned to the center of the boiler via a central pipe at the height of the strainers. In this way, the pre-evaporated black liquor will be mixed with other treatment liquid before it is added to the cellulose material before its boiling in the actual delignification phase. In such an addition, the pre-evaporated black liquor can be added in a position in the boiler where the bulk delignification phase starts.

In an alternative, the pre-evaporated black liquor can be added to the cooking liquids CLiq to be charged to the digester before cooking. This option displays dash-marked flow lines. In this way, the pre-evaporated black liquor can be mixed with other cooking liquids before the supply to the digester, and at such an early stage that the pre-evaporated black liquor forms part of the cooking liquid already starting from the initial delignification phase of the cooking.

The invention can be modified in a number of ways within the scope of the claims. For example, the invention can also be applied in batch cooking of the chips filled on the vessel according to the sequence; 1) Filling the vessel with chips 2) Heating the chips with steam 3) Heating / impregnation with hot black liquor 4) Heating / impregnation with hot black liquor 5) Boiling with boiling liquor 6) Washing pressure after cooking where the expelled cooking liquid is stored in a tank for hot black liquor ; 7) Washing constriction after the previous step with washing liquid where the pre-expelled liquid is stored in a tank for hot black liquor.

Emptying the cooked and washed chips. In this type of cooking sequence, the pre-evaporated black liquor can be added to the cooking liquor added to the chips in step 5 above. Alternatively, a modified impregnation step according to 4 above may be used, in which the pre-evaporated black liquor is charged to the hot black liquor supplied in step 4, alternatively by flowing through the vessel with pre-evaporated black liquor, or a mixture with pre-evaporated black liquor and hot black liquor, to expel residual amount As a further alternative, the pre-evaporated black liquor can be charged to the boiling during the start of step 5, the pre-evaporated black liquor being introduced into a boiler circulation for mixing with the cooking liquid which is under circulation in the vessel during cooking. 0228se.doc: uz .Säd-a-'Wíz-E. In accordance with the invention, the pre-evaporated black liquor consists of a heat-treated black liquor having a dry content (TS) exceeding the dentor content level which can be obtained in the black liquor drawn from the process and then depressurized. This corresponds to the black liquor referred to in Figure 1 as BL2 and which is then pressure relieved in at least one pressure relief vessel, FL in figure 1. Normally the dry content TS in this liquid can be around 17-20%, and according to the invention the dry content TS on the pre-evaporated black liquor should be increased by at least 10% from this level to 27-30%.

Already with this moderate increase in the dry matter content, an improved effect is obtained on the delignification process during cooking. Preferably, black liquor is further evaporated to a dry matter content of at least 30-40%, and most preferably at least 50%.

The higher the dry content, the less the optimal liquid-wood ratio is affected, while the necessary investments of other cooking liquid shrimp need to be reduced.

The black liquor which according to the invention is pre-evaporated may consist of pressurized black liquor drawn directly from the digester or which is drawn indirectly via single black liquor impregnation, which may be either pressurized or substantially atmospheric, and then pre-evaporated. The black liquor can also consist of such black liquor which has passed a so-called reboiler, where the black liquor is first used to generate steam, before it is pre-evaporated in an inventive manner.

With a dry content of 27-30%, twice as much recycled pre-evaporated black liquor is normally required as if the dry content were around 60% for similar conditions in the boiler in general.

The amounts that are relevant for return to the boiler depend on the current dry content of the pre-evaporated black liquor, the current liquid-wood ratio during the boil, and the amount of black liquor that accompanies the chips from a previous black liquor impregnation, - current raw material (hardwood, softwood, single- annual plants, eucalyptus etc.) ø necessary investment of white liquor and its alkali concentration ø which other types of cooking chemicals are added. 0228sedoc 1,5 .§1.d; -87i2.:. .

With liquid-wood ratios in the boiler in the lower range around 3-3.5: 1, ie. 3 to 3.5 kbm of liquid on each kbm of chips, the amount of pre-evaporated black liquor with a 40% dry content should be at least 5% of the total amount of liquid, which corresponds to a minimum added amount of pre-evaporated black liquor of about O.15-O.175 kbm, calculated per kbm chips, in order to achieve a significant effect on delignification.

With higher liquid-wood ratios in the boiler in the range from 7: 1 and up to 8: 1, a corresponding increase of the minimum added amount of pre-evaporated black liquor of about 0.35-0.40 kbm, calculated per kbm of chips, may be required in order to have a significant effect on delignification. to be achieved. If the increased liquid-wood ratio in the cooking zone is established by internal circulation of cooking liquid, the required increase to obtain an effect on the delignification becomes smaller.

Thus, relatively moderate amounts of recycled pre-evaporated black liquor are sufficient, which normally amounts to a single percent of the total amount of liquid in the boiler. In a boiler where the cooking process is established with a very low proportion of residual black liquor from the impregnation, as much as 20-40% of the total amount of liquid can consist of pre-evaporated black liquor.

However, the invention is not unambiguously determined by the amounts that are recycled, as previously stated, other process parameters during cooking can affect the required amount, as well as the type of cellulose pulp (hardwood / softwood / annual plants, etc.) that is boiled.

The invention is based on the principle of, in direct contrast to other cooking methods, returning released organic material to the cooking process, which released organic screen material is enriched by first undergoing substantial evaporation in order to increase the dry content of the black liquor. Thus, the concentration of the organic material increases, which surprisingly has been found to affect the delignification process favorably and contributes to increasing the yield by the fact that mainly the hemicellulose in the pre-evaporated black liquor can be reprecipitated on the cellulose fibers.

It is essential that in order to obtain the best effect, the pre-evaporated black liquor should be present during the bulk of the bulk delignification phase, more than 50% of the chip residence time in the bulk delignification phase, and preferably already at the initial delignification phase. The recycled pre-evaporated liquid can be further heat-treated in a separate step and / or mechanically distinguish certain fractions and adjusted with respect to other chemical contents. 0228se.doc '-_, = ¿§då = 9 ./. 1-2.:. .:. :