SE460543B - PROCEDURE FOR ACTIVATING LIGNOCELLULO MATERIAL WITH NITROGEN Dioxide CONTAINING GAS - Google Patents

Abstract

Method of activating aqueous lignocellulosic material with a gas containing nitrogen oxide, followed by a delignification stage for the purpose of obtaining a high quality end product with no or only a slight effect on the environment. In accordance with the inventive method, the lignocellulosic material is brought into contact (18) with an oxygen-containing gas (19), and activating gas is separated from the lignocellulosic material during and/or subsequent to the activating process (9). The method is characterized by a) controlling the supply of oxygen-containing gas (12) such that the separated gas (27) contains at least 2 kg nitric oxide (NO) calculated on 1000 kg of dry lignocellulosic material, and by b) reacting the separated gas (27) in one or more stages (26) with absorption solution (25) whose original pH lies within the range 3-13.5, and by passing (28) the gas purified with the aid of absorption solution to atmosphere, or to a destruction plant optionally subsequent ot further purification of the gas.

Description

460 545 The activation process is affected by a number of factors.

These include pulp consistency, amount of nitrogen dioxide invested, time and temperature. Also different temperature profiles, ie. different temperatures at different stages of activation affect the end result. In addition, the nitrate content and the hydrogen ion content during activation are of crucial importance for the activation process. By supplying nitrate and hydrogen ions to the activation step, the need for supply of expensive nitrogen dioxide can be drastically reduced. By optimizing, among other things, the above-mentioned parameters, one can also optimize the selectivity in the delignification of the cellulose pulp. This can be used to push delignification extremely far.

The activation process is also affected by the addition of oxygen-containing gas, for example oxygen. From an environmental point of view, it is always advantageous to add oxygen, as it has been shown. that the gas at the end of the activation then always has a low content of both nitrogen dioxide (NO2) and nitrogen oxide (NO). Furthermore, it is so. that under certain conditions during the activation of the cellulose mass, the oxygen addition contributes to an improved selectivity in the delignification of the cellulose mass.

Disclosure of the invention Technical problem However, it has been found that under other conditions the addition of too much oxygen can lead to a reduced selectivity in the delignification of the cellulose pulp. A low addition amount of oxygen in turn leads to environmental problems, since the gas after completion of activation 'in these cases has a high content of special nitric oxide (NO).

The solution of the present invention solves these problems and relates to a process for the production of cellulose pulp, in which 460,545 aqueous lignocellulosic material after and / or during the supply of oxygen-containing gas is subjected to activation in at least one step with nitrogen dioride ( NO 2) containing gas following delignification of the lignocellulosic material 1 at least one step and wherein gas is separated from the lignocellulosic material during and / or after activation. The process is characterized in that the supply of oxygen-containing gas is controlled so that the separated gas contains at least 2 kg of nitric oxide (NO) based on 1000 kg of dry-solid lignocellulosic material and that the separated gas is reacted in one or more steps with absorption solution. whose initial pH is in the range 3-13.5 and that the absorption solution after reaction with the separated gas is recovered and fed to the lignocellulosic material before and / or during the activation and that the gas purified by means of absorption solution after any further purification is carried to the atmosphere or to any destruction facility.

The mixture of the lignocellulosic material (hereinafter cellulosic pulp) with water must be such. that the pulp consistency upon activation is in the range of 2-80%. preferably 3-40%, preferably 5-30%.

Nitrogen dioxide (NO2) is added to the activation either as a substantially pure side or father is formed just before or in the activation reactor by adding nitrogen oxide (NO) and oxygen. Both nitrogen dioxide and nitric oxide can be added to the same cellulose pulp. Nitrogen dioxide (N20¿) and other polymeric forms of nitrogen oxides are also counted as nitrogen dioxide. One mole of nitrous oxide is counted as two moles of nitrogen dioxide. Additions products. in which nitrogen oxides are present. > Nitrogen dioxide (N2O3) is thus counted as one mole of nitrogen oxide and one mole of nitrogen dioxide. Addition products with the name Q 'oxygen probably occur as intermediates; Also counted - nitric acid (HN02). (as »active nitric oxide. The nitric acid, like nitrous oxide, is volatile and difficult: Vanal2tically distinguishes from frank nitrogen dioxide and nitrogen oxide. Nitric oxide '(N20), on the other hand, does not count as active nitric oxide. 460 543 depending on the lignin content of the cellulose pulp, tolerable attack on the carbohydrates of the pulp pulp and the desired degree of delignification.Calculated as monomers, the amount invested normally amounts to 0.1-2 kilograms per 100 kg of lignin in the cellulose pulp.

The temperature during activation can be selected relatively freely, for example in the range 20-110 ° C. If the activation is performed in one step, the optimum temperature is in the range 50-95 ° C. If the activation is divided into two steps, a temperature in the range 25-40 ° C in the first step and a temperature in the second step are preferred.

The time is partly dependent on the temperature. If the pH value is extremely low and the temperature is high, a short time must be selected. In other cases, activation results usually improve for a long time. l The amount of oxygen supplied before and / or during the activation of the cellulose mass must be kept low. According to a preferred embodiment of the invention, no intentional addition of oxygen-containing gas takes place during the activation of the cellulose mass. A certain amount of air always accompanies the cellulose mass into the activation unless special measures are taken and that amount of oxygen is often sufficient. In some cases, it may even be necessary to reduce the amount of air that accompanies the pulp. The removal of air can take place by compressing the cellulose pulp before introduction to the activation step or by heating and / or evacuating the cellulose pulp. In the event that nitric oxide (NO) is used as active nitric oxide, oxygen is preferably supplied only in an amount. which is less than the stoichiometric amount for the oxidation of nitric oxide (NO) to the nitrogen axis (NO2). within the range 80-100 ° C in the 'nen / or 460 543 The gas separated from the activation - which has a certain minimum content of nitrogen oxide (No) - is taken over for nano for treatment. This consists of at least two steps, namely the addition of oxygen-containing gas and the gas being reacted with an absorption solution. Usually the oxygen is added first, but it is quite possible to perform these two steps in one and the same treatment step.

In order to achieve optimal results, it has been shown that the amount of oxygen invested should amount to 0.10-0.35, preferably 0.20-0.28 mol O2 calculated per mole of nitric oxide (NO) in separated gas. As the absorption solution, any solution having a pH value in the range of 3-13.5 and which has the ability to highly liberate the separated gas from nitrogen oxides can be used.

According to preferred embodiments of the invention, either of the two solutions is used as the absorption solution. It is particularly convenient to use one solution in an absorption step followed by a second absorption step where the other solution is used.

One solution may be slightly acidic up to neutral and consists of liquor from the activation of the cellulose mass.

The solution may consist only of such effluent or also of said effluent mixed with another liquid. for example a liquid which raises the pH of the finished solution. The second solution has a pH value in the range 7-13.5 and is suitably constituted by liquor from delignification of the cellulose pulp by means of alkali. Particularly preferred is the effluent from an alkaline oxygen bleaching step and especially that from oxygen bleaching of nitrogen dioxide (NO 2) activated cellulose pulp according to the invention. This leads to low consumption of active nitrogen oxides and that precipitation of lignin avoids the solutions "can be used to advantage for impregnation dilution of the cellulose mass just before and .i.e. Treatment" with nitrogen goose. especially during axclvazlngen. ßfdiox (NO2) containing gas. 460 543 The pH values specified in this document refer to measurement with a glass electrode on a sample, which is cooled to room temperature (approx. 20 ° C) without evaporation. In samples taken during activation, the cellulose mass is separated before pH determination. If samples are taken at a pulp consistency exceeding 8%, dilute with pure water to 8% pulp consistency, after which the cellulose pulp is separated. stated pH values for absorption solutions, for example different liquors, refer to cooled undiluted samples.

With regard to the purification of the separated gas, good results are obtained if the relative amounts of the separated gas and the absorption solution and possibly the amount of oxygen (alternatively oxygen-containing gas) are adjusted so that the pH value of the absorption solution used is in the range 5-12.

The recovery of active nitric oxide is significantly beneficial when using absorbent solution within this pH range. When preparing this absorption solution, it is preferred that the absorption solution is only used for impregnation and / or dilution of the cellulose mass before the start of activation.

Advantages Through the process according to the invention it has been found satisfactory that in comparison with the prior art it keeps the consumption of both oxygen and newly supplied nitrogen dioxide (alternatively nitric oxide) at a low level while maintaining high selectivity during the delignification of the cellulose pulp.

Using certain conditions, it has even proved possible to achieve a slightly improved pulp quality. for example, improved strength properties despite low consumption of nitrogen oxides. in comparison with prior art technology. All this with little or no environmental impact.

Fig. schemes, a number of other parameters for the procedure according to the invention will be specified.

. In the method according to the invention shown in Figure 1, for example unbleached chemical cellulose pulp is conveyed via the line 1 to a lye removal plant 2. for example a press.

The cellulose mass can be filtered or unfiltered and is normally freed from the main part of the cooking liquor supplied from the digester. The boiling liquor is usually displaced with bleach liquors, for example oxygen bleach liquor. It is also possible to use a certain part of the liquor from the activation for this purpose.

When the cellulose pulp is introduced into the press 2, the pulp consistency is usually low (a few or a few percent). In press 2, the mass consistency is satisfied. for example to 30% and above.

Squeezed liquor is transported via line 3 for use in, for example, washing and / or dilution of unbleached pulp. Via the line 4, the cellulose pulp is conveyed to the dilution plant 5. Instead of a line 4, a precipice or a transport screw can be used in this position. In plant 5, the pulp consistency is reduced to, for example, St. The cellulose mass is then passed through line 6 (for example by means of a pump) to a mixer 7. Via line 8 a nitrogen oxide, for example nitrogen dioxide (NO 2), is supplied.

The desired temperature during activation is usually obtained by blowing steam into the cellulose pulp stream. The cellulose pulp is then passed upstream of the activation reactor 9. The temperature at which the activation is allowed to take place depends on a number of other parameters, but a temperature in the range 50-95 ° is usually used. At this embodiment of the invention, temperatures of about 90 DEG C. are particularly applicable to the actuation step shown in the actuation process. The height and flow rate of the reactor 9 determine the residence time (the bonding time), which is normally 60 DEG C. 360 minutes. At temperatures of about 90 ° C, temperatures of less than 90 ° C have been found to give a flow rate of 121 m. At the top of the reactor 9, nitric oxide (fl Ql ~ containing gas is separated from the cellulose pulp suspension. is fed via line 10 to an oxidation reactor 11. In this reactor, the gas containing, inter alia, nitric oxide (NO) is reacted with an oxygen-containing gas, preferably oxygen gas, which is supplied through line 12. The amount of oxygen charged amounts to 1 suitably to 0.10-0.35 mol O2 calculated per mole of nitric oxide (NO) in the gas transferred to the reactor II.

In this embodiment of the process according to the invention, no oxygen-containing gas is intentionally added to However, a greater or lesser amount of air always follows with the cellulose mass into the activation reactor. By means of the apparatus arrangement shown in Figure 1, it is possible to keep the amount of accompanying air (and thereby also the amount of oxygen supplied) at a suitable level. I 4 In order to obtain optimal activation effect, it is important to check and control the pH of the cellulose pulp before and during activation. In position 7, i.e. just before nitric oxide in any cellulose mass. form is supplied via line 8. the cellulose pulp normally has a pH value in the range 5-12. A pH value in the range 5.5-8 is preferred. After. the pH drops and it has been shown. that the cellulose mass during the final stage of activation and thereafter should have a pH value in the range 1.5-4.5. Particularly good results have been obtained at a final pH of 1.0-2.8; Appropriately low pH values can be achieved in several ways. One way is to add nitric acid or some other acid, preferably a strong mineral acid. Addition of an elite amount of oxygen, for example 0.5-2 kg per 1000 kg of the cellulose pulp provided with lignin can also be used to lower the pH value. that the nitric oxide has been charged, pressed and / or laid through line 23 for further treatment. 460 543 The cellulose pulp leaves the activation reactor 9 at its top and is fed via the line 13 to a lye separation plant 14. This can be constituted by a press by means of which a considerable part of the activation liquor is removed from the cellulose pulp. Instead of squeezing the liquor out of the cellulose pulp, it can be displaced by means of a liquid, preferably recycled activating liquor and / or fresh water. The displacement liquid may also contain bleach liquor, for example oxygen bleach liquor. The effluent is removed via line 15 to the dilution plant 5 and / or to the cellulose pulp in a position before said plant. The cellulose mass is then fed to the impregnation plant 16 where alkali of some kind, for example sodium hydroxide, is supplied. If oxygen bleach liquor has not already been added to the cellulosic pulp, it is suitable to add such, preferably in connection with the addition of a magnesium salt. Via line 17, the cellulose mass is caused to pass an intensive mixer 18 to which oxygen is introduced through line 19. The oxygen is dispersed in finely divided form in the cellulose mass suspension, the consistency of which is suitably in the range 5-10. The suspension passes upstream of the oxygen bleaching reactor 20. The oxygen pressure at the bottom of the reactor 20 is determined to some extent by the height of the reactor. The pressure of the supplied oxygen can be chosen freely. that the oxygen pressure in the top of the reactor 20 is level with the atmospheric pressure or higher than the atmospheric pressure.

It is quite possible to use a pulp consistency higher than 108 during the oxygen bleaching, including so-called high-consistency oxygen bleaching. The cellulose pulp is further transported through the line 21 to the lye separation plant 22, where the cellulose pulp is freed from oxygen bleach liquor in a known manner, for example by washing. The cellulose mass leaves, '46 Û 543 A certain part of the oxygen bleach liquor is transported by means of the pump 24 through the line 25 to the top of a gas absorption plant (scrubber) 26. The oxygen bleach liquor normally has a pH value in the range 9-12. The gas from the oxidation reactor 11 is fed to the bottom of the scrubber 26 via the line 27.

By contact between the gas and the absorption liquid, the gas is released from the majority of its content of nitrogen oxides. If a very high degree of absorption is achieved in the scrubber 26, it may be possible to transport the treated gas via the line 28 to the atmosphere. However, this is not preferable. without another 'alternative is to transport the gas to a recovery boiler where i.a. coke liquor is burned. In the case of very high environmental requirements, it may be necessary to subject the purified gas in line 28 to further purifying treatment before transport to the atmosphere or the recovery boiler.

The absorption liquid containing several nitrogen compounds is taken out at the bottom of the scrubber 26 and transported via the line 29 to the dilution plant S. A larger or smaller amount of the absorption liquid can also be used for other purposes. However, it is preferred. that the absorption liquid is supplied to the cellulose mass in some position before the activation reactor 9 for utilization of the existing nitrogen compounds during the activation process.

It may be advantageous to recycle both gases and liquids to the various stages even if this is not shown in the figure. For example, a part of the gas stream in the line 10 and / or the line 27 can be returned to the activation reactor 9.

'Furthermore, it is preferred. that a part of the gas discharged through the line 28 is returned to the activating reactor 9. On the way to the reactor 9, oxygen can be supplied to said gas stream.

In addition, a part of the absorption solution at the bottom of the scrubber 26 can be recycled to the scrubber and preferably pd-pl 'one or more places along the mantle surface of the scrubber in. <»§1fl l @@: akn - = - tte §fïVfDen shown in figure 2 The preferred embodiment of the method according to the invention initially corresponds to the embodiment of the method 1> 1 * 1 described in Figure 1; §.fi f ~ lr ° PP f.4 flfl i flflfi ß- í,,. Chemical pulp is transported via line 30 to a liquor removal plant 31. Lye removed from the pulp is transported away via line 32. Via line 33, the pulp is conveyed to a dilution plant 34. Thereafter, the pulp is passed through line 36 to a mixer pulp via a pulp 36. line 37 comes into contact with nitric oxide (NO) and oxygen (02). The molar proportion between these gases can be, for example, 2.S: 1. With the addition of these gases, the activation process starts. In this case, the activation is divided into two steps with intermediate dilution.

The cellulose mass at a consistency of, for example, 10-15% is caused to pass upstream of a first activation reactor 38. The temperature in this step can advantageously be comparatively low and the time comparatively short. For example, a temperature of 35 ° C and a time of 20 minutes can be used. Via the line 39, the cellulose pulp is transported to the diluting device 40 where the cellulose pulp is diluted, for example to a consistency of 4-9 μm. Thereafter, the pulp is passed through line 41 to a second activation reactor 42. In this second activation step, the temperature should be high (for example 90 ° C) and the time long (for example 180 minutes). This second treatment step can be called the maturation step. The cellulose pulp is then fed via line 43 to a gas separator 44, for example a cyclone. The separated nitric oxide-containing gas is passed via line 45 to an oxidation reactor 46, with a connected line 47 for supplying oxygen. å | ~, »d" ä ° ÜVf§§Bfter .gasavskiljandet 'is transported by the cellulose pulp in Ä ^ 30 through: line 48 _to a lye separation plant 49. In l v.anläggningen 50 the cellulose pulp is impregnated with alkali necessary for oxygen gasification. > for example "'in the form of' sodium- 'k .y. lÅÜhydro; id.joch_eventuell protector. for example in the form of an É,' g '*} f§mag§esium salt. The oxygen bleached_ken is also added to cellulose ... then the positions 49 and 50. The cellulosic mass is then transported to an intensive mixer through the line 52. where R without these parameters together with oxygen pressure and alkali investment can be selected according to conventional techniques.

From the oxygen bleaching reactor 54, the pulp mass is transported through line 55 to a lye separation plant 56. After separating oxygen bleach liquor from the pulp mass, it is transported via line S7 to some further treatment site, for example one or more final bleaching steps.

A pump 58, which has a pH value of 9-12, is transported by means of a pump 58. via line 59 to the top of a first gas absorption plant 60 (scrubber).

The liquid in question is atomized in a known manner with, for example, nozzles being caused to pass through the scrubber. as a thin hydrogen film on solid bodies inserted into the scrubber, for example saddle bodies or so-called Rapid rings. The gas from the oxidation plant 46 is passed via line 61 to the bottom of the scrubber 60. Purified gas is taken out of the scrubber 60 and is passed through line 62 to the bottom of a second gas absorption plant 63 (scrubber). Additional oxygen is supplied through line 64, which is connected to line 62. Just before the connection line 64 or thereafter, a portion of the gas flowing through line 62 may advantageously be separated and returned via a line to any of the (or both) activation reactors 38 and 42. Via line 65, absorption liquid is carried from the liquor separation plant 49 to the top of the scrubber 63. This liquid has a pH value which is within the pin range 3.5-6; 5. The cellulose pulp suspension introduced into plant 49 has “fl stlgezj3. 1 den fail. normally a pH value, which is clear while using g§§§f§y1gas§lekavlut as a displacement liquid, a pH f #; Y§rdeèpä avluten is obtained within the above specified range. If a "pressure" is used to "remove" the activation effluent from the ßfff35 "Lfcellulose mass, a fi. '“1: * =. 1f = arsn1 = * n« = l; i'_1 »íÉ; ~ _ = f» a2 _, - ï s * s “" fbra; ¿som Éabsorb fi iønsvatska -ii .... / \ - Vf ....., .., _._____..._ ~ _. ,, _ .., _ ,, _.___-, ___. «-.__-._, ._ tz; The gas purified in two stages is taken out of the system via the line 66 for transport to, for example, the atmosphere or a recovery boiler or other combustion plant. It is also possible that before the final removal of ' the gas purify the same in. any third purification step.

Absorption solution from the first scrubber 60 is passed through line 67 to the diluent 34 and absorption solution from the second scrubber 63 is passed through line 68 to the diluent 40.

In the process described above, a residual gas is obtained which is highly purified, i.e. its content of nitrogen oxides is almost negligible at the same time as two absorption solutions eznalles, which are not effectively utilized via the activation. By adding these to the cellulose pulp before and during the activation of the cellulose pulp with nitrogen dioxide (NO 2) containing gas, an activated cellulose pulp is obtained, which in a subsequent treatment step can be delignified 1 (for example an oxygen bleaching step) in a highly selective manner. It has been found possible to lower the lignin content of the cellulosic pulp from a kappa number of 30-35 to 3-4 at a maintained viscosity of about 950 dma / kg if the method described above is applied.

Claims (8)

10 15 20 25 30 460 543 PATENT REQUIREMENTS 1. A process for the production of cellulosic pulp, wherein aqueous lignocellulosic material and / or during the addition of oxygen-containing gas is subjected to activation in at least one step with nitrogen dioxide (NO 2) containing gas followed by delignification of the lignocellulosic material in at least one step and wherein gas is separated from the lignocellulosic material during and / or after the activation, characterized by the combination, that the supply of oxygen-containing gas is controlled so that the separated gas contains at least 2 kg of nitric oxide (NO) calculated on 1000 kg of dry-thinking lignocellulosic material and that the separated gas is reacted in one or more steps with absorption solution, the initial pH of which is in the range 3-13.5, and that the absorption solution after reaction with the separated gas is recovered and added to the lignocellulosic material before and / or during activation and that the gas purified by means of absorption solution after any further purification f to the atmosphere or to any destruction facility. Process according to Claim 1, characterized in that the separated gas is supplied with oxygen-containing gas before and / or during the treatment with the absorption solution. A method according to claims 1-2. k 8 n n e t e c k - n a t d 8 r a v. that the absorption solution as the main component contains effluent from the activation of the lignocellulosic material. 'A' V 'ï' 4. A method according to claims 1-2, characterized in that the initial p-value of the absorption solution is in the range 7-13.5. 10 15 k ä n n e t e c k - Process according to Claim 4, in which the absorption solution contains as a main component effluent from delignification in alkaline medium of lignin-cellulose material activated with nitrogen dioxide (NO 2) containing gas. 6. A method according to claim 5, characterized in that the absorption solution consists of effluent from alkaline oxygen delignification. 7. Process according to claims 1-6, characterized in that the relative amounts of the separated gas and the absorption solution and possibly the amount of oxygen-containing gas are adjusted so that the pH value of the absorption solution used is in the range 5-12 . A method according to claims 1-7. It is known from this that a part of the separated gas, which has been reacted with absorption solution in at least one step, is returned to the activation of the lignocellulosic material after any addition of oxygen.