SE511262C2 - Process for working up alkaline condensate emanated in cellulose pulp preparation - Google Patents

Abstract

The present invention solves the problem concerning the emission of considerable quantities of nitrogen oxides in the combustion of gases generated from stripper gases and/or generated methanol, and relates to a method of processing condensate generated in the manufacture of cellulose pulp by alkaline digestion. The invention is characterized in that acid is added to the condensate prior to stripper treatment, so as to lower the pH of the condensate to </=7.5 leading to retainment of the major part of the original nitrogen content of the condensate, and thereafter recovering stripper gases generated by the stripper treatment in a conventional manner, and passing cleansed, nitrogen rich condensate to a consumer station.

Description

15 20 25 30 35 511 262 stands for both batch and continuous digestion of lignocellulosic material. After digestion of the lignocellulosic material, the boiling liquor is separated from the resulting cellulose mass and this boiling liquor, commonly referred to as black liquor, (now usually mixed with bleached liquor) is evaporated before the black liquor in the form of thick liquor is burned, for example in a recovery boiler. Even when the black liquor is evaporated, gas mixtures of a similar type as the one described above are caused to leave the black liquor. These gas mixtures are caused to condense and a so-called evaporation condensate is formed.

These condensates must be disposed of, either individually or in admixture, and the present invention describes a process for working up said condensate.

Background Art The lignocellulosic material, for example wood, contains a number of chemical compounds including inorganic ones such as various nitrogen compounds. After the alkaline digestion, the majority of the nitrogen compounds are probably found as dissolved ammonia in the cooking liquid. Since both the cooking liquid and the boiling liquor are strongly alkaline, both the cooking steam and the evaporating steam contain a considerable amount of ammonia. Other volatiles included in the gas mixtures are, for example, methanol, various terpenes and sulfur compounds.

In addition to the substances mentioned, the gas mixtures naturally contain water vapor.

Both the boiler steam and the evaporating steam may, as previously stated, condense and the resulting condensate contains the above-mentioned chemicals and this content explains why the condensate is designated as impure.

The condensation takes place in such a way that the comparatively hot steam is brought into contact with a cooled surface in a condenser.

An alternative name for condenser is heat exchanger and it is usually a tube heat exchanger or spiral heat exchanger. Direct condensation with cold water also occurs.

The collected crude condensate, which is usually a mixture of boiler condensate and evaporation condensate, has a pH value exceeding 8 and the pH value is usually above 9.

Said collected impure condensate is passed to a steam stripper where the majority of said gaseous contaminants are evaporated from the liquid. This is called stripping and consists of the condensate being heated with steam in a column. The steam can be fresh steam or lye steam from some step in the lye evaporation plant. This results in typically over 90% of the methanol and sulfur compounds in the original, crude condensate being removed. Furthermore, almost the entire nitrogen content of the crude condensate is probably evaporated off, mainly as ammonia. The stripped compounds in the form of so-called stripper gases can either be led directly to destruction and / or combustion or caused to condense. The condensate formed in that position consists largely of methanol, which, however, is to some extent polluted by other components in the evaporated gas. This methanol is then used as fuel in a waste gas destruction furnace and / or in a lime kiln and / or in a recovery boiler or other boiler.

Upon combustion of stripper gases or methanol obtained in the manner described above, a significant part of the nitrogen content (probably mainly ammonia), often in the order of 40%, is converted to nitrogen oxides (NO 2). Emitting nitrogen oxides is negative from an environmental point of view, as they have both acidifying and eutrophication effects on the environment.

The purified condensate constitutes a very large volume of liquid and it is usually distributed to different positions in the pulp mill. One part usually goes to the chemical recycling and another part usually to washing the cellulose pulp, while the remaining amount goes to sewage.

Disclosure of the Invention Technical Problem There is a general effort to limit emissions of nitrogen oxides to the atmosphere as much as possible, as such emissions are negative from an environmental point of view for previously described reasons. For this reason, emissions of nitrogen oxides are taxed in some countries, which contributes to an increase in the cost of the pulp production process.

The solution The present invention contributes to a solution of the above-mentioned problems and relates to a process for working up condensate arising from the production of cellulose pulp by means of alkaline digestion, characterized in that the condensate is added acid before stripper treatment to lower the condensate pH to 7.5 leading to retention of the majority of the condensate's original nitrogen content and that after the stripper treatment stripper gases are disposed of in a conventional manner while purified nitrogen-rich condensate is discharged to the collection point.

By the term main part is meant that suitably at least 75% of the original nitrogen content of the condensate is retained in the condensate after the stripper treatment.

The term nitrogen-rich does not mean that the total amount of nitrogen in the purified condensate is large, but only that the amount of nitrogen is large in comparison with conventionally processed, purified condensate, where the amount of nitrogen is very small and sometimes almost zero. The pH of the crude or original condensate in the form of both boiler condensate and evaporative condensate and a mixture of these is above 8 and usually above 9.

It is preferred to add so much acid that the pH of the crude condensate is lowered to S 6.5. Any acid can be used and a suitable one is sulfuric acid.

According to the invention, it is quite possible to treat boiler condensate and evaporation condensate separately, respectively, in the manner described. However, it is preferred to first mix these condensates and then treat them in accordance with the invention.

The acid can be added to the two condensate streams separately or when these two are mixed. The acid can further be added to the mixing tank or in any position after the mixing tank, but before the stripper column. Of course, it is also possible to supply the acid in excess to one of the condensate streams so that when the two condensate streams are mixed a suitable pH value is obtained.

The lowering of the pH value of the condensate according to the invention leads to the dissolved ammonia present in the condensate apparently transferring gradually to ammonium ions (NH [), which predominantly remain in the condensate during the stripping.

All or part of the purified nitrogenous condensate can be taken to any collection point.

It is preferred that the condensate is transported to a place where nitrogen is needed, for example to a biological sewage treatment plant within the sulphate pulp mill in question, possibly in combination with a paper mill.

The purified nitrogen-rich condensate, or at least part of it, can also be added to the traditional collection points.

Advantages The process according to the invention prevents the formation of nitric oxide emissions from the combustion of nitrogen-rich stripper gases and / or nitrogen-rich methanol originating from crude condensate. This is especially important from an environmental point of view. If emissions of nitrogen oxides are subject to a fee, the advantage is that pulp production is cheaper to a certain extent.

In cases where the purified nitrogen-rich condensate can be used for some specific useful purpose, for example in biological treatment of sewage where there is a lack of nitrogen, the invention shows another advantage.

Figure description Figure 1 shows a flow chart in which condensate formed during alkaline pulp production is worked up in accordance with the invention.

Best Embodiment A preferred embodiment of the method according to the invention is described below with reference to Figure 10 15 20 25 30 35 511 262 1. In connection therewith, certain sub-steps of the method are described in more detail. Finally, an embodiment follows where the method according to the invention is simulated in a laboratory.

The process diagram according to Figure 1 shows a continuous boiler 1. To this boiler, lignocellulosic material, for example wood in the form of chips, and cooking liquid in the form of white liquor, possibly mixed with boiling liquor, are fed. The boiler has a high temperature, for example up to l70% L, which leads to a strong overpressure. The wood is digested by the cooking liquid under the described conditions into cellulose pulp, which is caused to leave the digester 1, usually in its bottom or in the vicinity of its bottom. The handling of the lignocellulosic material and subsequently the cellulose pulp during pulp production has not been included in the process schedule for reasons of space and clarity.

After digestion of the wood chips, ie at a certain place on the boiler l, the boiling liquor is removed under pressure. This is conveyed via the line 2 to a first flash tank 3 and further via the line 4 to a second flash tank 5. The main task of the flash tanks 3 and 5 is to gradually reduce the pressure of the boiling liquor from perhaps 0.7 MPa down to atmospheric pressure.

During this pressure drop, gas mixture of the type previously described is released. From the flash tank 5 the gas mixture is passed via the line 6 to a condenser 7. In the condenser 7, which can also be called heat exchanger, the hot gas mixture comes into contact with cold sheet metal walls or cold tubes (opposite side of the sheet metal wall or tube wall is usually cooled with comparatively cold water ) and the gas mixture condenses to a liquid or condensate. Another alternative is to exclude the said walls and tubes and bring cold water into direct contact with the gas mixture so that condensate arises. This condensate, called boiler condensate, is fed via line 8 to the collecting vessel or condensate tank 9. Gas mixtures of a similar type are taken out at several places in the boiler (not shown in the figure). An example of this is the basing vessel (not shown in the figure), which is located before the boiler itself 1. Since gas mixtures are taken out in several places in the boiler and all of these are caused to condense (and then taken to the condensate tank) so The resulting condensate is called cooker condensate.

At the bottom of the digester 1, bleach liquor is usually introduced which meets the digested lignocellulosic material in countercurrent.

By means of the bleaching liquor, the main part of the used cooking liquid, ie the boiling liquor, is displaced from the flowing cellulose pulp stream and out of the boiler 1 and the resulting mixture of bleaching liquor and boiling liquor, called black liquor and also thin liquor, is collected and then transferred to the evaporation plant. 10. In the evaporator 10, the dry matter content of the black liquor or barrel liquor is increased from perhaps about 18-20% to perhaps over 70%. This is done by steam from the recovery boiler indirectly heating the barrel liquor step by step, commonly referred to as power. In each power (or in direct connection thereto) a condensate called evaporative condensate is formed. In most effects (or in direct connection therewith) from unclean to heavily impure condensate occurs. From any power, or possibly in a pair, the condensate is relatively pure and then this condensate can be removed from the system and used for suitable purposes in the very extensive pulping process until the final bleached cellulose pulp.

For reasons of clarity and simplification, the process diagram shows the removal of only one crude gas mixture from the evaporator 10. This is passed via line 11 to a first condenser 12 and via line 13 to a second condenser 14. The condensate formed in these two positions is passed via line 15 to the condensate tank 9. The condensate described here is called evaporation condensate.

The distribution between boiler condensate and evaporation condensate varies from factory to factory and a certain variation over time can also occur in one and the same factory. The temperature of the condensate mixture in the tank 9 usually stays in the range 50-60¶3. The pH of the condensate mixture can vary, but is usually higher than 9 and for example around 9.5.

In accordance with the invention, an acid, for example sulfuric acid, is added to the condensate mixture via line 16. So much acid is added that the condensate mixture has a pH of 7.5 or lower. According to a preferred embodiment of the invention, so much acid is added that the condensate mixture has a pH of 6.5 or lower.

From the bottom of the tank 9 the pH-adjusted condensate mixture is discharged and fed via line 17 to the heat exchanger 18. From this the condensate is fed via line 19 to the stripper column 20. The temperature of the condensate in this position is typically 95-110% L Water vapor, fresh steam and / or steam from any position in the evaporation, the stripper column 20 is supplied at its bottom via the line 21 to strip (drive) of previously mentioned chemicals in vapor form, which chemicals are perceived as and constitute contaminants.

The temperature of the condensate at the top of the stripper column 20 is typically 100-120% L. The amount of, for example, fresh steam supplied via line 21 is called steam ratio and this can typically amount to 0.2, which means a supply of 40 tonnes of steam per hour at a supply of 200 tons of condensate per hour via line 19. The degree of stripping in the stripper column 20 is usually controlled by determining the content of chemical oxygen-consuming substance = COD (Chemical Oxygen Demand) partly in the incoming condensate in position 19 and partly in the outgoing purified the condensate in position 22. The mean value of nine measurements over a period of time showed that the COD in the incoming condensate was 10756 mg / l and in the outgoing condensate was 498 mg / l. In the case of impurities in the condensate measured as COD, according to this about 95% was removed by the stripper treatment and usually the stripper treatment is controlled so that at least 90% of the impurities are removed from the crude condensate, measured as COD.

The condensate present at the bottom of the stripper column 20, which is freed from the majority of said impurities and is therefore called pure condensate, is taken out of the column 20 at its bottom and passed via line 22 to the heat exchanger 18. Thanks to the heat exchange, the temperature of the outgoing purified the condensate to typically 65-70W3 and this is led via line 23 to a suitable pick-up point. Due to the lowering of the pH of the condensate included in the stripping in accordance with the invention, the majority of the nitrogen of the original and constituent constituents is retained in the outgoing purified condensate.

Outgoing stripper gases are passed via line 24 to condenser 25. Some of the gases are condensed in that position to condensate and this is returned to stripper column 20 via lines 26 and 19. As gas residues, condenser 25 leaves at its top and is passed via line 27 to methanol column 28 The amount of gas entering the methanol column 28 can be about 2 tons per hour. Inside the column 28 a part of the introduced gas changes to a liquid form, which means that somewhere inside the column there is a boundary between a lower liquid phase and an upper gas phase. The temperature at the top of the methanol column 28 is typically 70-75% L Water vapor, fresh steam and / or steam from any position in the evaporation, is fed to the methanol column 28 via line 29. The amount of water vapor supplied can be 500 kg per hour. Mainly gaseous methanol is evaporated from column 28 and passed via line 30 to partial condenser 31. In it some gas condenses to condensate and this is passed via line 32 back to methanol column 28. At the bottom of the methanol column 28 a certain amount of liquid is drawn off which via line 33 is passed to a decanter or turpentine separator 34. As the name implies, turpentine is separated in this decanter and the turpentine is passed via line 35 to the vessel or tank 40. Residual liquid in decanter 34 is passed via line 36 back to the stripper column 20.

The temperature in the partial condenser 31 is typically 63-67% 3. Residual substances in gaseous form are fed via line 37 to a so-called methanol condenser 38. The temperature in this methanol condenser is typically 30-35 ° C, which leads to essentially all the gaseous methanol condensing to a liquid which is fed via line 39 to the methanol tank 40. .

From the top of the methanol condenser 38, the gases which are not condensable at the described temperature are passed via line 41 to a gas boiler 42, where the malodorous, sulfur-containing substances which constitute the main part of said gas mixture are burned and destroyed. In the work-up of the crude condensate according to the invention described here, the nitrogen content (mainly in the form of ammonia) in said gas mixture is extremely small and possibly even down to zero.

The liquid supplied to the methanol tank 40 via line 39 usually consists of over eighty percent of methanol.

The methanol in liquid form is passed via lines 43 and 44 to the gas boiler 42 to serve as fuel in the gas boiler for combustion and destruction of the previously mentioned malodorous gases. As a rule, more methanol is formed than is needed as fuel in the gas boiler 42 and the excess is fed via lines 43 and 45 to the lime kiln 46, where it is used as a support fuel in the transfer of the unburned lime (mesa) to burnt lime. The said methanol is also substantially nitrogen-free.

Since both the malodorous gas mixture and the methanol are substantially nitrogen-free, said combustions of these substances lead to at most a very low formation and emission of environmentally unfriendly nitrogen oxides via the exhaust gases in the respective position.

The process diagram according to Figure 1 shows a common way of handling the stripper gases, i.e. from position 24 onwards in the process. There are alternative ways of handling said stripper gases and the method according to the invention is also applicable to these, although these methods are not described in detail in this patent document.

In said process diagram, the cooker condensate comes from a continuous cooker. Of course, the cooking condensate formed during batch cooking can also be treated in accordance with the invention.

Example 1 In a sulphate pulp mill a certain amount of crude condensate was taken out, which condensate was a mixture of boiler condensate and evaporation condensate. The condensate was divided in a laboratory into aliquots of 100 ml.

The original condensate had a pH of 9.57 and various amounts of 0.1 molar sulfuric acid were added to the aliquots leading to a series of samples with gradually decreasing pH values.

In the laboratory, a simulated stripping of each sample was performed by refluxing in a round-bottomed flask equipped with a cooling device for a period of 5 minutes. When the sample cooled, its pH value and its content of dissolved ammonia plus ammonium ions were measured. The ammonia plus ammonium content of the original condensate was also determined.

Ammonia plus the ammonium content in the samples and the original condensate were determined with Dr LANGE Kyvett test LCK 303, edition 95/07.

Measured pH values and both ammonia plus ammonium content and nitrogen content (recalculated) of the test series are shown in the table below.

N ma !! 1 (no acid additive) 57.2 2 9.24 112.0 87.0 3 9.13 122.0 94.7 I 4 8.98 140.0 109.0 5 8.85 152.0 118.0 6 806 1610 1250 7 8.48 173.0 134.0 8 7.78 212.0 165.0 9 6.98 221.0 172.0 10 6.61 236.0 183.0 11 SAS 2540 1970 I Originals of NH 3 + NH 4 'was 255.0 g / l and the content of N was 198.0 mg / l.

As can be seen, at a pH value of just under 8 eighty percent of the nitrogen remained in the condensate after the simulated stripping of the condensate. At a pH of 5.5, substantially all of the nitrogen remained in the condensate after the simulated stripping of the condensate.

The nitrogen which in the various samples departed from the samples and the round flask is assumed to have left the samples and the round flask in the form of gaseous ammonia.

The experiments indicate that even on a technical scale it is possible to maintain the main part of the nitrogen content of the original condensate in the condensate which has been subjected to stripper treatment, if it is carried out in accordance with the invention.

Claims (8)

10 15 20 25 13 .511 262 EUTTEIPPKIHNV Process for the work-up of condensate resulting from the production of cellulose pulp by means of alkaline digestion, characterized in that the condensate is added acid before stripper treatment to lower the pH of the condensate to S 7.5, leading to the retention of the majority of the condensate's original nitrogen content. that after the stripper treatment, stripper gases are disposed of in a conventional manner while purified nitrogen-rich condensate is discharged to the collection point. Process according to Claim 1, characterized in that the condensate is added to the condensate before stripper treatment to lower the pH of the condensate to s 6.5. 3. A method according to claims 1-2, wherein the condensate consists exclusively of evaporative condensate. 4. A method according to claims 1-2, wherein the condensate consists exclusively of boiler condensate. 5. A method according to which the condensate is according to claims 1-2, of a mixture of evaporating condensate and boiling condensate. 6. A method according to claims 3-5, characterized in that the acid is supplied to the respective condensate stream or when these two streams are mixed or in a position after the mixing point and before the stripper treatment. 511 262 14 7. A method according to claims 1-6, characterized in that all or part of the purified nitrogen-rich condensate is fed to a biological sewage treatment plant. A process according to claims 1-6, characterized in that all or part of the purified nitrogen-rich condensate is used as washing liquid in the cellulose pulp production and / or is used in the chemical recovery system.