FI85511C - Process for the recovery of aliphatic acids and sugars - Google Patents

Description

5 1 85511

This invention relates to a process for recovering lower aliphatic acids and sugars from the lignin digestion effluent of said acids, from which lignin has already been removed, and in particular to a process in which the waste liquor is heated to evaporate said acids.

The process according to the invention makes it possible to separate the lower aliphatic acids, and in particular formic acid, in order to recycle them and to recover the sugars for further processing from the lignin-depleted pulp of the pulp cooking process with these organic acids. It is already known to subject the waste liquor from such a pulp cooking process to direct evaporation, but it is not economically and technically possible to recover sugars from the lignin-sugar mixture obtained as a result of such direct evaporation. Direct evaporation of the waste broth has also failed to recover the acid esterified to the sugars, resulting in a loss of 25 acids.

Waste liquors from pulp cooking with inorganic cooking chemicals are known to be evaporated in several series-connected evaporation steps by indirectly heating each evaporation step with steam from one of the previous evaporation steps. However, in order to recover not only lower aliphatic acids but also sugars, the waste liquor from pulp cooking with these acids should be diluted prior to being passed to the next evaporation step to evaporate the organic acid in this next step. However, when water is used for dilution, large amounts of a very dilute acid solution are obtained, which it is not economically feasible to concentrate in order to recycle it back to the cooking process.

The object of the present invention is to obviate the above-mentioned drawbacks and to provide a process for recovering lower aliphatic acids and sugars from the waste liquor of pulp cooking with said acid, and evaporating according to the invention a concentrated effluent from the step, in which case either water or a very dilute aqueous solution can be used for the final dilution step. In this way, a concentrated acid solution is recovered from the first evaporation step, which can be recycled to the cooking process. In addition, the final evaporation step yields a valuable sugar-containing product which no longer contains substantial amounts of free or esterified acid and is therefore suitable as a raw material for sugar products. In this way, the acid content of the sugar solution decreases from step one to step and the last step results in a nearly acid-free sugar-lignin mixture and the acid is recovered from the first step in such a concentration that no expensive concentration methods are required.

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The energy costs of evaporation can be minimized in the countercurrent process according to the invention and other acid-water solutions can also be introduced for concentration as long as they are passed to a dilution step in which the concentration of acid-30 is almost the same as that of the solution to be concentrated.

In a preferred embodiment of the invention, the last evaporation step is heated with water vapor, the condensate of which is passed to the preceding final dilution step. The evaporation is preferably carried out at least in the first evaporation step under reduced pressure, whereby the temperatures can be kept low, preferably at a maximum of 70 ° C.

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With the process according to the invention, a concentrated aqueous formic acid solution can be separated from the first evaporation step after condensation, which can be recycled back to the pulp soup with formic acid.

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The invention is described in more detail in the following drawing, which shows a flow diagram of the process according to the invention.

In the accompanying figure, the evaporation steps are generally indicated by reference numerals 1, 2 and 3. It is clear that there may be several evaporation steps if the need so requires. The dilution step between evaporation steps 1 and 2 is denoted by reference numeral 4 and the dilution step between the second evaporation step 2 and the third evaporation step 3 is denoted by reference numeral 5.

The waste slurry, from which the lignin has preferably already been removed, is introduced in line 6 to a first evaporation stage 1, where it is indirectly heated by steam containing formic acid from the following evaporation stage 2. formic acid vapors 8 and concentrated formic acid solution 25 19. The sugar-containing effluent is then passed from the first evaporation stage 1 via line 7 to the first dilution stage 2 in the second dilution stage 10 and diluted with from there further in line 12 to a second dilution step 5, where it is diluted with water 14 before being passed through line 15 to a third evaporator. to stage 3. The steam 35 containing formic acid obtained from the third evaporation stage is passed in a pipeline 13 to the second evaporation stage 2.

From the third evaporation stage, a substantially CT) acid-free sugar solution 16 is recovered. The third evaporation stage 4 85511 3 is heated by water vapor 17, the condensate 18 of which can be passed to the second dilution stage 5 in the pipeline 14.

By the process described above, the formic acid 5 in the waste liquor 6 is evaporated and enriched in the steam 8 and condensate 19 leaving the first evaporation stage 1. At the same time, a substantially pure sugar solution 16 is recovered from the last evaporation stage 3.

The invention is described in more detail below by means of examples.

Example 1 57.1 g of lignin-depleted formic acid broth was evaporated at 70 ° C under reduced pressure to a dry matter content of 52.5% by weight. 38.2 g of 44.3% by weight of formic acid were recovered as a distillate. To the evaporation residue containing 41.4% by weight of formic acid was added 40.0 g of dilute (5% by weight) formic acid. The resulting mixture was stirred for about 20 and a half hours, after which it was evaporated to a dry matter content of 79.2% by weight. 34.7 g of 12.5% by weight of formic acid were recovered as a distillate. The liquid evaporation residue contained 22.3% by weight of free and 2.6% by weight of bound formic acid. The total formic acid yield in the distillates (including the lignin removal step) was 91.6% of the initial formic acid content of the waste liquor.

Example 2 360.9 g of lignin decantation broth was evaporated at 70 ° C under reduced pressure to a dry matter content of 33.8% by weight. 239.3 g of 34.1% by weight of formic acid were recovered as a distillate. To the evaporation residue containing 23.0% by weight of formic acid was added 250 ml of water. The resulting mixture was stirred for about an hour, after which it was evaporated to a dry matter content of 35.1% by weight. 252.9 g of 6.9% by weight of formic acid were recovered as a distillate. The residue with a formic acid content of 14.0% by weight was further evaporated to a dry matter content of 62.3% by weight.

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31.8 g of 14.3% by weight of formic acid were recovered as a distillate. The formic acid content of the liquid evaporation residue was 14.1% by weight. The total formic acid yield in the distillates (including the lignin removal step) was 95.7% of the initial formic acid content of the broth 5.

Example 3 440.3 g of lignin-depleted broth waste liquor were evaporated at 70 ° C under reduced pressure to a dry matter content of 31.6% by weight. 291.9 g of 38.9% by weight of formic acid were recovered as a distillate. To the evaporation residue containing 28.4% by weight of formic acid was added 150 g of dilute (15% by weight) formic acid. The resulting mixture was stirred for about half an hour, after which it was evaporated to a dry matter content of 52.4% by weight. 203.5 g of 26.1% by weight of formic acid were recovered as a distillate. To the evaporation residue containing 12.4% by weight of formic acid was added 85 g of water. The resulting mixture was stirred for about half an hour, after which it was evaporated to a dry matter content of 71.7% by weight. 109.0 g of 8.7% by weight of formic acid were recovered as distillate 20. The formic acid content of the liquid evaporation residue was 2.7% by weight. The total formic acid yield in the distillates (including the lignin removal step) was 94.7% of the initial formic acid content of the effluent.

Claims (4)

6 85511 A process for recovering lower aliphatic acids and sugars from the waste liquor of pulp cooking with said acids, from which lignin has preferably been removed, by heating the waste broth (6) to evaporate said acids, characterized in that the evaporation is carried out in several steps (1, 2, 3) countercurrently heating the acidic steam (9, 13) from the next step (2, 3) following the previous step (1, 2) and diluting the concentrated waste slurry (12) from the previous step (2) with the condensate (11) obtained from the previous step (2), the final dilution step (5) water or a very dilute aqueous solution (14) is used. Method according to Claim 1, characterized in that the last evaporation step (3) is heated by steam (17), the condensate (18) of which is passed to the last dilution step (5) preceding it. Process according to Claim 1 or 2, characterized in that the evaporation is carried out at least in the first evaporation step (1) under reduced pressure, preferably at a maximum of 70 ° C. Process according to one of the preceding claims, characterized in that a concentrated aqueous formic acid solution (8, 19) is separated from the first evaporation step (1) after condensation. 7 85511