WO1991018863A1

WO1991018863A1 - Process for the recovery of aliphatic acids and sugars

Info

Publication number: WO1991018863A1
Application number: PCT/FI1991/000167
Authority: WO
Inventors: Kari Saari; Antti Vuori; Tapio Mattila
Original assignee: Kemira Oy
Priority date: 1990-05-29
Filing date: 1991-05-28
Publication date: 1991-12-12
Also published as: FI902677A0; FI85511C; FR2672065A1; FI85511B; FI902677A

Abstract

The invention relates to a process to a process for the recovery of lower aliphatic acids, such as formic acid, and sugars from the spent cooking liquor from a pulping process carried out with these same acids, lignin having been removed from the spent liquor. According to the invention, spent liquor is evaporated indirectly in a plurality of steps (1, 2, 3) by heating a preceding step (1, 2) with an acid-containing vapor (9, 13) obtained from the subsequent step (2, 3) and by diluting (4, 5) with the obtained condensate (11) the concentrated spent liquor (12) from the preceding step (2), water or a very dilute aqueous solution (14) being used in the last dilution step (3).

Description

Process for the recovery of aliphatic acids and sugars

The present invention relates to a process for the recovery of lower aliphatic acids and sugars from spent cooking liquor from a pulping process carried out using the said acids, lignin having already been removed from the liquor, and particularly to a process in which the spent liquor is heated in order to evaporate the said acids.

By the process according to the invention, lower aliphatic acids, and particularly formic acid, can be separated for re¬ cycling, and the sugars can be recovered for further refining from a spent liquor from a pulping process carried out using these organic acids, the lignin having been removed from the liquor. It is previously known to subject spent liquor from such a pulping process to direct evaporation, but the recovery of sugars is not economically or technically feasible from a mixture of lignin and sugar obtained from such direct evapora¬ tion. It has also not been possible by direct evaporation of the spent liquor to recover the acid esterified in sugars; this has resulted in losses of acid.

It is^' known that spent liquors from pulping processes carried out using inorganic cooking chemicals have been evaporated in a plurality of evaporation steps coupled in series, by heating each evaporation step indirectly by means of vapor obtained from some previous evaporation step. In order to recover not only the lower aliphatic acids but also sugars, the spent li¬ quor from a pulping process carried out using these acids should, in a manner deviating from the state of the art, be diluted before it is directed to the subsequent evaporation step for evaporating the organic acid in this subsequent step. However, when water is used for the diluting, large amounts of a very dilute acid solution is obtained which is not economi¬ cally feasible to concentrate for recycling into the pulping process .

The object of the present invention is to eliminate the above- mentioned disadvantages and to provide a process for the recov¬ ery of lower aliphatic acids and sugars from spent liquor from a pulping process carried out using the said acid, and accord¬ ing to the invention the evaporation is carried out countercur- rently in a plurality of steps, by heating a preceding step with the acid-containing vapor obtained from the subsequent step and by diluting with the obtained condensate the concen¬ trated spent liquor from the preceding step, in which case either water- or a very dilute aqueous solution can be used for the last dilution step. In this manner, from the first evapora¬ tion step there is recovered a concentrated acid solution which can be recycled into the pulping process. In addition, from the last evaporation step there is recovered a valuable sugar- containing product which no longer contains substantial amounts of free or esterified acid and iε thus a.suitable raw material for sugar products. By- this procedure the acid content of the sugar solution decreases from one step to the next, and from the last step there is obtained an almost acid-free mixture of sugar and lignin, and the acid is recovered from the first step in so concentrated a form that no expensive concentrating proc¬ esses are necessary.

The energy costs of the evaporation can be minimized in the countercurrent process according to the invention, and other solutions of acid and water can be introduced into it for con¬ centration, as long as they are directed to a dilution step in which the degree of concentration of acid is almost the same as that of the solution to be concentrated.

In a preferred embodiment of the invention, the last evapora¬ tion step is heated using water vapor, the condensate of which is directed to the last dilution step, which precedes it. The evaporation is preferably carried out at least in the first evaporation step under a vacuum, in which case the temperatur can be maintained low, preferably at maximum at 70 °C.

By the process according to the invention it is possible to separate from the first evaporation step after condensation a concentrated aqueous solution of formic acid, which can be recycled into the pulping process carried out with formic aci

The invention is described below in greater detail with ref¬ erence to the accompanying drawing, which depicts a flowsheet of a process according to the invention.

In the accompanying figure, the evaporation steps in general are indicated by reference numerals 1, 2 and 3. It is evident that there may be even more evaporation steps, if necessary. The dilution step between evaporation steps 1 and 2 is indi¬ cated by reference numeral 4, and the dilution step between th second evaporation step 2 and the_.third evaporation step 3 by numeral 5.

Spent liquor from which lignin has preferably already been removed is introduced via a pipe 6 into the first evaporation step 1, where it is heated indirectly with a formic acid- containing vapor which is derived from the subsequent evapora¬ tion step 2 and which is directed from the second evaporation step 2 to the first evaporation step 1 via a pipe 9. From the first evaporation step 1 there are recovered formic acid- containing vapors 8 and a concentrated formic acid solution 19 Thereafter the sugar-containing spent liquor is directed from the first evaporation step 1 via a pipe 7 to the first dilutio step 4, where it is diluted with a formic acid-containing con¬ densate 11 derived from the second evaporation step 2, before it is directed via a pipe 10 to the second evaporation step 2 and from there further via a pipe 12 to the second dilution step 5, where it is diluted with water 14 before being directe via a pipe 15 to the third evaporation step 3. Formic acid- containing vapor obtained from the third evaporation step is directed via a pipe 13 to the second evaporation step 2. A substantially acid-free sugar solution 16 is recovered from the third evaporation step. The third evaporation step 3 is heated with water vapor 17, the condensate 18 of which can be directed to the second dilution step 5 via a pipe 14.

By the process described above, the formic acid present in spent cooking liquor 6 can be evaporated and concentrated into the vapor 8 and condensate 19 leaving the first evaporation step 1. At the same time a substantially pure sugar solution 16 can be recovered from the last evaporation step 3.

The invention is described below in greater detail with the help of examples.

Example 1

57.1 g of spent cooking liquor from formic acid pulping, ligni having been removed from the spent liquor, was evaporated unde a vacuum at 70 °C to a solids content of 52.5 % by weight. 38. g of a 44.3 weight-% formic acid was recovered as a distillate. 40.0 g of a dilute (5 % by weight) formic acid was added to th evaporation residue, which contained formic acid 41.4 % by weight. The obtained mixture was mixed for approx. half an hour, whereafter it was evaporated to a solids content of

79.2 % by weight. 34.7 g of a 12.5 weight-% formic acid was recovered as a distillate. The liquid evaporation residue con¬ tained free formic acid 22.3 % by weight and combined formic acid 2.6 % by weight of the solids. The total yield of formic acid in the distillates (including the lignin separation step) was 91.6 % of the original formic acid content of the spent cooking liquor.

Example 2

360.9 g of a spent cooking liquor from a formic acid pulping process, lignin having been removed from the liquor, was evapo rated under a vacuum at a temperature of 70 °C to a solids content of 33.8 % by weight. 239.3 g of a 34.1 weight-% formic acid was recovered as a distillate. 250 ml of water was added to the distillation residue, which contained formic acid 23.0 by weight. The obtained mixture was mixed for approx. one hour whereafter it was evaporated to a solids content of 35.1 % by weight. 252.9 g of a 6.9 weight-% formic acid was recovered as a distillate. The residue, the formic acid content of which wa

14.0 % by weight, was evaporated further to a solids content o 62.3 % by weight. 31.8 g of a 14.3 weight-% formic acid was obtained as a distillate. The formic acid content of the liqui distillation residue was 14.1 % by weight. The total yield of formic acid in the distillates (including the lignin-removing step) was 95.7 % of the original amount of formic acid in the spent cooking liquor.

Example 3

440.3 g of a spent cooking liquor from a formic acid pulping process, lignin having been removed from the liquor, was evapo rated under a vacuum at a temperature of 70 °C to a solids content of 31.6 % by weight. 291.9 g of a 38.9 weight-% formic acid was obtained as a distillate. 150 g of a dilute (15 % by weight) formic acid was added to the evaporation residue, whic contained formic acid 28.4 % by weight. The obtained mixture was mixed for approx. half an hour, whereafter it was evapo¬ rated to a solids content of 52.4 % by weight.- 203.5 g of a

26.1 weight-% formic acid was recovered as a distillate. 85 g of water was added to the evaporation residue, which contained formic acid 12.4 % by weight. The obtained mixture was mixed for approx. half an hour, whereafter it was evaporated to a solids content of 71.7 % by weight. 109.0 g of an 8.7 weight-% formic acid was obtained as a distillate. The formic acid con¬ tent of the liquid evaporation residue was 2.7 % by weight. Th total yield of formic acid in the distillates (including the lignin-removing step) was 94.7 % of the original formic acid amount in the spent liquor.

Claims

1. A process for the recovery of lower aliphatic acids and sugars from the spent cooking liquor from a pulping process carried out with the said acids, lignin having preferably been removed from the spent liquor, by heating the spent liquor (6) in order to evaporate the said acids, characterized in that the evaporation is carried out countercurrently in a plurality of steps (1, 2, 3) by heating a previous step (1, 2) with the acid-containing vapor (9, 13) obtained from the subsequent εtep (2, 3) and by diluting (4, 5) with the obtained condensate (11) the concentrated spent liquor (12) coming from the previous step (2), water or a very dilute aqueous solution (14) being used for the last dilution step (3) .

2. A process according to Claim 1, characterized in that the last evaporation step (3) is heated with water vapor (17) , the condensate (18) of which is directed to the last dilution step (5), which precedes it.

3. A process according to Claim 1 or 2, characterized in that the evaporation is carried out under a vacuum at least in the first evaporation step (1), preferably at maximum at 70 °C.

4. A process according to any of the above claims, charac¬ terized in that a concentrated aqueous solution (8, 19) of formic acid is separated after condensation from the first evaporation step (1) .