FI66035B - FOERFARANDE FOER FOERBAETTRING AV AVDUNSTBARHET AV AVFALLSVAETSKA UPPSTAODD UNDER CELLULOSAFRAMSTAELLNINGSPROCESS - Google Patents

Description

66035

The present invention relates to a process for improving the evaporability of a waste solution generated in a cellulose production process, mainly in its cooking step and separated from the fibrous fraction during washing of the pulp 5, and to increase the concentration of the concentrated waste solution by evaporation.

From the production of pulp by the sulphate process, for example from Finnish pine wood, the typical composition of the waste solution, calculated as a percentage by weight of organic dry matter, is as follows: lignin ** 7% - hydroxy acids 28% volatile acids 11% 15 - extracts 5% - other compounds 9%

The sulphate lignin dissolved from wood is mainly (60-90%) in the form of polymers. The average weight-weighted molecular weight of sulfate lignin when cooking softwood is 2,000 to 36,000, with a typical value of about 3-500.

From the production of pulp by the sulphite process, for example from Finnish spruce, the typical composition of the waste solution, calculated as a percentage by weight of organic dry matter, is as follows: 25 - lignosulphonic acids 55%

carbohydrates 28 X

aldonic acids 5% acetic acid ^% extractants 4% 30 - other compounds b%

The mass-weighted average molecular weight of lignosulfonic acids when boiling softwood is 3,000 to 50,000, with a typical value of about 4,000 to 5,000. The maximum molecular weight can be as high as 110,000.

It is known that the viscosity of a waste solution in practice imposes an upper limit on the volatility of the solution.

It is also well known that waste broths such as those described are used as fuel in a recovery boiler or similar other type of waste liquor combustion boiler, the functional purpose of which is to generate steam and also to regenerate or directly regenerate the cooking chemical.

The vapor evolution of a soda boiler or similar is proportional to the dry matter content of the waste solution to be incinerated. The lower the water content of the solution to be burned, the higher the net calorific value and the steam generation of the boiler.

Efforts have been made to increase the dry matter content of the waste solution to be incinerated by means of technical solutions in which more viscous solutions can be treated in the evaporation process. Among such hardware applications, the so-called "falling film" evaporators. These can achieve dry matter contents of more than 65%. In conventional evaporators, the practical upper limits are about 65% for sulphate waste solution and about 55% for sulphite waste solution.

As already stated above, the viscosity of the waste solution practically sets an upper limit on the concentration of the solution in the evaporation process.

The method according to the invention is characterized by what is defined in the claims. The basic idea is to lower the viscosity of the waste solution to improve its volatility.

It is known that the intrinsic viscosity of macromolecular compounds, i.e. the viscosity at infinite dilution, is proportional to the molecular size of the compound. It has now been found that a similar dependence can also be found at finite concentrations, as shown in the example case below.

3,65035

Case Study

Dependence between the dynamic viscosity (1? 0UC) and the dry matter content (3 ») of the acidic sodium sulphite soup effluent (80% pine, 20% spruce).

5 k.a./% Dyn. visc./cSt 40 Ι, ό 45 2,5 50 4,2 10 55 8

The mass-weighted Mole-weight distribution of the dry matter of said waste liquor is as follows: Μω% 15> 40,000 18.8> 30,000 21.8> 20,000 26.9> 10,000 34.1> 5,000 43.3 20 The mass-weighted average molecular weight of the dry matter is about 2,700.

When a part of the high molecular weight fraction is removed from said waste liquor so that the mass-weighted molecular weight distribution of the dry matter becomes as follows: Μω%> 40,000 0> 30.000 3> 20.000 8.1 30> 10.000 15.3> 5,000 24.5 mass-weighted average molecular weight of the dry matter is about 600.

Correspondingly, the measured dynamic viscosities at different average molecular weights at different dry matter concentrations are as follows: 66035 ka / ί v / cSt (120 ° C) _Μω = A2700 Μω = B1R00 Μω = ° 60Γ 40 1.6 1.0 0.66 45 2.5 1, * 0.76 5 50 4.2 2.6 1.26 55 8 5.0 2, * 60 13.0 8.1 3.9 65 22.1 13.6 6.6 70 - - 11.3 10 75 _-_ ~ _19.3

Looking at the figures in the table above in this example case, the following dependence can be found between the dynamic viscosity 15 and the average molecular weight of the dry weight of the pulp-weighted waste liquor. waste liquor.

o - °> £ v / cSt (120 ° C) CC Μω 20 On the other hand, the dependence between viscosity and dry matter content can be expressed in the form of an equation, exemplified by the following. dependence between the dynamic viscosity and the dry matter content of the untreated waste liquor (Μω = 2700) in the example = 25 v _ / cSt = 0.02118 exp (0.106942 * ka./%) M 2700 ω

If we now consider, for example, the solutions shown in the exemplary case, in which the mass-weighted average molecular weights of the dry matter 30 are 2700 for solution A and 600 for solution C, it can be seen that the dynamic viscosity of solution C is decisively lower, i.e. about 1/3 A- at dry solids contents corresponding to the viscosity value of the solution. Solutions A and C differ in that solution C is prepared from solution A by removing a high molecular weight dry matter fraction of about 18-19% of the total dry matter it contains. In this 66035 example case, the difference. the separation is made on an ultrasonic model.

If, with a conventional evaporator, solution A can be evaporated to a dry matter content of 55%, with a dynamic viscosity of about 8 cSt, correspondingly solution C can be evaporated to a dry matter content of about 67% with the same equipment.

For example, if it is assumed that solution A is fed to a recovery boiler at 1000 t / day in 55% dry matter, the fact that water is fed with fuel at 818 t / day is significant. Correspondingly, when solution C is prepared from a corresponding amount of solution A and fed to a recovery boiler at 810 t / day with a dry matter content of 67%, only 399 t / day of water is fed with the fuel, i.e. only 15 half of that of solution A.

If we look at the high molecular weight fraction that was separated from solution A in the preparation of solution C, it can be stated that its mass-weighted average Moieyl weight is about 64,000. This means that this solution, to be called solution D, can be separately evaporated to a dry matter content of about 30% (31.8%) by conventional hainduization equipment, based on the above discussion. Solution D can be used as a dispersing aid, flocculation aid, glue or as a raw material for the preparation of similar substances, either as a solution or as a dried product. Solution fraction D can also be thermally or chemically treated to cleave the lignosulfonate molecules, after which this solution is passed to concentration by evaporation and co-incineration with solution C. The thermal and / or chemical cleavage of the lignosulfonate molecules mentioned here can most preferably be carried out by returning the solution fraction D back to the pulp cooking stage, in which conditions favorable to lignin cleavage are maintained in accordance with the basic objectives of pulp production.

As a basic idea of the process according to the invention, lowering the viscosity of the waste solution in order to improve the volatility by separating the high molecular weight fraction from the solution, known methods can in principle be applied. Such methods include, for example, ultrafiltration, partial or total precipitation of the lignin fraction, and ion exclusion.

When considering ultrafiltration as a separation method, it is known that the cut-off values of the semipermeable membranes used in industrial ultrafiltration applications of sulphite process waste liquor aimed at the separation of lignosulfonates are of the order of 5 for lignosulfonates and lignins. Significantly fewer films with a cut-off value of the order of 10,000 to 100,000, but preferably 20,000 to 60,000, are used in the process of the present invention.

Using fewer films, the flux through the film per unit area is significantly increased and the separated high molecular weight fraction is recovered in purer with lower equipment and operating costs. The use of ultrafiltration is possible for waste liquors from both sulphite and sulphate-20 drop processes or similar cooking methods.

Lignin in sulphate waste liquor can be precipitated by acidification. At a pH of about 8, it is possible to precipitate 75 to 80 pounds of total lignin. The optimum dry matter content of the precipitation for the waste liquor is about 25% and the precipitation is favored by the high temperature. By acidifying to different pH levels, it is possible to fractionate lignin out of solution as a function of molecular size. The precipitate is separated from the mother liquor, for example by sedimentation and filtration or centrifugation. The mass-weighted average molecular weight of the mother liquor can be influenced by a procedure as described and at the same time significantly improved. the volatility of the solution. If desired, the precipitate can be further dried to a dry matter content of about 30-60% of the dry matter content, e.g. with a fluid bed dryer, and fed to 35 recovery boilers either as a separate feed or as a dispersed solution feed. Part of the lignin fraction can be used to prepare various lignin-based products.

Claims (8)

66035 A method for improving the wettability of a cellulose cooking waste solution, characterized in that organic matter, mainly lignin-based, is removed from said solution before or during evaporation. Process according to Claim 1, characterized in that the proportion of organic matter to be removed in the total dry matter content of the solution is 1 to 40% by weight, but preferably 5 to 25% by weight. Process according to Claims 1 to 2, characterized in that the organic matter to be removed is the most molecular fraction. Process according to Claims 1 to 3, characterized in that the organic matter to be removed is separated from said organic matter. from the solution by ultrafiltration and / or precipitation. Process according to Claims 1 to 4, characterized in that the organic matter to be removed has a major molecular weight of more than 10.00 ° C. Process according to Claims 1 to 5, characterized in that the high-molecular organic matter removed from the solution is treated thermally or chemically in such a way that the molecular size is substantially reduced. A method according to claims 1-6, characterized in that after said thermal or chemical treatment, the solution fraction is returned to the main solution stream. Process according to Claims 1 to 7, characterized in that said thermal / chemical treatment is carried out in connection with the cooking of cellulose.