FI62364C - APPARATUS AND EQUIPMENT FOR INDIVIDUAL OPERATION - Google Patents

Description

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The invention relates to a method and apparatus for evaporating waste liquor from pulp cooking in several successive stages, in which process the evaporated waste liquor is passed under pump pressure the water contained in the spent liquor is removed from the flash tank vapor and the expansion tank thus concentrated and the cooled waste liquor is derived from the following evaporation of the factor-bogie again heated by the heat exchangers and their associated also the pressure of heat exchangers at a lower pressure through the expansion vessels, wherein a part of the final expansion tank future the concentrated spent liquor is returned to the first heat exchanger being pumped evaporated spent liquor to increase its dry matter content.

Especially in the evaporation of black liquor from sulphate pulp cooking, tubular heat exchangers are commonly used, in which the liquor to be evaporated passes in vertical tubes inside the cylindrical casing, the outer surfaces of which are heated by steam introduced inside the cylindrical casing. When the liquor is then allowed to expand to a lower pressure in a separate expansion tank into which the liquor from the heat exchanger tubes is introduced, part of the water contained in the black liquor evaporates, since the temperature of the liquor is higher than the boiling point of water at this pressure. As a result of the removal of steam, the dry matter content of the liquor increases.

Et al. the evaporator operating by the expansion method has the function of operating-Yarma, since the temperature of the lye is not raised so much that boiling begins in the evaporator tubes, with the result that the dry matter contained in the lye adheres to the inner surfaces of the evaporator tubes. Utilization of the heating surfaces is also efficient, since in the expansion tank the lye cools to a temperature corresponding to the prevailing pressure, whereby the temperature difference between the heat exchanger and the expansion tank is completely utilized.

When only one heat exchanger is used for the evaporation based on the expansion method, a part of the 2 62364 concentrated broth generated in the expansion tank must be returned to the evaporative waste liquor in order to increase its dry matter content. In other words, the waste broth must be recycled several times through the heat exchanger. When the amount of broth to be recycled is usually large, a pump is required to supply the heat exchanger, which must have a large volume flow. When the head of the pump is low (the height of the evaporator and the flow resistances in the piping are relatively small), the power losses of the pump become large, because the type of pump operating at low head and high volume flow has poor efficiency. In addition, such a pump is expensive.

Another way to implement expansion evaporation is multi-stage. In this case, the waste liquor to be evaporated is pumped through several successive heat exchangers, and in order to raise the temperature, each heat exchanger is heated separately with steam. In this case, the disadvantage caused by the poor efficiency of the feed pump, etc., is avoided, because the volume flow of the pump has been reduced and at the same time the lifting height has been increased (the combined flow resistance of several heat exchangers is higher).

When each heat exchanger is heated separately with steam, the waste scrap in the last stage of the plant tends to boil over the heat surfaces, resulting in fouling of the waste surfaces as the dry matter separating from the waste liquor adheres to them. If, on the other hand, the boiling of the effluent at the end of the evaporator plant is prevented by boiling it only in the expansion tank after the last heat exchanger of the plant (there is no expansion tank at all between the other stages), the temperature difference very large heating surfaces. The plant will therefore become expensive.

In the waste liquor evaporation method according to the present invention, it is possible to achieve an efficient utilization of the heating surfaces without the above-mentioned disadvantages.

The invention is characterized by what is stated in the appended claims.

Various embodiments of the method according to the invention will now be described with reference to the accompanying drawings, in which Figure 1 shows an embodiment if several separate steam-heated heat exchangers are used and Figure 2 shows a cross-sectional view of one heat exchanger taken along line II-II in Figure 1. an embodiment utilizing a single heat exchanger divided into a plurality of separate phases by dividing the heat exchanger at its upper and lower ends by partitions into compartments according to Figure 4. Fig. 5 shows a cross-sectional view of the heat exchanger taken from Fig. 3 along the line V-V.

The waste liquor to be evaporated according to Figure 1, the so-called the weak broth is introduced into the evaporator plant by a pump 1 which presses it from the supply space 3 of the first heat exchanger 2 into the vertical evaporator tubes 4 connected to it (Fig. 2). Each of the heat exchanger 2 is brought to the cylindrical shell of the fitting raised by heating the vapor of the arrow. As a result of the heating steam, the temperature of the broth increases as it passes through the tubes 4 to the outlet space 6 at the top of the heat exchanger 6. The inlet space 3 and the outlet space 6 of each heat exchanger 2 are separated from the tube part by a horizontal partition 5. The rise in the temperature of the broth in the tubes is chosen so that, under the prevailing pressure conditions, the broth cannot boil in the tubes, which would result in the dry matter contained in the broth sticking to the inner surfaces of the tubes. In this case, it is suitable to use a temperature difference of 5-20 ° C between the steam supplied to the heat exchanger and the broth fed to it.

The outlet space 6 of each heat exchanger 2 is connected by a pipeline to an expansion tank 7 located above the heat exchanger 2 with a lower pressure than in the discharge space 6. When the broth heated in the pipes 4 it is removed as steam from the joint 8 of the expansion tank 7.

The concentrated broth formed in the expansion tank 7 in the above-mentioned manner and cooled to its boiling point is in turn passed via a pipeline 9 through the next heat exchanger 2 to an expansion tank 7 above it, which is located according to Fig. 1 a certain difference in height from the previous stage. When this height difference is dimensioned to be greater than the flow resistance in the piping of the second heat exchanger 2, the concentrated broth passes from the expansion tank of the first heat exchanger 2 through the second heat exchanger under the influence of its hydrostatic pressure alone, and thus no pump is required between the heat exchangers.

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As it passes through the second heat exchanger 2, the cooled broth from the first heat exchanger 2 is reheated, after which it is again allowed to swell to the lower pressure prevailing in the expansion tank 7 above the second heat exchanger 2, again resulting in water evaporation and broth solids. From the plant comprising the heat exchanger 4 according to Fig. 1, the steam streams from the expansion tanks 7 are collected in the collecting pipe 10 and the concentrated broth from the last heat exchanger is partly fed to the combustion 11. However, most of the concentrated broth is recycled through line 12 back to the suction of the weak broth pump 1 to increase the dry matter content of the evaporating broth.

Figure 3 shows the above-described expansion evaporation method applied to a single heat exchanger and the different steps are shown with the same reference numerals as above. Separate successive heat exchanger stages 2 are obtained by dividing the heat exchanger supply space 3 and the discharge space 6 by four partitions 13 into compartments in which the broth to be evaporated along the pipes 4 moves in each stage. into its own expansion tank 7, which are superimposed in a larger unitary tank. There is a height difference Δ between the expansion tanks 7. H, which corresponds to the flow resistance of the next stage.

The invention is not limited to the appended embodiments, but can be modified within the scope of the appended claims.

Claims (3)

A method for multi-stage evaporation of effluent arising from cellulose boil in which the effluent to be evaporated is fed under pressure from the pump (1) through a meadow-heated heat exchanger (2) forming the first stage of an expansion vessel (7). which exerts a lower pressure than in the heat exchanger (2), whereby the water entering the effluent entering the expansion vessel (7) may be partially removed as indicated, since the effluent at this pressure exceeds the boiling point of the water, and the concentrated and cooled liquor in the expansion vessel, which is concentrated and cooled, can be passed through the following evaporation steps, which consist of heat exchangers (2) which re-heat the liquor and to these connected expansion vessels (7), which also have a lower pressure than that which is present in the expansion vessel. the heat exchangers (2), whereby a portion of the concentrated effluent which is released from the expansion vessels (7) is returned to the first heat exchanger (2) to supply the effluent to be evaporated and pumped to increase its dry matter content, characterized in that there is such an elevation difference ΔΗ between the heat exchangers (2) of the different stages of the heat exchangers (2) that the effluent from each expansion vessel [7) moves through the next stage heat exchanger (2) to the expansion vessel (7). it only through its hydrostatic pressure. Apparatus intended for realization of the method according to claim 1, comprising a vertical heat exchanger (2) provided with vertical tubes (4), the lower and upper part of which is a feeding space (3) separated by a horizontal partition (5) and a drainage space. for the effluents, which are joined to each other by the pipes (4), characterized in that for the formation of successive evaporation steps, the feed space (3) and the drainage space (6) are divided into separate compartments by vertical partitions (13). .