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Title: Falling-film evaporator for spent liquor from chemical pulping, and process for manufacturing tube lamellae in the evaporator.

TECHNICAL FIELD:

The present invention relates to a further development of the known type of apparatus for evaporating spent liquor from chemical pulp cooking, so- called black liquor, which operates in accordance with the falling-film principle, i.e. that a film of the liquid which is to be evaporated is arranged to flow down along a surface which is heated with steam, with water in the film evaporating during its progress downwards. The invention relates, on the one hand, to a further development of the method of achieving evaporation and, on the other, to a method for manufacturing tube lamellae down which the falling film is to flow.

STATE OF THE ART:

Spent liquor from the cooking and bleaching departments of the chemical pulp industry consists mainly of water. Most of this water must be evaporated off, and the liquor concentrated, since, in a later stage, organic material in this liquor will be combusted and thermal energy and chemicals collected.

This water can be evaporated in different ways, in different types of apparatus, one of which ways is by means of so-called falling film evaporation, in which the liquor is arranged to flow down in the form of a film on a heated surface. This surface can consist of a wall having outwardly directed bulges, with two walls being welded together to form lamellae. It can also consist of a series of pipes. Steam for heating flows inside the lamellae, and the falling film flows down on the outside of these lamellae. In the case of pipes or tubes, these are placed vertically adjacent to each other and steam for heating is arranged to flow inside the pipes, and the falling film to flow down along the outside of these pipes. Other types of apparatus exist which have vertical

pipes where the liquor is arranged to flow inside the pipes and the heating steam to flow on the outside of these pipes. Concentric pipes which are arranged inside each other are also known. Aqueous steam which is at a pressure which can be between 1 and 12 bar above the atmospheric pressure is used depending on how concentrated the liquor is and the temperature which is required. As a result, the evaporator has to be made to be stable so that it can withstand the internal pressure. As a rule, the apparatuses are cylindrical, with a height of up to about 10 metres and a diameter of 2-5 metres. Normally, the evaporators are coupled in series, with steam which is formed in one apparatus being used for evaporation in another. The evaporation temperature in the different apparatuses differs, and the highest temperature is usually applied where the most concentrated liquor is being evaporated. This evaporator is then expediently heated with so-called fresh steam of the desired pressure.

TECHNICAL PROBLEM:

When the liquor film has reached a high concentration, precipitation can take place, so that deposits can form on the heat surfaces. Such deposits have a tendency to accumulate, especially when the liquor is flowing down inside narrow tubes. If the film is instead allowed to fall down on the outside of the tubes or lamellae, the problems of blocking and cleaning due to deposits can be reduced. Since the falling liquor film normally flows down over a long smooth surface without turbulence-generating irregularities, the heat transfer is often low. While the known types of evaporation apparatus function well per se, there is still a need to improve the heat economy of the evaporation process and reduce capital costs.

SOLUTION :

According to the present invention, the abovementioned problems have been solved and a falling- film evaporator for evaporating spent liquor from chemical pulp cooking, so-called black liquor, has been produced which consists of an elongate, upright container which has inlet and outlet openings for steam and spent liquor, and a tube system in the container through which evaporation steam is intended to flow and which is surrounded on the outside by a falling film of black liquor, which evaporator is characterized in that the tube system consists of horizontal tubes.

According to the invention, it is expedient for the tubes to be arranged in the form of lamellae with parallel tubes lying above each other, which tubes are connected to, and open out into, a vertical, pipe-shaped box at the ends of the tubes.

According to the invention, the tubes in the lamellae can be arranged n groups with, for example, three tubes in each group, which tubes are bent at 180° two or more times at the boxes so that steam in these tubes is obliged to pass backwards and forwards three or more times between the boxes.

According to the invention, it is expedient for the tube lamellae to be coupled together in one or more cassettes.

The invention furthermore includes a process for manufacturing tube lamellae as described above, and the process is characterized in that the tubes are firmly welded, at their ends, in an elongate sheet metal wall which is plane or curved in cross-section and which has cutouts for the tubes, after which the sheet metal wall is welded to the remaining part of the box so that the weld between the tubes and the box ends up inside the box.

DESCRIPTION OF THE FIGURES:

In that which follows, the invention will be described n more detail with reference to the attached drawings, where: F g. 1 shows, diagrammatically and n section, a falling-film evaporator according to the invention,

Fig. 2 shows one embodiment of a tube lamellae according to the invention, and

F g. 3 shows another embodiment of a tube lamella according to the present invention.

DETAILED DESCRIPTION:

Figure 1 shows, diagrammatically and in vertical section, a falling-film evaporator according to the invention. It consists of a closed, cylindrical container 1 which has a length of some 10 metres and a diameter of 4-5 metres. The gable ends are dome-shaped in order to be able to withstand pressure. Naturally, the container 1 can be of another design. At its lower part, it is provided with an inlet pipe 2 for liquor which is to be evaporated. This liquor mixes with liquor which has already finished being evaporated and which is present in the bottom part of the container 1. From the bottom part 1 there passes an outlet pipe 3 through which liquor which has finished being evaporated is pumped, with the aid of a pump 4 which is coupled into the pipe 3, to the pipe 5, with a part of the liquor being returned to the evaporator through a pipe 6. It is also possible to conceive of this pipe 6 being located inside the container 1. Valves 16 are arranged to regulate the return flow through the pipe 6 in relation to the outlet pipe 5.

The liquor which is reintroduced into the evaporator through the pipe 6 is distributed in a distribution box 7 immediately above the tube system 8. This distribution box 7 can be of a conventional type and has the task of spreading the liquor in a uniform stream over the whole of the tube system 8. The liquor then

flows uniformly as a film over the surfaces in the tube system 8 and eventually drips down into the lower part of the container 1. The aqueous steam which is to heat the pipe system 8 to evaporation temperature enters, under suitable pressure, through a supply pipe 9 and is distributed, in a pipe or box 10 which is arranged transversely in relation to the supply pipe 9, to the tube lamellae, which are placed adjacent to each other, in the tube system 8. The tube system 8 consists of tube lamellae which are arranged adjacent to each other and which, in turn, consist of horizontal tubes which open out, at each end, into two boxes 11. Only one lamella is shown in Figure 1, and the box on the right of the figure serves as the box for feeding the steam to the tubes, while the box on the left constitutes a discharge box for condensate and steam which then flows out into the outlet pipe 12 at the lower part of the container 1.

The liquor which is fed in at the upper part of the tube system 8 flows down, as has been mentioned above, along the tube system as a film. Due to the fact that the tubes are horizontal and arranged transversely in relation to the direction in which the liquor is flowing, the film will alter during the whole of its passage downwards and consequently acquire a new surface for each pipe. This facilitates evaporation and promotes heat transfer.

As the liquor film falls downwards, it is concentrated while emitting steam. This steam, which can also be at a higher pressure than that of the surrounding atmosphere and have a high heat content, is conducted away through the outlet opening 13 in the upper part of the container. It can then be used as heating steam in evaporators which are at a lower pressure. In order to avoid liquid drops accompanying the steam which leaves through the outlet pipe 13, a separating device can be arranged inside the container in front of the outlet opening.

Figure 2 shows a tube lamella according to the invention. It consists, in this embodiment, of straight tubes 14 which lie above each other and which are firmly welded into the boxes 11 at either end. The boxes 11 have openings which coincide with the ends of the tubes so that free passage can be provided for the steam, which flows into the tubes 14 through one box and out into the other box. The outlet for condensate and steam at the lower part of the left-hand box 11 is arranged in the same manner as the supply through the distribution box 10. However, the collecting box 15 is somewhat narrower than the distribution box 10 since some of the steam has condensed and consequently does not need to be conducted out through the collecting box 15. Naturally, it is possible to arrange the lamella in precisely the reverse manner.

While Figure 3 shows a design of the tubes 14 which is different from that in Figure 2, the arrangement is otherwise the same. In this case, the tubes 14 have, as is evident from the figure, been arranged in groups of three and have been bent at 180° (or 2 x 90°) at the opposite box, so that they pass back towards the first box, the feed-in box, where they have been bent in the same manner, and pass back to the outlet box on the left of the figure, where they are connected to this box. The steam which enters a tube from the feed-in box consequently has to pass three times between the boxes 11 before it flows out into the collecting box on the left- hand side. The boxes 11 can be square, rectangular, circular or oval in cross-section. If they are to withstand high pressure, they are expediently designed to be circular. A special procedure can be implemented for connecting the tubes 14 to the boxes 11. A long strip of sheet metal, which is straight or curved in cross-section and which is provided with openings corresponding to the external diameter of the tubes, is first firmly welded to these tubes. This weld is then executed on the inside of

the proposed box. It is a simple operation, which can readily be automated, to weld the pipes firmly to this elongate sheet metal strip in this manner. After the tubes have been firmly welded in this manner, the sheet metal strip, which includes the tubes and which is to be a part of the whole box, is then firmly welded to the remaining part of the box. This procedure consists of only two longitudinal external welds, and this operation, too, can readily be automated. As a result, it is very simple and cheap to manufacture the tube system 8 with tubes 14 and boxes 11.

The tube lamellae which are shown in Figure 1 or 2 are arranged adjacent to each other in the form of cassettes. The only connection which is required in principle is then the supply box 10 and the collecting box 15. A more detailed description of this arrangement will not be given here since it comes within well-known technology. The suspension of the pipe system 8 will not be shown either, since it, too, is of a well-known type. By means of the present invention, a very great advantage has been achieved in comparison with tube lamellae which contain vertical tubes in that substantially higher heat transfer numbers are obtained due to the continual rearrangements of the liquor film which take place as it flows from tube to tube, since the tubes according to the invention are horizontal. As a result of these mixing events, both the temperature gradient and the concentration gradient of the film will be equalized every time the liquor film flows over to a new tube. Depending on the tube di-uneters which are selected, the liquor film will, therefore, be equalized every 50 to 100 mm, while, in the case of tube lamellae having vertical tubes, the liquor film will flow undisturbed over the whole of the length of the tube (10 - 15 metres) . This improvement in the heat transfer is of interest both for achieving high dry matter contents in the liquor, since processing of the liquor film by means of the continual rearrangements affords great improve-

ments, and for evaporating very dilute solutions, for example bleaching department effluent, where very high heat transfer numbers are sought for economic reasons. Thus, the invention is not limited to spent liquors from chemical pulp cooking but can also be applied to bleaching department spent liquors and the like.

The invention is not limited to the embodiments shown but can be varied in different ways within the scope of the patent claims.