WO1998016685A1

WO1998016685A1 - Falling-film evaporator with horizontal tubes for evaporating waste liquid from pulp production

Info

Publication number: WO1998016685A1
Application number: PCT/SE1997/001706
Authority: WO
Inventors: Lars Olausson
Original assignee: Kvaerner Pulping Ab
Priority date: 1996-10-15
Filing date: 1997-10-10
Publication date: 1998-04-23
Also published as: AU4731897A; SE9603766L; SE9603766D0; SE512959C2

Abstract

The invention comprises a falling-film evaporator for evaporating wast liquor from pulp cooking, comprising a tube system (4, 5) with horizontal tubes (4). The invention is characterized in that the tubes (4) are, at each tube end, mounted in tube plates (5) provided with flow-through holes, and in that both the tube plates (5) are supportingly and sealingly fixed to the container wall (1) so that a closed space (18, 19) with an inlet (7) and outlets (10, 11) for the evaporation steam exists between the tube plates (5) and the wall (1).

Description

TITLE

Falling-film evaporator with horizontal tubes for evaporating waste liquid from pulp production.

TECHNICAL FIELD

The present invention relates to a further development of the known type of apparatus for evaporating waste liquor from pulp cooking, so-called black liquor, which works according to the falling-film principle, that is to say that a film of the liquid which is to be evaporated is allowed to run down along a surface which is heated by steam, the water in the film evaporating on its way downwards.

STATE OF THE ART

Waste liquids from the digesters and bleaching departments of the pulping industry contain mainly water. This water has to be evaporated for the most part and the liquor concentrated, as organic material in the latter will be combusted at a later stage and heat energy and chemicals will be utilized.

Evaporation of this water can take place in various ways in different apparatuses, one of which is by so-called falling-film evaporation, the liquor being allowed to run down in the form of a film on a heated surface. This surface can consist of a wall with protuberances, two walls being welded together into laminae. It may also consist of a row of tubes. Steam for heating flows inside the laminae and the falling film runs on the outside of these. In the case with pipes or tubes, these are usually placed vertically next to one another and steam for heating is allowed to flow inside the tubes and the falling film is allowed to flow down along these on the outside. There are also apparatuses with tubes in which the liquor is allowed to flow inside the tubes and heating steam on the outside of these. Concentric pipes inside one another can also be found.

Falling-film evaporators with tube systems with horizontal tubes have also been proposed recently. Swedish Patent Application 9501388-4, filed on 18 April 1995, describes such a construction.

Depending on how concentrated the liquor is and the temperature required, use is made of steam at a pressure which may lie between 1 and 12 bar above atmospheric pressure. This means that the evaporator must be made robust so that it can withstand the internal pressure. The apparatuses are as a rule cylindrical with a height of approximately 10 metres and with a diameter of 2-6 metres. The evaporators are usually connected in series, steam which is generated in one apparatus being used for evaporation in another. The evaporation temperature in the various apparatuses is different and the highest temperature is in most cases found where the most concentrated liquors are evaporated. This evaporator is then generally heated using so-called live steam at the required pressure.

TECHNICAL PROBLEM

When the liquor film has reached high concentration, precipitation may take place so that deposits may occur on the heating surfaces. Such deposits have a tendency to accumulate especially when the liquor flows down inside narrow tubes. If the film is instead allowed to fall down on the outside of the tubes or the laminae, problems of blockage and cleaning as a result of deposits can be reduced. As the falling liquor film usually flows down along a long smooth surface without turbulence-generating unevennesses, heat transfer is often low. The system with horizontal tubes as mentioned above solves the problem of too low heat transfer, but this new construction needed to be improved and made less expensive.

SOLUTION According to the present invention, the abovementioned problems have been solved and an improved and less expensive falling-film evaporator has been produced for evaporating waste liquid from pulp production, for example black liquor, consisting of an elongate, upright container with inlet and outlet openings for steam and waste liquor and a tube system with horizontal tubes in the container intended to be flowed through by the evaporation steam and to be surrounded on the outside by a falling film of waste liquor, which is characterized in that the tubes are, at each tube end, mounted in tube plates provided with flow- through holes, and in that both the tube plates are supportingly and sealingly fixed to the container wall so that a closed space with an inlet and outlets for the evaporation steam exists between the tube plates and the wall.

In order to make simple cleaning of the heating surfaces possible, it is expedient according to the invention that the tubes are arranged in the form of vertical rows with tubes lying parallel above one another.

According to the invention, the spaces next to each tube plate can expediently be divided by one or more horizontal partitions for deflecting the steam flow and dividing the condensate formed into a number of fractions of different degrees of purity.

According to the invention, it is expedient that reinforcing rods or the like are arranged between and are connected to the tube plates and the container wall to prevent deformation of the container wall at high internal pressure of the evaporation steam.

According to the invention, it is also expedient that the preferably rectangular tube plates are welded firmly, if appropriate via distance plates, to the container wall or guided down in grooves mounted on the container wall.

Seals between the container wall and the tube plates at the upper and lower edges of the tube plates are expediently arranged in the form of sealing plates.

According to the invention, it is expedient that the evaporated steam is removed at the top of the container.

When the evaporated steam is removed at the top of the container, it is expedient according to the invention that a drop separator for separating liquid drops from the evaporated steam is arranged above a distribution box for distributing liquor which is to be evaporated above the tube system.

DESCRIPTION OF THE FIGURES

The invention will be described in greater detail below with reference to the attached figures in which: Fig. 1 shows a vertical section of a falling-film evaporator according to the invention, Fig. 2 shows a vertical section along the line A-A in

Fig. 1, and Fig. 3 shows a horizontal section through the falling- film evaporator according to the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Fig. 1 shows a container 1 which is preferably cylindrical with a shell-shaped bottom part 2 and a correspondingly shaped upper part 3. Horizontal tubes 4 are arranged inside the container 1. These tubes are at both ends introduced into and sealingly fixed to vertical tube plates 5. The tubes 4 and the tube plates 5 thus constitute a tube system in the form of a unit or cassette. The tube plates 5 are expediently plane but they may also be curved or have another shape.

As will be shown in Fig. 3, the tube system is fixed directly or indirectly to the cylinder body 1 at the longitudinal vertical edges of the tube plates 5. A space is therefore created between the tube plates 5 and the cylinder body 1, which space is to be closed and seals 6 have therefore been arranged at the upper and lower horizontal edges of the tube plates against the cylinder body 1. These seals expediently consist of plates which can be welded to the upper and lower edges of the tube plates 5 and the cylinder wall 1 but they may also be fixed in another manner on condition that they constitute a permanent and durable seal which withstands overpressure from the spaces between the tube plates 5 and the container wall 1. These plate seals 6 are therefore expediently made curved in order to be capable of withstanding a high pressure and in order to be insusceptible to stresses as a result of thermal expansion.

A relatively large inlet opening 7 in the container wall 1 is made for the stream to flow into the space between one tube plate 5 and the cylinder wall 1. The steam will then flow into this space and subsequently through the tubes 4 and into the space on the other side of the tube system. According to the invention, a partition 8 has been made there in the upper part of the space and a corresponding partition 9 has been made in the first space, where the steam flows into the container, so that the steam will be deflected into the second space and flow in the opposite direction to the steam flowing in in the lower part of the tube system, after which it is deflected again in order then to be removed through the opening 10.

The steam which flows through the tubes 4 in the lower part of the tube system will in part condense and the condensate which is clean is removed via the outlet 11. The steam which is condensed in the upper part of the tube system comes out into the first space above the partition 9. In this, a flow-through possibility has expediently been arranged for this somewhat dirtier condensate into the space where the clean steam comes in. In the uppermost part of the tube system, steam is condensed which produces a dirty condensate which is removed together with uncondensed steam or separately at the outlet 10.

As is usual in this type of evaporator, the liquor which is to be evaporated is supplied to a distributor 12 above the tubes 4. The distributor 12 consists of a box with bottom holes through which the liquor flows evenly distributed over the pipes 4 and down to the bottom part 2 during evaporation. The evaporated liquid in the bottom part 2 is in part returned for evaporation again through the pipe 13 and is removed in part through the pipe 14. New liquid which is to be evaporated is supplied to the lower part of the container 1 through a supply pipe 15. The steam which has been generated during evaporation on the outside of the pipes 4 is according to the invention allowed to flow upwards and is removed through the outlet opening 16. It is therefore expedient to arrange a drop separator 17 in front of the outlet opening 16 so that collected drops can fall down into the distribution box 12. It is also possible to arrange for generated steam to flow out in another manner.

Fig. 2 shows a vertical section through the tubes 4. This figure shows how the steam flows up at the side of the tube system 4 to be removed finally at the outlet 16.

Fig. 3 shows a horizontal section through the tube system and the container 1. The tube system is, as shown by the figure, rectangular or square in section and therefore forms four spaces 18, 19 and 20, 21 with the cylindrical container 1. The tube plates 5 are preferably fixed by welding at their vertical edges. Fixing to the cylinder wall 1 can be effected by direct welding or with the aid of distance plates 22 which are designed to afford relatively flexible fixing. It is also possible to arrange grooves in the form of rails or the like on the cylinder wall 1 and lower the tube system 4-5 down in these grooves by making corresponding arrangements in the edges of the tube plates 5. It is important, however, that the connection between the tube plates 5 and the cylinder wall 1 is sealed. In the case of very large evaporation apparatuses, the horizontal tubes may need to be supported by one or more support plates similar to the tube plates 5, so-called baffles. The steam flowing in through the inlet 7 is usually at high pressure, for example 2 bar. The evaporated steam, however, is at a lower pressure, in most cases in the region of 1 bar but there may also be a vacuum in the case of this steam. A great pressure difference will therefore exist between the spaces 18 , 19 and 20, 21. This may lead to the cylinder wall 1 being deformed and becoming oval. In order to prevent this deformation of the cylinder wall, which may of course cause fractures or other inconveniences, reinforcing rods 23 have therefore, according to the invention, been arranged between the plates 5 and the cylinder wall 1. These rods 23, which may consist of, for example, flat iron bars, are expediently welded firmly in the tube plate 5 and the cylinder wall 1. By these means, the construction is reinforced and harmful outward bending of the cylinder wall 1 in the spaces 18 and 19 can thus be prevented. In certain loading cases, it may also be advantageous to use reinforcing rods 23 connected to a baffle to prevent harmful outward or inward bending of the cylinder walls 20 and 21.

By means of the invention, a falling-film evaporator has been produced, which utilizes the effective evaporation by the evaporation liquid being allowed to run on the outside of horizontal tubes and which has the great advantage that the construction is simple and inexpensive. Production of the tube packet itself is consequently very easy to carry out. Holes are drilled in the tube plates 5 in a simple, automated manner and the tubes are rolled or welded firmly in the holes in a likewise automatic and rapid manner.

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

Claims

1. Falling-film evaporator for evaporating waste liquid from pulp production, for example black liquor, consisting of an elongate upright container (1) with inlet and outlet openings (7, 15, 10, 11, 14, 16) for steam and waste liquor and a tube system (4, 5) with horizontal tubes (4) in the container (1) intended to be flowed through by the evaporation steam and to be surrounded on the outside by a falling film of waste liquor, c h a r a c t e r i z e d i n that the tubes (4) are, at each tube end, mounted in tube plates (5) provided with flow-through holes, and in that both the tube plates (5) are supportingly and sealingly fixed to the container wall (1) so that a closed space (18, 19) with an inlet (7) and outlets (10, 11) for the evaporation steam exists between the tube plates (5) and the wall (1) .

2. Falling-film evaporator according to Claim 1, c h a r a c t e r i z e d i n that the tubes (4) are arranged in the form of vertical rows with tubes (4) lying parallel above one another.

3. Falling-film evaporator according to Claim 1 or

2, c h a r a c t e r i z e d i n that the spaces (18, 19) next to each tube plate (5) are divided by one or more horizontal partitions (8, 9) in order to make possible condensate segregation by deflection of the steam flow.

4. Falling-film evaporator according to any one of Claims 1-3, c h a r a c t e r i z e d i n that reinforcing rods (23) or the like are arranged between and are connected to the tube plates (5) and the container wall (1) to prevent deformation of the container wall (1) at high internal pressure of the evaporation steam.

5. Falling-film evaporator according to any one of Claims 1-4, c h a r a c t e r i z e d i n that the preferably rectangular tube plates (5) are welded firmly, if appropriate via distance plates (22) , to the container wall (1) or guided down in grooves mounted on the container wall.

6. Falling-film evaporator according to any one of Claims 1-5, c h a r a c t e r i z e d i n that seals between the container wall and the tube plates (5) are arranged in the form of sealing plates (6) at the upper and lower edges of the tube plates (5) .

7. Falling-film evaporator according to any one of Claims 1-6, c h a r a c t e r i z e d i n that the evaporated steam is removed at the top (3) of the container (1) .

8. Falling-film evaporator according to Claim 7, c h a r a c t e r i z e d i n that a drop separator (17) for separating liquid drops from the evaporated steam is arranged above a distribution box (12) for distributing liquor which is to be evaporated above the tube system (4, 5).