WO1994026376A1

WO1994026376A1 - Evaporation plant

Info

Publication number: WO1994026376A1
Application number: PCT/FI1994/000185
Authority: WO
Inventors: Olof FÄGERLIND; Reino Havukainen; Jutta Nyblom; Åke SCHÖNBERG; Jarmo SÖDERMAN; Erik ÅGREN
Original assignee: A. Ahlstrom Corporation
Priority date: 1993-05-11
Filing date: 1994-05-11
Publication date: 1994-11-24
Also published as: AU6651194A; SE9301624L; ZA943245B; SE9301624D0

Abstract

The present invention relates to a multistage evaporation plant having a very compact compact construction. The evaporation stages are formed by dividing a box-type space consisting of outer walls, a ceiling and a bottom with partition walls to compartments which may be connected to each other forming consecutive evaporation stages. The compartments are most preferably rectangular. They may be positioned opposite to each other forming pairs so that a condensate channel (38) at the end of the compartments will be in the middle of the evaporation space.

Description

EVAPORATION PLANT

The multistage evaporation plants comprise as known several, consecutive evaporation stages, which usually operate in such a way that the pressure and temperature in each stage is lower than that in the preceding stage. Only the first stage is heated with fresh steam or like heating fluid, and the heating fluid in each subsequent stage is vapor evaporated in the previous stage. Each evaporation stage has at least one evaporator, typically a falling film evaporator, in which liquid to be evaporated is supplied to the upper portion of the evaporator, and flows downwards on the surface of the heat exchange elements, e.g. plates or tubes, and boils due to heat exchange contact with the heating fluid, which condenses and releases heat. Unevaporated liquid accumulates at the bottom of the evaporator, from where it is supplied to the upper portion of the next apparatus. The flow of vapor between the stages as well as the treatment of the condensate, requires complicated piping, since liquid is evaporated in a number of separate pressure vessels. Thus in a conventional multistage evaporation plant the piping and associated equipment are very costly and also consume large volumes of space.

The object of the present invention is to provide a multistage evaporation plant, an evaporator block, which has a simpler and more compact construction than conventional evaporation plants. Thus equipment costs and the demand of space may be diminished and thus also construction costs may be substan¬ tially reduced.

The present invention relates to a multistage evaporation plant, which includes a number of consecutive evaporation stages connected in series and provided with heat exchange elements and it is essential for the invention that a continuous, covered volume is divided by generally vertical partitions or like into compartments, of which at least a portion is operationally connected to each other to form evaporation stages operating at consecutively reducing temperatures and pressures, whereby the evaporation plant also comprises has means for feeding the heating fluid and the liquid to be evaporated to the stages and means for removing the evaporated liquid and the condensates formed in the evaporation stages as well as means for arranging the flow of the liquid to be evaporated between the stages and means for feeding the secondary vapor formed in the evaporation stage to the next stage.

An evaporation plant in accordance with the present invention is very compact of the construction and efficient in the use of space. The evaporation stages are formed when a box-like space having outer walls, roof and floor is divided with partitions into compartments. Each compartment corresponds to one evaporation stage.

Each stage, compartment, is divided most preferably into three volumes in the vertical direction of the compartment. The liquid to be evaporated is supplied to the uppermost volume to be distributed into the heat exchange elements in the intermediate volume for evaporation. The liquid, which is not evaporated accumulates to the bottom volume of the compartment. The intermediate volume operates as a distribution volume for the heating steam.

The heat exchange element utilized in the evaporation plant according to the invention is most preferably formed by heat exchange lamella, on the inner surface of which the liquid to be evaporated flows in a thin falling film, and boils and on the outer surface of which the heating fluid (e.g. steam) condenses. If so desired also tubes may be used as heat exchange elements.

As in a multistage evaporators usually, a pressure difference prevails between the consecutive compartments, i.e. stages, whereby the condensation temperatures of the heating vapor in different stages decrease towards the last stage. Fresh steam is brought through the outer wall of the evaporation plant to the stage, which has the highest temperature. The steam is supplied to the intermediate volume of the first stage. The liquid to be evaporated is brought to the desired stage, for example, to the second stage from outside the plant. The condensate which forms when the steam condensed on the heat exchange surfaces accumulates at the bottom of the intermediate volume of each stage, from where the condensate flows through a recovery opening to a condensate collecting channel common to all of the stages. In order to maintain the pressure differential between the stages a water lock is typically provided in the condensate channel at the junctions between stages. Thus no condensate piping as utilized in a conventional multistage evaporation plant is necessary in the evaporation plant in accordance with the present invention.

The secondary vapor generated in the evaporation of the liquid and the unevaporated liquid are supplied together to the bottom volume of each stage, in which the vapor is separated from the liquid and is led to act as heating fluid in the next stage through the opening in the partition wall of the stages. Since the consecutive evaporation stages are thus arranged into close proximity with each other, no steam piping between the stages is required as in conventional multistage evaporation plants.

The walls forming the evaporation stages may be manufactured of many different materials, which can withstand the pressure, temperature and corrosive conditions in an evaporator. Pre- stressed concrete is a very advantageous material for use in constructing both the outer walls (including floor and ceiling) of the evaporation plant and the partition walls between the stages. The floor may also be made of a material different than the walls and ceiling, e.g. of soil. The inner side of each pre-stressed concrete wall or partition is preferably coated with a material forming a moisture barrier. Steel may also be used to construct the plant according to the present invention.

In an evaporation plant in accordance with the present invention the volume of the stages substantially continuously increases from the first stage towards the last stage. This is due to an increasing vapor amount and the increase in the specific volume of the vapor when the pressure decreases.

The construction of the evaporation plant in accordance with the present invention is not in any way limited to the evaporation of any particular type of liquid. It may well be utilized to evaporat all different types of liquids, which may be evaporated also in conventional multistage evaporation plants, such as salt water, different effluents of the industry, for example, the spent liquors and bleaching effluents from a pulp mill, efflents from a paper mill and liquids in chemical industries that must be concentrated.

Other objects and advantages of the present invention are described more in detail below, by way of example, with reference to the accompanying drawings, of which

Fig. 1, 2 and 3 is a cross-sectional view of an exemplary evaporation plant in accordance with the present invention taken at the top volume, intermediate volume and bottom volume of each of the stages, respectively;

Fig. 4 is a cross-sectional view along line A-A of Fig. 1;

Fig. 5 is a cross-sectional view along line B-B of Fig. 1; and

Fig. 6 is an isometric of any exemplary heat exchange unit that may used in each of the steps of the evaporation plant of

Figs. 1 through 5.

Fig. 1 illustrates a top view of an evaporation plant 1 in accordance with the present invention. The outer walls 2 define a continuous space, which is divided by generally vertical partitions 4 and 5 into compartments, i.e. ten consecutive evaporation stages El - EX connected in series, through which the liquid to be evaporated flows. The first stage El operates at the highest temperature and pressure and the last stage EX at the lowest temperature and pressure. Each stage has a number of heat exchange units 6.

Fig. 4 illustrates a vertical sectional view of each of the compartments or stages. Each compartment (stage) is vertically divided into top volume 12, intermediate volume 14 and bottom volume 16 separated by upper partition 8 and lower partition 10. Fig. 1 illustrates a top view of the top volume 12, Fig. 2 of the intermediate volume 14 and Fig. 3 of the bottom volume 16.

The heat exchange units 6 are located in the intermediate volume of each of the compartments. Fig. 6 illustrates a preferred heat exchange unit. The heat exchange surface there is formed by lamellas 20, which are, as known, manufactured by joining two plate-like elements to each other along their periphery. The lamellas 20 are combined to form heat exchange units, i.e. modules 6. The modules 6 comprise an upper end 22 and a lower end 24 and lamellas 20 between the ends 22, 24. Each module 6 is brought to the top volume 12 of each compartment and is lowered down through an opening in the upper partition 8. The lower end 24 is placed in a special sealing opening in the lower partition 10 and the upper end 22 remains on top of the upper partition plane 8, and the ends 22, 24 are sealed to the partitions 8, 10.

The lamellas 20 may be produced of flexible material, for example, of metal plastic laminate, which is described in PCT- application PCT/FI92/00309. The lamella module may thus be transferred gently folded between the ends 22, 24 so that the transportation volume of the modules may be significantly decreased. When assembling the lower end 24 it is laid down by means of a special lift wire 26, whereby the folded heat surface straightens up. An internal support, such as a supporting mesh, may be placed inside the lamellas 20. The support maintains the heat surfaces apart during the evaporation, thus forming a flow channel in the lamella for the liquid to be evaporated. The supporting mesh must endure the pressure differential between the heat exchange surfaces.

The evaporation modules 6 may be lifted into the top space of the compartments EI-EX separately, and separately transported to the lift opening in the roof or ceiling 3 of the evaporation plant 1. Only one such service opening is necessary in each stage EI-EX to accomodate this.

The operation of an evaporation plant is described below more in detail.

The fresh steam (or other heating fluid) 18 (Fig. 2) and the liquid 27 to be evaporated (Fig. 3) is led through an outer wall 2 to the first evaporation stage El. The steam 18 is brought to the intermediate volume 14 of the first stage El operating as a vapor distribution volume, in which the heat exchange units 6 are also located, as described above. The liquid 27 to be evaporated is pumped to the upper level 22 of the stage, from which it is distributed to the inner surface of the lamella 20 and evaporated when flowing down in a falling film over a surface of the lamella 20. The upper end of the lamella 20 operates as a liquid distribution device. The upper end of the lamella 20 is provided with an opening for the inlet of the liquid and in the lower end of the lamella 20 with an opening for removing liquid and steam evaporated therefrom from the lamella.

The generated secondary vapor is led down to the bottom volume 16 of the compartment together with the unevaporated liquid, i.e. residual liquid. In the bottom volume the secondary vapor and the residual liquid are separated due to the gravitational force and the droplet separator 28. The clean vapor 30 flows through the opening 32 between the compartments which is almost of the length of a compartment to the next evaporation stage and rises, guided by a deflector 31 to the intermediate volume 14 of the compartment. This is repeated from one stage to another, until the secondary vapor 34 exiting from the last stage EX (Fig. 3) is condensed without new steam generation by a surface condenser (not shown) . Alternatively, for example, the last evaporation stage EX may operate as a secondary condenser, in which the vapor supplied from stage EIX is condensed by means of cooling water.

In each stage EI-EX the condensate generated from the heating fluid on the outer surface of the lamella accumulates to the bottom of the intermediate volume, i.e. at the bottom partition 10, from which it flows through the opening 36 at the end of the compartment to the condensate recovery channel 38 which is common to all compartments EI-EX. The channel 38 runs below the bottom partition 10 of each stage and to the water locks 40 located at the junctions between each stage. Thus the clean condensate may be accumulated and yet the pressure differential between the stages may be maintained. The condensate overflowing the water lock 40 is at the temperature slightly higher than the vaporizing temperature of the next stage so that some flash vapor is generated. This vapor rises through an opening in the bottom partition 10 back and mixes with the vapor of the previous stage. Energy is thus efficiently utilized.

The combined condensate 42 in the channel 38 from all the stages EI-EX of the evaporation plant 1 is led from the end of the last stage EX to a clean water storage vessel, from which it can be reutilized.

The residual liquid, which remains after the separation of the secondary vapor in the bottom volume 16 of each compartment

EI-EX, is led to the recirculation pump 44 located below the evaporator block. The purpose of the pump 44 is to raise the residual liquid to the top volume 12 of the compartment along vertical channels 46. A liquid circulation is thus generated, which is adjusted so that the inner surface of the lamella 20 remains wetted by the liquid film all the way from the top to the bottom. A portion of the circulation liquid is allowed to the next stage through a liquid lock 48. The residual liquid from the previous stage is at a slightly higher temperature than the vaporizing temperature of said stage so that flash vapor is generated. The flash vapor is mixed with the secondary vapor generated in said stage so that energy is efficiently utilized.

The liquid 50, the evaporation of which is completed, exits from the last stage EX, as seen in Fig. 3.

The discharge of the gases from each stage leads to a common recovery line (not shown) . The control of the gas discharge in each stage is provided at peripheral points relative to the steam flow direction, i.e. the end of the intermediate volume to the top and bottom portions. This ensures an efficient dis¬ charge of gases, which are either heavier or lighter than the water vapor. Vapor may be directed in each stage by vertical partitions or deflectors, over which there is no pressure differential and which thus may be very light of the construction. The light partitions ensure the flow of uncondensed gases to the recovery line for gas discharge. The gas exiting from each compartment may flow through an orifice plate stationarily mounted to the discharge line. The orifice plate may be dimensioned in the collecting line according to the pressure differential due to the lower pressure caused by a vacuum pump.

The gas flow is brought to a surface condenser from the recovery line to a condensation part for gases integrated in the surface condenser (in the second part the secondary vapor condenses from the last evaporation stage EX) . The water vapor and condensable gases therein, such as methanol, condense here. Gases which do not condense are drawn to a vacuum pump. By doing so the amount of condensate containing the condensable gases is minimized. The condensate is further treated in a separate cleaning column, which concentrates each gas to its own fraction. The column is, for example, a bubble bottom column and the drive steam thereof is taken as a substream from the first stage El of the evaporation plant 1. The condensation of the vapor also takes place in the evaporation plant 1 at a lower pressure and temperature in a separate heat exchange unit built in a compartment so that the energy of the vapor may be returned to be utilized in the evaporation plant.

The compartment forming the evaporation stage is preferably rectangular of the cross-sectional form (i.e. rectangular parallelepiped in construction) . Taking the demands on the strength into consideration, the width D of each compartment is preferably shorter than its length L. In the vertical direction the upper and lower partitions 8, 10 divide the compartments into strength controllable effective spans. A vertical intermediary support may also be provided.

In Fig. 1 the space defined by the outer walls is efficiently utilized by locating the compartments in pairs opposite to each other so that the ends of the compartments, the length of which is D, meet. Thus the condensate recovery channel, which is common to all compartments is located in the middle of the evaporation compartments and the channel 38 will be as short as possible.

The location of compartments opposite to each other is also otherwise advantageous. The size of the compartments must gradually increase from the first stage El towards the last stage EX, because both the amount of the vapor and the specific volume of the vapor increase. By locating the first stage El and the last stage EX immediately adjacent each other, the construction and layout of the evaporation plant is optimized: no excess space is utilized, although the walls are vertical. In the evaporation plant 1 of Fig. 1 stages El, II, III are of the same size and stages EX, IX, VIII opposite to them are of the same size, respectively. Stage EIV is larger than stage EIII immediately adjacent to it, whereby stage VII opposite to stage IV and next to stage EVIII is equally smaller than stage EVIII, respectively. Thus the total lengths L₃+L₈ and L₄+L₇ remain equal and the volume of the entire evaporation plant 1 will be completely utilized regardless of the rectangular form. The construction principle of the evaporation plant in accordance with the present invention is thus very flexible. The capacity of the evaporation plant may be increased in a very simple manner: by lengthening the compartments from the outer wall end of the compartments and by increasing as necessary the number of the heat exchange units 6 in the compartments, respectively.

The compartments EI-EX may also be disposed consecutively in a straight line, but this leaves empty space at the first stages, if the outer walls are desired to be maintained straight.

The • present invention has many advantages compared to the conventional multistage evaporation plants. A compartment-like evaporation plant 1 in accordance with the present invention is in principle one pressurized vessel, although it contains several evaporation stages (compartments) operating at different pressures. Usually a multistage evaporation plant comprises sevaral separate pressure vessels. The demand for piping and the equipment connected thereto is significantly decreased according to the present invention. The construction of the evaporation plant 1 enables a very efficient use of the space. Stage by stage the need for space required by the increasing amount of steam may be easily controlled, especially when the compartments are positioned in pairs opposite to each other, which enables the optimization of the size of each stage compartment relative to the use of space, All this results in considerable savings in the costs.

Claims

1. A multistage evaporation plant, comprising a plurality of consecutive evaporation stages (EI-EX) , provided with heat exchange elements (6) , connected in series, characterized in that a continuous covered volume (2, 3, 7) is divided with generally vertical partitions (4, 5) or like into compartments, of which at least a portion is operationally connected to each other to form evaporation stages (EI-EX) operating in successively decreasing temperatures and pressures, whereby the evaporation plant also comprises means for supplying heating fluid (18) and liquid (27) to be evaporated into the stages and means for removing evaporated liquid (50) and condensates (42) generated in the evaporation stages and means (44, 46, 48) for arranging the flow of the liquid to be evaporated between the stages and means (31, 32) for feeding the secondary vapor (30) generated in the evaporation stages (EI-EIX) into the next stage.

2. Evaporation plant in accordance with claim 1, characterized in that compartments (EI-EX) are substantially rectangular in cross-section.

3. Evaporation plant in accordance with claim 2, characterized in that volume (2, 3, 7) is divided by partitions (4, 5) into compartments so that several pairs of rectangulars (e.g. El, EX) located beside each other and with meeting ends are formed.

4. Evaporation plant in accordance with claim 2 or 3, characterized in that the width D of the rectangular is substantially smaller than the length L thereof.

5. Evaporation plant in accordance with claim 3, characterized in that the evaporation stages (EI-EX) share in the volume a common condensate recovery channel (38) for the discharge of the condensates from the stages, which channel is arranged in the middle of the space between the pairs of rectangular compartments (El and EX, etc.) .

6. Evaporation plant in accordance with one of the preceding claims, characterized in that the volume of the compartments increases from the first stage (El) towards the last stage (EX).

7. Evaporation plant in accordance with one of the preceding claims, characterized in that each stage is divided in the vertical direction of the volume into top volume (12) including liquid distribution means therein, an intermediate volume (14) including vapor distribution means therein, and bottom volume (16) including liquid recovery means therein, which volumes are separated by upper and lower partitions (8, 10) , respectively.

8. Evaporation plant in accordance with one of the preceding claims, characterized in that there is a water lock (48) between the bottom volumes of each of the consequtive stages for leading the liquid to be evaporated from stage (e.g. EIX) to the next stage (EX) and a recirculation pump (44) for leading the liquid further to the top volume of said stage (EX).

9. Evaporation plant in accordance with one of the preceding claims, characterized in that the bottom of each stage (e.g. EIX) is provided with means (28) for separating the secondary vapor generated in the heat exchange units from the evaporated liquid and an opening (32) for leading said vapor to the next stage (EX) to act as heating fluid.

10. Evaporation plant in accordance with one of the preceding claim, characterized in that the condensate generated in each stage from the heating fluid is supplied through the opening (36) in the bottom partition (10) of the stage to the condensate recovery channel (38) , along which the combined condensate flow (42) is led from the evaporation plant.

11. Evaporation plant in accordance with claim 10, characterized in that the condensate recovery channel is provided with water locks (40) at junctions between said stages to maintain a pressure differential between the stages.

12. Evaporation plant in accordance with one of the preceding claims, characterized in that the volume is provided with means (34) for leading the secondary vapor generated in the last evaporation stage to a surface condenser outside the volume or that the last stage operates as a secondary condenser.

13. Evaporation plant in accordance with one of the preceding claims, characterized in that the intermediary volume (14) of each stage is provided with means for removing non-condensed gases to the recovery line common to the stages.

14. Evaporation plant in accordance with one of the preceding claims, characterized in that the heat exchange units (6) of the evaporation stages have a lamella (20) as the heat exchange surface, and that the heat exchange unit comprises an upper (22) and a lower (24) end and lamellas between them.

15. Evaporation plant in accordance with one of the preceding claims, characterized in that the upper and lower partitions (8, 10) of the stages have openings for assembling heat exchange units (6) to the intermediate volume (14) of the stage.

16. Evaporation plant in accordance with one of the preceding claims, characterized in that the lamella (20) is of flexible material, whereby the heat exchange unit may be transported so that the lamella is folded between the ends (22, 24).

17. Evaporation plant in accordance with one of claims 1-13, characterized in that the heat exchange units of the evaporation stages have tubes as heat exchange surfaces.

18. Evaporation plant in accordance with one of the preceding claims, characterized in that the continuous covered volume includes exterior wall, floor, and ceiling components, and wherein at least said exterior wall and ceiling and generally vertical partition walls are constructed of pre-stressed concrete.