WO2002025201A1

WO2002025201A1 - Apparatus for carrying out a physical and/or chemical process, such as an evaporator

Info

Publication number: WO2002025201A1
Application number: PCT/NL2000/000679
Authority: WO
Inventors: Dick Gerrit Klaren
Original assignee: Klarex Beheer B.V.
Priority date: 2000-09-22
Filing date: 2000-09-22
Publication date: 2002-03-28
Also published as: AU2000279701A1

Abstract

This invention relates to an apparatus for carrying out a physical and/or chemical process, such as an evaporator. The apparatus comprises a reservoir provided with upwardly directed tubes, which tubes are accommodated, at top and bottom ends thereof, in tube plates and are in open communication with a top box and a bottom box, so that a medium can flow through them from the bottom box in an upward direction. The top box is connected with a first return line, provided with a compressor and connected with the reservoir, for discharging a vapor fraction of the medium, separated in the top box, from the top box and feeding it, in compressed condition, to the reservoir. The top box is further connected with a second return line for discharging a liquid fraction of the medium from the top box and feeding it back to the bottom box, under pump action, for recirculation. According to the invention, in the bottom box at least one distribution plate is arranged for supporting a fluidized bed of granular material, which is maintainable in a quasi-stationary, fluidized condition by the medium fed back via the bottom box. Besides, the top box is connected with a downcomer for feeding separated granular material back from the top box to the bottom box.

Description

Apparatus for carrying out a physical and/or chemical process, such as an evaporator.

This invention relates to an apparatus for carrying out a physical and/or chemical process, such as an evaporator, comprising a reservoir provided with upwardly directed tubes, which tubes are accommodated, at top and bottom ends thereof, in tube plates and are in open communication with a top box and a bottom box, so that a medium can flow through them from the bottom box in an upward direction, the top box being connected with a first return line, provided with a compressor and connected with the reservoir, for discharging a vapor fraction of the medium, separated in the top box, from the top box and feeding it, in compressed condition, to the reservoir, and the top box further being connected with a second return line for discharging a liquid fraction of the medium from the top box and feeding it back to the bottom box for recirculation.

Such apparatus is known from practice and is designated by those skilled in the art as a vertical rising film vapor (re)compression evaporator. In such apparatus, a condensate is separated, conventionally by evaporation, from a medium that is recycled, for instance fresh water, which, after distillation, is separated as a distillate from a flow of seawater, which is recycled a number of times. The reservoir with the upwardly directed tubes forms a heat exchanger, with which heat is supplied to a medium moving upwards through the tubes, so that in the tubes and/or the top box vapor is formed. In the top box of the apparatus, a separation of saturated vapor and liquid is effected. The saturated vapor is compressed by means of a fan and fed, in compressed condition, as a heat-carrying medium to the reservoir at a saturation temperature level increased with respect to the saturated vapor in the top box. In the reservoir, heat delivery to the tubes takes place, in particular by condensation of the supplied compressed saturated vapor on the outer surface of the tubes. The resulting condensate is then discharged as a distillate.

A drawback of the known apparatus is fouling of the surface of the heat exchanger by impurities contained in the liquid. Such impurities may consist of solid particles already contained in the medium at the beginning of the process. The surface of the heat exchanger may, however, also become fouled by substances dissolved in the liquid, which substances precipitate as a result of the increased temperature and/or increased concentration of the medium, in particular on the wall of the tubes of the heat exchanger. Such impurities may cause problems, in particular in evaporators, and can seriously impede a proper operation. In some cases, the formation of deposits can be reduced by operating the evaporator at a relatively low temperature level. Because of the large specific vapor volumes, this, however, leads to a considerable increase in the compressor power required. Besides, the evaporator, in particular the outlet box, must also be suitable for maintaining therein a reduced pressure, which, in particular in large plants, leads to a very heavy and hence expensive structure. Furthermore, in spite of the low temperature level, chemicals must be added to the medium to keep in solution the substances that give rise to the formation of deposits or to ensure that these substances deposit as a soft sludge, which is removed from the tubes by flowing the liquid. Consequently, the operational cost of the known apparatus is relatively high.

The object of the invention is to provide an apparatus of the type mentioned in the opening paragraph, in which the above drawbacks are removed. According to the invention, the apparatus is characterized in that in the bottom box at least one distribution plate is arranged for supporting a fluidized bed of granular material, which is maintainable in a quasi-stationary, fluidized condition by the medium fed back via the bottom box, and that the top box is further connected with a downcomer for feeding granular material back from the top box to the bottom box. By using the granular material, in this context also called fluidized bed particles, which form a fluidized bed in the quasi-stationary, fluidized condition, it is ensured, that deposition on the surface of the heat exchanger can be inhibited by the fluidized bed particles exerting a slightly scouring action on the surface of the heat exchanger. In particular, a portion of the particles from the fluidized bed is entrained by the medium, which has the result that the particles can exert a slightly scouring action on the inner walls of the upward tubes, so that deposition and fouling can be inhibited. Furthermore, by means of the fluidized bed the medium in the upward tubes can be brought into turbulence, with the result that the efficiency of the heat transfer of the heat exchanger can be considerably improved. By feeding the granular material from the top box back to the bottom box, by means of a downcomer, separated with respect to the main flow of flowing medium, it is ensured that the fluidized bed can be maintained and that damage to the pump, when feeding back the liquid fraction of the medium, through entrained particles can be avoided.

The downcomer is preferably placed externally, that is to say outside the reservoir, and is preferably communicable, through switching on and off, with the bottom box of the reservoir. It is observed that the use of a fluidized bed in a heat exchanger with upwardly directed tubes and the feedback of particles of granular material, via a downcomer, to the bottom box are known per se and can be carried out in many ways, for instance as described in the international patent applications PCT/NL94/O0081 and PCT/NL94/00082. In the known heat exchanger, the top box is, however, provided with one discharge, with which the total amount of medium with the granular material is discharged for further processing.

Through the scouring action of the fluidized bed particles, in particular aqueous mediums can be passed at high temperature through the upwardly directed tubes of the heat exchanger without the occurrence of fouling problems. Consequently, the temperature of these aqueous liquids in the upwardly directed tubes may be about 100 °C. In aqueous liquids, at this temperature saturation already occurs at a system pressure of about 1 atmosphere, so that the heat exchanger can be of relatively light design. In particular, the outer surface of the reservoir, the top and/or bottom box may be different from the form of a circular cylinder. Preferably, the outer surface of the reservoir, the top and/or bottom box is manufactured from flat plate material and the reservoir, the top and/or bottom box have a rectangular cross-section transverse to the upward direction. In a very advantageous manner, the reservoir, the top and/or bottom box can be accommodated in a box-shaped structure forming a common outer surface. Such a box-shaped structure is relatively inexpensive to manufacture.

Preferably, the apparatus is provided with a plurality of first and/or second return lines arranged in parallel, so that the circulation throughput can be increased.

Further advantageous embodiments are defined in the subclaims. The invention also relates to a method for carrying out a physical and/or chemical process. The invention will now be explained in more detail with reference to an exemplary embodiment shown in a drawing. In the drawing:

Fig. 1 is a diagrammatic cross-section of a vapor recompression evaporator; and

Fig. 2 is a diagrammatic perspective view of a vapor recompression evaporator with a plurality of propeller pumps operating in parallel;

Fig. 3 is a diagrammatic cross-section of an upper part of a vapor recompression evaporator, with a spray nozzle arranged in the top box;

Fig. 4 is a diagrammatic cross-section of a lower part of a vapor recompression evaporator with a centrifugal pump placed outside the bottom box; and Fig. 5 is a diagrammatic cross-section of a lower part of a vapor recompression evaporator with an eccentrically placed second discharge and a bend located outside the bottom box, in which bend a propeller pump is accommodated. The figures only relate to diagrammatic representations of preferred embodiments of the invention. In the figures, similar or corresponding parts are indicated by the same reference numerals.

Fig. 1 shows a vapor recompression evaporator, comprising a reservoir 1, in which a series of risers 2 are provided, which are accommodated, at the top end 2a and the bottom end 2b, in tube plates 2c, 2d. Located above the upper tube plate 2c is a top box 4 separated from the reservoir 1, in which top box the risers 2 open, while below the lower tube plate 2d a bottom box 3 separated from the reservoir 1 is present, in which one or more distribution plates 5 are arranged for supporting a fluidized bed of granular material. The risers 2 are provided at the bottom end 2b with a run-in piece 6, in which openings 7 are provided. Arranged above or at a short distance from the upper tube plate 2c is a throttling plate 9 provided with openings 8, which openings are in register with the openings of the risers 2 and have a diameter smaller than the internal diameter of the riser 2.

Located in the top box 4 is a funnel-shaped reservoir 10, which is provided at a top end with an opening 11. The bottom end of the funnel- shaped reservoir 10 opens into a tubular suction line 12, which extends from the top box 4 through the reservoir 1 into the bottom box 3. The suction line 12 extends through the tube plates 2c, 2d and is located inside the outer surface Bl of the reservoir of the vapor compression evaporator. The outer surface Bl of the reservoir 1, the outer surface B2 of the bottom box 3 and the outer surface B3 of the top box 4 form a common outer surface B, which, transversely to the upward direction, has a substantially rectangular cross-section. The outer surface B is built up from welded flat steel plates, which are supported by stiffening ribs not shown.

Included in the lower part of the tubular suction line 12 is a propeller 13 of a circulating pump. The propeller 13 is provided with a driving shaft 14, which is connected with a motor 15 of the pump, arranged outside the bottom box 3.

The tube 12 forms, in this exemplary embodiment, the second return line for discharging the liquid fraction of the medium from the top box 4 and feeding it back to the bottom box 3. By means of the circulating pump, formed by propeller 13, shaft 14 and motor 15, liquid medium is recirculated through the bottom box 3, #ιe risers 2, the top box 4, the reservoir 10, and the tubular suction line 12. As a result of this liquid flow, fluidized bed particles, as will hereinafter be explained in more detail, are fed above the distribution plates 5, fluidized to a fluidized bed and uniformly distributed with the medium over the risers 2. In the top box 4, above throttling plate 9, the fluidized bed particles are separated from the liquid fraction of the medium through the upward flow velocity of the medium, which is relatively lower with respect to the flow velocity in the tubes, under the action of gravity. The particles are discharged via one or more slanting lines 16 to an upper part 17A of an upwardly extending downcomer 17, arranged externally with respect to the housing B. To this end, the outlet box 4 is connected near the bottom end, in this case near the throttling plate, with the outlet line 16. Of course, several downcomers 17 may also be used. The transport of the fluidized bed particles through the lines 16 can be improved by discharging the fluidized bed particles from the top box 4 to the downcomer 17 with a partial flow of the liquid fraction of the medium. The driving force for this liquid flow can be obtained through the flow resistance as a result of the level h, over which the fluidized bed extends in the top box 4 above the inlet opening of line 16, and optionally a pressure difference over a resistance plate 18 in the top box. The partial flow of the liquid fraction of the medium leaving the top box 4 with the fluidized bed particles through the lines 16 is fed back to the top box 4 via the upper part 17A of the downcomer 17 by means of a line 19, above the level of the fluidized bed in the top box 4 and the resistance plate 18 optionally used. The resistance plate 18 may also be used to prevent boiling of the liquid fraction of the medium in the fluidized bed directly above the upper tube plate 2c, since boiling of the liquid hinders the feedback of the fluidized bed particles from the top box 4 to the downcomer 17. Referring to Fig. 3, it is shown that the top box may be provided with one or more nozzles for evaporating the liquid fraction of the medium by relaxation (flashing off). In the embodiment shown in Fig. 3, the top box is therefore provided with a cenj-ral plate 35 with holes in which upwardly directed tubes 36 are accommodated. The plate 35 is arranged above the fluidized bed, so that the liquid fraction of the medium, from which the granular material has already been separated, flows upwards via the tubes 36 and is sprayed by means of spray nozzles 37 in a part of the top box 4 located above the plate 35, where a portion of the liquid fraction of the medium evaporates by relaxation as a result of the lower pressure. The vapor fraction is, after passing the separator 25, discharged via line 26 to the compressor. The remaining portion of the liquid fraction flows via opening 11 and the funnel-shaped reservoir 12 into the suction line 10.

In the upper part 17A of the downcomer 17, the fluidized bed particles falling down under the action of gravity separate from the liquid fraction of the medium flowing upwards with relatively low velocity as a result of the pressure difference. To reduce the upward velocity of the liquid fraction, the upper part 17A of the downcomer 17 may be provided with a diameter increased with respect to the lower part.

In the lower part 20 of the downcomer 17, the fluidized bed particles collect as a packed bed. The packed bed is decomposed at the bottom end by feeding liquid via line 21 to the space 22 in the lower part 20 of the downcomer 17. The space 22 is separated from the rest of the bottom end 20 of the external downcomer 17 by means of a distribution plate 23. The distribution plate 23 is preferably slightly inclined and is provided with openings, through which the liquid fraction of the medium fed via line 21 can pass, so that the packed bed of fluidized bed particles is decomposed and the fluidized bed particles are fed to the bottom box 3 via one or more lines 24. The driving force for the liquid flow through line 21 can be obtained from the pressure difference over the lower distribution plate 5. Optionally, as an alternative to the line 21, a branch 19A of the line 19 may be provided between the upper part of the downcomer 17 and the upper part of the top box 4. This auxiliary line 19A is shown dotted in the figure and is provided with a relatively small pump 19B.

Further located in the top box 4 are a drop separator 25 and a line 26, connected with the top box in upward direction thereabove, for discharging a vapor fractioi} of the medium, released in the top box 4 as a result of the heating of the medium in the risers 2. The vapor fraction is compressed by means of a fan 27 and is then fed, in compressed condition at an increased temperature level, via line 28 to the reservoir 1. The lines 26 and 28 and the fan 27 form, in this exemplary embodiment, the first return line, provided with a compressor and connected with the top box 4 and the reservoir 1, for discharging a vapor fraction of the medium, separated in the top box 4, from the top box 4 and feeding it in compressed condition to the reservoir 3.

The inflow opening of the first return line is connected with the top box 4 in upward flow direction above the outflow opening of the second return line. The downcomer is connected with the top box 4 via the discharge line 16 in upward flow direction below the inflow openings of the second and the first return line.

The compressed vapor, fed from the top box 4 to the reservoir 1, condenses on the outer surface of the risers 2 in the reservoir 1. Consequently, the medium is heated in the risers 2 of the heat exchanger. The condensed vapor is discharged as a distillate from the reservoir 1 via line 29. To discharge non-condensed gases from the reservoir, a line 30 is provided. To feed liquid medium, the apparatus is provided with a feed line 31. Besides, a discharge line 32 is provided for discharging the medium from the apparatus. In practice, the amount of medium fed via the feed lines will be smaller than the amount of medium discharged via the discharge line, since a portion of the medium fed is evaporated and discharged as a condensate. In practice, the feed flow is selected substantially smaller than the recirculation flow, so that the medium can be recycled a number of times, before it is discharged via the discharge line.

By means of the above measures, the vapor recompression evaporator is provided with a fluidized bed heat exchanger with self-cleaning properties, so that fouling of the heat transferring surface is avoided and, by turbulence also with very low liquid velocities in the tubes, a proper heat transfer can be obtained. Because already at low liquid velocities a proper heat transfer takes place, the tubes can be of relatively short design. Consequently, the dimension in upward direction of the heat exchanger can be selected relatively small and the lift of the circulating pump can be relatively small.

This renders it possible to arrange in the top box, during use, a relatively high liquid level above the upper tube plate, which has the result that, by means of hydrostatic pressure, the occurrence of boiling of the medium in the tubes and the top box can be suppressed. This facilitates the feedback of fluidized bed particles from the top box to the upper part of the downcomer.

A further possibility of selecting the length of the tubes relatively small is the limitation of the temperature difference between the inlet and the outlet of the tubes. For instance, at a temperature difference of 2.75 ^σC over the tubes 2, about 0.5 % of an aqueous medium can be converted into vapor. With a distillation production of 100 tons per hour, a flow of about 20,000 m³ per hour must be recycled through the apparatus. The temperature in the top box is then about 100 °C, at a pressure of about 1 atmosphere. In the light of the above, it is preferable to use circulating pumps with a large throughput and to provide these circulating pumps with a suction line of simple construction. Preferably, the suction line is therefore, as shown in Fig. 1, designed as a straight tubular line, which, between the top box 4 and the bottom box 3, extends through the reservoir parallel to the tubes 2.

Having regard to the above large throughput of recirculating liquid, it is preferable to design the heat exchanger with a very large horizontal cross-section.

Preferably, the apparatus is therefore, as shown in Fig. 2, provided with several first and second return lines arranged in parallel. If, during use, the pressure in the reservoir 1 and the top box 4 is selected atmospheric, the structure of the outer housing BB may be of box-shaped design from relatively thin plate material, which, if necessary, is provided with light stiffenings, and which is secured to a frame built up from uprights and girders.

It may be clear that the invention is not limited to the preferred embodiments shown herein.

In particular, instead of propeller pumps, vane pumps or centrifugal pumps may also be used. In this connection, it is observed that the pump may also be arranged in another place, for instance below or beside the bottom box. The second return line may then extend partly outside the bottom box.

It is further observed that both the first return line and the second return line may optionally extend inside and/or outside the outer housing. This also applies to the downcomer. Furthermore, the vapor separator may be designed in many ways, for instance as a gauze package, or even be omitted.

Referring to Fig, 4, the second return line comprises a suction line 12, which extends through the bottom box 3 to beyond the outer surface B3 until a feed side of a centrifugal pump 40. The second return line further comprises a pressure line 41, extending from the discharge side of the centrifugal pump 40 and connected to the bottom box 3.

Referring to Fig. 5, it is shown that the second return line can be placed eccentrically with respect to the center line of the heat exchanger, so that it can be more easily connected to the entrance of a pump. In Fig. 5, the second return line comprises a bend located outside the outer surface B3 of the bottom box 3, in which bend a propeller 13 of a propeller pump is accommodated. The bend 42 is connected on an entrance side to an eccentrically placed suction line 12 and is connected on an exit side to the bottom box 3.

Many embodiments are possible within the scope of the invention as defined in the appended claims.

Claims

1. An apparatus for carrying out a physical and/or chemical process, such as an evaporator, comprising a reservoir provided with upwardly directed tubes, which tubes are accommodated, at top and bottom ends thereof, in tube plates and are in open communication with a top box and a bottom box, so that a medium can flow through them from the bottom box in an upward direction, the top box being connected with a first return line, provided with a compressor and connected with the reservoir, for discharging a vapor fraction of the medium, separated in the top box, from the top box and feeding it, in compressed condition, to the reservoir, and the top box further being connected with a second return line for discharging a liquid fraction of the medium from the top box and feeding it back to the bottom box for recirculation, characterized in that in the bottom box at least one distribution plate is arranged for supporting a fluidized bed of granular material, which is maintainable in a quasi-stationary, fluidized condition by the medium fed back via the bottom box, and that the apparatus is further provided with a downcomer for feeding granular material, separated in the separator, back from the top box to the bottom box.

2. An apparatus according to claim 1, wherein the second return line extends through the reservoir.

3. An apparatus according to claim 2, wherein the second return line comprises a tube extending from the top box through the reservoir into the bottom box.

4. An apparatus according to any one of the preceding claims, wherein the outer surfaces of the reservoir, the top and/or bottom box have a form different from a circular cylinder.

5. An apparatus according to any one of the preceding claims, wherein the outer surface of the reservoir, the top and/or bottom box is manufactured from flat plate material.

6. An apparatus according to any one of the preceding claims, wherein a plurality of second return lines, arranged in parallel, are provided, which are each provided with a pump.

7. An apparatus according to any one of the preceding claims, wherein a plurality of circulating pumps, arranged in parallel, are provided.

8. An apparatus according to any one of the preceding claims, wherein a plurality of first return lines, arranged in parallel, are provided.

9. An apparatus according to any one of the preceding claims, wherein a plurality of compressors, arranged in parallel, are provided.

10. An apparatus according to any one of the preceding claims, wherein a plurality of downcomer s, arranged in parallel, are provided.

11. An apparatus according to any one of the preceding claims, wherein the outer housing is box-shaped.

12. A method for carrying out a physical and/or chemical process, wherein a medium to be treated flows through a fluidized bed, and wherein the medium, after passing a first side of a heat-transferring surface of a heat exchanger, is passed in an upward direction through a space, in which the upward flow velocity of the medium is reduced to below the fall velocity of fluidized bed particles in the medium, and wherein, under the action of gravity, fluidized bed particles are separated from the medium and discharged from the space and fed back to the fluidized bed, wherein, furthermore, in the space a vapor fraction of the medium is separated, which, for condensation, is fed to a second side of the heat-transferring surface of the heat exchanger, and wherein, besides, from the space a liquid fraction of medium is discharged and is recirculated via the fluidized bed.