COMBINATION MULTI-EFFECT DISTILLATION AND MULTI-STAGE FLASH EVAPORATION SYSTEM

Abstract

The combination multi-effect distillation and multi-stage flash evaporation system integrates a multi-stage flash (MSF) evaporation system with a multi-effect distillation (MED) system such that the flashing temperature range of the MSF process is shifted upward on the temperature scale, while the MED distillation process operates in the lower temperature range. The multi-stage flash evaporation system includes a plurality of flash evaporation/condensation stages, such that the multi-stage flash evaporation system receives a volume of seawater or brine from an external source and produces distilled water. The multi-effect distillation system includes a plurality of condensation/evaporation effects, such that the multi-effect distillation system receives concentrated brine from the multi-stage flash desalination system and produces distilled water.

Claims

1. A combination multi-effect distillation and multi-stage flash evaporation system, comprising: a multi-stage flash evaporation system comprising a plurality of flash evaporation/condensation stages, said multi-stage flash evaporation system receiving a volume of saltwater from an external source and producing distilled water; and a multi-effect distillation system comprising a plurality of condensation/evaporation effects and a final condenser, said multi-effect distillation system receiving concentrated brine from said multi-stage flash evaporation system for further desalination thereof and producing a desalinated water distillate.

2. The combination multi-effect distillation and multi-stage flash evaporation system as recited in claim 1, wherein each of the flash evaporation/condensation stages comprises a flash chamber and a condenser, the condenser having at least one conduit having an inlet and an outlet, the at least one conduit passing through the plurality of flash chambers.

3. The combination multi-effect distillation and multi-stage flash evaporation system as recited in claim 2, further comprising means for extracting the volume of saltwater from the external source, passing it through the final condenser and feeding the volume of saltwater under pressure through the at least one conduit, the means being in fluid communication with the inlet of the at least one conduit.

4. The combination multi-effect distillation and multi-stage flash evaporation system as recited in claim 3, further comprising means for heating the volume of saltwater after the volume of saltwater has been delivered through the at least one conduit and prior to injection thereof into the flashing stage.

5. The combination multi-effect distillation and multi-stage flash evaporation system as recited in claim 4, further comprising means for extracting the distilled water from a last stage of the multi-stage flash evaporation system, wherein the heated volume of saltwater injected into the plurality of flash chambers is flashed into vapor within the plurality of flash chambers, and the vapor condenses on an external surface of the at least one conduit to form the distilled water.

6. The combination multi-effect distillation and multi-stage flash evaporation system as recited in claim 5, wherein the means for extracting the volume of saltwater from an external source and feeding the volume of saltwater under pressure through the at least one conduit comprises at least one pump in fluid communication with the inlet of the at least one conduit.

7. The combination multi-effect distillation and multi-stage flash evaporation system as recited in claim 6, wherein the means for heating the volume of saltwater comprises: a brine heater in fluid communication with the outlet of the at least one conduit; and a boiler for delivering first heating steam into the brine heater after the volume of saltwater has been pre-heated by the at least one conduit.

8. The combination multi-effect distillation and multi-stage flash evaporation system as recited in claim 7, further comprising a first desuperheater in communication with the heater for selectively cooling the first heating steam prior to delivery thereof into the brine heater, wherein a first portion of condensed steam produced by the heater is recycled for use in the first desuperheater and a second portion of the condensed steam produced by the heater is recycled for use in the boiler.

9. The combination multi-effect distillation and multi-stage flash evaporation system as recited in claim 8, further comprising a second desuperheater for selectively cooling a second heating steam produced by the boiler prior to delivery thereof into a first one of the plurality of condensation/evaporation effects of said multi-effect distillation system.

10. The combination multi-effect distillation and multi-stage flash evaporation system as recited in claim 9, further comprising: a plurality of feed heaters, wherein each said feed heater is in communication with the at least one conduit and a respective one of the effects; and a plurality of flash pots, each flash pot being in communication with a respective effect and configured for pressure equalization.

11. The combination multi-effect distillation and multi-stage flash evaporation system as recited in claim 5, further comprising a pre-treatment system for filtering the volume of saltwater prior to delivery thereof to said multi-stage flash evaporation system.

12. The combination multi-effect distillation and multi-stage flash evaporation system as recited in claim 11, wherein the pre-treatment system comprises a nanofiltration membrane filter for removal of hardness ions from the saltwater.

13. The combination multi-effect distillation and multi-stage flash evaporation system as recited in claim 11, wherein the pre-treatment system comprises a filtration system selected from the group consisting of a low pressure microfiltration system, an ultrafiltration membrane filtration system, and a combination thereof.

14. The combination multi-effect distillation and multi-stage flash evaporation system as recited in claim 11, wherein the pretreatment system further comprises one or more valves to regulate the flow of the saltwater stream passing through the membrane filtration system and the remainder of the saltwater stream that is bypassing the membrane filtration system.

15. The combination multi-effect distillation and multi-stage flash evaporation system as recited in claim 5, further comprising a pump in fluid communication with the at least one conduit and the inlet to the first one of the plurality of condensation/evaporation effects of the multi-effect distillation system, the pump being configured for extracting the remaining concentrated brine in the flash chamber of the last stage in the multi-stage flash evaporation and delivering it under pressure in two portions, wherein a first portion thereof circulates in the multi-stage flash evaporation system for further desalination, and a second portion thereof passes to the first effect of the multi-effect distillation system for further desalination.

16. The combination multi-effect distillation and multi-stage flash evaporation system as recited in claim 5, further comprising a thermal vapor compressor in fluid communication with a final one of the plurality of condensation/evaporation effects of said multi-effect distillation system.

17. The combination multi-effect distillation and multi-stage flash evaporation system as recited in claim 5, wherein the means for heating the volume of brine comprises: a brine heater in fluid communication with the outlet of the at least one conduit; and a mechanical vapor compressor in fluid communication with at least one of the flash evaporation stages for delivering vapor generated in the last flash evaporation stage into the brine heater as first heating steam for heating the volume of brine after the volume of brine has been heated by the at least one conduit.

18. The combination multi-effect distillation and multi-stage flash evaporation system as recited in claim 17, further comprising a desuperheater for selectively cooling the heating steam prior to delivery thereof into the brine heater.

19. The combination multi-effect distillation and multi-stage flash evaporation system as recited in claim 18, wherein the desuperheater is in communication with the brine heater and at least a portion of condensed steam produced by the brine heater is recycled for use in the desuperheater.

20. The combination multi-effect distillation and multi-stage flash evaporation system as recited in claim 19, further comprising a pre-treatment system for filtering the volume of saltwater prior to delivery thereof to said multi-stage flash evaporation system.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] FIG. 1 diagrammatically shows a combination multi-effect distillation and multi-stage flash evaporation system according to the present invention.

[0033] FIG. 2 diagrammatically shows a conventional prior art multi-effect distillation system.

[0034] FIG. 3 diagrammatically shows a conventional prior art multi-stage flash evaporation system.

[0035] FIG. 4 is a graph showing plots for vapor equilibrium temperature in a brine pool at a given depth below the surface for varying depths, taken over typical flashing ranges for a conventional prior art multi-stage flash evaporation process.

[0036] FIG. 5 is a graph showing plots of boiling point elevation loss, pressure drop loss and non-equilibrium loss, along with a resultant total thermodynamic loss, across the stages of a conventional prior art multi-stage flash evaporation system.

[0037] FIG. 6 is a graph showing the combined effect of the losses of FIG. 5, shown as per-stage and accumulated mass flow rates of product distillate, for a conventional prior art multi-stage flash evaporation system.

[0038] FIG. 7 diagrammatically shows an alternative embodiment of the combination multi-effect distillation and multi-stage flash evaporation system.

[0039] FIG. 8 diagrammatically shows a further alternative embodiment of the combination multi-effect distillation and multi-stage flash evaporation system.

[0040] Similar reference characters denote corresponding features consistently throughout the attached drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0041] The combination multi-effect distillation and multi-stage flash evaporation system 10, as shown in FIG. 1, combines a multi-effect distillation (MED) system, similar to the MED system 100 of FIG. 2, with a multi-stage flash (MSF) evaporation system, similar to the MSF evaporation system 200 of FIG. 3. The multi-stage flash evaporation portion of system 10, shown in FIG. 1, begins with a mixture of seawater feed and recycled concentrated brine entering the system under pressure, being drawn into conduits or pipes 32 via a mixer 68 or the like. The seawater feed is drawn from an outside source by a pump 28 and passes through the final condenser 24 of the multi-effect distillation portion of system 10, as will be described in detail below. Prior to injection into the MSF process, the total volume (or, alternatively, only a first portion) of seawater feed is preferably pre-treated by passage through a filtering system 40 using a nanofiltration (NF) membrane or the like. Selective flow control of the first portion of seawater feed passing to the filtering system 40 may be provided by any suitable type of valve 46. A second portion of seawater feed bypasses the filtering system 40 through any suitable type of valve 42, as shown in FIG. 1. Filtering system 40 may be coupled selectively with a secondary, low-pressure microfiltration (MF) or ultrafiltration (UF) membrane filtration system 44. Untreated seawater or rejected brine from the filtering system 40 may be expelled from the system via outlet 48. The pre-treated seawater is then passed to the MSF portion of system 10 through the feed heaters 20 of the multi-effect distillation portion of system 10, as will be described in detail below. It should be understood that while seawater is discussed herein, other kinds of saltwater, e.g., brine, can be treated by the multi-effect distillation and multi-stage flash evaporation system.

[0042] The pre-treated, pre-heated seawater stream joins the first portion of the recycled concentrated brine stream from conduit or pipe 64, and the two streams are then mixed together in mixer 68 or the like. The mixture of seawater and brine is transported, under pressure, through conduits or pipes 32 to a brine heater 14, which then delivers heated brine to flash chambers 16. A boiler 12, which combusts fuel to heat recycled condensate along with additional makeup water, acts as the steam generator, supplying the brine heater 14 with the heating steam needed to heat up the brine. Following heat transfer to the brine, the steam condenses, and this condensate follows conduit or pipe 52 back into boiler 12 for recycling as steam. The condensate is pressurized by a pump 50. Additionally, a desuperheater 54 may be provided, as shown. The desuperheater 54 is used to inject controlled amounts of cooling water (i.e., condensate selectively provided by pump 50 through pipe or conduit 56) into the superheated steam flow to reduce or control steam temperature.

[0043] The flash chambers 16 act in a manner similar to those of the conventional MSF system 200 of FIG. 3, yielding desalinated distilled water, which is drawn out of the MSF portion by pipe or conduit 58. Recycle pump 60 passes first portion of the concentrated brine through recycle pipe or conduit 64 to mix at 68 with the pre-treated, pre-heated seawater, under control of a valve 62. The remainder of the concentrated brine enters the feed water inlet 36 of the MED portion of system 10, under control of valve 66.

[0044] The MED portion includes multiple effects 18, which operate in a manner similar to the conventional multi-effect distillation evaporator 100 of FIG. 2. As shown in FIG. 1, a portion of the steam generated by boiler 12 may be diverted along pipe or conduit 70, supplying heated steam to the steam chest of the first effect. A desuperheater 72, similar to desuperheater 54, may be provided in pipe or conduit 70, with a portion of the condensate being delivered thereto via pipe or conduit 74 and being pressurized by pump 76.

[0045] Heating vapor for each further effect of the MED portion is provided from vapor generated in the previous effect, after passing through the feed heaters 20, to heat up a portion of the brine entering from the feed water inlet 36 and converting it to vapor. The condensed vapor from each effect 18, which is desalinated water, is collected in the respective receptacle 22 via a pipe or conduit 26, with the first receptacle receiving the distillate from the MSF portion of system 10 via pipe or conduit 58. The vapor from the final effect passes through the final condenser 24. After condensing in the final condenser, the condensate is mixed with the distillate stream from last receptacle 22 via pipe or conduit 34, forming the final distillate product (i.e., desalinated water), which is removed by distillate pump 80. Reject brine is removed from final effect 18 by pump 78. Receptacles 22 are preferably provided with flashing pots, such that the distillate from the MSF portion, along with distillate from each subsequent effect of the MED portion, is fed to each flashing pot associated with a particular effect, where the pressure in the flashing pot is maintained at a specific vacuum, thus causing flashing of the distillate to occur at a desired rate. For example, pressure in the first one of the plurality of flash pots is equalized with the pressure in the first effect via pipe or conduit 30, and the first flash pot receives the distillate from the MSF portion of system 10 via pipe or conduit 58. Pressure in the second flash pot is equalized with pressure in the second effect via pipe or conduit 30, and the second flash pot receives distillate from the first effect via pipe or conduit 26, along with remaining unevaporated distillate from the first flash pot. This process continues until the pressure in the last flash pot is equalized with the pressure in the final condenser 24 via pipe or conduit 30. The last flash pot receives distillate from the last effect via pipe or conduit 26 along with the remaining unevaporated distillate from the previous flash pot.

[0046] System 10 shifts the flashing temperature range of the MSF process upward on the temperature scale while incorporating an MED subunit into the MSF system in the lower temperature range. Typical MSF plants operate under normal conditions with a flashing temperature range between about 40 C. and 90 C. In system 10, though, maintaining a similar 50 C. flashing span, the same MSF portion of system 10 can be operated for a flashing temperature range between 60 C. and 110 C., leaving the low temperature range between 40 C. and 60 C. for the added MED portion. Effectively, this is an expansion of the flashing temperature range similar to that in the high temperature operation of some MSF plants, but with a far better utilization of the flashing temperature range, especially the lower temperature range.

[0047] In FIG. 1, the MED portion operates on the lower temperature side of system 10. As described above, a portion of the concentrated brine from the MSF and the entire MSF product distillate continue to produce vapor by boiling and flashing through the MED portion, while the remaining portion of the concentrated brine is recycled back in the MSF portion after mixing with the makeup seawater feed. It should be noted that apart from the necessary changes in the process temperature and pressure gradients, the MSF process in system 10 remains relatively unchanged from a conventional MSF system, with the exception of the heat rejection section in the conventional system, which is no longer required in system 10. Instead, the heat rejection stages are added to the heat recovery section.

[0048] In the alternative embodiment of FIG. 7, heat is recycled within the MED portion. In FIG. 7, vapor in the lowest temperature effect 18 and vapor flashed in the last receptacle 22 are recycled back by the thermal vapor compressor (TVC) 84 to be used as the heating steam driving the first effect. In the further alternative embodiment of FIG. 8, heat is recycled in both the MED and MSF portions. In FIG. 8, the system operates in a manner similar to that of the system of FIG. 7, however a further appropriate amount of vapor produced in the last few stages of the MSF is used to replace the heating steam supplied to the brine heater 14. In FIG. 8, this is shown being performed by a mechanical vapor compressor (MVC) 82, however it should be understood that any suitable type of compressor, including a TVC, may be used in this recycling process.

[0049] In the embodiments of FIGS. 7 and 8, the MSF portion is of recycle type, but the heat rejection stages are joined with the heat recovery stages. The MED portion still operates in a conventional manner, where feed water is heated in the MED feed heaters 20 and then mixed with recycled brine for further heating in the MSF stages 16 until it reaches the brine heater 14. The portion of the concentrated brine withdrawn from the MSF portion is fed to the first MED effect for further boiling, flashing, and evaporation.

[0050] The distillate from the MSF portion is fed to first flashing pots 22, where the pressure therein is maintained at the vacuum of the first effect, thus causing flashing of the distillate to occur at the desired rate. The vapor released by flashing of the distillate is passed on to join the vapor heating the feed in the respective feed heater 20. The brine reject from each effect 18, operating on the higher temperature side of the system, is passed on to the subsequent effect to allow further boiling and flashing and generation of vapors. Similarly, the product distillate of each effect 18 is passed on to the next lower temperature flash pot 22 to allow recovery of its excess heat by partial flashing. The vapor released in the last effect 18 can either be passed on to the final condenser 24, where it condenses at the lowest process temperature, and, thus, the lowest pressure, or alternatively be compressed by a thermal vapor compressor (TVC) 84 or the like for reuse, as in the alternative embodiments of FIGS. 7 and 8.

[0051] In order to show the effectiveness of the combination multi-effect distillation and multi-stage flash evaporation system 10, Tables 1A and 1B below show sample performance characteristics of conventional MED and MSF systems compared against the combination MED-MSF desalination system operating as a heat-driven system (i.e., the embodiment of FIG. 1); a TVC-driven MED portion and heat-driven MSF portion of the combination MED-MSF desalination system (i.e., the embodiment of FIG. 7); and a TVC-driven MED portion and a MVC-driven MSF portion of the combination MED-MSF desalination system (i.e., the embodiment of FIG. 8). Process performance indicators include equivalent gain output ratio (GOR) in unit mass of distillate per unit mass of equivalent amount of heating steam (e.g., kg distillate/kg equivalent heating steam), total energy input (thermal and electrical) in kWh per ton of distillate, total exergy input (including the sum of all actual useful energy based on the Second Law of thermodynamics) in kWh per ton of distillate, and product water recovery ratios (distillate mass/seawater makeup feed, or distillate mass/total seawater feed including cooling). As can be seen in Tables 1A and 1B, the GOR is significantly higher for the combination MED-MSF desalination system embodiments compared against the conventional MED process and the conventional MSF process. Further, the energy input and exergy input are significantly lower for the combination MED-MSF desalination system compared against the conventional MED process and the conventional MSF process. The most significant of these indicators is the exergy input, which shows that the heat-driven MED-MSF combination is the most efficient of all three embodiments. Furthermore, in terms of product water recovery ratio, the MED-MSF combination is clearly superior compared to the conventional MED process and the conventional MSF process. The product water recovery ratio for the heat-driven MED-MSF combination system is the highest in all three embodiments of the MED-MSF combination system. With such superior performances and higher product water recovery ratios, the combination MED-MSF desalination system clearly outperforms the other systems and techniques in terms of operating and overall product water unit costs.

TABLE-US-00001 TABLE 1A Comparison of the Combination MED-MSF Desalination System Against Conventional MSF Number TVC of TBT C./ Motive Stages or Last Stage or Effect Heating Steam Effects Temp C. Steam C. C. System Description MSF MED MSF MED MSF MED MED Conventional MED N/A 9 N/A 75/40 N/A 80 N/A System (FIG. 2) Conventional MSF 19 N/A 90.6/40.6 N/A 100 N/A N/A System (FIG. 3) Conventional MSF 23 N/A 110/40.6 N/A 120 N/A N/A System (FIG. 3) Combination MED-MSF 19 9 110/76.3 76.3/40 120 82.2 N/A System (FIG. 1) Combination MED-MSF 19 9 110/76.4 76.4/40.5 120 N/A 141.68 System (FIG. 7) Combination MED-MSF 19 9 110/82.2 82.2/46 N/A N/A 141.68 System (FIG. 8)

TABLE-US-00002 TABLE 1B Comparison of the Combination MED-MSF Desalination System Against Conventional MSF Equivalent Performance Exergy Input Product Water GOR Ratio (Thermal + Recovery Ratio (Kg distillate/ (kWh/ton Pumps, kWh/ton (ton distillate/ System Description kg steam) distillate) distillate) ton seawater) Conventional MED 7.71 82.87 11.56 0.398/0.0675 System (FIG. 2) Conventional MSF 7.14 316.97 61.37 0.3745/0.0914 System (FIG. 3) Conventional MSF 8.9 248.22 58.61 0.3745/0.1149 System (FIG. 3) Combination MED-MSF 10.57 57.79 3.69 0.495/0.107 System (FIG. 1) Combination MED-MSF 9.48 52.8 8.0 0.388/(N/A) System (FIG. 7) Combination MED-MSF 15.4 58.6 14.73 0.4762/(N/A) System (FIG. 8)

[0052] It is to be understood that the present invention is not limited to the embodiments described above, but encompasses any and all embodiments within the scope of the following claims.