Abstract:
Water can be separated from a liquid composition (e.g., salt water) by directing a carrier gas flow through an evaporator and directly contacting the carrier gas flow with the liquid composition in the evaporator to humidify the carrier gas with water evaporated from the liquid composition, producing a humidified gas flow, which is then compressed by injecting a fluid that includes steam and/or an organic compound at an elevated pressure at least five times greater than the pressure in the evaporator and at a temperature at least as high as a saturation temperature of the steam/organic compound at the elevated pressure of the fluid. After being compressed, the humidified gas flow is directed through at least one condenser where water is condensed from the compressed humidified gas flow and collected; and the dehumidified gas flow is re-circulated back through the evaporator for reuse as the carrier gas.

Description:
BACKGROUND 
     In this century, the shortage of fresh water may rival or surpass the shortage of energy as a global concern for humanity, and these two challenges are inexorably linked, as explained in the “Special Report on Water” in the 20 May 2010 issue of The Economist. Fresh water is one of the most fundamental needs of humans and other organisms; each human needs to consume a minimum of about two liters per day. The world also faces greater fresh water demands from farming and industrial processes. 
     The hazards posed by insufficient water supplies are particularly acute. A shortage of fresh water may lead to a variety of crises, including famine, disease, death, forced mass migration, cross-region conflict/war, and collapsed ecosystems. In spite of the criticality of the need for fresh water and the profound consequences of shortages, supplies of fresh water are particularly constrained. It is estimated that 97.5% of the water on Earth is salty, and about 70% of the remainder is locked up as ice (mostly in ice caps and glaciers), leaving only a fraction of all water on Earth as available fresh (non-saline) water. 
     Moreover, the Earth&#39;s water that is fresh and available is not evenly distributed. For example, heavily populated developing countries, such as India and China, have many regions that are subject to scarce supplies. Further still, the supply of fresh water is often seasonally inconsistent. Meanwhile, demands for fresh water are tightening across the globe. Reservoirs are drying up; aquifers are falling; rivers are dying; and glaciers and ice caps are retracting. Rising populations increase demand, as do shifts in farming and increased industrialization. Climate change poses even more threats in many regions. Consequently, the number of people facing water shortages is increasing. Naturally occurring fresh water, however, is typically confined to regional drainage basins; and transport of water is expensive and energy-intensive. 
     On the other hand, many of the existing processes for producing fresh water from seawater (or to a lesser degree, from brackish water) require massive amounts of energy. Reverse osmosis (RO) is currently the leading desalination technology, but it is energy intensive and still relatively inefficient due to the large pressures required to drive water through membranes and their tendency for fouling. In large-scale plants, the specific electricity required can be as low as 4 kWh/m 3  at 30% recovery, compared to the theoretical minimum around 1 kWh/m 3 , though smaller-scale RO systems (e.g., aboard ships) have much worse efficiency. 
     Other known systems used in existing seawater desalination systems include thermal-energy-based multi-stage flash (MSF) distillation, which typically also is an energy- and capital-intensive process, and multi-effect distillation (MED). In MSF and MED systems, however, the maximum brine temperature and the maximum temperature of heat input are limited in order to avoid calcium sulphate precipitation which leads to the formation of hard scales on the heat transfer equipment. 
     Humidification-dehumidification (HDH) desalination systems include an evaporator and a condenser as their main components and use a carrier gas (e.g., air) to communicate energy between the heat source and the brine. In the evaporator, hot seawater comes in direct contact with dry air and this air becomes heated and humidified. In the condenser, the heated and humidified air is brought into (indirect) contact with cold seawater and gets dehumidified, producing pure water and dehumidified air. Some of the present inventors were also named as inventors on the following patent applications that include additional discussion of HDH processes for purifying water: U.S. Pat. No. 8,292,272 B2; and U.S. Pat. No. 8,252,092 B2. 
     Although the importance of using a low entropic heat source is known in thermal applications, such as power production, use of a high-temperature energy source has not been feasible for seawater desalination until now because of the calcium sulphate fouling problems. In the last decade, the top brine temperature (and correspondingly the heating steam temperature) of thermal desalination systems has been increased from 70-90° C. to 90-120° C. using water softening technology, such as nanofiltration and hybrid systems. Further increase in heating steam temperature (and hence, decrease in total entropy entering the system) using such methods is thought to be unfeasible, however, due to the fouling problems. 
     SUMMARY 
     Methods and apparatus for high-efficiency thermal-energy-based water purification are described herein. The methods and apparatus can be used to produce substantially pure water from, e.g., salt water (such as sea water or brackish water) or from waste water, agricultural run-off water, rain water, and groundwater. Various embodiments of the device and method may include some or all of the elements, features and steps described below. For the purpose of providing a consistent and coherent exemplification, much of the discussion that follows is directed to desalination, though the approach can be readily employed in other contexts separating water from other liquid compositions. 
     Notwithstanding excellent energy recovery in previous multi-stage flash (MSF) and multi-effect distillation (MED) systems, their thermal performance is reduced because the lower temperature of the input heat results in introduction of a higher entropy rate into those systems. As an alternative, the thermal desalination methods and apparatus described herein can be driven using a much-higher-temperature and much-higher-pressure heat input, which is equivalent to heat input with a much lower total specific entropy rate. The methods and apparatus also include methods for advantageously recovering and reusing the energy input to the system, and apparatus therefore, in order to increase the efficiency of water production. 
     In a method of this disclosure, a flow of a carrier gas is directed through at least one evaporator operated at an evaporator pressure, which can be sub-atmospheric (i.e., less than that of the surrounding external environment). In the evaporator, the carrier gas flow directly contacts a liquid composition to humidify the carrier gas with water evaporated from the liquid composition, producing a humidified gas flow. Next, the humidified gas flow is compressed by injecting a fluid at an elevated pressure at least five times greater than the evaporator pressure. The injected fluid comprises a vapor selected from at least one of steam and an organic compound at a temperature at least as high as a saturation temperature of the vapor at the elevated pressure of the fluid. The compressed and humidified gas flow is then directed through at least one condenser, where the compressed humidified gas flow is dehumidified by condensing water from the compressed humidified gas, and that condensed water is collected. The dehumidified gas flow is then recirculated from the condenser back through the evaporator, where the dehumidified gas is reused as the carrier gas in a closed loop. 
     Accordingly, in embodiments of a variable-pressure HDH system for water desalination, high-quality heat, at a low specific entropy and a high specific enthalpy, can be input in the form of high-temperature and high-pressure steam [e.g., at a temperature over 120° C. or at least 200° C. and at a pressure of at least 10 bar (1 MPa), and in a particular embodiment, at 300° C. and 30 bar (3 MPa)] at the thermal vapor compressor to compress, heat, and further humidify the moist carrier gas exiting the evaporator. The energy can be used efficiently in the cycle to produce desalinated water, and the energy can be recovered in the form of mechanical work from the dehumidified moist air in an expander. To increase the water vapor content in the carrier gas, the evaporator can be operated under sub-atmospheric pressure conditions. By virtue of the lower total entropy rate of the steam entering the system, the sub-atmospheric operation and the energy recovery in the expander, much higher system efficiencies can be achieved, delivering much higher performance than conventional HDH systems. 
     The mechanical work recovered from the expander can be used, for example, in an auxiliary water-purification apparatus, such as a reverse osmosis (RO) unit, to further desalinate the brine exiting the HDH system to increase water production and the recovery ratio. This hybrid HDH-RO system can have higher energy efficiencies. The recovered mechanical work can also be reused in a mechanical compressor to compress moist air before or after the thermal compressor. 
     The methods and apparatus described herein can also improve performance, increasing the temperature and pressure of energy input without exceeding a top brine temperature of 70° C. and by advantageously recovering and reusing the high-quality energy input. In the methods and apparatus, the quality of steam can be increased such that the total entropy rate entering the system is reduced by up to 50%, which can potentially revolutionize the thermal desalination field. Applications of this technology includes desalination of seawater and has wide spread usage in other forms of water purification and extraction as well as for remediation of water-based waste products, e.g., from industrial oil production. The methods and apparatus are also especially applicable to water-and-power coproduction systems. And because embodiments of the system can be operated with lower energy demands than those of other desalination approaches, the system and methods can be powered entirely by an alternative energy source, such as solar panels and solar steam generators (as solar energy is often plentiful in arid regions), and can be operated free of a connection to an external power grid (for the area/region) or in the absence of any available or nearby power grids. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a high-temperature-and-high-pressure-steam-driven system for humidification-dehumidification (HDH) desalination using a thermal vapor compressor. 
         FIG. 2  is a general representation of the least thermal energy required to drive a thermal desalination system as a function of the temperature, T st,in , of the steam input to the thermal vapor compressor and as a function of the total specific entropy rate, {dot over (S)} in /{dot over (m)} pw , of the steam. The calculations here are for a reversible system. 
         FIG. 3  is a schematic diagram of a high-temperature-and-high-pressure-steam-driven system for humidification-dehumidification desalination using a thermal vapor compressor coupled with an auxiliary water production unit. 
         FIG. 4  is a psychometric representation of a representative embodiment of a thermal-vapor-compression-driven HDH system in accordance with this disclosure. 
         FIG. 5  is a plot of the effect of steam pressure on the performance of a representative embodiment of a thermal-vapor-compression-driven HDH system in accordance with this disclosure, wherein the gained output ratio and the equivalent electricity consumption are plotted as a function of the pressure of the steam input to the thermal vapor compressor. 
         FIG. 6  is a schematic diagram of a high-temperature-and-high-pressure-steam-driven system for humidification-dehumidification desalination using a thermal vapor compressor with a heat pump. 
         FIG. 7  is a schematic diagram of a high-temperature-and-high-pressure-steam-driven system for humidification-dehumidification desalination using a thermal vapor compressor with a refrigeration or air-conditioning unit. 
         FIG. 8  is a schematic diagram of a co-production, high-temperature-and-high-pressure-steam-driven system for humidification-dehumidification desalination using a thermal vapor compressor. 
         FIG. 9  is a schematic diagram of a high-temperature-and-high-pressure-steam-driven system for humidification-dehumidification desalination using a thermal vapor compressor and multiple extraction flows for the carrier gas. 
         FIG. 10  is a schematic diagram of a high-temperature-and-high-pressure-steam-driven system for humidification-dehumidification desalination using a thermal vapor compressor and multiple extraction flows for the brine and liquid mixture. 
     
    
    
     In the accompanying drawings, like reference characters refer to the same or similar parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating particular principles, discussed below. 
     DETAILED DESCRIPTION 
     The foregoing and other features and advantages of various aspects of the invention(s) will be apparent from the following, more-particular description of various concepts and specific embodiments within the broader bounds of the invention(s). Various aspects of the subject matter introduced above and discussed in greater detail below may be implemented in any of numerous ways, as the subject matter is not limited to any particular manner of implementation. Examples of specific implementations and applications are provided primarily for illustrative purposes. 
     Unless otherwise defined, used or characterized herein, terms that are used herein (including technical and scientific terms) are to be interpreted as having a meaning that is consistent with their accepted meaning in the context of the relevant art and are not to be interpreted in an idealized or overly formal sense unless expressly so defined herein. For example, if a particular composition is referenced, the composition may be substantially, though not perfectly pure, as practical and imperfect realities may apply; e.g., the potential presence of at least trace impurities (e.g., at less than 1 or 2% by weight or volume) can be understood as being within the scope of the description; likewise, if a particular shape is referenced, the shape is intended to include imperfect variations from ideal shapes, e.g., due to machining tolerances. 
     Spatially relative terms, such as “above,” “upper,” “beneath,” “below,” “lower,” and the like, may be used herein for ease of description to describe the relationship of one element to another element, as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the apparatus in use or operation in addition to the orientation depicted in the figures. For example, if the apparatus in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the exemplary term, “above,” may encompass both an orientation of above and below. The apparatus may be otherwise oriented (e.g., rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. 
     Further still, in this disclosure, when an element is referred to as being “on,” “connected to” or “coupled to” another element, it may be directly on, connected or coupled to the other element or intervening elements may be present unless otherwise specified. 
     The terminology used herein is for the purpose of describing particular embodiments and is not intended to be limiting of exemplary embodiments. As used herein, the singular forms, “a,” “an” and “the,” are intended to include the plural forms as well, unless the context clearly indicates otherwise. Additionally, the terms, “includes,” “including,” “comprises” and “comprising,” specify the presence of the stated elements or steps but do not preclude the presence or addition of one or more other elements or steps. 
     An embodiment of a high-temperature-and-high-pressure-steam-driven system for humidification-dehumidification (HDH) water purification using a thermal vapor compressor is illustrated in  FIG. 1 . This cycle involves the humidification of the carrier gas by water from the liquid composition followed by the dehumidification of the humidified carrier gas to release fresh water. The apparatus includes an evaporator  12  for humidification and a condenser  14  for dehumidification with conduits for the passage of a liquid composition and carrier gas therebetween. 
     The energy for this cycle is input into the carrier gas after humidification in the evaporator  12  in the form of compression, and the carrier gas is then dehumidified in the condenser  14 . The carrier gas after dehumidification can be expanded by an expander  20  to a lower pressure, wherein the expander  20  expands the carrier gas to reduce its pressure and its temperature before it is reintroduced into the evaporator  12 . 
     The separation of the humidification and dehumidification functions into distinct components (i.e., into the evaporator  12  and condenser  14 ) in the HDH apparatus can reduce thermal inefficiencies and improve overall performance. For example, recovery of the latent heat of condensation in the humidification-dehumidification process is affected in a separate heat exchanger (i.e., the condenser  14 ) in which the seawater, for example, can be preheated. The HDH process thus can provide high productivity due to the separation of the basic processes. 
     Using the apparatus and methods described herein, the principle of humidification-dehumidification of a carrier gas can be utilized to separate substantially pure water from a liquid composition. The liquid composition can be in the form of a water-based solution with dissolved components, such as salts, and/or a mixture containing solids and/or other liquids. The process is herein described in the context, for example, of water desalination, where purified water is separated from salt water, though the process and apparatus can likewise be utilized in the context of separating water from other liquid compositions. 
     In the humidification-dehumidification methods, an inert carrier gas that can hold water vapor (e.g., selected from air, nitrogen, helium, carbon dioxide, argon, hydrogen, etc.) having the ability to carry water vapor is used as the medium for separating substantially pure water from the liquid composition (e.g., seawater, brackish water, etc.). In the embodiment of  FIG. 1 , the carrier gas is circulated through the evaporator  12  and condenser  14  and therebetween via conduits  18  and  22  in a closed loop. 
     The evaporator  12  can be filled with a packing material in zone  32  in the form, e.g., of polyvinyl chloride (PVC) packing to facilitate the gas flow and to increase the liquid surface area that is in contact with the carrier gas. The body of the evaporator  12  (and the condenser  14 ) can be formed, e.g., of stainless steel and is substantially vapor impermeable; seals formed, e.g., of epoxy sealant, gaskets, O-rings, welding or similar techniques, are provided at the carrier gas and water inputs and outputs of the evaporator  12  and at the interfaces of each modular component and adjoining conduits to maintain vacuum in the system. In one embodiment, the evaporator  12  is substantially cylindrical with a height of about 1.5 m and a radius of about 0.25 m. 
     The evaporator  12  and condenser  14  are both of a modular construction (i.e., each in the form of a separate and discrete device) and are substantially thermally separated from one another. The characterization of the evaporator  12  and condenser  14  as being “substantially thermally separated” is to be understood as being structured for little or no direct conductive thermal energy transfer through the apparatus between the evaporator and condenser, though this characterization does not preclude a mass flow carrying thermal energy (via gas and/or liquid flow) between the chambers. This “substantial thermal separation” characterization thereby distinguishes the apparatus from, e.g., a dewvaporation apparatus, which includes a shared heat-transfer wall between the evaporator and the condenser. In the apparatus of this disclosure, the evaporator and condenser need not share any common walls that would facilitate conductive heat transfer therebetween. 
     The carrier gas flows upward through the evaporator  12  from the port for conduit  22  to the port for conduit  18 . Humidification of the carrier gas is achieved by spraying the liquid composition from one or more nozzles  34  into a spray zone  31  at the top of the evaporator  12  then through a zone  32  including a packing material, where some of the water in the liquid composition will evaporate, while a non-evaporated remnant of the liquid composition flows down through a rain zone  33  to a surface of collected remnant liquid composition in, e.g., a tray at the bottom of the chamber. Meanwhile, the carrier gas moves up through the evaporator  12 , as shown by arrow  26 , and is brought into contact with the liquid composition, particularly in the bed of packing material, to humidify the carrier gas with water vapor evaporated from the liquid composition. The carrier gas can consequently be saturated with water vapor before being withdrawn from the evaporator  12  via conduit  18 . 
     In conduit  18 , the moisture-laden low-pressure carrier gas is then compressed to a higher pressure and higher temperature in the thermal vapor compressor (also referred to as a “thermocompressor”)  16  and introduced into the condenser  14 , where the water is condensed from the gas via a dehumidification process. The thermal vapor compressor  16  can be, for example, in the form of a steam-jet ejector employing a Venturi with a conduit for steam input  36 . In an example of this embodiment, the thermal vapor compressor  16  can compress the carrier gas using saturated steam (e.g., from an associated power plant) at a pressure of 1 to 3 MPa. Optionally, a mechanical compressor can also be included in conduit  18 , either upstream or downstream of the thermal vapor compressor  16 , to provide additional compression of the carrier gas. 
       FIG. 2  shows the effect of increasing the heating steam temperature, T st,in , (and correspondingly reducing the total specific entropy rate of the steam entering the system) on the least thermal energy required to produce 1 kg/s of water in a thermal water purification system. The total specific entropy rate of the steam entering the system is represented as {dot over (S)} in /{dot over (m)} pw  [kJ/kg·K], where {dot over (S)} in  is the total entropy rate of the input steam (W/K), and {dot over (m)} pw  is the mass flow rate of the product water. For this illustration, the salinity, S 1 , of the seawater feed is set at 35,000 ppm; and the temperature, T 0 , of the seawater feed is set at of 30° C. Seawater properties are evaluated using the correlations presented by M. Sharqawy, et al., “Thermophysical properties of seawater: A review of existing correlations and data,” Desalination and Water Treatment (16), pages 354-380 (2010). The curves in  FIG. 2  are plotted at a recovery ratio (RR) of 50%. It is observed that by increasing the steam temperature from 90° C. to 120° C. (and correspondingly reducing {dot over (S)} in  by 31.6%) the least thermal energy required is reduced by 27%. If the steam temperature can be further increased to 200° C. (correspondingly reducing {dot over (S)} in  by 59%) the least thermal energy required is reduced by 50%. The specific least thermal energy consumed is represented as Δ{dot over (H)} in,least /{dot over (m)} pw  [kJ/kg], where {dot over (H)} is the total enthalpy rate (W). 
     The dehumidification process also results in heating of the liquid composition that is eventually used to irrigate the evaporator  12 . The liquid composition is pumped via a pump, not shown, at a substantially constant mass flow from a source  28 , which can be, for example, a tank fed by a sea, ocean, groundwater, waste pool or other body of water, through a liquid-feed conduit  30 , as shown in  FIG. 1 , that passes through the condenser  14 , wherein the liquid composition is preheated before entering the evaporator  12 , thus recovering some of the energy input to the thermal vapor compressor  16  in the form of thermal energy, which is given back to the carrier gas stream in the evaporator  12 . The liquid-feed conduit  30  can have a flow configuration inside the condenser  14  that will increase its surface area to allow for thermal energy transfer from the moisture-laden carrier gas (thereby driving precipitation of water from the carrier gas) through the walls of conduit  30  into the liquid composition. The water vapor in the carrier gas therefore condenses (along path  24 ) and is collected as substantially pure (fresh) water at the bottom of the condenser  14 , e.g., in a collection tray. The collected fresh water, can then be removed from the condenser  14  to, e.g., a storage tank  29  for use, e.g., as drinking water, for watering crops, for washing/cleaning, for cooking, for industrial use, etc. 
     The evaporator  12  and condenser  14  are operated at different pressures, wherein that pressure difference is maintained using the thermal vapor compressor (TVC)  16  to compress the humidified carrier gas with the steam supply and by using the expander  20  (e.g., in the form of a throttle valve, a nozzle, a turbine, a screw, a reciprocating expander, a centrifugal expander or a scroll expander) to expand the dehumidified carrier gas and recover energy in the form of work, {dot over (W)} out , as shown in  FIG. 1 , where the recovered work can be in the form of a mechanical energy or which can be converted to electricity by a generator, can be used in an auxiliary water-purification (e.g., desalination) apparatus  42 , such as a reverse osmosis unit (or a mechanical vapor-compression system or an electro-dialysis system), as shown in  FIG. 3  to desalinate the brine output from conduit  40  the evaporator  12 . The auxiliary water-purification apparatus  42 , in turn, produces additional fresh water, which is output to the pure water storage tank  29 , and a higher-concentration brine, which is output to a brine containment  44 . A small amount of water can be condensed out of the carrier gas in both the thermal vapor compressor  16  and expander  20  and be collected. 
     Alternative water-purification apparatus that can be driven by the recovered work, {dot over (W)} out , include a mechanical vapor compression system, which likewise can be driven by work in the form of a mechanical drive force or after conversion to electricity, or an electrodialysis system, which can be powered by work that has been converted to electricity. In other embodiments, the recovered work, {dot over (W)} out , can be used for a variety of other applications, including pumping the carrier gas or liquid composition, providing electricity to the electrical grid, powering associated electrical components (such as sensors or controllers), or compress moist air with a mechanical compressor either upstream or downstream of the thermal vapor compressor  16  in conduit  18 . 
     The carrier gas can operate in a closed loop and undergo the humidification process at a lower pressure and the dehumidification process at a higher pressure. The carrier gas and/or the liquid composition can also be heated in the system with a gas or liquid heater to achieve or maintain desired temperatures. The pressure ratio (i.e., the ratio of the absolute pressure in the condenser  14  to the absolute pressure in the evaporator  12 ) can be, for example, about 1.2. This pressure differential creates an opportunity for greater heat recovery for the following reasons: (1) the heat recovered in the condenser  14  from the carrier gas to pre-heat the liquid composition is of a higher grade (higher temperature); (2) the carrier gas itself is heated (apart from getting humidified) in the evaporator  12  by virtue of being at a lower temperature than the liquid composition; and (3) heat can be recovered as work output from the expander  20 , as described herein. 
     A part of the compressor work can be supplied by the work, {dot over (W)} out , extracted from the expansion process, for example, by coupling the expander  20  to a mechanical compressor positioned upstream from the thermal vapor compressor  16  or by using a motor-generator arrangement to transfer the work, {dot over (W)} out , from the expander  20  to the thermal vapor compressor  16  to compress the injected fluid. Alternatively or additionally, the work, {dot over (W)} out , extracted from the expander  20  can be used to drive a heat pump to heat the fluid injected by the thermal vapor compressor  16 . Cooling of carrier gas via expansion en route to the evaporator  12  results in a lower temperature in the evaporator  12 , which also improves the performance of the cycle. 
     A psychrometric chart for an exemplary embodiment of a desalination process conducted with this apparatus and in accord with these methods is provided in  FIG. 4 , where air is the carrier gas, and where the pressure for dehumidification, P D , in the condenser  14  is greater than the pressure for humidification, P H , in the evaporator  12 . Path  1 - 2  in  FIG. 4  is the carrier-gas humidification process in the evaporator  12 , wherein the process is approximated to follow the saturation line. Path  2 - 3  is the thermo-compression process in which the humidified carrier gas is compressed to a higher pressure and temperature in the thermal vapor compressor  16 . Path  3 - 4  is the dehumidification process in the condenser  14 , wherein this process also approximated to follow the saturation line at a higher pressure, P D . Path  4 - 1  is the air expansion process through the expander  20 , where some of the energy that was input in the compressor  16  is recovered. 
     High pressure and high temperature are possible because the heating steam is not brought in direct contact with the liquid composition (e.g., seawater), it is instead brought in contact with the vapor laden carrier gas. Thus, this new system can be designed such that the brine temperature does not exceed 60° C. For standard seawater concentrations, this is sufficient to avoid scale formation. 
     In order to reduce the heat required to run a thermal desalination system, the input heating steam needs to be at a lower entropy state or of a lower mass flow rate. We will first investigate the effect of using low entropy, higher pressure (saturated) steam.  FIG. 5  illustrates the increase in the Gained Output Ratio (GOR), which is the ratio of the latent heat of evaporation of the water produced to the net heat input to the cycle, and the decrease in equivalent electricity consumption when higher pressure steam is used in a simulated experiment. For this example, the component effectivenesses and efficiencies are fixed along with the operating pressures and feed seawater conditions. The air and water side pressure drops are assumed to be zero. The temperature of the incoming seawater is 30° C.; the energy based effectiveness of both the evaporator  12  and the condenser  14  is set at 80%; the isentropic efficiency of both the thermal vapor compressor  16  and the expander  20  is assumed to be 100%; the pressure in the evaporator  12  is 40 kPa; the pressure in the condenser  14  is 48 kPa; the quality of the input steam is assumed to be 1 (ideal); and the heat capacity ratio (HCR) of the evaporator is set at 1. 
     The strong impact that an increase in steam pressure can have on the performance of the humidification-dehumidification thermal-vapor-compressor (HDH-TVC) system can be clearly observed in  FIG. 5 . In this example, when the steam pressure is increased from 250 kPa to 1000 kPa (i.e., from a saturated steam temperature of 127.4° C. to 179.9° C.), the GOR is increased by 45%. Even though a higher pressure steam is used, the equivalent electricity consumption is still reduced by 9% for the aforementioned increase in steam pressure. This is because the mass flow rate of high-pressure steam extracted from the (fictious) steam turbine is small, and the corresponding work lost in the steam turbine is reduced. 
     As noted, above, the work recovered from an expander  20  can be used to drive a variety of processes within and/or outside the HDH apparatus.  FIG. 6  illustrates an embodiment of the system in which the recovered work, {dot over (W)} out , from the expander  20  is supplied in the form of heat energy to a heat pump  46  where the heat is recycled (a) to the superheated steam supply that is then fed into the thermal vapor compressor  16  or (b) to the steam generation unit for generating the superheated steam. In an alternative embodiment, illustrated in  FIG. 7 , the recovered work, {dot over (W)} out , from the expander  20  is supplied to a refrigeration or air-conditioning  48  unit where the work energy extracts heat from an object to be refrigerated via a vapor compression refrigeration device. 
     An embodiment where the steam is supplied from a Rankine or combined cycle power plant  50  is illustrated in  FIG. 8 . In this embodiment, heat  52  is input to the power plant  50 , which produces a work output, {dot over (W)} out , and an output of steam  36 . The output steam  36  is fed to the thermal vapor compressor  16  where it is injected as a low-entropy heat source into the carrier gas in conduit  18 . This embodiment can be combined with (a) the auxiliary water purification apparatus  42  (employing, e.g., a reverse osmosis apparatus), integrated as shown in  FIG. 3 ; (b) the heat pump  46 , integrated as shown in  FIG. 6 ; or (c) a refrigeration or air-conditioning unit, integrated as shown in  FIG. 7 . 
       FIG. 9  illustrates an embodiment of the invention that uses a multi-extraction humidification system, wherein the gas is extracted at multiple intermediate locations via intermediate conduits  56  from the evaporator  12  and/or from the condenser  14  and supplied at corresponding input locations to the condenser  14  or to the evaporator  12 , respectively, facilitating reduced entropy generation in the components and, hence, higher system performance. The gas can flow through the intermediate conduits  56  naturally, or the flow can be powered by a fan in one or more of the conduits. An expansion/compression device  58  is mounted in each intermediate conduit  56  to expand gas passing through a conduit  56  from the condenser  14  to the humidifier  12  or to compress gas passing through a conduit  56  from the humidifier  12  to the condenser  14 . The amount of gas extracted through an intermediate conduit  56  depends strongly on the operating temperatures, and this amount can be controlled by components, such as expanders and compressors that may be placed in the intermediate conduits  56 . Providing multiple extractions can serve to essentially break up the evaporator  12  and condenser  14  into a number of smaller parts with different values of mass flow ratio and to thereby balance heat capacity rates across the evaporator  12  and/or across the condenser  14 , allowing for manipulation of gas temperatures, pressures and mass flow rates. 
     In the system of  FIG. 10 , intermediate conduits  56  are provided for multi-extraction of the liquid composition. In this illustration, the inner and outer circuits for the liquid and gas flow in the system are reversed in comparison with the previous illustrations; this switch in configuration in  FIG. 10  is made for ease of illustration of the intermediate conduits  56 . In this embodiment, the packing material  60  is segregated into distinct beds separated by brine collection receptacles (e.g., trays)  62 , wherein the brine collection receptacles  62  are distributed across different heights of the humidifier  12 . Inside the humidifier  12 , brine is collected in each of the brine collection receptacles  62 . The brine can be extracted from the receptacles  62  and passed through intermediate conduits  56  to corresponding intermediate locations in the condenser, where the brine is injected into conduit  30 . Alternatively, the liquid composition (e.g., seawater) can be extracted from intermediate locations in conduit  30 , passed through conduits  56  and injected into corresponding brine receptacles  62  at the other ends of the conduits  56 . 
     In describing embodiments of the invention, specific terminology is used for the sake of clarity. For the purpose of description, specific terms are intended to at least include technical and functional equivalents that operate in a similar manner to accomplish a similar result. Additionally, in some instances where a particular embodiment of the invention includes a plurality of system elements or method steps, those elements or steps may be replaced with a single element or step; likewise, a single element or step may be replaced with a plurality of elements or steps that serve the same purpose. Further, where parameters for various properties are specified herein for embodiments of the invention, those parameters can be adjusted up or down by 1/100 th , 1/50 th , 1/20 th , 1/10 th , ⅕ th , ⅓ rd , ½, ¾ th , etc. (or up by a factor of 2, 5, 10, etc.), or by rounded-off approximations thereof, unless otherwise specified. Moreover, while this invention has been shown and described with references to particular embodiments thereof, those skilled in the art will understand that various substitutions and alterations in form and details may be made therein without departing from the scope of the invention. Further still, other aspects, functions and advantages are also within the scope of the invention; and all embodiments of the invention need not necessarily achieve all of the advantages or possess all of the characteristics described above. Additionally, steps, elements and features discussed herein in connection with one embodiment can likewise be used in conjunction with other embodiments. The contents of references, including reference texts, journal articles, patents, patent applications, etc., cited throughout the text are hereby incorporated by reference in their entirety; and appropriate components, steps, and characterizations from these references optionally may or may not be included in embodiments of this invention. Still further, the components and steps identified in the Background section are integral to this disclosure and can be used in conjunction with or substituted for components and steps described elsewhere in the disclosure within the scope of the invention. In method claims, where stages are recited in a particular order—with or without sequenced prefacing characters added for ease of reference—the stages are not to be interpreted as being temporally limited to the order in which they are recited unless otherwise specified or implied by the terms and phrasing.