ONE STEP INTEGRATION OF MEMBRANE DISTILLATION WITH DIRECT AIR-STRIPPING

Abstract

A membrane distillation (MD) system includes a sweep gas MD (SGMD) module and a knockout chamber. The MD module includes a feed inlet, a feed outlet, a condensing media inlet, and a condensing media outlet. The condensing media is sweep gas. The knockout chamber is positioned after the feed outlet. The knockout chamber includes a liquid inlet, a liquid outlet, and a vapor outlet. Direct gas phase stripping within the SGMD module leads to additional water evaporation at the knockout chamber and contributes to enhanced water or VOCs removal of the MD system.

Claims

1. A membrane distillation (MD) system comprising at least one MD module, at least one feed reservoir and at least one knockout chamber, the at least one MD module comprising at least one membrane, a sweep gas inlet operable to receive a sweep gas and a permeate outlet, a feed inlet operable to receive a feed media and a feed outlet, and at least one external heating element positioned and operable to heat the feed media to a first temperature prior to introduction of the feed media to the at least one MD module; and the at least one knockout chamber operable to receive a mixture of a portion of the sweep gas and the feed media exiting the feed outlet of corresponding at least one MD module, a first opening to release the portion of the sweep gas from the mixture, a second opening for the feed media to exit the at least one knockout chamber and recycle to the at least one feed reservoir.

2. The MD system of claim 1 further comprising more than one MD module.

3. The MD system of claim 1 further comprising a second external heating element positioned and operable to heat the stream exiting the first of the at least one MD module to at least the first inlet temperature.

4. The MD system of claim 1 further comprising more than two MD modules and more than two knockout chambers arranged serially and plural external heating elements in addition to the at least one external heating element, wherein each of the plural external heating elements is positioned and operable to heat the feed media exiting each of the more than two knockout chambers to the first temperature prior to the feed media being introduced to a successive MD module.

5. The MD system of claim 1 comprising more than two MD modules and more than two knockout chambers, wherein the at least one external heating element is a central heat exchanger positioned and operable to also heat the feed media exiting each of the more than two knockout chambers to at least the first inlet temperature prior to the feed media being introduced to a successive MD module.

6. The MD system of claim 1 wherein each of the MD modules is independently selected from the group consisting of a hollow fiber membrane module, a tubular module, a flat membrane module and a spiral wound membrane module.

8. The MD system of claim 1 wherein each of the MD modules is independently selected from the group consisting of a sweep gas membrane distillation (SGMD) module.

9. The MD system of claim 1 wherein at least one MD module comprises a spiral wound membrane module or a hollow fiber module.

10. The MD system of claim 1 wherein the at least one membrane is made of a material from the group consisting of hydrophobic polytetrafluoroethylene, hydrophobic polypropylene, hydrophobic polyvinylidene difluoride, hydrophobic polytetrafluoroethylene modified with carbon nanomaterials, carbon nanotubes, graphene oxide, hydrophobic polypropylene modified with carbon nanomaterials, and hydrophobic polyvinylidene difluoride modified with carbon nanomaterials.

11. A method for purifying a feed media comprising the steps of providing at least one MD module and at least one knockout chamber, circulating a sweep gas through at least one MD module, heating the feed media to a first temperature prior to introduction of the feed media to the at least one MD module using at least one external heating element, receiving a mixture of a portion of the sweep gas and the feed media in at least one knockout chamber, and condensing the sweep gas containing vaporized feed media exiting each MD module.

16. A membrane distillation (MD) system comprising: a MD module comprising at least one membrane, a sweep gas inlet operable to receive a sweep gas and a permeate outlet, and a feed inlet operable to receive a hot feed media and a feed outlet, and at least one external heating element positioned and operable to heat the feed media to a first inlet temperature prior to introduction of the feed media to the MD module; and a knockout chamber operable to receive a mixture of a portion of the sweep gas and the feed media containing mixture of partial water/solvent vapor and mainly liquid feed exiting the feed outlet of the MD module, the knockout chamber operably connected to a feed reservoir containing the feed media to allow recycling of the feed media, wherein the at least one external heating element is operable to maintain the feed media entering the MD module at the first inlet temperature.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] FIG. 1 illustrates a schematic diagram of an AS-SGMD system with one SGMD module coupled to a knockout chamber, wherein concentrated salt water or wastewater is recycled back to a feed reservoir in accordance with an embodiment of the inventive concepts;

[0020] FIG. 2 illustrates a schematic diagram of an AS-SGMD system with one SGMD module coupled to a knockout chamber, wherein concentrated salt water or wastewater is connected to a feed inlet of a next stage of MD system, or to an inlet of a MVC equipment in accordance with an embodiment of the inventive concepts;

[0021] FIG. 3 illustrates a SEM image of hydrophobic surface of pristine PTFE membrane showing porous structure of the pristine PTFE membrane in accordance with an embodiment of the inventive concepts;

[0022] FIG. 4 illustrates an SEM image of the hydrophobic surface modified with immobilized carbon nanotubes showing uniform distribution of CNTs over the entire membrane surface in accordance with an embodiment of the inventive concepts;

[0023] FIG. 5 is a chart illustrating correlation between linear velocity and flux, and linear velocity and recovery in accordance with embodiments of the inventive concepts; and

[0024] FIG. 6 illustrates a photograph of spiral wound AS-SGMD modules fabricated in accordance with an embodiment of the inventive concepts.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] Disclosed embodiments relate to an apparatus and methods for sweep gas membrane distillation with integrated air stripping (AS-SGMD).

[0026] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. As used herein, the singular terms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise.

[0027] Referring now to the drawings, where like elements are designated by like reference numerals, FIG. 1 is an exemplary AS-SGMD system includes one SGMD module (100) coupled to a knockout chamber (200), wherein the concentrated salt water or wastewater is recycled back to the feed reservoir (300). The SGMD module in FIG. 1 may employ membranes such as but not limited to flat membranes, hollow fiber membranes, tubular membranes, spiral wound membranes, etc. The AS-SGMD module (100) includes a feed inlet (101), a feed outlet (102), a condensing media inlet (103), a condensing media outlet (104), a feed channel (105), a permeate channel (106), and a MD membrane (107).

[0028] Feed solution in the feed reservoir (300) is heated using a heating unit (301). Conventional heating or microwave heating may be used for heating of the feed solution.

[0029] Compared to conventional heating method, microwave heating has several advantages including fast heating, simply controlled heating rate, and more efficient energy conversion. Saline water is significantly more heated than pure water due to rapid heating of salt ions, especially that of large salt ions CF, through field-induced motion by the microwave electric field and energy transfer by the interactions between salt ions and water molecules. Large salt ions that do not fit in the unit cell of the water network make defects to it and cleave the network through oscillations in response to the microwave electric field. The use of microwave heating not only promoted more efficient mass transfer in MD and thus water vapor flux enhancement, but it also consumed much less energy. Performance enhancement in MD with microwave heating can be attributed to non-thermal effects such as the generation of nanobubbles, localized superheating, and breaking down of the hydrogen-bonded salt-water or solvent-water clusters.

[0030] The heated feed solution is introduced through the feed inlet (101) into the feed channel (105), where water vapor is generated and moves to the permeate channel (106) through pores of the MD membrane (107). Condensing media, for example, sweep gas, is introduced to the SGMD module (100) via a condensing media inlet (103) at flow rate F.sub.A.

[0031] Preferably, compressed air is employed as the sweep gas and is supplied by an air pump (401). The air flow rate F.sub.A is regulated using a needle valve (402) and a flow meter (403). A mass flow controller may be employed to replace the needle valve (402) and the flow meter (403). Upon entering the SGMD module (100), a major portion of the sweep air F.sub.A1 (108) flows through the permeate channel (106) and swipes the MD generated water vapor out of the SGMD module (100) through the condensing media outlet (104) which is connected to a first condenser (404). When the air/vapor mixture passes through the first condenser (404), the vapor is condensed into liquid water and the air/water mixture flows together into a first separation unit (400). The liquid water is stored in the first separation unit (400) and can be used as potable water or process water. The air is released through an opening on the top cover of the first separation unit (404). A flow meter (405) and a needle valve (406) may be connected to the opening so as to regulate flow rate F.sub.A1 of the air flowing through the permeate channel.

[0032] Depending on the feed flow rate (not shown) and the supplied air flow rate F.sub.A, a portion of the sweep air (109) flows into the feed channels through the pores of the microporous MD membranes (107). After entering into the feed channel (105), the air is immediately brought into intimate contact with heated water in the feed channel (105) and leads to the formation of micro-sized air bubbles (110) which will eventually be saturated with water vapor due to air-stripping effect. Some of these air bubbles will be able to move together with the feed stream towards the feed outlet (102). The air/water mixture stream exiting the module (100) is routed into the knockout chamber (200) through a stream entrance at the top of the knockout chamber (200), i.e., liquid inlet. When the air/water mixture stream enters the knockout chamber (200), the air bubbles saturated with water vapor immediately separate from the liquid, break and thus release the water vapor inside them. While the air flows out of the knockout chamber (200) through an opening on the top cover, it carries not only the released vapor but also water vapor generated by evaporation of water in the knockout chamber (200). The vapor and air may be discharged directly to the environment.

[0033] Alternatively, the air/vapor mixture may be allowed to pass through a second condenser (201), wherein the vapor is condensed into liquid water and then collected in a second separation unit (202) for water reuse. A flow meter (203) may be connected to the air-releasing port of the second separation unit (202) to monitor flow rate F.sub.A2 of the air entering the feed channel (201).

[0034] After releasing the air, the concentrated feed water leaves the knockout chamber (200) through a liquid port at the lower portion of the chamber, i.e., liquid outlet, and is recycled back to the feed reservoir (300). This ensures no air enters into the feed reservoir (300) causing pressure build-up in the fully closed feed reservoir (300).

[0035] Alternatively, the feed water in the knockout chamber (200) may be directed to the next stage of MD processing or to a mechanical vapor compression (MVC) equipment for further concentration, as shown in FIG. 2.

[0036] The sweep air entering the feed channel (105) not only can cause air-stripping in the feed channel (105) but can also reduce or even prevent inorganic salt scaling (precipitation fouling) and particulate fouling.

[0037] The same system as illustrated in FIG. 1 or its variations may be used for a number of applications including but not limited to water desalination, wastewater concentration, separation of organic solvents from their aqueous mixtures, biofuels and petrochemical waste, etc.

[0038] Experiments were conducted using the system in FIG. 1.

Experiment 1

[0039] A hollow fiber SGMD module was constructed by placing Accurel PP Q3/2 hydrophobic hollow fiber membranes (about 0.6 mm inner diameter) through a #16 PTFE tubing connected to a compression tee connector at each end. Epoxy was used to seal and secure the hollow fibers at the far end of each tee connector. The effective membrane area of the module is about 37.7 cm.sup.2.

[0040] Now referring to FIG. 5 and Table 1, for module configuration with polypropylene hollow fiber membrane, equivalent flux J.sub.E and recovery R as a function of feed flow velocity (v) is shown. For these experiments, the feed water inlet temperature T.sub.1 was maintained at about 60° C. and no condenser was used in the testing system. Each test was conducted for about one (1) hour. Initial and final volume of the feed water were measured and their difference (volume reduction) was used for calculation of equivalent flux J.sub.E and recovery R. The latter (i.e., recovery R) is defined as the ratio of volume reduction over the total volume of feed water passed through the MD module.

[0041] It can be seen from FIG. 5 and Table 1 that the equivalent flux increases with increasing feed velocity but tends to level off at higher feed velocity. Both equivalent flux and recovery were calculated based on mass reduction of recycled feed water. There are two sources of mass reduction: (1) M1—mass loss of water vapor to the permeate side of hollow fiber membranes, and (2) M2—mass loss of water vapor in the knockout chamber.

TABLE-US-00001 TABLE 1 Feed Water Feed Water Supplied Collected Linear Velocity Outlet T.sub.2 ΔT Flux J.sub.E Recovery Heat Q Distillate v (cm/s) (° C.) (° C.) (Kg/m.sup.2 .Math. hr) R (%) (J/min) (mL) 3.7 27.91 32.50 7.15 8.99 681 N/A 7.4 34.12 25.64 7.43 4.67 1074 6.8 14.7 46.40 12.96 10.61 3.33 1086 12.6 22.1 50.96 9.01 11.67 2.44 1133 13.2 29.5 55.69 5.64 11.99 1.88 945 12.3

[0042] Permeate flux depends on both the temperature gradient across the membrane and the rate of heat supplied for water evaporation Q (J/min), which can be calculated as follows:

Q=F.sub.1×C.sub.p×ΔT  (1)

where F.sub.1 is the feed flow rate (g/min), C.sub.p is specific heat of water (4.19 kJ/kg.Math.° C.) and ΔT (° C.) is the temperature difference between the feed inlet and outlet.

[0043] As shown in Table 1, the rate of heat supply Q to the module initially increases with increased feed velocity. For example, 1074 J/min at 7.4 cm/s in comparison to 1133 J/min at 22.1 cm/s. However, further increase of feed velocity beyond certain value leads to decreased Q, for example, 945 J/min at 29.5 cm/s. Note that the change in the quantity of collected water at the permeate side (see Table 1) with feed velocity shows the same trend as that of Q. That is, more water was collected with increasing feed velocity but beyond certain point, the distillate amount was reduced with further increase of feed velocity (for example, 13.2 mL at 22.1 cm/s in comparison to 12.3 mL at 29.5 cm/s). Although no condenser was employed to completely condense water vapor into water, the amount of water collected at the permeate side can be used as a close approximation to the actual mass loss M1 of water vapor. Application of higher feed flow rate resulted in increased driving force for vapor mass transfer across the membrane. This indicates that, as compared to vapor transport across the membrane (dominated by temperature gradient), evaporation of water has more significant effect on MD (dominated by rate of heat supply Q).

[0044] Evaporation of water from the knockout chamber depends on water surface temperature, water surface area, air temperature, air humidity and air velocity above the water surface. Assuming that air temperature, air humidity and air velocity were constant, higher feed flow velocity resulted in higher water outlet temperature (T.sub.2) and thus higher water surface temperature in the knockout chamber, leading to higher rate of water loss in the knockout chamber (higher M.sub.2).

[0045] The influence of feed velocity on equivalent flux J.sub.E is a combination of its influence on M.sub.1 (mass loss of water vapor to the permeate side of membrane) and M.sub.2 (mass loss of water vapor in the knockout chamber). Initially, J.sub.E increased with increasing feed velocity due to increase of both M.sub.1 and M.sub.2. With further increase in feed velocity (v) beyond 22.1 cm/s, although M.sub.1 is reduced due to lower heat supply rate, it was compensated by the higher M.sub.2 and the total mass loss still increased but at a slower rate.

[0046] As shown in FIG. 5, the recovery R decreases monolithically with increasing feed velocity (v). This is explained by the fact that, at higher feed velocity, the residing time of feed flow in the module is less. Due to limited membrane surface area, the percentage of feed solution in contact with the membrane is therefore less.

Experiment 2

[0047] Now referring to Table 2, experiments were conducted regarding the influence of feed velocity (v) on the performance of the same hollow fiber module under DCMD mode and compared to that under AS-SGMD mode. The feed inlet temperature T.sub.1 (about 60° C.) was the same as in the case of AS-SGMD tests. Room temperature water (about 15-28° C.) was used as the cooling medium. Similar to the trend under AS-SGMD mode, the flux J increased but the recovery R decreased with increased feed velocity (v).

[0048] For application of MD in ZLD (zero liquid discharge) treatment of wastewater, a key process parameter is the concentration factor CF which can be used to indicate the degree at which the solids dissolved in the feed water are concentrated in the brine. It is related to the percentage recovery R by the following equation,

CF=1/(1−R)  (2)

[0049] As shown in Table 2, the MD module demonstrated better performance operated under AS-SGMD mode than under DCMD mode, especially at low feed velocity within the tested range. For example, at the highest feed velocity of about 29.5 cm/s, the percent recovery R and concentration factor CF achieved with AS-SGMD are only slightly higher than those achieved with DCMD. At the lowest feed velocity of about 7.4 cm/s, however, both R and CF of SGMD are significantly higher than those of DCMD.

TABLE-US-00002 TABLE 2 Feed Water SGMD DCMD Linear Velocity Flux J.sub.E Recovery Concentration Flux J.sub.E Recovery Concentration (cm/s) (Kg/m.sup.2 .Math. hr) R (%) Factor CF (Kg/m.sup.2 .Math. hr) R (%) Factor CF 7.4 7.43 4.67 104.9% 3.71 2.33 102.4% 14.7 10.61 3.33 103.4% 6.90 2.17 102.2% 22.1 11.67 2.44 102.5% 9.02 1.89 101.9% 29.5 11.99 1.88 101.9% 11.14 1.75 101.8%

Experiment 3

[0050] Now referring to Table 3, experiments were conducted regarding the influence of air flow into the feed side. Saline water at about 10,000 ppm salinity in a feed water reservoir was heated and transported to the AS-SGMD module using a peristaltic pump. Feed water flew through the bore side of the hollow fiber membranes. A knockout chamber was connected to the feed outlet to allow escape of additional water vapor from the feed stream before it returned back to the feed reservoir. The feed water inlet temperature was maintained at about 70±0.2° C. Counter-current sweep gas, for example, compressed air at about 20±0.2° C., was introduced into the shell-side, flushed through the space between the PTFE tubing and the hollow fibers, and exited together with water vapor from the module. A condenser was connected to the sweep gas outlet to condense the vapor moisture into cold water. The condensed water and the air were then separated in a separation unit.

TABLE-US-00003 TABLE 3 F.sub.A F.sub.A1 F.sub.A2 V.sub.1 V.sub.2 V.sub.T Test # (LPM) (LPM) (LPM) (ml) (ml) (ml) 1 11.0 10.5 0.5 52.3 5.3 57.5 2 11.5 10.5 1.0 52.5 8.6 61.1 3 12.5 10.5 2.0 52.1 12.4 64.5 4 13.5 10.5 3.0 51.9 15.8 67.7 5 14.0 10.5 3.5 50.7 18.6 69.3

[0051] For all experiments, the effective air flow F.sub.A1 through the permeate side was kept the same while the total air flow F.sub.A at the sweep air inlet was adjusted to allow varied air flow F.sub.A2 into the feed side. Each experiment was conducted for about one hour. At the end of each experiment, the volume V.sub.1 of condensed permeate water and the total volume reduction V.sub.T of the feed were measured. The difference between V.sub.T and V.sub.1 is thus the water loss V.sub.2 due to air-stripping.

[0052] The knockout chamber positioned between the feed outlet and the feed reservoir allows additional water evaporation represented by V.sub.2. With the increase of F.sub.A2, due to enhanced air-stripping effect, more water was removed from the feed stream leading to higher total volume reduction V. For example, comparing test 1 and test 2, when the gas flow towards the feed side was increased from about 0.5 LPM to 1.0 LPM, the amount of water removed from the feed stream due to air-stripping increased from about 5.3 ml to about 8.6 ml, and the total volume reduction increased from about 57.5 ml to about 61.1 ml.

[0053] The AS-SGMD permeate volume V.sub.1 remains stable as long as the effective air flow F.sub.A1 through the permeate channel is kept constant. Although further increase in F.sub.A2 still enhanced water evaporation at the knockout chamber and led to more water removal, it may slow down vapor transport across the membrane within the AS-SGMD module, as demonstrated by the slight decrease in the permeate volume. It is believed that when too much sweep gas is allowed to flow towards the feed side, the decrease in permeate flux will eventually overweigh the additional water evaporation at the knockout chamber, thus leading to reduced total water removal volume V.sub.T.

Experiment 4

[0054] Now referring to FIG. 6, spiral wound AS-SGMD elements are constructed in such a way that the feed enters and leaves the element through a perforated central tube. The hydrophobic sides of two membrane sheets and a feed spacer in between are sealed together to form a membrane envelope. A spiral wound AS-SGMD element is fabricated by rolling up one or more membrane envelopes and one or more sheet of permeate spacers around the central tube. The open permeate channels allow sweep air to remove generated vapor out of the module. A fabricated spiral wound AS-SGMD element is assembled into a membrane housing to form a spiral wound AS-SGMD module.

[0055] Now referring to Table 4, two spiral wound AS-SGMD elements were fabricated using pristine PTFE membrane and CNIM membrane, respectively. The two elements have the same effective membrane surface area. The two assembled spiral wound AS-SGMD modules were tested at two different feed flow rates and two different sweep air flow rates using the AS-SGMD system as illustrated in FIG. 1.

TABLE-US-00004 TABLE 4 Feed Flow Rate 100 ml/min 500 ml/min Air Flow Rate 30 LPM 15 LPM 30 LPM 15 LPM Recovery R (%) R.sub.P (%) R (%) R.sub.P (%) R (%) R.sub.P (%) R (%) R.sub.P (%) Pristine Membrane 4.32 3.28 3.20 2.58 1.17 0.93 0.86 0.78 CNIM Membrane 4.57 3.55 3.46 2.78 1.45 1.21 0.95 0.84

[0056] Experimental data of total water recovery R and permeate recovery R.sub.P of the two modules are listed in Table 4. The total water recovery R is the ratio of total volume change of the feed to the volume of feed passed through the module, while the permeate recovery R.sub.P is the ratio of the volume of condensed permeate to the volume of feed passed through the module.

[0057] It can be seen that, in all cases, R is always higher than R.sub.P. The difference can be attributed to the additional water vapor loss at the knockout chamber induced by air-stripping in the feed stream.

[0058] Data in Table 4 also reveal performance enhancement of AS-SGMD module fabricated using CNIM membrane (membrane modified with immobilized carbon nanotubes). Under the same testing conditions, the CNIM-MD module shows higher R and Rp than the module fabricated using unmodified membrane.

Experiment 5

[0059] Now referring to Table 5, a spiral wound AS-SGMD element fabricated using CNIM membrane can be used for extraction of organic solvent(s) and hydrocarbons (including gasoline) from its aqueous solution. For example, Isopropanol and ethanol are tested here but this is applicable to all solvents, biofuels and gasoline. All tests were conducted using the same sweep air flow rate and at relatively low feed temperature, preferably about 40±0.5° C. All tests lasted for about 40 minutes.

[0060] The separation factor is the measure of the efficiency of separation and is determined from the ratio of the concentration of the more permeable species (i.e., solvent) and the less permeable species (i.e., water) in the permeate divided by the same ratio in the feed side.

[0061] After treatment with AS-SGMD, the solvent concentration in the feed decreased and the solvent concentration in the permeate increased. At the end of the test, the solvent concentration in the permeate can be as much as 5.75 times that in the final feed solution and 3.89 that in the initial feed solution. This indicates that AS-SGMD is a viable method to efficiently extract organic solvent from its aqueous mixture.

TABLE-US-00005 TABLE 5 Solvent Solvent Concen- Concen- Solvent tration tration Concen- Feed in Initial in Final tration in Sepa- Flow Feed Feed Permeate ration System (ml/min) (wt %) (wt %) (wt %) Factor isopropanol- 200 12.02 9.18 43.0 5.52 water ethanol- 100 8.94 6.05 34.8 5.61 water ethanol- 300 8.41 5.13 24.9 3.61 water

[0062] While the inventive concepts described herein with reference to illustrative embodiments for particular applications, it should be understood that the inventive concepts are not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, embodiments and substitution of equivalents all fall within the scope of the inventive concepts. Accordingly, the inventive concepts are not to be considered as limited by the foregoing description.