Microwave-Induced Desalination via Direct Contact Membrane

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Microwave Induced Desalination via Direct Contact Membrane Distillation Sagar Roy, Madihah Saud Humoud, Worawit Intrchom, and Somenath Mitra ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02950 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 8, 2017

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ACS Sustainable Chemistry & Engineering

Microwave Induced Desalination via Direct Contact Membrane Distillation Sagar Roy, Madihah Saud Humoud, Worawit Intrchom and Somenath Mitra*

Department of Chemistry and Environmental Science New Jersey Institute of Technology Newark, NJ, 07102 USA

* Corresponding Author Somenath Mitra, 973-596-5611(t), 973-596-3586(F), [email protected]

ACS Paragon Plus Environment


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Abstract: Membrane distillation (MD) is emerging as an important desalination technology that can operate at relatively low temperatures and can handle high salt concentrations. In this paper, we present microwave induced membrane distillation (MIMD) where microwave radiation is used to heat the saline water for MD. Pure water vapor flux from MIMD was compared to that generated by conventional heating, and the enhancement reached as high as 52%. Due to the higher dielectric constants, flux enhancement was more significant at high salinity, and the mass transfer coefficient at 150,000 ppm was found to be nearly 99% higher than what was observed under conventional heating. Performance enhancement in MIMD was attributed to non-thermal

effects such as the generation of nanobubbles, localized superheating and breaking down of the hydrogen bonded salt-water clusters. These effects were investigated using FTIR, ion mobility measurements and dynamic light scattering. In addition, microwave heating consumed nearly 20% less energy to heat water to the same temperature. The combination of energy savings and higher flux represent a significant advancement over the state of the art for MD.

Keywords: Desalination; microwave; membrane distillation; water vapor flux


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Introduction With increasing demand for fresh water, the need for seawater desalination has been growing rapidly. Low energy consumption and small equipment foot prints makes membranebased techniques such as reverse osmosis, forward osmosis and membrane distillation (MD) attractive alternatives. Compared to thermal distillation, MD is a membrane based evaporative process where the driving force is a temperature induced vapor pressure gradient generated by having a hot feed and a cold permeate across a membrane. Typically, MD is carried out at 50– 90°C and at atmospheric pressure. Therefore, it has the potential to generate high quality drinking water using low temperature heat sources such as waste heat from industrial processes and solar energy. Although MD is considered a promising technology, there still exist many limitations such as low water vapor flux, high temperature and concentration polarization, fouling at high salt concentrations and high energy consumption 1-2. Besides developing more efficient membranes 3-6 efforts have been made to address the above issues by implementing novel designs such as the introduction of alternate turbulent-inducing spacers and baffles 7-11 and ultrasound that enhances membrane performance by reducing fouling 12-13. However, such devices tend to increase power consumption, damage membrane surfaces and mechanical vibrations modify the liquid-membrane interface.14. Microwave energy has been used as a heat source in many industrial processes, chemical synthesis and is extensively used in domestic kitchens 15-18. When a dielectric material is placed in a microwave field dipoles are formed which develop an orientation polarization and change directions at high frequency. The lag between dipole orientation and the electric field leads to a dielectric loss and subsequently heat generation 19. In addition, microwave processes are known



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to demonstrate non-thermal effects by causing local super heating, reducing activation energy of physical and chemical processes, breaking down hydrogen bonded structures in aqueous environments and generating nanobubbles at temperatures below the boiling point 19-25. There are only a few reports on for the use of microwaves in membrane processes. They have been employed in gas separation where they successfully enhanced gas transfer in membrane pores 26 and in vacuum-based membrane distillation 27-28. These processes have been carried out in microwave ovens where the membrane is also exposed to the microwave, and can be problematic if the latter absorbs microwave. The placing of the membrane module in a microwave cavity is not always feasible, and this is particularly true in processes such as direct contact membrane distillation (DCMD) where the microwave would also heat the water in the permeate side and reduce the vapor pressure gradient. In addition, putting the whole membrane modules in microwave heaters can be challenging during scale up. The objective of this paper is explore microwave heating of salt water for MD, especially DCMD. Another objective is to explore performance enhancement via non-thermal effects that are associated with microwave heating.

Experimental Section Chemicals, materials and membrane modules Sodium chloride (NaCl) was obtained from Sigma–Aldrich (St. Louis, MO) and deionized water (Barnstead 5023, Dubuque, Iowa) were used in all experiments. Flat polypropylene (Celgard, LLC, Charlotte, NC), polytetrafluoroethylene composite membranes with nonwoven polypropylene support (Advantec MFS, Inc.; Dublin, CA) membrane were used in the MD experiments. DCMD test cell was fabricated from polytetrafluoroethylene (PTFE). The details of the membranes are given in our previously published work 29-30.


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Experimental procedure The alteration of water and salt-water interactions with microwave heating were characterized using Fourier transform infrared spectroscopy (FTIR) (IRAffinity-1, Shimadzu), and dynamic light scattering to measure electrophoretic mobility (Zetasizer Nano-ZS90, Malvern Instrument Ltd, UK). The FTIR measurements and the ionic mobility were studied with 50 and 50-1000 ppm NaCl solutions, respectively. The change in particle size under microwave radiation was studied using 3000 ppm calcium sulfate (CaSO4) dispersion. The experimental set up for microwave MIMD is shown in Figure 1. Here the feed was circulated through a microwave heater where it was heated to the desired temperatures. The hot feed was then passed through the MD modules. This arrangement could be useful for all types of MD configurations namely direct contract, sweep gas, and vacuum membrane distillation. The set up comprised PTFE membrane cell (for flat membranes) having an effective membrane area of 14.5 cm2, Viton O-rings, PTFE tubing, PFA and PTFE connectors, feed and permeate flow pump (Cole Parmer, Vernon Hills, IL), circulating heated temperature bath (GP200) and circulating chiller (MGW Lauda RM6). The hot aqueous NaCl solution was circulated on one side of the membrane in the MD cell. For conventional MD, the feed brine was heated using a temperature regulated hot oil bath and in MIMD using an 1100-watt domestic microwave (Oster, OGZF1301). Cold distilled water was circulated on the permeate side at a constant flow rate of 200 mL/min and 15oC. Inlet and outlet temperatures of feed and permeate side were monitored using a K-type temperature probe (ColeParmer). Makeup water was added continuously to the feed side to maintain constant concentration throughout the experiment. The concentration of the feed brine and distillate were



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measured using a conductivity meter (Jenway, 4310). Each experiment was repeated at least three times to ensure reproducibility and relative standard deviation was found to be less than 1%.

Results and Discussion Microwave induced Membrane Distillation (MIMD) In this research microwave was used to heat the water before entering the membrane module. The results were compared to the MD carried out by conventional heating to the same bulk temperature. Figure 2a shows the influence of temperature on water vapor flux with conventional and microwave heating respectively at a flow rate of 40 mL/min. It was observed that the permeate flux in all cases increased with temperature. The increase in feed temperature increased the vapor pressure difference between feed and permeate side, hence the driving force for mass transport. Both membranes studied here exhibited higher water vapor flux in MIMD than via conventional heating. At 50°C the water vapor fluxes in MIMD were as high as 37.5 and 10.7 kg/m2.h for PTFE and PP membranes respectively, which were 35 and 43 % higher than MD via conventional heating 31-32. In a microwave field the water dipoles constantly attempt to reorient in the oscillating electric field. A ‘wait and switch’ process has been used to model the reorientation where a water molecule waits until a neighbor attains a favorable orientation and the hydrogen bonds switch accordingly. Therefore, the hydrogen-bonded aggregates are continuously formed and destroyed in an oscillating microwave field 33-35. This process weakens the hydrogen bonding structure in the bulk water and enhances molecular mobility. Another interesting feature of microwaveinduced heating is the formation of nanobubbles, where thermal and non-thermal effect of


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microwave radiation generates super-heated hot spots leading to the formation of gaseous and vapor bubbles at temperatures below the boiling point 25, 35-38. These have been confirmed by both atomic force microscopy (AFM) and dynamic light scattering studies where the nanobubbles formation increased with increasing microwave energy 25. In the context of MD the formation of nanobubbles is particularly relevant as it promotes the generation of water vapor that are permeated as pure water. These non-thermal effects are responsible for the enhancement in MIMD over the conventional MD at the same temperature. It is observed from Figure 2b that the flux enhancement decreased at higher temperatures. For example, the microwave induced enhancement for PP was found to decrease from 43% at 50oC to 14% at 80oC. This is because of the weakening of the hydrogen bonding and the decrease in the average molecular dipole moment and dielectric constant at higher temperatures 27, 39

. Therefore, non-thermal effects were less pronounced at higher temperatures. The effect of feed flow rate is shown in Figure 3a where the flow was varied between 20

and 320 mL/min at 50oC. Permeate flow rate was kept constant around 250 mL/min for all the experiments. As expected, the water vapor flux increased with feed velocity. The elevated flow rates increased turbulence and reduced the boundary layer which helped in lowering the temperature polarization and increased the driving force for MD. The percent enhancement of water vapor flux due to microwave heating is shown in Figure 2b. It can be seen from Figure 3b that the water vapor flux for MIMD was higher than that of the conventional MD at same feed flow rate. The feed flow rate influenced the residence time in the microwave chamber, which determines the microwave energy absorption. As observed from Figure 3b the effect of microwave irradiation was more pronounced at lower flow rates. The flux enhancement in the PTFE membrane was 42% at 20 ml/min and dropped to 10%



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at 280 mL/min. The longer residence time allowed the feed to absorb greater microwave energy that enhanced the superheating and other non-thermal effects that are commonly associated in microwave processes. This is in line with previous reports where microwaves effects such as nanobubbles generation are relatively slow to take effect 25.

Effect of Salt Concentration in MIMD The solvation of ion pairs in water involves a large number of water molecules, which strongly affects the water−water interactions and the corresponding salt−water clusters that are formed 40. It has been reported that the average hydrogen bond angles and lengths can vary depending upon the dissolved salt 40. Therefore, MIMD can expected to be strongly influenced by presence of salts. Figure 4 shows the effect of microwave irradiation on water vapor flux at various brine concentrations. The plot clearly demonstrates that microwave heating was more effective at higher salt concentration in feed. Under the same conditions, the flux enhancement at 150,000 ppm salt was 23 and 14% higher than pure water using the PTFE and PP membranes respectively. The conductivity of the distillate was measured and the salt rejection was found to be greater than 99%. This enhancement can be attributed to several factors. Temperature polarization is an important consideration in MD. Microwave irradiation can provide localized superheating, which can not only compensate for temperature polarization but also provide a higher vapor pressure gradient compared to what can be achieved via conventional heating. In saline solutions, the dielectric constant increases with salt concentration and the ions are able to interact more strongly with the changing potential 41.

The water vapor flux (Jw) can be expressed as:


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= − , =

−

1

where, k is the mass transfer coefficient (kg/m2.s.Pa), Pf and PP (Pa) is the partial water vapor pressure in feed and permeate side.

The mass transfer in DCMD process involves three general steps: 1) vaporization of the hot feed from the liquid/gas interface; 2) migration of water vapor across the membrane from hot to the cold interface driven by the vapor pressure gradient; 3) condensation of the vapor into the cold side stream42. Further, depending on the pore size and the mean free path of the transferring species, mass transport across the membrane mainly occurs in three regions: Knudsen region, continuum region (or ordinary-diffusion region) and transition region (or combined Knudsen/ordinary–diffusion region)43. In general, the overall mass transfer is controlled by the diffusion through the boundary layer. The increased turbulence in the feed reduces the boundary layer resistance at the membrane interface and enhances the overall mass transfer rate. The implementation of microwave induced heating does not affect the second and the third mass transport steps, but the weakening of the hydrogen bonded structure could potentially influence the rate of vaporization at the liquid/gas interface in the feed side.

The effect of microwave heating on the mass transfer coefficient at different salt concentration in feed is represented in Table 1. From the table, it is observed that the mass transfer coefficients decreased at higher concentrations due to the increase in concentration and temperature polarization. However, the rate of decrease in MIMD was lower compared to conventional MD. At a 150,000 ppm salt concentration the enhancement in mass transfer coefficient for PTFE and PP were close to 99 and 81% respectively compared to conventional



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MD. The enhancement in mass transfer coefficient in MIMD was mainly attributed to some of the non-thermal effects that reduced both boundary layer resistance and temperature/concentration polarization 41, 44. The heat transfer in DCMD comprises of convectional heat transfer at the feed side boundary layer, heat transfer due to the phase change related mass transfer at the liquid-vapor interface, conductive heat transfers across the membrane, heat transferred due to the movement of water vapor through the membrane pores and lastly heat transfer in the permeate side boundary layer associated with phase change as well as convection45. The microwave heating could only affect the feed side boundary layer region heat transfer via increasing the mass transfer rate and decreasing the temperature polarization effect as mentioned earlier.

Power Consumption in MIMD: Power consumption is an important consideration in MD as well as MIMD. The power required to heat waster was estimated by connecting the heating sources through a power meter. Conventional heating comprises of conduction through a heat exchanger or vessel followed by convection in the bulk water while microwave heating involves the direct heating of water molecules and salt-water clusters. Figure 5 shows the energy required to maintain the desired temperature in MD under steady conditions at a flow rate of 200 mL/min for 4 hr. It clearly shows that the MIMD was much more energy efficient than conventional heating. At low temperatures, the microwave induced heating consumed 20-22% less energy compared to conventional heating although this saving dropped somewhat at higher temperatures. However, it is noted that this work was carried out using us domestic microwave and heater designed with better distribution of the microwave radiation may improve this further.


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Proposed Mechanism for MIMD The microwave heating effects salt water in multiple ways. Hydrogen bonded (H2O)n and salt-water clusters are known to disintegrate during microwave heating 46. The Raman spectra of microwave treated water have shown significant alteration of the O-H stretch compared to the untreated water, and this state was reported to last for few hours after the microwave field has been removed 47-48. Similarly, nanobubble formation was initiated nearly thirty seconds after the exposure to microwave radiation 25. Therefore, the microwave heating in DCMD does not have to carry out in-situ to realize some of the advantages of MIMD. We believe that during MIMD not only was the water heated, but also hydrogen bonded H2O and salt-water clusters were destroyed. These enhanced the escaping tendency of water molecules from the bulk solution leading to higher flux 27. In this study, we investigated these phenomena by studying ionic mobility, IR spectra and colloid formation under microwave radiation. Electrophoretic Light Scattering (ELS) is used to measure the electrophoretic mobility of the molecules or ions in solution. Typically, the motion of the ions in a liquid medium depends on its charge, shape and size, and the bulkier ions move more slowly. Salts dissolved in water form hydrated species with a tightly bound inner sphere and a loose outer sphere. Electrophoretic mobility can be a measure of the effect of microwave on salt water clusters along with its hydration sphere. Figure 6a shows the ionic mobility of NaCl salt-water clusters at different salt concentrations. The mobility increased with temperature when heated by conventional means. It is clear from the figure that microwave irradiation increased the ionic mobility quite dramatically which was attributed to the ability to breakdown of the salt-water cluster and reduce the size of the hydration spheres. The measurements were taken 120 sec after the microwave heating which



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simulated real-world situation where the heated brine would flow into the membrane after microwave heating. The FTIR spectra of NaCl solution at room temperature as well after conventional and microwave heating are represented in Figure 6b. The water molecules are known to absorb the IR differently due to the variation in water-water or salt-water interactions under different conditions 49-52. From figure 6a it is evident that the distribution of cluster sizes were quite different. The peak at 2127 cm-1 resulted from the combination of bending and librations. The bending frequency at 1644 cm-1 was attributed to the hydrogen bonding, which was much weaker for microwave treated water. The main stretching band for water molecules was observed at ~3490 cm-1. The water clusters were hydrogen bonded to a slightly different extent that led to the broadening of the peaks. The effect of microwave was also investigated using an insoluble salt that would form a uniform colloidal dispersion. CaSO4 was selected because it is an important component of membrane fouling in real world situations. Figure 6c shows the influence of microwave irradiation on particle size distribution. The CaSO4 particle size was 322 nm at room temperature and dropped to 184 nm at 70oC. When the suspension was heated to 70oC by microwave rather than conventional heating, the particle size dropped further to 119 nm. This clearly demonstrates that microwave heating significantly lowered the average particle size of the CaSO4 clusters. This phenomenon is particularly important for MD with real world samples where sparingly soluble salts lead to fouling. Reduction in particle size significantly lowers the fouling tendency and can lead to higher flux as well as longer membrane life.

Conclusion


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Microwave irradiation was successfully employed as a means to heat the saline water in DCMD. Higher enhancement in vapor flux was observed at lower temperatures, flow rates and at higher salt concentrations. Compared to conventional DCMD, the increase in mass transfer coefficient during MIMD reached as high as 72%. These enhancements were attributed to factors such as localized super heating and the destruction of water and salt-water networks. An increase in ionic mobility and decrease in particle size were also observed during microwave heating. In general, the MD process requires a significant amount of energy to generate pure water The viability of this process greatly depends on the energy required per gallon of pure water, and its ability to couple to sustainable sources of energy. In this work, direct measurement have shown over 20% reduction in power consumption when water was heated using the microwave. Further, the microwave can also be combined with low grade heat such as industrial waste heat and solar energy to further reduce carbon footprint. Together these results encourage the coupling of microwave heating to MD and possibly other desalination techniques.

Acknowledgement

This study was partially supported by a grant from the Chemical, Bioengineering, Environmental, and Transport Systems Division, National Science Foundation (grant number CBET-1603314).



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Electric Field on the Characteristics of the Cyclic Water Trimer. Chinese Journal of Chemical Physics 2005, 18 (6), 952. 45.

Qtaishat, M.; Matsuura, T.; Kruczek, B.; Khayet, M., Heat and mass transfer analysis in

direct contact membrane distillation. Desalination 2008, 219 (1-3), 272-292. 46.

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dynamics study. The Journal of chemical physics 2007, 126 (3), 034509. 47.

Kryachko, E. S., Ab initio studies of the conformations of water hexamer: modelling the

penta-coordinated hydrogen-bonded pattern in liquid water. Chemical physics letters 1999, 314 (3), 353-363. 48.

Rao, M. L.; Sedlmayr, S. R.; Roy, R.; Kanzius, J., Polarized microwave and RF radiation

effects on the structure and stability of liquid water. Current Science 2010, 98 (11), 1500-1504. 49.

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dynamics in the infrared spectroscopy of water. Science 2003, 301 (5640), 1698-1702.



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Table 1. Mass transfer coefficients at different feed concentrations. Concentration

Mass transfer coefficient-PTFE

Mass transfer coefficient-PP

(thousand

Membrane (X 10-7 kg/m2.s.Pa)

Membrane (X 10-7 kg/m2.s.Pa)

ppm)

MD

MIMD

Enhancement

MD

MIMD

(%)

Enhancement (%)

0

11.3

16.6

46.9

2.21

3.33

50.7

3.5

10.6

15.9

50.0

2.05

3.2

56.1

10

9.54

15.4

61.4

1.96

3.14

60.2

20

9.05

15.0

65.7

1.85

2.85

54.1

35

8.58

13.8

60.8

1.79

2.79

55.9

60

7.69

13.6

76.8

1.7

2.73

60.6

90

7.18

13.2

83.8

1.52

2.68

76.3

120

6.83

12.6

84.5

1.49

2.55

71.1

150

6.14

12.2

98.7

1.36

2.46

80.9



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Figure captions: Figure 1. Schematic of microwave induced MD setup. Figure 2. a) Water vapor flux in PP and PTFE membranes at different temperatures in MIMD and conventional MD at 40 mL/min flow rate; b) Flux enhancement at different temperatures. Figure 3. a) Water vapor flux in PP and PTFE membranes at different flow raters in MIMD and conventional MD at 50oC; b) Flux enhancement at different flow rates. Figure 4. Effect of salt concentration on water vapor flux at 40 mL/min flow rate and 50oC. Figure 5. Comparison of energy consumption during MIMD and MD for 4 hours of continuous operation at different temperatures and at a flow rate of 200 mL/min. Figure 6.a. Ionic mobility of various NaCl solutions at 60oC. Figure 6.b. FTIR spectra of NaCl-water solution (50 ppm) under microwave and conventional heating. Figure 6.c. average particle size of CaSO4 dispersion under various conditions at 70oC.


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Permeate side water/ cooling media inlet Chiller Distillate outlet

Hot Feed inlet Feed outlet

Condenser

Distillate Make up water outlet Microwave Constant heating temperature bath Figure 1. Schematic of microwave induced MD setup.



Water vapor flux (kg/m2.hr)

60 50 40 30

PTFE-MIMD

PTFE-MD

PP-MIMD

PP-MD

20 10 0 45

55

65 Feed temperature (oC)

75

85

50 Flux enhancement (%)

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40 PP-MIMD

PTFE-MIMD

30 20 10 0 45

55

65 Feed temperature (oC)

75

85

Figure 2. a) Water vapor flux in PP and PTFE membranes at different temperatures in MIMD and conventional MD at 40 mL/min flow rate; b) Flux enhancement at different temperatures.


Page 23 of 29

Water vapor flux (kh/m2.hr)

40

30

20

PTFE-MIMD

PTFE-MD

PP-MIMD

PP-MD

10

0 0

50

100

150 200 250 Feed flow rate (mLmin)

300

70 60 Enhancement (%)

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PP-MIMD

50

PTFE-MIMD

40 30 20 10 0 0

50

100 150 200 Feed flow rate (mL/min)

250

300

Figure 3. a) Water vapor flux in PP and PTFE membranes at different flow raters in MIMD and conventional MD at 50oC; b) Flux enhancement at different flow rates.



50 PTFE-MIMD Water vapor flux (kg/m2.hr)

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Page 24 of 29

PTFE-MD

PP-MIMD

PP-MD

40

30

20

10

0 0

40 80 120 Salt concentration (× 1000 ppm)

Figure 4. Effect of salt concentration on water vapor flux at 40 mL/min flow rate and 50oC.


Page 25 of 29

2 Power requirement (kWh)

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Conventional heating

Microwave heating

1.6 1.2 0.8 0.4 0 50

60 70 Feed temperature (oC)

80

Figure 5. Comparison of energy consumption during MIMD and MD for 4 hours of continuous operation at different temperatures and at a flow rate of 200 mL/min.



Concentration (ppm) 0

200

400

600

800

1000

Mobility (um cm/Vs)

0 -0.5 -1 -1.5 -2 -2.5

Room temperature

Conventional

Microwave

Figure 6.a. Ionic mobility of various NaCl solutions at 60oC.

50

Room temperature Conventional heating

40 Transmitance (%)

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Page 26 of 29

Microwave heating

30 20 10 0 400

900

1400

1900 2400 2900 -1 Wave number (cm )

3400

3900

Figure 6.b. FTIR spectra of NaCl-water solution (50 ppm) under microwave and conventional heating.


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400 350 Particle size (nm)

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300 250 200 150 100 50 0 Room temperature

Conventional

Microwave

Figure 6.c. average particle size of CaSO4 dispersion under various conditions at 70oC.



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TOC Graphic:

Hydrogen bonded (H2O)n and salt-water clusters are known to disintegrate during microwave heating. This phenomenon is exploited to enhance pure water flux during desalination via microwave induced membrane distillation.


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Hydrogen bonded (H2O)n and salt-water clusters are known to disintegrate during microwave heating. This phenomenon is exploited to enhance pure water flux during desalination via microwave induced membrane distillation. 165x86mm (96 x 96 DPI)


Microwave-Induced Desalination via Direct Contact Membrane

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