Effect of bubble characteristics and nozzle size on the membrane distillation enhanced by gas – liquid two-phase ﬂ ow

This study investigates the membrane performance and fouling control in bubble-assisted sweeping gas membrane distillation with high concentration saline (333 K saturated solution) as feed. The results show that a longer bubbling interval (3 min) at a ﬁ xed bubbling duration of 30 s can most ef ﬁ ciently increase the ﬂ ux enhancement ratio up to 1.518. Next, the ﬂ ux increases with the gas ﬂ ow rate under a relatively lower level, but tends to plateau after the threshold level (1.2 L·min (cid:1) 1 ). Compared to the non-bubbling case, the permeate ﬂ ux reaches up to 1.623-fold at a higher bubble relative humidity of 80%. It was also found that greater ﬂ ux enhancement can be achieved and, meanwhile, dramatic ﬂ ux decline can be delayed for an intermittent bubbling system with a smaller nozzle size. These results accord well with the observations of fouling deposition in situ on the membrane surface with scanning electron microscope (SEM). This study investigates the membrane performance and fouling control in This study investigates the performance and


INTRODUCTION
Membrane distillation (MD) is an innovative separation technology for desalination, and water and wastewater treatment, due to its merits of mild operation temperature and pressure, with appropriate penetration rate, high rejection rate for nonvolatile components and small footprint when In recent years, there has been a keen interest in the MD process enhanced by gas-liquid two-phase flow for general desalination applications. For instance, Ding et al. () observed that the cleaning efficiency of gas bubbling is improved with the increase of gas flow rate and gas bubbling duration, and the decrease of membrane fouling when introducing intermittent gas bubbling during the concentration of traditional Chinese medicines (TCM) by direct contact membrane distillation (DCMD). Chen et al. () achieved 26% permeation flux enhancement and later appearance of major flux decline by incorporating gas bubbling into DCMD when salt solution was concentrated from 18% to saturation. Also, it was found that heat-transfer coefficient and temperature polarization coefficient (TPC) reached up to 2.30-and 2.13-fold in comparison with non-bubbling DCMD (Chen et al. ). A recent air-bubbling vacuum membrane distillation (AVMD) study proposed that the flux was doubled at a certain feed velocity and gas/liquid proportion (Wu et al. ).
As an extension of the intermittent bubble-enhanced MD process, this paper aims to research the bubble characteristics (i.e., bubble velocity, bubble relative humidity) and nozzle size on mass transfer intensification and scaling mitigation for supersaturated saline solution as feed. Meanwhile, the anti-fouling efficiency in MD brine processing with gasliquid two-phase can be achieved through the evaluation of the local fouling status on the membrane surface.

Materials and membrane module
Feed solution: saturated NaCl solutions at a temperature of 333 K as feed stream were prepared by magnetically stirring solid sodium chloride (supplied by Guangzhou Chemical Reagent Fac., China) in 1,000 mL of deionized water for 30 min.
A hollow-fiber hydrophobic MD membrane (Jack Co. Ltd, China) was employed in our bubble-assisted sweeping gas membrane distillation (SGMD) experiments. Each membrane is made of polyvinylidene fluoride (PVDF) with 78% porosity, 113 ± 1.7 contact angle, 3.07 N breaking strength, 4.038 bar LEPw, 0.22 μm mean pore size, and its inner and outer diameters are 1.2 mm and 0.9 mm, respectively. All data on membrane properties were provided by the manufacturer.

Experimental set-up
The experimental set-up is shown schematically in Figure 1.
The bubble-assisted SGMD system can be divided into two parts: thermal cycle and cooling cycle.
In the thermal cycle, the hot feed, maintained by a heater at constant temperature, was circulated by a selfpriming pump. The discharge pressure is manually adjusted by means of a 2/3 way valve on the pump's loop line. The bubble flow introduced by an air pump joins the feed flow at the entrance of the membrane module, and therefore a gas-liquid two-phase flow is injected vertically upward into the membrane module. The velocity and relative humidity (RH) of the bubble is controlled by a gas flow meter and humidifier, respectively. The velocity, temperature, and pressure of feed were individually monitored by temperature indicator (TI), pressure indicator (PI), and rotameter.
In the cooling cycle, condensation water prepared from a cooler is recycled into the condenser pipe. Water vapor turns into water droplets when swept straight down to the condenser pipe by the air pump. The weight and conductivity of the penetrant is measured by a balance and conductivity indicator (CI), respectively. The air pump not only acts as an aid to sweep gas into the membrane module, it also supplies gas bubbling into the feed side.
The bubble nozzle mounted at the feed side entrance of the membrane module is used for dispersion of bubbles.
Experiments were also carried out to investigate the effect of different nozzle sizes on the enhancement of critical flux and membrane fouling control. Nozzles with a diameter (D n ) of 0 mm, 2.2 mm, 3.5 mm, 6.0 mm, and 10.0 mm were employed to produce bubbles.

Water quality analyses
The permeation flux (J, Kg·m À2 ·h À1 ) in the MD was calculated by Equation (1): where m (Kg) is the weight of permeation, A (m 2 ) is the total effective membrane area, and t (h) is the operation time (Liu The normalized/relative flux (%) before and after fouling was calculated by Equation (2): is the instantaneous flux during the filtration of real industrial samples, which could cause flux decline due to The energy consumption of the MD system is affected by the membrane, mainly by its thermal energy efficiency (E).
This parameter evaluates the heat transfer due to flux (Q N in W m À2 ) and heat total due to conduction through the membrane (Q in W m À2 ) (Eykens et al. ).
The rejection (R) of solute was calculated by Equation (4): The permeate quality is determined by the separation efficiency. In MD it is defined as the retention of non-volatiles in the feed solution and is calculated based on the concentration in feed (c f , g/L) and permeate (c p , g/L) (Eykens et al. ).
Trans-membrane flux enhancement ratio (Φ) was calculated by Equation (5): where J S (Kg·m À2 ·h À1 ) is the steady-flow membrane distillation (single-phase flow) flux, and J U (Kg·m À2 ·h À1 ) is the unsteady-flow membrane distillation (continuous gas-liquid two-phase flow, intermittent gas-liquid two-phase flow with three bubble on/off ratios (30 s/1 min, 30 s/2 min, 30 s/3 min)) flux obtained by different flow regimes samples.
The other operating parameters were kept constant.

Influence of bubble characteristics on mass transfer
Bubble on/off ratio Consequently, the temperature/concentration layer at the membrane surface is reduced, and then a higher flux is obtained in a bubbling SGMD process. Clearly, four J curves follow a similar trend, i.e., the J initially increases with increasing gas flow rate (0 Q g 1.2 L·min À1 ) and then reaches a plateau at higher gas flow rate

Bubble relative humidity
The relationship between the flux enhancement ratio and the bubble relative humidity is plotted in Figure 4. The 60 min experiment is run at fixed parameters of Q g ¼ 0.5 L·h À1 , D n ¼ 10.0 mm, and bubble on/off ratio ¼ 30 s/ 3 min.
It can be seen that the Φ value increases dramatically from 1.228 to 1.552 at a range of RH g from 58% to 80%.
As the bubble relative humidity increases, small bubbles are not burst easily and tend to aggregate into the formation of gaseous mass. Subsequently, gaseous mass flows with the feed flow in the hot feed side to develop slug flow (intermittent large bullet-shaped bubbles with less clear phase boundaries). The better turbulent effect is caused by the slug flow, and then the shear intensity at the membrane surface increases. Thereby, better membrane permeate performance can be attained in a relatively higher relative humidity.

Influence of nozzle size on bubble-assisted SGMD process
The enhancement of critical flux

Scaling control
To further investigate the influence of gas bubbling with different D n on fouling control, the crystal deposition on the membrane surface is examined by SEM. Figure 6 shows SEM images of surfaces of membrane for six membrane systems: clean membrane, fouled membrane with gas-liquid two-phase flow (D n ¼ 2.2, 3.5, 6.0, 10.0 mm), and fouled membrane with single-phase flow.
In Figure 6 Also, the enhanced sheer stress can reduce the formation of crystals on the low membrane surface. Therefore, fouling limitation is improved by gas bubbling at the feed side in SGMD.
With the decrease of D n , the fouling layer on the surface of the membrane is much thinner. Additionally, the scaling deposition is close to less uniform cubic crystals and the crystal face is much rougher. This is consistent with the ten-  Regarding the conductivity and retention rate of permeate in SGMD, the repeat test indicated the good reproducibility of the permeate flux and high hydrophobic property with conductivity of over 19.2 μs·cm À1 and retention rate of over 99.7%.
To sum up, intermittent bubbling can not only improve the permeate flux, but also remove the deposited salt and foulants from the membrane surface. It is available to resist the fouling formation and deposition for a high concentration SGMD process.