Fluoride removal from aqueous solution by direct contact membrane ...

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Abstract Direct contact membrane distillation (DCMD) pro- cess using polyvinylidene ... Laboratory of Wastewater Treatment, Center of Researches and. Water Technologies, PB ..... AFM analysis software, characterize the surface of studied.
Environ Sci Pollut Res DOI 10.1007/s11356-014-2858-z

RESEARCH ARTICLE

Fluoride removal from aqueous solution by direct contact membrane distillation: theoretical and experimental studies Ali Boubakri & Raja Bouchrit & Amor Hafiane & Salah Al-Tahar Bouguecha

Received: 24 January 2014 / Accepted: 31 March 2014 # Springer-Verlag Berlin Heidelberg 2014

Abstract Direct contact membrane distillation (DCMD) process using polyvinylidene fluoride (PVDF) membrane was used for fluoride removal from aqueous solution. This study has been carried out on heat and mass transfer analyses in DCMD. The dusty-gas model was used to analyze the mass transfer mechanism and to calculate the permeate flux. The heat transfer is analyzed based on energy balance, and the different layers are considered as a series of thermal resistances. Mass transfer analysis showed that the transition Knudsen-molecular diffusion is the dominant mechanism to describe the transport of water vapor through the pores of the PVDF membrane. The most significant operating parameter is the feed temperature. The permeate increases sensitively with feed temperature and velocity, and it shows insignificant change with feed salts concentration. Heat transfer analysis showed the conduction through the matrix of the membrane presents the major part of available energy. The increasing feed temperature leads to increase thermal efficiency (TE) and decrease temperature polarization coefficient (TPC). The experimental results are in good agreement with theoretical values. Therefore, it is suggested to work at high feed temperature, which will benefit both the thermal efficiency and permeate flux. The

Responsible editor: Bingcai Pan A. Boubakri (*) : R. Bouchrit : A. Hafiane Laboratory of Wastewater Treatment, Center of Researches and Water Technologies, PB 273, 8020 Soliman, Tunisia e-mail: [email protected] S. Al-Tahar Bouguecha Department of Mechanical Engineering, Faculty of Engineer, King Abdul-Aziz University, PB 80204, Jeddah 21589, Kingdom of Saudi Arabia e-mail: [email protected]

experimental results proved that DCMD process is able to produce almost fluoride-free water suitable for many beneficial uses. Keywords Fluoride removal . Direct contact membrane distillation . PVDF membrane . Diffusion model . Mass and heat transfer Nomenclature aw Water activity Bm Membrane distillation coefficient (kg/m2 s Pa) Dh Hydraulic diameter (m) dp Pore diameter (m) Dw Diffusion coefficient for water (m2/s) hf Heat transfer coefficient of the heat transfer boundary layer in feed side (W/m2 K) hm Heat transfer coefficient of membrane (W/m2 K) hp Heat transfer coefficient of the heat transfer boundary layer in permeate side (W/m2 K) ΔHv Latent heat of vaporization (J/kg) J Permeate flux (kg/m2 h) k Thermal conductivity (W/m K) kB Boltzman constant (1.380 10−23 J/K) Kn Knudsen number M Molecular weight (kg/kmol) Nu Nusselt number P Total pressure inside the pore (Pa) Pmf Vapor pressure at the feed side of the membrane surface (Pa) Pmp Vapor pressure at the permeate side of the membrane surface (Pa) Pr Prandtl number Q Heat flux (W/m2) R Gas constant (8.314 J/mol K) Re Reynolds number T Temperature (K)

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U xm

Total heat transfer coefficient (W/m2 K) Mole fraction at the membrane interface

Greek letters δ Membrane thickness (m) ε Membrane porosity τ Membrane tortuosity λ Mean free path (m) σ Collision diameter (m) ρ Density of water (kg/m3) ν Linear velocity of water (m/s) μ Dynamic viscosity of water (kg/m s) Subscripts f Feed p Permeate m Membrane a Air

Introduction Fluoride is a persistent and highly reactive natural mineral element. It can be found in water and wastewater through various medium such as food industry, pharmaceuticals, cosmetics, semiconductor manufacturing, coal power plants, ceramic production, electroplating, fertilizer manufacturing, and natural sources (Kemera et al. 2009). According to the World Health Organization (WHO), the maximum allowable limit for fluoride in drinking water is 1.5 mg/L (WHO 2006). Small quantities of fluoride are beneficial for mineralization of bones and protection of teeth. The higher concentration of fluoride can cause dental and skeletal fluorosis. Apart from that, it also causes cancer, neurological problems, muscular manifestations, urinary tract problem, gastrointestinal problems, and lesions of the thyroid (Mohapatra et al. 2009). Consequently, the removal of excess fluoride from water is necessary in order to safeguard public health and environment. Wide varieties of technologies are available for the removal of fluoride from water such as adsorption, ion exchange, chemical precipitation, reverse osmosis, nanofiltration, electrodialysis, and Donnan dialysis (Mohapatra et al. 2009; Osterwalder et al. 2013; Boubakri et al. 2013a). However, the shortcomings of most of these methods are high operational and maintenance costs, pretreatment required to maintain the pH, regeneration, waste disposal, and secondary pollution such as generation of toxic sludge (Poursaberi et al. 2012). Recently, an emerging technology, membrane distillation (MD), has been investigated as a possible alternative process for the water contaminated by fluoride. MD is a thermally driven process that involves transport of vapor through the pores of hydrophobic microporous membranes combining simultaneous mass and heat transfer (El-Bourawi et al.

2006). The role of the porous hydrophobic membrane does not contribute to the separation through its selectivity but it rather acts as barrier for a liquid-vapor interface (Qtaishat et al. 2009). The main advantage of MD is the ability to operate at lower hydrostatic pressure than conventional pressure-driven membrane processes and a lower operating feed temperature, considerably below its boiling point, than conventional distillation (Boubakri et al. 2013b). Direct contact membrane distillation (DCMD) has been the most frequently studied configuration. It is simple to operate and requires the least equipment, in which the feed and the permeate are directly separated by the hydrophobic membrane. The MD process has been applied for water desalination (Boubakri et al. 2014; Shirazia et al. 2014a), wastewater treatment (Shirazi et al. 2014b), and in the food industry (Kozak et al. 2009). Mass and heat transfer mechanisms govern the permeation flux of membrane distillation, and several theoretical studies have been developed to predict the performance of DCMD (Martíneza and Rodríguez-Maroto 2007; Andrjesdóttir et al. 2013; Ali et al. 2013). Mass transfer in MD is controlled by three models, including Knudsen diffusion, viscous flow, and molecular diffusion (Phattaranawik et al. 2003). The combination of Knudsen-molecular diffusion transition and Knudsen-viscous flow transition offer better agreement with the experimental data. Many researchers have focused on modeling of membrane distillation using the dusty-gas model in order to expand the application of membrane distillation process (Andrjesdóttir et al. 2013; Liua and Wang 2013; Jensen et al. 2011), The aim of this work was to investigate the feasibility of DCMD process using flat sheet polyvinylidene fluoride (PVDF) membrane for fluoride removal. The main objectives of this study are as follows: (a) Elucidate the mechanism of mass transfer across the micro-porous PVDF membrane (b) Study the effect of relevant operating parameters on DCMD performance (c) Compare the experimental results with the predicted permeate fluxes (d) Investigate the DCMD thermal analysis as function of feed temperature

Theoretical background MD process is based upon using a microporous hydrophobic membrane. A hot aqueous solution (contains non-volatile solute), named feed side, flows in direct contact with the membrane, and a cold pure water flows in the other side as permeate. The hydrophobic nature of the membrane prevents the penetration of the liquid solution into the pores unless an

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applied pressure exceeds the liquid-entry pressure (LEP). As result, liquid/vapor interfaces are formed at the membrane pores feed side. The temperature and composition differences across the solution layers adjusted to the membrane create a vapor-pressure difference, the driving force in DCMD process, since both mechanisms occur simultaneously across the membrane including mass and heat transfer. Mass transfer The mass flux (J) of water vapor diffusing through the dry porous membrane is proportional to the vapor pressure difference across the membrane, and can be expressed by Darcy’s law for laminar flow in packed beds (Pal and Manna 2010):   J ¼ Bm Pm f − Pmp ð1Þ where Bm is the membrane coefficient and Pmf and Pmp are the vapor pressures at the feed and permeate vapor/liquid interface, respectively. Pmf and Pmp at the temperature Tmf and Tmp, respectively, are related to the activity of the solution by: Pmi ¼ awi P0mi

i ¼ f;p

ð2Þ

Where awi is the water activity and P0mi is pure water vapor and can be evaluated by using Antoine equation (Bouguecha et al. 2002):   3841; 2 0 Pmi ¼ exp 23:273 − ð3Þ T mi 45

λ¼

kBT m sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi    σ σ 2 Mw w a π 2 P 1þ Ma

ð6Þ

where kB is the Boltzman constant, P is the total pressure inside the pore, Tm is the average membrane temperature, σw and σa are the collision diameters for water vapor and air, respectively, and Mw and Ma are the molecular weights of water and air, respectively. Table 1 shows the relative equations dominating the mass transfer mechanism based on Kn in a gas mixture with a uniform pressure throughout the system (Geancoplis 1993).where dp, ε, τ, and δ are the pore diameter, membrane porosity, pore tortuosity, and membrane thickness, respectively, Pa is the air pressure, and D is the water diffusion. The value of PDw (Pa m2/s) for water-air can be calculated from the following expression (Phattaranawik et al. 2003; Qtaishat et al. 2008): PDw ¼ 1:895 10 5 T 2:072

ð10Þ

The value of the tortuosity factor can be estimated by the following relationship (Iversen et al. 1997): τ¼

ε Þ2

ð2

ð11Þ

ε

Heat transfer

P0mi in Pascal and Tmi in Kelvin. The vapor pressure composition can be estimated using Raoult’s law, which can be written in case of dilute solutions:

Heat transport mechanisms in DCMD are mainly three: Heat flux from the hot stream to the hot membrane surface, referred as temperature polarization in the hot side:

Pmi ¼ ð1−xmi ÞP0mi

Q f ¼ h f ðT f

ð4Þ

where xmi is the mole fraction of the solute at the membrane interface. According to the dusty-gas model, the diffusive mass transfer through microporous membrane can be divided into three mechanisms, including Knudsen diffusion, viscous flow, and molecular or transition mechanism (Andrjesdóttir et al. 2013; Alkhudhiri et al. 2012; Kurdian et al. 2013). These models relate the transport with collisions between molecules and/or molecules with membrane. To judge the dominating mechanism of the mass transfer in the pores, the Kn number is used: Kn ¼

λ dp

ð5Þ

where dp is the membrane pore diameter and λ is the mean free path of transported molecules which can be calculated as follows (Phattaranawik et al. 2003):

T f mÞ

ð12Þ

Heat flux thought the membrane, which is the sum of conductive flux through membrane material and pores and latent heat flux related to the diffusion of the vapor:   ð13Þ Qm ¼ Qv þ Qc ¼ JΔH v þ hm T fm T pm

Table 1 Mass transfer mechanism and membrane coefficient as function of Knudsen number Kn