Membrane distillation-photocatalysis

Membrane distillation - photocatalysis To solve the problem of membrane fouling observed in the pressure-driven membrane photoreactor, Mozia et al. [90] studied a new type of PMR in which photocatalysis was combined with a direct contact membrane distillation (DCMD). MD can be used for the preparation of ultrapure water or for the separation and concentration of organic matter, acids and salt solutions. In the M D the feed volatile components are separated by means of a porous hydrophobic membrane thanks to a vapor-pressure difference that acts as driving force and then they are condensed in cold distillate (distilled water), whereas the nonvolatile compounds were retained on the feed side. 

Tremendous opportunity exists for hybrid processes consisting solely of membrane processes or a combination of membrane and non-membrane processes. Of the large number of potential combinations, studies of several are reported in the literature including nanofiltration with reverse osmosis [99] nanofiltration with electrodialysis [100] ultrafiltration with nanofiltration and reverse osmosis [101] ultrafiltration with membrane distillation [102] nanofiltration with reverse osmosis and a microfiltration membrane-based sorbent [103] microfiltration with flotation [104] microfiltration and ultrafiltration with ozone and activated carbon adsorption [105] and membrane processes with photocatalysis [106-107]. Despite the activity in this area, a comprehensive approach to designing hybrid systems does not exist future work would benefit from the development of such a design framework. 

Mozia S, Morawski A W, Toyoda M and Tsumura T (2010), Integration of photocatalysis and membrane distillation for removal of mono- and poly-azo dyes from water . Desalination, 250,666-672. 

The most common configurations of PMRs with suspended photocatalyst are those utilizing pressure driven membrane processes, such as microfiltration (MF), ultraflltration (UF) and NF. In other types of PMRs, photocatalysis is combined with dialysis, pervaporation (PV) or direct contact membrane distillation (DCMD, MD) (Mozia, 2010). 

Mozia S and Morawski AW (2006), Hybridization of photocatalysis and membrane distillation for purification of wastewater , Catal Today, 118,181-188. 

Mozia S, Tomaszewska M and Morawski A W (2007), Photocatalytic membrane reactor (PMR) coupling photocatalysis and membrane distillation—Effectiveness of removal of three azo dyes from water , Catal Today, 129,3-8. 

Mozia S, Morawski A W,Toyoda M and Tsnmura T (2009), Effect of process parameters on photodegradation of Add Yellow 36 in a hybrid photocatalysis-membrane distillation system , Chem Eng J, 150,152-159. 

Mozia S and Morawski A W (2009), Integration of photocatalysis with ultrafiltration or membrane distillation for removal of azo dye Direct Green 99 from water ,/A4v Oxid Technol, 12,111-121. 

For water purification, not only the separation of photocatalyst particles is required but also the exclusion of some degradation products. A new type of photocatalytic membrane reactor (PMR), combining photocatalysis with direct contact membrane distillation (DCMD), was constructed in the laboratory and tested by using carbon-coated Ti02 [300-305]. It is schematically illustrated in Figure 3.44. 

Unlike in the case of the PMRs utilizing pressure driven membrane techniques, no membrane fouhng due to the presence of TiOj particles was observed in case of the hybrid photocatalysis-MD system. It was reported (Mozia and Morawski, 2009) that after more than 340 h of operation with a suspension of IIO2 in distilled water, the flux was equal to the maximum permeate flux (i.e., measured for distilled water) and amounted to about 0.16,0.26 and 0.39 m /m. day for the inlet feed temperatures (F, ) of 50,60, and 70°C, respectively. The observed lack of the influence of 1102 on the permeate flux could be explained by the mechanism of mass transport in MD which is due to a difference between the vapor pressure on the two sides of the membrane. Since the process can be conducted without application of pressure difference as a driving force, the main factor responsible for membrane fouling is excluded. In the absence of higher pressure, unlike in the case of pressure driven techniques, cake formation on the membrane surface could be avoided. 

Coupling of photocatalysis and MD could avoid fouling problems related to the use of pressure-driven membrane separations (Mozia, Tomaszewska, Morawski, 2007). MD is a separation process that is based on the principle of vapor—liquid equilibrium. The nonvolatile components (e.g. ions, macromolecules, etc.) are retained on the feed side, whereas the volatile components pass through a porous hydrophobic membrane and then they condense in a cold distillate (usually distilled water). 

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