Theoretical and Experimental Approaches of Liquid Entry Pressure Determination in Membrane Distillation Processes

Abstract

Membrane distillation (MD) is a thermally driven separation process that employs a hydrophobic membrane as a barrier for IntroductionMembrane distillation (MD) is a thermally driven separation technique using microporous hydrophobic membranes and performing on the principles of vapor-liquid equilibrium under different configurations. In this process, only volatile compounds (mainly water) of the feed stream evaporate at the membrane pore entrance, cross the membrane pores in vapor phase to finally be either condensed or removed as a vapor from a membrane module. The hydrophobic nature of the membrane prevents the pores from wetting by capillary forces. MD is known as a promising technology for many applications such as desalting seawater, brackish water, highly saline water [1,2], and removing organic compounds and heavy metals from aqueous solutions [3,4]. MD has also been used to manage waste water such as radioactive waste waters, oily waste waters [5], where the product could be safely discharged to the environment or the waste streams could be reused in an appropriate industrial activity. In biotechnology and food processing applications, MD has also been found as a promising tool, for instance, for removing ethanol and other metabolites from fermentation broths [6], for gentle concentration of valuable compounds in fruit juices [7], and in herb extract such as Ginseng [8].MD has many attractive features as compared to conventional separation processes. Low operating temperatures (~30-70°C) is one of them since the feed is not necessarily heated up to the boiling point like in thermal distillation. Thus, MD may advantageously utilize alternative energy sources, such as solar energy, geothermal energy, waste heats from power plant, etc. [9]. Compared with pressure driven membrane filtration processes such as nanofiltration or reverse osmosis, lower operating pressure translates to lower equipment costs and increased process safety. It is worth highlighting that membrane fouling in MD seems to be less of a problem for many applications than that in pressure-driven filtration processes [10].MD is, however, attended by some drawbacks. Compared to reverse osmosis, MD is known to have a lower permeate flux, and the susceptibility of permeate flux to processing conditions, particularly to temperature and concentration, is considerably high. Also, the trapped air within the membrane pores

Enhanced flux in direct contact membrane distillation using superhydrophobic PVDF nanofibre membranes embedded with organically modified SiO2 nanoparticles

“…The LEP tests were conducted to determine the minimum pressure required to eject water (in liquid state) through the SiO 2 NP‐embedded PVDF nanofibre membranes. The LEP is affected by several parameters including pore size, geometry and membrane hydrophilicity . The LEP values gradually increased from 58.9 ± 2.2 kPa to 84.2 ± 2.8 kPa for pristine PVDF nanofibre membrane (M1) and PVDF nanofibre membrane embedded with ODTS‐modified SiO 2 NPs (M3), respectively (Fig.…”

“…The LEP is related to the hydrophobicity of the membrane surface (i.e. measured by contact angle, CA) as well as the shape, geometry and size of the pores . Polyvinylidene fluoride (PVDF) has been used widely in MD owing to its high thermal stability, low thermal conductivity and minimal wetting propensity .…”

Poly(vinylidene difluoride) Membrane Assisted by Modified ZnO/ZIF Nanoparticles for Membrane Distillation

“…The liquid entry pressure (LEP), also known as wetting pressure, is defined as the pressure (Pa) that must be used for pure water before it penetrates into a hydrophobic membrane with the symbol of LEP of water being LEP w . One way to examine LEP w is the dead‐end filtration setup . The membrane is placed between an upper and a lower chamber in the testing cell and an upper chamber which is connected to a nitrogen gas cylinder, filled with distilled water.…”

“…The LEP is one of the important characteristics in MD as it provides information about the anti‐wetting ability of the membrane surface . A higher hydrophobicity of the membrane surface increases LEP w because the surface of the hydrophobic membrane will have a lower tendency to let water penetrate.…”

Enhanced desalination using a three-layer OTMS based superhydrophobic membrane for a membrane distillation process

“…As earlier mentioned, membrane wetting pores leads to reduced permeate quality; hence, it is necessary to utilize membranes with greater LEP value. 37 Franken et al 38 has suggested a model to evaluate LEP value based on Cantor-Laplace equation…”