Abstract
Membrane distillation (MD) is a thermally-based membrane operation in which the distillation of aqueous feeds occurs through a membrane. Although first works on MD appeared in the literature in the 1960s, the application of MD has been limited, mainly due to the lack of membranes and modules specifically designed, as well as to the thermal demand. Nevertheless, MD attracted the attention of many researchers, and significant improvements of the MD performance have been achieved. In this chapter, MD features, drawbacks, and possible solutions are presented and discussed.
TopMain Principles
The core of membrane distillation is the membrane, which has to be hydrophobic and microporous. Typical materials used to prepare the membrane are polypropylene (PP), polytetrafluoroethylene (PTFE), and polyvinylidene fluoride (PVDF). The hydrophobicity is fundamental to avoid the aqueous feed could permeate as liquid through the membrane, while the presence of micropores is needed to establish the liquid-vapor equilibrium. Each micropore acts as an interface, and a well-defined area for evaporation is created. From this interface, the vapor and volatiles migrate through the micropore and reach the other side of the membrane to be, then, recovered. In this way, all non-volatiles present in the feed are retained, therefore allowing the treatment of feeds containing different non-volatile species in the same unit, as sketched in Figure 1.
Figure 1.
Liquid-vapor interface at the micropore mouth
It has to be pointed out that, in addition to the membrane hydrophobicity, to carry out MD experiments, it is important to never exceed the so-called Liquid Entry Pressure (LEP), which is the pressure value at which the aqueous feed starts to penetrate the membrane as liquid, no matter its hydrophobic character. The LEP value is directly linked to the membrane hydrophobicity and to the liquid surface tension, while it is inversely proportional to the membrane pore size.
The MD process is characterized by a simultaneous transfer of mass and heat. Both depend on the boundary layers and the membrane resistance.
In particular, while the boundary layers resistances are affected by the fluid dynamics, the membrane resistance is a function of the membrane properties. Specifically, the mass transfer encounters three main resistances in the membrane:
- 1.
viscous resistance (given by the momentum transferred to the membrane)
- 2.
Knudsen diffusion resistance (due to collision molecules-membrane)
- 3.
molecular resistance (due to collision among molecules)
The heat transfer in the membrane is due to both conduction through the membrane material and to the latent heat of vaporization removed by the vapor flux. The trans-membrane flux of the vapor and volatiles is a function not only of the membrane resistance, but also of the difference of vapor pressure across the two membrane sides, which represents the driving force of the process. To create this driving force, the feed to be treated is heated (typical temperatures range between 40 °C and 80 °C) while at the permeate side of the membrane various options can be applied, leading to different MD configurations. Main MD configurations are:
Key Terms in this Chapter
Membrane: A barrier able to be selectively permeated by some species, while rejecting the others.
Hydrophobic Membrane: A membrane that repels liquid water.
Brine: A stream highly concentrated in salts.
Temperature Polarization: Temperature gradient between the solution bulk and the membrane surface.
Wastewater: A polluted water stream, often produced by industries.
Desalination: A process that recovers fresh water starting from brackish and seawater.
Concentration Polarization: Concentration gradient between the solution bulk and the membrane surface.
Rejection: The amount of species which does not permeate the membrane with respect to their amount in the initial feed (%).
Flux: The amount of distillate per unit of membrane area and time (L/m 2 h).
Membrane Distillation: A process in which the distillation of a hot feed occurs through a membrane.