Thermo-osmosis in Membrane Systems: A Review

Abstract

We give a first review of experimental results for a phenomenon little explored in the literature, namely thermal osmosis or thermo-osmosis. Such systems are now getting increased attention because of their ability to use waste heat for separation purposes. We show that this volume transport of a solution or a pure liquid caused by a temperature difference across a membrane can be understood as a property of the membrane system, i. e. the membrane with its adjacent solutions. We present experimental values found in the literature of thermo-osmotic coefficients of neutral and hydrophobic as well as charged and hydrophilic membranes, with water and other permeant fluids as well as electrolyte solutions. We propose that the coefficient can be qualitatively explained by a formula that contains the entropy of adsorption of permeant into the membrane, the hydraulic permeability, and a factor that depends on the interface resistance to heat transfer. A variation in the entropy of adsorption with hydrophobic/hydrophilic membranes and structure breaking/structure making cations could then explain the sign of the permeant flux. Systematic experiments in the field are lacking and we propose an experimental program to mend this situation.

Perspectives on Thermoelectric Energy Conversion in Ion-Exchange Membranes

“…A temperature difference can lead to a thermo-osmotic pressure. The magnitude as well as the direction of the thermo-osmotic water transfer depends on the membrane [ 86 ]. This additional term is little investigated.…”

Thermo-osmosis in hydrophilic nanochannels: mechanism and size effect

“…Experimental and theoretical works have shown that the thermo-osmotic coefficient can be described as a function of the interfacial excess enthalpy distribution of the fluid and its hydrodynamic properties at the interface. 24 However, the influence of various factors on the mechanism of thermo-osmosis is still unclear. For example, the excess enthalpy distribution is affected by temperature, pressure, liquid polarity, 25 and liquid structure in the boundary layer.…”

Thermo-osmotic pressure and resistance to mass transport in a vapor-gap membrane

“…This invention uses the fact that condensation of fluid on the receiving side gives rise to a hydrostatic pressure on this side, caused by a phenomenon called thermal osmosis. 14 The principle is illustrated in Fig. 1.…”

“…8,[23][24][25] Moreover, the interaction of the fluid with the membrane is widely ignored, even though it has been shown that the properties of the membrane can influence the flow direction. 14,26 For precise control of the flow, it is clearly important to distinguish between independent and dependent driving forces. For a quantitative description it is furthermore essential to account for the coupling between heat and mass transfer, as mass can also be transported by a temperature gradient, the so-called Soret effect.…”