Temperature polarisation

In comparison to isothermal membrane processes, little attention has been paid to date to polarisation phenomena in non-isothermal processes. In non-isothermal processes such as membrane distillation and thermo-osmosis, transport through the membrane Occurs when a temperature difference is applied across the membrane. Temperature polarisation will occur in both membrane processes although both differ considerably in membrane structure, separation principle and practical-application. In a similar manner to concentration polarisation in pressure-driven membrane processes, coupled heat and mass transfer contribute towards temperature polarisation. 

The concept of temperature polarisation will be described using membrane distillation as an example. A detailed description of membrane distillation has already been given in chapter VI and a schematical representation of temperature polarisation in such a process i.s depicted in figure VII - 22. Two compartments filled with water are separated by a hydrophobic porous membrane (e.g. teflon). As the membrane is not wetted by 

2 (the temperature difference between the bulk feed and the bulk permeate) 

The heat conductivity of the solid material (polymer) vp is, in general, 10 to 100 times greater than Xg, the heat conductivity through the pores. Because of entrainment with water vapour molecules the convective heat flow through the membrane pores, is given 

VII - 64 demonstrates that an increase in the volume flux (increase in the driving force, i.e. the temperature difference across the membrane) leads to an increase in temperature polarisation. Furthermore, a higher heat conductivity for the solid (polimier) also increases temperamre polarisation, whereas an increase in the heat transfer coefficient and an increase in membrane thickness reduce this effect. 

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Figure 2.16 shows both temperature profile and temperature polarisation in MD Tf> Tp. During heat transfer, two boundary layers are formed 5 and 8p.A% a consequence, temperature polarisation occurs < 7) and Tp > Tp. 

An important effect making the MD process different from traditional heat exchanges, the temperature polarisation and concentration polarisation occur in the membrane wall due to the transfer of both water vapour and latent heat. As previously stated, the heat and mass transfer across the membrane move from the hot feed stream to the cold permeate one. The temperature gradients cause a difference in temperature between the Uquid-vapour interfaces and the bulk temperatures on both sides of the membrane. This effect, in membrane science called temperature polarisation, reduces the water vapour flux and in literature it is measured by the so-called temperature polarisation coefficient (t), given by ... 

The latent heat for vaporisation determines the amount of vapour to be transported from the feed to the permeate side increasing heat for vaporisation increases permeate product. Nevertheless, the amount of latent heat depends on the extent of temperature polarisation and heat conduction (which is heat loss, thus should be minimised).This heat loss is influenced by membrane porosity and membrane thickness, and can be reduced by increasing the membrane thickness. However, such an increase results in a decrease of the resulting mass transfer. As such, this trade-off phenomenon could be solved by identifying and using an optimised thickness. 

Many studies are today focused on producing a better understanding of the influence of the concentration and temperature polarisation effects during the OMD process (Ravindra Babu et al., 2008).These authors found that the polarisation phenomenon depends on various parameters, such as the type of osmotic agent, the osmotic agent concentration, and the feed flow... 

Furthermore, the same authors also showed that at any concentration used in their experiments, the driving force reduction due to the concentration polarisation effect was higher than that of the temperature polarisation effect. For example, at 8 mol/kg, CaCl2 corresponds to 225 Pa of reduction in driving force due to concentration polarisation, whereas the reduction due to temperature polarisation effect is only 75 Pa. The best result obtained by Ravindra Babu et al. (2008) in terms of pineapple juice concentration was up to 62° Brix, preserving the ascorbic acid content of the fruit. 

Other parameters of interest are the hydrodynamic conditions (flow velocity) and module design, because they determine the effect of temperature polarisation and hence influence the driving force (see chapter VII). 

The following equation for temperature polarisation can be derived for this process. (It should he noted that this equation is similar to eq.VII - 61. e.xcept that the enthalpies of vaporisation and condensation are not included since no phase transitions occur). 

The heat conductivity in the membrane, appears in both eqs. VII - 64 and VII - 65. However, both values are not equal the value in eq.VII 65 (thermo-osmosis) will be greater so that this factor will have a stronger effect on the temperature polarisation. Because a convective term which mainly depends on the volume flu.x appears in eq.VII -58, the net result is that the effect of temperature polarisation is always greater in membrane distillation even when the temperature difference across the membrane is the same in both processes and when the same membrane material is used. 

With all polarisation phenomena (concentration, temperature polarisation), the flux at a finite time is always less than the original value. When steady state conditions have been attained a further decrease in flux will not be observed, i.e. the flux will become constant as a function of time. Polarisation phenomena are reversible processes, but in practice, a continuous decline in flux decline can often be observed. This is shown schematically in figure VH - 23. 

Calculate the temperature polarisation (assume heat of vapourization = heat of... 

At low operation temperatures, polarisation losses and the importance of catalysis of the electrode reactions Increase. At the cathode, mixed potentials can arise when traces of combustible substances determine the electrode potential in competition with oxygen, an effect, mentioned near the end of Section 2.3, whose cause was recognised by Hartung in 1981 [143], today the basis of the development of hydrocarbon sensors. For the anodes growing interest is directed to materials which accelerate the electrochemical oxidation of CO and hydrocarbons and is stable against fuel impurities. 

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