Pervaporative aroma recovery during beverage processing
Abstract: In this study, the fundamental mass and heat transfer properties of the membrane separation process called pervaporation were investigated. The suitability of pervaporation for aroma compound recovery was especially evaluated. The mass transfer in pervaporation is affected by an external mass transfer resistance in the liquid feed boundary layer, i.e. the concentration polarization phenomenon. The fluxes of the selectively permeated compounds can often be reduced by several tens of percent if the feed flow characteristics are unfavourable. The presence of a certain aroma compound in the pervaporated mixture does not seem to affect the mass transfer of other compounds in the mixture. This behaviour is mainly due to the normally very low concentrations of aroma compounds in beverages. The presence of other compounds, e.g. ethanol, at high concentrations in the separated mixture seems, in some cases, to affect the mass transfer of the aroma compounds. The temperature at which the pervaporation process is carried out strongly affects the performance of the process. The most important effect of the process temperature is, however, that mass fluxes increase exponentially as the temperature increases, a fact which is important for process economics. However, some aroma compounds are heat sensitive and the actual process temperature is therefore a trade-off between process economics and the heat sensitivity of aroma compounds. The membrane material used in pervaporation determines, to a large extent, the performance of the process. A number of commercially available membranes exist, and three appear to be suitable for aroma recovery. The basis of the heat transfer in pervaporation is the change in state that permeants are subjected to as they are transported from the liquid feed to the vapour permeate. The pervaporation process consequently consumes heat, and the heat transfer in the process can be described in a similar way as for the mass transfer. Computer simulations of the behaviour of large-scale pervaporation units for aroma recovery indicate that pervaporation could be used as a technique for aroma recovery. For four of the five aroma compounds investigated the recovery degrees are very high. Both the investment and running costs for a pervaporative aroma recovery unit are extremely dependent on the choice of membrane material. High-flux membranes give lower costs but poorer process performance.
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