NAMS 2002 Workshop - ICOM 2008

Experimental
The experimental set-up involves a pervaporation cell and two condensers in
series operated under controlled vacuum and temperature. A POMS-PEI
membrane gently provided by GKSS, Germany was used. For on-line MS
monitoring, the partial permeate pressure of each compound i is acquired in realtime.
The permeate is sampled through a split line with a needle valve.
Previously, independent mono component calibrations were obtained in order to
correlate the characteristic MS signal of compound i with its partial permeate
pressure.
Results
The experimental work developed for validation of the MS monitoring technique
was performed with the MS coupled to the pervaporation-condensation system,
varying the temperature in the first condenser. This work allowed to define
suitable operating conditions where the MS signals were sensitive, reproducible
and independent, validating the MS as an on-line monitoring tool.
The mathematical model we developed was based on phase equilibrium in the
condensers and it allowed us to obtain pre-defined condensates using
Pervaporation and Condensation-in-series under constant operating conditions,
such as upstream conditions and permeate pressure. The model allows for
prediction of the percentage of condensation of each compound and the
composition of the condensates in each condenser, at variable temperature in
the condenser. After an independent measurement of dissolved gases, the
model was firstly developed for systems with water and different amounts of
dissolved gas as feed solution, taking into account inert gas permeation. The
effect of inert gases on the performance of the condensation process are
discussed comprehensively and predicted mathematically. This model was also
applied to complex feed streams: dilute aroma compounds in aqueous solutions
and dilute aroma compounds in hydro alcoholic solutions, comprising different
amounts of dissolved gas. In all steps, with increasing complexity of the feed
solution, this modelling toolbox was experimentally validated and the results
obtained interpreted. As a final result, the model developed proved to be
adequate to all feed streams studied, including feed solutions with complex
composition, as happens in biological and fermentation media (with ethanol and
dissolved gases).
This model enables to predict correctly the degree of condensation of the
different feed components as a function of the condenser’s temperature, making
possible the design of the best strategies for aroma recovery and fractionation.
References
[1] T. Schäfer, J. Vital and J.G. Crespo, Coupled pervaporation/mass spectrometry for
investigating membrane mass transport phenomena, J. Membrane Sci., 2004, 241 (2004) 197.