Desalination 199 (2006) 334–336

Presented at EUROMEMBRANE 2006, 24–28 September 2006, Giardini Naxos, Italy.

0011-9164/06/$– See front matter © 2006 Elsevier B.V. All rights reserved.

Perspectives and limitations of using on-line mass spectrometry for monitoring of pervaporation processes

Carla Brazinha*, João G. CrespoREQUIMTE/CQFB, Department of Chemistry, FCT, Universidade Nova de Lisboa,

P-2829-516 Caparica, Portugal Tel. +351212948385; Fax +351212948385; email: carla.brazinha@dq.fct.unl.pt

Received 25 October 2005; accepted 6 March 2006

1. Introduction

Organophilic pervaporation has a high potentialfor aroma recovery from dilute aqueous solu-tions because it involves a low energy inputwhen compared with other separation processessuch as distillation; also, it operates at mildconditions allowing for a direct recovery ofaroma compounds from fermentation processesor other complex media.

In traditional pervaporation systems, acomplete condensation of permeate occurs in asingle condenser. Aiming to fractionate the per-meate stream, in order to obtain defined aromaprofiles, this study involves the use of a pervapo-ration module followed by two condensers inseries operating at different temperatures, allow-ing for a fractionated condensation. Due to thedifferent relative volatilities of the different com-ponents of the system, an increase on the globalseparation factor may be achieved. This approachrequires an optimisation of the temperature(s) ofcondensation in the downstream circuit.

Mass spectrometry (MS) has proved to be apowerful analytical tool with high precision and

sensitivity. In this work, we evaluate its domainof applicability as an on-line monitoring toolcoupled with pervaporation (PV). Coupled MS/PVhas a high potential for the understanding of PVsystems because it allows for on-line monitoringof each component of the permeate stream,enabling transient studies and reducing experi-mental workload significantly when varying anyexperimental parameter.

2. Experimental

For this purpose, an installation equipped foron-line MS monitoring was designed and built.This rig involves a pervaporation cell withdefined flow channel geometry and a series ofvacuum condensers operated under controlledvacuum and temperature (Fig. 1). A POMS–PEImembrane gently provided by GKSS, Germany,was used during these studies.

This unit is equipped with a capacitancepressure gauge, with readings independent fromthe nature of the gas or vapour present; the con-centration of the permeates is acquired in real-time by on-line monitoring with a MS system,where the permeate is sampled through a splitline with a needle valve and an on–off valve. *Corresponding author.

28doi:10.1016/j.desal.2006.03.1

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3. Results and discussion

Coupling MS with PV is not a trivial issue,nor is the selection of the most adequate calibra-tion procedure. For studying the impact of thesolute(s) concentration in the feed, and of feedfluid dynamics, on the composition of the per-meate, MS proved to be able to correctly moni-tor the system changes, as mentioned by Schaferet al. [1]. In this case, as the main component ofthe permeate (solvent) changes slightly duringthe whole experiment, an in situ calibration wasperformed with the standard mixture, within the

range of concentrations of the minor solutecomponent (the aroma) (Fig. 2). When perform-ing this type of calibration, the MS does notneed to respond independently to each com-pound because, even if the major componentinterferes with the aroma ion signal, the interfer-ence is reproducible.

In order to study PV process associated withfractionated condensation, it is necessary toevaluate the impact of operating the condensersat different temperatures. Under these circum-stances, the permeate composition changes sig-nificantly along the permeation circuit duringthe experiment and, therefore, a single compo-nent calibration must be performed with thearoma of interest, providing that its characteris-tic ion signal increases linearly with its partialpressure (Fig. 3). This type of calibration is onlyvalidated if the MS responds independently,which has to be verified.

It was observed that the source pressure of theMS is an essential parameter in assuring an inde-pendent response of the MS to each permeatecomponent (data not shown). It was also found thatfor source pressure values higher than 10–5 mbar

Fig. 1. Schematic diagram of a pervaporation unit equipped with two vacuum condensers-in-series and a split line to acoupled mass spectrometer.

Recirculation pump

FII-3

TT(perm)

PPressure gauge

Vacuum pump

MS

V-6

V-7

V-8

V-9

P-2 P-3

Condenser 1 Condenser 2

TT(feed)

Feed vessel

PV module

Fig. 2. In situ MS calibration of ethyl acetate for differentpperm (feed Re = 5500).

0

20

40

60

80

100

120

140

160

0[etac]feed (ppm)

M/z

43

× 1

0–4

pperm = 2.5 mbar

pperm = 0.68 mbarpperm = 0.69 mbar

5 10 15 20 25 30 35 40 45 50

336 C. Brazinha, J.G. Crespo / Desalination 199 (2006) 334–336

deviations to linearity of the MS signal will occur,which is in accordance with Ref. [2].

4. Conclusions

MS proved to be a valuable technique formonitoring on-line and in real time the responseof PV systems, when operated under variablefeed composition and fluid dynamics, using anin situ calibration procedure. MS has also proved

to be able of monitoring the impact of differentcondensation temperature strategies on the per-meate composition along the downstream circuit,using a single component calibration procedure,while keeping the source pressure of the MSbelow 10–5 mbar. This opens the domain ofapplicability of MS coupled with PV, allowingfor an on-line monitoring of permeate streamsbefore and after each condenser, making abletransient studies and reducing the experimentalworkload significantly when varying any exper-imental parameter.

References

[1] T. Schafer, J. Vital and J.G. Crespo, Coupledpervaporation/mass spectrometry for investigatingmembrane mass transport phenomena, J. Membr.Sci., 241 (2004) 197–205.

[2] J.A. Basford, M.D. Boeckmann, R.E. Ellefson,D.H. Holkeboer, L. Lieszkovszky and C.M. Stupak,Recommended practice for the calibration of massspectrometers for partial pressure analysis, J. Vac.Sci. Tech. A, 11(3) (1993) A22–A40.

Fig. 3. Single MS calibration of ethyl acetate.

0

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160

0p (mbar) × 104

M/z

43

× 1

0–4

200 400 600 800 1000 1200