DESALINATION

ELSEVIER Desalination 148 (2002) 11 l-1 14 www.elsevier.com/locate/desal

Hollow fibre modules for orange juice aroma recovery using pervaporation

Andrew Shepherd”, Albert0 Claudio Habertb*, Cristiano P. BorgesC

Chemical Engineering Program /COPPE, Federal University of Rio de Janeiro, P O.Box 68.502, 21.945-970 Rio de Janeiro, Brazil

Tel./Fax +55 (212) 5628301/300; email habert@peq.coppe.ufrj. br

Received 1 February 2002; accepted 31 March 2002

Abstract

The use of poly (dimethyl siloxane) PDMS hollow fibres was investigated in orange juice aroma recovery. Modules for pervaporation were designed and built for this purpose, scrutinized with binary ester-water feeds and compared to other module geometries using experimental enrichment factors and mass transfer coefficient values. Operational variables tested for the aroma separation included feed flow rate and aroma feed concentration. Further experimental runs were carried out with an industrial multicomponent feed consisting of an aqueous orange juice by-product. The results show the good potential of the modules for dilute aroma recovery.

Keywords: Pervaporation; Hollow fibres modules; Aroma; Orange juice

1. Introduction separations [I]. In addition, since many of the

The use of membrane technology in the food industry has increased dramatically in the last decade. Pervaporation separations have been analysed for the recovery of natural occurring aromas and have the advantages of being favoured by the international food legislation, which prohibits recovery methods other than physical

*Corresponding author.

aroma compounds are sensitive to high temp- eratures, pervaporation may offer an effective way for a selective recovery with reduced loss of aroma since it can operate at mild temperatures. Most published research in aroma concentration was carried out using commercial composite poly (dimethyl siloxane), PDMS membranes in the plate and frame geometry, namely PervapB 1060 or Pervap 1070 which contains incorporated

Presented at the International Congress on Membranes and Membrane Processes (ICOM). Toulouse, France, July 7-12, 2002.

0011-9 164/02/$- See front matter 0 2002 Elsevier Science B.V. All rights reserved PII: SO0 I l-9 I64(02)00662-8

112 A. Shepherd et al. /Desalination 148 (2002) 111-114

silicates [2]. PDMS’s use is justified by the fact that this polymer exhibits high selectivity and permeability towards aroma compounds with low water solubility. Nevertheless, dilute component recovery is affected by the concentration polarisation phenomena. This is due to the inherent difficulty in promoting turbulent feed conditions. Hollow fibre modules have been subject of recent studies as a means of diminishing the concentration polarisation and resulting feed flow limitations for the aroma recovery [3]. This work aims at using special well-spaced hollow fibre modules constructed in our laboratory for the selective recovery of aroma compounds. Details of module construction are listed elsewhere [4].

The Solution-Diffusion Model commonly describes the mechanism of aroma transport in pervaporation. However, an alternative approach for trace organics is to use a global mass transfer coefficient, derived from the film theory. This can be applied to the aroma transport in pervaporation [3]. The aroma permeate flux can be related to the bulk liquid and permeate streams using the following equation:

Where, K, is the global mass transfer coefficient, $,,f is the aroma volumetric fraction and f and p represent feed and permeate phases respectively. The overall mass transfer coefficient is commonly used to evaluate membrane module efficiency. Another parameter is the enrichment factor, described as the ratio of liquid phase and permeate phase aroma mass fractions.

p.,. =$ or

(2)

Temperature influence on the recovery of aromas by pervaporation is also an important process parameter to be analysed. For water it is only expected an increase in its diffusion coefficient while for the aroma components variation in activity coefficient is also important [5]. This, in

turn, increases the driving force for permeation of the aroma components. The dependence of permeate flux with temperature can be expressed by an Arrhenius type equation.

J = Jo exp(- E, / RT) (3)

E, is the apparent activation energy for permeation.

2. Experimental

The membranes used in the pervaporation tests consisted of two WSLO (well-spaced longitudinal outflow) hollow fibre modules containing dense PDMS fibres, in series, with a total area of 0.257 m2. A transversal hollow fibre module [3] was also tested for comparison. Pervaporation experiments were carried out using a standard set- up described in detail elsewhere [4]. Feed and permeate compositions were measured by gas chromatography. For the feed temperature experiments, thermal isolation was provided by wrapping the system with a non-conductive material. The multicomponent feed consisted of an orange juice by-product known as water-phase. This by-product contains water, ethanol and a large number of aroma compounds with concen- tration in the ppm range. These samples were supplied by Cargill (Brazil). The analytical strategy consisted of considering all low concentration aroma compounds in the feed and permeate streams as one single pseudo component. Thus, three components were chosen to be monitored, namely, water, ethanol and the remaining aroma pseudo component. Furthermore, binary synthetic solutions consisting of water and one model solute, ethyl butyrate, were chosen to characterize the module performance.

3. Results

Figs. 1 and 2 show the influence ethyl butyrate feed concentration and the feed flow rate at 298 K. The well-spaced module was compared with a

A. Shepherd et al. /Desalination 148 (2002) 111-114 113

Ethyl butirate concentration in feed (ppm)

9.OE-07 I I I

0 100 200 300 400

Feed flow rate (L/h)

Fig. 1. Ethyl butyrate permeate concentration as a function Fig. 2. Ethyl butyrate global mass transfer coefficient as a of ethyl butyrate feed concentration. Water-ester feed, 298 function of feed flow rate. Water-ester feed, 298 K. 1, Well

K, 300 L/h. 1, Well spaced hollow fibre module; 2, Hollow spaced hollow fibre module; 2, Hollow fibre module with

fibre module with no spacers; 3, Transverse flow module. no spacers; 3, Transverse flow module.

hollow fibre not containing spacers and with the transverse flow module. The well-spaced modules showed higher product concentration in permeate streams as compared to other modules. An asymptotic value of Kg obtained with lower feed flow rate using this module, Fig. 2, indicates a more efficient mass transfer.

Figs. 3 and 4 show that, besides the permeate fluxes increase, the aroma concentration and enrichment factor also increase with temperature. Ethanol enrichment factors, however, remained constant. Another important remark is that phase

295 305 315

Feed Temperature (K)

325

Fig. 3. Aroma permeate concentration (l), aroma and ethanol (2,3) enrichment factors as a function of feed temperature. Water-phase feed, feed flow 300 L/h.

1.7E-06

c B - 1.3E-06

2 i

separation occurred after permeate condensation for the runs at 298 and 308 K. In addition, Table 1 presents the apparent activation energy for each component. It is seen that the aroma pseudo com- ponent is more influenced by feed temperature changes, reflected in higher apparent energies. The values agree with the results presented by Adgo [6].

The feed flow rate influence, at 298 K and 20, 100 and 300 L/h, on aroma mass transfer was evaluated in Fig. 5. It can be seen an increase in the aroma permeate concentration and in the

30 ( ( 8

Fig. 4. Water (I), ethanol (2) and aroma (3) permeate fluxes as a function of feed temperature. Water-phase feed, 300 L/h.

114 A. Shepherd et al. /Desalination 148 (2002) Ill-114

Table 1 Apparent activation permeation energies

Reference E,, kJ/mol

Aroma Water Ethanol

This study 49.1 19.5 27.8 [61 35.7 29.0 37.1

z

B

3 4 0 Z+.

1 0 100 200 300

Feed flow rate (L/h)

Fig. 5. Ethanol (1) and aroma (2) enrichment factors and aroma permeate concentration (3) as a function of feed flow rate. Water-phase feed, 298 K.

enrichment factor when the feed flow was varied from 100 to 300 L/h. The ethanol enrichment factor remained constant at 3.8. These results indicate that a flux coupling mechanism is not present. The assumption is further supported by constant ethanol permeate flux at different aroma feed concentrations.

4. Conclusions

The well-spaced modules were showed to be a feasible option for aromas recovery from water phase orange juice stream. The influence of feed flow conditions suggests that the WSLO modules may have an important role in aroma recovery, through the decrease of liquid phase boundary layers.

The water-phase aromas can be enriched up to 8% w/w in the pervaporation permeate stream. Since phase separations were observed in the permeate streams, operational temperatures must necessarily consider the possibility of enriched aroma recycling.

References

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