Modeling of fouling layer deposition in cross-flow microfiltration during tomato juice clarification

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Abstract

The effect of parameters, such as transmembrane pressure and axial flow rate, on membrane fouling during tomato juice clarification were studied by cross-flow microfiltration using flat sheet polyvinylidenefluoride membranes. The effect of fouling on permeate flux was modeled using a classical constant pressure dead-end filtration equation and its modified form for cross-flow filtration. The main physico-chemical properties of tomato juice were evaluated. The clarified juice was very similar to the feed except for insoluble solids and lycopene, which were concentrated in the retentate. Cake formation was identified as the main reason for flux decline. At different axial flow rates, the fouling mechanism evolves from cake filtration to an intermediate pore blocking mechanism with increasing pressure.

Highlights

► The pseudo-steady state filtrate flux increased by increasing Reynolds number and operating pressure up to an optimum value. ► Field and Hérmia models were used to predict flux decline mechanism as two different mathematical methods. ► New concept of fouling resistance factor (R) was used as function of compression and thickness of fouling layer. ► Operating pressure and Reynolds number affect on the compression and thickness. ► A second-order polynomial correlation was being used to represent the dependency of K to Reynolds number and pressure values.

Introduction

Tomato (Lecopersicon esculentum) originated in America and was taken to Europe at the beginning of the sixteenth century, and from this area spread out to the rest of the world (Yilmaz, 2001, Razi et al., 2011). Tomato juice, one of the most popular tomato products, is rich in vitamins, minerals and other beneficial components such as lycopene which is one of the most important and abundant carotenoids (Roldan-Gutierrez and Luque de Castro, 2007). In addition to the lycopene's benefits to human health due to its roles as antioxidant, it is also applied as food colorant in food industry in order to adjust the red color (Ravelo-Pérez et al., 2008).

Traditionally, some thermal treatments such as Cold break or Hot break have been applied in industrial processes in order to inactivation some enzymes such as pectinmethylesterase (PMS) and polygalacturanase (PG) and destroy spoilage bacteria to minimize quality loss (Xu et al., 1986, Gould, 1992). Although thermal treatment is available public domain, however it causes several unpleasant effects on it, such as flavor and aroma loss, nutritional degradation and color change to brownish red (Wu et al., 2008, Cassano et al., 2007a).

A good alternative to traditional methods will be membrane processes including microfiltration (MF) and ultrafiltration (UF) which need lower energy and capital cost. Membrane processes are non-thermal separation methods which cause no phase change, and minimal loss of proteins, vitamins, sugars and salts (Bottino et al., 2002, Pereira et al., 2002, Koseoglu et al., 1990).

Nevertheless, when membrane processes started, permeate flux reduces constantly until it reaches to a uniform rate which is also known as steady-state (Mohammadi et al., 2002, Cassano et al., 2003). The reason for such a phenomenon, called fouling, is the collection of cell wall polysaccharids such as lignin, cellulose and pectin, on the membrane surface or inside its pores, which may lead to a considerable value of fruits and energy waste, reduction of product rate and juice quality (Barros et al., 2003, Mohammadi et al., 2003, Yu et al., 1986).

There are four modes of fouling that are categorized according to different blockage mechanisms as follows (Grace, 1956):

  • i)

    Complete blocking.

  • ii)

    Standard blocking.

  • iii)

    Intermediate blocking.

  • iv)

    Cake filtration.

Based on experimental observation, shown in Fig. 1, these mechanisms may occur individually or in some cases are combinations of two or more modes. For each mechanism, a mathematical model has been developed to predict permeate flux decline rate and its limit value, which leads to prevent fouling. Rai et al. (2008) studied flux decline mechanism during microfiltration of watermelon and identified cake formation was the main reason for flux decline. Also Barros et al. (2003) described the flux behavior of ceramic and polysulfone membranes during cross-flow ultrafiltration of pineapple juice by two mathematical methods. Cassano et al. (2007b) estimated fouling mechanism during ultrafiltration of blood orange in fixed operation conditions of pressure and temperature, and identified the fouling mechanism in dependence of the axial velocity. Although studies on prediction of fouling mechanism have been reported, there is no report on simultaneous effects of operation conditions on the fouling mechanism and clarification of the tomato juice by microfiltration.

In this study, cross-flow microfiltration using polyvinylidenefluoride (PVDF) membrane was applied for clarification of tomato juice and the effects of axial flow rate and pressure were investigated on flux decline mechanism. Hérmia theory (Hérmia, 1982) and its modified version for cross-flow filtration by Field et al. (1995) are two mathematical methods which were used to predict flux decline mechanism.

Section snippets

Tomato juice preparation

Ripe tomatoes were purchased from the local market in Tehran. Unpeeled fruits were manually washed in water and cut in pieces. Tomato juice was produced by juicer and stored at −18 °C and defrosted to room temperature before use.

MF unit and procedure

After defrosting, the juice was filtered by a fine nylon cloth and submitted to laboratory pilot unit that equipped with stainless steel module, containing a polymeric flat sheet membrane (material: polyvinylidenefluoride (Durapore, HVLP, Millipore, Billerica, MA), mean

Theory

The convective flux, direct from the bulk solution toward the membrane prevails on the rate of shear-induced back diffusion of the rejected material, this leading to the formation of a fouling layer on the membrane surface and flux decline (Todisco et al., 1996).

The mode of flux decline during filtration for power law non-Newtonian fluids can be identified (Hérmia, 1982):d2tdV2=βdtdVnwhere V is the cumulative volume of filtrate, t the time of operation, and β is a constant. The permeate flux is

Hérmia model evaluation

Fig. 3 shows the changes of permeate flux versus time at constant pressure values. It must be mentioned that all the experiments have been carried out at temperature of 50 °C and Reynolds number of 300. The pore blocking of PVDF porous membrane and gel layer formation over the membrane surface by pectic materials present in the juice causes the flux to decrease with time. The permeate flux increased from 9.74 kg/m2 h to 12.2 kg/m2 h at the end of 5 h of experimental when ΔP was increased from 1 to 3 

Conclusion

Identification of flux decline mechanism of tomato juice was carried out using cross-flow microfiltration at different values of pressure and axial flow rate. From the above discussions the following conclusions can be drawn:

  • -

    The cake filtration was the main reason for flux decline at different pressure values based on Hérmia theory.

  • -

    The mathematical analyses of the flux behavior using Field et al. (1995) equation revealed that by increasing axial flow rate, fouling mechanism evolves from the

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