FU Xiao-qin, LI Lin, WANG Wen-zong, CHEN Ling, LI Bing

(School of Light Industry and Food Sciences, South China University of Technology, Guangzhou 510640,China)

0 Introduction

Pumpkin, a kind of widely-planted crop in China, possesses many biological functions. It is generally used as a vegetable or dried as crude food material. Due to its insufficient processing, the economic benefit of pumpkin is relatively low. It is necessary to find a way to the effective utilization of pumpkin and to the improvement of its additional value.

Pumpkin polysaccharide (PP) is an important functional active component in pumpkin. It has been widely used in the fields of medicine and health care due to its good hypoglycemic effect and strong immunity-improving and blood lipid-reducing ability[1,2].As the traditional production method of PP, namely the water extracting-alcohol precipitating method, is of high energy consumption and cost as well as low efficiency, in recent years, some researchers adopted ultrafiltration method to separate and purify PP[3].However, serious concentration polarization and membrane fouling may occur during the ultrafiltration because PP is of high relative molecular mass and PP solution is of high viscosity. In order to solve this problem, some methods are proposed to enhance the ultrafiltration process, such as the enhancement with physical fields. And, due to its wide adaptability and safety, more and more attention is paid to ultrasonic enhancement.

As for the ultrasonic enhancement of ultrafiltration, some works had been reported. Kobayashietal[4]adopted ultrasonic with 28 kHz and 8～33 W to enhance the ultrafiltration of peptone and found out that ultrasonic can effectively overcome the membrane fouling and enhance the ultrafiltration. Similar results had been obtained from the ultrafiltration of peptone by Chaietal[5]at various ultrasonic frequency and power. Moreover, in Muthukumaran′s research on the ultrafiltration of leucosin[6], the membrane flux was found to increase by 1.2～1.7 times. All these results indicate that ultrasonic is effective in enhancing the ultrafiltration of biological macromolecules. In order to promote the industrialization of PP and the application of ultrasonic enhancement technology, this paper introduces ultrasonic field in the ultrafiltration process, and develops a pilot ultrasonic-enhanced ultrafiltration system to concentrate pumpkin polysaccharides. By analyzing the effects of ultrasonic parameters and operation parameters on the ultrafiltration properties, the mechanism of ultrasonic enhancement is investigated.

1 Materials and Methods

1.1 Main materials and equipment

The main materials and equipment used in the investigation are as follows:

Crude PP:purchased from Tai′an Zhonghui Plant Biochemical Co., Ltd.. Polysulphone (PS) membrane: with a molecular cut-off of 30 000，purchased form Shanghai Filter Co., Ltd.. 3,5-dinitrosalicylic acid: analytical reagent, purchased from Shanghai Richjoint Chemicals Co., Ltd.. Phenol: analytical reagent, purchased from Guangdong Shantou Guanghua Chemicals Co., Ltd.. Other chemical are all analytical reagents. PCS-type UV-2102 UV-Vis spectrophotometer: produced by UNICO (Shanghai) Instruments Co., Ltd.. Pilot ultrasonic-enhanced ultrafiltration system: self-developed.

1.2 Experimental methods

1.2.1 Ultrafiltration method

The self-developed ultrafiltration system was used to perform experiments. In the ultrafiltration, 10 L of crude PP solution was fed in the system, and the ultrasonic generator was simultaneously opened to form an ultrasonic field in the membrane chamber. The ultrafiltration properties were measured with membrane flux and rejection at various pressure, temperature, ultrasonic frequency and power.

1.2.2 Calculation of membrane flux

Membrane flux is calculated as follows:

(1)

WhereJis the membrane flux(L·m-2·h-1),Qis the volume of permeate(L),tis the ultrafiltration time(h) andAis the effective membrane area(m2).

1.2.3 Calculation of membrane rejection

Membrane rejection is an important index representing the rejection performance of membrane. It is calculated according to Equation (2):

Ra=(1-Cp/Cb)×100%

(2)

WhereRais the membrane rejection(%),Cpis the mass concentration of the crude solution(mg/mL) andCbis the mass concentration of the permeate(mg/mL).

2 Pilot Ultrasonic-Enhanced Ultrafiltration System

2.1 System procedure

The self-developed pilot ultrasound-enhanced ultrafiltration system is a kind of automatic electromechanical equipment with a crude filtration loop, two ultrafiltration loops, a cleaning loop and a heat-exchange loop. The control module of the system provides a human-machine interface, and is of the functions of dynamic simulation and on-line display. An industrial computer is used as the host computer and SIEMENS S7-200 PLC is used as the slave computer to implement a layered automatic control. The process parameters such as temperature and pressure can be detected on line by the sensors distributed in the loops and the control signals can be fed back in real time. Thus, the automation degree and ultrafiltration efficiency of the system are both greatly improved.

Fig.1 illustrates the flow chart of the ultrafiltration system. Fig.2 is the photograph of the system.

Fig.1 Flow chart of the self-developed system Fig.2 Photograph of the self-developed system

2.2 Ultrafiltration membrane module with ultrasonic enhancement

Membrane module is the most important component of an ultrafiltration system. In the developed system, nine ultrasonic resonators are uniformly distributed on the upper metal panel of the plate membrane module. Thus, ultrasonic can directly affect the liquid in the chamber. Fig.3 shows the overhead view of the membrane module with ultrasonic resonators.

Fig.3Overhead view of the membrane module 1-stud; 2-ultrasonic resonator; 3-module panel

Fig.4Variation of membrane flux with time at different ultrasonic powers

The main parameters of the ultrasonic generator are as follows: input voltage, 220 V; working current, 0.4～0.9 A; output frequency, 28 and 40 kHz; working temperature, 0～50 ℃; relative working humidity, less than 80%. Both the output frequency and the duty cycle of the generator can be automatically or manually adjusted.

3 Results and Discussions

3.1 Effect of ultrasonic power on ultrafiltration properties

Fig.4 shows the variation of membrane flux with ultrafiltration time at different ultrasonic powers.The experiments were performed for 1.2% crude PP solution with a flowrate of 500 L/h at 0.45 MPa and 45 ℃. It can be seen from Fig.4 that, during the ultrafiltration with or without ultrasonic enhancement, the membrane fluxes all decrease with time; and that, with ultrasonic enhancement, the decrease amplitude of flux in the whole ultrafiltration process obviously reduces. After the ultrafiltration for 120 min, the flux decreases by about 30% without enhancement but only by less than 11% with enhancement.

Concentration polarization and membrane fouling are two problems that affect the ultrafiltration property. With the prolonging of ultrafiltration time, the concentration polarization becomes more and more serious, many PP macromolecules deposit on the membrane surface to form a gel layer[7], which results in the decrease of flux. As a kind of mechanical wave, ultrasonic is of high energy. When it propagates in liquid, ultrasonic cavitation may occur, bubbles with large amount form. The bubbles experience a process from formation to expansion and further to rupture. The rupture of bubbles causes intensive agitation to the liquid[8], which smoothens the PP gel layer and cleans the membrane holes. Thicker the gel layer is, more obvious of the loosening effect of ultrasonic. Thus, the ultrasonic enhancement effect becomes stronger with the ultrafiltration time.

3.2 Effect of ultrasonic frequency on ultrafiltration properties

The ultrasonic enhancement of ultrafiltration relays on not only the ultrasonic power but also the frequency[9]. Figs.5 and 6 show the variations of average membrane flux and rejection with ultrasonic power at different frequencies. The experiments were performed for 1.2% crude PP solution with a flowrate of 500 L/h at 0.45 MPa and 45 ℃ for 120 min. As illustrated in Figs.5 and 6, both the average membrane flux and the rejection increase with ultrasonic power, especially at a low ultrasonic frequency (28 kHz). It thus comes to the conclusion that the ultrasonic with low frequency is more effective in enhancing the ultrafiltration.

Fig.5Variation of average membrane flux with ultrasonic power at different frequencies

Fig.6Variation of membrane rejection with ultrasonic power at different frequencies

Ultrasonic cavitation is highly related to the frequency. At high ultrasonic frequency, the cavitation threshold increases, so that higher ultrasonic power is needed to form cavitation bubbles. In other word, at a constant ultrasonic power, bubbles are easier to form at low ultrasonic frequency[10]. So, the enhancement of ultrafiltration with 28 kHz ultrasonic is more obvious.

It is also found that the average membrane flux at 0.75 W/cm2greatly increases while the rejection of PP suddenly decreases (see Figs.5 and 6). This may probably due to the damage of PS membrane caused by ultrasonic vibration.

3.3 Effect of temperature and pressure on ultrasonic properties

In order to reveal the effects of temperature and pressure on the ultrasonic enhancement, some experiments were performed for 1.2% PP solution with a flowrate of 500 L/h at 0.45 MPa and 45 ℃ for 120 min, in which 28 kHz ultrasonic with a power of 0.63 W/cm2was adopted. The results are shown in Figs.7 and 8.

Fig.7Relationship between average membrane flux and temperature with ultrasonic enhancement

Fig.8Relationship between average membrane flux and pressure with ultrasonic enhancement

As shown in Fig.7, the average membrane flux of PP approximately linearly increases with temperature, and the increase amplitude becomes higher at high temperature, which means that temperature is of a positive effect on the ultrafiltration and that high temperature is more helpful to the ultrasonic enhancement. From Fig.7, it is also found that low-frequency ultrasonic is more helpful to the enhancement, which accords well with the conclusion drawn from Fig.5.

Fig.8 reveals the relationship between average membrane flux and ultrafiltration pressure. Similar to the phenomenon without ultrasonic enhancement, the average membrane flux with ultrasonic enhancement first increases with the pressure and then becomes approximately constant at 0.45 MPa. Moreover, it is found that, the increase amplitude of average membrane flux (as compared with that without ultrasonic enhancement) at low pressure is higher than that at high pressure, which means low pressure is more suitable for the ultrasonic enhancement.

4 Conclusions

In order to overcome the concentration polarization and membrane fouling during ultrafiltration, ultrasonic was introduced and a pilot ultrasonic-enhanced ultrafiltration system was developed, which was then successfully applied to the concentration of PP. From the experimental results, it can be found that:

(1)Ultrasonic effectively overcome the concentration polarization and membrane fouling and improves the membrane flux, especially at high ultrasonic power.

(2)Low-frequency ultrasonic is superior to the high-frequency one in terms of enhancing the ultrafiltration.

(3)Temperature is of a positive effect on the ultrafiltration and high temperature is more helpful to the ultrasonic enhancement of PP ultrafiltration.

(4)Low pressure is more suitable for the ultrasonic enhancement of PP ultrafiltration.

The application of ultrasonic enhancement technology to ultrafiltration has attracted more and more attention. Further researches may emphasize the enhancement mechanism and the coupling of ultrasonic parameters with operation conditions.

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