A novel method for spinning hollow fiber membrane and its application for treatment of turbid water

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Abstract

A novel method using phase inversion technique, to spin hollow fiber membrane was proposed in this work. Using polyacrylonitrile, the hollow fibers are spun. The characterization of the hollow fibers indicated that they are in the range of microfiltration membranes. Turbid water with high turbidity (from 200 to 1000 NTU) was treated using various cross flow velocities and transmembrane pressure drop values. Effects of these operating conditions on the permeate flux were studied in detail. Two modes of operating conditions were studied, namely, total recycle mode and batch mode. In all the cases, the turbidity of the permeate was found to be zero. A simple resistance in series model was used to quantify the cake resistance and its dependence on the operating conditions.

Highlights

► A novel method is developed to spin the hollow fiber membranes. ► Water with high turbidity can be treated successfully by these membranes. ► The permeate contains zero turbidity. ► Cake resistance is analyzed by resistance-in-series model.

Introduction

Microfiltration is a pressure driven membrane separation process and is used to separate micron sized particles from aqueous solution [1]. It is becoming an essential unit operation for various processes. The commercial applications include, pretreatment of water [2], treatment of biological suspension [3], beer and malt processing [4], dilute slurries [5], removal of dyes [6], fractionation of whey protein [7], etc.

Among various configurations of membrane modules, namely, plate and frame, hollow fiber, tubular, spiral wound, etc., hollow fiber modules have several distinct advantages. These include (i) large membrane area per unit volume leading to saving of space, (ii) ease of back flushing after the operation, (iii) ease of handling and operation [8].

A series of methods for production of various hollow fibers, relevant processes, equipment and with specific properties have been appeared in the literature. In general, there are four methods of preparation of hollow fibers, namely, (i) melt spinning; (ii) dry spinning; (iii) wet spinning; and (iv) combination of dry and wet spinning. In all these methods, polymers or their blends are extruded through a spinneret nozzle and the core is formed using water or similar liquids, leading to the formation of the hollow fibers. Various designs of spinneret nozzles are available [9], [10], [11]. The spinneret diameters vary typically in the range 1–2 mm. The shape of the spinneret nozzle largely determines the quality and characteristics of the spun hollow fibres. The dimensions of the holes in the spinneret are fixed. Therefore, a particular spinneret can produce a fixed size of the fibers. Spinnerets being costly, these designs are expensive. A number of patents and papers are available on spinnerette based hollow fiber spinning in various ranges of ultrafiltration and microfiltration [12], [13], [14], [15], [16], [17], [18], [19], [20], [21].

In the present work, a simple and novel hollow fiber spinning technique using two coaxial disposable needles has been developed and demonstrated. The produced hollow fibers using polyacrylonitrile polymer has been characterized and applied to the treatment of turbid water. There are number of reports available for treatment of turbid water using microfiltration. Jones et al. [22] reported removal of clay suspension using flat sheet microfiltration module. From 402 NTU turbid solution they produced a permeate less than 1.2 NTU for various operating conditions. They used pressure pulsation to improve the permeate flux. Wittmann et al. used dead end hollow fiber unit for treatment of turbid water [23]. From 105 to 184 NTU feed samples, the permeate turbidity was brought down to less than 0.1–0.2 NTU. Swart et al. [24] studied the effects of operating conditions on microfiltration of turbid water. They observed that cross flow microfiltration is more economical than tubular filter press. Dillon et al. [25] reported the treatment of turbid water from an Australian aquifer using microfiltration and quantified the membrane fouling by defining a modified index. Kim et al. [26] undertook a study of size ranges of natural organic matters (NOM) responsible for fouling of microfiltration membrane during filtration of turbid water. They concluded that the NOMs with molecular weight range between 300–2000 and 20000–40000 Da are responsible for fouling of the hydrophobic membrane. Petala and Zouboulis [27] used vibratory shear enhanced processing of turbid water using microfiltration, ultrafiltration and nanofiltration membranes. They reported more than 97% removal of turbidity. Kim et al. [28] used microfiltration as a pretreatment process to produce drinking water from raw turbid water. They reported rapid fouling of the membrane by formation of cake layer over the membrane surface.

In this paper, the utility of hollow fiber microfiltration membranes produced by the novel method outlined earlier is investigated for the treatment of turbid water. The effects of operating conditions on the permeate flux have been studied in detail. Studies on performance of both, complete recycle and batch mode of operation have been undertaken. A simple resistance-in-series model is used to quantify the fouling of the membrane.

Section snippets

Materials

Polyacrylonitrile (PAN) polymer (average molecular weight 250,000 g/gmol) was procured from M/s, Technorbital, Kanpur, India. N,N-dimethyl formamide (DMF) was purchased from M/s, Merck (India) Ltd., Mumbai, India. The polymer (PAN) was heated to 70 °C for 2 h so that the moisture in the polymer was evaporated. The polymer melt consisting of 15% by wt of PAN dissolved in solvent DMF was used for spinning.

Hollow fiber membrane and module preparation

Fig. 1a describes the complete set up of producing the hollow fiber. The heart of this set up

Membrane characterization

The cross section and the surface morphology of the hollow fiber membrane were studied using scanning electron micrograph as mentioned earlier. Inner diameter of hollow fibers was 0.7 mm and outer diameter 1.3 mm. These figures are presented in Figs. 3a and b. Fig. 3a clearly shows the interior hollow core of the fiber produced by the method as explained in Section 2.1. SEM photograph of the surface of the membrane presented in Fig. 3b shows the porous structure of the membrane. It is also

Conclusions

Microfiltration grade of hollow fiber membranes were spun using an inexpensive, novel method. The hollow fiber produced were characterized using scanning electron micrograph, contact angle and membrane permeability. A detailed analysis was performed for treatment of turbid water using these membranes. The permeate was free of turbidity and had characteristics close to distilled water. The particles in the range of size 700 nm onwards were removed by these membranes and that fraction was

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