Effect of inner-layer thermal conductivity on flux enhancement of dual-layer hollow fiber membranes in direct contact membrane distillation

Abstract

The thermal conductivity of hydrophobic/hydrophilic hollow fiber membranes is a key factor affecting flux in direct contact membrane distillation processes. In this paper both experimental work and mathematical modeling are performed to investigate the effect of the hydrophilic layer's thermal conductivity on vapor flux. Modeling predicts a significant increase in vapor flux from 31.4 to 78.5 kg m⁻² h⁻¹ under the feed and distillate temperatures of 80 and 20 °C, respectively, when the thermal conductivity of the inner hydrophilic layer is varied from 0.2 to 1.4 W m⁻¹ K⁻¹. To investigate this experimentally, graphite particles and multiwall carbon nanotubes (MWNT) were embedded for the first time into the hollow fiber hydrophilic layer to improve its thermal conductivity. It was found that incorporating graphite alone led to only a minor improvement in thermal conductivity. However, by incorporating both graphite and MWNT the thermal conductivity was increased from 0.59 to 1.30 W m⁻¹ K⁻¹. This improvement is attributed to the conducting network formed by the MWNT which bridges the polymer nodules. The improved thermal conductivity led to a significant increase in vapor flux from 41.2 to 66.9 kg m⁻² h⁻¹, under the inlet feed and distillate temperatures of 80.4 and 15.3 °C, respectively, in general agreement with the mathematical modeling.

Introduction

Compared with multi-effect evaporation or multi-stage flash vaporization, membrane distillation (MD) has the advantage of operation at relatively lower feed temperatures in water production or the dehydration of aqueous feeds. Low production costs are achievable to employ MD in some circumstances where industrial waste steams or low-cost heats are readily available [1]. Especially, MD is favorable on the dehydration of feed solutions with high osmotic pressures which cannot be handled easily by reverse osmosis [2]. Therefore, MD is regarded as a promising technology for seawater desalination and dehydration of aqueous solutions in the chemical industry. Over the past four decades, the progress of MD has experienced patent registration, theoretical study, system and module development based on commercially available hydrophobic membranes originally designed for microfiltration, and investigation and development of novel MD membranes especially for MD applications.

There are four types of MD, such as Direct Contact Membrane Distillation (DCMD), Air-Gap Membrane Distillation (AGMD), Sweeping Gas Membrane Distillation (SGMD) and Vacuum Membrane Distillation (VMD). Among them DCMD is the most attractive one due to its simple configuration and low requirements for operation [3]. In the case of water as the permeate, i.e. desalination of brackish water or dehydration of aqueous solutions, DCMD is considered to be one of the most suitable types due to the easy accessibility of cold water as coolant. As far as the membrane configurations are concerned, flat-sheet membranes are widely utilized because of the simple module design and easy processability. However, it needs a much larger space to install the flat-sheet membrane modules compared with hollow fiber membrane modules in order to achieve a certain production rate. Hence, hollow fiber membranes are more practical and desirable in large-scale MD applications.

Many hydrophobic microfiltration hollow fiber membranes have been applied in the MD research and most of them exhibited moderate flux. For instance, Teoh et al. applied Hyflux™ polypropylene (PP) hollow fiber membranes to study the effect of different module designs on the flux. It was found that the water fluxes under a feed temperature of 75 °C were in the range of 5.4–8.5 kg m⁻² h⁻¹ for different module designs [4]. Gryta [5] employed Accurel PP S6/2 membrane supplied by Membrana GmbH on concentrating tap water and obtained a flux of 34 kg m⁻² h⁻¹ under a feed temperature of 90 °C. In the dry-jet wet-spinning process, polyvinylidene fluoride (PVDF) is the frequently used material for MD membrane fabrication due to its high solubility in common solvents, i.e. N-methyl pyrrolindone (NMP). However, the hydrophobicity of PVDF is moderate and needs further improvement by importing highly hydrophobic fine particles to prevent membrane wetting. Teoh and Chung [6] increased the contact angle of PVDF hollow fiber membranes from 88° to 103° by incorporating polytetrafluoroethylene (PTFE) particles into the outer layer. In the study of Wang et al., the PVDF/Cloisite 20A composite hollow fiber membranes with a void fraction of up to 90% exhibited an even higher flux of 79.2 kg m⁻² h⁻¹ under a feed temperature of 81.3 °C [7].

It seems that hollow fiber membranes with a thinner thickness would exhibit a higher vapor flux due to the low resistance for mass transfer. However, hollow fiber membranes fabricated by the wet-spinning process with such thin wall is too vulnerable to withstand the hydraulic pressure from either lumen or shell side. Taking into account the effect of heat conduction across the membrane, the optimum wall thickness of MD single-layer hollow fiber membranes to achieve the highest flux was suggested in the range of 30–60 μm [8]. Therefore, hydrophobic–hydrophilic dual-layer hollow fiber membranes were developed with the hydrophilic inner layer serving as the mechanical support for the brittle outer layer while the thin outer layer aiming to generate a high vapor flux. Bonyadi and Chung fabricated the first hydrophobic–hydrophilic dual-layer hollow fiber MD membranes based on PVDF. With incorporating polyacrylonitrile (PAN) and hydrophilic Cloisite NA⁺ particles into the inner layer and hydrophobic Cloisite 15A particles into the outer layer, the dual-layer hollow fiber membranes exhibited vapor flux of 55 kg m⁻² h⁻¹ under a feed temperature of 90 °C [9]. The hydrophobic–hydrophilic concept was also adopted by Wu et al. on flat membranes by surface grafting on flat cellulose acetate (CA) membranes through the radiation of a gas mixture consisting of styrene, pyridine and CCl₄ [10]. However, the modified CA membranes exhibited a high salt rejection but a low flux in the DCMD process. Qtaishat et al. synthesized hydrophobic surface modifying macromolecules (SMMs) and incorporated them into the polyetherimide hydrophilic polymer for membrane fabrication [11]. The resultant flat hydrophobic–hydrophilic dual-layer membranes obtained a flux of 28 kg m⁻² h⁻¹ under a feed temperature of 65 °C. Based on the modeling work carried out by the same group, these flat-sheet hydrophobic–hydrophilic dual-layer membranes would increase 4- to 5-fold with an increase in thermal conductivity of the inner hydrophilic layer from 1 to 20 W m⁻¹ K⁻¹ [12].

Because the vapor condensation occurs at the interface of hydrophilic/hydrophobic layer, the existence of hydrophilic layer may greatly reduce the mass transfer resistance between the evaporation side and the condensation side comparing with mere hydrophobic membranes with the same thickness. However, this design may cause high thermal resistance within the inner hydrophilic layer. To solve this problem, importing fillers with high thermal conductivity was adopted in the membrane fabrication. In addition, increasing the thermal conductivity of the hydrophilic layer will enhance the temperature gradient across the hydrophobic layer as shown in Fig. 1. As a result, the effect of temperature polarization will be highly decreased [12]. In the study of Ishida and Rimdusit, a thermal conductivity as high as 33 W m⁻¹ K⁻¹ was obtained from polybenzoxazine blended with boron nitride (80 vol.%) [13]. The fillers at high content tended to constitute heat flow paths within the matrix, thus increasing the thermal conductivity.

In the present work, we predict the flux increment for hydrophobic–hydrophilic dual-layer hollow fiber membranes with respect to the thermal conductivity improvement in the inner hydrophilic layer. This is the first time that the flux of hydrophobic–hydrophilic dual-layer hollow fiber membranes is modeled to facilitate the membrane design. In addition to employing hydrophilic graphite as the bulk filler to increase the thermal conductivity of the inner layer, MWNT with the extremely high thermal conductivity, ∼2000 W m⁻¹ K⁻¹, is employed to bridge the particles and polymer nodules to enhance the thermal conductivity. A remarkable flux is achieved by the thermal property adjustment.