Membrane Distillation

Membranes, Hybrid Systems and Pilot Studies

Kang-Jia Lu, Tai-Shung Chung

• Publisher: CRC Press
• Year: 2019
• ISBN:
  9781000691009
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Industrial & Technical Chemistry

This book aims to elaborate the basics and recent advances of membrane distillation (MD) as the same shows promise for seawater desalination and wastewater treatment. Starting with fundamentals of MD processes, including the heat and mass transfer analysis, energy evaluation and mathematical modelling, text includes engineering and molecular design of MD membranes. Various types of hybrid systems, including freeze desalination (FD)-MD, MD-crystallization (MDC), pressure retarded osmosis (PRO)-MD and forward osmosis (FO)-MD, will be discussed in this book. Further, it summarizes the future of MD from both industrial and academic perspectives along with energy sources and economic analysis.

Section A

Fundamentals

1Introduction to Membrane Distillation

Kang-Jia Lu and Tai-Shung Chung

1.1 MEMBRANE DISTILLATION AND ITS HISTORICAL DEVELOPMENT

Water is one of the most essential natural resources, yet it is depleting rapidly due to human activities. An increasing number of countries, especially in arid regions, have reached the limit at which sustainable water can be hardly delivered (United Nations, 2018). In order to mitigate water scarcity, various desalination technologies have been developed in past few decades. Among them, desalination based on membrane technologies contributes to nearly 68% of the total desalination capacity, and this figure is constantly increasing as the membrane market grows rapidly (Nakoa et al., 2016). Among various types of membrane technologies, membrane distillation (MD) has drawn increasing attention in past few decades due to its unique characteristics.

MD is a thermally driven separation process that involves the transport of water vapor across a hydrophobic membrane (Alkhudhiri et al., 2012; Alklaibi and Lior, 2005; Camacho et al., 2013; Drioli et al., 2015; Johnson and Nguyen, 2017; Khayet, 2011; Khayet and Matsuura, 2011; Lawson and Lloyd, 1997; Schneider et al., 1988; Wang and Chung, 2015). As its name suggests, the driving force of MD is similar to that of the conventional distillation process, namely, the water vapor pressure difference. Unlike the traditional distillation, MD uses hydrophobic membranes as the contact surfaces where water evaporation takes place. The concept was first brought up by Bodell in his U.S. patent entitled “Silicone rubber vapor diffusion in saline water distillation” in June 1963 (Bodell, 1963). Five years later, another patent entitled “Distillation of saline water using silicone rubber membrane” was published by Bodell as a continuation-in-part of his first MD patent (Bodel, 1968). In his patents, Bodell described an apparatus that extracted potable water from unpotable aqueous mediums through membrane-assisted distillation. The apparatus consisted of several vapor permeable, liquid impermeable tubular silicone rubber membranes positioned in a hot feed tank. Water vapor that diffused into the lumen side of the membranes was collected by an air stream and condensed externally to a salt-free fluid. This invention gave rise to the first sweeping gas membrane distillation (SGMD) configuration. Bodell also mentioned an alternative method to collect water vapor by applying vacuum on the membrane lumen side, which was regarded as the first description of vacuum membrane distillation (VMD). More detailed descriptions of various types of MD configurations will be summarized in Chapter 2.

The first paper on MD was published in 1967 by Findley (1967). He introduced an “infinite-stage flash evaporation” apparatus where water evaporated from one interface of “a nonwettable porous membrane” and condensed at the other interface of the membrane, giving rise to the first direct contact membrane distillation (DCMD) configuration. Findley listed several advantages of this technology, including the mild operating temperature and pressure, low tendency to scale, space saving, and the flexibility to carry out the heat exchange either inside or outside of the evaporation system. Findley also pointed out that the porous medium would introduce resistance to vapor diffusion and the heat conduction across the membrane would cause energy loss. In addition, Findley specified the desired properties of membranes for this application. According to him, the suitable membrane should have (1) a high resistance to conductive heat flow; (2) a sufficient but not excessive thickness; (3) a negligible permeability to liquids and nonvolatile compounds; (4) a low absorptivity of moisture; (5) a high porosity and relatively straight-through pores to reduce the mass transfer resistance; and (6) a uniform porosity. These prospective views are still providing important guidance today in developing suitable membranes for MD (Alkhudhiri et al., 2012; Camacho et al., 2013; Drioli et al., 2015a; Khayet, 2011; Suk and Matsuura, 2006; Wang and Chung, 2015). In the same year, a U.S. patent was filed by Weyl et al. (Weyl, 1967)using a polytetrafluoroethylene (PTFE) membrane to recover demineralized water from saline waters. The membrane was 3175 µm thick and had a porosity and mean pore size of 42% and 9 µm, respectively. Weyl et al. addressed another important advantage of MD, namely, 100% theoretical rejections to nonvolatile solutes if wetting did not occur. Other than PTFE, polyethylene (PE), polypropylene (PP), and polyvinyl chloride (PVC) and hydrophobic ceramic composition were also suggested. They also proposed to fabricate composite membranes that consisted of a hydrophilic support and a hydrophobic coating to achieve desirable antiwetting properties. In addition, two setup configurations were proposed to improve the heat recovery of the systems. One was a five-stage system consisting of alternating membrane and metal plate with heat supplied through the bottom plate. The heat released in the condensed water compartment of a stage was transferred to the saline water in the next stage by heat conduction through the metal plates. The other design was to have the hydrophobic flat sheet membrane coiled up into a hot cylinder to give a multistage spiral wound module.

1967 was an exciting year for MD. Another two papers were published by Henderyckx et al. from Belgium (Henderyckx, 1967; Van Haute et al., 1967). In one of the works, they established a curve relating the vapor permeability to the temperature for a given membrane in an SGMD configuration (Van Haute et al., 1967). In their second study, a diffusion doublet that consisted of a vapor permeable membrane and a plastic condensation surface that were separated by a thin layer of gas was investigated, giving rise to the air gap membrane distillation (AGMD) configuration (Henderyckx, 1967). They also stated the possibility of utilizing waste hot water and solar energy to power the system, though no further experiment was conducted. In 1969, Findley et al. published another fundamental study on mass and heat transfer relations involved in evaporation through porous membranes (Findley et al., 1969). The study showed that the major factor influencing the rate of vapor transfer was the diffusion through the stagnant gas inside the membrane pores. After that, the passion toward MD quickly faded away, which could partially be ascribed to the lack of suitable membranes and the relatively low production rate compared to other membrane-based desalination technologies such as reverse osmosis (RO).

In early 1980s, with the advances in membrane fabrication techniques, the interest in MD was recovered. Especially, the academic interest in MD was fueled by its versatility and fundamental engineering concepts embodied in MD processes. In 1969, Bob Gore accidentally applied a sudden and accelerating stretch to the heated PTFE rods of about 800% and formed a microporous structure with a porosity over 70%. This happy accident led to the creation of the expanded polytetrafluoroethylene (ePTFE) under the trademark Gore-Tex (Schneider et al., 1988). In 1982, Gore & Associated Co. proposed to apply the Gore-Tex membrane for MD in a liquid-gap MD configuration. The adopted membrane had a thickness of 25 µm, a porosity up to 80%, and a pore size between 0.2 and 0.45 µm (Gore, 1982). During the same period, Cheng et al. filed a series of U.S. patents on multilayer hydrophobic–hydrophilic composite membranes for MD (Cheng, 1981; Cheng and Wiersma, 1982, 1983a, 1983b). They claimed that the attachment of hydrophilic support layers to hydrophobic support layer could significantly enhance the water vapor flux. It was reported that a composite membrane with the maximum hydrophobic pore diameter of 0.48 µm generated a flux of 75.2 kg m⁻² day⁻¹ for a feed temperature of 62.8 °C and a distillate temperature of 56.7 °C. That was one order of magnitude higher than that reported by Weyl et al. in 1967. At the same time, several other companies such as the Swedish Development Co. and Enka AG also conducted research on MD actively....

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• About the Editors
• Contributors
• SECTION A     Fundamentals
• SECTION B     Hybrid Systems
• SECTION C     Pilots & Economic Studies
• Index