MEMBRANE DISTILLATION

Membrane Distillation

Table of Content

MEMBRANE DISTILLATION1

COMPREHENSIVE LITERATURE REVIEW1

Introduction1

Approaches followed for MD improvements1

Modeling in MD4

Feed temperature5

Feed inlet concentration6

Permeate inlet temperature7

Temperature difference and mean temperature effect8

Permeate flow velocity8

Vapor pressure difference10

Membrane thickness10

Membrane porosity11

Membrane pore size11

Pore size distribution12

Pore Tortuosity12

Membrane surface chemistry13

Membrane module geometry14

Rate controlling step in MD process16

Crystallization fouling17

Biological fouling18

Particulate and corrosion fouling19

Long-term MD performance21

Energy and maintenance cost evaluation in MD23

Conclusions24

REFERENCES25

Membrane Distillation

COMPREHENSIVE LITERATURE REVIEW

Introduction

Membrane distillation (MD) is the thermally propelled method, in which only vapor substances are conveyed through porous hydrophobic membranes. The fluid feed to be treated by MD should be in direct communicating with one edge of membrane and does not penetrate interior dry pores of membranes. The hydrophobic environment of membrane stops fluid answers from going into its pores due to exterior stress forces. (Termpiyakul 2004)

Approaches followed for MD improvements

Various experimental methods have been applied for improvements of MD process. The first attempt was claimed in 1967. A multi-stage MD system capable of reusing latent heat of vaporization many times has been proposed. (Tay 1995)The permeate of first stage of MD process can be used to heat feed of the second stage and permeate of second stage is applied to heat feed of third and so on. Years later, Findley concluded in his published paper that if in MD process high temperature is applied, low cost system and long-life membranes with adequate characteristics can be obtained, MD could be an economical method for desalination of seawater. Since then, numerous studies have been performed to investigate effect on MD permeate flux enhancement and selectivity of different operating variables such as temperature, flow rate, vacuum pressure, etc. (Aron 2009)

It is generally acknowledged that used membranes in most of MD researches are actually made for other process (i.e. microfiltration) rather than MD. Despite that most of required features to MD membranes are met with these commercially available membranes, need for new membranes fabricated especially for MD (Moziaa 2009)purposes have been also widely accepted by MD investigators. However, only few investigators have worked on design of MD membranes , and . With recent advancement in this field, new promising generation of membranes has appeared that not only provides higher permeability, but also can increase transmembrane pressure difference through minimizing heat loss by conduction through non-porous portion (i.e. matrix) of membrane(Manna 2010). Within this area, porous homogenous and porous composite hydrophobic/hydrophilic membranes have been made and tested for MD applications and . On other hand, advancements in module design, let to emerge of new modules for different MD configurations, that can provide improved fluid flow dynamics and can yield high reasonable transmembrane fluxes, through minimizing boundary layer heat and mass transfer resistances , , and . Nevertheless, more must be done before getting adequate membrane for MD applications and MD configurations as well as appropriate MD modules (Lawson 1996).

Another approach of making MD the competitive and the viable process is to find new areas where MD can be successfully applied. The increasing academic interests towards MD process have lead to recognition of wide variety ...