**2. Principles of liquid membranes**

New technologies and developments in membranes can be accessed from journals (e.g., J. Membr. Sci., Sep. Sci. Technol., Sep. Purif. Technol., J. Alloy. Compd.), vendor communications (via websites), patents and conference proceedings, e.g., annual ACS (Prudich et al., 2008). Theories and applications of liquid membranes (LMs) are stated in (Baker, 2007; Baker & Blume (1990); Kislik, 2010; Scott & Hughes, 1996). Refer to Kislik (Kislik, 2010), LMs are classified in different criteria as follows:


hollow fiber supported liquid membrane (HFSLM) is renowned as a favorable system to separate valuable compounds or pollutants at a very low concentration and has specific characteristics of simultaneous extraction and stripping of the low-concentration target species in one single stage, non-equilibrium mass transfer, high selectivity and low solvent

This chapter describes transport mechanisms in HFSLM and shows some applications with




New technologies and developments in membranes can be accessed from journals (e.g., J. Membr. Sci., Sep. Sci. Technol., Sep. Purif. Technol., J. Alloy. Compd.), vendor communications (via websites), patents and conference proceedings, e.g., annual ACS (Prudich et al., 2008). Theories and applications of liquid membranes (LMs) are stated in (Baker, 2007; Baker & Blume (1990); Kislik, 2010; Scott & Hughes, 1996). Refer to Kislik

2. Facilitated or carrier-mediated transport (The chemical aspects of complexation

reactions to the performance of facilitated transport will be discussed later.)

data at the average percentage of deviation of 2%. (Pancharoen et al., in press).

reference to our up-to-date publications, for example

were reported (Wannachod et al., 2011).

(Lothongkum et al., 2009).

**2. Principles of liquid membranes** 

1. Bulk liquid membrane (BLM)

1. Simple support

4. Active transport

2. Water-soluble polymers

3. Emulsion liquid membrane (ELM)

3. Coupled counter- or cotransport


3. Electrostatic, ion-exchange carriers 4. Neutral, but polarizable carriers



(Kislik, 2010), LMs are classified in different criteria as follows: - Classification based on module design configurations

2. Supported or immobilized liquid membrane (SLM or ILM)

used.


## **2.1 Membrane structures, materials and modules**

The performance of membrane relates closely to its structure, material and module. It is known that porous membranes can be classified according to their structures into microporous and asymmetric membranes. Microporous membranes are designed to reject the species above their ratings. They can get blocked easily compared to asymmetric membranes.

In case of membrane materials, polymeric or organic membranes made of various polymers (e.g., cellulose acetate, polyamide, polypropylene, etc) are cheap, easy to manufacture and available of a wide range of pore sizes. However, some limitations like pH, temperature, pressure, etc can impede the applications of polymeric membranes. On the other hand, ceramic or inorganic membranes have advantages of high mechanical strength, high chemical and thermal stability over the polymeric membranes but they are brittle and more expensive.

In terms of membrane modules, the development of membrane module with large surface areas of membrane at a relatively low manufacturing cost is very important. Resistance to fouling, which is a particularly critical problem in liquid separation, depends on the membrane module. Of four types of the SLM modules (spiral wound, hollow fiber, tubular and flat sheet or plate and frame), hollow fiber module (Fig.1) has the greatest surface areato-volume ratio resulting in high mass transfer coefficient and is the most efficient type of

Fig. 1. The hollow fiber module (http://www.liquicel.com/product-information/gastransfer.cfm)

Roles of Facilitated Transport Through HFSLM in Engineering Applications 181

2. simultaneous extraction and stripping of very low-concentration target species (either

4. compact and modular design for easy installation and scaling up for industrial

7. lower operating cost (consuming small amounts of extractant and solvent and low

As stated, the extremely important disadvantage of HFSLM is the fouling of the hollow fibers causing a reduction in the active area of the membrane and therefore a reduction in flux and process productivity over time. Fouling can be minimized by regular cleaning intervals. The concepts of membrane fouling and cleaning were explained by Li & Chen (Li & Chen, 2010). Active research includes, for example, membrane surface modification (to reduce fouling, increase flux and retention), new module designs (to increase flux, cleanability), etc should be further studied. In short, flux enhancement and fouling control were suggested by different approaches separately or in combination (Cui et al., 2010; Scott

Mass transfer plays significant role in membrane separation. The productivity of the membrane separation processes is identified by the permeate flux, which represents rate of target species transported across the membrane. In general practice, high selectivity of membranes for specific solutes attracts commercial interest as the membranes can move the specific solutes from a region of low concentration to a region of high concentration. For example, membranes containing tertiary amines are much more selective for copper than for nickel and other metal ions. They can move copper ions from a solution whose concentration is about 10 ppm into a solution whose concentration is 800 times higher. The mechanisms of these highly selective membranes are certainly different from common membranes which function by solubility mechanism or diffusion. The selectivity of these membranes is, therefore, dominated by differences in solubility. These membranes sometimes not only function by diffusion and solubility but also by chemical reaction. In this case, the transport combines diffusion and reaction, namely facilitated diffusion or

For an in-depth understanding of the facilitated transport through liquid membrane, we recommend to read (Kislik, 2010). The facilitated transport mechanisms can be described by solute species partitioning (dissolving), ion complexation, and diffusion. The detailed steps

facilitated transport or carrier-mediated transport (Cussler, 1997).

precious species or toxic species) in one single stage; 3. mild product treatment due to moderate temperature operation;

maintenance cost due to a few moving parts);

1. hydrodynamic management on feed side; 2. back flushing or reversed flow and pulsing;

3. membrane surface modification;

**3. Mass transfer across HFSLM** 

6. regular effective membrane cleaning.

applications;

8. higher flux;

& Hughes, 1996):

are as follows:

4. feed pretreatment; 5. flux control;

5. low energy consumption; 6. lower capital cost;

9. non-equilibrium mass transfer.

membrane separation. Hollow fiber module is obviously the lowest cost design per unit membrane area. Compared to flat sheet modules, hollow fiber modules can be operated at higher pressure operation and their manufacturing cost is lower. However, the resistance to fouling of the hollow fibers is poor so the module requires feed pretreatment to reduce large particle sizes. The properties of module designs are shown in Table 1.


Table 1. Properties of membrane module designs (modified from Table 2. p. 419, Baker, 2007)

In principle, important performance characteristics of membranes are 1) permeability, 2) selectivity and retention efficiency, 3) electrical resistance, 4) exchange capacity, 5) chemical resistance, 6) wetting behavior and swelling degree, 7) temperature limits, 8) mechanical strength, 9) cleanliness, and 10) adsorption properties (Kislik, 2010). We will discuss in section 3 relevant to permeability, resistance and mass transfer across the HFSLM.

### **2.2 Hollow fiber supported liquid membrane (HFSLM)**

The characteristics of the hollow fiber module are shown in Table 2.


Table 2. Characteristics of hollow fiber module

Hollow fiber modules are recommended to operate with the Reynolds number from 500-3,000 in the laminar flow region. They are one of high economical modules in terms of energy consumption. Other advantages of HFSLM over conventional separations are:

1. high selectivity based on a unique coupled facilitated transport mechanisms and sometimes by using synergistic extractant;


membrane separation. Hollow fiber module is obviously the lowest cost design per unit membrane area. Compared to flat sheet modules, hollow fiber modules can be operated at higher pressure operation and their manufacturing cost is lower. However, the resistance to fouling of the hollow fibers is poor so the module requires feed pretreatment to reduce large

Manufacturing cost moderate high high high Resistance to fouling very poor moderate good very good Parasitic pressure drop high moderate low low High pressure operation yes yes difficult difficult Limit to specific membranes yes no no no Table 1. Properties of membrane module designs (modified from Table 2. p. 419, Baker,

In principle, important performance characteristics of membranes are 1) permeability, 2) selectivity and retention efficiency, 3) electrical resistance, 4) exchange capacity, 5) chemical resistance, 6) wetting behavior and swelling degree, 7) temperature limits, 8) mechanical strength, 9) cleanliness, and 10) adsorption properties (Kislik, 2010). We will discuss in

Hollow fiber modules are recommended to operate with the Reynolds number from 500-3,000 in the laminar flow region. They are one of high economical modules in terms of energy consumption. Other advantages of HFSLM over conventional separations are: 1. high selectivity based on a unique coupled facilitated transport mechanisms and

section 3 relevant to permeability, resistance and mass transfer across the HFSLM.

**Characteristics Description**  Material Polypropylene

**2.2 Hollow fiber supported liquid membrane (HFSLM)** 

Table 2. Characteristics of hollow fiber module

sometimes by using synergistic extractant;

The characteristics of the hollow fiber module are shown in Table 2.

Fiber ID (m) 140 Fiber OD (m) 300 Number of fibers 10,000 Module diameter (cm) 6.3 Module length (cm) 20.3 Effective surface area or contact area (m2) 1.4 Area per unit volume (cm2 /cm3) 29.3 Pore size (m) 0.03 Porosity (%) 30 Tortuosity 2.6

**fibers Spiral-wound Flat sheet Tubular** 

particle sizes. The properties of module designs are shown in Table 1.

**Properties Hollow** 

2007)

9. non-equilibrium mass transfer.

As stated, the extremely important disadvantage of HFSLM is the fouling of the hollow fibers causing a reduction in the active area of the membrane and therefore a reduction in flux and process productivity over time. Fouling can be minimized by regular cleaning intervals. The concepts of membrane fouling and cleaning were explained by Li & Chen (Li & Chen, 2010). Active research includes, for example, membrane surface modification (to reduce fouling, increase flux and retention), new module designs (to increase flux, cleanability), etc should be further studied. In short, flux enhancement and fouling control were suggested by different approaches separately or in combination (Cui et al., 2010; Scott & Hughes, 1996):

