An Overview Of Membrane Processes

The following tables will give you a basic appreciation for the technologies. Table 1 provides a comparison of the factors that affect the performance of the five technologies.

Table 1. Factors Impacting on the Performance of Membrane Processes

Technology

Driving Force

Influencing Factors

Size

Diffusivit y

Ionic Charge

Solubility

Microfiltration

Pressure

+ + +

-

-

-

Ultrafiltration

Pressure

+ + +

-

+

-

Nanofiltration

Pressure

+ + +

+

+

-

Reverse Osmosis

Pressure

+

+ + +

+

+ + +

Electrodialysis

Electrica 1

+

+

+ + +

-

Table 2 provides a comparison of membrane structures. Between these two tables you should get an idea of the operating conditions viz., membrane structural types the driving forces involved in separation, and the separation mechanisms.

Common types of membrane materials used are listed in Table 3. This gets us into the concept of geometry. There are three types of modules generally used, namely: Tubular, Spiral wound, and Hollow fiber. A comparison of the various geometries is given in Table 4.

Table 2. Compares Membrane Structures

Technology

Structure

Driving Force

Mechnism

Microfiltration

Symmetric microporous (0.02 - 10 n

Pressure, 1-5 atm

Sieving

Ultrafiltration

Asymmetric microporous (1 -20 nm)

Pressure, 2-10 atm

Sieving

Nanofiltration

Asymmetric microporous (0.01 - 5 nm)

Pressure, 5 - 50 atm

Sieving

Reverse Osmosis

Asymmetric with homogeneous skin and microporous support

Pressure, 10 -100 atm

Solution diffusion

Electrodialysis

Electrostatically charged membranes (cation and anion)

Electrical potential

Electrostatic diffusion

Table 3. Summary of Common Membrane materials and Their Characteristics

Technology

Membrane Materials

Polar Character

Microfiltration

Polypropylene (PP)

Polyethylene (PE)

Non polar

Polycarbonate (PC)

Non polar

Ceramic (CC)

Non polar

Technology

Membrane Materials

Polar Character

Ultrafiltration

Polysulfone (PSUF)

Non polar

Dynel

Non polar

Cellulose acetate (CA)

Non polar

Nano filtration

Polyvinylidene fluoride

Polar

(PVDF)

Cellulose acetate

Polar

Reverse Osmosis

Polyamide

Polar

Nylon

Polar

Electrodialysis

Styrene/vinylpyridene

-

Divinyl benzene

Table 4. A Comparison Between Module Types

Parameter

Module Type

Tubular

Spiral wound

Hollow fiber

Surface area, m2/m3

300

1000

15,000

Inner dia./spacing, mm

20-50

4-20

0.5-2

Feed flow rate, Liter/m2-day

300 - 1000

300 - 1000

30 - 100

Production, m3/m3 of membrane per day

100 - 1000

300 - 1000

450 - 1500

Pretreatment requirements

Simple

Average

High

Extent of clogging

Little

Average

High

Parameter

Module Type

Tubular

Spiral wound

Hollow fiber

Mechanical

Cleaning

Methods

Possible

Not possible

Not possible

Chemical Cleaning Methods

Possible

Possible

Possible

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