Dairy Process Engineering

Module 6. Membrane processing

Lesson 29
MATERIALS FOR MEMBRANE CONSTRUCTION

Introduction

The selection of membranes offered by the various suppliers in the business may appear to be confusing since many materials may be used to make membranes, and they are provided under an array of trade names. In reality, relatively few materials are actually used in quantity, and only a few basic membrane types form the bulk of the membranes being sold and used.

Different materials for membrane construction

Following materials are used for the membrane construction

1. Cellulose Acetate

· Contains 38 – 40 % by weight of acetyl group.

· Membranes are strong and flexible.

· Used for RO, NF and UF applications.

· Advantages

o Low price

o Less prone to fouling due to its hydrophilic nature.

· Limitations

o Low resistance to pH change.

o Maximum operating temperature is 35°C.

o Low resistance to chemicals normally used for cleaning and sanitization (e.g. chlorine)

o Can be eaten by microorganisms.

2. Synthetic polymers

1. Poysulphones

· Used for UF and MF membrane since 1975.

· Advantages

o Exceptional temperature and pH resistance

· These membranes do not tolerate oil, grease, fat and polar solvents.

· Aromatic polysulphones are used as they are resistant to oxidation, dimensionally stable and resistant to acids, alkali, salt solutions and detergents even at elevated temperatures or moderate pressure.

2. Polyvinylidenefluoride

· These are resistant to hydrocarbons and oxidizing environments.

· These have excellent abrasion resistance and good resistance against temperature and chemicals.

3. Polyacrylonitrile
4. Polytetrafluoroethylene

It has extraordinary chemical resistance and can be used in a wide range of temperature.

5. Aliphatic and aromatic polyamides

3. Composite membranes

· Made to replace cellulose acetate RO membranes.

· The main advantage is the combination of relatively high flux and very high salt rejection.

· They also have good temperature and pH resistance.

· They do not tolerate oxidizing environments.

· Composite membranes are made in two-layer and three-layer designs, the precise composition of which is proprietary.

· Around 1980, FilmTec marketed the two-layer design which immediately became the industry standard for water desalination.

· In the mid-1980s Desalination Systems, Inc. (DSI) began making composite membranes with a three-layer design.

· The three-layer design is available for RO and NF, and it is still the best choice for treating a vast array of difficult process streams.

4. Others

. ABS :Useful material in water desalination systems.
. Delrin: Also called POM; it is an excellent material. Its biggest weakness is limited resistance in low pH environments.
. PE Polyethylene has good chemical stability but very limited temperature stability.
. PP Polypropylene is chemically a very resistant polymer. The temperature stability is limited, and it has a tendency to creep.
. PVC PVC is mostly used for low pressure piping. It is inexpensive, but has severe temperature limitations. It can only be used for un-demanding applications, such as the desalination of ground water.
. C-PVC Chlorinated PVC has significantly better temperature stability than ordinary PVC. The present political lobbying against the PVC industry may preclude it from being an acceptable construction material in the future.
. PVDF An excellent but rather expensive material. It has good heat stability and is chemically almost as resistant as Teflon.
. FRP Glass fiber reinforced polyester is widely used for housings. It has become the standard in water desalination in spite of its obvious shortcomings. Corrosion resistance is its prime advantage, closely followed by low price.
. Epoxy A very special type made only in Germany by Membratec. The housings are thin walled, but can nevertheless tolerate very high pressure.

 

Membrane structure

Membranes have a tight top layer facing the product to be treated. This layer is also called the skin layer. It is thin, typically <0.1 micron. The membrane itself is 150 - 250 micron, the bulk of the membrane simply providing structural support for the skin layer.

Pore size differs for different types of processes:

MF – 5 – 0.1 micron

UF – 0.1 – 0.01 micron

NF, RO – 0.001 micron (theoretical)

Selection of membrane materials

1. Based on chemical resistance

2. Based on type of process