3. Microfiltration and Ultrafiltration

Define microfiltration

• Microfiltration is a pressure-driven process for removing micron-sized particles, such as bacteria, yeast cells, colloids, and smoke particles.

• microfiltration membranes have pore sizes ranging from 0.1 to 10 µm

• Microfiltration often operate with Pressure (< 2 bar)

What are the factors that affect the structure of the membrane?

• Pore size (Pore size on a membrane refers to the diameter of the openings/pores within the membrane material)

• Pore size distribution (refers to the variation in sizes of the pores within a membrane)

• Porosity (refers to the void volume fraction of the membrane)
  ɛ = (Volume of Voids / Total Volume)

What are microfiltration membranes prepared with?

Microfiltration membranes can be prepared from two main classes of materials:

• Organic materials (polymers)

• Inorganic materials (ceramics, metals, glasses)

What are the advantages and disadvantages of polymer membrane film & tubular ceramic membranes?

• for an ideal membrane, we need to make the porosity as high as possible and the pore size distribution as narrow as possible

• inorganic membrane usually has narrow pore size distribution

Microfiltration membrane manufacturing methods

What is dead-end filtration?

• The main problem affecting microfiltration is the decline in flux over time

• Dead-end filtration form thicker and more compacted cake with filtration time, which decrease membrane flux

What is cross-flow filtration?

• cross-flow can reduce the membrane surface fouling

• the cross-flow velocity should be high enough to maintain a cleaner surface

• in practice, most membranes still require periodic cleaning

What are the applications of microfiltration?

• Juice/beer industry (sterilisation or clarification of drinks)

• Dairy industry (increase shelf life of dairy products)

• Water industry (used to remove suspended material and micro-organisms, improving water quality)

Define ultrafiltration

• Ultrafiltration lies between nano- and micro-filtration, usually used to retain macromolecules from a solution.

• The lower limit is ∽2000 Da molecular weight cut-off (the smallest molecular weight this filtration can deal with, 1Da=1g/mol)

• Ultrafiltration membranes have pore sizes ranging from 5 nm to 0.1 µm.

• Ultrafiltration often operates with pressure (1-10 bar)

Describe the structure of ultrafiltration membranes

What are the applications of ultrafiltration?

• Food industry (concentration of milk, clarification of beer, concentration of egg products, recovery of potato starch, etc.)

• Dairy industry

• Waste water treatment (oil/water emulsion separation and colour removal)

• Pharmaceuticals (macromolecules)
  Textiles (colour removal/dye recovery)

• Metallurgy (mineral precipitates)

How do we determine the flux based on pore structure?

How do we determine the flux for straight capillaries and pressure-driven membranes?

What is the tortuosity factor?

A measure of how “bendy” or tortuous a path is:

• 𝜏 close to 1 (more bendy/tortuous)

• 𝜏 > 1 (more straight)

What are nodular structures?

How do we determine the flux for nodular structures?

Summary of microfiltration and ultrafiltration membranes