Membranes Flashcards
(38 cards)
What is a membrane in separation processes?
A semipermeable barrier that selectively allows passage of certain particles or molecules based on size or properties.
How are membrane separations different from distillation?
They are rate-based, not equilibrium-stage processes.
What does Ultrafiltration (UF) retain?
Soluble macromolecules (> a few thousand Daltons); allows small molecules and solvents to pass.
What does Microfiltration (MF) remove?
Bacteria, microorganisms, and large macromolecules (pore size 0.1–10 μm)
What does Nanofiltration (NF) reject?
Multivalent ions (e.g., SO₄²⁻, PO₄³⁻) almost completely; partial NaCl rejection; separation is influenced by size and charge.
What is the typical structure of a membrane used in practice?
A thin selective skin layer (0.1–1 μm) on a thicker porous support for mechanical strength.
What is Reverse Osmosis (RO)?
A process where applied pressure reverses osmotic flow, allowing solvent to pass through the membrane while rejecting salts.
What are common types of membrane modules?
Stirred cell,
Flat sheet
Spiral wound
Tubular
Hollow fibre modules
Which module has the highest packing density?
Hollow fibre module.
Which module handles viscous fluids and suspended solids best?
Tubular membrane module.
What is membrane fouling?
Accumulation of particles or chemicals that block membrane pores and reduce performance.
What are the four pore blocking mechanisms?
Complete pore blocking
Internal blocking
Partial blocking
Cake filtration
What factors contribute to membrane fouling?
Feed properties
Membrane material
Operating conditions (e.g. pressure, temperature, cross-flow velocity).
How can fouling be minimized?
Pre-filtration
pH/ionic strength control
Cross-flow velocity
Backflushing (for some membranes)
Where is RO commonly used?
Desalination of seawater or brackish water for high-quality treated water.
What is a major advantage of membranes?
Can separate based on size, shape, and sometimes charge — with compact high-surface-area designs.
What is a major disadvantage of membranes?
Lower throughput than distillation; cost scales almost linearly with membrane area.
What are six key characteristics of an ideal membrane for separation?
High selectivity for desired permeate component
High mechanical strength
Chemical resistance
Low pressure drop
Low fouling tendency / easy to clean
Cheap to make
What are four desirable properties of the membrane backing material?
High mechanical strength to support membrane
High porosity / low pressure drop
Chemically resistant
Bonds well to membrane
Why are most separation membranes used in a composite form with backing material?
Most separation membranes are made very thin because only the top surface is active in separation, and thin membranes reduce pressure drop. However, this also makes them mechanically weak. Therefore, they are attached to a porous backing material that:
Provides mechanical strength
Allows for easier handling
Reduces the risk of defects
Minimises pressure drop while maintaining separation performance
What are four primary advantages of hollow fibre membranes?
High concentrations in the retentate
High packing density
Cleaned by backflushing
Can operate at low pressure / low pumping lower required
What are three primary disadvantages of hollow fibre membranes?
Fragile structure of fibres
Difficult to clean
In straight designs, damage on one fibre requires replacement of entire module.
Describe the structure of a hollow fibre membrane module.
A shell packed with thousands of fine hollow fibres (like drinking straws). Feed flows either through or around the fibres, and permeate passes through the membrane walls. Offers high surface area in a small volume. Cylindrical casing.
What is the main difference between porous and non-porous membranes?
Porous membranes separate by size exclusion (sieving), while non-porous membranes separate by diffusion through the membrane material.
