Introduction to Separations Flashcards
What is the cost of separation?
Separation is 40% – 70% of total plant capital and operating costs
What is the relationship with concentration and cost of separation?
The lower the concentration of a substrate in its natural form → the higher the cost of that substrate.
Cost is driven by the cost of the separation
Mechanism of separation
- Mixing of chemicals is spontaneous as it increases entropy and randomness
- Separation is the opposite of mixing of chemicals ∴ ,
- Separation of chemicals requires the uses of energy
Homogeneous separation
all components in one continuous system (more difficult).
Sometimes, work is put in to go from homogeneous to heterogenous separation
Heterogeneous separation
- components can be in different phases.
- If two or more immiscible phases exist mechanical separation is preferable
• e.g. centrifuge, pressure reduction, electrical/magnetic field
Classifying separations -Property
• Molecular properties ○ Molecular weight ○ Van der Waals volume ○ Van der Waals area ○ Molecular shape (acentric factor) ○ Dipole moment ○ Polarizability ○ Dielectric constant ○ Electric charge ○ Radius of gyration • Thermodynamic and transport properties ○ Vapour pressure ○ Solubility ○ Absorptivity ○ Diffusivity
Classifying separations - Method
• Method ○ Phase creation ○ Phase addition ○ Barrier ○ Solid Agent ○ Force field or gradient
Phase creation
○ Involves the creation of a second phase that is immiscible with the feed
○ Accomplished by energy or pressure reduction
○ Suitable for mixture that have tendency to vaporise
○ e.g. evaporation, sublimation, crystallisation, distillation
Phase addition
○ For homogenous separation, single phase mixture, a second immiscible phase must be developed
○ This is achieved by:
§ Creation of energy separating agent (ESA)
§ Mass separating agent (MSA)
○ Disadvantages of MSA:
§ Need additional separator to recover MSA
§ Need for MSA makeup
§ Possible contamination of the product
§ more difficult design procedure
○ When 2 immiscible fluid phases are contacted, intimate mixing of the 2 phases is important in enhancing mass transfer rates
○ After phase contact, employing gravity and/or enhanced techniques completes the separation process
Separation by barrier
○ Includes the use of microporous and nonporous membrane as semipermeable barriers
○ Membrane are fabricated from polymer, natural fibre, ceramic, metal etc.
○ Microporous membrane- separation occur at different diffusion rate
○ Nonporous - separation based on the solubility
Solid agent
○ Process that use solid mass-separating agents
○ Solid normally in the form of a granular material or packing, e.g.
§ Activated carbon
§ Aluminium oxide
§ Silica gel ,or
§ Calcium aluminosilicate zeolite
Force field/gradient
○ External fields can take advantage of differing degrees of response of molecules and ions to force fields
○ Can be used to enhance current separation techniques
When selecting a separation process, what are the options to think about?
- Feed conditions
- Product conditions
- Property differences
- Characteristics of the operation
- Economics
- Opinion
Separation rule of thumbs
○ Remove the most plentiful impurities first
○ Remove the easiest-to-remove impurities first
○ Select processes that makes uses of the greatest differences in the properties of the product and its impurities
○ select and sequence processes that exploit different separation driving forces
Key components
○ Light key- high relative volatility, low b.p.
○ Heavy key- low relative volatility, high b.p.
