After fermentation, a pretreatment of the fermentation broth is required to separate biomass, proteins, and cells. The multivalent salts also need to be removed for purification. he separation of inorganic salts and proteins/cells presents a special problem with the production of valuable substances from renewable raw materials. Therefore, the methods used for downstream processing will play a very important role.
Membrane Process is essentially a separation process based on molecular properties. The advanteges of membrane process include:
- It reduces the number of unit processes in treatment systems
- Potential for process automation and plant compactness
- Much smaller foot print than the conventional plants of the same capacity
- Easy scale-up, expansion and retrofication
- Less or no chemical use and provides highest quality water
- No formation of secondary chemical by-products
- Less sludge production
- Water reuse and recycling
Ceretainly, the disadvantges exist, includng membrane fouling, low membrane life time, low selectivity, and high capital and operating cost.
Based on the driving forces, the following processes are defined as:
Pressure driven membrane processes
Microfiltration (MF)
-Simple screening mechanism
Pore size 0.01 μm - 10 μm
DP » 0.01 to 0.5 MPa
-Low pressure process
-Most effectively remove particles and microorganisms (bacteria)
-High flux
-Colloids/Macromole ---> theoretically pass through the membrane
Ultrafiltration (UF)
-Screening and Adsorption
Pore size 1 - 100 nm
DP 0.1 to 1 MPa
-Membrane is classified in terms of Molecular Weight-Cut off (MWCO) : 1000 - 100,000
-Two layers: a thin (0.1 to 0.5 µm), skin layer and a porous substructure support layer
-Separation of macromolecules
-Only surface deposition: no internal pore plugging; so, relatively easy to remove, irreversible
Nanofiltration (NF)
- NF Removes molecules in the 0.001 micron range
DP 0.5 to 6 MPa
MWCO: 0.2 to 200
-NF is essentially a lower-pressure version of reverse osmosis
-NF performance characteristics between reverse osmosis and ultrafiltration
-Membrane: similar to UF, thin active layer; porous support layer
Electrical driven membrane processes
-Electrodialysis (ED)
-Dialysis
-Osmosis
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Based on Modules, membrane module refers to the device which houses the membrane element:
•
Tubular membrane module
- Membrane is cast inside the support tube
- Tubular membranes have a diameter of 5 - 15 mm
- High SS tolerance
- Flow is usually inside out
-Mainly MF and UF
- Low packing density, high prices per module
• Hollow fibre membrane module
- Consists of a bundle of hundreds and thousands of hallow fiber
- Entire assembly is inserted into a pressure vessel
- Feed can be applied inside of the fiber (inside-out flow) outside (outside-in flow)
- Highest packing density of all.
- Hollow fiber is used mainly for NF and RO
•Spiral wound membrane module
- Flexible permeate spacer is provided between two flat sheet membranes
- Membrane: sealed three side and open side is attached to perforated pipe
- Flow is in a spiral pattern.
- Membrane envelop is spirally wound along with a feed spacer
- Filtrate is collected within the envelop and piped out
- Packing density:high
- RO and NF
• Plate and frame
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Membrane Fouling: Deposition or accumulation of solids on the membrane.
Fouling causes resistance to flow through the membrane and eventual decline in overall flux.
Three major mechanisms of resistance flow:
-Pore narrowing
-Pore plugging
-Gel/cake formation due to concentration polarization
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