Lecture 4: Downstream Processing Flashcards
What is mass transfer?
“process of transport from one phase to another”
Phases can be: liquids, solids or gases
Phases can be miscible or separate e.g., gas & liquid, solid & liquid, oil & water
Concentration difference typically drives transferW
What are the applications of mass transfer?
Distillation
Crystallisation
Gas adsorption
Nutrient supply
What are the phases of mass transfer
- Transfer within gas/oil to liquid interface
- Transfer across gas/solid/oil interface into liquid phase
- Transfer from liquid to cell boundary
- Diffusion across cell boundary to cell surface
- Diffusion into cell to reaction center
What are the principles of mass transfer?
- Mass transfer can limit growth by restricting nutrient supply
- Defined by Ficks Law
“J = -D deltaC/deltaX” - Growth is limited via:
A. in phase transfer: slow
diffusion or long
distance
B. Between phase transfer:
exchange across
interface can be rate
limiting. Large clusters
of cells can deprive
middle cells of nutrients
C. Across cell membrane:
unless transporters,
diffusion can be slow
What are the effects of limited mass transfer?
- limitation of output:
a. overall reaction rate = slower. Eg. formation of Glucoronic acid from glucose by Gluconobacteroxydans
b. Alternate metabolic pathways switched on. E.g., preparation of bakers yeast
- Large scale bioreactors designed to optimise mass transfer
How can mass transfer be optimised in bioreactors?
- mixing: without shear force, heterogeneity in the chemostat
- Particle size: small separate granules dissolve faster, smaller gas bubbles increase SA
- Pressure: can help with gas/liquid driving force
What are the different types of bioreactor?
1 Stirred tank reactors
2 bubble column bioreactors
3 airlift bioreactors
4 solid & fluid phase bed reactors
What is the stirred tank bioreactor?
- stirred tank vessels
- aeration via gas exchange
- baffles connected to stirrer
- mixing through stirring, controls nutrient and gas distribution
- Typically multiple baffles to increase mixing
- impeller type, length, and liquid viscosity are related to “shear rate” (measure of mixing efficiency)
- Shear rate is related to “shear stress” (which can damage cells) & also E req to turn impellers in viscose liquids
- Shear stress can be minimised by larger fermenters, stirring at 120rpm for animal and plant cells, filamentous and mycelical cultures. Planktonic microbials >300rpm
What is a bubble column bioreactor?
- Gas stream forms tiny bubbles at base of reactor
- density causes bubbles to rise
- diffusion of gas through the reactor from bubbles
- mixing of nutrients through bubbling
What are the pros and cons of bubble column bioreactors?
+ high aeration with mixing
- Foaming a problem
+ suited for wastewater and less viscous fermentations
- poorly controlled and can be poor mixing
What are airlift bioreactors?
- Based on bubble column
- gas used to create a flow, causing nutrient mixing
- concentric is most common
- multiple types of ALR:
a. internal-loop split
b. internal-loop concentric tube reactor
c. External loop
What are fluid and solid phase bioreactors?
- Particles used to prod product from feed
- Particles sink away from effluent which is harvested
- solid phase can contain bacterial biofilms, enzymes, etc
- no gas, so generally anaerobic
What are hollow fiber bioreactos?
- often used for mammalian cells
- porous fibers stimulate blood vessels
- nutrients and waste flow through hollow fibers
- cells can grow on surface of fibers
- cells and secreted proteins can accumulate in the interfiber space
What is downstream processing?
- Products of microbial activity include: biomass & or products trapped inside cells, soluble & secreted products
- most of processing can determine viability of commercial process
- yield is lost at each step of an isolation or purification
- multiple products can sometimes be obtained from a single fermentation
What is clarification
- separation of cells from liquid
- multiple methods depending on cells:
A. waste water/brewing use settling/floccation
B. centrifuging can be used on smaller cells
C. Filtration often used to remove small/trace cells
