Chemostat - Mass and Kinetic Balances Flashcards
What makes a continuous fermentation process a chemostat?
Keeping one element (typically the substrate) constant as the limiting reactant.
What assumptions do we make for a Chemostat reactor?
Ideal mixing, no concentration gradients
When the substrate is the limiting agent, what is the specific growth rate equal to?
The dilution rate (D)
What is the dilution rate and what are it’s units?
The dilution rate is the flux (flow in OR flow out) divided by the reactor volume D = F/V. The units are time^-1
What is the equation that shows that D = u?
dx/dt - ux - Dx (dx/dt = 0) => D = u
What is the general balances for substrate, product and biomass concentration?
Assume they are constant
Sub: 0 = -rs x + D(csf - cs)
Prod: 0 = rpi x + D(cpf - cp)
Bio: 0 = (u - D)x
What are the solutions for substrate, product, and biomass concentration?
Assume they are constant
cs = (DKs)/(Umax - D)
cp = (rp x)/D
x = D(cfs - cs)/rs
What is the critical dilution rate?
It is the maximum Flow over reactor volume that can be used. If this amount is exceeded, biomass will be washed out of the reactor faster than it can replicate.
What are the 8 characteristics of a chemostat reactor?
Chemostat characteristics:
Volume is constant (Fout > 0)
One limiting substrate, D = u
Both flow in and flow out. Inflow contains no biomass, outflow contains cells and product in same concentration as the reactor.
Steady state: x, Cs, Cp, etc… are constant
Cs depends on D not on Csf (not on substrate concentration in feed: Csf)
High productivity can maintained for long periods of time.
Most often lower product concentration than in a fed-batch
Problems with contamination and strain stability (random mutations affect productivity)
How does a chemostat start?
It starts as a batch reactor (no feed or outflow) and eventually the outflow and feed streams are started.
