Reactor Theory and Practice
Continuous Flow Stirred Tank Reactor (CFSTR)
A Continuous Flow Stirred Tank Reactor (CFSTR) is one in which the contents are stirred so uniformly that it is assumed that no variation or concentration gradients exist within the vessel. In theory, any sample taken from the overflow of the reactor will be identical with any sample taken from within the vessel. In this reactor there is an in-flow of nutrient and an equal out-flow of nutrient, plus microbial waste products and microbial cells.
In order to describe the nature of a CFSTR one should understand three parameters affecting reactor dynamics, these are 1) flow rate, 2) residence time and 3) dilution time.
1) Flow rate Q=V/RT
That is Flow rate (Q) in ml/min is equal to the reactor volume (V) in ml, divided by the Residence Time (RT) in minutes. It can be measured empirically in a reactor in steady state, as the volume of effluent from the reactor per unit of time. In steady state the inflow and outflow of the reactor are equal and the culture volume of the reactor does not change.
2) Retention time RT = V/Q
The residence time (RT) in minutes is the time it takes to entirely exchange the volume of the reactor and is expressed as shown above, where V = the volume of the reactor and Q is the flow rate of the effluent leaving the system.
1.1) Flow Rate Q = V/RT
The flow rate (Q) (in ml/min) then is expressed as shown above, where V is the reactor volume and RT is the residence time. If for example the volume of the reactor is 400 ml and the flow rate is 40ml/min. then the Residence time R is 10 minutes.
3) Dilution Rate D = Q/V
The dilution rate equals the flow rate divided by the reactor volume.
In the example above, the dilution rate is:
D = 40 ml/min = 0.1 / min
400 ml
That is 0.1 or 10% of the volume of the vessel is changed every minute.
In microbiology classes a CFSTR is called a chemostat. Adjusting the flow rate can alter the rate of growth of the culture therein. Reducing the flow rate permits the exhaustion of some nutrients and the accumulation of wastes slowing growth as the culture approaches stationary phase. Increasing the flow rate increases the nutrient concentration and reduces wastes bringing the culture closer to exponential phase. In a chemostat at steady state u (the instantaneous growth rate of the culture is equal to D (the dilution rate).
If one increases the flow rate such that D > u then the planktonic culture is flushed from the system leaving only attached cells within the reactor. At this point the reactor is operating as a biofilm reactor. Of course planktonic cells are constantly being released from the biofilm by erosion or scheduled release (see chapter 2) but these cells are rapidly washed from the reactor.
Typically in setting up a reactor, the vessel is filled to capacity with nutrient medium, coupons are inserted and the reactor is seeded (inoculated) with the organism or combination of organisms desired. A time interval with the nutrient inflow closed is permitted in order for cells to attach to the coupons and then the influent is adjusted to a level which will rapidly clear the planktonic population (i.e. growth rate > residence time).
Exercise 3 – Building and Using a Biofilm Continuous Flow Stirred Reactor, a part of the Hypertext biofilm laboratory exercise collection describes the construction of a CFSTR.
This reactor may be fed either by gravity or by a pump from a nutrient medium reservoir. The waste medium is collected in a container and disinfected or autoclaved prior to disposal. The waste medium container should be placed in a tray or basin with sufficient capacity to hold the entire contents of the system in the unlikely event of a catastrophic failure. Here again the coupons used are typically glass microscope slides. The preprinted slides distributed by various companies are ideal for this purpose since they have printed on them slightly raised circles of uniform dimension so that, if one wishes to harvest cells from a known surface area, this can be readily done using the expression (Area = πr2) .
