76. In an external recycle reactor (CSTR with external recycle), the outlet stream is concentrated 20 fold with respect to biomass and half of this concentrated cells is fed back into the reactor. In such a situation, washout will take place when dilution rate (D) –
A. is equal to 𝜇𝑚𝑎𝑥
B. is around 2 𝜇𝑚𝑎𝑥
C. is around 0.5 𝜇𝑚𝑎𝑥
D. Washout will never take place

Introduction:

In the design of Continuous Stirred-Tank Reactors (CSTR), especially those with external recycle, a common challenge is the risk of washout. Washout occurs when the microbial population in the reactor is washed out faster than it can grow, leading to a loss of the desired biomass. This phenomenon is highly influenced by the dilution rate, the flow rate at which the culture medium enters and exits the reactor, as well as the maximum specific growth rate (μmax\mu_{max}μmax​) of the organisms involved.

In this context, an external recycle reactor uses a mechanism where a portion of the output stream is recycled back into the reactor, which increases the concentration of biomass in the input. The problem arises when the dilution rate becomes too high for the system to sustain, resulting in washout. Let’s explore the conditions under which washout can occur in such reactors.

Key Concepts:

1. Dilution Rate (D):
   The dilution rate is defined as the flow rate into the reactor divided by the reactor volume. It is crucial in determining how much of the biomass is removed from the reactor.

2. Maximum Specific Growth Rate (μmax\mu_{max}μmax​):
   The maximum rate at which microorganisms grow when nutrients are abundant. It represents the upper limit of the microbial growth rate in a given environment.

3. Washout Condition:
   Washout occurs when the dilution rate exceeds the rate at which microorganisms can grow. In such cases, microorganisms are washed out of the reactor faster than they can reproduce, causing a complete collapse of the microbial population.

Problem Setup:

In the case of an external recycle reactor, the outlet stream is concentrated 20-fold with respect to biomass, and half of this concentrated biomass is fed back into the reactor. This setup increases the effective biomass concentration in the reactor.

The question asks at what dilution rate (D) washout will occur in this specific reactor configuration.

Conditions for Washout:

Washout typically happens when the dilution rate (D) exceeds the maximum specific growth rate (μmax\mu_{max}μmax​) of the microorganisms in the reactor. However, the presence of external recycle affects the dynamics, as the biomass concentration is effectively increased by recycling.

Understanding the Situation:

• If the dilution rate (D) becomes too large relative to the growth rate (μmax\mu_{max}μmax​), the biomass will be removed faster than it can regenerate, leading to washout.

• Since the biomass concentration is increased by the recycle process, the system can withstand a higher dilution rate before washout occurs.

Correct Answer:

In this case, washout will occur when the dilution rate (D) is around twice the maximum specific growth rate (μmax\mu_{max}μmax​), due to the increased effective biomass concentration from the recycle process.

Thus, the correct answer is:

B. is around 2 μmax\mu_{max}μmax​

Conclusion:

Washout in an external recycle reactor occurs when the dilution rate exceeds the maximum growth rate of the microorganisms. In this particular scenario, the dilution rate (D) needs to be about twice the maximum specific growth rate (μmax\mu_{max}μmax​) for washout to occur, taking into account the increased biomass concentration from the external recycle mechanism. Understanding this balance is essential in reactor design and operation to avoid washout and maintain stable microbial populations.