# DBT JRF 2022 — Question 92

Exam: DBT JRF 2022 · Section: B · Topic: Bioprocess Engineering › Bioreactor Design - CSTR with Recycle · Difficulty: Medium

## Question

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) -

## Options

- **A.** is equal to $\mu_{max}$
- **B.** is around 2 $\mu_{max}$
- **C.** is around 0.5 $\mu_{max}$
- **D.** Washout will never take place

## Answer

Option B

## Explanation

Cell recycle bioreactor and washout dilution rate: In a conventional chemostat without cell recycle: $\mu = D$ at steady state, and washout occurs when $D > \mu_{max}$ . With 20-fold cell concentration and 50% cell recycle: The effective dilution rate experienced by cells is reduced because cells are recycled back. With cell retention factor: If $c$ = concentration factor (20-fold), $R$ = recycle ratio (0.5): Approximate washout condition occurs at $D \approx 2\mu_{max}$ (twice the conventional washout dilution rate). Intuition: With 50% recycle and 20-fold concentration, cells are retained more effectively, allowing higher throughput (higher D) before washout compared to a standard chemostat. Answer: With 20-fold concentration and 50% recycle, washout occurs at approximately $D \approx 2\mu_{max}$ .

Source: https://mypyqbuddy.com/questions/in-an-external-recycle-reactor-cstr-with-external-6a104cd5920734408bc0c76b