Recycle, Purge, and Bypass

Key points

1. 01

   Recycle returns part of the product stream to the feed to increase residence time and overall conversion — but pushed too far, it can reduce productivity.

2. 02

   Recycle raises overall conversion of fresh feed, not necessarily single-pass conversion — per-pass performance typically drops toward CSTR-like behavior as the recycle ratio grows.

3. 03

   Cell recycle decouples hydraulic retention time ($1/D$) from biomass retention time — returning cells that would otherwise wash out lets a bioreactor run at a dilution rate above $\mu_{max}$ without washing out.

4. 04

   Bypass diverts part of the feed around the process and rejoins it with the product, used when the product is too concentrated or when temperature control is needed for an exothermic process.

5. 05

   Purge bleeds off part of the recycle stream so inerts or impurities entering with the feed, or forming inside the reactor, don't build up.