Recycling and Bypass Reference Page

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Intro to Recycling and Bypass

Recycle and bypass streams are common features of real process flowsheets. They make processes more efficient, improve conversion, control composition, and reduce waste, but they also make material balances more complicated.

A recycle stream returns part of a process stream to an earlier point in the process. A bypass stream routes part of the feed around a unit operation and recombines it later. A purge stream is often added to a recycle loop to prevent the buildup of inert or unwanted components.

The videos on this page introduce recycle and bypass systems, show how to solve balances around them, and demonstrate both numerical and symbolic solution approaches.

Recycling Before Recycling Was Cool

This video introduces recycle streams and explains why recycle is used in process engineering. It shows how recycle loops affect material balances and how to set up the equations needed to solve them.

Visuals

Leaving Radiator Springs In the Dust

This video introduces bypass streams and works through an example involving both bypass and recycle. It emphasizes how to identify the correct balances and how to simplify the solution strategy.

Visuals

The Symbolic Meaning of Recycling

This video solves a recycle problem symbolically so that the algebraic structure of the balances is easier to see. The symbolic form helps make clear how recycle affects the overall system relationships.

Visuals

Examples and Definitions

Definitions

PFD (Process Flow Diagram)
A schematic drawing of the mixers, dividers, separators, reactors, and other unit operations used in a process.
Recycle
A portion of a process stream that is returned to an earlier point in the process.
Bypass
A portion of the feed routed around part of a process and mixed with the outlet stream later.
Purge
A stream withdrawn from a recycle loop to prevent buildup of trace, inert, or unwanted components.
Tie Component
A component whose amount is unchanged by the process. It is often useful for simplifying calculations, but is never strictly necessary.
Overall Balance
Treating an entire process flowsheet as a single black box with only overall inputs and outputs.
Mixer
A unit that combines multiple inlet streams into a single outlet stream.
Divider
A unit that splits one inlet stream into multiple outlet streams, each with the same composition as the inlet.
Separator
A unit that splits one inlet stream into multiple outlet streams with different compositions.
Split Fraction
The fraction of a given species in the inlet stream that exits in a particular outlet stream.

For two outlet streams, the split fractions for a given species sum to 1. For more than two outlet streams, the split fractions across all outlets sum to 1.

Composition
The composition of a stream is given by the mass fractions or mole fractions of its components. See What the Frac? for more details.