Objective
This article explains the bottom tray and below-tray feed stage naming convention in OLI Flowsheet: ESP and clarifies how to correctly connect vapor feeds introduced below the bottom tray to avoid convergence issues.
Overview
In OLI Flowsheet: ESP, column stages are numbered using a consistent internal convention. However, the labeling of the bottom tray and below-bottom tray feed stages can be unintuitive for users modeling vapor feeds that enter the column below the lowest tray.
This article describes the distinction between Feed, Stage 1 (Bottom) and Feed, Stage −1, and explains when each should be used.
Bottom Tray Naming Convention
In a column object, the lowest tray is labeled:
Stage 1 (Bottom)
This label refers to the physical bottom tray itself, not the space below the tray.
Connecting a feed stream to Stage 1 (Bottom) means the stream is introduced onto the bottom tray.
Modeling Vapor Feeds Below the Bottom Tray
In many tower configurations, a vapor feed enters the column below the bottom tray, rather than directly onto the tray.
In this case, the feed should be connected to:
Feed Stage −1
This stage represents the region below the bottom tray, allowing the vapor to rise through the tray as expected in the physical system.
Recommended Feed Connection
To correctly model a vapor feed introduced below the bottom tray:
Open the column feed configuration.
Select Feed, Stage −1 for the incoming vapor stream.
Verify that the vapor flows upward into Stage 1 (Bottom) during the simulation.
Introducing a vapor feed at Stage 1 (Bottom) when the physical feed is below the tray may lead to convergence difficulties in some cases.
Conclusion
When modeling vapor feeds that enter a column below the bottom tray in OLI Flowsheet: ESP, the feed should be connected to Feed, Stage −1, not Feed, Stage 1 (Bottom). Understanding this distinction helps ensure the column behaves as intended and can improve simulation convergence.