(195d) Gas Entrainment By Liquid Jets and Cascades Can Unsettle Towers

Non-condensable gases are often present in distillation towers, expectedly or unexpectedly. When they do, or when the quantity of non-condensable gases varies from the design or expectation, they can adversely affect condensation, cause instability, or lead to excessive venting and product loss. Some of the causes of gas trapping issues are well-known, while others remain obscure. This article briefly describes the better-known issues, while focusing on those that are less well-known, presenting experiences in which non-condensable gas trapping led to major operating problems.

Gas trapping issues in distillation towers can be classified to three groups:

• Gas trapping by tower or auxiliary equipment hardware. These are well-known.
• Gas trapping by liquid head – usually unappreciated, and include gas trapping by liquid level at reflux accumulators, condensers, or bottom sump baffles, as well as trapping of gas generated by liquid jets inside the tower.
• Unexpected quantity of gas (high or low) entering the tower usually unappreciated, and includes gas absorption by feed or reflux drum liquid or deficient quantity of gas entering the tower.

Each of these will be addressed and discussed, with some guidelines for preventing and overcoming gas trapping issues.