Distillation Columns Risk Assessment "When the Regular HAZOP Evaluation Is Not Enough"

The HAZOP method (Hazard operability study) was developed by ICI in the early 70s. In the 1980s risk studies gradually came into use in petrochemicals, oil, chemicals, rail transport, automobiles and other industries. This methodology has been used to assess the safety of new projects or existing units and their modifications. The purpose of this risk analysis is to identify potential accident scenarios that can occur at a facility and to lower the corresponding risks to an acceptable level. Risk analysis is performed by a multi-disciplinary group.

At Rhodia the HAZOP methodology principles were implemented in the beginning of 80's. The risk analysis is mandatory and performed at each 5 years for some facilities and at each 3 years for some others.

Some of the common initiating causes analyzed by a HAZOP group are the faults affecting condenser capacity of distillation systems. Usually, in the HAZOP analysis, the risk of this scenario is reduced at an acceptable level by introducing active safeguards (such as steam valve closure). Nevertheless, in some cases, the regular safety system (instrumental safety chain) is not enough to guarantee a safe unit shut down.

After further analysis, one possible scenario has been identified for distillation systems where the difference of volatility of the compounds is high. In this case the safety barriers must to take into account the intrinsic dynamic effect of the heat accumulation that occurs after the steam feed valve closure. By not considering this situation, an important amount of organics can be released to the atmosphere through the column vent system.

Based on the process operation information and column design, a simplified method is proposed to quantify the amount of volatiles that can be released after a condenser failure, considering the existent safety barriers.