Cracker Feedstock Flexibility from Naphtha to Ethane: Design Criteria and Concepts for Quench Water and Quench Oil System

Driven by the US shale gas boom and associated ethane supply and pricing dynamics, cracking ethane has become much more attractive than liquid feedstocks in the last years. Owners of liquid crackers have correspondingly minimized liquid feedstocks or even fully replaced with gaseous feedstocks. However, the recent decline of global crude oil prices has shown how unpredictable the market is. The key to success, therefore, is to maximize cracker feedstock flexibility.  

Nearly twenty years ago, in 1996, an expansion and ethane conversion project for the Qenos Naphtha/LPG Cracker was executed by Linde Engineering. The plant originally was designed for Naphtha Cracking in 1983. After the conversion project, the cracker was then able to crack the full range of feedstocks: from 100% Naphtha/LPG to 100% Ethane.

This paper briefly describes challenges in process engineering when converting an olefin cracker from liquid to gaseous feedstocks. Process unit concepts and associated modifications are explained in more detail, particularly for the hot section containing the quench oil column unit.

In order to stably operate the quench oil system given the lack of liquid cracking products when processing gaseous feedstocks, hydrocarbons must be added with similar boiling range and aromaticity. This concept has been optimized in the past two decades to a maximum extent possible. In the same way, prediction methods could be developed and validated thanks to long term operational experience, as this paper will elaborate.