(81f) Decoking and Cracking in a Twin CELL Furnace – Design and Operational Considerations

Senior Process Supervisor

Technip Stone & Webster Process Technology, Claremont, CA

Joel Guillaume

Ethylene Technology Manager

Technip Stone & Webster Process Technology, Houston, TX

Bruce Evans

Director of Pyrolysis Technology

Technip Stone & Webster Process Technology, Claremont, CA

Abstract: Cracking furnaces are the heart of the ethylene plant. As crackers has increased in size, so has the cracking furnace capacity. Ethylene producers purchase various feed stocks ranging from “Gas” feeds to “Liquid” feeds and up to “Heavy Liquid” feeds. Economically, it is important to minimize the number of furnaces in the design of a modern ethylene plant, to have multiple feedstock flexibility and also to have partial decoking and partial cracking features in a twin cell furnace design.

Co-cracking or blended cracking is often used to handle two or more feed stocks in a single furnace. Segregated or “hybrid” cracking (two or more feeds cracked separately in a single cell or in a twin cell furnace) becomes important in modern furnace design for achieving feedstock flexibility while maintaining furnace availability, when it is desired to crack each feed at a specific conversion or severity. In addition, due to the large capacity of modern twin cell furnaces, it is important to consider decoking in one cell and normal cracking in another cell in the same twin cell furnace to increase furnace availability.

In 2011, Technip presented a paper in AIChE Spring Meeting (EPC Conference, Paper Number 96E) titled “Co-cracking vs Hybrid (Segregated) Cracking in Individual Furnace - Design and Operational Considerations”. This abstract is a continuation (Part 2) of the paper presented in 2011.

Simulation of decoking and cracking in a twin cell furnace is an important advancement to the simulation of hybrid cracking or co-cracking in an individual furnace. A significant challenge in the design of decoking and cracking in a twin cell cracking furnace is how to properly simulate the complete furnace performance due to a large difference in the furnace fired duty and flue gas flow from each cell, as well as a large difference in bridge wall temperature from each cell. In addition to the radiant coils, simulation of furnace firing, flue gas mixing and performance of convection banks present a significant challenge. Convergence of the simulation of the complete system presents a challenge due to the complexity of the system. How to control temperatures in the decoking cell during this operation is also an important consideration. Material selection of process convection banks which supply the decoking cell is to be carefully evaluated. For decoking and cracking in the same twin cell furnace, a cracking furnace model has been developed by Technip called SPLIT-MIXING to simulate decoking and cracking using an integrated EFPS (firebox and radiant section) model and PROVISION (heat exchanger) software. An improved methodology is to utilize Technip’s new generation of furnace simulation software: SPYRO^® Suite 7, which is capable to rigorously simulate decoking and cracking in the same twin cell furnace in a single converged file. Additionally, CFD is utilized to verify flue gas flow patterns.

This paper will also discuss key operational aspects of individual cell decoking.