Feed Flexible Furnaces: What Are the Advantages, Constraints, and Considerations?

In the ethylene industry, cracking furnaces are typically designed for dedicated feedstocks. The designs of the radiant coil in the firebox, as well as the coils of the convection section, are catered to the feedstock to be cracked. Furnace designs have become flexible enough to fully crack a gaseous feedstock or a liquid feedstock.

Because of the constantly varying costs of feedstocks, ethylene plant owners may purchase an array of gaseous and liquid feedstocks. Sometimes this is a usage of products from the owner’s refineries or the import of liquefied petroleum gas (LPG).

Grassroot designs of today’s plants require furnace counts with large ethylene production capacities to meet their needs. Distribution among these large capacity furnaces becomes difficult, when typically the furnace loading is of one type of feedstock. In a revamping design, increased cracking results in increased recycle cracking. However, the recycle flow is not substantial enough to load one of the new higher capacity furnaces. Such was the case at YNCC plant #2 where two (2) new feed flexible 192U were designed and installed at site.

The YNCC#2 plant feedstocks ranged from Heavy Naphtha to light gas feedstock, including the recycle ethane. The furnace design allowed for simultaneous cracking of the liquid feedstock and the gaseous recycle ethane in the same furnace. The design encompasses the practice of independent cell operation (ICO) where different feedstocks can be cracked in the different cells, to which one of the cells can also be in decoking mode – independent cell decoking (ICD). Even further, when needed, the recycle ethane can be cracked in just a quarter of the furnace by using Quad-cracking, where only half of one cell is cracking a particular feed. With all the considerations to be feed-flexible, the furnace was also designed with the Gas Turbine Integration (GTI). The furnace convection section heat integration design allows the furnace burners to be capable of utilizing ambient air as well as gas turbine exhaust as the combustion air source.

This presentation will discuss the considerations that went into the design of the 192U feed-flexible furnace while incorporating the use of GT exhaust and address the furnace availability concerns when the other recycle gas furnaces are decoking.