Cracking furnaces are the heart of the ethylene plant. Ethylene producers purchase various feedstocks ranging from “Gas” feeds: ethane, propane, butane and ethane propane mix, “Liquid” feeds: NGL’s, naphtha and up to “Heavy liquid” feeds: condensates, atmospheric gas oil (AGO), and vacuum gas oil (VGO). However, in a modern feed flexible ethylene plant, the quantity of each feedstock received varies. As plants have become larger, furnaces have also become significantly larger. Therefore, economically, it is important to minimize the number of furnaces in the design of an ethylene plant, and also to have multiple feedstock flexibility in the furnace design. In addition, modern plants may contain identical furnaces.
Co-cracking is often used to handle two or more feedstocks in a single furnace. Typical applications include naphtha with C4 and C5 mixtures, ethane and propane, and even naphtha and ethane. However, in co-cracking or blended cracking, each feedstock often cannot be cracked at its optimal conversion for gas cracking or optimal severity for liquid cracking. In this paper, governing feeds, feedstock interactions and impact on yields will be discussed.
Segregated or “hybrid” cracking (two or more feeds cracked separately in a single cell or in a twin cell furnace) becomes very 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.
Simulation of co-cracking of different feeds requires free radical based yield prediction and rigorous radiant coil and TLE coking predictions. Accurate simulation of the firebox, convection section and TLE’s is also essential.
A significant challenge in the design of hybrid cracking furnaces is how to properly simulate the complete furnace performance due to the following reasons:
- Process convection banks and radiant coil banks must process different feed and feed/dilution steam streams, whereas utility banks (economizers, HP steam superheat) are common. Accurate prediction of heat transfer, process side and flue gas side temperature profiles in the convection and radiant sections is a challenge.
- Fired duty in each cell of a twin cell furnace or in each section of a single cell furnace can be different. Challenges in the design are the prediction of flue gas mixing in the transition zone from the radiant section of each cell or section to the common convection section and associated impact on the firebox. Prediction of the performance of the coils and passes of the convection section including tube wall and fin temperatures is essential.
For segregated cracking, a new cracking furnace model has been developed by Technip called SPLIT-MIXING to simulate hybrid cracking using an integrated EFPS model and PROVISION software. By integration of PROVISION-EFPS, radiant section kinetic model and firebox model within the EFPS program were successfully linked with PROVISION heat exchange models to perform hybrid cracking furnace calculations.
Technip has also developed a new generation of furnace simulation software: SPYRO SUITE 7, and this tool is also able to rigorously simulate hybrid cracking furnaces.
The salient differences between SPYRO 7 simulations and the PROVISION-EFPS simulations for hybrid cracking are summarized as follows:
- Rigorous convection section and TLE heat transfer calculations for hybrid cracking with different feeds can be done automatically.
- Simulation of cracking in the shock bank, cross-over volume and TLE section for each feed pass is accomplished with SPYRO 7.
- SPYRO 7 can perform radiant coil run length predictions for different feeds cracking in the same firebox.
In actual operation, co-cracking can be rather straightforward, assuming the cracking severity is properly determined. In general, relatively small amounts of gas feed are cracked with liquids. Co-cracking must be established to maximize furnace capability and achieve the target total olefin production.
Hybrid cracking can mean two gas feedstocks, one gas and one liquid feedstock, or two different liquid feedstocks. In segregated or hybrid cracking, firing and dilution steam ratios are considerations. The furnace must be configured to allow introduction of different feeds through the convection sections to zones in the radiant and TLE section.
For both mixed feed cracking types, introduction and control of the feeds and effective liquid feed vaporization are critical.
A case study for a large modern furnace is included to compare co-cracking and segregated cracking.
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