Dual Riser FCC System Olefin Yield Optimization
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Solution Overview
Problem
Dual-riser fluidized catalytic cracking (FCC) units face challenges in maximizing propylene and ethylene yields while maintaining heat balance, particularly when processing light feeds, which do not produce sufficient coke to sustain the reaction temperature, requiring external heat sources and leading to inefficiencies.
Innovation Solution
A dual-riser FCC system is employed, where light hydrocarbon feeds are processed in each riser under independently optimized conditions, with a coke precursor added to one riser to enhance coke production and reduce the need for supplemental fuel, allowing for improved heat balance and increased olefin production.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If light feeds are processed in a conventional single-riser FCC unit, then olefin production is improved, but heat balance is worsened due to insufficient coke production
Solution Approach 1:
The single riser is divided into two separate risers, each optimized for different feed types and product objectives. The first riser processes light feeds for maximum olefin production, while the second riser processes heavy feeds for coke generation and heat supply, resolving the heat balance problem while maintaining high olefin yields
Solution Approach 2:
Different operating conditions and catalyst formulations are applied to different risers based on their specific functions. The first riser uses conditions optimized for light feed cracking and olefin production, while the second riser uses conditions optimized for heavy feed conversion and coke generation, allowing each zone to perform its specialized function efficiently
2Productivity
If heavy feeds are cracked at high temperatures to maximize propylene yield, then propylene production is improved, but coke production increases leading to heat imbalance
Solution Approach 1:
The heavy feed stream is extracted and directed to a dedicated second riser where it is processed separately from light feeds. This allows the heavy feed to be cracked at high temperatures for maximum propylene yield while the excess coke produced is utilized as a beneficial heat source in the regenerator, eliminating the heat imbalance problem
Solution Approach 2:
The excess coke that would normally be considered a harmful byproduct causing heat imbalance is converted into a beneficial heat source. The coke is combusted in the regenerator to provide the necessary heat for the endothermic cracking reactions, transforming a problem into a solution for the overall process energy balance
3Use of energy by moving object
If external fuel is added to heat balance the FCC unit processing light feeds, then heat balance is improved, but process complexity increases
Solution Approach 1:
The system achieves self-service for heat balance by using the coke naturally produced from heavy feed cracking in the second riser to supply heat to the regenerator. This eliminates the need for external fuel addition and complex heat balance control systems, simplifying the overall process operation
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enhances selectivity and conversion rates, achieving a 15% relative increase in total ethylene and propylene yields by optimizing conditions for each feed type and reducing the reliance on external heat sources, thereby simplifying the regenerator operation and reducing costs.
Implementation Method 1
The coke is deposited on the catalyst and ultimately burned with an oxygen source such as air in a regenerator. Burning of the coke is an exothermic process that can supply the heat needed for the cracking reaction.
Implementation Method 2
The cracking reaction is endothermic, meaning that heat must be supplied to the reactor process to heat the feedstock and maintain reaction temperature.
Implementation Method 3
The FCC process uses a reactor called a riser, essentially a pipe, in which a hydrocarbon feed gas is intimately contacted with small catalyst particles to effect the conversion of the feed to more valuable products. The resulting hydrocarbon gas and catalyst mixture both flow in the riser, hence the term fluid catalytic cracking.
Data Source
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AI summary
A dual riser FCC process is disclosed wherein first and second hydrocarbon feeds (5, 6) are supplied to the respective first and second risers (2, 4) to make an effluent rich in ethylene, propylene and/or aromatics. Where the hydrocarbon feeds are different, the respective risers can have different conditions to favor conversion to ethylene and/or propylene. A minor amount of a coke precursor (80, 82) can be added to one or both of the hydrocarbon feeds (5, 6) to reduce or eliminate the amount of supplemental fuel needed to heat balance the system. The different feeds, including the coke precursor and any recycle streams (36, 44) can be segregated by type to improve olefin yields, including an embodiment where the paraffinic feeds are supplied to one riser and the olefinic feeds to the other.