Dual Riser FCC Unit for Bimodal LCO and Propylene Yield
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Solution Overview
Problem
Conventional fluid catalytic cracking (FCC) units face challenges in achieving a bimodal yield distribution of light cycle oil (LCO) and propylene, with difficulty in increasing diesel and light olefin production while reducing gasoline output, due to limitations in a single reaction zone.
Innovation Solution
A dual riser FCC unit is proposed, where the main riser operates at low severity for LCO yield maximization, and a second riser at high severity for LCO and light olefin production, using a low zeolite-to-matrix catalyst and high-propylene producing ZSM-5 catalyst, respectively, with separate reactor vessels and fractionators to compartmentalize catalysts and limit exposure.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a single FCC reactor is used with conventional catalyst, then the process is simple to operate, but it cannot achieve bimodal yield distribution of LCO and propylene simultaneously
Solution Approach 1:
The single FCC reactor is segmented into two separate riser reactors (first riser and second riser) that operate in series. The first riser uses a low-zeolite catalyst to maximize LCO yield, while the second riser uses a high-ZSM-5 catalyst to maximize propylene yield. This segmentation allows each reactor to be optimized for its specific function, achieving bimodal yield distribution that cannot be obtained in a single reactor.
2Productivity
If high zeolite content catalyst is used to increase conversion, then productivity improves, but selectivity to LCO decreases
Solution Approach 1:
Different catalyst compositions are used in different locations (risers) of the process. The first riser employs a catalyst with low zeolite content (0-50 wt% zeolite Y) to preserve LCO selectivity, while the second riser uses a catalyst with high ZSM-5 content (50-100 wt% ZSM-5) to achieve high propylene selectivity. This local differentiation of catalyst quality allows each stage to optimize for its specific product target.
3Adaptability or versatility
If single catalyst type is used throughout the process, then catalyst management is simplified, but the ability to optimize for multiple products is limited
Solution Approach 1:
The catalyst system is segmented into two distinct catalyst streams that circulate through separate risers. Catalyst A (low zeolite) is dedicated to the first riser for LCO production, while Catalyst B (high ZSM-5) is dedicated to the second riser for propylene production. Each catalyst type is optimized for its specific function, and the segmented circulation system maintains this specialization throughout the process.
Solution Approach 2:
Different catalyst compositions are deployed at different stages of the cracking process to match the specific requirements of each reaction zone. The first riser receives catalyst optimized for mild cracking and LCO preservation, while the second riser receives catalyst optimized for severe cracking and propylene formation. This local optimization of catalyst quality enables versatile product distribution control.
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 LCO and light olefin yields, achieving a bimodal yield distribution and improving diesel and propylene production, while maintaining catalyst activity by recycling spent catalyst and using MFI zeolite in the second riser for increased selectivity.
Implementation Method 1
Hydrocarbon feed contacts catalyst in the reactor to crack the hydrocarbons down to smaller molecular weight products
Implementation Method 2
During this process, coke tends to accumulate on the catalyst
Implementation Method 3
coke tends to accumulate on the catalyst which is burned off in the regenerator
Implementation Method 4
The first cracked stream is fractionated to provide a heavy cycle oil stream
Data Source
AI summary
A process and apparatus is for recycling HCO and/or naphtha from a first FCC unit to a second FCC unit to recover additional distillate and/or light olefins. A first catalyst stream for the first FCC unit may be isolated from a second catalyst stream for the second FCC unit. Fractionation of second cracked products from the second FCC unit may be separate from fractionation of first cracked products from the first FCC unit.