Two-Stage Hydrocarbon Cracking for Propylene Yield and Cycle Life
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Solution Overview
Problem
Existing propylene production processes suffer from low yield and high coke formation on the catalyst, leading to short cycle lengths in cracking reactors.
Innovation Solution
A two-step process involving a low acidic density cracking catalyst followed by a high acidic density cracking catalyst, with the addition of aromatics to stabilize activity and minimize coke formation, is employed to convert hydrocarbon mixtures into propylene.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a single high acidic density cracking catalyst is used to maximize propylene yield, then propylene production increases, but coke formation on the catalyst increases leading to short cycle lengths
Solution Approach 1:
The cracking process is divided into two sequential steps using different catalysts: first a low acidic density catalyst to convert olefins to propylene with minimal coke, then a high acidic density catalyst to crack paraffins and naphthenes to propylene. This segmentation allows each catalyst to operate under optimized conditions for its specific function, achieving high overall propylene yield while maintaining acceptable cycle lengths.
Solution Approach 2:
The low acidic density catalyst performs preliminary conversion of olefinic compounds to propylene before the high acidic density catalyst processes the remaining paraffinic and naphthenic compounds. This preliminary action reduces the coke-forming burden on the second catalyst, allowing it to operate longer before requiring decoking.
2Productivity
If a medium pore zeolite catalyst is added to FCC catalyst to increase propylene yield, then propylene production increases, but the medium pore zeolite catalyst degenerates due to regeneration steps
Solution Approach 1:
The function of the medium pore zeolite catalyst is extracted and separated into a dedicated second reactor unit that processes only the high boiling fraction. This extraction allows the zeolite catalyst to operate without the harsh regeneration conditions of the FCC unit, improving its stability and longevity while maintaining high propylene yield.
Solution Approach 2:
The catalytic cracking function is segmented between two separate reactor units with different catalyst types: the first unit handles olefin conversion with a low acidic density catalyst, while the second unit handles paraffin and naphthene cracking with a high acidic density catalyst. This segmentation protects the zeolite catalyst from degenerating regeneration cycles.
3Productivity
If cracking severity is increased to achieve higher propylene yield, then propylene production increases, but coke formation on the catalyst increases
Solution Approach 1:
The cracking process is segmented into two stages with different severity levels: the first stage uses mild cracking conditions with a low acidic density catalyst to convert olefins to propylene with minimal coke formation. The second stage uses more severe cracking conditions with a high acidic density catalyst to crack the remaining heavy fractions. This segmentation allows high overall propylene yield while limiting coke formation to acceptable levels.
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
The process achieves a high yield of propylene while maintaining acceptable cycle lengths by minimizing coke formation, using a two-step cracking process with controlled reaction conditions and aromatic addition.
Implementation Method 1
by contacting the feed with a cracking catalyst in a reactor
Implementation Method 2
cracking catalyst at a hydrocarbon partial pressure of below 3 bar and at a weight hourly space velocity of between 0.5 and 100 h−1
Data Source
AI summary
The invention is directed to a process to prepare propylene from a mixture of hydrocarbons having an olefin content of between 5 and 50 wt. % and boiling for more than 90 vol. % between 35 and 280° C. or from a hydrocarbon feed comprising paraffins, naphthenics and/or aromatics and optionally up to 10 wt. % of olefins, by first contacting the feed with a low acidic density cracking catalyst in a reactor, separating propylene and subsequently contacting the residue with a high acidic density cracking catalyst in a reactor at a more elevated temperature, separating propylene and recycling the residue to first and second cracking reactors. Aromatics may be added to first and second cracking step to improve cycle length.