Sulfiding Agent Prevents Metal Catalyzed Coking in FCC Risers
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Solution Overview
Problem
Dual-riser Fluid Catalytic Cracking (FCC) processes face excessive coke formation in the secondary riser due to Metal Catalyzed Coking (MCC), which is exacerbated by insufficient sulfur species to form hydrogen sulfide, leading to inadequate passivation of active metals and increased coke deposition.
Innovation Solution
Incorporating a sulfiding agent, such as hydrogen sulfide or organic sulfur compounds, into the FCC riser to form a metal sulfide layer that isolates coke precursors from active metal sites, preventing MCC by maintaining a sufficient sulfur concentration, typically between 20 to 2000 wppm, to inhibit coking.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If dual-riser FCC process is used to maximize propylene selectivity, then productivity is improved, but excessive coke formation occurs in the secondary riser
Solution Approach 1:
Hydrogen sulfide acts as an intermediary substance that adsorbs onto active metal sites on reactor internals, forming a protective layer that prevents coke precursors from directly contacting and reacting with the metal surfaces. This mediator approach allows the dual-riser process to operate at high temperatures for propylene production while preventing metal-catalyzed coking in the secondary riser.
Solution Approach 2:
The invention changes the chemical environment in the secondary riser by introducing hydrogen sulfide, which alters the surface properties of metal internals through sulfidation. This parameter change transforms the metal surface from a coking-promoting state to a coking-resistant state, enabling sustained operation at the high temperatures (538-593°C) required for propylene maximization.
2Productivity
If high temperature operation is used in secondary riser to promote light olefin formation, then productivity is improved, but metal catalyzed coking is exacerbated
Solution Approach 1:
Hydrogen sulfide serves as a protective intermediary that forms a sulfide layer on metal surfaces, preventing direct contact between coke precursors and active metal sites. This allows the secondary riser to operate at high temperatures (538-593°C) necessary for light olefin production without suffering from accelerated metal-catalyzed coking that would otherwise occur at these temperatures.
Solution Approach 2:
The invention applies preliminary anti-action by pre-treating metal surfaces with hydrogen sulfide to form protective sulfide layers before coke formation can occur. This preventive measure counteracts the coking tendency inherent in high-temperature operation, allowing sustained productivity without the harmful effects of metal-catalyzed coking.
3Object-generated harmful factors
If sulfur species are increased to passivate metals, then coke formation is reduced, but hydrogen sulfide concentration becomes insufficient
Solution Approach 1:
The invention employs self-service by using hydrocarbons already present in the feedstock as the source of hydrogen sulfide through controlled decomposition. Rather than requiring external sulfur addition, the system utilizes the feedstock's own hydrocarbon content to generate the necessary hydrogen sulfide in situ, which then automatically passivates metal surfaces and prevents coking.
Solution Approach 2:
The invention converts the harmful effect of hydrocarbon decomposition (which can lead to coking) into a beneficial source of hydrogen sulfide. By controlling the decomposition of hydrocarbons to release hydrogen sulfide, the system transforms a potential coking precursor into a protective agent that passivates metal surfaces and prevents metal-catalyzed coking.
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 sulfiding agent effectively reduces coke formation in the secondary riser, extending operational capability beyond weeks to months by passivating active metals and preventing filamentous carbon deposition, thereby enhancing the stability and productivity of the FCC process.
Implementation Method 1
hydrogen sulfide or provides a source of hydrogen sulfide, either by decomposition, liberation, or other chemical reaction, that subsequently forms a metal sulfide layer on the interior metal surface of the reactor internals
Implementation Method 2
Hydrogen sulfide subsequently passivates the active metals in the FCC unit
Implementation Method 3
sulfur species that decompose to form hydrogen sulfide in an FCC riser
Data Source
AI summary
A process and apparatus is described in which a sulfiding agent is added to a catalytic conversion reactor to prevent metal catalyzed coking. The catalytic reactor may be downstream from a first fluid catalytic cracking reactor that provides C10— hydrocarbons as feed to the downstream catalytic reactor.