Two-Stage Vapor-Liquid Separation for Heavy Hydrocarbon Pyrolysis
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Solution Overview
Problem
Current pyrolytic cracking processes for producing lower olefins face challenges in achieving higher yields due to coke formation and vaporization issues with heavy hydrocarbon feedstocks containing high boiling point or non-vaporizable coke precursors, such as crude oil and its fractions, which lead to fouling and coking in the convection section and vapor-liquid separation equipment.
Innovation Solution
A process involving the separation of unvaporized portions of the feedstock containing coke precursors and high boiling pitch fractions, followed by heating and vaporizing a portion of the hydrocarbon feedstock in a pyrolysis furnace using a two-cyclone system, where the heated gas is superheated and routed to a second vapor-liquid separator, and the remaining liquid is heated with superheated steam before being pyrolyzed in the radiant zone, optimizing conditions to minimize coking and maximize olefin production.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If heavy hydrocarbon feedstock containing coke precursors is fed directly to a pyrolysis furnace, then lower olefin production is achieved, but coke formation occurs in the convection section and vapor-liquid separator causing fouling and reduced on-stream time
Solution Approach 1:
The vapor-liquid separation process is divided into two sequential stages: a first vapor-liquid separator operating at lower temperature to remove bulk liquid, followed by a second vapor-liquid separator operating at higher temperature to remove remaining liquid. This segmentation allows each separator to operate under optimized conditions, preventing coke formation while maximizing hydrocarbon gas recovery for pyrolysis.
Solution Approach 2:
The first vapor-liquid separator performs preliminary separation at a temperature below the dew point of heavy hydrocarbons but above the dew point of water. This preliminary action removes the bulk of liquid condensate before the gas stream enters the second separator, reducing the liquid load and temperature requirements for the second separator and minimizing coke formation risk in both units.
2Productivity
If separation temperature is increased to increase hydrocarbon gas feed rates to the radiant section, then more olefins are produced, but coke formation increases in the convection section and vapor-liquid separator
Solution Approach 1:
The two-stage vapor-liquid separation system segments the temperature increase required for complete vaporization into two progressive stages. The first separator operates at moderate temperature to remove bulk liquid, and the second separator operates at higher temperature to remove remaining liquid. This segmentation achieves complete vaporization and maximum hydrocarbon gas feed rate to the radiant section while distributing the thermal stress and preventing excessive coke formation that would occur with a single high-temperature separator.
3Device complexity
If a single vapor-liquid separator is used, then the process is simpler, but complete vaporization of heavy hydrocarbon feedstock cannot be achieved without excessive coking
Solution Approach 1:
The use of two vapor-liquid separators in series segments the vaporization process into two controlled stages. The first separator handles the bulk separation at lower temperature, and the second separator completes the vaporization at higher temperature. This segmentation achieves complete vaporization of heavy hydrocarbon feedstock with minimal coking, overcoming the limitations of a single separator while maintaining reasonable process complexity.
Solution Approach 2:
The first vapor-liquid separator performs preliminary separation at optimized temperature conditions, removing the bulk of liquid condensate. This preliminary action reduces the liquid load and simplifies the requirements for the second separator, enabling complete vaporization without excessive temperature increase that would cause coking. The staged approach achieves superior vaporization efficiency compared to a single separator.
4Ease of manufacture
If heavy hydrocarbon feedstock is used instead of traditional feedstocks, then production costs are reduced, but the feedstock cannot be completely vaporized in the convection section
Solution Approach 1:
The two-stage vapor-liquid separation system enables complete vaporization of low-cost heavy hydrocarbon feedstocks containing high boiling point components. The first separator operates at moderate temperature to remove bulk liquid, and the second separator operates at higher temperature to ensure complete vaporization. This segmentation makes it economically viable to use heavy hydrocarbon feedstocks by achieving the vaporization completeness required for efficient pyrolysis.
Solution Approach 2:
The first vapor-liquid separator performs preliminary separation at optimized temperature, removing the bulk of heavy hydrocarbon condensate. This preliminary action reduces the temperature requirement for complete vaporization in the second separator, enabling complete vaporization of expensive heavy feedstocks without excessive energy input or coke formation, thereby making low-cost feedstock utilization economically viable.
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 process allows for the economical processing of heavy hydrocarbon feedstocks to produce lower olefins in higher yields while reducing fouling and coking, enhancing energy efficiency and extending the on-stream time of pyrolysis furnaces by effectively vaporizing and pyrolyzing the feedstock without the need for additional distillation units.
Implementation Method 1
heating said feedstock within the first stage preheater to produce a heated gas-liquid mixture
Implementation Method 2
heating the gas in a vapor phase superheater provided in said convection zone to a temperature of about 450 to 700°C
Implementation Method 3
pyrolyzing the gas to produce olefins and other pyrolysis products
Implementation Method 4
separating and removing the gas from the liquid in the vapor-liquid separator
Data Source
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AI summary
A process for making lower olefins from a heavy hydrocarbon feed by use of a combination of two vapor-liquid separation devices, and, then, pyrolytically cracking the light fraction of the heavy hydrocarbon feed to thereby produce a lower olefin product.