Natural Gas Ethane Extraction with Flash Gas Recycle for Stable Reflux
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Solution Overview
Problem
Existing processes for extracting ethane and C3+ hydrocarbons from natural gas are susceptible to significant reductions in ethane extraction rates due to fluctuations in the quality of the separation column head, leading to a 'snowball' effect that worsens methane depletion and reduces ethane recovery efficiency.
Innovation Solution
Introducing a bottom stream rich in C2+ hydrocarbons into a fractionation column, separating the compressed flash gas into a fuel and recycle stream, cooling and partially expanding the recycle stream, and reintroducing it into the separation column overhead stage to maintain a high methane content reflux, thereby stabilizing ethane extraction rates.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a recirculation stream is taken from the recompressed gas exiting the top of the methane-ethane separation column and cooled counter-currently to form the main reflux, then high ethane extraction rates can be achieved, but the quality of the main reflux deteriorates in temperature and/or composition under certain operating conditions, leading to a snowball effect that significantly decreases the ethane extraction rate
Solution Approach 1:
The patent introduces an intermediary stream (the recycled compressed flash gas stream) that acts as a buffer to stabilize the main reflux composition. This intermediary stream, rich in methane and recovered from the flash gas compression process, is mixed with the recirculation stream to maintain consistent reflux quality and prevent the snowball effect of composition deterioration.
Solution Approach 2:
The patent implements a feedback mechanism by recycling the compressed flash gas stream back to the separation column inlet. This closed-loop feedback ensures that variations in head stream quality are compensated for, maintaining stable operating conditions and consistent ethane extraction rates despite fluctuations in the separation column performance.
2Quantity of substance
If the main reflux becomes depleted in methane to achieve high ethane extraction, then ethane recovery improves, but the quality of the overhead stream deteriorates further, exacerbating methane depletion and creating a snowball effect
Solution Approach 1:
The patent recovers and再利用s the methane that would otherwise be discarded in the flash gas stream. By compressing the flash gas and recycling it to the separation column, the system recovers valuable methane and uses it to replenish the main reflux, preventing methane depletion and maintaining stable composition without sacrificing ethane recovery.
3Manufacturing precision
If liquid is carried over to the upper trays of the separation column, then separation efficiency decreases, but the snowball effect leads to significant decrease in ethane extraction rate
Solution Approach 1:
The patent applies preliminary action by pre-cooling the recycled flash gas stream in the heat exchanger before reintroducing it to the separation column. This preliminary cooling prevents thermal disturbances that could cause liquid carryover to upper trays, maintaining separation efficiency and preventing the snowball effect before it can occur.
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 maintains a consistent ethane extraction rate and operational flexibility, reducing energy consumption and investment costs by optimizing the ethane extraction and liquefaction units.
Implementation Method 1
expansion of the turbine feed stream in a dynamic expansion turbine
Implementation Method 2
cooling of the starting natural gas stream in at least one first upstream heat exchanger
Implementation Method 3
separation of the cooled natural gas stream into a liquid stream and a gas stream
Implementation Method 4
introduction of a hydrocarbon-rich C2+ bottom stream recovered from the separation column into a fractionation column
Implementation Method 5
compression of the flash gas stream(s)
Implementation Method 6
Liquefaction of the purified compressed natural gas stream in a liquefaction unit to form a pressurized liquefied natural gas stream
Implementation Method 7
flash expansion of the pressurized liquefied natural gas stream and recovery of the expanded liquefied natural gas in a storage facility
Data Source
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AI summary
This method comprises the following steps: - recovering and compressing a top stream (98) from a separating column (34) in order to form a compressed purified natural gas stream (102); - liquefying the compressed purified natural gas stream (102) in a liquefaction unit (24) in order to form a stream (120) of pressurized liquefied natural gas; - flash expansion of the pressurized liquefied natural gas stream (120) and recovery in a storage tank (66); - recovery and compression of a flash gas stream (126) resulting from the expansion; - separating the compressed flash gas stream (132) into a fuel stream (20) and into a recycle stream (134); - cooling and expansion of the recycle stream (134), then introducing the cooled and expanded recycle stream at a top stage of the separating column (34).