Method for extracting ethane from an initial natural gas stream and corresponding plant
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Solution Overview
Problem
Existing methods for extracting ethane from natural gas are prone to significant decreases in ethane extraction rates due to fluctuations in the quality of the main reflux, leading to a 'snowball' effect that deteriorates the separation process and results in operational issues such as liquid plugs and shut-downs during transport or liquefaction.
Innovation Solution
A method that involves separating the compressed flash gas into a fuel stream and a recycle stream, with the recycle stream being cooled and partially expanded to maintain a high methane concentration, introduced at the top of the separation column, and used as a reflux to stabilize the ethane extraction process, thereby preventing the deterioration of the reflux quality and maintaining a constant ethane extraction rate.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a recirculation stream is taken from the recompressed gas coming from the head of the separation column for methane and ethane, then the ethane extraction rate can be maintained high, but the quality of the main reflux deteriorates under certain operating conditions leading to a 'snowball' effect that significantly decreases the ethane extraction rate
Solution Approach 1:
The recirculation stream is segmented into two separate streams: a first recirculation stream introduced at an intermediate level of the separation column and a second recirculation stream introduced at the head level. This segmentation prevents the snowball effect by distributing the recirculation impact across different column zones, maintaining both ethane extraction efficiency and reflux quality stability.
Solution Approach 2:
An intermediate recirculation stream is introduced at an intermediate level of the separation column, acting as a mediator between the head recirculation and the bottom feed. This intermediate stream stabilizes the column operation by providing a buffer that prevents quality deterioration while maintaining high ethane extraction rates.
2Productivity
If the main reflux becomes depleted in methane, then the ethane separation rate in the column decreases, but this leads to further deterioration of the head stream quality, aggravating the methane depletion of the main reflux
Solution Approach 1:
The system implements feedback control by continuously monitoring the composition of recirculation streams and adjusting the recirculation rates and introduction levels accordingly. This feedback mechanism prevents the snowball effect by detecting early signs of composition deterioration and correcting them before they propagate through the system.
Solution Approach 2:
The recirculation streams are pre-cooled and pre-conditioned before being introduced into the separation column. This preliminary action ensures that the recirculation streams have the optimal temperature and composition characteristics needed to maintain stable reflux quality without causing downstream composition deterioration.
3Manufacturing precision
If heavy hydrocarbons such as ethane, propane and butane are extracted from natural gas, then they can be marketed separately with high purity, but they may condensate during transport or freeze in liquefaction exchangers
Solution Approach 1:
The system carefully controls temperature and pressure parameters throughout the extraction, separation, and liquefaction processes. By maintaining hydrocarbon concentrations below freezing points and controlling dew points, the process prevents condensation and freezing while achieving high purity extraction of ethane, propane, and butane for separate marketing.
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 ensures a stable ethane extraction rate and operational flexibility, reducing energy consumption and investment costs by maintaining a high methane concentration in the reflux, thus preventing the 'snowball' effect and optimizing energy usage between the ethane extraction and liquefaction units.
Implementation Method 1
expansion of the turbine feed stream in a dynamic expansion turbine
Implementation Method 2
cooling the stream of initial natural gas in at least one first upstream heat exchanger
Implementation Method 3
separation of the stream of cooled natural gas into a liquid flow and a gas flow
Implementation Method 4
introduction of a bottom stream rich in C2+ hydrocarbons recovered from the separation column into a fractionation column, and recovery from the fractionation column of a flow of ethane
Implementation Method 5
recovery and compression of at least a part of a head stream coming from the separation column, for forming a stream of compressed purified natural gas
Implementation Method 6
liquefaction of the compressed purified natural gas stream in a liquefaction unit, for forming a stream of pressurized liquefied natural gas
Implementation Method 7
flash expansion of the stream of pressurized liquefied natural gas and recovery, in a storage, of expanded liquefied natural gas
Data Source
AI summary
A method includes:recovering and compressing a head stream coming from a separation column, to form a stream of compressed purified natural gas;liquefying the stream of compressed purified natural gas in a liquefaction unit to form a stream of a pressurized liquefied natural gas;flash expanding of the stream of pressurized liquefied natural gas and recovering in a storage;recovering and compressing of a flow of flash gas coming from the expanding;separating the flow of compressed flash gas (132) into a fuel stream and a recycle stream;cooling and expanding the recycle stream, then introducing the cooled and expanded recycle stream at a head stage of the separation column.