Methane Liquefaction Recycle Expansion for Heavy Hydrocarbon Removal
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Solution Overview
Problem
Existing methods for liquefying methane-rich gas, such as LNG, are insufficient in reducing C5+ hydrocarbons and aromatic compounds to desired levels when the feed gas pressure exceeds 50 bar, necessitating additional steps like pressure reduction and recompression.
Innovation Solution
A process involving a dual methane expander system where a stream of feed methane-rich gas is mixed with recycle gas, expanded to reduce pressure, and the vapor stream is reheated and recompressed to form a recycle gas, while a second part of the recycle gas is cooled and passed through a liquefaction unit to produce liquefied methane, effectively separating and reducing higher hydrocarbons.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional cooling or scrubbing methods are used to remove higher hydrocarbons, then the process is simple, but the removal efficiency is insufficient when feed gas pressure exceeds 50 bar
Solution Approach 1:
The process segments the hydrocarbon removal into two stages: first, a work expander performs initial pressure reduction and partial condensation to remove bulk heavy hydrocarbons; second, a scrub column performs final polishing to achieve the required 0.1 mol% C5+ and 1 mol ppm aromatic specifications. This segmentation allows each unit to be optimized for its specific function.
Solution Approach 2:
The process changes the pressure parameter dynamically: feed gas enters the work expander at high pressure (50-120 bar), is expanded to intermediate pressure (5-30 bar) for condensation, then the vapor stream is recompressed back to high pressure (40-120 bar) before entering the scrub column. This parameter change enables efficient hydrocarbon removal while maintaining processability.
2Manufacturing precision
If pressure is reduced significantly in a work expander to remove heavy hydrocarbons, then hydrocarbon removal improves, but energy consumption increases due to recompression
Solution Approach 1:
The work expander, which normally consumes energy to reduce pressure, is operated in reverse as a compression device to re-compress the vapor stream back to high pressure. This converts the energy-consuming pressure reduction step into a beneficial compression step, significantly reducing the net energy requirement for the process.
Solution Approach 2:
The work expander serves dual functions: first as a pressure reduction device for hydrocarbon condensation and removal, then as a compression device to repressurize the vapor stream. This multi-functionality eliminates the need for separate compression equipment and reduces overall energy consumption.
3Use of energy by moving object
If feed gas pressure is maintained high to avoid additional compression steps, then energy consumption is reduced, but hydrocarbon removal efficiency deteriorates
Solution Approach 1:
The work expander performs preliminary pressure reduction and hydrocarbon condensation before the gas enters the scrub column. This preliminary action removes the bulk of heavy hydrocarbons at intermediate pressure, making the subsequent scrubbing operation more effective and energy-efficient while maintaining overall high removal efficiency.
Solution Approach 2:
The scrub column acts as an intermediary device that receives the vapor stream from the work expander at intermediate pressure, performs final hydrocarbon removal, and outputs gas at the required high pressure. This intermediary step enables efficient hydrocarbon removal without requiring the entire process to operate at low pressure.
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 efficiently reduces the concentration of C5+ hydrocarbons and aromatic compounds, achieving desired levels of purity in LNG production, particularly suitable for floating and small-scale land-based LNG production from higher pressure gases.
Implementation Method 1
passing the resulting mixture to a gas expander, the expander outlet having a pressure of between 3 bar and 50 bar, so as to form a mixture of vapor and a condensed liquid containing higher hydrocarbons
Implementation Method 2
reheating and compressing said vapor stream to a pressure of from 40 to 120 bar to form a first constituent of the above-said recycle gas
Implementation Method 3
cooling a second part of the said recycle gas to a temperature higher than the outlet temperature of the said gas expander
Implementation Method 4
passing said cooled second part of the recycle gas into a liquefaction unit to form liquefied methane and a second vapor stream
Data Source
AI summary
A process for liquefying methane-rich gases comprisingproviding a stream of feed methane-rich gas at a pressure of from 40 bar to 120 bar and containing higher hydrocarbons;providing a stream of methane-rich recycle gas at a pressure of from 40 bar to 120 bar;mixing the feed gas with a first part of the recycle gas;passing the resulting mixture to a gas expander, the expander outlet having a pressure of between 3 bar and 50 bar, so as to form a mixture of vapor and a condensed liquid containing higher hydrocarbons;separating the expander outlet stream into a liquid stream and a vapor stream;reheating and compressing said vapor stream to a pressure of from 40 bar to 120 bar to form a first constituent of the above-said recycle gas;cooling a second part of the said recycle gas to a temperature higher than the outlet temperature of the said expander;passing said cooled second part of the recycle gas into a liquefaction unit to form liquefied methane and a second vapor stream;reheating and compressing said second vapor stream to a pressure of from 40 bar to 120 bar to form a second constituent of the above-said recycle gas.