LNG Nitrogen Removal Using Integrated Recycle Heat Exchanger Circuit
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Current methods for removing nitrogen from natural gas feeds with low nitrogen concentrations are inefficient, complex, and costly, especially in small and mid-scale LNG facilities, due to the need for extensive equipment and high capital costs, and struggle with maintaining purity requirements for vented nitrogen products.
Innovation Solution
A method involving a main heat exchanger that cools and liquefies a natural gas feed stream, with a recycle stream of nitrogen-enriched vapor being compressed and reintroduced to enhance liquefaction, allowing for efficient separation of nitrogen-rich vapor products while minimizing methane loss and meeting purity standards.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a dedicated nitrogen rejection unit (NRU) is used to remove nitrogen efficiently and produce high-purity nitrogen product, then nitrogen removal efficiency and product purity are improved, but capital cost and device complexity increase due to additional equipment
Solution Approach 1:
The main heat exchanger in the LNG liquefaction process is made to serve dual functions: cooling the natural gas feed stream and cooling the compressed nitrogen-rich recycle stream. This eliminates the need for separate cooling equipment for the nitrogen rejection function, reducing capital cost and device complexity while maintaining high nitrogen removal efficiency
Solution Approach 2:
The nitrogen rejection function is merged with the existing LNG liquefaction process by integrating the cooling of the nitrogen-rich recycle stream into the main heat exchanger. The recycle stream is introduced separately into the heat exchanger and cooled alongside the feed stream, combining two functions into one unified system
2Manufacturing precision
If nitrogen product purity is increased to meet environmental specifications for venting, then environmental compliance is improved, but methane loss increases due to stricter separation requirements
Solution Approach 1:
A nitrogen-rich stream is withdrawn from the liquefaction process, compressed, and recycled back to the main heat exchanger for re-liquefaction. This feedback loop allows nitrogen to be concentrated progressively with each cycle, achieving high purity (>95 mol%) in the final vented product while recovering methane that would otherwise be lost
Solution Approach 2:
Instead of discarding nitrogen-rich streams that contain valuable methane, the process recycles these streams back through the heat exchanger and separation system. The methane is recovered and returned to the LNG product, while only the highly purified nitrogen is vented, minimizing methane loss while meeting purity specifications
3Productivity
If complex separation processes are used to achieve high nitrogen removal efficiency, then nitrogen removal efficiency is improved, but power consumption and operational complexity increase
Solution Approach 1:
The nitrogen-rich stream, when compressed and reintroduced to the heat exchanger, serves as its own cooling medium for the feed stream while being re-liquefied. The system uses the thermal energy already present in the process streams to achieve further separation, minimizing the need for additional external power input
Solution Approach 2:
The process exploits changes in temperature, pressure, and phase state of the nitrogen-rich recycle stream as it passes through the heat exchanger and expansion valve. By carefully controlling these parameters, the system achieves enhanced nitrogen separation efficiency using the natural thermodynamic behavior of the gases rather than requiring additional energy-intensive equipment
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 simplifies the nitrogen removal process, reduces capital costs, and effectively produces high-purity nitrogen-depleted LNG and nitrogen-rich vapor products, even at low nitrogen concentrations, while maintaining environmental purity requirements.
Implementation Method 1
passing a natural gas feed stream through a main heat exchanger to cool the natural gas feed stream and liquefy all or a portion of said stream
Implementation Method 2
cool the natural gas feed stream and liquefy all or a portion of said stream, thereby producing a first LNG stream
Implementation Method 3
compressing the recycle stream to form a compressed recycle stream
Implementation Method 4
passing the compressed recycle stream through the main heat exchanger, separately from and in parallel with the natural gas feed stream, to cool the compressed recycle stream and at least partially liquefy all or a portion thereof
Implementation Method 5
expanding, partially vaporizing and separating the first LNG stream, or an LNG stream formed from part of the first LNG stream, to form a nitrogen-depleted LNG product and a recycle stream composed of nitrogen-enriched natural gas vapor
Implementation Method 6
expanding, partially vaporizing and separating
Data Source
AI summary
A method and apparatus for liquefying a natural gas feed stream and removing nitrogen therefrom to produce a nitrogen-depleted LNG product, in which a natural gas feed stream is passed through main heat exchanger to produce a first LNG stream, which is separated to form a nitrogen-depleted LNG product and a recycle stream composed of nitrogen-enriched natural gas vapor, and in which the recycle stream is passed through main heat exchanger to produce a first LNG stream, separately from and in parallel with the natural gas feed stream, to produce a first at least partially liquefied nitrogen-enriched natural gas stream that is separated to provide a nitrogen-rich vapor product.