LNG Nitrogen Removal Using Intermediate Feed Gas Separation
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Solution Overview
Problem
Existing methods for removing nitrogen from natural gas feeds with low nitrogen concentrations are inefficient, complex, and costly, particularly in small and mid-scale LNG facilities, where the nitrogen concentration varies over time, posing challenges in achieving high purity nitrogen vent streams while minimizing methane loss.
Innovation Solution
A method involving a main heat exchanger with multiple cooling passages and separation systems that cool, liquefy, and separate natural gas streams to produce a nitrogen-depleted LNG product, utilizing a closed-loop refrigeration system and recycle streams to enhance efficiency and purity, while minimizing equipment and power consumption.
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 patent combines the nitrogen rejection function with the existing LNG liquefaction process by integrating a nitrogen rejection column into the main distillation system. The nitrogen rejection column is integrated with the LNG distillation column, allowing nitrogen removal to occur during the normal liquefaction process without requiring completely separate dedicated equipment. This merging approach achieves high purity nitrogen product while avoiding the capital cost and complexity of a fully separate NRU system.
Solution Approach 2:
The distillation column system performs multiple functions simultaneously: it conducts the primary LNG liquefaction and separation while also functioning as a nitrogen rejection unit. By configuring the column to produce both LNG product and high purity nitrogen stream from the same separation system, the equipment achieves multi-functionality, eliminating the need for separate dedicated nitrogen removal equipment and reducing overall system complexity.
2Manufacturing precision
If prior art methods such as nitrogen recycle stream or dedicated rectifier column are used, then nitrogen removal capability is improved, but device complexity and capital cost increase due to large amount of equipment
Solution Approach 1:
The patent merges the nitrogen rejection function with the existing LNG distillation column by integrating a nitrogen rejection column into the same system. Instead of adding a completely separate rectifier column or recycle system, the invention configures the distillation column to perform both LNG production and nitrogen rejection functions, thereby achieving effective nitrogen removal without proportionally increasing equipment quantity or complexity.
3Adaptability or versatility
If nitrogen product is vented to atmosphere to meet environmental requirements, then nitrogen product purity must be >95 mol% which poses separation challenges, but if used as fuel gas with >30 mol% methane then separation is easier but heating value requirements must be met
Solution Approach 1:
The patent implements a dynamic system where the nitrogen product specification can adapt to different operational requirements. The distillation column configuration allows the system to produce nitrogen streams of varying purity levels depending on market conditions and environmental requirements. When high purity is needed for venting, the column operates to achieve >95 mol% purity; when fuel gas is acceptable, the system can produce nitrogen with 30-50 mol% methane content, providing flexibility in meeting different specifications.
Solution Approach 2:
The invention changes the operational parameters of the distillation column to produce nitrogen products with different compositions based on requirements. By adjusting the draw-off points, reflux ratios, and column operating conditions, the system can dynamically produce nitrogen streams ranging from high purity (>95 mol%) suitable for atmospheric venting to lower purity streams (30-50 mol% nitrogen) suitable for fuel gas applications, thereby adapting to different market and environmental conditions.
4Ease of manufacture
If nitrogen concentration in feed is 1-10 mol% (typical range), then NRU applicability is hindered by high capital cost, but alternative solutions proposed in prior art are very complicated and inefficient
Solution Approach 1:
The patent merges the nitrogen rejection function with the existing LNG distillation column, allowing the system to handle typical natural gas nitrogen concentrations (1-10 mol%) effectively. By integrating the nitrogen rejection capability into the main separation system rather than requiring a fully dedicated NRU, the invention achieves efficient nitrogen removal at lower capital cost, making the solution economically viable for typical feed compositions.
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
The method effectively removes nitrogen from natural gas feeds with low concentrations, achieving high purity nitrogen products with reduced capital and operational costs, and is adaptable to varying nitrogen levels in the feed stream.
Implementation Method 1
introducing a natural gas feed stream into a warm end of a main heat exchanger, cooling and at least partially liquefying the natural gas feed stream
Implementation Method 2
expanding, partially vaporizing and separating the cooled and at least partially liquefied stream
Implementation Method 3
a refrigeration system for supplying refrigerant to the main heat exchanger for cooling the cooling passages
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 fed into the warm end of a main heat exchanger, cooled and at least partially liquefied, withdrawn from an intermediate location of the main heat exchanger and separated to form a nitrogen-enriched natural gas vapor stream and a nitrogen-depleted natural gas liquid stream, the liquid and vapor streams being reintroduced into an intermediate location of the main heat exchanger and further cooled in parallel to form a first LNG stream and a first at least partially liquefied nitrogen-enriched natural gas stream, respectively.