LNG Nitrogen Rejection with Integrated Fuel Stream Control

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Solution Overview

Problem

Existing systems for nitrogen removal in LNG liquefaction processes lack control over nitrogen concentration in both the LNG product and the fuel stream, often resulting in excessive nitrogen rejection, unnecessary power consumption, and reduced gas turbine power output.

Innovation Solution

An integrated system that selectively rejects nitrogen from a portion of the flash gas, using a rectification column and heat exchanger to control the nitrogen content in the fuel stream and vented gases, allowing for precise adjustment of nitrogen levels in both the fuel and LNG product.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Manufacturing precision

If nitrogen rejection is increased to meet LNG product specifications, then nitrogen content in LNG is reduced, but nitrogen content in fuel stream becomes too low causing unnecessary power consumption and reduced gas turbine output

Engineering Contradiction:

Improvenitrogen content control in LNG productVSAvoidgas turbine power output

Core Design Contradiction:

Manufacturing precisionVSProductivity

Solution Approach 1:

The flash gas stream is divided into two separate streams: one directed to the nitrogen rejection unit for producing high-purity nitrogen to be vented, and another bypassing the NRU to be mixed with the nitrogen-depleted fuel stream. This segmentation allows independent control of nitrogen content in both LNG product and fuel gas, resolving the contradiction between meeting LNG specs and maintaining gas turbine output

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The system dynamically adjusts the flow rate of the bypass stream mixed with the fuel stream to control the final nitrogen content in the fuel gas. By varying this mixing parameter, the system can maintain fuel nitrogen content within the optimal 15-50% range for gas turbine operation while still meeting LNG product specifications

Inventive Principle:

Principle #35Parameter changes

2Reliability

If nitrogen rejection is increased to ensure fuel stream meets specifications, then nitrogen content in fuel is reduced, but excessive nitrogen is rejected causing unnecessary power consumption

Engineering Contradiction:

Improvefuel stream nitrogen specification complianceVSAvoidpower consumption

Core Design Contradiction:

ReliabilityVSLoss of energy

Solution Approach 1:

Instead of removing all excess nitrogen from the fuel stream, the system performs partial nitrogen rejection only to the extent needed to meet the maximum nitrogen specification. The bypass stream provides additional nitrogen to maintain optimal fuel quality, avoiding the excessive energy consumption that would result from complete nitrogen removal

Inventive Principle:

Principle #16Partial or excessive action

3Manufacturing precision

If existing nitrogen rejection solutions are used, then on-specification LNG is produced, but control over nitrogen content in both LNG and fuel stream is not achieved

Engineering Contradiction:

ImproveLNG product nitrogen specificationVSAvoidcontrol flexibility for both product and fuel streams

Core Design Contradiction:

Manufacturing precisionVSAdaptability or versatility

Solution Approach 1:

The system incorporates flow control valves and monitoring that allow dynamic adjustment of the bypass stream flow rate based on the desired fuel gas nitrogen content. This feedback mechanism enables independent optimization of both LNG product quality and fuel stream composition, providing the adaptability and control flexibility that existing solutions lack

Inventive Principle:

Principle #23Feedback

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 enables tight control over nitrogen concentrations, reducing unnecessary nitrogen rejection, optimizing power usage, and maximizing gas turbine power output while maintaining LNG product specifications.

Implementation Method 1

A first portion of the flash gas from a nitrogen stripper or endflash drum is sent to a rectification column where a nitrogen-enriched vapor phase stream containing only trace quantities of hydrocarbons is withdrawn from the top of the rectification column

Methodology Applied

Scientific EffectVapor-liquid equilibrium:

Implementation Method 2

A first portion of the flash gas from a nitrogen stripper or endflash drum is sent to a rectification column

Methodology Applied

Scientific EffectDistillation: Distillation

Implementation Method 3

The nitrogen-enriched vapor phase stream is warmed in a heat exchanger against high pressure recycle vapor and warm feed gas

Methodology Applied

Scientific EffectHeat exchange: Heat Exchanger

Implementation Method 4

The first portion is compressed to form a high-pressure vapor stream that is recycled

Methodology Applied

Scientific EffectCompression: Compression

Implementation Method 5

cooling and at least partially liquefying a natural gas feed stream to form a cooled LNG stream

Methodology Applied

Scientific EffectRefrigeration: Cooling

Data Source

PatentUS20230076428A1Integrated nitrogen rejection for liquefaction of natural gas

Publication Date: 2023.03.09 HONEYWELL LNG LLC

AI summary

A method and system for controlling the nitrogen concentration in an LNG product and fuel from flash gas within preferred ranges. A cooled LNG stream is separated into a nitrogen-enriched vapor stream, a fuel stream, and an LNG product stream using a plurality of phase separating devices, such as flash drum or rectifying column. A portion of the vapor stream is recycled to the rectifying column as reflux. A portion of a stream having a higher concentration of nitrogen is combined with the fuel stream to maintain the fuel stream within a desired nitrogen concentration range.