Closed Loop Methane Refrigeration for High-Nitrogen LNG Feed Gas

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Solution Overview

Problem

Open loop methane refrigeration cycles face inefficiencies due to accumulation of lighter components like nitrogen in the refrigerant recycle stream, increasing compression power requirements and reducing LNG production.

Innovation Solution

A closed loop methane refrigeration cycle is implemented, which separates the methane refrigerant from the natural gas feed, using a cascading process with ethylene and methane refrigeration cycles to subcool and expand the gas, reducing nitrogen accumulation and eliminating the need for a nitrogen removal unit in some cases.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Productivity

If an open loop methane refrigeration cycle is used, then the system can process natural gas feed, but nitrogen accumulates in the refrigerant recycle stream increasing compression power requirements

Engineering Contradiction:

ImproveLNG productionVSAvoidcompression power

Core Design Contradiction:

ProductivityVSUse of energy by moving object

Solution Approach 1:

The system separates the refrigerant loop from the natural gas feed stream, creating distinct functional segments. The methane refrigerant circulates in a closed loop through heat exchangers to cool the natural gas, then is recompressed and recycled, preventing nitrogen from the feed gas from contaminating the refrigerant stream and accumulating in the recycle loop.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The methane refrigerant acts as an intermediary substance that transfers thermal energy from the natural gas feed to the compression system without direct mixing. This mediator enables cooling while maintaining separation between the nitrogen-containing feed gas and the refrigerant recycle stream, avoiding nitrogen accumulation.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Power

If nitrogen accumulates in the refrigerant stream, then compression power increases, but system efficiency decreases

Engineering Contradiction:

Improvecompression powerVSAvoidsystem efficiency

Core Design Contradiction:

PowerVSLoss of energy

Solution Approach 1:

The closed loop system continuously monitors and recycles the methane refrigerant, creating a feedback mechanism where the refrigerant is constantly purified by separation from the natural gas feed. This feedback loop prevents nitrogen accumulation that would otherwise increase compression power requirements and reduce overall system efficiency.

Inventive Principle:

Principle #23Feedback

3Reliability

If a closed loop methane refrigeration cycle is implemented, then nitrogen accumulation is prevented, but system complexity increases

Engineering Contradiction:

Improveprocess efficiencyVSAvoidsystem complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The system merges the refrigeration function with the natural gas processing flow by integrating heat exchangers into the existing process train. The closed loop methane refrigeration cycle combines cooling, heating, and compression functions in an integrated system that maintains reliability while managing complexity through functional consolidation.

Inventive Principle:

Principle #5Merging (Combining)

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 enhances process efficiency, reduces refrigerant compression power, and increases LNG production by preventing nitrogen accumulation in the methane cooling cycle, especially for medium and high nitrogen content feed gases.

Implementation Method 1

subcooling the natural gas feed by indirect heat exchange with a methane refrigerant at one or more methane refrigerant heat exchangers

Methodology Applied

Scientific EffectIndirect heat exchange: Heat Exchanger

Implementation Method 2

cooling the natural gas to near atmospheric vapor pressure through indirect heat exchange with one or more refrigerants

Methodology Applied

Scientific EffectHeat transfer: Conduction (thermal)

Implementation Method 3

reducing a pressure of the subcooled natural gas feed through an expansion device

Methodology Applied

Scientific EffectPressure reduction: Depressurisation

Implementation Method 4

expanded to near atmospheric pressure

Methodology Applied

Scientific EffectExpansion cooling: Joule-Thomson Effect

Implementation Method 5

separating a two phase stream, resulting from reducing the pressure, in an end flash vessel

Methodology Applied

Scientific EffectPhase separation: Density Gradient

Implementation Method 6

condensing a methane refrigerant discharge in an ethylene heat exchanger to form a condensed methane refrigerant

Methodology Applied

Scientific EffectCondensation: Condensation

Data Source

PatentUS20230375261A1Closed loop LNG process for a feed gas with nitrogen

Publication Date: 2023.11.23 CONOCOPHILLIPS CO

AI summary

Systems and methods for processing liquefied natural gas (LNG) can include an LNG production system with a methane refrigeration cycle downstream from an ethylene refrigeration cycle. The methane refrigeration cycle can be a closed loop methane refrigeration cycle that maintains a methane refrigerant separate from a natural gas feed, (e.g., compared to an open loop methane refrigeration cycle that extracts the methane refrigerant from the natural gas feed and recombines the methane refrigerant with the natural gas feed). The natural gas feed can be a medium or high nitrogen gas feed having a nitrogen content greater than 1.0% molarity.