Small-Scale LNG Production With Rectifier-Based Methane Purification
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Solution Overview
Problem
Existing methods for producing LNG from natural gas/NGL processing plants struggle to achieve high methane purity (>95%) when the sales gas stream contains less than 95% methane, often requiring complex heat exchange systems that increase capital costs and impact NGL recovery, and are insufficient for producing substantial quantities needed for onsite power generation.
Innovation Solution
A system and method utilizing an LNG fractionation column with an internal knockback condenser, separating heavier components before liquefaction, and employing a refrigeration loop with a nitrogen-methane flash vapor stream to achieve high methane purity without requiring heat exchange with the main natural gas/NGL plant process streams, allowing for efficient production of up to 100,000 gallons per day of 95%+ methane LNG.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If heat exchange with demethanizer overhead stream is used to liquefy sales gas, then LNG production is achieved, but NGL recovery decreases and system complexity increases
Solution Approach 1:
The patent divides the heat exchange system into two independent parts: (1) a main plant heat exchange system for NGL separation using demethanizer overhead stream, and (2) a separate LNG production heat exchange system using dedicated refrigerant streams. This segmentation allows each system to operate independently without interfering with the other, resolving the contradiction between LNG production and NGL recovery while reducing overall system complexity.
Solution Approach 2:
The patent extracts the LNG production function from the main NGL processing heat exchange system by introducing a separate refrigeration loop with dedicated refrigerant streams (propane, ethylene, nitrogen-methane). This extraction allows LNG production to proceed without compromising the main plant's NGL recovery operations, eliminating the harmful coupling between the two processes.
2Manufacturing precision
If multiple flash stages are used to achieve high methane purity LNG, then LNG purity improves, but capital costs increase
Solution Approach 1:
The patent changes the refrigerant parameters (temperature, pressure, composition) in a single flash stage to achieve high methane purity LNG. By using optimized refrigerant streams with specific temperature-pressure characteristics and controlling the flash expansion parameters, the system achieves 95%+ methane purity without requiring multiple flash stages, thereby reducing capital costs while maintaining manufacturing precision.
Solution Approach 2:
The patent employs composite refrigerant mixtures (propane-ethylene, nitrogen-methane) with specific compositional ratios to achieve the desired cooling effect and methane separation in a single flash stage. These composite refrigerant materials provide enhanced separation efficiency compared to single-component refrigerants, enabling high purity LNG production with reduced system complexity.
3Ease of operation
If sales gas stream with less than 95% methane is liquefied directly, then processing simplicity is maintained, but LNG methane purity remains below 95%
Solution Approach 1:
The patent introduces an intermediary fractionation column between the sales gas feed and the LNG product stream. This fractionation column acts as a mediator that selectively separates heavier hydrocarbon components (ethane, propane, butanes) from the methane-rich stream. The fractionation process maintains relative processing simplicity while achieving the required 95%+ methane purity in the final LNG product.
Solution Approach 2:
The patent utilizes phase transition (condensation) of heavier hydrocarbon components in the fractionation column at controlled temperatures and pressures. By exploiting the different condensation points of methane versus heavier hydrocarbons, the system efficiently separates these components, allowing simple processing to yield high-purity LNG after the phase separation step.
4Reliability
If diesel engines are used for power generation, then power supply reliability is maintained, but environmental harm and operating costs increase
Solution Approach 1:
The patent changes the fuel parameter from diesel to high-purity methane (natural gas), fundamentally altering the combustion characteristics. Methane combustion produces significantly lower emissions of particulate matter, nitrogen oxides, and carbon monoxide compared to diesel. This parameter change maintains power supply reliability through natural gas turbine generators while dramatically reducing environmental harm and operating costs associated with diesel fuel procurement and emission compliance.
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 solution enables the production of high-purity LNG with 95%+ methane from sales gas streams containing less than 95% methane, reducing diesel engine emissions and costs by providing a sufficient fuel source for natural gas turbine-driven generators, while integrating seamlessly with existing natural gas/NGL processing plants without affecting NGL production or plant feed stream cooling.
Implementation Method 1
cooling the overhead stream from the fractionation column and liquefying it to produce an LNG product stream
Implementation Method 2
The cooled overhead stream is then flashed in a flash stage to form a flash vapor stream and an LNG product stream
Implementation Method 3
separating the feed stream in a first fractionation column or rectifier into a first overhead stream and a first bottoms stream
Implementation Method 4
the first recycle stream is compressed in the first compressor expander unit, and preferably also compressed in the second compressor expander unit, to form the compressed recycle stream
Data Source
AI summary
A system and method for producing an LNG product stream to provide fuel to generators, as an alternative to diesel, to power drilling and other equipment. Using sales gas from a natural gas/NGL plant containing less than 95% methane as a feed stream, production of LNG having 95% or more methane in quantities of 100,000 GPD or more LNG product are achievable with the system and method. The system and method preferably combine use of strategic heat exchange between the feed and a nitrogen-methane flash vapor stream and other streams within the LNG processing system without requiring heat exchange with process streams in the natural gas/NGL plant and a rectifier column that uses an internal knockback condenser and does not require a reboiler to remove heavier components from the sales gas feed.