As the name specified, Liquefied natural gas or LNG is natural gas with the primary element as methane. The LNG is converted to liquid form for ease of transport and storage. While in liquid form, LNG takes up around 1/600th the volume of its gaseous form. Naturally, liquefaction is advantageous as it can be transported or stored to a greater quantity. LNG Process is the process of liquefaction. The process of cooling the gaseous LNG to -259°F or -162°C for transforming it into liquid is known as LNG Process. The process is actually a chain of methods, hence popularly known as LNG Process Chain.
Origin of Natural Gas
- Natural gas exists in nature under pressure in rock reservoirs in the Earth’s crust, either in conjunction with and dissolved in heavier hydrocarbons and water or by itself.
- It is produced from the reservoir similarly to or in conjunction with crude oil.
- Natural gas has been formed by the degradation of organic matter accumulated in the past millions of years. Two main mechanisms (biogenic and thermogenic) are responsible for this degradation.
- Natural gas produced from geological formations comes in a wide array of compositions. The varieties of gas compositions can be broadly categorized into three distinct groups:
- Non-associated gas – it occurs in conventional gas fields
- Associated gas – it occurs in conventional oil fields.
It occurs outside of the former two. Most common types of unconventional gas are:
- Tight gas – natural gas produced from reservoir rocks with such low permeability that massive hydraulic fracturing is necessary to produce the well at economic rates;
- Coalbed methane – methane adsorbed into the solid matrix of the coal;
- Natural gas from geo-pressurized aquifers;
- Gas hydrates – methane clathrate is a solid clathrate compound in which a large amount of methane is trapped within a crystal structure of water, forming a solid similar to ice;
- Deep gas
Composition of Natural Gas
Natural gas is a complex mixture of hydrocarbon and non-hydrocarbon constituents and exists as a gas under atmospheric conditions.
Raw natural gas typically consists primarily of methane (CH4), the shortest and lightest hydrocarbon molecule. It also contains varying amounts of:
- Heavier gaseous hydrocarbons: ethane (C2H6), propane (C3H8), normal butane (n-C4H10), iso-butane (i-C4H10), pentanes, and even higher molecular weight hydrocarbons.
- Acid gases: carbon dioxide (CO2), hydrogen sulfide (H2S), and mercaptans such as methanethiol (CH3SH) and ethanethiol (C2H5SH).
- Other gases: nitrogen (N2) and helium(He).
- Water: water vapor and liquid.
- Liquid hydrocarbons: crude oil and/or gas condensates.
- Mercury: only trace amounts.
Refer to the table in Figure 1 for a typical composition of Natural gas.
Why Liquefied Natural Gas (LNG)?
- Liquefied natural gas (LNG) is natural gas (predominantly methane, CH4, with some mixture of ethane C2H6) that has been converted to liquid form for ease and safety of non-pressurized storage or transport.
- Liquefaction is required to achieve natural gas transport over the seas where laying pipelines is not feasible technically and economically.
- 1/600th the volume of natural gas in the gaseous state.
- odorless, colorless, non-toxic, and non-corrosive.
Natural Gas – Exploration to End-User
Typical 2-Train LNG Plant
Liquefaction Temperatures of LNG
LNG Liquefaction Process Flow
Figure 5 shows a Schematic of a Simple Refrigeration Cycle (LNG Process Flow)
Liquefaction Techniques/LNG Process Techniques
Different Liquefaction techniques include:
- Single Refrigeration cycle
- Multiple Refrigeration cycles
- Self Refrigerated cycles
- Cascade Processes
- The cooling of natural gas involves the use of refrigerants which could either be pure component refrigerants or mixed component refrigerants.
LNG process liquefaction is performed using various technologies mentioned below:
- CASCADE PROCESS by ConocoPhillips
- C3MR or AP-X by Air Products
- DMR by Shell
- Mixed Fluid Cascade – MFC by Linde
- Liquefin by Axens / Air Liquide
- Most Straight Forward of All Processes
- Kenai Plant Continuous Operation 1969
- CoP License, Plant Build by Bechtel.
- The raw gas is first treated to remove typical contaminants.
- Next, the treated gas is chilled, cooled, and condensed to -162 ˚C in succession using propane, ethylene, and methane.
- The last stage is pumping LNG to storage tanks and awaiting shipment.
- Pure component Refrigerants
- Specific operating ranges for each component
- Mixed Refrigerants
- Modified to meet specific cooling demands.
- Helps improve the process efficiency
- Mixed refrigerants are mainly composed of hydrocarbons ranging from methane to pentane, Nitrogen, and CO2. Typically, Methane – 25-30%, Ethane – 45-55%, Propane – 15-20%, Nitrogen – 1-5%, Butane – 1-2%.
Single MR Process for LNG Liquefaction
- Significant improvement from Cascade Process
- Use of Coil wound Heat Exchangers & MR refrigerant simplified the process.
- Mixed Refrigerant offered a way to provide the required refrigeration over the temperature range required.
C3MR process of Liquefaction of LNG
- Introduction of Propane as Pre-cooling to liquefication
- Improved Efficiency, increased single train capacity
- Reduction in MR refrigerant volumetric flow due to pre-cooling by Propane
- Train size continued to grow with larger drivers & larger compressors
- Liquefication capacity up to 5 MMTPA.
AP-X Hybrid LNG Process
- Improved C3-MR process – pre-cooling by Propane, liquefaction using MR, and sub-cooling using Nitrogen Cycle.
- Nitrogen Cycle has a simple & efficient expander loop.
- Increased capacity by a reduction in volume flow of MR (40%of C3MR) & Propane (20% of C3MR).
- Liquefication capacity up to 8.0 MMTPA.
- Nitrogen Cycle is a simplified version of the cycle employed by Air Products in Air Separation plants.
- Higher vapor pressure at the required liquefication temperature of Natural Gas
- Relatively smaller volumetric flowrate in low-pressure Nitrogen circuit.
- Improved efficiency by reducing pressure losses
DMR LNG Liquefaction Process
- DMR – Dual Mixed Refrigerant is very similar to C3MR
- The difference is in the utilization of a second pre-cooling refrigerant component.
- The use of two mixed refrigerant cycles allows full utilization of power in a design with two mechanically driven compressors.
- It allows keeping the compressors at their best efficiency point over a very wide range of ambient temperature variations and changes in feed gas composition.
- The natural gas stream is cooled via two stages. The first stage cools natural gas to -50°C while the second column cools natural gas to LNG at -160°C.
Liquefin by Axens (Air Liquide)
- Developed by IFPEN and AXENS, now owned by Air Liquide.
- a highly efficient process and provides the most cost-competitive LNG product per ton.
- is optimized best with the Brazed Aluminium Heat Exchanger, leading to further cost reductions and scalable output.
- Compact and modular design
- Balanced refrigeration power allowing for identical refrigerant compressor drivers
- Very cost-effective solution