Liquified Natural Gas | Introduction | Process | Knowledge today

 Liquified Natural Gas

Liquefied Natural Gas (LNG) has become an increasingly important energy source in recent years as countries worldwide seek to diversify their energy mix and reduce reliance on traditional fossil fuels. Liquefied Natural Gas (LNG) is a natural gas that has been cooled to -162 degrees Celsius, at which point it becomes a liquid. Cooling natural gas to LNG reduces its volume by 600 times, making it much easier to transport and store. Producing LNG involves compressing and cooling natural gas until it reaches a liquid state, making it much easier to transport and store. This innovative process has revolutionized how natural gas is used, making it a cleaner and more efficient fuel for various applications.

The introduction of LNG has significantly impacted the global energy landscape. It has enabled countries to tap into new sources of natural gas, which were previously uneconomical to extract or transport. This has helped increase the natural gas supply, leading to greater energy security and stability. LNG has also brought environmental benefits, with lower emissions of greenhouse gases and other pollutants compared to traditional fossil fuels. This has made LNG an attractive option for countries seeking to reduce their carbon footprint and meet climate change targets.

LNG has transformed the transportation industry, providing cleaner and more efficient fuel for ships and trucks. This has led to a significant reduction in emissions from the transportation sector, which is one of the most significant contributors to global greenhouse gas emissions. The growth of the LNG industry has been supported by significant investments in infrastructure, including liquefaction plants, storage facilities, and import terminals. This has created new business and employment opportunities and has helped stimulate economic growth in many regions.

Liquefication Process 

Natural gas liquefaction is a complex process that involves cooling the gas to a temperature of -162°C (-260°F) to transform it into a liquid state. The natural gas liquefaction process is the most critical and expensive component in liquefied natural gas (LNG) plants, and understanding its design and operational requirements is crucial for plant success.

Several licensed natural gas liquefaction processes are available, each with advantages and disadvantages. These processes are categorized into cascade, mixed refrigerant, and expander processes.

Cascade processes use multiple refrigeration cycles to heat the natural gas, allowing them to achieve high efficiencies but requiring a high degree of process control. Mixed refrigerant methods use a mixture of hydrocarbon refrigerants to cool the gas, making them more straightforward and robust than cascade processes. Still, they may not achieve the same low temperatures. Expander processes use a refrigeration cycle that relies on gas expansion, making them less expensive and simpler but potentially less efficient.

Selecting a natural gas liquefaction process requires project developers to consider plant capacity, reliability, operability, energy efficiency, and environmental impact. Equipment selection and configuration are also crucial factors in determining plant success.

The differences between gas liquefaction processes are mainly associated with the type of refrigeration cycles employed. These processes can be broadly classified into three groups:

1. cascade liquefaction processes, 
2. mixed refrigerant processes, and 
3. expansion-based processes. 

Onshore Liquefaction Process

1) APCI propane pre-cooled mixed refrigerant process.

The APCI (Air Products and Chemicals Inc.) propane pre-cooled mixed refrigerant process is a method used for natural gas liquefaction. It is a highly efficient and cost-effective process that involves the use of a mixed refrigerant system. Here is a detailed description of the process:

• Compression and Pre-cooling: The natural gas is first compressed and then pre-cooled by propane. The propane is cooled to around -42°C by a propane chiller, which is driven by a gas turbine or an electric motor. The pre-cooled natural gas is then sent to the next stage.
• LNG Production: The pre-cooled natural gas is sent to a heat exchanger, where it is further cooled by the mixed refrigerant. The mixed refrigerant is a combination of propane, methane, ethane, and nitrogen. The propane in the mixed refrigerant absorbs the heat from the natural gas, causing it to condense into liquid form.
• Separation: The liquefied natural gas is then separated from the mixed refrigerant. The mixed refrigerant is then sent to a compressor, where it is compressed to a higher pressure and sent back to the heat exchanger to cool the natural gas again.
• Liquefied Natural Gas Storage and Loading: The liquefied natural gas is stored in cryogenic tanks at a temperature of around -162°C. The LNG is then loaded onto tanker ships or trucks for transportation to its destination. 

2) Phillips Optimized Cascade Process

The Phillips Optimized Cascade LNG process is a technology used in the liquefaction of natural gas, developed by Phillips Petroleum in the 1960s. The process involves a series of refrigeration cycles that use propane, ethylene, and methane to cool and condense natural gas into a liquid state.

The process starts with pre-cooling the natural gas feed to remove any impurities, such as water and carbon dioxide. The gas is then compressed, passed through a heat exchanger, and cooled by propane refrigerant. The propane refrigerant is then compressed and cooled by a second refrigerant, ethylene, which is also compressed and cooled by methane.

The methane refrigerant, the coldest of the three, cools the ethylene refrigerant to a shallow temperature, causing it to condense into a liquid state. The ethylene is then used to cool the propane refrigerant to a similar temperature, causing it to condense as well.

The cold propane is then used to cool the natural gas feed to a shallow temperature, causing it to condense into a liquid state. The liquefied natural gas (LNG) is then stored in insulated tanks, ready for transport.

One of the advantages of the Phillips Optimized Cascade LNG process is its high energy efficiency. The process uses the cold temperatures generated by each refrigeration cycle to cool the next refrigerant, minimizing the energy needed to produce LNG. The process is also highly scalable and can be adapted to different production capacities, making it suitable for large and small LNG plants. The process is complex and requires high technical expertise to operate and maintain. It also requires a significant investment in equipment and infrastructure, making it a costly option compared to other LNG processes.

3) Statoil/Linde Mixed Fluid Cascade Process 

The Statoil/Linde mixed fluid cascade process is a cryogenic process for producing liquefied natural gas (LNG) from natural gas. The process is named after the two companies that developed it: Statoil and Linde.

The mixed fluid cascade process uses a combination of propane and mixed refrigerant streams to cool and liquefy natural gas. The process involves several steps:

• Gas Pre-Treatment: Natural gas is first treated to remove any impurities, such as water, carbon dioxide, and hydrogen sulfide, that could interfere with the liquefaction process.
  
• Refrigeration: The natural gas is then cooled to around -162°C (-260°F) using a mixture of propane and refrigerant streams. The propane is used to cool the natural gas to around -42°C (-44°F), while the mixed refrigerant streams are used further to cool the natural gas to its liquefaction temperature. 
• Separation: The cooled and liquefied natural gas is then separated into components using a fractionation column. The column separates the LNG into its main components: methane, ethane, propane, butane, and heavier hydrocarbons.
• Product Treatment: The separate LNG components are then treated to remove any remaining impurities and to adjust the product specifications to meet customer requirements.

The mixed fluid cascade process is a flexible and efficient technology that can produce various LNG products with varying compositions. The process is particularly well-suited to handling natural gas streams with high impurities, such as nitrogen and carbon dioxide, and can be easily integrated into existing gas processing facilities. The Statoil/Linde mixed fluid cascade process is a cryogenic technology that uses a combination of propane and mixed refrigerant streams to liquefy natural gas. The process involves gas pre- treatment, refrigeration, separation, and product treatment and is flexible and efficient, producing a range of LNG products with varying compositions.