LNG is a gas transport product. The gas, which is primarily methane (CH4), is converted to liquid form for ease of storage or transport, as its volume is about 1/600th the volume of natural gas in the gaseous state. It is produced close to the production facilities in an LNG liquefaction plant, stored, transported in cryogenic tanks on an LNG carrier, and delivered to an LNG regasification terminal for storage and delivery to a pipeline system.
LNG carriers are used when the transport distance does not justify the cost of a pipeline. The main drawback is the cost of the liquefaction, calculated as how much of the total energy content of the gas is used for liquefaction. About 6% of energy content is used to produce LNG in a large modern plant, due to overall thermal efficiency. More than 10% could be consumed with smaller, gas turbine-driven trains. This compares to losses of about 0.9% per 1,000 km of transport distance for large pipeline systems.
The LNG feedstock comes from a gas plant as outlined above. It must satisfy sales gas specifications. Ethane, propane and butane all have freezing points of less than -180 °C and can be part of the LNG, but the concentration of methane is generally above 90%. Some NGLs are also needed as refrigerant for the cryogenic process.
LNG liquefaction
LNG processes are generally patented by large engineering, oil and gas companies, but are generally based on a one- two- or three-stage cooling process with pure or mixed refrigerants. The three main process types of LNG process with some examples of process licensors are:
Ø Cascade cycle:
ü Separate refrigerant cycles with propane, ethylene and methane (ConocoPhillips)
Ø Mixed refrigerant cycle:
ü Single mixed refrigerant (SMR) (PRICO)
ü Single mixed refrigerant (LIMUM® ) (Linde)
ü Propane pre-cooled mixed refrigerant: C3MR (sometimes referred to as APCI: Air Products & Chemicals, Inc.)
ü Shell dual-mixed process (DMR) (Shell)
ü Dual mixed refrigerant (Liquefin Axens)
ü Mixed fluid cascade process (MFCP) (Statoil/Linde)
Ø Expander cycle
ü Kryopak EXP® process
Each process has different characteristics in scalability, investment cost and energy efficiency. For smaller installations, e.g., to handle stranded gas or isolated small gas fields, a single cycle process is preferable due to its low CAPEX (and possibly lower weight for floating LNG), even if energy efficiency is significantly lower than the best cascade or DMR processes, which cost more but also allow the largest trains typically, 7.8 million tons per annum and lowest energy consumed per energy unit LNG produced.
Most processes use a mixed refrigerant (MR) design. The reason is that the gas has a heat load to temperature (Q/T) curve that, if closely matched by the refrigerant, will improve stability, throughput and efficiency (see the figure below). The curve tends to show three distinct regions, matching the precooling, liquefaction and sub-coiling stages. The refrigerant gas composition will vary based on the individual design, as will the power requirement of each stage, and is often a patented, location-specific combination of one or two main components and several smaller, together with careful selection of the compressed pressure and expanded pressure of the refrigerant, to match the LNG gas stream.
Typical LNG train power use is about 28 MW per million tons of LNG per annum (mtpa), corresponding to typically 200 MW for a large trains of 7.2 mtpa, or 65 MW per stage for three cycles. In addition, other consumers in gas treatment and pre-compression add to total power consumption and bring it to some 35-40 MW per mtpa, and over 50 for small LNG facilities well under 1 mtpa capacity.
Some examples are given here. (Please note that these process flow diagrams are simplified to illustrate the principle and do not give a complete design.) All designs are shown with heat exchangers to the sea for comparison. This is generally needed for high capacity, but for smaller plants air fin heat exchangers are normally used.
A triple cycle mixed refrigerant cascade claims to have the highest energy efficiency. It is represented here by the Linde design, co-developed with Statoil.
The actual design varies considerably with the different processes. The most critical component is the heat exchanger, also called the cold box, which is designed for optimum cooling efficiency. Designs may use separate cold boxes, or two or three cycles may combine into one complex common heat exchanger. This particular deign uses the patented Linde coil wound heat exchanger, also called the “rocket design,” due to its exterior resemblance to a classic launch vehicle.
For each train, the cooling medium is first passed through its cooling compressor. Since pressure times volume over temperature (PV/T) remains constant, it results in a significant temperature rise which has to be dissipated, typically in a seawater heat exchanger as shown in the figure above (indicated by the blue wavy line). It then goes though one or more heat exchangers/cold boxes before it expands, either though a valve or a turbo-expander, causing the temperature to drop significantly. It is then returned to cool its cold box before going on to the compressor.
The pre-cooling stage cools the gas to a temperature of about -30 to -50 ºC in the precooling cold box. The cooling element is generally propane or a mixture of propane and ethane and small quantities of other gases. The precooling cold box also cools the cooling medium for the liquefaction and sub cooling stage.
The liquefaction process takes the gas down from -30 ºC to about -100-125 ºC, typically based on a mixture of methane and ethane and other gases. It cools the LNG stream as well as the refrigerant for the final stage. Sub-cooling serves to bring the gas to final stable LNG state at around 162 ºC. The refrigerant is usually methane and/or nitrogen.
The ConocoPhillips optimized cascade process was developed around 1970. It has three cycles with a single refrigerant gas (propane, ethylene and methane) in each.
The dual cycle mixed refrigerant (DMR), developed by Shell and others, may look simpler but the overall design will be similar in complexity as multistage compressors are typically needed. It is shown on the left with the C3MR on the right for comparison.
For small and micro LNG, single cycle designs are often preferred. There are literally hundreds of patented solutions, but only a handful of mainline licensors, that have solved the challenge of achieving single cycle refrigeration. However, this means multiple internal stages in the process flow and the heat exchanger itself. The PRICO SMR is shown on the left and the Linde LIMUM® (on the right).
