Big Chemistry: Liquefied Natural Gas [Hackaday]

View Article on Hackaday

The topic of energy has been top-of-mind for us since the first of our ancestors came down out of the trees looking for something to eat that wouldn’t eat them. But in a world where the neverending struggle for energy has been abstracted away to the flick of a finger on a light switch or thermostat, thanks to geopolitical forces many of us are now facing the wrath of winter with a completely different outlook on what it takes to stay warm.

The problem isn’t necessarily that we don’t have enough energy, it’s more that what we have is neither evenly distributed nor easily obtained. Moving energy from where it’s produced to where it’s needed is rarely a simple matter, and often poses significant and interesting engineering challenges. This is especially true for sources of energy that don’t pack a lot of punch into a small space, like natural gas. Getting it across a continent is challenging enough; getting it across an ocean is another thing altogether, and that’s where liquefied natural gas, or LNG, comes into the picture.

Liquefaction

Before we start looking at how LNG is made, it’s best to start by asking why we even need LNG in the first place. After all, we’ve got an incredibly complex, continent-spanning infrastructure of pipelines that’s optimized for the long-haul bulk transportation of natural gas. Why bother to go through all the effort and expense of liquefying natural gas?

In a word: oceans. Those vast networks of pipelines pretty much stop at the water’s edge, and while there certainly are some undersea natural gas pipelines, recent events have shown us just how vulnerable those can be. So shipping natural gas by sea has become a necessary means of moving energy from point A to point B. And to do that efficiently, you need to dramatically reduce its volume. Turning natural gas into a liquid does exactly that: it increases its density 600 times, making it feasible to ship in bulk.

The feedstock for liquefied natural gas is, of course, natural gas. We’ve already covered a fair amount about the process of harvesting and distributing natural gas, but briefly, natural gas is a mixture of hydrocarbons like methane and ethane produced from the decay of ancient biomass in geological formations. Along with liquid hydrocarbons and contaminants like nitrogen, carbon dioxide, sulfur-containing compounds, and water vapor, the gas accumulates in underground reservoirs that are tapped by drilling.

Raw natural gas is transported, under its natural pressure or with the help of enormous compressors, via pipelines to plants that clean the gas. Recovering sulfur and helium, both valuable chemical elements, from the raw gas is especially important, but it’s also important to scrub low-value contaminants like water and CO₂ from the natural gas, since they can both cause freezing problems down the line. Water is removed by bubbling the raw natural gas through triethylene glycol (TEG), an extremely hygroscopic solution, while CO₂ is removed using an amine scrubber, which exposes the acidic raw gas to nitrogen-containing amine solutions like diethylamine (DEA), which adsorbs the CO₂. After further purification, which removes any remaining heavier hydrocarbons and contaminants like mercury, which will not behave when exposed to aluminum and stainless steel, the natural gas feedstock is about 85% to 90% methane (CH₄), with the rest being a mix of ethane (C₂H₆), propane (C₃H₈), and butane (C₄H₁₀).

The clean, dry natural gas is then ready for liquefaction. Like most gases, natural gas will condense into a liquid when its temperature drops below its boiling point, which is -161.5°C for methane. So, to make LNG, an industrial-scale cryogenic process is required. Most LNG today is made with a process called C3MR, which is a dual-loop progressive cooling system. The “C3” refers to propane, a three-carbon compound used as the refrigerant in the pre-cooling loop. Each half of the cycle is essentially the same as found in any refrigerator, although vastly different in scale. In the precooling stage, liquid propane is passed through an expansion valve, which causes a phase change and a sudden drop in temperature. The cooled propane removes heat from the natural gas via a heat exchanger, the propane is compressed with a three-stage compressor, and the heat is removed via a condenser so the cycle can begin again.

After pre-cooling, the natural gas is at about -33°C — cold, but not cold enough. The cooled gas now enters the “MR,” or “mixed-refrigerant” loop, with a mix of propane, pentane, methane, and ethylene used. Again, the thermal cycle is familiar, but the scale is even more massive — the coil-type heat exchangers used in some MR loops can have thousands of kilometers of tubing coiled up inside them, with a heat exchange area of 40,000 square meters. There are also plate-fin heat exchangers, which have multiple layers of corrugated fins sandwiched between flat aluminum plates. Plate-fin heat exchangers are installed in insulation-filled enclosures called cold boxes. Both types of heat exchangers are usually deployed in parallel assemblies called trains to achieve massive throughput and redundancy.

After the MR loop, the natural gas has dropped to about -160°C and is now a colorless, odorless, non-toxic liquid. The energy input to get to this point has been considerable — something like 13 kilowatts to yield a single tonne of LNG. As of 2021, the global liquefaction capacity was over 450 million tonnes per year, with more capacity still under construction.

Storage and Transportation

But all of this production means nothing without somewhere to put all this LNG. The cryogenic nature of LNG presents certain engineering challenges. While not as cold as liquid nitrogen or oxygen, LNG can still cause steel embrittlement, which is why special alloys of stainless steel are used for LNG piping and tanks. Storage tanks, which are used to hold LNG temporarily between production and shipping, also have to be carefully engineered. These tanks are often built partially or even fully underground; while the subsoil provides insulation that reduces heat transfer into the LNG, the cold can freeze groundwater and cause frost heave underneath the tanks. Storage tanks also have to allow for some boil-off of the LNG, with the resulting natural gas either captured and sold off through regular distribution channels, or piped back to the beginning of the process and reliquefied.