Liquefaction, LNG, and cryogenics is both a complex topic and an area intrinsically linked to the broader gases in refinery subject. Here, Professor Ralph Scurlock explores the history of this industry and much more.

Cryogenics is the technology of “cold”, including liquefaction, refrigeration, storage, handling and applications of cryogenic liquids and their cold, on an ever increasing scale.

Until about 50 years ago, around 1959, cryogenics was largely concerned with LHe, LH2 and the air component gases in liquid form, LOX, LIN and LArgon, at temperatures below about 100 K (or -173°C).

At that time (1959), the first successful trial shipments of liquefied natural gas, LNG, were made using the British technology of North Thames Gas Board, across the Atlantic from the Gulf of Mexico to Canvey Island in the Thames estuary.

These trials heralded the gigantic changes in the energy markets which have taken place since then. Very large natural gas fields have been discovered and developed all around the world, usually in places remote from user markets.

While pipelines are cheaper, they are vulnerable to political changes and prove unreliable; so the principle of independent refrigeration chains of LNG export terminal, sea-going tankers, input terminal, storage tanks and regasifier have become more and more attractive in the modern world.

Very soon, the technical problems of storing and handling large quantities of LNG at around 112 to 120 K (-161 to -153°C) started to appear, and the cryogenics industry rapidly became involved with these problems as well as with developing the highly efficient, mixed refrigerant liquefaction cycle.

At that time also, (ie. around 1959), the enormous waste of energy arising from gas flares at oilwells, and also at oil refineries, was beginning to strike everyone as a bad example of the waste of one of the world’s energy resources.

The problem was what to do at the well-head with the low MW hydrocarbon gases, or liquefied petroleum gases LPG, associated with the depressurisation of crude oil during extraction. The markets for LPG were relatively small, so that it was far simpler to flare it at the well-head; and this became standard practice for many years.

Since collecting the gases and storing it under pressure up to 30 bars was technically difficult and expensive, refrigerating the gas so as to liquefy it at temperatures down to around 231 K (-42°C) under 1 bar pressure was more attractive. The development of refrigerated LPG tankers enabled independent refrigeration chains to be established, for capturing the flare gases and transporting them in liquid form at 1 bar around the world.

Today, energy wasting flares at oil well-heads are disappearing while there are only small flares at oil refineries as safety devices, while the market for LPG has expanded to require enormous 100 m diameter insulated storage tanks each holding up to 400,000 m3 of cold LPG at temperatures down to -42°C. The technical problems are similar to those met with LNG at the lower temperatures of -161 to -153°C, and the cryogenic industry has become involved again with storage and handling problems.

LNG and LPG as Multi-component mixtures
The composition of natural gas varies with locality, just like crude petroleum. After removal of water and carbon dioxide, all natural gas remains a mixture of largely methane, with some ethane and propane and smaller proportions of nitrogen.

The production of Liquid Natural Gas LNG involves the cooling and condensation over a considerable temperature range of the natural gas mixture, yielding a liquid mixture with a number of unexpected properties under storage conditions.

Likewise, the composition of Liquid Petroleum Gas LPG varies with locality. After removal of water and carbon dioxide, the LPG remains as a mixture of largely propane, with some normal-butane together with smaller quantities of iso-butane and ethane.

It can be seen, for example, that the liquid densities of the LNG and LPG components are significantly different.

For LNG components, liquid ethane is over 25% more dense than methane, whilst nitrogen is about twice the density of methane. For LPG, the component liquid n-butane is over 30% more dense than liquid propane, while ethane is 25% less dense than liquid propane.

If the liquids, both LNG and LPG, are not mechanically mixed uniformly by composition, then density differences between layers of liquid, called composition stratification, will occur.

In single component liquids, like LIN and LOX, the density decreases with increasing temperature by about 1% for a rise of between 2.0 and 2.5 K. As a consequence, stable stratification can occur between liquid layers differing in density by no more than 1%, with a hot layer at the saturation temperature corresponding to the ullage pressure, on top of a colder, denser layer at a subcooled temperature, under normal storage conditions.

It is well known that this temperature stratification is difficult to get rid of by normal filling and decanting operations; there is no convective mixing between the layers.
In liquid mixtures, the liquid density varies with composition as well as temperature.

For example, the variation in density of liquid propane mixtures is complicated by the facts that propane at its NBP at -42°C has a density of 581 kg/m3 while n-butane at its NBP at -4°C has a density of 601 kg/m3. This density difference of 3.3% is more than sufficient for a colder liquid propane to stratify stably on top of warmer liquid n-butane, with no convective mixing between the layers.

Properties of LNG and LPG in common with other cryogenic liquids
1. For storage and handling at 1 bar pressure, LNG and LPG are boiling liquid mixtures at temperatures of 112-120 K for LNG and 232- 270 K for LPG, and therefore require storage in insulated tanks and transfer in insulated lines.

2. For controlled storage, all heat entering the liquid through the tank insulations should be removed by the latent heat of the boil-off gas.

3. If the heat entering the liquid is NOT removed by the boil-off gas, the heat energy will be stored in the liquid as “thermal overfill”.

4. If thermal overfill is allowed to build up, an unstable storage condition will result. The thermal overfill energy will eventually remove itself spontaneously via a large, uncontrollable increase in the rate of boil-off over a short period, described variously by the terms vapour explosion or rollover.

5. This uncontrolled boil-off must be avoided by correct tank management, to prevent release of flammable gas into the environment and possible damage to
the tank.

6. The precursor to thermal overfill is stratification between 2 (or more) liquid layers of different density.

7. There are 3 types of stratification, (a) temperature stratification, in a mixture of uniform composition throughout, (b) composition stratification, in an isothermal mixture with differing composition between the layers, and (c) complex combinations of (a) and (b) which are subject to double-diffusive convection, with both temperature and composition gradients contributing to convective stability or mixing.

8. Evaporation studies have shown that all cryogenic liquids, including LNG and LPG, do not suffer nucleate boiling as a result of heat inflows through the insulation. The heat fluxes are too low by an order of magnitude to generate nucleate boiling.

9. Instead, the heat inflows are carried to the surface by boundary layer liquid flows at the tank walls. The superheated liquid at the surface then undergoes surface evaporation controlled by heat conduction through a thin surface layer.

10. Under stratification, the wall boundary layer flow does not have enough buoyancy to cross the cold/warm liquid interface if the density difference is greater than 1%. The heat inflow cannot be released by surface evaporation and remains locked in the lower layer as thermal overfill.

11. This is a potential disaster situation, because the lower layer will heat up, reducing in density until it approaches that of the upper surface. As density equilibrium is approached, the liquids start to mix by spontaneous natural convection plumes, as a so-called rollover event. The associated heat of mixing, together with the thermal overfill being released from the lower layer, produces large quantities of vapour.

12. The rate of generation of vapour during a spontaneous mixing event or rollover event is difficult to estimate because every event is different, as was found in some 100 instrumented rollovers produced at the Southampton IOC.

13. From measurement of the variation with time of the density, composition and temperature of the two layers, it was possible to predict the time of rollover. For further information, see Chapter 5 – Multi-component Liquids, in “Storage and Handling of Cryogenic Liquids: The Application of Cryogenic Fluid Dynamics” by R.G.Scurlock, (2007). (Published by Kryos Publications, )

14. From the ease with which it was possible to set up rollover events in the laboratory with many liquid mixtures, including nitrogen/oxygen, oxygen/argon, methane/ethane, refrigerants R12/R22, it is clear that spontaneous convective mixing or rollover between stratified layers in a storage tank is not confined to LNG and LPG mixtures.

15. The peak evaporation rate appears to be determined by unimpeded molecular evaporation, and hence the size of the large emergency vents to atmosphere can be determined for LNG and LPG storage tanks.

Stratification by thermal underfill
If a tank is filled or topped with sub-cooled liquid, then auto-stratification will occur immediately, with an upper layer at a higher temperature and lower density in equilibrium with the vapour at ullage pressure, on top of the sub-cooled liquid.

Sub-cooled liquid, ie. cooled to below its boiling point at tank pressure, is dangerous to have in a storage tank. Any mixing will lead to sub-atmospheric pressure in the tank and consequential ingress of atmospheric air and the formation of an explosive vapour mixture inside the tank; or collapse of the tank not designed for negative internal pressure.

Subcooled liquid must NOT be loaded into sea-going LNG or refrigerated LPG tankers. All will appear to be well when they set sail, with all tank pressures registering slightly above atmospheric pressure in the normal way.

However, when the sea gets rough, the motion of the tanker will induce mixing and the nightmare of a negative pressure may register in one or more tanks. Emergency purging will be necessary, with nitrogen gas, if carried; if not, with propulsion engine exhaust gas.

Prevention and Removal of Stratification
Stratification is the starting point for a rollover or vapour explosion event. Safe management of an LNG or LPG tank requires the prevention and/or removal of stratification, by mechanical mixing, which may need to be continuous in the absence of accurate instrumentation inside the tank.

There are a number of ways in which the stratification can occur and be detected, and these are discussed in the same book.

These ways include custody management mistakes, and auto-stratification, for example, from high MW volatile components, and monomolecular layers of surface impurities.

Path dependent mixing
One strange phenomenon which is observed, particularly with handling and mixing large volumes of low temperature hydrocarbon liquids, is that the heat of mixing between 2 liquids of different composition depends on the path chosen to obtain the final mixture.

From a thermodynamic point of view, mixing is an irreversible process and one might expect that mixing should be path-dependent. This is not usually noticeable when the heat of mixing is small and the latent heat of vaporisation is large.

However, this is not true for cryogenic liquids in general and low temperature hydrocarbon liquids in particular.

For example, when propane-rich and butane-rich liquids are mixed at sea, (it’s not allowed in harbour) to make marketable commercial LPG on board refrigerated LPG tankers, the heat of mixing will evaporate 5-10% of the liquid, corresponding to 15-30 m3 of vapour for every cubic metre of liquid mixture.

To avoid excess pressure being generated, or vapour being vented, it is important to control the rate of mixing to match the refrigerating capacity on board.

If cold propane-rich liquid is added to warmer butane, then the vapour generated for recondensing by the refrigeration plant is TWICE the vapour generated when warm butane-rich liquid is added to cold propane.

This path dependent mixing phenomenon was
studied and confirmed at Southampton.

So, the message is “ When mixing HOT and COLD, always add HOT to COLD for minimum boil-off”.