Gas Processing – what is it?

This article is intended to provide background and context to non-specialists in gas processing and thereby enable constructive dialogue with engineers and suppliers.

Let’s first try to answer this fundamental question – what is gas Processing, and why do it?

And to start with – what is gas?

Well, gas is one of the 4 known ‘states’ of matter (stuff), these being solid, liquid, gas and plasma.

A ‘State’ is like a condition in which something (or someone) ‘is’.  Could be likened to a human physical or emotional state - asleep, calm, active, hyper, hot, cold.

Plasma is very high temperature gas with the electrons stripped off to wander where they will, in other words a soup of charged ions.  Gas, liquid and solid are fairly familiar to most of us , so I leave this to your personal experience to ponder.

I say 4 states, but there might be other states we have not encountered yet – such as ‘dark matter’. Furthermore thanks to Einstein – mass (matter) and energy can be interchanged under the right (extreme) conditions – such as in a star or a nuclear reactor. The matter of a neutron star or within a black hole is (we believe) far far far more dense than any normal solid – but we are (I am) digressing - well away from ‘gas processing’.

Pretty much all substances can be made to exist in any of the 3 (or 4) states of matter, depending what pressure is imposed on them and what their temperature is.  The temperature is just a measure of their ‘internal energy’ with which the molecules vibrate spin around and zoom about crashing into things (other molecules and the walls of a container).  The pressure is the caused by force exerted by the molecules bombarding the walls of a container.  It’s actually the total force divided by the inside wall area of the container. 

See Article on Pressure - a most important basic ‘state property’ of a gas.

If we ignore plasma, gas has the highest energetic state of normal matter. We can also state that the molecules are in their lowest ‘state of aggregation’.  This rather clumsy descriptor means that the molecules are energetic fast moving free spirits with usually little tendency to stay with or close to neighbouring molecules.  The density of a substance in its gas state is several hundred times less than when it’s in the more ordered liquid or solid state (i.e. higher states of aggregation).

Types of Gas
Gas can be natural gas or synthetic (man-made) gas, or Industrial Gas, including products from atmospheric air.

Apart from hydrogen, the Industrial gases are typically sought and produced for reasons other than their fuel value. The products of Air Separation - Oxygen, Nitrogen, Argon, Neon, Krypton, Xenon have numerous applications from medical, electronics, steel production, arc welding, inerting, fluorescent lighting.

Helium is also present in air, but at only 5.2 parts per million. As a result it is rarely economic to recover from air, but is instead obtained from some natural gas sources where it is sometimes found at a few percent. But it is a finite resource which is often in short supply due to increasing demand from electronics, for cooling superconducting magnets of MRI scanners and low temperature physics.

A separate article on industrial gases and helium is in preparation, so the remained of this article is focussed on natural gas and hydrogen.

Natural gas can be extracted from the porous rocks where it has been trapped since it was formed from living vegetation millions of years ago.  This is a useful energy source for heating and power generation.  It mainly comprises methane which when burned produces much less CO2 than coal or oil (or oil products).   However, it does produce CO2 on combustion and, if released to the atmosphere methane itself is a much more potent greenhouse gas than CO2.

Challenges in utilising Natural gas:

a) It’s often found in inhospitable or uninhabited locations where there is no local demand for it.

b) It’s a gas, so its relatively low density makes it awkward to transport to the population centres where it is needed.

c) It often contains impurities which are a nuisance or make it hazardous in its natural state, or inert components (like nitrogen or carbon dioxide) that reduce its value as a fuel. The removal of unwanted nitrogen from natural gas has spawned a range of cryogenic separation processes, termed Nitrogen Removal Units (NRUs).

The technologies used in NRUs are primarily distillation processes. They sometimes arise as part of an LNG process - see below. At other times the NRU may exist as a free-standing cryogenic unit to improve the gas quality. They are described in a separate Article.

Gas Treating
Processing of natural gas involves cleaning it up to remove those undesirable or unnecessary impurities; compressing it to transport by pipeline to the places that need it; or cooling it to the liquid state (600 times higher density) so that it can be transported greater distances where a pipeline would be impracticable. 

Ustream of any cryogenic process, some commonly occurring but troublesome impurities such as CO2 and water and aromatic hydrocarbons must be dealt with first as they would freeze and block the heat exchangers of  the low temperature processing unit.   Others like mercury are found in traces in natural gases and will attack the aluminium heat exchangers widely used in cryogenic processes under certain conditions leading to failure.

LNG plant treating requirements

One of the main cryogenic gas processes is production of LNG = Liquefied Natural Gas.  Before liquefying natural gas it is necessary to remove water to a fraction of a ppm and CO2 usually to below 50ppm.

Heavier hydrocarbons C5+ are removed to a safe level by scrubbing within the cooling process.

If there is excess nitrogen in the natural gas feeding an LNG plant (over 1%) this is normally stripped out in an NRU at the cold end of the process and leaves with the flash gas passing to the site fuel.

Absorption and adsorption
Different gas processing / treating technologies to remove these ‘contaminants’ include absorption in a solvent for acid gases like H2S (rotten eggs gas) and CO2, adsorption on one or more ‘beds’ of small solid particles for water and mercury. 

Adsorption is a semi-batch process with two (or more) vessels that switch between being in service purifying the gas and being regenerated, usually using a heated gas stream.

Membranes

So called semi-permeable membranes have also gained acceptance as a technology that allows removal of certain (undesirable) components which permeate quickly through the thousands of tiny hollow fibre membranes housed in a pressure vessel.  The impurities flow down the core of the fibre at low pressure and are collected and disposed of.  The valuable components are (by and large) held back on the high pressure feed side of the membrane. The performance of polymer and composite membrane materials are steadily improving but do not provide perfect separation. There is a compromise between permeability and selectivity.

Permeable components include ‘polar’ molecules such as CO2 and H2O which readiliy ‘dissolve’ in the polymer material comprising the membranes active layer.

But this is not always the case.  Hydrogen is one of the most permeable gases because of its small molecular size, so if membrane separation is used for hydrogen separations  the hydrogen is produced as the low pressure product. The same applies to the small molecule helium.

A unique type of membrane permeable only to hydrogen is based upon the metal Palladium often in alloy with Silver.  The perfect selectivity arises because hydrogen adsorbs and passes through the very thin active layer as hydrogen atoms, recombining to form H2 molecules downstream of the membrane.

An article on membrane systems and their basic design is in preparation.

Syngas
Syngas meaning Synthetic gas is man-made gas produced usually by a chemical reaction between a carbon containing feedstock,  water (as steam) and oxygen.  The resulting mixture is mainly hydrogen, carbon monoxide and some carbon dioxide.  The carbon containing feed-stock can be anything from coal to biomass to municipal waste, coke or hydrocarbons or bottom of the barrel from a refinery.

The term ‘syngas’ can also mean mixtures of CO and H2 are known as Synthesis gas because from it various valuable chemicals may be produced (synthesised) by catalysed reactions.  These include methanol, ammonia, linear paraffins (Fischer-Tropsch process).

Synthesis gas is a potential building block for producing many petrochemicals and fertilisers as well hydrogen as a fuel. The syngas production by reforming of fossil fuels (reacting with steam and or oxygen), the process can include steps to capture all or most of the carbon dioxide for subsequent use or sequestration. When the end product is hydrogen, this is termed blue hydrogen, to distinguish it from green hydrogen produced by electrolysis using renewable electricity and from grey hydrogen produced from fossil fuel reforming without CO2 capture.

Summary

The potential scope of the gas processing sector is clearly large – covering technology for production of gases of various types – industrial , natural, synthetic / synthesis. People working in the sector are involved in the design, optimisation, construction and operation of equipment and processes to capture, purify, separate, liquefy, store, transport and make use of gases of all types.
The technologies for processing and separation of gases apply many of the standard chemical engineering unit operations – Distillation, Absorption, Adsorption. Some like Membranes are less well established but growing in importance as new materials are developed.

Cryogenic process engineering is dealt with in more detail in a separate article.