THT for gas odorisation: what purity, cloud point and evaporation residue actually control

In short

Natural gas has no smell. Everything a distribution network relies on for leak detection is added on purpose, at a rate somebody calculated, from a drum somebody accepted. That makes an odorant specification a different kind of document from a specification for a material that ends up in a product: nothing downstream corrects it, and the failure it prevents is not a bad batch.

The criterion behind all of it is the same everywhere, whatever the national rules say in detail. Gas must be detectable by a person with a normal sense of smell at a concentration well below the point at which it becomes flammable — commonly set at one fifth of the lower explosive limit. Odorisation is dosed to meet that, and the specification of the odorant is what makes the dose predictable.

The specification, line by line

These are the values published for the tetrahydrothiophene we supply, CAS 110-01-0. The use conditions for THT as a natural gas odorant are set out in ISO 13734.

Parameter Specification What it controls
Appearance Colourless transparent liquid Haze means water or solids; colour means the material has changed
Purity 99.0% min. Odour per unit mass, odour character, and the sulfur budget
Cloud point −30 °C max. Behaviour in outdoor tankage and injection lines in winter
Evaporation residue 0.20% max. Deposits in the vessel, the pump and the injection nozzle

Four lines is a short specification, and each one is carrying more than it looks.

Purity, and whether the other 1% smells

For most materials, a purity figure asks how much of the product is product. For an odorant it asks something narrower: is the balance odour-active, and does it smell like the same thing?

Those are two different failures.

If the remaining fraction is odour-dead — water, residual hydrocarbon, anything without an active sulfur function — then the effective odour per kilogram is lower than the dose calculation assumes. The network is injecting to a mass rate and getting less intensity than the rate implies. Nothing alarms; the margin just narrows.

If the remaining fraction is odour-active but chemically different, the problem is the opposite and worse. Intensity may be fine while the character shifts. Gas networks train people, and in some countries the public, to recognise a particular smell. An odorant that smells strongly of something else is still an odorant and is not doing the job it was specified for.

There is a third consequence that has nothing to do with smell. Odorant is a deliberate addition of sulfur to a gas stream that is otherwise required to be low in it, and many networks run a total sulfur limit that the odorisation dose eats into. Purity is part of that budget. A lot that is 99.0% THT and 1% other sulfur compounds is not equivalent to one that is 99.0% THT and 1% water, even if both pass the same line on the same sheet.

Cloud point is a water specification

Cloud point is the temperature at which a clear liquid stops being clear. For a hydrophobic organic liquid stored in outdoor tankage, what generally comes out first is dissolved water: solubility falls as the temperature drops, and the water separates as a haze before it separates as a phase.

That is why the limit is set at −30 °C maximum rather than somewhere near the freezing point of the material. It is not describing how cold THT can get before it stops flowing. It is describing how cold it can get before the water it is carrying comes out of solution — and an odorant injection skid is a piece of outdoor equipment, often in a location chosen for the pipeline rather than for the weather.

Free water in an odorant system does two things, neither of them visible from the gas side. It corrodes, in a system built for a dry organic liquid. And at low enough temperature it freezes, in lines whose internal diameter is small because the injected volume is small.

This is also the line that connects back to appearance. Haze in a drum at ambient temperature is the same observation as a failed cloud point, arriving early and for free at goods-in.

Evaporation residue is a maintenance number

THT is metered as a liquid and has to leave as a vapour. Whatever will not evaporate stays behind.

Evaporation residue at 0.20% maximum measures that fraction directly. Per drum it is a small number. Per year of continuous injection it is not, and it does not distribute itself evenly: it concentrates at the point where liquid odorant meets flowing gas, which is the injection nozzle — the smallest orifice in the system and the one whose partial blockage changes the dose without stopping it.

That failure mode is worth stating plainly, because it is the one that does not announce itself. A blocked nozzle stops injection and gets noticed. A partially fouled nozzle keeps injecting at a lower rate than the setpoint, and the system reports that it is dosing.

THT is not the low-dose option, and that is not why it is chosen

This is where the common description of THT gets it backwards, including in older copy on this site, which is why it is worth being explicit.

THT has a higher odour threshold than mercaptan-based odorants, not a lower one. More of it is needed, by mass, to reach the same detectability. If minimum dose were the deciding criterion, THT would lose.

It is chosen for stability, and the reason is structural. Mercaptans carry an S–H bond and oxidise readily to disulfides. Disulfides are far less odorous than the mercaptans they came from, so the oxidation reaction converts an odorant into something close to an odourless compound — which is the mechanism behind odour fade, the failure in which gas leaves the plant correctly odorised and arrives at the far end of the network smelling of much less. New pipe, oxygen ingress and long residence times all make it worse.

THT is a cyclic sulfide. It has no S–H bond and no comparable oxidation pathway, which is why the site's product page describes it as the most stable gas odorant in current use. It is also less aggressive toward the materials it is stored and injected with than mercaptans are.

So the trade is a real one and it runs in a definite direction: a higher mass dose, in exchange for an odorant that still smells the same at the end of the network as it did at the injection point. A supplier claiming both is describing two different products.

What a certificate of analysis should name

The method, for every line. ISO 13734 covers requirements and test methods for organic sulfur odorants. A certificate reporting values against it should say which methods produced them. One that reports four numbers and a standard number is citing the standard, not applying it.

Whether the values were measured on this lot. Some certificates report the specification limit in the results column. Those are different claims and the difference only surfaces when something goes wrong.

Lot traceability back to a campaign. A certificate tied to a lot number is a different document from one issued against a product code.

Water content, if you can get it. It is not on the four-line specification, and cloud point is a proxy for it rather than a measurement of it. On a delivery going into winter service, the direct number is worth asking for.

Sources. Specification values are those published for the tetrahydrothiophene supplied by Palica Chem, CAS 110-01-0, as shown on /tetrahydrothiophene-tht/; ISO 13734 is cited on that page as the standard governing use conditions. The description of THT as the most stable gas odorant in current use is quoted from the same page. Odour threshold, oxidation and odour-fade behaviour describe general chemistry, not measurements on any particular lot. Dosing rates are set by the network operator against national rules and are not specified here; figures for a specific delivery come from that delivery's certificate of analysis.