Production of liquefied natural gas (LNG) requires the gas stream to be purified first, hence numerous sweetening processes have been developed to remove carbon dioxide (CO2), hydrogen sulphide (H2S), carbonyl sulphide (COS) and mercaptans (thiols). One of the most widely used processes for feedstocks with a high acid loading is Sulfinol, developed by Shell Research, with around 200 units licensed around the world since the 1960s.

Within the Sulfinol process, CO2 and H2S are removed using an absorption medium called Sulfinol, which is an organic solvent mixture made up of sulfolane (tetrahydrothiophene) and alkanolamines in an aqueous mixture.

Feedstock

The feedstock is subjected to absorption, regeneration, heat exchange and recycling, and simultaneous physical and chemical absorption of the acid gases occurs under conditions of elevated pressure and moderate pressure.

This reduces the water content of the feedstock – and the water content after processing can be used as an indication of the efficiency of the process.

Traditionally these moisture measurements have been made off-line using laboratory gas chromatography techniques, but this has several drawbacks.

For example, it results in a significant delay between the sample being taken and the results being available – which is potentially costly if the measurement indicates that the Sulfinol process is not operating efficiently.

Furthermore, the sampling and testing regime is time-consuming, which, itself, is inefficient. Another point to note is that if the Sulfinol process is not optimised, foaming can occur, leading to additional problems.

Online measurements, on the other hand, enable the process to be controlled in

near-real-time, leading to improved efficiency. Moreover, if the measurements are fed back into an automatic control system, it reduces the number of tasks that have to be performed by the human operator.

Online monitoring

Servomex has developed just such an online system for continuously monitoring the water in Sulfinol. This comprises a sampling/conditioning system and a Servomex2500 infrared gas analyser (Fig.1).

The customer for whom the system was developed has confirmed that the Sulfinol process now runs more efficiently, with consequent cost savings. The incidence of Sulfinol foaming has also been reduced.

For many other water measurements, the Servomex2500 analyser can measure from trace levels (0–50ppm by weight) to 0–5percent by weight with what the company describes as excellent reliability and long-term stability.

The analyser achieves this high performance by using single beam dual wavelength (SBDW) infrared photometry with a single optical path for the infrared light beam and a robust solid-state pyroelectric detector.

A motorised chopper wheel alternately places a measure filter and a reference filter in the optical path.

The reference filter provides compensation against measurement errors due to build up of dirt on the optics or reduction in light output from the infrared light source.

This is all proven technology that is operating reliably in numerous process plants around the world in process control, plant safety and legislative compliance applications.

Shell’s Sulfinol process is not the only gas sweetening process used in LNG production. Others include aMDEA and ADEG from BASF, DGA from Huntsman Corporation, Gas/Spec from INEOS, Amine Guard from UOP and Ucarsol from Union Carbide and the Dow Chemical Company.

All of these can benefit from continuous online analysis in a similar way to the Sulfinol process.

The monetary savings available will depend on a number of factors, including which process is being employed, how efficiently the process operates without the online measurement, the capacity of the plant, the level of impurities in the feed gas and the specification of the purified gas.

Nevertheless, says Servomex, it is almost certain that plant operators will find the benefits provided by online continuous analysis will enable payback to be achieved, making this upgrade an extremely attractive one.

Although the level of water in Sulfinol gives an indication of the process efficiency, it is also important to measure the CO2 in the gas either side of the scrubber.

Once again, infrared photometry offers a combination of rapid response, reliability, accuracy, stability and good rejection of cross-interfering compounds.

The Servomex2500 gas analyser is suitable for this application, but the company has recently launched a more cost-effective infrared instrument that is capable of measuring CO2 in natural gas.

Known as the Servomex1900IR, the new instrument is offered in a number of variants, including models with CO2 measurement ranges of 0–25percent and 0–50percent suitable for monitoring the feedstock gas and ranges of 0–1percent, 0–2.5percent, 0–5percent and 0–10percent for use downstream of the scrubber.

All CO2 ranges from 0–1percent up to 0–10percent are available in solvent resistant versions if required. This instrument is intrinsically safe, making it suitable for use with flammable samples in Zone1 hazardous areas (ATEX Category2) without the need for a purge.

Not only does this make the unit simpler and less costly to install, but it makes a significant difference to the cost of ownership over extended periods. In addition, the analyser’s casing is divided into two compartments, which enables the sample side compartment to be approved as intrinsically safe sample: significantly reducing the hazards present when working using a ‘hot work permit’.

On the other side the flameproof/explosion-proof terminals compartment enables the instrument to be located in hazardous areas without the need for a purge.

According to Servomex, the 1900IR is the first gas analyser to incorporate an intrinsically safe infrared transducer.

Custom systems

As with the Servomex2500 for monitoring water in Sulfinol, Servomex can design, manufacture and supply custom sampling and conditioning systems to complement the Servomex1900IR gas analyser, so customers can procure a complete system from one source, which certainly has its advantages.

Monitoring the CO2 either side of the scrubber is essential if the process is to be fully visible and efficiently controlled.

Specifications for natural gas processing are extremely tight, so producers need to be certain that they are fulfilling the specification while operating the plant as efficiently as possible.

Gas analysers for this application need to be capable of delivering the performance levels required, but plant operators need to be aware that the cost of ownership can vary significantly from one type of analyser to another, so choosing the correct model is vital.