The Risks of Moisture in Sour Gas and How to Eliminate Them

Measuring moisture to reduce operational and safety risks in sour gas processing

Sour gas is the term applied to natural gas in gas fields that contains high levels of hydrogen sulfide (H₂S) – typically above 4 ppmv at standard temperature and pressure – or a combination of hydrogen sulfide and carbon dioxide (CO2); gas with a predominance of CO2 is commonly referred to as ‘acid gas’.  Where it is commercially viable, sour and acid gases can be extracted and sweetened using a process such as amine gas treatment to remove the unwanted contaminants and produce pipeline quality or sweet gas, suitable for use as a fuel or in power generation. 

However, sour gas can also be a valuable feedstock for the production of sulfuric acid and elemental sulphur; these substances are widely used in the manufacture of fertilizers, detergents, dyes and other chemical compounds. 

Regardless of application, it is critical for process efficiency, quality and safety that the moisture content of the sour gas is measured accurately. 

Why moisture creates a risk in sour gas processing 

The presence of moisture in sour gas can create several problems.  It can react with hydrogen sulfide to produce sulfuric acid, which is highly corrosive.  Similarly, carbon dioxide condensing with water vapor on metal surfaces will form a corrosive environment.  In each case, corrosion will damage pipelines, equipment and infrastructure, potentially leading to gas leaks, failures and safety hazards.

Carbon dioxide in the presence of moisture at certain temperatures and pressures can also contribute to the formation of hydrates or solid crystalline compounds.  These can potentially restrict or block pipework, valves or other processing systems.   

Moisture in processed sour gas, or sweet gas, will present the same problems as those described above.  It should be noted, however, that the amount of moisture required to reach saturation water vapor pressure in gases that are rich in hydrogen sulfide and carbon dioxide is considerably higher than that for moisture in methane or a sweet natural gas at the same temperature.  As such, the water dew point measured in sour gas, irrespective of the measurement principle applied, will be significantly lower than for a sweet gas with the same moisture content. 

The moisture content of sweet gas can be affected by the treatment processes being used.  For example, amine treatment to selectively absorb acid gases uses aqueous solvents such as DEA (Diethanolamine), MDA (Monoethanolamine) and MDEA (methyl diethanolamine).  These can also absorb some of the moisture present in the gas flow, which even after regeneration can adversely affect the long-term efficacy of the solvents. 

Reducing the moisture content of processed gas to a predetermined level is also critical to ensure that the gas meets quality and custody transfer specification, while effective monitoring of moisture levels throughout the sour gas and sweet gas processing, transport and distribution network is a key factor in enabling processors and transport operators to optimize production efficiencies and control costs. 

How to measure moisture content in sour gas processing

To measure the moisture content in sour gas, various instruments are employed, such as moisture analyzers, dew-point analyzers and hygrometers. These devices use different principles, such as absorption, condensation or electrical properties, to determine the amount of moisture present in the gas.  

One of the most widely used instruments is based on tunable diode laser absorption spectroscopy (TDLAS), such as the Michell OptiPEAK TDL600.   

In simple terms, this works by using an infra-red laser tuned to the absorption characteristics of the gas being measured.  This is focused through a gas sample, where the interaction between photons of light and gas and moisture molecules causes the latter to absorb light at specific colors, or absorption lines.  The intensity of light that passes through the gas is then measured by a photo detector.  By scanning the wavelength of the laser across a range of specific wavelengths, it is possible to create an absorption spectrum, which shows the characteristics of the target gas species, allowing them to be identified and quantified.  This can all be achieved rapidly and extremely accurately.   

For more information about TDLAS, see our two-part blog series which gives a useful introduction to this technology.

One of the challenges of using TDLAS when measuring the volume of water molecules in a sour gas sample is the uncertainty associated with the spectra of hydrogen sulfide in the near infrared region, making precise measurements difficult to achieve.  In addition, if there are relatively high concentrations of carbon dioxide – typically between 3% and 15% – then the absorption spectra of the gas and water partially overlap, with the carbon dioxide exerting a pressure-like effect, which suppresses the height of the absorption peak and broadens the peak width. 

In both cases, the solution is to apply advanced software algorithms that automatically compensate for the effects of each gas, to produce consistent and accurate results. 

One of the advantages of TDLAS is that it is a non-contact technology.  In some sour gas processing applications, however, our ceramic metal-oxide sensors, such as those used in the Michell Promet EExD moisture analyzers for sour gas processing, provide an effective option.   

These devices are not affected by cross-sensitivity with hydrogen sulfide or carbon dioxide and are capable of providing extremely accurate and consistent results, especially in hazardous areas.  Although the sensor interface is affected over time by contact with acidic gases, this can easily be overcome by increasing the frequency of recalibration – something that is made even easier by using our sensor exchange program.  

Related Information

Petrochemical Applications 

Related Blogs

What is Tunable Diode Laser Absorption Spectroscopy (TDLAS)? Part One

What is Tunable Diode Laser Absorption Spectroscopy (TDLAS)? Part Two

Precise Measurement of Moisture in Natural Gas

How to Make the Best Choice of Moisture Sensor for your Natural Gas Process

Related Categories

Trace Moisture Analyzers for Natural Gas Quality and Petrochemical Applications