The determination of moisture in natural gas and its products is often required at various stages of the production, processing, storage and transportation.
Typically water is separated from raw natural gas in the early stages of production and processing. However some water will remain present in the gas fraction as water vapour. If the gas is then cooled or comes in contact with a surface that is colder than the prevailing water dew point temperature of the gas, water will condense in the form of liquid or ice if cooled sufficiently. Under pressure, water also has the unique property of being able to form what are known as solid hydrates, these are lattice structures around hydrocarbons. Ice or solid hydrates can cause blockage in pipelines.
In addition, water combines with gases such as hydrogen sulphide (H2S) and carbon dioxide (CO2) to form corrosive acids. Water in natural gas also increases the cost of transportation in pipelines by adding unnecessary mass and as water vapour has no calorific or heating value it also adds to the expense of compression and transportation.
The first stage of dehydration of natural gas is often accomplished by passing the gas through Tri-ethylene Glycol (TEG). TEG is very effective at removing water but can introduce a trace of TEG residue into the natural gas. If the gas is to be processed cryogenically (for LPG or LNG production), then dehydration down to very low concentrations is required and for this, molecular sieves are most commonly employed. The drying process has a significantly high cost associated with it, therefore it is important to dry the gas but not to expend excessive resources to over dry it. Thus, it becomes important to perform moisture content measurements on the natural gas after it has passed thought the dehydration equipment. Insufficient dehydration may lead to processing issues whilst over dehydration is a waste of resources.
When LNG is “re-gassed” at a receiving facility, it is passed through heat exchangers which can be prone to leakage and may introduce unwanted moisture into dry gas. Therefore it is often a requirement to monitor the moisture content after the heat exchangers to confirm the gas is still dry.
There have been many documented cases where insufficient dehydration has led to processing issues which have ultimately resulted in plant shutdown.
Qa3 determine the moisture content of gas by at-line electronic moisture analyser employing a silicon capacitance type cell, in accordance with ASTM D5454-11; using a methane gas standard with a certified water content of 20 ppm v/v as a quality control (QC) in order to validate the calibration before and after each analyses.