MCM’s heated Silicon Sensor Moisture Analyser against Aluminium Oxide and Oscillating Crystal Moisture Analysers, during Molecular Sieve Test Runs at St Fergus, Scotland


Courtesy of Moisture Control & Measurement Ltd

Methods for dehydrating natural gas using molecular sieves (mol sieve) are well established. Many strategies have been developed with the intention of extending mol sieve lifetime and preventing unplanned shut-downs of dehydration units. The amount of adsorbent required in such dehydration processes is determined by the water content in the feed, the end-of-run (EOR) water adsorption capacity of the mol sieve inventory and the minimum time required for regeneration of the mol sieve bed. When the capacity of the adsorbent falls below the level where all water in the feed can be adsorbed during the minimum adsorption time, then the adsorbent must be replaced.

Improving the speed of response and accuracy of moisture measurement within dehydration processes provides significant commercial benefit to production:

  • In cases where feed gas is unsaturated, the use of the adsorbent in gas plants can be maximised by improving the measuring accuracy of the feed inlet moisture. This helps prevent unplanned costly shut-downs, which in many cases can exceed the cost of mol sieve inventory.
  • Detecting moisture breakthrough on the beds as soon as possible reduces operating costs by optimising changeover and the regeneration schedule of those beds.

LNG plants often employ Aluminium Oxide probes on the mol sieve dryers, usually located both on the feed inlet and lower mol sieve beds. For a variety of reasons, these analysers suffer from drift, insensitivity and slow response (see Reference 9.1, pp17-20; T.K. Mehrhoff, General Electric Company - Comparison of Moisture Analysers at Concentrations 1 to 15 ppm).

Aluminium Oxide analysers are simple devices that provide no mechanism to automatically correct for drift. As a result these instruments produce unreliable data, often long before scheduled servicing is called up (see Reference 9.2, pp21-26; Saburo Hasegawa, National Bureau of Standards - Performance Characteristics of a Thin-Film Aluminium Oxide Humidity Sensor).

In order to boost confidence in the collected data, many production plants protect against the deficiencies of the Aluminium Oxide technology by also utilising an Oscillating Crystal analyser which is generally considered to be more sensitive and responsive. Often, the Oscillating Crystal instrument is positioned at the common outlet of the mol sieve dryer. However, a 'dual technology' approach such as this costs more to operate and support. If a faster, more accurate and reliable analyser could be identified, and the support overhead involved with dual technologies could be reduced to a single, more reliable system, then long-term cost savings could be achieved through common spares, training and validation (see Reference 9.3, pp27-28; SEIC Terms of Reference {Rev 1 - March 13th, 2008).

Shell's internal procedures already state that Silicon sensors are preferred over Aluminium Oxide technology (see Reference 9.4, pp29-31; Design and Engineering Practice Document DEP - On-Line Process Stream Analysis - Analysers). The goal of the evaluation was to confirm that MCM's heated Silicon Sensor was:

  • Inherently faster and more accurate than either of the currently installed systems.
  • Of commercial benefit to production plants using such dehydration processes.

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