Water Measurements in Desiccant Dryer Outlets

Application Note

 Desiccant Dryers
Molecular Sieves, Activated Alumina and Silica Gel desiccants are common drying techniques for Natural Gas Processing plants. Typically, two or three desiccant vessels are operated in parallel, using a piping system that allows a bed to be removed from the process for regeneration. As a bed becomes saturated, it is put into the regeneration circuit where hot, dry gas drives the absorbed moisture out. The freshly regenerated bed is then ready for service.

 Critical Control of Moisture
Normally, when the inlet gas moisture concentration is constant or can be measured, the desiccant dryers have a predictable operational period between regenerations. Due to desiccant aging, or overheating, or contamination in the circuit, the desiccant can lose capacity. Desiccant beds can also channel, allowing small concentrations of wet gas to flow through the bed without being dried. Regardless, the effectiveness of the drying process must be confirmed to avoid potential problems in downstream processes.
 
Traditional Measurements Solution
Electrochemical and vibrating quartz crystal moisture analyzers have been the traditional method for monitoring the levels of H2O in Desiccant Dryer Outlets Unfortunately, trace hydrocarbons and other contaminants present in the stream contaminate the electrochemical probes, resulting in high maintenance costs due to the need to be constantly replaced. It’s not uncommon for moisture probes to be replaced multiple times a year. Quartz crystal moisture meters have slow wet-up and dry-down times that allow water downstream and prolong regeneration times.

 SpectraSensors' Solution
SpectraSensors TDL analyzers are the ideal solution for this challenging application. The non-contact laser and detector is impervious to damage from contaminants. Tunable Diode Laser technology means that measurement interferences from other infrared absorbing compounds are avoided. There are no wet-up or dry-down delays, resulting in fast updates even when the concentration changes dramatically. Only SpectraSensors employs Differential Spectroscopy, which incorporates a dryer using metal getter technology to subtract the spectrum of the dry gas from the wet spectrum. This allows the analyzer to measure streams whose background gas compositions may change.

 Validation
The pre-calibrated analyzer provides robust sub-ppm measurements of H2O certified across the full range of applicable natural gas stream compositions. With the TDL technology, the calibration accuracy does not change over time. Periodic validation with either an internal permeation system or an external cylinder gas verifies that the factory calibration is maintained.

Application Data

Target Components	H2O in Desiccant Dryer Outlet
Typical Measurement Ranges	0 – 10ppm* (other ranges available by request)
Typical Precision	±0.05 ppm or 2% of Full Scale* (whichever is greater)
Measurement Response Time	1 to ~60 sec
Principle of Measurement	Differential Tunable Diode Laser Absorption Spectroscopy (H2O Scrubber included)
Validation	Integrated Permeation System - recommended Cylinder Gas - optional

Typical Background Stream Composition

Component	Minimum (Mole%)	Typical (Mole%)	Maximum (Mole%)
Water	0	<1 ppm	10 ppm
Nitrogen	0	0.1	3
Oxygen	0	0	1
Methane	75	95	100
Carbon Dioxide	0	0	3
Ethane	0	3	10
Propane	0	1	5
Butanes	0	0.5	2
C5+	0	0.4	0.5

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