Moisture Measurement for Natural Gas Liquid Extraction

Natural Gas Liquids Extraction Plant

Plant Protection, Energy Efficiency and Cost Savings

The separation of Natural Gas Liquids (NGL) is an important stage in the purification of natural gas, before it can be considered fit for use as a dry pipeline-quality product. The removal of NGL generally uses a complex fractionation train, which demands precise control of process conditions; in turn, this requires the use of extremely sensitive monitoring instruments, with trace moisture analyzers, in particular, being essential for the reliability and energy efficiency of key production systems. 

The NGL process train 

Natural Gas Liquids (NGL) occur as a low-density mix of gaseous hydrocarbons in raw natural gas extracted from gas fields. These hydrocarbons are separated as liquid components during initial gas processing and are normally removed as a combined process stream, generally called Y-grade NGL. This is then processed via a fractionation plant to separate the stream into ethane, liquefied petroleum gases (propane and butanes) and natural gasoline. 

A typical process flow starts with the raw wellhead gas passing through filter coalescers; these operate at normal process temperature to reduce the content of bulk entrained hydrocarbon condensates and water.

The resulting moisture-saturated gas/liquid mixture is then dehydrated through a glycol contactor (glycol dehydration unit), where liquid tri-ethylene glycol (TEG) is spray injected as desiccant into the gas flow rising through the process column. 

At this stage the gas is dry, with a water dew point between -30 and -10 ˚C at process pressure, but it is still laden with liquid hydrocarbons. Joule-Thomson cooling is therefore used as the next step in NGL extraction, with a partial pressure reduction of the gas stream by 8-10 bars producing a 7-8 ˚C drop in temperature at the inlet of a liquid separator. 

A series of molecular sieve dehydration columns is then used, to reduce the process gas moisture content by between 0.01 and 0.05 ppmV, at less than -80 ˚C dew point at process pressure; this is essential for reliable operation of the primary extraction method for lighter hydrocarbon liquids. 

The process stream next passes through cryogenic turbo-expanders, which extract kinetic energy (work) from the gas flow by expanding it from 55 to 17 bar in a near isentropic (constant entropy) process. A sequence of interlinking heat exchangers lowers the temperature of the process stream to below -80 ˚C, to liquify the non-methane content. The resultant cold gas/liquid mixed phase fluid enters a de-ethanizer column, which separates the two phases into NGL and residual gas. 

NGL process train
Caption: NGL process train

The NGL is subsequently delivered to fractionation plants for separation into individual high-purity hydrocarbon components, while the residual gas (comprising >98 % methane) is compressed for pipeline transmission as industrial and domestic fuel. 

The importance of trace moisture measurement 

Accurate trace moisture measurement is a critical part of the monitoring and control process of NGL separation.

In particular, reliable operation of the cryogenic turbo-expanders depends on the natural gas feedstock from the molecular sieve dehydration columns having a consistently low trace moisture content. This is essential to prevent the rapid formation of ice crystals that would damage the high-speed rotating variable inlet guide vanes in the turbo-expander. An alarm limit of 0.1 ppmV is typical; to minimize risk, the acceptable level of trace moisture is generally set between 0.02 and 0.05 ppmV under normal operating conditions.  

Trace moisture measurement is also vital for the optimal performance of the molecular sieve dehydration system. This has multiple dehydration columns that function simultaneously, with columns being regenerated in sequence by back-flushing dry purge gas across the desiccant bed at 300 ˚C. In most installations, this uses considerable levels of energy and, to ensure continuous delivery of dry gas to the turbo-expanders, is set to operate in what might be called an extremely cautious configuration. For example, it’s not uncommon for regeneration to take up to 12 hours, including cool-down, to release the adsorbed moisture. 

By introducing an advanced in-line moisture measurement analyzer, capable of providing precise results almost in real-time, it becomes possible to optimize regeneration cycles. In practice, this means extending the period between cycles, allowing each column to remain on-line for longer – reducing the proportion of overall process time that each column is being regenerated – without affecting the quality of the feedstock gas or the safety and reliability of the cryogenic turbo-extractors. Based on our experience with a number of customers in the natural gas sector, it is possible to increase the productivity of each column by up to 100 %, while achieving considerable reductions in energy costs. 

Precise trace moisture monitoring also allows detailed performance profiles to be created for each dryer column. The quality of this data gives many plant operators the confidence to extend the functional lifetime of desiccant materials, often well beyond scheduled replacement dates.  This has a direct benefit in terms of plant availability and reduced operating costs. 

Trace moisture measurement systems

At Michell Instruments we have many years of experience designing and manufacturing advanced in-line trace moisture measurement systems, for use in NGL molecular sieve dehydration and other trace moisture measurement requirements in LNG processes. 

Our systems are custom engineered and include sophisticated analysis instruments, based on fast-response QCM (quartz crystal microbalance) technology, plus all ancillary equipment and fittings. These systems are capable of providing near real-time measurement of ultra-low trace moisture content down to 0.01 ppmV. 

To learn more about our solutions for natural gas processing, click here

With over 40 years of experience in the development of innovative precision instruments, we are the experts in trace moisture measurements for all Natural Gas Liquids applications. If you would like to discuss your requirements, then please contact our team today.

Found this article interesting? Check out our blog on Precise Measurement of Moisture in Natural Gas

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