Natural gas is the answer to much of the nation’s energy needs, according to many analysts. Widespread availability of shale gas in the U.S. and resulting low costs are the main reasons.

However, before raw natural gas is suitable for distribution to gas customers, contaminants such as moisture and hydrogen sulfide must be removed. Additionally, NGL are recovered for use by petrochemical plants and others.

That processing involves many steps, including cryogenic fractionation, which is used to separate the various hydrocarbons. This cryogenic process demands bone-dry gas, with moisture levels below 100 parts per billion. To remove moisture down to that level, processing plants pass the gas through vessels containing molecular sieve desiccants that selectively adsorb the water. When the desiccants become saturated, they are regenerated with hot gas, which drives the water off so the molecular sieve is back in its original state and the adsorption process can begin again.

“Cryogenic plants are allergic to moisture, we have to keep it out at all cost,” Charles Bates, plant supervisor at an Okarche, Okla., facility for DCP Midstream, told Midstream Business. “When moisture is not managed properly, it causes loss of efficiency and earnings.”

Proper operation of the molecular sieve dehydration system is therefore critical to the operation of any gas processing or LNG plant. In theory, it all works fine. But the whole process hinges on one key element: the analyzer that tests the desiccant dryer outlet gas for moisture content. If a bad measurement is made there, it can become a major economic blow or even shut the plant down. And there are lots of ways to make poor operational choices with bad information.

As a result, plant managers are turning to tunable diode laser (TDL) analyzers to take those mistakes out of the process.

Maintenance methods

In the past, gas processors turned to electrochemical probes such as aluminum oxide capacitance or quartz crystal microbalance (QCM, or vibrating quartz crystal), to measure H2O in the desiccant dryer outlets. But these always came with inherent problems and costs.

For instance, trace hydrocarbons and other impurities (methanol, glycols and amines) present in the stream can easily contaminate the measurement sensor, resulting in drift and ultimate loss of analyzer response. Plant operators had three solutions—maintenance, maintenance and more maintenance driving their cost of ownership higher and higher and keeping their staff busier than necessary. Meanwhile, these operators were left operating without meaningful data.

QCMs also suffer from sensitivity to vapor impurities, resulting in false readings. And both types of probes are slow in reporting data; they simply need too much time to react. That can translate into wet gas passing undetected for an hour or more.

All of these drawbacks have led many plant managers to turn to TDL analyzers as the answer for fast, accurate readings with virtually no maintenance costs. SpectraSensors Inc., based in Houston, is the world’s leading provider of extractive TDL analyzers with more than 90% of the installed units.

Moisture on Mars

NASA scientists developed tunable diode lasers in the 1990s to look for moisture on Mars. SpectraSensors became the first company to develop and manufacture online TDL analyzers based on that technology for use in hydrocarbon streams. Since a laser beam is used, there is no physical contact between the gas stream and the sensor.

The result is fast, accurate measurement from an analyzer that is nearly maintenance-free for many years of operation. There is a built-in verification system that emits an accurate level of moisture for field validation purposes. Coupled with a high-efficiency dryer, this validation system solves the long-standing problem of a lack of accurate field references for trace moisture analyzers.

“I’ve tried a lot of analyzers over the years, but they never gave me the accuracy or response I had to have. With the SpectraSensors TDL, all those problems go away,” added Bates.

Cost savings

One of the first cost savings realized at the processing plant involves desiccant drying regeneration. Since the TDL analyzer provides near-instant and accurate information, plants have been able to extend time between regenerations by up to 20%. Fewer regeneration cycles means less thermal cycling, less thermal damage to the sieve and longer sieve life.

If a plant can eliminate even one extra recharge in 10 years, the savings could add up to $100,000 in sieve replacement costs. And since the entire system can be down for two days during each reload, the cost for that reload could reach $400,000 in lost revenue at today’s gas prices. For larger processing plants, it could be much greater. There is also the added expense of loading contractors.

“I really like that it does self-validation,” said Bates. “I am confident enough to do my own breakthrough test without an on-site rep from the sieve company. Since I had multiple beds each with a TDL on the outlet, I could be testing one while the other was in regeneration. That meant the plant was always in operation, you don’t make money if it isn’t running.”

Knowing more about how the desiccant dryers are running means operators can avoid upsets and keep producing product.

Another plant manager who turned to this technology said that TDL analyzers are paying for themselves and they have “definitely prevented a freeze-up or two and allowed us to see how much water the beds are seeing. I calculated the beds were only adsorbing 25% of their design capacity with the old sensors.”

From then on, he lengthened his cycle times, extending the time until his next sieve recharge.

Speed of light

Because of the physical property changes that are required for other technologies to do their measurements, “electrochemical" sensors can take up to an hour or more to report changes in the moisture level. Instead, TDL analyzers measure the absorption of light. TDL analyzers report a fresh reading every 16 seconds.

A TDL analyzer also has the advantage of reducing the potential for measurement interference. Because there is no contact with the gas, there is nothing to get saturated and fouled as happens with traditional electrochemical sensors. This also means with TDL measurement, there is very little scheduled maintenance.

The verification system on the TDL is designed for more than five years of service before the dryer and moisture emitter needs to be replaced, which is a very simple procedure. However, aluminum oxide sensors, vibrating quartz crystal sensors and other methods require regular recalibration and replacement of the measurement cells due to their sensitivity to the harsh environment.

The reason is simple: these sensors have to come in direct contact with the gas. The only alternative with the older units is regular maintenance—and the associated manpower costs.

Daily validation is an important aspect of the measurement because it helps operators confirm the analyzer’s performance on a regular basis. The consumables associated with the validation systems are minimal at most.

“The only complaint I’ve heard,” Greg Lankford, SpectraSensors’ business development director, told Midstream Business, “is that plant managers say it’s so long between times that anyone checks the analyzer that when we decide to do it, we have to go back to the manual … nobody remembers how.”

If there are other types of chemicals present in the process, especially methanol or glycol, traditional measurement techniques can mistakenly read these as moisture and might prompt unnecessary process changes or plant shut-in. Quartz crystal analyzers are particularly susceptible to this. TDL also comes with superior sensitivity and the ability to measure moisture in the parts per billion levels accurately.

For gas processing plants, the result is TDL analyzers are offering a host of advantages including new advancements in speed, accuracy and reliability.

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