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PinPoint Precision - Sam Miller, SpectraSensors Inc., USA, examines how a TDL based analyser pinpoints moisture intrusion and trace H2S levels in refinery operations.

 

The emergence of tunable diode laser (TDL) based technologies has been a major development in many industries, and has created valuable opportunities for refineries and chemical plants to maximise productivity while avoiding high maintenance costs.

 

By instantly and accurately pinpointing the presence of moisture, HCl, H2S and other impurities in feedstock and fuel gas streams, the TDL based gas analyser is able to provide significant benefits. Among those are the prevention of foreshortened lifespan of costly catalysts, enhanced assurance of product quality, reduced turnaround recovery times, and avoidance of unscheduled shutdowns.

 

What is a TDL?

A tunable diode laser is a type of semiconductor based laser that can be tuned to optically select a very specific wavelength (or colour) of light. NASA’s jet propulsion laboratory (JPL) developed long wavelength (near infrared) tuneable lasers and processor controls that enable users to precisely ‘target’ specific modules and detect trace elements in gas. This spectroscopy technology was vital to research and operations in the Space Station and Mars Polar Lander programs as well as studies on global warming, emissions, weather and climates throughout the world.

 

TDLs emit near infrared light at wavelengths that can be absorbed by the gas being measured (e.g. CO2, H2S, H2O, HCl). The laser’s light passes though the gas sample, and then is measured by a solid state detector. As the laser’s wavelength is swept or ‘tuned’ across a specific wavelength, energy is absorbed, reducing the amount of light arriving at the detector at that specific wavelength only. The result is a sharp peak shaped curve (Figure 1). The fraction of light absorbed (the peak height) is directly proportional to the concentration of the associated target gas.

 

As the laser is tuned across the line multiple times per second, it can be used for fast, accurate measurements and control of process parameters. TDL gas analysers have the ability to react quickly to upset conditions without the risk of false alarms.

 

Design

The TDL based spectrometer’s optical design is a critical factor in terms of accuracy, repeatability and durability. The nature of the optical design developed by SpectraSensors is such that the sample flows though the chamber and the sensor is isolated behind a window, rather than in the stream of gas. By doing so, the accuracy of the analyser is not affected by the gas or impurities, which are harsh enough to debilitate the accuracy and service life of conventional electromechanical analysers.

 

For example, the presence of moisture or hydrogen chloride in a reformer of a refinery can ‘poison’ the catalysts. Undetected and untreated moisture slugs can result in the premature degradation of catalysts, costly unscheduled process downtime or possibly requiring replacement of this exceedingly expensive material. Slugs of moisture can render conventional electrochemical sensors useless for a period due to a required ‘dry down’ time. Such an event makes those sensors inoperative at a time when they are needed most.

 

For refineries, the ultimate danger of dry down lapses is not only potential damage to the reactor catalyst, but also that ‘off spec’ materials might be inadvertently produced, both of which could cost a refinery hundreds of thousands of dollars or more.

 

In the same situation the TDL based analyser is not affected by the moisture slug, and has zero dry down time. Rather, this technology utilises absorption spectroscopy that safely measures moisture from behind the non wetted side of an optical window.

 

H2S measurement

For H2S measurement, the hazardous waste, costs, and maintenance issues associated with ‘traditional’ analysers, such as lead, acetate, tape systems, UV analysers, chromatographs and other technologies, are also now being avoided via the TDL based H2S analyser. One of the major benefits of the TDL based analyser for this application is the avoidance of hazardous waste, a major drawback of lead acetate tape systems. Lead acetate tape is a consumable that must be changed weekly or monthly. Once used, the tape must be disposed of in a manner that is both costly and inconvenient. If there is a system leak, the entire tape is exposed, causing considerable added expense.

 

UV analysers used for H2S measurement require light source replacement, and are often ‘confused’ by interfering background gas components. Fuel gases have varying percentages of H2S as well as other gas components.

UV photometer measurement results are susceptible to changes in composition: that is, when the fuel gas background matrix changes, erroneous readings, and consequential problems, often occur.

 

The other traditional H2S measurement technology, gas chromatography, is a highly complex system and requires trained experts to operate. Those factors are contrary to the trend of refineries and chemical plants, which is to reduce required manpower for maintenance through automation. Here again the TDL based analyser measures both sweet and sour gas though a window outside the stream, avoiding contact with caustic gases. The changing gas matrix does not confuse the tuneable laser.

 

For H2S analysis, the TDL based analyser measures sour gas headed for processing and sweetened gas coming out. Because it provides faster readings, process control is improved by instantly letting operators know how much processing is needed. Also, when exit gases do not meet specifications, shutoff can be performed immediately, while false alarms and unwarranted shutdowns can be virtually eliminated.

 

In addition to enhanced accuracy and process control, the TDL based H2S analyser offers consistent repeatability as well as low operational costs. This greatly reduces the cost of ownership and virtually eliminates technician support while providing accurate and stable calibration throughout the life of the analyser. Each SpectraSensors unit is preset at the factory, so it is truly ‘plug and play’ and requires little training.

 

While initial purchase price is somewhat higher, the reduced operating costs of this technology are likely to overcome this initial investment quickly. These devices can run for years without requiring maintenance, recalibration or replacement. Therefore, costs such as analyser technicians, repair or replacement of probes, as well as an inventory of spare sensor heads are eliminated.

 

The TDL based gas analyser has become the de facto standard in the natural gas industry, where producers, processors and pipeline operators have been upgrading to this technology for several years.

Jennifer Soto
Author: Jennifer Soto
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Category: tdl technology