Tunable diode laser (TDL)-based analyzers (Fig. 1) are increasingly being used in these “pain-point” analytical applications to measure contaminants reliably while reducing maintenance and operating costs.
TDL absorption spectroscopy uses a laser mounted behind a window that protects it from the wear and tear of caustic gas contents while enabling the analyzer to accurately and quickly read varying gas concentrations. The laser does not come into contact with the gas and calibration does not change or drift over time.
Dependable H2O analysis.
Impurities, such as H2O and corrosive acids found in many gas streams, are the nemesis of conventional sensors that are directly exposed to the gas stream and those harmful elements. Over time—and sometimes within a few weeks—those probes become damaged and inaccurate, resulting in costly repair or replacement. In the meantime, corrosion and contamination from problematic impurities can damage equipment, catalysts, desiccants and processes.
The TDL-based analyzer allows gas processors, whether they are sweetening, dehydrating or removing various impurities, to monitor and measure the inputs and outputs of their processes for CO2, acids, H2O and H2S. Additionally, it accurately takes repeatable measurements within seconds, whereas conventional devices often take minutes.
Excessive dehydration costs may be incurred due to inaccurate H2O readings or concerns that the gas may be too wet. In many cases, it is necessary to over-process the gas in order to achieve quality specifications for multiple customers when taking into consideration the risk of inaccurate measurements and potential false shut-ins.
Fast and reliable measurement availability allows the plant to confidently deliver gas that is within specifications. TDL-based technology is very fast and does not drift, thus enabling operators to keep dehydration costs to a minimum—without fear of a shut-in.
Accurate H2S sweetening.
For monitoring and measuring H2S in gas streams, lead acetate tape analyzers are frequently the technology of choice. In that design, gas is sent directly through the lead acetate tape, which changes color when reacting with sulfur. The operator then analyzes the color change with a photometer. However, the lead acetate tape is a consumable that is directly exposed to the gas stream. Tape reels must be changed periodically (weekly or monthly), and if there is a system leak, it exposes the whole tape. Also, the used tape is considered hazardous waste and must be disposed of accordingly.
Some processors use an ultraviolet (UV) photometer for measuring H2S. Natural gas may have varying percentages of methane, propane and possibly more than a dozen other components. UV photometer measurement results are susceptible to gas composition changes. When the gas background matrix changes, it often causes erroneous readings and consequential problems.
For H2S applications, the TDL-based analyzer measures sour gas going into processing, and sweetened gas coming out. Faster analyzers improve processes control by letting operations know instantly how much processing is needed and providing for faster shutoff when the sweetened gas does not meet specifications. Changes in gas concentrations can be seen immediately, with TDL analyzers preventing false alarms, inaccurate readings and unwarranted shutdowns.
Liquefied natural gas (LNG) applications.
Even trace amounts of H2O or CO2 can threaten processing equipment integrity during natural gas compression and liquefaction due to ice formation. It is essential to have very fast H2O or CO2 detection in order to improve the life of the desiccant while retaining product quality.
In the past, LNG and natural gas liquids processors have relied on surface-based analyzers such as electrochemical and quartz crystal cells, to measure trace H2O in process streams. Although these devices may perform with acceptable accuracy at first, the confidence level in their measurements soon becomes low, due to drift, the inability to read high H2O concentrations and the tendency for the sensor probes to become desensitized by the gas streams they are measuring. Such situations have led to excessive maintenance and high operational costs.
The TDL-based gas detection method does not experience aging effects, making its factory calibration a constant. The result is a very low-maintenance analyzer that does not require consumables or scheduled calibration.
Emphasis on process industries.
Although TDL-based analyzers have been widely accepted by the natural gas production and pipeline transmission industry, this technology is fairly new to the processing industry. Because the need for improved process control and reduced costs is equally in demand among processors, there has been immediate interest in applying this technology throughout the petrochemical industry. Thousands of TDL analyzers are now installed from offshore production to processing and refining, chemical plants and emissions monitoring.