Confronted with increasing costs and product integrity issues, the hydrocarbon processing industry has become increasingly dependent on sensors for detection and measurement of impurities in gas streams. For some sensing technologies this can be a hit and miss situation that can compromise process control and result in costly actions including high maintenance costs, unplanned shutdowns and unanticipated shut-ins where the buyer can block the seller from shipping gas.
Gas processing plants require fast and accurate measurements of moisture (H2O), hydrogen sulfide (H2S), and carbon dioxide (CO2) because these contaminants damage expensive equipment such as turbo-machinery and pipelines, shorten desiccant, contactor, or mole sieve lifetimes, and threaten the operator’s ability to maintain tight process control and product quality. In some cases, the gas processor’s downstream customer can shut in their supplier which costs hundreds of thousands of dollars. Tunable Diode Laser (TDL) based analyzers are increasingly being used in these “pain-point” analytical applications to measure contaminants reliably while reducing maintenance and operating costs. TDL absorption spectroscopy employs 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 (Figure 1 and 2).
Dependable H2O Analysis
Impurities such as moisture and corrosive acids found in many gas streams are the nemesis of conventional sensors, which 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 the 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, moisture and H₂S. Furthermore, it accurately takes repeatable measurements within seconds, whereas conventional devices often take many minutes.
Excessive dehydration costs may be incurred because of inaccurate moisture readings or concern on the part of the operator that the gas may be too wet. In many cases, it is necessary to over-process the gas in order to achieve quality specs for multiple customers when taking into consideration the risk of inaccurate measurements and potential false shut-ins. The availability of a fast and reliable measurement allows the plant to confidently deliver gas that is within specifications.
The TDL-based technology is very fast and does not drift, thereby enabling operators to keep dehydration costs to a minimum—without fear of a shut-in. (Figure 3)
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 system 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 a UV photometer for measuring H₂S. Natural gas may have varying percentages of methane, propane and possibly more than a dozen other components. The measurements results of UV photometers are susceptible to gas composition changes. When the background matrix of the gas changes, that 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. Fast analyzers enable better control of processes by letting the processor know instantly how much processing is needed and provides for faster shut-off when the sweetened gas is not up to required standards. Changes in gas concentrations can be seen immediately with TDL analyzers preventing false alarms, false readings and unwarranted shutdowns.
The presence of even trace amounts of H₂O or CO₂ can threaten the integrity of processing equipment during compression and liquefaction of natural gas 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 (liquid natural gas) and NGL (natural gas liquids) gas processors have relied on surface based analyzers such as electrochemical and quartz crystal cells to measure trace amounts of moisture 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 concentrations of moisture, and the tendency for their 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 timeless 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 process industry. However, because the need for improved process control and reduced costs are equally if not even greater among processors, there has been immediate interest in applying this technology throughout the petrochemical industry. Thousands of TDL analyzers are now installed at operations from off shore production to processing and refining, to chemical plants and emissions monitoring.
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