Advancements in Trace Moisture Measurement of Desiccant Dryer Output Reduce Various Operating Costs in Natural Gas Cryogenic Process Greg Lankford Charles Bates Dr. Xiang (Sherry) Liu Dir. Business Development Plant Supervisor Director R&D SpectraSensors, Inc. DCP Midstream SpectraSensors, Inc. Houston, TX Okarche, OK Rancho Cucamonga, CA. HsuHung (Steven) Huang KuanTing (Gary) Yeh Senior R&D Engineer Senior R&D Engineer SpectraSensors, Inc. SpectraSensors, Inc.

ABSTRACT Trace H2O measurement at desiccant dryer output is crucial for the natural gas processing plant. SpectraSensors TDL (Tunable Diode Laser) trace H2O analyzer has positioned itself as a most desirable analytic technique for this challenging application, due to its superior sensitivity, operation reliability, speed of response, low maintenance and immunity to vapor impurities. This paper describes the analyzer design, system configuration, calibration and validation. Field operation data fully demonstrated that SpectraSensors TDL trace H2O analyzer has enabled a natural gas processing plant to optimize their processes by reducing dryer regeneration frequency, extending molecular sieve bed life, energy savings, reducing various operational costs and preventing freeze up.

INTRODUCTION

Natural Gas (NG) has been used as one of the primary energy sources for decades due to its low cost and widespread availability. NG is processed for purification before it is distributed to customers through pipeline networks. In natural gas processing plants, molecular sieve desiccants are typically used to dry the natural gas stream before its liquefied. Typically, two to four desiccant vessels are operated in parallel, using a piping system that allows a bed to be removed from the process for regeneration. When the bed becomes saturated, it is regenerated where hot, dry gas drives the adsorbed moisture out. The freshly regenerated bed is then ready for service. Normally, when the inlet gas moisture concentration is constant or can be measured, the desiccant dryers have a predictable operational period between regenerations. But due to aging, overheating, or contamination in theunit, the desiccant can lose capacity. Desiccant beds can also channel, allowing small concentrations of wet gas to flow through the bed without being dried. The presence of even trace amounts of moisture can threaten the integrity of compression and expansion equipment due to the formation of ice particles, resulting in serious safety hazards and substantial shutdown costs. Therefore, the H2O level at the

desiccant dryer outlet must be monitored by trace H2O analyzers to avoid potential problems in the downstream process, maximize process efficiency and reduce variable operation costs. Traditional analyzers for monitoring H2O level in desiccant dryer outlets use either electrochemical probes (e.g. aluminum oxide capacitance probes) or Quartz Crystal Microbalance (QCM, vibrating quartz crystal). Trace hydrocarbons and other vapor impurities (methanol, glycols, amines and oils) present in the stream can easily contaminate the electrochemical probes, resulting in drift and ultimate loss of sensor response. The electrochemical probes require intensive maintenance such as periodic refurbishment or replacement of the sensor heads, resulting in high maintenance costs. QCMs also suffer from sensitivity to vapor impurities, resulting in false detection of dryer breakthrough. QCMs’ and electrochemical probes slower wet-up response may allow moisture to pass downstream. The slower dry-down response of QCMs and electrochemical probes can lead to prolonged regeneration time and unnecessary costs. Based on tens of years experience in Tunable Diode Laser (TDL) based gas absorption spectroscopy [1-3], SpectraSensors developed the first TDL based sub-ppmv H2O analyzer in 2007 for the natural gas cryogenic process. The TDL analyzers overcome the shortcomings of the traditional techniques and provide superior sensitivity and repeatability, fast response times, operational robustness, long-term reliability under harsh conditions, no retention or wet-up/dry-down delays, no sensitivity or drift due to contamination from vapor impurities, no field calibration and low maintenance. Compared to other vendor’s TDL H2O analyzers that came to market later and only in recent couple of years, only SpectraSensors TDL analyzer uses a patented spectral subtraction technique which subtracts the dry spectrum from the wet spectrum periodically to eliminate measurement interference from background absorbing species, improve detection sensitivity, and reduce measurement errors due to background changes. These significant advantages have positioned SpectraSensors TDL analyzer as the most desirable analytic technique for trace H2O detection at desiccant dryer outlets in natural gas cryogenic processes, as well as in other challenging petrochemical applications. [4-9] This paper describes the fundamental design, system configuration; calibration and validation of SpectraSensors TDL trace H2O analyzers. Laboratory test data are presented to demonstrate the analyzer’s excellent sensitivity, repeatability and speed of response time. Field operation data from two installation sites are presented and analyzed to demonstrate that SpectraSensors TDL trace H2O analyzers perform much better than traditional techniques and competing TDL analyzers in preventing freeze up, increasing process efficiency and reducing various operation costs.

SPECTROSCOPY DESIGN

The fundamentals of TDL absorption spectroscopy including wavelength modulation spectroscopy with 2nd harmonic detection (WMS-2f) which is used in SpectraSensors analyzer can be found in multiple literature [1-9]. A H2O absorption line near 1877 nm is selected for measuring trace H2O in natural gas stream [10] due to its strong absorption strength and its relative isolation from the background methane (CH4) absorption interference. However, even using this line, the dominant background absorption still prevents any direct measurement of the target 2f signal from sub-ppm H2O, as illustrated by Fig. 1(A). A patented spectral subtraction technique [11-12] has been developed to enable the extraction and measurement of the unobstructed trace contaminant spectrum, as shown in Fig. 1(B), from the background natural gas spectrum which is more than 2 orders of magnitude stronger than 100 ppbv H2O spectrum.

FIGURE 1. 2F SPECTRA OF TRACE H2O IN NATURAL GAS STREAM: (A) DRY AND WET SPECTRA; (B) DIFFERENTIAL SPECTRA. A schematic of the stream switching and conditioning which underpins this spectral subtraction methodology is illustrated by Fig. 2. Each measurement cycle starts with a dry cycle as shown in Fig. 2(A). In this cycle, the sample stream flows through a high-efficiency dryer which selectively removes the trace H2O without altering the background natural gas stream itself. The dry spectrum is recorded at the end of the dry cycle. Then the analyzer automatically switches back to bypass the dryer, as shown in Fig. 2(B), and measure the unmodified wet sample stream. Subtraction of the recorded dry spectrum from the live wet spectrum generates a live, unobstructed differential spectrum of the trace H2O, which is free of background interferences, as illustrated in Fig. 1 (A) & (B). The H2O concentration of the natural gas stream is then easily calculated from the live differential spectrum.

(A) (B)

FIGURE 2. ILLUSTRATION OF THE STREAM SWITCHING SCHEME FORSPECTRAL SUBTRACTION TECHNIQUE: (A) DRY CYCLE; (B) WET CYCLE. This technique is an efficient tool not only to eliminate obscuring background interference but also to cancel common mode electronic and optical noises in the wet and dry spectra, increase the measurement signal-to-noise ratio and improve detection sensitivity. The dry spectra update interval can be user programmed from minutes to hours, as required by the desired long term measurement repeatability. In addition, measurement errors due to changes in temperature (T), pressure (P) or stream composition can be effectively corrected by automatically initiating dry spectra updates when the measured T or P change, or spectral distortion exceeds a user preset level. The dryer is a metal getter designed to last over 3-5 years for 100ppbv H2O under standard analyzer operating conditions. Dry cycles can be optionally deactivated, for example, when high concentrations of H2O occur during process upsets, or when high levels of H2O need to be measured. The analyzer data algorithm automatically calculates the dryer consumption based on the measured H2O concentration, the sample flow rate and the time usage of the dryer. The analyzer will give an alarm when the dryer approaches the pre-programmed percentage of its lifetime. This methodology is further enhanced by proprietary peak tracking algorithm. Pressure and background compensation algorithms correct for spectra shift, back pressure variation and background composition change within one measurement cycle without triggering dry cycles and increasing blind time. The details have been described in previous papers [7, 9].

SYSTEM CONFIGURATION The analyzer is designed to operate near atmospheric sample pressure so that the sample gas can be easily vented to flare. Operating the sample stream near atmospheric pressure also avoids the drawbacks arising from vacuum operation, such as unreliability and high maintenance of vacuum pumps, sample handling system (SHS) susceptibility to actual

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and/or virtual leaks, and the vulnerability of WMS-2f signal to pressure and/or modulation current variations. The analyzer has the SHS and the spectrometer cell mounted in an insulated and electrically heated stainless steel enclosure, as shown in Fig. 3(A). The temperature inside the enclosure is usually controlled at 50±0.1ºC to cover potential ambient T change of -20ºC to 50ºC. Maintaining the sample stream at this elevated temperature improves measurement repeatability by minimizing H2O adsorption to and desorption from the SHS tubing walls, by stabilizing the optical alignment of the spectrometer cell, and by eliminating any variation of the 2f signal with temperature. The layout of the SHS components inside the enclosure has been optimized to provide the best analyzer performance. The assemblies of electronics and SHS have different configurations according to different hazardous location certification requirements. Currently, the following models are available: SS2100 certified for CSA Class 1 Division 2, SS2100a certified for ATEX zone 2, SS2100i-2 certified for IECEX, ATEX and GOST-R zone 1.

FIGURE 3. SYSTEM CONFIGURATIONS: (A) SHS & SPECTROMETER CELL INSIDE A HEATED ENCLOSURE; (B) SS2100; (C) SS2100A; (D) SS2100I-2.

ANALYZER CALIBRATION AND VALIDATION Each analyzer is calibrated in the factory using gas mixtures that best reproduce the real natural gas stream compositions with H2O concentration variation range of 0-10ppmv and pressure variation range of 950-1700mb. The calibration gas mixture is obtained by diluting the premixed H2O in methane (C1) sample with high purity gases that may present in a natural gas stream, including ethane (C2), propane (C3), butane (C4), nitrogen (N2) and carbon dioxide (CO2). The premix is gravimetrically certified by the gas vendor to be NIST traceable. The dilution ratio of the premix and the mixing ratio of the background gases are controlled by digital mass flow controllers which also have NIST traceable certifications. The analyzer calibration is certified in the factory based on these NIST traceable standards, as illustrated by Fig. 4. Due to the intrinsic stability of TDL analyzers, no field calibration is needed throughout the life of the analyzer. Each analyzer is equipped with a permeation tube based validation system, as shown in Fig. 3(A). The permeation tube permeation rate is certified as NIST traceable by the tube

(A) (B) (C) (D)

vendor. It allows the user to verify the validity of the original calibration certificate throughout the life of the analyzer. The on-board permeation tube delivers about 1 ppmv H2O to the dried natural gas stream generated by the dryer, which is being used in the stream switching system. Gas temperature and gas flow rate across the permeation tube output are both tightly controlled to guarantee consistency of the generated H2O level. -The analyzer reading on the validation stream is calibrated in the factory based on previously certified calibration. An analyzer constant is established to link the validation reading to the permeation rate gravimetrically certified by the vendor. After replacing the permeation tube in the field, the validation reading can be simply scaled based on the certified permeation rate of the new permeation tube and the analyzer constant, as illustrated by Fig. 4.

FIGURE 4. CALIBRATION AND VALIDATION SCHEME.

LAB TEST RESULTS

Figure 5 shows the concentration step test results of a typical trace H2O analyzer for desiccant dryer outlet in natural gas cryogenic process. The superior linearity of the concentration measurement is being illustrated by the inset plot of the averaged analyzer reading on each step versus the real H2O concentration. The reading repeatability on each step ranges from ±4 to ±10 ppbv over the entire 0 – 10 ppmv range. Here, repeatability is defined as twice the standard deviation of the analyzer readings (2σ), which corresponds to a 95% confidence interval. The repeatability of the concentration measurement can be improved by applying moving averages to the raw readings shown in Fig. 6. The analyzer always updates the concentration measurement every 17s, irrespective of any data averaging. The response time of the raw readings shown in Fig. 5 ranges from 51s – 68s, most of which is due to the sample transport time through the sampling lines and the spectrometer cell.

Working Standards

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FIGURE 5. STEP AND LINEARITY TEST RESULTS. Figure 6 shows over 2 hour long test results at 1 ppmv and 0 ppmv. The repeatability will be improved by 25% by applying a moving data average of 16. Even without moving average, the analyzer performs well within its reading repeatability spec of ±50 ppbv, which is highlighted by the green bar. It should be noted that the measurement cycle for these lab tests was set to about half an hour in order to examine the cycle to cycle reading repeatability, which is dominated by the SHS design. During field operation, the measurement cycle can be extended to support less blind time and even better measurement repeatability.

FIGURE 6. LONG TERM 1 PPMV AND 0 PPMV TEST RESULTS.

FIELD OPERATION RESULTS Monitored Molecular Sieve Bed Breakthrough Test The DCP Midstream, Okarche Oklahoma Plant (150 MMSCFD) had previously agreed to Beta Test a TDL analyzer from another supplier. After approximately 9 months of unsuccessful operation it was determined that this particular TDL was not capable of measuring in the sub ppm region required by the inlet to the Cryogenic unit. The TDL technology was replaced by QCM technology from the same analyzer supplier. The QCM technology suffered interference due to Methanol which is purposefully injected into the pipeline system feeding the plant (depicted in Figure 7). Understandably DCP was very skeptical concerning TDL technology specifically and moisture analyzers in general due to previous experience. It was agreed to install an online SpectraSensors TDL in parallel to the existing QCM while having the molecular sieve supplier visit the site and operate their typical field analyzers used to characterize the operation. Therefore the test set up was three moisture analyzers in parallel off of the same sample point. The following graph is a DCS screen shot of both the QCM and TDL. The molecular sieve supplier analyzer system was not connected to DCS and therefore not available for display but confirmed the below results.

FIGURE 7. COMPARISON OF TDL & QCM FIELD MEASUREMENTS ON ONE BREAKTHOUGH EVENT IN THE SAME NATURAL GAS STREAM. The QCM registers a false base line reading of 3 ppm due to the influence of methanol in the gas stream. The TDL began to respond to moisture breakthrough twenty minutes ahead of the QCM. The 1.5 ppmv spike at the beginning of breakthrough event is the

QCM

TDL

daily validation via integrated permeation device programmed for 7:00 am (coincidental that bed breakthrough began just before 7:00 am) Decision was not made to switch out of the bed until all three online analyzers confirmed moisture breakthrough. When bed switch was initiated note that the TDL drops to zero immediately. The QCM responds quickly to the bed switch as well but never dries to zero before Methanol pushes through the second bed and returns to the false base line reading. DCP is extremely happy with the results and immediately extended their bed regeneration cycle from 12 hours to 18 hours (33% increase). SAVINGS ASSOCIATED WITH RELIABLE TRACE MOISTURE MEASUREMENT (150 MMSCFD gas processing plant with two bed molecular sieve dehydration system)

• Longer time between molecular sieve bed regenerations. Use of the SSI TDL analyzer has allowed DCP to extend time between regenerations by ~ 20% to 18 hours. Overall cycle time is now 36 hours.

• Extending the cycle time will extend the life of the sieve. Eliminating even one extra recharge in ten years saves ~$100K in sieve replacement costs.

• Avoidance of 2 days downtime for reload saves over $400K in lost revenue at today’s gas prices. More importantly, the supply chain is not disrupted, which can have significant financial impact on upstream gas suppliers.

• Less natural gas is consumed due to fewer regenerations over time. • Exchangers are better protected. If damaged, ethane recovery drops from 90 to

80% recovery at a cost of $3 – 4K/day. • Capability to conduct in-house bed performance tests saves ~$100K over ten

years. • Less wear and tear on rotating equipment, electric power savings, reduction of

emissions, all can improve the profitability of the gas processing plant. CONTINUED ONLINE PLANT RESULTS Figures 8 & 9 are DCS screen shots from a second Processing Plant in Oklahoma. Comment from plant engineer: “The SpectraSensors analyzers are really paying for themselves. They have definitely prevented a freeze up or two and allowed us to see how much water the beds are seeing. This is how I calculated the beds were only adsorbing 25% of their design capacity”. After this discovery fresh sieve material was installed in both beds and the plant is now actually switching based upon the breakthrough as monitored by the TDL analyzer referenced by the following screen shots. (Spike is daily automatic validation.)

FIGURE 8. FIELD MEASUREMENT RESULTS: (A) TWO BREAKTHROUGH EVENTS FOLLOWED BY DAILY VALIDATIONS; (B) ZOOM IN HALF AN HOUR TO ILLUSTRATE THE FAST DRY DOWN (from 0.5 ppm to 0.05 ppm in 2 minutes).

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FIGURE 9. FIELD MEASUREMENT RESULTS: TOP GRAPH BED (A) BREAKTHROUGH OF FRESH BED. BOTTOM GRAPH BED (B). BED SWITCH DECISION MADE AT 0.2 PPM (200 PPB)

CONCLUSIONS

Trace H2O measurement at desiccant dryer output is crucial for natural gas processing plant. SpectraSensors TDL trace H2O analyzer has positioned itself as the most desirable analytic technique for this challenging application, due to its superior sensitivity, operation reliability, fast response time, low maintenance and superior immunity to vapor impurities. Field operation data fully demonstrated that SpectraSensors TDL trace H2O analyzer has helped natural gas processing plant tremendously in preventing freeze up, extending molecular sieve life, saving energy and reducing various operation costs.

ACKNOWLEDGEMENTS

The authors thank Dr. Alfred Feitisch, Matthew Kinsey, Paul Silva, Nathan St John, Dr. Mathias Schrempel, Bill Jenko, Mohamad Dabboussi, Dr. Wenhai Ji, Adam Chaimowitz, and other colleagues for their contributions to the development and field testing of all these analyzers.

REFERENCES 1. May, R., “System and method for water vapor detection in natural gas”, US patent

6657198B1, Dec. 2, 2003. 2. Liu, X., Jeffries, J.B., Hanson, R.K., and etc., “Development of a tunable diode

laser sensor for measurements of gas turbine exhaust temperature,” Applied Physics B, 82, pp. 469-472, 2006.

3. Zhou, X., Liu, X., Jeffries, J.B., and Hanson, R.K., “Development of a sensor for temperature and water vapor concentration in combustion gases using a single tunable diode laser”, Meas. Sci. Technol. 14, pp. 1459-1468, 2003.

4. Liu, X., Zhou, X., Feitisch, A. and Sanger, G., “Tunable diode laser absorption spectroscopy based trace moisture detection in natural gases”, 52nd Analysis Division Symposium of the Instrumentation, Systems and Automation Society, Houston, TX, April 15-19, 2007.

5. Liu, X., Zhou, X. and Feitisch, A., “Tunable diode laser analyzers for ethylene production and quality control”, 53rd Analysis Division Symposium of the Instrumentation, Systems and Automation Society, Calgary, Alberta, CA, April 20-24, 2008.

6. Liu, X., Zhou, X., Ji, W. and Feitisch, A., “Advanced NH3 and CO2 TDL gas analyzers for petrochemical process control and product qualification”, 54th Analysis Division Symposium of the Instrumentation, Systems and Automation Society, Houston, TX, April 20-23, 2009.

7. Liu, X., Ji, W., and Feitisch, A., “Development of H2S, H2O, NH3 and C2H2 TDL analyzers for petrochemical applications in optically interfering hydrocarbon streams”, 55th Analysis Division Symposium of the Instrumentation, Systems and Automation Society, New Orleans, LA, April 25-29, 2010.

8. Liu, X., Ji, W., Feitisch, A., Jenko, B. And Trygstad M., “Advancing TDL Technology: Applied spectroscopy in service of process control objectives”, 56th Analysis Division Symposium of the Instrumentation, Systems and Automation Society, Houston, Texas, April, 2011.

9. Liu, X., Huang, H., Yeh, K, etc., “Assuring ethylene purity: reliable sub-ppm measurement of ammonia and water with online TDL analyzers”, 57th Analysis Division Symposium of the Instrumentation, Systems and Automation Society, Anaheim, California, April, 2012.

10. R. May, “System and method for water vapor detection in natural gas,” US patent 6657198B1, Dec. 2, 2003.

11. Zhou, X., Liu, X., Feitisch, A. and Sanger, G., “Reactive gas detection in complex backgrounds,” US Patent # 7,704,301, Apr. 27, 2010.

12. Zhou, X., Liu, X., Feitisch, A. and Sanger, G., “Reactive gas detection in complex backgrounds,” US Patent # 7,819,946, Oct. 26, 2010.

Advancements in Trace Moisture Measurement of Desiccant Dryer Output Reduce Various Operating Costs in Natural Gas Cryogenic ProcessABSTRACTINTRODUCTIONSPECTROSCOPY DESIGNSYSTEM CONFIGURATIONFIGURE 4. CALIBRATION AND VALIDATION SCHEME.LAB TEST RESULTSFIGURE 5. STEP AND LINEARITY TEST RESULTS.FIELD OPERATION RESULTSFIGURE 8. FIELD MEASUREMENT RESULTS: (A) TWO BREAKTHROUGH EVENTS FOLLOWED BY DAILY VALIDATIONS; (B) ZOOM IN HALF AN HOUR TO ILLUSTRATE THE FAST DRY DOWN (from 0.5 ppm to 0.05 ppm in2 minutes).FIGURE 9. FIELD MEASUREMENT RESULTS: TOP GRAPH BED (A) BREAKTHROUGH OF FRESH BED. BOTTOM GRAPH BED (B). BED SWITCH DECISION MADE AT 0.2 PPM (200 PPB)conclusionSACKNOWLEDGEMENTSREFERENCES