The MIDAC Low Moisture in Corrosive Gases Process Analyzer for Continuous On-Line Applications

Transcription

1 AP-156 The MIDAC Low Moisture in Corrosive Gases Process Analyzer for Continuous On-Line Applications Dr. Peter G. Zemek MIDAC Corporation Fitch Avenue Irvine, California USA Abstract: Midac has developed a Titan On-Line Process Analyzer for measuring moisture in corrosive gasses. The system can also be used to simultaneously analyze other target gases at different orders of magnitude in association with low moisture levels. Inlet sample pressures can range from ambient to 5000 PSI. Moisture analytical minimum detection limits (MDL) is approximately 500 ppbv (0.5 ppmv). Other configurations can be used to detect moisture levels at very low ppb concentrations. Sample flow rate range is 0.1 to 2.0 lpm. The system requires plant nitrogen and power. Systems have been incorporated into ammonia plants where low moisture levels in the ppm range and ammonia levels in the percent level concentrations are needed in near-real time. Results from installed systems prove high accuracy and precision. Process streams can be accommodated at high pressure and high concentrations. Other contaminants and by-products such as methane and other organics may also be analyzed simultaneously. The system can simultaneously measure ppm water, percent ammonia, and percent methane. The system can operate in Class I Div II environments. Key Words: FTIR, MIDAC, infrared, process analysis, continuous monitoring, explosive atmospheres, ammonia, water, H 2 O, moisture, methane, on-site analysis, corrosive gases, dewpoint, Titan. Introduction: Analysis for moisture in corrosive gasses has been a challenge for many instrumental technologies. Ammonia is not compatible with many of the moisture measuring techniques employed by ammonia plants such as chemiluminescent dewpoint meters and cold mirror devices. Additionally, simultaneous measurements of moisture, ammonia, and methane can be performed simultaneously and at different orders of magnitude.

2 The system receives high-pressure sample gas from a single sample line or multiple sample lines and reduces the pressure to a manageable level ( psi). The analysis at higher pressure eliminates the refrigeration effects of ammonia pressure reduction and increases minimum detection levels from an increased pressure in the sample cell. The system incorporates an Indium Arsenide (InAs) detector that is thermo-electrically cooled that eliminates the need for liquid nitrogen. The InAs detector provides approximately one order of magnitude more sensitivity than the more commonly used Mercury Cadmium Telluride (MCT) detector. The increased sensitivity of the InAs detector allows for much smaller pathlengths than would typically be used with MCT detectors. All components in the system are non-reactive to hot anhydrous ammonia (stainless steel, Kalrez, and PEEK). All components on the inlet side of the sample cell, including the cell, are heated to prevent condensation and adsorption onto the walls of components in contact with the sample gas. The high-pressure sample stream allows for the use of a very small pathlength (15 cm). The cell is a once through IR signal design with a volume of approximately 150cc. The system may also be incorporated with a 4-meter sample cell with a volume of 190cc to provide even lower moisture detection levels. The sample cell incorporates quartz windows and Kalrez o-rings to provide very strong and chemically inert materials for sample containment. The thick quartz allows for pressures up to 5000 PSI. 2

3 The once through signal design of the cell provides that no mirrors are incorporated into the cell to reduce maintenance downtime. System downtime can be expected to be a tune-up once per year and relatively low cost component replacement every 3-5 years. The system also incorporates pressure relief valving and venting for emergency pressure relief should the process overpressure or a component fails. The system is optionally designed to work in a Class I Div II explosive atmosphere. The component box is positively purged with plant nitrogen or instrument air. The use of the synthetic background eliminates the need for periodic background measurements. The system is designed to run continuously and provide near real time concentration readout and a continuous emission-monitoring graph. The signal can provide up to eight 4-20 milliamp signal output channels for control room monitoring and datalogging. 3

4 The system uses a touchscreen Class I Div II door mounted computer that continuously reads out target compound concentrations. The software can be programmed to take a sample every 5 seconds up to 1 month. The software uses revolving directories to overwrite data or archive data for whatever period the user selects. Calibrations are performed in one of Midac s Laboratories or on-site to user specified levels. The calibration will remain viable for as long as the system components stay functioning. Accuracy of target compound concentrations can be achieved within 2-3%. All systems come with optional on-site training for both components and software. Users are also trained in method development and calibration point generation should they want to change the range of the calibration or target compounds. 4

5 Although the system is designed to output exhaust at no more than 30 psi during normal operation, exhaust line pressure relief valves are incorporated into the system. If this valve is activated, high-pressure sample gas will be directed to a suitable exhaust area or control device. 5

6 Pressure Relief Valve Heated cell 15 cm sample cell The analyzer enclosure measures 4 feet tall by 3 feet wide by 13 inches deep and is equipped with the following inputs and outputs: Inputs: 2 Inlet Sample Lines or More (bottom, ¼ Swagelok ss tubing) Compressed Air In (right hand side, ¼ Swagelok) Purge Gas In (right hand side, ¼ Swagelok) Cooling Water In (left hand Side, 3/8 Swagelok) Data Cabling In/Out (left hand side, 1 female Union) 110V/220V Power (top) Computer keyboard (front) Outputs: Purge from cabinet to sensor (right hand side) Used Gas Exhaust (left hand side, ¼ Swagelok) Cooling Water Return (left hand side, 3/8 Swagelok) Cabinet diaphragm vent (top) Components within the analyzer enclosure include: 2 three way manual valves 1 flow meter 2 SS 2-stage heated pressure regulators 1 Moore pressure regulator for FTIR box purge 1 Pressure transducer and readout w/backup analog pressure gauge 1 Temperature sensor and readout 1 needle back pressure valve 1 pressure relief valve 1 single pass 15cm heated gas cell 316 ss, quartz windows, Kalrez O-rings 6

7 Purge gas continuously flows into the spectrometer compartment maintains an optical density within the electronics box free of moisture and contamination achieved by mole sieve nitrogen. Excess nitrogen is then exhausted to a vent or within the larger cabinet through exhaust openings in the spectrometer compartment. Heated Pressure Regulators FTIR Spectrometer Box w/cell HydroQuant The HydroQuant method of FTIR gas analysis was developed at MIDAC for the specific application of measuring moisture in corrosive gases. Because of the corrosive and dangerous nature of most sample gases, a technique had to be developed that allowed data to be taken without collecting a background spectrum. Since very low levels of moisture are to be measured, even small amounts of water in the background could adversely affect results. The HydroQuant algorithm creates a synthetic background from each sample interferogram. The sample interferogram is multiplied by a Gaussian function that can be described as: 7

8 x e k 2 2 After multiplying the interferogram by this function, the resulting interferogram has been smoothed out. The function approaches zero as you travel down the wing of the interferogram. The resulting single beam is a smooth instrument response profile without the sharp lines from the water. It is very similar to a fully purged system background. A sample single-beam (top) and its corresponding synthetic background (bottom) are shown. The sample is air and was collected at 2cm -1, 256 scans with a K-value of The k value is a user selectable parameter to modify the behavior of the Gaussian function. The higher the K-value, the faster the Gaussian decays and the more deresolved the interferogram appears. Using a synthetic background saves an enormous amount of time that would be required to generate a background spectrum. Typical moisture analysis consists of low ppb measurements, and it would take many hours to dry a gas cell to below that level. This approach works great with compounds with narrow features. 8

9 File # 1 = C:\AUTOQ3\METHODS\NH3\C0007.SPC 1921 Rows H2O 1 ppm-m full spectrum.01 Absorbance Wavenumber (cm-1) Typical absorbance spectrum collected with the instrument. Absorbance spectra collected from the instrument depict a clean level baseline with very distinct absorbance features. Signal to noise peak to peak measurements were absorbance units. The detector cuts off at approximately 3000 cm-1. The spectrum depicted above demonstrates the noise associated below this wavenumber. A typical spectrum collected of moisture from the Autoquant software. Enlargement of the boxed region reveals the very small absorbance features of moisture. File # 1 = C:\AUTOQ3\METHODS\NH3\C0007.SPC 1921 Rows 40E-05 30E-05 H2O (1 ppm-m) ~0.25 ppm on 4M cell 20E-05 Absorbance 10E E-05-20E Wavenumber (cm-1) 9

10 The problem typically encountered with moisture in ammonia gasses is overlap of ammonia absorbance bands with moisture bands and foreign gas peak broadening. This system incorporating the higher pressures, InAs detector, and Midac s patented Autoquant/Hydroquant software eliminates these features as seen in the following spectra. The top spectrum above depicts the quantitative moisture bands collected in nitrogen. The bottom spectrum depicts a 40% ammonia spectrum containing approximately 30 ppm of moisture. Many moisture bands used for quantification are not interfered with from the ammonia absorbance bands. The moisture spectral features also show no significant peak broadening. 10

11 2 NH3 methane Absorbance / Wavenumber (cm-1) X-Zoom CURSOR File # 4 : NH3NOH2OC Jsuction,32sc,121C,1atm,001gain 10/2/00 4:54 PM Res=None Other contaminant and by-product features are depicted in the above spectra. Several moisture bands not interfered with by either ammonia or methane were used for quantification. This is an actual spectrum from an ammonia plant. The spectrum are very clean and normal from a spectrum generated from a synthetic background. File # 1 = C:\AUTOQ3\METHODS\NH3\C0021.SPC Rows.02 Ammonia Plant.01 0 Absorbance ppm-m (0.25 ppm) Wavenumber (cm-1) The spectra above were generated n the lab and from an ammonia plant. There is no significant difference between spectral features. Additional On-Line process instruments can also be designed to fit your analytical and process needs. 11

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