Equilibrator for Rapid and Continuous Detection of a Gas in a Liquid
20200094201 ยท 2020-03-26
Assignee
Inventors
Cpc classification
B01D19/0005
PERFORMING OPERATIONS; TRANSPORTING
B01F23/23
PERFORMING OPERATIONS; TRANSPORTING
C02F1/20
CHEMISTRY; METALLURGY
B01F25/312
PERFORMING OPERATIONS; TRANSPORTING
International classification
Abstract
A rapid and continuous separator or equilibrator to separate a gas from a liquid includes a venturi and injector, a mixer and a free overfall stream to separate a gas from a liquid. The injector introduces a carrier medium into the liquid which provides a reservoir for the gas to diffuse into as the liquid and carrier make a single transit through the apparatus. The separator was developed to enable real-time estimation of methane concentrations in ground water during purging. Real-time monitoring allows evaluation of trends during water well purging, spatial trends between water wells, and temporal comparisons between sampling events. These trends may be a result of removal of stored casing water, pre-purge ambient borehole flow, formation physical and chemical heterogeneity, or vertical flow outside of well casing due to poor bentonite or cement seals. Real-time information in the field can help focus an investigation, aid in determining when to collect a sample, save money by limiting costs (e.g. analytical, sample transport and storage), and provide an immediate assessment of local methane concentrations, Four domestic water wells, one municipal water well, and one agricultural water well were sampled for traditional laboratory analysis and compared to the field separator or equilibrator results. Applying a paired t-test comparing the new separator or equilibrator method and traditional laboratory analysis yielded a p-value 0.383, suggesting no significant difference between the two methods for the current study. Additional field and laboratory-based experimentation and potential modification of this device are necessary to justify use beyond screening at this time. However, early separator or equilibrator use suggests promising results and applications.
Claims
1. An apparatus to separate at least one constituent from a liquid comprising multiple constituents, comprising: a venturi tube comprising a venturi orifice and configured to pass a liquid through the venturi tube and further configured to introduce a carrier medium into the liquid, whereby a first portion of a first constituent in the liquid diffuses into the carrier medium; a mixer configured to receive the liquid and the carrier medium from the venturi tube, whereby the mixer mixes the liquid and the carrier medium; and a plenum comprising a first exit and configured to receive the liquid and the carrier medium from the mixer, wherein the plenum is further configured to pass the liquid and the carrier medium through a free overfall stream within the plenum, whereby the carrier medium exits the liquid, and the carrier medium is discharged from the plenum through the first exit.
2. The apparatus of claim 1, wherein the first constituent continuously diffuses from the liquid into the carrier medium.
3. The apparatus of claim 2, wherein carrier medium comprising the first constituent is discharged from the plenum through the first exit in less than about one minute from the carrier medium being introduced into the liquid.
4. The apparatus of claim 2, wherein carrier medium comprising the first constituent is discharged from the plenum through the first exit in less than about ten seconds from the carrier medium being introduced into the liquid.
5. The apparatus of claim 4, wherein the carrier medium makes a single pass through the apparatus from the venturi orifice to the first exit of the plenum.
6. The apparatus of claim 1, wherein the first constituent comprises a hydrocarbon gas.
7. The apparatus of claim 6, wherein the liquid comprises water and the hydrocarbon gas comprises methane.
8. The apparatus of claim 7, wherein the venturi tube comprises an injection orifice configured to inject the carrier medium into the liquid.
9. The method of claim 8, wherein the carrier medium comprises a gas.
10. The apparatus of claim 1, wherein a second portion of the first constituent diffuses from the liquid into the carrier medium in the mixer.
10. The apparatus of claim 10, wherein the mixer comprises a static mixer configured to mix the carrier medium and the liquid.
12. The apparatus of claim 1, wherein a third portion of the first constituent diffuses into the carrier medium from the liquid within the plenum.
13. The apparatus of claim 12, wherein the carrier medium comprising the first constituent is directed into a gas analyzer.
14. The apparatus of claim 13, wherein the plenum comprises a second exit to discharge the liquid from the plenum.
15. An apparatus to separate at least one constituent from a liquid comprising multiple constituents, comprising a venturi tube connected in series to a mixer and the mixer is connected in series to a plenum, whereby the liquid flows through the venturi tube, through the mixer and into the plenum, wherein a first constituent of the liquid is continuously separated from the liquid into a carrier medium as the liquid passes through the venturi tube, the mixer and into the plenum.
16. The apparatus of claim 15, whereby the liquid passes through the venturi tube, the mixer and into the plenum in less than about one minute.
17. The apparatus of claim 15, whereby the liquid passes through the venturi tube, the mixer and into the plenum in less than about ten seconds.
18. The apparatus of claim 15, wherein the first constituent continuously diffuses from the liquid into the carrier medium as the liquid passes through the venturi tube, the mixer and the into the plenum.
19. The apparatus of claim 15, wherein the carrier medium comprising the first constituent is discharged from the plenum and into a gas analyzer.
20. The apparatus of claim 15, wherein the liquid comprises water and the first constituent comprises a hydrocarbon.
21. The apparatus of claim 15, wherein the carrier medium is introduced into the liquid at the venture tube.
22. The apparatus of claim 15, wherein the carrier medium comprises a gas.
23. A method to separate at least one constituent from a liquid comprising multiple constituents, comprising: causing a pressure drop in the liquid; introducing a carrier medium into the liquid; diffusing a first constituent of the liquid's multiple constituents into the carrier medium; mixing the liquid and the carrier medium; and, moving a portion of the carrier medium out of the liquid.
24. The method of claim 23, comprising passing the liquid through a venturi orifice.
25. The method of claim 24, comprising injecting the carrier medium into the liquid in proximity of the venture orifice.
26. The method of claim 23, wherein the carrier medium comprises a gas in the form of bubbles, and comprising breaking the bubbles into smaller bubbles.
27. The method of claim 26, comprising breaking the bubbles into smaller bubbles using a static mixer.
28. The method of claim 26, comprising moving a portion of the bubbles out of the liquid by passing the liquid and bubbles through a free overfall stream.
29. The method of claim 28, comprising collecting the bubbles as a free gas over the liquid and passing the free gas into a gas analyzer.
Description
BRIEF DESCRIPTION OF THE FIGURES
[0049] Embodiments of the invention may be used to separate or equilibrate most any gas contained within most any liquid. In the following examples, the separated gas will be a hydrocarbon, and more specifically methane, and the liquid will be water.
[0050] Referring to
[0051] Water is pumped from water source 145 at water acquisition 240, which could be a well containing ground water, or a stream, lake, or other water source, and is directed to water distribution 235, which includes hydrant or faucet 140. The water stream is then split four ways using manifold splitter 135. A splitter of another configuration that splits more or less ways may be used. Water directed to sample collection 260 through tube 121 is used for collecting laboratory samples. Water directed to geochemical monitoring 255 through tube 122 passes through a commercial flow cell fitted with multi-parameter probes that are connected to the appropriate instrument for monitoring of pH, specific conductance, conductivity, total dissolved solids, oxidation reduction potential, dissolved oxygen, temperature, and any other parameters necessary during parameter recording 250. Water directed to excess discharge 245 through tube 123 is any additional water not being directed to sample collection 260, geochemical monitoring 255, and water gaging 230 and is discharged for disposal. The water directed towards the separator or equilibrator processes through tube 120, first passes through water gaging 230 which contains water flowmeter 40 that regulates the flow rate to the desired rate. Water then enters the venturi ejector 225 inlet, within venturi tube 220, and is constricted to a small diameter opening causing the water to pass through at a high velocity. The increased velocity is accompanied by a pressure drop at the opening, which is less than atmospheric, that passively draws in air through a small diameter tube at the bottom of venture ejector 225. Air that enters venture ejector 225 first begins at air entry 265. Atmospheric air is passively injected at air entry 265, whereas the rate is controlled by the suction created within venture ejector 225. Before entering venturi tube 220, the atmospheric air passes through air cleaning 270, which includes hydrocarbon trap 275 and inlet tube 310, to ensure no outside hydrocarbons are introduced to the system. The rate of injection is monitored at gas flow rate 280, using flowmeter 285, which is connected by tube 354 and tube 355. The atmospheric air then enters venturi tube 220 through a small diameter tube at the bottom of venture ejector 225 using tube 350. Passive flow of atmospheric air into venturi tube 220 eliminates the need for injection of gas or air via a pump into the water stream. Other inert gases such as nitrogen, argon, etc. may be used in place of atmospheric air that is passively injected at air entry 265. Flow through venturi ejector 225 is highly turbulent and rapidly creates new interfaces for transfer of methane from the water to gas phase. During venturi tube 220, air is introduced into the water via a large number of small bubbles resulting in turbulent two phase, i.e. gas and water, bubble flow and a large area of gas-water contact around the bubbles to enhance the transfer of methane from the water to the gas phase. The pressure differential across venturi ejector 225 must remain great enough to passively inject atmospheric air. Transfer rates of constituents, such as methane, within venturi ejectors, such as venture ejector 225, exceed conventional gas-liquid mixing systems such as stirred tanks, bubble columns, and packed columns. The venturi ejector 225 performance is controlled by inlet and throat diameters, downstream pipe length, and air/water flow rates and varies by commercial venture ejectors. After passing through venturi ejector 225, the air-water mixture flows through static mixer 215 at gas-water mixing 210. Venturi ejector 225 and static mixer 215 are connected by coupler 12. Static mixers, or motionless mixers, contain internal elements, e.g. blades or helices, installed in pipes, columns, and reactors that provide increased areas of gas-water contact, a uniformed distribution of concentration and temperature, radial mixing, and lengthened gas-liquid contact times. The effectiveness of redistribution is dependent on the design feature of the elements, e.g.
[0052] blades or helices, and number of elements used. Similar to venturi devices, concurrent water and air flow in static mixers, e.g. static mixer 215, results in generation of small relatively uniform bubbles resulting in bubble flow and enhanced transfer of methane or other constituents between water and air. Air-water mixtures with bubbles have the potential to coalescence, i.e. lump or group together, but are broken to smaller bubbles upon contact with elements, e.g. blades or helices, within static mixers, e.g. static mixer 215, because of shear, therefore enhancing volumetric gas-liquid mass transfer rates, i.e. the transfer of a constituent from water to air, and coefficients. Use of venturi ejector 225 preceding static mixer 215 eliminates the need to inject air into the static mixer static mixer 215.
[0053] Water exiting static mixer 215, as part of gas-water mixing 210, is discharged into gas-tight plenum 20 at separation 200 for additional gas-water separation as water drops into gas-tight plenum 20, i.e. via a free overfall jet stream, causing turbulence within the inner tube that penetrates the water barrier within gas-tight plenum 20, and allowing methane, or other constituents such as carbon dioxide, hydrogen sulfide, or nitrogen, to transfer to the gas phase. A free overfall jet plunging into water downstream further enhances mass transfer, i.e. transfer of compound from water to air, and the mass transfer decreases with increasing downstream pipe length from a venturi device, i.e. the shorter the lengths of pipe/connections, e.g. coupler 12, after venture ejector 225 the better the mass transfer.
[0054] Water exits gas-tight plenum 20 by moving downward through the inner tube, then upward inbetween the inner and outer tube, and finally overflowing over the top of outer tube and moving downward exiting through the bottom. The gas phase, which contains the methane, or constituent of interest, exist gas-tight plenum 20 through the top. Water that exits gas-tight plenum 20 is collected at collection 295 using container 30, and is then disposed of at disposal 290. The configuration of gas-tight plenum 20 can also be altered such that it consists of a single tube at a desired length with a regulating valve on the bottom used to keep water in the bottom of the tube and a gas-tight setup.
[0055] Before entering the measurement instrumentation for analysis, the gas stream that left the top of gas-tight plenum 20, at separation 200, passes through moisture trap 15 at initial moisture removal 185. Moisture trap 15 removes water moisture from the gas stream, preventing any interference and measurement instrumentation damage. A gas sample for laboratory analysis can be collected at gas sample collection 175 using port 505. To use port 505, toggle valve 490 is closed, directing the entire gas stream to port 505. Port 505 and toggle valve 490 are connected to assembly 300. Assembly 300 is the combination of port 504, toggle valve 495, coupler 507, port 505, coupler 508, port 475, coupler 509, toggle valve 490, and port 506 which are connected in sequence.
[0056] Gas pressure is monitored with differential pressure gage 181 or another pressure measuring device, at system pressure 180 and connected to gas stream at port 475 using tube 370, and adjusted through relief valve 170 and relief valve 171 to prevent gas-tight chamber 20 from dewatering, due to excess pressure in the inner tube, and to prevent excessive pressure buildup near instrumentation intakes at gas analysis 115 and gas analysis 150. An additional safety factor is built in by using toggle valve 495 which stops all flow from continuing in the process setup, thereby protecting that gas measurement instrumentation from water damage. Toggle valve 495 is used primarily if water floods gas-tight plenum 20, usually due to the measurement instrumentation gas demand being larger than the available gas phase, and water begins to exit out of the top. When toggle valve 495 is closed, relief valves 170 and 171 are opened to allow atmospheric air to flow to the measurement instrumentation, preventing damage to the instrumentation due to the restricting gas flow and low pressure. Port 475 and toggle valve 495 are connected to assembly 300. After gas passes through assembly 300, it enters splitter 400 through tube 344 and tube 345 which is connected to port 506. Splitter 400 allows for the gas stream to be bypassed towards relief valve 170 and relief valve 171 through tube 395 and tube 397. When relief valve 170 is open, the gas stream exits the system through tube 396, and when relief valve 171 is open, the gas stream exits through tube 398. The other outlet on splitter 400 is connected to gas dryer 90 through tube 390.
[0057] Gas dryer 90, which can consist of materials such as Nafion, is also used to remove water moisture from the gas stream, preventing any interference and instrumentation damage. Gas dryer 90 contains an inner tube (such as Nafion) that carries the gas sample through the inside, but wicks water through the tube by passing a dry gas on the outside of the tube that is counter current. The dry gas begins at drying entry 55 which is atmospheric, or ambient, air. Peristaltic pump 50 draws in air at pumping 65 from drying entry 55 into tube 375 which is connected to either hydrocarbon trap 75 at contamination mitigation 70 or moisture trap 85 at air dryer 80 through tube 376. Similar to hydrocarbon trap 275 at air cleaning 270, hydrocarbon trap 75 ensures no outside hydrocarbons are introduced to the system or gas stream running counter current to the inner tube in gas dryer 90. If hydrocarbon trap 75 is used, it is connected to moisture trap 85 at air dryer 80 using additional tubes similar to tube 375, tube 376, tube 359, and tube 360. Moisture trap 85 removes the moisture from the incoming atmospheric air before passing into gas dryer 90. Moisture trap 85 is connected to gas dryer 90 using tube 359 and tube 360. After the dry air passes across the inner tube in gas dryer 90, it exits at dry gas vent 165.
[0058] After the gas sample passes through gas dryer 90, the rate at which the measurement instrumentation is sampling the gas is monitored at instrumentation demand 95 using flowmeter 100 connected by tube 385. Measurement of the flow rate using flowmeter 100 is important to understanding measurement instrumentation readings if the nominal flow rate is not achieved. An optional hydrocarbon trap 110 may be introduced at sample filter 105 to remove additional hydrocarbons if methane is the sole constituent of interest since methane isn't removed by some hydrocarbon traps. The gas phase sample then leaves flowmeter 100 and enters splitter 330 via tube 335. Splitter 330 allows for the gas stream to be directed to ports 500 through tube 365, tube 366, and tube 367. The ports 500 are used to connect to measurement instrumentation, e.g. various analytical instrumentation for measuring constituents in the gas phase. Any number of the ports 500 may be used as long as the measurement instrumentation gas demand is satisfied. More or less of the port 500s and connecting tubes may be used depending on the splitter 330 configuration.
[0059] The gas stream is then directed, using tube 130 and tube 131, to measurement instrumentation such as an IRGA 25, e.g. LandTec GEM 2000, at gas analysis 115 and TVA 35, e.g. Thermo Scientific Toxic Vapor Analyzer (TVA-1000B), at gas analysis 150 for real-time data analysis. The GEM2000 Plus, i.e. IRGA 25, uses an infrared cell to measure methane in %-volume and is accurate for gas-phase concentration measurements greater than 1.0%. The GEM2000 Plus, e.g. IRGA 25, also has additional sensors capable of measuring H.sub.2S, CO, CO.sub.2, and O.sub.2. These additional gases are simultaneously screened for along with methane. The TVA-1000B, e.g. TVA 35, is used to measure lower concentrations of methane on a flame ionization detector below 10,000 ppmv. The instrument also contains a photoionization detector allowing simultaneous detection of additional compounds to methane which may be present. Readings from the field instrumentation are recorded at data logging 155 and allowed to vent as exhaust at exhaust exit 160. Other instrumentation for measuring compounds in the gas-phase may substitute or compliment IRGA 25 and TVA 35.
[0060] Field gas concentrations measured at gas analysis 115, gas analysis 150, and data logging 155 are then used in the derived mass transfer equation to calculate the initial aqueous concentration.
[0061] The principles of the separator or equilibrator are similar to batch air stripping. Batch air stripping is a widely used method for determining Henry's law constants and relies on a dynamic principle developed by Mackay et al. 1979. An inert gas is purged through water, releasing a dissolved compound. Relative concentrations of one phase are measured over time and it is assumed that the exiting gas is in equilibrium. The separator or equilibrator however, increases gas-water interface contact times for mass transfer, decreases equilibrium times, and equilibrates a continuous flow of sample water unlike batch air stripping and other equilibrators.
[0062] Tube 120, tube 121, tube 122, and tube 123 may consist of a high density polyethylene (HDPE) tubing that is replaceable to prevent cross contamination from different sampling locations that the separator or equilibrator has been used. Other tubing materials may be used such that it is inert and doesn't cross contaminate the water or gas stream. Tube 130, tube 131, tube 335, tube 344, tube 350, tube 355, tube 360, tube 370, tube 375, tube 385, and tube 390 can consist of a inch tube, such as R-3603 Tygon tubing that is easily replaceable. Other inert tubing materials such as Teflon or stainless steel may be used and the tubing size may change depending on user preference. Tube 345, tube 354, tube 359, tube 365, tube 366, tube 367, tube 376, tube 395, tube 396, tube 397, tube 398, and inlet tube 310 are made of inch stainless tubes. Other tube sizes or materials that do not leach or emit constituents, such as volatile organics, to the gas stream may also be used. All tube connections and ports should be gas tight. Gas tape may be used for fittings and Teflon tape may be used for water connections. Background testing of all tubes, hydrocarbon traps, and moisture removing materials should be conducted using the measurement instrumentation that will be used when the separator or equilibrator is operational to prevent cross contamination, inaccurate results, and understand background concentrations.
[0063] Potential modifications include the use of additional venturi ejector and static mixers in various configurations to further enhance mass transfer and enable real-time aqueous analyses of less volatile compounds and introduction of the gas stream directly to a mass spectrometer or other device enabling rapid compound identification.
[0064] The gas-water equilibrator was designed to increase gas-water mass exchange rates beyond rates characteristic of commercially available equilibrators. Monitoring of concentration trends during purging allows for a more rigorous comparison of temporal trends between sampling events and comparison of baseline conditions with potential post-impact conditions. Other benefits of the device include real-time information and decision making in the field to help focus an investigation, aid in determining when to collect a sample, save money by limiting costs (e.g. analytical, sample transport, sample storage), and provide an immediate assessment of local methane concentrations, or concentrations of other constituents, relative to the action level for additional investigation.
EXAMPLES
[0065] Embodiments of the invention may be used to separate or equilibrate most any gas contained within most any liquid. In the following examples, the separated gas will be a hydrocarbon, and more specifically methane, the liquid will be water. The water tap closest to each wellhead being sampled that, did not pass through any treatment system was used to collect dissolved gas ground water samples and monitor field parameters (pH, specific conductance, dissolved oxygen, oxidation reduction potential, temperature). Samples were collected by securing polyethylene tubing (for example Nalgene 489 LDPE ID No. 14476-120) to the water tap and placing into an inverted 60 mL serum bottle (for example Wheaton Science #223746) containing a 0.5 g trisodium phosphate (TSP) pellet (sodium phosphate dodecahydrate, ACS, 98-102%) to maintain pH10 for sample preservation. The inverted sample bottle and water line were submerged in a 5-gal plastic bucket containing purged ground water. The bottle was slowly turned to an upright position as it filled from the water line. When the bottle was completely filled, the water line was placed near the bottom of the sample bottle and several bottle volumes were replaced before the sample line was removed. All bubbles were allowed to escape (no headspace) as the butyl rubber Teflon-faced septum and aluminum crimp cap were placed on the bottle and sealed while submerged, preventing any atmospheric contact with the sample. The aluminum caps were then crimped for permanent seal. Bottles were filled in duplicates to safeguard against bottle breakage and loss of samples. Samples were stored at 6 C. and analyzed within a 14 day recommended holding time.
[0066] Methods (RSK175v5 and RSK194v4) developed internally at EPA's Robert S. Kerr Environmental Research Center in Ada, Okla. were used to analyze methane, ethane, and propane.
[0067] Collected samples were allowed to approach room temperature, typically between 18 and 22 C., before laboratory analysis. Headspace generation methods followed Kampbell and Vandegrift and RSK194v4 and 175v5 (Kampbell and Vandegrift 1998; RSKSOP-175v5 ; RSKSOP-194v4). The initial aqueous concentration is determined by combining the mass of compound in gas and aqueous phases (as determined using Henry's Law Constant) per volume of water. A series of equations are provided in RSK175v5 to determine aqueous concentration from gas phase analysis. We condensed these equations into a single equation:
C.sub.W=C.sub.GC(RMTP.sub.LABHS)[1+(VV.sub.DILHSPP.sub.DILHS)][K_1H+VV.sub.HSW](6)
[0068] where:
[0069] C.sub.W=Aqueous concentration (g L.sup.1)
[0070] C.sub.GC=Gas concentration determined from gas chromatography analysis (ppmv)
[0071] M=Molecular weight of compound of interest (g/mol)
[0072] P.sub.DIL=Absolute pressure of dilution gas added to headspace (atm)
[0073] P.sub.HS=Absolute pressure of headspace gas (atm) [P.sub.ABSOLUTE=P.sub.ATMOSPHERIC+P.sub.GAUGE]
[0074] R=Gas constant (0.08206 L atm mol.sup.1 K.sup.1)
[0075] T.sub.LAB=Laboratory temperature (K)
[0076] V.sub.DIL=Volume of dilution gas added to headspace (mL)
[0077] V.sub.HS=Volume of headspace (mL)
[0078] V.sub.W=Volume of water in serum bottle (mL) [V.sub.W=Vol. serum bottleV.sub.HS]
[0079] K.sub.H=Dimensionless Henry's Law Constant at laboratory temperature (g L.sup.gas.sup.1/g L.sup.Water .sup.1)
[0080] The LandTec GEM2000 Plus uses an infrared cell to measure methane (CH.sub.4) in %-volume and is accurate for gas-phase concentration measurements greater than 1.0%. The GEM2000 Plus also has additional sensors capable of screening for H.sub.2S, CO, CO.sub.2, and O.sub.2. H.sub.2S can be important in sour stray gas investigations such as in Alberta and west Texas. In addition to methane, CO.sub.2 is also measured on a dual wavelength infrared cell with reference channel. The CO2 reading is filtered to an infrared absorption frequency 4.29 m (nominal), the frequency specific to CO.sub.2. The CH.sub.4 reading is filtered to an infrared absorption frequency of 3.41 m (nominal), the frequency specific to hydrocarbon bonds. The presence of other light hydrocarbons (e.g. ethane, propane, butane) will result in higher readings of CH.sub.4 than is actually present. Oxygen, CO, and H.sub.2S are measured (Hydrogen compensated) on an internal electrochemical cells (EC Cell). The O.sub.2 cell is a galvanic cell type with no influence from CO.sub.2, CO, H.sub.2S, SO.sub.2, or H.sub.2. The Thermo Scientific Toxic Vapor Analyzer (TVA-1000B) is a portable instrument that was used to measure lower concentrations of methane on a flame ionization detector (FID) below 10,000 ppmv. The instrument also contained a photoionization detector (PID) allowing simultaneous detection of additional compounds to methane which may be present.
[0081] The instruments were calibrated each day before use and verified against known methane gas standards. The PID was calibrated with isobutylene. Mid-day and end of day calibration verifications were conducted to ensure accurate measurements. All calibrations and verifications were within QC performance with the exception of the PID detector which consistently failed to maintain calibration and provided a low reading bias. Recalibration necessitated removing and cleaning the PID lamp window. Since estimation of methane concentrations was based on FID readings, this had no impact on data quality for methane. The inability to maintain calibration of the PID was surprising given the highly conditioned nature (e.g., GAC filtration of introduced air, moisture removal) of the air stream prior to entry to the PID. PIDs are known to be very sensitive (negative bias) to relative humidity in an air stream. A detailed outline of QC requirements, instrument specifications, and calibration and verification performance is summarized in Table 1.
[0082] A YSI multiparameter probe was used for the measurement of pH, oxidation reduction potential, specific conductance, dissolved oxygen, and temperature of ground water during a well purge. Before field use, the instrument was calibrated and verified against known standards. Performance of each probe was also verified mid-day and at the end of the day versus standards.
[0083] Aqueous samples were collected and analyzed on an Agilent Micro 3000 Gas Chromatograph (GC) equipped with a thermal conductivity detector (TCD) to analyze fixed gases (H.sub.2, O.sub.2, N.sub.2, CO.sub.2, CO) and light hydrocarbons (C1-C9) in straight chain, branched, or cyclic forms. The micro GC is comprised of four modules, each having a sample loop, injector, pre-column, analytical column, and detector. The column, injector, and detector temperatures are all independently controlled resulting in four simultaneous independent measurements for each sample. A gas phase helium or argon blank is analyzed prior to and after analysis of standards. These blanks are used to detect the presence of background analyte concentrations or interferences in the analytical system. Field, trip, and equipment blanks are prepared and analyzed exactly the same way as samples. Gas phase reporting limits for methane, ethane, ethylene, acetylene, propane, and butane are near their lowest calibration standards of 10 ppmv using multiple point calibration. The method detection limit (MDL) for each compound is determined from seven runs at the lowest calibration standard using a Student's t-test at a 99% confidence level with n-1 degrees of freedom. MDLs are typically between 0.5-1.0 ppmv. Similarly, reporting limits for H.sub.2 and CO.sub.2 are 20 ppmv and 100 ppmv, respectively. Samples analyzed on the GC had an aqueous MDL and reporting limit for methane of 0.3 g/L and 1.3 g/L dissolved methane, respectively. A summary of quality control information is provided in Table 1.
[0084] A series of QC samples were collected which included trip blanks, field blanks, and equipment blanks (Table 2). These QC samples were filled with Barnstead NANOpure Diamond UV water and were preserved with a trisodium phosphate (TSP) pellet (sodium phosphate dodecahydrate, ACS, 98-102%) and stored and analyzed in an identical method to the field samples. Trip blanks were used to assess potential contamination from sampling, storage, and shipment to and from the field. Field blanks were used to assess potential contamination from sample bottles and environmental sources. Equipment blanks were used to assess potential contamination from sampling equipment, cleaning procedures, or sample preservation. Field QC also included field duplicate samples meant to represent the precision of sampling, analysis, and site heterogeneity. Temperature blanks were included to measure the temperature of samples in storage until analysis.
[0085] Methane was detected in field blank FieldBlk02 (12 g/L) and equipment blank EquipBlk02 (12 g/L) and were collected on Apr. 18, 2012. These blanks were shipped with field samples PGDW05 (Apr. 18, 2012) and PGPW02 (Apr. 20, 2012). These two field samples have been flagged since the methane in the blank samples was above the quantitation limit and the sample concentrations for methane were less than 10 times the concentration found in the blank. Detection of propane in FieldBlk02, and ethane and propane in EquipBlk02 likely indicate laboratory contamination of these blanks since neither ethane nor propane was detected in field samples PGDW05 and PGPW02. PGPW02 was collected two days after the blank samples and PGDW05 was collected the same day as the blank samples. Samples from PGDW20 were collected on Apr. 16, 2012, samples PGDW23 and PGDW30 were collected on Apr. 17, 2012, and sample PGDW50 was collected on Apr. 19, 2012. Field and equipment blanks associated with these samples and dates show no detection of methane, ethane, or propane.
[0086] Domestic water wells were sampled as part of a larger ground water investigation in the Pavillion oil and gas field near Pavillion, Wyoming, and are within the Wind River Basin (Illustration 1) (DiGiulio et al. 2011). Ground water samples from domestic wells were collected to evaluate potential stray gas migration as a result of gas production well completion activities. The Wind River Formation is the main formation used for domestic, agricultural, industrial, and municipal water supply.
[0087] Four domestic wells (PGDW05, PGDW20, PGDW23, PGDW30), one municipal water well (PGPW02), and one agricultural well (PGDW50) were sampled. Aqueous methane concentrations measured on the separator or equilibrator ranged from non-detect to 1470 m/L. Homeowner's existing submersible pumps were used to pump water from the wells.
[0088] Well PGDW05. PGDW05 is a domestic well with a depth of 64 m. Methane concentrations exhibited periodic variations with an overall increasing trend with purge volume (Graph 2A). The homeowner's water-well setup directed water into a 50 gallon (189 L) storage tank at outlet of the well. After the tank volume was exchanged, the well was allowed to recover for 45 minutes before re-purging. Approximately 2.6 tank volumes were purged before collecting the fixed laboratory dissolved gas sample for comparison to the separator or equilibrator. The final three separator or equilibrator field aqueous measurements had an average of 57.9 g/L dissolved methane. Fixed laboratory analysis reported 53 g/L dissolved methane. This value however was flagged due to the presence of methane in field and equipment blanks. Methane had been detected at this domestic well during previous sampling events at 17, 5.4, and 65 g/L. FID readings corrected for background (4.1 ppmv) and flow rate were as high as 160 ppmv. The pH and specific conductivity rapidly stabilized during the purge and dissolved oxygen rapidly decreased to be between 0.01 and 0.11 mg/L (Graph 3).
[0089] Additional samples collected as part of a larger sampling scheme detected the presence of lowlevel gasoline range organics (GRO) and diesel range organics (DRO) at 48 and 63.5 g/L, respectively. During operation of the separator or equilibrator, the PID detector on the TVA1000B had fluctuating responses up to 1.29 ppmv (above background) indicating additional dissolved constituents besides methane (likely the GRO and DRO). Because methane has an ionization potential around 12.5 eV, it is not detected by the PID. The PID response was low relative to the FID, so the influence on the FID reading was insignificant.
[0090] Well PGDW20. PGDW20 is a domestic well with a depth of 140 m. Measurements on the separator or equilibrator started at approximately 3,400 L purge volume (Graph 2B). The data indicate no increasing or decreasing trends, but show some variability with purge volume. The final three separator or equilibrator field aqueous methane measurements had an average of 115.4 g/L dissolved methane. Fixed laboratory analysis reported 111 g/L and 108 g/L (duplicate sample) dissolved methane. FID readings corrected for background (0.34 ppmv) and flow rate were as high as 381 ppmv. Dissolved oxygen was initially elevated and rapidly decreased to <0.1 mg/L. Specific conductivity and pH stabilized after 2,000 L purge volume (Graph 4).
[0091] Well PGDW23. PGDW23 is a domestic well with a depth of 152 m. Aqueous methane concentration increased until reaching a purge volume of 1,000 L (Graph 2C). After a 1,000 L purge volume, separator or equilibrator methane concentrations gradually decreased for the remainder of the well purge. This could indicate potential pre-purge ambient flow across the screened interval, short-circuiting across the cement sheath above the screened interval, or effects of physical and chemical heterogeneity. The final three separator or equilibrator field aqueous methane measurements had an average of 184.6 g/L dissolved methane. Fixed laboratory analysis reported 226 g/L dissolved methane. FID readings corrected for flow rate were as high as 771 ppmv (background=0.0 ppmv). The pH, specific conductivity, and dissolved oxygen rapidly stabilized (Graph 5).
[0092] Well PGDW30. PGDW30 is a domestic well with a depth of 79 m. Among all the wells sampled, PGDW30 had the highest aqueous methane concentrations (Graph 2D). Unlike the locations mentioned earlier, the aqueous methane concentration initially decreased before increasing. A significant difference existed between field and laboratory analysis. The final three separator or equilibrator field aqueous methane measurements had an average of 1,234.7 /L dissolved methane. Fixed laboratory analysis reported 384 g/L dissolved methane. FID readings corrected for background (1.16 ppmv) and flow rate were as high as 4,027 ppmv.
[0093] After 800 L purge volume, dissolved oxygen began to rapidly increase from around 0.05 mg/L to 0.44 mg/L and FID readings started to decrease. Specific conductivity and pH remained stabilized (Graph 6). Possible reasons for discrepancy between the separator or equilibrator and laboratory values include: (1) potential water from another source (not representative of the water already purged) entered the system and had lower aqueous methane concentrations when the laboratory sample was collected, (2) an improper seal on the crimp cap for the sample bottle caused a loss of methane before laboratory analysis (which would not be related to the increase in dissolved oxygen), or (3) methane exsolved from the ground water as it was pumped to the surface because of changes in partial pressure which resulted in a decreased aqueous concentration in the laboratory sample. The separator or equilibrator is capable of measuring dissolved or free gas, therefore would still measure exsolved methane.
[0094] Additional samples collected as part of a larger sampling scheme detected the presence of gasoline range organics (GRO) and diesel range organics (DRO) at 27.3 g/L, and 43.8 g/L, respectively. During operation of the separator or equilibrator, the PID detector on the TVA1000B had an increasing response up to 0.56 ppmv (above background), indicated additional dissolved constituents besides methane (likely the GRO and DRO). The PID response was low relative to the FID, so the influence on the FID reading was insignificant.
[0095] Well PGDW50. PGDW50 is an agricultural well with a depth of 61 m. Aqueous methane measurements on the separator or equilibrator began after 159 L purge volume and indicated the presence of dissolved methane (Graph 2E). Aqueous concentration decreased to levels below detection. The final three separator or equilibrator field aqueous methane measurements were all non-detect. Fixed laboratory analysis of dissolved methane also reported a non-detect value (<1.3 g/L). FID readings corrected for background (2.25 ppmv) and flow rate were as high as 166 ppmv which occurred at the beginning of purging.
[0096] PGDW50 had the highest specific conductivity of all wells sampled and rapidly stabilized. Dissolved oxygen levels were also the highest in all wells sampled and stabilized near 1 mg/L and continued to drop when methane was no longer detected (Graph 7). This water-well is located in an area with heavy cattle traffic and manure, and could explain initial detection of methane. Methane may not be a characteristic of the local water formation, but rather due to potential connectivity (infiltration) between the surface and casing well-water. It was not until the stagnant casing water was removed that actual aquifer properties were measured.
[0097] Well PGPW02. PGPW02 is a municipal well with a depth near 154 m. Aqueous methane concentrations were fairly stable for the entire purge (Graph 2F). The final three separator or equilibrator field aqueous methane measurements had an average of 8.6 g/L dissolved methane. Fixed laboratory analysis reported 8 /L dissolved methane for both the sample and sample duplicate. These values however were flagged due to detection of methane in blanks. FID readings corrected for background (4.5 ppmv) and flow rate were as high as 17 ppmv. The pH, specific conductivity, and dissolved oxygen rapidly stabilized (Graph 8).
[0098] A paired t-test was used to determine if a significant difference exists between the field separator or equilibrator method and the fixed laboratory method. It is assumed that the differences in field and laboratory methods are normally distributed and the null hypothesis is defined as no difference between the separator or equilibrator method and fixed laboratory method. Applying the paired t-test yields a t-statistic of 0.9555 and a p-value of 0.38321. At the 0.05 level, the pvalue>0.05. We fail to reject the null hypothesis and conclude that there is not enough evidence to suggest a significant difference between the separator or equilibrator method and fixed laboratory method.
[0099] In this investigation, FID response to the presence of other light hydrocarbons (ethane, propane, butane) and organic compounds evident in GRO and DRO analyses was insignificant compared to methane eliminating the need for a hydrocarbon trap (granular activated carbon) prior to the FID. However, a carbon trap will be used in future studies to ensure that other hydrocarbons do not interfere with estimating of methane concentrations.
[0100] Practical use of this equilibrator is dependent upon rapid mass transfer of methane from water to air. Non-attainment of equilibrium would result in a negative bias in field estimation of aqueous methane concentrations compared to fixed laboratory values. A negative bias was not observed in this study (Graph 9) suggesting that the combined use of the venturi ejector, static mixer, and free overfall jet stream resulted in rapid and near equilibrium conditions for liquid-gas exchange for methane.
[0101] Mass transfer can be evaluated by determining the mass transfer coefficient necessary for attainment of near equilibrium conditions using this separator or equilibrator. Mass transfer of methane from water to gas can be described by
V.sub.ddtC.sup.W=A(C.sub.WK.sup.C.sup.
[0102] This equation does not incorporate source/sink terms for methane since rates of CH.sub.4 production or degradation are likely insignificant compared to the mass exchange rate. The term A/V or a (cm.sup.1) is often called the specific surface area or interfacial area. The mass transfer coefficient (cm s.sup.1) is broken into two terms representing liquid (K.sub.L) and gas resistance (K.sub.G)
[0103] For noncondensable gases such as oxygen and methane, or compounds with a high Henry's Law Constant greater than 10.sup.3 atm-m.sup.3/mol, resistance to mass transfer is liquid phase controlled and =.sub.L (Thomas 1982).
[0104] Integration of equation 3 gives
C.sub.W(t)C.sub.G/K.sub.H
C.sub.W(i)C.sub.G/K.sub.H=exp(.sub.Lat)(5)
[0105] where .sub.La (s.sup.1) is a lumped parameter combining .sub.L and the interfacial area. This equation has been used with upstream and downstream dissolved oxygen concentrations to estimate .sub.La values for venturi devices and static mixers (Chisti et al. 1990; Goto and Gaspillo 1992; Heyouni et al. 2002). The Henry's Law Constant and enthalpy of solution (temperature adjustment) for oxygen are virtually identical to methane (Sander 1999). For oxygen aeration studies using venturi ejectors, this equation is rearranged and expressed in terms of an oxygen transfer efficiency (Baylar and Ozkan 2006) factor E or a collection efficiency factor (Agrawal 2013).
E=CC.sub.G.sup.W/(K.sup.t.sub.H).sup..sup.CC.sup.WW.sup.(.sub.((.sup.i.sup.
[0106] Rapid mass transfer is denoted by values of E approaching 1.0. In this equilibrator, time for mass transfer was approximately 10 seconds which included exchange in the plenum, venturi ejector and static mixer. For an efficiency factor of 0.95, this corresponds to a .sub.La value of 0.3 s.sup.1.
[0107] Mass transfer coefficients for venturi ejectors and static mixers in single use have achieved mass transfer coefficients in excess of 0.3 s.sup.1 (Marquez et al. 1994; Heyouni et al. 2002). However, reported .sub.La values for venturi devices and static mixtures vary by orders of magnitude and are highly dependent on design factors and temperature. Evans et al. (2001) measured .sub.La values ranging from 0.1 s.sup.1 to 0.5 s.sup.1 for a jet venturi. However, a number of studies though indicate poorer performance. Cramers and Beenackers (2001) measured .sub.La values from 0.015 to 0.03 s.sup.1. Dong et al. (2012) compared one to three venturi devices in parallel and series configurations (6 trials) and measured mass transfer coefficients for oxygen from 0.0009 s.sup.1 to 0.0033 s.sup.1 at 20 C. with the most efficient design having three venturi devices in parallel. Ozkan et al. (2006a) conducted extensive testing (72 trials) of venturi devices having design features and mass transfer coefficients for oxygen from 0.0002 s.sup.1 to 0.0187 s.sup.1 at 20 C. Park and Yang (2013) tested 10 tube tip and annular nozzle area configurations and measured mass transfer coefficients for oxygen ranging from approximately 0.0008 s.sup.1 to 0.008 s.sup.1. Rodriguez et al. (2012) measured a .sub.La value of 0.007 s.sup.1 for a venturi ejector in oxygen aeration experiments. Utomo et al. (2008) examined mass transfer coefficients for oxygen for 5 design configurations. Mass transfer coefficients ranged from approximately 0.05 s.sup.1 to 0.07 s.sup.1.
[0108] Static mixers are often combined with airlift systems to enhance aeration of water. Zhu et al. (1992) summarized data from Middleton (1978) and plotted mass transfer coefficients for bubble flow using motionless mixers. K.sub.La values increased with energy dissipation from 0.1 s.sup.1 to 5 Heyouni et al. (2002) evaluated the effect of increasing water and gas velocity on effectiveness of static mixers. K.sub.La values varied from approximately 0.1 s.sup.1 for water and gas velocities of 0.70 m/s and 0.016 m/s, respectively to 2.2 s.sup.1 for water and gas velocities of 1.30 m/s and 0.437 m/s, respectively. Water and air velocities through the static mixer in this equilibrator were approximately 0.5 m/s indicating that a mass transfer coefficient of 0.3 s.sup.1 was achievable. However, similar to venturi devices, other studies indicate poorer performance. Chisti et al. (1990) evaluated the effect of gas velocity on aeration of aqueous salt solutions. K.sub.La values for oxygen varied from 0.006 s.sup.1 to 0.03 s.sup.1. K.sub.La values increased with increased gas velocity. Goto and Gaspillo (1992) evaluated the effect of water and gas velocity on aeration of water. K.sub.LA values for oxygen varied from 0.004 s.sup.1 to 0.05 s.sup.1. K.sub.La values increased with increased gas and water velocity.
[0109] We could find no published mass transfer coefficients for the combined use of venturi ejectors and static mixtures as used in this device. With such extreme variability in reported mass transfer coefficients in the literature, it is apparent that mass transfer studies specific to this design are necessary for rigorous evaluation of attainment or near attainment of equilibrium under various operating conditions. However, good agreement between equilibrator and fixed laboratory values suggest the configuration used here achieved rapid mass transfer.
[0110] To enable real-time monitoring of aqueous methane concentrations during ground water purging, a gas-water equilibrator was designed to increase gas-water mass exchange rates beyond rates characteristic of commercially available equilibrators. Real-time monitoring of methane during purging is necessary to evaluate the effect of inadequate removal of stagnant casing water prior to sample collection and the potential effects of pre-purge ambient borehole flow and surrounding physical and chemical heterogeneityeffects common to long screened domestic and monitoring wells. Monitoring of concentration trends during purging allows for a more rigorous comparison of temporal trends between sampling events and comparison of baseline conditions with potential post-impact conditions. Other benefits of the device include real-time information and decision making in the field to help focus an investigation, aid in determining when to collect a sample, save money by limiting costs (e.g. analytical, sample transport, sample storage), and provide an immediate assessment of local methane concentrations relative to the action level for additional investigation.
[0111] Dissolved methane concentrations determined on the separator or equilibrator were in excellent agreement to reported fixed laboratory data with the exception of PGDW30 (Graph 9) which may be due to changing conditions toward the end of purging. Variations in methane concentrations during purging were observed for all locations. Similar observations have also been reported by other researchers (Harder et al. 1965).
[0112] While: (1) a negative bias for field separator or equilibrator data in comparison to fixed laboratory data was not evident, (2) a significant decrease in gas phase oxygen concentration occurred subsequent to gas-water mixing, and (3) a number of literature studies indicate that mass transfer from water to gas in venturi ejectors and static mixers can be sufficiently rapid to ensure attainment of equilibrium. There is nevertheless extreme variation in literature values for mass transfer coefficients which indicate attainment of equilibrium cannot be guaranteed using these devices. Additional field and laboratory-based experimentation and potential modification of this device are necessary to justify use beyond screening at this time.
[0113] Potential modifications include the use of additional venturi ejector and static mixers in various configurations to further enhance mass transfer and enable real-time aqueous analyses of less volatile compounds and introduction of the gas stream directly to a mass spectrometer or other device enabling rapid compound identification.
TABLE-US-00001 TABLE 1 Instrumentation and Quality Control (QC) QC Instrument Calibration Verification Requirements Stated Analyte (Detector) Range Standard Standard (Frequency) Accuracy CH4 LandTec 0-100% 50% 2.5%, 50% 1% of readings 0.3% (0-5%) GEM2000 Plus (beginning & end of 1.0% (5-15%) (IRGA) each sample event) 3.0% (15-100%) CO2 LandTec 0-100% 35% 5%, 20%, 35% 1% of readings 0.3% (0-5%) GEM2000 Plus (beginning & end of 1.0% (5-15%) (IRGA) each sample event) CO LandTec 0-2000 ppmv 100 ppmv 100, 500, 1000 10% of readings 10% (0-2000 ppmv) GEM2000 Plus ppmv (beginning & end of (EC Cell) each sample event) O2 LandTec 0-21% 20.9% 4%, 10%, 20.9% 1% of readings 1.0% (0-5%) GEM2000 Plus (beginning & end of 1.0% (5-21%) (EC Cell) each sample event) H2S LandTec 0-500 ppmv 25 ppmv 25, 100 ppmv 10% of readings 10% (0-500 ppmv) GEM2000 Plus (beginning & end of (EC Cell) each sample event) VOCs Thermo 1.0-10,000 ppmv 0.0, 10, 100, 1000, 10, 100, 1000, 90-110% of known 25% or 2.5 Scientific 10000 ppmv CH4 10000 ppmv CH4 values (after ppmv, whichever TVA-1000B calibration, beginning is greater, from (FID) & end of each sample 1.0 to 10000 event) ppmv VOCs Thermo 0.5-500 ppmv 0.0, 10, 100, 225 10, 100, 225 80-120% of known 25% or 2.5 Scientific ppmv Isobutylene ppmv Isobutylene values (after ppmv, whichever TVA-1000B calibration, beginning is greater, from (PID) & end of each sample 0.5 to 500 ppmv event) Dissolved Agilent Micro ~0.001-100 Refinery Gas Refinery Gas 85-115% of 85-115% Gases 3000 Gas MOLE % Standards #7 Standards #7 known values (After Chromatograph determined (Methane UHP & #5 and He/Ar blank at first (TCD) by calibration 26.864%) & #5 Natural Gas of analysis queue, RSKSOP 194v4 (Methane UHP Standard #1 before He/Ar blank & 175v5 4.979%) and at end of sample set, Natural Gas and every 15 Standard #1 samples) (Methane UHP 94.686%)
TABLE-US-00002 TABLE 2 Fixed laboratory results for collected dissolved gas samples and blanks Methane Methane Ethane Ethane Propane Propane Butane Butane Sample ID Date (74-82-8) QC (74-84-0) QC (74-98-6) QC (106-97-8) QC Units g/L g/L g/L g/L MDL 0.3 0.5 0.7 0.7 QL 1.3 2.7 3.8 4.7 PGDW05-0412 Apr. 18, 2012 53 B (Blk02) <2.7 U <3.8 U <4.7 U PGDW20-0412 Apr. 16, 2012 111 8 <3.8 U <4.7 U PGDW20d-0412 Apr. 16, 2012 108 7 <3.8 U <4.7 U PGDW23-0412 Apr. 17, 2012 226 19 11.4 0.9 J PGDW30-0412 Apr. 17, 2012 384 3 <3.8 U <4.7 U PGDW50-0412 Apr. 19, 2012 <1.3 U <2.7 U <3.8 U <4.7 U PGPW02-0412 Apr. 20, 2012 8 B (Blk02) <2.7 U <3.8 U <4.7 U FieldBlk01 Apr. 16, 2012 <1.3 U <2.7 U <3.8 U <4.7 U FieldBlk02 Apr. 18, 2012 12 <2.7 U 1.1 J <4.7 U FieldBlk03 Apr. 22, 2012 <1.3 U <2.7 U <3.8 U <4.7 U FieldBlk04 Apr. 24, 2012 <1.3 U <2.7 U <3.8 U <4.7 U EquipBlk01 Apr. 16, 2012 <1.3 U <2.7 U <3.8 U <4.7 U EquipBlk02 Apr. 18, 2012 12 2 J 0.8 J <4.7 U EquipBlk04 Apr. 24, 2012 <1.3 U <2.7 U <3.8 U <4.7 U TripBlk01 Apr. 16, 2012 <1.3 U <2.7 U <3.8 U <4.7 U TripBlk02 Apr. 18, 2012 <1.3 U <2.7 U <3.8 U <4.7 U TripBlk03 Apr. 22, 2012 <1.3 U <2.7 U <3.8 U <4.7 U TripBlk04 Apr. 24, 2012 <1.3 U <2.7 U <3.8 U <4.7 U QC Flags: U The analyte was analyzed for, but was not detected above the reported quantitation limit (QL) J The analyte was positively identified. The associated numerical value is the approximate concentration of the analyte in the sample (due either to the quality of the data generated because certain quality control criteria were not met, or the concentration of the analyte was below the (QL) B The analyte was found in a blank sample above the QL and the concentration found in the sample was less than 10 times the concentration found in the blank