Method for a portable sampling system

Abstract

A method for drying a gas sample comprises flowing purge gas over the exterior of tubes of a perfluorosulfonic acid membrane and flowing the gas sample through interior of tubes, wherein the drying operation is conducted under deep vacuum and with the purge gas flowing at a rate that is typically less than that of the gas sample being dried.

Claims

1. A method for drying a gas-phase sample with a purge gas, the method comprising: obtaining a gas-phase sample from a flue stack of a combustion device using a sample probe, the gas-phase sample has a first mass flow rate; filtering particulates out of the gas-phase sample at the sample probe passing the gas-phase sample through a very flexible conduit, wherein the conduit comprises: (a) a single dryer comprising plural tubes of perfluorosulfonic acid membrane that dries the gas-phase sample, and (b) a heating jacket that covers a portion of the length of the plural tubes, wherein the portion is less than about 20% of a total length of the conduit; and flowing purge gas, under deep vacuum or near-deep vacuum over an exterior surface of the plural tubes, and at a mass flow rate that is less than the first mass flow rate.

2. The method of claim 1 wherein flowing a purge gas over an exterior surface of the tubes further comprises flowing the purge gas at a flow rate that, in addition to being less than the mass flow rate of the gas-phase sample, is sufficient to reduce a dew point thereof to 4° C. or less.

3. The method of claim 1 wherein the mass flow rate of the purge gas is in a range of about 0.12 to 0.3 of the mass flow rate of the gas-phase sample.

4. The method of claim 1 wherein the tubes and the conduit have substantially the same length.

5. The method of claim 1 wherein a dew point of the purge gas is in a range of 10° C. to 35° C.

6. The method of claim 1 wherein the purge gas is a wet purge gas.

7. The method of claim 1 wherein the tubes have a length of about 1.5 meters.

8. The method of claim 1 wherein the portion is less than about 10 percent of the length of the conduit.

9. A method for drying a gas-phase sample with a wet purge gas, the method comprising: obtaining a gas-phase sample from a flue stack of a combustion device using a sample probe; filtering particulates out of the gas-phase sample at the sample probe; drying the gas-phase sample in a single dryer having less than about twenty tubes of perfluorosulfonic acid membrane disposed in tubing, the tubes and the tubing being very flexible, and wherein less than about 20 percent of a length of the tubing is covered by a heating jacket, wherein drying further comprises: (a) flowing the gas-phase sample through an interior of the tubes, and (b) flowing the wet purge gas under deep-vacuum or near-deep vacuum over an exterior of the tubes, wherein a mass flow rate of the purge gas is less than a mass flow rate of the gas-phase sample and is sufficient for reducing the dew point of the gas-phase sample to 4° C. or less.

10. The method of claim 9 and further wherein flowing the wet purge gas comprises flowing the wet purge gas at a mass flow rate in a range of about 0.12 to 0.3 of the mass flow rate of the gas-phase sample.

11. The method of claim 9 wherein less than about 10 percent of the length of the tubing is covered by the heating jacket.

12. The method of claim 9 wherein the tubes have a length of about 1.5 meters.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 depicts a portable sampling system in the prior art.

(2) FIG. 2 depicts a portable sampling system in accordance with an illustrative embodiment of the present invention.

(3) FIG. 3 depicts a cross-sectional view of a membrane dryer for use in the portable sampling system of FIG. 2.

(4) FIG. 4 depicts a method for drying in accordance with an illustrative embodiment of the present invention.

(5) FIG. 5 depicts a plot of dryer length as function of the ratio of the purge-gas flow rate to the sample-gas flow rate.

DETAILED DESCRIPTION

(6) The terms below are provided with the following explicit definitions for use in this disclosure and the appended claims: The term “wet,” when used to describe the purge gas or ambient air, means having a dew point in the range of 15 to 30° C. The phrase “deep vacuum” means a vacuum of 0.1 bar absolute or less. The phrase “near deep vacuum” means a vacuum of greater than 0.1 bar absolute and less than or equal to 0.2 bar absolute. The phrase “shell side” of a membrane dryer means the region on the outside of the tubes of the drying membrane (e.g., PFSA, etc.) and inside of the outer tube that contains the tubes of the drying membrane. The phrase “tube side” of a membrane dryer means the region within the tubes of the drying membrane (e.g., PFSA, etc.). The phrase “purge gas” means a gas that is introduced to the shell side of a membrane dryer, which is to be swept over the outside of the tubes for assisting in removing moisture from a gas that flows within the tubes. The phrase “very flexible” means something that is capable of being formed into a loop (i.e., a circle) having a diameter of 8 inches or less. The term “substantially” means within +/−15 percent of a nominal value. For example, if a first member and a second member are described as being “substantially the same length,” then the first member can have a length that is in the range of 15% less to 15% greater than the length of the second member. The term “about” means within +/−15 percent of a nominal value; that is, synonymous with “substantially.” The term “gas” means one or more gases (a substance having a single defined thermodynamic state at room temperature) and/or one or more vapors (a substance in which the gas phase and liquid phase can co-exist). For example, “flue gas” typically includes both gas and vapor, and more than one of each.

(7) FIG. 2 depicts portable sampling system 200 in accordance with the illustrative embodiment of the present invention. The salient features of portable sampling system 200 include sample probe 204, dryer 226, and I/O system 222, interconnected as shown.

(8) Sample probe 204 obtains sample 101 of flue gas from a flue stack (in conjunction with pump 124 in I/O system 222). The sample probe includes probe shaft (or “stinger”) 106 and probe body 208. Probe shaft 106, which is inserted into a flue stack to obtain a sample, comprises a metal, such as stainless steel or Hastelloy, suitable for exposure to high temperatures and the corrosive nature of flue gas.

(9) Filter 102, which is intended to filter out particulates from the flue gas sample, is fitted to the distal end of probe shaft 106 Filter 102 is typically a sintered metal or wire-mesh filter capable of filtering out particles as small as 10 microns.

(10) In the illustrative embodiment, probe body 208 includes heater 210, ammonia scrubber 240, optional inlet dew-point sensor 242, and vacuum pump 228. Heater 210 heats the probe body to prevent condensation from occurring, which would knock sulfur dioxide out of the gas sample. Also, heater 210 is used to heat ammonia scrubber 240.

(11) Ammonia scrubber 240 is used to remove ammonia from flue-gas sample 101. The ammonia scrubber protects the downstream gas analyzer from clogging due to the formation of ammonium salts. Ammonia, which is a highly reactive gas, is occasionally added to stack gases to reduce the nitrogen oxide content of the gases by conversion to nitrogen and water. But when present in gas samples, ammonia will readily react with other components in the gas sample, such as sulfur dioxide, to form ammonium salts. This salt is relatively low-boiling, so it is present as a gas at the higher temperatures in the stack. But when the flue-gas sample cools down while passing dryer 226, the salt precipitates out as a solid, clogging the dryer or downstream analyzer.

(12) Ammonia scrubber 240 comprises a polysulfone shell that surrounds a stainless-steel-shell housing or, alternatively, both the shell and housing are stainless steel. The housing contains a phosphoric-based scrubbing media and inert ceramic burl saddles. The water vapor in the sample activates the scrubbing media to produce phosphoric acid. The phosphoric acid reacts with the ammonia, in an acid-base neutralization reaction, producing a phosphate of ammonia. This compound is a solid even at elevated temperatures, and deposits immediately within the ammonia scrubber as a visible salt residue. For proper operation, the ammonia scrubber should be kept at a temperature above the sample dew point to avoid the loss of water-soluble analytes. Heater 240, or a heater integrated (e.g., an electrical resistance strip heater, etc.) with ammonia scrubber 240, is used for this purpose.

(13) Inlet dew point sensor 242, if present, determines the dew point of flue-gas sample 101.

(14) Vacuum pump 228, which is commercially available, is used to pull a near-deep vacuum (i.e., greater than 0.1 bar absolute to about 0.2 bar absolute) or, preferably, a deep vacuum (0.1 bar absolute or less) on the outside of the Nafion™ tubes within dryer 226.

(15) Dryer 226 provides at least a dual functionality; in addition to drying, it fluidically couples the sample probe 204 to I/O system 222 and provides the requisite 1.5 meters or more of length. A short length of heating jacket 114 encloses the first 0.1 m or so of dryer 226 to ensure that no cooling—and hence no condensation—occurs.

(16) In accordance with the present teachings, dryer 226 comprises relatively few tubes (e.g., less than twenty and more typically in the range of 6 to 12 tubes) of a suitable PFSA membrane disposed within a thermally insulated “shell.” In the illustrative embodiment, six tubes of 050 Nafion™ (inner diameter of 0.05 inches) is used. In the illustrative embodiment, the shell comprises tubing made of fluorinated ethylene propylene (FEP). In the illustrative embodiment in which six tubes of 050 Nafion™ are used, the FEP tubing has an inner diameter of ⅜ of inch. In some embodiments, both the Nafion™ tubes and the shell are about the same length; in the illustrative embodiment, that length is about 1.5 m, but more generally is in the range of about 1.5 m to 5 m.

(17) FIG. 3 depicts a cross-sectional view of dryer 226, wherein six Nafion™ tubes 350 are disposed within shell 352. In some other embodiments, other PFSA membranes may suitably be used. Furthermore, by virtue of the length of the dryer, the relatively low number of Nafion™ tubes in the dryer, and the low value for Young's modulus of the materials used (i.e., Nafion™ and FEP tubing), the dryer is very flexible.

(18) In conjunction with the present disclosure, those skilled in the art will be able to design, make, and use dryers incorporating PFSA membranes.

(19) The distal end of dryer 226 enters I/O system 222. I/O system 222 includes vacuum pump 124, optional dew point sensor 246, flow meter 248, and particulates filter 244. The I/O system is contained in a housing, typically made of metal or hard plastic.

(20) Through the action of vacuum pump 228, I/O system 222 draws in ambient moist air 105, which serves as the purge gas for dryer 226. In the illustrative embodiment, vacuum pump 228 is situated in probe body 208. However, in some other embodiments (not depicted), vacuum pump 228 is disposed in I/O system 222 rather than the probe body, wherein a tube running alongside dryer 226 conveys the purge gas back to the vacuum pump.

(21) Air 105 is filtered in filter 244, for removal of particulates. Air filter 244, which comprises a fluorocarbon borosilicate glass microfiber element (commercially available from United Filtration Systems of Sterling Heights, Mich. or others), is suitable for removing particulates having a size 1 micron or greater. The filtered air is delivered to distal end 227 of dryer 226 via tubing 132, which includes a flow restriction (not depicted) to throttle the flow of air into the dryer. The flow restriction can be implemented as a hole in a plug (i.e., an orifice), a needle valve, or the like. Alternatively, restriction tubing can be used, with employs a longer length of tubing of larger diameter, which is advantageous because it is less likely to be blocked by a particulate that passes filter 244.

(22) Air 105, which is not dried as in prior-art system 100, is delivered to the “shell” side of dryer 226, passing over the outside of the Nafion™ tubes therein. In some other embodiments, air 105 is subjected to moisture removal, but not via a PFSA-based dryer. For example, in such other embodiments, air 105 is passed through a dessicator or a condensing cooler.

(23) In addition to drawing in ambient air, I/O system 222 outputs the conditioned flue-gas sample 103 to the gas analyzers (not depicted). The conditioned flue-gas sample exits dryer 226 into conduit 134 and is drawn through vacuum pump 124 (which provides the suction for drawing flue gas sample 101 from a flue-gas stack). Conditioned flue-gas sample 103 then passes through in-line optional outlet dew-point sensor 246 (if present), flow meter 248, and exits I/O system 222 to gas analyzers for analysis.

(24) FIG. 4 depicts method 400 for drying a gas stream to a low dew point, such as 4° C. or less, using a wet gas source, such as ambient air. Method 400 can be used for drying flue-gas gas samples, for subsequent analysis, as in the illustrative embodiment. Furthermore, method 400 can be used for any application in which a dry purge gas is unavailable and a gas stream must be dried to low dew points (e.g., 4° C. or less). It is to be understood that although portable sampling system 200 was developed for use with gas sources that are wet (e.g., wet ambient air, etc.), the sampling systems and methods disclosed herein can be used for drying a sample gas when a dry gas source (e.g., dry instrument air, relatively dry ambient air, etc.) is available.

(25) In accordance with operation 401, a membrane dryer is provided. The membrane preferably, but not necessarily, comprises PFSA. As previously noted, PFSA dryers include Nafion™ dryers, among others. A sample gas that requires drying is drawn through the inside of the drying membranes (the tubes of PFSA membrane, etc.), per operation 402. In operation 403, a gas, such as moist ambient air, is accessed for use as the purge gas. The gas is drawn over the shell-side of the membrane dryer at either near-deep or deep vacuum, in accordance with operation 404.

(26) FIG. 5 depicts a plot of dryer length as a function of the ratio of the mass flow rates of the purge gas to the sample gas (“gas-flow ratio”). More specifically, the y-axis provides the length, in inches, of PD-6T-based dryer required to achieve −10° C. dew point for 1 liter per minute (lpm) of sample gas flow. FIG. 5 is based on the following conditions: dryer used: PD-6T (6 tubes of 050 Nafion™ (inner diameter of 0.05 inches)); vacuum level: deep vacuum (0.1 bar absolute or less) on the outside of the tubes within the dryer; sample gas: inlet: 70° C. dew point outlet: −10° C. dew point; purge gas: inlet: 15° C. dew point.

(27) FIG. 5 shows that for a PD-6T-based dryer having a length of 1.5 m (59 inches) and under the stated conditions, the target moisture level (i.e., −10° C. dew point) of the sample gas is met, surprisingly, at a gas-flow ratio as low as about 0.15. The length-versus-gas-flow-ratio relation has a generally exponential form at ratios below about 0.2. Per FIG. 5, for PFSA dryers running at 0.1 bar absolute, there is a “knee” in the curve between 0.1 and 0.2 gas flow ratio where performance drops off dramatically and the length of the drying membrane required to achieve acceptably low dew point becomes unacceptably large. A significant amount of performance benefit is achieved by increasing the gas flow ratio to 0.3, but above that, there is a relatively minor performance increase while, at the same time, the size of the vacuum pump increases tremendously due to the increased gas flow.

(28) Assuming the gas-flow ratio remains constant, the length of drying membrane required to reach the target dew point increases linearly with the flow rate of the gas sample. In embodiments in which a PD-6T-based dryer shorter than 1.5 m is used, one can reduce the sample flow rate to achieve the target dew point. In this regard, most gas analyzers require 1 lpm of gas flow and, consequently, most portable sampling/conditioning systems are sized for 1.5 lpm (to provide margin for the analyzer).

(29) It will be clear to those skilled in the art that in embodiments in which a dryer contains more than six tubes of 050 Nafion™, the length of the dryer required for a given set of conditions would be reduced relative to a PD-6T dryer. As a first order approximation, the requirement scales linearly. That is, for a Nafion™-based dryer with twice the number of tubes (i.e., twelve) than a PD-6T dryer, the length of the dryer is reduced by a factor of two. The shape of the plot depicted in FIG. 5 applies to all PFSA membrane dryers when purged at 0.1 bar absolute, although for PFSA membranes other than Nafion™, additional tubes or length will required.

(30) There is no specific limitation on the maximum gas-flow ratio, except that, as previously explained, high purge-gas flow rates might cool the dryer to the extent that condensation occurs therein. In that regard, optional dew-point sensor 246 can be used in a control loop that adjusts purge-gas flow rate, such as to optimize the cooling/temperature gradient across the dryer while avoiding any condensation. As previously noted, the flow rate of the purge gas is dictated, to some extent, by the operation of the vacuum pump (i.e., the pump's operating curve). Additionally, as purge-gas flow rates increase, the size of the vacuum pump will increase, as well, incurring cost and weight penalties.

(31) In some embodiments, the gas flow ratio is less than 1 (i.e., the mass flow rate of the purge gas is less than the mass flow rate of the gas sample). More particularly, in some embodiments, the gas flow ratio should be in the range of about 0.12 to about 0.99. In some other embodiments, the gas flow ratio is less than about 0.50. And in some additional embodiments, the gas flow ratio is in the range of about 0.15 to about 0.50. In some preferred embodiments, the gas flow ratio is in the range of about 0.15 to 0.3. In some further embodiments, the gas flow ratio is in a range of about 0.12 to 0.2.

(32) It is to be understood that the disclosure teaches just one example of the illustrative embodiment and that many variations of the invention can easily be devised by those skilled in the art after reading this disclosure and that the scope of the present invention is to be determined by the following claims.