WO1994028402A1

WO1994028402A1 - Apparatus for generating known concentrations of gases

Info

Publication number: WO1994028402A1
Application number: PCT/US1993/005130
Authority: WO
Inventors: Stephen A. Noding
Original assignee: The Dow Chemical Company
Priority date: 1993-05-28
Filing date: 1993-05-28
Publication date: 1994-12-08
Also published as: AU4397993A

Abstract

A device is provided for reliably generating a wide range of known concentrations of an oxidizing or a reducing gas in an inert carrier gas. The gas-generating electrode (42) is sandwiched between a porous diffusion membrane (44) and an ion-exchange membrane (38) which has been preconditioned by exposure to a source of a precursor ion for the gas to be generated, and the minimal void volume defined between the ion-exchange membrane and porous diffusion membrane and surrounding the gas-generating electrode is filled with a solution of the precursor ion for the gas to be generated.

Description

APPARATUS FOR GENERATING KNOWN CONCENTRATIONS OF GASES

This invention relates to apparatus for producing known concentrations of gases, principally as used for the calibration of associated sensing or detecting apparatus or for generating such gases for experimental uses

Two types of apparatus for the dynamic generation of low-level concentrations of gases in gas mixtures are widely used, i e , permeation-tube apparatus and electrolytic apparatus These are most often used to calibrate ambient monitors for determining the safe or unsafe condition of a gaseous environment, but are also used to calibrate instruments for 0 determining the concentration of a low-level gas in any gaseous stream for purposes of process control The reliability and accuracy of these apparatus when used as calibration apparatus is of critical importance if the ambient monitors and instruments are to perform their intended functions in support of a safe working environment and effective processes of manufacture, respectively 5 Permeation-tube calibrators utilize a sealed tube containing the gaseous material of interest The tube is sealed with a polytetrafluoroethylene membrane, and the gas in the tube is maintained in the liquid state or in solution Since polytetrafluoroethylene is permeable to a wide range of substances, molecules of the contained gas dissolve and diffuse through the membrane at a fixed rate into the surrounding atmosphere If a carrier gas is passed around o the permeation tube, precise and accurate concentration standards of specific gases can be generated Disadvantages of permeation-tube calibrators include limited portability, limited useful life of the tube, and limited concentration output, i e , it is difficult to produce gas levels of greater than 50 parts per million by volume

In an electrolytic calibrator, the gas in question is generated by passage of an 5 electric current through a reagent electrolyte solution The gas thus generated escapes as bubbles from the solution and is dispersed in a carrier gas stream to form a standard gas mixture of known concentration Until the early 1980's, electrolytic calibrators had the disadvantage of limited portability, as well as a lack of stability and sensitivity An improved calibrator of the early 1980's solved the portability problem, but could not generate high 0 concentrations of gas reliably and at high efficiencies

The present invention, in contrast, provides a novel and inventive electrochemical device for reliably producing a wide range of known concentrations of an oxidizing or reducing gas in an inert carrier gas An "inert carrier gas" as employed herein refers to any gas that will remain inert and not react with the oxidizing or reducing gas under the conditions of 5 generation, i e , those prevailing when the calibration process is carried out or when the oxidizing or reducing gas is generated for use in a particular experimental process

An electrochemical cell is provided in the apparatus for electrolytically generating oxidizing or reducing gases The cell includes a hollow body containing a first electrode and an electrolyte solution, which electrolyte solution provides the ions that make up the oxidizing or reducing gases The electrochemical cell also includes a second electrode at which the oxidizing or reducing gases are formed, a preconditioned ion-exchange membrane disposed between and in contact with the electrolyte solution and the second electrode, which by reason of such preconditioning permits transfer therethrough of the ions forming the oxidizing or reducing gases (from the electrolyte) to the second electrode at a rate exceeding the diffusion rate of such ions through the ion-exchange membrane in the absence of such preconditioning, while also substantially containing the electrolyte in the hollow body, a porous membrane in contact with and separating the second electrode from a mixing chamber outside the electrochemical cell, through which the oxidizing or reducing gases diffuse to the mixing chamber, and apparatus for passing a known quantity of direct electrical current between the first and second electrodes to generate a known quantity of oxidizing or reducing gas at the second electrode

In addition to the electrochemical cell, the present gas generation apparatus further includes the aforementioned mixing chamber and apparatus for passing a known quantity of optionally humidified, inert carrier gas into the mixing chamber to entrain the generated gas in the inert carrier gas, thereby generating a gaseous mixture having a known concentration of an oxidizing or reducing gas

Figure 1 is a partial cross-sectional view of an apparatus of the present invention in a preferred embodiment

The description that follows of the thus-illustrated preferred embodiment indicates the manner in which the principles of the present invention are applied, but is not to be construed as limiting the present invention to a particular construction or constructions Turning now to Figure 1, a preferred gas generation device 10 is illustrated A hollow cylindrical body 12 contains an electrolyte solution 14 in contact with a first eleαrode 16, the first electrode 16 being connected via a terminal connection 18 covered with electrical insulation 20 to a source 22 of direct current (for example, a battery or an alternating power source which has been stepped down with a direct current transformer or rectifier), to some conventional current measuring apparatus 24 (for example, an ammeter, a microammeter, or a resistor in parallel combination with a voltmeter), and optionally to conventional recording and display apparatus 26 Feedback circuitry may be used in a conventional manner to ensure current regulation in the device 10 A vented seal cap 28 seals off the top of the body 12, in part via an O-ring 30 positioned between the seal cap 28 and the top of the body 12

An ion-exchange membrane sealing assembly is located at the bottom of the body 12 for further containing the electrolyte 14 in the body 12 (the body 12 having a perforated bottom portion 32 against which the ion-exchange membrane sealing assembly is sea ngly pressed), and includes a boltable compression fitting 34 held to the body 12 via one or more bolts 36, a preconditioned ion-exchange membrane 38, and an O-πng 40 positioned between the compression fitting 34 and the body 12 A gasket, not shown, of a suitably inert material can also be placed between the ion-exchange membrane 38 and the perforated bottom portion 32 of the body 12 to improve sealing against leaks of the electrolyte solution 14 from the device 10 Sandwiched against the ion-exchange membrane 38 are a second, metallic mesh electrode 42, a porous membrane 44, and a perforated disk 46 which may suitably be constructed of the same material as the body 12, with the second electrode 42 being positioned between and in contact with the ion-exchange membrane 38 and porous membrane 44 A gasket of a suitably inert material can again be placed between the porous membrane 44 and the underlying perforated support disk 46 for improving sealing in the device 10 The second electrode 42 is connected via terminal connection 48 covered with electrical insulation 50 to the first electrode 16 and to the electrical elements described in conjunction with the electrode 16 above in a complete circuit

A flow-through mixing chamber 52 is positioned to receive gases electrolytically generated at the second electrode 42, and passing through the porous membrane 44 and through the underlying perforated disk 46 An inlet 54 communicates with the mixing chamber 52 for carrying a precisely metered and flow-controlled stream of inert carrier gas from a source thereof (not shown) into the mixing chamber 52, while an outlet 56 is provided to remove the mixed stream of gas (consisting of the carrier gas and the generated gas) from the mixing chamber 52 A drain means 58 communicates with the mixing chamber 52, and removes any water that might condense in the mixing chamber and into which the generated gas (for example, chlorine) might dissolve, thereby reducing the concentration of the generated gas in the mixed gas stream leaving the mixing chamber 52 via outlet 56

The ion-exchange membrane 38 in the device 10 has been preconditioned (prior to assembly and use of the device 10) by contacting it for a time with a source of the precursor ion of the oxidizing or reducing gas to be produced at the second electrode 42 and passed into the mixing chamber 52 By this preconditioning, the membrane 38 is effectively infused with these ions for providing an effective pathway for ion-exchange in operation of the device 10 For example, in a preferred application wherein chlorine is the desired gas, the membrane 38 can be suitably preconditioned by boiling the membrane 38 for an hour in an aqueous 5 weight percent solution of lithium chloride or hydrogen chloride

The vented seal cap 28 is vented to allow any gases generated at the first electrode 16 to pass out of the device 10, but retains any electrolyte solution 14 that might otherwise be carried over with the evolved gases For example, when chlorine or bromine is produced in the device 10, hydrogen can be produced from an aqueous electrolyte solution 14 at the first electrode 16 The vented cap 28 allows the hydrogen to escape the hollow cylindrical body 12 Electrolyte solution 14 is retained, however, by a porous membrane (not shown) carried on the seal cap 28, for example, a membrane made of porous polytetrafluoroethylene, which permits the hydrogen to pass from device 10 through vented cap 28 but which keeps the electrolyte solution 14 in hollow body 12 should the device 10 be tipped over accidentally

The size of the first electrode 16 is preferably sufficient to prevent high current densities, and resultant heat build up, from occurring The surface area of the first electrode 16 preferably is from 0 5 to 1 5 times the surface area of the second electrode 42 More preferably, the surface area of the first electrode is from 0 8 to 1 2 times the surface area of the second electrode Most preferably, the electrodes 16 and 42 have surface areas which are approximately equal The electrolyte solution 14 is generally a solution of an inorganic compound which, when eleclrolyzed, forms the desired oxidizing or reducing gas For example, and as has been suggested above, chlorine is beneficially generated by electrolyzing an aqueous solution of lithium chloride or hydrochloric acid Similarly, bromine can be generated by electrolyzing an aqueous solution of lithium bromide or hydrogen bromide It has already been mentioned that the electrolysis of these aqueous electrolyte solutions produces hydrogen gas, the companion product is hydroxyl ions, which are then neutralized with hydrochloric acid in the electrolyte solution 14 to return water Should the hydroxyl ion concentration increase, however, over time, these hydroxyl ions can instead electrolytically produce oxygen as the evolved gas and degrade the first electrode 16 Consequently, it will be necessary to periodically check the pH of these aqueous electrolyte solutions 14 and to assure that an excess of hydroxyl ions is not built up How often the pH will need to be checked will depend on the usage of the device 10, thus, where a 10 part per million (ppm, by volume) flow of chlorine is produced daily, the pH will generally need to be checked on a quarter-annual basis Where a 1000 ppm flow of chlorine is generated on a daily basis, the pH of the solution 14 may need to be checked before each use, or should be checked at least once a week

In terms of materials of construction, the first electrode 16 and the second electrode 42 are each preferably a substantially flat platinum wire mesh or screen The ion- exchange membrane 38 substantially separates the electrolyte 1 from the second electrode 42 while permitting a small amount of solution to be carried with the precursor ion generated at the first electrode 16 to the second electrode 42, and also acts as a reverse ion barrier for ions which would poison the electrolyte 14 or electrode 16 The membrane 38 can be formed from any known ion-exchange membrane material, and in general, a membrane is selected which will permit the transfer of the ion from which the oxidizing or reducing gas is generated If a halogen is to be generated, an anion-exchange membrane is chosen to permit a flow of negative hahde ions from the electrolyte 14 through the membrane 38 to the second electrode 42 If hydrogen is to be generated, a cation-exchange membrane is used to permit a flow of positive hydronium ions (H₃0 ' ) from the electrolyte 14 through the ion-exchange membrane 38 to the second electrode 42 Preferably, if a cation-exchange membrane is desired, the membrane 38 is made from a material having a polytetrafluoroethylene backbone and perfluonnated two-carbon sulfonated side chains, such as that marketed by E I du Pont de Nemours and Company, Inc , under the tradename "Nafion" If an anion-exchange membrane

5 is desired, the membrane 38 is preferably made from a styrene-divinylbenzene copolymer having quaternary ammonium side chains

The porous membrane 44 is principally designed to retain the electrolyte solution 14 in the body 12, while still permitting gases generated at the electrode 42 to be passed to the mixing chamber 52 Generally, the porous membrane 44 does not control the rate of release of

1 o the oxidizing or reducing gas from the cell Preferably, the average pore diameter of the porous membrane 44 is between 50 microns and 100 microns, and more preferably is 75 microns, which is about one thousand times the molecular size of the most common gases of interest such as hydrogen and chlorine And although the porous membrane 44 is not rate- controlling, it does provide to a degree an added function of regulating the flow of the

15 generated gas to the mixing chamber 52 so that this flow is very uniform

As previously disclosed, the ion-exchange membrane 38, the second electrode 42, and the porous membrane 44 are preferably in physical contact and form a sandwich-type structure It has been found that this structure has a direct and substantial bearing on the accuracy and sensitivity of device 10

20 If the membranes 38 and 44 are not positioned closely together, a large layer of solution will form around the second electrode 42 during operation of device 10 in which gas bubbles will form and be released as erratic bursts Moreover, the gas pressure build-up that would occur with a device having such a large layer of solution around the second electrode 42 would cause solution to be forced through the porous diffusion membrane 44 into the mixing

25 chamber 52, further decreasing the sensitivity and accuracy of the device 10.

Thus, it has been found that the void volume between the ion-exchange membrane 38 and the porous membrane 44 (that is, that volume between the ion-exchange membrane 38 and the porous membrane 44 not occupied by the second electrode 42) should be minimized to the point that gas bubbles will not form in and be released as erratic bursts

30 from the liquid layer around the second electrode 42 during operation of the device 10

In this regard, it has been found that a void volume of about ten microhters per square centimeter of the membranes 38 and 44 is the maximum volume which can be used without a substantial loss of accuracy and sensitivity in device 10 Advantageously, the void volume is essentially only comprised of the interstices of the second, metallic mesh electrode

35 42

It has also been found advantageous in a particularly-preferred embodiment of the device 10 to fill this void volume with a solution of the gas precursor ion For example, in the generation of chlorine the void volume in the interstices of the metallic mesh electrode 42 can preferably be filled with a dilute solution of lithium chloride and hydrochloric acid As the chloride ions in this small volume of anolyte are initially oxidized to chlorine, a partial positive charge is built up Because the resulting lithium ions cannot back-migrate through the anion- exchange membrane 38 into the catholyte/electrolyte solution 14, chloride ions from the 5 solution 14 are in effect pulled through the membrane 38 via the enhanced ion exchange pathways provided therein by preconditioning These chloride ions are continuously oxidized at the anode 42, thereby pulling additional chloride ions across the membrane 38

A particularly preferred use of the device 10 in this particularly-preferred embodiment is for preparing standard gas samples containing trace concentrations of chlorine, 10 using air as the inert carrier gas For example, gas concentrations of from 0 1 ppm by volume to 3 000 ppm by volume of chlorine in air can be produced reliably with device 10

For this use an anion-exchange membrane 38 is required The preferred material for forming the anion-exchange membrane 38 is again a styrene-divinylbenzene copolymer having quaternary ammonium side chains The electrolyte solution 14 is preferably an aqueous 15 solution of sodium chloride, lithium chloride or hydrochloric acid

Upon passage of a current through the device 10, chlorine gas is initially generated at the second electrode 42 from a thin film of solution at the surface of the anion- exchange membrane 38, and chlorine generation proceeds as discussed above Air as the carrier gas flows through inlet 54 into the chamber 52 and across the surface of the membrane 20 44, where it entrains and mixes with the chlorine generated at the second electrode 42

The direct current flowing through the electrodes 16 and 42 is accurately measured by known apparatus and methods and the rate at which chlorine is generated may be readily calculated through titration with potassium iodide solution, for example, and through the known electrochemical stoichiometπc equivalence The rate of air flow can be 25 likewise accurately controlled and measured, for example, with an accurate gas pump and flowmeter From the known rate of air flow and the known rate of chlorine generation, the concentration of chlorine in the air leaving the device 10 through the outlet 56 can be calculated with a high degree of accuracy, thereby providing a useful gas standard for calibrating other instruments or for use in experimental processes where very accurate known 30 concentrations of a desired gas is required

Under certain circumstances, it may be desirable to calibrate an associated sensing or detecting device with a gas stream at 100 percent relative humidity High humidities can conveniently be achieved in the present device 10 by purging the carrier air flow through water before mixing this air with the newly-generated chlorine 35 The present invention is further illustrated by the following examples EXAMPLE 1

The calibration device 10 of the present invention was constructed with an anion- exchange membrane 44 made from a styrene-divinylbenzene copolymer with quaternary- ammonium side chains Chlorine was electrolytically generated at a known, constant, and controlled rate from an electrolyte solution 14 consisting of a 3 molar aqueous hydrochloric acid solution including 10 weight percent of lithium chloride, while metering a constant flow of air as a carrier gas through the device 10 Current from a constant-current power source was passed through the electrolytic cell and the current measured in milliamps (mA) The data obtained are shown in Table I

10

TABLE I CHLORINE GENERATION

(a) Air flow of 445 cm3/minute

(b) Air flow of 105 cm3/minute

In Table I, "Theoretical ppm" (or the concentration of chlorine that could

25 theoretically be generated from application of the current and at the given air flow rate) was determined through the known electrochemical stoichiometnc equivalence for chlorine of 193,000 coulombs per mole of chlorine produced "Actual ppm" was determined, as suggested above, by titration of the chlorine produced and collected over the elapsed time with 0 01 N potassium iodide solution The "Elapsed Time" is an indication of the stability and reliability of

30 the device for generating a given concentration of chlorine over a period of time Under actual use conditions, the time on line typically would be 30 minutes or less As shown in Table I, a wide range of gas concentrations can be accurately and reliably produced over long periods by the apparatus of the present invention

While certain representative embodiments and details have been shown and/or

35 exemplified for the purpose of illustrating this invention, it will be apparent to those skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope of the invention, as more particularly defined in the claims thereto

Claims

C l a lm s :

1 Apparatus for generating a gaseous mixture of a known concentration of an oxidizing or reducing gas in an inert carrier gas, comprising an electrochemical cell which includes a hollow body containing a first electrode and an electrolyte solution of precursor ions for the oxidizing or reducing gas to be generated, a second electrode at which the oxidizing or reducing gases are formed from the precursor ions from the electrolyte solution, a preconditioned ion-exchange membrane disposed between and in contact with the electrolyte solution and the second electrode, a porous membrane in contact with the second electrode on an opposite side of the second electrode from the preconditioned ion-exchange membrane, and through which the oxidizing or reducing gas diffuses, and apparatus for passing a known quantity of direct electrical current between the first and second electrodes to generate a known quantity of the oxidizing or reducing gas at the second electrode, a mixing chamber in communication with the porous membrane for receiving the oxidizing or reducing gas diffusing therethrough, and apparatus for passing a known quantity of an inert carrier gas into the mixing chamber to entrain the oxidizing or reducing gas received therein in the inert carrier gas The device of Claim 1 , further comprising apparatus for humidifying the inert carrier gas prior to introduction into the mixing chamber

3 The device of Claim 1 , wherein the void volume not occupied by the second electrode between the ion-exchange membrane and the porous membrane is minimized to the point that gas bubbles will not form in and be released as erratic bursts from the liquid layer that is formed in this void volume during operation of the device

4 The device of Claim 2, wherein the void volume not occupied by the second electrode between the ion-exchange membrane and the porous membrane is minimized to the point that gas bubbles will not form in and be released as erratic bursts from the liquid layer that is formed in this void volume during operation of the device 5 The device of Claim 3, wherein the void volume has a maximum volume of about 10 microliters per square centimeter of surface area of the membranes

6 The device of Claim 4, wherein the void volume has a maximum volume of about 10 microliters per square centimeter of surface area of the membranes

7 The device of Claim 5, further comprising an amount of a solution containing the gas precursor ion in the void volume between the ion-exchange membrane and the porous membrane 8. The device of Claim 6, further comprising an amount of a solution containing the gas precursor ion in the void volume between the ion-exchange membrane and the porous membrane.