US2980513A - Combustibles-in-air instrument - Google Patents

Description

April 18, 1961 s. F. KAPFF coMusnLEs-lN-AIR INSTRUMENT 4 Sheets-Sheet 1 Filed Oct. 28, 1957 :7A/ding April 18, 1961 S. F. KAPF F COMBUSTIBLES-IN-AIR INSTRUMENT Filed OCT.. 28. 1957 4 Sheets-Sheet 2 GEL v//fe/vo col/nf j L April 18, 1961 s. F. KAPFF COMBUSTIBLES-IN-AIR INSTRUMENT 4 Sheets-Sheet 5 Filed Oct. 28, 1957 D. c. Pon/E2 Q7 SUPPL Y April 18, 1961 s. F. KAPFF 2,980,513

COMBUSTIBLES-IN-AIR INSTRUMENT Filed oct. 28, 1957 4 sheets-sheet 4 Unite 2,980,513 'COMBUSTIBLES-NMR INSTRUMENT 2 Claims. (Cl. 23-255) This invention relates to the operation of an air separation process. More particularly, the invention relates to a method and means for detecting total C2-C4 hydrocarbons present in air charged to an oxygen plant.

The operation of plants for the production of pure oxygen or nitrogen from air basically involves a lique- -action of the air and subsequent fractionation of the ycomponents in a distillation tower. This distillation is performed at low temperatures and produces an overhead product of pure nitrogen and a bottoms product of liquid loxygen. In the liquid oxygen will be found any sub- ;stances present in the entering air which are not trapped out in the early cooling stages and have boiling points lhigher than oxygen. Some of the materials of interest are the hydrocarbons in the C2-C4 range of molecular weights. The presence of sizable quantities of these in .the liquid oxygen presents a serious explosive hazard.

The safe operation of such a plant therefore involves maintaining the hydrocarbon concentrations as low as possible in the liquid oxygen. The concentrations convsidered tolerable vary for the diierent hydrocarbons. Acetylene is by far the most dangerous since in concentrations of a few p.p.m. it reaches its solubility limit in liquid oxygen. The solid acetylene which then separates -can detonate under certain conditions causing serious explosions. The concentrations of other hydrocarbons from ethylene to the butanes are not so critical since these are much more soluble in liquid oxygen. However, it is desirable to hold their concentrations as low as possible, since, if any explosion should occur, thesek would furnish an additional fuel supply.

Monitoring of the air stream at various points in the process is desirable for early detection of hydrocarbon contamination. An object of this invention is, therefore, to provide an instrument capable of analyzing for hydro carbons in air or oxygen streams in the concentration range of -10 ppm.

Detection of hydrocarbons in air in this concentration range normally requires highly accurate instrumentation and extremely sensitive detectors. It is, however, a further object of this invention to provide means whereby the hydrocarbon can be concentrated from a large volumek of air into a small volume for detection and measurement. According to my invention, CO2 and H2O are removed from a monitoring sample stream. The sampling point is generally chosen so that this has been done by the plant. If not, then removal means must 'be provided in connection With the instrument. The sample stream is then passed through a cold silica gel rcolumn for con centration of the hydrocarbons on the cold silica gel. Periodically the gel is -heated to remove the adsorbed hydrocarbons as a sudden burst which, can readily 4be detected. y

Such detection of the hydrocarbons in the air stream after desorptionV from the gel is by means of a catalytic States arent e l CC nurn wire. A Wheatstone bridge circuit containing the catalytically active platinum filament and an identical but non-catalytic filament permits accurate measurement of such changes. Both lilaments are exposed to the air stream but only one increases in temperature.

Further details and advantages of my invention Will be described by reference to a preferred embodiment illustrated in the accompanying drawings wherein:

Figure 1 is al diagrammatic illustration of one system according to my invention;

Figure 2 is an elevation, partly in section, of the hydrocarbon concentration cell; k

`Figure 3 is a circuit diagram of the system including the hydrocarbon detector; and

Figures 4, 5 and 6 are views illustrating the hydrocarbon detector apparatus.

Referring to the drawings, the air sample, after drying and CO2 removal, is pumped by pump 10 at a constant rate from line 11 through the refrigerated hydrocarbon accumulator 12 and then through the detector cell 13. A timing -mechanism 14, -which is started when the gel 1,5` in column 16 becomes cold enough to adsorb hydrocarbons such as ethylene, permits the passage of air through the gel 15 for a period of about an hour. The gel is then heated by heater 17 and the adsorbed material desorbed into the air stream 18 and measured by the detector cell 13. Just prior to the desorption, a small amount of hydrocarbon is injected into the air stream 18 to check the detector .13 and to supply `a standardization signal.

The temperature of the gel column 16 is controlled from a thermocouple 20 located in the air space 21 between the cooling helix 22 and the gel column 16. The thermocouple output is fed to a controller 23 which operates valve 24 to introduce liquid nitrogen into -coil 22.

Details of the gel column 16, the refrigeration coil 22, and accumulator 12 are shown in Figure 2. The accumulator 12 is a double-Walled vacuum vessel (Dewar flask) closed at its top by plate 12a which is secured by clamping rings 12b and 12e and bolts 12d to the flanged opening of vessel 12.

The gel 15 is contained in the central glass helix 16 which has a length of about 4 ft. over-al1 and an inside diameter of 6 mm. About 2% ft. of this length is filled with Davison #70 silica gel 15 of about 8 mesh. Around this glass tube 16 is tightly wound a coil 17 of bare Nichrome wire (0.032" diameter and 31 ohms total resistance) for heating the column 16 during the desorption step. At the outlet of gel column 16 is the thermocouple 56 to measure the temperature of the air leaving the gel 15. This thermocouple 56, through suitable relays 58 and 60, starts the timer 14 when the air temperature reaches F. The coil 22 of 1%@ inch copper tubing fitted closely to the inner Wall of the insulated vessel 112 serves to conduct coolant (liquid nitrogen) into the system whenever the temperature at the thermocouple Ztl is above 255 F. f

The system used for pumping the Iliquid nitrogen into the low temperature bath comprises a reservoir 32 for liquid nitrogen which is delivered by lines 33 and 34 and solenoid valve 24 to the insulated low temperature bath 12. A constant pressure controlled by valve 39 is maintained on the surface Aof the liquid nitrogen 36 by cornpressed gas introduced into reservoir 32 by line 37 from compressed gas cylinder 38. A 15 p.s.i. pressure relief valve 37a is provided on valved line 37.

Another system for pumping the liquid nitrogen may comprise a solenoid valve normally open to the atmosphere and in series with a heater (not shown) immersed in the liquid nitrogen 36. vUpona signal from the con troller 23, this circuit is energized, closing the valve and turning on the heater. The evaporation of liquid nitro1 gen produces a pressure 'in the closed space forcing liquid nitrogen out of the reservoir 32 to the low temperature bath 12. When the accumulator 12 has cooled to a preselected temperature, the heater is turned off and the valve opens reducing the pressure to atmospheric.

The detector cell 13 shown in some detail inFigures 4, and 6, includes two platinum wires 37 and 38 mounted in chambers 43 and 44 and exposed to the gas stream 18 by short diffusion paths. The use of two filaments 37 and 38, one catalytically active and one rendered inactive, 'in the same st-ream reduces the effects` of flow rate and thermal conductivity changes in cell 13. The filaments 37l and 38 may comprise identical platinum wires, one of which has been deactivated by suitable treatment, such as by treating with silicone vapors according to a procedure described in the literature (I. Chem. Physics 16 (1948), p. 237).

For rapid response the diffusion path from thel stream 18 to the filaments 37 and 38 should be short and of large area. However, at air flow rates of 0.05 to 0.1 c.f.m., considerable turbulence near the wires 37 and 38 makes such an arrangement unsatisfactory unless special precautions are taken. The most satisfactory arrangement for giving fast response without the undesirable turbulence at the laments 37 and 38 has been found lto involve the insertion of a single layer of fine mesh Monel screen 42, as shown, around the gas stream as it passes through the cell 13.

Figure 3 illustrates the control circuit for the system including the ilaments 37 and 38 in the hydrocarbon detector circuit. The detector circuit includes the catalytically active lament 37 and the inactive tilament 33 in adjacent arms of the Wheatstone bridge circuit 45 including the 500 ohm bridge resistor 71, the 500 ohm bridge resistor 72, the 100 ohm potentiometer 73 provided with adjustable slide wire contact 74, and the recorder 49. The diierences in resistance in the filaments 37 and 38 resulting from the occurrence of catalytic oxidation of trace amounts of hydrocarbons on catalytically active filament 37 unbalance the bridge circuit 45 which unbalance is recorded on recorder 49. Potentiometer 73 serves as a zero adjustment.

Closing the switch 46 energizes the D.C. power supply 47, the detector circuit 45, the sample feed pump 10, the recorder 49, the refrigeration-temperature control 23, and the pilot light 51. Refrigeration control 23 operates relay 52 providing power to valve 53 and pilot light 54 as long as thermocouple 20 indicates that the adsorption assembly 12 is too warm. When the proper temperature is reached, controller 23 operates relay switch 52 intermittently to maintain substantially constant temperature in adsorption assembly 12.

When the air stream 18 leaving gel column 16 of Figure 2 has reached a predetermined temperature, this is sensed by thermocouple 56 which actuates sensitive relay 57 which provides power to a heavy duty relay 58 starting timer 14 by closing relay 6i). Relay 60 provides power to timer motor 61 and pilot light 62. Near the conclusion of the timing cycle, switch 63 is actuated by a timer cam (not shown) providing power to open standardizing gas valve 64 and illuminate pilot light 65. Switch 63 is next returned to its original position by timer .cam de-energizing valve 64. Switch 66 next operates from a timer cam removing power from the refrigeration valve 53 and supplying power to the gel heater 17, pilot light 68, andthe reset 69 and accompanying resistor 70 for sensitive relay 57. v

The introduction of a standardization gas to check the activity of the platinum laments 37 and 38 before starting desorption of the gel may present diiculties because of the small volume of gas to be introduced into the air stream. The use of cylinders of pure gases is most desirable since then no calibration need be made when a new cylinder is required.

A system for introducing standardizing gas, such as isobutane or any other combustible gas of high purity, is provided which is shown in Figure l. The cylinder gas system 76 is maintained at 1.5 p.s.i.g. by valve 76a up to the solenoid valve 64. Between this valve 64 and the detector cell 13 is ahigh resistance leak which may comprise a needle valve 77, a calibrated glass capillary of proper ow, and the like. When the valve 64 is opened by the timer 59, a small ow of gas (1.0 cc./ min.) passes through valve 77 into the air stream 1S in line 77a.

A second system for introducing the standardizing gas is to provide two valves arranged to dene a small volurne of gas between them at a pressure slightly greater than atmospheric. The valve on the cylinder side is normally open and the valve toward the system is nor- A fourth system for standardization might involve they use of a 3-way valve and a liquid of low vapor pressure, such as dodecane. When standardizing, the entire air stream may be diverted to bubble through a layer of this liquid. Thermostating maintains the liquid at constant temperature (Le. constant vapor pressure).

Calibration of the equipment has been accomplished by injection of 'small quantitiesl of pure gases into the Y air stream followed by a desorption of same from the gel.

lt has been determined experimentally that such an' addition is retained on the cooled gel for at least one hour so that the addition of hydrocarbon in aV single burst is equivalent to small additions all during the adsorption period. Hence the injectedv sample can be considered to have been contained in the total volume of air passed through the gel during the adsorption period. As 4mentioned earlier, the calibration will vary somewhat for dilferent hydrocarbon because of their different behavior during combustion on the detecting ilaments.

For obtaining increased sensitivity, the operation of the instrument may be changed to permit desorbing adsorbed hydrocarbons into an air stream moving through the apparatus at lower velocity than that obtained during the collection period. This has the double effect of producing a greater hydrocarbon concentration by desorbing into a smaller air volume and of passing the sample through the detection cell 13 more slowly, permitting more time for diffusion into the filament chambers 43 and 44. Each of these elects will result in increased signals.

In a typical record, large standardizationpeaks occur at timed intervals while the peaks following closely thereafter are those arising from the desorption step.

It has been found that if the gel 15 is heated gradually, the various hydrocarbons adsorbed thereon will desorb at characteristic temperatures. This gradual heating may be accomplished, for example, by inserting between switch 66 and gel heater 17, by means of a stepping relay, suitable resistances in predetermined sequence to varyl the current flowing in heater 17. Hence calibration with known gases will permit qualitative and quantitative estimation of the gases adsorbed.

If an analysis of the gas being desorbed from a gel is desired, the output of the detector 13 is plotted on one axis of an X-Y recorder with the other axis representing the temperature of the silica gel column. The record so obtained indicates hydrocarbon concentration as a function of the temperature of desorption.

It is contemplated that adsorbents other than silica gel may be used including molecular sieve-type adsorbents, activated carbon and alumina, and the like.` A double type adsorbents may be used to accumulate methane after the C22-C5 hydrocarbons have been retained by the silica gel column. The desorption of the two columns may be suitably programmed and two desorption signals shown on the recorder.

The invention has been described in terms of a system for concentrating dilute hydrocarbons from oxygen-rich streams. The invention has further been described in connection with a detector for the concentrated hydrocarbons comprising a device wherein they are oxidized. However, in some circumstances, it may be desirable t0 preserve the hydrocarbon components and the' detector may comprise non-destructivedetectors, such as thermal conductivity cells, gas gravity balances, chromatographic analyzers, and the like. In any event, the concentration of the hydrocarbons as taught herein makes possible the use of rugged and less sensitive detectors and makes unnecessary expensive apparatus utilizing techniques, such as involved in mass spectrometers, infrared analyzers, and the like. y

Although the invention has been described with reference to embodiments thereof, it should be understood that these are by way of illustration only and that the invention is not necessarily limited to such embodiments. Alternative components and operating techniques will become apparent to those skilled in the art in view of the foregoing disclosure and, accordingly, modications in the construction and operation of the apparatus are contemplated without departing from the spirit of my invention.

What I claim is:

1.v An apparatus for detecting micro amounts of combustibles in air which comprises accumulator chamber means, hydrocarbon adsorption means in said accumulator chamber means, means for cooling said accumulator chamber means while flowing a sample air stream containing micro quantities of combustibles through said adsorption means, heater means lfor said adsorption means, timing means adapted to terminate the cooling of .the accumulator chamber means and for energizing the heater for the adsorption means, whereby accumulated combustibles are desorbed from said adsorption means, combustibles detector means, conduit means for flowing the desorbed combustibles from the accumulator chamber means through the combustibles detector means,

l and means actuated by said timing means for injecting into said conduit'en route to said detector means a small amount of a standardizing combustible gas as a reference peak for indicating the accumulated amount of desorbed hydrocarbon combustibles.

2. An apparatus for detecting the concentration of combustibles in a ilowing air stream which comprises a silica gel column Ifor accumulaing hydrocarbons from said ilowing air stream, means for controllably refrigerating said silica gel column during an adsorption cycle, means for injecting a standardizing combustible gas into said owing'air stream downstream of said silica gel column, said means for injectingr including a supply of combustible gas maintained under pressure and valve means controlled by a timer hereinafter described and a calibrated leak means for injecting the combustible gas into the flowing air stream at a uniform rate for a selected period of time, combustibles detector means through which said air stream ilows, said detector means being adapted to indicate the amount of combustibles owing therethrough, controllable heating means for said silica gel column adapted to heat the column during a desorption cycle, whereby combustibles are accumulated to detectable concentration during the adsoprtion and refrigeration cycle and are released in relatively high concentration in a burst of short duration during the desorption cycle, and program timer means adapted to con- -trol1 the relative length of said adsorption and desorption cyc es.

References Cited in the file of this patent UNITED STATES PATENTS 2,222,828 Guthrie Nov. 26, 1940 2,398,818 Turner Apr. 23, 1946 2,694,923 Carpenter Nov. 23, 1954 2,813,010 Hutchins Nov. l2, 1957 OTHER REFERENCES Separation and Analysis of Gases by Gas Chromatography, by Patton, Lewis and Kaye, published in Analytical Chemistry, Vol. 27, No. 2 February 1955, pages 170- 174.

Claims (1)

1. AN APPARATUS FOR DETECTING MICRO AMOUNTS OF COMBUSTIBLES IN AIR WHICH COMPRISES ACCUMULATOR CHAMBER MEANS, HYDROCARBON ADSORPTION MEANS IN SAID ACCUMULATOR CHAMBER MEANS, MEANS FOR COOLING SAID ACCUMULATOR CHAMBER MEANS WHILE FLOWING A SAMPLE AIR STREAM CONTAINING MICRO QUANTITIES OF COMBUSTIBLES THROUGH SAID ADSORPTION MEANS, HEATER MEANS FOR SAID ADSORPTION MEANS, TIMING MEANS ADATPED TO TERMINATE THE COOLING OF THE ACCUMULATOR CHAMBER MEANS AND FOR ENERGIZING THE HEATER FOR THE ADSORPTION MEANS, WHEREBY ACCUMULATED COMBUSTIBLES ARE DESORBED FROM SAID ADSORPTION MEANS, COMBUSTIBLES DETECTOR MEANS, CONDUIT MEANS FOR FLOWING THE DESORBED COMBUSTIBLES FROM THE ACCUMULATOR CHAMBER MEANS THROUGH THE COMBUSTIBLES DETECTOR MEANS,

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