June, 19661 WALKER AND CAMPION 347 A Continuous Monitor for Hydrogen in Gases BY J. A. J. WALKER AND P. CAMPIOX ( U . K.A .E. A ., Reactor MateriaZs,Laboratory, Cztlcheth, Warrington, Lancashire) The construction and use of an instrument for the continuous monitoring of hydrogen in gas streams is described. The principles upor! which the instrument is based are the Catalytic oxidation of hydrogen to water, and the subsequent determination of the water with an electrolytic hygrometer. Factors relevant to the efficient operation of the instrument for laboratory and plant conditions are discussed. Results indicate a coefficient of variation of the order of $5 per cent. for the 80 to 1000-v.p.m. hydrogen range. IN a previous paper1 a technique was outlined for the continuous monitoring of hydrogen in carbon dioxide based gas mixtures.The work reported here is a description of the con- struction, operation and performance of an instrument used for monitoring hydrogen, both in the laboratory and under plant conditions. The principles upon which the instrument are based is the catalytic oxidation of hydrogen to water, and the determination of the latter with an electrolytic hygrometer. The instru- ment may be used for hydrogen monitoring of inert-gas streams other than carbon dioxide. EXPEKIMENTAL The apparatus is shown schematically in Fig. 1. APPARATUS- I t is housed in an 18 x 18 x 9-inch aluminium cabinet. The front panel contains a variable auto-transformer and pyrometer for controlling and indicating the temperature of the catalytic furnace, gas-input connections, flow controllers, rotameters and the hydrogen concentration meter.Two molecular-sieve 5 A driers, the electrolvtic cell and the catalvtic furnace are fixed on the rear Danel. The concent ration recorder. A = B = c = of hydr&en is recorded continuously by means of a 10-mV poientiometric L Recorart + To v e n t Molecular sieve, S A , I /16-inch D = Catalyst tube and furnace, 300' C Millaflow flow controllers F = Electrolytic cell Rotameters for sample and oxygen G = Electronics Schematic diagram of hydrogen monitor pel lets, ambient temperature E = Total-flow volameter Fig. 1 . The catalytic furnace consists of a stainless-steel vessel that is resistance heated and packed with small pieces of platinum ribbon. The heater is rated to give a maximum temperature of 800" C at the platinum surface.At the time of assembly, the stainless-steel pipework, desiccant containers and catalyst tube (together with its platinum) must be cleaned according to a standard procedure,l and all gas lines must be thoroughly dried. All couplings are of the de-mountable O-ring or compression-seal type.348 WALKER AND CAMPION: A CONTINUOUS [Analyst, 1701. 91 PROCEDURE- Connect the sample gas and oxygen supply to their respective pxitions on the front panel of the instrument, and adjust the gas pressure to 20 p.s.i. The optimum flow-rates are 100 ml per minute and 10 ml per minute for sample and oxygen, respectively, at a furnace temperature of 500" C. With a new instrument it has been found that clean-up periods of 3 days are required, with argon as the purge gas.A blank reading for the instrument is determined by passing oxygen or helium through the system. Under the above operating conditions, hydrogen values of less than 5 v.p.m. are observed, which are considered to be negligible. Replace the sample gas periodically with a standard hydrogen - carbon dioxide mixture to check the efficiency of the catalyst and electrolytic cell. Exposure of the electrolytic cell to an excess of moisture (most likely to occur during the commissioning of the instrument) leads to "flooding" and, possibly, to damage of the phosphorus pentoxide film. Rapid re- coating of the cell may be carried out according to the manufacturer's instructions. RESULTS AND DISCUSSION The following important factors were considered in the design and operation of the instrument.Cleanliness of s w faces- I t is of paramount importance to ensure that all internal stainless-steel surfaces in contact with the sample gas or gases are thoroughly clean with respect to grease and oxide. Surfaces not subjected to the cleaning technique previously describedl may act as a "sponge" to moisture and a high instrument blank may result. This stricture particularly applies to the catalytic unit and the tubing between the latter and the electrolytic cell. Minimzim delay time- The time required to register an alteration of hydrogen concentration in the sample gas is dependent on the delay volume and the flow-rate of sample gas. This delay volume of the pipework, valves and catalyst vessel must be as small as possible, consistent with efficient operation of the instrument.In the present design, the sample gas drier, immediately prior to the Millaflow flow controller, constitutes 95 per cent. of the delay volume. The equilibra- tion time required for the instrument described is of the order of 10 minutes. Minimum maintenance- The instrument has been used over plant operating periods of several weeks during which time no maintenance has been required. The equilibrium moisture capacity of molecular sieve 5 at 26" C and inlet moisture concentration* of 100 v.p.m. is approximately 30 per cent. w/w. Therefore, for a sample gas flow-rate of 100 ml per minute, containing 100 \'.p.m. of moisture, the life of the sample gas drier will be o f the order of 5 years.Frequent reduction in oxidation eficiency of the catalyst, for example, by the deposition of carbon from thcrmally unstable organic compounds or irreversible poisoning 1 ) j r sulphur compounds, must be avoided. In addition, potential catalyst poiwns, such as sulphur compoundy, are polar molecules and are therefore likeljv to be retained firmly on the molecular-+xe drier. Calibralinu -- Although the electrolytic hygrometer is a quantitative instrument and, therefore, frequent calibration is not necessary, the provision of a standard hydrogen gas supply facilitates rapid calibration of both the platinum catalyst and the electrolytic cell. A series of four hydrogen - carbon dioxide gas mixtures was prepared and analysed by means of helium gas cl-~romatography.~ The hydrogen concentration5 were 80, 160, 510 and 920 v.p.m.The effect of varying the catalyst temperature over the range of 150" to 500" C and halving the sample gas flow-rate to 50 ml per minute was investigated with the 920 v.p.m. hydrogen standard-gas mixture. The results (shown in Table I) indicate that complete oxidation of the hydrogen was achieved over the temperature range 300" to 500" C for a 100 ml per minute sample flow-rate and 200" to 500" C for a 50 ml per minute sample flow-rate. In addition, it was shown that varying the oxygen flow-rate from 2 to 30 ml per minute did not affect the indicated hydrogen concentration.June, 19661 MONITOR FOR HYDROGEN IN GASES 349 TABLE I VARIATION OF CATALYST TEMPERATURE AND SAMPLE FLOW-RATE Catalyst Standard gas Hydrogen monitor Indicated hydrogen temperature, flow-rate, reading, concentration, " C ml per minute v.p.m. v.p.m.500 50 470 940 500 100 910 910 450 50 480 960 450 100 910 910 400 50 470 940 400 100 930 930 350 50 470 940 350 100 890 890 300 50 470 940 300 100 900 900 250 50 460 920 250 100 359 359 200 50 465 930 200 100 138 138 150 50 83 166 150 100 83 83 A typical calibration of the instrument over the range 0 to 1000 v.p.m. of hydrogen, for a sample flow-rate of 100 ml per minute, oxygen flow-rate of 10 ml per minute and catalyst temperature of 300" C, is shown below- Hydrogen in standard gas, v.p.m. 920 513 160 80 Hydrogen monitor, v.p.m. . . 930 490 170 85 Calibration of this type, together with the determination of hydrogen in a gas mixture that had been analysed by helium gas chromatography, have indicated a maximum error of t-5 per cent.for the instrument. TNTEKFERENCE- Any hydrogenous compound that passes through the molecular sieve and is oxidised to water on the catalyst will create a positive error in the recorded hydrogen content. The lower alkanes and alkenes are examples. The four lowest alkanes were used to investigate the interference effect on the instrument. Methane, ethane, propane and butane were added to carbon dioxide to provide four sources of the basic gas with 500 v.p.m. of alkane as impurity. A mixture of alkane and carbon dioxide, flow-rate 100ml per minute, together with oxygen, flow-rate 10ml per minute, were passed through the instrument at a catalyst temperature of 300" C.The results are shown below- Methane Ethane Propane Butane Percentage of oxidation . . . . <0.001 0.5 1.0 2.0 On the basis of these results, the operating temperature of 300" C was selected in order to minimise interference by oxidation of higher hydrocarbons. 0 PE 1 ~ 4 T I 0 N l-1 N D E R PLAN T C 0 N D IT1 0 pi S- The hydrogen content o f the gas coolant in the Advanced Gas-Cooled Reactor at M'ind- scale was continuously monitored by gas chromatography and hydrogen monitor for a period of 6 weeks. The gas chromatograph and hydrogen monitor values were compared for a nominal 400-v.p.m. hydrogen concentration. A relative deviation of 7 per cent. was found for 137 paired determinations. 'The difference between this deviation and the deviation obtained with hydrogen standards (i.e., 5 per cent.) was attributed to a slight difference in coolant-sampling position and t o the deviation normally found in routine helium iunisation chromatography . C o N c LI! SI ON An instrument has been devised for the continuous monitoring of hydrogen in carbon Operation parameters have been determined for maximum efficiency An assessment for continuous monitoring under plant conditions has been dioxide and inert gases. and specificity. made and the instrument found to be satisfactory. REFERENCES 1. 2. Linde. Data Sheet No. 9890-E. 3. Ltd., London, 1962, p. 321. Walker, J . -4. J., and Campion, P., 14naZyst, 1965, 88, 280. Berry, R., in van Swaay, M., Editor, "Gas Chromatography 1962,'' Buttcrworths & Co. (Publishers) Received Oclohev 29th, 1965