238 Ind. Eng. Chem. Fundam., Vol. 18, No. 3, 1979
The Peak Flammability Limits of Hydrogen Sulfide, Carbon Dioxide, and Air for Upward Propagation Edward J. Anthony" and Margaret
F. Powell
Department of the Environment, Building Research Establishment, Fire Research Station, Borehamwood, Hertfordshire, United Kingdom WD6 2BL
The minimum concentrations of carbon dioxide, to prevent the combustion of any concentration of hydrogen sulfide in air, called the peak concentration, have been determined at 20, 100, and 150 OC. They are 34.9 f 0.2, 40.2 f 0.6, and 40.7 f 0.8% carbon dioxide in air, respectively. The effects of carbon dioxide have been shown to be purely thermal up to stoichiometry. For fuel-rich mixtures, however, the carbon dioxide also acts as a weak oxidant.
Introduction There are at present no data in the literature on the peak flammability limits of hydrogen sulfide, carbon dioxide, and air at atmospheric pressure for upward propagation over a range of temperatures below the auto-ignition temperature of hydrogen sulfide at 212 O C . Limits have been determined here in a large diameter tube (i.d. 70 mm) for which it may be assumed that quenching effects due to the walls are negligible (Zabetakis, 1965). The results are analyzed in terms of the thermal capacity, adiabatic flame gas temperature, and burnt gas composition calculated for the limit curves (Anthony and Powell, 1977). Experimental Section Apparatus. The apparatus consisted of a flammability tube (1.26 m in length and 70 mm i.d.1 connected to a gas supply line and placed in a thermostatically controlled oven. The oven, in which the atmosphere was stirred to ensure uniform temperature, could be held at any desired temperature from ambient to 250 "C. It was fronted by a full-length glazed observation panel which permitted the observation of tests. The base of the flammability limit tube, which protruded into a lower vented compartment, was covered by a removable hollow cap connected to an exhaust system. Ignition was attempted by an ac spark across a gap of 5 mm between a pair of tungsten electrodes which were positioned centrally 180 mm above the base of the flammability limit tube. The spark energy was provided by a transformer with an input voltage variable from 0 to 240 V and a maximum of 15 kV. The actual output voltage employed was 11 kV, which was equivalent to 477 J/s, the spark being maintained for a period of approximately 2 s. As the flame fronts produced by the combustion of hydrogen sulfide, carbon dioxide, and air mixtures were a very faint blue, the entire front of the observation panel was surrounded by a light-proof screen. This permitted the observer to determine the extent of the flame propagation, in the tube, in darkness. High-purity hydrogen sulfide and carbon dioxide (9970 pure) drawn from cylinders were used in this work. The air was supplied from a compressor and was partially dried. Procedure. A known percent mixture (by volume) was passed through the flammability limit tube via a stainless steel coil to ensure that the temperature of the gas was brought to that of the oven. When the atmosphere in the tube had been displaced not less than six times, the gas 0019-7874/79/1018-0238$01 .OO/O
supply was switched off. The exhaust system was switched off and the hollow cap removed 30 s before the flow of gas mixture was stopped to prevent the gas at the open end of the tube being disturbed immediately prior to ignition. The flow of gas was then stopped, ignition was attempted, and observation was made as to whether or not the gas mixture in the tube was flammable. The criterion of flammability was taken as a flame which propagated 0.6 m or more. The procedure was repeated with different mixtures of known composition until the peak concentrations of carbon dioxide in the carbon dioxidelair mixtures were evident.
Results These are shown graphically in Figure 1. The peak concentrations are 34.9 f 0.2, 40.2 f 0.6, and 40.7 f 0.8% carbon dioxide in air for 20, 100, and 150 O C . The only comparable studies are due to Zabetakis (1965) and a Russian one (Popov and Bezzub, 1939) for the peak limit of hydrogen sulfide, carbon dioxide and air for horizontal propagation in a closed tube. They give values of approximately 32.4 and 32.6% carbon dioxide in the carbon dioxidelair mixture to inert hydrogen sulfide. The figure quoted by Zabetakis is low when compared with the present result (although there are differences in the experimental condition; e.g., a 51 mm diameter tube is used and the ambient temperature is 25 OC). One would also expect Zabetakis' figure to be greater than that of the Russians since more diluent is always required to inert mixtures for upward propagation than either horizontal or downward propagation. Nevertheless, agreement between the results may be regarded as fair. Calculations of equilibrium burnt gas composition indicated that below stoichiometry the volumetric proportion of carbon dioxide dissociated to carbon monoxide and oxygen in the burnt gas mixtures was typically of the order of to lo4%. This is negligible, being comparable with free radical concentrations. Above stoichiometry the percentages increased by three orders of magnitude and calculations for the extrapolated flammability limit curve (Figure 3), determined at 20 "C, indicated that up to 2% of the carbon dioxide can be dissociated. Over the fuel-rich portion of the experimentally determined limit curves the dissociation is of the order 0.14.9%. This behavior is not surprising in view of the considerable similarities between the combustion chemistry of hydrogen sulfide/oxygen and of hydrogenloxygen (Gray and Sherrington, 1974) for which carbon dioxide also acts as a weak oxidant above stoichiometry (Kelly and Padley, 1971). Departure from
Published 1979 by the American Chemical Society
Ind. Eng. Chem. Fundam., Vol. 18,
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stoichiometry is expressed in Figures 2 and 3 in terms of the equivalence ratio which is the ratio of the actual amount of oxygen to the stoichiometric amount just required to burn the fuel completely. Discussion Initially certain workers believed that flammability limits for fuel/oxidant and fuel/oxidant/diluent systems corresponded to some fixed flame gas temperature (Egerton and Powling, 1948). This view was not surprising in view of the fact that most of the early determinations were carried out on hydrocarbons, expecially for saturated hydrocarbons, for which this is often the case although the reasons for this are still not fully clear (Gaydon and Wolfhard, 1973; Hertzberg and Burgess 1974). That this is not the case for the hydrogen sulfide, carbon dioxide, and air system is clear from the adiabatic flame
No. 3, 1979 239
temperatures. These were calculated from points taken from the experimentally derived curves of Figure 1 and shown in Figures 2 and 3 (Anthony and Powell 1977). Instead, regardless of the initial ambient temperatures, the final flame gas temperature for any given equivalence ratio is almost constant (for any equivalence ratio less than 1). This demonstrates that the effect of the carbon dioxide is to reduce the final flame gas temperature to a limit value, which is what one would expect for a purely thermal effect on limits. The higher the initial ambient temperature the more carbon dioxide is needed to reduce the final flame gas temperature to the limit for the equivalence ratio. A certain degree of scatter must be expected in the calculated flame temperatures determined from the experimentally derived curves because of the scatter in the results themselves. In addition, the abrupt change in flame temperatures for the hydrogen sulfide, carbon dioxide, air system at 373 K between 1.05 5 X I1.15 is almost certainly due to the difficulties in drawing the limit curve around the “peak region” and is unlikely to reflect a real physical change. This uncertainty is reflected in the relatively large confidence limits given for the peak curves at 373 and 423 K. The change in gradient, on the other hand, is quite reasonable given the change in flame chemistry which will inevitably occur in changing from a fuel-lean to fuel-rich flame. More recently, Larsen has observed that the ratio of thermal capacity (KCp) of inerts to the total thermal capacity per mole of mixture was always constant at 0.80 f 0.03 (Larsen, 1975a,b). For Larsen’s analysis excess oxidant is regarded as an inert for fuel-lean flames and excess fuel an inert for fuel-rich flames. His analysis was limited to methane and n-pentane with air and a wide range of diluents. However, he concluded that “The limits of flammability of methane and n-pentane and probably all fuels in atmospheres composed of oxygen and the inert gas agent, i.e., He, Ar, N P ,COPetc., are a function of the heat capacities of both the inert gases and any fuel, or oxygen, in excess of that required by the chemical stoichiometry of the combustion process”. Calculation of KCp for mixtures of hydrogen sulfide, carbon dioxide, and air (using both the present results and those of Zabetakis) indicates that it varies continuously between 0.89 and 0.76 for lower and upper limits, respectively. This is a larger deviation than indicated with hydrocarbon/air mixtures and it suggests that caution must be used in applying this approach to other nonhydrocarbon systems. The cause for the deviation is not clear and it has been suggested to the authors that it may lie in uncertainties in the chemistry of the oxidation (Larsen, 1978). The present consensus of opinion would appear to be that upward flammability limits are not fundamental physicochemical properties of the mixture (Lovachev et al., 1973). Instead it would appear that they result from an interplay between the buoyancy due to natural convection and combustion forces due to the fuel’s reactivity (Lovachev et al., 1973; Andrews and Bradley, 1973; Crescitelli et al., 1977). Hertzberg (1976) has, for instance, determined the minimum flame speed for limit mixtures by relating the velocity of the hot bubble of gas associated with the flame front to the combustion forces and regards the limits as essentially a blow-off limit involving flame stretch. Flame speeds for the limit mixtures were not determined in this study and it is not possible to analyze our results in these terms. All that can be said is that the buoyancy
240
Ind. Eng. Chem. Fundam., Vol. 18, No. 3, 1979
force F per cubic meter (where pu and pb are the unburnt and burnt gas density and g is the acceleration due to gravity)
F =
( p , - pb)g
(N/m3)
is remarkably constant for the limit curves having values of 11.30 and u = 0.40, 10.70 and u = 0.30, and 10.3 and u = 0.2 (N/m3) (a being the standard deviation) for determinations at 20, 100, and 150 O C , respectively.
Conclusions The peak limit for hydrogen sulfide, carbon dioxide, and air for upward propagation at 20, 100, and 150 " C have been determined. Adiabatic flame gas temperature calculations indicate that the carbon dioxide's primary activity is to bring the final flame gas temperature to a particular value for any given equivalence ratio. Above stoichiometry the carbon dioxide also acts as a weak oxidant. Acknowledgment This paper forms part of the work of the Fire Research Station, Building Research Establishment, Department of
the Environment, UK. It is contributed by permission of the Director of Building Research Establishment. The authors would like to thank Mr. C. Finch for assistance with the experiments. Literature Cited Andrews, G. E.; Bradley, D. Fourteenth Symposium(International)on Combustion 1973, p 1119. Anthony, E. J.; Powell, M. F. Riv. Combust. 1977, 3 7 , 361. Crescitelii, S.;Russo, G.; Tufano, V.; Napolitank, F.; Tranchino, L. Combust. Sci. Techno/. 1977, 15, 201. Egerton, A.; Powling, J. Proc. R. SOC. London, Ser. A 1948, A793, 190. Gaydon, A. G.; Wolfhard, H. G. "Flames. Their Structure Radiation and Temperature", 3rd ed, Chapman and Hall: London, 1973. Grav. P.: Sherrinaton. M. E. J. Chem. Soc..Faradav Trans. 1 . 1974. 70. 2338. Herkberg, M.; Burgess, D. Twentieth Annual ISA Analysis Instrumentation Symposium, 1974. Hertzberg, M. U . S . Bur. Mines Rep. Invest. 8727, 1976. Kelly, R.; Padley, P. J. Trans. Faraday SOC. 1971, 6 7 , 740. Larsen, E. JFFIFire Ref. Chem. 1975a, 5 . Larsen, E. ACS Symp. Ser. 1975, No. 16, 376. Larsen, E. Private communication, 1978. Lovachev, L. A.; Babkim, V. S.: Bunev, V. A,; V'Yum, A. V.; Kriwlln, V. N.; Baratov, A. N. Combust. Flame 1973, 2 0 , 259. Popov, P. V.; Beuub, K. E. Trans. Sci. Fertilizers Insectofungkbs USSR 1939, 135, 92. Zabetakis, M. G. U . S . Bur. Mines Bull., 1965, No. 627.
Received for review May 8, Accepted February 22,
1978 1979
Oxygen Probe Dynamics in Flowing Fluids V. Linek,' P. Bene5, and V. Vacek Department of Chemical Engineering, Institute of Chemical Technology, 16628 Prague, Czechoslovakia
r(
The relation between dissolved oxygen concentration changes G(t ) and the responses t ) of membrane-covered polarographic oxygen probes is investigated. This relation is described by a model considering nonsteady oxygen diffusion through a membrane and an electrolyte layer and convective oxygen transfer from the bulk of fluid to the outer membrane surface. Various methods of evaluating the process parameters from the probe response are reviewed with regard to the terms in which the function of G ( t ) is defined and with regard to the procedures available for experimental determination of the transient characteristics of the probe. The description of probe dynamics has been amplified by a treatment of those probes which exhibit a slowdown under conditions such that the probe reading is significantly influenced by hydrodynamics. The description of probes with spherical cathodes was shown to be equivalent to that of probes with planar cathodes. Problems which may arise in the application of oxygen probes in various situations are discussed.
Introduction Membrane-covered polarographic oxygen probes are used ever more frequently for the determination of kinetics and/or transport characteristics of various processes by dynamic methods. It is a unique advantage of these probes that the measurements can be taken in any medium unless the probe material is chemically attacked. The dynamic method consists of monitoring the changes of oxygen concentration G ( t ) due to a step change of oxygen concentration in one of the input inflowing streams. The probe reading is proportional to the oxygen flux to the cathode. Thus it is quite possible that some instruments will not be able to follow the rapidly changing oxygen concentration, owing to the dynamics of the probe proper. The relation between the concentration change studied, G ( t ) , and probe response r ( t )can be derived from the mechanism of oxygen transfer from bulk to the probe cathode. Probes with planar cathodes are used most often, and for these probes the analysis of the relation between G ( t )
and r(t)has advanced the most (Heineken, 1970; Benedek and Heideger, 1970; Linek et gl., 1978a). The cathode is circular (probes by Clark, Cerkasov, Hospodka, and CBslavskY, the Beckman probe, the WTW probe) or ring shaped (e.g., YSI probe) or is of a lattice form (e.g., the Borkowski-Johnson probe). Probes with spherical cathodes, having tip diameters of 0.2 to 0.6 ym, were constructed recently by Lee et al. (1978) for measuring fluctuations of dissolved gas concentration in the liquid layer adjacent to the gas phase; this concerned fluctuations induced by liquid phase mixing. In all studies published so far, the oxygen transfer to the probe cathode was considered to be a consequence of one-dimensional molecular oxygen diffusion through a membrane of uniform thickness and eventually also through an electrolyte layer between membrane and cathode (the so-called one-region, one-dimensional diffusion model). The cathode geometry is the dominant factor affecting the quality of approximation by this type of model. The deviations in behavior of real probes from
0019-7874/79/1018-0240$01.00/00 1979 American Chemical Society
