Gas evolution rates

Janssen and Hoogland (J3, J4a) made an extensive study of mass transfer during gas evolution at vertical and horizontal electrodes. Hydrogen, oxygen, and chlorine evolution were visually recorded and mass-transfer rates measured. The mass-transfer rate and its dependence on the current density, that is, the gas evolution rate, were found to depend strongly on the nature of the gas evolved and the pH of the electrolytic solution, and only slightly on the position of the electrode. It was concluded that the rate of flow of solution in a thin layer near the electrode, much smaller than the bubble diameter, determines the mass-transfer rate. This flow is affected in turn by the incidence and frequency of bubble formation and detachment. However, in this study the mass-transfer rates could not be correlated with the square root of the free-bubble diameter as in the surface renewal theory proposed by Ibl (18). 

Consequence of runaway reaction Temperature rise rates Gas evolution rates Adiabatic Dewar Adiabatic calorimetry Pressure ARC VSP/RSST RC1 pressure vessel... 

What the consequences are of a runaway in terms of the heat and gas evolution rates. 

The peak gas evolution rate, QGmax, can be obtained from calorimetric measurements (see Annex 2 and equations (A2.3) and (A2.4)). It is important that such calorimetric tests are performed so as to minimise the amount of dissolved gas in the test. "Open" tests are therefore preferred to "closed" tests[2]. 

Adiabatic Dewar calorimeters are usually used in the closed mode. However, it is possible to incorporate a vent line to either an external containment vessel or to a burette for measuring the permanent gas evolution rate. This vent line contains an automatic valve to simulate the operation of the pressure relief system. 

It is potentially more of a problem if the measurement pressure is higher than the maximum accumulated pressure. In this case there is the potential for more gas to dissolve in the sample under pressure than would be the case for the reactor and this can lead to the gas evolution rate being underestimated1131. In such cases, it may be possible to correct for dissolved gas if the kinetics are well-understood. A method for doing this is given in references 14 and 15. 

An alternative method of obtaining the gas evolution rate is to use an open test, venting to a constant pressure automated gas burette or to a thermal mass flowmeter111. However, these techniques have been developed to characterise the normal chemical reaction by measuring gas flow rates from a heat flow calorimeter... 

A similar test to that for gassy systems (see A2.5 above) should be used to determine the permanent gas evolution rate. The rate of temperature rise should also be measured so that the rate of vaporisation can be calculated using equation (A2.7) above. 

The interpretation of peaks and rates of formation need to be carefully examined since the rates of change in concentration vary due to both the rate of sulfide formation and the rate of carbon dioxide evolution. Increases in concentration can (and do) occur under conditions of low gas evolution, even when the rate of sulfide formation is constant. This becomes especially important when a decrease in the concentration in mid fermentation can be due to the gas evolution rate alone. In some literature (and perhaps in practice) this leads to the mistaken interpretation that the formation has diminished. The same logic can be applied to attribute high concentration at the beginning and end of fermentation to higher formation rates when these are the points of the lowest gas evolution rates. 

A small hold-up volume can be provided between main reservoir and the condenser, allowing small liquid samples to be taken, an analysis of which will yield independent reaction rates. The recycling of even small concentration percentages of liquid products can be eliminated by interposing several reservoirs containing about two or three times the amount of liquid circulated in an individual experiment. The gas evolution rates are measured by determining the rate of pressure rise with a manometer when the system is temporarily closed off from the atmosphere. 

Because the amount of Ca2+ evolved by 10 minutes CO2 bubbhng was larger than 1000 ppm, the present system may sufficiently fix CO2 from flue gas. displacement by H+ will proceed fast under acidic condition. This indicates that CO2 fixation rate will be enhanced when the pH value of resin-dispersed solution is low. Flue gas contains not only CO2 but also nitrogen oxide compounds, so that when flue gas is bubbled into the resin-dispersed solution instead of ptue CO2 gas, evolution rate of Ca from the resin is expected to be enhanced. At the same time, if the pH value of resin-dispersed solution is too low, CO2 wiU not be dissolved in the solution. Fiuther study on the reaction kinetics is necessary. 

Many effects of gas bubbles released at electrodes (on electrolyte flow, mass and heat transport, conduction, etc.) have been well studied in the past. A text with an extensive treatment of this topic is that of Hine [38]. However, in Hall-Heroult cells these effects are worthy of special mention because the relatively high current density, of the order of 1 A cm-2, and temperature make the volumetric gas evolution rate from the anode large. Furthermore, difficulties of measurement on actual cells mean less knowledge of these effects than in many other electrochemical cells. Finally, one effect of the bubble is to make the task difficult in reducing the enormous... 

The Hazard Evaluation Chemist quantifies the actual potential hazards involved in an operation - heats of reaction, gas evolution rates, minimum decomposition temperatures, etc. 

Continuous dissolution is especially advantageous when fuel and cladding are to be dissolved completely, as there is then no problem in removing undissolved solids from the dissolver. In such a case, fuel may be charged continuously at the top, dissolvent may be fed continuously, and dissolved solution removed continuously. The volume of undissolved fuel in the dissolver adjusts itself automatically so that the rate of solution balances the rate of addition. The big advantages over batch dissolution are smaller dissolver volume, more uniform product solution composition, steady gas evolution rate, and smaller and more efficient absorption system. It is estimated [B12] that the volume of a continuous dissolver may be from one-tenth to one-twentieth that of a batch dissolver of the same average dissolving rate. 

Figuie 25. Dependence of gas-evolution rate on temperature showing activation energies for KN3, Ba(N3>2, and Pb(N3>2-... 

Figure 26. Gas-evolution-rate curves for irradiation with a low-pressure mercury lamp.

Figure 27. Gas-evolution rate of KN3 iingle crystal in ultrahigh vacuum, using a 150-watt...

Figure 28. Gas-evolution rate vs. wavelength for KN3 single crystal.

Figure 29. Activation energies for Ba(N3)2 gas-evolution rate measured at different times...

Figure 30. Typical gas-evolution rates for Pb(N3)2 single crystals in ultra-high vacuum using monochromatic light.

As with lead azide, impurities were found to alter photodecomposition rates in AgNa [237]. The addition of Pb " retarded the gas evolution rate while CO2 enhanced it. No explanation for the effect was given, but it may involve changes in stoichiometric charge-compensating defects. The authors claim that photolysis is not concentrated in disordered regions of the crystal as is observed for lead and thallous azides (see Section D). 

The first school began with Venczel s dissertation in Zurich in 196190 on the transport of ferric ion to an electrode evolving hydrogen gas from one molar sulfuric acid. Venczel found that mass transfer increased rapidly with the onset of gas evolution. Ibl and Venczel26 reported mass transfer at gas-evolving electrodes as Nemst boundary layer thicknesses that are functions of gas evolution rate... 

It is important to measure the gas evolution rate during the normal reaction. This information is required to design the venting or scrubbing system so that the reactor does not become pressurized as the reaction proceeds. 

Gas evolution rate can be measured by very simple methods. The gas generated in a reaction can be collected in an upturned measuring cylinder filled with a suitable collecting fluid (water for insoluble gases, silicone oil for water-soluble gases). Timing the rate of gas collection gives a quick assessment of the evolution rate. 

Gas evolution rates, like heat evolution rates, follow an Arrhenius dependence on temperature. If the temperature increases by 10 K, the volumetric rate of gas evolution will more than double the reaction will speed up and the volume of gas will also be greater due to the higher temperature. 

The thermolysis kinetics of 1-12 have the following features in common In all of the samples, the gas evolution rate decreases steadily with time, the considerable gas evolution was observed already on the initial stage of the heating of sample. Figure 10.8 illustrates typical gas evolution kinetics during thermolysis. 

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