Helium volume approximation

In conclusion we want to emphasize that in principle it is possible to measure the absolute amount of a sorptive gas adsorbed on porous solids without using the so-called helium volume approximation by combined... 

Thermal desorption. Substances collected on to the adsorbent are analysed by thermal desorption at a temperature up to 300°C using dry helium carrier gas to transport the substances into a small volume (approximately 500 il) cold trap . The cold trap is then flash heated, the desorbed sample being transferred to a GC for analysis using a WCOT column. 

AIR - A substance containing by volume approximately 78 - 79% nitrogen 20.95% oxygen,. 94% argon, traces of carbon dioxide, helium, etc. 

The approximation of the void volume (V ) of a sorbent material most often used in practice today is the so-called helium volume (V J, i. e. the proposition... 

From eq. (3.5) the mass adsorbed (m ) can be calculated if a model for the sorbent/sorbate volume (V ) is introduced and all the necessary measurements have been performed, i. e. (fio) is known according to eq. (3.10). As outlined in Chap. 1, V is often approximated by the so-called helium volume. 

Here V is the net volume of the adsorption chamber, i. e. its geometric volume minus the volume of all fixed parts of the magnetic suspension and of the other elements which permanently are included in the chamber as for example wires of thermocouples, the basket for sorbent sample etc. Also (V ) indicates the volume of the sorbent / sorbate sample which here again is approximated by the helium volume, cp. Chaps. 2 and 3 ... 

Activity measurements. Activity and selectivity measurements were performed at 10 psig in a 14-mm internal diameter glass fluid bed reactor using 25 grams of 90 to 38 micron catalyst particles. A reactant mixture of approximately 18 volume % 02 7 volume X NH3 and 7 volume % CH3OH and the balance of helium was fed to the catalyst, and temperature and contact time were varied to find the optimum yield of HCN. Optimum reaction temperatures were found to range from 425° to 475°C with contact times of 3 to 5 seconds (calculated at STP>. Fixed bed reactor studies produced similar results. The yields reported in this paper are based on carbon fed, unless otherwise noted. More details on catalyst performance can be found in our patents (7,8). 

BOYLE S LAW. This law, attributed to Robert Boyle (1662) but also known as Mariottc s law, expresses the isothermal pressure-volume relation for abody of ideal gas. That is, if the gas is kept at constant temperature, the pressure and volume are in inverse proportion, or have a constant product. The law is only approximately true, even for such gases as hydrogen and helium nevertheless it is very useful. Graphically, it is represented by an equilateral hyperbola (see Fig. I). If the temperature is not constant, the behavior of die ideal gas must be expressed by die Boyle-Charles law. 

The original statement of Charles law was made in this way For every degree rise or fall in temperature the volume of a gas increases or decreases by an amount equal to its volume at 0°C. If this law held rigidly all the way down the scale of course the volume of a gas would become zero at —273°C. This point, — 273°C., would be the absolute zero below which substances could not be cooled. As different gases were studied it was found that they obeyed this law quite exactly until they approached the temperature at which they would condense to a liquid. The more difficultly condensible a gas, the further down the scale it would follow this law. Helium, which was the last gas to succumb to efforts at liquefaction, follows the law with a good deal of accuracy to within a few degrees of — 273°C. Hence, since it was found that the less condensible a gas the more nearly it approximated a certain ideal behavior, an imaginary perfect gas was postulated which would have exactly the ideal behavior. The absolute zero then is defined as the temperature at which the volume of this perfect gas would become zero, that is — 273°C. 

Reaction Conditions. Each of the four SRC II distillation cuts was pyrolyzed at 500, 700, 900, and 1100°C at approximately 7 seconds residence time in helium. The fuel vapor concentration averaged 1.4 volume percent. Oxidative pyrolyses were done at the same temperatures and residence times using MD-4 at average stoichiometric oxygen concentrations of 6.6, 20.4, and 73.0 percent. The range of concentrations at each level was large. 

Lighter hydrocarbons such as acetylene and 1,3-butadiene were metered at atmospheric pressure to the tubular reactors at a flowrate of about 30 mL/min. Benzene was introduced to the reactor in a mixture containing benzene and helium helium was bubbled through liquid benzene maintained at approximately 25 °C to produce a mixture containing about 12% benzene by volume. The flowrates of the inlet feed streams were such that the residence times of the hydrocarbons in the heated section of the tubular reactors varied from about 25 to 30 sec. The variations of residence times were caused primarily by the differences in the temperature levels used in the reactor and by the variations in the conversions. 

It can be calculated if we have at least an approximate value for V, e.g. by determining the sample volume by means of Helium pycnometry and assuming that Helium is not adsorbed ... 

For monatomic gases such as helium, argon, or xenon, etc., the molar heat capacity at constant volume is approximately 3(cal)/(g mole)(K). In the same system of units R is about 2 so the heat capacity at constant pressure for an ideal, monatomic gas is around 5(cal)/(g mole)(K). 

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