Pore volume helium-mercury

A much more accurate method of determining the pore volume of a catalyst sample is the helium-mercury method. One places a known weight of catalyst (IT) in a chamber of known volume. After the chamber has been evacuated, a known quantity of helium is admitted. From the gas laws and measurements of the temperature and pressure, one may then proceed to... 

A container of V = 50 cc is loaded with W = 53 g of porous pellets. It is first evacuated then connected to a supply of helium. A gas volume V +Vg = 29 cc is introduced this way. V is the volume between the pellets and Vg is that of the pores. Then mercury is introduced, the amount to fill the spaces between the pellets being Vg = 19 cc. Find the true density and the porosity of the pellets. 

Measurement of other properties also is treated in Chapter 6. Pore volume is measured with helium and mercury pososimeters that together measure the empty space between particles and within particles. P6.01.05 is an example... 

The total pore volume, Vp, sometimes called specific pore volume when referred to unit mass, is the total internal volume per unit mass of catalysts. Some of this pore volume may be completely enclosed and thus inaccessible to molecules participating in a catalytic reaction. The total accessible pore volume is often derived from the amount of vapour adsorbed at a relative pressure close to unity, by assuming that the pores are then filled with liquid adsorptive. The accessible pore volume may be different for molecules of different sizes. It may be useful to determine the dead space by means of a nonsorbable gas (normally helium) in conjuction with the determination of the bulk volume of the catalyst by means of a non-wetting liquid (mercury). 

The porosity of a catalyst or support can be determined simply by measuring the particle density and solid (skeletal) density or the particle and pore volumes. Particle density pp is defined as the mass of catalyst per unit volume of particle, whereas the solid density p, as the mass per unit volume of solid catalyst. The particle volume Vp is determined by the use of a liquid that does not penetrate in the interior pores of the particle. The measurement involves the determination by picnometry of the volume of liquid displaced by the porous sample. Mercury is usually used as the liquid it does not penetrate in pores smaller than 1.2/m at atmospheric pressure. The particle weight and volume give its density pp. The solid density can usually be found from tables in handbooks only in rare cases is an experimental determination required. The same devices as for the determination of the particle density can be used to measure the pore volume V, but instead of mercury a different liquid that more readily penetrates the pores is used, such as benzene. More accurate results are obtained if helium is used as a filling medium [10]. The porosity of the particle can be calculated as ... 

A more accurate procedure is the helium-mercury method. The volume of helium displaced by a sample of catalyst is measured then the helium is removed, and the volume of mercury displaced is measured. Since mercury will not fill the pores of most catalysts at atmospheric pressure, the difference in volumes gives the pore volume of the catalyst sample. The volume of helium displaced is a measure of the volume occupied by the solid material. From this and the weight of the sample, the density of the solid phase, P5, can be obtained. Then the void fraction, or porosity, of the particle, p, may be calculated from the equation... 

From the helium-mercury measurements the pore volume, the solid density, and the porosity of the catalyst particle can be determined. Values of p are of the order of 0.5, indicating that the particle is about half void space and half solid material. Since overall void fractions in packed beds are about 0.4, a rule of thumb for a fixed-bed catalytic reactor is that about 30% of the volume is pore space, 30% is solid catalyst and carrier, and 40% is void space between catalyst particles. Individual catalysts may show results considerably different from these average values, as indicated in Examples 8-4 and 8-5. 

Solution The volume of mercury displaced, minus the helium-displacement volume, is the pore volume. Hence... 

Table I compares the pore properties for the starting metakaolln and the LSX pellets, and Figure 6 plots differential pore volume distributions for both materials. The pore distribution data show that the macroporosity of the LSX pellets 1s approximately half that of the starting metakaolln. The difference between pore volumes measured with helium and mercury 1s used as an Indicator for the microporosity of the sample.

Pore Volume Measurement The pore volume was measured from the differences of mercury and helium displacement values. 

A helium-mercury displacement method is sometimes used to check the pore volume obtained from the adsorption-desorption isotherm at the saturation pressure (Ries, Van Nordstrand, Johnson, and Bauer-meister, 50). The helium measurement which gives the solid volume and solid density is obtained by means of a miniature isotherm type apparatus. This part of the determination is essentially the same as that described by Smith and Howard (58) and Schumb and Rittner (55). Mercury displacement yields the pellet volume and pellet density and is measured volumetrically by means of a mercury buret attached directly to the same sample bulb in which the helium measurement is performed. The difference between the pellet volume and the solid volume is obviously the pore volume. 

The pore volume—for pores with a radius smaller than 7.5 u—is mostly estimated by subtracting the specific volumes measured with mercury and with helium. When applied to microporous systems with a large surface area, however, the latter volume needs to be corrected because of the fact that the helium atoms have a volume of their own (45, 48). As the acting radius of the helium atom is not known and as a possible adsorption of helium slightly compensates the effect, it is difficult to estimate the actual value of the correction. It may, however, amount to a few per cent of the density. 

A much more accurate method of determining the pore volnme of a catalyst sample is the helium-mercury method. One places a known weight of catalyst (W) in a chamber of known volume. After the chamber has been evacnated, a known quantity of helium is admitted. From the gas laws and measurements of the temperature and pressure, one may then proceed to determine the volume occupied by the helium (Vne)- This volume is equal to the sum of the volume exterior to the pellets proper and the void volume within the pellets (Vv,jj<,). The helium is then pumped out and the chamber is filled with mercury at atmospheric pressure. Since the mercury will not penetrate the pores of most catalysts at atmospheric pressure, the mercury will occupy only the volume exterior to the pellets proper (Eng)- Hence,... 

Another useful technique is mercury pycnometry which can be used to determine the geometrical volume of the dry gel (pores and solid), hence giving complementary data to helium pycnometry which measures only the pore volume. In this characterization method, a volume-calibrated chamber containing the sample is filled with mercury the mercury volume (or weight) is measured to compute the sample volume. Unlike mercury porosimetry, no pressure is exerted so that mercury neither enters the pores nor crushes the sample. 

In order to characterize the structure of RF and carbon xerogels, a combination of nitrogen adsorption (for micro- and meso-pores) and mercury porosimetry (for pore diameters from 7.5 to 150 nm) was used to obtain the BET surface area and pore volume (microporous and total) helium and mercury pycnometry were applied to determine the skeletal and bulk density. 

Fig. 5.19 Change of density with pore volume during rednction HeD - density of helium displace HgD - density of mercury displace...

Active carbons and the core samples were investigated by densimetry, mercury porosimetry and adsorption. This made it possible to obtain a synthetic image of their texture within the micro- and macropores. Pore volume, accessible to helium was determined from measurements of true and apparent densities. Mercury porosimetry was used to analyse pores between 100-7500 nm. Benzene adsorption isotherm and the Kelvin equation were used to determine pore radii 1.5-100 nm. 

The void volume (pore volume), solid density, and porosity of the catalyst particle can be determined from the helium-mercury method. In this method, a container of known volume V is filled with a known weight of pellets or particles fV. After evacuation, helium is admitted, and the sum of the volume of the space between the pellets V and the void volume inside the particles Vg is calculated using the ideal gas law. The tx-ue density of the solid is ... 

Total poie volume may be determined by measuring the density of the material with helium and then with mercury. The difference between the respective specific volumes gives the pore volume. 

Surface area, helium and mercury density, pore volume, mean pore radius. 

Surface area/pore size Calculated from the volume of a gas monolayer adsorbed by the catalyst and the known area covered by a gas molecule. Pore volume can be calculated from the helium density (helium fills the pores) and the mercury density (mercury does not fill the pores). The average pore radius assuming cylindrical pores is calculated as twice the pore volume divided hy the surface area. 

The evolution of pore volume distributions, calculated from mercury porosimetry and helium densities, of activated carbons... 

Textural properties were obtained from measurement of true (helium) and apparent (mercury) densities, total open pore volumes and pore volume distributions. For determination of the helium densities, a Micromeritics Autopycnometer 1320 was used. Apparent densities were determined in a Carlo Erba Macropore Unit 120. The pore volume distributions were evaluated with a mercury porosimeter. Carlo Erba 2000. Specific surface areas were determined by physical adsorption in a Omnisorb 360 and a Sorptomatic Carlo Erba 1900. Nj at 77 K and CO2 at 273 K were used. We assumed a cross-section for a molecule of N2 of 0.162 nm and of 0.187 nm for a molecule of COj. All textural properties are e.xpressed on a dry ash free basis (daf). 

Figure 2 shows the evolution of pore volume distributions, calculated from mercury porosimetry and helium densities, of chars obtained from oxidized coal and activated at the above mentioned COj flow rates. In general, an important enhancement in pore volume can be observed when coal preoxidation is increased and when low flow... 

The true density of coal is usually determined by helium displacement and therefore is often referred to as the helium density. Helium is used because it has the ability to penetrate all the pores of a given sample of coal without (presumably) any chemical interaction. In the direct-pressure method, a known quantity of helium and a weighed sample of coal are introduced into an apparatus of known volume, whereupon the pressure of the helium at a given temperature allows calculation of the volume of the coal. In the indirect method, mercury is used to compensate for the helium displaced by the introduction of the coal. 

Calculate the porosity and the mean pore radius. The particle porosity may be readily determined by a helium pycnometer and a mercury porosimeter. In the pycnometer, the solid skeletal volume Vs is obtained. The skeletal density ps is found from the sample weight Ws ... 

---