Adsorbent volume

The retention time of the non-adsorbing methane (ti) is the measure of the column void volume or holdup. Ethylene is adsorbed by the catalyst, hence it does not reach the detector until the available surface is saturated, at which point ethylene breaks through and is detected by the sensor (t2). The adsorbed volume of ethylene is given simply by ... 

Enhancement of gas storage capacity through adsorption occins when the overall storage density is increased above that of the normal gas density at a given pressure. The adsorbed phase has a greater density than the gas phase in equilibrium with it. However, enhancement in a storage system of fixed volume can only happen if a greater amount of gas is adsorbed compared to the volume of gas displaced by the adsorbent volume. 

The integrated terms are simply the specific heat of the unit mass of adsorbent and its associated adsorbate. The specific heat at constant volume has been used for the adsorbate since, theoretically, there is no expansion of the adsorbate volume and the heat required to raise the temperature is the change in internal energy. In practice there will be some expansion and a pessimistically high estimate could use the specific heat at constant pressure The specific heat of the adsorbed phase is in any case difficult to estimate and it is common to approximate it to that of saturated liquid adsorbate at the same temperature. 

For multipoint BET measurements, a plot of P/V(P0-P) versus PIPq is obtained using various concentrations of adsorbate and measuring the adsorbed volume. Since the BET equation is in the form of a straight line ... 

Calculate C and the specific surface area As of a material from the nitrogen adsorption isotherm according to the BET equation from the data points given in the figure. Use the ideal gas equation to convert the adsorbed volumes into moles (STP indicates that the volumes adsorbed are given for standard temperature and pressure, i.e., 273 K and 101.3 kPa). 

In this theory the adsorbed layers are considered to be contained in an adsorption space above the adsorbent surface. The space is composed of equipotential contours, the separation of the contours corresponding to a certain adsorbed volume, as shown in Figure 17.7. The theory was postulated in 1914 by Polanyi(18), who regarded the potential of a point in adsorption space as a measure of the work carried out by surface forces in bringing one mole of adsorbate to that point from infinity, or a point at such a distance from the surface that those forces exert no attraction. The work carried out depends on the phases involved. Polanyi considered three possibilities (a) that the temperature of the system was well below the critical temperature of the adsorbate and the adsorbed phase could be regarded as liquid, (b) that the temperature was just below the critical temperature and the adsorbed phase was a mixture of vapour and liquid, (c) that the temperature was above the critical temperature and the adsorbed phase was a gas. Only the first possibility, the simplest and most common, is considered here. 

The potential theory postulates a unique relationship between the adsorption potential ep and the volume of adsorbed phase contained between that equipotential surface and the solid. It is convenient to express the adsorbed volume as the corresponding volume in the gas phase. 

The surface area occupied by this adsorbed volume... 

Here P is the equilibrium pressure during the adsorption measurements, V the adsorbed volume of N2, the monolayer volume and is a constant. 

Following mono-layer uptake, further increase in pressure results in multi-layer adsorption of N2. For this part of the isotherm, condensation-evaporation equilibrium is assumed to take place, instead of adsorption-desorption equilibrium for each individual layer other than the first layer. This dynamic equiUbria for the first and higher layers and some simplifying assumptions form the basis for the B ET treatment of the multi-layer adsorption isotherm. A lengthy derivation leads to the BET relation between adsorbed volume of N2 and relative pressure. Here relative pressure is defined as the ratio of the equilibrium pressure to the... 

To calculate the total pore volume it is necessary to measure the total adsorbate volume at a relative pressure above the point where the hysteresis loop closes. In general, a relative pressure as close to unity as possible should be chosen in order to include the large radii pores in the measurement. For example, grams of nitrogen are adsorbed at P/Pq = 0.99 then the corresponding volume of pores Fp and the largest pore radii r are given by... 

To calculate the pore size distribution consider the work sheet shown as Table 8.1, and the corresponding explanation of each column. The adsorbed volumes are from a hypothetical isotherm. The procedure used... 

The shape of the curve shown in Fig. 15.6 is fortuitous in as far as the continuous flow method is concerned. For reasons to be discussed later, the desorption signal (see Fig. 15.3) is generally used to calculate the adsorbed volume. When, for example, 1.0 cm of nitrogen is desorbed into... 

Volumetric measurements require that adsorbed volumes be corrected for ideality. The continuous flow method does not require this correction because the detector senses the gas concentration change at ambient temperature and pressure. Gravimetric apparatus measure the adsorbed weight directly and needs no correction for deviations from ideality. 

The sharp increase in the adsorbed volume of nitrogen due to capillary condensation for the sample obtained with decane is relatively vertical, reflecting the homogeneity of the sample which has a high specific surface area of 750 m2/g. For other compounds, the capillary condensation is less pronounced, meaning that only a part of material is well crystallized. This is confirmed by the low value of nitrogen adsorbed and low specific surface areas (500 m2/g for heptane, 405 m2/g for undecane). 

Figure 1. Adsorption-desorption nitrogen isotherms for AC-ref SC-100 and SC-155 Adsorbed volume (cnvVg) vs. p/p .

True adsorbent volume. Vft. The weight of the adsorbent packing divided by adsorbent density. 

Kuge and Yoshikawa (3) related a change in the gas chromatographic peak shape to the beginning of multilayer adsorption on the surface of the solid. For small adsorbate volumes, the peak shape is symmetrical. As the adsorbate volume is increased, a sharp front, diffuse tail, and a defect at the front of the peak top is observed (Figure 11.2). It then acquires a diffuse front and sharp tail. This point corresponds to the B point of the BET Type II adsorption isotherm at which the relative surface area may be calculated. 

Fig. 3. Cycling operation with pure methane pressure profiles (continuous line and left hand scale), adsorbed volume of methane per volume of composite block (symbol ( ) and right-hand scale).

Efficiency of the ANG process The performance of ANG systems is usually defined as the volume of methane or natural gas delivered, measured at standard conditions per adsorbent volume. The impact of the cycling operation on performances is evaluated by the efficiency (Fig. 6) defined as follows,... 

Table 1. As the relative pressure increases, the thickness of adsorbed molecule layers on the pore surfaces increases as well, and then the pore filling process caused by capillary condensation occurs first in the small pores simultaneously with the multilayer adsorption on the larger pores. For specimen III, the value of 4d,max is significantly larger than those values for the other specimens, which is ascribed to the fact that as a result of the pore filling process in the larger macropores the adsorbed volume starts to increase abruptly only near the saturation vapor pressure in the gas adsorption isotherm. From the above results, they suggested that 4d,max / 4d,mm is closely related to rmax, that is, the larger rmax, the wider ranges the length-scale of the fractal regime in value.

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