Adsorbed volume, determination

In Chapter 7.4, empty reactor volume determination of a perfect CSTR is described by following the discharge concentration from the sudden step-change injection of a non-adsorbing inert gas (solid line in the picture.) Next the same experiment is discussed if made with a chemisorbing gas and shown on the previous picture with a dotted line. In this second case, the reactor... 

Two general cases are considered (1) adsorption under conditions of constant or nearly constant external solution concentration (equivalent to infinite fluid volume) and (2) adsorption in a batch with finite volume. In the latter case, the fluid concentration varies from c°t to c7 when equilibrium is eventually attained. A = (c° - c /c = Ms(h7 — h0i)/(Vfc0i) is a partition ratio that represents the fraction of adsorbate that is ultimately adsorbed. It determines which general case should be considered in the analysis of experimental systems. Generally, when A90 > 0.1, solutions for the second case are required. 

Attempts have been made, using helium, to measure the density of the adsorbed phase 108-110) to try to find out whether the films are to be thought of as gaslike or liquidlike. The volume of the adsorbent was determined before adsorption, and then after a known amount of gas had been adsorbed. It was concluded 109) that the adsorption of helium, although small, was finite, introducing uncertainty in the results. Furthermore, while the concept of density is useful when multilayers are considered, it is not necessarily so at coverages less than unity. 

The classical method involves admitting a known quantity of gas to the sample chamber, which is usually maintained near the condensation point of the gas. Adsorption of the gas on the surface of the solid occurs, decreasing the pressure in the chamber until the adsorbed gas is in equilibrium with the free gas phase. The volume of gas adsorbed is determined by subtracting the volume of gas required to fill the free space (dead space) at equilibrium pressure from the volume of gas admitted. The dead space is obtained by precalibration of the chamber volume or by repeating the determination with a sample of negligible adsorption. The specific surface area (S), in m2/g, is given by the following... 

Two main routes are available for evaluating the volume up to that surface the direct one of measuring the volume accessible to a gas, which is not adsorbed at the temperature and pressure of the dead volume determination, and the indirect one, of simply subtracting from the volume of the empty bulb the estimated volume of the sample. 

Various gas and liquid adsorption techniques are used to determine the porosity of a specimen. They are mostly based on the Brunauer-Emmett-Teller method (BET) [14]. Atoms or molecules penetrate into a sample through interconnected pores with links to the sample surface. The adsorbed volume and temperature and pressure dependent data are used to quantify the porosity and surface to volume ratios, which contain information about the pore size distribution [15]. A recent review is published by Schneider [11], Care must be taken that the used probe (gas or liquid) does not react with the sample. When pores become too small, the probe may not penetrate into them. Pores or interconnected pores are not detected, when no connection to the sample surface exists. For example, thin capping layers would close all pores and render the technique useless, even though the pores may be totally interconnected below the cap. 

Interparticle volume could be measured using GPC as the total exclusion volume of high-molecular-weight polymers, and the void volume could be accurately measured as the elution volume of deuterated acetonitrile eluted with neat acetonitrile. The example of these measurements and comparison with the adsorbent mass determined by unpacking the column and weighing the dried adsorbent are shown in Table 3-5. 

In this section we discuss briefly some experimental methods for investigating adsorbed polymers. Determination of adsorbed amounts is for polymers not much different from that discussed in chapter 2. We therefore concentrate on three aspects which are typical for polymers. These are the (relative) number of segments in contact with the surface (l.e., the trains) (sec. 5.6a), the extension (thickness) of the adsorbed layer (sec. 5.6b), and the volume fraction profile normal to the surface (sec. 5.6c). 

This result is in a qualitative agreement with the experimental t-plot of Ar adsorption at 87 K on MCM 41 samples (see Figure 2(b)) using the data given in reference [13], As for simulation data, we assume that the density of the adsorbate equals that of the 3D-liquid and we have determined the thickness of the adsorbed film as the ratio of the adsorbed volume with the surface of the sample. Assuming pores of MCM 41 are cylinders, the specific surface S of each sample was determined via the relation between the porous volume V (given by the adsorbed amount after capillary condensation) and the diameter d of the pores S = 4V/d. Comparison of the different t-curves indicates that there is a pore size (5.1 nm) above which no confinement effect occurs on multilayer adsorption. Below this value, the thickness of the adsorbed film increases as the pore diameter decreases, t-curves are often analysed with the Frenkel-Halsey-Hill equation [14] /n... 

The isotherm model is compared directly to the experimental adsorption data in Figures 2 and 3. The plots show that there is good agreement between model predictions and the experimental data for all of the adsorbates. Furthermore, the regressed value of ft s is in excellent agreement with the total pore volume determined from N 2 adsorption at 77 K. It is also somewhat surprising to observe that most of the TBM measurements were in the nonlinear range of the isotherms even at the low concentrations considered in this study. 

Nitrogen isotherms were measured by using an ASAP (Micromeritics) at 77K. Prior to each analysis, the samples were outgassed at S73K for 10 - 12 h to obtain a residual pressure of less than 10 torr. The amount on nitrogen adsorbed was used to calculate specific surface area, and the micro pore volumes determined from the BET equation [14] and t-plot method [15], respectively. Also, the Horvath-Kawazoe model [16] was applied to the experimental nitrogen isotherms for pore size distribution. 

Figure 1 illustrates typical N2 adsorption isotherms determined on the char samples produced from the dififerent wastes, providing information about samples larger pores, mainly macropores, mesopores and larger micropores. Nitrogen adsorbed volumes expressed in standard conditions of temperature and pressure (STP) per sample mass unit, V as a function of die relative pressure (p/po) are shown in the figure. 

Table 1 summarizes the thermodynamic results obtained from these curves. As explained in the experimental section, the irreversibly adsorbed volume (Vjn-) is determined from the difference of volume between the primary and secondary isotherms. This volume corresponds to the amount held by strong chemisorption at the adsorption temperature over these samples. 

Table 1 presents the textural parameters of the different materials studied using adsorption/ desorption isotherms before and after modifications or catalytic testing, corresponding to BET surface area, the total pore volume and the proportion of the micropore volume. The adsorption isotherm was found to be in agreement with the ones reported for MCM-41 materials with similar pore sizes [5]. Pore condensation of N, signified by a steep increase in the adsorbed volume in the N2 adsorption isotherm, was observed at a relative pressure (P/Po) of 0.26. Using Kelvin s equation, compensating for the multilayer adsorption the pore size was determined to be 2.5 nm. 

The adsorption of molecules on a solid surface can reach significant loads. The surface concentration may be defined as the quotient of the adsorbed amount of substance and the surface of the adsorbent in mol m 2. However the effective internal surface area of an adsorbent is especially difficult to determine because it depends on the nature and size of the solutes. It is therefore advisable to use the mass or the volume of the adsorbent instead of the internal surface. The loading is then expressed as rnolg-1 or gl 1 adsorbent. Adsorbent volume can be expressed as total adsorbent volume Vads or as solid-phase volume (Vads - Vpore). According to Eq. 2.35 both values can be transformed into each other. In Eq. 2.35 Cp represents the concentration of component i within the pore system. The total load of the adsorbent is given as qi and the pure solid load as qi. 

The extent of adsorption of gases onto solid surfaces can be determined experimentally using a wide variety of apparatus and techniques, and the literature on this subject is extensive. In general, measurements fall into one of two categories either the volume of the gas adsorbed is determined manometrically, or gravimetric methods are used, where the mass adsorbed on the solid is determined directly. 

The value of % ZnO are reflective of the relative amounts of zinc present in given mass of samples. The sample Zi (%ZnO = 72.8) is indicative of high porosity and the difference 27.2% could be largely attributed to the presence of adsorbed water. Determination of approximate pore volume by adsorption of water, has confirmed the highest porosity for Zj. 

Microwaves are an alternative heat source. A high frequency generator is necessary and focusing of tire waves is required to produce heating on a determined adsorbent volume. This technology is applicable for grains and for more recent porous material such as activated carbon cloths or felts. 

Breakthrough volume depends on the retention power of the SPE adsorbing material and determines the volume of the water sample, which can be percolated through the pre-column until the analyte arrives at the pre-column outlet. Thereby, the breakthrough volume determines the extent of analyte enrichment. Obviously, to achieve a high extent of pre-enrichment, the retention of the analyte should be a maximum at the sample loading step (but it should be a minimum at its elution step). 

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