Swelling sorbent materials

In Chap. 5 measurements of gas adsorption by slow rotational oscillations of the sorbent material are discussed. This method uses the inertia of mass to detect changes caused by gas adsorption. Combined with gravimetric or volumetric measurements it allows the measurement of gas solubilities in non-rigid, i. e. swelling sorbent materials as for example polymers. 

In practice combined volumetric-gravimetric measurements have been fairly successful [0.31]. Also densimetric-volumetric and densimetric gravimetric measurements using magnetic suspension balances (2 positions and 3 positions types respectively) can be recommended. If swelling sorbent materials are considered (slow) oscillometric measurements are recommended. Chap. 5. In case of multicomponent sorption systems (N > 2) a gas analyzing system has to be used in any case. 

Abstract The physical and experimental technique of gas adsorption measurements by slow oscillations of a rotational pendulum or, hkewise, the relaxational motion of a freely floating rotator are described. Combinations of the pendulum with either gravimetric or volumetric measurements are outhned. These especially are suited to measure the absorption or solubility of gases in non-rigid or swelling sorbent materials like polymers. Pros and cons of these methods are discussed in brief. List of symbols. References. 

Combined oscillometric and gravimetric measurements provide a basis to determine simultaneously the mass and the volume of a swelling sorbent material like polymers or resins in a sorptive gas atmosphere. However measurements seem in practice to be restricted to determinations of sorption equilibria of these materials as the kinetics of mass uptake often is very slow. To give an example we mention that in case of sorption of CH4 on pellets of Makrolon 2400 at 35 °C, p = 2 MPa, it took more than 4 days till equilibrium was reached. 

In this section we will provide the reader with the basic equations allowing one to calculate from combined oscillometric and gravimetric measurements both the mass of gas (m ) adsorbed or absorbed in a swelling sorbent material of mass (m ) and its volume (V ) in the sorbate state at given pressure (p) and temperature (T) of the sorptive gas and sorbent material. We start by mentioning the result of oscillometric measurements, cp. Sect. 2.2, namely the relation (5.25)... 

Figure 5.14. Training instrument for oscUlometric, volumetric, gravimetric, and dielectric measurements of gas adsorption equilibria in rigid and swelling sorbent materials. The pendulum (left) is covered by a plexiglass vessel allowing direct optical observations of its rotational oscillations.

In Sections 3 and 4 we will outline more sophisticated methods namely osciUomeny for handling sorption equilibria in swelling sorbent materials like polymers and adsorption calorimetry for determining the heat of adsorption which is set free upon adsorption of a gas but needed for desorption of the adsorbed molecules form the sorbent material. Finally in Section 5 we will mention in brief impedance measurements in gas adsorption systems which still have potential to improve control of adsorption reactors on a commercial / industrial scale. Also hints are given for choosing a measurement method if the purpose of the measurements and requirements for the accuracy of data are given. 

Today there are several experimental methods available to measure pure gas and gas mixture adsorption equilibria on porous rigid or swelling sorbent materials. All these methods have their specific advantages and disadvantages [1]. Choice of any of them depends mainly on the purpose of measurement and/or accuracy and reliability of data needed. For quick measurements of restricted accuracy gas expansion experiments or volumetric measurements are recommended. If high accuracy data are needed, weighing procedures, i. e. gravimetry should be used... 

Polymeric materials can absorb considerable amounts of gas, for example CO2 especially at elevated pressures (p) and temperatures (T) above the so-called glass transition temperature [5.19]. This often causes changes in size and volume of the polymer, which have to be taken into account in industrial processing situations, for example in gas separation processes using polymeric sorbent materials [5.20]. Sorption phenomena of swelling polymers cannot be measured adequately by either gravimetric or volumetric methods. 

Functionalized polysiloxane sorbents (FPS) are spatially cross-linked organosilicon polymers with a controllable porous structure, which are not soluble, and do not swell in known common solvents. Their intermediate position among typical inorganic materials (silicas, silicates, quartz, etc.) and organic polymers results from their chemical compo-... 

A number of sorbents have been proposed to clean water surfaces from oil [318]. The use of hydrophobic aerosil was proposed for this purpose, which, however, can hardly be accomplished for economic reasons. More promising seems to be the proposal to use natural materials for oil absorption, such as turf, diatomite, vermiculite, swelled perlite. A method has been proposed for the modification of perlite by a consequent treatment with cationic surfactants and higher carboxylic acid salts. Such modification of swelled perlite increases its oil capacity up to 600%, the water absorption decreases 10 -100-fold, and the sinkability decreases considerably. The degree of oil removal from the water surface is, according to in vitro tests data, 98 - 99%. Methods have been found to use oil-saturated sorbents. 

In conclusion, it should be also said that the origin of the hysteresis loop of the adsorption—desorption isotherms of porous polymers is stiU debated and can be interpreted in different ways. For example, there exists an opinion that hysteresis is not related to traditional capillary condensation in the pores, but may be a consequence of the out-of-equihbrium character of phase transitions in real disordered mesoporous polymers [255]. A failure to reach equilibrium under the given experimental conditions may be caused by the slow diffusion rate of the sorbate [256] or slow swelling of the polymeric sorbent on adsorption and slow relaxation of its swollen structure on desorption. Quite often, a subsequent adsorption on the same material results in larger adsorption capacity values. It is the so-called conditioning effect [256] that may imply a nonequihbrium character of the process. Even the reproducibihty of the shape and location of a hysteresis loop of the isotherms may indicate the estabhshment of fast... 

Sorption isotherms for -valeric acid on hypercrosslinked micropor-ous Styrosorbs 1 crosslinked with p-xylylene dichloride, biporous Styrosorb IBP, and, for comparison, macroporous Amberlite XAD-2 are given in Fig. 11.3. Again, one cannot see any correlation between the specific surface area of dry hypercrosslinked sorbents and their capability to take up the acid. Indeed, Styrosorb 1 with X — 25% is a non-porous material when in the dry state, but it swells in water (0.15 mL/g) and, accordingly, retains a noticeable amount of the sorbate. On the other hand, the apparent specific surface area of dry Styrosorb 1 and Styrosorb IBP, both having a 100% degree of crosslinking, are of the same order of magnitude, about lOOOm /g, but the biporous Styrosorb IBP takes up twice as much water as Styrosorb 1 does (2.90 and 1.31 mL/g, respectively) and exhibits an exceptional sorption capacity at equal equilibrium... 

In the first mode, known portions of the polymer were equilibrated with solutions of CaCl2 and HCl, as well as with their mixtures of known concentrations. The final composition of the bulk solutions in equilibrium with the polymeric phase was determined by titrating the excess HCl acid with NaOH and hy complexometric titration of the Ca ions with ethylenediamine tetraacetate (EDTA). From these data the concentrations of the electrolytes within the porous space of the polymeric material were calculated and then the apparent phase distribution coefficients k of HCl and CaCl2, defined as the ratio between the equihbrium concentrations of the corresponding electrolytes within and outside the polymeric beads. These calculations are strongly facifitated by the outstanding property of the neutral hypercrossfinked polystyrene sorbents, namely that their swelling does not depend on the electrolyte concentration, so that the volume of the porous space remains constant in all experiments. Thus,... 

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