Pressure drop technique

In the pressure drop technique, a known number of moles of gas (determined by accurate measurements of temperature and pressure in a precalibrated volume) is secured in one section of the apparatus. The IL is metered into another section, whose volume is also known accurately. Then a valve is opened and the gas is allowed to expand into the entire apparatus, dissolving in the IL. Measurement of the full pressure drop when equilibrium is reached allows the number of moles of gas to be determined in the vapor phase and, subsequently, the number of moles in the liquid phase to be determined by difference. 

Data determined by the pressure drop technique [9] are shown in Figure 8.4. The solubility of CO2, CH4, and Oj in l-butyl-3-methylimidazolium tetra-fluoroborate [C4CiIm][BF4]) at 303 K shows the much greater solubility of CO2 compared to CH4 and Oj. Henry s law constants can be determined from the slopes of these graphs. 

Figure 8.4 Solubility of CO2, CH4, and O2 in [C4CjIm][BF4] at 303 K, as determined by the pressure drop technique. (From Jacquemin, J. et al., J. Chem. Thermodyn., 38, 490, 2006.)...

The capillary pressure pa in a foam can be significantly increased by the Foam Pressure Drop Technique employed to produce dry foams [32-35]. The principle of this method is that the foam is brought into contact with a porous plate (usually sintered glass filter) under which a reduced pressure p 0 is created, and this pressure difference Ap = p0 - p 0 should not exceed the capillary pressure in filter pores 2acosd / rn (where 6 is the contact... 

The quantity xp is a much better defined characteristic of foam stability (since the pressure in the borders along the height of the foam column remains constant during its destruction). This parameter is also much more sensitive to the kind of surfactant, electrolyte concentration and other additives, compared to the lifetime of the foam in gravitational field, with an averaged pressure value from 0 to pgH. Estimation of the stability of foams from different surfactants by xp and by the Ross-Miles test has been reported in [16]. The results are discusses in Section 7.6.1. The advantages of the Foam Pressure Drop Technique and, respectively, xp as a characteristic of the foam stability, are clearly shown. 

As already mentioned (see Chapter 3), at the instant of foam formation the films and borders are in non-equilibrium state. The films thin mainly due to the capillary pressure, while the borders thin due to gravity or a pressure drop (when the foam is dried by the Foam Pressure Drop Technique [21-23]). The surfactant adsorption layers decrease the flow rate through the borders and films and the process of thinning becomes similar to the flow in thin gaps with solid surfaces. As indicated in Sections 3.2.1 and 5.3 the degree of retardation of the flow depends on the surfactant type and concentration as well as on the film type. A complete immobility at the film and border surfaces usually is not reached. 

These two examples with the homologous series of alkylsulphonates and alkylsulphates indicate the undoubted advantages of Foam Pressure Drop Technique for determining the foam stabilising properties of surfactants. This technique allows to distinguish small differences in the foam stabilising ability of surfactants. 

Though not quantitative, the comparison between the two techniques provides information about the effect of the pressure in the foam liquid phase as well as the effect of the foam film type. The advantages of the Foam Pressure Drop Technique for estimating the foam stabilising ability of the surfactants is indisputable. 

KI1 Kiran, E. and Zhuang, W., A new experimental method to study kinetics of phase separation in high-pressure polymer solutions. Mirltiple rapid pressure drop technique - MRPD, J. Supercrit. Fluids, 1, 1, 1994. 

Deposition Accumuiation Monitors Methods for monitoring deposits can provide an indication of the accumulation of biofilm and other solids on surfaces or in orifices. For example, monitoring the pressure drop across an orifice provides a simple method for continuous monitoring of deposit accumulation and biofilm accumulation. The main disadvantage of the pressure drop technique is that it is not specific to the biofilm buildup since it detects the total scaling and deposition effects in a line [12]. 

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