Absorption into Liquids

As discussed by Bidleman (1988), there is a variety of evidence from both laboratory and field studies that the gas-particle partitioning in many cases in the atmosphere is consistent with partitioning to a liquid rather than adsorption on a solid i.e., the gaseous SOC is absorbed into a liquid particle or a liquid on the surface of a solid. For example, tetrachlorodibenzo-p-dioxin (TCDD) is found in both the vapor and particle phases, whereas none would be found in the gas phase 

If an SOC is absorbed into a liquid organic layer on the particle, a relationship between Kp and pL that is similar to that developed for adsorption onto a solid can be derived (Pankow et al., 1994a, 1994b). In this case, the gas-particle partitioning coefficient for the / th compound is defined as 

FIGURE 9.62 Plots of log (F TSP) A against T 1 for some PAH from Pankow (1991) based on data of Yamasaki et al. (1982). 

The expression for the gas-particle partitioning coefficient Kp given by Eq. (DDD) in units of m3 per p.g is developed by Pankow (1994a, 1994b) as 

(KKK) and the analogous logarithmic form of Eq. (Ill) in Box 9.3 predicts that a plot of log Kp against log pL for the partitioning of a series of compounds into liquid particles or into a liquid layer on particles should be a straight line with a slope of 1 if the activity coefficients in the liquid phase, yom, remain constant. 

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Condensed phase interactions can be divided roughly into two further categories chemical and physical. The latter involves all purely physical processes such as condensation of species of low volatility onto the surfaces of aerosol particles, adsorption, and absorption into liquid cloud and rainwater. Here, the interactions may be quite complex. For example, cloud droplets require a CCN, which in many instances is a particle of sulfate produced from SO2 and gas-particle conversion. If this particle is strongly acidic (as is often the case) HNO3 will not deposit on the aerosol particle rather, it will be dissolved in liquid water in clouds and rain. Thus, even though HNO3 is not very soluble in... 

For the case of purely physical absorption into liquid initially gas-free (Cao = 0), the amount of component A absorbed per unit area during the time 6 is (Section III,A,2)... 

Ion Exchange Adsorption Process. Supercritical fluid extracts of Crotalaria spectabilis contain monocrotaline, a basic alkaloid, and non-polar lipid material. In the separation of caffeine from coffee, Zosel (3) recommended separation of the caffeine from the fluid phase by either adsorption onto activated carbon or absorption into liquid water. Activated carbon adsorption would be undesirable in the present case because the lipids would also be adsorbed and because desorption from activated carbon is quite difficult. Liquid water would absorb... 

Many examples of gas-liquid contacting operations are found in the process industries, often involving gas incorporation or absorption into liquid, perhaps with chemical reaction in the liquid, washing or humidifying a gas stream, removal of gas from liquid, and so forth. 

Laboratory absorbers for studying absorption into liquids may be divided into two groups. Some of them have effectively a quiescent liquid which comes into contact with gas for a desired time which can also be changed with relative ease. Strictly speaking these absorbers simulate the conditions which are foreseen by the Higbie model. Table 7 shows the main types and Danckwerts (2) discusses in some detail design characteristics and proper operation of many of these absorbers. These absorbers may also be used to obtain "process specific data" -such as, reaction kinetics, diffusivities etc. (2,73-75). 

Gaseous chlorine, under room temperature, was bubbled into liquid bromine maintained at -5°C. Excess chlorine left the reactor through a vent into an absorption column. The chlorine addition rate was adjusted to the reactor s cooling capacity, to prevent the temperature from rising above 0°C. 

Ammonia is recovered from an air-ammonia gas mixture by absorption into water, using a countercurrent packed column. The absorption of ammonia in water is accompanied by the evolution of heat which causes a rise in the liquid water temperature and hence a change in the equilibrium relationship. The problem and data values are taken from Backhurst and Harker (1990). 

One way to control gaseous pollutants like SO2 and SO3 is to remove the gases from fuel exhaust systems by absorption into a liquid solution or by adsorption onto a solid material. Absorption involves dissolving the gas in a liquid while adsorption is a surface phenomenon. In each case, a subsequent chemical reaction can occur to further trap the pollutant. Lime and limestone are two solid materials that effectively attract sulfur dioxide gas to their surfaces. The ensuing chemical reaction converts the gaseous pollutant to a solid nontoxic substance that can be collected and disposed or used in another industry. 

Excess electrons can be introduced into liquids by absorption of high-energy radiation, by photoionization, or by photoinjection from metal surfaces. The electron s chemical and physical properties can then be measured, but this requires that the electrons remain free. That is, the liquid must be sufficiently free of electron attaching impurities for these studies. The drift mobility as well as other transport properties of the electron are discussed here as well as electron reactions, free-ion yields, and energy levels. 

The earliest work in this area assumed that particles in the atmosphere were solid and that the uptake of SOC involved adsorption to a solid or solid-like surface. It was subsequently recognized that many atmospheric particles are liquid or have liquid-like outer layers, and hence the uptake of gases could be treated as absorption into a liquid. These approaches are summarized in the following. It should be noted that these treat the equilibria between the gas- and condensed-phase species i.e., it is assumed that thermodynamics rather than kinetics controls the distribution between the phases. The implications of this assumption are discussed later. 

Given these caveats, we first treat the case of adsorption to a solid surface, and then absorption into a liquid particle or liquid surface layer on a particle. As we shall see, the distribution of SOC between the gas and condensed phases can be used to infer the nature of the sorbent sites. 

In short, differentiating between adsorption on a solid and absorption into a liquid for partitioning of semivolatile compounds in the atmosphere is often difficult to do in an unambiguous manner. However, the use of a combination of approaches can help to differentiate these two mechanisms and, perhaps more important, give some insight into the mechanisms of interaction of the SOC with the condensed-phase material. 

One problem with the use of pL as a key parameter in both adsorption and absorption is the difficulty in obtaining accurate values for pL for solid SOCs, since they are not experimentally accessible and must be estimated (e.g., see Finizio et al., 1997, and references therein). In addition, as discussed in the preceding section with respect to absorption into a liquid phase, slopes of 1 for plots of log Kp against log pL are only expected if the activity coefficients, y, do not change along a series of compounds. 

As discussed elsewhere in this book, there is increasing evidence for reactions at the air-water interface in the atmosphere. Pankow (1997) has treated partitioning of gases to the interface as well and predicts that as for adsorption on a solid and absorption into a liquid, there should be a linear relationship between log Kp and In pL with a slope of approximately 1. 

Rates of gas-liquid, liquid-liquid, and solid-liquid mass transfer are important and often control the overall rates in bioprocesses. For example, the rates of oxygen absorption into fermentation broth often control the overall rates of aerobic fermentation. Ihe extraction of some products from a fermentation broth, using an immiscible solvent, represents a case of liquid-liquid mass transfer. Solid-liquid mass transfer is important in some bioreactors using immobilized enzymes. 

Thus, when deahng with gas transfer in aerobic fermentors, it is important to consider only the resistance at the gas-liquid interface, usually at the surface of gas bubbles. As the solubihty of oxygen in water is relatively low (cf. Section 6.2 and Table 6.1), we can neglect the gas-phase resistance when dealing with oxygen absorption into the aqueous media, and consider only the liquid film mass transfer coefficient Aj and the volumetric coefficient k a, which are practically equal to and K a, respectively. Although carbon dioxide is considerably more soluble in water than oxygen, we can also consider that the liquid film resistance will control the rate of carbon dioxide desorption from the aqueous media. 

Fermentation broths are suspensions of microbial cells in a culture media. Although we need not consider the enhancement factor E for respiration reactions (as noted above), the physical presence per se of microbial cells in the broth will affect the k a values in bubbling-type fermentors. The rates of oxygen absorption into aqueous suspensions of sterilized yeast cells were measured in (i) an unaerated stirred tank with a known free gas-liquid interfacial area (ii) a bubble column and (iii) an aerated stirred tank [6]. Data acquired with scheme (i) showed that the A l values were only minimally affected by the presence of cells, whereas for schemes (ii) and (iii), the gas holdup and k a values were decreased somewhat with increasing cell concentrations, because of smaller a due to increased bubble sizes. 

The value of the liquid phase mass transfer coefficient can be obtained from the experimental data for physical absorption of oxygen into blood saturated with oxygen, or estimated from the data with the same apparatus for physical oxygen absorption into water or a reference liquid or solution with known physical properties. Mass transfer coefficients for liquids flowing through or across tubes or hollow fibers can usually be correlated by equations, such as Equation 6.26a for... 

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