Ammonia absorption from a bubble expanding at a submerged orifice into water
Introduction
Bubble columns are widely used as reactors, gas–liquid contactors and absorbers in the chemical industry. Due to their simple design, the bubble column absorbers have the advantage of cost for energy and construction. However, the behavior of bubbles in bubble columns is complicated and difficult to predict. In refrigeration systems, for example, the performance strongly depends on the design of the bubble column. There are many studies concerning the design of tubular bubble absorbers.
Infante Ferreira, Keizerm, and Machielsen (1984) proposed a model for the calculation of heat and mass transfers in vertical tubular bubble column for the ammonia–water system. The mass transfer coefficients experimentally obtained were then correlated using a modified Sherwood equation.
Sujatha, Mani, and Srinivasa Murthy (1997) also analyzed the mass transfer for several organic absorbents as well as ammonia using a vertical tubular bubble absorber. A correlation equation for the liquid phase volumetric mass transfer coefficients was then proposed.
Elperin and Fominykh (1999a) studied gas absorption from a rising spherical bubble. They showed that the mass transfer rate and bubble collapse rate depend on the concentration of the absorbate in the liquid phase. Elperin and Fominykh (1999b) then investigated the mass and heat transfers during non-isothermal absorption in slug flow with small bubbles and presented a theoretical discussion.
In the cases of very soluble gaseous absorbates such as NH3, SO2 and HCl, however, the solute in a bubble rapidly dissolves into the absorbent. Therefore, it is essential for the design of the absorbers to know the absorption rate from a growing bubble connected to a gas distributor.
Guedes de Carvalho, Rocha, Vasconcelos, Silvia, and Oliveira (1986) separated the gas absorption in a bubble column into three regions: bubble formation in a gas distributor, rising bubbles, and bubble breakage at a free surface: and then experimentally investigated the mass transfer coefficient for each region.
Filla, Davidson, Bates, and Eccles (1976) investigated the mass transfer resistance in the gas phase for the ammonia absorption from a slug bubble rising in water.
Terasaka, Hieda, and Tsuge (1999) measured the SO2 absorption rate into water during pure SO2 bubble formation at an orifice in a bubble column. A theoretical model for non-spherical bubble formation accompanied by mass transfer was then proposed. Elperin and Fominykh (2001) theoretically investigated the effect of the absorbate concentration in the liquid phase on coupled mass and heat transfers for the experimental results reported by Terasaka et al. (1999).
In previous research, the mass transfer in the gas phase and interface was usually neglected although the mass transfer in the liquid phase must be very important because it often becomes a control stage.
In terms of the liquid phase mass transfer, many useful theories have been proposed. For the situation when the Reynolds number for the terminal velocity of a spherical single bubble is much greater than unity, that is, Re≫1, Boussinesq (1905) theoretically described the mass transfer coefficient kLusing a potential flow theory. For Re<1, Levich (1962) investigated in term of mass transfer from a rising spherical single bubble. To estimate kL for 10<Re<103, Lochiel and Calderbank (1964) modified the Levich equation using the velocity profile around a spherical bubble. Higbie (1935) proposed the penetration theory based on Fick's one-dimensional unsteady diffusion equation for unsteady mass transfer at the gas–liquid interface. In the unsteady cases, such as bubble growth and an accelerating rising bubble, Higbie's penetration theory is suitable rather than those of Boussinesq (1905) and Levich (1962) and Lochiel and Calderbank (1964).
In this study, therefore, the absorption from a gas bubble consisting of both ammonia as a soluble component and nitrogen as an insoluble component into water as an absorbent was investigated. To clarify the mechanism of mass transfer of the soluble component from a growing bubble, the non-spherical bubble formation model proposed by Terasaka et al. (1999) was modified for application to this situation. The simulated results such as bubble growth curve, bubble surface area and bubble shape were compared with the experimental ones.
Section snippets
Mechanism of mass transfer
Two gaseous components are contained in a bubble. One is a very soluble gas and the other does not dissolve in the absorbent. When a bubble begins to be generated at an orifice, the solute gas suddenly contacts with fresh absorption liquid, where mass transfer through the gas–liquid interface to the liquid phase is not steady.
The mass transfer from a bubble to the bulk of the liquid consists of three mass transports, i.e., a mass transfer from the bulk of the gas in a bubble to the gas–liquid
Physicochemical data
In this study, a gaseous mixture of NH3 and N2 was used as the feed gas into the gas distributor. The absorbent is distilled water. The ammonia component is more soluble in water but nitrogen does not dissolve in water. The physicochemical properties of the gas mixture and liquid are shown in Table 1.
Ammonia reversibly dissociates in water as follows:
The forward and reverse rate constants for ammonia dissociation in water are very fast, i.e., and ,
Bubble growth curves, surface area and shapes
Fig. 4 shows the bubble growth curves as a parameter of the NH3 composition in the feed gas, yF. For the same total gas flow rate, QF, and gas chamber volume, VC, the first stage of the bubble growth curves had very similar behavior, whereas the bubble expansion rate decreased with the increasing concentration of NH3 after the rapid expansion period. Finally, the volume of the bubble detached from an orifice, VB, decreased with increasing yF.
The lines in Fig. 4 describe the estimation of the
Conclusions
The mass transfer mechanism of the soluble component from the bulk of the gas phase in a bubble to the bulk of the static liquid phase was elucidated. The mass transfer was described by separating it into three mass transfer resistances for the gas phase, interface and liquid phase.
For the beginning of bubble formation, the resistance at the interface was important rather than the other resistances. However, the mass transfer in the liquid phase governed the mass transfer rate throughout the
Notation
gas–liquid contacting area, concentration of in bulk of liquid, concentration of at interface, saturated concentration in liquid, concentration difference of , diffusivity in gas phase, diffusivity in liquid phase, maximum horizontal diameter of bubble, m orifice diameter, m parameter in Eq. (15), total molar feed, mol Henry's constant, kinetic constant, mass transfer coefficient for transport through
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