Turbulent contactors

A comparison of different gas-hquid contactors best starts with exploring the amount of energy that is needed to generate inter facial area. In conventional turbulent contactors, the bubble size is determined by bubble break-up and coalescence. In essence, one can set up a balance between (1) surface tension, which 

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Figure 15 shows data characterizing gas-liquid mass transfer (represented by values of kid) as a function of the power input for monoliths and typical turbulent contactors (agitated tanks and bubble columns). It is... 

So, for laminar contactors we predict that the mass transfer is a much weaker function of the amount of energy dissipated than for turbulent contactors. The fact that energy is not required for interfacial area generation is in perfect agreement with this scaling analysis. Of course, there still is an impact of velocity on... 

For low power input, e.g. (P/V) = 1 kW/m this gives ki a 0.5, which is an order of magnitude higher than can be obtained in turbulent contactors at the same power input. Again, this result can be understood by reahzing that no power is required for bubble break-up. 

In Fig. 6.4, experimental mass-transfer data for the adsorption of oxygen are compared for turbulent contactors and monoliths. The data for stirred tanks and bubble column follow the trend predicted by Eq. (6.3), and the different lines correspond to different Uu/V). 

Figure 6.4 Gas-liquid mass transfer versus power input for monoliths and turbulent contactors. All data are for the Oj/ water system. The dotted line corresponds to Eq. (6.10), the shaded area gives the range of kiO vs. P/V) for monoliths. The five larger dots in the monolith data are for a set of experiments where the gas holdup c was kept at 0.6.

The experimental bubble-column data from Linek et al. [27] and stirred-tank data from Schliiter and Deckwer [28] are also plotted in Fig. 6.5. If we ignore shuttling of catalyst particles to the film surrounding bubbles in turbulent contactors, then the highest gas-to-sohd mass-transfer rate is obtained at high catalyst loading, and in that case to a first approximation the gas-liquid mass-transfer rate, as defined by Eq. (6.3), can be used to estimate the overall mass transfer. 

Using a simple scaling analysis, involving (1) viscous pressure drop, (2) hydrostatic pressure drop, (3) interfacial pressure drop and (4) penetration theory for mass transfer, it has been demonstrated that two-phase laminar bubble-train flow in small channels can exhibit better mass transfer for a given power input than turbulent contactors. 

Slurry reactors can be classified according to the phases where the reactants are present. Table II gives an overview. The most important distinction is whether the solid phase is a reactant or a catalyst. In principle, the solids could also be inert and only present to increase mass transfer between phases as is often the case, e.g., in trickle flow reactors. In slurry reactors the introduction of solids for this purpose only is not worthwhile, with the exception of solids like zeolites and activated carbon for enhancement of mass transfer or improvement of selectivity [21, 22] but in such a system the solid is not really inert. Another example is the turbulent contactor in which large but light balls are moved by a gas flow and irrigated by a liquid phase. However, this regime falls outside the scope of the present presentation. If the solid is a reactant as well as the gas phase and liquid phase, the situation becomes rather complex nevertheless, it corresponds to many practical situations (see e.g. Shah [2]). A rather exceptional... 

In many types of contactors, such as stirred tanks, rotary agitated columns, and pulsed columns, mechanical energy is appHed externally in order to reduce the drop si2e far below the values estimated from equations 36 and 37 and thereby increase the rate of mass transfer. The theory of local isotropic turbulence can be appHed to the breakup of a large drop into smaller ones (66), resulting in an expression of the form... 

Breakup in a highly turbulent field (L/velocity) ". This appears to be the dominant breakup process in distillation trays in the spray regime, pneumatic atomizers, and high-velocity pipehne contactors. 

The term three-phase fluidization requires some explanation, as it can be used to describe a variety of rather different operations. The three phases are gas, liquid and particulate solids, although other variations such as two immiscible liquids and particulate solids may exist in special applications. As in the case of a fixed-bed operation, both co-current and counter- current gas-liquid flow are permissible and, for each of these, both bubble flow, in which the liquid is the continuous phase and the gas dispersed, and trickle flow, in which the gas forms a continuous phase and the liquid is more or less dispersed, takes place. A well established device for countercurrent trickle flow, in which low-density solid spheres are fluidized by an upward current of gas and irrigated by a downward flow of liquid, is variously known as the turbulent bed, mobile bed and fluidized packing contactor, or the turbulent contact absorber when it is specifically used for gas absorption and/or dust removal. Still another variation is a three-phase spouted bed contactor. 

The 1970 s also brought about increased use of three-phase systems in environmental applications. A three-phase fluidized bed system, known as the Turbulent Bed Contactor, was commercially used in the 1970 s to remove sulfur dioxide and particulates from flue gas generated by coal combustion processes. This wet scrubbing process experienced several... 

A Study of a Fluidized Turbulent Bed Contactor with Application to Cooling Towers... 

Performance and Design of a Turbulent Bed (Contactor (TBC)) Cooling Tower... 

A short cocurrent horizontal pipeline contactor gives 86 percent removal of NH3. There is no bypassing because of the highly turbulent gas flow and injection of liquid into the center of the pipe. What would we expect the exit gas temperature to be ... 

In this case, E is the axial dispersion coefficient and z refers to the axial direction in the contactor. The axial dispersion coefficient is a function of flow rates, turbulence, etc., and has a value far in excess of the molecular diffusivity D. Design methods that allow for axial dispersion are described in the research literature, but there is an acute need for more data on values of E for large-scale equipment. 

Horizontal contactors are essentially bubble columns with an aspect ratio less than one, and the gas is sparged at the bottom as turbulent jets. In order to get fairly uniform gas-liquid dispersion, multiple injection points are employed for the gas. The gas-liquid contact can be further improved using impellers (Fig. 33). The impellers are of a modified propeller type and are mounted on a horizontal shaft. 

As a gas passes through the contactor unit into the liquid, a large amount of turbulence is set up, and liquid particles can become entrained with the gas. Carry-over of these liquid particles from one tray to the tray above is known as entrainment. It is often defined as the weight of liquid entrained per unit weight of gas. Liquid can be entrained by the gas as a result of violent splashing of the liquid or because of extensive foaming or frothing. 

Calabrese RV, Chang TPK, and Dang PT. Drop breakup in turbulent stirred-tank contactors. Part I Effect of dispersed-phase viscosity. AIChE J 1986 32 657-666. 

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