Transition regime mass transfer

The mass transfer coefficient increases only slightly with temperature, so above a certain temperature the reaction becomes mass transfer controlled. Further increases in temperature give almost no change in conversion. The transition to mass transfer control occurs at a lower temperature for very reactive species, such as H2 and CO, than for hydrocarbons, but the kinetics of oxidation are often not known. The design temperature and flow rate are based on lab tests or experience with similar materials. The reactor is usually operated in the mass transfer control regime, where the conversion depends on the rate of mass transfer and the gas flow rate. 

Mishima, K., and M. Ishii, 1984, Flow Regime Transition Criteria for Upward Two-Phase Flow in Vertical Tubes, Int. J. Heat Mass Transfer 27 723, (3)... 

Overall mass-transfer rates at a sphere in forced flow, and mass-transfer rate distribution over a sphere as a function of the polar angle have been measured by Gibert, Angelino, and co-workers (G2, G4a) for a wide range of Reynolds numbers. The overall rate dependence on Re exhibited two distinct regimes with a sharp transition at Re = 1250. Local mass-transfer rates were deduced from measurements in which the sphere was progressively coated by an insulator, starting from the rear. 

Ultimately at high frequencies the pulses overlap and we arrive in the dispersed bubble flow regime. Thus we consider the pulses to be zones of the bed already in the dispersed bubble flow, spaced by moving compartments that are still in the gas-continuous flow regime. This concept is very helpful in calculating mass transfer and mixing phenomena, as well as in pressure drop relations (9) where it appears that above the transition point the pressure drop can be correlated linearly with the pulse frequency. Pulses are to be considered as porous to the gas flow as is shown when we plot the pulse velocity versus the real gas flow rate, figure 5. 

While the classical hydraulic model provides a reasonable approximation for the froth and emulsion regimes, different mechanisms determine the hydraulics and mass transfer in the spray regime. The transition from froth to spray is gradual, and so is the change in the hydraulic and mass transfer behavior (110,111,113,114). 

In this study, a and kLa are measured in a TBR in the pressure range [0.3-3.2 MPa] using fast and slow chemical absorption of carbon dioxide into diethanolamine (DEA) aqueous and organic solutions. Only the trickling regime and trickling/pulsing transition have been explored. A simple model to explain the increase of interfacial area and mass transfer... 

The transition from the liquid- to the gas-phase reaction regime is often accompanied by a marked increase in the reaction rate, because the gas phase surrounding the catalyst pellet offers less mass-transfer resistance than the liquid phase. For the case of an exothermic reaction, this may have an undesirable effect, as it gives rise to a rather narrow reaction zone with steep temperature gradients. Thus, the catalyst may be exposed to local overheating, which results in subsequent deactivation of the bed or the occurrence of a number of undesirable side reactions. Furthermore, if the heat removed is insufficient, the hot-spot temperature could occur. 

Experimental evidence has demonstrated that Dean vortices can be effective for enhancement of membrane performance under laminar conditions [18]. As flow conditions approach the transition and turbulent flow regimes, straight membranes have a better mass transfer and higher wall shear rate than in flows with curved membrane channels. The effects of Dean vortices on the performance of membrane filtration have been studied experimentally and theoretically by Belfort and coworkers [19-22]. Mallubhotla and Belfort [21] assessed the filtration of suspensions of polydispersed polystyrene particles (mean diameter 25 pm) and silica particles (mean diameter 20 pm) with and without the presence of Dean flow using an 180° U-bend channel... 

There is some controversy as to how the mass transfer rate varies with particle size, with the transition mechanism between boundary layer transfer and molecular diffusion appearing to depend on particle density. Levins and Glastonbury simply sum the molecular diffusion and boundary layer terms, giving a smooth transition between the two regimes. Brucato et al. (1990) present experimental results with dense particles (pp > 2 kg/m ), showing little effect of particle size down to a 15-p particle radius (the Levins and Glastonbury correlation predicts an increase in dissolution rate of about 66% compared to large... 

The transition between spray and froth regimes is usually detected by light-scattering tech-niqnes. Mass transfer models discussed later are based on the froth regime, the one that normally prevails. 

The solids and the fluid have similar densities in liquid fluidization. The consequence is that most liquidized beds operate in the particulate regime where there is a smooth transition from incipient fluidization to pneumatic transport without bubble formation or slugging. They typically operate at near isothermal conditions and have good mass transfer between the liquid and the suspended solids. As a first approximation, the solid phase is well mixed and the liquid phase is in piston flow. There may also be a gas phase. Typical applications are in cell culture, including wastewater treatment. The specialized literature gives details. 

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