Mass transfer extreme turbulence

Figures 4.34 and 4.35 represent two extreme cases. Drying processes represent the case shown in Fig. 4.34 and distillation processes represent Fig. 4.35. Neither case represents a convective mass transfer case while the gas flow is in the boundary layer, other flows are Stefan flow and turbulence. Thus Eqs. (4.243) and (4.244) can seldom be used in practice, but their forms are used in determining the mass transfer factor for different cases.

The existence of turbulent flow can be advantageous in the sense of providing increased heat and mass transfer rates. However, the motion is extremely complicated and difficult to describe theoretically [3, 8]. In dealing with turbulent flow it is customary to speak of a total shear stress and total fluxes normal to the main flow direction (the main flow is in the x direction, and the y axis is normal to the flow direction), which are defined as... 

A chemical reaction is initiated by bringing the required reactants into intimate contact. Efficient execution of this process often defines the efficiency of the reaction and, therefore, requires considered examination when designing a reaction system. The physical transport of a component along a concentration gradient by molecular diffusion and turbulent convection is known as mass transfer. The transport of mass through an interface between two media or phases of the same medium is extremely important, since chemical reactions are normally coupled to the mass transfer efficiency. The mass transfer coefficient is used to quantify the efficiency of mixing in macrofluidic systems and is given by... 

In contrast to either the membrane reactor or the fixed-bed reactor, turbulent fluidized beds have excellent heat transfer characteristics. In addition, they have good mass transfer rates and the ability to operate at extremely large scales. While a typical fixed-bed reactor may weigh up to 740 kt, a fluidized bed with a similar capacity... 

Mass transfer by molecular diffusion resides at the other end of the spectrum. It yields the lowest possible mass transfer rate and sets an upper limit on time requirements. The solvent spill considered in Practice Problem 4.4, for example, requires several days to complete evaporation by diffusion into stagnant air. The same data applied to turbulent air flow at the same temperature yield an estimate of several minutes, lower by three orders of magnitude. The factor of 1000 can thus be viewed as separating the two extremes of diffusive and turbulent mass transfer. 

The expression predicts high mass transfer coefficients during the first fractions of a second when the boundary layer is still extremely thin. Thereafter they decline rapidly, falling to 1/100th to 1/1000th of the turbulent flow values after 3 h of elapsed time. 

---