Gas backmixing

Figure 16. Mixing coefficients for different vessel diameters Ml longitudinal dispersion of fluid, MB gas backmixing, Ms longitudinal solid dispersion. (From Van Deemter, 1980.)...

A common feature of all models for the upper part of circulating fluidized beds is the description of the mass exchange between dense phase and dilute phase. Analogously to low-velocity fluidized beds, the product of the local specific mass-transfer area a and the mass-transfer coefficient k may be used for this purpose. Many different methods for determining values for these important variables have been reported, such as tracer gas backmixing experiments [112], non-steady-state tracer gas experiments [117], model reactions [115], and theoretical calculations [114],... 

Burovoi, I. A., Dracheva, T. V, and Ibraev, A. Kh., Semi-Industrial Testing of the Fluidized-Bed Acid Leaching of Zinc Calcine, Tsvet, Metal. 44(10), 16(1971) Chem. Abs. 76,36,301 (1972). Cankurt, N. T., and Yerushalmi, J., Gas Backmixing in High Velocity Fluidized Beds, Second Intern. Conf. Fluidization. Cambridge, pp. 387-393 (1978). 

Low gas backmixing and axial dispersion favor improved conversion and selectivity of chemical reactions, especially for fast and complex reactions. 

Fig. 32. Influence of solids circulation rate on gas backmixing at a velocity higher than the incipient fast fluidization velocity (after Li and Wu, 1991).

Owing to the rapid formation and dissolution of particle clusters which contribute to high slip velocities and solid backmixing but preserve a limited extent of gas backmixing, the fast fluidized bed regenerator exhibits unique axial and radial profiles for voidage, temperature and carbon concentration (see Figs. 9 and 11 and Table VIII). 

This class of reactions, carried out in fluidized beds, involves parallel and series reactions, with reaction intermediates being the desired products. Industrial examples include partial oxidation of n-butane to maleic anhydride and o-xylene to phthalic anhydride. The vigorous solid mixing of fluidized beds is valuable for these reactions because they are highly exothermic. However, gas backmixing must be minimized to avoid extended gas residence times that lead to the formation of products of total combustion (i.e., CO2 and H2O). For this reason, fluidized bed catalytic partial oxidation reactors are operated in the higher velocity regimes of turbulent and fast-fluidization. 

Many small bubbles are required in a dispersion to give a large gas-liquid interfacial area and thus effective mass transfer. Small bubbles are more readily entrained into the circulating liquid stream and so give more gas backmixing than larger bubbles under similar hydrodynamic conditions. 

The degree of gas backmixing is mostly related to the proportion of the gas flow which recirculates through the agitator. As has already been seen in... 

The degree of gas backmixing therefore depends upon the amount of gas recirculating (which is a function of local liquid velocity and turbulence, flow pattern and bubble size) as compared with the amount fed to the vessel. Of the gas generally recirculating, that which backmixes does so by being drawn into the cavities. There is very little information available on the amount of recirculation but some preliminary data for 6-blade disc turbines (C=H/3, H = T) is presented in Figure 15.21. 

Weinstein and co-workers developed elaborate sampling and injection manifolds to measure gas backmixing in the entrance region, or dense zone and fully... 

The most attractive feature of a turbulent fluidized bed is its uniform temperature field. Thus the conversion can be obviously improved at fully developed turbulent fluidization for most isothermal catalytic fluidized bed reactors compared with bubbling fluidization. At the same time, the solids backmixing is also violent in turbulent fluidization, which in turn intensifies the gas backmixing. A wide gas RTD is very harmful to the selectivity of a chemical process with side reactions. 

First, gas backmixing is substantially reduced by immersed obstacles. The higher the gas velocity the smaller the gas backmixing (Ye, 1987). There is little gas backmixing between the top and the bottom of each horizontal baffle. The axial gas-mixing coefficient is lowered only in beds of small particles or at low gas velocities. The larger the particles and the higher the gas velocity, the less effective is the horizontal baffles. 

Gas backmixing (dispersion) High Pea = 0.1-0.4 Pea = 5-8, close to plug flow in central zone strong backmixing near the wall Plug flow... 

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