Dense-phase fluidized beds mass transfer

Mass Transfer in Dense-Phase Fluidized Beds... 

In a dense-phase fluidized bed (see Chapter 9), mass transfer can take place between the particle and the gas, between the bubble and the emulsion, and between the bed and the surface. These processes are discussed in the following. 

The book is arranged in two parts Part I deals with basic relationships and phenomena, including particle size and properties, collision mechanics of solids, momentum transfer and charge transfer, heat and mass transfer, basic equations, and intrinsic phenomena in gas-solid flows. Part II discusses the characteristics of selected gas-solid flow systems such as gas-solid separators, hopper and standpipe flows, dense-phase fluidized beds, circulating fluidized beds, pneumatic conveying systems, and heat and mass transfer in fluidization systems. 

The best known model for slugging fluidized beds is that of Hovmand and Davidson (45,46). This is a variant on the Orcutt model (17,18) (see also Table 2) which assumes plug flow of gas in the dense phase. The interface mass transfer is again composed of two parts, a throughflow component and a diffusional component, with the dimensionless mass transfer coefficient given approximately by... 

Using the two-phase model, a fluidized catalytic bed reactor can be divided into two regions, one for the dense phase, i.e., the emulsion phase, and another for the bubble phase, with associated mass and heat transfer between the two regions and phases. 

More recent correlations for gas holdup and mass transfer include the effect of pressure and bimodal bubble size distribution (small and large bubbles), in a manner analogous to the treatment of dilute and dense phases in fluidized beds [see, e.g., Letzel et ah, Chem. Eng. Set, 54 (13) 2237 (1999)]. 

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],... 

The two-phase flow theory is adopted in the model it consists in two phases, a dense phase and a bubble phase separated by a film through which the mass transfer occurs. Gases move upward in both bubble and dense phase with plug flow, which proves to be adequate to describe the flow in a bubbling fluidized bed gasifier [17],... 

Gas in the dense and bubble phases plays different roles in a bubbling bed reactor. When gas enters the fluidized bed reactor, the gas in the bed flows to the dense and bubble phases. The gas reactant reacts in the dense phase upon contact with the particles. Interphase mass transfer allows gas reactant and product transfer between the bubble phase and the dense phase. As a bubble rises through a dense or emulsion phase region... 

From the point of view of mass transfer, a fluidized bed without bubbles is very effective, since the volumetric mass transfer coefficient is as a rule very high (this is the product of the mass transfer coefficient at the particle surface, and the surface area of the particles in the bed). On the other hand, in bubbling beds the mass transfer between the fluid and the solid is usually limited by the mass transfer between the bubbles and the dense phase. This process can be described by another volumetric mass transfer coefficient, that is the product of the specific area of the bubbles, which is quite small due to the relatively large bubble diameters, and the mass transfer coefficient between the bubbles and the dense phase, which is relatively large, due to the effective interchange of gas in the bubbles and gas in the dense phase. The bubbles also contribute to a large residence time distribution of the fluid phase (compare section 7.2.4) and this reduces further the effectivity of the mass transfer between the fluid phase and the solid. In bubbling beds the fluid is usually a gas. 

The prediction of the effects of axial mixing in fluidized beds is very difficult. In practical models both residence time distribution, chemically enhanced mass transfer from bubbles to dense phase, and intraparticle diffusion are taken into account. 

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