Heat transfer transition zones

Transition Zone III is of utmost importance, since the formation of dry spots is accompanied by a dramatic change in the heat transfer mechanism. In such units as gas-fired boilers, the dry spots may cause the tube wall temperature to approach the temperature of the heating gas. However, before the tube wall temperature reaches a steady-state value, the tensile strength of the tube wall is reduced, and rupture may occur. This phenomenon, called burn-out, may also occur at any point along the tube wall if the wall heat flux qmt is large enough so that a vapor film forms between the tube wall and the liquid surface. 

In the transition zone and the freeboard region, heat transfer between bed and wall is a function of bed density. Shirai et al. (S9, Sll) studied heat transfer from a sphere immersed in the fluidized bed and showed the trend of decreasing heat-transfer coefiicient with decreasing bed density. 

Rate of cooling also is important in systems where crystallization is undesired. For amorphous products, cooling must be sufficiently rapid that the mass passes through the crystallization zone between the solubility curve and glass transition zone before nucleation occurs. In this case, heat transfer must occur more... 

When the Reynolds number is between 2,100 and 10,000 it is called the transitional zone. In this zone one equation is not sufficient. Here we must resort to breaking the heat transfer data into intervals to fit the TEMA curves as closely as possible. 

For a system with n components (including nonad-sorbable inert species) there are n — 1 differential mass balance equations of type (17) and n — 1 rate equations [Eq. (18)]. The solution to this set of equations is a set of n — 1 concentration fronts or mass transfer zones separated by plateau regions and with each mass transfer zone propagating through the column at its characteristic velocity as determined by the equilibrium relationship. In addition, if the system is nonisothermal, there will be the differential column heat balance and the particle heat balance equations, which are coupled to the adsorption rate equation through the temperature dependence of the rate and equilibrium constants. The solution for a nonisothermal system will therefore contain an additional mass transfer zone traveling with the characteristic velocity of the temperature front, which is determined by the heat capacities of adsorbent and fluid and the heat of adsorption. A nonisothermal or adiabatic system with n components will therefore have n transitions or mass transfer zones and as such can be considered formally similar to an (n + 1)-component isothermal system. 

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. 

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