Heat transfer bed-surface

The heat transfer coefficient hjp which characterises heat transfer between the bed wall or an immersed surface and the fluidized bed is defined by 

The high rates of heat transfer obtainable are due to a number of reasons. Firsf, fhe presence of particles in a fluidized bed increases the heat transfer coefficient by up to two orders of magnitude, compared with the value obtained with gas alone at the same velocity. This is because the particles tend to reduce the thickness of the boundary layer at the heat transfer surface (Jowitt, 1977). The bed particles are responsible for fhe fransfer of heat and, because of the high rate of particle movement (and very short residence times close to the heat transfer 

The second reason for high rates of heat transfer is that the volumetric particle heat capacity is about 1000 times greater than that of a gas and therefore approximates to that of a liquid. As Botterill (1975) has pointed out, a fluidized bed is effectively a fluid of high heat capacity but very low vapour pressure. Third, the very high specific surface of the particles results in high heat fluxes. 

In a bubbling fluidized bed the coefficient of heat transfer between bed and immersed surfaces (vertical bed walls or tubes) can be considered to be made up of three components which are approximately additive (Botterill, 1975). 

Botterill (1986) recommends the Zabrodsky (1966) correlation for hmax for Group B powders  

For temperatures beyond 600 C radiative heat transfer plays an increasing role and must be accounted for in calculations. The reader is referred to Botterill (1986) or Kunii and Levenspiel (1990) for treatment of radiative heat transfer or for a more detailed look at heat transfer in fluidized beds. 

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Figure 7.16 Effect of fluidizing gas velocity on bed-surface heat transfer coefficient in a fluidized bed...

In addition, the rising bubbles cause the motion of particles, which obviously intensifies the solids mixing on the macro scale and leads to the temperature uniformity and high bed/surface heat transfer characteristic. This is another attractive advantage of G-S fluidization. 

Bed-to-Surface Heat Transfer. Bed-to-surface heat-transfer coefficients in fluidized beds are high. In a fast-fluidized bed combustor containing mostly Group B limestone particles, the dense bed-to-boiling water heat-transfer coefficient is on the order of 250 W/(m -K). For an FCC catalyst cooler (Group A particles), this heat-transfer coefficient is around 600 W/(600 -K). 

These are used for adiabatic processing or when it is practical to embed heat-transfer surface in the bed. Usually, heat transfer is more... 

Give the detailed chemical engineering design for the fluidness bed and heat transfer surfaces. Select a suitable heat transfer fluid and give reasons for your selection. Do not attempt to specify the filters or to design the condenser/cooler in detail. 

In general, gas-to-particle or particle-to-gas heat transfer is not limiting in fluidized beds (Botterill, 1986). Therefore, bed-to-surface heat transfer coefficients are generally limiting, and are of most interest. The overall heat transfer coefficient (h) can be viewed as the sum of the particle convective heat transfer coefficient (h ), the gas convective heat transfer coefficient (h ), and the radiant heat transfer coefficient (hr). 

Overall bed-to-surface heat transfer coefficient = Gas convective heat transfer coefficient = Particle convective heat transfer coefficient = Radiant heat transfer coefficient = Jet penetration length = Width of cyclone inlet = Number of spirals in cyclone = Elasticity modulus for a fluidized bed = Elasticity modulus at minimum bubbling = Richardson-Zaki exponent... 

GLS fluidized with a stable level of catalyst. Only the fluid mixture leaves the vessel. Gas and liquid enter at the bottom. Liquid is continuous, gas is dispersed. Particles are larger than in bubble columns, 0.2-1.0 mm. Bed expansion is small. Bed temperatures are uniform within 2 C in medium size beds, and heat transfer to embedded surfaces is excellent. Catalyst may be bled off and replenished continuously, or reactivated continuously. 

Bed-to-surface heat transfer, 11 809—810 Beer, 3 561-589. See also Beer brewing Brewing entries brewing process for, 3 563, 564,... 

Normally, the heat-transfer rate is between 5 and 25 times that for the gas alone. Bed-to-surface-heat transfer coefficients vary according to the type of solids in the bed. Group A solids have bed-to-surface heat-transfer coefficients of approximately 300 J/(m2s-K) [150 Btu/(h-ft2-°F)]. Group B solids h ave bed-to-surface heat-transfer coefficients of approximately 100 J/(m2- s-K) [50 Btu/(h-ft2-°F)], while group D solids have bed-to-surface heat-transfer coefficients of 60 J/(m2-s-K) [30 Btu/(hft2 oF)]. 

Whilst fluidised bed furnaces can be operated in the range 1075-1225 K, most operate close to 1175 K. Some of the tubes are immersed in the bed and others are above the free surface. Heat transfer to the immersed tubes is good. Tube areas are usually 6-10 m2/m3 of furnace, and transfer coefficients usually range from 300 to 500 W/m2 K. The radiation component of heat transfer is highly important and heat releases in large furnaces are about 106W/m3 of furnace. 

In gas-solid suspensions and fluidized beds, the heat transfer between particles and the wall surface or between a particle at one temperature and a group of other particles at another temperature is largely due to particle impacts. Thus, the average rate of heat transfer may be expressed in terms of a collisional heat transfer coefficient hc, which is defined by... 

For the bed-to-surface heat transfer in a dense-phase fluidized bed, the particle circulation induced by bubble motion plays an important role. This can be seen in a study of heat transfer properties around a single bubble rising in a gas-solid suspension conducted... 

Figure 12.13. Effect of temperature and pressure on h, hgc, and hmm for bed-to-surface heat transfer (from Botterill et al., 1981).

The suspension-to-wall surface heat transfer mechanism in a circulating fluidized bed (see Chapter 10) comprises various modes, including conduction due to particle clusters on the surface or particles falling along the walls, thermal radiation, and convection due to... 

The particle convection is in general important in the overall bed-to-surface heat transfer. When particles or particle clusters contact the surface, relatively large local temperature gradients are developed. This rate of heat transfer can be enhanced with increased surface renewal rate or decreased cluster residence time in the convective flow of particles in contact with the surface. The particle-convective component hpc can be expressed by the following equation, which is an alternative form of Eq. (12.39) ... 

Compared to the fluidized bed, a spouted bed with immersed heat exchangers is less frequently encountered. Thus, the bed-to-surface heat transfer in a spouted bed mainly is related to bed-to-wall heat transfer. The bed-to-immersed-object heat transfer coefficient reaches a maximum at the spout-annulus interface and increases with the particle diameter [Epstein and Grace, 1997]. 

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