Packings heat conductivity

Patten studied the change on heat conductivity C, diffusivity C/Kpy and specific volume (the reciprocal of the apparent density) of various packings with varying moisture contents. Figure 58 shows some of the data obtained by Patten an coarse and line quartz. The curves, with the... 

The heat-conductivity data obtained by Patten were by no means uniform in character. This may be attributed to a number of factors, but particularly to the nature of the packings used. Conductivity when plotted against moisture content as illustrated in Figure 58 was linear, concave upward or concave downward, depending upon the material used. 

C = coefficient of heat conductivity measured in heat units per unit time per unit of packing volume per deg temperature difference usual units are cal per sec per deg C per cc m = a constant for limestone packings, m = 0.0073 for iron ore, 0.0105 anthracite and other coals, 0.0050 blast-furnace charge, 0.0072. The values are in metric units. q = volume of gas flow per unit-time per square unit of cross-sectional packing area, expressed as liters per sec per sq cm T = temperature, deg abs, C = fractional voids (dimensionless) d = diameter of particles, cm... 

Limestone having an average diameter of 3 cm is packed in a 100-cm cylindrical column. The voids are calculated to be 34.5 percent. Air having a temperature of 320 deg C is made to flow through the column at a rate of 500 liters per sec. Determine the coefficient of heat conductivity. 

Sample size and weight, packing density, heat capacity, and heat conductivity are among the sample parameters that may impact the outcome of a thermal analysis. It is important that a small sample be used and that the sample pan fits the sample shape. 

Except the recycle loop reactor, real reactors, either packed beds or monoliths, are neither plug flow nor mixed flow reactors. However, in small laboratory reactors, heat conduction tln-ough the solid phase probably makes the temperature to be unifonn as in mixed flow. Conversely, the concentration profiles are those of plug flow. Real LO curves are thus intennediate between tliose of plug and mixed flow. 

Foams, in addition to being useful as cushioning, can be used to provide thermal insulation for products. A frozen product, for example, might be packaged with ice (or dry ice or gel packs) to provide cooling, and encased in a foam container to help reduce the conduction of heat from the surroundings into the container. Often the temperature inside and outside the container can be regarded as relatively constant, and the heat transfer process can considered essentially one-dimensional. In such cases, Fourier s law of heat conduction reduces to its one-dimensional steady-state form ... 

In the packed bed reactor, there is also the influence of heat conduction from particle. As we know, the temperature affects substantially the rate constant and therefore the reaction rate. In parallel, there are the effects of mass transfer by convection and diffusion in the pores of the particles. Therefore, these effects change the kinetics considerably and hence the chemical reaction rate. 

Increasing the heat conductivity of the reactor material and/or of the catalytic or inert packing in the reactor. 

It is very well possible that the axial heat conduction in the wall has an influence on the temperature in the region close to the wall. A detailed description of the temperature in the bed as a function of the radial and axial positions can be obtained by extending the model to two dimensions. However, it is also possible to get more insights into the behaviour by adjusting the one-dimensional model. Instead of accounting for the heat capacity of the wall in the accumulation term, an extra energy balance for the wall is included in the model, in which heat is exchanged with the packed bed ... 

The length and diameter of the tube and the particle size (hydratdic diameter) also affect flow distrihution within the packed tube. If the ratio of the tube diameter to that of the particle diameter is above 30, radial variations in velocity can be n lected, and plug (piston) flow behavior can be assumed. The ratio of the tube length to particle diameter is also important if this ratio exceeds 50, axial dispersion and axial heat conduction effects can be ignored. These efiects bring notable simplifications into the modeling of PBRs, which are discussed in Chapter 3. 

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