Transfer height

Experimental results for particles in the millimeter range are shown in Fig. 12 in terms of unitary heat transfer height z/TV for different average particle diameters for both empty columns and columns with internal baffles. The various data give a range ofNH values between 2 and 7 for the 7.2-m experimental apparatus, corresponding to particle-to-gas heat transfer coefficients between 300 and 1,000 kcal/irf hr °C. The measured pressure drops for the two columns were of the order of 10 mm water gage. 

Figure 12. Experimental values for unitary heat-transfer height. (Kwauk and Tai, 1964.)...

Fig. 46. Experimental values of unitary heat-transfer height. Circles empty tube, solids heated circles with bar empty tube, solids cooled triangles baffles, solids heated. [After Kwauk and Tai, 1964.]...

Voidage lb per size range. apparatus and diam.. heat-transfer height. lb/(hr) Temp,... 

For condensation of methylene chloride in water, in cocurrent downflow 4-in. and 6-in. diam columns packed with i-in. Intalox saddles, the volumetric transfer coefficients reported (HlOa) were less than half those obtained with the sieve-plate column. The difference may be due partially to the different definition of the temperature driving-force applied for these two columns. (The log-mean AT was used for the packed bed, and a 2-in. transfer height was assumed.) The volumetric heat transfer coefficients increased with the 0.4-0.6 power of the liquid rate from 65,000 to 150,000 Btu/hr/ft /°F with the liquid rate increasing from 1 to 4 x 10" Ib/hr/ft. Contrary to the sieve-plate and spray-column studies, no effects of the vapor flow rate (from 1100 to 2500 Ib/hr/ft ) on the heat-transfer coefficient were noted in the packed bed study. 

The mass transfer height available in the column is 102 ft from tray 1 to tray 60. A simulation of the present column operation indicates that the bottom of the rectifying section has a higher loading than the top of that section. Likewise, the bottom of the stripping section has a higher loading than the top of that section. 

HETP value—Mass transfer height that provides one theoretical stage of separation. 

Proton transfers from strong acids to water and alcohols rank among the most rapid chemical processes and occur almost as fast as the molecules collide with one another Thus the height of the energy barrier the activation energy for proton transfer must be quite low... 

To determine how the height of a theoretical plate can be decreased, it is necessary to understand the experimental factors contributing to the broadening of a solute s chromatographic band. Several theoretical treatments of band broadening have been proposed. We will consider one approach in which the height of a theoretical plate is determined by four contributions multiple paths, longitudinal diffusion, mass transfer in the stationary phase, and mass transfer in the mobile phase. 

To minimize the multiple path and mass transfer contributions to plate height (equations 12.23 and 12.26), the packing material should be of as small a diameter as is practical and loaded with a thin film of stationary phase (equation 12.25). Compared with capillary columns, which are discussed in the next section, packed columns can handle larger amounts of sample. Samples of 0.1-10 )J,L are routinely analyzed with a packed column. Column efficiencies are typically several hundred to 2000 plates/m, providing columns with 3000-10,000 theoretical plates. Assuming Wiax/Wiin is approximately 50, a packed column with 10,000 theoretical plates has a peak capacity (equation 12.18) of... 

Design Procedure. The packed height of the tower required to reduce the concentration of the solute in the gas stream from to acceptable residual level ofjy 2 may be calculated by combining point values of the mass transfer rate and a differential material balance for the absorbed component. Referring to a sHce dh of the absorber (Fig. 5),... 

Gj /k aPh.3.s the dimension of length or height and is thus designated the gas-phase height of one transfer unit, The integral is dimensionless and indicates how many of these transfer units it takes to make up the whole tower. Consequently, it is called the number of gas-phase transfer units, N. Equation 40 may therefore be written as... 

Hq and aie called the overall gas-phase height of a transfer unit and the number of overall gas-phase transfer units, respectively. In the case of a straight equiUbrium line, is often neady concentration-independent as explained earher. In such cases, use of equation 47 is especially convenient... 

To use all of these equations, the heights of the transfer units or the mass transfer coefficients and must be known. Transfer data for packed columns are often measured and reported direcdy in terms of and and correlated in this form against and... 

Therefore, in this case, one transfer unit corresponds to the height of packing required to effect a composition change just equal to the average driving force. 

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