Adiabatic Design Method

Classical Adiabatic Design Method The classical adiabatic method assumes that the heat of solution serves only to heat up the liquid stream and that there is no vaporization of solvent. This assumption makes it feasible to relate increases in the hquid-phase temperature to the solute concentration x by a simple eutnalpy balance. The equihbrium curve can then be adjusted to account For the corresponding temperature rise on an xy diagram. The adjusted equilibrium curve will become more concave upward as the concentration increases, tending to decrease the driving forces near the bottom of the tower, as illustrated in Fig. 14-8 in Example 6. 

Comparisons by von Stockar and Wilke [Ind. Eng. Chem. Fundam., 16, 89 (1977)] between the rigorous and the classical adiabatic design methods for packed towers indicate that the simple adiabatic method underestimates the packing depths by as much as a factor of 1.25 to 1.5. Thus, when using the (dassical adiabatic method, one should consider the possible need to apply a design safety factor. 

Applicability of Physical Design Methods Physical design methods such as the classical isothermal design method or the classical adiabatic design method may be applicable for systems in which chemical reac tions are either extremely fast or extremely slow or when chemical equihbrium is achieved between the gas and liqmd phases. 

Finally, note that the adiabatic and isothermal pipe methods produce results that are reasonably close. For most real situations the heat transfer characteristics cannot be easily determined. Thus the adiabatic pipe method is the method of choice it will always produce the larger number for a conservative safety design. 

The recommended design method for Mach numbers up to supersonic (Ma < 4) and adiabatic wall conditions (Tw = Taw) is arbitrary since all the methods yield essentially the same results The design methods for hypersonic Mach numbers (Ma, > 4) and cold wall conditions (Tw< T w) should be based on conservatism. Comparison of the available skin friction data reveals differences of 20 percent on surfaces near adiabatic wall temperature and as much as a factor of 2 for highly cooled walls (T 0.2Taw). If only the most recent skin friction data for... 

The heat of reaction and the rate of heat production in a reaction mixture as a function of temperature are important quantities for the design of reactors in chemical industry. Presently, several methods for the determination of these quantities are available, such as Differential Scanning Calorimetry, Differential Thermal Analysis, Bench Scale Calorimetry / / and adiabatic calorimetric methods. 

The validity of this widely used procedure depends on the conditions in the laboratory column being similar to those in the full-scale unit. In particular, since large-diameter columns commonly operate adiabatically it is very important that the small-scale column should be very well insulated. If heat loss from the laboratory column is significant, the apparent LUB will be erroneously low leading to underdesign of the full-scale unit. Other factors such as axial dispersion, which may not be the same in the large and small column can also lead to discrepancies, but if caution is exercised the LUB concept provides a simple and practically useful design method. 

This design procedure is widely used and its validity depends on the conditions in the laboratory column beilig similar to those for the full-scale unit. The small-diameter unit must be well insulated to be similar to the large-diameter tower, which operates adiabatically. The mass velocity in both units must be the same and the bed of sufficient length to contain a steady-state mass transfer zone (LI). Axial dispersion or axial mixing may not be exactly the same in both towers, but if caution is exercised, this method is a useful design method. 

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