Column design methods

Effect of Axial Dispersion on Column Performance. Another assumption underlying standard design methods is that the gas and the Hquid phases move in plug-flow fashion through the column. In reaHty, considerable departure from this ideal flow assumption exists (4) and different fluid... 

Whereas there is extensive Hterature on design methods for azeotropic and extractive distillation, much less has been pubUshed on operabiUty and control. It is, however, widely recognized that azeotropic distillation columns are difficult to operate and control because these columns exhibit complex dynamic behavior and parametric sensitivity (2—11). In contrast, extractive distillations do not exhibit such complex behavior and even highly optimized columns are no more difficult to control than ordinary distillation columns producing high purity products (12). 

The FUG method is convenient for new-column design with the following specifications ... 

A more quantitative and lengthy method, but still very useful for checking of the type required here is the Smith-Brinkley method (Reference 5). It uses two sets of separation factors for the top and bottom parts of the column for a fractionator or reboiled absorber and one overall separation factor for a simple absorber. The method is tailor-made for analysis of a column design or a field installed column. The Smith-Brinkley method starts with the column parameters and calculates the resulting product compositions unlike other methods that require knowing the compositions to determine the required reflux. 

The design of a distillation column is based on information derived from the VLE diagram describing the mixtures to be separated. The vapor-liquid equilibrium characteristics are indicated by the characteristic shapes of the equilibrium curves. This is what determines the number of stages, and hence the number of trays needed for a separation. Although column designs are often proprietary, the classical method of McCabe-Thiele for binary columns is instructive on the principles of design. 

Although this method appears tedious, it is not so unwieldy as to be impractical. It does require close attention to detail. However, a value of (L/D)miii can be obtained with one trial that may be satisfactory for order of magnitude use, which is quite often what is desired before proceeding with detailed column design and establishment of operating L/D... 

Mlcrochromatographlc Methods During the past two years rapid. Inexpensive, miniaturized column chromatographic methods for the separation of hemoglobins have been developed These methods are designed for the qualitative detection and quantitative determination of hemoglobins In normal and abnormal conditions and cover the quantitation of Hb-A2 the detection of Hb-S, Hb-C other abnormal Hbs differentiation of various conditions In adults and the detection of hemoglobinopathies especially sickle cell anemia at birth (27, 28, 29, 30) ... 

The amount of a component absorbed or stripped in a column is dependent on the column design (the number of stages), the component solubility, and the gas and liquid rates. The fraction absorbed can be estimated using the absorption factor method, attributed to Kremser (1930) (see Volume 2, Chapter 12). If the concentration of solute in the solvent feed to the column is zero, or can be neglected, then for the solute component the fraction absorbed =... 

This chapter covers the design of separating columns. Though the emphasis is on distillation processes, the basic construction features, and many of the design methods, also apply to other multistage processes such as stripping, absorption and extraction. 

This method is basically a rating method, suitable for determining the performance of an existing column, rather than a design method, as the number of stages must be known. 

Relaxation methods are not competitive with the steady-state methods in the use of computer time, because of slow convergence. However, because they model the actual operation of the column, convergence should be achieved for all practical problems. The method has the potential of development for the study of the transient behaviour of column designs, and for the analysis and design of batch distillation columns. 

An estimate of the overall column efficiency will be needed when the design method used gives an estimate of the number of ideal stages required for the separation. 

Plate design, like most engineering design, is a combination theory and practice. The design methods use semi-empirical correlations derived from fundamental research work combined with practical experience obtained from the operation of commercial columns. Proven layouts are used, and the plate dimensions are kept within the range of values known to give satisfactory performance. 

Packed columns are used for distillation, gas absorption, and liquid-liquid extraction only distillation and absorption will be considered in this section. Stripping (desorption) is the reverse of absorption and the same design methods will apply. 

The design methods given in this section can be used for reboilers and vaporisers. Reboilers are used with distillation columns to vaporise a fraction of the bottom product whereas in a vaporiser essentially all the feed is vaporised. 

Practical Aspects There are a number of process-specific concerns that are accounted for in good design. In regenerate systems, sorbents age, losing capacity because of fouling by heavy contaminants, loss of surface area or crystallinity, oxidation, and the like. Mass-transfer resistances may increase over time. Because of particle shape, size distribution, or column packing method,... 

Fluorescence detection can be up to four orders of magnitude more sensitive than UV absorbance, especially where laser induced excitation is used, mass detection limits being as low as 10-20—10 21 mole. Pre- and post-column derivatization methods are being developed to extend the applicability of fluorescence detection to non-fluorescent substances. Several types of electrochemical and mass spectrometric detector have also been designed. Detector characteristics are summarized in Table 4.21. 

For the absorption of soluble gases or particulates from the gas streams, scrubbers can be used. These are generally simple packed columns, spray towers, or Venturi units. Design methods are well established [236]. Attention must be paid to the prevention of sprays, mists, and carryover when wet scrubbers are used. 

In this chapter consideration is given to the theory of the process, methods of distillation and calculation of the number of stages required for both binary and multicomponent systems, and discussion on design methods is included for plate and packed columns incorporating a variety of column internals. 

Thus, there are eight design-method/descriptor-set combinations to compare, as shown in the first two columns of Table 1. We use UCC to measure diversity, as it provides a comprehensive assessment of coverage in all low-dimensional subsets of variables. Recall that a small value of UCC is better. Furthermore, no matter how the design is generated, UCC can be measured according to the BCUT or constitutional descriptors (6 or 20 PCs). The results are very similar for the two replicates, hence only the first replicate is reported. 

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