Theoretical plate extraction

In the example, the minimum reflux ratio and minimum number of theoretical plates decreased 14- to 33-fold, respectively, when the relative volatiHty increased from 1.1 to 4. Other distillation systems would have different specific reflux ratios and numbers of theoretical plates, but the trend would be the same. As the relative volatiHty approaches unity, distillation separations rapidly become more cosdy in terms of both capital and operating costs. The relative volatiHty can sometimes be improved through the use of an extraneous solvent that modifies the VLE. Binary azeotropic systems are impossible to separate into pure components in a single column, but the azeotrope can often be broken by an extraneous entrainer (see Distillation, A7EOTROPTC AND EXTRACTIVE). 

It is seen that by a simple curve fitting process, the individual contributions to the total variance per unit length can be easily extracted. It is also seen that there is minimum value for the HETP at a particular velocity. Thus, the maximum number of theoretical plates obtainable from a given column (the maximum efficiency) can only be obtained by operating at the optimum mobile phase velocity. 

Figure 1.17 Separation of large ring polycyclic aroaatic hydrocarbons extracted from carbon black on a 1.8 x 0.2 n I.D. fused silica capillary column packed with 3 micrometer spherical octadecylsllanized silica gel eluted with a stepwise solvent gradient at a flow rate of 1.1 mlcroliters/min with an inlet pressure of about 360 atmospheres. Under isocratic conditions this column yielded ca. 225,000 theoretical plates. (Reproduced with permission from ref. 238. Copyright Friedr. Vieweg t Sohn).

The sample volume initially introduced onto the sorbent, the choice of sorbent and solvent system and careful control of the amount of solvent used are of paramount importance for effective pre-concentration and/or clean-up of the analyte in the sample. The number of theoretical plates in an SPE column is low (/V = 10-25). SPE is a multistage separation method and as such requires only a reasonable difference in extractability to separate two solutes. In SPE concentration factors of 1000 or more are possible, as compared to up to 100 for LLE with vortex mixing. 

Paprika can be extracted to recover carotenoids, not only with CO2 but also with other gases. For example, by using ethane or ethylene, better results were obtained for the yield, extraction time, and quality of product. The solubilities of carotenoids are better in these gases, which is why the consumption of solvent and the extraction time were reduced. Practically water-free dye-concentrate was recovered by supercritical fluid ethane (under the conditions extraction 250 bar, 45°C separation 46 bar, 45 °C). The separation of pungent substances (capsaicinoids, free fatty acids) from the pigments can be carried out effectively in a continuous, counter-current extraction column with a large number of theoretical plates. 

GC-Mass Spectrometry. The ether extracts were concentrated using an Oldershaw column with 30 theoretical plates followed by a 200-plate spinning band distillation apparatus (Kontes Glass Co., Vineland,... 

The operation is most easily understood by reference to the equivalent true countercurrent system (Fig. 15). If we consider a feed containing two species A and B, with A the more strongly adsorbed, and a desorbent C, then in order to obtain separation the net flow directions in each section must be as indicated. With the equilibrium isotherms and the feed composition and flow rate specified, this requirement in effect fixes all flow rates throughout the system as well as the adsorbent recirculation rate or switch time. From simple theoretical considerations it can be easily shown that the affinity of the adsorbent for the desorbent should be intermediate between that for the strongly and weakly adsorbed feed compounds (i.e., a c > 1 -0, bc < 1 -0). The heights of the individualized bed sections are then determined by the requirement that each section contain sufficient theoretical plates to achieve the required purity of raffinate and extract products. For a linear system the analysis is straightforward since simple expressions for the concentration profile are available in terms of the kinetic and equilibrium... 

Efficiencies for the two types of columns were both somewhat low at about 1500 theoretical plates for the plastic bead column and only about 1000 for the silica particle columns the latter were more prone to deterioration from crop co-extractives, dropping to only about 600 after months of use, while the plastic bead column was not measurably affected. 

The steady state of a TMB is calculated for different numbers of theoretical plates, where an identical number of plates in each zone is assumed. The extract and raffinate purities are derived from each numerical simulation. 

Martin and Synge (3) introduced the important concept of theoretical plates into chromatography. Their concept was derived from partition theory and random statistics, and was related to similar ideas developed for extraction and fractional distillation. They supposed that the column could be divided into a number of sections called theoretical plates, and that solutes (dissolved compounds) could be expected to achieve equilibrium between the two phases (mobile and stationary) that exist within each plate. The chromatographic process, like an extraction process, can be visualized to occur when mobile phase (solvent) is transferred to the next plate, where a new equilibrium is established. Theoretical plate numbers of 1000 or more are common for HPLC columns, which means that 1000 separate equilibria must be established to obtain the same degree of separation by solvent... 

Extraction chromatography (reversed phase partition chromatography) has been used in analytical and biochemistry to effect chemical separations. It is a method which combines the simplicity of ion exchange and the selectivity of solvent extraction. Ion exchange theory may be used to calculate the number of theoretical plates in the column and the enrichment coefficient. Extraction chromatography as a separation method has been recently reviewed by Cerrai (J) and Katykhin (7). 

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