Separating efficiency

Another important characteristic of a gas chromatographic column is the thickness of the stationary phase. As shown in equation 12.25, separation efficiency improves with thinner films. The most common film thickness is 0.25 pm. Thicker films are used for highly volatile solutes, such as gases, because they have a greater capacity for retaining such solutes. Thinner films are used when separating solutes of low volatility, such as steroids. 

The electroosmotic flow profile is very different from that for a phase moving under forced pressure. Figure 12.40 compares the flow profile for electroosmosis with that for hydrodynamic pressure. The uniform, flat profile for electroosmosis helps to minimize band broadening in capillary electrophoresis, thus improving separation efficiency. 

Two gas chromatograms showing the effect of polarity of the stationary phase on the separation efficiency for three substances of increasing polarity toluene, pyridine, and benzaldehyde. (a) Separation on silicone SE-30, a nonpolar phase, and (b) separation on elastomer OV-351, a more polar phase. Note the greatly changed absolute and relative retention times the more polar pyridine and benzaldehyde are affected most by the move to a more polar stationary phase. 

To use the formula in Equation 38.2, it is necessary to define at what stage the two peaks representing the two masses are actually separate (Figure 38.4). The depth of the valley between the two peaks serves this purpose, with valley definitions of 5, 10, or 50%. A 5% valley definition is a much stricter criterion of separation efficiency than the 50% definition. 

Determination of separation efficiencies from pilot-plant data is also affected by axial dispersion. Neglecting it yields high or values. Literature data for this parameter have usually not been corrected for this effect. 

Separation Efficiency. Similarly to other unit operations in chemical engineering, filtration is never complete. Some soflds may leave in the hquid stream, and some Hquid will be entrained with the separated soHds. As emphasis on the separation efficiency of soHds or Hquid varies with application, the two are usually measured separately. Separation of solids is measured by total or fractional recovery, ie, how much of the incoming solids is coUected by the filter. Separation of Hquid usually is measured in how much of it has been left in the filtration cake for a surface filter, ie, moisture content, or in the concentrated slurry for a filter-thickener, ie, solids concentration. 

Table 2. MSW Separation Efficiencies for Trommels as a Function of Waste Component, Wt %...

Units are available in stainless steel or protected mild steel, often prefabricated, up to 12.5 m in diameter, capable of processing >5 m /s depending on the separation efficiency required. When the separator is used for classification of granular soflds, smaller-diameter (<4 m) units are used, separating nearly all particles coarser than - 150 fim. 

There are relationships between the independent size separation device parameters and the dependent size separation efficiencies. For example, the apparent bypass value does not affect the size distribution of the fine stream but does affect the circulation ratio, ie, the ratio of the coarse stream flow rate to the fine stream flow rate. The circulation ratio increases as the apparent bypass increases and the sharpness index decreases. Consequendy, the yield, the inverse of the circulating load (the ratio of the feed stream flow rate to the fine stream flow rate or the circulation ratio plus one), decreases hence the efficiencies decrease. For a device having a sharpness index of 1, the recovery efficiency is equal to (1 — a). 

At best only one equilibrium stage achievable for each WFE or SPE unit separation efficiency decreases as pressure decreases. 

The individual membrane filtration processes are defined chiefly by pore size although there is some overlap. The smallest membrane pore size is used in reverse osmosis (0.0005—0.002 microns), followed by nanofiltration (0.001—0.01 microns), ultrafHtration (0.002—0.1 microns), and microfiltration (0.1—1.0 microns). Electro dialysis uses electric current to transport ionic species across a membrane. Micro- and ultrafHtration rely on pore size for material separation, reverse osmosis on pore size and diffusion, and electro dialysis on diffusion. Separation efficiency does not reach 100% for any of these membrane processes. For example, when used to desalinate—soften water for industrial processes, the concentrated salt stream (reject) from reverse osmosis can be 20% of the total flow. These concentrated, yet stiH dilute streams, may require additional treatment or special disposal methods. 

The lye boHer is usuaHy steam heated but may be direct-fired. Separation efficiency may be iacreased by adding a tower section with bubble-cap trays. To permit the bicarbonate content of the solution to buHd up, many plants are designed to recirculate the lye over the absorber tower with only 20—25% of the solution flowing over this tower passiag through the boHer. Several absorbers may also be used ia series to iacrease absorptioa efficieacies. 

Maximum Separative Capaeity and the Separative Effieieney. The separative efficiency of a gas centrifuge used for isotope separation is best defined in terms of separative work. Thus, the separative efficiency E is defined by... 

Theoretical Formulation of the Separative Efficiency. The separative efficiency E of a countercurrent gas centrifuge maybe considered to be the product of four factors, all but one of which can be evaluated on the basis of theoretical considerations. In this formulation the separative efficiency is defined by... 

Design data for separation of the particular or similar mixture in a packea column are not available. Design procedures are better estabhshed for tray-type columns than for packed columns. This is particularly so with respect to separation efficiency since tray efficiency can be estimated more accurately than packed height equivalent to a theoretical stage (HETP). 

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