Efficiency of Separation

Rotational flow in the forced vortex within the cyclone body gives rise to a radial pressure gradient. This pressure gradient, combined with the frictional pressure losses at the gas inlet and outlet and losses due to changes in flow direction, make up the total pressure drop. This pressure drop, measured between the inlet and the gas outlet, is usually proportional to the square of gas flow rate through the cyclone. A resistance coefficient, the Euler number Eu, relates the cyclone pressure drop Ap to a characteristic velocity v  

The characteristic velocity v can be defined for gas cyclones in various ways but the simplest and most appropriate definition is based on the cross-section of the cylindrical body of the cyclone, so that  

The Euler number represents the ratio of pressure forces to the inertial forces acting on a fluid element. Its value is practically constant for a given cyclone geometry, independent of the cyclone body diameter (see Section 9.4). 

Two factors contribute to how well compounds are separated by chromatography. One is the difference in elution times between peaks The farther apart, the better their separation. The other factor is how broad the peaks are The wider the peaks, the poorer their separation. This section discusses how we measure the efficiency of a separation. 

In chromatography, the resolution of two peaks from each other is defined as 

Volume is proportional to time, so any ratio of times can be written as the corresponding ratio of volumes. For example, 

Volume flow2 /radiuSjW Volume flow, V radius,) 

A peak with a retention time of 407 s has a width at the base of 13 s. A neighboring peak is eluted at 424 s with a width of 16 s. Find the resolution for these two components. 

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Reflux ratio. This is defined as the ratio between the number of moles of vapour returned as refluxed liquid to the fractionating column and the number of moles of final product (collected as distillate), both per unit time. The reflux ratio should be varied according to the difficulty of fractionation, rather than be maintained constant a high efficiency of separation requires a liigh reflux ratio. ... 

The efficiency of separation of solvent from solute varies with their nature and the rate of flow of liquid from the HPLC into the interface. Volatile solvents like hexane can be evaporated quickly and tend not to form large clusters, and therefore rates of flow of about 1 ml/min can be accepted from the HPLC apparatus. For less-volatile solvents like water, evaporation is slower, clusters are less easily broken down, and maximum flow rates are about 0.1-0.5 ml/min. Because separation of solvent from solute depends on relative volatilities and rates of diffusion, the greater the molecular mass difference between them, the better is the efficiency of separation. Generally, HPLC is used for substances that are nonvolatile or are thermally labile, as they would otherwise be analyzed by the practically simpler GC method the nonvolatile substances usually have molecular masses considerably larger than those of commonly used HPLC solvents, so separation is good. 

As a rule of thumb, one can say that the efficiency of separation of mixtures and the simplicity of operating and maintaining apparatus are much greater for GC than for LC. Hence, other things being equal, GC is most often the technique of first choice and can be used with a very wide variety of compound types. However, for nonvolatile or thermally labile substances like peptides, proteins, nucleotides, sugars, carbohydrates, and many organometallics, GC may be ruled out completely... 

High-pressure liquid chromatography (HPLC) is simply a variant on LC in which the moving liquid stream is forced along under high pressure to obtain greater efficiency of separation. 

The main advantage of HGMS is high efficiency of separation even at relatively high dow rates and minimum pressure drops across the filter. The capital cost is very high, and only large installations are attractive economically because capacity increases with the square of the diameter of the canister while the weight of copper conductor increases linearly with diameter. 

There is stiR no obvious reason to beheve that the efficiency of separating a mixture and an a (relative volatihty) of 1.1 is related to that for an a of 2 however, it is known that when a is small, the required reflux and are large, but (T,.ondenser reboiier ) small (see Distillation). 

Density. The density of saturated water and steam is shown in Figure 2 as a function of temperature on the saturation line. As the temperature approaches the critical point, the densities of the Hquid and vapor phase approach each other. This fact is cmcial to boiler constmction and steam purity because the efficiency of separation of water from steam depends on the density difference. 

Operating holdup contributes effectively to mass-transfer rate, since it provides residence time for phase contact and surface regeneration via agglomeration and dispersion. Static holdup is hmited in its contribution to mass-transfer rates, as indicated by Thoenes and Kramers [Chem. Eng. ScL, 8, 271 (1958)]. In laminar regions holdup in general has a negative effecl on the efficiency of separation. 

Other Cycle Steps A PSA cycle may have several other steps in addition to the basic adsorption, depressurization, and repressuriza-tion. Cocurrent depressurization, purge, and pressure-equalization steps are normally added to increase efficiency of separation and recoveiy of product. At the end of the adsorption step, the more weakly adsorbed species have been recovered as product, but there is still a significant amount held up in the bed in the inter- and intra-... 

Efficiency of separation is weakly dependent on the size of the vessel. 

Types of columns and packings. A slow distillation rate is necessary to ensure that equilibrium conditions operate and also that the vapour does not become superheated so that the temperature rises above the boiling point. Efficiency is improved if the column is heat insulated (either by vacuum jacketing or by lagging) and, if necessary, heated to Just below the boiling point of the most volatile component. Efficiency of separation also improves with increase in the heat of vaporisation of the liquids concerned (because fractionation depends on heat equilibration at multiple liquid-gas boundaries). Water and alcohols are more easily purified by distillation for this reason. 

The efficiency of separation will obviously affect the purity of the liquid, and it may be necessary to provide a series of separate stages to meet the standard required by the specification. 

Some units will maintain a reasonable efficiency of separation over a range of 60%-120% of normal performance rating while other types will not. This flexibility is... 

The hot feed enters the fractionator, which normally contains 30-50 fractionation trays. Steam is introduced at the bottom of the fractionator to strip off light components. The efficiency of separation is a function of the number of theoretical plates of the fractionating tower and the reflux ratio. Reflux is provided by condensing part of the tower overhead vapors. Reflux ratio is the ratio of vapors condensing back to the still to vapors condensing out of the still (distillate). The higher the reflux ratio, the better the separation of the mixture. 

The most-nsed stationary phase in PLC is sihca gel, with type 60 taking preference. In the fnture, other sorbents snch as the RP materials will also most probably be increasingly nsed. This will also be trae for the case of special PLC plates consisting of layer combinations snch as precoated plates with concentrating zones, resnlting in simphfication of sample application as well as an increase in the efficiency of separation. 

A high electric field inside the capillary generates heat. This so-called Joule heat can destroy the efficiency of separation. An electric field corresponds to a well defined current value for a specific electrolyte system. An ohm plot can be constructed for each electrolyte system by measuring the current at increasing electric fields. For an ideal system, the ohm plot is a... 

It is important to note that all flotation clarifiers63,64 may be used for treatment of nickel-chromium plating wastes regardless of their shapes (rectangular or circular) or manufacturers. A filtration unit is an optional step for final polishing. The treatment efficiency of separate flotation and filtration units65 will be similar to that of a combined flotation-filtration unit (Figure 6.8). 

Figure 8.4 Inertial separators increase the efficiency of separation by giving the particles downward momentum.

One drawback is the small amount of space along the sorting conveyor, which can reduce the efficiency of separating the material (Fig. 1). 

Cocurrent depressurization, purge, and pressure-equalization steps are normally added to increase efficiency of separation and recovery of product. At the end of the adsorption step, the more weakly adsorbed species have been recovered as product, but there is still a significant amount held up in the bed in the inter- and intraparticle void spaces. A cocurrent depressurization step can be added before the blowdown step, which is countercurrent to adsorption. This increases the amount of product produced each cycle. In some applications, the purity of the more strongly adsorbed components has also been shown to be heavily dependent on the cocurrent depressurization step [Cen and Yang, Ina. Eng. Chem. Fundam., 25, 758-767 (1986)]. This cocurrent blowdown is optional because there is always a countercurrent one. Skarstrom developed criteria to determine when the use of both is justified [Skarstrom in Li, Recent Developments in Separation Science, vol. II, CRC Press, Boca Raton, pp. 95-106 (1975)]. 

Successive extractions, whilst increasing the efficiency of extraction of both solutes, may lead to a poorer separation. For example, if DA = 102 and I)v = 10 one extraction will remove 99.0% of A and 9.1% of B whereas two extractions will remove 99.99% of A but 17% of B. In practice, a compromise must frequently be sought between completeness of extraction and efficiency of separation. It is often possible to enhance or suppress the extraction of a particular solute by adjustment of pH or by complexation. This introduces the added complication of several interrelated chemical equilibria which makes a complete theoretical treatment more difficult. Complexation and pH control are discussed more fully in Chapter 3. 

To improve the efficiency of separation based on conventional porous particle technology, new particles and techniques have been introduced, mainly focusing on the improvement (acceleration) of the exchange of the solute between the mo-... 

Qu and Qc, and solids concentrations of Cu and Ca, respectively. The most desirable division of solids in a device for solid/liquid separation, such as a thickener (described in Chapter 5), is where all of the solids report to the underflow so that the overflow is clarified liquid and consequently Ca is zero. If the efficiency of separation (E) is defined as the mass ratio of solids (of all sizes) in the underflow to that in the feed, then a clarified overflow corresponds to an efficiency of 100 per cent. 

In processes where classification or separation of particles is required, the efficiency of separation will be a function of one or more distributed properties of the particles. The function which describes the efficiency with which particles are separated by size (d) is usually termed the grade efficiency, G(d). For particles in a narrow size interval between d and d + Ad, G(d) is defined as the mass ratio of such particles in the underflow to that in the feed. The overall separation efficiency E corresponds to the particle size d for which G(d) equals E. 

During electrophoretic separation, the analyte ions displace background co-ions equivalent to their charge. The difference in the conductivity between the analytes and the BGE co-ions induces a signal recorded by the C D detector. It is, therefore, important for optimal sensitivity that the difference in conductance between the analyte and the electrolyte be as high as possible. On the other hand, optimal efficiency of separation is attained when the /tBCE and /tanaiyte are matched. Howevei the ji and the equivalent conductance leqmv are linked ... 

While the external electrical field approach is a method directly modifying the zeta-potential of the capillary wall, it is not applicable with commercial apparatuses. The back-pressure technique, on the other hand, has the disadvantage that the flat electroosmotic flow profile is disrupted by superposition of a pressure-driven laminar flow profile hence, the efficiency of separation deteriorates. 

Chromatography is the primary analytical method in chemical analysis of organic molecules. This technique is used to analyze reaction products in most of the processes we have been describing. The analysis of reaction products and of the efficiency of separation units usually is done by analytical chemists (who earn lower salaries), but chemical engineers need to be aware of the methods of analysis used and their reliability. 

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