Columns overall efficiency

Efficiency (overall column efficiency) Ratio of the number of theoretical stages required to effect a distillation separation to the number of actual trays. 

The actual number of stages is equal to the number of equihbrimn stages divided by the fractionator efficiency(overall column efficiency). Although the tray efficiency will vary, we will use the fractionator efficiency. The fiactionator efficiency is obtained from the O Coimel correlation given in Figure 6.17. Vital et al. [46] have reviewed and tabulated fractionator and absorber efficiencies for many systems. These data may help to arrive at a reasonable fractionator efficiency. 

The required number of actual plates, A/p, is larger than the number of theoretical plates, because it would take an infinite contacting time at each stage to estabhsh equihbrium. The ratio is called the overall column efficiency. This parameter is difficult to predict from theoretical... 

This is the one case where the overall column efficiency can be related analytically to the Murphree plate efficiency, so that the actual number of plates is calculable by dividing the number of theoretical plates through equation 86 ... 

EmpiricalEfficieny Prediction Methods. Numerous empirical methods for predicting plate efficiency have been proposed. Probably the most widely used method correlates overall column efficiency as a function of feed viscosity and relative volatiHty (64). A statistical correlation of efficiency and system variables has been developed from numerous plate efficiency data (65). 

FIG. 14-7 O Connell correlation for overall column efficiency for absorption. To convert ffP/ i in pound-moles per cubic foot-centipoise to Idlogram-moles per cubic meter-pascal-second, multiply by 1.60 X 10 . [O Connell, Trans. Am. Inst. Chem. Eng., 42, 741 (1946).]... 

FIG. 14-37 Overall column efficiency of 25-mm Oldersbaw column compared with point efficiency of 1,22-m-diameter-sieve sieve-plate column of Fractionation Research, Inc, System = cyclohexane-n-heptane, [(Fair, Null, and Bolles, Ind, Eng, Chem, Process Des, Dev, 22, 53 (i.982),]... 

Tbe best-established theoretical method for predicting E is that of tbe AlCbE [Buhhle-Tray Design Manual, American Institute of Chemical Engineers, New York, 1958). It is based on tbe sequential prediction of point efficiency, Murpbree efficiency, and overall column efficiency ... 

Overall Column Efficiency Calculated values of E, , must be corrected for entrainment, if any, by the Colburn equation [Eq. (14-101)]. The resiJting corrected efficiency is then converted to column efficiency by the relationship of Lewis [Ind. Eng. Chem., 28, 399 (1936)] ... 

Provide the overall treatment efficiency (in column E) for the entire sequence by entering that value in connection with the last treatment step in the sequence only. Enter NA in column E for the efficiency of all preceding steps in the sequence. 

While ethyl chloride is one of the least toxic of all chlorinated hydrocarbons, CE is a toxic pollutant. The off-gas from the reactor is scrubbed with water in two absoiption columns. The first column is intended to recover the majority of unreacted ethanol, hydrogen chloride, and CE. The second scrubber purifies the product fiom traces of unreacted materials and acts as a back-up column in case the first scrubber is out of operation. Each scrubber contains two sieve plates and has an overall column efficiency of 65% (i.e., NTP = 1.3). Following the scrubber, ethyl chloride is finished and sold. The aqueous streams leaving the scrubbers are mixed and recycled to the reactor. A fraction of the CE recycled to the reactor is reduced to ethyl chloride. This side reaction will be called the reduction reaction. The rate of CE depletion in the reactor due to this reaction can be approximated by the following pseudo first order expression ... 

Biddulph [90] emphasizes the importance of using point efficiencies rather than tray efficiencies or overall column efficiencies, due to the wide fluctuations that often exist. 

Mols of distillate or overhead product, lb mols/hr or batch distillation, mols Mols component, i, in distillate Vaporization efficiency of steam distillation Overall column efficiency Overall tray efficiency Eqg = Murphree point efficiency, fraction Murphree plate/tray efficiency, = E ... 

Unfortimately, the efficiencies for tray and overall column operation are incomplete and nullify to a certain extent some very high quality theoretical performance design. Tray efficiencies may be estimated by Figure 8-29 or Table 8-11. 

Stirred tanks are modeled assuming that both phases are well mixed. Tray columns are usually modeled as well mixed on each tray so that the overall column is modeled as a series of two-phase, stirred tanks. (Distillation trays with tray efficiencies greater than 100% have some progressive flow within a tray.) When reaction is confined to a single, well-mixed phase, the flow regime for the other phase makes little difference but when the reacting phase approximates piston flow, the flow regime in the other phase must be considered. The important cases are where both phases approximate piston flow, either countercurrent or cocurrent. 

Overall column efficiency. This is sometimes confusingly referred to as the overall plate efficiency. 

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. 

For the idealised situation where the operating and equilibrium lines are straight, the overall column efficiency and the Murphree plate efficiency are related by an equation derived by Lewis (1936) ... 

Plate, and overall column, efficiencies will normally be between 30 per cent and 70 per cent, and as a rough guide a figure of 50 per cent can be assumed for preliminary designs. 

Kmv = Murphree plate efficiency, E0 = Overall column efficiency. ... 

A quick estimate of the overall column efficiency can be obtained from the correlation given by O Connell (1946), which is shown in Figure 11.13. The overall column efficiency is correlated with the product of the relative volatility of the light key component (relative to the heavy key) and the molar average viscosity of the feed, estimated at the average column temperature. The correlation was based mainly on data obtained with hydrocarbon systems, but includes some values for chlorinated solvents and water-alcohol mixtures. It has been found to give reliable estimates of the overall column efficiency for hydrocarbon systems and can be used to make an approximate estimate of the efficiency for other systems. The method takes no account of the plate design parameters and includes only two physical property variables. 

Using O Connell s correlation, estimate the overall column efficiency and the number of real stages required for the separation given in Example 11.5. 

Figure 11.57. Overall column efficiency E as function of viscosity-relative volatility product (fiixf)(65)...

The situation described in Equation 9.1 is reversed at a reduced temperature. The overall column efficiency decreases rather dramatically for most samples, but successful separations are still practical with the correct choice of parameters. The reduced longitudinal diffusion in the first term means that the optimal flow rate shifts to lower flow rates. The increased viscosity of the mobile phase requires lower flow rates as well. While at high temperatures one often operates the HPLC at flow rates many times the optimal value, in subambient work, it is best to sacrifice speed and work close to the optimal flow rate. 

Overall column efficiency can be calculated from the Murphree tray efficiency by using the relationship developed by Lewis [Ind. Eng. Chem. 28, 399 (1936)]. 

Overall Column Efficiency This is the ratio of the number of... 

A mixing model can be used to convert the Oldershaw point efficiencies to overall column efficiencies. This enhances the commercial column efficiency estimates. A conservative approach suggested by Fair et al. is to apply the Oldershaw column efficiency as the estimate for the overall column efficiency of the commercial column, taking no credit for the greater plug-flow character upon scale-up. The author prefers this conservative approach, considering the poor reliability of mixing models. 

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