Fractionator theoretical stage

Fractional vacuum distillation is the method used to separate terpene mixtures into their components. The terpene chemist usually has in the laboratory a range of columns with differing numbers of theoretical stages. Experimental distillation in the laboratory is useful in providing data for manufacturing plants that produce commercial quantities of terpene products. 

FIG. 22-43 Graphical determination of theoretical stages for a foam-fractionation stripping column. 

Unless the hquid pool is purposely lengthened vertically in order to give additional separation via bubble fractionation, it is usually taken to represent one theoretical stage. A bubbler submergence or 30 cm or so is usually ample for a solute with a molecular weight that does not exceed several hundred. 

LK = subscript for light key Nn, = minimum theoretical stages at total reflux Xhk = mol fraction of heavy key component Xlk = mol fraction of the light key component otLK/HK = relative volatility of component vs the heavy key component... 

After actual theoretical trays are determined (see Actual reflux and theoretical stages) one needs to estimate the actual physical number of trays required in the distillation column. This is usually done by dividing the actual theoretical trays by the overall average fractional tray efficiency. Then a few extra trays are normally added for offload conditions, such as a change in feed composition. 

N,n = Minimum theoretical stages at total reflux Q = Heat transferred, Btu/hr U - Overall heat transfer coefficient, Btu/hrfP"F u = Vapor velocity, ft/sec U d = Velocity under downcomer, ft/sec VD(js = Downcomer design velocity, GPM/fL Vioad = Column vapor load factor W = Condensate rate, Ibs/hr Xhk = Mol fraction of heavy key component Xlk = Mol fraction of the light key component a, = Relative volatility of component i versus the heavy key component... 

Gilliland" tells how his famous correlation was developed for relating actual and minimum reflux to actual and minimum theoretical stages for a fractionating column. Numerous plate-to-plate calculations were made and the results plotted using his well-known correlating parameters. The best curve was then drawn through points. 

Packed fractional distillation columns run in the batch mode are often used for low-pressure drop vacuum separation. With a trayed column, the liquid holdup on the trays contributes directly to the hydraulic head required to pass through the column, and with twenty theoretical stages that static pressure drop is very high, e.g., as much as 100-200 mm Hg. 

Figure 13.14. A centrifugal packed fractionator, trade name HIGEE, Imperial Chemical Industries. Units have been operated with 500 times gravitational acceleration, with 3-18 theoretical stages, up to 36 in. dia, employing perforated metal packing. For distillation, one unit is needed for rectification and one for stripping. Units have been used primarily for gas stripping and on offshore platforms because of compactness [Ramshaw, Chem. Eng., 13-14 (Feb. 1983)].

What does ETF do ETF simply tells you how many actual trays make up one theoretical stage of a fractionation column. This is an agonizing question I have heard repeated many times. I personally have gained much income by resolving this question as a consultant. (In my case, the IRS has also gained considerable income ) The quest continues even now for a good and reliable answer. My answer is ETF. 

Determination of theoretical stage number and stage efficiency, or more simply HETS, has been established for any extraction process. The next item of order is the packed extractor column flooding limit. Just as fractionation columns must be sized for vapor and liquid, liquid-liquid extraction columns must be sized for flood limits. 

FIGURE 14 Theoretical stages versus reflux ratio (benzene-toluene at 1.0 bar). xD, mole fraction benzene in overhead xB, mole fraction toluene in bottoms. 

Example 5.3 A mixture containing 50 moles each of benzene and toluene is to be distilled under conditions of constant reflux ratio until mole fraction of the residual benzene is less than 0.20. The column contains three theoretical stages. Calculate the material balance for this separation. 

These values are plotted in Fig. 5,6a to c. Using a bottoms mole fraction of 0.100 and three theoretical stages in Fig, 5.6a it is only possible to achieve an overheads mole fraction of C of approximately 0.883 at total reflux, Thus, it may be concluded that essentially pure C cannot be recovered with this particular distillation system. Therefore, C will be removed in the foreruns cut until the distillation produces an acceptable R,... 

Connecting 0.147 with = 0.883 in Fig. 5.66 and stepping off three theoretical stages shows residual mole fraction of C is 0.160. 

Fig.13 Theoretical stages per meter packings as a fraction of the Bodenstein number for near-critical extraction of monoglycerides from a mixture of mono-, di-, and triglycerides at 12 MPa and 313 K in the droplet regime.

Example 2 Estimation of overall column efficiency. A continuous fractionation unit has been designed to operate on a liquid feed containing components, A, B, C, and D. Calculations have shown that 20 theoretical stages are necessary in the column, not including the reboiler. On the basis of the following data, estimate the overall column efficiency and the number of actual trays needed in the column by (a) Fig. 16-9, (b) Eq. (4), and (c) Eq. (6) ... 

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