Trays, fractionating

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. 

Considerable work on methods for pre-predicting fractionator tray efficiency continues to the present. Shortcut methods from the past differed rather widely.The... 

Better examples of shortcut design methods developed from property data are fractionator tray efficiency, from viscosity " and the Clausius-Clapeyron equation which is useful for approximating vapor pressure at a given temperature if the vapor pressure at a different temperature is known. The reference states that all vapor pressure equations can be traced back to this one. 

Toluene is recovered as a high purity product by fractionating the mixed aromatics obtained from the extraction of catalytic reformate or powerformate. About 70 fractionation trays are required to produce toluene having a purity of 99.7 percent. Toluene is consumed principally as a feedstock for hydrodealkylation plants. Toluene is used in a number of solvent applications. 

Open live Steam Distillation—With Fractionation Trays, Binary... 

Figure 8-152A. Mechanical performance correlations for high-pressure fractionation trays. Used by permission, Capps, R. W., The American institute of Chemical Engineers, Chem. Eng. Prog. V. 89, No. 3, (1993), p. 35, all rights reserved.

Greater capacity and efficiency than fractionation trays and other dumped packings. 

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 fourth desorbent characteristic is that the desorbent material should be readily available (at a sustainable cost). It is important that the desorbent be readily available and not cost-prohibitive because desorbent is gradually lost during operations. Since the desorbent is separated from both the extract and raffinate components by fractionation, trace quantities (parts per million) of desorbent are present in the respective product streams due to fractionation tray efficiencies and fractionation control optimization of reflux to feed ratio. Ultimately, each n-paraffn separation facility must balance the operating expenses (utilities consumed during fractionation) to minimize desorbent loss against the replacement value of desorbent to maintain inventory. 

Most atmospheric columns contain from 30 to 50 fractionation trays. For each sidestream desired, about five to eight trays are required, plus additional trays above and below the primary trays. The various sidestreams collected from the distillation column contain lighter boiling products that must be removed. Smaller reboiling units are used to remove lighter products and direct them back into the distillation column as vapor. Also, refluxing units are sometimes employed to condense and remove heavy end products from collected fractions. These condensed heavier products are reintroduced into the lower trays. 

Now that I ve confused you with the preceding description (though I believe it to be very much to the point in a simple way), I present a very easy way to understand tray hydraulics. Only three inputs are necessary to design any fractionation tray ... 

The Erwin two-film (ETF) tray efficiency method is listed in the following four steps. This is a program subroutine copied from the fractionation tray computer program Chemcalc 13 [1], This tray efficiency method has been used successfully for over a decade and has hundreds of successful applications. It is presented here for you to apply as a supplemental to the other, more expensive computer simulation programs to make them more complete. Key variable nomenclatures follow. 

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