Rectifying column

Equation (14-135) is based on the assumption of constant molar overflow and a constant value of Emv from tray to tray. It needs to be applied separately to each section of the column (rectifying and stripping) because GM/LM, and therefore X, varies from section to section. Where molar overflow or Murphree efficiencies vary throughout a section of column, the section needs to be divided into subsections small enough to render the variations negligible. 

Fig. 4.12. Design diagrams for isopropyl acetate (IPOAc) reactive distillation column and comparison with simulation results (solid curves simulated column profile markers = stage composition for column rectifying section + = stage composition for column stripping section).

The general definition given in Equation 3.20 can be written specifically for a simple column rectifying section using Equation 3.11 to give the familiar definition of reflux... 

Rectification takes place in a twin-column rectifier. The air fed to the pressure column is rectified to pure nitrogen which leaves the column over-head and is liquefied in the condenser. The major part of the liquid nitrogen is used as reflux to keep rectification going while a smaller percentage can be withdrawn as liquid product. The low-pressure column derives its reflux also from the pressure column. Liquid streams are withdrawn from the central and bottom part and flashed into the top column. 

C.v. Linde invents tiie double-column rectifier for simultaneous... 

The reflux ratio is a measure of the relative flows of liquid and vapor counter-flowing in the column. It is commonly defined as R = L ID, where Lj is the reflux rate or rate of liquid flowing from the condenser back to the column and D is the distillate rate. The operating line of the column rectifying section given by Equation 5.13 has a slope of L IV, where and are the liquid and vapor flow rates in the rectifying section. Since the molar flow is assumed constant in each column section. 

The column vapor and liquid flows at this reflux ratio are estimated from the relationships V, = L + D and R = L, D, where and L, are the vapor and liquid molar flows in the column rectifying section (assuming constant molar flows in the section). The distillate rate, D, is 60 Ibmol/hr (Example 12.1). 

Rectifying Columns. Rectifying, or enriching, columns are columns in which the feed enters at the bottom of the column (see Fig. 6.13). For the sake of discussion, assume the feed enters as a saturated vapor at the rate of F kgmol/h and the products are saturated liquids produced at D and B kg mol/h. The composition of the saturated liquid produced by the condenser at the top is... 

Fig. 5.146, the four column sections are rearranged to form a side-rectifier arrangement. ... 

Both the side-rectifier and side-stripper arrangements have been shown to reduce the energy consumption compared with simple two-column arrangements. This results from reduced mixing losses in the first (main) column. As with the first column of the simple sequence, a peak in composition occurs with the middle product. Now, however, advantage of the peak is taken by transferring material to the side-rectifier or side-stripper. 

Place 50 g. of anhydrous calcium chloride and 260 g. (323 ml.) of rectified spirit (95 per cent, ethyl alcohol) in a 1-litre narrow neck bottle, and cool the mixture to 8° or below by immersion in ice water. Introduce slowly 125 g. (155 ml.) of freshly distilled acetaldehyde, b.p. 20-22° (Section 111,65) down the sides of the bottle so that it forms a layer on the alcoholic solution. Close the bottle with a tightly fitting cork and shake vigorously for 3-4 minutes a considerable rise in temperature occurs so that the stopper must be held well down to prevent the volatilisation of the acetaldehyde. Allow the stoppered bottle to stand for 24-30 hours with intermittent shaking. (After 1-2 hours the mixture separates into two layers.) Separate the upper layer ca. 320 g.) and wash it three times with 80 ml. portions of water. Dry for several hours over 6 g. of anhydrous potassium carbonate and fractionate with an efficient column (compare Section 11,17). Collect the fraction, b.p. 101-104°, as pure acetal. The yield is 200 g. 

Place 200 g, (250 ml.) of rectified spirit in a 1-litre round-bottomed flask fitted with a reflux condenser. Cool in ice and run in, slowly and with frequent shaking, 200 g. (109 ml.) of concentrated sulphuric acid. Add 83 g. (104 ml.) of -butyl cyanide (Section 111,113) to the mixture and reflux the whole for 10 hours. Allow to cool, pour the reaction mixture into ice water, separate the upper layer of ester and alcohol, and dry over anhydrous magnesium or calcium sulphate. Distil through a fractionating column and collect the ethyl n-valerate at 143-146°. A further amovmt of the pure ester may be obtained by redrying the fraction of low boiling point and redistilling. The yield is 110 g. 

Fig. 5. UOP cumene process Rx = reactor R = rectifier BC = benzene column CC = cumene column T = transalkylation.

The rectifying section contains three or four bubble cap (wine) plates in the top section of the stiU to produce distillates up to 160° proof. Whiskey stills are usually made of copper, especially in the rectifying section, which often yields a superior product. Additional copper surface in the upper section of the column may be provided by a demister, a flat disk of copper mesh. Stainless steel is also used in some stills. 

Fig. 1. UOP catalytic condensation process for cumene synthesis. R = reactor RECT = rectifier DP = depropanizer RC = recycle column ...

Ydibierschky Three-Column Sequence. If only simple columns are used, ie, no side-streams, side-rectifiers/strippers etc, then the separation sequence can be completed by adding an entrainer recovery column, column 3 in Figure 19a, to recycle the entrainer, and a preconcentrator column (column 1) to bring the feed to the azeotropic column up to the composition of the binary azeotrope. 

Ethylene Oxide Purification. The main impurities ia ethylene oxide are water, carbon dioxide, and both acetaldehyde and formaldehyde. Water and carbon dioxide are removed by distillation ia columns containing only rectifying or stripping sections. Aldehydes are separated from ethylene... 

In the use of temperature measurement for control of the separation in a distillation column, repeatability is crucial but accuracy is not. Composition control for the overhead product would be based on a measurement of the temperature on one of the trays in the rectifying section. A target would be provided for this temperature. However, at periodic intervals, a sample of the overhead product is analyzed in the laboratory and the information provided to the process operator. Should this analysis be outside acceptable limits, the operator would adjust the set point for the temperature. This procedure effectively compensates for an inaccurate temperature measurement however, the success of this approach requires good repeatability from the temperature measurement. 

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