External reflux

A mixture of 12-ketostearamide, triethylamine, and tetrahydrofuran stirred and heated to reflux, external heating discontinued, POClg in tetrahydrofuran added at a rate to maintain gentle reflux, which is continued an additional 0.5 hr. -> 12-ketostearonitrile (startg. m. f. 362). Y 84%. B. Freedman and G. Fuller, J. Am. Oil Chemists Soc. 49,188 (1972) with phosphonitrile chloride s. J. C. Graham and D. H. Marr, Can. J. Chem. 50, 3857 (1972) with tris(diethylamino)phosphine s. T. Sodeyama, M. Kodomari, and K. Itabashi, Chem. Lett. 1973, 577. 

Consider again the simple process shown in Fig. 4.4d in which FEED is reacted to PRODUCT. If the process usbs a distillation column as separator, there is a tradeofi" between refiux ratio and the number of plates if the feed and products to the distillation column are fixed, as discussed in Chap. 3 (Fig. 3.7). This, of course, assumes that the reboiler and/or condenser are not heat integrated. If the reboiler and/or condenser are heat integrated, the, tradeoff is quite different from that shown in Fig. 3.7, but we shall return to this point later in Chap. 14. The important thing to note for now is that if the reboiler and condenser are using external utilities, then the tradeoff between reflux ratio and the number of plates does not affect other operations in the flowsheet. It is a local tradeoff. 

Hydrolysis of benzyl cyanide to phenylacetamide. In a 1500 ml. three-necked flask, provided with a thermometer, reflux condenser and efficient mechanical stirrer, place 100 g. (98 ml.) of benzyl]cyanide and 400 ml. of concentrated hydrochloric acid. Immerse the flask in a water bath at 40°. and stir the mixture vigorously the benzyl cyanide passes into solution within 20-40 minutes and the temperature of the reaction mixture rises to about 50°, Continue the stirring for an additional 20-30 minutes after the mixture is homogeneous. Replace the warm water in the bath by tap water at 15°, replace the thermometer by a dropping funnel charged with 400 ml. of cold distilled water, and add the latter with stirring crystals commence to separate after about 50-75 ml. have been introduced. When all the water has been run in, cool the mixture externally with ice water for 30 minutes (1), and collect the crude phenylacetamide by filtration at the pump. Remove traces of phenylacetic acid by stirring the wet sohd for about 30 minutes with two 50 ml. portions of cold water dry the crystals at 50-80°. The yield of phenylacetamide, m.p. 154-155°, is 95 g. RecrystaUisation from benzene or rectified spirit raises the m.p. to 156°. 

Note 1. The heat developed by the reaction is just enough to cause gentle refluxing, provided that the mixture is not stirred too vigorously. If refluxing stops during the addition, external heating must be applied. 

The slope L/V of the operating line is termea the internal-reflux ratio. This ratio in the operating-line equation for the top section of the column [see Eq. (13-21)] is related to the external-reflux ratio R = L + i/D by... 

The overhead purity is specified as X/ ) = 0.95. The reflux temperature is the bnbble-point temperature (saturated reflux), and the external-reflnx ratio is set at 71 = 4.5. 

However, the total number of equilibrium stages N, N/N,n, or the external-reflux ratio can be substituted for one of these three specifications. It should be noted that the feed location is automatically specified as the optimum one this is assumed in the Underwood equations. The assumption of saturated reflux is also inherent in the Fenske and Underwood equations. An important limitation on the Underwood equations is the assumption of constant molar overflow. As discussed by Henley and Seader (op. cit.), this assumption can lead to a prediction of the minimum reflux that is considerably lower than the actual value. No such assumption is inherent in the Fenske equation. An exact calculational technique for minimum reflux is given by Tavana and Hansen [Jnd. E/ig. Chem. Process Des. Dev., 18, 154 (1979)]. A computer program for the FUG method is given by Chang [Hydrocarbon Process., 60(8), 79 (1980)]. The method is best applied to mixtures that form ideal or nearly ideal solutions. 

The thermal quality of the solvent feed has no effect on the value of (S/F)mjn, but does affect the minimum reflux to some extent, especially as the (S/F) ratio increases. R nax occurs at higher values of the reflux ratio as the upper-feed quality decreases a subcooled upper feed provides additional refluxing capacity and less external reflux is required for the same separation. It is also sometimes advantageous to introduce the primary feed to the extractive distillation column as a vapor to help maintain a higher solvent concentration on the feed tray and the trays immediately below... 

External reflux can be furnished by returning some of the externally... 

The same end may be achieved by continuous operation at total external reflux with a small U bend in the reflux line for foamate holdup [Rubin and Melech, Can. ]. Chem. Eng., 50, 748 (1972)]. 

Foam Breaking It is usually desirable to collapse the overflowing foam. This can be accomphshed by chemical means (Bikerman, op. cit.) if external reflux is not employed or by thermal means [Kishi-moto, Kolloid Z., 192, 66 (1963)] if degradation of the overhead product is not a fac tor. 

By using an anionic collector and external reflux in a combined (enriching and stripping) column of 3.8-cm (1.5-in) diameter with a feed rate of 1.63 ni/n [40 gal/(h ft )] based on column cross section, D/F was reduced to 0.00027 with C JCp for Sr below 0.001 [Shou-feld and Kibbey, Nucl. AppL, 3, 353 (1967)]. Reports of the adsubble separation of 29 heavy metals, radioactive and otheiwise, have been tabulated [Lemlich, The Adsorptive Bubble Separation Techniques, in Sabadell (ed.), Froc. Conf. Traces Heavy Met. Water, 211-223, Princeton University, 1973, EPA 902/9-74-001, U.S. EPA, Reg. 11, 1974). Some separation of N from by foam fractionation has been reported [Hitchcock, Ph.D. dissertation. University of Missouri, RoUa, 1982]. 

FIG. 23-1 Heat transfer to stirred tank reactors, a) Jacket, (h) Internal coils, (c) Internal tubes, (d) External heat exchanger, (e) External reflux condenser. if) Fired heater. (Walas, Reaction Kinetics for Chemical Engineers, McGraw-Hill, 1959). 

In a 5-I. round-bottom flask, fitted with a stirrer, separatory funnel and a reflux condenser to the upper end of which a calcium chloride tube is attached, is placed 150 g. of magnesium turnings. A small crystal of iodine (Note i) and about 100 cc, of a mixture of 822 g. (6 moles) of M-butyl bromide and 2 1. of anhydrous ethyl ether are added. As soon as the reaction starts, 350 cc. of anhydrous ether is added and the remainder of the -butyl bromide solution is dropped in at such a rate that the mixture boils continuously. The time of addition (one and one-half hours) may be decreased by cooling the flask externally. Stirring is started as soon as enough liquid is present in the flask. 

To the acid chloride, mechanically stirred and heated on the steam bath, is added 2.5 kg. (805 ml. 15.6 moles) of dry bromine as rapidly as it will react (Note 5). The addition requires about 12 hours. The contents of the flask are stirred and heated an additional 2 hours, transferred to a dropping funnel (Note 6), and added in a thin stream to 5 1. of absolute ethyl alcohol, which has previously been placed in a 12-1. flask provided with a stopper carrying an effleient reflux condenser, a separatory funnel, and a mechanical stirrer. The resulting vigorous reaction is controlled by external cooling. After the dibromoacid chloride has been added, the reaction mixture is allowed to stand at room temperature overnight and is then poured into 5 1. of cold water. The top alcoholic aqueous layer is decanted and extracted once with 8 1. of ether. The oily bottom layer is dissolved in the ether extract, washed first with 1 1. of a 2% sodium bisulfite solution, then with two 1-1. portions of 3% sodium carbonate solution, and finally with several portions of water. The ether solution is dried over 175 g. of potassium carbonate the solvent is distilled on the steam bath. The yield of residual ester (Note 7) amounts to 2260-2400 g. (91-97% of the theoretical amount). 

In a 2-1. three-necked, round-bottomed flask fitted with a liquid-sealed mechanical stirrer, a dropping funnel, and an efficient reflux condenser are placed 720 g. (226 cc., 4.5 moles) of bromine (Note i) and 1.5 g. of sulfur (Note 2). A glass tube is connected to the top of the condenser to carry the evolved hydrogen bromide to a gas trap (Org. Syn. 14, 2). Sixty-nine grams (69 cc., 0.52 mole) of dry paraldehyde (Note r) is added slowly, with stirring, over a period of about four hours. The reaction proceeds under its own heat during the addition of the paraldehyde subsequently the mixture is heated externally for two hours at 60-80°. The solution is distilled and a fraction collected over the range 155-175° (Note 3). 

Internal reflux is induced by means of externally cooled liquid pumparounds. A pumparound simply removes hot liquid from the tower, pumps it through a heat exchanger and then introduces this cooled liquid into the tower a few trays above. Use of pumparounds allows a better distribution of tower loadings than if all the heat were removed from the VPS using an overhead condenser. Four to six trays between sidestreams and two pumparounds are normally specified for a lube VPS. The three liquid sidestream products to be used as lube plant feed stocks are steam stripped to remove lighter boiling components which condense with tire sidestreams. 

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