Mixtures minimum reflux

Example 5.3 Using Eq. (5.8), determine the best sequence for the mixture of alkanes in Table 5.2. Assume the ratio of actual to minimum reflux to be 1.1. 

Example. Dissolve 0 3 g. of />-chlorobenzoic ncid in a small quantity of warm ethanol (about 10 ml.), and ctlrefully add 5 o aqueous sodium hydroxide drop- wise until the solution is just pink to phenolphthalein. Evaporate to dryness on a water-bath. Dissolve the sodium -chlorobenzoate in a minimum of water, add a solution of 0-5 g. of phenacyl bromide in ethanol (about 5 ml.), and boil the mixture under reflux for i hour, and then cool. The phenacyl ester usually ciy stallises on cooling if it does not, add water dropnise with stirring to the chilled solution until separation of the ester just begins. Filter the ester, wash on the filter with water, drain and recrystallise from ethanol m.p. 90 . The /)-bromophenacyl ester is similarly prepared, and after recrystallisation from aqueous ethanol has m.p. 128 . (M.ps., pp. 543-545.)... 

Mix together 1 0 g. of pure p-naphthol and the theoretical quantity of 50 per cent, potassium hydroxide solution, add 0-5 g. of the halide, followed by sufficient rectified spirit to produce a clear solution. For alkyl chlorides, the addition of a little potassium iodide is recommended. Heat the mixture under reflux for 15 minutes, and dissolve any potassium halide by the addition of a few drops of water. The p-naphthyl ether usually crystallises out on cooling if it does not, dilute the solution with 10 per cent, sodium hydroxide solution untU precipitation occurs. Dissolve the p-naphthyl ether in the minimum volume of hot alcohol and add the calculated quantity of picric acid dissolved in hot alcohol. The picrate separates out on cooling. Recrystallise it from rectified spirit. 

Simple analytical methods are available for determining minimum stages and minimum reflux ratio. Although developed for binary mixtures, they can often be applied to multicomponent mixtures if the two key components are used. These are the components between which the specification separation must be made frequendy the heavy key is the component with a maximum allowable composition in the distillate and the light key is the component with a maximum allowable specification in the bottoms. On this basis, minimum stages may be calculated by means of the Fenske relationship (34) ... 

Favorable Vapoi Liquid Equilibria. The suitabiHty of distiUation as a separation method is strongly dependent on favorable vapor—Hquid equiHbria. The absolute value of the key relative volatiHties direcdy determines the ease and economics of a distillation. The energy requirements and the number of plates required for any given separation increase rapidly as the relative volatiHty becomes lower and approaches unity. For example given an ideal binary mixture having a 50 mol % feed and a distillate and bottoms requirement of 99.8% purity each, the minimum reflux and minimum number of theoretical plates for assumed relative volatiHties of 1.1,1.5, and 4, are... 

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. 

Methylphosphonic acid [993-13-5] M 96.0, m 104-106 , 105-107 , 108 , pK, 2.12, pK j 7.29. If it tests for Cf, add H2O and evaporate to dryness repeat several times till free from Cl". The residue solidifies to a wax-like solid. Alternatively, dissolve the acid in the minimum volume of H2O, add charcoal, warm, filter and evaporate to dryness in a vacuum over P2O5. [J Am Chem Soc 75 3379 1953.] The di-Na salt is prepared from 24g of acid in 50mL of dry EtOH and a solution of 23g Na dissolved in 400mL EtOH is added. A white ppte is formed but the mixture is refluxed for 30min to complete the reaction. Filter off and recrystallise from 50% EtOH. Dry crystals in a vacuum desiccator. [7 Chem Soc 3292 1952.]... 

The mixture was refluxed gently on a steam bath for VA hours. Fifteen minutes after initiating the reaction, the reaction mixture gave a negative ferric chloride test. Most of the ethanol and acetic acid were removed by distillation in vacuo, 300 ml of water and 300 ml of ether were added to the concentrate, and the mixture was shaken. The layers were separated, the aqueous layer extracted with fresh ether, and the combined ether extracts were washed with water, dried over anhydrous sodium sulfate, filtered and evaporated to dryness in vacuo. The residue was crystalli2ed by trituration with ether, and the crystals were collected by filtration, washed with hexane and dried. The mother liquors were concentrated to dryness and dissolved in a minimum amount of acetone, whereupon a second crop was obtained. The two crops were combined, dissolved in ethyl acetate, decolori2ed with activated charcoal, and recovered by concentration. 

This graphical representation is easier to use for nonideal systems than the calculation method. This is another limiting condition for column operation, i.e., below this ratio the specified separation cannot be made even with infinite plates. This minimum reflux ratio can be determined graphically from Figure 8-23, as the line with smallest slope from xp intersecting the equilibrium line at the same point as the q line for mixture following Raoul t s Law. 

Figure 8-23. Fractionation of binary mixture at minimum reflux condition.

The following is a short approximation method for minimum reflux ratios for multicomponent mixtures [98] ... 

Figure 8-37. x-y diagram for ethanol-water mixture, showing minimum reflux. Used by pemiission, Block, B. Chem. Eng. Feb. 6 (1961), p. 87. 

Minimum reflux for binary or psuedobinary mixtures is given by the following when separation is essentially complete (Xd 1) and D / F is the ratio of overhead product and feed rates RmD/F = 1/( a-1), when feed is at the bubblepoint (Rm + 1)D/F = a/(a-1), when feed is at the dewpoint. 

Colburn, A. P. (1941) Trans. Am. Inst. Chem. Eng. 37, 805. The calculation of minimum reflux ratio in the distillation of multicomponent mixtures. 

TOTAL AND MINIMUM REFLUX CONDITIONS FOR MULTICOMPONENT MIXTURES... 

A distillation column separating 150 kmol-h 1 of a four-component mixture is estimated to have a minimum reflux... 

Underwood AJV (1946) Fractional Distillation of Multicomponent Mixtures - Calculation of Minimum Reflux Ratio, 7 Inst Petrol, 32 614. 

Example 11.2 Using the Underwood Equations, determine the best distillation sequence, in terms of overall vapor load, to separate the mixture of alkanes in Table 11.2 into relatively pure products. The recoveries are to be assumed to be 100%. Assume the ratio of actual to minimum reflux ratio to be 1.1 and all columns are fed with a saturated liquid. Neglect pressure drop across each column. Relative volatilities can be calculated from the Peng-Robinson Equation of State with interaction parameters assumed to be zero (see Chapter 4). Determine the rank order of the distillation sequences on the basis of total vapor load for ... 

The reduction in the required number of plates as R is increased beyond Rm will tend to reduce the cost of the column. For a column separating a benzene-toluene mixture, for example, where Xf = 0.79, xd = 0.99 and xw = 0.01, the numbers of theoretical plates as given by the McCabe-Thiele method for various values of R are given as follows. The minimum reflux ratio for this case is 0.81. 

If A and B are the light and heavy key components of a multicomponent mixture, then applying the method given earlier for binary mixtures, equation 11.53, the minimum reflux... 

A mixture of n -C4 to w-C7 hydrocarbons is to be distilled to give top and bottom products as follows. The distillation is effected at 800 kN/m2 and the feed is at 372 K. The equilibrium values, K, are shown in Figure 11.39. It is required to find the minimum reflux ratio. No cooling occurs in the condenser. 

A mixture of hexane, heptane, and octane is to be separated to give the following products. What will be the value of the minimum reflux ratio, if the feed is liquid at its boiling point ... 

UNDERWOOD, A. J. V. J. Inst. Petroleum 32 (1946) 614. Fractional distillation of multi-component mixtures — calculation of minimum reflux ratio. 

To a flask equipped with a reflux condenser are added 68 gm (0.5 mole) of phenylurea and 120 ml (1.0 mole) of 42 % hydrazine hydrate solution. The mixture is refluxed for 12 hr, filtered hot using decolorizing charcoal, and then concentrated to 100 ml. On cooling, the crystalline product separates and it is washed with the minimum amount of water. The filtrate is concentrated to 25 ml and another crop of crystals is collected. The total yield of the crude... 

Numbers of Theoretical Trays and of Transfer Units with Two Values of kL/kG for a Distillation Process An equimolal mixture at its boiling point is to be separated into 95 and 5% contents of the lighter component in the top and bottoms products. The relative volatility is a- = 2, the minimum reflux is 1.714, and the operating reflux is 50% greater. The two values of kjJkQ to be examined are —1 and... 

We stress that this equation dictates the minimum reflux ratio based purely on thermodynamic arguments. As a1 3 = 1.11 in our case, the value of rmin = 12.28. In general, the mixture will not be equimolar, and if the products are not pure but satisfy a less strict specification, the value of rmin will be smaller. Now, a column operated under these conditions will have an efficiency of 100% since it is using the minimum amount of work necessary to separate the components. 

To 64.0 g of l-p-chlorophenyl-2,5-dimethyl-3-pyrrole acetonitrile, 64.0 g of KOH and 300 ml of ethanol are added and the mixture is refluxed for 15 h. The alcohol is evaporated and one dilutes with 300 ml of water. The aqueous phase is washed with ether, and then acidified with 20% HCI. The precipitated obtained is filtered and one washes with petroleum ether and a minimum of ether. 58.5 g (yield 85%) of l-p-chlorophenyl-2,5-dimethyl-3-pyrroleacetic acid are thus obtained, melting point 99.5°-101°C. 

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