Distillation columns reflux considerations

Distillation capital costs. The classic optimization in distillation is to tradeoff capital cost of the column against energy cost for the distillation, as shown in Fig. 3.7. This wpuld be carried out with distillation columns operating on utilities and not integrated with the rest of the process. Typically, the optimal ratio of actual to minimum reflux ratio lies in the range 1.05 to 1.1. Practical considerations often prevent a ratio of less than 1.1 being used, as discussed in Chap. 3. 

The magnitudes of various flowrates also come into consideration. For example, temperature (or bottoms product purity) in a distillation column is typically controlled by manipulating steam flow to the reboiler (column boilup) and base level is controlled with bottoms product flowrate. However, in columns with a large boilup ratio and small bottoms flowrate, these loops should be reversed because boilup has a larger effect on base level than bottoms flow (Richardson rule). However, inverse response problems in some columns may occur when base level is controlled by heat input. High reflux ratios at the top of a column require similar analysis in selecting reflux or distillate to control overhead product purity. 

Similar considerations apply to distillation columns. Entropy is generated on the trays due to mixing and heat transfer between dissimilar streams. We can reduce the entropy production by adding more trays to the column. With more trays there is less of a gradient across each tray and the entropy production goes down. A column with many trays requires less reflux and therefore less heat to the reboiler. The energy consumption is reduced only at the expense of added capital investment for the taller column. 

An important part of the design of any distillation column is the choice of reflux ratio. There are a few qualitative considerations which must be kept in mind. 

Eollowing are two examples (16.1 and 16.2) of a distillation column that demonstrate the effect of applying different pairing strategies. In both examples the control loops for the column pressure and the liquid levels in the condenser accumulator and the column bottom are determined independently based on practical considerations. Thus, the column pressure is controlled by various techniques that may involve the condenser coolant rate, and the liquid levels are controlled by the product flow rates. What remains to be decided is how to pair the distillate and bottoms compositions with the reflux rate and the reboiler heat duty. The same distillation column is used in both examples, having a total condenser and a reboiler, one feed and two products. The column is designed to separate a benzene-toluene mixture into benzene and toluene products with specified purities. 

Figure 5.7 is a sketch of the plant under consideration. Fresh feed enters the reactor at a flow rate Fq and composition Zo,a 1 (pure component A in the fresh feed). We assume that the relative volatilities of components A, B, and C, aAloLsIoic, are 4/2/1, respectively, so unreacted component A comes overhead in the first distillation column and is recycled back to the reactor at a rate D and composition Reactor effluent F is fed into the first distillation column. The flow rates of reflux and vapor boilup in this column are R and V. Bottoms B from the first column is fed into the second column, in which components B and C are separated into product streams with about 1 percent impurity levels. 

Figure 5.21 shows that the distillation boundary has indeed been crossed (only Just) by merely operating at a finite reflux value (note X/> and lie on opposite ends of the distillation boundary). Wahnschafft and coworkers have attempted to explain this phenomenon and have concluded for distillation boundary crossing to be possible that (1) the distillation boundary has to display considerable curvature, (2) a distillate or bottoms composition has to close to or on the concave side of the boundary, and (3) the distillation column needs to operate at a certain range of reflux ratios for it to be feasible as over-refluxing the column will cause the topology to resemble that of the RCM, hence rendering the product specifications to be infeasible. 

From a feed that comprises only vapor that is introduced at the bottom of a column, one could expect to achieve a top distillate stream highly purified in the more volatile species if there is a condenser at the top and there is reflux of the condensate at the top. Therefore such a distillation column has only an enriching section (Figure 8.1.26(a)) which can produce a highly purified distillate. However, there will be considerable loss of the more volatile species in the liquid leaving the column bottom where the vapor feed is introduced since there is no stripping section in the column. Correspondingly if a liquid feed is... 

For a typical distillation column, five input variables can be manipulated product flow rates, B and D, reflux flow rate P, coolant flow rate qc, and heating medium flow rate qh. Thus, according to the General Rule, Npc = 5. This result can also be obtained from Eqs. 13-1 and 13-2, but considerable effort is required to develop the required dynamic model. Although five output variables could be selected as CVs, xo, xp, hp, ho, and P, for many distillation control problems, it is not necessary to control all five. Also, if it not feasible to measure the product compositions on-line, temperatures near the top and bottom of the column B often controlled inftad, as discussed in the next secticW ... 

A sodium ethoxide solution freshly prepared from 2.5 liters of anhydrous ethanol and 115 gm (5 gm atom) of sodium is warmed to 50°C and stirred while 825 gm of diethyl malonate is added. To the clear solution is added slowly 685 gm of n-butyl bromide. The reaction commences almost immediately and considerable heat is generated. The addition rate is adjusted so that the reaction does not become violent. Cooling may be necessary. After the addition, the reaction mixture is refluxed until neutral to moist litmus (about 2 hr). Then a distillation column is attached to the flask and approximately 2 liters of alcohol are distilled off in 6 hr, using a water bath. The... 

Hydrochloric acid [7647-01-0], which is formed as by-product from unreacted chloroacetic acid, is fed into an absorption column. After the addition of acid and alcohol is complete, the mixture is heated at reflux for 6—8 h, whereby the intermediate malonic acid ester monoamide is hydroly2ed to a dialkyl malonate. The pure ester is obtained from the mixture of cmde esters by extraction with ben2ene [71-43-2], toluene [108-88-3], or xylene [1330-20-7]. The organic phase is washed with dilute sodium hydroxide [1310-73-2] to remove small amounts of the monoester. The diester is then separated from solvent by distillation at atmospheric pressure, and the malonic ester obtained by redistillation under vacuum as a colorless Hquid with a minimum assay of 99%. The aqueous phase contains considerable amounts of mineral acid and salts and must be treated before being fed to the waste treatment plant. The process is suitable for both the dimethyl and diethyl esters. The yield based on sodium chloroacetate is 75—85%. Various low molecular mass hydrocarbons, some of them partially chlorinated, are formed as by-products. Although a relatively simple plant is sufficient for the reaction itself, a si2eable investment is required for treatment of the wastewater and exhaust gas. 

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