Varying reflux ratio

Comtant reflux, varying overhead composition. The reflux is set at a predetermined value at which it is maintained for the entire run. Since the pot liquid composition is changing, the instantaneous composition of the distillate also changes. The progress of the distillate and pot compositions in a particular binary separation is illustrated in Fig. 13-120. The variation of the distillate composition for a multicomponent batch distillation is shown in Fig. 13-121 (these distillate product cuts have relatively low purity). The shapes of the curves are functions of volatility, reflux ratio, and number of theoretical plates. The distillation is continued until the average distillate... 

Reflux ratio. This is defined as the ratio between the number of moles of vapour returned as refluxed liquid to the fractionating column and the number of moles of final product (collected as distillate), both per unit time. The reflux ratio should be varied according to the difficulty of fractionation, rather than be maintained constant a high efficiency of separation requires a liigh reflux ratio. ... 

Colburn relationship found that the optimum number of trays varies from 2 to 3 times the number at total reflux. Gilliland [Ind. Eng. Chem, . 32, 1220 (1940)] from the establishment of an empirical relationship between reflux ratio and theoretical trays based on a study of existing columns indicated that... 

The effect of utilities costs on optimum operation was noted by Kiguchi and Ridgway [Pet. Refiner,. 35(12), 179 (1956)], who indicated that in petroleum-distillation columns the optimum reflux ratio varies between 1.1 and 1.5 times the minimum reflux ratio. When refrigeration is involved, 1. IRmm < flopt < 1 is used in the condensers, 1.2Rrniii < fLpt < 1 -4Rrn... 

The heat duty of amine reboilers varies with the system design. The higher the reboiler duty, the higher the overhead condenser duty, the higher the reflux ratio, and thus the lower the number of trays required. The lower the reboiler duty, the lower the reflux ratio will be and the more trays the tower must have. 

Batch with Constant Reflux Ratio, Fixed Number Theoretical Plates in Column, Overhead Comporation Varies... 

Distillation at constant reflux ratio but varying top product composition. 

Distillation at constant top product composition but varying reflux ratio. 

The program starts up the column at total reflux (R very high). After steady state is reached on all plates, vary the reflux ratio interactively and attempt to carry out the distillation in minimum time, while attempting to... 

Study the effect of varying reflux ratio on the separation. 

Selectivity and side reactions - Typically, the alcohol-to-acid ratio inside an RD unit may vary over several orders of magnitude. Especially for stages with an excess of alcohol, the use of a SAC may lead to side reactions. Selectivity was assessed by testing the formation of side products in a suspension of SZ in pure alcohol under reflux for 24 h. No ethers or dehydration products were detected by GC analysis. 

In the distillation column example, the manipulated variables correspond to all the process parameters that affect its dynamic behavior and they are normally set by the operator, for example, reflux ratio, column pressure, feed rate, etc. These variables could be constant or time varying. In both cases however, it is assumed that their values are known precisely. 

Top temperatures are usually controlled by varying the reflux ratio, and bottom temperatures by varying the boil-up rate. If reliable on-line analysers are available they can be incorporated in the control loop, but more complex control equipment will be needed. 

Variable reflux, where the reflux rate is varied throughout the distillation to produce a fixed overhead composition. The reflux ratio will need to be progressively increased as the fraction of the more volatile component in the base of the still decreases. 

Equations, determine sequences of simple and complex columns that minimize the overall vapor load. The recoveries will be assumed to be 100%. Assume the actual to minimum reflux ratio to be 1.1 and all the columns, with the exception of thermal coupling and prefractionator links, are fed with saturated liquid. Neglect pressure drop through columns. Relative volatilities can be calculated from the Peng-Robinson Equation of State with interaction parameters set to zero. Pressures are allowed to vary through the sequence with relative volatilities recalculated on the basis of the feed composition for each column. Pressures of each column are allowed to vary to a minimum such that the bubble point of the overhead product is 10°C above the cooling water return temperature of 35°C (i.e. 45°C) or a minimum of atmospheric pressure. 

Figure 14.7 illustrates what happens if the reflux ratio is fixed for a binary separation. Because the reflux ratio is fixed, the slope of the operating line is fixed, but its position varies through time. For a fixed number of stages, the distillate and the bottoms gradually become more concentrated in the heavier component. This means... 

In this case, the energy requirement is a variable as reflux ratio varies. For a differential time interval ... 

Operation at constant reflux ratio is better than operation with constant distillate composition for high-yield batch separations. However, operation with constant distillate composition might be necessary if high product purity is required. In fact, it is not necessary to operate in one of these two special cases of constant reflux ratio or constant distillate composition. Given the appropriate control scheme, the reflux ratio can be varied through the batch... 

Figure 14.9 Reflux ratio can be varied in batch distillation to maintain overhead product purity. (Reproduced from Smith R and...

Although this problem is one of multicomponent batch distillation, the product remains of constant composition so that normal methods can be used for plate-to-plate calculations at a point value of the varying reflux ratio. 

Note that the total number of plates, the feed plate location and reflux ratio can all be varied during simulation. The array form of the program also allows graphing the axial steady state tray-by-tray concentration profile. Part of the program is shown below. 

When a lies close to 1 the minimum reflux ratio is large, but since Xj varies with stage number so does [n(i+1)/P]MiN- At the feed point in a 235U plant enriching to 90% 235U, (nf/P)MiN is 29,100, but at the product end of the cascade it approaches zero. 

Spontaneous nucleation. Monodisperse micrometer-size Co or Ni particles have been obtained as follows (16,18) cobalt or nickel hydroxide was suspended in EG (250 mL) the molar ratio hydroxide/polyol was varied from 0.01 to 0.15. The suspension was stirred and brought to boil (195°C). The metal precipitation does not occur immediately when the boiling point is reached but only after an induction time. The water and the volatile products resulting from the oxidation of ethylene glycol were distilled off, while the main part of the polyol was refluxed. For Co the reduction was completed in a few hours. For nickel hydroxide it was difficult to achieve a complete reaction due to an incomplete dissolution of this hydroxide. This could be overcome by adding a small amount (a few milliliters) of... 

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