Minimum Reflux Ratio, Rmin

Using Figure 8-33 the separation from Xq, initial kettle volatile material to X3 as the distillate of more volatile overhead requires three theoretical plates/stages at total reflux. Using finite reflux R4, and four theoretical plates the same separation can be achieved with infinite theoretical plates and the minimum reflux ratio, Rmin- The values of reflux ratio, R, can be determined from the graph with the operating line equation as,... 

R = Reflux ratio = External reflux ratio for a given separation, = L/D, L = liquid rectifying column R = Actual reflux ratio, O/D Rra = Minimum reflux ratio, O/D R = Pseudo minimum reflux ratio Rmin = Minimum external reflux ratio for a given separation... 

For each column, the recoveries of the light and heavy key components have been specified. Then, the minimum number of trays Nmill and the minimum reflux ratio Rmin have been calculated using the shortcut distillation model DSTWU with Winn-Underwood-Gilliland method in Aspen Plus. The reflux ratio was set to... 

It uses activity coefficient methods to calculate vapor-liquid equilibrium. It will estimate a minimum reflux ratio (Rmin) assuming no pinch point in the rectifying section. The actual reflux ratio (RR) is set as a multiple of Rmin (R = C Rmin). 

To achieve a separation one needs to use at least a minimum reflux ratio (-Rmin)- Using simplifying assumptions of constant volatility and equal molar latent heat for the components,... 

The most important limiting values of BED are the minimum feed flow rate (Fmn) at given reflux ratios (R), particularly at infinite reflux ratio (R=° ), minimum reflux ratio (Rmin). minimum number of theoretical stages in the extracting section and... 

Smin = minimum number of theoretical plates R = any reflux ratio Rmin = minimum reflux ratio... 

Determine the minimum reflux ratio candidate for each feed Rinin.l. Rmin,2. Rniin.nf... 

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. 

The easiest way to use a shortcut distillation model is to start by estimating the minimum reflux and number of stages. The optimum reflux ratio is usually between 1.05 and 1.25 times the minimum reflux ratio, Rmm, so 1.15 x Rmin is often used as an initial estimate. Once the reflux ratio is specified, the number of stages and optimum feed stage can be determined. The shortcut model results can then be used to set up and initialize a rigorous distillation simulation. 

In the second step, an operating line is drawn from distillate composition y = X = through the point of intersection of the <7-line with the equilibrium curve, which results in a pinch and represents minimum reflux conditions. The intercept of this line on the ordinate is given by /(RMin + 1) establishes the minimum reflux ratio. We have... 

The indirect effects add to those of Fig. 14-42, widening the gap between true and apparent efficiency. The indirect effects exponentially escalate as minimum reflux is approached. Small errors in VLE or reflux ratio measurement (this includes column material balance as well as reflux rate) alter R/Rmin. Near minimum reflux, even small R/Rmin errors induce huge errors in the number of stages, and therefore in tray efficiency. Efficiency data obtained near minimum reflux are therefore meaningless and potentially misleading. 

When these are entered into the shortcut column as specifications, the minimum reflux is calculated to be Rmin = 2.130. The actual reflux ratio can then be specified as 2.13 X 1.15 = 2.45, as shown in Figure 4.20. 

For a given specification, a reflux ratio can be selected anywhere from the minimum, Rmin, to an infinite value (total reflux) where all the overhead vapor is condensed and returned to the top stage (thus, no distillate is withdrawn ). The minimum reflux corresponds to an infinite number of stages, while an infinite reflux ratio corresponds to the minimum number of stages. 

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