Reflux ratio rectifying column sections

To redistribute the stages in the remaining sections, a shortcut simulation is used to find out the required number of trays, the feed tray location and the minimum reflux ratio for each column in the sequence. To make use of the existing column with the same number of trays (24 trays) iterations are required to adjust the sum of the rectifying sections in each column equal to 24 (number of trays in the main column). Finally, the sequence of the simple columns is merged into a complex column. The main column is not changed, but the side strippers and pump arounds need to be relocated or adjusted. 

The number of trays in each column section (rectifying or stripping) is determined by the required fractionation in that section at a given L/V ratio. This ratio, which is directly related to the reflux ratio, is usually limited by practical considerations such as economically acceptable reboiler and condenser duties. Also, the reflux ratio is limited by tray hydraulics considerations, discussed in Chapters 14 and 15. Column flooding can occur if the vapor or liquid velocities become excessive. The total column trays and feed location are set once the number of trays in each section is known. 

Even though the column has no condenser and the reflux is from an external feed, the column, including its rectifying section, behaves more like a distillation column than an absorber. The reason is that the two feeds—the external reflux and the main feed—have similar compositions and average boiling points. Also, the vapor and liquid compositions on each tray are not much different from each other so that mass transfer takes place both ways between the phases. As a result, the UV ratio variation from tray to tray is fairly small in each column section, and the temperature rises steadily from tray to tray going down the column. The column profiles are shown in Table 9.3. 

Eor the purpose of estimating column pressure drops, vapor and liquid flow rates are estimated from the reflux ratio. Calculations are performed at two points in the column one in the rectifying section and the other in the stripping section. 

Eor the trayed column, the reflux ratio is 2.75 and the distillate rate is 55.5 kmol/h. The liquid flow in the rectifying section,... 

Typically, the quantity of distillate product that is being condensed and returned to the top of the column needs to be specified. It can be defined as either an internal or external quantity depending upon which variables are used. The internal reflux ratio is defined in terms of flowrates within the rectifying section of the colunm ... 

In Table 1, the column entitled "reflux ratio" is the external reflux ratio -i.e., the ratio of liquid condensate returned as reflux to that kept as product. Having chosen this ratio and the concentration of stillage, feed and product, the ideal number of stages required in the rectifying and stripping sections, as well as the energy, are then determined. 

Total reflux of type 1 may be approached in a continuous distillation column by approaching total reflux conditions in both the rectifying and stripping sections. The designer approaches this type of operation of a continuous distillation column as the reflux ratio is increased indefinitely at a fixed feed rate and nonzero product rates. In the limit, this type of operation is recognized as total reflux of type 1, continuous distillation columns at total reflux. The necessary conditions for an equivalence to exist between columns at the operation conditions of total reflux of types 1 and 2 are presented in Sec. 10-2. Total reflux of types 1 and 2 are of significant interest because columns at these types of operation produce the best possible separations. 

REFLUX E TIO. The analysis of fractionating columns is facilitated by the use of a quantity called the reflux ratio. Two such quantities are used. One is the ratio of the reflux to the overhead product, and the other is the ratio of the reflux to the vapor. Both ratios refer to quantities in the rectifying section. The equations for these ratios are... 

The liquid entering the top stage is the external reflux rate, L0, and its ratio to the distillate rate, LQ/D, is the reflux ratio, R. For the case of a total condenser, with reflux returned to the column at its bubble point, L0 = L and R = LID, a constant in the rectifying section. Since V = L + D,... 

It is also useful to note that the generalized reflux in the rectifying section, R r, reduces to exactly the classical reflux ratio of r = IJD. However, the reflux ratio in the stripping section, is almost always expressed as a reboil ratio in classical column design as 5 = V B. In this case, the relationship between the classical parameter and our generalized one is given as R/ s = — 5 — 1. 

At the same time, the transformation eliminates the reaction term in the balance equation. The operating line for the rectifying section of a reaction column is formally identical to the operating line of a non-reactive column. An infinite reflux ratio gives an expression that is formally identical to the one for calculating conventional distillation lines [5, 6]. Accordingly, we will refer to lines that have been calculated by this procedure as RD lines. These analogies are found for all the relationships that are important in distillation [7, 8]. 

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