Amount of reflux

Constant overhead composition, vaiying reflux. If it is desired to maintain a constant overhead composition in the case of a binaiy, the amount of reflux returned to the column must be constantly increased throughout the run. As time proceeds, the pot is gradually depleted of the hghter component. Finally, a point is reached at which the reflux ratio has attained a veiy high value. The receivers are then changed, the reflux is reduced, and an intermediate cut is taken as before. This technique can also be extended to a multi-component mixture. 

However, before the above set of equations can be solved, many important decisions must be made about the distillation column. The thermal condition of the feed, the number of equilibrium stages, feed location, operating pressure, amount of reflux, and so on, all must be chosen. 

Some designers suggest adding 10% to the calculated liquid horsepower to allow for changes in the process. For reflux pumps the LHP should be increased 35% beyond that calculated.26 This is because the amount of reflux is a determinant in the amount of separation possible and the pump should not limit the options available to the plant operator. 

The upper portion of the ice bed emerges as a drained bed. A proper balance of feed rate, spent-brine discharge rate, and rate of outflow through the probe permits the column liquid level to be maintained at a desired distance above the probe screen. The drained bed is washed with a small amount of reflux (10% or less of the ice rate) to yield a product of less than 300 p.p.m. in salt content. 

During the slow distillation, continuous supervision is necessary. The temperature rises from -10° (Se02F2) slowly to +47°. A sufficient amount of reflux must be maintained for a good separation. Above +30° the HSeOFs should be collected, though spectroscopically pure HSeOFs appears only at the end of... 

The ratio L/V depends on the amount of reflux in the column. At minimum reflux, the operating lines intersect each other and the q line at the point where the q line intersects the xy curve (in Fig. 8.2, at point D). 

In this example, the approach used for constructing the operating lines is based on the relationship that typically holds, in practice, between the minimum reflux and the amount of reflux actually employed. First, draw the operating line that would pertain for minimum reflux in the enriching section. As noted earlier, this line is defined by points D and E. Next, measure its slope (L/V) on the diagram. This is found to be 0.5. 

Separation constraints The separation in a column can be expressed as the impurity levels of the key components in the two products xg.LK in the bottoms and xD Hx in the distillate. Separation is limited by the minimum reflux ratio and the minimum number of trays. We must always have more trays than the minimum and a higher reflux ratio than the minimum. If the number of trays in the column is not large enough for the desired separation, no amount of reflux will be able to attain it and no control system will work. In extractive distillation columns, there is also a maximum reflux ratio limitation, above which the overhead stream becomes less pure as the reflux increases. 

With higher bottoms flow rates, lower reboiler duties are required at minimum reflux since less liquid must be vaporized. For any given bottoms rate, a minimum amount of reflux is required to maintain both liquid and vapor phases on all the trays. Curve AB in Figure 6.5 is a plot of the minimum reflux ratio vs. bottoms rate. This minimum is not to be confused with the minimum reflux ratio required to bring about a specified separation with a given number of stages. The minimum reflux curve in Figure 6.5 represents reflux ratios below which either the liquid or the vapor dries up on some of the trays. The curve characteristics depend mainly on the feed... 

The factors on which the efficiency of a fractionating column in achieving separations depends are (i) Amount of contact between vapour and liquid (ii) amount of reflux, and (iii) rate of distillation. 

Calculate Required Stages or Transfer Units. After the model is selected, the number of theoretical stages or transfer units is compaled. This is an index of the difficulty of the. separation and is dependent on the amount of reflux that is used. It is in this step that the familiar stages/reflux relationship is developed, with the Anal combination of these two paramaters dependent on economics. 

Operation. Typical operating conditions for butadiene recovery plants have been described by Buell and Boatright (I). These data were used as a guide for setting the conditions used in this study. At the start of a run, the column was preheated electrically to the desired column temperature profile. The C4 feed was pumped into the column until the pressure reached 30-35 psig. Solvent (preheated to 130°F) was then circulated through the column, and the reboiler temperature increased to about 275°F. The hydrocarbon feed was pumped in at a rate which gave the desired solvent-to-hydrocarbon feed ratio. A portion of the butene stream was returned to the column to provide reflux. Temperature of the solvent feed tray was used to control the amount of reflux. Only that portion of C4 s which dissolved in the solvent could be carried down the column. The excess C4 s were revaporized to the hydrocarbon trays. 

The above-mentioned determination of the minimum reflux ratio for a continuous distillation assumes that the mixture to be separated is fed in at its boiling (bubble) point. In this case the amount of reflux in the lower column section is increased by the amount of the feed, i.e. 

Reflux ratio and amount of reflux referred to the free cross-section (ml/cm h) ... 

The determination of the amount of reflux is in principle performed at the column head. If the column were to operate adiabaticaUy, that is to say without gain or loss of heat, and the molar heats of evaporation of the components were identical, the amount of reflux measured just above the still pot would be equal to that measured at the top. In practice, however, the quantity of bottom reflux is generally... 

The question of the distillation pressure will be fully discussed in section 5.4. The number of stages and the minimum reflux ratio are calculated as described in 4.7.2 the temperature of the feedstock is usually kept as near as possible to its boiling point. The dimensions of the column and the heat requirements can be found as detailed in sections 4.11 and 4.12, respectively. When calculating the column diameter one should remember that the lower half-column is loaded with an amount of reflux... 

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