Internal reflux ratio, increase

Note that when f = 0, Eqs. 14-651 and 14-661 both say L1/V2 = L(/Vi- As the fraction condensed increases (reflux is subcooled more), the internal reflux ratio, L1/V2, becomes larger. Thus the net result of subcooled reflux is equivalent to increasing the reflux ratio. Numerical calculations (such as Problem 4.D51 show that a large amount of subcooling is required to have a significant effect on 17V. With highly subcooled reflux, an extra tray should be added for heating the reflux IKister. 19901. 

The HETP determined at total reflux is then used at the actual reflux ratio. Ellis and Brooks (1971) found that there is an increase in the HETP for internal reflux ratios below 1.0, but the increase is usually quite small until L/V approaches 1/2. Thus, the usual measurement procedure can be used for most design situations. 

An increase in condenser duty to 250 kW has the effect of increasing both the reflux ratio and the internal reflux ratio. Thus, these quantities must be redetermined. A simple way to establish the L/V ratio is to make an energy balance around the condenser to determine the overhead vapor that is condensed ... 

This is not unusual to select an operating reflux ratio ten, or even fifty times such a low minimum. Selecting a higher reflux can reduce the number of trays required, and this becomes a balance of the reduction in trays versus operating and capital expense in handling the increased liquid both external to the column and internally. 

Turndown on a PAN type tray should be limited to 2 1 (ratio of high to low flow rates), which results in a reasonable design. Bonilla [131] points out that it is cosdy to design for short-term high turndown rates such as start-up, shutdown, or for other short term periods, because it is better to increase reflux ratio to increase internal loads for such periods rather than design the distributor for large turndown. 

In an operating column the effective reflux ratio will be increased by vapour condensed within the column due to heat leakage through the walls. With a well-lagged column the heat loss will be small and no allowance is normally made for this increased flow in design calculations. If a column is poorly insulated, changes in the internal reflux due to sudden changes in the external conditions, such as a sudden rain storm, can have a noticeable effect on the column operation and control. 

Increased reflux ratio has the effect of decreasing the required number of theoretical contacts, but it increases the internal flow, equipment diameter, and energy requirements. It is frequently necessary to solve the separation problem a considerable number of times, imposing various operating conditions and carefully ascertaining the effect on the total cost of the particular solution proposed. Optimum design conditions are discussed in Chapter 13. 

Discussion—When distilling a mixture of compounds as is the case of crude petroleum, there will be a normal increase in reflux ratio down the column. In the case where heat losses occur in the column, the internal reflux is abnormally greater than the reflux in the head. The opposite is true when the column gains heat, as with an overheated mantle. 

Note that the Zob(C) = 0.05 disturbance (Fig. 11.23), which crashed the two-temperature control scheme, causes no problem in the internal composition control structure and that the time scale has been increased to 300 min to make sure that there is long-term instability. The internal composition controller detects the increase in reactant A composition on tray 6 because there is less B coming into the column in the Fqa feed. The composition controller reduces the flowrate of Fqa. The temperature on tray 3 increases as less D is produced because less B is entering the column. The temperature controller cuts back on the vapor boilup, and the reflux ratio cuts back on the reflux. The final steady state produces somewhat lower purity bottoms and less of it. Distillate purity is higher with only a slightly lower flowrate because of the C coming in with the Fqb feed. 

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