Subcooled reflux

Subroutine conden is used to calculate all condensers. It operates in very much the same fashion as subroutine eeboil, except that more possibilities occur. If reflux has been fixed, the amount of flfees is examined to see if it is larger than fixre. If not the product is set to zero and temperature is determined by bubble point. If the condenser is a two product condenser with a set temperature, tcset, this temperature is accepted if it is below the bubble point, indicating subcooled reflux. If flfees is large enough, the remainder after subtraction of fixre is taken as the appropriate product, and temperature is determined either by isovFL or BUBPT for the cases of partial and total condenser respectively. For two-product condensers, the division of the total product at tcset must be determined and calculation transfers to statement 21. 

Subcooled reflux to maintain low ethanol vapour pressure... 

The discontinuity of flowrates on the 17 stage occurs because this is the feed tray. Because the feed is 30% liquid in the bottom cascade is higher than in the top because the feed is 70% vapor flowrate in the top cascade is higher than in the bottom. There is also a drop in both liquid and vapor flows at the top of the column. This drop occurs because of the subcooled reflux. 

In certain cases, the desired manipulated variable for a particular process cannot be directly adjusted. Distillation columns can be particularly sensitive to sharp changes in ambient conditions due to weather fronts or thundershowers, since both of these cases can cause significant increases in the reflux subcooling. Equating the heat lost by the condensing vapor to the heat required by the subcooled reflux results in the following equation ... 

One hundred pound-moles per hour of a mixture of 60 mole% methanol in water at 30°C and 1 atm is to be separated by distillation at the same pressure into a distillate containing 98 mole% methanol and a bottoms product containing % mole% water. The overhead condenser will produce a subcooled reflux at 40°C. Determine by the Ponchon method ... 

Most of the vapor from the top of the column is condensed and subcooled in a condenser. This subcooled liquid then flows into the base of the reflux drum. There is heat transfer between the hot vapor and the cooler vapor-liquid interface (at temperature T) and between the vapor-liquid interface and the cooler-subcooled reflux in the tank (at temperature Tr). The vapor film coefficient is 10 Btu/hr °C ft, and the liquid film coefficient is 30 Btu/hr °C The heat transfer area on the surface of the liquid is 72 ft". The heat of vaporization of isobutane is 120 Btu/lbm. 

When an upward loop is not vented (Fig. 5.16, c, e with a closed vent valve or no vent line), inerts may accumulate at the high point, causing intermittent siphon action and reflux flow oscillation. Cases were reported (68) where hot vapor was sucked back from the column into the high point and caused hammering upon contacting the subcooled reflux. This ruptured the reflux line and nozzle. The phenomenon is most troublesome in vacuum, where slight air leaks occur. 

Subcooling. A subcooled reflux condenses some of the vapor rising up the column. This condensation will not persist during coolant failure because the reflux drum will either pump dry quickly, or reflux will be reduced by the drum level controller. Credit for subcooling is not taken toward the relief capacity when coolant or reflux fail (60). 

Upon entry into a column, a subcooled reflux quickly heats up to its boiling point by condensing column vapor. The liquid downflow (or "internal reflux ) in the top section therefore equals the external reflux plus that condensate. Frequently, the condensate makes up about 10 to 30 percent of the internal reflux flow, and it is not uncommon to find situations where it exceeds 40 percent of the internal reflux. Subcooling variations alter the quantity of condensate formed and are therefore reflected as changes in internal reflux. Inside the column, only internal reflux matters, and this will fluctuate even when the external reflux stream is flow-controlled. 

In azeotropic distillation, subcooled reflux can lower column ce iacity. 

Subcooled reflux often occurs if the condenser is at ground level. Then a punp is required to return the reflux to the top of the column. A saturated liquid will cause cavitation and destroy the pmn) thus, the liquid must be subcooled if it is to be punped. To analyze the effect of subcooled reflux, consider the top of the column shown in Figure 4-29. The cold liquid stream, Lq, must be heated up to its boiling point. 

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. 

Include the effects of subcooled reflux or superheated boilup in your McCabe-Thiele and Lewis analyses... 

A15. Develop a key relations chart for binary McCabe-Thiele distillation. That is, on one sheet of paper summarize everything you need to know about binary distillation. You will probably want to include information about operating lines, feed lines, efficiencies, subcooled reflux, and so forth. 

D5. A distillation column is operating with a subcooled reflux. The vapor streams have an enthalpy of Hi = H2 = 17,500 Btu/lbmol, while the saturated liquid hi = 3100 Btu/lbmol. Enthalpy of the reflux stream is hg = 1500 Btu/lbmol. The external reflux ratio is set at Lq/D = 1.1. Calculate the internal reflux ratio inside the column, L1/V2. 

Air recirculation Condenser liquid drainage Subcooling reflux Vapor-bound condensers... 

Recycle ratio, 48-49 Reducing liquid subcooling (reflux problems), 279... 

Condensers for vacuum towers often are operated at temperatures substantially below the dew point of the overhead vapor from the column. This is done to reduce the vapor pressure above the liquid distillate and minimize loss of distillate product in the noncondensable inerts that must be vented. The condenser must be operated above the solidification temperature of any components present in the entering vapor, which may require the use of tempered cooling water. If highly subcooled reflux is returned to the column, one additional theoretical stage should be added to the rectifying section to allow for reheating of the reflux liquid to its boiling temperature. 

This is also a commonly encountered control scheme. Both level controls may be calculated by the method of Chapter 16 care should be taken to include the subcooled reflux effect on overhead level control. 

Figures 23d and 2.4c show a Type R system which is a rectifying section employing subcooled reflux. The most economical Type R design uses crude oil charge as the cooling medium for the pumpback reflux streams and, thus, also obtains the benefit of feed preheat. This is employed at all draw trays. There is the additional advantage that all liquids within the tower are true equilibrium reflux liquids, and, thus, all trays can be considered as being effective for fractionation purposes. A fairly uniform vapor and liquid traffic exists in all sections of the tower.

The overflash liquid condensing section is calculated as a single-stage flash condensation. The heat removal across this portion of the tower is accomplished internally by reva-porizing subcooled reflux which is pumped back from the next draw tray up in the tower. 

Calculate the reflux induced on Tray (D2 —1) by the use of subcooled reflux. This induced reflux is the amount of vapor from Tray (D2 - 2) which enters and is condensed on Tray (D2 - 1) for the purpose of converting the cooled pumpback reflux to bubble point liquid. 

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