Underwood minimum reflux binary

Binary minimum reflux so calculated implies feed enthalpy just equal to the above started vapor V and liquid L. Any increase or decrease in that enthalpy must be matehed by inerease or decrease in total heat content of overhead reflux. Note that the Underwood" binary reflux equation essentially computes the flash versus specifi-eation composition relationship along with enthalpy correction. 

For binary or near binary minimum reflux ratio, L/D min, use the Underwood equations.10... 

The Underwood and Fenske equations may be used to find the minimum number of plates and the minimum reflux ratio for a binary system. For a multicomponent system nm may be found by using the two key components in place of the binary system and the relative volatility between those components in equation 11.56 enables the minimum reflux ratio Rm to be found. Using the feed and top compositions of component A ... 

An analytical approach for the estimation of minimum reflux ratio han been pablished by Underwood and is useful for malticomponant as well as binary systems. There are three basic assumptions made by Underwood ... 

For binary systems, the pinch point usually occurs at the feed plate. When this occurs, an analytical solution for the limiting flows can be derived fKing. 19801 that is also valid for multiconponent systems as long as the pinch point occurs at the feed stage. Unfortunately, for multiconponent systems there will be separate pinch points in both the stripping and enriching sections if there are nondistributing conponents. In this case an alternative analysis procedure developed by Underwood (1248) is used to find the minimum reflux ratio. 

Occasionally there is a need to perform some preliminary but rapid estimates for a specific separation without resorting to the tedious graphical or plate by plate calculations. In such instances one can turn to some of the short-cut methods that have been developed specifically for multicomponent separations in the chemical process industry but which also work reasonably well with binary and multicomponent separations at low temperatures. These are the Fenske-Underwood method for obtaining the minimum number of plates at total reflux, the Underwood method for obtaining the minimum reflux, and the Gilliland correlation to determine the theoretical number of plates based on the information provided by the two prior methods. 

This method is based on the assumption of constant flow rates, R and G, constant relative volatility O] 2 and an infinite reflux ratio V. According to Fenske and Underwood [2.64] the required minimum number of theoretical separation stages for binary rectification is... 

The basic assumption of the Fenske-Underwood relation is that the ratio of the equilibrium constants or the relative volatility, as defined by Eq. (6.19), in a binary mixture or the two key components present in a multicomponent mixture remain constant over the temperatures encountered in the distillation column. If this can be assumed without the introduction of excessive error, the minimum number of plates at total reflux can be determined from... 

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