Reflux, enthalpy internal

In the column model discussed in Chapters 3 through 13, the internal liquid and vapor flows were considered only from the standpoint of their effect on the thermodynamic performance of the column. The performance parameters that were investigated included quantities such as compositions, temperatures, pressures, enthalpies, and A -values. The column was assumed capable of physically handling any liquid or vapor flow rate, regardless of hydraulic effects or pressure drops. The only flow limitations that were taken into account involved minimum reflux ratio and conditions under which the liquid or vapor dried up, or approached zero flow in certain parts of the column. 

With the internal reflux ratio (I )min known, the external reflux ratio is computed by enthalpy balance with (12-29). This requires a knowledge of the rectifying section pinch-point compositions. Underwood shows that... 

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. 

One can often calculate unmetered internal unit flows, such as reflux rates, from an enthalpy balance if only a reboiler or preheater duty is known... 

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