Condensate subcooling

To select the proper heat transfer relations to represent the functions, you need to analyze the heat transfer functions that will take place in the unit-tube and/or on the shell side. Some units may have several functions, such as the example in Rubin s recommendations on this subject that is, steam desuperheating and hydrocarbon condensing steam and hydrocarbon condensing, and condensate subcooling. Rubin presents an excellent interpretation of multizone operation for several different sets of conditions. See Figures 10-91Aand 10-91B. 

A mixture containing 12 mol % water is to be separated by distillation into products with 99.5 and 0.5 mol % butanol. The accompanying flowsketch of a suitable process utilizes two columns with condensing-subcooling to 40°C. The 53% saturated solution is refluxed to the first column, and the 98% is fed to the second column. The overhead of the second column contains a small amount of butanol that is recycled to the condenser for recovery. The recycle material balance is shown with the sketch. 

The cooling water (or other medium) must absorb enough heat to balance the heat of vaporization and condensate subcooling. Piping and hot wells must be sized based upon the maximum condenser requirement. The following example illustrates the method of calculating the quantity of cooling water for a specific service. 

Test data were obtained on a vertical condenser-subcooler for a pure component. Compare the observed heat-transfer coefficient to that expected, and calculate the apparent fouling coefficient. Data were obtained in late autumn and the water flow was cocurrent with the process flow in order to minimize the amount of subcooling. Design inlet water temperature was 30°C (86°F). 

The Nusselt analysis of laminar film condensation has been shown to be reasonably accurate for a variety of ordinary fluids such as steam and organic vapors, despite the approximations made in the model. Measured heat transfer coefficients are about 15-20 percent higher than predicted values. Numerous studies have been conducted to explain the observed differences. For example, in Eq. 14.9, a correction may be made to the latent heat of evaporation to take into account condensate subcooling ... 

The combination of distillation and decantation is used in the well-known azeotropic distillation process shown in Fig. 11.4-1. The separation of the homogeneous azeotropic feed (e.g., ethanol/water) is achieved by the admixture of an entrainer (e.g., toluene) which forms a large mixing gap with one of the feed components (here water). In colunrn C-1, water is recovered as bottoms B. The azeotropic overhead fraction t> is mixed with the fraction S2 (toluene-rich) from the decanter. In colunrn C-2 pure ethanol is recovered as bottoms B2. The overhead fraction D2 is condensed, subcooled, and split into the fractions 51 (water-rich) and 52 (toluene-rich) in the decanter. Both fractions are recycled within the pro-... 

Condensate Subcooling For vertical condensers, condensate can be readily subcooled if required. The subcooling occurs as falling-film heat transfer. Heat transfer coefficients can be calculated as presented in a later section. 

With few exceptions, vertical downdraft condensers provide better heat transfer than updraft condensers. Occasionally, updraft condensers are used when minimum condensate subcooling is desired or when the remaining vapors will not condense in the presence of previously obtained condensate. Sonte-tinaes corrosion is minimized by preventing condensate and uncondensed vapors from contacting. Chemical reactions may also be avoided. 

Condensate subcooling occurs when the distillation tower overhead product is all liquid. This means the contents of the reflux drum must be at or below its bubble point. The condenser operation will automatically adjust itself to satisfy this condition. 

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