Ammonia And Water Relative Volatility

Oxygen-Nitrogen and Ammonia-Water Example. Tables 7-3 and 7-4 give similar comparisons for other systems. In Table 7-3 the comparison is for the oxygen-nitrogen system at 10 atm., and again the agreement between the two methods is good. The other data are for ammonia-water at 10 atm., and even in this case the two methods are in fair agreement.

Ammonia-Water Example. In order to illustrate the fact that the assumption of constant molal overflow rate is not always justified, consider the following example in which the molal latent heat of vaporization varies approximately twofold over the tower. An aqueous solution containing 20 weight per cent ammonia is to be separated into a distillate containing 98 mol per cent ammonia and a bottoms containing 0.1 mol per cent ammonia. The tower and total condenser will operate at an absolute pressure of 20 atm. The feed will enter the system at 20°C. and be heated to 40°C in the condenser as shown in Fig. 7-20. Using the data and notes given below, calculate:

1. The minimum reflux ratio O/D, using the enthalpy-composition method.

2. The number of theoretical plates required at O/D equal to 1.5 times the minimum value found in Part 1, using the enthalpy-composition method.

3. The number of theoretical plates required at total reflux.

4. The minimum reflux ratio O/D, assuming constant molal overflow rates.

5. The number of theoretical plates required for O/D equal to 1.5 times the value obtained in Part 4, using constant molal overflow rates.

Data and Notes. As a basis, the enthalpies of pure liquid water and pure liquid ammonia are taken as zero at their boiling points. On this basis a 20 weight per cent ammonia solution at 40°C. has an enthalpy equal to —3,180 cal per g. mol. Other enthalpy and equilibrium data (Ref. 2) for 20 atm. follow.

Mol fraction NH3, in liquid or vapor

Enthalpy liquid cal. per g. mol vs. mol fraction in liquid

Enthalpy vapor cal. per g mol vs. mol fraction in vapor

T, °C. vs. mol fraction in liquid

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