7.43: Untitled Page 188

Chapter 7

7‐36. In Figure 7.36 we have illustrated a process in which NaHCO is fed to 3

a combined drying and calcining unit in which Na CO , H O and CO are 2

3

2

2

produced. The partial pressure of water vapor in the entering air stream is equivalent

to 12.7 mm Hg and the system operates at one atmosphere (760

mm Hg). The exit gas leaves at 300°F and a relative humidity of 5%. The NaHCO is 70% solids and 30% water (mass a

b sis) when fed to the system.

3

The Na CO leaves with a water content of 3% (mass

basis). To improve the

2

3

character of the solids entering the system, 50% (mass basis) of the dry material is moistened

to produce Na CO  H O and then recycled.

2

3

2

Figure 7.36. Dryer‐

ciner

cal

Calculate the following quantities per ton of Na CO product.

2

3

a) mass of wet NaHCO fed at 30% water.

3

b) mass of

water fed to moisten recycle material.

c) cubic feet of dry air fed at 1 atm and 273K.

d) total volume of exit gas at 1 atm and 300F.

7‐37. In Figure 7‐5 we have illustrated an ammonia “converter” in which the unconverted gas is recycled to the reactor. In this problem we consider a process in which the feed stream is a

metri

stoichio

c mixture of nitrogen and hydrogen

ontaini

c

ng 0.2% argon. In the reactor, 10% of the entering actants

re

(nitrogen

and hydrogen) are converted to ammonia which is removed in a condenser. To be explicit, the conversion is given by

 RN

2

C  Conversion of N



 0 . 10

2

( M



)

N2 1

The unconverted gas is recycled to the converter, and in order to avoid the buildup of argon in the system, a purge stream is incorporated in the recycle

Material Balances for Complex Systems

353

tream.

s

In this problem we want to determine the fraction of recycle gas that must be purged if e

th argon entering the reactor is limited to 0.5% on a molar basis.

Section 7.4

7‐38. Solve Problem 7‐37 using one of the following methods described in Appendix B.

a). The bi‐section method

b). The false position method

c). Newton’s method

d). Picard’s method

e). Wegstein’s method