Chapter 4, Problem 4.81P

The fresh feed to an ammonia production process contains nitrogen and hydrogen in stoichiometric proportion, along with an inert gas (I). The feed is combined with a recycle stream containing the same three species, and the combined stream is fed to a reactor in which a low single-pass conversion of nitrogen is achieved. The reactor effluent flows to a condenser. A liquid stream containing essentially all of the ammonia formed in the reactor and a gas stream containing all the inerts and the unreacted nitrogen and hydrogen leave the condenser. The gas stream is split into two fractions with the same composition: one is removed from the process as a purge stream, and the other is the recycle stream combined w ith the fresh feed. In every stream containing nitrogen and hydrogen, the two species are in stoichiometric proportion.

RecyclePurge

Fresh feedN,. Hj. 1				Nj,H2
	REACTOR		CONDENSER	CondensateMl.

1. Let .¥io be the mole fraction of inerts in the fresh feed, /_sp the single-pass conversion of nitrogen (and of hydrogen) in the reactor, and y the fraction of the gas leaving the condenser that is purged (mol purged/mol total). Taking a basis of 1 mol fresh feed, draw and fully label a process flowchart, incorporating .tio,/sp, and y_p in the labeling to the greatest possible extent. Then, assuming that the values of these three variables are given, write a set of equations for the total moles led to the reactor (n_r), moles of ammonia produced (n_p), and overall nitrogen conversion (f_m). Each equation should involve only one unknown variable, which should be circled.
2. Solve the equations of Part (a) for .vw = 0.01,= 0.20. and y_p = 0.10.
3. Briefly explain in your ow n words the reasons for including (i) the recycle stream and (ii) the purge stream in the process design.

Prepare a spreadsheet to perform the calculations of Part (a) for given values of .vio, /_sp, and y . Test it with the values in Part (b). Then in successive rows of the spreadsheet, vary each of the three input variables two or three times, holding the other two constant. The first six columns and first five rows of the spreadsheet should appear as follows:

xio	fsp	yp	nr	np	fov
0.01	0.20	0.10
0.05	0.20	0.10
0.10	0.20	0.10
0.01	0.30	0.10

Summarize the effects on ammonia production (/tp) and reactor throughput (n_r) of changing each of the three input variables.