Question

7. A process takes hydrogen and nitrogen to make ammonia. It consists of a feed stream (Hz: 330 kmol/h and N2: 110 kmol/h), a mixer in which the feed stream is mixed with the recycle stream, and a reactor (25% conversion), followed by a separator where the product is removed and the reactants are recycled. The recycle stream takes 95% of the unreacted H2 and N, and 2% of the produced ammonia. Draw the BFD for the process and number all the streams; Perform the process mass balance and add the streams table at the bottom of the BFD 8. Solve problem 7 assuming 25% conversion in the reactor and a feed containing (Hz: 300 kmol/h; Nz: 100 kmol/h and CO2: 1 kmol/h). Consider of purge of 1%.

Answer 1

7(A)

The block diagram of the process is shown below

The reaction is

$N_{2}+ 3H_{2}\rightarrow 2NH_{3}$

The number of streams

F , M, E, P, R = 5 streams

Single pass conversion = 25%

F stream analysis

Component	Kmol/h	mol%	M.W	Kg/h
N​​​​​​2	110	25	28	3080
H​​​​​​2	330	75	2	660
Total	440	100		3740

The feed ratio is stiochiometric

Ratio of H2: N2 = 3:1

95% of H2 and N2 are recycled and also 2 % of ammonia

R(x )- amount of N2 in recycle

95% of N2 in E stream is recycled

Doing N2 balance around mixer

M = 110+ R(x)

Doing N2 balance around reactor

(1-0.25)(110+ R(x))= (R(x)/0.95)

R(x)= 272.608 Kmol/h

Amount of nitrogen in recycle = 272.608 Kmol/h

Since ratio of H2: N2 = 3:1 remains constant

Amount of H2 in recycle = 272.608(3)=

817.826 kmol/h

Amount of ammonia produced according to stiochiometry in the reactor

= 2(0.25)(110+ R(x))= 2(0.25)(110+272.608)= 191.3043 Kmol/h

P analysis

Component	Kmol/h	mol%	M.W	Kg/h
N​​​​​​2	272.608(0.05/0.95)=14.3247		28	401.0916
H​​​​​​2	43.0433		2	86.086
NH​​​​​3	191.342		17	3252.822
Total	248.710			3740

Overall mass of system is conserved

Ammonia in product = 191.342 Kmol/h

98% is in product and 2% is in recycle

Ammonia in recycle = (191.342)(0.02)/0.98 = 3.9049 Kmol/h

R analysis

Component	Kmol/h	mol%
N​​​​​​2	272.608	24.910
H​​​​​​2	817.824	74.732
NH​​​​​3	3.9049	0.358
Total	1094.3369	100

M analysis

Component	Kmol/h	mol%
N​​​​​​2	272.608+110= 382.608	24.936
H​​​​​​2	817.824+ 330= 1147.824	74.809
NH​​​​​3	3.9049	0.255
Total	1534.3369	100

E analysis

Component	Kmol/h	mol%
N2	382.608(0.75)= 286.956	21.428
H2	860.868	64.285
NH3	191.3043	14.287
Total	1339.128	100

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