Recycle
Recycle stream
1. Increased reactant conversion.
1. Increase the overall
Fresh
conversion
Feed
2. Allow the reactor to be
operated at low conversion
3. Improved selectivity
2. Continunous catalyst regeneration.
3. Circulation of a working fluid.
Process
Product
Separator
Mixer
Bypass
Bypass stream
To control composition of a final exit
stream from a unit by mixing the
bypass stream and the unit exit
stream.
Fresh
Feed
Purge stream
A stream bled off from the process to
remove an accumulation of inerts or
unwanted material that might
otherwise build up in the recycle Fresh
Feed
stream.
Process
Mixer
Divider
Recycle
Process
Mixer
Product
Divider
Separator
Purge
Product
Example 12.1 ; 7th ed. A Continuous crystallizer involving a recycle
stream (a process for the production of flake NaOH).
<3> H2O
<1> 10000 lb/hr
40 wt.% NaOH
<2>
Evaporator
<4>
50 wt.%
NaOH
Crystallizer
and Filter
95 wt.% NaOH Filter
cake <5>
5 wt.% solution (45
wt.% NaOH)
Crystallizer
and Filter
95 wt.% NaOH Filter
cake <4>
5 wt.% solution (45
wt.% NaOH)
Recycle
<6> 45 wt.% NaOH
<2> H2O
<1> 10000 lb/hr
40 wt.% NaOH
Evaporator
<3>
50 wt.%
NaOH
<5> 45 wt.% NaOH
<3> H2O
<1> 10000 lb/hr
40 wt.% NaOH
<2>
Evaporator
<4>
50 wt.%
NaOH
Crystallizer
and Filter
95 wt.% NaOH Filter
cake <5>
5 wt.% solution (45
wt.% NaOH)
Recycle
<6> 45 wt.% NaOH
<3> H2O
<1> 10000 lb/hr
40 wt.% NaOH
<2>
Crystallizer
<4>
Evaporator 50 wt.%
and Filter
OverallNaOH
System
Recycle
<6> 45 wt.% NaOH
<5>
95 wt.% NaOH Filter
cake
5 wt.% solution (45
wt.% NaOH)
<3> H2O
<1> 10000 lb/hr
40 wt.% NaOH
Overall System
<5>
97.25 wt.% NaOH
Nsp = 2; NS = 3
NU = 3(2+1) – (3+2+2) = 2
NE = 1 + 1 = 2
ND = 2 – 2 = 0
Basis :
1 hr
<1>
<2>
Component mass frac. mass mass frac. mass
NaOH
0.4
4000
H2O
0.6
6000
total
1.0
10000
<3>
<4>
mass frac.
mass
mass frac. mass
0
0
0.5
1 5886.8895
0.5
1 5886.8895
1.0
<5>
<6>
mass frac.
mass
mass frac. mass
0.9725 4000.0000
0.45
0.0275 113.1105
0.55
1.0000 4113.1105
1.00
Nsp = 2; NS = 3
NU = 3(2+1) – (3+2+2) = 2
NE = 1 + 1 = 2
ND = 2 – 2 = 0
<4>
50 wt.%
NaOH
Crystallizer
and Filter
<5> 4113.1105 lb
97.25 wt.% NaOH
Recycle
<6> 45 wt.% NaOH
<4>
<5>
<6>
mass frac. mass mass frac.
mass
mass frac. mass
0.5
21491
0.9725 4000.0000
0.45 17491.00
0.5
21491
0.0275 113.1105
0.55 21377.89
1.0 42982.01
1.0000 4113.1105
1.00 38868.89
0.000000
0.000000
0.000000
<3> H2O
<1> 10000 lb/hr
40 wt.% NaOH
<2>
Evaporator
<4>
50 wt.%
NaOH
Crystallizer
and Filter
95 wt.% NaOH Filter
cake <5>
5 wt.% solution (45
wt.% NaOH)
Recycle
<6> 45 wt.% NaOH
Basis :
1 hr
<1>
<2>
<3>
<4>
<5>
<6>
Component mass frac. mass mass frac. mass mass frac.
mass
mass frac. mass mass frac.
mass
mass frac. mass
NaOH
0.4
4000
0.44 21491.00
0
0
0.5
21491
0.9725 4000.0000
0.45 17491.00
H2O
0.6
6000
0.56 27377.89
1 5886.8895
0.5
21491
0.0275 113.1105
0.55 21377.89
total
1.0
10000
1.00 48868.89
1 5886.8895
1.0 42982.01
1.0000 4113.1105
1.00 38868.89
<2> H2O
<1>
40 wt.% NaOH
Evaporator
<3>
50 wt.%
NaOH
Crystallizer
and Filter
95 wt.% NaOH Filter
cake <4> 4113.1105 lb
5 wt.% solution (45
wt.% NaOH)
<5> 45 wt.% NaOH
<2> H2O
Nsp = 2; NS = 4
NU = 4(2+1) – (3+2+2+2) = 3 <1> 10000 lb/hr
NE = 1 + 1 = 2
40 wt.% NaOH
ND = 3 – 2 = 1
Overall System
<4> 4113.1105 lb
97.25 wt.% NaOH
<5> 45 wt.% NaOH
<3>
50 wt.%
NaOH
Crystallizer
and Filter
<4> 4113.1105 lb
97.25 wt.% NaOH
<5> 45 wt.% NaOH
Nsp = 2; NS = 3
NU = 3(2+1) – (2+2+3) = 2
NE = 1 + 1 = 2
ND = 2 – 2 = 0
<2> H2O
<1>
40 wt.% NaOH
Evaporator
<3>
50 wt.%
NaOH
Crystallizer
and Filter
95 wt.% NaOH Filter
cake <4> 4113.1105 lb
5 wt.% solution (45
wt.% NaOH)
<5> 45 wt.% NaOH
Basis :
1 hr
<1>
Component mass frac.
NaOH
0.4
H2O
0.6
total
1.0
<2>
<3>
mass mass frac. mass mass frac.
21491.0
0.00
0.00
0.5
32236.5
1.00 10745.50
0.5
53727.5
1.00 10745.50
1.0
<4>
<5>
mass mass frac.
mass mass frac.
mass
21491.0
0.9725 4000.0000
0.45 17491.0026
21491.0
0.0275 113.1105
0.55 21377.8920
42982.0
1.0000 4113.1105
1.00 38868.8946
0.00000
0.00000
0.00000
Example 12.2 ; 7th ed. Recycle in a process in which a reaction occurs
<1> Fresh feed
C6H6
H2
<2>
Reactor
<3>
Separator
<4>
C6H6
H2
C6H12
Recycle
<5>
22.74 mol% C6H6
77.26 mol% H2
Cyclohexane (C6H12) can be made by the reaction of benzene (C6H6) with
hydrogen according to the following reaction:
C6H6 + 3H2  C6H12
where overall conversion of benzene is 95%, and the single pass conversion is
20%. In the fresh feed hydrogen is 20% excess, and the composition of the
recycle stream is 22.74 mol% benzene and 77.26 mol% hydrogen. Determine
the ratio of the recycle stream to the fresh feed.
Basis : 100 mol C6H6 in the fresh feed.
<1> Fresh feed
100 mol C6H6
360 mol H2
<2>
Reactor
<3>
Separator
<4>
C6H6
H2
C6H12
Recycle
<5>
22.74 mol% C6H6
77.26 mol% H2
Element balance (C and H)
Nsp = 2; NS = 2
NU = 2(2+1) – (3) = 3
NE = 1 + 1 + 1 = 3
ND = 3 – 3 = 0
<1> Fresh feed
100 mol C6H6
360 mol H2
Overall System
<4>
C6H6
H2
C6H12
<1> Fresh feed
600 mol C
1320 mol H
Overall System
<4>
600 mol C
1320 mol H
C6H6 in <4> = (1-0.95). C6H6 in <1> = 0.05 . 100 = 5 mol (or 30 mol C from C6H6 in <4>)
mol C from C6H12 in <4> = 600 – 30 = 570 mol (or 570/6 mol as C6H12)
mol H from C6H6 and C6H12 in <4> = 5.6 + (570/6).12 = 30 + 1140 = 1170 mol, thus there is H2 in <4>.
mol H from H2 in <4> = 1320 – 1170 = 150 (or 150/2 mol as H2)
No. Component
1 C6H6
2
H2
3
C6H12
total
MW
mol frac.
78
0.217
<1>
mol
100
mass
7800
mol frac.
0.0286
<4>
mol
5
mass
390
2
0.783
360
720
0.4286
75
150
84
0
1.000
0
460
0
8520
0.5429
1.0000
95
175
7980
8520
Nsp = 3; NS = 3
NU = 3(3+1) – (4+3) = 5
NE = 1 + 2 + 1 + 1 = 3
ND = 5 – 5 = 0
<3>
C6H6
H2
C6H12
<4>
5 mol C6H6
75 mol H2
95 mol C6H12
Separator
Recycle
total of mol
fraction in
<3> = 1
mol frac.
0.2174
<5>
22.74 mol% C6H6
77.26 mol% H2
C6H6 / C6H12 = (1-0.2)/0.2
<3>
mol
mass
mol frac.
760.000 59280.0000
0.0286
<4>
mol
5
mass mol frac.
390
0.2274
<5>
mol
mass
755.000 58890.0000
0.7554 2640.141 5280.2814
0.4286
75
150
0.7726 2565.141 5130.2814
0.0272
95.000 7980.0000
1.0000 3495.141 72540.2814
0.5429
1.0000
95
175
7980
8520
0
0
0
1.0000 3320.141 64020.2814
<1> Fresh feed
C6H6
H2
<2>
<3>
Reactor
<4>
C6H6
H2
C6H12
Separator
Recycle
<5>
22.74 mol% C6H6
77.26 mol% H2
No. Component
1 C6H6
2
H2
3
C6H12
total
MW mol frac.
78
0.217
<1>
mol
100
mass mol frac.
7800
0.2252
<2>
mol
475.000
Conversion =
0.2
<Reaction>
mass
mol frac.
mol
mass mol frac.
37050.000
-1
-95
-7410
0.2083
<3>
mol
mass
mol frac.
380.000 29640.0000
0.0286
<4>
mol
5
mass mol frac.
390
0.2274
2
0.783
360
720
0.7748 1634.077
3268.153
-3
-285
-570
0.7396 1349.077 2698.1530
0.4286
75
150
84
0
1.000
0
460
0
8520
0.0000
0.000
1.0000 2109.077
0.000
40318.153
1
95
7980
0.0521
95.000 7980.0000
1.0000 1824.077 40318.1530
0.5429
1.0000
95
175
7980
8520
<5>
mol
mass
375.000 29250.0000
0.7726 1274.077
2548.153
0
0
0
1.0000 1649.077 31798.1530
Example 12.5 ; 7th ed. Bypass calculations.
<3> Bypass
<1> 100 kg/hr
Butane-free
gasoline feed
80 wt.% n-C5H12
20 wt.% i-C5H12
<2>
isopentane
tower
<4>
100 wt.% n-C5H12
<6> to natural gasoline plant
90 wt.% n-C5H12
10 wt.% i-C5H12
<5> iso-pentane side stream
100 wt.% i-C5H12
In the feedstock preparation of a plant manufacturing natural gasoline, iso-pentane
is removed from butane-free gasoline as shown in the schematic diagram. What
fraction of butane-free gasoline is passed through the iso-pentane tower?
<3> Bypass
<1> 100 kg/hr
Butane-free
gasoline feed
80 wt.% n-C5H12
20 wt.% i-C5H12
<2>
isopentane
tower
<4>
100 wt.% n-C5H12
<5> iso-pentane side stream
100 wt.% i-C5H12
<1> 100 kg/hr
Butane-free
gasoline feed
80 wt.% n-C5H12
20 wt.% i-C5H12
Overall
system
<6> to natural gasoline plant
90 wt.% n-C5H12
10 wt.% i-C5H12
Nsp = 2; NS = 3
NU = 3(2+1) – (3+2+2) = 2
NE = 1 + 1 = 2
ND = 2 – 2 = 0
<6> to natural gasoline plant
90 wt.% n-C5H12
10 wt.% i-C5H12
<5> iso-pentane side stream
100 wt.% i-C5H12
<1> 100 kg/hr
Butane-free
gasoline feed
80 wt.% n-C5H12
20 wt.% i-C5H12
Basis :
Overall
system
<6> to natural gasoline plant
90 wt.% n-C5H12
10 wt.% i-C5H12
<5> iso-pentane side stream
100 wt.% i-C5H12
1 hr
Component
n-C5H12
i-C5H12
total
<1>
<2>
mass frac. mass mass frac. mass
0.8
80
0.2
1.0
20
100
<3>
mass frac. mass
0.8
<4>
mass frac.
mass
1
0.2
1
0
1.0
<5>
<6>
mass frac.
mass
mass frac.
mass
0
0
0.9
80.00
0
1
1
11.11
11.11
0.1
1.00
8.89
88.89
<3> Bypass
<1> 100 kg/hr
Butane-free
gasoline feed
80 wt.% n-C5H12
20 wt.% i-C5H12
<2>
isopentane
tower
<4>
100 wt.% n-C5H12
<5> iso-pentane side stream
100 wt.% i-C5H12
<3>
80 wt.% n-C5H12
20 wt.% i-C5H12
<4>
100 wt.% n-C5H12
Mixer
<6> to natural gasoline plant
90 wt.% n-C5H12
10 wt.% i-C5H12
Nsp = 2; NS = 3
NU = 3(2+1) – (2+2+3) = 2
NE = 1 + 1 = 2
ND = 2 – 2 = 0
<6> 88.89 kg
90 wt.% n-C5H12
10 wt.% i-C5H12
<3>
80 wt.% n-C5H12
20 wt.% i-C5H12
<4>
100 wt.% n-C5H12
Basis :
<6> 88.89 kg
90 wt.% n-C5H12
10 wt.% i-C5H12
Mixer
1 hr
Component
n-C5H12
i-C5H12
total
<1>
<2>
<3>
<4>
<5>
<6>
mass frac. mass mass frac. mass mass frac. mass mass frac.
mass mass frac.
mass
mass frac.
mass
0.8
80
0.80
44.44
0.8
35.56
1
44.44
0
0
0.9
80.00
0.2
1.0
20
100
0.20
1.00
11.11
55.56
0.2
1
8.89
44.44
0
1.0
0
44.44
1
1
11.11
11.11
0.1
1.00
8.89
88.89
Example 12.6 ; 7th ed. Purge calculations.
<7> Recycle
<5>
<1> Fresh feed
100 mol/hr
67.3 mol% H2
32.5 mol% CO
0.2% mol% CH4
<2>
Reactor
<3>
Separator
<6> Purge
H2
CO
3.2 mol% CH4
<4> 100 mol% CH3OH
Considerable interest exits in the conversion of coal into more convenient liquid products for
subsequent ptoduction of chemical. Two of the main gases that can be generated under suitable
conditions from insitu (in the ground) coal combustion in nthe presence of steam (as occurs
naturally in the presence of ground water are H2 and CO. After cleanup, these two gases can be
combined to yield methanol according to the following equation
CO + 2H2  CH3OH
Some CH4 that enters the process does not participate in the reaction. A purge stream is used to
maintain the CH4 concentration in the exit from the separator at no more than 3.2 mol%, and
prevent hydrogen buildup as well. The once-through conversion of the CO in the reactor is 18%.
Compute the moles of recycle, CH3OH, and purge per mole of feed, and also compute the purge
gas composition.
<6> Purge
H2
CO
3.2 mol% CH4
<7> Recycle
<5>
<1> Fresh feed
100 mol/hr
67.3 mol% H2
32.5 mol% CO
0.2% mol% CH4
<2>
Reactor
<3>
Separator
<4> 100 mol% CH3OH
<1>
H2
CO
CH4
total
frac.
0.673
0.325
0.002
1.000
mol
67.3
32.5
0.2
100
total
mol
32.7
32.5
135.4
200.6
frac.
0.163
0.162
0.675
1.000
total
mol
100
100
400
600
frac.
0.167
0.167
0.667
1.000
C
O
H
<4>
frac.
CH3OH
total
1
1
mol
100
100
C
O
H
<6> Purge
H2
CO
3.2 mol% CH4
<7> Recycle
<5>
<1> Fresh feed
100 mol/hr
0.163 C
0.162 O
0.675 H
<2>
<1> Fresh feed
100 mol/hr
0.163 C
0.162 O
0.675 H
Reactor
<3>
Separator
<6>
H2
CO
3.2 mol% CH4
Overall
system
<4>
0.167 C
0.167 O
0.667 H
Nsp = 3; NS = 3
NU = 3(3+1) – (4+3) = 5
NE = 1 + 2 + 1 + 1 = 5
ND = 2 – 2 = 0
<4>
0.167 C
0.167 O
0.667 H
total of mol
fraction in
<6> = 1
mol CH4 in <1> =
mol CH4 in <6>
<1>
H2
CO
CH4
total
frac.
0.673
0.325
0.002
1.000
mol
67.3
32.5
0.2
100
total
mol
32.7
32.5
135.4
200.6
frac.
0.163
0.162
0.675
1.000
total
mol
100
100
400
600
frac.
0.167
0.167
0.667
1.000
frac.
C
O
mol
1.450
1.250
H
10.400
C
O
H
<4>
frac.
CH3OH
total
1
1
mol
100
100
C
O
H
<6>
CO
H2
CH4
total
Elements
C
O
H
total
frac.
0.200
0.768
mol
1.250
4.800
0.032
1.000
0.200
6.25
MW
12
16
1
y
0.163
0.162
0.675
1.000
total
<1>
mol
32.7
32.5
135.4
200.6
mass
392.4
520
135.4
1047.8
y
0.167
0.167
0.667
1.000
<4>
mol
mass
31.250 375.000
31.250 500.000
125.000 125.000
187.5 1000.000
y
0.111
0.095
0.794
1.000
<6>
mol
1.450
1.250
10.400
13.100
mass
17.400
20.000
10.400
47.800
Nsp = 4; NS = 3
NU = 3(4+1) – (5+4) = 6
NE = 1 + 3 + 1 + 1 = 5
ND = 2 – 2 = 0
total of mol
fraction in
<3> = 1
<5>
0.768 H2
0.2 CO
0.032 CH4
<3>
H2
CO
CH4
CH3OH
Separator
<4> 31.25 mol
100 mol% CH3OH
CO/CH3OH = (1-0.18)/0.18
y
0.657
0.171
0.027
<3>
mol
mass
142.361 284.722
37.073 1038.050
5.932
94.907
0.144
1.000
31.250 1000.000
216.616 2417.679
<4>
mol
y
0
0
0
1
1
0
0
0
y
0.768
0.200
0.032
<5>
mol
mass
142.361 284.722
37.073 1038.050
5.932
94.907
31.250 1000.000
31.250 1000.000
0
1.000
0
0
185.366 1417.679
mass
0
0
0
<2>
y
0.756
0.216
0.028
<2>
mol
mass
609.167 1218.333
173.611 4861.111
22.778 364.4444
0
0
0
1.000 805.5556 6443.889
Reactor
Conversion =
<Reactor>
stoic.
mol
-2 -62.500
-1 -31.250
0
0
1
31.250
<3>
0.18
mass
-125
-875
0
y
0.736
0.192
0.031
<3>
mol
mass
546.667 1093.333
142.361 3986.111
22.778 364.444
1000
0.042
1.000
31.250 1000.000
743.056 6443.889