Thermal cracking of ethane in tubular reactor

Thermal cracking of ethane in tubular reactor
‣ Ethylene demands – polyethylene, ethylene oxide,
ethylene glycol – 20 million tons per annum
‣ Main Reaction CH → CH + H
moles so steam as inert
2 6 2 4 2
increase in number of
‣ Endothermic reaction - ∆H 34.5 kcal/mol, high
temperature for high equilibrium conversions, increasing
temperatures along the length of the reactor
2C2H6 → CH
3 8
+ CH4
‣ Side reactions
yield of side products higher
higher conversion
Indian Institute of Science

Yield conversion diagram for ethane cracking
Indian Institute of Science

Ethane cracking reactor
Typical operating condn
•L = 95 m
•G = 68.68 kg/m2/s
•P inlet 2.99 atm, outlet 1.2 atm
•T inlet 680, outlet 820 C
•Production 10000 tons/coil
Indian Institute of Science

Balances
mass
dF
dz
j
=
R
j
4
2
t
2
dT 1 ⎡ πd
t
⎤
energy = ⎢qz ( ) πdt + ∑−( ∆Hi)
ri⎥
dz ∑FC
j pj ⎣
4 i ⎦
j
πd
dp ⎡2f ξ ⎤ du
ρ ρ
dz ⎣ d r ⎦ dz
2
momentum − = ⎢ + ⎥ fu +
fu
t
π
b i
∑
⎡RT
F' 1 j
u = =
⎢
A A⎢
p
⎢⎣
F
j
⎤
⎥
⎥
⎥⎦
Indian Institute of Science

Simulation of ethane reactor
Indian Institute of Science

Hydrogenation of oil
‣ Major demand – margarine, shortenings, vanaspati
‣ Vegetable oils – mixture of triglycerides - glycerol and
fatty acids
‣ Fatty acids – saturated (S) , monosaturated (cis, R1 and
trans, R2) and diunsaturated (B). Hydrogenation to
reduce odor or color, improve stability and increase
melting point.
‣ Product requirements – some polyunsaturated (health)
and R2 ( consistency and higher melting points)
Indian Institute of Science

Reactions
1/2
,
1 4 H 5 6
r −r ∝C r −r ∝C
H
2 2
implies selectivity of monounsaturates over saturates
proportional to (C H2 ) 1/2
Indian Institute of Science

Yield conversion diagrams
Indian Institute of Science

Balances
dC
j
mass = Rj
j = BR ,
1, R2,
M
dt
0
2
, b
( ) ( )
, ,
,
,
−ka C − C = R C C
dC
L v H g H s H j H s
H
2 2 2 2
( ) ( )
, , , ,
( )
, ,
1 1 1
= +
ka ka ka
L
dt
v
= ka C −C −ka C −C
L v H g H b S S H b H s
= ka C − C + R
S S H b H s H
2 2 2 2
2 2 2
L v S S
Indian Institute of Science

Stirred tank batch reactor
‣ Desired conversion –
batch time
‣ Desired production rate
and batch time – volume
‣ Based on volume –
internal design
‣ Cooling load
∑
i
( ) ( )
− Q = V −∆ H r = AUT −T
i i K r
Indian Institute of Science

Ammonia synthesis
‣ Major demand – Fertilizer, chemicals,
explosives, polyamides, pharmaceuticals; 150
million tons per annum
‣ Main reaction
1 3
N2 + H2 NH3,
∆ H298K
=−45.7 kJ/
mol −
2 2
‣ High pressure, low temperatures favorable
‣ Catalytic reaction – iron, promoted ruthenium
1
Indian Institute of Science

Ammonia synthesis – equilibrium
Ammonia Mol fraction
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
1
P = 300 atm
200
100
50
10
3
(A)
0.0
500 600 700 800 900
Temperature (K)
Ammonia Mol fraction
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
(B)
T = 473 K
523
573
623
673
723
773
0 100 200 300
Pressure (Atm)
Indian Institute of Science

Ammonia synthesis - balances
mass
dC
j
u ( , )
s
= ηRj Cj
T
dz
energy
dT 4U
ρ
fuc s p
= ( Tr
− T) + η( −∆H)
r
dz d
catalyst
1
2
r dr
'
2 i
' '
ie i i
'
1 d 2 dT
2 ⎜ r λe
⎟ Hr
r dr dr
t
d ⎛ dC ⎞
⎜ rD ⎟ =−R
C T
⎝ dr ⎠
⎛ ⎞ =∆
⎝ ⎠
( , )
Indian Institute of Science

Reactor simulation
mol fraction
0.7
0.6
0.5
0.4
0.3
0.2
0.1
N 2
H 2
NH 3
0.0
0 1 2 3 4 5
Length (m)
temperature
860
840
820
800
780
760
740
720
700
680
0 1 2 3 4 5
Length (m)
120
100
80
rate at bulk conditions
observed rate
rate
60
40
20
0
0 1 2 3 4 5
Length (m)
Indian Institute of Science

Optimal temperature
4//
/-8
34/
/-7
3//
/-6
24/
Dwsdms
/-5
/-4
/-3
2//
14/
1//
/-2
04/
/-1
0//
/-0
4/
34/ 4// 44/ 5// 54/ 6//
Sdl odq`stqd J
/
Indian Institute of Science

Fixed bed reactors
Indian Institute of Science

Ammonia – Multibed reactor
Conversion
1.0
0.8
0.6
0.4
0.2
r=0
r/T=0
Reactor 1
Reactor 2
Reactor 3
0.0
400 500 600 700 800 900
Temperature
Indian Institute of Science

Fixed bed reactors
Indian Institute of Science

Ammonia – Autothermal
300
1000
T top
-T 1L
DT
250
200
150
100
T top
-T feed
473
491
513
523
578
623
T feed
K
900
800
700
600
50
500
0
400 500 600 700 800 900 1000
T top
400
400 500 600 700 800 900 1000
T top
K
Indian Institute of Science

Ammonia – Autothermal
800
750
Conversion
700
650
600
550
reactor
heat exchanger
Conversion
0.8
0.7
0.6
0.5
0.4
0.3
0.2
r=0
∂r/∂T=0
500
0 1 2 3 4 5 6
Length
0.1
0.0
600 650 700 750 800 850 900
Temperature
Indian Institute of Science