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21
+ CH3OH
+ O2
+ CH4/O2
CH3NH2
(CH3)2NH
(CH3)3N
Methyl
amines
Acrylo
nitrile
Hexa methylene tetramine
Di-isopropyl amine
Nitric acid
HCN
+ CO2 + HNO3 + H3PO4 + H2SO4
Urea NH2CONH2
NH3 from synthesis gas (CO + H2)
Chemicals and fertilizers from ammonia (Moulijn et al.)
1. Name at least two chemicals that are produced from synthesis
gas?
2. Most of the synthesis gas today is produced from natural gas
by steam reforming it.
What is natural gas? What are its components?
Synthesis gas is produced from natural gas by reacting it with
steam. This reaction is called
steam reforming reaction. Although natural gas contains many
chemical compounds, we will
assume that it consists of only CH4. Steam reforming reactions
of CH4 occur over a Ni-based
catalyst. The reactions are as follows:
Main reaction: 4 + 2 + 3 2 = +206 /
+ 2 2 + 2 = 41 /
4 + 2 2 + 2 = +247 / Side reactions:
Ammonia NH3
MEA, DEA, TEA
Ammonium Nitrate, NH4NO3
Ammonium Phosphates, NH4H2PO2 (NH4)2HPO4, (NH4)3PO3
Ammonium Sulphate, (NH4)2SO4
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22
4 + 22 = 75
20 + 2 = 173 /
Natural gas also contains small amounts of H2S (< 1%). For
the Ni catalyst, H2S is a poison
and needs to be removed down to few hundred parts per million.
This removal is called
sweetening of natural gas. Which unit operation is recommended
for removing H2S from a
concentration of about, say, 10,000 ppm in natural gas to, say,
100 ppm?
__________________.
Which liquid do you recommend for the unit operation?
A) Water [low solubility for H2S]
B) Adsorption
C) Ethanol amines [basic in nature; reacts reversibly with H2S;
heating liberates H2S]
(Sourced from: Wikipedia)
Explain the working of the above natural gas sweetening
plant.
Coking
reactions:
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23
Lets return to the steam reforming reactions.
Thermodynamics favors ____________ pressures (high/low) and
_________ temperatures
(high/low) for the main reaction. N2 and H2 react at 100-250 bar
pressure to produce
ammonia. We have two options for the choice of pressure:
1. Produce synthesis gas at low pressures and compress it before
it enters NH3 reactor
2. Compress the natural gas, produce synthesis gas at high
pressures and feed it to the
NH3 reactor.
Obviously CO is removed before the reactor. Discuss the
implications of the above choices
on capital and operating costs.
Were in a similar situation as sulphuric acid manufacture: we
choose conditions NOT
favoured by thermodynamics.
In actual practice, excess steam, three times the theoretical
amount is added to natural gas for
steam reforming. By adding more steam,
CH4 conversion _____________________
Carbon formation ___________________
As steam reforming reactions are endothermic (net), heat needs
to be supplied. These
reactions are carried out at high temperatures, about 800 900
oC. Even at such
temperatures, a catalyst is required because CH4 is a stable
gas.
Study the flow sheet below and comment on the energy recovery
and energy integration. The
reformer actually consists of several tubes, shown as a dotted
line here. The tubes are heated
by a fuel supplied to the outside of the tubes. .
What is the fuel in the above figure?
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24
CH4 + steam
Synthesis gas
Fuel + air
Flue gas
CH4 + steam
Synthesis gas
Fuel + air
Flue gas
Air
Boiler water
Steam
Natural gas
Superheated steam
Exhaust gas to stack
Synthesis gas
Select one of the options for synthesis gas reactor. Justify
your selection with cogent
arguments.
A) Multi tubular reactor 500-600 tubes, each tube 7 to 12 meters
long, 3 to 5 inches in
diameter.
B) Several huge bed of catalyst (in parallel) heated outside by
a fuel. Only 1 bed is
shown below. Diameter = few meters; height = 10-15 meters
Schematic diagram of reformer (adapted from Felder and
Rousseau)
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25
Typically, about 90-95% conversion of methane is achieved by
steam reforming. Ideally, we
want all the methane to be converted. For close to 100%
conversion, what can be done?
A) Increase temperature [high temperature may be harmful to the
catalyst]
B) Have longer catalyst filled tubes [its an equilibrium limited
reaction]
C) Use higher H2O/ CH4 ratios
D) Reform the remaining CH4 by adding air to a second reformer
to which effluents of
primary reformer are fed. [ammonia needs N2]
In addition to reaction in previous discussion, the following
reactions will also occur when air
is introduced.
4 +1
22 + 2 2 = 36 /
4 + 22 + 22 = 803 /
+1
22 2 = 284 /
2 +1
22 2 = 242 /
1. What is a downside of the secondary reformer?
2. Write all the compounds present in gases exiting secondary
reformer?
3. What are goals of the next few steps to prepare the feed for
NH3 synthesis reactor?
4. CO2 gas can be absorbed in liquid solvents but not CO. Why?
(Find an answer to this
question). Refer to the reactions in previous pages and suggest
a method to remove
CO.
Draw a block diagram for the ammonia process discussed so far.
Apparently this diagram is
incomplete. We will build it further as we move along.
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26
Water gas shift reaction:
+2 2 + 2 = 41 /
Low temperatures favor the shift reaction. In the past,
catalysts were active > 600 K.
More active catalysts at 500 K were developed. The water-gas
shift reaction is carried out in
two adiabatic reactors: in the first high temperature (HT)
reactor, ironoxide is used as the
catalyst; in the second low temperature (LT) reactor, copper
based catalysts are used.
0
100
200
300
400
500
600
700
400 450 500 550 600 650 700 750 800
Kp
Temperature, K
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27
Syn gas
Shifted gas
High temperature reactor
Low temperature reactor
640 K
710 K
490 K
510 K
Why do you think two reactors are used instead of one large
reactor with superior LT
catalyst?
Again, note that exothermic reversible reactions are carried out
at high temperatures. What is
the implication of high pressure on reactor size? [these
reactors are operated almost at the
same pressure as reformers].
Most of the CO is converted to CO2. The exit gas from LT reactor
contains about 0.3 mole %
CO. The exit gas from LT reactor is at about 500 K.
What is the unit operation precedes absorption of CO2 for
effective absorption?
CO2 is absorbed in hot K2CO3 solution at ~ 70 oC. This solution
is regenerated by heating it.
Complete the below flow chart used for absorption of CO2.
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28
steam
CO2 lean gas
K2CO3 + CO2
Gas from LT reactor
What are the components of gas stream leaving absorption
column?
CO and CO2 are poisonous to ironbased ammonia synthesis
catalyst. They need to be
removed to vanishing levels before synthesis gas can be fed to
NH3 reactor.
Which of the following is a good choice of CO and CO2 removal
from the gas exiting the
absorption system? Why?
A) Additional water-gas shift reaction and absorption
B) Reverse steam reforming reaction
+ 32 4 + 2 = 206 /
+ 42 4 + 22 = 165 /
Water is condensed from the gas exiting the methanation reactor.
Now, this gas contains N2,
H2, CH4, Ar. The first two react to make ammonia; the last two
are inerts.
NH3 production is a landmark in the manufacture of bulk
chemicals. It is the first chemical
produced at high pressure (>100 bar) and high temperature (~
700 K or 430 o
C). N2 is very
stable and inert, hence suitable catalyst needs to be developed
for NH3 synthesis. Haber
developed a commercial process during 1908 1913 in collaboration
with scientists and
engineers at BASF. The group at BASF tested a whopping 6500
catalysts before discovering
the correct iron based catalysts. Haber received the Nobel Prize
for chemistry in 1919 for
ammonia synthesis.
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29
Thermodynamics:
2 + 32 23 = 91.44 /
Clearly ____________ temperatures and ____________ pressures are
favorable for NH3
production. However, ___________ temperatures are used in
practice because
_______________________
Typical reactor conditions:
Temperatures: 400 oC (inlet), 400 500
oC (outlet); pressures: 100 250 bar
Commercial NH3 synthesis reactors:
Few commercial reactors for NH3 synthesis are shown. Discuss
salient features of the
reactors and compare their designs.
0
5
10
15
20
25
30
35
40
45
50
200 250 300 350 400 450 500 550 600
% c
on
ver
sion
Temperature, oC
200
300industrial plants
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30
Why the feed gas is flowing through the annulus?
Why is quenching done?
What could be one disadvantage of indirect cooling in separate
heat exchanger to
produce the steam?
Typical conversion of N2/H2 is ~15-20 %, why such low
conversions?
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31
Whats immediately downstream of the reactor?
In H2SO4 production, cooling of exit gas and removal of SO3
ensured a high conversion of >
99.5%.
Could we use same principle in NH3 production? Both are
equilibrium limited reactions.
[Hint: feed gas for NH3 synthesis contains an inter gas CH4]
Draw block diagram below for NH3 synthesis.
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32
Study the flow-sheet for NH3 synthesis.
Purge gas from the plant contains significant H2. How can we
remove this H2?
A) Adsorption
B) Distillation
C) Membrane
D) Absorption