16 February 2019
Week 2- Group Discussion
Synthesis of Ammonia
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16 February 2019
Synthesis of Ammonia
Let’s design a chemical process
based on the chemical change of
N2 and H2 into ammonia, NH3.
(Ammonia used primarily as a
fertilizer). The chemical reaction is
N2 + 3H2  2NH3
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Now, let’s synthesis
production of ammonia using
5 key synthesis steps.
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But, before we start, what are
5 key synthesis steps?
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5 key Synthesis Step
Eliminate differences in molecular types
Distribute chemicals by matching sources and sinks
Eliminate differences in composition
Eliminate differences in temperature, pressure and
phase
Integrate tasks (combine tasks into unit operations)
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Synthesis Steps
Synthesis Step
 Process Operation
 Eliminate differences in
molecular types
 Chemical reaction
 Distribute chemicals by
matching sources and sinks
 Mixing/Recycle
 Eliminate differences in
composition
 Separation
 Eliminate differences in
 Temperature, pressure and
temperature, pressure and phase
phase change
 Integrate tasks (combine tasks
into unit operations)
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16 February 2019
Synthesis Steps
Synthesis Step
 Process Operation
 Eliminate differences in
molecular types
 Chemical reaction
 Distribute chemicals by
matching sources and sinks
 Mixing/Recycle
 Eliminate differences in
composition
 Separation
 Eliminate differences in
 Temperature, pressure and
temperature, pressure and phase
phase change
 Integrate tasks (combine tasks
into unit operations)
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But, before we start
synthesize, what are the
critical elements need to be
included?
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 Process Synthesis involves:
 Selection of processing mode: continuous or batch
 Fixing the chemical state of raw materials, products, and byproducts, noting the differences between them.
 Process operations (unit operations) - flowsheet building
blocks
 Synthesis steps Eliminate differences in molecular types (chemical
reaction)
Distribute chemicals by matching sources and sinks
(mixing & recycle)
Eliminate differences in composition (separation)
Eliminate differences in temperature, pressure and
phase
Integrate tasks (combine tasks into unit operations)
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Read the ammonia design
process article.
Develop a summary or
flowsheet on how the
ammonia plant was
developed.
(1 hour)
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Assumptions:
1) Conversion <100%
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Boiling points for the ammonia
process
Compund
NH3
H2
N2
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Boiling point at 1
atm, in C
-33
-253
-196
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Assumptions:
1) Conversion <100%
2) Separation of NH3 from N2 &
H2 is perfect
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If the gaseous mixture below -33C, NH3
will condense
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Assumptions:
1) Conversion <100%
2) Separation of NH3 from N2 &
H2 is perfect
3) What we need to do with N2
& H2 exiting condenser?
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Recycle
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Assumptions:
4) Assume air is source of N2
Compound
NH3
H2
N2
O2
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Boiling point at 1
atm, in C
-33
-253
-196
-183
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Thus, add condenser to
provide N2
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5) What is good source of H2?
Water ‘cracking’
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Still can be
improved
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Consider chemical energy
stored in fuels such as CH4
Each molecule of CH4 provides enough energy
to decompose 3.3 moles of H2O. (803kJ/mole
CH4 divided by 242 kJ/mole H2O equals 3.3
moles of H2O)
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As usual, not 100% conversion
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Crucial to remove O2 because it will ultimately react with
H2 (& perhaps N2) in the ammonia synthesis reactor.
The best way, supply CH4 in excess. (O2 will be limiting
reactant)
O2 eliminated, therefore, the requirement of
condenser is set by methane. Operate condenser
below -164C to condense methane.
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Another component need to
be removed, CO2. Why?
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Compare electrolysis &
burning CH4
1) CH4 process require additional unit
2) CH4 impurity enters the ammonia synthesis
reactor
3) We no longer produce O2 as by-product. We
instead consume some O2
Therefore, replace electrolytic reactor with
methane burner.
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PFD become:
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Drawback of recycle? How to
solve?
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Auxiliary equipment (splitter &
HEX)
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