CHE2162: Assignment 1
TOTAL = 40 Marks (13% of the unit)
DUE DATE: 9th September 2022, 11:55 pm
Part A
Ammonia (NH3) is an important feedstock to produce fertilisers. It is generally produced via the HaberBosch process, causing substantial CO2 emissions. A team at Monash University has recently developed
an alternative process in which ammonia can be formed directly from nitrogen and hydrogen via an
electrochemical process. The hydrogen feed comes from a renewable source. A prototype plant for the
electrochemical production of ammonia is being built at Monash. The following paragraph describes
the process:
A gas stream is entering an electrochemical cell used to produce ammonia from hydrogen and nitrogen
according to the following reaction:
𝑁2 + 3𝐻2 → 2 𝑁𝐻3
The fresh feed to the process is a mixture of nitrogen and hydrogen gas in a molar ratio of 1:3 at a
pressure of 12 bar. A recycling gas stream originating from a condenser is mixed with the fresh feed
and added to the cell. Most of the hydrogen and nitrogen gas entering the electrochemical cell is
converted to ammonia. There are no side reactions. The system is operating in steady state.
A stream of the solvent tetrahydrofuran (THF; C4H4O) is passed through the electrochemical cell at a
flow rate of 8 L/min. As the THF passes through the cell it dissolves all the ammonia produced in the
reaction. At the same time, it is saturated with nitrogen and hydrogen gas. The molar concentrations of
nitrogen and hydrogen gas dissolved in the THF stream leaving the electrochemical cell are 25 mM and
75 mM, respectively. Assume that all nitrogen and hydrogen entering the electrochemical cell either
react electrochemically or leave the cell as dissolved gasses in the outgoing THF stream. The THF
stream carrying the ammonia and unreacted gases leaving the electrochemical cell then enters a
degassing unit operated at a lower pressure (0.10 bar). The liquid stream leaving the degassing unit is
recycled back to the electrochemical cell, which consists mostly of THF with a small residue of
ammonia and no other components. The gas outlet that carries all N2, H2 and NH3 (assume no THF
being present in the gas stream) is compressed back to 12 bar and sent to a condenser unit. The
condenser produces liquid ammonia at -25°C and a gas stream which is recycled back to the mixer
immediately before the electrochemical cell, where it is mixed with the fresh feed before entering the
electrochemical cell. The gas stream leaving the condenser is in equilibrium with the condensed
ammonia. The process produces liquid ammonia at a rate of 104 kg/s. The electrochemical cell and the
degassing unit operate at a temperature of 20°C.
Additional Information:
● The concentration unit 1 mM = 1 mmol/L
● You may use Antoine Equation to estimate the saturation vapor pressure of ammonia at -25°C.
Answer the following questions:
a) Draw a process flow diagram, indicating all process streams and unknown variables.
[5 marks]
b) Calculate the molar flow rate of the fresh feed stream.
[2 marks]
c) What is the composition of the recycle gas stream leaving the condenser?
[3 marks]
d) Determine the composition (mol fractions) of the gas stream leaving the degassing unit.
[6 marks]
e) In a separate experiment to measure the solubility of ammonia in THF at gas-liquid
equilibrium, you find that the vapour pressure of ammonia at 0.15 M ammonia solution in
THF at 20°C is 0.12 bar. Use this information to estimate the concentration of ammonia in the
THF stream leaving the degassing station, assuming it is in equilibrium with the gas stream
leaving the degassing unit. Ignore any volume expansions of the THF solutions due to the
dissolution of gasses.
[6 marks]
f) Estimate the concentration of ammonia in the THF stream leaving the electrochemical cell.
Ignore any volume expansions of the THF solutions due to the dissolution of gasses.
[4 mark]
Part B
Ammonium sulphate is used commonly in fertilizers for alkaline soil. It is a good source of N for the
plants (containing 21wt% N) and also useful for pH balance. Other uses of ammonium sulphate are as
a food additive and treatment of drinking water in combination with chlorine.
Gaseous ammonia and 50 wt% aqueous sulphuric acid solution are fed to a reactor in stoichiometric
proportions where they react completely to form aqueous ammonium sulphate:
2NH3 + H2SO4→ (NH4)2SO4
The aqueous ammonium sulphate solution leaving the reactor is mixed with a recycle stream coming
from a crystalliser. The mixed stream passes through an evaporator where some of the water is removed.
The concentrated solution leaving the evaporator is saturated at 80°C and passes through a crystalliser
and filter unit which produce anhydrous (NH4)2SO4 crystals at 10°C. The saturated (NH4)2SO4 solution
at 10°C leaving the crystalliser/filter is recycled to be mixed with reactor product stream. The wet filter
cake, containing 95 wt% (NH4)2SO4 crystals and the remaining saturated (NH4)2SO4 solution at 10°C
is sent to a dryer where all of the water is removed.
Taking a basis of ammonia produced in Part A of this assignment, answer the following questions –
(a) The mass flow rate of the recycle stream (kg/h).
[4 marks]
(b) Mass flow rate of the water evaporated in the evaporator and the dryer (kg/h).
[4 marks]
(c) If the water leaving the evaporator is condensed to be mixed with a concentrated sulphuric acid
solution to make the feed solution of 50 wt% sulphuric acid what would be the mass
composition of the concentrated sulphuric acid solution. Comment on why it is important to
recycle the water leaving the evaporator.
[6 marks]
Solubility Data of (NH4)2SO4 in water :
T (°C) Solubility (g/100mL water)
0 70.6
10 73.0
20 75.5
30 78.1
40 81.2
50 84.5
60 87.4
80 94.1
100 103.8
Marking criteria: Your solutions are expected to satisfy the following criteria:
1. Schematic. Draw a useful diagram of the system. Symbols, units, flows of materials/energy and
other relevant properties should be defined on the diagram.
2. Define the problem. State what you will determine, e.g. mass flow of product stream.
3. Assumptions. List and justify assumptions and simplifications.
4. Data. Clearly state data needed for calculations, including units (reference source where necessary)
5. Analysis. Define appropriate equations, define all symbols.
6. Calculation. After completely developing the analysis in symbols, substitute numerical values and
calculate results. Clearly show and explain working. Final answer should be stated clearly, with
correct units and significant figures.
7. Interpretation & Reflection. Evaluate your results. How reliable, realistic are they? Are they
constrained by assumptions and simplifications? How could this be addressed? What are the most
important processes, based on your calculations? Can you make recommendations on design or
operational parameters? This is to be a concise statement – no more than a paragraph.

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