CHE2162: Assignment 1
\nTOTAL = 40 Marks (13% of the unit)
\nDUE DATE: 9th September 2022, 11:55 pm
\nPart A
\nAmmonia (NH3) is an important feedstock to produce fertilisers. It is generally produced via the Haber\u0002Bosch process, causing substantial CO2 emissions. A team at Monash University has recently developed
\nan alternative process in which ammonia can be formed directly from nitrogen and hydrogen via an
\nelectrochemical process. The hydrogen feed comes from a renewable source. A prototype plant for the
\nelectrochemical production of ammonia is being built at Monash. The following paragraph describes
\nthe process:
\nA gas stream is entering an electrochemical cell used to produce ammonia from hydrogen and nitrogen
\naccording to the following reaction:
\n\ud835\udc412 + 3\ud835\udc3b2 \u2192 2 \ud835\udc41\ud835\udc3b3
\nThe fresh feed to the process is a mixture of nitrogen and hydrogen gas in a molar ratio of 1:3 at a
\npressure of 12 bar. A recycling gas stream originating from a condenser is mixed with the fresh feed
\nand added to the cell. Most of the hydrogen and nitrogen gas entering the electrochemical cell is
\nconverted to ammonia. There are no side reactions. The system is operating in steady state.
\nA stream of the solvent tetrahydrofuran (THF; C4H4O) is passed through the electrochemical cell at a
\nflow rate of 8 L\/min. As the THF passes through the cell it dissolves all the ammonia produced in the
\nreaction. At the same time, it is saturated with nitrogen and hydrogen gas. The molar concentrations of
\nnitrogen and hydrogen gas dissolved in the THF stream leaving the electrochemical cell are 25 mM and
\n75 mM, respectively. Assume that all nitrogen and hydrogen entering the electrochemical cell either
\nreact electrochemically or leave the cell as dissolved gasses in the outgoing THF stream. The THF
\nstream carrying the ammonia and unreacted gases leaving the electrochemical cell then enters a
\ndegassing unit operated at a lower pressure (0.10 bar). The liquid stream leaving the degassing unit is
\nrecycled back to the electrochemical cell, which consists mostly of THF with a small residue of
\nammonia and no other components. The gas outlet that carries all N2, H2 and NH3 (assume no THF
\nbeing present in the gas stream) is compressed back to 12 bar and sent to a condenser unit. The
\ncondenser produces liquid ammonia at -25\u00b0C and a gas stream which is recycled back to the mixer
\nimmediately before the electrochemical cell, where it is mixed with the fresh feed before entering the
\nelectrochemical cell. The gas stream leaving the condenser is in equilibrium with the condensed
\nammonia. The process produces liquid ammonia at a rate of 104 kg\/s. The electrochemical cell and the
\ndegassing unit operate at a temperature of 20\u00b0C.
\nAdditional Information:
\n\u25cf The concentration unit 1 mM = 1 mmol\/L
\n\u25cf You may use Antoine Equation to estimate the saturation vapor pressure of ammonia at -25\u00b0C.
\nAnswer the following questions:
\na) Draw a process flow diagram, indicating all process streams and unknown variables.
\n[5 marks]
\nb) Calculate the molar flow rate of the fresh feed stream.
\n[2 marks]
\nc) What is the composition of the recycle gas stream leaving the condenser?
\n[3 marks]
\nd) Determine the composition (mol fractions) of the gas stream leaving the degassing unit.
\n[6 marks]
\ne) In a separate experiment to measure the solubility of ammonia in THF at gas-liquid
\nequilibrium, you find that the vapour pressure of ammonia at 0.15 M ammonia solution in
\nTHF at 20\u00b0C is 0.12 bar. Use this information to estimate the concentration of ammonia in the
\nTHF stream leaving the degassing station, assuming it is in equilibrium with the gas stream
\nleaving the degassing unit. Ignore any volume expansions of the THF solutions due to the
\ndissolution of gasses.
\n[6 marks]
\nf) Estimate the concentration of ammonia in the THF stream leaving the electrochemical cell.
\nIgnore any volume expansions of the THF solutions due to the dissolution of gasses.
\n[4 mark]
\nPart B
\nAmmonium sulphate is used commonly in fertilizers for alkaline soil. It is a good source of N for the
\nplants (containing 21wt% N) and also useful for pH balance. Other uses of ammonium sulphate are as
\na food additive and treatment of drinking water in combination with chlorine.
\nGaseous ammonia and 50 wt% aqueous sulphuric acid solution are fed to a reactor in stoichiometric
\nproportions where they react completely to form aqueous ammonium sulphate:
\n2NH3 + H2SO4\u2192 (NH4)2SO4
\nThe aqueous ammonium sulphate solution leaving the reactor is mixed with a recycle stream coming
\nfrom a crystalliser. The mixed stream passes through an evaporator where some of the water is removed.
\nThe concentrated solution leaving the evaporator is saturated at 80\u00b0C and passes through a crystalliser
\nand filter unit which produce anhydrous (NH4)2SO4 crystals at 10\u00b0C. The saturated (NH4)2SO4 solution
\nat 10\u00b0C leaving the crystalliser\/filter is recycled to be mixed with reactor product stream. The wet filter
\ncake, containing 95 wt% (NH4)2SO4 crystals and the remaining saturated (NH4)2SO4 solution at 10\u00b0C
\nis sent to a dryer where all of the water is removed.
\nTaking a basis of ammonia produced in Part A of this assignment, answer the following questions \u2013
\n(a) The mass flow rate of the recycle stream (kg\/h).
\n[4 marks]
\n(b) Mass flow rate of the water evaporated in the evaporator and the dryer (kg\/h).
\n[4 marks]
\n(c) If the water leaving the evaporator is condensed to be mixed with a concentrated sulphuric acid
\nsolution to make the feed solution of 50 wt% sulphuric acid what would be the mass
\ncomposition of the concentrated sulphuric acid solution. Comment on why it is important to
\nrecycle the water leaving the evaporator.
\n[6 marks]
\nSolubility Data of (NH4)2SO4 in water :
\nT (\u00b0C) Solubility (g\/100mL water)
\n0 70.6
\n10 73.0
\n20 75.5
\n30 78.1
\n40 81.2
\n50 84.5
\n60 87.4
\n80 94.1
\n100 103.8
\nMarking criteria: Your solutions are expected to satisfy the following criteria:
\n1. Schematic. Draw a useful diagram of the system. Symbols, units, flows of materials\/energy and
\nother relevant properties should be defined on the diagram.
\n2. Define the problem. State what you will determine, e.g. mass flow of product stream.
\n3. Assumptions. List and justify assumptions and simplifications.
\n4. Data. Clearly state data needed for calculations, including units (reference source where necessary)
\n5. Analysis. Define appropriate equations, define all symbols.
\n6. Calculation. After completely developing the analysis in symbols, substitute numerical values and
\ncalculate results. Clearly show and explain working. Final answer should be stated clearly, with
\ncorrect units and significant figures.
\n7. Interpretation & Reflection. Evaluate your results. How reliable, realistic are they? Are they
\nconstrained by assumptions and simplifications? How could this be addressed? What are the most
\nimportant processes, based on your calculations? Can you make recommendations on design or
\noperational parameters? This is to be a concise statement \u2013 no more than a paragraph.<\/p>\n","protected":false},"excerpt":{"rendered":"
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