Dissolving substances in liquids requires the consideration of the solution enth
ID: 702503 • Letter: D
Question
Dissolving substances in liquids requires the consideration of the solution enthalpy (or "heat of solution"). ???(T,r) is the change in enthalpy for a process in which 1 mole of a solute is dissolved in r moles of a liquid solvent at a constant temperature T. If ??, is negative, the solution process is exothermic. This means that, if the process is adiabatic, the temperature will ncrease In a continuous adiabatic process, 100 kg/h of an aqueous anmonia solution with 3.0 wt% concentration is produced at 1 atm. The feed consists of a pure ammonia (gas) stream and a pure water stream, both at 25° C. a) Explain in 2-3 sentences what the molecular origin of the heat of solution is. b) Work out the mass balance in mass and molar terms and draw a completely labelled flow diagram. What is the value of r? c) Due to the heat of solution, the temperature of the mixture Tmix will not be the same as the mixture component temperature (25° C), unless heat is supplied/removed to keep the process at constant temperature. Use UniSim to find the temperature of the mixture. Apply the UNIQAC and Antoine models for the liquid and vapour phases, respectively (see the week 9 self-study material for instructions). You should find that the mixed stream is a liquid stream. What is its temperature Tmix? d) Using Tmix from c) and assuming the heat capacity Cp 4.2 kJ/(kg K) for the mixture calculate the heat of solution ??.(25°C) (as kJ//mol ammonia). Start by writing the energy 12] e) If the product specification is changed to 4.0 wt% ann monia. UniSim will show that the mixed stream is a liquid/vapour mixture. Which vapour fraction does UniSim compute? What is the temperature now? What could be the reason for the occurrence of the vapour balance for this problem. phase?Explanation / Answer
(a) Heat of solution involves the energy released due to solute-solute molecule bond breaking, solvent-solvent molecule bond breaking in vicinity of solute molecules. The energy interaction associate with solute -solvent interaction (due to different size, charge and polarity).
(b) Mol wt of Ammonia = 17 g/mol and water = 18 g/mol
(c), (d), (e) need the UniSim simulator.