Abstract
The evaluation of a new air water generator (AWG) based on absorption and reverse osmosis is described. For the evaluation, an aqueous lithium bromide solution has been selected from a wide range of liquids as the absorbent. At high salt mass fractions, the aqueous lithium bromide solution has a low vapour pressure and a high osmotic pressure. The low vapour pressure ensures that the water vapour can be absorbed from the air, but the high osmotic pressure leads to high pressures over the membrane. Due to the high osmotic pressures, several reverse osmosis membrane modules are necessary and salt solution has to be present on both sides of the membrane, which leads to an additional inlet on the permeate side. Models for the absorber, the reverse osmosis membrane module and the complete multi-stage reverse osmosis system have been developed in Python. The model of the complete system has then been used to simulate the performance of the AWG at different boundary conditions. The simulations have shown that based on the defined assumptions, extracting water from the air with absorption and reverse osmosis is possible and that the energy demand per litre of pure water is similar to AWG systems which use condensation.
Highlights
Water scarcity has been a global problem for many decades
Reverse osmosis If the solvents are to be moved from the concentrated solution through the membrane into the less concentrated solution, this can be achieved by applying a pressure that is greater than the osmotic pressure Π
This is a conflict of interest, because the higher the mass fraction of the solute, the better the water vapour can be absorbed from the air, and the lower, the better the water can be recovered by reverse osmosis
Summary
Water scarcity has been a global problem for many decades. Due to population growth and climate change, competition for access to fresh water is expected to increase further. In many regions affected by water shortages, large facilities for the provision of sufficient quantities of clean water do not exist and permanent transport is not possible. Another possible source, which has not yet been truly exploited, is the water that is stored in the atmosphere. The earth’s atmosphere contains such a large amount of water vapour that the liquid state of the water results in a volume of about 13’000 km, which is about one seventh of the volume of fresh water on the earth’s surface [4]. With a hygroscopic aqueous solution, very probably a salt solution, water has first to be extracted from ambient air and with reverse osmosis instead of desorption the water has to be expulsed again
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