Evaluation of a new air water generator based on absorption and reverse osmosis

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.

Modeling and Simulation of Either Co-Current or Countercurrent Operated Reverse-Osmosis-Based Air Water Generator

“…For this reason, a variety of different absorbents were evaluated, with aqueous salt solutions containing lithium bromide (LiBr), lithium chloride (LiCl), sodium hydroxide (NaOH), and potassium hydroxide (KOH) proving to be particularly effective. Lithium bromide was eventually chosen for the further investigations because it can attain the lowest water vapor partial pressures and has the lowest toxicity values [9].…”

“…Conventional reverse osmosis membrane modules have no inlet on the permeate side and can usually only be operated up to a maximal pressure of about 120 bar [10]. Since osmotic pressures of up to 2000 bar have to be overcome for salt solutions with salt mass fractions of up to w ≈ 0.5, Fill et al presented a configuration that still allows reverse osmosis to be used [9]. In order to reduce the osmotic pressure, an inlet is added on the permeate side, where a salt solution of lower concentration than in the feed is introduced.…”

“…With a salt solution, water is extracted from the ambient air and subsequently expelled from the aqueous salt solution using multiple reverse osmosis stages instead of desorption. Fill et al have shown the feasibility of water extraction from the ambient air using this combination [9]. The results in terms of specific energy demand were promising but roughly comparable to those of conventional processes.…”

“…The inlet mass flow rate of the solution and the inlet volume flow rate of the air are assumed to be constant and are calculated according to Appendices B and C of [9].…”