No abstract

The conventional working fluids used in absorption chillers (water/lithium bromide and ammonia/water) present several disadvantages that limit their effective application. Recently, some works have reported the addition of NaOH to the ammonia/water working pair to improve the separation of ammonia in the generator, reducing the chiller driving temperature by taking advantage of the common ion effect. However, the presence of NaOH in the absorber has a negative impact on the absorption process. This study analyzes the technical viability of separating NaOH from ammonia/water/NaOH mixtures by using reverse osmosis membranes to incorporate this separation method into future chiller designs that work with these mixtures. The concentration range analyzed covers the solution concentration values of interest for absorption chiller applications (approximate 0.02−0.05 mass fraction of NaOH and 0.3 mass fraction of NH3). The results obtained show that, by using an in-series configuration of the modules, reverse osmosis technology is suitable for separating NaOH from the ternary mixtures studied.

New simple and accurate correlations of the equilibrium properties and solution enthalpies for NH3/H2O/alkaline hydroxide mixtures are presented for use in the modeling and design of absorption chillers in a temperature range between 273.15 and 353.15 K. The correlations are used to calculate the P−T−x diagrams and show the effect of hydroxide on the liquid−vapor equilibrium. Finally, the correlations presented are used to estimate the improvement in the cycle performance in absorption chillers when NH3/H2O mixtures containing NaOH are used instead of the conventional NH3/H2O mixture. Results show that the addition of hydroxide can significantly decrease the generator temperature of ammonia/water absorption refrigeration cycles while simultaneously increasing the Coefficient of Performance (COP). For a NaOH concentration of 12%, the temperature at the generator decreases by about 17 °C, the COP increases by 28%, and the size of the rectifier can be up to 46% less than that required when the conventional NH3/H2O cycle is used.