Determining the composition of ammonia/water mixtures using short-wave near-infrared spectroscopy

• A model for determine ammonia in solution with high concentration is established.
• The absorbance and density of aqueous solutions has been “in-situ” measured.
• The influence of temperature, pressure and concentration is simultaneously evaluated.
• The model has been validated using a reference method based on density measurements.

This paper proposes a methodology based on short-wave near-infrared spectroscopy to determine the ammonia content of ammonia/water mixtures with ammonia mass fraction in the range 0.35–0.65. Establishing this methodology meant modeling the relationship between the pressure bar (15–25) bar, temperature (20–50) °C and composition of the ammonia–water in the mixture (0.35–0.65 in ammonia mass fraction) with absorbance at 1033 nm. The experiments were designed to optimize experimental work. A 23 factorial design+3 center points was used to establish and analyze the significance of the variables in the absorbance using analysis of variance (ANOVA). A linear model for absorbance was obtained using the least squares method. The trueness of the results versus the values obtained was assessed using a reference method; density measurement was chosen for this study. The accuracy of the results in terms of root-mean-square deviation (RMSD) was 3.7%. The methodology proposed represents a fast alternative for the “in-situ” measurement of the ammonia composition of ammonia–water mixtures in absorption refrigeration systems.

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