A Study of the Bubble Column Evaporator Method for Improved Ammonium Bicarbonate Decomposition in Aqueous Solutions: Desalination and Other Techniques

From Firenze University Press Journal: Substantia

Muhammad Shahid, School of Science, University of New South Wales

Mojtaba Taseidifar, School of Science, University of New South Wales

Richard M. Pashley, School of Science, UNSW Canberra

A bubble column was used to study the improved thermal decomposition of NH4HCO3 in aqueous solution using a continuous flow of hot gas bubbles of opti-mum sizes (1–3 mm) produced via controlled bubble coalescence to maintain bubble size. The rapid transfer of heat from small, hot (dry) gas bubbles to the surrounding water, i.e. into a transient hot surface layer, was used as an effective and energy efficient method of decomposing ammonium bicarbonate in aqueous solution. It is shown that the continuous flow of (dry) hot gases, even at 275 °C, only heat the aqueous solution in the bubble column to about 57 °C, at which it was also established that NH4HCO3 has a negligible decomposition rate even with long-term exposure to this solution temperature. Hence, the effects observed appeared to be caused entirely by the effective collisions between the hot gas bubbles and the solute. It was also established that the use of high gas inlet temperatures can reduce the thermal energy requirement to only about 50% (i.e. about 575 kJ/L) of that reported in previous studies and less than 25% of solution boiling.

This paper is concerned with optimising a range of applications that use water, be they desalination, sterilisation, reactions and more using a bubble column evaporator. The overarching goal of water treatment by decomposition is to remove unwanted substances or solutes from water affordably and robustly. For example, the decomposition of ammonium bicarbonate (with chemical formula NH4HCO3) in aqueous solution is an important and energy-intensive process in the application of forward osmosis1 and, in the regeneration of ion-exchange resins. For the latter application, the ion-exchange resins comprising carboxylic acid and tertiary amine groups for desalination can be thermally regenerated using the BCE process at a lower energy cost than with conventional methods. More recently, Shahid et al.3 studied that solutes, ammonium bicarbonate (NH4HCO3) and potassium persulphate (K2S2O8) can be thermally decomposed in aqueous solutions using a bubble column evaporator (BCE) process at sub-boiling condition (around 45 °C). Fulks et al.4 and Gokel5 studied, ammonium bicarbonate decomposition in solution over the temperature range 30−85 °C. Complete decomposition into ammonia, carbon dioxide and water was observed above 60 °C.

The main decomposition reaction is:

NH4 HCO3 (aq)≜NH3 (g)+CO2 (g)+H2O

The decomposition rate of this solute can be readily measured from simple electrical-conductivity measurements. The decomposition of ammonium bicarbonate using the hot-gas BCE process is examined in this study.

DOI: https://doi.org/10.36253/Substantia-833

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