Substantia. An International Journal of the History of Chemistry 4(2) Suppl.: 49-55, 2020 Firenze University Press www.fupress.com/substantia ISSN 2532-3997 (online) | DOI: 10.36253/Substantia-833 Citation: M. Shahid, M. Taseidifar, R.M. Pashley (2020) A Study of the Bub- ble Column Evaporator Method for Improved Ammonium Bicarbonate Decomposition in Aqueous Solutions: Desalination and Other Techniques. Substantia 4(2) Suppl.: 49-55. doi: 10.36253/Substantia-833 Copyright: © 2020 M. Shahid, M. Tasei- difar, R.M. Pashley. This is an open access, peer-reviewed article pub- lished by Firenze University Press (http://www.fupress.com/substantia) and distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distri- bution, and reproduction in any medi- um, provided the original author and source are credited. Data Availability Statement: All rel- evant data are within the paper and its Supporting Information files. Competing Interests: The Author(s) declare(s) no conflict of interest. A Study of the Bubble Column Evaporator Method for Improved Ammonium Bicarbonate Decomposition in Aqueous Solutions: Desalination and Other Techniques Muhammad Shahid, Mojtaba Taseidifar, Richard M. Pashley* School of Science, University of New South Wales, Northcott Drive, Canberra, Australia *Corresponding author: r.pashley@adfa.edu.au Abstract. 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 tem- perature. 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. Keywords: non-boiling decomposition, bubble coalescence, transient collisions, ammonium bicarbonate. 1. INTRODUCTION 1.1. Significance of solute decomposition 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 decomposi- tion 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 ener- gy-intensive process in the application of forward osmosis1 and, in the regen- eration of ion-exchange resins.2 For the latter application, the ion-exchange resins comprising carboxylic acid and tertiary amine groups for desalination 50 Muhammad Shahid, Mojtaba Taseidifar, Richard M. Pashley 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, ammo- nium bicarbonate (NH4HCO3) and potassium persul- phate (K2S2O8) can be thermally decomposed in aque- ous solutions using a bubble column evaporator (BCE) process at sub-boiling condition (around 45 °C). Fulks et al.4 and Gokel5 studied, ammonium bicarbonate decom- position 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 (1) The decomposition rate of this solute can be readily measured from simple electrical-conductivity measure- ments. The decomposition of ammonium bicarbonate using the hot-gas BCE process is examined in this study. 1.2. Significance of the bubble column evaporator (BCE) The bubble column evaporator (BCE) offers a good illustration of the use of a gas-liquid interface to drive fundamental processes involving heat and mass transfer. Bubble columns are devices in which a gas, often dry air, is pumped through a multi-porous sinter disc to form gas bubbles which are continuously replenished and come into intimate contact with the column solution. Dry gas bubbles in the column solution may be used simply to mix the liquid phase homogenously to attain a uniform temperature distribution or to saturate dis- solved gases in the column solution. Substances can also be transferred from one phase to the other, for example when liquid reaction products are stripped from a gas: both mass- and heat-transfer processes can occur simul- taneously.6 Recently, aqueous bubble column evaporators have been used for a range of new applications. These exploit the long known, but still unexplained effect of bubble bubble coalescence inhibition that occurs systematical- ly with many salts. The effect is both ion pair and con- centration dependent. In combination with the effect are size dependent bubble rise rates and rapid water- vapour uptake into the bubbles.3,7-9 These phenomena together offer a variety of applications. The most strik- ing of these we have developed are in desalination. Some of a wide range of other useful applications of the BCE10 are: a new method for the precise determination of enthalpies of vaporisation (ΔHvap) of concentrated salt solutions;7,11 evaporative cooling;8 a new method for thermal desalination;12-14 a novel method for sub- boiling thermal sterilization;7,15-19 a novel method for the low-temperature thermal decomposition of dif- ferent solutes in aqueous solution;3 a new approach to aqueous solute precipitation in a controlled manner.20 The efficient removal of heav y metal ions in an ion- specific, ion-flotation process is a specially noteworthy advance.21 In addition, a bubble column condenser has also been designed for the production of high-quality water as condensate.22-24 1.3. Proposed mechanism of BCE thermal decomposition The application of the BCE process opens up a new approach to the thermal decomposition of degra- dable salts in aqueous solution. The hot surface layer produced transiently on the surface of hot bubbles (see Figure 1) created in the BCE appears to play a signifi- cant role in providing high heat- and mass-transfer effi- ciency, since the BCE is a direct-contact evaporator.25 Degradable chemicals exposed to this hot layer can be efficiently decomposed. In addition, gaseous products are rapidly captured by the rising bubbles, due to the internal gas/vapour rotational flow produced within the rising bubbles. Here the effectiveness of the BCE as a method for solute decomposition was assessed and quantified. Experiments were conducted using ammonium bicar- Figure 1. Schematic diagram of BCE thermal decomposition using a hot-air bubble layer. (Reprinted with permission from Ref. 3. Copyright 2015 American Chemical Society). 51A Study of the Bubble Column Evaporator Method for Improved Ammonium Bicarbonate Decomposition in Aqueous Solutions: Desalination and Other Techniques bonate at high inlet gas temperatures and a comparative study of energy cost was determined. 2. MATERIALS AND METHODS 2.1. Materials Certified reagent-grade (≥ 99% purity), ammonium bicarbonate (NH4HCO3) was supplied by May & Bak- er Ltd and used without further purification. Aqueous solutions were prepared using deionized, ultrafiltered water (Milli-Q). At room temperature, the deionized water had a conductivity < 2.0 µS/cm and a natural equi- librium pH of 5.7. All concentrations are given in molal- ity (m) units. 2.2.2. Electrical conductivity measurements in standard NH4HCO3 solutions Ammonium bicarbonate solutions were prepared in the range: 0.5 to 2 m. Electrical conductivity values of all the solutions were measured using a Thermo Fisher Sci- entific (Waltham, MA, USA) conductivity meter at 25 °C. 2.3. BCE system for thermal decomposition A high-surface-area gas/water interface was pro- duced continuously by pumping dry gases (laboratory grade air and nitrogen separately), through a 40−100 micron pore-size glass sinter into a 120 mm diameter open-top glass column (Büchner type, Pyrex® Boro- silicate, VWR) filled with 250 mL solution. The BCE apparatus used to study improved decomposition with a high-temperature gas (air) flow is shown in Figure 2. This system enables the use of inlet dry gas temperatures of more than 275 °C. The inlet air temperature was var- ied using a Tempco air heater (300W) with a thermo- couple temperature monitor and an AC Variac electrical supply. The actual temperature of the dry gas flowing into the solution was measured at the centre of the sinter by a Tenmars thermometer (±1.5 °C) without any solu- tion in the column. The gases (air and nitrogen) were produced by cylinder (Coregas Pty Ltd, Australia) and a BOC gas flow meter. The temperature of the column solution was also continuously monitored using a ther- mocouple positioned at the centre of the column solu- tion. The air flow at temperatures of 300−600 °C, was needed to produce gas temperatures just above the glass sinter up to 275 °C, and this necessitated the use of steel and brass connectors for the downstream output from the heater and the use of FM Insulation Rock Wool as an insulating material. For comparison, the effects of solution tempera- ture on the decomposition of NH4HCO3 solutions was studied over time using stirred samples in a Tamson (Beiswijk, The Netherlands) heating bath at temperatures matching those of the BCE tests. During these experi- ments, samples were regularly taken out from the col- umn and water bath, and their electrical conductivities and pH values measured using a EUTECH CON 700 pH 700 Bench meter (Eutech Instruments Pte Ltd.). 3. RESULTS AND DISCUSSION 3.1. Thermal decomposition of ammonium bicarbonate solutions using a BCE 3.1.1. Measurement of the electrical conductivity of NH4HCO3 solutions at different concentrations As the NH4HCO3 salt thermally decomposed into NH3 and CO2 gases, the concentration of NH4+ and HCO3- reduced in the aqueous solution. So the decom- position process could be monitored through the meas- urement of the electrical conductivity of the samples taken from the bubble column. The pH of aqueous solutions were also measured and found to be basic. As hot dry bubbles enter the column, water vapori- sation occurs, and water vapour passes into the bubbles. Variac AC Gas heater Thermometers Gas in Gas cylinder Heat gas Warm/hot gas in Bubble column evaporator Air flow meter Figure 2. Schematic diagram of the bubble column evaporator (BCE) for solute decomposition. (Reprinted with permission from Ref. 3. Copyright 2015 American Chemical Society). 52 Muhammad Shahid, Mojtaba Taseidifar, Richard M. Pashley The amount of vaporised water removed, mv (g) after time t (sec), during a typical BCE process was estimated using the following relation: (2) where, rf (L/s) is the room-temperature gas flow rate, measured just prior to the heater, about 22.5 L/min in this study; Tc, Tf are the gas temperatures (in K) at the top of the column solution and at the flow meter; and Pc, Pf are the corresponding pressures at the same positions. These are the factors used to estimate the “bubble col- umn flow rise” rate. Here is the water-vapour density in g/L at the temperature of the solution at the top of the column, which was calculated from the vapour pressure of the solution using the ideal gas equation. Using the measured electrical conductivity of the NH4HCO3 solutions at different time intervals, the per- cent decomposition of NH4HCO3 at time (t) in the BCE process was calculated: Decomposition%=[1– ]×100 (3) Here [NH4HCO3]t is the concentration of NH4HCO3 at time (t) during the BCE operation and [NH4HCO3]0 is the initial concentration of NH4HCO3, just before pour- ing the solution into the bubble columns. 3.1.2. Decomposition of NH4HCO3 Solutions Some typical decomposition results (using air and nitrogen) obtained under different solution conditions are given in Figure 3. These results clearly demonstrate that the improved BCE process is much more efficient for NH4HCO3 decomposition than the standard method using a simple stirred heating (without BCE) at the same solution temperature, here around 57 °C. The results in Figure 3 obtained for quite different stirring conditions showed that the decomposition rates for simple heating (without the BCE) remained the same. This shows that the continuous mixing by the bubbling process in the BCE did not itself contribute to the NH4HCO3 decompo- sition. Different concentrations of NH4HCO3 were also studied, as shown in Table 1. It was observed that the presence of NH4HCO3 at concentrations above about 0.5 m inhibited bubble coalescence to a similar degree as that at 0.17M. (This critical concentration is the same for all 1:1 salts like NaCl that exhibit the fusion inhibition phenomenon9) It was also observed that fine (1−3 mm diameter) bub- bles were produced in the BCE process (see Figure 4a). It was clear from the photos taken during the decompo- sition of 2 m NH4HCO3 solution that, after bubbling for 10 min, the average bubble size started to increase (Fig- ure 4b). That is expected as electrolyte concentration reduces.9 Finally, after almost complete decomposition of NH4HCO3 at around 20 min, the bubble size became the same as in pure water. This provides explicit visual indication of the complete decomposition of ammo- nium bicarbonate in the aqueous solution (Figure 4c). The thermal decomposition of ammonium bicarbonate solutions into ammonia and carbon dioxide gas and the resulting reduction in NH4HCO3 concentration reflects the increase in bubble size. Table 1. Decomposition efficiency for an initial solution of 2 m NH4HCO3 solution using a heated (dry) air inlet in the BCE pro- cess. Time (min) Column Solution Temperature (°C) Electrical Conductivity (mS/cm) pH 0 Room Temp. 85 7.74 5 54.2 52.1 9.25 10 56.7 28.3 9.44 15 57.1 8.95 9.33 20 57.8 1.57 8.87 Figure 3. Percent decomposition of NH4HCO3 solutions at different concentrations in the BCE with an inlet gas (air and nitrogen) tem- perature of 275 °C and column solution temperature of 57 °C and in a stirred vessel for simple heating at around 57 °C. 53A Study of the Bubble Column Evaporator Method for Improved Ammonium Bicarbonate Decomposition in Aqueous Solutions: Desalination and Other Techniques 3.1.3. Effect of initial bubble temperature on NH4HCO3 decomposition in a BCE It appears that the decomposition of NH4HCO3 in aqueous solutions within a hot air BCE system occurs due to the hot surface layer initially present around the stream of hot air bubbles released from the frit. We con- sider the likely thickness of this transient heated layer as a function of inlet air temperature. The maximum extent of the layer can be estimated for a given temperature, assuming that it is uniform, from the total heat avail- able from the freshly released bubble. For example, for a 1mm bubble we can estimate the maximum layer thick- ness of water heated to, say, 80°C by the bubbles with an initial release temperature of about 150 °C (i.e. the inlet gas temperature) as follows. This bubble layer thickness varies with bubble size (V) and the temperatures of the inlet air, the steady state column temperature and the average temperature of the heated surface film surrounding the bubbles. The maxi- mum heated layer thickness can be estimated using the thermal energy balance equation: Cp∆TV=Cwater∆t4πr2ρwz (4) where Cp, Cwater are the air and water heat capacities, in units of J/(m3×K) and J/(kg×K), respectively, and ρw is the liquid water mass density (in kg/m3). ∆t, ∆T are the transient temperature increase in the water layer and the temperature reduction within the cooling bubbles, in units of K, respectively. The volume of a layer of thickness z around a bub- ble is given by 4πr2ρwz, when z is much smaller than r. Hence the cooling of the bubble by ∆T must determine the thickness z. For example, for bubbles cooling by 100 °C, the maximum heated water layer thickness, heated from 20 to 80 °C, is about 70 nm. So ammonium bicar- bonate could be decomposed in this surface region, as illustrated in Figure 5. When the inlet gas temperature is increased, the thickness of the surface hot water layer would also be increased, provided the mean tempera- ture of the film and other assumptions are fixed. Con- sequently, the volume of the decomposition area (in the hot bubble layer) is correspondingly increased, leading directly to improved decomposition. It is useful to estimate the thermal energy cost to produce decomposition of an ammonium bicarbonate solution. We have done this by passing 22.5 L/min of air heated to 275 °C through a bubble column for 20 mins with 1-3 mm size bubbles. Different concentra- tions of ammonium bicarbonate solution were used. The heat capacity Cp at a constant pressure of air in units of J/gK can be calculated from gas heat capacity per mole. This is fairly constant with temperature. At 275 °C this corresponds to about 1.017 J/gK respectively. For a flow rate of 22.5 L/min, this requires a total heat to raise the temperature of gas from 20 to 275 °C of about 144 kJ per 250 mL of solution or 575 kJ/L. By comparison, heating a litre of ammonium bicarbonate solution using different inlet gas temperatures is shown in Figure 6. Figure 4. Photographs of the bubble sizes in NH4HCO3 solutions in a BCE with an inlet of (dry) air at 275 °C, at experimental times: (a) 0 min (b) 10 min; and (c) 20 min. 54 Muhammad Shahid, Mojtaba Taseidifar, Richard M. Pashley 4. CONCLUSIONS AND FUTURE WORK A method for sub-boiling, thermal decomposition of ammonium bicarbonate solutions was presented. This is a considerable improvement on standard methods. We have shown that a bubble column can rapidly exchange heat from hot gas bubbles to the nearby water surround- ing the bubbles. This can be used as an effective and energy efficient method of decomposing ammonium bicarbonate in solution. It can be readily scaled up to treat industrial ammonium bicarbonate draw solutions used in forward osmosis desalination. An important inference is this: The BCE process, in addition to those applications already mentioned, might also be used to destroy unwanted solutes. Hormones and pharmaceutical compounds present in wastewater pre- sent intractable problems for city water recycling. Con- ventional technologies do not effectively treat these con- taminants. Antibiotic residues from human consump- tion or intensive farming can contribute to the develop- ment of antibiotic-resistant bacteria. Protozoa, residual pharmaceutical compounds and hormones are believed to have potential risks to humans and the environment. Examples of such treatments with CO2 are given in oth- er papers in this volume. The possibilities of achieving very high temperatures within bubbles say with oxygen to carry out reactions at low cost are open. 4. NOMENCLATURE Abbreviations AB Ammonium bicarbonate BCE Bubble column evaporator Symbols °C Degree Celsius M Concentration in mol/L m Concentration in mol/kg n Number of moles T Temperature ∆Hvap Enthalpy of vaporisation ∆P Pressure difference between inside and outside of the bubble ∆T Temperature difference between inlet gas and bubble column solution 5. ACKNOWLEDGMENT We would like to thank the Australian Research Council for funding this project. 6. REFERENCES 1. J. R. McCutcheon, R. L. McGinnis, and M. Elimelech, A novel ammonia—carbon dioxide forward (direct) osmosis desalination process, Desalination, 2005, 174(1), 1-11. Figure 5. Relationship between the temperature of the inlet gas and the estimated thickness of the transient hot bubble surface layer around a 1 mm radius bubble in pure water, 0.5 m NaCl and 5.0 m CaCl2. Reprinted by permission of the publisher (Taylor &Francis Ltd, http://www.tandfonline.com) from Ref. 10. Figure 6. Comparison of thermal energy cost using air as carrier gas; A indicates energy cost at 150 °C inlet gas temperature using 2 m AB solution; B shows at 150 °C inlet gas temperature with 1 m AB solu- tion; C illustrates at 150 °C inlet gas temperature with 0.5 m AB solu- tion and D explains energy cost at 275 °C using 2 m AB solution. 55A Study of the Bubble Column Evaporator Method for Improved Ammonium Bicarbonate Decomposition in Aqueous Solutions: Desalination and Other Techniques 2. N. P. G. N. Chandrasekara and R. M. Pashley, Study of a new process for the efficient regeneration of ion exchange resins, Desalination, 2015, 357, 131-139. 3. M. Shahid, X. Xue, C. Fan, B.W. Ninham, and R.M. Pashley, Study of a novel method for the thermoly- sis of solutes in aqueous solution using a low tem- perature bubble column evaporator, J. Phys. Chem. B, 2015, 119 (25), 8072–8079. 4. G. Fulks, G. B. Fisher, K. Rahmoeller, M. C. Wu, E. D’Herde, J. Tan, A review of solid materials as alter- native ammonia sources for lean NOx reduction with SCR, 2009, SAE Technical Paper. 5. G. W. Gokel, Dean’s handbook of organic chemistry. McGraw-Hill New York, 2004, 71375937. 6. P. Zehner and M. Kraume, Bubble columns, Ull- mann’s Encyclopedia of Industrial Chemistry, 2000. 7. C. Fan, M. Shahid, R.M. Pashley, Studies on bubble column evaporation in various salt solutions, J. Sol. Chem., 2014, 43(8), 1297-1312. 8. M. Francis, R. Pashley, Application of a Bubble Col- umn for Evaporative Cooling and a Simple Procedure for Determining the Latent Heat of Vaporization of Aqueous Salt Solutions, J. Phys. Chem. B, 2009, 113(27), 9311-9315. 9. V. S. J. Craig, B. W. Ninham, R. M. Pashley, The effect of electrolytes on bubble coalescence in water, J. Phys. Chem. , 1993, 97(39), 10192-10197. 10. M. Shahid, C. Fan, R. M. Pashley, Insight into the bubble column evaporator and its applications, Int. Rev. Phys. Chem. , 2016, 35(1), 143-185. 11. C. Fan and R. M. Pashley, Precise Method for Deter- mining the Enthalpy of Vaporisation of Concentrated Salt Solutions Using a Bubble Column Evaporator, J. Sol. Chem., 2015, 44(1), 131-145. 12. M. J. Francis, R. M. Pashley, Thermal desalination using a non-boiling bubble column, Desalin. Water Treat., 2009, 12(1-3), 155-161. 13. M. Shahid, R. M. Pashley, A study of the bubble col- umn evaporator method for thermal desalination, Desalination, 2014, 351, 236-242. 14. M. Taseidifar, M. Shahid, R. M. Pashley, A study of the bubble column evaporator method for improved thermal desalination, Desalination, 2018, 432, 97-103. 15. X. Xue and R.M. Pashley, A study of low temperature inactivation of fecal coliforms in electrolyte solutions using hot air bubbles, Desalin. Water Treat., 2015, 1–11. 16. M. Shahid, A study of the bubble column evaporator method for improved sterilization, J. Water Process. Eng., 2015, 8, 1–6. 17. A. G. Sanchis, M. Shahid, and R. M. Pashley, Improved virus inactivation using a hot bubble col- umn evaporator (HBCE), Colloids and Surfaces B: Biointerfaces, 2018, 165, 293-302. 18. A. G. Sanchis, R. M. Pashley, B. W. Ninham, Virus and bacteria inactivation by CO2 bubbles in solution, NPJ Clean Water, 2019, 2(1), 5. 19. M. Shahid, R. M. Pashley, M. Rahman, Use of a high density, low temperature, bubble column for thermal- ly efficient water sterilisation, Desalin. Water Treat., 2014, 52, 4444–4452. 20. C. Fan, R. M. Pashley, The controlled growth of cal- cium sulfate dihydrate (gypsum) in aqueous solution using the inhibition effect of a bubble column evapo- rator, Chem. Eng. Sci., 2016, 142, 23-31. 21. M. Taseidifar, F. Makavipour, R. M. Pashley, A. F. M. M. Rahman, Removal of heavy metal ions from water using ion flotation, Environ. Technol. Innov., 2017, 8, 182-190. 22. P. N. Govindan, G. P. Thiel, R. K. McGovern, J. H. Lienhard, M. H. Elsharqawy, Bubble-Column Vapor Mixture Condenser. Google Patents. 23. G. P. Narayan, J. H. Lienhard, Thermal Design of Humidification–Dehumidification Systems for Affordable Small-Scale Desalination, IDA J. Desalin. Water Reuse, 2012, 4(3), 24-34. 24. M. Schmack, H. Goen, and A. Martin, A Bubble Col- umn Evaporator with Basic Flat-plate Condenser for Brackish and Seawater Desalination, Environ. Tech- nol., 2015 37(1), 74–85. 25. C. P. Ribeiro, P. L. C. Lage, Gas‐Liquid Direct‐Con- tact Evaporation: A Review, Chem. Eng. Technol., 2005, 28(10), 1081-1107.