Bubble Column Evaporator Research Articles

Nucleation of PFAS adsorbents using a bubble column evaporator (BCE)

Perfluorooctanoic acid (PFOA) and, in general, derivatives of per- and polyfluoroalkyl substances (PFAS) have been used in various products and industries due to their unique physical and chemical properties such as hydrophobicity, wetting properties and oleophobicity (lack of affinity to oils). Unfortunately, they are resistant to biodegradation and accumulate as environmental contaminants widely found in water, soil and air. This study puts forward several environmentally friendly and simply produced adsorbents which remove PFOA from aqueous solutions. Calcium fluoride (CaF2), aluminium fluoride (AlF3) and calcium-ferric layered double hydroxide (LDH Ca-Fe) were prepared using solution nucleation in a bubble column evaporator (BCE). The efficiency of these adsorbents was measured based on particle growth in the BCE and PFOA-removal rates using solid-phase extraction (SPE) and liquid chromatography/mass spectrometry (USEPA 537). LDH Ca-Fe successfully removed 90 % of PFOA (initially at 10 ppm). This study suggests that the BCE method offers cost-effective and simply produced adsorbents, which appear promising in removing PFAS from the environment.

A new commercial prototype of a bubble column evaporator (BCE) for high-quality water production in livestock (piggery) farms

The bubble column evaporator (BCE) offers a novel thermal process with enhanced evaporation efficiency, due to the enhanced heat and mass transfer efficiency from a continuous stream of hot dense bubbles that rise into a column of solution. This work evaluates the viability of the BCE to produce purified water from two different experimental solutions: synthetic piggery effluent and 0.5 m NaCl solution, when sparging with either hot air or combustion gas flow. When the BCE prototype was operated with combustion gas, an increased heat and mass transfer efficiency was observed due to the heat released by the condensation of hot water vapour in the combustion gas mixture, therefore reducing the energy cost when using combustion gas to evaporate the column solutions. For the first time, the quality of the condensed water from the BCE prototype was analysed and the report showed that high-quality condensed water could meet the most rigorous international drinking water quality standards, thus being safe to reuse in the piggery farm. In general, this paper provides a simple and affordable water treatment process that can be applied in the livestock (piggery) farm for efficient water reuse.

Size control of precipitated particles of amino acids using a bubble column evaporator

The precipitation of five amino acids: DL-alanine, L-arginine, L-leucine, DL-methionine and L-tyrosine was studied at their solubility limits and isoelectric point by using a bubble column evaporator (BCE). The precipitation of amino acids via a bubble column evaporator and a standard stirring method were compared via turbidity measurements. Particle size, zeta potential and polydispersity index (PDI) were also measured using a Malvern Zeta-sizer and the particle morphology was examined using Scanning Electron Microscopy (SEM). The novel BCE process emerges as a much more effective method than precipitation by standard stirring methods. Better control of fine particle size and growth rates is achieved. The amino acids in zwitterionic form exhibit the same unexplained bubble coalescence inhibition phenomenon as do common salts. This suggests obvious applications in flotation technologies.

The effect of amino acids on bubble coalescence in aqueous solution

Many salts reduce the size of bubbles and inhibit bubble coalescence. The phenomenon occurs above the same concentration for some ion pairs (0.17 M for 1:1 salts). For other ion pairs no such effect occurs. The phenomenon remains inexplicable by standard theories of colloid science. It occurs for a range of sugar isomers and not for others. Again, why this occurs is not known. The present work extends the effect to a wider range of solutes. Amino acids were found to inhibit bubble coalescence and reduce the size of bubbles. The effect is the same as that for inorganic salts but it is dramatically more efficient. A series of amino acids: dl-Alanine, l-Arginine, l-Leucine, dl-Methionine and l-Tyrosine was studied using a bubble column evaporator (BCE). The amino acid solutions were prepared at different concentrations and beyond their solubility limits. This remarkable effect also provides new hints into how bubble interactions affect crystallisation.

Feasibility analysis of using a bubble column evaporator as a solvent swap device

A feasibility analysis was performed on the use of a bubble column evaporator (BCE) as a solvent swap device. Batch BCE simulations successfully proved that the BCE was capable of performing a solvent swap for both the dcm - methanol and the water - methanol systems with the aid of computational fluid dynamics (CFD). This was achieved by saturating the inlet gas with the new solvent which condensed inside the column. The old solvent then evaporated into the gas phase which exited via the column outlet. Progression of the solvent swap therefore caused the liquid mass fraction of the species to vary. The effect of bubble size, gas flowrate, gas inlet temperature and liquid mass fraction on the solvent swap efficiency was also investigated. The liquid mass fraction was found to have the greatest influence on the solvent swap efficiency. The evaporation rate decreased proportionately with the mass fraction of the evaporating species in the liquid phase which was a result of Raoult’s law. The thermodynamic model was used to validate the CFD solution and estimate the time required for a batch solvent swap. A smaller bubble size was found to enhance mass transfer in the solvent swap by increasing the evaporation and condensation thermodynamic efficiencies. The higher gas flowrate also improved the mass transfer in the system and reduced the time required for a solvent swap as there was a greater throughput of gas, however it resulted in a decreased evaporation efficiency due to a lower gas residence time in the column. The higher gas inlet temperature increased the rate of condensation and the time required for the a solvent swap. The increased condensation of the new solvent had a negative effect on the evaporation of the old solvent as a result of raoults law which resulted in a longer solvent swap operation. The increased gas inlet temperature also caused the evaporation rate to decrease because of the relationship between the density of an ideal gas and temperature. An increased inlet gas temperature resulted in a lower inlet gas density. There was therefore a reduced mass throughput of gas into the column and consequentially the rate of evaporation decreased. The liquid mass fraction was found to have the greatest influence on the solvent swap efficiency. The evaporation rate decreased proportionately with the mass fraction of the evaporating species in the liquid phase which was a result of Raoult’s law.

CFD Simulation of a bubble column evaporator

The combined principles of computational fluid dynamics (CFD) and heat and mass transfer are applied to design process equipment capable of performing both batch and continuous solvent swap operations at low temperature in the pharmaceutical industry. A bubble column evaporator was chosen a low temperature evaporator as it is a primitive, scalable and robust unit operation employed in a broad range of industries. A CFD analysis using OpenFOAM was subsequently performed on batch and continuous bubble column evaporators (BCE) for the methanol air system to provide data which experimental analysis cannot collect (with the same degree of detail) such as temperature distributions, concentration gradients and velocities across the column, which can then be used to predict methods of enhancing efficiency and evaporation rate. The effects of the gas flowrate, gas temperature and liquid flowrate on mass transfer in a BCE were investigated and the resulting system was analysed. Detailed analysis revealed that the gas was saturated almost instantaneously upon entering the column. It was concluded that both the gas flowrate and gas temperature are important factors affecting the rate of evaporation. An increased outlet gas temperature enhanced the rate of mass transfer in the system by up to 100% by increasing the gas phase saturation concentration of the solvent. This method proved particularly successful for the methanol-air system as the liquid had a significantly higher specific heat capacity than the gas and therefore the increased inlet gas temperatures had negligible effects on the liquid temperature.

A Review and Update of Bubble Column Evaporator Processes

This paper gives an updated review of the bubble column evaporator (BCE) and its various new processes. These include recent work on helium gas desalination and high temperature inlet gas decomposition. The BCE process offers a continuously produced source of high gas-water interface and consequently provides high overall heat and mass transfer coefficients. Very different results have been obtained using nitrogen, oxygen, carbon dioxide and helium inlet gases. Although the bubbling process itself is both simple to use and apply, our understanding of the fundamental physical and chemical principles involved is surprisingly limited and there are many issues yet to be explained. Recently the process has been used to develop new methods for the precise determination of enthalpies of vaporisation of concentrated salt solutions, as an evaporative cooling system, a sub-boiling thermal desalination unit, for sub-boiling thermal sterilization, for low temperature thermal decomposition of different solutes in aqueous solution and for the inhibition of particle precipitation in supersaturated solutions. These novel applications can be very useful in many industrial practices, such as desalination, water/wastewater treatment, thermolysis of ammonium bicarbonate (NH4HCO3) for the regeneration in forward osmosis and refrigeration related industries. The background theories and models use to explain the BCE process are also reviewed and this fundamental knowledge is applied to the design of BCE systems and to explain recently explored applications, as well as potential improvements. Many other prospective applications of the BCE process are also reported in this paper.

Efficient Dewatering of Slimes and Sludges with a Bubble Column Evaporator

The recalcitrant nightmare of de-watering slime/sludge is a major issue, for both industry and the environment. A simple process is developed that solves the problem. It uses a bubble column evaporator (BCE) with heated dry air. The model slime to illustrate the de-watering process was a concentrated dispersion of spherical 5 micron silica particles in pure water. Typical slime samples were de-watered in the range 20-35% colloid/water (w/w) using dry inlet gases pre-heated to temperatures of 150°C and 250°C. The BCE process was run at sub-boiling temperatures, with the column solution in the range, 43°C and 74°C, with those two inlet temperatures operating for de-watering the slime. A significant bonus is that the pure water vapour produced can be condensed and used as a source of high-quality water for reuse. The BCE process offers simplicity, resilience to slime feed quality, and a pure water biproduct. It also offers a continuous and controlled low-maintenance process. These are clear advantages in de-watering a wide variety of industrial slimes and sludges. In addition, the process involves the passage of a continuous flow of hot dry gases. This causes the dispersion to remain sufficiently fluid to allow easy transportation. However, once the hot gas flow ceased, the dispersion immediately solidified. The success of the bubble column process for dewatering and validation of the mechanism is even more enhanced if helium is used instead of air. It appears that hot helium atoms can disrupt water hydrogen-bonding in the liquid surrounding the hot bubbles and this enhances water vapour collection efficiency. The bubble method appears to offer more than significant advantages over other methods, such as hydrocyclone methods, which are often used to de-water mining wastes.

Controlled Growth of Strontium Sulfate Particles in Aqueous Solution: Inhibition Effects of a Bubble Column Evaporator

In the oil industry, strontium sulfate (SrSO4) scale deposits have long plagued oilfield and gas production operations. This remains an unsolved problem. We here show how the bubble column evaporator (BCE) can be used to control aqueous precipitation from salt solutions. Mixtures of strontium nitrate and sodium sulfate in the BCE system were used to precipitate strontium sulfate at different degrees of supersaturation. The effectiveness of the BCE system was compared to standard mechanical stirring. The precipitation of strontium sulfate in both processes was monitored through turbidimeter, particle counting, Dynamic Light Scattering (DLS) and Scanning Electron Microscopy (SEM). The results show that the BCE system has a significant inhibition effect and so can be used to control precipitation growth rate, even from supersaturated solutions. This remarkable effect also provides new insights into mechanisms of crystallisation, of bubble interactions and mineral flotation.

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

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 optimum 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.

The “molecule diode” effect caused by adsorbed surfactant layers improves evaporation efficiency in the bubble column evaporator (BCE) for seawater desalination

The bubble column evaporator (BCE) can be used as an improved thermal desalination technology simply based on pumping a heated gas, such as helium, through a porous sinter to produce hot gas bubbles to transfer heat and mass across the water-gas interface. In this paper, a “molecule diode” effect caused by surfactant monolayers has been proposed, because adding surfactants into the column solution produced an improved evaporation efficiency. The mechanism involved appears to be based on enhancing water molecule transport across the bubble surfaces from the column solution via the hydrophilic headgroups, whilst restricting water vapour molecules returning to the solution because of the layer of hydrophobic tails coating the inside bubble surface. In this paper, the enthalpy of vaporization (△Hv) was estimated, and the bubble saturation period and the bubbling and foaming behaviour was observed, to study the performance of surfactants on improving the evaporation efficiency achieved by the BCE process. Myristyltrimethylammonium bromide (MAB), a typical cationic surfactant, combined with the moderately heated helium bubbles generated in the BCE column, performed best by reducing the apparent △Hv value for water by 15 % and increasing the evaporation efficiency by 2.3 times, compared with the value for non-surfactant column solutions pumped with air, which demonstrates the potential for a low cost thermal desalination technology based on this sub-boiling process.

An improved evaporation process with helium inlet in a bubble column evaporator for seawater desalination

The bubble column evaporator (BCE) process is a promising method for seawater desalination with improved evaporation efficiency. In this paper, for the first time, the enthalpy of water vaporization (ΔHv) in a helium sparged aqueous solution was measured and calculated using an energy balance equation for a BCE by applying an improved column design. A significant drop in the measured ΔHv values correlated with the observation of a larger weight loss of water vapor from the salt solution suggest that pre-heated helium inlet gases can facilitate the evaporation process, apparently by disrupting the hydrogen bonding among water molecule clusters, probably in the regions close to the surface of the rising hot bubbles. The increased evaporation efficiency of about 3.1 times with 75 °C heated helium inlet illustrates its potential for future seawater desalination because of its relatively low energy consumption compared with other commercial desalination methods and suggests that solar energy or waste vent gases could be used as suitable sources of thermal energy, and the helium carrier gas can be recycled during the condensation process to reduce the commercial cost.

Spatial Decoupling of Light Absorption and Scattering Centers in Plasmon-assisted Bubble Column Evaporator for Solar Steam Generation

Spatial Decoupling of Light Absorption and Scattering Centers in Plasmon-assisted Bubble Column Evaporator for Solar Steam Generation

Water sterilisation using different hot gases in a bubble column reactor

This study compares the effect of using a range of different hot gases to sterilise water at atmospheric pressure within a hot bubble column evaporator (HBCE) reactor. Water containing bacteria (E.coli ATCC 15597) and viruses (MS2 ATCC 15597-B1) was sterilized with 6 different gases (CO2, N2, O2, Ar, air and combustion gas) at inlet temperatures of 200 °C (for viruses) and 150 °C (for bacteria). The results obtained suggest three different gas dependent sterilisation mechanisms: a thermal inactivation mechanism, based on heat transfer between hot gas bubbles for argon, air and nitrogen; a CO2 specific inactivation mechanism, based on the penetration of CO2 molecules through bacterial membranes and virus capsids; and an oxidation/combustion mechanism that inactivates only E.coli, essentially by direct contact with oxygen or through reactive oxygen species. CO2 gas and combustion gases gave the best inactivation results with viruses and O2 gas was found to be most effective for the destruction of E.coli.

Improved virus inactivation using a hot bubble column evaporator (HBCE).

An improved hot bubble column evaporator (HBCE) was used to study virus inactivation rates using hot bubble-virus interactions in two different conditions: (1) using the bubble coalescence inhibition phenomenon of monovalent electrolytes and (2) with reducing the electrostatic repulsive forces between virus and bubble, by the addition of divalent electrolytes. It is shown that the continuous flow of (dry) air, even at 150-250 °C, only heats the aqueous solution in the bubble column to about 45°-55 °C and it was also established that viruses are not significantly affected by even long term exposure to this solution temperature, as confirmed separately from water bath experiments. Hence, the effects observed appeared to be caused entirely by collisions between the hot air bubbles and the virus organisms. It was also established that the use of high air inlet temperatures, for short periods of time, can reduce the thermal energy requirement to only about 25% (about 114 kJ/L) of that required for boiling (about 450 kJ/L).

A study of the bubble column evaporator method for improved thermal desalination

A bubble column evaporator (BCE) was used to study the enhanced evaporation of water from 0.5m NaCl solutions, using a range of pre-heated carrier gases (air, CO2, Ar and He), each at an inlet temperature of 150°C. Surprisingly, the use of hot helium as a carrier gas substantially improved water evaporation efficiency, by a factor of about 3 times, compared with, for example, dry air. It is suggested that this higher efficiency could be due to a slightly reduced water hydrogen bonding, and hence enthalpy of vaporisation, due to the continuous sparging of hot He gas atoms into the aqueous solution. These results suggest that this process could form a basis for the development of a more efficient, sub-boiling desalination process. The effect of added cationic and anionic surfactants on BCE water evaporation rate was also studied. The cationic surfactant myristyltrimethylammonium bromide (MAB) and the anionic surfactant Sodium dodecyl sulfate (SDS) each gave a slightly enhanced water evaporation rates, in agreement with previous studies, probably due to enhanced water vapor retention within surfactant coated bubbles.

The energy balance within a bubble column evaporator

Bubble column evaporator (BCE) systems have been studied and developed for many applications, such as thermal desalination, sterilization, evaporative cooling and controlled precipitation. The heat supplied from warm/hot dry bubbles is to vaporize the water in various salt solutions until the solution temperature reaches steady state, which was derived into the energy balance of the BCE. The energy balance and utilization involved in each BCE process form the fundamental theory of these applications. More importantly, it opened a new field for the thermodynamics study in the form of heat and vapor transfer in the bubbles. In this paper, the originally derived energy balance was reviewed on the basis of its physics in the BCE process and compared with new proposed energy balance equations in terms of obtained the enthalpy of vaporization (ΔH vap) values of salt solutions from BCE experiments. Based on the analysis of derivation and ΔH vap values comparison, it is demonstrated that the original balance equation has high accuracy and precision, within 2% over 19–55 °C using improved systems. Also, the experimental and theoretical techniques used for determining ΔH vap values of salt solutions were reviewed for the operation conditions and their accuracies compared to the literature data. The BCE method, as one of the most simple and accurate techniques, offers a novel way to determine ΔH vap values of salt solutions based on its energy balance equation, which had error less than 3%. The thermal energy required to heat the inlet gas, the energy used for water evaporation in the BCE and the energy conserved from water vapor condensation were estimated in an overall energy balance analysis. The good agreement observed between input and potential vapor condensation energy illustrates the efficiency of the BCE system. Typical energy consumption levels for thermal desalination for producing pure water using the BCE process was also analyzed for different inlet air temperatures, and indicated the better energy efficiency, of 7.55 kW·h per m3 of pure water, compared to traditional thermal desalination techniques.

Regeneration of strong acid/strong base mixed-bed resins using ammonium bicarbonate (AB) for a sustainable desalination process

Mixed-bed systems comprising both strong-acid (cation) and strong-base (anion) ion exchange (IEX) resins can be used for the removal of salts to produce drinking quality water from saline feedwater. In this study, complete regeneration in situ was achieved by washing with concentrated ammonium bicarbonate (AB) solution, which produces complete regeneration of the mixed resins, without the need for their separation. This process was found to be more effective than regeneration with AB using a resin heat-decomposition process, previously studied. The results presented here show that a mixed-bed resin can desalt 0.1M NaCl solution, producing about 6× the resin mass of drinking quality water. The resin can then be regenerated using concentrated AB and then re-used again to remove NaCl, producing ammonium bicarbonate solution. The product AB solution can then be readily converted into drinking water (DW) simply by heating to 60–80°C or by using a recently developed bubble column evaporator (BCE) process or by using multiple distillation columns (MDC). These processes decompose the AB solution to both produce drinking water product and gaseous NH3 and CO2, which can then be collected and used to re-form the AB solution. In this way the resin can be regenerated in a continuous process, without the need for further acid and base consumption, minimizing chemical waste, environmental impact, and costs.

Low temperature MS2 (ATCC15597-B1) virus inactivation using a hot bubble column evaporator (HBCE)

In the treatment of household wastewater viruses are hard to eliminate. A new technique is described which tackles this major problem. The MS2 (ATCC15597-B1) virus was used as a surrogate to estimate the inactivation rates for enteric viruses by a hot (150°C) air bubble column evaporator (HBCE) system Its surface charging properties obtained by dynamic light scattering, have been studied in a range of aqueous salt solutions and secondary treated synthetic sewage water. A combination of MS2 virus surface charge properties with thermal inactivation rates, and an improved double layer plaque assay technique, allows an assessment of the efficiency of the HBCE process for virus removal in water. The system is a new energy efficient treatment for water reuse applications.

Determining the Enthalpy of Vaporization of Salt Solutions Using the Cooling Effect of a Bubble Column Evaporator

The enthalpy of vaporization (ΔHvap) of salt solutions is not easily measured, as a certain quantity of pure water has to be evaporated from a solution, at constant composition, and at a fixed temperature and pressure; then the corresponding heat input has to be measured. However, a simple bubble column evaporator (BCE) was used as a novel method that allowed undergraduate students to determine the ΔHvap value of a salt solution within an hour or so, because it only required temperature measurement (at a steady state) of the column solution and the inlet gas flowing into the column. In this article, by way of illustration, the ΔHvap value of a 0.5 M NaCl solution was determined using the BCE process. In this experiment the evaporative cooling effects of the column were also easily demonstrated, as the inlet air at room temperature cooled the column solution to less than 10 °C. The change that students measured directly demonstrated the endothermic process of water vaporization. The role of several basic physical chemistry concepts, such as vapor pressure, phase transition, and enthalpy, involved in this experiment can be reinforced. An unexpected property of some common salt solutions, of bubble coalescence inhibition, was also observed in this experiment.