Numerical simulation on the falling film absorption process in a counter-flow absorber

Falling liquid film is commonly employed in variety of industrial systems, such as LiBr/H₂O absorption heat pump or chiller. In this paper, the falling film absorption in aqueous lithium bromide solution was investigated numerically using CFD software package-Fluent. The practical convective boundary condition at the cooling water side was considered. The heat transfer coefficient is assumed constant, and the coolant temperature changes linearly along its flow path. The numerical results indicate that the profile of temperature is exponential and their gradients are high due to the distinct heat effect associated with the absorption at the interface and the cooling effect of coolant at the wall at small downstream distance. As the downstream distance increases, the profile of temperature is nearly linear. The absorption heat and mass fluxes reach a maximum at the inlet region and decrease at the outside of the inlet region. Specially, the effect of variable physical properties on the absorption process was considered and discussed. The prediction of the total absorption mass transfer rate is about 6.5% higher when assuming constant properties.     

Heat capacity measurement and cycle simulation of the trifluoroethanol (TFE) +quinoline mixture as a new organic working fluid used in absorption heat pump

The 2,2,2-trifluoroethanol (TFE)+quinoline mixture was chosen as a potential working fluid used in an absorption chiller. Heat capacity was measured and correlated by a polynomial equation as a function of temperature and concentration, and the parameters of the regression equation were determined by a least-squares method. The cycle simulation for a double effect absorption cycle was carried out at various operation conditions. The appropriate operation condition and COP values were calculated for the absorption cycle by using the proposed working fluid. TFE+quinoline solution could be a promising working fluid as an alternative to the LiBr+H₂O and H₂O+NH₃ systems based on the operation range and simulation results.     

Surfactant effects on gas absorption in a coke-oven gas treatment process

The effect of organic impurities in industrial stripped coal water (SCW) on the absorption of CO₂ was measured experimentally. Removal of these impurities via activated carbon showed a marked improvement in interphase mass transfer of a vertical wetted-wall column absorber. However, this benefit was not found in a stirred-cell absorber, in which a different flow pattern from that in wetted-wall column absorber is expected. An ad hoc systematic study on the effects of three deliberately added surfactants on gas absorption by pure water in three different absorbers with different flow patterns was thereafter conducted. The experimental results reveal that absorption deterioration also prevails only in a vertical, wetted-wall column absorber and the reduction in liquid phase mass transfer by the addition of surfactant can be satisfactorily correlated with surface pressure of solutions. This indicates that the effect of the industrial impurities in SCW on gas absorption may successfully be simulated under the same flow pattern by a surfactant solution with the same surface pressure. A possible modification of the existing coke-oven gas (COG) treatment process for the benefit of absorption enhancement was finally proposed.     

New method for the determination of surfactant solubility and partitioning between CO2 and brine

A novel method is presented for measuring solubility in supercritical CO₂ (scCO₂), which can be used in conjunction with traditional cloud point measurements to obtain information directly on the soluble portion of a given sample and, ultimately, a much more informative data set. In this method, surfactant from a known amount of CO₂ solution was transferred into an aqueous solution and the surfactant concentration of the aqueous solution was measured directly by HPLC (high-performance liquid chromatography). In this work, the partitioning of a series of 2-ethylhexanol (2-EH) alkoxylate surfactants among an aqueous phase (water or brine) and scCO₂ as a function of electrolyte concentration, temperature, and pressure was also investigated. Surfactant partition coefficient was determined based on the reduction of HPLC measured surfactant concentration in the aqueous phase due to surfactant partitioning into CO₂. An understanding of surfactant partitioning between brine and scCO₂ is particularly important in the design of CO₂ foam processes, particularly for surfactant stabilized foam in subsurface systems, where it can affect surfactant transport and foam propagation. In general, the solubility in scCO₂ increased with pressure and decreased with temperature. The partitioning of the surfactants between CO₂ and water phases was almost proportional to pressure, and decreased as temperature increased, where the latter held more sensitivity. The partition coefficient was very sensitive to surfactant formula. For the 2-EH-PO5-EO_x series, the partition coefficient between scCO₂ and the aqueous phase increased with decreasing EO content.     

Gemini Surfactants with Polymethylene Spacer: Supramolecular Structures at Solid Surface and Aggregation in Aqueous Solution

Aggregation of α,ω-bisammonium cationic gemini surfactants with a variable polymethylene spacer and two dodecyl chains has been studied on a solid surface and in aqueous solution. Scanning electron microscopy and dynamic light scattering with the time-resolved fluorescence quenching technique were used for the experiments on the solid surface and in aqueous solution, respectively. As the results from the scanning electron microscopy indicate, the morphology of supramolecular structures of gemini surfactants at the solid surface depends on the spacer length. In aqueous solution, gemini surfactants with spacers consisting of 4, 6, 8, 10, and 12 CH₂ groups form spherical micelles with diameters between 2 and 3.5 nm. Micelles of gemini surfactant with a short ethylene spacer show an increase in size up to 13 nm at the maximum concentration investigated. The aggregation number of micelles determined by time resolved fluorescence quenching was found to be in the range 14–25 for the spacer lengths from 6 to 12 CH₂ groups with only a moderate increase with surfactant concentration. For micelles of gemini surfactants with the short ethylene spacer, the increase of the aggregation number up to 50 at the maximum concentration was observed. The findings support micellar growth of gemini surfactants with short ethylene spacer.     

Carbon dioxide absorption in a cross-flow rotating packed bed

This work investigates the feasibility of applying the cross-flow rotating packed bed (RPB) to the removal of carbon dioxide (CO₂) by absorption from gaseous streams. Monoethanolamine (MEA) aqueous solution was used as the model absorbent. Also, other absorbents such as the NaOH and 2-amino-2-methyl-1-propanol (AMP) aqueous solutions were compared with the MEA aqueous solution. The CO₂ removal efficiency was observed as functions of rotor speed, gas flow rate, liquid flow rate, MEA concentration, and CO₂ concentration. Experimental results indicated that the rotor speed positively affects the CO₂ removal efficiency. Our results further demonstrated that the CO₂ removal efficiency increased with the liquid flow rate and the MEA concentration; however, decreased with the gas flow rate and the CO₂ concentration. Additionally, the CO₂ removal efficiency for the MEA aqueous solution was superior to that for the NaOH and AMP aqueous solutions. Based on the performance comparison with the conventional packed bed and the countercurrent-flow RPB, the cross-flow RPB is an effective absorber for CO₂ absorption process.     

Absorption of carbon dioxide by the absorbent composed of piperazine and 2-amino-2-methyl-1-propanol in PVDF membrane contactor

This paper tests the performance of microporous polyvinylidinefluoride (PVDF) hollow fiber in a gas absorption membrane process (GAM) using the aqueous solutions of piperazine (PZ) and 2-amino-2-methyl-1-propanol (AMP). Experiments were conducted at various gas flow rates, liquid flow rates and absorbent concentrations. Experimental results showed that wetting ratio was about 0.036% when used with the aqueous alkanolamine solutions, while that was 0.39% with aqueous piperazine solutions. The CO₂ absorption rates increased with increasing both liquid and gas flow rates at N_Re < 20. The increase of the PZ concentration showed an increase of absorption rate of CO₂. The CO₂ absorption rate was much enhanced by the addition of PZ promoter. The resistance of membrane was predominated as using a low reactivity absorbent and can be neglected as using absorbent of AMP aqueous solution. The resistance of gas-film diffusion was dominated as using the mixed absorbents of AMP and PZ. An increase of PZ concentration, the resistance of liquid-film diffusion decreased but resistance of gas-film increased. Overall, GAM systems were shown to be an effective technology for absorbing CO₂ from simulated flue gas streams, but the viscosity and solvent-membrane relationship were critical factors that can significantly affect system performance.     

Interfacial area in cocurrent gas—liquid upward flow in tubes of various size

Interfacial area was measured for the case of cocurrent gas—liquid upward flow in tubes 10, 15 and 20 mm i.d. using the technique of absorption with fast chemical reaction of carbon dioxide into 1 N aqueous solution of sodium hydroxide. Measured data on pressure drop and void fraction are also reported. The interfacial area showed a maximum at a different superficial velocity for different tubes. The effect of liquid superficial velocity on the interfacial area was rather small. A successful correlation was obtained in terms of the parameter aQ_L/α_L against the frictional part of the pressure drop. There is an indication that there is a difference in the flow conditions between the 1O mm tube and the larger 15 and 2O mm tubes.     

Performance and pressure drop of CO2 absorption into task-specific and halide-free ionic liquids in a microchannel

The gas–liquid two-phase flow pattern, absorption rate and pressure drop of CO₂ absorbed into the aqueous solution of the task-specific ionic liquids (1-aminopropyl-3-methylimidazole tetrafluoroborate [Apmim][BF₄] and 1-hydroxyethyl-3-methylimidazole tetrafluoroborate [OHemim][BF₄]) and halide-free ionic liquid 1-butyl-3-methylimidazolium methylsulfate [Bmim][CH₃SO₄] were investigated in a microreactor. The absorption mechanism of the three ionic liquids was analyzed employing the ¹³C NMR spectroscopy. The [Apmim][BF₄] was found to have the best ability of CO₂ capture compared with the other two ionic liquids, as chemical absorption occurred between [Apmim][BF₄] and CO₂, while only physical absorption took place between [OHemim][BF₄]/[Bmim][CH₃SO₄] and CO₂. The sequence of CO₂ absorption rate in three ionic liquids aqueous solutions is: [Apmim][BF₄] > [Bmim][CH₃SO₄] > [OHemim][BF₄]. Furthermore, the effects of gas–liquid flow rate and ionic liquids concentration on CO₂ absorption rate and pressure drop were studied, the pressure drop models based on various flow patterns were proposed.     

Absorption of SO2 by aqueous NaOH solutions in the presence of a surfactant

A report is presented on the influence of liquid flow rate, NaOH concentration, column length and presence of 5 × 10^-3 wt-% of the surfactant SLS on the rate of absorption of pure SO₂ by aqueous NaOH solution in a sphere-and-cylinder column. The presence of sodium lauryl sulphate (SLS) prevented axial turbulence which increased mass transfer in longer columns and was almost independent of the flow rate. The enhancement factor due to the reaction between SO₂ and NaOH with respect to the process of physical absorption was analyzed for the systems with excess OH^- in the outflow. The results obtained in the presence of a surfactant are satisfactorily explained by film theory with a single reaction plane model. Those obtained in the absence of surfactant are best described by a two-plane model using renewal theory.     

Synthesis and Properties of Quaternary Ammonium Gemini Surfactants with Hydroxyethyl at Head Groups

A series of cationic gemini surfactants C_mH_{2m + 1}N⁺(CH₂CH₂OH)₂ (CH₂)_s N⁺(CH₂CH₂OH)₂C_mH_{2m + 1} 2Br⁻, referred to as m-s-m (OH) (m = 8,10,12, s = 3,4), were prepared by quaternization of dihydroxyethyl tertiary amines with dibromoalkane. The dihydroxyethyl tertiary amines were synthesized by nucleophilic substitution of diethanolamine with bromoalkane. The characterization of the m-s-m (OH) surfactants was performed using ¹H NMR and MS. The surface activities and aggregation behavior in aqueous solution of the m-s-m (OH) surfactants were studied using surface tension measurements, dynamic light scattering (DLS), and transmission electron microscopy (TEM). The surface tension and critical micellar concentration of these surfactants in aqueous solution decreased dramatically due to the introduction of hydroxyethyl at the head group. The micelles and/or vesicles formed in the aqueous solution of m-s-m (OH) surfactants were strongly dependent upon the lengths of spacer chains and carbon chains. The number of vesicles increases and that of micelles decreases when the lengths of spacer chains and carbon chains increase.     

Simultaneous absorption of CO<Subscript>2</Subscript> and SO<Subscript>2</Subscript> into aqueous AMP/NH<Subscript>3</Subscript> solutions in binary composite absorption system

To enhance the absorption rate for CO₂ and SO₂, aqueous ammonia (NH₃) solution was added to an aqueous 2-amino-2-methyl-1-propanol (AMP) solution. The simultaneous absorption rates of AMP and a blend of AMP+ NH₃ for CO₂ and SO₂ were measured by using a stirred-cell reactor at 303 K. The process operating parameters of interest in this study were the solvent and concentration, and the partial pressures of CO₂ and SO₂. As a result, the addition of NH₃ solution into aqueous AMP solution increased the reaction rate constants of CO₂ and SO₂ by 144 and 109%, respectively, compared to that of AMP solution alone. The simultaneous absorption rate of CO₂/SO₂ on the addition of 1 wt% NH₃ into 10 wt% AMP at a p _A1 of 15 kPa and p _A2 of 1 kPa was 5.50×10⁻⁶ kmol m⁻² s⁻¹, with an increase of 15.5% compared to 10 wt% AMP alone. Consequently, the addition of NH₃ solution into an aqueous AMP solution would be expected to be an excellent absorbent for the simultaneous removal of CO₂/SO₂ from the composition of flue gas emitted from thermoelectric power plants.     

Absorption of SO<Subscript>2</Subscript> using PVDF hollow fiber membranes with PEG as an additive

In this study, removal of SO₂ from gas stream was carried out by using microporous polyvinylidene fluoride (PVDF) asymmetric hollow fiber membrane modules as gas-liquid contactor. The asymmetric hollow fiber membranes used in this study were prepared polyvinylidene fluoride by a wet phase inversion method. Water was used as an internal coagulant and external coagulation bath for all spinning runs. An aqueous solution containing 0.02 M NaOH was used as the absorbent. This study attempts to assess the influence of PEG additive, absorbent flow rate, SO₂ concentration, gas flow rate and gas flow direction on the SO₂ removal efficiency and overall mass transfer coefficient. The effect of liquid flow rate on SO₂ removal efficiency shows that at very low liquid flow rate, the NaOH available at the membrane surface for reacting with SO₂ is limited due to the liquid phase resistance. As liquid flow rate is above the minimum flow rate which overcomes the liquid phase resistance, the SO₂ absorption rate is controlled by resistance in the gas phase and the membrane. The SO₂ absorption rate with inlet SO₂ concentration was sharply increased by using hollow fiber membranes compared to a conventional wetted wall column because the former has higher gas liquid contacting area than the latter. The mass transfer coefficient is independent of pressure. When the gas mixture was fed in the shell side, the removal efficiency of SO₂ declined because of channeling problems on the shell side. Also, the addition of PEG in polymer dopes increased SO₂ removal efficiency. This work was presented at the 6 ^th Korea-China Workshop on Clean Energy Technology held at Busan, Korea, July 4–7, 2006.     

Integration of reverse osmosis and membrane crystallization for sodium sulphate recovery

Reverse osmosis and membrane crystallization are evaluated in this work as stand-alone and integrated technologies for the recovery of Na₂SO₄ from aqueous solutions. When SO₂ is removed from flue gases by absorption in an aqueous solution and reacts with NaOH, a reusable product (i.e., Na₂SO₄) of industrial interest can be obtained.For stand-alone reverse osmosis, the effect of the concentration of the feed solution and pressure is studied. For membrane crystallization, the influence of the concentration and flow rate of the feed and osmotic solutions on the process performance has been determined. The characterization of the obtained crystals shows that Na₂SO₄·10H₂O is obtained. From the experimental results, the potential for integration of reverse osmosis and membrane crystallization is simulated. It was concluded that using a reverse osmosis unit prior to the membrane crystallization unit minimizes the total membrane area in comparison with the stand-alone processes.     

Kinetics of CO2 absorption into a novel 1‐diethylamino‐2‐propanol solvent using stopped‐flow technique

A stopped‐flow apparatus was used to measure the kinetics of carbon dioxide (CO₂) absorption into aqueous solution of 1‐diethylamino‐2‐propanol (1DEA2P) in terms of observed pseudo‐first‐order rate constant (k_o) and second‐order reaction rate constant (k₂), in this work. The experiments were conducted over a 1DEA2P concentration range of 120–751 mol/m³, and a temperature range of 298–313 K. As 1DEA2P is a tertiary amine, the base‐catalyzed hydration mechanism was, then, applied to correlate the experimental CO₂ absorption rate constants obtained from stopped‐flow apparatus. In addition, the pK_a of 1DEA2P was experimentally measured over a temperature range of 278–333 K. The Brønsted relationship between reaction rate constant (obtained from stopped‐flow apparatus) and pK_a was, then, studied. The results showed that the correlation based on the Brønsted relationship performed very well for predicting the absorption rate constant with an absolute average deviation of 5.2%, which is in an acceptable range of less than 10%. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3502–3510, 2014     

Removal of nitric oxide and sulfur dioxide from flue gases using a FeII-ethylenediamineteraacetate solution

The combined absorption of NO and SO₂ into the Fe(II)-ethylenediamineteraacetate(EDTA) solution has been realized. Activated carbon is used to catalyze the reduction of Fe^III-EDTA to Fe^II-EDTA to maintain the ability to remove NO with the Fe-EDTA solution. The reductant is the sulfite/bisulfite ions produced by SO₂ dissolved into the aqueous solution. Experiments have been performed to determine the effects of activated carbon of coconut shell, pH value, temperature of absorption and regeneration, O₂ partial pressure, sulfite/bisulfite and chloride concentration on the combined elimination of NO and SO₂ with Fe^II-EDTA solution coupled with the Fe^II-EDTA regeneration catalyzed by activated carbon. The experimental results indicate that NO removal efficiency increases with activated carbon mass. There is an optimum pH of 7.5 for this process. The NO removal efficiency increases with the liquid flow rate but it is not necessary to increase the liquid flow rate beyond 25 ml min^?1. The NO removal efficiency decreases with the absorption temperature as the temperature is over 35 °C. The Fe²⁺ regeneration rate may be speeded up with temperature. The NO removal efficiency decreases with O₂ partial pressure in the gas streams. The NO removal efficiency is enhanced with the sulfite/bisulfite concentration. Chloride does not affect the NO removal. Ca(OH)₂ and MgO slurries have little influence on NO removal. High NO and SO₂ removal efficiencies can be maintained at a high level for a long period of time with this heterogeneous catalytic process.     

Simultaneous Absorption of Hydrogen Sulfide and Carbon Dioxide into di-isopropanolamine Solution

The simultaneous absorption of hydrogen sulfide and carbon dioxide into di-isopropanolamine (DIPA) solution was investigated in a 183 cm long, 2.72 cm OD wetted-wall column at atmospheric pressure. The influence of liquid flow rate, gas flow rate, temperature and liquid concentration on the absorption rate, overall gas-phase mass transfer coefficient and selectivity factor were studied at a constant gas feed ratio. The results show that the absorption rate of CO₂ increases rapidly with increasing liquid flow rate (the Reynolds number of the turbulent liquid film ranges from 2600 to 4350) but increases moderately with increasing gas flow rate (G = 18-91 L/min), indicating that it is liquid-phase mass transfer controlled. In contrast, the absorption rate of H₂S increases very slowly with increasing liquid flow rate but increases rapidly with increasing gas flow rate, indicating that it is gas-phase mass transfer controlled. The absorption rate of CO₂ also increases with increasing temperature (26-80°C) but H₂S absorption rate decreases with increasing temperature. When the concentration of DIPA solution increases from 0.2 to 2.6 mol/L, the absorption rate of both CO₂ and H₂S increases but with a larger rate of increase for CO₂ For selective H₂S removal, it is preferable to operate at low liquid and high gas flow rates, low temperatures and low DIPA concentrations.     

Investigation of interfacial area and void fraction in upward,cocurrent gas-liquid flow

Interfacial area was measured in cocurrent, gas-liquid upward flow through a vertical, 2.54 cm I.D. and 2.67 m long tube by absorption of CO₂ diluted in air into aqueous sodium hydroxide solution. Void fraction and pressure drop were also measured for superficial liquid velocities up to 1.3 m/s. Although the interfacial area exhibited a maximum and minimum with increasing superficial gas velocities at relatively high liquid flow rates as in previous experiments, several significant differences were found. Since the dissipation parameters proposed in the past to correlate interfacial area were found to be less than satisfactory, a new empirical relation is proposed, which can correlate most of the present data within ± 20%.     

Mass transfer of carbon dioxide in aqueous colloidal silica solution containing <Emphasis Type="Italic">N</Emphasis>-methyldiethanolamine

The absorption rate (R _A ) of carbon dioxide was measured into an aqueous nanometer sized colloidal silica solution of 0–31 wt% and N-methyldiethanolamine of 0–2 kmol/m3 in a flat-stirred vessel for the various sizes and speeds of at 25 °C and 0.101 MPa to obtain the volumetric liquid-side mass transfer coefficient (k _L a) of CO₂. The film theory accompanied by chemical reaction between CO₂ and N-methyldiethanolamine was used to estimate the theoretical value of absorption rate of CO₂. An empirical correlation formula containing the relationship between k _L a and rheological property of the aqueous colloidal silica solution was presented. The value of R _A in the aqueous colloidal silica solution was decreased by the reduction of k _L a due to elasticity of the solution.     

CO<Subscript>2</Subscript> capture using aqueous solutions of K<Subscript>2</Subscript>CO<Subscript>3</Subscript>+2-methylpiperazine and monoethanolamine: Specific heat capacity and heat of absorption

The specific heat capacity, heat of CCO₂ absorption, and CCO₂ absorption capacity of aqueous solutions of potassium carbonate (K₂CO₃)+2-methylpiperazine (2-MPZ) and monoethanolamine (MEA) were measured over various temperatures. An aqueous solution of K₂CO₃+2-MPZ is a promising absorbent for CCO₂ capture because it has high CCO₂ absorption capacity with improved absorption rate and degradation stability. Aqueous solution of MEA was used as a reference absorbent for comprison of the thermodynamic characteristics. Specific heat capacity was measured using a differential scanning calorimeter (DSC), and heat of CCO₂ absorption and CCO₂ absorption capacity were measured using a differential reaction calorimeter (DRC). The CCO₂-loaded solutions had lower specific heat capacities than those of fresh solutions. Aqueous solutions of K₂CO₃+2-MPZ had lower specific heat capacity than those of MEA over the temperature ranges of 303-353 K. Under the typical operating conditions for the process (CCO₂ loading=0.23mol-CCO₂·mol^?1-solute in fresh solution, T=313 K), the heat of absorption (?ΔHabs) of aqueous solutions of K₂CO₃+2-MPZ and MEA were approximately 49 and 75 kJ·mol-CO₂, respectively. The thermodynamic data from this study can be used to design a process for CCO₂ capture.