Liquid Film Mass Transfer Coefficient Research Articles

Dynamic modeling and control of an intercooled absorber for post-combustion CO2 capture

A medium-order dynamic model was developed in gPROMS® for an intercooled CO2 absorber column with piperazine solvent. This model represents an improvement from previously published rate-based dynamic models because it uses a regressed eNRTL thermodynamic package and a liquid film mass transfer coefficient with an experimentally measured CO2 loading dependence. Off-design simulation results were validated against a high-order steady state model, and the maximum predicted column temperature differed by less than 1.2°C for 100–85% flue gas load. Operating the absorber at a constant liquid to gas (L/G) ratio results in favorable dynamic behavior with disturbances in flue gas load, but a commercial system will not have a reliable flue gas flow measurement. The medium-order model was used to identify a liquid phase temperature within the top bed of absorber packing as a control variable that is sensitive to changes in L/G ratio. As the flue gas load was ramped from 100% to 80%, a maximum L/G ratio deviation of 0.9% was observed when controlling the column temperature with the solvent flowrate. Feedforward control may be necessary to account for other process disturbances that influence the temperature profile and was shown to be effective at counteracting a step change in intercooling temperature.

Adsorption of Cefocelis Hydrochloride on Macroporous Resin: Kinetics, Equilibrium, and Thermodynamic Studies

The adsorptions of Cefocelis hydrochloride in aqueous solution on macroporous resin (HP-20) were studied. On the basis of static experiments with HP-20 resin, the adsorption kinetics, isotherms, and thermodynamics of Cefocelis hydrochloride in aqueous solution on macroporous resin (HP-20) were investigated. Adsorption equilibrium data were correlated with the Langmuir and Freundlich equations. Adsorption data of kinetic were modeled using the Crank equation and fitting for the diffusion coefficient (De), and the value of the liquid film mass transfer coefficient (kf) was estimated from Carberry equation. Dynamic adsorption was performed on HP-20 resin packed in a glass column to obtain optimal parameters. The initial concentration, bed height, and residence time were considered to determine the dynamic test for the breakthrough curve. The values of thermodynamic parameters including the isosteric adsorption enthalpy (ΔH), free energy (ΔG), entropy (ΔS), and adsorption activation energy (Ea) demonstrated t...

Measuring Nitrous Oxide Mass Transfer into Non-Aqueous CO2BOL CO2 Capture Solvents

This paper investigates CO2 absorption behavior in CO2BOL solvents by decoupling the physical and chemical effects using N2O as a nonreactive mimic. Absorption measurements were performed using a wetted-wall contactor. Testing was performed using a “first generation” CO2-binding organic liquid (CO2BOL), composed of an independent base and alcohol. Measurements were made with N2O at a lean (0.06 mol CO2/mol BOL) and rich (0.26 mol CO2/mol BOL) loading, each at three temperatures (35, 45, and 55 °C). Liquid-film mass transfer coefficients (kg′) were calculated by subtracting the gas film resistance-determined from a correlation from literature–from the overall mass transfer measurement. The resulting kg′ values for N2O in CO2BOLs were found to be higher than that of 5 M aqueous MEA under comparable conditions, which is supported by published measurements of Henry’s coefficients for N2O in various solvents. These results suggest that the physical solubility contribution for CO2 absorption in CO2BOLs is great...

A simple and timesaving method for the mass-transfer assessment of solvents used in physical absorption

A simple dynamic absorption procedure to assess the mass-transfer performances of a solvent toward a selected gaseous solute is presented. Absorption was operated semi-continuously at transient state until the equilibrium was reached without solvent recirculation. Four volatile organic compounds (VOC) more or less hydrophobic (toluene, acetone, dichloromethane, isopropanol) were absorbed in water and two heavy organic solvents (Bis(2-ethylhexyl) adipate DEHA and polydimethylsiloxane PDMS). A numerical resolution procedure was developed to simulate the gas–liquid mass-transfer and to deduce the VOC partition coefficients, expressed as the Henry’s law constants, as well as the overall liquid-phase mass-transfer coefficients. The overall liquid-phase mass-transfer coefficients were correlated to the diffusion coefficients using the Higbie penetration theory. The results confirm the high selectivity of water whereas the two organic solvents, especially DEHA, exhibit rather good affinity with all VOC even if the Henry’s law constants of the most soluble and the less soluble compounds for those solvents differ by 1 or 2 orders of magnitude. The liquid-film mass-transfer coefficients in the two organic solvents, even being more viscous, are larger than in water which confirms their good potential for hydrophobic VOC treatment.

Measuring the Absorption Rate of CO2 in Nonaqueous CO2-Binding Organic Liquid Solvents with a Wetted-Wall Apparatus.

The kinetics of the absorption of CO2 into two nonaqueous CO2-binding organic liquid (CO2 BOL) solvents were measured at T=35, 45, and 55 °C with a wetted-wall column. Selected CO2 loadings were run with a so-called "first-generation" CO2 BOL, comprising an independent base and alcohol, and a "second-generation" CO2 BOL, in which the base and alcohol were conjoined. Liquid-film mass-transfer coefficient (k'g ) values for both solvents were measured to be comparable to values for monoethanolamine and piperazine aqueous solvents under a comparable driving force, in spite of far higher solution viscosities. An inverse temperature dependence of the k'g value was also observed, which suggests that the physical solubility of CO2 in organic liquids may be making CO2 mass transfer faster than expected. Aspen Plus software was used to model the kinetic data and compare the CO2 absorption behavior of nonaqueous solvents with that of aqueous solvent platforms. This work continues our development of the CO2 BOL solvents. Previous work established the thermodynamic properties related to CO2 capture. The present paper quantitatively studies the kinetics of CO2 capture and develops a rate-based model.

Oxygen transfer in bubble columns at industrially relevant superficial velocities: Experimental work and CFD modelling

In this study, the oxygen transfer rate (OTR), bubble size distribution, interfacial area per unit volume and liquid film mass transfer coefficient have all been measured in a 0.39m diameter pilot-scale bubble column operated at industrially relevant superficial velocities between 0.14 and 0.28ms−1 for which few data exist in the literature. The measured OTR ranged from 6.5 to 8.8kgm−3h−1, with values generally increasing with superficial velocity. The mean bubble size in the column was 3–5mm, giving an interfacial area per unit volume for mass transfer of some 380–570m2m−3. Experimental data were used to validate a computational fluid dynamics (CFD) based model of the bubble column; it was found that this model could predict the experimentally measured OTR within 20% across the range of superficial velocities examined.

Impact of Surfactant Addition on Oxygen Mass Transfer in a Bubble Column

AbstractThe impact of sparger design and surfactant addition on the oxygen transfer rate in a bubble column was examined. Additionally, measurements were also made of the holdup and bubble size distribution, allowing both the interfacial area for mass transfer and the liquid film mass transfer coefficient to be determined for a range of industrially relevant superficial velocities. It was found that for the velocity range examined changes in the superficial velocity had a minimal impact on the observed value of liquid film mass transfer coefficient. In contrast, addition of both hydrophilic and hydrophobic surface‐active compounds led to an approximately threefold reduction in liquid film mass transfer coefficient.

Hydrodynamics and mass transfer coefficients for a modified Raschig ring packed column

The pressure drop, the liquid holdup, as well as the liquid film mass transfer coefficients (kL) for a modified Raschig packing, with turbulence promoters, used in absorption columns, were determined experimentally. The aim of this work is to verify the improved mass transfer properties of this new packing for the randomly and, particularly, for the arranged packed columns. The experiments were performed at gas velocities ranging from 800 to 2,000 m h−1 and liquid velocities scaling between 2.5 and 8.11 m h−1, ranges that cover most of the absorption column operation conditions. Experimental data and correlations for the pressure drop, the liquid holdup and the gas–liquid mass transfer coefficients (kL) for modified Raschig ring packed columns are presented. The influence of the gas and the liquid velocities on the column hydrodynamics and the mass transfer coefficients have been obtained experimentally and also, have been compared with literature data.

Packing Characterization for Post Combustion CO2 Capture: Mass Transfer Model Development

Three mass transfer properties: the effective area (ae), liquid film and gas film mass transfer coefficients (kL and kG) are measured consistently for eleven packings with different surface area and corrugation angle. The effective area and liquid film mass transfer coefficient increases with liquid velocity while gas film mass transfer coefficient increases with gas velocity. The fractional effective area decreases with surface area and barely changes with corrugation angle. kL and kG increase as surface area increases or corrugation angle decreases. A new concept-mixing point density is proposed to represent the packing geometry influence on kL and kG. Correlations for ae, kL, and kG are developed including effects of gas and liquid rates and packing geometry.

CO2 Mass Transfer and Solubility in Aqueous Primary and Secondary Amine

Abstract Three amines were tested as solvents for CO 2 capture: monoisopropanolamine (MIPA) and 3 amino 1 propanol (MPA), which are primary amines, and diethanolamine (DEA), which is a secondary amine. The three amines were tested at 7 molal in water, which corresponds to 30 wt % monoethanolamine (MEA). The CO 2 absorption rate and solubility in the solvents was measured using a bench scale wetted wall column (WWC) at 20–100 °C and over the range of CO 2 loading expected for coal flue gas. The experimental data are analyzed to suggest performance in a real process. The performance of the three new amines are compared against literature results for 7 m MEA and other solvents with amines of related structures, such as 10 m Diglycolamine (DGA) ® and 4.8 m 2-amino-2-methyl-1-propanol (AMP). The mass transfer performance and the CO 2 carrying ability of the amine are related to the structural hindrance and the pKa. The CO 2 absorption rate of 7 m MIPA and 7 m DEA are competitive with 7 m MEA. For 7 m MPA, the measured liquid film mass transfer coefficient is about 20% lower than 7 m MEA. The CO 2 solubility data are used to generate a semi-empirical model for each of the new solvents. The results for CO 2 solubility at 40 °C suggest that MIPA and MPA have similar CO 2 capacity to MEA, whereas DEA has double the CO 2 capacity of the three primary amines. Based on the comparison of data collected in this work and literature results, it is generalized for unhindered amines that the overall CO 2 mass transfer performance of the solvent is inversely related to the pKa of the amine. Also, the CO 2 capacity of the amine increases with the relative hindrance of the nitrogen group in the amine molecule.

Modeling and Experimental Studies of Adsorptive Dewatering of Selected Aliphatic Alcohols in Temperature Swing Adsorption System

Based on experimental work, closely coupled with mathematical modeling, an analysis was performed of the adsorptive drying of selected water-aliphatic alcohol solutions. The experimental work involved ethanol, n-propanol, and n-butanol dehydration using two adsorbents, 3A and 4A zeolite molecular sieves. The isothermal model developed in this paper incorporates an overall linear mass driving force, including liquid film mass transfer and intraparticle diffusion coefficients, a variable axial diffusivity, and experimentally determined Langmuir-Freundlich isotherms. Breakthrough curves generated by the model were compared with those obtained experimentally, giving a fairly good fit. The model simulations enabled determination of the water content profiles in adsorbent beds.

Modeling of CO2 Absorption Kinetics in Aqueous 2-Methylpiperazine

CO2 absorption into 8 molal (m) 2-methylpiperazine (2MPZ) is modeled in Aspen Plus using a previously developed rigorous thermodynamic model. With the regression of three reaction rate constants for carbamate and bicarbonate and one parameter for diffusion activation energy, the kinetic model represents CO2 flux measured in a wetted-wall column at 40–100 °C with 0.1–0.4 mol CO2/mol alkalinity with a relative deviation from −20% to 20%. The kinetic reaction rate is interpreted with activity-based termolecular kinetics and the rate constants are correlated with the Bro̷nsted theory. The liquid film mass transfer coefficient (kg′) is well represented by the model at medium to rich CO2 loading. The power to which kg′ is dependent on the rate constant of 2MPZ carbamate formation (k2MPZ–2MPZ) decreases from ∼0.5 at very lean loading to nearly 0 at rich loading, whereas it is increasingly dependent upon the diffusion coefficient of the reactants and products and the physical liquid film mass transfer coefficient (kl0) as loading and temperature increases. In the liquid film, 2MPZ and 2MPZCOO– are the major reactants and protonated 2MPZ carbamate and bicarbonate are the major products. The pseudo-first-order region for 2MPZ narrows and shifts to higher value of kl0 as temperature increases. Under typical industrial conditions, the gas film resistance contributes less than 25% of the total resistance.

Characterization of Novel Structured Packings for CO2 Capture

Structured packing is widely being considered for post-combustion CO2 capture because of its high mass transfer per unit of pressure drop. The hydraulic and mass transfer performances of structured packings with different corrugation angles were measured. The effective mass transfer area was measured by absorption of CO2 with 0.1 gmol/L NaOH. The liquid film mass transfer coefficient was measured by desorption of toluene into air. The gas film mass transfer coefficient was measured by absorption of SO2 from air into 0.1 gmol/L NaOH. The effective area and the liquid film mass transfer coefficient are dependent on liquid velocity while the gas film mass transfer coefficient is a function of gas velocity. Increasing the corrugation angle from 45° to 60° (Mellapak 250Y to Mellapak 250X) resulted in (1) a 54% decrease in pressure drop, (2) a 6% decrease in effective area, (3) a 17% decrease in kG, and (4) a 18% decrease in kL. Increasing the corrugation angle from 45° to 70°, a larger difference was found in pressure drop, kG and kL. However, the impact of corrugation angle on effective area is not significant. In conclusion, increasing the corrugation angle will have a benefit on pressure drop with a negative impact on mass transfer properties. Future work will be focused on economic analysis between operation costs (related to pressure drop) and capital costs (related to mass transfer properties) to determine the optimum corrugation angle for a particular separation task.

Absorption rates and CO2 solubility in new piperazine blends

The absorption rate and CO2 solubility of four new blends using concentrated piperazine (PZ) were measured. The blends are 6 m PZ/2 m hexamethylenediamine (HMDA), 6 m PZ/2 m diaminobutane (DAB), 6 m PZ/2 m bis(aminoethyl)ether (BAE), and 5 m PZ/2 m N-2(aminoethy)lpiperazine (AEP). The liquid film mass transfer coefficient (kg’) of the blends was measured at 20 100°C using a wetted wall column (WWC). PCO2* was measured at 20–100°C using the WWC, and at 100–160°C using a total pressure apparatus. A semi-empirical VLE model was regressed for each blend using measured PCO2*, and the models show good agreement with experimental data. The process performance of the new blends is compared to 5 m PZ/2.3 m AMP, 2 m PZ/4 m AMP, 8 m PZ, and 7 m MEA. The high pKa of primary diamines contributes to high lean and rich loading and low solvent capacity for 6 m PZ/2 m HMDA, 6 m PZ/2 m DAB, 6 m PZ/2 m BAE. The Habs of PZ/AEP and PZ/AMP is competitive with 7 m MEA. PZ/BAE and PZ/HMDA have Habs higher than 8 m PZ but lower than 7 m MEA. The energy performance of the solvent depends on CO2 VLE and thermal stability. 6 m PZ/2 m HMDA, 6 m PZ/2 m AEP, and 5 m PZ/2 m BAE show good energy performance and are competitive with 8 m PZ. At 40°C the absorption rate of 5 m PZ/2 m AEP is about the same as 8 m PZ; 6 m PZ/2 m DAB and 6 m PZ/2 m BAE have 10% lower rates than 8 m PZ.

Modeling of Adsorptive Drying of N-Propanol

On the basis of experimental work connected with mathematical modeling, a parameter analysis was conducted on the adsorptive drying of water-miscible organic liquids. The experimental work involved dehydration of n-propanol using two desiccants, 3A and 4A zeolite molecular sieves. The equilibrium relationship and fixed bed breakthrough curves were determined experimentally. The parameters required by the model are the equilibrium constants, liquid film mass transfer coefficient, intraparticle diffusion coefficient, and axial diffusivity. A linear driving force (LDF) relation was applied to represent the mass transfer rate in the particle liquid film and inside the adsorbent particle. The fit of the model to experimental breakthrough data was quite good.

Mass Transfer Mechanism of a Water-Sparged Aerocyclone Reactor

Air stripping of ammonia is a widely used process for the pretreatment of wastewater. Scaling and fouling on the packing surface in packed towers and a lower stripping efficiency are the two major problems in this process. New patented equipment that is suitable for the air stripping of wastewater with suspended solids has been developed. Air stripping of ammonia from water with Ca(OH)2, was performed in the newly designed gas-liquid contactor, a water-sparged aerocyclone (WSA). The mechanism of the mass transfer process in the WSA was investigated using a CO2—NaOH rapid pseudo first order reaction system. The results indicated that there is a critical gas phase inlet velocity Ug. When Ug is lower than this value, the increase of the inlet velocity has a double function of both intensifying the liquid side film mass transfer coefficient kL and increasing the specific mass transfer area a; whereas when Ug is larger than this value, the major function of Ug increase is to make the water drops in the WSA broken, increasing the mass transfer area of gas-liquid phases.

CO2Absorption Rate into Concentrated Aqueous Monoethanolamine and Piperazine

The CO2 equilibrium partial pressure and liquid film mass transfer coefficient (kg′) in (7, 9, 11, and 13) m monoethanolamine (MEA) and (2, 5, 8, and 12) m piperazine (PZ) were measured in a wetted wall column. Also examined was 7 m MEA/2 m PZ. Absorption and desorption experiments were performed at (40, 60, 80, and 100) °C over a range of CO2 loading. Amine concentration does not affect the CO2 partial pressure of PZ or MEA solutions as a function of CO2 loading with less than 0.45 mols CO2/mol alkalinity. Changes in amine concentration and temperature often do not affect the measured value of kg′. At higher temperature and CO2 loading in PZ, the diffusion of reactants and products limits CO2 transfer, and kg′ is depressed. PZ (8 m) exhibits a 70 % greater CO2 capacity than 7 m MEA and a 50 % greater CO2 capacity than 11 m MEA. kg′ decreases by a factor of 30 in aqueous MEA with 0.23 to 0.50 CO2 loading. kg′ decreases by a factor of 20 in aqueous PZ with 0.21 to 0.41 CO2 loading. PZ is shown to absorb CO2 2 to 3 times faster than MEA.

Aqueous piperazine derivatives for CO 2 capture: Accurate screening by a wetted wall column

Concentrated aqueous piperazine (PZ) has been identified as a better solvent for CO 2 capture than monoethanolamine (MEA), because it has a greater rate of CO 2 absorption and greater CO 2 capacity. This work evaluates the effect of substitute groups on PZ performance. Many previous screening studies measured absorption/desorption with CO 2/N 2 sparging, which lacks accuracy and cannot be used to estimate actual absorber performance. In this work a wetted wall column was used to accurately measure absorption/desorption rate at typical rich and lean CO 2 loading ( α) and simulate performance of real packing. The method also provides accurate measurement of CO 2 solubility at 40–100 °C. This study provided rate and solubility data at 40–100 °C and practical ranges of CO 2 loading for 8 m 1-methylpiperazine (1-MPZ), 8 m 2-methylpiperazine (2-MPZ), 4 m 2-MPZ/4 m PZ, 7.7 m N-(2-hydroxyethyl)piperazine (HEP), 6 m 1-(2-aminoethyl)piperazine (AEP), 8 m 2-piperidine ethanol (2-PE), and 2 m trans-2,5-dimethylpiperazine (2,5-DMPZ). With the measurements of CO 2 flux ( N C O 2 ) and equilibrium driving force, liquid film mass transfer coefficients ( k g′) are calculated. The rate decreases as of 1-MPZ = PZ > 2-MPZ/PZ > 2-MPZ > HEP > MEA > AEP = 2-PE. This method also allows bracketing and determination of equilibrium CO 2 partial pressure ( P C O 2 * ) at each condition. Semi-empirical solubility models of CO 2 for each amine were regressed from experimental solubility data to find the lean and rich CO 2 loading corresponding to 0.500 kPa and 5 kPa CO 2 partial pressure respectively. Based on the solubility model, the actual operating capacity of the solvents without overstripping decreases in the sequence of 2-PE > 2-MPZ > 2-MPZ/PZ > 1-MPZ > PZ > HEP > AEP > MEA. The enthalpy of CO 2 absorption (Δ H abs) of all the piperazine derivatives is around 70 kJ/mol CO 2.

Effects of surfactants on hydrodynamics and mass transfer in a split‐cylinder airlift reactor

AbstractEffects of various concentrations (0–5 ppm) of anionic (sodium dodecyl sulfate, SDS) and non‐ionic (Tween‐80 and Triton X‐405) surfactants on gas hold‐up and gas–liquid mass transfer in a split‐cylinder airlift reactor are reported for air–water. Surfactants were found to strongly enhance gas hold‐up. Non‐ionic surfactants were more effective in enhancing gas hold‐up compared to the anionic surfactant SDS. An enhanced gas hold‐up and a visually reduced bubble size in the presence of surfactants implied an enhanced gas–liquid interfacial area for mass transfer. Nevertheless, the overall gas–liquid volumetric mass transfer coefficient was reduced in the presence of surfactants, suggesting that surfactants greatly reduced the true liquid film mass transfer coefficient and this reduction outweighed the interfacial area enhancing effect. Presence of surfactants did not substantially affect the induced liquid circulation rate in the airlift vessel.

Accurate screening of amines by the Wetted Wall Column

Abstract To screen amine solvents accurately for CO 2 capture, a Wetted Wall Column (WWC) was used to measure equilibrium CO 2 partial pressure and CO 2 absorption/desorption rate at variable CO 2 loading from 40 to 100 °C. The solvents included 10 m diglycolamine (DGA ® ), 4.8 m 2-amino-2-methyl-propane (AMP), 8 m N-methyl-1,3-propanediamine (MAPA), 7 m/2 m and 5 m/5 m methyldiethanolamine (MDEA)/piperazine (PZ). With a semi-empirical VLE model assuming a lean/rich CO 2 loading corresponding to 500 Pa/5000 Pa CO 2 partial pressure, cyclic capacity and heat of CO 2 absorption was determined. Liquid film mass transfer coefficients are reported for each solvent, which allows estimation of packing area required for 90 removal via a simple absorber design. The results show that the capacity of DGA ® and MAPA is 10–20% less than 7 m MEA with a 5 to 15% slower rate. 4.8 m AMP has a capacity twice as great as 7 m MEA, but the rate is lower by 45%. 7 m/2 m MDEA/PZ has a similar capacity to 8 m PZ but slightly slower rate. 5 m /5 m MDEA/PZ has a capacity 20% greater than 8 m PZ and a comparable rate. The heat of CO 2 absorption in the primary amines is about 80 kJ/mol CO 2 . The value for PZ and its blend with MDEA is about 70 kJ/mol CO 2 .