Liquid-side Resistance Research Articles

PEBAX polymer inclusion hydrophobic deep eutectic solvent membranes for pervaporation of biobutanol: Mass transfer and COSMO-RS analysis

Deep Eutectic Solvent(DES) and specifically Hydrophobic Deep Eutectic Solvents (HDES) are considered a relatively novel class of solvents, which show good features to include them in pervaporation membranes. Polymer inclusion HDES membranes offer a separation media, which shows a faster molecular diffusion than polymeric membranes, combining the best properties of liquid and polymer membranes, such a high selectivity with high burst pressure and durability.The aim of this work focuses on the development of different PEBAX/ lidocaine-thymol [2:1] (Lidol HDES) membranes by the temperature-induced phase-inversion technique to be used in the pervaporation process to recover butanol from ABE model solution. The mass transfer modeling through the membrane using the resistances-in-series approach was implemented to find the mass transfer resistance distribution.The polymer inclusion membranes showed improved results for the butanol/water selectivity compared to the single PEBAX membrane used as a reference. The flux of butanol obtained with the 30 % w/w Lidol HDES membrane was 0.112 kg m−2 hr−1. The flux of water was 0.445 kg m−2 hr−1 showing a selectivity value for butanol/water of 20 % bigger than single PEBAX membrane. The improvement in the selectivity can be explained by the synergic effect of the HDES in the membrane.After use, the polymer inclusion membrane consistently demonstrated a stable performance in effectively separating butanol from the ABE solution. It was seen that the overall resistance decreases as the liquid flow rate increases; regarding the liquid side resistance, it becomes important at smaller flow rates and is almost negligible for turbulent regimen.

Capillary microbioreactors for VOC vapor treatment: Impacts of operating conditions

Micro-capillary bioreactors (1 mm ID, 10 cm long) were investigated for the biodegradation of toluene vapors as a model volatile organic compound (VOC). The intended application is the removal of VOCs from indoor air, when such microbioreactor is coupled with a microconcentrator that intermittently delivers high concentrations of VOCs to the bioreactor for effective treatment. The effects of key operating conditions were investigated. Specifically, gas film and liquid film mass transfer coefficients were determined for different gas and liquid velocities. Both mass transfer coefficients increased with gas or liquid velocity, respectively, and the overall gas-liquid mass transfer was dominated by the liquid-side resistance. Experiments with the microbioreactors focused on the effects of gas velocity, liquid velocity and mineral medium renewal rate on the treatment of toluene vapors at different inlet concentrations. The best performance in terms of toluene removal and mineralization to CO2 was obtained when the gas and liquid velocity ratio was close to one and achieving Taylor or slug flow pattern. Sustained treatment over extended periods of time with toluene elimination capacities ranging from 4000 to over 9000 g m−3 h−1 were obtained, which is orders of magnitude greater than conventional biofilters and biotrickling filters. Biological limitations generally played a more important role than mass transfer limitation. Continuous mineral medium supply at a high rate (10 h liquid retention time) enabled pH control and provided ample nutrient supply and therefore resulted in better toluene elimination and mineralization. Overall, these studies helped select the most suitable conditions for high performance and sustained operation.

Mass transfer characteristics of a continuously operated hollow-fiber membrane contactor and stripper unit for CO2 capture

Mass transfer performance of a polypropylene hollow-fiber membrane contactor as part of a continuously operated CO2 capture unit with amino acid salt (potassium glycinate) absorbent and vacuum solvent regeneration was studied. The effects of key operating parameters on the absorption mass transfer characteristics were explored. Without vacuum stripping, absorption rate was found to be limited by low CO2 desorption efficiency from the loaded absorbent solution in the stripping unit, resulting in high solvent CO2 loadings and limited chemical absorption rates. Introduction of vacuum stripping greatly improved desorption performance, resulting in improved steady-state absorption performance. The overall mass transfer coefficient increased at higher stripping temperatures and lower vacuum pressures in the range of 60−80 °C and 300−800 mbar (abs). The overall mass transfer coefficient increased with increasing liquid flow rate, and the highest value reached was 1.8 ∙ 10−4 m s-1. The individual mass transfer coefficients in absorption were calculated based on mass transfer correlations and experimental data, including estimation of the enhancement factor for chemical absorption. The overall mass transfer resistance was found to be dominated by the liquid-side resistance, at almost 90 % of the total resistance. The estimated membrane mass transfer coefficient was low compared to a theoretical value assuming non-wetted operation, suggesting potential partial wetting of the membrane. Stable performance of the unit and the membrane contactor was demonstrated during a stability test with over 30 h of operation.

Modeling and experimental studies of ammonia absorption in a spray tower

We did an experimental study on ammonia absorption in a spray tower. The kinetic characteristics of droplets were investigated by considering the forces acting on a droplet. The gas-phase mass transfer coefficient was deduced with the Colburn analogy method. A simplified model for predicting NH3 capture with a spray tower has been presented based on mass transfer and ionic equilibrium, which was successfully validated against experimental data. The influences and the sensitivity analysis of main operating parameters on the absorption efficiency were analyzed. As the most sensitive parameter, the mean droplet diameter was obtained by fitting experimental data with an empirical correlation. The distributions of two typical process parameters along the absorber were also simulated. Decreasing the pH value of absorbent is an effective but restrictive way to strengthen the mass transfer rate on account of insufficient liquid-side resistance.

CO2 quality control through scrubbing in oxy-fuel combustion: An evaluation of operational pH impacts, and prediction of SO2 absorption rate at steady state

Oxy-fuel combustion is a promising CCS technology which is being demonstrated prior to commercialisation. While the flue gas in oxy-fuel combustion is concentrated in CO2, it contains impurities such as SO2. The elimination of SO2 can provide a clean CO2 stream ready for storage. This paper is to understand the absorption of SO2 in scrubbing relevant to those used in oxy-fuel technology.Steady state experiments were conducted in a continuous well stirred reactor to understand the absorption rate of SO2/CO2 into a total concentration of 0.28M of mixtures of NaHSO3 and NaHCO3 simulating liquids formed by scrubbers using NaOH as the reagent at solution pH values from 4 to 7 with the exiting gas concentrations of SO2 from 19ppm to 1500ppm and a constant CO2 concentration of 70%. Online measurement included gas phase SO2 and liquid pH, and offline measurement included CO2 (aq), HCO3−, S (IV), SO32− and S (VI) after each experiment. Three aspects investigated were the impacts of pH on the solution chemistry, the significance of solution pH and the concentration of gas phase SO2 on the absorption rate of SO2.The total sulphur concentration in liquid was found to be related to the effectiveness of Na+. The effective ratio of Na+ can be defined as the total sulphur to Na+ ratio and this effectiveness ratio of Na+ is pH dependent. At pH<5, Na+ is 99% effective. It reduces dramatically from 99% at a pH 5 to less than 15% at pH above 7. With regard to carbon based species also absorbed, super saturation of CO2 (aq) was observed at pH>5.5. The concentration of HCO3− increases dramatically above pH 6 and below this pH, the concentration of HCO3− is negligible.The absorption rate of SO2 was found to increase with pH with some increase with the concentration of SO2. The operational pH window for scrubbing may be defined by an upper limit pH where the absorption rate of SO2 starts to decreases from the maximum absorption rate of SO2 and the lower limit pH where the absorption rate of SO2 reduces to half of the maximum absorption rate of SO2. Both the upper limit and the lower limit decrease initially and stay stable with the concentration of SO2. This decrease is caused by the reversible reaction of the hydrolysis of SO2 and confirmed by equilibrium experiments of SO2 and sodium solutions. Operation within region 2 (pH 5–6) is recommended, depending on the scrubber design. The operation exit pH of the produced liquid can be varied within the region.The absorption rates of SO2 obtained in the steady state experiments were predicted by a model based on the instantaneous reaction assumption. This model generally overestimates the absorption rates of SO2 at pH values below 6 indicating a kinetic limitation of SO2 and water reaction at low pH values. The analysis on the controlling regions indicates that the gas side mass transfer resistance decreases with the concentration of SO2. Liquid side resistance becomes more important at a lower pH and a higher concentration of SO2.

Contribution of Liquid- and Gas-Side Mass Transfer Coefficients to Overall Mass Transfer Coefficient in Taylor Flow in a Microreactor

Abstract Gas-liquid microreactors find an increasing range of applications both in production, and for chemical analysis. The most often employed flow regime in these microreactors is Taylor flow. The rate of absorption of gases in liquids depends on gas-side and liquid-side resistances. There are several publications about liquid-side mass transfer coefficients in Taylor flow, but the data about gas-side mass transfer coefficients are practically non existent. We analysed the problem of gas-side mass transfer resistance in Taylor flow and determined conditions, in which it may influence the overall mass transfer rate. Investigations were performed using numerical simulations. The influence of the gas diffusivity, gas viscosity, channel diameter, bubble length and gas bubble velocity has been determined. It was found that in some case the mass transfer resistances in both phases are comparable and the gas-side resistance may be significant. In such cases, neglecting the gas-side coefficient may lead to errors in the experimental data interpretation.

Effect of fiber packing density on physical CO2 absorption performance in gas–liquid membrane contactor

Microporous polyvinylidene fluoride (PVDF) hollow fiber membranes were fabricated via wet-spinning process. The prepared fibers were characterized in terms of gas permeation, wetting pressure and membrane morphology. Physical absorption of CO2 from distilled water was conducted through the gas–liquid membrane contactors. The overall mass transfer resistance of the membrane system was evaluated using Wilson plot analysis. The spun membranes have good gas permeability, high wetting pressure and reasonably effective surface porosity which is appropriate for mass transfer between gas and liquid phases. It was found that by increasing the packing density of the membrane module, CO2 absorption flux decreases with reductions in liquid velocity. At increasing packing densities of up to 13%, liquid side resistance contributed to more than 90% of the overall mass transfer resistance due to ‘dead zone’ phenomena. The overall mass transfer coefficient of the module increased as the packing density increased from 2.6% to 13%. A clear trend of mass transfer resistance can be observed when the fiber packing density was varied accordingly in gas–liquid membrane contactor systems.

Influence of pH on gas phase controlled mass transfer in a membrane contactor for hydrogen sulphide absorption

The influence of pH on mass transfer in a hollow fibre membrane contactor applied to a dilute hydrogen sulphide (H2S) gas stream has been studied. When using sodium hydroxide to control pH, a threshold was established at pH 10 where the reaction between sodium hydroxide and H2S could be described as pseudo-first order with respect to H2S, indicating an excess of reactive hydroxide. For solvent pH above 10, the reactivity counteracted the low H2S solubility constant and negated liquid side resistance thus the process was predominantly gas phase controlled. Whilst the gas side hydrodynamics were firmly within the laminar regime, the experimentally determined overall mass transfer coefficient recorded in this study compared well to previous investigations at higher solvent concentrations. Based on the experimental data, a solvent pH of 11 was deemed most economically attractive. This sodium hydroxide concentration facilitates efficient H2S removal at solvent concentrations several orders of magnitude below those proposed in previous contactor studies. The data therefore illustrates that less concentrated chemical solvents can be used without demanding further membrane area to counterbalance the lower reactivity since [OH−] is already in excess at these solvent concentrations.

CO2 desorption from rich alkanolamine solution by using membrane vacuum regeneration technology

Aiming to achieve lower regeneration energy consumption and less absorbent loss, membrane vacuum regeneration technology was developed as the novel regeneration technology for CO2 enrichment in the chemical absorption process. In this study, monoethanolamine (MEA) was selected as absorbent and polypropylene (PP) hollow fiber membrane contactors were used to test the regeneration performance. Two different operating modes, solution flowing in tube side (mode A) and in shell side (mode B) of membrane module were compared by investigating mass transfer individual resistances and the influences of key operating parameters on CO2 desorption flux for both modes. The experimental results showed that mode A was superior to mode B, and liquid side resistance was dominant in overall mass transfer resistance for both modes when membrane was gas-filled. By optimizing the operating condition, better regeneration performance could be achieved to CO2 desorption flux 2.2×10−4mol/m2s and lower total equivalent work compared to thermal regeneration. In addition, amine loss problem, MEA concentration variation after regeneration and pressure drop on liquid side were also discussed, it showed MEA loss could be limited at 0.5g MEA/kg CO2 in Mode A. However, after the long-term running of membrane vacuum regeneration, it was also supposed that risk of membrane wetting will be increased due to the increase of membrane pore size.

Simultaneous separation of H 2S and CO 2 from natural gas by hollow fiber membrane contactor using mixture of alkanolamines

In the present work we studied the use of a membrane contactor system for simultaneous absorption of H 2S and CO 2 from a gas mixture similar to natural gas. MDEA solution and mixtures of MDEA/DEA and MDEA/MEA were used as absorbent liquid. PVDF and PSf hollow fiber membranes were used in the membrane contactor modules. Design of experiments were carried out by Taguchi method in a manner that a systematical investigation of the effect of operational parameters (temperature, pressure, gas and liquid flow rates, absorbent concentration and acid gas content of the feed), in addition to the effect of membrane material and absorbent liquid, was implemented on process performance parameters including removal efficiencies, overall mass transfer coefficients and H 2S to CO 2 selectivity. The results indicate that liquid side resistance is negligible in comparison with membrane and gas side resistances. Both membranes were wetted by the liquid, but PSf suffered more. The presence of a dense layer in membrane side adjacent to the liquid, changes the influence of operational parameters. It was observed that increasing absorbent concentration had a different effect in the presence or absence of CO 2. Addition of MEA enhanced the removal efficiencies while DEA had no significant effect.

Coupled heat and mass transfer in a counter flow hollow fiber membrane module for air humidification

Hollow fiber membrane based air humidification offers great advantages over the traditional methods because the liquid water droplets are prevented from mixing with the process air, while water vapor can permeate through the membranes effectively. The novelty in this research is that the coupled heat and moisture transport in a hollow fiber membrane module for air humidification is investigated, both numerically and experimentally. The air stream and the water stream flow in a counter flow arrangement. It is found that the membranes play a key role in humidification performances. For sensible heat transfer, both the liquid side and the membrane side resistance can be neglected, while the total heat transfer coefficients are determined by the air side heat transfer coefficients. In contrast, in mass transfer, only the liquid side resistance can be neglected, while the total mass transfer coefficients are co-determined by membrane properties and the air side convective mass transfer coefficients.

Scrubbing of CO2 Greenhouse Gases, Accompanied by Precipitation in a Continuous Bubble-Column Scrubber

A continuous bubble-column scrubber that absorbs carbon dioxide (CO2) gas using an alkaline solution under a pH-stat condition was used to explore the effects of the pH of the solution, gas concentration, gas-superficial velocity, and liquid-flow rate on the absorption rate and volumetric mass-transfer coefficient of CO2. When the scrubbing factor was evaluated, this scrubber outperformed other scrubbers. Correlations of local volumetric mass-transfer coefficients with process variables are presented here and the gas−liquid interfacial area and individual mass-transfer coefficient are discussed to provide insight into the mass-transfer mechanism. Investigation of the liquid-side mass transfer coefficient revealed that the kL/[OH−]0.5 value was a constant, thus demonstrating that the system obeyed second-order kinetics. Evidence also shows that liquid-side resistances are within the range of 27%−99%, indicating that gas-side mass-transfer resistance cannot be neglected in some cases. This implies that the absorption mechanism is flexible and can be shifted by adjusting the process parameters.

Desulfurization Rate during the Copper Blow in a Peirce–Smith Converter

The desulfurization rate in a Peirce–Smith converter (which works as a well-mixed reactor) was obtained during the copper blow. Conversion data of the copper sulfide as a function of the operation time were correlated distinguishing between two kinetic steps; in both cases, the conversion was proportional to time. The variation of the matte temperature was not influential, so the chemical resistance in the bulk of the melt can be considered negligible. The variation in the global volumetric coefficient deduced from the experimental data was justified by a model having two resistances to mass transfer, the gas-side resistance being constant and the liquid-side resistance increasing when the sulfide concentration decreased.

Hydrodynamics and mass transfer in a packed column: Case of toluene absorption with a viscous absorbent

Volatile organic compounds (VOCs) cause nuisance to humans and the environment. Recent legislation encourages industrialists to set up equipment for treating their VOC-loaded gaseous effluents. This piece of research studies the absorption process, using a viscous organic absorbent ( di(2-ethylhexyl) adipate = DEHA ) to treat a toluene-loaded vent gas, in terms of hydrodynamics and mass transfer. It is shown that DEHA does not lead to an excessive pressure drop. Correlations predicting hydrodynamic parameters from previous literature are summarised and tested against experimental results. It is shown that acceptable prediction accuracy can be achieved for counter-current pressure drop and liquid hold-up. Treatment efficiency for the toluene-loaded vent gas is shown to be very good. Calculation of mass transfer constants ( k L a ) enables to test literature correlations against the experimental results. The mass transfer is supposed to be limited by the liquid-side resistance. Our experimental results showed that the k L a of the system depends on the liquid velocity but also on the gas velocity. This behaviour has also been observed by the few authors who have used viscous fluids in their experiments, but is contrary to all the authors who have work on low-viscosity fluids. It is therefore clear that the influence of viscosity on the phenomenon is considerable. Not one current correlation is currently accurate in the case of a viscous absorbent.

Gas Induction Characteristics of Hollow Self-Inducing Impeller

The mechanism of self induction has been investigated for the case of hollow pitched blade downflow turbines. The pressure difference between the orifice and the reactor head space results into self induction of gas. The dependence of pressure difference as well as the rates of gas induction on various parameters like impeller speed, blade angle, orifice location and the number of open orifices was investigated. Gas side and liquid side resistances were experimentally measured for blade angles of 30°, 45° and 60°. An attempt has been made to propose a predictive procedure for the rate of gas induction on the basis of pressure driving force as well as the gas and the liquid side resistances. The gas induction rates for wide range of blade angle (viz. PBTD 30°, 45° and 60°), various hole combinations (viz. 1; 1,2 and 1,2,3,4,5,6) and for submergence of S = 2T/3 and for 1,2,3,4,5,6 holes at S = T/3 submergences have been predicted with this procedure. The predictions are in good agreement with the experimental results.

Mass Transfer in Structured Packing: A Wetted-Wall Study

A short wetted-wall column was used to measure and correlate gas-phase mass transfer coefficients for various pure components evaporating counter current into an air stream. Mass transfer coefficients were also measured for binary mixtures. Both a smooth and a complex surface, similar to the surface of the structured packing Mellapak, were used in the study. The gas-phase mass transfer coefficients for the smooth surface were correlated with Shg = 0.0044RegScg0.5Wel0.111. The results for the complex surface were correlated with Shg = 0.0036Reg0.76Scg0.5Rel0.41Bo-0.13. Mass transfer coefficients for binary mixtures were compared with these gas-phase mass transfer coefficients. Enhanced mass transfer was observed for systems with large differences in pure-component surface tensions. This was not the case for binary systems with small differences in the pure-component surface tensions. Negligible liquid-side resistance to mass transfer was found in all systems in this study. High-viscosity liquids deviated from the proposed correlations.

Individual and simultaneous desorption of H 2S and CO 2 from synthetic green liquor

Selective desorption of H 2S from green liquor has been investigated using a batch stirred tank reactor. It was found that the gas and liquid side resistances are dependent on the stirrer velocity. In this equipment for physical absorption and desorption of H 2S or CO 2 the resistance in the liquid side is dominant. Within the pH interval researched, the individual desorption rate of CO 2 as well as H 2S is mainly determined by the liquid side mass transfer rate and is not enhanced by any reaction in the film. For the desorption of H 2S from a Na 2S solution, an equilibrium model based on the film theory was evaluated. Another model also based on the film theory with finite reaction rate in the film as well as in the bulk was evaluated for the desorption of CO 2 from NaHCO 3 solution. Both models show values which are in good agreement with the experimental results. For the desorption of H 2S from synthetic green liquor a model was taken from the literature [M. Wallin, S. Olausson, Chem. Eng. Commun. 123 (1993) 43–59] and modified. The modified model takes into account that the desorption of H 2S from synthetic green liquor is enhanced by the chemical reaction from hydrogencarbonate to carbonate. The desorption of CO 2 from synthetic green liquor is well described by the model evaluated for the individual desorption of CO 2 from NaHCO 3 solution.

New correlation for sieve‐tray point efficiency, entrainmnt, and section efficiency

AbstractA comprehensive composite database for distillation sieve‐tray efficiency is used to develop point efficiency and entrainment correlations based on a model that considers the fluid on the distillation tray to be contained in a liquid‐continuous region near the tray deck and a vapor‐continuous region on top of the liquid‐continuous region. This model allows estimates of the portion of the mass transfer that occurs in each region and the mass‐transfer resistance that occurs on the liquid side and vapor side of the interface. For most cases, most of the mass transfer occurs within the liquid‐continuous region. The liquid side resistance is often significant.The entrainment correlation is consistent with the work of Bennett et al., which relates entrainment to the ratios of the liquid to vapor density and the froth height to the tray spacing. A simple liquid continuous‐only mass‐transfer model containing only four empirical parameters correlates the point efficiency data to within 6.4%. Despite a twofold change in vapor Schmidt number, no dependency on vapor Schmidt number is seen. Important dimensionless groupings are the Reynolds number based on the hole velocity, effective froth density, ratio of the liquid inventory to the perforation diameter, and fraction of the tray area perforated. Mathematically simple and accurate methods allow the prediction of the section efficiency for trays operating in cross or parallel flow. They address vapor and liquid mixing, entrainment and a criterion to avoid significant degradation of the tray efficiency due to weeping.

Evaluation of a new type of membrane for blood oxygenation.

Results of tests on oxygen transfer rates and on a series of membranes, both in saline solutions and blood are reported. A series of hematological tests was carried out during the tests with blood, in order to assess the damage caused to the blood by contact with the membrane. All the results were compared with results obtained with the same equipment using Celgard membranes. A series of measurements at high liquid flow rate was carried out both with Celgard and the MFT membrane in order to eliminate the liquid-side resistance and evaluate the permeability of the membranes. The results of oxygen transfer rate into blood, when extrapolated by the "constant slope method" showed excellent agreement with the data obtained with saline solutions, justifying the use of oxygen absorption into saline during the process of screening membranes for use in blood oxygenators.

The distribution of gas—side and liquid—side resistance in the absorption of chlorine into benzene in a wetted-wall column

Measurements of the rate of absorption of Cl 2 into liquid benzene from a mixture of N 2 and Cl 2 have been carried out using a wetted-wall column as a model reactor. It is shown that the results for both the gas and the liquid phase can be interpreted by means of an unsteady state diffusion model The gas—side resistance and the liquid—side resistance each exert about half the total resistance. Absorption rates measured at a bulk gas velocity differing from the gas—liquid interfacial velocity by up to ±20% show no significant deviations from those predicted by the model for equal bulk gas and interfacial velocities.