Contactor Length Research Articles

Ambient wind conditions impact on energy requirements of an offshore direct air capture plant

This study proposes an off-grid direct air (carbon) capture (DAC) plant installed on the deck of an offshore floating wind turbine. The main objective is to understand detailed flow characteristics and CO2 dispersion around air contactors when placed in close proximity to one another. A solid sorbent DAC design is implemented using a commercially deployed air contactor configuration and sorbent. The sorbent is assumed to undergo a temperature vacuum swing adsorption cycle. Computational fluid dynamics (CFD) is used to determine the local conditions entering each unit based on varying wind speed and angle. Two-dimensional (2D) simulations were used to determine the pressure drop through a detailed air contactor design considering the sorbents APDES-NFC, Tri-PE-MCM, MIL-101(Cr)-PEI-800, and Lewatit VP OC 106. Only APDES-NFC was explored for further analysis in the three dimensional (3D) CFD dispersion model. 3D simulations were used to model flow patterns and CO2 dispersion using passive scalars. A worst case scenario is analyzed for all DAC units in adsorption mode with fans running simultaneously. 2D simulations show an under utilization of contactor length, and quantify pressure loss curves for four common sorbents. One commercially deployed sorbent is considered for further analysis; a pressure drop of 390.62 Pa is experienced for a flow velocity of 0.73 m s−1 through a 1.5m×1.5m×1.5m contactor. Using 3D simulations, fan energy demands are computed based on flow velocities and applied pressure gradients. There is found to be a decrease in overall fan power demand as wind speed increases. High wind speeds can passively drive the adsorption process with fans shut off at certain wind directions. This occurs at an average contactor inlet velocity of 17.5 m s−1, correlating to a hub height (150 m) wind speed of 24 m s−1. Thermal energy demands are computed based on inlet CO2 concentrations entering downstream units. Thermodynamic work for desorption based on an assumed second law efficiency is compared to thermal energy for desorption computed using a simplified isotherm method, taking into account the impacts of humidity on CO2 adsorption. Going from 414.72 ppm to 300 ppm CO2 inlet concentration requires an additional 63.9 kWh (t- CO2)−1 using the 2nd law thermodynamic efficiency method and 25.9 kWh (t- CO2)−1 using the isotherm method. Contactor arrangement, wind angles, and wind speeds have a significant impact on flow patterns experienced, and resulting CO2 dispersion. High wind speeds assist in CO2 dispersion, resulting in higher inlet concentrations to downstream DAC units and decreased thermal energy requirement.

3-D computational fluid dynamics modeling of a hollow fiber membrane contactor ozonation process

Ozonation processes have been successfully used to remove organic micropollutants. Membrane contactors can transfer the ozone via a bubbleless process, making them promising devices for many applications thanks to a good control of the transferred ozone. Here we studied the local transfer of ozone into water through a single and a multi-fiber membrane contactor by developing a 3-D model in a Comsol Multiphysics® workflow. To our knowledge, this is the first model to combine 3-D analysis and a multi-fiber membrane contactor for an ozonation application. The model established here could serve as a startpoint for optimizing bubbleless processes and scaling up membrane contactor processes, including the ozonation of micropollutants. Dedicated experiments were performed to check the model. Using of a 3-D modeling approach makes it possible to account for the non-ideal effects that occur in hollow fiber membrane contactors. The simulations highlighted hydrodynamics as a very important factor to ensure good distribution of ozone and therefore good treatment of pollutants. Moreover, the transferred ozone does not increase proportionally with increasing membrane length or residence time. Finding the right compromise between contactor length and fibers number can help optimize the contactor geometry and thus make the process as effective as possible.

Rigorous non-isothermal modeling approach for mass and energy transport during CO2 absorption into aqueous solution of amino acid ionic liquids in hollow fiber membrane contactors

In the current study, a rigorous modeling approach was used to develop a mathematical model for the non-isothermal absorption process of CO2 in hollow fiber membrane contactors (HFMC). Four amino acid-based ionic liquids (ILs) namely, tetramethylammonium glycinate [N1111][Gly], 1-ethyl-3-methylimidazolium glycinate [C2mim][Gly], 1-butyl-3-methylimidazolium glycinate [C4mim][Gly] and 1-hexyl-3-methylimidazolium glycinate [C₆mim][Gly] were used as absorbents. These ILs have never been implemented for the membrane contactor CO2 absorption operations both on the lab scale and industrial scale. Wide ranges of operating conditions were investigated for freshly unloaded absorbents and partial conversion of absorbents considering steady-state conditions for the model. Non-wetting conditions were adopted by keeping very low transmembrane pressure. The non-isothermal model predicted a significant temperature rise along the contactor length, ranging from 10 to 25 K, which further affected the absorption and reaction kinetics. Simulations confirmed the strong influence of absorbent concentration and mild influence of process temperature on the separation performance of ILs, CO2 boundary flux, and reaction kinetics between CO2 and ILs. Very fast absorption of CO2 near the interface justified the dominancy of the chemisorption for the current absorption. Finally, the comparison with experimental data evidenced the identical trends and hence validated the model.

Mass transfer performance and correlations for CO2 absorption into aqueous blended PG/MEA in PTFE membrane contactor

AbstractBACKGROUNDThe method for capturing carbon dioxide (CO2) with amine solution in a hollow fiber membrane contactor has been identified as a potential large‐scale application technology for capturing CO2 from flue gas. The mass transfer performance of CO2 relies in large extent on the operating conditions, the types of absorbents and the properties of membrane materials. Therefore selecting the appropriate absorbent and studying the effect of operating parameters on the mass transfer performance become of paramount importance.RESULTSThe experimental results showed that the blended potassium glycinate/monoethanolamine solution has better CO2 absorption performance and higher surface tension than the single monoethanolamine solution under the same conditions. Furthermore, the experimental results illustrated that the CO2 absorption flux increases as the gas velocity, CO2 partial pressure and liquid velocity increase, and decreases with the increase of CO2 loading and membrane contactor length; and the CO2 removal efficiency increases as the liquid velocity and membrane contactor length increase, and decreases with the increase of CO2 loading, gas velocity and CO2 partial pressure. Additionally, new correlations for the prediction of the CO2 absorption flux JCO2 and the CO2 removal efficiency ηCO2 are proposed in this work. The calculated data of CO2 absorption flux and CO2 removal efficiency were found to be in satisfactory agreement with experimental results, with absolute average deviation (AAD) lower than 4.42%.CONCLUSIONThe correlations for the prediction of JCO2 and ηCO2 based on the experimental results are of significance for predicting the operating parameters and design of a polytetrafluoroethylene (PTFE) hollow fiber membrane contactor. © 2019 Society of Chemical Industry

Design optimizations of the thermal performance and cost in an air-gap hollow fiber membrane contactor (AHFMC)

An air–gap hollow fiber membrane contactor (AHFMC) has been proposed and applied for fluid heating in the HVAC applications. The water and the liquid desiccant (solution) streams flow inside the neighboring hollow fiber membranes in a cross-flow arrangement. The solution is heated due to the absorption and mixing heats by absorbing the water vapor from the water stream. A mathematical model of the coupled heat and mass transports in the AHFMC is established by converting it into a parallel-plate one. The optimal design of the AHFMC is studied by using the multi-objective particle swarm optimization (MOPSO) algorithm to obtain the maximum thermal performance and minimum total annual cost together with the coupled heat and mass transfer model. A set of multiple optimum solutions, which are named as “Pareto optimal solutions”, are then obtained. Influences of geometric variables of the contactor length, width, height, fiber outer diameter and fiber distances in different directions on the heating performance and total annual cost are studied. The optimally geometrical parameters of the AHFMC are obtained using the MOPSO algorithm. These results are useful for the design of the AHFMC based on the desired temperature lift, the contactor constraint, and the available investment in the real applications.

Thermodynamic analyses and optimizations of extraction process of CO2 from acidic seawater by using hollow fiber membrane contactor

Extraction process of CO2 from acidic seawater by using hollow fiber membrane contactor is studied in this paper by using finite time thermodynamics or entropy generation minimization theory. Analytical formulae of the extraction rate of CO2 and entropy generation rate (EGR) caused by mass transfer (MT) and the flow of the process are obtained when the acidic seawater (liquid phase) flows in the lumen side and the H2 (gaseous phase) flows in the shell side conversely. Relations between the concentration of CO2 and contactor length in both liquid phase and gaseous phase, between the MT flux and contactor length, between the pressure of the two phases and the contactor length, as well as between the local EGR and contactor length are obtained by numerical examples. The influences of the volume flow rates of the acidic seawater and the H2 at the inlet, the number and the length of hollow fibers on the extraction rate of CO2 and EGR per MT rate are also analyzed. The results show that the extraction rate of CO2 can reach up to more than 98%. The EGRs caused by MT and fluid flow account for 85.99% and 14.01% of the total EGR, respectively. Furthermore, the total EGR caused by MT and fluid flow is minimized by using nonlinear programming method with fixed extraction rate of CO2 when the gas mixture of H2 and CO2 flows in the shell side and the concentration of CO2 in the gaseous phase is completely controllable. The EGR of the minimum EGR MT strategy can be decreased by 36.57% than that of the MT strategy that H2 flows in shell side, in which the EGR caused by MT is decreased by 42.52% and the EGR caused by the flow remain basically unchanged. The EGR caused by MT is minimized by using optimal control theory and the analytical solution of the optimal concentration configuration of CO2 in the gaseous phase is obtained.

Investigation of carbon dioxide (CO2) capture in a falling film contactor by computer simulation

In this work, mathematical models of carbon dioxide (CO2) absorption by monoethanolamine amine (MEA) in a falling film contactor are developed. The proposed models aim to predict conversion of the gas–liquid reaction along the contactor, gas–liquid interface temperature profile (axial and radial), liquid film thickness along the contactor length, axial and radial concentration profiles of reactants in liquid film, and axial and radial profiles of velocity in the liquid film. A code written in MatLab was used to obtain these profiles based on multi grid method through programming of kinetic and thermodynamic equations and physical properties of the absorption system. The mathematical model is validated by an experimental measurement based on absorption of CO2 gas by MEA solution. Four parameters are studied as independent variables namely, mole fraction of carbon dioxide in gaseous mixture, molar concentration of absorbent (MEA, volumetric flow rate of MEA, and its temperature. It is found that the entrance effect of the falling film contactor is related to axial distance from the contactor entrance exponentially: E=B0exp(−B1y) An optimization technique based on minimization of the sum of the squared error between the experimental and predicted composition of absorption process is used to obtain B0 and B1. It is found that reaction between carbon dioxide and MEA is instantaneous, and the axial conversion of carbon dioxide in the gas phase varies exponentially with the contactor length.

Intensification of volatile organic compound absorption in a compact wet scrubber at co-current flow

Three volatile organic compounds (VOC) with no acidic or basic function (butanol, butyraldehyde, methylethylketone), encountered at low concentrations in odorous effluents, were absorbed in water in a compact wet scrubber. This gas-liquid contactor consisted of a wire mesh packing structure where the gas phase flows at high velocity (>12 m s−1). A very turbulent two-phase downward flow could be observed in the scrubber with dispersed fine droplets (around 10 μm). For compounds showing a good affinity for water, such as butanol, removal efficiencies up to 90% were measured for a short contactor length of 32 cm leading to a gas residence time of 20 ms. However, the removal efficiency of butyraldehyde, which is poorly soluble in water, ranged between 10 and 30%. Mass-transfer modeling was achieved and underlined that working with several small scrubbers in series, fed with an unloaded solution, is effective to improve the removal efficiency. The influences of the VOC/solvent affinity, the contactor length, and the mass-transfer and hydrodynamic parameters on the removal efficiency were evaluated through a sensitivity analysis.

Mathematical modeling of aroma compound recovery from natural sources using hollow fiber membrane contactors with small packing fraction

A numerical simulation was carried out in order to investigate aroma compounds recovery from aqueous phase using an organic solvent. A hollow-fiber membrane contactor with small packing fraction was selected to be used in the study. Momentum, continuity and mass transfer equations were solved simultaneously utilizing finite element method. The simulation results showed that solving the Navier–Stokes equations has significantly diminished average deviation about 55% compared to Happle’s velocity model. Simulation results revealed that mass transfer resistance increases along the contactor length in both tube and shell sides. It seems that it is due to the increasing thickness of the boundary layer. Enhancement of the solvent velocity could result in increasing mass transfer rate through the membrane. However, it reduced the extraction efficiency of aroma compounds. Moreover, recovery of 2-hexanal was higher than benzaldehyde due to its higher partition coefficient. Furthermore, n-hexane has higher extraction efficiency than miglyol because of lower Schmidt number.

Scrubbing intensification for sulphur and ammonia compounds removal

Operating conditions were optimised in a new compact scrubber in order to remove odorous sulphur (H 2S and CH 3SH) and ammonia compounds. The influence of the superficial gas and liquid velocities, pH, contactor length, inlet concentrations (sulphur compounds, ammonia, chlorine), and the mixing effects was characterised. Whereas abatement increased with velocities, pH and the chlorine concentration, an increase of inlet CH 3SH concentration drove to a worse efficiency of process. Moreover, the contactor length and the presence of another pollutant in the gas phase only played a role on the methylmercaptan removal. Finally, the reactive consumptions were estimated at the outlet of the reactor. The chlorination by-product quantification permitted to understand the under-stoichiometry.

Application of an Air-Operated Impinging Streams Contactor in Liquid-Liquid Extraction

An experimental study was conducted on the liquid-liquid extraction of isobutyric acid by means of cumene from its aqueous solutions in an air-operated impinging streams contactor (AOISC) with spray nozzles. Aqueous and organic streams may or may not contact one another inside the spray nozzles depending on the modes of operation. The overall volumetric mass transfer coefficient, KLa, determined enabled us to evaluate the performance capability of the contactor. The effects of air and solution flow rates, contactor length and diameter, impingement zone as well as modes of operation have been investigated. These experimental results verify the capability of the method of impinging streams in liquid-liquid extraction.

Extractive bioconversion in a four‐phase external‐loop airlift bioreactor

AbstractThe integration of biosynthesis and product separation can increase the productivity of immobilized plant cells in airlift bioreactors. Extractive bioconversion of anthraquinones was studied in an external‐loop airlift bioreactor consisting of a riser, a downcomer, and two horizontal sections, while containing alginate‐immobilized Frangula alnus cells, a continuous aqueous phase (nutrient solution), dispersed solvent phase (n‐hexadecane or silicone oil), and gas bubbles. A simple mathematical model was developed to describe the cocurrent liquid‐liquid extraction in the riser section of the bioreactor and to rationalize the measured product concentrations in the aqueous and solvent phase. The model equations were solved analytically in a dimensionless form and used to study the effects of flow conditions, solvent properties, product formation rate, droplet size, and contactor length on the extraction efficiency and product concentration profiles in the continuous and dispersed phase.

Modeling of venturi scrubber efficiency

The parameters affecting venturi scrubber performance have been rationally examined and modifications to the current modeling theory have been developed. The modified model has been validated with available experimental data for a range of throat gas velocities, liquid-to-gas ratios and particle diameters and is used to study the effect of some design parameters on collection efficiency. Most striking among the observations is the prediction of a new design parameter termed the minimum contactor length. Also noted is the prediction of little effect on collection efficiency with increasing liquid-to-gas ratio above about 2ℓ m −3. Indeed, for some cases a decrease in collection efficiency is predicted for liquid rates above this value.

Counter-current flow of dispersed and continuous phase—I: Discrete polydispersed model

A discrete mathematical model has been proposed of the counter-current gravitational flow of a continuous and a polydispersed phase in vertical column contactors. Allowances have been made for particle entrainment, newly defined back flow of particles in relation to the back flow of the continuous phase, particle dispersion and coalescence. Appropriate solutions enable profiles to be obtained of local hold-up of the dispersed phase as well as those of particle size distribution along the contactor's length. The model with the parameters distributed with respect to particle size has served to derive in turn a lumped parameter modification whose limits of applicability have been shown.

Mass transfer in liquid‐liquid (horizontal) pipeline contactors

AbstractThe mass transfer characteristics of liquid‐liquid (horizontal) pipeline contactors were studied for the bubble and plug flow regimes. The contactors were operated co‐currently. The theory of mass transfer with chemical reaction was used to obtain values of physical mass transfer coefficient (kLa) and effective interfacial area (a) for a number of systems. In all the experiments the resistance to mass transfer was confined to the continuous phase. The effect of the flow rates and viscosities of the two phases, interfacial tension, contactor length, pipe diameter, the presence of solids, etc. on both, kLa and a was studied. The values of kLa and a were found to vary from 1 to 50 × 10−3 sec−1, and 0.4 to 25 cm−1, respectively.

Mass transfer between liquids in cocurrent pipeline flow

AbstractMass transfer between n‐butanol and water has been studied in cocurrent pipeline flow. Phase NTU's correlated with individual phase velocities, and also depended on the input ratio of the two phases and the contactor length. Mass transfer stage efficiencies varied from near zero to essentially 100%. End effects were large. Pressure drop data agreed with the correlation of Charles and Lilleleht. The pipeline contactor showed contactor effectiveness and energy requirements similar to those of other experimental liquid‐liquid extraction devices.