CO2 Mass Transfer Coefficient Research Articles

Control Relevant Model of Amine Scrubbing for CO2 Capture from Power Plants

A low-order amine scrubbing model was developed for an intercooled absorber and advanced flash stripper configuration with piperazine solvent. The low-order lumped parameter model uses semi-empirical thermodynamics and rate-based mass transfer and embeds reaction kinetics in a constant overall transfer coefficient. The predicted off-design steady state of this model is compared to a high-order Aspen Plus simulation for 100–85% power plant load. The difference in CO2 removal rate between the two models is less than 1% when power plant load is greater than 94%. The removal rate is systematically overpredicted in the low-order model because a constant CO2 mass transfer coefficient in the absorber leads to an overprediction of absorber performance at part-load operation. Compared to pilot plant data, the low-order model captures the dynamic response of a step change in the stripper pressure control valve. The characteristic time of the total CO2 inventory is found to be 77 min, compared to a total liquid resi...

Mass transfer and kinetics study on combined CO2 and SO2 absorption using aqueous ammonia

Combined capture of CO2 and SO2 using aqueous ammonia (NH3) is a good option to reduce capital and operational costs for post-combustion capture process. The mass transfer and kinetics of combined CO2 and SO2 absorption in aqueous NH3 solvent were determined at various SO2 concentrations, temperatures and NH3 concentrations using wetted wall column. A bi-liquid film assumption was proposed to analyze the mass transfer and kinetic characteristics by dividing the liquid film to SO2 instantaneous reaction region and CO2 pseudo-first-order reaction region. The high SO2 concentration significantly reduces the CO2 mass transfer coefficient in overall liquid phase and the SO2 instantaneous reaction region, but the high temperature and aqueous NH3 concentration benefit the CO2 mass transfer. In the CO2 pseudo-first-order reaction region, the apparent and second-order reaction rate constants of CO2 absorption were calculated and the second-order reaction rate constant can match very well with the previous work.

Exploiting an Alternative CO2 Absorption Process by Efficient Solvent Mixture

CO2 capture greatly helps with greenhouse gas mitigation. Chemical and physical absorption can control CO2 emission, but these methods are costly. To reduce the cost, an efficient solvent mixture of tetramethylammonium hydroxide (TAMH), tetramethylene sulfone (TMS), and ethylene glycol (EG) is assessed. Gas–liquid equilibrium, reaction kinetics, and mass transfer models are developed and validated by experiments. Henry’s constant, reaction kinetics, and mass transfer coefficients between CO2 and TAMH-TMS-EG are identified. CO2 loading and mass transfer coefficient are, respectively, obtained as 0.55 mol/molTAMH and 4.02kmol/m2/s/kPa, which are on average 25% and 34% higher than the typical MEA process. The theoretical energy consumption amount for desorption of TAMH-TMS-EG-CO2 solutions is identified as 1.11 GJ/t to 1.34GJ/t. Minimum mass transfer resistance is determined at 40% to 80% TMS fraction. A temperature bulge shift and improvement in the interface characteristics enhance mass transfer due to uni...

Radon, CO2 and CH4 as environmental tracers in groundwater/surface water interaction studies − comparative theoretical evaluation of the gas specific water/air phase transfer kinetics

The applicability of radon as environmental tracer in groundwater/surface water interaction studies has been documented in a considerable number of publications. In some of these reports it has also been suggested to validate the radon based results by using CO2 and CH4 as supplementary tracers. The on-site measurement of the three gaseous parameters relies on their extraction from the water followed by the measurement of their concentration by means of mobile gas-in-air detectors. Since most related practical applications require the recording of time series, a continuous extraction of the gases from (e.g.) a permanently pumped water stream is necessary. A precondition for the sound combined interpretation of the resulting time series is that the individual temporal responses of the extracted gas-in-air concentrations to instantaneously changing gas-in-water concentrations are either identical or in reproducible relation to each other. The aim of our theoretical study was the comparison of the extraction behavior of the three gaseous solutes with focus on the individual temporal responses to changing gas-in-water concentrations considering in particular the gas specific water/air phase transfer kinetics. We could show that the overall mass transfer coefficients of radon, CO2 and CH4 result in a virtually similar temporal response to aqueous concentration changes, thus confirming the straightforward combined measurement/utilization of the dissolved gases as environmental tracers in groundwater/surface water interaction studies.

CO2 Dissolution and Design Aspects of a Multiorifice Oscillatory Baffled Column

Dissolution of CO2 in water was studied for a batch vertical multiorifice baffled column (MOBC) with varying orifice diameters (d0) of 6.4–30 mm and baffle open area (α) of 15–42%. Bubble size distributions (BSDs) and the overall volumetric CO2 mass transfer coefficient (KLa) were experimentally evaluated for very low superficial gas velocities, UG of 0.12–0.81 mm s–1, using 5% v/v CO2 in the inlet gas stream at a range of fluid oscillations (f = 0–10 Hz and x0 = 0–10 mm). Remarkably, baffles presenting large do = 30 mm and α = 36%, therefore in the range typically found for single-orifice oscillatory baffled columns, were outperformed with respect to BSD control and CO2 dissolution by the other baffle designs or the same aerated column operating without baffles or fluid oscillations. Flow visualization and bubble tracking experiments also presented in this study established that a small do of 10.5 mm combined with a small value of α = 15% generates sufficient, strong eddy mixing capable of generating and...

Analysis of Hydrated Lime Consumption in Circumneutral Underground Coal Mine Drainage Treatment

Treatment evaluations were performed at four Pennsylvania hydrated lime treatment sites and one quicklime treatment site designed to remove ferrous iron from underground coal mine drainage. Two of the sites included a pretreatment decarbonation step to exsolve CO2(aq) and reduce hydrate consumption due to hydroxylation and calcite formation. Decarbonation reduced the daily hydrated lime dose by 22 % at one site and 28 % at the other. Field-measured CO2 mass transfer coefficients were determined for both decarbonation systems. CO2 mass transfer modeling predicted that Ca(OH)2 use would be reduced by an additional 19 and 28 % at these sites if the decarbonation systems were optimized. Hydroxylation of CO2 species and calcite formation consume between 40 and 90 % of the Ca(OH)2 dose. In terms of cost, more money is being spent on consumption due to hydroxylation and calcite formation than on removing the targeted parameter, ferrous iron. These processes can be minimized by improving decarbonation and oversizing ferrous reactor tanks to lower treatment pH.

CO2 Absorption into Aqueous Solutions Containing 3-Piperidinemethanol: CO2 Mass Transfer, Stopped-Flow Kinetics, 1H/13C NMR, and Vapor–Liquid Equilibrium Investigations

Global efforts to reduce carbon dioxide emissions stemming from the combustion of fossil fuels have acknowledged and focused on the implementation of post combustion capture (PCC) technologies utilizing aqueous amine solvents to fulfill this role. The cyclic diamine solvent piperazine has received significant attention for application as a CO2 capture solvent, predominantly for its rapid reactivity with CO2. A thorough investigation of alternative but simpler cyclic amines incorporating a single amine group into the cyclic structure may reveal further insight into the superior kinetic performance of piperazine and the wider applicability of such cyclic solvents for PCC processes. One such example is the cyclic monoamine 3-piperidinemethanol (3-PM). To facilitate the evaluation of 3-PM as a capture solvent requires knowledge of the fundamental chemical parameters describing the kinetic and equilibrium of the reactions occurring in solutions containing CO2 and 3-PM. Additionally, in parallel with the preceding, experimental measurements of CO2 absorption into 3-PM solutions, including mass transfer and vapor–liquid equilibrium measurements, can be used to validate the CO2 absorption performance in 3-PM solutions and compared to that of monoethanolamine (MEA) under similar conditions. The present study is focused in two parts on (a) determination of fundamental kinetic and equilibrium constants via the analysis of stopped-flow kinetic and quantitative equilibrium measurements via 1H/13C nuclear magnetic resonance (NMR) spectroscopy and (b) experimental measurements of CO2 absorption into 3-PM solutions via wetted wall column kinetic measurements, vapor–liquid equilibrium measurements, and corresponding physical property data including densities and viscosities of the amine solutions over a range of concentrations and CO2 loadings. Fundamental kinetic rate constants describing the reaction of CO2 with 3-PM are significantly faster than MEA at similar temperatures (3-PM = 32 × 103 M–1 s–1, extrapolated to 40 °C from kinetic data between 15.0 and 35.0 °C; MEA = 13 × 103 M–1 s–1, 40 °C). Conversely, the equilibrium constants describing the reaction between bicarbonate and amine, often termed carbamate stability constants, are significantly lower for 3-PM than MEA at similar temperatures. Overall CO2 absorption rates in 3.0 M solutions of 3-PM and MEA, assessed in overall CO2 mass transfer coefficients, are lower in the former case over the entire range of CO2 loadings from 0.0 to 0.4 mol of CO2 per mol of amine. The reduced absorption rates in the 3-PM solutions can be attributed to higher solution viscosities and thus corresponding reductions in CO2 diffusion. CO2 absorption and cyclic capacities in 3.0 M solutions of 3-PM and MEA were found to be significantly higher in the case of 3-PM. The larger CO2 capacities are attributed to the lower stability 3-PM carbamate and the formation of larger amounts of bicarbonate compared to MEA. Overall, the larger CO2 absorption capacity, cyclic capacity, and rapid kinetics with CO2 position 3-PM as an attractive CO2 capture solvent.

Experimental study on volumetric mass transfer coefficient of CO2 absorption into MEA aqueous solution in a rectangular microchannel reactor

Abstract Monoethanol amine (MEA) aqueous solution is one of the most widely used absorbents for CO2 in industry. The mass transfer behavior of CO2 absorption into MEA aqueous solution in a microreactor was investigated experimentally. Liquid side volumetric mass transfer coefficients (kLa) of CO2 were measured in horizontal microreactor with rectangular cross section. The influences of gas and liquid phase flow rates on kLa for Taylor and annular flow were primarily studied. The concentration of CO2 in liquid phase at the outlet of microreactor was obtained by titration method and then kLa was calculated. Results indicated that: under the same liquid side flow rate, kLa increased gradually up to a stable value with increasing gas phase flow rate; when the gas flow rate was relatively small, kLa decreased with the increase of the liquid flow rate; while the gas flow rate reached a critical flow rate, kLa increased with increasing liquid phase flow rate. Several dimensionless criterions were correlated to predict kLa, and the calculated values coincided well with experimental data.

Efficient storage and utilization of CO2 in open raceway ponds for cultivation of microalgae

For efficient storage and utilization of CO2 in open raceway ponds, the effects of cultural and operational parameters were studied. A 10 m2 indoors raceway pond was operated to determine CO2 storage capacity, average rate of absorbed CO2 losses and mass transfer coefficient for CO2 outgassing from various pH, salinity and alkalinity regimes of culture medium; mixing velocities and culture depths. Average rate of CO2 outgassing for saltwater (35 ppt salinity) at 40 meq/L alkalinity was 40-fold higher than seawater (35 ppt salinity and 2.3 meq/L alkalinity) at pH 8. Operating at lower pHs or salinities aggravated CO2 outgassing. An empirical equation for CO2 outgassing average mass transfer coefficient, \(\bar K_L \), was developed as a function of mixing velocity and depth. Nannochloropsis sp. PTCC6016 was cultivated in the pond for 14 days. Due to higher amount of outgassing, CO2 utilization efficiency declined as the productivity in the pond decreased.

Experimental Study on CO2 Absorption into Aqueous Ammonia-based Blended Absorbents

A crucial problem for the promising absorbent aqueous ammonia (NH3) is the low CO2 absorption rate and high ammonia volatile loss rate. This work investigated the effect of potential promoters for CO2 absorption into aqueous ammonia solutions on a wetted wall column. Monoethanolamine, sarcosinate, taurinate, glycinate, piperazine and 1-methyl piperazine were introduced to NH3 solutions as promoters. We found sarcosinate, piperazine and 1-methyl piperazine could increase the mass transfer coefficient of CO2 into the ammonia-based solutions significantly. We further undertook a detailed mass transfer study of CO2 into ammonia based blended solutions to investigate the effects of temperature and additive concentration. Besides, a novel ammonia separation method from low concentration ammonia solution by membrane vacuum regeneration was developed which could recover more than 80% of ammonia gas from low concentration ammonia.

Amino acids/NH3 Mixtures for CO2 Capture: Effect of Neutralization Methods on CO2 Mass Transfer and NH3 Vapour Loss

Amino acid salts are effective promoters to improve CO2 absorption in NH3–based solutions, but at the expense of an increase in NH3 vapour loss. To address this issue, we proposed the neutralisation of amino acids using NH3 instead of KOH and investigated the effect of neutralisation methods on NH3 vapour loss and mass transfer coefficient of CO2 in amino acids/NH3 mixtures at 15oC. It has been found that NH3 neutralized amino acids solutions can enhance KG of CO2 in the NH3 solution and suppress NH3 loss at the same time. Among three amino acids investigated, taurine is most suitable for the NH3 neutralisation. We developed the chemical equilibrium model for amino acid–NH3–CO2–H2O and used the model to predict the species profiles in the mixture and explain the experimental results.

CO2 Absorption into Aqueous Amine Blends containing Benzylamine (BZA), Monoethanolamine (MEA), and Sterically Hindered/Tertiary Amines

Abstract To date, a single mono-amine solvent with ideal properties for CO2 capture processes is yet to be found. As a compromise, solvent formulations incorporating two or more individual amines into a single solvent blend have been proposed and are gaining momentum as next generation capture solvents. Recently we have identified benzylamine (BZA) for use as a CO2 capture solvent. A significant drawback with BZA is the formation of stable carbamate salts at high CO2 loadings which has severely limited the concentration of BZA which can be used. Thus, BZA is ideally placed as a candidate for use in formulations with other amines. In this study, we investigate CO2 absorption into aqueous solvent blends containing BZA, MEA, sterically hindered, and tertiary amines using a wetted-wall column contactor at 40oC, and simulations of the vapour liquid equilibrium behaviour of the blends using a software tool developed in Matlab. Overall CO2 mass transfer coefficients, KG, at 40oC are reported for blends containing 3.0 M BZA together with 3.0 M of the second amine component. Several of the formulations were found to possess significantly faster absorption rates than in standalone MEA solutions at similar total amine concentrations, and CO2 loadings, as in the blends. A formulation containing BZA and MEA was found to display the fastest CO2 absorption rates due to the blend containing two fast reacting components. Highly branched and sterically hindered amines in formulations with BZA limit the CO2 absorption rate due to the parallel impact of the amine structure on the physical properties of these solutions. Absorption capacities at 40oC are similar in each of the blends with 6.0 M MEA, however cyclic capacities are significantly higher in the majority of the BZA blends with the largest improvement observed being 92% in a blend containing 3.0 M BZA and 3.0 M AMP. Overall absorption enthalpies in all of the BZA blends here were found to be larger than that observed in standalone MEA due largely to the contribution from the larger protonation enthalpy of BZA.

Optimize Flue Gas Settings to Promote Microalgae Growth in Photobioreactors via Computer Simulations

Flue gas from power plants can promote algal cultivation and reduce greenhouse gas emissions(1). Microalgae not only capture solar energy more efficiently than plants(3), but also synthesize advanced biofuels(2-4). Generally, atmospheric CO2 is not a sufficient source for supporting maximal algal growth(5). On the other hand, the high concentrations of CO2 in industrial exhaust gases have adverse effects on algal physiology. Consequently, both cultivation conditions (such as nutrients and light) and the control of the flue gas flow into the photo-bioreactors are important to develop an efficient "flue gas to algae" system. Researchers have proposed different photobioreactor configurations(4,6) and cultivation strategies(7,8) with flue gas. Here, we present a protocol that demonstrates how to use models to predict the microalgal growth in response to flue gas settings. We perform both experimental illustration and model simulations to determine the favorable conditions for algal growth with flue gas. We develop a Monod-based model coupled with mass transfer and light intensity equations to simulate the microalgal growth in a homogenous photo-bioreactor. The model simulation compares algal growth and flue gas consumptions under different flue-gas settings. The model illustrates: 1) how algal growth is influenced by different volumetric mass transfer coefficients of CO2; 2) how we can find optimal CO2 concentration for algal growth via the dynamic optimization approach (DOA); 3) how we can design a rectangular on-off flue gas pulse to promote algal biomass growth and to reduce the usage of flue gas. On the experimental side, we present a protocol for growing Chlorella under the flue gas (generated by natural gas combustion). The experimental results qualitatively validate the model predictions that the high frequency flue gas pulses can significantly improve algal cultivation.

Experimental studies on suppression of ammonia vaporization by additives

Aqueous ammonia (NH3) based CO2 capture process has been regarded as a promising technology to capture CO2 from many industrial sources including coal-fired power stations. However, this technology has been limited by the high volatility of ammonia and its relatively low reaction rate with carbon dioxide. This study selected nine kinds of chemicals and added them into ammonia solution as additives to investigate their potential to suppress ammonia vaporization. A wetted-wall column reactor was used to study the effect of additives on ammonia loss rate and overall gas mass transfer coefficient of CO2 at the absorption temperature of 15°C and solution of 3 M (mol/L) aqueous ammonia. Among all additives tested, Sulfolane (TMS) and Propylene carbonate (PC) showed the highest potential to suppress ammonia vaporization and enhance the reaction at the same time. The best scenario in this study was the addition of 0.3 M PC to 3 M aqueous ammonia under the CO2 loading of 0.4 which led to a reduction of ammonia loss by 38% and an increase of overall gas mass transfer coefficient by 10% compared to the case of no additive. © 2013 Society of Chemical Industry and John Wiley & Sons, Ltd

Effect of CO2 stripping on pH in open‐recirculating cooling water systems

A simplified mathematical model has been developed to evaluate the effect of CO2 stripping on pH of recirculating cooling water in power plant cooling systems. Experimental data from three pilot‐scale cooling towers using treated municipal wastewater as makeup water were used to determine the CO2 mass transfer coefficients in the gas phase kg and aqueous phase kw. The optimum values of the gas film mass transfer coefficient and water film mass transfer coefficient were determined by regression to be kg = 8.4 × 10−6 m/s and kw=8.4 × 10−8m/s, respectively. The results also indicate that the model is capable of predicting the pH of the cooling loop as a function of makeup water alkalinity and cooling loop alkalinity (in case of salt formation) as well as the operating parameters of the cooling system. © 2013 American Institute of Chemical Engineers Environ Prog, 33: 275–282, 2014

CO2 absorption by piperazine promoted aqueous ammonia solution: absorption kinetics and ammonia loss

AbstractAqueous ammonia (NH3) solution has been proposed as a promising absorbent for post‐combustion capture of carbon dioxide (CO2). However, when aqueous NH3 is used alone, the capture process suffers from a low CO2 absorption rate and high NH3 loss. In this study, we measured CO2 mass transfer coefficients and NH3 loss rate in a wetted wall column study of CO2 absorption in piperazine (PZ) promoted aqueous NH3 solution, at NH3 concentrations of 0.59–4.12 mol/L (M), CO2 loadings (molar ratio of CO2/NH3) of 0 to 0.5, temperatures of 283.15–298.15 K. The second‐order rate constants for the reactions of CO2 with PZ and NH3, which are determined based on the kinetic results from the wetted wall column, agree reasonably well with published values. Addition of PZ to aqueous NH3 can significantly increase mass transfer coefficients of CO2 in aqueous NH3, but at the expenses of an increase in NH3 loss. The enhancement by addition of PZ is due to the fast reaction of CO2 with PZ and PZ carbamate. However, addition of PZ to aqueous NH3 increases the concentration of free NH3 in the solvent, thereby increasing NH3 loss.

Mass transfer coefficients considering effects of steam in oxy-fuel combustion of coal char

Considering the effects of Stefan flow and steam in the oxy-fuel combustion of char, correction factors for the mass transfer coefficients of gas reactants, O2, CO2, and H2O, are derived in the present work. By comparisons with the experimental data, the rigid continuous-film model, and the uncorrected single-film model, it is concluded that the corrections greatly improve the predictions of the particle temperature, combustion rates and burnout time. The correction factors are related to the gas components, reaction numbers and rate ratios. Generally, the increase of number and rate of surface reaction will result in the enhancement of Stefan flow and hence decrease the mass transfer coefficients. But discussion under the typical conditions of oxy-fuel combustion shows that different reaction has different impacts on the mass transfer of reactive gases. In the presence of steam, the correction for O2 is nearly the same as cases neglecting the steam, owing to the low gasification rate of H2O and the accelerative diffusion transfer of O2 in H2O and H2. The correction for CO2 increases slightly by about 3% because the gasification rate of H2O is only a little higher than that of CO2. On the contrary, the participation of O2 and CO2 reactions plays considerable role in the mass transfer coefficient of H2O. Especially, the high rate reaction, like O2 oxidation, has more remarkable effects on the mass transfer of gas reactants. Therefore, more attention should be paid to the modification of mass transfer coefficients when the O2 oxidation reaction is prevailing in the char oxy-fuel combustion, for the transfer coefficients will be greatly changed.

Utilization of municipal wastewater for cooling in thermoelectric power plants

A process simulation model has been developed using Aspen Plus® with the OLI (OLI System, Inc.) water chemistry model to predict water quality in the recirculating cooling loop utilizing secondary- and tertiary-treated municipal wastewater as the source of makeup water. Simulation results were compared with pilot-scale experimental data on makeup water alkalinity, loop pH, and ammonia evaporation. The effects of various parameters including makeup water quality, salt formation, NH3 and CO2 evaporation mass transfer coefficients, heat load, and operating temperatures were investigated. The results indicate that, although the simulation model can capture the general trends in the loop pH, experimental data on the rates of salt precipitation in the system are needed for more accurate prediction of the loop pH. It was also found that stripping of ammonia and carbon dioxide in the cooling tower can influence the cooling loop pH significantly. The effects of the NH3 mass transfer coefficient on cooling loop pH appear to be more significant at lower values (e.g., kNH3<4×10−3m/s) when the makeup water alkalinity is low (e.g., <90mg/L as CaCO3). The effect of the CO2 mass transfer coefficient was found to be significant only at lower alkalinity values (e.g., kCO2<4×10−6m/s).

Carbon dioxide adsorption on polyacrylamide-impregnated silica gel and breakthrough modeling

Polyacrylamide-impregnated silica gel was prepared to capture CO2 from flue gas. The polymerization of acrylamide was carried out in AN solvent using AIBN as initiator and EGDMA as crosslinker. The adsorbents were characterized by N2 adsorption, FTIR analysis, SEM analysis, and thermal gravimetric analysis. The results showed that the polymer was not only occupying the porosity of the silica, but necessarily surrounding silica particles, and the amide groups was successfully loaded on the support silica. The impregnated silica displayed good thermal-stability at 250°C. The CO2 adsorption isotherms were measured to examine CO2 adsorption on adsorbents, and the results showed that the capacity was increased significantly after modification. The CO2 isosteric adsorption heats calculated from the isotherms showed that the adsorption interaction of CO2 with the functionalized material may be mainly an intermolecular force or hydrogen bond. Fixed-bed breakthrough model of CO2 adsorption on functionalized silica was successfully developed to describe the breakthrough curves under different adsorption temperature, CO2 concentration, and gas flow rate. The mass transfer coefficients of CO2 were calculated from the breakthrough model, the results showed that adsorption rate could be promoted by increasing temperature, flow rate and CO2 concentration, among which the effect of gas flow rate is the most obvious.

Promoted CO2 absorption in aqueous ammonia

AbstractAqueous ammonia is a promising solvent for post combustion CO2 capture but the CO2 absorption rate in aqueous ammonia alone is low. This short communication investigates the effect of seven additives as potential promoters of CO2 absorption in aqueous ammonia, and aims to inspire more research to develop effective promoters to increase CO2 absorption in aqueous ammonia and further advance the aqueous ammonia‐based capture process. The additives were monoethanolamine, piperazine, 1‐methyl piperazine, 4‐amino piperidine and the sodium salts of three amino acids: sarcosine, taurine and glycine. Experiments were performed on a wetted wall column at an absorption temperature of 15 °C using 3 M (mol/L) NH3 in the presence and absence of 0. 3 M of each additive. CO2 mass transfer coefficients increased to different extents in the presence of different additives. Piperazine, 1‐methyl piperazine and sarcosine sodium salt promoted CO2 absorption in aqueous NH3 to a greater extent than the other additives. © 2012 Society of Chemical Industry and John Wiley &amp; Sons, Ltd