Multi-field synergy study of CO2 capture process by chemical absorption

Carbon dioxide capturing from the flue gas of power stations is an effective way to mitigate the global warming. In order to predict the performance from startup to stable operation in CO₂ absorption process, a multi-field synergy model was developed based on CO₂ capture process in a packed column by means of monoethanolamine (MEA). The model suggests that the integral diffusion–reaction coefficient plays an important role in the diffusion, fluid flow, heat transfer and chemical reaction processes. The influences of the fluid flow, heat transfer and chemical reaction can be justified using corresponding synergy numbers, quantifying multi-field interactive dynamic characteristics of the CO₂ capture process. The simulation shows a good agreement compared with data in the literature. The results show that the packing Reynolds number can be used as a criterion to choose the proper packing. The less the Reynolds number is, the more efficient the reaction absorption is. The average synergy number F_dc would be decreased by 20% with 6 K temperature drop and be descended by 7% with the 2.5% solvent weight percentage increment, which improved the efficiency of CO₂ capture by about 5% and 14%, and lowered the energy consumption by about 5%. The average synergy number F_dh would be decreased by about 8% with the 0.062 mol/molMEA lean solvent loading increment, which improved the efficiency by about 15% and lowered the energy consumption by 5%. After comparing with CMR-2, Raschig rings, Berl saddles and Pall rings, the 33% less average synergy number F_df of the CMR-2 packing with about 5% drop in energy consumption yields the highest efficiency of 71%, which is 10% higher than that of the Berl saddle packing. The results indicate that the proposed multi-field synergy model is an effective way to intensify the capture process as a guideline with the priority of precision and simplicity.     

Lime enhanced gasification of solid fuels: Examination of a process for simultaneous hydrogen production and CO2 capture

The lime enhanced gasification (LEGS) process uses CaO as a CO₂ carrier and consists of two coupled reactors: a gasifier in which CO₂ absorption by CaO produces a hydrogen-rich product gas, and a regenerator in which the sorbent is calcined producing a high purity CO₂ gas stream suitable for storage. The LEGS process operates at a pressure of 2.0 MPa and temperatures less than 800 °C and therefore requires a reactive fuel such as brown coal. The brown coal ash and sulfur are purged from the regenerator together with CaO which is replaced by fresh limestone in order to maintain a steady-state CaO carbonation activity (a_ave). Equilibrium calculations show the influence of process conditions and coal sulfur content on the gasifier carbon capture (>95% is possible). Material balance calculations of the core process show that the required solid purge of the sorbent cycle is mainly attributed to the necessary removal of ash and CaSO₄ if the solid purge is used as a pre-calcined feedstock for cement production. The decay in the CaO capture capacity over many calcination–carbonation cycles demands a high sorbent circulation ratio but does not dictate the purge fraction. A thermodynamic analysis of a LEGS-based combined power and cement production process, where the LEGS purge is directly used in the cement industry, results in an electric efficiency of 42% using a state of the art combined cycle.     

Analysis of carbon dioxide-to-methanol direct electrochemical conversion mediated by an ionic liquid

An intensified process for carbon dioxide capture and conversion is proposed and analyzed, considering an electrochemical parallel plate reactor which processes a CO₂-charged stream from an absorption unit at 40 °C and atmospheric pressure and where the target product of the conversion is methanol.The task-specific ionic liquid 1-(3-aminopropyl)-3-methylimidazolium bromide was selected, synthesized and characterized. This ionic liquid has shown a good absorption capacity, high ionic conductivity, high chemical–electrochemical stability and acts as a charged intermediate (CO₂^*−) stabilizer, enabling the electrochemical reduction of absorbed CO₂.The electrical energy in the electrochemical reactor was estimated to be 8.683 kWh kg (CO₂)⁻¹ or 115.16 g (CO₂) kWh⁻¹, too high to ensure the environmental sustainability of the process. A low concentration of carbon dioxide in the liquid phase, at ambient conditions, implies the need for a high electrode area for the process and is a major hindrance to improving the economy of the process.     

CO2 absorption into a phase change absorbent: Water-lean potassium prolinate/ethanol solution

Phase change solvents are attractive energy-efficient absorbents for carbon dioxide (CO₂) capture due to CO₂-rich phase formation. Potassium prolinate + water + ethanol (ProK/W/Eth) solution has shown good capture characteristics as a promising one in our previous work. In this work, absorption rate of CO₂, solubility of N₂O, and heat of absorption for ProK/W/Eth solution were investigated using a stirred cell reactor and a CPA201 reaction calorimeter and these results were also compared with the aqueous ProK and 30 mass% MEA solutions. Using ethanol as a solvent can substantially increase the CO₂ physical solubility and the absorption rate of CO₂ in ProK/W/Eth solutions is far higher than that in aqueous 30 mass% MEA solutions especially at a low CO₂ loading range. Solid precipitation, obtained from the liquid-to-solid phase change absorption, was analyzed by ¹³C NMR and DSC-TGA. The enthalpy change for ProK/W/Eth solutions at various CO₂ loading was also discussed.     

Spherical potassium intercalated activated carbon beads for pulverised fuel CO2 post-combustion capture

Spherical carbon beads with a uniform diameter of ca. 0.6–0.8 mm and high mechanical strength can be prepared by hydrothermal synthesis. To optimise the performance of these adsorbents for pulverised fuel post-combustion capture, the efficacy of potassium intercalation via a KOH treatment has been investigated, deliberately using nitrogen-free phenolic resin derived activated carbon (AC) beads so that the enhanced CO₂ adsorption achieved by potassium intercalation could be delineated from any other effects. At 25 °C and CO₂ partial pressure of 0.15 bar, the adsorption capacity of K-intercalated ACs nearly doubled from 0.79 mmol/g for the untreated carbons to 1.51 mmol/g whilst the effect on the morphology and mechanical strength is relatively small. It was found that only slightly more than ca. 1 wt.% of K is required to give the maximum benefit from intercalation that increases the surface polarity and the affinity towards CO₂. The notably increased CO₂ uptake of the K-AC beads as a result of modest increase in adsorption heat (32–40 kJ/mol compared to 27 kJ/mol for the original AC), coupled with the fast adsorption kinetics, suggest that the overall energy penalty is potentially superior to strongly basic polyethyleneimine and other amine-based solid adsorbent systems for carbon capture.     

Hydrogen production by sorption-enhanced steam methane reforming process using CaO-Zr/Ni bifunctional sorbent–catalyst

A bifunctional CaO-Zr/Ni (13, 18, and 20.5 wt% NiO) sorbent–catalyst was developed using the wet-mixing/sonication technique and applied for hydrogen production by sorption-enhanced steam methane reforming (SESMR), an intensified process that integrates hydrogen production with CO₂ capture. The material was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and N₂ physisorption (BET). CO₂ sorption efficiency of the developed materials was evaluated during 25 CO₂ sorption/regeneration cycles. The prepared sorbent–catalysts were then applied in the SESMR during 10 reaction cycles. The results showed that the bifunctional sorbent–catalyst with 20.5 wt% NiO loading presented the most suitable activity. The H₂ yield of ∼91% at the end of the 10th SESMR cycle is considerably higher than equilibrium H₂ yield that could be obtained by traditional steam methane reforming.     

Sulphation and carbonation properties of hydrated sorbents from a fluidized bed CO2 looping cycle reactor

Sulphation and carbonation have been performed on hydrated spent residues from a 75 kW_th dual fluidized bed combustion (FBC) pilot plant operating as a CO₂ looping cycle unit. The sulphation and carbonation tests were done in an atmospheric pressure thermogravimetric analyzer (TGA), with the sulphation performed using synthetic flue gas (0.45% SO₂, 3% O₂, 15% CO₂ and N₂ balance). Additional tests were carried out in a tube furnace (TF) with a higher SO₂ concentration (1%) and conversions were determined by quantitative X-ray diffraction (QXRD) analyses. The morphology of the sulphated samples from the TF was examined by scanning electron microscopy (SEM). Sulphation tests were performed at 850 °C for 150 min and carbonation tests at 750 °C, 10 cycles for 15 min (7.5 min calcination + 7.5 min carbonation). Sulphation conversions obtained for the hydrated samples depended on sample type: in the TGA, they were ～75–85% (higher values were obtained for samples from the carbonator); and in the TF, values around 90% and 70% for sample from carbonator and calciner, respectively, were achieved, in comparison to the 40% conversion seen with the original sample. The SEM analyses showed significant residual porosity that can increase total conversion with longer sulphation time. The carbonation tests showed a smaller influence of the sample type and typical conversions after 10 cycles were 50% – about 10% higher than that for the original sample. The influence of hydration duration, in the range of 15–60 min, is not apparent, indicating that samples are ready for use for either SO₂ retention, or further CO₂ capture after at most 15 min using saturated steam. The present results show that, upon hydration, spent residues from FBC CO₂ capture cycles are good sorbents for both SO₂ retention and additional CO₂ capture.     

Static mixers as heat exchangers in supercritical fluid extraction processes

The performance of a Kenics static mixer as a heat-transfer device for supercritical carbon dioxide (CO₂) flow is studied and compared with conventional tube-in-tube heat exchangers. Measurements were carried out at pressures ranging from 8 to 21 MPa, temperatures from 283 to 323 K, and mass flowrates from 2 to 15 kg/h. The corresponding Reynolds and Prandtl numbers, at bulk conditions, ranged between 10³ and 2 × 10⁴ and between 2 and 7, respectively. The temperature increase experienced by the supercritical CO₂ stream varied between 10 and 35 K. The heat fluxes obtained with the static mixer are one order of magnitude higher than the ones observed with a tube-in-tube heat exchanger for the same set of operating conditions. The heat-transfer enhancement is caused by the cross-sectional mixing of the fluid and to a lesser extent by conduction across the metallic mixing elements. Heat-transfer is also affected by temperature-induced variation of physical properties, especially in the pseudocritical region of the fluid. From the experimental data, a correlation was developed for convective heat-transfer to supercritical CO₂ in terms of the Nusselt number.     

Extraction of bioactive enriched fractions from Eruca sativa leaves by supercritical CO2 technology using different co-solvents

Supercritical fluid extraction from freeze-dried Eruca sativa leaves is assessed with the aim of studying the feasibility to obtain bioactive enriched fractions containing different classes of valuable compounds. Total extraction yields and compositions using pure CO₂ and CO₂ + selected co-solvents are compared. Overall extraction curves, fitted by the model of broken and intact cells developed by Sovová, are reported and the influence of the main parameters that affect the extraction process is analysed. The extract with the highest content in glucosinolates and phenols was collected at 30 MPa and 75 °C using 8% (w/w) of water with respect to the CO₂ flow rate, whereas the fraction richest in lipids was obtained using 8% (w/w) of ethanol as co-solvent at 45 °C and 30 MPa. A process including a first step with supercritical CO₂ extraction using water as co-solvent followed by a second step, where a fraction rich in lipids is extracted using ethanol as co-solvent, is proposed. SCCO₂ results are compared with Soxhlet and other methods that combine organic solvents with ultrasounds.     

Electrochemical cycling behaviour of lithium carbonate (Li2CO3) pre-treated graphite anodes – SEI formation and graphite damage mechanisms

Graphite electrode surfaces were treated using a simple process of sedimentation in aqueous solutions containing 0.5 and 1.0 wt.% Li₂CO₃ with particle sizes of ∼1–2 μm. During the first cycle of voltammetry tests (vs. Li/Li⁺), the graphite surface was subjected to electrochemical degradation as a result of fracture and removal of near-surface graphite particles. Surface degradation was accompanied by a 0.4% strain in the graphite lattice as determined by in situ Raman spectroscopy. Pre-treated electrodes experienced a capacity drop of 3% in the first cycle, compared to a 40% drop observed in case of untreated graphite electrodes. After testing for 100 cycles, a capacity of 0.54 mAh cm⁻² was recorded for the pre-treated electrodes as opposed to a significant drop to 0.11 mAh cm⁻² for the untreated graphite. Cross-sectional HR-TEM indicated that the SEI formed on the pre-treated electrodes primarily consisted of Li₂CO₃ crystals of 14.6 ± 6.9 nm in size distributed within an amorphous matrix. The results suggested that the Li₂CO₃ enriched SEI formed on the pre-treated electrodes reduced the intensity of solvent co-intercalation induced surface damage. It is proposed that the Li₂CO₃ enriched SEI facilitated Li⁺ diffusion and hence improved the capacity retention during long-term cycling.     

Chitosan nanoparticles generation using CO2 assisted processes

The current concerns of sustainable development make the biobased polymers the object of many studies. Chitosan is a biobased, biocompatible and biodegradable polysaccharide with antibacterial and cytocompatible properties. In this study, we aimed to generate chitosan particles with two processes using CO₂ under pressure, in order to decrease the use of organic solvent and to obtain nanoparticles.The first is a supercritical anti-solvent process: CO₂ acts as an anti-solvent toward an acetic acid aqueous solution of dissolved chitosan in which ethanol was added to enhance the anti-solvent effect. The reciprocal miscibility of CO₂ with the solvents induces the reduction of their solvating power, leading to supersaturation at the capillary outlet and causing the crystallization of the particles.This process led to the generation of more or less agglomerated chitosan nanoparticles with an individual average size of 378 nm.In the second process, the pressurized CO₂ is dissolved in water to lower the pH. This in turn allows the chitosan to be dissolved and the resulting solution is sprayed, thanks to the pressurized CO₂, into a hot air stream. This new process allowed the generation of dried chitosan nanoparticles with a median size of 390 nm.     

Effect of the porous structure in carbon materials for CO2 capture at atmospheric and high-pressure

Activated carbons prepared from petroleum pitch and using KOH as activating agent exhibit an excellent behavior in CO₂ capture both at atmospheric (∼168 mg CO₂/g at 298 K) and high pressure (∼1500 mg CO₂/g at 298 K and 4.5 MPa). However, an exhaustive evaluation of the adsorption process shows that the optimum carbon structure, in terms of adsorption capacity, depends on the final application. Whereas narrow micropores (pores below 0.6 nm) govern the sorption behavior at 0.1 MPa, large micropores/small mesopores (pores below 2.0–3.0 nm) govern the sorption behavior at high pressure (4.5 MPa). Consequently, an optimum sorbent exhibiting a high working capacity for high pressure applications, e.g., pressure-swing adsorption units, will require a poorly-developed narrow microporous structure together with a highly-developed wide microporous and small mesoporous network. The appropriate design of the preparation conditions gives rise to carbon materials with an extremely high delivery capacity ∼1388 mg CO₂/g between 4.5 MPa and 0.1 MPa. Consequently, this study provides guidelines for the design of carbon materials with an improved ability to remove carbon dioxide from the environment at atmospheric and high pressure.     

太阳能有机朗肯循环低温热发电关键因素分析

分析了一种以有机朗肯循环（ORC）和复合抛物面集热器（CPC）为主要组成的太阳能低温热发电系统,设计了旨在增强ORC发电的稳定性及减小换热流体与有机工质传热温差的新型系统结构,建立了系统的数学模型,并结合合肥地区的气象数据进行了数值模拟。结果指出有机工质蒸发温度、集热器倾斜角调整方式、蒸发器级数等参数是系统优化的关键因素,当工质蒸发温度为119℃,蒸发器为两级,集热器倾斜角年调整6次时,系统年发电能力为87.1kW•h•m^-2。     

Modeling and optimization of industrial Fischer–Tropsch synthesis with the slurry bubble column reactor and iron-based catalyst

To optimize industrial Fischer–Tropsch(FT) synthesis with the slurry bubble column reactor(SBCR) and ironbased catalyst, a comprehensive process model for FT synthesis that includes a detailed SBCR model, gas liquid separation model, simplified CO_2 removal model and tail gas cycle model was developed. An effective iteration algorithm was proposed to solve this process model, and the model was validated by industrial demonstration experiments data(SBCR with 5.8 m diameter and 30 m height), with a maximum relative error b 10% for predicting the SBCR performances. Subsequently, the proposed model was adopted to optimize the industrial SBCR performances simultaneously considering process and reactor parameters variations. The results show that C_(5+) yield increases as catalyst loading increases within 10–70 ton and syngas H_2/CO value decreases within1.3–1.6, but it doesn't increase obviously when the catalyst loading exceeds 45 ton(about 15 wt% concentration).Higher catalyst loading will result in higher difficulty for wax/catalyst separation and higher catalyst cost. Therefore, the catalyst loading(45 ton) is recommended for the industrial demonstration SBCR operation at syngas H_2/CO = 1.3, and the C_(5+) yield is about 402 ton" per day, which has an about 16% increase than the industrial demonstration run result.     

Analysis of CO2–NH3 reaction dynamics in an aqueous phase by PCA and 2D IR COS

Both carbamation and bicarbonation are of prime importance in the absorption reactions of CO₂ in an aqueous NH₃ solution, as they are related to the CO₂ working capacity, regeneration energy, and the critical problem of blocking the gas pathway for the CO₂ capture process. Herein, the influence of reaction temperature on the CO₂ and NH₃ reaction in an aqueous solution is demonstrated by a principal component analysis (PCA) and a two dimensional correlation analysis (2D IR COS) obtained from FT-IR, dependent on the reaction time. In contrast to the reaction at 298 K, conversion of the dominant reaction from carbamation to bicarbonation and respective conformational changes were observed at 278 K by PCA and 2D IR COS. The PCA results elucidate that two major reactions following the dependence of reaction time were divided into two regions, I and II. The turnover point was subsequently tracked in these two regions, where precipitation of ammonium bicarbonate occurred due to the limitation of solubility at this turning point. The interrelation and sequential variation of conformations in regions I and II were investigated by synchronous and asynchronous 2D correlation analyses. The combination of PCA and 2D IR COS provides a powerful and useful analytic method to capture and monitor the dynamics of complex chemical reactions.     

Tape-cast asymmetric membranes for hydrogen separation

Ceramic hydrogen separation membrane is a promising technology for obtaining pure hydrogen in a wide range of processes including power generation with pre-combustion CO₂ capture, water-gas shift, methane reforming, etc. This work presents for the first time the production of cer-cer asymmetrical composite membranes. BaCe_0.65Zr_0.20Y_0.15O_3-δ (BCZY) supported BCZY- Gd_0.2Ce_0.8O_2-δ (GDC) membranes were produced by tape casting. Three different sintering aid incorporation methods were investigated to enhance the final density of the BCZY-GDC layer. The optimization of the whole process leads to produce planar crack-free asymmetrical proton conductive membranes with Ø=12 mm, constituted by a porous 350 µm thick BCZY substrate with an open porosity of 48%, and a 20 µm thick gas tight BCZY-GDC layer.     

Preparation and characterization of polystyrene foams from limonene solutions

The foaming process has been traditionally performed at high temperature because the CO₂ and the polymer should behave as a homogeneous solution. The addition of a solvent could avoid the high working temperature while the homogeneity is ensured. Among the terpene oils, limonene outlines as a good candidate to carry out the dissolution of polystyrene because it respects the green chemistry principle, it is highly soluble in CO₂ and very compatible with the polymer.The sorption of CO₂ is the first step of the foaming process. The presence of the terpene oil enhances the solubility of the gas which is solubilized in the Polystyrene as well as in the limonene. During the foaming process, many parameters can be tuned to customize the foams. In this work, a fractional factorial design of experiment was proposed to determine the effect of pressure, temperature, concentration of the solution, contact time and vent time over the diameter of cells, its standard deviation and the cells density. The proposed foaming process can be simply performed at mild pressure and temperature thanks to the presence of the solvent. The results showed that the most suitable conditions to foam polystyrene from limonene solutions are 90 bar, 30 °C, 0.1 gPS/ml Lim, 240 min contacting and 30 min venting. Finally, the samples were characterized to determine the amount of residual solvent, their glass transition and degradation temperature checking that the foams presented around 5% of solvent traces but did not show any evidence of degradation.     

Visual and Raman spectroscopic observations of hot compressed water oxidation of guaiacol in fused silica capillary reactors

Using fused silica capillary reactors (FSCRs), we investigated the decomposition of guaiacol during hot compressed water oxidation (HCWO), with H₂O₂ added in stoichiometric ratios from 100 to 300%. Reactions were performed between 180 and 300 °C for durations from 2 to 10 min while the concurrent generation of CO₂ during the oxidation process was followed by Raman spectroscopy and the phase behavior of guaiacol in HCW, with or without H₂O₂, was observed visually under a polarized microscope configured with a heating/cooling stage. We found that complete conversion of guaiacol and 100% yield of CO₂ were achieved with a 150% stoichiometric ratio of oxidizer after 10 min at 200 and 300 °C, respectively. Based on the global reaction kinetics for the complete conversion of guaiacol to CO₂, the reaction is considered to be first order. The activation energy and pre-exponential factor for CO₂ formation are 18.62 kJ mol⁻¹ and 12.81 s⁻¹, respectively.     

Experimental measurement and modelling with Aspen Plus® of the phase behaviour of supercritical CO2 + (n-dodecane + 1-decanol + 3,7-dimethyl-1-octanol)

Experimental phase equilibrium data for the systems CO₂ + n-dodecane, CO₂ + 1-decanol and CO₂ + 3,7-dimethyl-1-octanol were used to determine values for binary interaction parameters for use in the RK-ASPEN thermodynamic model in Aspen Plus^®. Bubble and dew point data of the mixtures CO₂ + (n-dodecane + 1-decanol), CO₂ + (n-dodecane + 3,7-dimethyl-1-octanol), CO₂ + (1-decanol + 3,7-dimethyl-1-octanol) and CO₂ + (n-dodecane + 1-decanol + 3,7-dimethyl-1-octanol) were measured experimentally in a static synthetic view cell, and compared to the data predicted by the RK-ASPEN model. The model predicted the phase equilibrium data reasonably well in the low solute concentration region; significant deviation of model predictions from experimental data occurred in the mixture critical and high solute concentration regions due to the exclusion of solute–solute interaction parameters in the model. Distribution coefficients and separation factors were determined for the multi-component mixture and separation of the alkane from the alcohol mixture with a supercritical fluid extraction process was found to be possible.     

Polymeric coating of fluidizing nano-curcumin via anti-solvent supercritical method for sustained release

Nano-curcumin was coated by poly(lactic-co-glycolic acid) (PLGA) using a novel fluidization assisted supercritical anti-solvent procedure. PLGA solution was sprayed into supercritical CO₂ media, in which nano-curcumin particles were fluidized by ultrasonic vibration. The influences of process parameters, such as solvent types, solution concentrations, CO₂ flow rates, the ratio of PLGA to curcumin, and ultrasonic power on the particles size and the curcumin loading were investigated. Scanning electron microscopy, laser particle size analyzer, and differential scanning calorimetry were used to characterize as-produced samples in terms of the structure, morphology and particle size distribution. The PLGA-curcumin nano-capsules were obtained with the average size of 63 nm and the loading of 38%, under the ultrasonic power of 210 W, and with the average size of 40 nm and 36% loading, at the ultrasonic power of 350 W. In vitro studies prove that proposed method is successful in preparing sustained release systems.