CO2-空气微通道蒸发器两相区熵产分析

微通道已成为换热器研究领域的热点,以CO₂微通道蒸发器为研究对象,建立了CO₂微通道蒸发器两相区内、外侧均有相变的熵产模型,通过建立的CO₂微通道蒸发器二维分布参数模型求解系统熵产数.分析CO₂与空气侧质量流率、空气入口温度及CO₂蒸发温度对系统熵产数的影响.结果表明:CO₂质量流率对系统熵产数影响很小;系统熵产数主要由CO₂与空气两侧温差传热引起;系统熵产数随空气入口温度的增大而增大,随CO₂的蒸发温度的增大而减小;随着空气质量流率的增大,系统熵产数增大,且蒸发温度越高,空气质量流率对系统熵产数的影响越大.     

万吨级低温多效蒸发海水淡化能量及经济性能模拟分析

通过VC++编制了海水淡化的模拟工艺程序,模拟分析了蒸发器总效数、首效蒸发器温度、末效蒸发器温度、浓缩比等因素对低温多效蒸发海水淡化工艺的能耗和技术经济的影响,为海水淡化系统的设计和运行提供了参考依据.     

小管径CO2蒸发器换热性能

为进一步优化适用于低蒸发温度的CO₂蒸发器性能，建立CO₂翅片管式蒸发器数学模型，探究在低蒸发温度-35℃、制冷剂流量0.06 kg/s、进风干球温度-25℃、风速1—3.5m/s条件下，5种不同换热管径及流路数对CO₂蒸发器的换热量、传热特性、压降特性的影响，并搭建CO₂跨临界双级压缩制冷试验台对蒸发器进行性能测试，验证蒸发器模型的准确性与可靠性。研究结果表明：迎面风速的增加可以提高蒸发器换热特性和增大管内外侧压降，但迎面风速过大反而会使管内换热系数降低；小管径在CO₂蒸发器的应用，可以显著提高换热性能，5 mm管径蒸发器空气侧与制冷剂侧换热系数相比于6.35 mm管径换热器分别最大提高15%和45.91%;通过增加流路数可以明显改善小管径蒸发器压降过大问题，为用于低温制冷的CO₂蒸发器性能优化与推广提供参考。     

己二酸对MEA吸收-解吸CO2过程的影响<

实验研究了己二酸对MEA水溶液吸收-解吸CO₂的影响。在0.4 mol·L-1 MEA的CO₂吸收富液解吸过程加入一定量的己二酸并分析了其对CO₂解吸能耗和解吸速率的影响,发现解吸速率明显升高,析出单位体积CO₂的能耗显著降低;对解吸还原后的贫液进行了CO₂二次吸收的实验,发现因加酸引起的CO₂二次吸收量变化小于7%;为去除不确定因素对CO₂相似文献   

水平管降膜蒸发器的传热性能

在换热面积为2.375m2的水平管降膜蒸发试验平台上,采用5052铝合金管作为换热管,以实际海水为原料,进行了低温多效海水淡化中水平管降膜蒸发器传热性能研究试验。研究了料液喷淋密度、管外蒸发温度、总传热温差、海水盐度以及管内蒸汽中不凝气含量等因素对海水淡化过程降膜蒸发器总传热系数的影响。结果表明,在试验条件范围内,总传热系数随着料液喷淋密度和管外蒸发温度的升高而增加,随着传热温差的增大而降低;冷凝侧有不凝气存在时,总传热系数下降幅度较大;海水浓度对传热系数影响较小;在控制不凝气含量的条件下,传热系数在3500W/(m2?℃)以上。试验结果为海水淡化的工程设计和生产优化提供了依据。     

金属离子促进乙醇胺捕集二氧化碳研究

为了解决醇胺法燃烧后捕集二氧化碳再生能耗过高的问题,研究了一种向胺溶液中添加金属离子以降低其CO₂解吸能耗的方法,称之为金属离子络合物热缓冲自热利用技术。以广泛商业化应用的单乙醇胺（MEA）溶液为研究载体,并在MEA溶液中分别添加金属离子铜或镍, 通过建立含有金属离子的MEA捕集CO₂体系的化学反应模型,解释金属离子热缓冲剂效应的内在机理。机理显示在MEA-金属离子-CO₂-H₂O体系中,金属-MEA络合物作为一种有效的反应热缓冲剂,将有机胺吸收CO₂过程中释放的反应热（放热反应）存储于金属络合物的解离键能中（吸热反应）,在CO₂高温解吸中通过其络合放热反应将储存的能量释放出来用于CO₂解吸,形成自热再生低能耗CO₂捕集技术,从而降低了MEA再生的能耗。本文进行了综合的实验测定来评价金属离子对MEA溶液捕集CO₂过程的性能提升影响,包括CO₂反应热、解吸速率、吸收-解吸循环负载、汽液平衡溶解度等。实验结果表明铜离子或镍离子作为添加剂,能增加MEA的CO₂平衡循环负载14%~20%或7%~10%,同时能够降低MEA的CO₂反应热值6.6%~24%或6.0%~20%。     

环丁砜对乙醇胺溶液吸收和解吸CO2的影响

利用物理溶剂环丁砜替代部分水,采用气液搅拌实验装置和真实热流量热法测定了环丁砜对乙醇胺（MEA）溶液吸收和解吸二氧化碳（CO₂）过程的影响,考察了CO₂循环负载、吸收速率、吸收热和解吸热等性质变化。研究表明：环丁砜对MEA溶液负载CO₂的吸收热影响较小,但对吸收速率、循环吸收容量和解吸过程影响较大。环丁砜可降低MEA溶液对CO₂的表观吸收速率,且随CO₂负载量的增大,降幅也逐渐变大。环丁砜有利于富液解吸过程,加快解吸速率,增大CO₂解吸程度,同时单位热流负荷、单位冷流负荷和单位能耗均有不同程度的降低。在燃煤电厂烟气条件下,20% MEA+20% sulfolane体系相对20% MEA体系,其表观吸收速率平均降低约10%,CO₂循环吸收容量增加24%,单位CO₂解吸能耗降低18%。     

低温多效蒸发海水淡化系统热力分析

建立了喷射器低温多效蒸发海水淡化系统的数学模型,计算分析了各种温度损失随温度的变化,并研究了顶值盐水温度、蒸发器效数和动力蒸汽等参数对系统的造水比和生产单位质量淡水所需传热面积的影响。结果表明各种温度损失在末效蒸发器内显著增加;喷射器低温多效蒸发系统的热力特性明显优于多效蒸发系统;通过增加顶值盐水温度、蒸发器效数和动力蒸汽温度,可以实现系统的优化运行。     

带压环境中CO2/H2O/N2气体稀释对合成气层流火焰速度的影响

合成气稀释燃烧是燃气轮机高效低污染燃烧的重要运行方式。本文以CO₂、H₂O和N₂为稀释气体,利用数值模拟方法研究稀释比对不同压力下合成气（CO/H₂/CH₄）层流火焰速度（S_L）的影响规律,并从自由基浓度变化、敏感性数值和生成速率（rate of production,ROP）三个方面解析三种气体的物理和化学作用机理。结果表明,S_L随燃烧压力和稀释比的增大而不断减小,其中CO₂对层流火焰速度的抑制最为显著。稀释气体的物理效应对层流火焰速度的影响远大于化学效应,但CO₂和H₂O的化学效应不能忽略。化学效应则是通过改变H和OH自由基浓度影响S_L,其中CO₂稀释降低H和OH自由基浓度,H₂O稀释则是降低H自由基浓度,从而降低合成气的层流火焰速度。进一步反应动力学分析发现了H/OH浓度变化在低压、加压下的主要化学反应路径,且受H₂O稀释的化学反应速率对压力较CO₂更为敏感。     

基于CO2循环的低碳高效白云石煅烧新工艺

白云石是一种广泛应用的冶金、建材和化工原料。针对白云石煅烧过程中CO₂排放严重等问题,构建了基于CO₂循环载热与资源化回收的白云石低碳煅烧竖窑新工艺。通过白云石（CaCO₃·MgCO₃）煅烧过程的Gibbs自由能变计算,发现提高煅烧温度（50~100 K）可有效克服CO₂对反应的抑制作用;通过纯CO₂环境中CaCO₃分解过程的热重实验分析,验证了CO₂循环煅烧白云石煅烧的可行性;通过化学反应动力学计算,解析了全CO₂组分环境下CO₂压力对CaCO₃·MgCO₃高温分解过程的影响,并发现提高CO₂压力可促进气固传热,从而提升分解速率和改善矿料分解均匀性;对CO₂循环煅烧工艺系统能-质平衡计算表明：该工艺理论能耗仅为140 kg/(t 煅白),且煅烧过程的CO₂排放降低70%以上,环境效益显著。     

Study on the Mechanism of CO Formation in Reverse Water Gas Shift Reaction Over Cu/SiO2 Catalyst by Pulse Reaction, TPD and TPR

The mechanism of reverse water gas shift reaction over Cu catalyst was studied by pulse reaction with QMS monitoring, temperature programmed desorption (TPD) and temperature programmed reduction (TPR) of Cu/SiO₂ catalyst. The reduced and/or oxidized copper offered low catalytic activity for the dissociation of CO₂ to CO in the pulse reaction study with 1 ml volume of He/CO₂, but the rate of CO formation was significantly enhanced with H₂ participating in the reaction. The TPD spectra of CO₂ obtained by feeding H₂/CO₂ over copper at 773 K provided strong evidence of the formation of formate at high temperature. The formate derived from the association of H₂ and CO₂ is proposed to be the key intermediate for CO production. The formate dissociation mechanism is the major reaction route for CO production.     

CO2 adsorption performance of polyethyleneimine-modified ion-exchange resin

A series of solid amine adsorbents were prepared by the template method with ion-exchange resin (D001) as the carrier and polyethyleneimine (PEI) as the modifier. The absorbents were characterized by energy disperse spectroscopy (EDS), scanning electron microscope (SEM), N₂ adsorption–desorption, Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA) techniques. The effects of PEI loading, adsorption temperature and influent velocities on CO₂ adsorption capacity in a fixed-bed reactor were investigated. The results show that the solid amine adsorbent prepared by the template method had a better PEI dispersion, stability and CO₂ adsorption capacity. The maximum CO₂ adsorption capacity was 3.98 mmol·g^?1 when PEI loading was 30%, the adsorption temperature was 65°C and the influent velocity was 40 mL·min^?1. The CO₂ adsorption capacity decreased only by 9.50% after 10 cycles of adsorption–desorption tests. The study of kinetics indicates that both chemical adsorption and physical adsorption occurred in the CO₂ adsorption process. The CO₂ adsorption process included fast breakthrough adsorption and gradually approaching equilibrium stage. The particle internal diffusion process was the control step for CO₂ adsorption.     

Comprehensive sulfation model verified for T-T sorbent clusters during flue gas desulfurization at moderate temperatures

An empirical sulfation model for T-T sorbent clusters was developed based on amassed experimental results under moderate temperatures (300-800 °C). In the model, the reaction rate is a function of clusters mass, SO₂ concentration, CO₂ concentration, calcium conversion and temperature. The smaller pore volume partly results in a lower reaction rate at lower temperatures. The exponent on SO₂ concentration is 0.88 in the rapid reaction stage and then decreases gradually as reaction progresses. The exponent on the fraction of the unreacted calcium is 1/3 in the first stage and then increases significantly in the second stage. The CO₂ concentration has a negative influence on SO₂ removal, especially for the temperature range of 400-650 °C, which should be avoided to achieve a high effective calcium conversion. The sulfation model has been verified for the T-T sorbent clusters and has also been applied to CaO particles. Over extensive reaction conditions, the predictions agree well with experimental data.     

Catalytic Ring Hydrogenation of Benzoic Acid with Supported Transition Metal Catalysts in scCO2

The ring hydrogenation of benzoic acid to cyclohexanecarboxylic acid over charcoal-supported transition metal catalysts in supercritical CO₂ medium has been studied in the present work. The cyclohexanecarboxylic acid can be produced efficiently in supercritical CO₂ at the low reaction temperature of 323 K. The presence of CO₂ increases the reaction rate and several parameters have been discussed.     

Effects of potassium on the chemisorption of CO2 and CO on the Mo2C/Mo(100) surface

The interaction of CO₂ with K-promoted Mo₂C/Mo(100) has been studied with high-resolution electron energy loss spectroscopy, work function measurements and temperature-programmed desorption. Pre-adsorbed potassium dramatically affects the adsorption behavior of CO₂ on the Mo₂C/Mo(100) surface. It increases the rate of adsorption, the binding energy of CO₂ and it induces the dissociation of CO₂ through the formation of negatively charged CO₂. Potassium adatoms also promote the dissociation of adsorbed CO over Mo₂C. This revised version was published online in July 2006 with corrections to the Cover Date.     

Characterization of Cu/CeO2/γ-Al2O3 Thin Film Catalysts by Thermal Desorption Spectroscopy

Thermal desorption spectroscopy (TDS) under ultra high vacuum (UHV) condition has been used to investigate the desorption characteristics of Cu/CeO₂/γ-Al₂O₃ thin film catalysts coated onto the microchannel of a microreactor. TDS results demonstrate that surface desorption profiles and chemical properties (acid–base and redox properties) are remarkably influenced by the catalyst composition, i.e. the loading of copper and ceria. The enhanced basicity with the increase of ceria loading and the decrease of copper loading is evident from the shifted desorption maximum of CO₂ in TDS spectra. Three oxygen species, ranging from weakly bound oxygen desorbed at low temperature to the strongly held lattice oxygen desorbed at high temperature, are easily discernible and clearly identified by O₂ TDS spectra, depending on the catalyst compositions. The concomitant thermal desorption of O₂, CO₂, and H₂O at low temperature indicates the unique chemical properties of copper/ceria catalyst with appropriate copper and ceria contents. The observed low-temperature feature is ascribed to the role of porthole of copper/ceria interfacial area for several desorbed species. The weakly bound oxygen species is attributed to the enhanced abundance of copper/ceria interfacial anionic vacancies created by the intimate contact between copper and ceria entities and its impact on steam reforming of methanol (SRM) reaction is tentatively discussed in terms of reverse oxygen spillover.     

Pathways for CO2 Formation and Conversion During Fischer–Tropsch Synthesis on Iron-Based Catalysts

CO₂ reaction and formation pathways during Fischer–Tropsch synthesis (FTS) on a co-precipitated Fe–Zn catalyst promoted with Cu and K were studied using a kinetic analysis of reversible reactions and with the addition of ¹³C-labeled and unlabeled CO₂ to synthesis gas. Primary pathways for the removal of adsorbed oxygen formed in CO dissociation steps include reactions with adsorbed hydrogen to form H₂O and with adsorbed CO to form CO₂. The H₂O selectivity for these pathways is much higher than that predicted from WGS reaction equilibrium; therefore readsorption of H₂O followed by its subsequent reaction with CO-derived intermediates leads to the net formation of CO₂ with increasing reactor residence time. The forward rate of CO₂ formation increases with increasing residence time as H₂O concentration increases, but the net CO₂ formation rate decreases because of the gradual approach to WGS reaction equilibrium. CO₂ addition to synthesis gas does not influence CO₂ forward rates, but increases the rate of their reverse steps in the manner predicted by kinetic analyses of reversible reactions using non-equilibrium thermodynamic treatments. Thus the addition of CO₂ could lead to the minimization of CO₂ formation during FTS and to the preferential removal of oxygen as H₂O. This, in turn, leads to lower average H_2/CO ratios throughout the catalyst bed and to higher olefin content and C₅₊ selectivity among reaction products. The addition of ¹³CO₂ to H₂/¹²CO reactants did not lead to significant isotopic enrichment in hydrocarbon products, indicating that CO₂ is much less reactive than CO in chain initiation and growth. We find no evidence of competitive reactions of CO₂ to form hydrocarbons during reactions of H₂/CO/CO₂ mixtures, except via gas phase and adsorbed CO intermediates, which become kinetically indistinguishable from CO₂ as the chemical interconversion of CO and CO₂ becomes rapid at WGS reaction equilibrium.     

Optimization of CO2 capture process with aqueous amines using response surface methodology

Amine is one of candidate solvents that can be used for CO₂ recovery from the flue gas by conventional chemical absorption/desorption process. In this work, we analyzed the impact of different amine absorbents and their concentrations, the absorber and stripper column heights and the operating conditions on the cost of CO₂ recovery plant for post-combustion CO₂ removal. For each amine solvent, the optimum number of stages for the absorber and stripper columns, and the optimum absorbent concentration, i.e., the ones that give the minimum cost for CO₂ removed, is determined by response surface optimization. Our results suggest that CO₂ recovery with 48 wt% DGA requires the lowest CO₂ removal cost of $43.06/ton of CO₂ with the following design and operating conditions: a 20-stage absorber column and a 7-stage stripper column, 26 m³/h of solvent circulation rate, 1903 kW of reboiler duty, and 99°C as the regenerator-inlet temperature.     

CaCO3–CO2–H2O system in falling film on a bank of horizontal tubes: Model verification

The purpose of this work is to model the CaCO₃–CO₂–H₂O system in falling film on a bank of horizontal tubes. The model was applied on a 5-effects reference thermal vapor compression multiple-effect distiller (MED-TVC) operating at top brine temperatures (TBT) of 60–70 °C. The model can predict pH values, CaCO₃ deposition and fouling resistance with greater accuracy. Through the MED stages, the HCO₃^? and CO₂ concentrations slightly increased while the CO₃^2? concentration slightly decreased. The pH decreased from 8.8 in the first stage to 8.4 in the 5th stage. The CO₂ release rates as well as the CaCO₃ deposition rates increase with increasing top brine temperature (TBT). CO₂ release rates decrease from 36.4 g/t feed water in the first stage to 32.5 g/t in the last stage. The specific CaCO₃ deposition decreases from 127.3 g/t feed water in the first stage to 100.1 g/t in the last stage.     

SrCo0.8Fe0.2O3−δ sorbent for high-temperature production of oxygen-enriched carbon dioxide stream

Perovskite-type SrCo_0.8Fe_0.2O_3−δ (SCF) has been prepared by a liquid citrate method and used to produce O₂-CO₂ gas mixture for oxyfuel combustion. Oxygen is desorbed and an oxygen-enriched carbon dioxide stream is obtained when SCF is exposed in a carbon dioxide stream at high temperature. Oxygen is adsorbed when SCF is regenerated in an air stream. A carbonation-reaction mechanism for O₂-desorption has been identified with the evidences of XRD and TGA analysis. It is found that the theoretical oxygen sorption capacity decreases with the increase of temperature. The sorption kinetics over a temperature range of 700-900 °C has been examined by TGA experiment. Both desorption and sorption processes exhibit a high reaction rate in an initial stage followed by a slower rate in a second stage. It is difficult to reach the theoretical oxygen sorption capacity during the whole temperature range due to the slow oxygen desorption rate. Optimal temperatures for oxygen sorption and desorption processes are determined to be 900 and 850 °C, respectively. Multiple sorption and desorption cycles indicate that SCF sorbent has high reactivity and cyclic stability. Comparison with the reference La_0.1Sr_0.9Co_0.5Fe_0.5O_2.6 (LSCF) and Sr_0.5Ca_0.5Co_0.5Fe_0.5O_2.47 (SCCF) sorbents shows that SCF has faster carbonation reaction at high temperature, i.e., 850 and 900 °C, and much higher theoretical oxygen sorption capacities.