Transcritical CO2 Heat Pump Dryer: Part 2. Validation and Simulation Results

The simulation model of a transcritical CO₂ heat pump dryer presented in Part 1 has been first validated with available experimental data in this part and then used to simulate the heat pump dryer to study the variation of performance parameters such as heating COP, moisture extraction rate, and specific moisture extraction rate. The validation with experimental data shows that the model slightly over predicts the system performance. The possible reasons for the difference between experimental and numerical results are explained. Simulation results show the effect of key operating parameters such as bypass air ratio, re-circulation air ratio, dryer efficiency, ambient condition (temperature and relative humidity), and air mass flow rate. Results show that unlike bypass air ratio and ambient relative humidity, the effect of dryer efficiency, recirculation air ratio, ambient temperature, and air mass flow rate are very significant as far as the system performance is concerned.     

Transcritical CO2 Heat Pump Dryer: Part 1. Mathematical Model and Simulation

In this study, a mathematical model and simulation code has been developed to investigate the performance of a transcritical CO₂ heat pump dryer. The model takes into account detailed heat and mass transfer and pressure drop phenomena occurring in each component of the system. To take care of the variable heat transfer properties, the heat exchanger components were divided into several infinitesimal segments to examine the state, heat and mass balance and pressure drop for both refrigerant and air, and hence accurate results are expected. In Part 2 of the article, the model developed has been validated with experimental data and then the model was used to investigate effects of important operating parameters on the performance.     

热泵干燥装置的应用和发展分析

在简要介绍热泵干燥装置基本结构和工作原理的基础上，对热泵干燥装置的特点、市场拓展、应用与推广中的主要问题和解决思路、技术发展等进行了较全面的分析，为科学地把握和应用热泵干燥技术提供了较好的参考。     

Transcritical CO2 Heat Pump Dryer: Part 1. Mathematical Model and Simulation

In this study, a mathematical model and simulation code has been developed to investigate the performance of a transcritical CO₂ heat pump dryer. The model takes into account detailed heat and mass transfer and pressure drop phenomena occurring in each component of the system. To take care of the variable heat transfer properties, the heat exchanger components were divided into several infinitesimal segments to examine the state, heat and mass balance and pressure drop for both refrigerant and air, and hence accurate results are expected. In Part 2 of the article, the model developed has been validated with experimental data and then the model was used to investigate effects of important operating parameters on the performance.     

跨临界二氧化碳热泵喷射循环实验

在跨临界CO₂热泵热水器系统中引入优化设计的喷射器,对系统进行实验研究,分析了制热系数、引射比、升压比、喷射器效率等参数随热水体积流量和出口温度及高压侧压力的变化趋势以及优化设计的喷射器对系统的影响。实验结果表明:随着热水体积流量减小或其出口温度增加,引射比将逐渐减小,而喷射器效率逐渐升高;在测试工况范围内升压比基本保持不变,系统COP_h最高将近3.5;系统高压侧的压力因优化喷射器的引入而明显降低,有利于系统的安全运行;跨临界二氧化碳热泵喷射循环系统存在一个最优运行压力,值得注意的是在最优运行压力下,热水出水温度虽未达到最高,但依旧超过55℃。系统稳定运行在最优高压侧压力下,不仅系统性能大幅度提高,而且保证了热水的出水温度。     

低温热泵干燥装置的技术方案分析

低温热泵干燥装置在化学品、食品、药品、农副产品的干燥中具有广泛需要。给出了基本型、真空型和吸附剂型三类低温热泵干燥装置,对其结构、工作原理和特性进行了简要分析。分析表明,在初投资方面,基本型最低,真空型最高;在能源效率方面,真空型最高,基本型最低;吸附剂型在初投资和能源效率方面均居中。上述工作可为低温热泵干燥装置的设计提供较好的参考。     

Effects of Component Arrangement and Ambient and Drying Conditions on the Performance of Heat Pump Dryers

Performances of five heat pump dryer (HPD) configurations were studied by computer simulation. The component arrangement covers the fully open, the partially open, and the fully closed systems with external condenser or external cooler. The HPD performance was investigated for high and low drying rate and within the ambient temperature range of 20°C to 40°C. The best operating mode of the HPD depends on both drying rate and ambient condition. For the case of tropical climate and high drying rate, the partially open system is the optimum operating mode in general, except at some particular conditions where the fully open and fully closed systems are recommended. However, the operation for the low drying rate is more complicated, as the optimum configuration is sensitive to the change of ambient temperature.     

热泵干燥装置的特性方程及其应用分析

以应用最广泛的封闭式热泵干燥装置为例,建立了其SMER计算方程。在此基础上,对SMER随干燥器进口空气温度、干燥器出口空气温度和蒸发器出口空气温度而变化的规律进行了计算和分析。结果表明,其他两个参数一定时,SMER随干燥器出口空气温度上升而降低,随蒸发器出口空气温度上升而增加,但当干燥器进口空气温度取适宜值时,SMER存在最大值。     

PREDICTION OF CO2 FREEZE-OUT CONDITIONS AND CO2 HYDRATE FORMATION CONDITIONS WITH A SINGLE EQUATION OF STATE

A recent study by Eggeman and Chaffin (2005 Eggeman , T. , and Chafin , S. ( 2005 ). Beware of pitfalls of CO₂ freezing predictions , Chem. Eng. Prog. , 101 ( 3 ), 39 – 44 . [Google Scholar]), which showed large discrepancies in CO₂ freeze-out conditions as predicted by several commercial simulators, prompted a reexamination of using the TBS equation of state for phase equilibrium calculations involving solids. Salim and Trebble (1994 Salim , P. , and Trebble , M. A. ( 1994 ). Modelling of solid phases in thermodynamic calculations via translation of a cubic equation of state at the triple point , Fluid Phase Equilib. , 93 , 75 – 99 .[Crossref] , [Google Scholar]) had previously presented a methodology for extending the Trebble-Bishnoi-Salim (TBS) equation of state (Salim, 1990 Salim , P. ( 1990 ). A modified Trebble-Bishnoi equation of state, M.Sc. thesis , University of Calgary . [Google Scholar]) to calculations involving a solid phase. In this study, the CO₂ freeze-out conditions in CO₂/CH₄ and CO₂/C₂H₆ mixtures are calculated from the TBS equation of state, and it is shown that they provide a better data fit than the traditional Poynting correction method. Furthermore, since the use of an equation of state in SLE/SVE calculations does not require the explicit assumption of a pure solid phase, the model was assessed for its ability to correlate CO₂ gas hydrate equilibrium conditions. Gas hydrates were simply treated as an impure solid phase, and it was seen that the predictions of gas hydrate equilibrium were in very good agreement with the experimental data. Computationally, the use of the TBS equation of state has the advantage, over the model of Yokozeki (2005 Yokozeki , A. ( 2005 ). Methane gas hydrates viewed through unified solid-liquid-vapour equation of state , Int. J. Thermophys. , 26 ( 3 ), 743 – 765 . [Google Scholar]), that it does not require a modifying factor (c_b) in the repulsive term to handle the presence of hydrates; they are instead handled using a unique binary interaction parameter for the hydrate phase.     

Numerical Investigation of the Effect of Using CO2 as the Refrigerant in a Heat Pump Tumble Dryer System

CO₂ appears to be a suitable refrigerant for dryer operation conditions due to its thermophysical and environmentally friendly properties. In this study, a heat pump tumble dryer system using CO₂ as the refrigerant was theoretically investigated, and a computer model was developed using MATLAB software. The changes in the energy consumption and drying time, depending on the inlet CO₂ pressure into the gas cooler, inlet CO₂ temperature into the evaporator, air mass flow rate, and dryer efficiency, are presented. The results show that the optimum operating parameters are extremely effective in reducing the drying time and energy consumed.     

Drying Kinetics of Tomato by Using Electric Resistance and Heat Pump Dryers

Abstract

Drying kinetics of tomato was studied by using heat pump dryer (HPD) and electric resistance dryers with parallel and crossed airflow. The performance of both systems was evaluated and compared and the influence of temperature, air velocity, and tomato type on the drying kinetics was analyzed. The use of HPD showed to be adequate in the drying process of tomatoes, mainly in relation to the conversion rate of electric energy into thermal energy. The heat pump effective coefficient of performance (COP_HT,EF) was between 2.56 and 2.68, with an energy economy of about 40% when compared to the drying system with electric resistance. The Page model could be used to predict drying time of tomato and statistical analysis showed that the model parameters were mainly affected by drying temperature.     

Drying Kinetics of Tomato by Using Electric Resistance and Heat Pump Dryers

Drying kinetics of tomato was studied by using heat pump dryer (HPD) and electric resistance dryers with parallel and crossed airflow. The performance of both systems was evaluated and compared and the influence of temperature, air velocity, and tomato type on the drying kinetics was analyzed. The use of HPD showed to be adequate in the drying process of tomatoes, mainly in relation to the conversion rate of electric energy into thermal energy. The heat pump effective coefficient of performance (COP_HT,EF) was between 2.56 and 2.68, with an energy economy of about 40% when compared to the drying system with electric resistance. The Page model could be used to predict drying time of tomato and statistical analysis showed that the model parameters were mainly affected by drying temperature.     

CO2跨临界循环特性对热泵干燥系统的影响

对CO2跨临界循环的超临界放热特性和运行规律进行了分析,根据这些特性,对其应用于干燥热泵系统进行了探讨,并对蒸发温度、过热度及气体冷却器出口温度对系统除湿率的影响做出分析。研究表明CO2跨临界循环放热段存在大的温度滑移,气体冷却器出口温度低有利于系统运行,这些特点非常适合干燥系统,而且干燥系统除湿率也受到这些特性的影响,系统存在最优运行压力和最佳的蒸发温度与气体冷却器出口温度组合。     

EFFICIENT DESULFURIZATION IN O2/CO2 COMBUSTION: DEPENDENCE ON COMBUSTION CONDITIONS AND SORBENT PROPERTIES

Based on experiments on desulfurization, CaSO₄ decomposition, and a system approach using theoretical analysis, efficient in-furnace desulfurization in O₂/CO₂ combustion was investigated. The influence of combustion conditions and sorbent properties on system desulfurization efficiency was clarified. The global desulfurization efficiency was found to increase with O₂ purity. The global desulfurization efficiency in a dry recycle was higher than that in a wet recycle. The global efficiency of in-furnace desulfurization decreased with initial O₂ concentration. As the temperature increased, the global desulfurization efficiency increased first and then decreased due to the decomposition of CaSO₄. In the temperature range investigated, the global desulfurization efficiency in O₂/CO₂ coal combustion was much higher than that of conventional coal combustion in air. The global desulfurization efficiency decreased with sorbent size. When the particle radius decreased to one quarter, the global desulfurization efficiency doubled, becoming as high as 80%. The global desulfurization efficiency was very different among the three sorbents investigated, whether in O₂/CO₂ combustion or in conventional air combustion. The global desulfurization efficiency increased in the order of Ca(OH)₂, scallop, and limestone in O₂/CO₂ combustion, but in the order of scallop, Ca(OH)₂, and limestone in conventional air combustion. Nevertheless, all three sorbents demonstrated much higher desulfurization efficiency in O₂/CO₂ combustion than in conventional air combustion.     

SOLUBILITY OF CO2AND H2S IN A MIXED SOLVENT

Mixed solvents are a combination of chemical and physical solvents and have some advantages over traditional treating solvents for the removal of acid gases from gas streams. The solubility of H₂S and CO₂in a mixed solvent consisting of AMP (2-amino-2-methyl-l-propanol), sulfolane, and water has been measured at 40 and 100°C at partial pressures of the acid gas to 6000 kPa. The solubility in the mixed solvent was compared with the solubility in an aqueous solution of equivalent amine concentration. At solution loadings less than 1 mol acid gas/mol amine, the solubility of the acid gas is lower in the mixed solvent than in the corresponding amine solvent. At higher loadings, the trend is reversed and the solubility is greater in the mixed solvent. The results are rationalized in terms of the effect of the physical solvent component on the chemical reaction and physical vapor-liquid equilibria. The solubility model of Deshmukh and Mather was used to correlate the data.     

A Fourier-transform infrared study of CO2 and CO2/H2 interactions with caesium-doped copper catalysts

FTIR spectra are reported of CO₂ and CO₂/H₂ on a silica-supported caesium-doped copper catalyst. Adsorption of CO₂ on a “caesium”/silica surface resulted in the formation of CO₂ ⁻ and complexed CO species. Exposure of CO₂ to a caesium-doped reduced copper catalyst produced not only these species but also two forms of adsorbed carboxylate giving bands at 1550, 1510, 1365 and 1345 cm⁻¹. Reaction of carboxylate species with hydrogen at 388 K gave formate species on copper and caesium oxide in addition to methoxy groups associated with caesium oxide. Methoxy species were not detected on undoped copper catalyst suggesting that caesium may be a promoter for the methanol synthesis reaction. Methanol decomposition on a caesium-doped copper catalyst produced a small number of formate species on copper and caesium oxide. Methoxy groups on caesium oxide decomposed to CO and H₂, and subsequent reaction between CO and adsorbed oxygen resulted in carboxylate formation. Methoxy species located at interfacial sites appeared to exhibit unusual adsorption properties.     

Comparative CO2 Adsorption Analysis in Pure and Amine-Modified Composite Membranes

In this study, the CO₂ adsorption analysis in cellulose acetate–TiO₂- and cellulose acetate–3-aminopropyl-trimethoxysilane TiO₂-blended membranes was performed. The membranes were also characterized using scanning electron microscopy and Fourier transform infrared analysis techniques. The adsorption results indicated that 120 and 90°C were considered as optimized temperatures for regeneration of cellulose acetate–TiO₂ and cellulose acetate–3-aminopropyl-trimethoxysilane-modified TiO₂ membranes. The testing results revealed that adsorption capacity reached maximum at 3.0 bars. Validation of experimental results was performed by pseudo-first-order, second-order and intraparticle diffusion models. The correlation factor R² represented that the second-order model was fitted well with the experimental data. The intraparticle diffusion model represented that adsorption is not a single-step process.     

The kinetics of CO2 hydrogenation on a Rh foil promoted by titania overlayers

Submonolayer deposits of titania on a Rh foil have been found to increase the rate of CO₂ hydrogenation. The primary product, methane, exhibits a maximum rate at a TiO _x coverage of 0.5 ML which is a factor of 15 higher than that over the clean Rh surface. The rate of ethane formation displays a maximum which is 70 times that over the unpromoted Rh foil; however, the selectivity for methane remains in excess of 99%. The apparent activation energy for methane formation and the dependence of the rate on H₂ and CO₂ partial pressure have been determined both for the bare Rh surface and the titania-promoted surface. These rate parameters show very small variations as titania is added to the Rh catalyst. The methanation of CO₂ is proposed to start with the dissociation of CO₂ into CO_(a) and O_(a), and then proceed through steps which are identical to those for the hydrogenation of CO. The increase in the rate of CO₂ hydrogenation in the presence of titania is attributed to an interaction between the adsorbed CO, released by CO₂ dissociation, and Ti³⁺ ions located at the edge of TiO _x islands covering the surface. Differences in the effects of titania promotion on the methanation of CO₂ and CO are discussed in terms of the mechanisms that have been proposed for these two reactions.     

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.     

Comparative Proteomic Analysis of Puccinellia tenuiflora Leaves under Na2CO3 Stress

Soil salt-alkalinization is a widespread environmental stress that limits crop growth and agricultural productivity. The influence of soil alkalization caused by Na₂CO₃ on plants is more severe than that of soil salinization. Plants have evolved some unique mechanisms to cope with alkali stress; however, the plant alkaline-responsive signaling and molecular pathways are still unknown. In the present study, Na₂CO₃ responsive characteristics in leaves from 50-day-old seedlings of halophyte Puccinellia tenuiflora were investigated using physiological and proteomic approaches. Comparative proteomics revealed 43 differentially expressed proteins in P. tenuiflora leaves in response to Na₂CO₃ treatment for seven days. These proteins were mainly involved in photosynthesis, stress and defense, carbohydrate/energy metabolism, protein metabolism, signaling, membrane and transport. By integrating the changes of photosynthesis, ion contents, and stress-related enzyme activities, some unique Na₂CO₃ responsive mechanisms have been discovered in P. tenuiflora. This study provides new molecular information toward improving the alkali tolerance of cereals.