共查询到20条相似文献,搜索用时 15 毫秒
1.
The methodology of response reactions (RERs) introduced earlier from thermodynamic and kinetic considerations is used in this work to develop a new algorithm for the classification and enumeration of unique/direct reaction routes (RRs) and overall reactions (ORs). According to the RERs approach, a unique set of both RRs and ORs may be generated starting from any conceivable set of linearly independent RRs and ORs. In particular, the direct ORs may be most conveniently enumerated starting from the formula matrix of the terminal species (reactants and products), i.e., without any relation to the elementary reactions comprising the detailed mechanism. Depending on the type of ORs produced by the RRs one can distinguish between two distinct types of direct RRs. Namely, one option is to define a direct RR by specifying the intermediate species that need to be eliminated. This type of RR is referred to as Milner RRs. The other option is to require the direct RRs to produce RERs, thus resulting in RRs referred to as Happel-Sellers RRs. 相似文献
2.
A new process is proposed which converts CO 2 and CH 4 containing gas streams to synthesis gas, a mixture of CO and H 2 via the catalytic reaction scheme of steam-carbon dioxide reforming of methane or the respective one of only carbon dioxide reforming of methane, in permeable (membrane) reactors. The membrane reformer (permreactor) can be made by reactive or inert materials such as metal alloys, microporous ceramics, glasses and composites which all are hydrogen permselective. The rejected CO reacts with steam and converted catalytically to CO 2 and H 2 via the water gas shift in a consecutive permreactor made by similar to the reformer materials and alternatively by high glass transition temperature polymers. Both permreactors can recover H 2 in permeate by using metal membranes, and H 2 rich mixtures by using ceramic, glass and composite type permselective membranes. H 2 and CO 2 can be recovered simultaneously in water gas shift step after steam condensation by using organic polymer membranes. Product yields are increased through permreactor equilibrium shift and reaction separation process integration.
CO and H 2 can be combined in first step to be used for chemical synthesis or as fuel in power generation cycles. Mixtures of CO 2 and H 2 in second step can be used for synthesis as well (e.g., alternative methanol synthesis) and as direct feed in molten carbonate fuel cells. Pure H 2 from the above processes can be used also for synthesis or as fuel in power systems and fuel cells. The overall process can be considered environmentally benign because it offers an in-situ abatement of the greenhouse CO 2 and CH 4 gases and related hydrocarbon-CO 2 feedstocks (e.g., coal, landfill, natural, flue gases), through chemical reactions, to the upgraded calorific value synthesis gas and H 2, H 2 mixture products. 相似文献
3.
Thermodynamic chemical equilibrium analysis of steam reforming of glycerol(SRG)for selective hydrogen production was performed based on the Gibbs free energy minimisation method.The ideal SRG reaction(C 3H 8O 3+3H 2O→3CO 2+7H 2)and a comprehensive set of side reactions during SRG are considered for the formation of a wide range of products.Specifically,this work focused on the analysis of formation of H 2,CO 2,CO and CH4 in the gas phase and determination of the carbon free region in SRG under the conditions at atmospheric pressure,600 K–1100 K and 1.013×10 5–1.013×10 6 Pa with the steam-to-glycerol feed ratios(SGFR)of 1:5–10.The reaction conditions which favoured SRG for H 2 production with minimum coke formation were identifies as:atmospheric pressure,temperatures of 900 K–1050 K and SGFR of 10:1.The influence of using the inert carrier gas(i.e.,N 2)in SRG was studied as well at atmospheric pressure.Although the presence of N 2 in the stream decreased the partial pressure of reactants,it was beneficial to improve the equilibrium yield of H 2.Under both conditions of SRG(with/without inert gas),the CH4 production is minimised,and carbon formation was thermodynamically unfavoured at steam rich conditions of SGFR>5:1. 相似文献
4.
This paper describes a technique that can be used to analyze the reactions that take place in a reactor via mass balances, a graphical representation and interpretation of basic thermodynamic principles. This allows for the chemical species taking part in the reactions being considered, to create lines that define attainable boundaries in a GH plot. The end result is an attainable area that satisfies the conditions for an attainable region in a GH plot. The simultaneous methanol synthesis from syngas with the Water Gas Shift (WGS) reaction is used to illustrate this approach. Considering the investigation, one can readily see that at higher temperatures the reaction was not feasible thermodynamically at 1 bar, but at increased pressure the reaction could again become favorable and thermodynamically feasible. This paper also shows that the introduction of either water or CO 2, or both, to the feed opens up the mass balance region, resulting in WGS activity and generating more reaction path alternatives. Again, the change in Gibbs free energy across the reactor and the reaction pathways leading to product are interlinked. 相似文献
5.
The selective catalytic reduction of NO by H 2 under strongly oxidizing conditions (H 2-SCR) in the low-temperature range of 100–200 °C has been studied over Pt supported on a series of metal oxides (e.g., La 2O 3, MgO, Y 2O 3, CaO, CeO 2, TiO 2, SiO 2 and MgO-CeO 2). The Pt/MgO and Pt/CeO 2 solids showed the best catalytic behavior with respect to N 2 yield and the widest temperature window of operation compared with the other single metal oxide-supported Pt solids. An optimum 50 wt% MgO-50wt% CeO 2 support composition and 0.3 wt% Pt loading (in the 0.1–2.0 wt% range) were found in terms of specific reaction rate of N 2 production (mols N 2/g cat s). High NO conversions (70–95%) and N 2 selectivities (80–85%) were also obtained in the 100–200 °C range at a GHSV of 80,000 h −1 with the lowest 0.1 wt% Pt loading and using a feed stream of 0.25 vol% NO, 1 vol% H 2, 5 vol% O 2 and He as balance gas. Addition of 5 vol% H 2O in the latter feed stream had a positive influence on the catalytic performance and practically no effect on the stability of the 0.1 wt% Pt/MgO-CeO 2 during 24 h on reaction stream. Moreover, the latter catalytic system exhibited a high stability in the presence of 25–40 ppm SO 2 in the feed stream following a given support pretreatment. N 2 selectivity values in the 80–85% range were obtained over the 0.1 wt% Pt/MgO-CeO 2 catalyst in the 100–200 °C range in the presence of water and SO 2 in the feed stream. The above-mentioned results led to the obtainment of patents for the commercial exploitation of Pt/MgO-CeO 2 catalyst towards a new NO x control technology in the low-temperature range of 100–200 °C using H 2 as reducing agent. Temperature-programmed desorption (TPD) of NO, and transient titration of the adsorbed surface intermediate NO x species with H 2 experiments, following reaction, have revealed important information towards the understanding of basic mechanistic issues of the present catalytic system (e.g., surface coverage, number and location of active NO x intermediate species, NO x spillover). 相似文献
6.
This paper describes a pair of chemical reaction experiments developed for Rowan University's introductory course in chemical reaction engineering: an esterification reaction carried out in a packed bed, and a competitive reaction in which the kinetics were influenced by micromixing. The first experiment is the esterification of ethanol and acetic acid to form ethyl acetate. Students first examine this reaction in their organic chemistry class. The experiment developed in this project re-examines this reaction from a chemical engineering perspective. For example, the reaction is reversible and equilibrium-limited, but in the organic chemistry lab, there is no examination of the kinetics. The complementary chemical engineering experiment examines the relationship between residence time and conversion. The second experiment is a competitive system involving two reactions: 5I− + IO3− + 6H+ → 3I2 + 3H2O The first reaction is essentially instantaneous. Thus, when H+ is added as the limiting reagent, a perfectly mixed system would produce essentially no I2. Production of a significant quantity of I2 is attributed to a local excess of H+; a condition in which all H2BO3− in a region is consumed and H+ remains to react with I− and IO3−. In the spring of 2005, for the first time, both experiments were integrated into the undergraduate chemical reaction engineering course. This paper describes the use of the experiments in the classroom and compares the performance of the 2005 students to the 2004 cohort, for whom the course included no wet labs at all. 相似文献
7.
The influence of the addition of 5 vol.% of carbon monoxide, hydrogen, carbon dioxide or water to the feed of partial oxidation of methane was investigated over Ni/γ-Al 2O 3 and Rh/γ-Al 2O 3 catalysts. In addition to catalytic tests, thermodynamic calculations were performed to predict the effect of these gas co-feeds. Compared to the thermodynamic trends, differences in the influence of the co-feeding on catalytic performances were observed between both catalysts. Co-feeding of CO, H 2, CO 2 or H 2O can modify the oxidation state and dispersion of the metal component of the catalysts during reaction, and as a consequence, their performances. Changes in catalysts can be due to dynamic processes occurring during reaction. It is suggested to take these processes into account in a more complex kinetic equation for the reactions involved. 相似文献
8.
Gradient-corrected density functional theory was used to investigate the adsorption of H 2S on Pd(1 1 1) surface. Molecular adsorption was found to be stable with H 2S binding preferentially at top sites. In addition, the adsorption of other S moieties (SH and S) was investigated. SH and S were found to be preferentially bind at the bridge and fcc sites, respectively. The reaction pathways and energy profiles for H 2S decomposition giving rise to adsorbed S and H were determined. Both H 2S (ad) → SH (ad) + H (ad) and SH (ad) → S (ad) + H (ad) reactions were found to have low barriers and high exothermicities. This reveals that the decomposition of H 2S on Pd(1 1 1) surface is a facile process. 相似文献
9.
Thermochemical analyses of interfacial reactions in titanium, zirconium, and hafnium diboride reinforced oxidematrix composites have been carried out to evaluate the chemical compatibility. The chemical reactivity of these diborides with oxygen and the high volatility of B 2O 3( l ) at reduced oxygen pressures are concerns during processing and operating conditions. The thermochemical stability and the vaporization behavior of B 2O 3( l ) are discussed in terms of partial pressures of dominant gaseous species of the boron-oxygen system at 1700 and 2300 K. The TiB 2/ZrO 2 and TiB 2/HfO 2 systems are thermodynamically stable in a limited oxygen pressure range. The TiB 2/Al 2O 3 system is stable, but the reactions in this system may apparently be accompanied by formation of gaseous products (B 2O 3, AlO, Al 2O, and lower boron oxides) in the presence of elemental oxygen. These thermochemical considerations are very useful in evaluating the effectiveness of oxides as diffusion barrier coatings on diboride reinforcements. 相似文献
10.
The rate of propane dehydrocyclodimerization to form C 6 aromatics is limited by a sequence of irreversible dehydrogenation reactions leading to propene, higher alkenes, dienes, trienes, and aromatics. Quasi-equilibrated acid—catalyzed cracking, oligometization, and cyclization reactions of alkene intermediates occur in sequence with these dehydrogenation reactions. Each dehydrogenation reaction is in turn limited by the rate of elementary steps that dispose of H-atoms formed in C-H bond activation steps. The rate of C-H bond activation, recombinative hydrogen desorption, and propane chemical conversion have been measured from the rates of isotopic redistribution and chemical conversion during reactions of C 3H 8/C 3/D 8and D 2/C 3/H 8 mixtures on H-ZSM5, Ga/H-ZSM5, and Zn/H-ZSM5. Isotopic studies show that C-H activation steps are fast during steady-state propane dehydrocyclodimerization on H-ZSM5, Ga/H-ZSM5, and Zn/H-ZSM5. Ga and Zn species increase the rates of propane chemical conversion, recombinative hydrogen desorption, and deuterium incorporation from D2into reaction products. Disposal of hydrogen formed in C-H bond activation steps occurs by transfer of H-atoms to unsaturated species to form alkanes or to Ga and Zn species, which catalyze the recombinative desorption of H-atoms to form dihydrogen (H2). The sequential release of several H-atoms during a propane dehydrocyclodimerization turnover limits the rate and selectivity of this reaction on H-ZSM5. In-situ X-ray absorption studies suggest that Ga and Zn species reside at cation exchange sites as monomeric cations and that recombinative desorption involve reduction—oxidation cycles of such cations during each dehydrocyclodimerization turnover. These monomeric species form directly during exchange of Zn ions from solution onto H-ZSM5. Ga3+species, however, do not exchange directly from solution onto H-ZSM5, but instead form extrazeolitic Ga2O3 crystals. Ion exchange occurs during subsequent contact with propane or hydrogen at 700-800 K via vapor phase exchange of volatile Ga1+ species. 相似文献
11.
We report results on the adsorption and desorption of H 2S on polycrystalline UO 2 at 100 and 300 K, using ultrahigh vacuum X-ray photoelectron spectroscopy (XPS), low energy ion scattering (LEIS), and temperature programmed desorption (TPD). Our work is motivated by the potential for using the large stockpiles of depleted uranium in industrial applications, e.g., in catalytic processes, such as hydrodesulfurization (HDS) of petroleum. H 2S is found to adsorb molecularly at 100 K on the polycrystalline surface, and desorption of molecular H 2S occurs at a peak temperature of 140 K in TPD. Adsorption rates of sulfur as a function of H 2S exposure are measured using XPS at 100 K; the S 2p intensity and lineshapes demonstrate that the saturation coverage of S-containing species is 1 monolayer (ML) at 100 K, and is 0.3–0.4 ML of dissociation fragments at 300 K. LEIS measurements of adsorption rates agree with XPS measurements. Atomic S is found to be stable to >500 K on the oxide surface, and desorbs at 580 K. Evidence for a recombination reaction of dissociative S species is also observed. We suggest that O-vacancies, defects, and surface termination atoms in the oxide surface are of importance in the adsorption and decomposition of S-containing molecules. 相似文献
12.
Sharp NO and O 2 desorption peaks, which were caused by the decomposition of nitro and nitrate species over Fe species, were observed in the range of 520–673 K in temperature-programmed desorption (TPD) from Fe-MFI after H 2 treatment at 773 K or high-temperature (HT) treatment at 1073 K followed by N 2O treatment. The amounts of O 2 and NO desorption were dependent on the pretreatment pressure of N 2O in the H 2 and N 2O treatment. The adsorbed species could be regenerated by the H 2 and N 2O treatment after TPD, and might be considered to be active oxygen species in selective catalytic reduction (SCR) of N 2O with CH 4. However, the reaction rate of CH 4 activation by the adsorbed species formed after the H 2 and N 2O or the HT and N 2O treatment was not so high as that of the CH 4 + N 2O reaction over the catalyst after O 2 treatment. The simultaneous presence of CH 4 and N 2O is essential for the high activity of the reaction, which suggests that nascent oxygen species formed by N 2O dissociation can activate CH 4 in the SCR of N 2O with CH 4. 相似文献
13.
Porous combustors have been studied intensively concerning the combustion of natural gas. The advantages of combustion in porous inert media, such as low emissions, high power turndown ratio of typically 10:1 and compactness, can also be used for different chemical gas phase reactions, e.g. the HCl synthesis from H 2 and Cl 2. The advantages of porous reactors result from the heat transport properties of the porous medium, i.e. emissivity and conductivity. Heat transport mechanisms and chemical reactions were implemented in a numerical code in order to investigate the H 2/Cl 2 system. Important parameters of the reaction, e.g. the laminar flame speed and the adiabatic flame temperature, are higher for the H 2/Cl 2 reaction compared to the CH 4/air combustion. By studying the influence of H 2O and HCl as inert components it was shown by numerical investigations that the maximum temperature could be decreased to a level, which makes the usage of a porous reactor feasible. A porous reactor for laboratory use was tested with O 2/CH 4/N 2 combustion, which delivers even higher adiabatic temperatures and flame speeds than the H 2/Cl 2 reaction. Finally, experiments with H 2/Cl 2/HCl reaction were carried out and first results are presented. 相似文献
14.
基于Gibbs自由能最小原理,对硅和四氯化硅(SiCl 4,STC)耦合加氢反应体系进行了热力学分析。通过化学平衡产物组成分布的分析,确定了反应体系主要产物为三氯硅烷(HSiCl 3,TCS)、二氯硅烷(H 2SiCl 2,DCS)、盐酸(HCl),并构造了3个相应的独立反应,讨论了对应的反应热(ΔrHθm)、自由能(ΔrGθm)和平衡常数(Kθp)与温度的关系。计算所采用的温度为673~923 K,压力为101.325 ~2 026.5 kPa,原料H 2与SiCl 4物质的量比为1~5。结果表明,生成TCS和DCS的反应为体系随着温度升高,四氯化硅平衡转化率及三氯硅烷产率降低;高压和适中的原料配比(H 2与SiCl 4物质的量比)有利于四氯化硅转化率及三氯硅烷产率的提高。 相似文献
15.
The adsorption and desorption of thiophene and the reactions of thiophene-derived adsorbed species in He, H 2, and O 2 were examined on H-ZSM5, H-Beta, and H-Y with varying Si/Al ratios. Thiophene adsorption uptakes (per Al) were independent of Al content, but were above unity and influenced by zeolite structure (1.7, 2.2, and 2.9 on H-ZSM5, H-Beta, and H-Y). These data indicate that thiophene oligomers form during adsorption and that their size depends on spatial constraints within zeolite channels. Adsorption and oligomerization occur on Brønsted acid sites at 363 K. Thiophene/toluene adsorption from their mixtures show significant thiophene selectivity ratios (10.3, 7.9, and 6.4, for H-ZSM5, H-Beta, and H-Y zeolites), which exceed those expected from van der Waals interactions and reflect specific interactions with Brønsted acid sites and formation of toluene–thiophene reaction products. Treatment of thiophene-derived adsorbed species above 363 K in He or H 2 led to depolymerization of thiophene oligomers and to the formation of unsaturated adsorbed species with a 1:1 thiophene/Al stoichiometry on all zeolites and at all Si/Al ratios. These unsaturated species desorb as stable molecules, such as H 2S, hydrocarbons, and larger organosulfur compounds, formed via ring opening and hydrogen transfer from H 2 or co-adsorbed species, and also form stranded unsaturated organic deposits. Smaller channels and higher Al contents preferentially formed H 2S, benzotiophenes, and arene products during treatment in He or H 2, as a result of diffusion-enhanced of secondary reactions of desorbed thiophene molecules with adsorbed thiophene-derived species. Only oxidative regeneration treatments led to full recovery of thiophene uptake capacities. A preceding treatment in H 2, however, led to the partial recovery of thiophene-derived carbon atoms as useful hydrocarbons and decreased the amount of CO 2 and SO 2 formed during subsequent oxidative treatments required for regeneration. 相似文献
16.
The direct synthesis of methanethiol, CH 3SH, from CO and H 2S was investigated using sulfided vanadium catalysts based on TiO 2 and Al 2O 3. These catalysts yield high activity and selectivity to methanethiol at an optimized temperature of 615 K. Carbonyl sulfide and hydrogen are predominant products below 615 K, whereas above this temperature methane becomes the preferred product. Methanethiol is formed by hydrogenation of COS, via surface thioformic acid and methylthiolate intermediates. Water produced in this reaction step is rapidly converted into CO 2 and H 2S by COS hydrolysis. Titania was found to be a good catalyst for methanethiol formation. The effect of vanadium addition was to increase CO and H2S conversion at the expense of methanethiol selectivity. High activities and selectivities to methanethiol were obtained using a sulfided vanadium catalyst supported on Al2O3. The TiO2, V2O5/TiO2 and V2O5/Al2O3 catalysts have been characterized by temperature programmed sulfidation (TPS). TPS profiles suggest a role of V2O5 in the sulfur exchange reactions taking place in the reaction network of H2S and CO. 相似文献
17.
The oxidation and reforming kinetics of methane by O 2, CO 2 and H 2O were studied on a stepped Pt(5 5 7) single crystal from 623 to 1050 K under methane rich conditions. The rate of carbon deposition was followed by ex-situ Auger electron spectroscopy under non-oxidative conditions. The apparent activation energy for methane decomposition was significantly lower than the apparent barriers measured for both total oxidation, CO 2 and H 2O reforming. Total oxidation of methane to CO 2 and H 2O followed by combined dry and steam reforming (combined combustion-reforming) led to CO production rates which were higher than direct CO 2 or H 2O reforming rates. The enhanced rates are most likely due to the ability of adsorbed oxygen to prevent carbon nucleation and/or scavenge carbon enabling the reforming reaction to turnover on a larger fraction of sites. Comparable amounts of carbon were found by Auger electron spectroscopy measurements after both direct dry or steam reforming, while combined oxidation-reforming had considerable less carbon. During direct dry or steam reforming, CO 2 and H 2O serve only to scavenge adsorbed atomic carbon, while in the presence of oxygen, carbon is removed by both combustion and reforming routes. 相似文献
18.
Heterogeneous photocatalysis (HP) using UV/TiO 2, photo-Fenton (PF) reaction using UV/Fe/H 2O 2 and the combination UV/TiO 2/Fe/H 2O 2 (HP–PF) were tested as processes to degrade oxalic acid (Ox) at relatively high concentrations (0.032 M). PF reactions were generally more efficient than HP including the reaction in the absence of H 2O 2. Oppositely to previous results (e.g., with EDTA), HP–PF combinations did not result, in the case of oxalate, better techniques for degradation than systems in the absence of TiO 2. The kinetic behavior was not unique and two parameters were taken to evaluate the efficiency of each system: initial rates ( R0) and time to 95% of total mineralization (TOC 95). Addition of hydrogen peroxide improves the initial HP reaction rate and reduces TOC 95. Addition of Fe 3+ also affects the reaction parameters but the effect of H 2O 2 seems to be higher, at least under the present conditions. When both H 2O 2 and iron were added simultaneously, the efficiency was higher. The optimal H 2O 2:Ox:Fe molar ratio was established and the results indicated that, at a fixed iron concentration, H 2O 2 increased R0 until a limit beyond which it did not cause any effect. No intermediates were formed in the reaction, oxalate being degraded directly to CO 2. Analogies and differences with the EDTA system are presented. 相似文献
19.
Catalytic performance for partial oxidation of methane (POM) to synthesis gas was studied over the Rh/Al 2O 3 catalysts with Rh loadings between 0.1 and 3 wt%. It was found that the ignition temperature of POM reaction increased with the decreasing of the Rh loadings in the catalysts. For the POM reaction over the catalysts with high (≥1 wt%) Rh loadings, steady-state reactivity was observed. For the reaction over the catalysts with low (≤0.25 wt%) Rh loadings, however, oscillations in CH 4 and reaction products (CO, H 2, and CO 2) were observed. Comparative studies using H 2-TPR, O 2-TPD and high temperature in situ Raman spectroscopy techniques were carried out in order to elucidate the relation between the redox property of the Rh species in the Rh/Al 2O 3 with different Rh loadings and the performance of the catalysts for the reaction. Three kinds of oxidized rhodium species, i.e. the rhodium oxide species insignificantly affected by the support (RhO x), that intimately interacting with the Al 2O 3 surface (Rh iO x) and the Rh(AlO 2) y species formed by diffusion of rhodium oxides in to sublayers of Al 2O 3 [C.P. Hwang, C.T. Yeh, Q.M. Zhu, Catal. Today, 51 (1999) 93.], were identified by H 2-TPR and O 2-TPD experiments. Among them, the first two species can be easily reduced by H 2 at temperature below 350 °C, while the last one can only be reduced by H 2 at temperature above 500 °C. The ignition temperatures of POM reaction over the catalysts are closely related to the temperature at which most of the RhO x and Rh iO x species can be reduced by CH 4 in the reaction mixture. Compared to the Rh/Al 2O 3 with high Rh loadings, the catalysts with low Rh loadings contain more Rh iO x species which possess stronger RhO bond strength and are more difficult to be reduced than RhO x by the reaction mixture. Higher temperature is therefore required to ignite the POM reaction over the catalysts with lower Rh loadings. The oscillation during the POM reaction over the Rh/Al 2O 3 with low Rh loadings can be related to the behaviour of Rh(AlO 2) y species in the catalyst switching cyclically from the oxidized state to the reduced state during the reaction. 相似文献
20.
Heterogeneous catalysts based on magnetic mixed iron oxides (MO·Fe 2O 3; M: Fe, Co, Cu, Mn) were used for the decolorization of several synthetic dyes (Bromophenol Blue, Chicago Sky Blue, Cu Phthalocyanine, Eosin Yellowish, Evans Blue, Naphthol Blue Black, Phenol Red, Poly B-411, and Reactive Orange 16). All the catalysts decomposed H 2O 2 yielding highly reactive hydroxyl radicals, and were able to decolorize the synthetic dyes. The most effective catalyst FeO·Fe 2O 3 (25 mg mL −1 with 100 mmol L −1 H 2O 2) produced more than 90% decolorization of 50 mg L −1 Bromophenol Blue, Chicago Sky Blue, Evans Blue and Naphthol Blue Black within 24 h. The fastest decomposition proceeded during the first hour of the reaction. In addition to dye decolorization, all the catalysts also caused a significant decrease of chemical oxygen demand (COD). Individual catalysts were active in the pH range 2–10 depending on their structure and were able to perform sequential catalytic cycles with low metal leaching. 相似文献
|