Systematic generation of thermochemical cycles for water splitting

Based on the observation that a thermochemical cycle is tantamount to a reaction route (RR) we employ the well-developed concepts of response reactions (RERs) and direct reaction routes (RRs) to develop an algorithm for a systematic enumeration of thermodynamically feasible thermochemical cycles for water splitting. The input is a set of chemical species along with their thermodynamic characteristics. A unique set of reactions is next generated from a list of species using the formalism of RERs. These are further screened and reduced to a subset of thermodynamically favorable RERs. Alternatively, the reactions may be selected from a separately build database of chemical reactions. The reactions are next assembled into direct RRs (thermochemical cycles), i.e., linear combinations of reactions that produce the desired overall reaction (OR), i.e., 2H₂O = 2H₂ + O₂. Only the direct RRs that satisfy the thermodynamic feasibility condition are further retained. The proposed algorithm is a new powerful tool for automatic generation and screening of thermodynamically feasible thermochemical cycles.     

DEVELOPMENT OF A NEW REACTOR FOR COMBINED COMMINUTION AND CHEMICAL REACTION

The combining of the two process steps comminution and chemical reaction is realized in a newly developed apparatus, the so-called reaction-mill. It is an up to 450°C heatable vibration-mill, that is resistant to pressures up to 2 MPa and that has a gas inlet and outlet for running heterogeneous chemical reactions during the process of comminution. The possible accelerations of the vibration-mill exceed up to 65 g and relative amplitudes of maximum 0.14 are adjustable. The advantage of the combination of the two process steps is the utilization of the effects of comminution exceeding the changes in the dispersity of the solid. The examined heterogeneous non-catalytic gas-solid reaction is the conversion of metallurgical grade (MG) silicon with gaseous hydrogen chloride to give trichlorosilane and silicontetrachloride. The examinations of the combined hydrochlorination and comminution have shown, that the induction periods in the starting phase of the reaction, carried out singularly, can be reduced to a minimum. Furthermore it could be noticed, that the lower temperature limit, which is necessary to start the reaction, could be lowered from 300°C to 100°C and that the selectivity of the reaction products is influenced by the mechanical energy. On one hand the effects are explained by the changes in the dispersity of the solid during comminution and on the other hand by a mechanical activation of the MG silicon.     

煤油共处理过程中的反应机理

主要从煤中键的裂解、氢转移机理以及逆反应等方面论述了近年来在煤液化过程特别是煤油共处理过程反应机理研究的新进展, 着重介绍了溶剂、催化剂对氢转移机理的影响, 指出今后应加强对煤油共处理反应机理的研究, 用于指导将来可能的工业实践．     

DEPENDENCE OF THE MULTIPLICITY FEATURES OF AN ISOTHERMAL CATALYTIC REACTION ON EXTERNAL AND INTERNAL TRANSPORT RESISTANCES

A mathematical model (Robin) which accounts for both internal and external transport resistances is used to determine the multiplicity features of a porous catalytic pellet in which an isothermal Langmuir-Hinshelwood reaction occurs. At most three solutions exist for a slab, but an arbitrarily large number of solutions can be found for an infinite cylinder or a spherical pellet. The maximal number of solutions for any finite external mass transfer resistance is bounded between that existing for a model which ignores the external mass transfer resistance and one which ignores intra-particle concentration gradients. The approximate shape of the cross section of the bifurcation set and of the uniqueness boundary of the Robin model can be estimated from the knowledge of the multiplicity features of three simplified (lumped, Dirichlet and Neumann) models, each containing one less parameter.     

Phototransduction in Anuran Green Rods: Origins of Extra-Sensitivity

Green rods (GRs) represent a unique type of photoreceptor to be found in the retinas of anuran amphibians. These cells harbor a cone-specific blue-sensitive visual pigment but exhibit morphology of the outer segment typical for classic red rods (RRs), which makes them a perspective model object for studying cone–rod transmutation. In the present study, we performed detailed electrophysiological examination of the light sensitivity, response kinetics and parameters of discrete and continuous dark noise in GRs of the two anuran species: cane toad and marsh frog. Our results confirm that anuran GRs are highly specialized nocturnal vision receptors. Moreover, their rate of phototransduction quenching appeared to be about two-times slower than in RRs, which makes them even more efficient single photon detectors. The operating intensity ranges for two rod types widely overlap supposedly allowing amphibians to discriminate colors in the scotopic region. Unexpectedly for typical cone pigments but in line with some previous reports, the spontaneous isomerization rate of the GR visual pigment was found to be the same as for rhodopsin of RRs. Thus, our results expand the knowledge on anuran GRs and show that these are even more specialized single photon catchers than RRs, which allows us to assign them a status of “super-rods”.     

KINETICS OF THE REACTION OF DIBROMOMETHANE AND TWO MIXED n-ALCOHOLS UNDER PHASE TRANSFER CATALYSIS

A novel method was provided to synthesize unsymmetric formaldehyde acetal by reacting two mixed alcohols (e.g., 1-butanol and 1-octanol) and dibromomethane via phase transfer catalysis. Only three final unique products with two alkoxide substituents (one unsymmetric and two symmetric acetals) were observed during the reaction for using higher reactivities of alcohols as the reactants for using higher reactivities of alcohols as the reactants. This result indicated that the rates of the four second reactions in the organic phase are larger than those of the two first reactions in the organic phase. Mass transfer of the catalyst and the active catalyst are very rapid, compared to the reaction in the organic phase. The reaction in the organic phase is a rate-determining step. Based on the fact that the intermediate products were not observed, a kinetic model was developed and simplified. A pseudo-first-order rate law was sufficient to describe the kinetics of reaction. High yields of the two symmteric acetals and the one unsymmetric acetal were produced by employing tetrabutylammonium bromide (TBAB) as a phase transfer catalyst in a high alkaline concentration of KOH/chlorobenzene two-phase medium.     

两股无预混流体湍流混合与反应过程的数值模拟

旋转填充床作为新型的高效反应传质设备,广泛应用于快速反应过程,如制备纳米粉体材料.但是旋转床内的混合机理以及混合与反应的相互关系尚不清楚.作为旋转填充床内流动、混合与反应过程一种初步的简化,针对两股无预混反应物料在通过丝网之后的湍流混合、扩散及反应行为进行了数值模拟研究,以求为实际应用中涉及多层丝网的旋转床内的混合反应机理的研究提供基础.对两股无预混反应物料的湍流流动与组分混合过程,应用双方程k-ε湍流模型进行了模拟计算.对于所涉及的化学反应过程,建立了瞬时封闭模型进行模拟.模拟的混合层厚度、平均浓度及浓度协方差等与文献公认结果进行了对比,二者吻合良好,平均浓度的偏差不超过10%.     

STUDY OF SOME UNIQUE FEATURES OF RATIO-DEPENDENT MODELS FOR PREDATOR-PREY-SUBSTRATE INTERACTIONS IN CONTINUOUS CULTURES

This article studies the dynamics of ratio-dependent models for continuous bioreactors involving interactions between predator, prey, and a limiting substrate. Ratio-dependent models, for which the growth rate of predator is a function of the ratio of prey to predator abundance, have recently received growing attention and were shown to exhibit more interesting behavior than the classical (strictly) prey-dependent models. It is shown that in addition to predicting areas of predator washout and areas of total washout, the studied examples of ratio-dependent models predict areas for which the species coexist either at steady state or in oscillatory mode for any initial population values. The studied models also predict a unique feature for which the interacting species can coexist or wash out depending on their initial values. The objective of this article is to study in some detail this interesting behavior that makes ratio-dependent models better candidates in predicting predator-prey interactions in real biological life.     

STUDY OF SOME UNIQUE FEATURES OF RATIO-DEPENDENT MODELS FOR PREDATOR-PREY-SUBSTRATE INTERACTIONS IN CONTINUOUS CULTURES

This article studies the dynamics of ratio-dependent models for continuous bioreactors involving interactions between predator, prey, and a limiting substrate. Ratio-dependent models, for which the growth rate of predator is a function of the ratio of prey to predator abundance, have recently received growing attention and were shown to exhibit more interesting behavior than the classical (strictly) prey-dependent models. It is shown that in addition to predicting areas of predator washout and areas of total washout, the studied examples of ratio-dependent models predict areas for which the species coexist either at steady state or in oscillatory mode for any initial population values. The studied models also predict a unique feature for which the interacting species can coexist or wash out depending on their initial values. The objective of this article is to study in some detail this interesting behavior that makes ratio-dependent models better candidates in predicting predator-prey interactions in real biological life.     

Simplified point defect model for growth of anodic passive films on iron

We present a simplified point defect model to describe the growth of the primary passive oxide film on the surface of iron. The model postulates a reduced set of elementary interfacial reactions to describe the formation and dissolution of the oxide film. By casting the model in dimensionless form, we obtain a relatively small set of parameters that must be assigned values. Parameter values are set by matching the film thickness predicted by the model with one experimental data point. The model is then used to predict variations in film thickness with time, temperature, and applied potential, yielding reasonable agreement with experimental data. The model also gives the correct qualitative trend in the dependence of film thickness on electrolyte pH. Although the model parameters used in our comparisons are probably not unique, they suggest that physical picture embodied in the model provides a suitable starting point for modeling the growth of passive films on Fe.     

Calculation of multicomponent multiphase equilibria

An algorithm which gives very good first trial values for the computation of chemical equilibrium composition is proposed. It is based on writing the chemical reactions in “canonical form” and changing the independent component set until it is formed by the species present in greatest quantity at equilibrium. The advancement degrees ξ_r of the reactions are calculated considering the reactions to occur independently of any other. By applying a similar procedure to the series reactor method, a new powerful solution algorithm of the Rosenbrook's type is obtained. In the first method we linearize the equilibrium equations written making null the gradient of the free energy of the system with respect to the corresponding ξ_r whilst in the second one we make successive unidimensional searches solving the equations for ξ_r     

氯与植物纤维原料中木素的反应速度及反应性质的探讨

<正> 在氯化法制造纸浆的过程中,氯气与经过碱液处理的植物纤维原料作用,使其中的木素成为氯化木素,以便用稀碱除去.本工作的目的是寻求在氯化法的条件下氯与木素的反应速度,并试图明确该反应的性质.     

REACTOR-MEMBRANE PERMEATOR CASCADE FOR ENHANCED PRODUCTION AND RECOVERY OF H2 AND CO2 FROM THE CATALYTIC METHANE-STEAM REFORMING REACTION

The catalytic reforming of methane by steam is an important industrial process that produces H₂, CO and CO₂, thus chemically transforming natural gas, coal gas and light hydrocarbon feedstocks to synthesis gas or hydrogen fuel. Methane-steam reforming may consist of a number of reactions depending on the reforming catalyst, operating conditions and feedstock composition, The typical industrially desirable reactions are the reverse of methanation (CH₄ + H₂O = CO + 3H₂) and the water-gas shift (CO + H₂O = CO₂ + H₂). Both reactions are equilibrium limited and the composition of the mixture that exits the reformer is in accordance with the one calculated thermodynarmically. Removal of reaction products at the reactor exit by means of selective membrane permeation can offer improved CH₄ conversions and CO₂ and H₂ yields, assuming the subsequent utilization of the reject streams by a second methane-steam reformer. We numerically investigated the feasibility of a system of two tubular methane-steam reformers, in series with an intermediate permselective polyimide membrane permeator, as means of improving the overall CH₄ conversion and the H₂, CO₂ yields over conventional methane-steam reforming equilibrium reaction-separation schemes that are currently in industrial practice. The unique feature of the permselective polyimide separator is the simultaneous removal of H₂ and CO₂ versus CH₄ and CO from the reformed streams. The utilized 6FDA-3,3', 5,5'-TMB aromatic polyimide was reportedly characterized [10] and found to exhibit superior permselective properties compared with other polyimides of the same or different dianhydride sequence. Conversion and yield of the designed reactor-membrane permeator reforming system can be maximized by optimizing the permselective properties of the membrane material and the design variables of the reactors and the permeator. Product recovery and purity in the permeate stream need to be compromised to overall enhance methane conversion and product yield. The operating variables that were varied to investigate their effect on the magnitude of conversion and yield included the inlet pressure of the first reformer, the temperature of both reformers, and the permeator dimensionless Pe' number (variation of the first two variables results to a drastic change in the composition of the reformed stream that enters into the permeator). The numerical results show that the new reformer-membrane permeator cascade process can be more effective (it can offer increased CH₄ conversions and H₂, CO₂ yields) than conventional equilibrium methane-steam reforming reaction-separation processes currently in practice.     

FUNDAMENTAL MODEL OF TRANSPORT AND REACTION IN A CYLINDRICAL CATALYST PORE

The kinetic theory of gases is used to arrive at integral equations which govern free molecular flow and heterogeneous kinetics in a single, cylindrical, isothermal, uniform catalytic pore. The formulation is in terms of reactant fluxes to the catalytic surface and overall or reactive sticking factors. Focus is on simple m-th order reactions which depend on the surface concentration of a single species. Effectiveness factors for the pore, as well as reactant flux profiles obtained using this fundamental model, are compared to conventional continuum diffusion-reaction model predictions of effectiveness factors and reactant concentration profiles. There are distinct differences between the predictions of the two models for moderate to high sticking factors (0·1 to 1). For very small sticking factors, as commonly found in practical catalytic reaction systems, predictions of both models are essentially the same. The widely used continuum diffusion-reaction method provides good predictions of effectiveness factors and reactant concentration profiles, although it is not rigorous in its treatment of species transport.     

Flow Chemistry – A Key Enabling Technology for (Multistep) Organic Synthesis

Laboratory scaled flow‐through processes have seen an explosive development over the past decade and have become an enabling technology for improving synthetic efficiency through automation and process optimization. Practically, flow devices are a crucial link between bench chemists and process engineers. The present review focuses on two unique aspects of modern flow chemistry where substantial advantages over the corresponding batch processes have become evident. Flow chemistry being one out of several enabling technologies can ideally be combined with other enabling technologies such as energy input. This may be achieved in form of heat to create supercritical conditions. Here, indirect methods such as microwave irradiation and inductive heating have seen widespread applications. Also radiation can efficiently be used to carry out photochemical reactions in a highly practical and scalable manner. A second unique aspect of flow chemistry compared to batch chemistry is associated with the option to carry out multistep synthesis by designing a flow set‐up composed of several flow reactors. Besides their role as chemical reactors these can act as elements for purification or solvent switch.     

Total asymmetric synthesis of monosaccharides and analogues

Since the discovery of the 'formose reaction' by Butlerow, total synthesis of carbohydrates has undergone rapid development. The most important methods for the asymmetric synthesis of monosaccharides and analogues of biological importance are presented. Nowadays any natural and non-natural monosaccharide can be prepared pure in both enantiomeric forms starting from inexpensive starting materials. Metal-based asymmetric catalysis and organocatalysis have been successfully applied, alone or in combination with chemoenzymatic methods. Alternative methods rely upon substrate- or reagent-controlled diastereo- and enantioselective reactions. Suitably protected carbohydrates have been prepared by total synthesis, thus allowing their direct use in the preparation of oligosaccharides and analogues.     

甲烷低温催化直接合成乙酸的新途径

对甲烷经合成气路线间接制乙酸的现状及在温和条件下直接转化制乙酸的研究进展作了述评．通过对间接与直接路线的比较．以及在直接路线中,甲烷低温氧化羰化和直接羧化制乙酸均相与非均相催化体系的分析。指出了CH4-CO2低温直接合成乙酸在工艺过程上的显著优势,尤其是采用非均相催化体系．该工艺为乙酸合成和CH4与CO2的绿色化学利用开辟了新途径。其研究将会丰富C1化学化工的理论与实践．并为实现热力学不利反应提供实验方法和理论依据。     

Acid–base surface free energies of solids and the definition of scales in the Good–van Oss–Chaudhury theory

The overwhelming basicity of all analysed surfaces strongly dependent on the choice of liquid triplet used for contact angle measurements and the negative values sometimes obtained for the square roots of the acid-base parameters can be summarized as the main problems arising from the application of the Good-van Oss-Chaudhury (GvOC) theory to the calculation of Lewis acid-base properties of polymer surfaces from contact angle data. This paper tries to account for these problems, namely: (1) the Lewis base, or electron donor component, is much greater than the Lewis acid or electron-acceptor component because of the reference values for water chosen in the original GvOC theory. A direct comparison of the acidic component with the basic one of the same materials has no meaning. A new reference scale for water which is able to overcome this problem is suggested. For the calculation of acid-base components, a best-fit approach is proposed which does not require any starting information about the liquids or polymers and can yield estimates of the acid-base parameters for both the liquids and the polymers involved; (2) the strong dependence of the value of the acid-base components on the three liquids employed is due to ill-conditioning of the related set of equations, an intrinsic and purely mathematical feature which cannot be completely cured by any realistic improvement in experimental accuracy. To reduce or eliminate the effect, one only needs a proper set of liquids, representative of all kinds of different solvents; (3) the negative coefficients appear as a simple consequence of measurement uncertainty, combined with the possible ill-conditioning of the equation set. We cannot exclude, however, that in some cases they could have a different origin.