Catalytic oxidation of glycerol in alkaline medium – Influence of mass transport limitations

Glycerol upgradation via oxidation is characterized by a complex reaction network, further complicated by the heterogeneous (gas/liquid/solid) nature of the reaction. Thus, rates and selectivities could be significantly modified by transport limitations, especially in industrial reactors. In this work, we have studied the reaction on supported Pd and Au catalysts under alkaline conditions, both in the absence and presence of transport resistances, both external and internal. Reaction pathways proposed on the basis of data under kinetic control have been used to rationalize the findings. Interestingly and counterintuitively, the selectivity to C₃ compounds is better under the influence of mass transfer. This behaviour is because of the nature of the reaction, which involves solution-mediated reactions in addition to the metal-catalyzed reactions on the catalyst surface. These findings have important implications for the scale up of this reaction and process design methodology.     

Enhanced Reaction Efficiency in Continuous Flow

Continuous flow reactors are enabling tools that can significantly benefit chemical reactions, especially those that are path length dependent (e.g., photochemical), mixing or transport dependent (e.g., gas-liquid), exothermic, or utilize hazardous or unstable intermediates. In this review, it is demonstrated how the nearly instantaneous mixing, exceptionally fast mass transfer, safe access to high temperatures and pressures, and high surface area to volume ratio can be leveraged to improve product yield, reaction rates and/or selectivity. By showcasing five synthetic methodologies examined by our group, it is hoped that the reader will gain an appreciation of the accessible and transformative nature of flow chemistry for improving existing transformations, enabling rapid optimization, and for developing new methodologies that depend on precise parameter controls.     

Mass transfer effects in polycondensation reactors wherein functional groups are not equally reactive

Three simplified models of polycondensation reactors are considered in which the condensation product is continuously removed by application of vacuum. Reversible polycondensation reactions of monomers violating the equal reactivity hypothesis have been simulated in these reactors. The effect of various rate and reactor design variables on the molecular weight distribution (MWD) and its moments is studed. It is observed that when the reverse reactions are rapid, the results are fairly sensitive to the level of vacuum applied and to the mass transfer resistance; whereas when the forward reactions predominate, results lie very close to earlier plots for the corresponding irreversible polymerizations. These reactor variables then have relatively small influence on the MWD. Splitting of the MWD curves for odd and even values on n is observed under certain conditions, the effects being more pronounced in the presence of mass transfer than in its absence.     

Electrochemistry of non-aged 90-10 copper-nickel alloy (UNS C70610) as a function of fluid flow: Part 1: Cathodic and anodic characteristics

The polarisation behaviour of freshly polished UNS C70610 (CN 102) 90-10 copper-nickel has been examined in fully characterised seawaters using the rotating disc electrode (RDE) and the rotating cylinder electrode (RCE) geometries. The charge and mass transport controlled responses of both the cathodic and anodic reactions are presented as a function of both laminar and turbulent undisturbed fluid flow. At low values of polarisation applied over short exposure periods (<1 h), the anodic behaviour of non-aged material is controlled by the selective dissolution of the copper component of the alloy. Under conditions of complete mass transport control, oxygen reduction proceeds via the irreversible direct four-electron reduction to the hydroxide ion. Above a critical range of Reynolds numbers the rate of both the reduction and oxidation reactions tended towards a more reversible character. This change in mechanism, however, was not observed for unalloyed copper and was attributed to a convective-diffusion-based modification of the corrosion product film.     

Reactor modelling for electrochemical processes

This paper presents a relatively straightforward approach to the modelling of electrochemical reactors operated in batch or continuous modes. The models are based on ideal flow assumptions of either well-mixed or plug flow and incorporate reaction rate models based on electrochemical kinetics and mass transport at one electrode. General characteristics of the reactor models are described, particularly with regard to the need for good mass transport in metal recovery applications. An example is given on the use of the model in the recovery of a heavy metal (Cd²⁺) from an acidified solution containing Cd(II) and Fe(III) ions. The reaction rate model is based on experimental data.     

APPLICATIONS OF A NON-PERMSELECTIVE, CATALYTICALLY ACTIVE MEMBRANE, A MODEL STUDY

A theoretical investigation has been presented for applications and features of a non-permselective, catalytic membrane reactor with separated feed of reactors [12-14, 17, 18]. Transmembrane fluxes were calculated from the dusty gas model as a function of a great number of parameters and operation conditions. This study shows that the non-permselective, catalytic membrane reactor with separated feed of reactants (CMRSR) has attractive features to use this reactor in fast and highly exothermic reactions and selectivity improvement in multiple reactions.

When the CMRSR is operated in the transport controlled regime, the process is easy to control and even possesses some self-controllability. Due to the transport conversion, thermal runaway cannot occur which allows operation with concentrated feed of reactants. Furthermore, a transmembrane pressure difference increases both the fluxes and the selectivity, because the reaction products are preferentially directed towards one side of the membrane. The simultaneous increase of both selectivity and fluxes is a remarkable feature of a CMRSR which is in contrast with conventional reactors.     

Analysis and optimization of cross-flow reactors with distributed reactant feed and product removal

A systematic and general model was proposed for the simulation of cross-flow reactors with product removal and reactant feed policies. Six types of cross-flow reactors were analyzed for reversible series-parallel reaction systems and their optimal feed distributions were determined by maximizing the desired product yield at the outlet of the reactor. The performances of reactors with different types of feed policies were compared at their optimal operating conditions. For irreversible reaction systems with lower order in distributed reactant for the desired reaction than those for undesired reactions, a higher yield and selectivity of the desired product could be achieved with the reactors with staged feed than with conventional co-feed reactors and a sufficiently high residence time was required by staged feed reactors to significantly improve the desired product yields and selectivities over those obtained by a co-feed reactor. However, for reversible reaction systems, the desired product yield always reached a maximum value, and then dropped down as the residence time increased. In addition to the kinetic order and residence time requirements, the rate constants of the reactions involved have to fall within certain ranges for the distributed feed reactor to obtain a higher maximum yield than one-stage co-feed reactors. Optimally distributed feed reactors always give higher maximum product yields than evenly distributed reactors with the same number of feed points. However, the improvement of yields is not as great as that between co-feed reactors and evenly distributed reactors. On the other hand, for reaction systems with higher order with respect to the distributed reactant in the desired reaction than the undesired reactions, co-feed reactors always give higher yield than staged feed reactors.     

Multi-mode low-dimensional models for non-isothermal homogeneous and catalytic reactors

A systematic procedure based on the Liapunov-Schmidt method of bifurcation theory is used to derive low-dimensional models for different types of non-isothermal homogeneous, catalytic and coupled homogeneous-heterogeneous reactors. These low-dimensional models are described by multiple concentration and temperature modes (variables), each of which is representative of a physical scale of the system. These “multi-mode models” capture mass and thermal micromixing as exchange of material and energy, respectively, between the modes (scales). The multi-mode models retain all the parameters and most of the qualitative features of the full convection-diffusion-reaction equations. While in the limit of vanishingly small local heat and mass diffusion times, they reduce to the classical ideal pseudo-homogeneous reactor models, they are also capable of capturing the mixing or mass (and/or heat) transfer-limited asymptotes for the case of fast reactions. We illustrate the usefulness of the multi-mode models in predicting mixing and selectivity effects on reactor performance and the influence of local transport effects on reactor runaway and bifurcation behavior for the case of non-isothermal homogeneous and catalytic reactors.     

Mass transfer and selectivity of ozone reactions

The present research concerns the behavior of mass transport in absorption of ozone accompanied by decomposition and ozonation reactions in aqueous solutions. On the basis of the film theory, a mathematical model has been formulated for the transport process taking into account molecular diffusion and the simultaneous decomposition and ozonation reactions of different orders. Analytical approximate and finite difference methods have been employed to predict concentration profiles, enhancement factor and selectivity of the chemical mass transfer in gas-liquid systems. The rate of mass transfer is enhanced by the chemical reactions as well as the availability of liquid reactant in the aqueous phase. When the ozonation reaction is much faster than the decomposition reaction, a large fraction of the absorbed ozone is utilized effectively in reacting with the liquid constituent and a high selectivity of the gas-liquid reactions can be obtained. Calculated results indicate that removal of certain organic pollutants by the oxidation process may be effective only in acidic or neutral solutions where a high mass transfer rate and selectivity is achieved.     

Catalytic hydrogenation in a packed bed bubble column reactor

The cocurrent downflow contactor reactor (CDCR) has been found to give low mass transfer resistances both in slurry and packed bed catalytic operation. The hydrogenation of propan-2-ol solutions of itaconic acid in the range 100–300 kPa and 20–70°C and of soyabean oil in the range 100–500 kPa and 130–160°C was studied using slurry (5% w/w Pd/C) and packed bed (3% w/w Pd/Al₂O₃ Raschig ring) catalyst. Mass transport and kinetic parameters were evaluated for both operational modes and while the slurry CDCR gave better mass transfer properties than the packed bed CDCR, the latter gave better mass transfer than conventional reactors and superior selectivity to the slurry CDCR. As has been observed with the slurry CDCR, the packed bed CDCR was found to operate under surface reaction rate control with negligible transport resistances. This was particularly evident for soyabean oil hydrogenation, which is well known to be transport controlled in conventional reactors.     

Application of electrochemical oxidation for treatment of industrial wastewater — the influence of reactor hydrodynamics on direct and mediated processes

The influence of kinetic and hydrodynamic factors in electrochemical reactors used in the removal of pollutants from industrial wastewater is shown, distinguishing between the two main types of reactions, namely direct and mediated electro‐oxidation. The effect of stirring during treatment of four different types of wastewater is reported. Whilst for direct electro‐oxidation of pollutants, the influence of agitation on the performance of the reactor can be easily predicted from a mass transfer correlation, its effect during electro‐oxidation mediated in the homogeneous phase by a redox couple is not straightforward. The Hatta number can be a useful criterion to apply to electrochemical reactors performing mediated oxidation of compounds (in analogy to gas–liquid reactions), so as to define whether the reaction occurs in the bulk of the reactor or near the electrode, and thus can be affected differently by stirring. The hydrodynamic conditions in the reactor for treatment of industrial wastewater can affect the differential selectivity of the removal of pollutants and this can be used for optimising the performance of the reactor with respect to a target pollutant. Copyright © 2006 Society of Chemical Industry     

微反应技术在氢化反应中的应用进展

氢化反应是有机合成中的一类重要反应,应用广泛.目前,氢化反应大多是在高压反应釜中间歇进行,有存在爆炸风险、转化率和选择性低等缺点,因此,开发新的、安全、高效的氢化工艺是必要的.微通道反应器能精确控制温度和反应时间,并且混合均匀,传质性能高,能极大提高反应选择性和生产产量,减少催化剂损耗.总结了微通道反应器中烯烃、炔烃、...     

Conversion of methane through dielectric-barrier discharge plasma

Methane coupling to produce C₂ hydrocarbons through a dielectric-barrier discharge (DBD) plasma reaction was studied in four DBD reactors. The effects of high voltage electrode position, different discharge gap, types of inner electrode, volume ratio of hydrogen to methane and air cooling method on the conversion of methane and distribution of products were investigated. Conversion of methane is obviously lower when a high voltage electrode acts as an outer electrode than when it acts as an inner electrode. The lifting of reaction temperature becomes slow due to cooling of outer electrode and the temperature can be controlled in the expected range of 60°C–150°C for ensuring better methane conversion and safe operation. The parameters of reactors have obvious effects on methane conversion, but it only slightly affects distribution of the products. The main products are ethylene, ethane and propane. The selectivity of C₂ hydrocarbons can reach 74.50% when volume ratio of hydrogen to methane is 1.50.     

Solvent effects on mechanisms and characteristics of electrode reactions

Some interesting results of recent experimental studies on electrode potentials, kinetic parameters and reaction mechanisms of electrode reactions in various organic and mixed solvents are presented. The relationship which was revealed between the R₊ value of the modified Born equation and donor number is introduced. Solvent effects are discussed on the mass transport of reactant and product, the electron transfer at the electrode-solution interface, the preceding or the following chemical reaction of electron transfer and the oxidation states.     

The mixing sensitivity of polysulphide generation

The effect of mixing and mass transfer on polysulphide generation by catalytic oxidation of sodium sulphide was studied using two batch‐operated reactors. One was a sparged reactor operated at atmospheric pressure and low mixing intensities (0.37 to 28 W/kg), the other was an unsparged pressurized reactor characterized by high mixing intensities (17 to 3100 W/kg). The reaction parameters examined included the impeller speed, sparged gas flowrate, oxygen partial pressure, and catalyst loading and type. In both reactors the maximum polysulphide yield, selectivity and rate of formation increased with increasing energy dissipation. Increased gas sparging increased the rate of reaction, but had little effect on either yield or selectivity. Increased oxygen partial pressure increased the rate of oxidation but decreased both the yield and selectivity. The type of catalyst dramatically affected the yield of polysulphide produced for a given set of reaction conditions with improved mixing increasing reaction rate, yield and selectivity.     

Falsification of Kinetic Parameters by Transport Limitations and Its Role in Discerning the Controlling Regime

Reactions occurring on the surface of a porous catalyst are accompanied by transport of heat and mass in the pores of the catalyst and across the boundary layer at the external surface. Under the conditions normally encountered in catalytic reactors, heat and mass fluxes can be large enough to cause finite gradients of concentration and temperature in the solid as well as in the film. In such instances the resulting rate of reaction is governed by both the kinetics of the reaction and the transport process (or processes) which gives rise to the gradient. Hence the dependence of the overall or global rate on temperature and the partial pressures of the reacting species can no more be expressed by the intrinsic kinetics of the reaction but is influenced also by the transport parameters of the system. In other words, in the presence of finite transport limitations the catalyst exhibits kinetics falsified by transport processes. This has been referred to as “disguised kinetics” by Wei [l]. Carberry [2] has examined the implications of this disguise in determining the operating regime of a process.     

Entwicklung membranunterstützter Reaktionsprozesse

The use of membrane technology is developping from the solely work-up of product and waste streams all the way to integration into processes. The membrane reactor offers, by analogy with biological cells, great possibilities for product-integrated environmental protection. Two principal areas of application of membranes in reactors are becoming apparent. Use for removal of products or by-products from bioreactors and the coupling with chemical reactions considered in this article. The first such membrane reactors served for the removal of water from esterification reaction mixtures. Significant advances for membrane reactor technology came with the recent development of membranes of enhanced selectivity and flow density as well as improved thermal and chemical stability. In addition to the availability of high-performance membranes, fundamental knowledge and methods are required to assure efficient reaction-engineering utilization of membrane reactors. This paper discusses fundamental concepts relating to the use of various membrane reactors in parallel, consecutive, and equilibrium reactions. In general, in the case of membrane-supported parallel reactions, controlled addition of reactant can raise the reaction selectivity. Selective removal of primary and side products from consecutive or equilibrium reactions can increase yields. Comparison of membrane-supported reactor types (batch, loop, and plug-flow membrane reactors) indicate that the membrane-supported loop reactor will prove most effective in the majority of cases thanks to its pronounced flexibility.     

Novel Gas-Liquid-Solid Reactors

Multiphase reactors involving gas, liquid, and solid phases have several important applications in the chemical industry, particularly in catalytic processes. Some of the well-known examples are: hydrogenation and oxidation of organic compounds, hydro-processing coal-derived and petroleum oils, Fischer-Tropsch synthesis, and methanation reactions. Due to the presence of three phases, the problem of reactor design is often important to achieve effective mass and heat transfer as well as a mixing pattern favorable to the particular process. The reactors are mainly of two types: (a) solid catalyst is suspended either by mechanical agitation or gas-induced agitation and (b) solid catalyst is in a fixed bed with concurrent or countercurrent feed of gas and liquid re-actants. The reactor types conventionally used in industry are: (a) mechanically agitated or bubble column slurry reactors and (b) trickle-bed or packed-bed bubble reactor. The various design and modeling aspects of these reactors have been reviewed by Satterfield [1], Chaudhari and Ramachandran [2], Shah [3,4], Ramachandran and Chaudhari [5], Shah et al. [6], and Herskowitz and Smith [7]. In several industrial processes these reactor designs are modified to achieve a certain specific objective, such as better heat or mass transfer, higher catalyst efficiency, better reactor performance and selectivity, etc. Similarly, specially designed reactors are often used for laboratory kinetic studies or to understand a certain phenomenon. Thus, novel multiphase reactors are becoming important from both academic and industrial viewpoints. Some of the recently introduced novel gas-liquid-solid reactor types are: (a) loop recycle slurry reactors, (b) basket-type reactors, (c) ebullated-bed reactors, (d) internal or external recycle reactors, (e) multistage slurry or packed-bed reactors, (f) column reactors with sieve trays or multiple agitators, (g) gas-induced agitated reactors, and (h) horizontal-packed-bed reactors. are being used in several new commercial processes, and various design aspects, such as hydrodynamics and mass and heat transfer, have been the subject of investigations in the last few years. However, no attempt to review the scattered information on these novel gas-liquid-solid reactors has been made. Therefore, the main objective of this paper is to review important developments in novel gas-liquid-solid reactors. For each type of reactor, advantages, disadvantages, and applications are discussed. Further, the status of information on hydrodynamics and mass transfer parameters and scale-up considerations is reviewed. These novel reactor designs are being used in several new commercial processes, and various design aspects, such as hydrodynamics and mass and heat transfer, have been the subject of investigations in the last few years. However, no attempt to review the scattered information on these novel gas-liquid-solid reactors has been made. Therefore, the main objective of this paper is to review important developments in novel gas-liquid-solid reactors. For each type of reactor, advantages, disadvantages, and applications are discussed. Further, the status of information on hydrodynamics and mass transfer parameters and scale-up considerations is reviewed.     

Novel Gas-Liquid-Solid Reactors

Multiphase reactors involving gas, liquid, and solid phases have several important applications in the chemical industry, particularly in catalytic processes. Some of the well-known examples are: hydrogenation and oxidation of organic compounds, hydro-processing coal-derived and petroleum oils, Fischer-Tropsch synthesis, and methanation reactions. Due to the presence of three phases, the problem of reactor design is often important to achieve effective mass and heat transfer as well as a mixing pattern favorable to the particular process. The reactors are mainly of two types: (a) solid catalyst is suspended either by mechanical agitation or gas-induced agitation and (b) solid catalyst is in a fixed bed with concurrent or countercurrent feed of gas and liquid re-actants. The reactor types conventionally used in industry are: (a) mechanically agitated or bubble column slurry reactors and (b) trickle-bed or packed-bed bubble reactor. The various design and modeling aspects of these reactors have been reviewed by Satterfield [1], Chaudhari and Ramachandran [2], Shah [3,4], Ramachandran and Chaudhari [5], Shah et al. [6], and Herskowitz and Smith [7]. In several industrial processes these reactor designs are modified to achieve a certain specific objective, such as better heat or mass transfer, higher catalyst efficiency, better reactor performance and selectivity, etc. Similarly, specially designed reactors are often used for laboratory kinetic studies or to understand a certain phenomenon. Thus, novel multiphase reactors are becoming important from both academic and industrial viewpoints. Some of the recently introduced novel gas-liquid-solid reactor types are: (a) loop recycle slurry reactors, (b) basket-type reactors, (c) ebullated-bed reactors, (d) internal or external recycle reactors, (e) multistage slurry or packed-bed reactors, (f) column reactors with sieve trays or multiple agitators, (g) gas-induced agitated reactors, and (h) horizontal-packed-bed reactors. are being used in several new commercial processes, and various design aspects, such as hydrodynamics and mass and heat transfer, have been the subject of investigations in the last few years. However, no attempt to review the scattered information on these novel gas-liquid-solid reactors has been made. Therefore, the main objective of this paper is to review important developments in novel gas-liquid-solid reactors. For each type of reactor, advantages, disadvantages, and applications are discussed. Further, the status of information on hydrodynamics and mass transfer parameters and scale-up considerations is reviewed. These novel reactor designs are being used in several new commercial processes, and various design aspects, such as hydrodynamics and mass and heat transfer, have been the subject of investigations in the last few years. However, no attempt to review the scattered information on these novel gas-liquid-solid reactors has been made. Therefore, the main objective of this paper is to review important developments in novel gas-liquid-solid reactors. For each type of reactor, advantages, disadvantages, and applications are discussed. Further, the status of information on hydrodynamics and mass transfer parameters and scale-up considerations is reviewed.     

Selectivity characteristics of the electrohydrodimerization of acrylonitrile

A mathematical model of a reaction scheme for the electrohydrodymerization of acrylonitrile to adiponitrile in a loop reactor is presented. This model, which is based on a plug flow reactor with a recycle loop and continuous removal of product, is used to simulate steady-state operation at various operating conditions. The effect of flowrate, current density and mass transport are investigated in terms of their effect on product distributions and selectivity. Overall, the reaction model deals with the formation of five products from the cathodic reactions.