New Catalyst Synthesis and Multifunctional Reactor Concepts for Emerging Technologies in the Process Industry

Abstract

In order to promote the introduction of emerging technologies in the process industry, it has been established the two types of supporting R&D activities can be effectively pursued in parallel: (1) new catalyst synthesis methods that eliminate or minimize mass transfer limitations; and (2) multi‐functional reactors by integrating catalysis, heat transfer and/or separation. Catalyst Synthesis: In this paper, several methods of catalyst tailoring will be described that can minimize mass transfer limitations at industrially relevant conversion levels. Three (3) specific examples have been selected to demonstrate what can be achieved: (1) micro‐engineered catalyst that enables enhanced inter‐phase transfer; (2) new mesoporous catalysts with ultra large pores to accommodate slowly diffusing reactants; and (3) customized zeolites of extremely small particles to achieve high effectiveness factors while retaining the virtues of shape selectivity. Multi‐functional Reactors: Applying process intensification principles, mature high‐volume petrochemical processes can be improved dramatically, beyond the expected progress. This will be described using three (3) specific examples: (1) intra‐reactor oxidative reheat for the production of styrene, by staging endothermic and exothermic reactions in series; (2) simultaneous operation of endothermic, dissociative adsorption of methane with exothermic, oxidative removal of carbon during catalytic partial oxidation; and (3) catalytic distillation for the production of ethers, ethyl benzene and the selective hydrogenation of highly unsaturated components in olefins streams.     

A novel reverse flow reactor coupling endothermic and exothermic reactions: Part II: Sequential reactor configuration for reversible endothermic reactions

The new reactor concept for highly endothermic reactions at elevated temperatures with possible rapid catalyst deactivation based on the indirect coupling of endothermic and exothermic reactions in reverse flow, developed for irreversible reactions in Part I, has been extended to reversible endothermic reactions for the sequential reactor configuration. In the sequential reactor configuration, the endothermic and exothermic reactants are fed discontinuously and sequentially to the same catalyst bed, which acts as an energy repository delivering energy during the endothermic reaction phase and storing energy during the consecutive exothermic reaction phase. The periodic flow reversals to incorporate recuperative heat exchange result in low temperatures at both reactor ends, while high temperatures prevail in the centre of the reactor. For reversible endothermic reactions, these low exit temperatures can shift the equilibrium back towards the reactants side, causing ‘back-conversion’ at the reactor outlet.The extent of back-conversion is investigated for the propane dehydrogenation/methane combustion reaction system, considering a worst case scenario for the kinetics by assuming that the propylene hydrogenation reaction rate at low temperatures is only limited by mass transfer. It is shown for this reaction system that full equilibrium conversion of the endothermic reactants cannot be combined with recuperative heat exchange, if the reactor is filled entirely with active catalyst. Inactive sections installed at the reactor ends can reduce this back-conversion, but cannot completely prevent it. Furthermore, undesired high temperature peaks can be formed at the transition point between the inactive and active sections, exceeding the maximum allowable temperature (at least for the relatively fast combustion reactions).A new solution is introduced to achieve both full equilibrium conversion and recuperative heat exchange while simultaneously avoiding too high temperatures, even for the worst case scenario of very fast propylene hydrogenation and fuel combustion reaction rates. The proposed solution utilises the movement of the temperature fronts in the sequential reactor configuration and employs less active sections installed at either end of the active catalyst bed and completely inactive sections at the reactor ends, whereas propane combustion is used for energy supply. Finally, it is shown that the plateau temperature can be effectively controlled by simultaneous combustion of propane and methane during the exothermic reaction phase.     

Behaviour of smooth catalysts at high reaction rates

In evaluating effective reaction rates in catalysts subject to heat and mass transport limitations, the size of the catalytic body is best defined by the so-called characteristic length ℓ, the ratio between catalyst volume and its external surface area, ℓ = V_p/S_p. This result follows from the limiting behaviour at very high reaction rates, when the effective reaction rates are proportional to 1/ℓ (e.g., Aris [R. Aris, The Mathematical Theory of Diffusion and Reaction in Permeable Catalysts, Oxford University Press, London, 1975.]) or, in dimensionless form, to the inverse of the Thiele modulus Φ. It is further known from simple geometrical shapes that a series solution can be written in terms of powers of (1/Φ) and that the second order term [in(1/Φ)²] depends on the shape of the catalytic body. It is the aim of this paper to develop expressions of such second order term for 2D or 3D catalytic bodies showing arbitrary smooth external surfaces.

In a similar way as the first order term allows to define the proper size of a catalyst, the second order term provides a characterization for the catalyst shape. This and other applications of the second order term are discussed.     

微结构反应器气固相催化过程强化的研究与工业化进展

轻烃的气固相催化转化与合成是重要的基础能源化工过程,其苛刻的反应条件与显著的热效应严重影响了反应器生产效率、过程能耗与排放。微结构催化反应器传递性能优越,可兼顾紧凑性与低压降,能在高反应通量下精确调控轻烃气固相催化转化与合成过程,适应日益增长的分布式生产需求。介绍了微结构催化反应器的制造并重点讨论了催化剂-反应器集成方式,以强吸热的甲烷蒸汽重整过程和强放热的甲烷化、乙烷氧化脱氢过程为例,综述了微结构反应器气固催化过程强化的研究与工业化进展,展望了新技术的未来发展方向。     

Simultaneous oxidative conversion and CO2 or steam reforming of methane to syngas over CoO–NiO–MgO catalyst

CO₂ reforming, oxidative conversion and simultaneous oxidative conversion and CO₂ or steam reforming of methane to syngas (CO and H₂) over NiO–CoO–MgO (Co: Ni: Mg=0·5: 0·5:1·0) solid solution at 700–850°C and high space velocity (5·1×10⁵ cm³ g⁻¹ h⁻¹ for oxidative conversion and 4·5×10⁴ cm³ g⁻¹ h⁻¹ for oxy-steam or oxy-CO₂ reforming) for different CH₄/O₂ (1·8–8·0) and CH₄/CO₂ or H₂O (1·5–8·4) ratios have been thoroughly investigated. Because of the replacement of 50 mol% of the NiO by CoO in NiO–MgO (Ni/Mg=1·0), the performance of the catalyst in the methane to syngas conversion process is improved; the carbon formation on the catalyst is drastically reduced. The CoO–NiO–MgO catalyst shows high methane conversion activity (methane conversion >80%) and high selectivity for both CO and H₂ in the oxy-CO₂ reforming and oxy-steam reforming processes at ⩾800°C. The oxy-steam or CO₂ reforming process involves the coupling of the exothermic oxidative conversion and endothermic CO₂ or steam reforming reactions, making these processes highly energy efficient and also safe to operate. These processes can be made thermoneutral or mildly exothermic or mildly endothermic by manipulating the process conditions (viz. temperature and/or CH₄/O₂ ratio in the feed). © 1998 Society of Chemistry Industry     

Kinetic measurements of CH4 combustion over a 10% PdO/ZrO2 catalyst using an annular flow microreactor

A kinetic study on CH₄ combustion over a PdO/ZrO₂ (10%, w/w) catalyst has been performed in a temperature range between 400 and 550 °C by means of an annular catalytic microreactor.

The role of mass transfer phenomena including diffusion in the catalyst pore, gas–solid diffusion and axial diffusion in the gas phase, has been preliminary addressed by means of mathematical modeling. Simulation results have pointed out the key role of internal diffusion showing that thicknesses of the active catalyst layer as thin as 10–15 μm are required to minimize the impact of mass transfer limitations. The thermal behavior of the reactor has been also addressed by means of catalytic combustion tests with CH₄ and CO–H₂ mixtures as fuels. The results have demonstrated the possibility to obtain nearly isothermal temperature profiles under severe conditions (up to 3% of CH₄) thanks to effective dissipation of reaction heat by radiation from the catalyst outer skin.

Finally the effect of reactants (CH₄ and O₂) and products (H₂O and CO₂) on CH₄ combustion rate has been addressed. The results have shown that both H₂O and CO₂ markedly inhibit the reaction up to 550 °C. The data have been fitted by the following simple power law expression r=k_rP_CH4P_H2O^−0.32P_CO2^−0.25 with an apparent activation energy of 108 kJ/mol.

Evidences have been found and discussed indicating a key role of the support on the extent of such inhibition effects.     

Coupling of highly exothermic and endothermic reactions in a metallic monolith catalyst reactor: A preliminary experimental study

An LaFe_0.5Mg_0.5O₃/Al₂O₃/FeCrAl metallic monolith catalyst for the exothermic catalytic combustion of methane and an Ni/SBA-15/Al₂O₃/FeCrAl metallic monolith catalyst for the endothermic reforming of methane with CO₂ have been prepared. A laboratory-scale tubular jacket reactor with the Ni/SBA-15/Al₂O₃/FeCrAl catalyst packed into its outer jacket and the LaFe_0.5Mg_0.5O₃/Al₂O₃/FeCrAl catalyst packed into its inner tube was devised and constructed. The reactor allows a coupling of the exothermic and endothermic reactions by virtue of their thermal matching. An experimental study in which the temperature difference between the chamber of the external electric furnace and the metallic monolith catalyst bed in the jacket was kept very small, by adjusting the power supply to the furnace, confirmed that the heat absorbed in the reforming reaction does indeed partly come from that evolved in the catalytic combustion of methane, and that the direct thermal coupling of the two reactions in the reactor can be realized in practice. When the temperature of the electric furnace chamber was 1088 K, and the gas hourly space velocities (GHSVs) of the reactant mixtures passed through the inner tube and the jacket were 382 h⁻¹ and 40 h⁻¹, respectively, the conversions of methane and CO₂ in the reforming reaction were 93.6% and 91.7%, respectively, and the heat efficiency reached 81.9%. Stability tests showed that neither catalyst underwent deactivation during 150 h on stream.     

Multi-scale catalyst design

Catalyst design has long been sought in catalysis and reaction engineering research. In this work, multi-scale analysis and strategy is explored to take a holistic view toward catalyst design perspective and elucidate impacts of designs at different scales to a catalytic reaction process performance. A few promising design concepts are introduced to break the compromise that often needs to be made in the conventional design approach. In the catalyst bed scale, micro- or mini-structured catalyst designs can be used to potentially eliminate all mass transfer resistance and realize intrinsic catalytic performance. At the particle level, incorporation of membrane separation functions into the catalyst unit enables regulation of mass transfer rate of individual reactant or product molecules that high reaction selectivity is achieved. At the level of intrinsic catalyst structures, three-dimensional (3-D) catalyst design models are outlined here to outweigh limitations or constraints imposed by the conventional way of thinking 2-D catalyst surface. Examples of exceptional catalytic activity or concerted effects are shown by incorporating different materials into nano-composite catalysts and optimizing size and/or shape of a catalyst material at the nano-scale.     

催化裂化汽油改质降烯烃反应规律及反应热

利用催化裂化催化剂在小型固定流化床实验装置上对催化裂化汽油催化改质降烯烃过程的反应规律进行了实验研究,详细考察了反应温度、剂油比和重时空速对产物收率和汽油辛烷值的影响,得到了催化裂化汽油改质过程的最佳实验操作条件：反应温度为400～430℃,剂油比为7左右,重时空速为20～30 h-1。在此基础上,计算了汽油改质过程的反应热,分析了反应条件对反应热的影响,揭示了反应热的变化规律。结果表明,低温改质为放热过程,高温改质为吸热过程。改质条件对反应热影响的强弱顺序为反应温度>剂油比>重时空速。     

Performance simulation of a wall-type reactor in which exothermic and endothermic reactions proceed simultaneously, comparing with that of a fixed-bed reactor

By combining endothermic and exothermic reactions in one reactor, a mutual utilization of thermal energy involved in reactions is expected to produce a saving energy and a cost-down for running in industrial reaction process. In this case, a wall-type reaction system is thought to be suitable because such reaction system is good at exchangeability of thermal energy by conductive heat transfer. This study supposed a wall-type reaction system consisting of endothermic and exothermic reaction channels stacked up and a fixed-bed reaction system of the same configuration, and compared them by numerical simulation in the case where endothermic and exothermic reactions progress simultaneously.

In the fixed-bed reaction system, heat transfer in the catalyst bed takes place by convection, and this transfer becomes the rate-limiting process. Accordingly, occurrence of hot spot in the exothermic channel and shortage of thermal energy in the endothermic channel were predicted. This trend became distinct by making the feed gas directions flowing in the two channels countercurrent and by stacking the channels in multiple tiers. In the wall-type reaction system, however, the temperature distributions in the exothermic and endothermic channels almost conformed to the set temperatures, and the temperature difference between channels was small. Even if the feed gases flowed in countercurrent and even if the channels were stacked several deep, this trend did not change. In the wall-type reaction system, the exchange of thermal energy would take place efficiently by conductive heat transfer between the endothermic and exothermic channels. Furthermore, it was inferred that the wall-type reaction system would provide a stable operation in mutual utilization of thermal energy.     

Molecular description of active sites in oxidation reactions: Acid-base and redox properties, and role of water

Oxidation reactions in heterogeneous catalysis usually involve a Mars and van Krevelen mechanism which includes activation of the substrate on a metallic cation, insertion of oxygen from lattice oxygen ions, a redox mechanism on the catalyst surface, and the transfer of several electrons. It follows that such a reaction necessitates both acid-base and redox properties of a catalyst the acid site being of Lewis type (cations) and the basic sites being the surface O^2- or OH^- species which could exhibit electrophilic or nucleophilic properties.

The active site should be able to fulfil the following requirements: H abstraction from the substrate, oxygen insertion, and electron transfer. It has been shown to correspond to an ensemble of atoms of limited size in an inorganic molecular complex. It could correspond to local structural defects including steps, kinks, coordinatively unsaturated cations or to clusters of atoms on the surface. Some examples are described namely:

1. (i) n-butane oxidation to maleic anhydride on (VO)₂P₂O₇ catalyst where four dimers of vanadyl cations on the (100) face were suggested to form the active site;

2. (ii) isobutyric acid oxidative dehydrogenation to methacrylic acid on iron hydroxy phosphates where trimers of iron oxide octahedra were shown to constitute the most efficient and selective catalytic site while water was observed to be absolutely necessary to facilitate the reaction which corresponds to hydroxylated surface sites ensuring the redox mechanism;

3. (iii) propane oxidative dehydrogenation to propene on VMgO samples which was shown to depend both on VO_x arrangements with respect to MgO and on the basicity of the material induced by MgO while vanadium cations induced acidic features.

     

甲烷催化燃烧与十二烷脱氢反应的间接热耦合

将甲烷催化燃烧的Pd-Zr/SBA-15/Al2 O3/FeCrA1金属整体式催化剂和十二烷催化脱氢的Pt-Sn-Li/Al2 O3/FeCrA1金属整体式催化剂分别填充在套管式反应器的内管和环隙中,在电炉伴温条件下研究了这两个反应的热耦合.实验结果表明,炉温略低于催化剂床层温度,说明吸热反应所需的热量来源于放热反应,...     

Structured hierarchical Mn-Co mixed oxides supported on silicalite-1 foam catalyst for catalytic combustion

Silicalite-1 (S1) foam was functionalized by supporting manganese-cobalt (Mn-Co) mixed oxides to develop the structured hierarchical catalyst (Mn-Co@S1F) for catalytic combustion for the first time. The self-supporting S1 foam with hierarchical porosity was prepared via hydrothermal synthesis with polyurethane (PU) foam as the template. Subsequently, Mn-Co oxide nano sheets were uniformly grown on the surface of S1 foams under hydrothermal conditions to prepare the structured hierarchical catalyst with specific surface area of 354 m²·g^-1, micropore volume of 0.141 cm³·g^-1 and total pore volume of 0.217 cm³·g^-1, as well as a good capacity to adsorb toluene (1.7 mmol·g^-1 at p/p₀ = 0.99). Comparative catalytic combustion of toluene of over developed structured catalyst Mn-Co@S1F was performed against the control catalysts of bulk Mn-Co@S1 (i.e., the crushed Mn-Co@S1F) and unsupported Mn-Co oxides (i.e., Mn-Co). Mn-Co@S1F exhibited comparatively the best catalytic performance, that is, complete and stable toluene conversion at 248 °C over 65 h due to the synergy between Mn-Co oxides and S1 foam, which provided a large number of oxygen vacancies, high redox capacity. In addition, the hierarchical porous structure also improved the accessibility of active sites and facilitated the global mass transfer across the catalyst bed, being beneficial to the catalysis and catalyst longevity.     

Modelling propane dehydrogenation in a rotating monolith reactor

The catalytic dehydrogenation of propane is equilibrium limited, strongly endothermic and normally carried out at high temperatures. The catalyst deactivates due to the laydown of carbonaceous species on the surface. This is conventionally countered by subjecting the catalyst to periodic regeneration. In commercially available processes, the catalyst time on line for a given cycle is in the order of 10–10,000 min.

In this study, the catalyst has been observed to exhibit very high activity and selectivity in the short period after regeneration. Conceptual and model development of a reactor with structured catalyst to capitalise on this beneficial early activity is presented.

The preferred reactor comprises a cylindrical block of honeycomb monolith that rotates past various feed zones, subjecting the catalyst successively to propane and regenerating gas. The exothermic nature of the regeneration reactions is used at least in part to provide heat to the endothermic dehydrogenation reaction via the regenerative heat transfer facilitated by the movement of the solid monolith. Specifically, it is noted that an oxidisible catalyst provides operating advantage due to the additional exotherms associated with the regeneration stage.

The process modelling shows the design to be feasible in terms of matching the heats of reactions and achieving high conversions, but questions are raised over its practicability from mechanical design and process stability viewpoints.     

微藻/核桃壳混合热解需热量及制备芳烃研究

利用热重分析仪（TG）、气相色谱-质谱联用仪（GC/MS）与固定床反应器,考察了微藻、核桃壳及混合物主要热解阶段的需热量特性,以及混合比、温度、催化剂种类对二者混合热解制备芳烃的影响规律。结果表明：在不同升温速率下,核桃壳热解在180～270℃和380～485℃处存在两个吸热峰,整体表现为吸热效应（378.56～596.45 kJ·kg^-1）;微藻热解在280～450℃处存在一个放热峰,整体表现为放热效应（-814.76～-1191.52 kJ·kg^-1）。微藻/核桃壳热解呈现较低的放热效应（-99.05～-158.04 kJ·kg^-1）,表明二者混合热解可以实现一定程度的热量耦合。微藻/核桃壳热解在制备芳烃上表现出明显的协同效应,且芳烃相对含量在600℃、混合比1：1下达到最大值,为20.51%;加入Cu/HZSM-5可进一步提高混合热解的芳烃相对含量,达到35.74%。为微藻与核桃壳的高值化利用提供了新思路。     

Optimizing the catalyst distribution for countercurrent methane steam reforming in plate reactors

Microscale autothermal reactors remain one of the most promising technologies for efficient hydrogen generation. The typical reactor design alternates microchannels where reforming and catalytic combustion of methane occur, so that exothermic and endothermic reactions take place in close proximity. The influence of flow arrangement on the autothermal coupling of methane steam reforming and methane catalytic combustion in catalytic plate reactors is investigated. The reactor thermal behavior and performance for cocurrent and countercurrent are simulated and compared. A partial overlapping of the catalyst zones in adjacent exothermic and endothermic channels is shown to avoid both severe temperature excursions and reactor extinction. Using an innovative, optimization‐based approach for determining the catalyst zone overlap, a solution is provided to the problem of determining the maximum reactor conversion within specified temperature bounds, designed to preserve reactor integrity and operational safety. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Investigation of alumina-supported Ni and Ni-Pd catalysts by partial oxidation and steam reforming of n-octane

Aseries of nickel and nickel-palladium supported upon alumina catalysts were prepared in order to obtain a suitable catalyst that could be used in the process of producing hydrogen by partial oxidation and steam reforming of n-octane. Hydrogen production by partial oxidation and steam reforming (POSR) of n-octane was investigated over alumina-supported Ni and Ni-Pd catalysts. The process occurred by a combination of exothermic partial oxidation and endothermic steam reforming of n-octane. It was found that Ni/Al₂O₃ catalyst activity was high at high temperatures and increased with the Ni loadings. Its activity, however, was not obviously increased when Ni loadings were over 5.0 wt%. Compared with nickel catalyst, the bimetallic catalyst of Ni-Pd/A1₂O₃ showed markedly increased activity and hydrogen selectivity at experimental conditions. The catalytic performance also became more stable when the palladium was added, which indicated that palladium plays an essential role in the catalytic action. The used catalysts of Ni-Pd/A1₂O₃ were regenerated three times by using air at space velocity of 2,000 h⁻¹ to obtain a long duration catalyst. Also, the typical catalyst was characterized by using SEM, BET, TG and ICP methods in detail.     

A novel reverse flow reactor coupling endothermic and exothermic reactions. Part I: comparison of reactor configurations for irreversible endothermic reactions

A new reactor concept is studied for highly endothermic heterogeneously catalysed gas phase reactions at high temperatures with rapid but reversible catalyst deactivation. The reactor concept aims to achieve an indirect coupling of energy necessary for endothermic reactions and energy released by exothermic reactions, without mixing of the endothermic and exothermic reactants, in closed-loop reverse flow operation. Periodic gas flow reversal incorporates regenerative heat exchange inside the reactor. The reactor concept is studied for the coupling between the non-oxidative propane dehydrogenation and methane combustion over a monolithic catalyst.Two different reactor configurations are considered: the sequential reactor configuration, where the endothermic and exothermic reactants are fed sequentially to the same catalyst bed acting as an energy repository and the simultaneous reactor configuration, where the endothermic and exothermic reactants are fed continuously to two different compartments directly exchanging energy. The dynamic reactor behaviour is studied by detailed simulation for both reactor configurations. Energy constraints, relating the endothermic and exothermic operating conditions, to achieve a cyclic steady state are discussed. Furthermore, it is indicated how the operating conditions should be matched in order to control the maximum temperature. Also, it is shown that for a single first order exothermic reaction the maximum dimensionless temperature in reverse flow reactors depends on a single dimensionless number. Finally, both reactor configurations are compared based on their operating conditions. It is shown that only in the sequential reactor configuration the endothermic inlet concentration can be optimised independently of the gas velocities at high throughput and maximum reaction coupling energy efficiency, by the choice of a proper switching scheme with inherently zero differential creep velocity and using the ratio of the cycle times.In this first part, both the propane dehydrogenation and the methane combustion have been considered as first order irreversible reactions. However, the propane dehydrogenation is an equilibrium reaction and the low exit temperatures resulting from the reverse flow concept entail considerable propane conversion losses. How this ‘back-conversion’ can be counteracted is discussed in part II Chemical Engineering Science, 57, (2002), 855-872.     

Investigation on catalytic distillation for ethyl acetate production with different catalytic packing structures

The catalytic packing is the core component of the catalytic distillation, and how the catalyst exists in the packing has significant influence on the process. To investigate the effect of catalyst packings on the catalytic distillation process, the classical ethyl acetate reactive distillation system was utilized, and a supported catalytic packing(SCP) was prepared in comparison with the conventional tea-bag catalytic packing(TBP). Laboratory scale experiments showed that the ethyl acetate conv...