Honeycomb supports with high thermal conductivity for gas/solid chemical processes

The scope of this review article is to address the use of novel monolithic catalysts with high thermal conductivity in externally cooled tubular reactors for gas/solid exothermic chemical processes in place of conventional packed beds of catalyst pellets.

After discussing the analysis and the implications of heat conduction in honeycomb monolith structures, we review herein simulation studies and experimental investigations showing that near-isothermal reactor operation can be achieved even under very high thermal loads by adopting specific materials and designs of the honeycomb supports associated with high effective radial thermal conductivities. For such monoliths, the limiting thermal resistance is located at the interface between the monolith and the inner tube wall (“gap resistance”). Recent measurements of the “gap” heat transfer coefficient point to very large values (>400 W/(m² K)), which are controlled both by the tube–monolith clearance at the actual operating conditions and by the thermal conductivity of the process gas.     

Simulation of structured catalytic reactors with enhanced thermal conductivity for selective oxidation reactions

The replacement of conventional-packed beds of pellets with “high conductivity” honeycomb catalysts in industrial externally cooled multitubular fixed-bed reactors is investigated by modeling and simulation for the oxidation of methanol to formaldehyde and for the epoxidation of ethylene to ethylene oxide, which involve a consecutive and a parallel reaction scheme, respectively. Results suggest that near-isothermal operation of the fixed-bed reactors can be achieved using monolithic catalyst supports based on relatively large volume fractions of highly conductive materials. Pressure drops are reduced to less than 1%. The selectivity is favored by the excellent control of the intraporous diffusional resistances resulting from the thin catalytic washcoats. Reactor designs based on larger tubes are feasible at the expense of greater volume fractions of catalyst support. A critical aspect is represented by the restrictions on the specific load of catalyst per reactor volume resulting from the poor adhesion of very thick catalyst layers onto metallic surfaces. Such a difficulty can be circumvented by maximizing the geometric surface area of the monolith (e.g. minimizing the honeycomb pitch), enhancing the catalytic activity (e.g. increasing the load of active components), and increasing the coolant temperature (if the selectivity is not adversely affected).     

Methyltrimethoxysilane based monolithic silica aerogels via ambient pressure drying

We have developed a novel route to monolithic silica aerogels via ambient pressure drying by the acid–base sol–gel polymerization of methyltrimethoxysilane (MTMS) precursor. An extent of silica polymerization in the alcogels plays a crucial role in obtaining the monolithic aerogels which could be optimized by a proper control over the MeOH/MTMS molar ratio (S) during the sol–gel synthesis. The alcogel undergoes the distinct “spring-back effect” at the critical stage of the drying and thereby preserving the highly porous silica network without collapse. The process yields silica aerogels exhibiting very low bulk density and high specific surface area of 0.062 g/cm³ and 520 m²/g, respectively. The average pore diameter and the cumulative pore volume varied from 4.5 to 12.1 nm and 0.58 to 1.58 cc/g, respectively. In addition, the aerogels are superhydrophobic with contact angle as high as 152°. We anticipate that the new route of the monolithic silica aerogel production will greatly expand the commercial exploitation of these materials.     

Kinetics of the selective catalytic reduction of NO by NH3 on a Cu-faujasite catalyst

The kinetics of the selective catalytic reduction (SCR) of NO by NH₃ in the presence of O₂ has been studied on a 5.5% Cu-faujasite (Cu-FAU) catalyst. Cu-FAU was composed of cationic and oxocationic Cu species. The SCR was studied in a gas phase-flowing reactor operating at atmospheric pressure. The reaction conditions explored were: 458<T_R<513 K, 2503 (ppm) < 4000, 12 (%) < 4. The kinetic orders were 0.8–1 with respect to NO, 0.5–1 with respect to O₂, and essentially 0 with respect to NH₃. Based on these kinetic partial orders of reactions and elementary chemistry, a wide variety of mechanisms were explored, and different rate laws were derived. The best fit between the measured and calculated rates for the SCR of NO by NH₃ was obtained with a rate law derived from a redox Mars and van Krevelen mechanism. The catalytic cycle is described by a sequence of three reactions: (i) Cu^I is oxidized by O₂ to “Cu^II-oxo”, (ii) “Cu^II-oxo” reacts with NO to yield “Cu^II-N_xO_y”, and (iii) finally “Cu^II-N_xO_y” is reduced by NH₃ to give N₂, H₂O, and the regeneration of Cu^I (closing of the catalytic cycle). The rate constants of the three steps have been determined at 458, 483, and 513 K. It is shown that Cu^I or “Cu^II-oxo” species constitute the rate-determining active center.     

Photocatalytic degradation of organic pollutants with simultaneous production of hydrogen

The photocatalytic degradation of a number of organic compounds in solution, including alcohols and organic acids, has been investigated under unaerated conditions with the use of Pt/TiO₂ photocatalyst and solar or UV irradiation. It has been found that production of CO₂ is in all cases accompanied by evolution of hydrogen, the production rate of which is significantly enhanced, compared with that obtained in the absence of organic additives in solution. Results are explained by considering that organic compounds act as sacrificial electron donors, which become progressively oxidized toward CO₂ by consuming photogenerated holes and/or oxygen. This results in decreased rates of electron–hole recombination and oxygen–hydrogen back reaction and, concomitantly, in increased H₂-production rates. The rate of photoinduced hydrogen production depends strongly on the concentration of the sacrificial agent employed and to a lesser extent on solution pH and temperature. When complete mineralization of the sacrificial agent is achieved, photogenerated oxygen can no longer be removed from the photocatalyst surface and the H₂-production rate drops to steady-state values, comparable to those obtained in the absence of the organic compound in solution. The amounts of carbon dioxide and “additional” hydrogen produced depend on the nature of the organic additive and are directly proportional to its initial concentration in solution. Quantification of results shows that the overall process may be described as “photoinduced reforming of organic compounds at room temperature”. It is concluded that mineralization of organic pollutants such as alcohols and organic acids, which are common waste products of biomass processing industries, can be achieved with simultaneous production of H₂ fuel. The process may provide an efficient and cost effective method for cleaning up waste streams.     

Miniaturization of screening devices for the combinatorial development of heterogeneous catalysts

The present work is focused on the determination of the advantages, bottlenecks and challenges of miniaturized screening systems which are essential to the success of combinatorial high-throughput methodologies in heterogeneous catalysis. Two different reactor configurations with different degrees of miniaturization were developed for the parallel and fast screening of heterogeneously catalyzed gas phase reactions: a monolithic reactor system acting as a multichannel reactor and a microreaction system based on microfabrication techniques. In both cases, a scanning mass spectrometry technique was successfully applied for quantitative product analysis within 60 s per catalyst. Due to its flexibility and high spatial resolution, this three dimensional scanning MS can be used with different and highly parallel reactor arrays. Many experiments were carried out to study the efficiency and reliability of the different screening systems, with the oxidation of methane, the oxidation of CO, and the oxidative dehydrogenation of i-butane as model reactions. Moreover, chip modules in silicon–glass technology having a number of parallel microchannels were developed, each of them containing a different catalyst. Using this approach, “catalysis-on-a-chip” proved in methane oxidation was possible. Finally, a multibatch reactor consisting of a number of parallel mini autoclaves was developed and tested in the liquid-phase hydrogenation of citral in order to overcome the lack of parallel and fast screening procedures for heterogeneously catalyzed gas–liquid reactions widely spread in the chemical industry.     

Experimental and theoretical method for the determination of the daily output of a solar still: input-output method

In the present work the thermal behavior of a typical solar still is examined experimentally and theoretically and its basic characteristics are analyzed. It is concluded from a larger number of experimental measurement that the operation of such stills during daytime is characterized by three phases: (1) starting, (2) pseudo steady-state, and (3) saturation. A mathematical equation is formed for the instantaneous simulation of the solar still during the pseudo steady-state phase. From this the equation in its integrated form determines satisfactorily the daily output by the daily solar radiation (solar energy input), the average ambient air temperature during the day, T_a(av), and the temperature of the water in the basin at the beginning of the day, T_win, as parameters. This method is called the “input-output” method.     

Synthesis, Characterization and Catalytic Applications of Organized Mesoporous Aluminas

The synthesis of organized mesoporous aluminas has opened a very interesting area for application of this type of materials, particularly as catalysts or catalyst supports. This review focuses on the individual synthesis routes to produce organized mesoporous aluminas with large surface areas and narrow pore size distributions, and on the evaluation of their textural, chemical and thermal properties and outlines examples of catalytic applications of organized mesoporous alumina-based catalysts. We tried to rationalize the synthetic approaches to prepare organized mesoporous aluminas, to relate their properties to synthetic procedures used as well as to their catalytic behavior in different reactions. Utilization of various structure-directing agents for “cationic,” “neutral,” “anionic,” “nanocasting,” and special approaches leading to scaffolding and lathlike organized mesoporous aluminas is discussed in the first part of this review, as well as textural and structural characterization and thermal stability of mesoporous aluminas synthesized by different synthetic approaches. In the second part, catalytic applications of organized mesoporous aluminas described in the open literature are evaluated from the standpoint of the importance of these reactions for technological applications.     

Analysis of Moisture Content and Density of Pears During Drying

Solar dried pears of the “S. Bartolomeu” variety are a very much appreciated and preferred dried food product in Portugal. Nevertheless, the traditional solar drying is carried out at open air during the months of 07 and 08, and this nowadays is a disadvantage for larger productions. This work is to evaluate the possibility of producing dried pears from this and other varieties, maintaining the characteristics of the traditional dried pears. In this study four different types of pears were studied, including “S. Bartolomeu” as a basis for comparison and the drying method employed was the traditional one. From the results it was concluded that, although the behaviour of the four varieties do not vary significantly, one particular variety (“D. Joaquina”) is a good alternative to the “S. Bartolomeu” pear.     

Chemical composition and bond structure of aerosol particles of amorphous hydrogenated silicon forming from thermal decomposition of silane

Aerosol particles of amorphous hydrogenated silicon resulting from thermal decomposition of silane were investigated by hydrogen evolution, IR-, EPR-, NMR spectroscopy, and transmission electron microscopy.

The experimental data show that aerosol particles contain to a various extent {SiH₂}_n polymer structures and two types of monohydride groups SiH- “clustered” and “dilute” monohydride groups. The hydrogen atoms of the “clustered” monohydride groups are located close to each other. The “clustered” monohydride groups are inaccessible to the ambient because they are embedded in the amorphous network. The “dilute” monohydride groups are relatively isolated from each other. The majority of “dilute” monohydride groups are open to the ambient. They are located on the surface of preferentially interconnected microchannels and microvoids.

Interaction between the “dilute” SiH groups and atmospheric oxygen results in formation of OSiH groups in which hydrogen and oxygen are bonded to a common silicon atom. Evidently, the interaction occurs throw the oxygen reaction with weak bonds associated with “dilute” monohydride groups. There is no interaction between oxygen and both “clustered” SiH groups and {SiH₂}_n chain because the former are inaccessible to atmospheric oxygen and the latter has presumably no weak bonds in the chains.     

The Body's Response to Deliberate Implants: Phagocytic Cell Responses to Large Substrata Vs. Small Particles

It is important to characterize possible inflammatory responses to small particles, and to separate clearly these effects from responses to larger objects nearby. This research used a chemiluminescent assay, scanning electron micrographs, and energy dispersive X-ray spectra to monitor inflammation-related reactive oxygen intermediate (ROI) production and morphological alterations of human monocyte-derived macrophages interacting with the walls of apolar and polar polystyrene cuvettes, in the absence and presence of small particles of surface-characterized Teflon™, polyethylene, Co-Cr-Mo alloy, titanium and alumina. The two types of polystyrene substrata represent the “bacterial” (as produced) and “tissue culture” (gas-plasma-treated [GPT]) materials widely used in biological testing and tissue culture. Monocyte-derived macrophage spreading during contact with the higher-surface-energy, more polar substratum suppressed “oxidative bursts” to lower levels than expressed from rounded cells in contact with the lower-energy, apolar substratum. Particulate matter engulfed by both rounded and spread cells did not significantly enhance ROI production beyond levels observed for no-particle controls during the one-hour exposure time. Biocompatibility of some implants might be related to cell-spreading-induced suppression of ROI production, improving the tissue integration of GPT implants.     

React: self-sufficient renewable energy air-conditioning system for Mediterranean countries

The REACt project (INCO-CT-2003-015434-REACt) aims at the introduction in target Mediterranean Partner Countries (MPC) of an advanced and innovative hybrid solar hot water and air-conditioning system. Taking into account the climatic, geographical and economic situation of each country, we propose to operate on two systems based on linear parabolic trough collectors. Both systems will be calibrated for two different applications and will act as “test bed” for innovative technologies such as direct steam generation, never before utilized on systems of this size and purpose; new diathermic fluids, optimized for high thermal capacity and high performance doubleeffect ammonia chillers. Specifically, we propose to operate in view of the placement of the systems in two test areas: a public hospital in Morocco and a tourist resort in Jordan. A further site that will be examined is a public hospital in the city of Baabda, Lebanon, where many of the activities (country analysis, assessments, dissemination) will be performed even if no system will be installed in such site in the framework of this project.

The action envisaged here is the introduction of a renewable energy system (RES) based co-generation system able to produce heat and air conditioning using solar power. The system works by means of an ammonia-based “chiller” which uses heat as input and produces refrigeration and heat as output. The input heat will come from thermal solar linear parabolic collectors. The main socio-economical objective is to generate nodes of good practice, accelerate local skill development, and promote and encourage relevant stakeholders, on all aspects of an innovative certified technology that is efficient, robust and suitable for standardized production and replication. The proposed system is a pilot system that will meet different needs and climate conditions under the national strategies and socio-economic conditions of the MPC.

The REACt project is based on the transfer of a hybrid RES, which respond to the third key area of activities, proposed by the European Commission sixth Framework Programme (International Cooperation) – INCO), in particular: “Cost-effective Renewable Energy (RE) for tourist resorts and commercial centers: Improvement of solar-collectors and their integration into buildings which themselves should be of passive solar design”.     

Oxidative dehydrogenation of propane over monoliths at short contact times

A specially designed tubular microreactor with independent control of feed preheat as well as catalyst temperature and allowing to rapidly quench reaction products was used to test performance of supported Pt-based monolithic catalysts in the reaction of propane oxidative dehydrogenation at short contact times. To minimize the impact of undesired homogeneous reactions capable of decreasing propylene selectivity, proprietary straight-channel thin-wall high cell density corundum micromonoliths were chosen as supports. Catalytic properties of supported platinum were modified by using promoters known as dehydrogenation catalysts (tin, zinc aluminate spinel, transition metal pyrophosphates) as well as by tuning reaction mixture composition (propane/oxygen ratio, water and hydrogen content). In the operation temperature range up to 900°C with contact times 0.03–0.1 s, ethylene/propylene selectivities were found to strongly depend upon the chemical composition of the active component and type of feed. The results thus obtained demonstrate that for Pt-based catalysts, propylene yield can be substantially improved by suppressing secondary reactions of deep oxidation and cracking.     

Organic–silicate hybrid catalysts based on various defined structures for Knoevenagel condensation

Two types of organic–inorganic hybrid base catalysts are prepared. Organic-functionalized molecular sieves (OFMSs), particularly “amine-immobilized porous silicates”, are designed based on common idea to immobilize catalytic active sites on silicate surface. Silicate–organic composite materials (SOCMs), such as “ordered porous silicate–quaternary ammonium composite materials”, are the precursors of ordered porous silicates obtained during the synthesis. Both the OFMS and the SOCM are used as the catalysts for Knoevenagel condensation. Among the OFMSs, there is clear tendency that the use of molecular sieve with larger pore volume and/or surface area gives the product in higher yield. Aminopropylsilyl (AP)-functionalized mesoporous silicates such as AP-MCM-41 gives the product in high yield under mild conditions. No loss of activity is observed after repeated use for three times. The SOCMs are also active for the same reaction. The precursors of the mesoporous silicates are more active than those of microporous silicates. This material can be repeatedly used without significant loss of activity. High activity is not due to the leached species. The active sites of the SOCM catalysts are considered to be SiO⁻ moieties located on the pore-mouth. Activity of the SOCM increases when the reaction is carried out without solvent, whereas decrease in activity of the OFMS is observed in the solvent-free system.     

Modification of faujasites to generate novel hosts for “ship-in-a-bottle” complexes

Zeolites X, Y, and DAY have been modified by a post-synthetical dealumination procedure to generate mesopores that are completely surrounded by micropores. In these novel host materials several bulky transition metal salen complexes have been occluded via the “ship-in-a-bottle” synthesis approach. Both the host materials and the “ship-in-a-bottle” catalysts have been characterized by FTIR spectroscopy and nitrogen adsorption. Additionally, the “ship-in-a-bottle” catalysts have been characterized by thermogravimetric analysis and ICP-AES spectroscopy. The catalysts have been tested in the stereoselective epoxidation of R-(+)-limonene and (−)--pinene with molecular oxygen as oxidant. The best results so far — 100% conversion, 96% selectivity and 91% de — were achieved with the immobilized (R,R)-(N,N′)-bis(3,5-di-tert-butylsalicylidene)-1,2-diphenylethylene-1,2-diaminocobalt complex in the epoxidation of (−)--pinene.     

Combustion of methane lean mixtures in reverse flow reactors: Comparison between packed and structured catalyst beds

The scope of this work is to compare systematically the performance of particle beds and monolithic beds in catalytic reverse flow reactors used for combustion of lean methane/air mixtures, using alumina-supported palladium as catalyst. Different values of gas surface velocity (0.1–0.3 m/s), particle diameter (3–6 mm, for particle bed), cell density (200–400 cpsi, for structured bed) and catalyst/inert ratio (0.4–1) were used for the simulation of the combustion of 3500 ppm methane in both kinds of reverse flow reactor. An unsteady one-dimensional heterogeneous model has been developed and solved using a MATLAB code. The model, physical parameters and transport properties used had been experimentally validated in a previous work, operating with a particle bed reverse flow reactor. Results obtained indicate that the reverse flow reactor is more stable when the catalyst particle beds are use, although the difference with the monolith bed decreases as surface velocity increases. In contrast, pressure drops in the bed are higher for the particle bed.     

A GRAPHICAL DESIGN METHOD FOR A PARTIAL SOLAR DRYER FOR CORN

This paper presents the application of a design method for a partial solar heating system of polyvalent modular dryers called “GJ-ABAQUE” to the drying of thick layers of grains.

This method is based on the use of charts or polynomial correlations. In the actual case where the drying air is not recycled, we only need one chart which allows one to determine the fraction of the monthly heating load supply by solar energy as a function of two dimensionless parameters. The latter implies the use of monthly average radiation data, the collector surface and estimates of drying loads.

The “GJ-ABAQUE” method was applied for drying 777 kg of corn, corresponding to 1 m³ of fresh product, in a thick layer in each modular dryer.     

A study of the use of solar concentrating plants for the atmospheric water vapour extraction from ambient air in the Middle East and Northern Africa region

In this paper an insight of two different methods for the production of fresh water is given within the framework of the FP6 project “AQUASOLIS” aimed at exploring the use of solar concentrating plants in Mediterranean countries for the supply of renewable water. The method presented in this paper is the extraction of water from air by direct cooling of humid air below the dew point. The energy consumption of the system is calculated and the possibility to use the solar cooling system for supplying the required refrigerating power is explored. Quantitative calculations are carried out simulating different weather scenarios in the three target Mediterranean partner countries: Jordan, Lebanon, and Morocco.     

Nanograin magnetoresistive manganite coatings for EMI shielding against directed energy pulses

Two magnetoresistive manganites, La_0.83Sr_0.17MnO₃ and La_0.7Sr_0.3MnO₃, are synthesized by the environmentally friendly “deposition by aqueous acetate solution (DAAS)” technique. The manganite film has a grain size of 100 nm, and can be processed as thinly as 0.03 μm per layer, while the powder form has a crystallite size of 40 nm. These magnetoresistive materials are shown to be effective and inexpensive electromagnetic interference (EMI) shield for the extremely low frequency (ELF) EM fields. The electrical resistance of manganites is very sensitive to external influences, such as temperature and electromagnetic fields. Both permeability (μ) and conductivity (σ) of manganites tend to increase with increasing applied magnetic field. The manganites have been shown to “react” to field increases in a way that is particularly useful for shielding EMI field fluctuations (e.g., due to current or voltage spikes).

The manganite properties, e.g., crystal structure, film morphology, radiation absorption and reflection, electrical resistivity, and magnetization, etc., have been measured. The ceramic manganites have a metal–insulator transition at 300 K or higher, and are suitable for a room temperature operation. A thin film (approx. 0.1 μm) of La_0.83Sr_0.17MnO₃ was fabricated on a quartz tube or refractory ceramic fiber blanket. Using this thin manganite film, the EMI shielding effectiveness for the measured E-field attenuation is similar to that of a 25 μm thickness of copper tube, aluminum foil, and silver–nickel particle-dispersed paper. The silver–nickel impregnated paper has an EMI shielding effectiveness of 35 dB at 10 kHz, and 15 dB at 60 Hz (or frequency above 1 MHz). The ceramic manganites are chemically inert, thermally stable, and mechanically flexible. They provide low cost EMI shielding against directed energy pulses and may serve as a “signature reduction” barrier.     

A new gas to liquids (GTL) or gas to ethylene (GTE) technology

Natural gas is a clean-burning and abundant energy resource, but much of it resides in locations remote from an economic means of transporting it to market. A logical solution for the problem would be to liquefy the natural gas, but this option requires very low temperatures and involves considerable costs. Another solution is to convert the natural gas into hydrocarbon liquids using chemical processing. Fischer-Tropsch technology converts the natural gas into “syngas” (a mixture of carbon monoxide and hydrogen) followed by reaction to liquid fuels. Unfortunately, Fischer-Tropsch technology is expensive.

At Texas A&M University, a research team has conceived a radically new process for converting natural gas into hydrocarbon liquids. It is a “direct” conversion method that does not require producing syngas. The process is essentially three reaction steps and two separation steps to produce hydrocarbon liquids. The process consists of two reaction steps and one separation step to produce ethylene. The process can operate economically with natural gas flows of as low as 300 kSCMD up to any desired capacity.

It is possible to use the GTL technology essentially anywhere natural gas exists from offshore platforms to relatively uninhabited onshore sites. This technology offers an alternative to flaring natural gas when pipelines do not exist. The liquids can be transported in liquid pipelines or in trucks or in tankers. Thus, it offers the opportunity to monetize a resource as well as to reduce undesirable emissions into the atmosphere. The GTE technology is more nearly suited to a location near an existing chemical industry that requires ethylene and/or hydrogen.

SynFuels International Inc. has licensed the technology to commercialize it, and the company has constructed a pilot plant capable of processing 3 kMCMD. The cost of a commercial 300 kSCMD plant should be in the US$ 50–75 million range. The cost of the liquids should be about US$ 25–28 per barrel. Of course, larger capacity plants would require a larger investment but produce a less expensive product.