Reactor Zone Research Articles

Modeling the Woody Biomass Combustion in a Vortexing Fluidized-Bed Combustor

This study develops an isothermal one-dimensional model to easily predict the performance of biomass combustion in a vortexing fluidized-bed combustor (VFBC). This model comprises a series of three reactor zones: a fluidized-bed combustion chamber, a lower freeboard, and an upper freeboard. This model can calculate the composition of the flue gas exhausted from a combustor and the concentration profiles of individual gas species at various combustor heights. This study verifies the proposed model by comparing its predictions with actual experimental data obtained from a series of systematic combustion experiments burning pinewood sawdust in a pilot-scale VFBC. This comparison revealed an air-leakage problem in the pilot-scale VFBC. The simulated results of CO2 and O2 concentrations are most sensitive to EA but almost non-sensitive to λ. As for the operating temperature, the simulation results for CO, CO2, and O2 are only sensitive at temperatures lower than 750 °C. The CO concentration in the exhaust is more affected than CO2 and O2 by the secondary air added. The fresh O2 added by secondary air can enhance CO consumption, but too much secondary air shortens the residence time of CO in the upper freeboard. Although there are obvious gaps between experimental and simulation results, the predicted change tendencies of gas concentrations correspond to the actual data. This model makes it possible to obtain more information in the preliminary steps of designing a larger scale VFBC for burning highly volatile fuel, e.g., biomass.

Enhanced CO 2-mineral sequestration by cyclic hydraulic fracturing and Si-rich fluid infiltration into serpentinites at Malentrata (Tuscany, Italy)

The magnesite deposits of Malentrata (Tuscany, Italy) were derived from serpentinite silicification–carbonation of the Ligurian ophiolite, and represent a natural analogue of in situ CO 2 mineral sequestration. Carbonation of magnesium silicate minerals (e.g. serpentine, olivine) at temperatures below 200 °C is an exothermal process, involving incorporation of carbon dioxide into stable carbonates (e.g. magnesite, dolomite). Serpentinites at Malentrata were transformed to a brownish friable rock characterized by the occurrence of opal, chromian montmorillonite, Fe-rich magnesite and minor iron sulfides and oxides. Widespread lenses of cohesive rocks occur within the altered serpentines, resulting from the complete silicification of the protolith. The pervasive alteration of serpentinite was accompanied by the formation of a network of magnesite and dolomite veinlets, and large magnesite–dolomite veins along major structures. Field observations, petrography and mineral chemistry define the following crystallization sequence in the major veins: i) early microgranular Fe-poor magnesite, ii) comb-textured Fe-rich magnesite and dolomite cementing the early brecciated magnesite vein infill, and iii) late quartz, chalcedony, and rare opal in the cavities. The mineral assemblage observed both in veins and in host rock is indicative of low-temperature hydrothermal alteration driven by Si- and CO 2-rich fluids under relatively low pH conditions. Pervasive and cyclic hydraulic fracturing maintained a high structural permeability during the whole hydrothermal event, creating conduits for the input and output fluids. The concomitant presence of CO 2 and silica in the fluid enhanced the congruent dissolution of serpentine and hampered immediate carbonate deposition. The Mg-bearing solutions were focused out of the reactor zone, and promoted massive carbonate veining along the main structural discontinuities. This two-steps reaction (dissolution of serpentinite followed by carbonate precipitation outside the reacting volume) seems to be very efficient and provide new insights on in situ and ex situ induced mineral carbonation and its industrial applications.

Preparation of 48Ca and its use in experiments on synthesis of new superheavy elements

A procedure was developed for preparing metallic 48Ca in a high yield. The starting 48CaCO3 was first calcined to 48CaO and then mixed with Al powder. The charge was fired at 1200–1300°C in a specially designed reactor. The vapor of the released 48Ca was condensed in the reactor zone cooled to room temperature. The process was monitored with 47Ca tracer. Much attention was given to recovery of expensive 48Ca from products of the aluminothermic process. The metallic 48Ca obtained was used for generating a beam of 48Ca5+ ions on an accelerator complex of FLNR JINR in experiments on synthesis of new superheavy elements.

Combustion of toluene–hexane binary mixtures in a reverse flow catalytic reactor

This work is focused on the application of reverse flow reactors to the combustion of lean mixtures of aliphatic and aromatic hydrocarbons in air. For this purpose, hexane and toluene were chosen as model compounds. The combustion of binary mixtures of these compounds (up to 500 ppmV total hydrocarbon concentration) over a commercial Pt / Al 2 O 3 catalyst in reverse flow reactors has been studied both experimentally, in a bench-scale unit, and by simulations, using a heterogeneous mono-dimensional dynamic model, good correspondence being observed between both approaches. As general trend, it was observed that the behaviour of the reactor is determined mainly by the combustion enthalpies and reactivities of toluene and hexane. Hence, increasing total concentration and increasing fraction of toluene (the most reactive compound) lead to more stable operation. Regarding the kinetic inhibition effects, in the conditions studied no influence on the reactor performance was observed, probably because the hydrocarbons combust in different reactor zones. This behaviour can be extended to the combustion of aromatic and C 5 – C 8 alkanes, characterised by their relatively low concentrations (determined by their vapour pressure) and high reaction rates.

Mathematical modeling and simulation of hydrotreating reactors: Cocurrent versus countercurrent operations

This paper describes a model to predict the behavior of trickle-bed reactors used for catalytic hydrotreating of oil fractions with cocurrent and countercurrent operation modes. A dynamic plug-flow heterogeneous one-dimensional model, which has been previously validated with experimental data obtained in an isothermal pilot plant reactor with cocurrent operation, was employed to compare both modes of operation. The reactor model considers the main reactions present in the hydrotreating process: hydrodesulfurization, hydrodenitrogenation, and hydrodearomatization. Simulations were performed for both pilot and commercial trickle-bed reactors, and the results are discussed in terms of variations with time and axial position of partial pressure, temperature and concentrations in liquid phase. A superior performance of countercurrent operation mode was found over cocurrent mode. It was recognized that countercurrent mode can have great potential to be used for deep hydrodesulfurization of oil fractions since it minimizes the inhibiting effect of some products (e.g. H2S) in reactor zones where these species tend to concentrate in cocurrent operation, i.e. at the bottom of the catalytic bed.

The role of mass spectrometry to study the Oklo–Bangombé natural reactors

The discovery of the existence of chain reactions at the Oklo natural reactors in Gabon, Central Africa in 1972 was a triumph for the accuracy of mass spectrometric measurements, in that a 0.5% anomaly in the (235)U/(238)U ratio of certain U ore samples indicated a depletion in (235)U. Mass spectrometric techniques thereafter played a dominant role in determining the nuclear parameters of the reactor zones themselves, and in deciphering the geochemical characteristics of various elements in the U-rich ore and in the surrounding rock strata. The variations in the isotopic composition of a large number of elements, caused by a combination of nuclear fission, neutron capture and radioactive decay, provide a powerful tool for investigating this unique geological environment. Mass spectrometry can be used to measure the present-day elemental and isotopic abundances of numerous elements, so as to decipher the past history of the reactors and examine the retentivity/mobility of these elements. Many of the fission products have a radioactive decay history that have been used to date the age and duration of the reactor zones, and to provide insight into their nuclear and geochemical behavior as a function of time. The Oklo fission reactors and their near neighbor at Bangombé, some 30 km to the south-east of Oklo, are unique in that not only did they become critical some 2 x 10(9) years ago, but also the deposits have been preserved over this period of geological time. The long-term geochemical behavior of actinides and fission products have been extensively studied by a variety of mass spectrometric techniques over the past 30 years to provide us with significant information on the mobility/retentivity of this material in a natural geological repository. The Oklo-Bangombé natural reactors are therefore geological analogs that can be evaluated in terms of possible radioactive waste containment sites. As more reactor zones were discovered, it was realized that they could be classified into two groups according to their burial depth in the Oklo mine-site. Reactor Zones 10, 13, and 16 were buried more deeply, and were therefore less weathered than the other zones. The less-weathered zones are of great importance in mobility/retentivity studies and therefore to the question of radioactive waste containment. Isotopic studies of these natural reactors are also of value in physics to examine possible variations in fundamental constants over the past 2 billion years.

Assessment of a new carbon tetrachloride destruction system based on a microwave plasma torch operating at atmospheric pressure

A new system for destroying volatile organic waste based on a microwave plasma torch that operates at atmospheric pressure and is coupled to a reactor affording isolation of output gases and adjustment of the plasma discharge atmosphere is proposed. The system was assessed by using carbon tetrachloride as the target volatile organic compound (VOC) and argon as the main gas in a helium atmosphere. Under optimal conditions, a microwave power of less than 1000 W was found to reduce the CCl 4 concentration at the reactor outlet to the parts-per-billion level and hence to virtually completely destroy the VOC. With high argon flow-rates and CCl 4 concentrations, the energy efficiency can reach levels in excess of 3000 g/kW h. Output gases and species in the plasma, which were identified by gas chromatography and light emission spectroscopy, respectively, were found to include no halogen-containing derivatives resulting from the potential cleavage of CCl 4. In fact, the main gaseous byproducts obtained were CO 2, NO and N 2O, in addition to small traces of Cl 2, and the solid byproducts Cl 2Cu and various derivatives depending on the particular reactor zone.

Numerical Modeling of an Ar–H2 Radio-Frequency Plasma Reactor under Thermal and Chemical Nonequilibrium Conditions

The species densities and the thermal and chemical nonequilibrium phenomena in an Ar–H2 radio frequency inductively coupled plasma reactor used for hydrogenation of materials have been investigated through numerical simulation. The mathematical model consists of a two-temperature fluid dynamics model and a chemical kinetics model that takes into account the effect of local chemical nonequilibrium. Computations are carried out for the rf plasma running at 11.7 kW and 27 kPa for different Ar–H2 mixtures and for pure argon. Predicted results for the electron and heavy-species temperatures, the species densities, as well as the degree of thermal and chemical nonequilibrium, are presented in detail. It is found that the electron and hydrogen atom densities in the reactor and in the near-wall region of the torch are strongly altered by nonequilibrium effects. The hydrogen atom density remains high in the reactor zone, and peaks in a region that has been found to be attractive for material processing. Deviations from thermal and chemical equilibrium are greatly reduced by the addition of hydrogen to an argon plasma.

The fate of the epsilon phase (Mo-Ru-Pd-Tc-Rh) in the UO2 of the Oklo natural fission reactors

In spent nuclear fuel (SNF), the micrometer- to nanometer-sized epsilon phase (Mo-Ru-Pd-Tc-Rh) is an important host of 99Tc which has a long half life (2.13×105 years) and can be an important contributor to dose in safety assessments of nuclear waste repositories. In order to examine the occurrence and the fate of the epsilon phase during the corrosion of SNF over long time periods, samples of uraninite from the Oklo natural reactors (∼ 2.0 Ga) have been investigated using transmission electron microscopy (TEM). Because essentially all of the 99Tc has decayed to 99Ru, this study focuses on 4d-elements of the epsilon phase. Samples were obtained from the research collection at University of Michigan representing reactor zone (RZ) 10 (836, 819, 687) and from RZ 13 (864, 910). Several phases with 4d-metals have been identified within UO2 matrix at the scale of 50−700 nm; froodite, PdBi2, with trace amounts of As, Fe, and Te, and palladodymite or rhodarsenide, (Pd,Rh)2As. The most abundant 4d-metal phase is ruthenarsenite, (Ru,Ni)As, which has a representative composition: As, 59.9; Co, 2.5; Ni, 5.2; Ru, 18.6; Rh, 8.4; Pd, 3.1; Sb, 2.4 in atomic %. Ruthenarsenite nanoparticles are typically surrounded by Pb-rich domains, galena in most cases; whereas, some particles reveal a complexly zoned composition within the grain, such as a Pb-rich domain at the core and enrichment of Ni, Co, and As at the rim. Some ruthenarsenites and Rh-Bi-particles are embedded in surrounding alteration products, e.g., chlorite, adjacent to uraninite (no further than ∼5 μm). A few of those particles are still coated by a Pb-rich layer. Based on these results, the history that epsilon phases have experienced can be described as follows: (i) The original epsilon phase was changed to, in most cases, ruthenarsenite, by As-rich fluids with other trace metals. Dissolution and a simultaneous precipitation may be responsible for the phase change. (ii) All Mo and most of the Tc were released from the epsilon phase. Galena precipitated surrounding the 4d-metal phases. (iii) Once the uraninite matrix has dissolved, the epsilon nanoparticles were released and “captured” within alteration phases that are immediately adjacent to the uraninite.

Dynamic simulation and operation of a high pressure ethylene-vinyl acetate (EVA) copolymerization autoclave reactor

Polyethylene (PE) is one of the most widely used polymers in chemical industry. In all the PE processes, low density polyethylene (LDPE) process is generally considered as the most profitable segment of the polyethylene business worldwide. There are two basic processes used for the manufacturing of LDPE: autoclave and tubular reactor, where autoclave reactor is often used to produce high-value specialty-type grades including ethylene-vinyl acetate (EVA) copolymer. Depending on different grades of the product, reactor zone temperatures in this autoclave process can be in the range 150–230 °C and pressure in the range of 1500–2000 kg/cm 2. In this paper, dynamic simulation of an industrial EVA copolymerization reactor will be established. Industrial operating condition data of seven product grades with melt index ranging from ones to over four hundred will be used as fitted data to obtain the key kinetic parameters in the model. The predicted outputs of melt index, vinyl acetate wt.% in polymer, and polymer production rate will be compared to plant data. This dynamic model was used in determining the controller tuning constants of the three important zone temperature control loops of the autoclave reactor and was also used for comparative study of various operating policies during grade transition.

Analysis of thermal radiation from laser-heated nanoparticles formed by laser-induced decomposition of ferrocene

Thermal radiation, originating from laser-heated gas-phase nanoparticles, was detected in the 400–700 nm wavelength range by means of optical emission spectroscopy. The particles were formed upon laser-induced photolytic decomposition of ferrocene (Fe(C5H5)2) and consisted of an iron core surrounded by a carbon shell. The laser-induced excitation was performed as the particles were still within the reactor zone, and the temperature of the particles could be determined from thermal emission. Both the temperature of the nanoparticles and the relative intensity changes of the emission were monitored as a function of time (with respect to the laser pulse), laser fluence and Ar ambient pressure. At high laser fluences, the particles reached high temperatures, and evidence was found for boiling of iron. Modeling of possible energy-releasing mechanisms such as black-body radiation, thermionic electron emission, evaporation and heat transfer by the ambient gas was also performed. The dominant cooling mechanisms at different ranges of temperature were clarified, together with a determination of the accommodation factor for the Ar–nanoparticle collisions. The strong evaporation at elevated temperatures also led to significant iron loss from the produced particles.

Neutron-Physical Calculation of a Fresh Zone in the Natural Nuclear Reactor at Oklo

A full-scale computer model of zone 2 of the Oklo reactor with a realistic materials composition is constructed. Modern Monte Carlo programs are used to calculate the multiplication factor, the excess reactivity, and the neutron flux for the fresh reactor zone 2. The calculations are performed in a wide range of variation of the zone parameters: the uranium content in the contemporary Oklo zone is varied from 35 to 55 mass% and the water content from 0.355 to 455 g/cm3. The power effect is determined. The temperature of the fresh zone is found to be 725 ± 55 K, at which the reactor is critical and can operate in a stable manner for a long time. The power of the reactor is maintained by negative feedback. The neutron spectrum, over which the cross section of strong absorbers must be averaged, for example, 62 149 Sm, in order to obtain the most accurate bounds on the possible shift of the samarium resonance as a result of a change in the fundamental constants, is found for 700 K.

Uraninite recrystallization and Pb loss in the Oklo and Bangombé natural fission reactors, Gabon

The Oklo and Bangombé natural fossil fission reactors formed ca. 2 Ga ago in the Franceville basin, Gabon. The response of uraninite in the natural reactors to different geological conditions has implications for the disposal of the UO 2 in spent nuclear fuel. Uraninite and galena from two reactor zones, RZ16 at Oklo and RZB at Bangombé, were studied to clarify the chronology and effect of alteration events on the reactor zones. In addition, ion microprobe U-Pb analysis of zircons from a dolerite dyke in the Oklo deposit were completed to better constrain the age of the dyke, and thereby testing the link between the dyke and an important alteration event in the reactor zones. The analyzed uraninite from RZ16 and RZB contains ca. 6 wt% PbO, indicating a substantial loss of radiogenic Pb. Transmission electron microscopy showed that microscopic uraninite grains in the reactor zones consist of mainly defect-free nanocrystalline to microcrystalline aggregates. However, the nanocrystalline regions have elevated Si contents and lower Pb contents than coarser uraninite crystallites. Single stage model ages of large, millimeter-sized galena grains at both RZ16 and RZB correlate well with the age of the Oklo dolerite dyke, 860 ± 39 Ma (2σ). Thus, the first major Pb loss from uraninite occurred at both Oklo and Bangombé during regional extension and the intrusion of a dyke swarm in the Franceville basin, ∼860–890 Ma ago. Uraninite Pb isotopes from RZ16 and RZB give lower ages of ca. 500 Ma. These ages agree with the “chemical” ages of the uraninite, and show that an ancient Pb loss occurred after the intrusion of the dolerite dykes. The presence of nanocrystallites in the reactor uraninite indicates internal recrystallization, which may have occurred around 500 Ma, resulting in the 6wt% PbO uraninite. It is suggested that leaching by fluid interaction triggered by the Pan-African orogeny was important during this second Pb-loss event. Thus, there are indications that uraninite at both the Oklo and Bangombé natural reactors has experienced at least two ancient episodes of Pb loss associated with internal recrystallization. These recrystallization events have occurred without significantly depleting the 2 Ga fission products compatible with the uraninite structure.

RIGOROUS MODELING OF A HIGH PRESSURE ETHYLENE-VINYL ACETATE (EVA) COPOLYMERIZATION AUTOCLAVE REACTOR

Polyethylene (PE) is one of the most widely used polymers in chemical industry. In all the PE processes, high-pressure autoclave process is generally considered as the most profitable segment of the polyethylene business worldwide in recent years. Depending on different grades of the product, reactor zone temperatures in this autoclave process can be in the range 150-230°C and pressure in the range of 1500-2000kg/cm2. In this paper, rigorous dynamic model of an industrial ethylene and vinyl acetate (EVA) copolymerization reactor will be established. Industrial operating condition data of seven product grades with melt index ranging from ones to over four hundred will be used as fitted or test data to obtain the key kinetic parameters in the model. The predicted outputs of melt index, vinyl acetate wt. % in polymer, and polymer production rate will be compared to plant data.

04/01292 Mobilization and mechanisms of retardation in the Oklo natural reactor zone 2 (Gabon) — inferences from U, REE, Zr, Mo and Se isotopes: Bros, R. et al. Applied Geochemistry, 2003, 18, (12), 1807–1824

04/01292 Mobilization and mechanisms of retardation in the Oklo natural reactor zone 2 (Gabon) — inferences from U, REE, Zr, Mo and Se isotopes: Bros, R. et al. Applied Geochemistry, 2003, 18, (12), 1807–1824

Micro-structured string-reactor for autothermal production of hydrogen

Novel micro-structured string-reactor designed as catalytically active wires placed in parallel into a tube was developed. The small diameter of the channels (∼100 μm) leads to a short radial diffusion time, a narrow residence time distribution (RTD), and a low pressure drop. This reactor was applied for the oxidative steam-reforming of methanol (OSRM) to produce hydrogen in autothermal mode for fuel cells. The heat generated during methanol oxidation at the reactor entrance is axially transferred to the reactor zone of the endothermic steam-reforming. The brass metal wires (Cu/Zn=4/1) were used as precursors for the preparation of string-catalysts. The brass wires have high thermal conductivity (120 W/(m K)) and the chemical composition is similar to the active phase of the Cu/ZnO/Al 2O 3 traditional catalyst during the steam-reforming of methanol. Brass-based string catalysts are obtained by metal/aluminium alloy formation on the outer surface of wires followed by an acid treatment leaching out aluminium. This treatment leads to an increase of the specific surface area (SSA) due to the formation of porous outer layer on the wire surface. The porous outer layer has the morphology of Raney metals. The catalysts were first tested for the steam-reforming of methanol and showed high activity together with selectivities close to 100% towards hydrogen and carbon dioxide. Then, the optimized catalyst was tested during the methanol partial oxidation (POX) and during OSRM. Oxygen was observed to be totally converted via total oxidation and therefore, higher methanol conversion in the OSRM together with CO 2 selectivity of 99% and H 2 selectivity of 60% were obtained.

Duplex Al-based thermal spray coatings for corrosion protection in high temperature refinery applications

The application of thermal spray coatings has been effective in preventing corrosion of steel and iron products. It has been used in a wide range of applications spreading from the petroleum to the food industry. In this work, the performance and effectiveness of a two-layered aluminum-based thermal spray coating applied to an ASTM A387 G11 steel was evaluated. The coating structure was comprised of an inner Al-Fe-Cr layer and an outer layer of aluminum. Coated samples were tested in the reactor zone of a fluid catalytic cracking unit (FCCU) of a petrochemical plant for 2.5 years. The reactor zone temperature was about 793 K (520 °C) and the environment was a mixed gas containing sulfur, oxygen and carbon. Laboratory-scale tests were also conducted on the coated samples in order to gain a better understanding of the corrosive effect of the gaseous species present in the FCCU atmosphere. Porosity present in the thermal spray coatings allowed the penetration of the atmosphere corrodents, which instigated intergranular corrosion of the steel substrate. The presence of an inner Al-Fe-Cr layer did not prevent coating spallation, which further contributed to the internal corrosion process.

Determination of the neutron flux in the reactor zones with the strong neutron absorption and leakage

The procedures for the numerical and experimental determination of the neutron flux in the zones with the strong neutron absorption and leakage are described in this paper. Numerical procedure is based on the application of the SCALE-4.4a code system where the Dancoff factors are determined by the VEGA2DAN code. Two main parts of the experimental methodology are measurement of the activity of irradiated foils and determination of the averaged neutron absorption cross-section in the foils by the SCALE-4.4a calculation procedure. The proposed procedures have been applied for the determination of the neutron flux in the internal neutron converter used with the RB reactor core configuration number 114.

Isotopic evidence for trapped fissiogenic REE and nucleogenic Pu in apatite and Pb evolution at the Oklo natural reactor

A part of the boundary layer of reactor zone 10 at the Oklo natural reactor shows a unique petrologic texture, which contains high-grade uraninite and massive apatite concretions. In order to study distribution behavior of fission products around the boundary between the reactor zone and the wall rock and to clarify the relation of migration mechanisms of fission products with geochemical factors, in-situ isotopic analyses of Nd, Sm, Gd, Pb and U in uraninite and apatite from the sample were performed by Sensitive High Resolution Ion Microprobe (SHRIMP). Sm and Gd isotopic ratios of uraninite and apatite show evidence of neutron irradiation with fluence between 4.4–6.8×10 19 n/cm 2. Judging from the isotopic anomalies of Nd and U, the apatite coexisting with the uraninite plays an important role in trapping fissiogenic LREE and nucleogenic 239Pu into the structure. Systematic Pb isotopic data from apatite, uraninite, galena and minium suggest the following chronological interpretations. 1. The apatite formed 1.92±0.01 Ga ago and trapped fissiogenic light REE and nucleogenic 239Pu that migrated from the reactor during the criticality. 2. The uraninite around the boundary between reactor and sandstone dissolved once 1.1∼1.2 Ga ago. 3. Galena grains were formed by U-Pb mobilization in association with the intrusion of dolerite dyke 0.45∼0.83 Ga ago. 4. Minium was derived from recent dissolution of galena under locally oxidizing conditions.

Oxygen isotopic composition of nano-scale uraninite at the Oklo-Okélobondo natural fission reactors, Gabon

High spatial resolution (10-30 μm), in situ oxygen isotopic analyses by secondary ion mass spectrometry (SIMS), coupled with high-resolution transmission electron microscopy (HRTEM), were used to show that uraninite from the Oklo-Okélobondo natural fission reactors that occur in near surface environments, have low δ 18 O values and nanotextures that are consistent with interaction with ground water. These low δ 18 O values (-14.4 to -8.5‰) suggest that the minerals exchanged with meteoric groundwater. In contrast, reactor zones that occur at depth have largely retained their original O isotopic composition (-10.2 to -5.6‰) and uraninites are well-crystallized and essentially defect-free. These observations clearly demonstrate that by combining both HRTEM and in situ O isotopic analyses by SIMS, it is possible to characterize the nano-scale porosity and postdepositional alteration of U-bearing phases.