The gas-solid trickle-flow reactor for the catalytic oxidation of hydrogen sulphide: a trickle-phase model

The oxidation of H2S by O2 producing elemental sulphur has been studied at temperatures of 100–300°C and at atmospheric pressure in a laboratory-scale gas-solid trickle-flow reactor. In this reactor one of the reaction products, i.e. sulphur, is removed continuously by flowing solids. A porous, free-flowing catalyst carrier has been used which contains a NaX zeolite acting as a catalyst as well as a sulphur adsorbent. In order to describe mass transfer in the trickle-flow reactor, a reactor model has been developed in which a particle-free, upflowing gas phase and a dense, downflowing gas-solids suspension, the so-called trickle phase, are distinguished. Within the trickle phase, diffusion of the reactants parallel to reaction in the catalyst particles takes place. The mass transfer rate from the gas phase to the trickle phase has been evaluated by the reaction of H2S with SO2, which is a much faster reaction than the reaction with O2. From the experiments and from the reactor model calculations it appears that for the H2S-O2 reaction no mass transfer limitations occur at temperatures up to about 200°C, whereas at 300°C gas-phase mass transfer and diffusion within the dense solids suspension offer resistance to reaction

The influence of the reactor pressure on the hydrodynamics in a cocurrent gas-liquid trickle-bed reactor

The influence of the reactor pressure on the liquid hold-up in the trickle-flow regime and on the transition between trickle-flow and pulse-flow has been investigated in a trickle-flow column operating up to 6.0 MPa with water, and nitrogen or helium as the gas phase.\ud \ud The effect of the gas velocity and gas density on the hold-up has been explained by means of the modified Galileo number Ga{1+ΔP/(ρlgL)}. At the transition between trickle- and pulse-flow the liquid hold-up is - for a given value of the superficial gas velocity - nearly the same at each gas density. Therefore, at elevated gas densities the transition occurs at higher liquid throughputs. From a comparison of the experiments with water-nitrogen and water-helium it has been concluded that at an equal gas density - for given values of vl and vg - the hydrodynamic behaviour is the same

Observability of Dark Matter Substructure with Pulsar Timing Correlations

Dark matter substructure on small scales is currently weakly constrained, and its study may shed light on the nature of the dark matter. In this work we study the gravitational effects of dark matter substructure on measured pulsar phases in pulsar timing arrays (PTAs). Due to the stability of pulse phases observed over several years, dark matter substructure around the Earth-pulsar system can imprint discernible signatures in gravitational Doppler and Shapiro delays. We compute pulsar phase correlations induced by general dark matter substructure, and project constraints for a few models such as monochromatic primordial black holes (PBHs), and Cold Dark Matter (CDM)-like NFW subhalos. This work extends our previous analysis, which focused on static or single transiting events, to a stochastic analysis of multiple transiting events. We find that stochastic correlations, in a PTA similar to the Square Kilometer Array (SKA), are uniquely powerful to constrain subhalos as light as $\sim 10^{-13}~M_\odot$, with concentrations as low as that predicted by standard CDM.Comment: 45 pages, 12 figure

Absorption accompanied with chemical reaction in trickle-bed reactors.

A new development in the field of internals in packed columns is the use of structured packing types. Recently, a new structured packing type coated with a thin alumina layer (KATAPAKTM) has been developed. In this report, the results of an experimental and theoretical study concerning the possible applicability of this new packing material for hydrogenation processes in a trickle-bed reactor is presented. The palladium catalyzed hydrogenation of α-methylstryrene is used as a model reaction to study hydrodynamics and mass transfer characteristics in a trickle-bed reactor under reactive conditions. Converstions at several process conditions are measured in a pilot plant in which 3 mm spheres as well KATAPAKTM is applied as packing materials. A comparison of the results of some physical absorption experiments with the results of hydrogenation experiments showed that the resistance in series model—in which the total resistance against mass transfer is calculated from the separate resistances—is not valid in systems where heterogeneous reactions at the solid surface can enhance the mass transfer-rate at the gas-liquid interphase. With the aid of a developed trickle-bed reactor model, based on liquid diffusion, simultaneous reaction at the solid surface and zero volume mixing points, the mass transfer phenomena in trickle-bed reactors in conditions where the resistance in series model fails can be explained and described. The numerically solved model calculates the hydrogen profiles in the liquid films of the reactor and over all single pass conversions at several process conditions. These conclusions are confirmed by the results of the simulation of a model reactor, i.e. the laminar film reactor with a catalytically active wall. From the results of the measurements it could be concluded that in trickle-flow conditions, the application of KATAPAKTM does not significantly improved on the overall performance of trickle-bed reactors. The increase of the physical absorption rate due to better mass transfer characteristics of structured packings compared to dumped packing types—as reported in literature—will be eliminated to a certain extent in reactive systems due to the enhancement effect of heterogeneous reactions in trickle-flow operation

On the Scalability and Message Count of Trickle-based Broadcasting Schemes

As the use of wireless sensor networks increases, the need for efficient and reliable broadcasting algorithms grows. Ideally, a broadcasting algorithm should have the ability to quickly disseminate data, while keeping the number of transmissions low. In this paper, we analyze the popular Trickle algorithm, which has been proposed as a suitable communication protocol for code maintenance and propagation in wireless sensor networks. We show that the broadcasting process of a network using Trickle can be modeled by a Markov chain and that this chain falls under a class of Markov chains, closely related to residual lifetime distributions. It is then shown that this class of Markov chains admits a stationary distribution of a special form. These results are used to analyze the Trickle algorithm and its message count. Our results prove conjectures made in the literature concerning the effect of a listen-only period. Besides providing a mathematical analysis of the algorithm, we propose a generalized version of Trickle, with an additional parameter defining the length of a listen-only period.Comment: arXiv admin note: substantial text overlap with arXiv:1407.603

Selective hydrogenation of 1,5,9-cyclododecatriene in up- and down-flow fixed-bed reactors: experimental observations and modeling

The performance of trickle and flooded-bed reactors has been investigated and compared for an exothermic multi-step catalytic reaction. Selective hydrogenation of cyclododecatriene over Pd/Al2O3 has been studied in both up- and down-flow modes of operation in the same pilot reactor. In the down-flow mode, hot spots and runaway could not be avoided without diluting both catalyst bed and liquid reactant. With this diluted system, the up-flow reactor leads to a higher productivity and a much better selectivity. A non-isothermal plug-flow reactor model predicts the performances of the up-flow reactor satisfactorily, but is found to be unsuitable to the case of a trickle-bed reactor. In the latter case, the productivity was underestimated, when complete wetting of catalyst particles was assumed. On the other hand, when partial wetting effect was incorporated, the calculated selectivity was always much higher than that observed actually in a trickle bed, due to heterogeneities of liquid velocity and partial wetting (poorly irrigated zones

Hydrodynamics in a cocurrent gas-liquid trickle bed at elevated pressures

Data on design and operation of trickle beds at elevated pressures are scarce. In this study the influence of the gas density on the liquid holdup, the pressure drop, and the transition between trickle and pulse flow has been investigated in a tricklebed reactor operating up to 7.5 MPa and with nitrogen or helium as the gas phase. Gas-liquid interfacial areas have been determined up to 5.0 MPa by means of CO2 absorption from CO2/N2 gas mixtures into amine solutions. \ud A comparison of the results from nitrogen as the gas phase to those of helium shows that at equal gas densities the hydrodynamic states are the same. The gas-liquid interfacial area increases when operating at higher gas densities. When the determined dimensionless interfacial areas agl/as are all within the range 0.25-0.8, the trickle-bed reactor is suggested to operate in the trickle-flow regime. The gas density has a strong influence on the liquid holdup. Due to the higher pressure gradients at elevated gas densities, the liquid holdup decreases noticeably. Besides, the boundary between the trickle-flow and pulse-flow regime shifts toward higher liquid throughputs: the region for trickle-flow operationg becomes larger. For the liquid holdup and the pressure gradient in the trickle-flow regime, correlations derived based on dimensionless numbers can be applied to high-prssure trickle beds

Some aspects of soil physics applicable to trickle irrigation : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Soil Science at Massey University

Irrigation of crops is one of the more widely used techniques to increase yeilds. Trickle irrigation is one such method and is more suited to horticultural crops. In New Zealand, with horticulture assuming more importance, appropriate methods of design and operation of trickle irrigation systems are required. In this study a simple approximation to Wooding's solution for steady infiltration from a shallow ponded source, much like that found under trickle emitters is examined. This may aid in irrigation design and practice. The approximation also allowed for the development of a method to concurrently measure the saturated hydraulic conductivity and sorptivity from simple field infiltration measurements with a minimum of soil disturbance. Saturated hydraulic conductivities and sorptivities are of great use in soil water studies in general. A commercial trickle irrigation system was also examined to determine the suitability of such irrigation systems to particular soils, and to examine the present irrigation scheduling. The approximation to Wooding's solution was found to perform well in the field in many respects, particularly in determining steady ponded zone sizes. Ponded zone sizes are important in that they control the volume of soil wetted by irrigation to a large degree. Much of this agreement is due to the use of parameters determined by the simple field method developed from this theory. Sorptivities and saturated hydraulic conductivities obtained by this method were found to be more realistic for trickle irrigation than those determined by other existing methods. Systematic errors in these other methods, mainly soil disturbance and the concomitant creation of continuous flow paths for water, as well as soil smearing, are thought to be the main cause of this difference. Temporal and spatial variation in soil physical properties are however, found to hinder the use of soil physics theory in the field. Macropores (due to soil biological activity) were found to profoundly influence infiltration processes and soil-water distribution. These effects were particularly marked for the site with a commercial trickle irrigation system. Here the efficiency of the present system is thought to be low, and evidence indicates that irrigation was in excess of plant requirements. The utility of Wooding's solution, and the method to measure soil physical parameters developed from this, is further demonstrated in this orchard

Data Dissemination Performance in Large-Scale Sensor Networks

As the use of wireless sensor networks increases, the need for (energy-)efficient and reliable broadcasting algorithms grows. Ideally, a broadcasting algorithm should have the ability to quickly disseminate data, while keeping the number of transmissions low. In this paper we develop a model describing the message count in large-scale wireless sensor networks. We focus our attention on the popular Trickle algorithm, which has been proposed as a suitable communication protocol for code maintenance and propagation in wireless sensor networks. Besides providing a mathematical analysis of the algorithm, we propose a generalized version of Trickle, with an additional parameter defining the length of a listen-only period. This generalization proves to be useful for optimizing the design and usage of the algorithm. For single-cell networks we show how the message count increases with the size of the network and how this depends on the Trickle parameters. Furthermore, we derive distributions of inter-broadcasting times and investigate their asymptotic behavior. Our results prove conjectures made in the literature concerning the effect of a listen-only period. Additionally, we develop an approximation for the expected number of transmissions in multi-cell networks. All results are validated by simulations