Safe design and operation of fluidized-bed reactors: Choice between reactor models

For three different catalytic fluidized bed reactor models, two models presented by Werther and a model presented by van Deemter, the region of safe and unique operation for a chosen reaction system was investigated. Three reaction systems were used: the oxidation of benzene to maleic anhydride, the oxidation of naphthalene to phthalic anhydride, and the oxidation of ethylene to ethylene oxide. Predictions of the optimal yield, the operating temperature and the conversion were also subjects of our study. It appeared that for reactions carried out in a fluidized bed operating under conditions of good fluidization all models predicted the same region of safe and unique operation. For a well-designed fluidized bed only the constraint of uniqueness is affected by the reactor model chosen. Predictions of the yield, conversion and operating temperature appeared to fit slightly less well. But still a good indication can be obtained from any of the models since the deviation in the results was less then a few percent for all three reaction systems. The strongest deviations between the models occurs in the region of gas loads only slightly higher than the minimum fluidization velocity. As the heat transfer characteristics are bad at low gas loads this region is unsuitable for highly exothermic reactions where large amounts of heat have to be removed by the coolant. In the region of good heat transfer with gas loads at least several times higher than the minimum the three models predict the same results. For this reason we finally recommed the use of simple models

Safe design of cooled tubular reactors for exothermic multiple reactions: Multiple-reaction networks

The model of the pseudo-homogeneous, one-dimensional cooled tubular reactor is applied to a multiple-reaction network. It is demonstrated for a network which consists of two parallel and two consecutive reactions. Three criteria are developed to obtain an integral yield which does not deviate more than a chosen fraction from the maximum yield that can be obtained in an isothermal reactor. The criteria enable us to choose relevant design and operating conditions for the safe execution of a reaction network in a tubular reactor. The method is illustrated for the production of maleic anhydride by air oxidation of benzene

Safe design and operation of tank reactors for multiple-reaction networks: uniqueness and multiplicity

A method is developed to design a tank reactor in which a network of reactions is carried out. The network is a combination of parallel and consecutive reactions. The method ensures unique operation. Dimensionless groups are used which are either representative of properties of the reaction system or exclusively of the design and operating variables. In a plot of the optimal yield vs the dimensionless operating temperature the region is indicated where operation under conditions of uniqueness is feasible. The method is illustrated with an example: the air oxidation of benzene of maleic anhydride

Dynamic p-enrichment schemes for multicomponent reactive flows

We present a family of p-enrichment schemes. These schemes may be separated into two basic classes: the first, called \emph{fixed tolerance schemes}, rely on setting global scalar tolerances on the local regularity of the solution, and the second, called \emph{dioristic schemes}, rely on time-evolving bounds on the local variation in the solution. Each class of $p$-enrichment scheme is further divided into two basic types. The first type (the Type I schemes) enrich along lines of maximal variation, striving to enhance stable solutions in "areas of highest interest." The second type (the Type II schemes) enrich along lines of maximal regularity in order to maximize the stability of the enrichment process. Each of these schemes are tested over a pair of model problems arising in coastal hydrology. The first is a contaminant transport model, which addresses a declinature problem for a contaminant plume with respect to a bay inlet setting. The second is a multicomponent chemically reactive flow model of estuary eutrophication arising in the Gulf of Mexico.Comment: 29 pages, 7 figures, 3 table

A basin- to channel-scale unstructured grid hurricane storm surge model applied to southern Louisiana

Southern Louisiana is characterized by low-lying topography and an extensive network of sounds, bays, marshes, lakes, rivers, and inlets that permit widespread inundation during hurricanes. A basin- to channel-scale implementation of the Advanced Circulation (ADCIRC) unstructured grid hydrodynamic model has been developed that accurately simulates hurricane storm surge, tides, and river flow in this complex region. This is accomplished by defining a domain and computational resolution appropriate for the relevant processes, specifying realistic boundary conditions, and implementing accurate, robust, and highly parallel unstructured grid numerical algorithms. The model domain incorporates the western North Atlantic, the Gulf of Mexico, and the Caribbean Sea so that interactions between basins and the shelf are explicitly modeled and the boundary condition specification of tidal and hurricane processes can be readily defined at the deep water open boundary. The unstructured grid enables highly refined resolution of the complex overland region for modeling localized scales of flow while minimizing computational cost. Kinematic data assimilative or validated dynamic-modeled wind fields provide the hurricane wind and pressure field forcing. Wind fields are modified to incorporate directional boundary layer changes due to overland increases in surface roughness, reduction in effective land roughness due to inundation, and sheltering due to forested canopies. Validation of the model is achieved through hindcasts of Hurricanes Betsy and Andrew. A model skill assessment indicates that the computed peak storm surge height has a mean absolute error of 0.30 m

Belastinggeld voor vrede en veiligheid: Een analyse van de Nederlandse defensie-uitgaven van 1990-2007

Dit paper analyseert de Nederlandse defensie-uitgaven van 1990 tot en met 2007. In 1990 bedroegen deze uitgaven nog 2,7% van het Bruto Nationaal Product (BNP). In 2007 was dit meer dan een procent lager. De Nederlandse krijgsmacht heeft zich in de afgelopen jaren in termen van taakuitvoering, grootte en samenstelling aanzienlijk gereorganiseerd. Deze veranderingen weerspiegelen zich echter niet in de samenstelling van de defensie-uitgaven. Zowel de verdeling over de operationele commando’s, als de verdeling over de uitgavensoorten, salarissen, exploitatie en investeringen, zijn ongeveer constant gebleven. De verwachte groei van de economie, gecombineerd met een gelijkblijvend Defensiebudget, leidt de komende jaren tot een verdere daling. Naar verwachting zullen de defensie-uitgaven in 2008 1,5% van het BNP bedragen. Dit percentage bedraagt, uitgaande van het huidige beleid, in 2011 naar verwachting 1,33%. Of het budget dan voldoende is om het huidige ambitieniveau te halen, is door stijgende loonkosten en geringe financiële flexibiliteit onzeker

The choice between cooled tubular reactor models: analysis of the hot spot

The applicability of the one-dimensional pseudo-homogeneous model of the cooled tubular reactor is studied. Using the two-dimensional model as the more accurate one we compared both models by studying the influence of the design and operating variables on the conditions in the hot spot of the reactor. The effects were studied on an analytical basis, and a relation is derived that describes the radial temperature profile in the hot spot of the reactor. In the first section we present the model equations and discuss the results obtained from a numerical evaluation. In the second section we compare mean and maximum radial temperatures and reaction rates in case a single exothermic reaction is carried out. We conclude that—for reactors operating in the steady state—in the hot spot the one-dimensional model predicts the proper temperature when it is compared with the average temperature calculated by the two-dimensional model, although large differences may arise between maximum and mean radial temperature. A new method is presented to obtain the maximum radial temperature in the hot spot directly from the results of the one-dimensional model. It was found that there can be large differences between the actual average reaction rate and the reaction rate at mean temperature as obtained from the one-dimensional model

The choice between cooled tubular reactor models: analysis of the hot spot

The applicability of the one-dimensional pseudo-homogeneous model of the cooled tubular reactor is studied. Using the two-dimensional model as the more accurate one we compared both models by studying the influence of the design and operating variables on the conditions in the hot spot of the reactor. The effects were studied on an analytical basis, and a relation is derived that describes the radial temperature profile in the hot spot of the reactor. In the first section we present the model equations and discuss the results obtained from a numerical evaluation. In the second section we compare mean and maximum radial temperatures and reaction rates in case a single exothermic reaction is carried out. We conclude that—for reactors operating in the steady state—in the hot spot the one-dimensional model predicts the proper temperature when it is compared with the average temperature calculated by the two-dimensional model, although large differences may arise between maximum and mean radial temperature. A new method is presented to obtain the maximum radial temperature in the hot spot directly from the results of the one-dimensional model. It was found that there can be large differences between the actual average reaction rate and the reaction rate at mean temperature as obtained from the one-dimensional model

The lumping of heat transfer parameters in cooled packed beds: effect of the bed entry

The lumping of the heat transfer parameters of the one- and the two-dimensional pseudo-homogeneous model of a cooled fixed bed were compared. It appeared that the lumping of the two-dimensional parameters, being the effective radial conductivity h-eff and the heat transfer coefficient at the wall (alpha)w, into the one-dimensional overall heat transfer coefficient U results in a length dependence of U. It is shown that the ratio (alpha)w/U develops from unity at the bed inlet to a final value. The magnitude of this final value depends on the Biot number, whereas the length of this transition section is affected by the Peclet number. A new relation to lump the effective conductivity and the heat transfer coefficient at the wall into an overall heat transfer coefficient which depends on the values of the Biot number and the Peclet number is presented. This relation accounts for both the magnitude and the transition length of the ratio (alpha)w/U