Flow simulations in arbitrarily complex cardiovascular anatomies – An unstructured Cartesian grid approach

Image guided computational fluid dynamics is attracting increasing attention as a tool for refining in vivo flow measurements or predicting the outcome of different surgical scenarios. Sharp interface Cartesian/Immersed-Boundary methods constitute an attractive option for handling complex in vivo geometries but their capability to carry out fine-mesh simulations in the branching, multi-vessel configurations typically encountered in cardiovascular anatomies or pulmonary airways has yet to be demonstrated. A major computational challenge stems from the fact that when such a complex geometry is immersed in a rectangular Cartesian box the excessively large number of grid nodes in the exterior of the flow domain imposes an unnecessary burden on both memory and computational overhead of the Cartesian solver without enhancing the numerical resolution in the region of interest. For many anatomies, this added burden could be large enough to render comprehensive mesh refinement studies impossible. To remedy this situation, we recast the original structured Cartesian formulation of Gilmanov and Sotiropoulos [Gilmanov A, Sotiropoulos F. A hybrid Cartesian/immersed boundary method for simulating flows with 3D, geometrically complex, moving bodies. J Comput Phys 2005;207(2):457–92] into an unstructured Cartesian grid layout. This simple yet powerful approach retains the simplicity and computational efficiency of a Cartesian grid solver, while drastically reducing its memory footprint. The method is applied to carry out systematic mesh refinement studies for several internal flow problems ranging in complexity from flow in a 90° pipe bend to flow in an actual, patient-specific anatomy reconstructed from magnetic resonance images. Finally, we tackle the challenging clinical scenario of a single-ventricle patient with severe arterio-venous malformations, seeking to provide a fluid dynamics prospective on a clinical problem and suggestions for procedure improvements. Results from these simulations demonstrate very complex cardiovascular flow dynamics and underscore the need for high-resolution simulations prior to drawing any clinical recommendations.     

Pilot tests of a new domestic ZhKD catalyst for the dehydrogenation of isoamilenes into isoprene

At the Synthetic Rubber Plant of OAO Nizhnekamskneftekhim, the dehydrogenation of isoamilenes into isoprene is currently performed on KDOM-08 catalysts with an insufficiently high yield of isoprene throughout the period of its industrial operation. More stable and highly active catalysts must be used to make the process more efficient. Under Russian Federation Government Decree No. 218, ZhKD-1 and ZhKD-2 iron-potassium catalysts have been developed by improving their formulas and optimizing their phase composition through selecting the proper ratio of initial compounds. To evaluate the possibility of transitioning to the new domestically-produced iron-potassium catalysts, we have performed pilot tests of the ZhKD-1 and ZhKD-2 catalysts in the dehydrogenation of methylbutenes into isoprene in adiabatic flow fixed-bed reactors at the Synthetic Rubber Plant of OAO Nizhnekamskneftekhim. The KDOM-08 catalyst used in the amount of 25 t in reactor 1 of the first system is taken as a base for comparison. The ZhKD-1 and ZhKD-2 catalysts are loaded into parallel reactors 7 and 8 of the fourth system. The KDOM-08 catalyst is shown to operate more efficiently under industrial conditions at loads of 1.0–2.0 t/h for 1000–3000 h, after which its performance characteristics deteriorate due to its gradual deactivation. The ZhKD-1 and ZhKD-2 catalysts are substantially superior to their industrial analogues in isoprene yield. It has been found that the ZhKD-2 catalysts operate more efficiently at even longer runs (4000–5000 h) and feedstock flow rates of 1.0–2.0 t/h, and the ZhKD-1 catalysts exhibits better activity (30–33 %) and selectivity (87–92 %) at higher loads of 2.3–3.0 t/h for up to 5000 h. From our analysis of the catalysts’ operation over the last 1000 h, it follows that at the same process temperatures (619°C) and feedstock loads (2.5 t/h), the ZhKD-1 and ZhKD-2 catalysts operate at a lower steam dilution coefficient (6.1 t/t) than the KDOM-08 catalyst (6.8 t/t). The rebuilding of reactors 7 and 8 allows the loaded catalyst mass to be reduced from 25 to 17 t, thereby almost doubling the daily output of isoprene per ton of catalyst. It is obvious that higher activity and selectivity along with smaller loads makes the use of the ZhKD-1 and ZhKD-2 catalysts economically profitable.     

Upgrading the industrial process flowsheet for the dehydration of methylbutenes to isoprene. II: Analysis of plant test results and industrial implementation of the upgraded design

Pilot tests of technology for the dehydration of methylbutenes to isoprene are performed in a tworeactor system with an additional supply of an overheated gas into the interreactor space. The tests are performed on a pilot plant with two adiabatic reactors. The total volume of the catalyst charge is 60 dm³, the temperatures are 565–620°С, the contact time is 0.18–0.25 s, the raw material is diluted with steam in a weight ratio of С₅Н₁₀: Н₂О = 1.0: (6.0–30.0), and the excess pressure is 0.6–0.7 kgf/cm². The dependence of the isoprene concentration in the contact gas on the heat energy supplied by the raw material and steam is determined under conventional conditions of the process and in a pseudo-isothermal mode via an additional supply of overheated gas into the interreactor space. It is shown that the isoprene yield is increased by 10–12% by using the upgraded mode. The conditions for conducting the industrial process are determined based on the obtained results. After upgrading the design, tests are performed at the synthetic rubber factory of PAO Nizhnekamskneftekhim on a plant for the dehydrogenation of methylbutenes in the reactor with a doublelayer catalyst bed (nine tons per layer). The patterns established during the pilot tests generally prove to be true, but the selectivity of the process is reduced due to a number of design flaws. Corrective measures are outlined. Comparison of the experimental results and the calculated values confirm the accuracy of the mathematical model.     

Effect of cerium on the transformation and reactivity of hematite in a reducing atmosphere

As is well known, cerium compounds are widely used as promoters in iron-potassium catalysts for the dehydrogenation of alkylaromatic and olefinic hydrocarbons. To determine the mechanism and role of cerium oxide in formation of catalytically active phases (potrassium ferrites), we must first study the effect of cerium on transformation and reactivity of iron oxide, which comprises 50 to 80% of the catalyst and participates in ferrite formation. In this work, the thermal behavior of cerium oxalate and the Fe₂O₃-CeO₂ model system, the main components in the production of iron-potassium catalysts, is examined by X-ray diffraction, thermal analysis, particle size distribution analysis, low-temperature nitrogen adsorption, and temperature-programmed hydrogen reduction under heating in air. It is shown that cerianite exhibits greater lability than hematite. It is established that introducing cerium into hematite improves the reactivity of the Fe₂O₃-CeO₂ system, and the partial reduction of Fe₂O₃ takes place upon its heating. The results from this work will be used in developing new iron-potassium catalysts with enhanced catalytic activity in the dehydrogenation of isoamilenes into isoprene.     

Investigating the mechanism of the effect of cerium additives on the properties of the iron-potassium system-the active component of dehydrogenation catalysts of hydrocarbons Report 2

The properties of the Fe₂O₃-K₂O and Fe₂O₃-K₂O-CeO₂ model systems with weight ratios of 80 : 20 and 50 : 20: 30, respectively, are studied by means of thermal, magnetic, X-ray and dispersion analysis, and low-temperature nitrogen adsorption. It is found that the successive formation of mono- and polyferrite phase occurs during the interaction of iron oxide and potassium carbonate. It is proposed that the activity of the iron-potassium catalyst is proportional to the content of the surface monoferrite phase. It is found that introducing cerium into the iron-potassium system leads to a redistribution of potassium mono- and polyferrites in the ferrite phase, raising the proportion of monoferrite. Introducing cerium therefore promotes the activity of the catalyst system. The results from this study will be used to develop new iron-potassium catalysts with high catalytic activity in the dehydrogenation of isoamylenes into isoprene.     

Polymerization of propylene in liquid monomer using state-of-the-art high-performance titanium-magnesium catalysts

A comparative study of propylene polymerization in liquid monomer is performed under laboratory conditions using the IK-8-21 Ti-Mg catalyst designed at the Boreskov Institute of Catalysis and imported industrial catalysts (conditionally labeled TMC-1, -2, and -3). The activity and stereospecificity of the catalysts are estimated along with properties of the resulting polypropylene (granular composition and physicomechanical characteristics). It is shown that the IK-8-21 catalyst is not inferior to imported counterparts in terms of catalytic properties in the synthesis of polypropylene. The polypropylene powder formed on IK-8-21 is homogeneous and has good morphology. The physicomechanical characteristics of polypropylene synthesized on the domestic IK-8-21 catalyst are similar to those for polypropylene prepared with the imported TMK-1 catalyst.     

Pilot tests of the microspherical aluminochromium KDI-M catalyst for <Emphasis Type="Italic">iso</Emphasis>-butane dehydrogenation

Results from pilot tests of microspherical aluminochromium KDI-M catalyst mixed with IM-2201 in a large-scale unit (Nizhnekamskneftekhim) for iso-butane dehydrogenation are discussed. Compared to KDI catalyst, its modified analogue KDI-M is more active and selective; the optimized grain-size composition and mechanical strength ensures higher yields of iso-butylene and longer nonstop operation (up to 400 days) of the reactor unit.     

Variation of heavy oil composition during thermolysis with the addition of kerosene fraction of hydrocracking in flow reactor

Thermolysis experiments of heavy oil have been carried out in flow reactor with the addition of 2.5–5.0% kerosene fraction of hydrocracking of vacuum gas oil prepared from conventional oil. It has been shown that the additive increases the degree of heavy oil conversion, restricts coke formation in the products of thermolysis as compared to the process without the additive, and provides a decrease in the viscosity of the products. Thermolysis conditions with the addition of kerosene fraction in flow reactor, which give liquid products in more than 97% yield and the viscosity of less than 75?mm²/s, have been determined in the case of the heavy oil of Ashal’ chinskoe deposit.     

The deactivating effect of carbon dioxide on iron-oxide catalyst in the dehydrogenation of methylbutenes

Characteristics of the most energy-intensive second stage of the two-stage production of isoprene from isopentane, i.e., dehydrogenation of methylbutenes are studied to improve the technical and economic performance of the process. The effect of the carbon dioxide formed during self-regeneration of the ironoxide catalyst (according to the reaction C_coke + 2H₂O → 2H₂ + CO₂) on the conversion of methylbutenes and selectivity with respect to isoprene is investigated. It is found that the presence of CO₂ in the reaction batch has a considerable effect on the conversion of methylbutenes; when the content of CO₂ in the raw material feed is 1.5 wt %, conversion of methylbutenes is reduced by 5–6%. It is demonstrated that CO₂ reversibly deactivates the catalyst and the catalyst activity is restored when its influx is discontined (the yield of isoprene returns gradually to its original value). The recovery rate depends on the concentration and duration of exposure to carbon dioxide. Treatment of the catalyst by steam in the absence of the reaction mixture leads to rapid regeneration of the catalyst. It is concluded that measures to continuously monitor CO₂ in the contact gas during the first stage of dehydrogenation, and to select the optimum modes (temperature, steam/raw materials ratio, etc.) for reducing carbon residue during the operation of iron-oxide catalyst in order to implement the two-stage technology for the dehydrogenation of methylbutenes (the main goal of which is to raise the conversion of methylbutenes to 35–40%) are of special importance.