AICHE Journal

Using the zeolite ZSM‐5, new technology has been developed for catalytically oligomerizing light olefins (C₃ to C₄) to gasoline (C₅ to C₁₀) and diesel (C₁₀ to C₂₀) range product. This reaction produces product constrained by both the shape selectivity of the zeolite catalyst and the thermodynamics governing the oligomerization reaction.

Many subgrid drag modifications have been put forth to account for the effect of small unresolved scales on the resolved mesoscales in dense gas‐particle flows. These subgrid drag modifications significantly differ in terms of their dependencies on the void fraction and the particle slip velocity. We, therefore, compare the hydrodynamics of a three‐dimensional bubbling fluidized bed computed on a coarse grid using the drag correlations of the groups of (i) EMMS, (ii) Kuipers, (iii) Sundaresan, (iv) Simonin, and the homogenous drag law of (v) Wen and Yu with fine grid simulations for two different superficial gas velocities. Furthermore, we present an (vi) alternative approach, which distinguishes between resolved and unresolved particle clusters revealing a grid and slip velocity dependent heterogeneity index. Numerical results are analyzed with respect to the time‐averaged solids volume fraction and its standard deviation, gas and solid flow patterns, bubble size, number density, and rise velocities. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4077–4099, 2013

The mixing due to helical flows in curved micro channels is investigated. A new chaotic mixing mechanism is presented relying on alternately switching between different flow patterns exhibiting four Dean vortices. Flow patterns and interfacial stretching factors are numerically computed for various Dean numbers. For experimental studies a prototype of a chaotic mixer with curved channels was fabricated. The experimental evaluation of the mixing performance corroborates the numerical prediction: the mixing performance found for Dean numbers above 140 is qualitatively different from that at lower Dean numbers; the periodic switching between different vortex patterns leads to efficient mixing, manifesting itself in an exponential growth of interfacial area. In addition to the studies on mixing, residence‐time distributions in the mixing channel are computed numerically. These investigations indicate that due to mass‐transfer enhancement originating from the transversal redistribution of matter in the chaotic flow, hydrodynamic dispersion is substantially reduced relative to a straight channel. © 2004 American Institute of Chemical Engineers AIChE J, 50: 2297–2305, 2004

The axial dispersion coefficient in a fluid in laminar flow in a tube is generally smaller in a curved tube than in a straight tube, because of the enhacement of lateral transport by secondary flows. In this paper the extent of this reduction is computed using Horn's modification of Aris's method of moments. Dimensional analysis suggests that the most convenient form in which to represent the results is that obtained empirically by Trivedi and Vasudeva. The results presented here cover the whole laminar flow regime, and show the region of applicability of previously computed dispersion coefficients to be limited. The computed dispersion coefficients agree well with previously reported experimental results for small Schmidt numbers; however, discrepancies are present at large Schmidt numbers. Possible causes are discussed.

Flatter velocity profiles and more uniform thermal environents are extremely desirous factors for improved performance in flow reactors and heat exchangers. One means of achieving this in laminar flow systems is to use mixers and flow inverters. These improve performance but at higher initial and operating costs. This paper introduces a new and more effective device for flow inversion which is achieved by changing the direction of centrifugal force in helically‐coiled tubes. Transient response experiments carried out under the conditions of both negligible and significant molecular diffusion reveal drastic narrowing of the residence time distribution (RTD). The effectiveness of the present device can be assessed by the fact that even at a Dean number of 3 the value of dispersion number as low as 0.0013 is obtained under the condition of significant diffusion, and in the case of negligible diffusion the value of dimensionless time at which the first element of tracer appears at the outlet is as high as 0.85.

Helical flows are investigated in structured microchannels with regard to micromixing by means of computational fluid dynamics (CFD). In the case of bas‐relief structured channels the numerical results are found to be in good agreement with experimental findings. The magnitude of the transverse flow is computed for various Reynolds numbers and for different geometries including channels with bas‐relief structures on two opposite walls. Transverse flows in structured microchannels are compared to secondary flow patterns in curved square channels. The corresponding helical flows are analyzed for Dean numbers ranging from 1 to 900. Special attention is paid to the occurrence of additional vortices close to the center of the outer channel wall. Based on the results a new type of micromixer is proposed that relies on the transition of the secondary flow pattern from two to four vortices. © 2004 American Institute of Chemical Engineers AIChE J, 50: 771–778, 2004

A computational study has been performed to determine the rates of mixing in a curved square duct at low Reynolds numbers of interest to microfluidic applications. Two flow streams with inlet scalar concentrations of zero and unity in the two halves of a duct perpendicular to the plane of curvature were allowed to mix by convection and diffusion. Concentration distributions and unmixedness coefficients are presented for several Reynolds and Schmidt numbers and are compared with values for a straight channel of equivalent length. It is seen that for large Schmidt number fluids, mixing is considerably enhanced at moderately low Reynolds numbers (Re ∼ 10), but is not enhanced at Reynolds numbers of the order of 0.1. © 2004 American Institute of Chemical Engineers AIChE J, 50: 2359–2368, 2004

Hollow fiber MFI zeolite membranes were modified by catalytic cracking deposition of methyldiethoxysilane to enhance their H₂/CO₂ separation performance and further used in high temperature water gas shift membrane reactor. Steam was used as the sweep gas in the MR for the production of pure H₂. Extensive investigations were conducted on MR performance by variations of temperature, feed pressure, sweep steam flow rate, and steam‐to‐CO ratio. CO conversion was obviously enhanced in the MR as compared with conventional packed‐bed reactor (PBR) due to the coupled effects of H₂ removal as well as counter‐diffusion of sweep steam. Significant increment in CO conversion for MR vs. PBR was obtained at relatively low temperature and steam‐to‐CO ratio. A high H₂ permeate purity of 98.2% could be achieved in the MR swept by steam. Moreover, the MR exhibited an excellent long‐term operating stability for 100 h in despite of the membrane quality. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3459–3469, 2015

Defect‐free Pd membranes were prepared by an electroless plating technique on porous stainless‐steel tubes. The effective surface of Pd membranes was up to 75 cm². The helium flux was not detected at room temperature and pressure difference of 3 atm. At 350°C hydrogen permeances of up to \documentclass{article}\pagestyle{empty}\begin{document}$ 8\ m^3 /(m^2 \cdot \overrightarrow h \cdot atm^{0.5}) $\end{document} were obtained. A t a pressure difference of 1 atm and 350°C, selectivity coefficients as high as J(H₂/JN₂) = 5,000 were observed. Fluxes of gases, impermeable through the Pd, were quantitatively described as a combination of Knudsen diffusion and viscous flow through the gaps created in the Pd layer at high temperatures. The stability of the prepared membranes at temperatures up to 700°C was investigated. The membranes were stable at 350°C over a period of 1,100 h, with no significant changes in the steady‐state hydrogen flux and with a recrystallization texture and aggregation of Pd grains.

The permeation behavior of the high‐flux asymmetric membrane differs from that of the conventional symmetric membrane. A calculation method for predicting the gas separation performance of a permeator with asymmetric membrane is presented. The model takes into account the permeate pressure drop and is applicable to both hollow‐fiber and spiral‐wound modules. The effect of permeate‐feed flow pattern on module performance is analyzed. It is shown that for the high‐flux asymmetric membrane, the countercurrent flow pattern is not necessarily always the preferred operating mode. The mathematical model is verified by large‐scale field pilot‐plant experiments for helium recovery from natural gas using large hollow‐fiber modules (220 m²/unit).