Chem Eng Sci Research Articles

CFD study of weeping rate in the rectangular sieve trays

Sieve trays are widely used fractionating devices existing in separation and purification industries. The weeping phenomenon that has a critical effect on the efficiency of tray towers was studied by a numerical model in a rectangular sieve tray. An Eulerian–Eulerian Computational Fluid Dynamics (CFD) method was performed. The obtained CFD results are in a good agreement with the experimental data and empirical correlation of Zuiderweg [Zuiderweg FJ. Sieve trays: a view on the state of the art, Chem Eng Sci 1982;37:1441–64] in terms of weeping rate and pressure drop, respectively. Furthermore, several hydraulic parameters are properly predicted by the model.

A simple confined impingement jets mixer for flash nanoprecipitation

Johnson and Prud'homme (2003. AICHE J 49:2264-2282) introduced the confined impingement jets (CIJ) mixer to prepare nanoparticles loaded with hydrophobic compounds (e.g., drugs, inks, fragrances, or pheromones) via flash nanoprecipitation (FNP). We have modified the original CIJ design to allow hand operation, eliminating the need for a syringe pump, and we added a second antisolvent dilution stage. Impingement mixing requires equal flow momentum from two opposing jets, one containing the drug in organic solvent and the other containing an antisolvent, typically water. The subsequent dilution step in the new design allows rapid quenching with high antisolvent concentration that enhances nanoparticle stability. This new CIJ with dilution (CIJ-D) mixer is a simple, cheap, and efficient device to produce nanoparticles. We have made 55 nm diameter β-carotene nanoparticles using the CIJ-D mixer. They are stable and reproducible in terms of particle size and distribution. We have also compared the performance of our CIJ-D mixer with the vortex mixer, which can operate at unequal flow rates (Liu et al., 2008. Chem Eng Sci 63:2829-2842), to make β-carotene-containing particles over a series of turbulent conditions. On the basis of dynamic light scattering measurements, the new CIJ-D mixer produces stable particles of a size similar to the vortex mixer. Our CIJ-D design requires less volume and provides an easily operated and inexpensive tool to produce nanoparticles via FNP and to evaluate new nanoparticle formulation.

Particle image velocimetry in a foam-like porous structure using refractive index matching: a method to characterize the hydrodynamic performance of porous structures

We present a method to measure two-dimensional velocity fields inside an artificial foam-like porous structure using particle image velocimetry and a refractive index matching technique to avoid optical distortion. The porous structure is manufactured by stereolithography with the epoxy resin WaterShed® XC 11122 as solid material, and anisole is used as refractive index-matched fluid. It was found that the direction of build-up of the stereolithographic structure plays an important role for the quality of the recorded images. The velocity fields measured in this study and the turbulent statistics derived thereof allow to characterize the hydrodynamic performance of the artificial foam-like structure and clarify the mechanisms of mixing. Results from this study compare well to results from a large eddy simulation reported by Hutter et al. (Chem Eng Sci 66:519–529, 2011b) and hence reinforce these simulations.

Flow-induced crystallization of high-density polyethylene: the effects of shear and uniaxial extension

The effects of shear, uniaxial extension and temperature on the flow-induced crystallization of two different types of high-density polyethylene (a metallocene and a ZN-HDPE) are examined using rheometry. Shear and uniaxial extension experiments were performed at temperatures below and well above the peak melting point of the polyethylenes in order to characterize their flow-induced crystallization behavior at rates relevant to processing (elongational rates up to 30 s − 1 and shear rates 1 to 1,000 s − 1 depending on the application). Generally, strain and strain rate found to enhance crystallization in both shear and elongation. In particular, extensional flow was found to be a much stronger stimulus for polymer crystallization compared to shear. At temperatures well above the melting peak point (up to 25°C), polymer crystallized under elongational flow, while there was no sign of crystallization under simple shear. A modified Kolmogorov crystallization model (Kolmogorov, Bull Akad Sci USSR, Class Sci, Math Nat 1:355–359, 1937) proposed by Tanner and Qi (Chem Eng Sci 64:4576–4579, 2009) was used to describe the crystallization kinetics under both shear and elongational flow at different temperatures.

Hydrodynamics of gas–liquid Taylor flow in rectangular microchannels

The effect of fluid properties and operating conditions on the generation of gas–liquid Taylor flow in microchannels has been investigated experimentally and numerically. Visualisation experiments and 2D numerical simulations have been performed to study bubble and slug lengths, liquid film hold-up and bubble velocities. The results show that the bubble and slug lengths increase as a function of the gas and liquid flow rate ratios. The bubble and slug lengths follow the model developed by Garstecki et al. (Lab chip 6:437–446, 2006) and van Steijn et al. (Chem Eng Sci 62:7505–7514, 2007), however, the model coefficients appear to be dependent on the liquid properties and flow conditions in some cases. The ratio of the bubble velocity to superficial two-phase velocity is close to unity, which confirms a thin liquid film under the assumption of a stagnant liquid film. Numerical simulations confirm the hypothesis of a stagnant liquid film and provide information on the thickness of the liquid film.

Numerical study of hydrogen production by the sorption-enhanced steam methane reforming process with online CO2 capture as operated in fluidized bed reactors

A three-dimensional (3D) Eulerian two-fluid model with an in-house code was developed to simulate the gas-particle two-phase flow in the fluidized bed reactors. The CO2 capture with Ca-based sorbents in the steam methane reforming (SMR) process was studied with such model combined with the reaction kinetics. The sorption-enhanced steam methane reforming (SE-SMR) process, i.e., the integration of the process of SMR and the adsorption of CO2, was carried out in a bubbling fluidized bed reactor. The very high production of hydrogen in SE-SMR was obtained compared with the standard SMR process. The hydrogen molar fraction in gas phase was near the equilibrium. The breakthrough of the sorbent and the variation of the composition in the breakthrough period were studied. The effects of inlet gas superficial velocity and steam-to-carbon ratio (mass ratio of steam to methane in the inlet gas phase) on the reactions were studied. The simulated results are in agreement with the experimental results presented by Johnsen et al. (2006a, Chem Eng Sci 61:1195–1202).

Continuous precipitation of L‐asparagine monohydrate in a micromixer: Estimation of nucleation and growth kinetics

AbstractAn experimental setup based on a static micromixer is used to determine nucleation and growth kinetics of L‐asparagine monohydrate precipitated via antisolvent addition. Mixing in static micromixers was characterized previously using competitive‐parallel reactions and computational fluid dynamics (Lindenberg et al., Chem Eng Sci. 2008;63:4135–4149). In this work, the mixer setup is used to determine nucleation and growth kinetics of L‐asparagine at high supersaturations, i.e., true kinetics which are not affected by transport limitations. The method is based on measuring the particle size distribution obtained at different residence times. A population balance equation model of the process is used for the design of a continuous precipitation process. Finally, an analysis of the characteristic time scales of nucleation, growth, and mixing shows that, under the conditions in this study, mixing is much faster than precipitation and that the two processes can be decoupled. © 2010 American Institute of Chemical Engineers AIChE J, 2011

A new framework and a simpler method for the development of batch crystallization recipes

AbstractWe use a new generalized dimensionless model of a seeded batch crystallization process to compare results from four crystal growth rate/concentration trajectories: a numerically‐computed optimal trajectory, a constant growth rate trajectory, a trajectory proposed by Mullin and Nyvlt [Chem Eng Sci. 1971;26:369–377] that can be calculated without a kinetic model, and a linear concentration‐time trajectory. Because the model is generalized and dimensionless it is not specific to any particular solute–solvent system. We show that if seed properties are good, all trajectories achieve a good result, whereas if seed properties are poor all trajectories achieve a poor result. If seed properties are intermediate, the linear concentration trajectory performs much worse than the other trajectories. On the basis of this, we conclude: (1) Linear trajectories and natural cooling trajectories are poor and should be avoided. When evaluating the benefit derived from an optimization of a temperature/saturation concentration trajectory, the appropriate benchmark should be the Mullin‐Nyvlt trajectory, which typically performs much better than the linear trajectory and can be calculated knowing only the mass of seeds at the beginning of the batch. (2) Changing seed properties is more likely to improve batch crystallizer performance than optimizing the growth/saturation concentration trajectory. (3) For rapid process development, the most reasonable approach is to use the Mullin‐Nyvlt trajectory and seek to improve process performance by adjusting seed properties. © 2010 American Institute of Chemical Engineers AIChE J, 2011

Liquid phase diffusion of branched alkanes in silicalite

AbstractThis work provides a new mass transfer model based on the Maxwell–Stefan theory, especially adapted to represent adsorbed phase multicomponent diffusion at high‐adsorbent loading. In our model—contrarily to the well‐known model developed by Krishna et al. (Chem Eng Sci. 1990;45:7:1779–1791; Gas Sep Purif. 1993;7:91–104; J Phys Chem B. 2005;109:6386–6396)—the hypothesis that the micropores are saturated does not imply a dependency between the adsorbed phase diffusion coefficients. Experimental liquid phase breakthrough curves of 2‐methylpentane (2MP), 3‐methylpentane (3MP), 2,3‐dimethylbutane (23DMB), and 2,2‐dimethylbutane (22DMB) were measured at 458 K in silicalite. The self‐diffusion coefficients and Langmuir parameters of the different species were determined using binary exchange breakthrough curves. The Maxwell–Stefan diffusion coefficients obtained for the different isomers are in the order D3MP,nc+1 > D2MP,nc+1 ≫ D23DMB,nc+1, and vary between 4 × 10−15 m2 s−1 for 3MP to 6 × 10−16 m2 s−1 for 23DMB. The 22DMB diffusion coefficient is so low that it could not be estimated (the quantity of 22DMB entering silicalite during the experiment is not significant). The model was then validated by comparing experimental breakthrough curves at different feed concentrations and simulations using the independently estimated parameters. Even though the diffusion coefficients of the different isomers vary by one order of magnitude, the agreement between simulated and experimental curves is very satisfactory, showing the good predictive power of our model. © 2010 American Institute of Chemical Engineers AIChE J, 2011

Steam reforming of methane in a tapered membrane – Assisted fluidized – Bed reactor: Modeling and simulation

A compartment model was developed to describe the flow pattern of gas within the dense zone of a tapered membrane-assisted fluidized-bed reactor (TMAFBR), in the bubbling mode of operation for steam reforming of methane under wall heat flux. The parameters of the developed model (i.e., number of compartments for the bubble and emulsion phases) were determined using the experimental data reported elsewhere [Adris AM, Lim CJ, Grace JR. The fluidized bed membrane reactor system: a pilot scale experimental study. Chem Eng Sci 1994; 49:5833–43.] and good agreements were obtained between model predictions and corresponding experimental data. The developed model was then utilized to predict the behavior of TMAFBR under various operating and design conditions. Moreover, the influences of tapered angle, bed operating temperature and pressure, and feed temperature on the methane conversion and the total yield of hydrogen were carefully investigated. Furthermore, the performance capability of the TMAFBR was compared with that of a columnar one under identical operating conditions.

A novel search framework for multi‐stage process scheduling with tight due dates

AbstractThis article improves the original genetic algorithm developed by He and Hui (Chem Eng Sci. 2007; 62:1504–1527) and proposes a novel global search framework (GSF) for the large‐size multi‐stage process scheduling problems. This work first constructs a comprehensive set of position selection rules according to the impact factors analysis presented by He and Hui (in this publication in 2007), and then selects suitable rules for schedule synthesis. In coping with infeasibility emerging during the search, a penalty function is adopted to force the algorithm to approach the feasible solutions. The large‐size problems with tight due dates are challenging to the current solution techniques. Inspired by the gradient used in numerical analysis, we treat the deviation existing among the computational tests of the algorithm as evolutionary gradient. Based on this concept, a GSF is laid out to fully utilize the search ability of the current algorithm. Numerical experiments indicate that the proposed search framework solves such problems with satisfactory solutions. © 2010 American Institute of Chemical Engineers AIChE J, 2010

A novel approach to scheduling multipurpose batch plants using unit‐slots

AbstractSeveral models for scheduling multipurpose batch plants exist in the literature. The models using unit‐specific event points have shown better solution efficiency on various literature examples. This article presents a novel approach to scheduling multipurpose batch plants, which uses unit‐slots instead of process‐slots to manage shared resources such as material storage. We develop two slightly different models that are even more compact and simpler than that of Sundaramoorthy and Karimi, Chem Eng Sci. 2005;60:2679–2702. Although we focus on material as a shared resource, our multi‐grid approach rationalizes, generalizes, and improves the current multi‐grid approaches for scheduling with shared resources. Our models allow nonsimultaneous transfers of materials into and out of a batch. We show by an example that this flexibility can give better schedules than those from existing models in some cases. Furthermore, our approach uses fewer slots (event‐points) on some examples than even those required by the most recent unit‐specific event‐based model. Numerical evaluation using literature examples shows significant gains in solution efficiency from the use of unit‐slots except where the number of unit‐slots required for the optimal solution equals that of process slots. We also highlight the importance of constraint sequencing in GAMS implementation for evaluating mixed‐integer linear programming based scheduling models fairly. © 2009 American Institute of Chemical Engineers AIChE J, 2010

Modeling of turbulent precipitation: A transported population balance‐PDF method

AbstractTurbulent precipitation is a complex problem, whose mathematical description of precipitation requires a coupling of fluid dynamics with the population balance equation (PBE). In the case of turbulent flow, this coupling results in unclosed equations due to the nonlinear nature of precipitation kinetics. In this article, we present a methodology for modeling turbulent precipitation using the concept of the transported probability density function (PDF) in conjunction with a discretized PBE, simulated via a Lagrangian stochastic method. The transported PBE‐PDF approach resolves the closure problem of turbulent precipitation for arbitrarily complex precipitation kinetics, while retrieving the full particle size distribution (PSD). The method is applied to the precipitation of BaSO4 in a turbulent pipe flow and comparisons are made with the experimental results of Baldyga and Orciuch (Chem Eng Sci. 2001;56:2435‐2444) showing excellent agreement, while insight is drawn into the mechanisms that determine the evolution of the product PSD. © 2009 American Institute of Chemical Engineers AIChE J, 2010

Bubble lengths in the gas–liquid Taylor flow in microchannels

Experimental results of measurements of the bubble and slug lengths in Taylor (slug) flow are presented. The experiments were carried out using 3 different straight microchannels (microreactor with square cross-section made of polydimethyloxosilane (PDMS); microreactor with circular cross-section made of glass; microreactor with rectangular cross-section made of polyethylene terephthalate modified by glycol (PETg)) and 4 different liquids (water, ethanol propanol and heptane). The results have been compared with the available literature correlations. It is concluded, that the values obtained from the correlation proposed by Laborie et al. [Laborie, S., Cabassud, C., Durant-Bourlier, L., Laine, J.M., 1999. Characterization of gas–liquid two-phase flow inside capillaries. Chem Eng Sci 54, 5723–5735] do not agree with the results of measurements, while the agreement of these results with the predictions obtained using the correlation proposed by Qian and Lawal [Qian, D., Lawal, A., 2006. Numerical study on gas and liquid slugs for Taylor flow in a T-junction microchannel. Chem Eng Sci 61, 7609–7625] is good. New, corrected values of the pre-exponential constant and the exponents in the Qian and Lawal [Qian, D., Lawal, A., 2006. Numerical study on gas and liquid slugs for Taylor flow in a T-junction microchannel. Chem Eng Sci 61, 7609–7625] correlation are proposed.

A quantitative sub-grid air entrainment model for bubbly flows – plunging jets

The accurate prediction of air entrainment is critical in simulating various important multiphase (air/water) flows. In this paper, we present a sub-grid air entrainment model that quantitatively predicts the rate of air entrainment and subsequent disperse bubbly flow for a plunging jet. The derivation of this model is based on the two-stage (i.e., low and high liquid jet velocity) air entrainment mechanisms suggested by Sene [Sene KJ. Air entrainment by plunging jets. Chem Eng Sci 1988;43(10):2615–23]. This model was validated against extensive experimental data for water jets in air over a wide range of liquid velocities (from around 1 to 10 m/s) for the total rate of air entrainment. It was then implemented into an Eulerian/Eulerian two-fluid computational multiphase fluid dynamics (CMFD) model, wherein the liquid and the bubbles are modeled as two distinct continua. This multiphase model, supplemented by the new sub-grid air entrainment model, was used to predict the void fraction distribution underneath plunging water jets at different depths and water jet velocities. It was found that this approach yields results that match the experimental observations very well.

Incremental identification of fluid multi-phase reaction systems

Despite their importance, rigorous process models are rarely available for reaction and especially multi-phase reaction systems. The high complexity of these systems, which is due to the superposed effects of mass transfer and intrinsic reaction, is the major barrier for the development of process models. A methodology that allows thesystematic decomposition of mass transfer and chemical reaction and thus enables the efficient identification of multi-phase reaction systems is proposed in this work. The method is based on the so-called Incremental Identification Method, recently presented by Brendel et al., Chem Eng Sci. 2006;61:5404-5420. The method allows to easily test the identifiability of a system based on the available measurement data. If identifiability is given, the intrinsic reaction kinetics can be identified in a sound and numerically robust manner. These benefits are illustrated using a simulated 2-phase enzyme reaction system. © 2009 American Institute of Chemical Engineers AIChE J, 2009

Supercritical fluid extraction of carqueja ( Baccharis trimera) oil: Process parameters and composition profiles

Carqueja ( Baccharis trimer a Less) is a common plant native from South America. In this work, the global yield ( X 0) of carqueja oil, extracted by means of supercritical CO 2 at 30 and 40 °C and from 100 to 300 bar, was investigated. The maximum solvent flow rate employed was 8.33 gCO 2/min, with extraction times up to 6 h. The global yields varied from 0.34 to 2.07% (w/w). Additionally, the supercritical process was compared with the organic solvent extraction in terms of yield and oil composition profile. Carqueja extracts were analyzed by gas chromatography–mass spectrometry (GC–MS) and 14 different components were identified. The mathematical models proposed by Tan and Liou [Tan, C. and Liou, D., 1989, Modeling of desorption at supercritical conditions, AIChE J, 35(6): 1029–1031], Brunner [Brunner, G., 1994, Gas Extraction: An Introduction to Fundamentals of Supercritical Fluids and the Application to Separation Processes (Springer, New York)], Sovová [Sovová, H., 1994, Rate of the vegetable oil extraction with supercritical CO 2. Modelling of extraction curves, Chem Eng Sci, 49(3): 409–414] and Esquível et al. [Esquível, M.M., Bernardo-Gil, M.G. and King, M.B., 1999, Mathematical models for supercritical extraction of olive husk oil, J Supercritical Fluids, 16: 43–58] were used for the description of the kinetic aspects of the supercritical process. The modeled results showed good agreement with experimental data especially for the constant extraction rate period (CER).

Modelling of mass transfer in gas–liquid stirred tanks agitated by Rushton turbine and CD-6 impeller: A scale-up study

A combined computational fluid dynamics (CFD) and population balance model (PBM) approach has been applied to the simulation of gas–liquid stirred tanks agitated by (i) a Rushton turbine or (ii) a CD-6 impeller, operating at aeration numbers from 0.017 to 0.038. The multiphase simulations were realised via an Eulerian–Eulerian two-fluid model and the drag coefficient of spherical and distorted bubbles was modelled using the Ishii–Zuber equations. The effect of the void fraction on the drag coefficient was modelled using the correlation by Behzadi et al. [Behzadi, A., Issa, R.I. and Rusche, H., 2004, Modelling of dispersed bubble and droplet flow at high phase fractions, Chem Eng Sci, 59: 759–770]. The local bubble size distribution was obtained by solving the PBM using the quadrature method of moments (QMOM). The local k L a was estimated using both the Higbie penetration theory and the surface renewal model. The predicted gas–liquid hydrodynamics, local bubble sizes and dissolved oxygen concentration were in good agreement with experimental measurements reported in the literature. A slight improvement in the prediction of the aerated power number was obtained using the non-uniform bubble size distribution resulting from the coupled CFD–PBM simulation. Evaluation of the prospective scale-up approaches indicates a higher probability of maintaining a similar level of mass transfer in a larger tank by keeping the P g / V and VVM constant. Considering its predictive capability, the method outlined in this work can provide a useful scale-up evaluation of gas–liquid stirred tanks.

Compartment model for steam reforming of methane in a membrane-assisted bubbling fluidized-bed reactor

Abstract A compartment model was developed to describe the flow pattern of gas within the dense zone of a membrane-assisted fluidized-bed reactor (MAFBR), in the bubbling mode of operation for steam reforming of methane both with (adiabatic) and without (isothermal) entering oxygen. Considering such a flow pattern and using the experimental data reported elsewhere [Roy S, Pruden BB, Adris AM, Grace JR, Lim CJ. Fluidized-bed steam methane reforming with oxygen input. Chem Eng Sci 1999; 54:2095–2102.], the parameters of the developed model (i.e., number of compartments for the bubble and emulsion phases) were determined and fair agreements were obtained between model predictions and experimental data. The developed model was utilized to describe the behavior of an industrial scale adiabatic and isothermal MAFBR. Moreover, the influences of various operating and design parameters such as steam-to-methane ratio (SMR), oxygen-to-methane ratio (OMR), operating temperature and pressure, and the number of hydrogen membrane tubes on the performance capability of the MAFBR were investigated. Furthermore, the performance capability of the MAFBR was optimized subject to the various operating and design constraints, including 1 ≤ SMR ≤ 4 and 500 ≤ T ≤ 1250 K, in the bubbling regime.

Optimum design of cooling water systems for energy and water conservation

Re-circulating cooling water systems (RCWSs) are widely used to reject waste process heat to the environment, conserve fresh water and reduce thermal pollution relative to once-through systems. Research on RCWS has mostly focused on individual components, cooling tower and heat-exchanger network. Kim and Smith [Kim, J.K. and Smith, R., 2001, Cooling water system design, Chem Eng Sci, 56(12): 3641–3658] developed a grass-root design method of RCWS (KSD). In this paper, the KSD method is expanded and a comprehensive simulation model of RCWS is developed accounting for interaction between cooling tower and heat-exchanger network. Regarding this model, a modern grass-root design method of RCWS, we call it Advanced Pinch Design (APD), based on combined pinch technology and mathematical programming is developed for minimum cost achievement. Having considered cycle water quality through introducing ozone treatment technology, APD methodology is further improved. This technique that we call Enhanced Cooling Water System Design (ECWSD), as the APD supplementary methodology, is provided water and energy conservation, minimum cost and environmental impacts. Related coding in MATLAB version 7.1 is developed for the illustrative example to get optimal values in RCWS design method computations. Finally the results of the introduced grass-root design methodologies, APD and ECWSD, are compared with KSD.