Farhad Ein-Mozaffari

CFD modeling of the mixing of water in oil emulsions

Abstract A computational fluid dynamics (CFD) model was developed for the mixing of water in oil emulsion in a lab-scale mixing tank equipped with a Rushton turbine impeller. Multiple reference frames (MRF) technique, k – ɛ model, and Eulerian–Eulerian approach were employed to model the impeller rotation, turbulence, and multiphase flow, respectively. The droplet size distribution within the mixing tank was estimated by means of the population balance approach, which employs the discrete method to describe coalescence and breakage of water droplets. To validate the CFD model, the cumulative probability size distribution computed using the model was compared with the experimentally determined values reported in the literature. This validated CFD model was then utilized to explore the effects of the impeller speed, oil type, and volume fraction of water on the cumulative probability droplet size distribution, number density, and distribution of local volume fraction of the dispersed phase.

CFD analysis of mixing in thermal polymerization of styrene

Abstract Thermal polymerization of styrene in a lab-scale CSTR equipped with a pitched blade turbine impeller was simulated using a computational fluid dynamics (CFD) software package. The rotation of the reactor impeller was modeled using the multiple reference frames (MRF) technique. The path lines of the particles, released at the reactor inlet, were also generated to analyze the reaction progress throughout the reactor vessel. The effects of the impeller speed, the input–output stream locations and the residence time were investigated. The simulation showed the formation of a well-mixed region around the impeller and stagnant or slow moving fluids elsewhere in the reactor due to high viscosity of the polymer mass. The monomer conversion computed using the CFD model was in good agreement with that obtained from the CSTR model at low residence time. The input–output locations have a significant impact on the monomer conversion and the system homogeneity in the CSTR.

Effect of ozone pretreatment on hydrogen production from barley straw

Application of ozone technology to lignocellulosic biohydrogen production was explored with a barley straw. Ozone pretreatment effectively degraded the straw lignin and increased reducing sugar yield. A simultaneous enzyme hydrolysis and dark fermentation experiment was conducted using a mixed anaerobic consortium together with saccharification enzymes. Both untreated and ozonated samples produced hydrogen. Compared to the untreated group, hydrogen produced by the groups ozonated for 15, 30, 45 and 90 min increased 99%, 133%, 166% and 94%, respectively. Some inhibitory effect on hydrogen production was observed with the samples ozonated for 90 min, and the inhibition was on the fermentative microorganisms, not the saccharification enzymes. These results demonstrate that production of biohydrogen from barley straw, a lignocellulosic biomass, can be significantly enhanced by ozone pretreatment.

Macromixing hydrodynamic study in draft-tube airlift reactors using electrical resistance tomography

The present study summarizes results of mixing characteristics in a draft tube airlift bioreactor using ERT. This technique offers the possibility for noninvasive and nonintrusive visualization of flow fields in the bioreactor and has rarely been utilized previously to analyze operating parameters and mixing characteristics in this type of bioreactors. Several operating parameters and geometric characteristics were examined. In general, results showed that the increase in superficial gas velocity corresponds to an increase in energy applied and thus, to a decrease in mixing time. This generally corresponded to an increase in liquid circulation velocity and shear rate values. Bottom clearances and draft tube diameters affected flow resistance and frictional losses. The influence of sparger configurations on mixing time and liquid circulation velocity was significant due to their effect on gas distribution. However, the effect of sparger configuration on shear rate was not significant, with 20% reduction in shear rates using the cross-shaped sparger. Fluid viscosity showed a marked influence on both mixing times and circulation velocity especially in the coalescing media of sugar and xanthan gum (XG) solutions. Results from this work will help to develop a clear pattern for operation and mixing that can help to improve several industrial processes, especially the ones related to emerging fields of technology such as the biotechnology industry.

Recent Achievements in Combination of Ultrasonolysis and Other Advanced Oxidation Processes for Wastewater Treatment

A review of recent advancements in the combination of ultrasonolysis with other advanced oxidation processes is studied. This study is based on the recent achievements and developments in the field of water and wastewater treatment using ultrasonic irradiation by means of acoustic cavitation with other advanced oxidation technologies. The most important parameter in combined methods is the synergetic effect which is a variable relative to the type of contaminant(s) and other physicochemical properties. The synergetic effect has the key role in process intensification so that a higher synergetic effect provides a higher intensified process efficiency. Results showed that sonolysis is a method that can increase the synergetic percentage in lowering fixed and even operating cost of the wastewater remediation. The combination of photolysis, photocatalysis, Fenton, photo-Fenton, and ozonation processes with ultrasonolysis results in hydroxyl radical production and therefore, the degradation of the organic chemicals. The effect of various parameters on the efficiency of combined processes is investigated in the present study. The combined process performance and the synergetic effect depend on the operating conditions and wastewater characteristics. The synergetic effect can enhance the degradation of organic compounds up to 400%. For example, it has been proven that the sonophotocatalytic process shows over 80% improvement with respect to the photocatalysis alone.

Characteristics of Local Flow Dynamics and Macro-Mixing in Airlift Column Reactors for Reliable Design and Scale-Up

There has been tremendous development within mixing operations in industry. Incomplete knowledge of this process caused serious economic losses to process industries. For optimum yields and the economic potential that goes with better understanding of mixing, research in this field continues to grow. The major forms of mixing in industry are either by mechanical or pneumatic agitation. Airlift bioreactors achieve mixing through pneumatic agitation and have gained attention over two decades for their fluid dynamic characteristics and low power consumption. It has been widely applied in bioprocess industries for production of biochemicals, to wastewater treatment in which the performance of this reactor has been overwhelming with respect to its production levels as compared to the conventional mechanical agitation.In this review, mixing through mechanical and pneumatic agitation is compared. An extensive literature is distilled from various investigators on the hydrodynamics and mixing characteristics of airlift bioreactors. This review has emphasis on factors that affect mixing such as the geometrical parameters of the vessel, gas flow rate, properties of the liquid medium, sparger design and measuring techniques employed. In an attempt to understand process related issues, sophisticated advances in the measuring techniques provides more insight into mixing in this reactor. Thus extensive correlations have been proposed by various investigators to predict the hydrodynamic and mixing parameters. Some design modifications proposed by several scholars have also been reviewed.

EXPERIMENTAL AND NUMERICAL STUDIES ON MIXING OF YIELD-PSEUDOPLASTIC FLUIDS WITH A COAXIAL MIXER

An anchor coaxial mixer is a combination of an anchor impeller and a central impeller. The central and anchor impellers provide the shear and bulk flows within the mixing tank, respectively. The agitation efficiency is governed by the proper design of this mixing system. In this study, the effects of the central impeller speed, anchor impeller speed, operation mode, and the speed ratio of an anchor-Scaba coaxial impeller were evaluated on its power consumption, mixing time, and flow pattern using electrical resistance tomography (ERT) and computational fluid dynamics (CFD). An ERT system with a five-plane assembly of peripheral sensing rings, each containing 16 stainless steel electrodes, was utilized to measure the mixing time for this coaxial mixer. The sliding mesh (SM) technique in the CFD analysis with the modified Herschel-Bulkley model was applied to simulate the impeller rotation and the rheological behavior of the xanthan gum solution (a pseudoplastic fluid possessing yield stress). To validate the model, the CFD results for power consumption were compared to the experimental data. The mixing times were correlated using the specific power consumption model.

Nonlinear Modeling for the Degradation of Aqueous Azo Dyes by Combined Advanced Oxidation Processes Using Artificial Neural Networks

One-hidden-layer artificial neural networks (ANNs) using a back-propagation structure have been trained on different sets of experimental data to identify and evaluate the degradation of different azo dyes (Reactive Yellow 84, Reactive Blue 19, Direct Red 23, Direct Red 28, and Acid Blue 193) by photo-Fenton process and combined ozonation and ultrasonolysis processes. Different input variables such as pH, initial concentrations of dyes and ozone, reaction time, ultrasonic power density, and initial concentrations of hydrogen peroxide and ferrous in aqueous solution were employed to model the degradation rates of azo dyes based on the decolorization efficiency and the removal rate using chemical oxygen demand (COD) and total organic carbon (TOC). A new model expression is developed to find the effect of individual parameters and their interactions on the efficiency of organic degradation by advanced oxidation processes.

Biomass processing into ethanol: pretreatment, enzymatic hydrolysis, fermentation, rheology, and mixing

Abstract Alternate energy resources need to be developed to amend for depleting fossil fuel reserves. Lignocellulosic biomass is a globally available renewable feedstock that contains a rich sugar platform that can be converted into bioethanol through appropriate processing. The key steps of the process, pretreatment, enzymatic hydrolysis, and fermentation, have undergone considerable amount of research and development over the past decades nearing the process to commercialization. In order for the commercialization to be successful, the process needs to be operated at high dry matter content of biomass, especially in the enzymatic hydrolysis stage that influences ethanol concentration in the final fermentation broth. Biomass becomes a thick paste with challenging rheology for mixing to be effective. As the biomass consistency increases, yield stress increases which limits efficiency of mixing with conventional stirred tanks. The purpose of this review is to provide features and perspectives on processing of biomass into ethanol. Emphasis is placed on rheology and mixing of biomass in the enzymatic hydrolysis step as one of the forefront issues in the field.

Using tomography images to study the mixing of wheat straw slurries

ABSTRACT Wheat straw is a good renewable source for the production of bioethanol. However, the mixing of wheat straw slurry is a challenging task due to its complex rheology. This type of slurry behaves as a non-Newtonian fluid possessing a yield stress. In mixing operations, the presence of a yield stress creates a region of active motion (cavern) around the impeller and stagnant zones in the remainder of the vessel. In this paper, electrical resistance tomography (ERT) was employed to measure the dimensions of the cavern around the impeller as a function of the wheat straw concentration, fiber size, impeller speed, and impeller type. These data were then utilized to estimate the yield stress of wheat straw slurries. To test the accuracy of this technique, the yield stress of a xanthan gum solution calculated from the tomography method was compared to that achieved using a rheometer. This study is the first novel application of ERT to estimate the yield stress of wheat straw slurries, as opposed to directly measuring it using rheometry. Average yield stresses of 5, 7, and 10 wt% slurries were found to be 1.31 Pa, 4.2 Pa, and 14.8 Pa, respectively, for fiber size of ≤ 2 mm, and 3.4 Pa, 6.8 Pa, and 16.7 Pa, respectively, when fiber size was 8 mm.