Bandaru V. Ramarao

On instabilities arising during sedimentation of two-component mixtures of solids

Experimental evidence is presented which shows that both uniform and differential sedimentation are unstable flows for relatively concentrated fluid suspensions containing particles of two different densities or sizes. The instability takes the form of a new type of fingering phenomenon which develops as a result of the two kinds of particles segregating laterally from each other.

Calculation of single fiber efficiencies for interception and impaction with superposed brownian motion

Abstract The single fiber efficiency of fibrous filters was studied by using a Brownian dynamics simulation technique. This simulation technique permits a rigorous and simultaneous consideration of all the important mechanisms of particle capture including interception, inertial impaction and Brownian motion. Simulation results obtained for a variety of situations indicate that diffusion tends to act synergistically with particle inertia leading to an increase in the collection efficiency as compared to when the mechanisms are considered individually. In particular, significant increases in collection efficiency are found in the critical range of the minimum single fiber efficiency.

Separation and Purification Technologies in Biorefineries

Separation and purification processes play a critical role in biorefineries and their optimal selection, design and operation to maximise product yields and improve overall process efficiency. Separations and purifications are necessary for upstream processes as well as in maximising and improving product recovery in downstream processes. These processes account for a significant fraction of the total capital and operating costs and also are highly energy intensive. Consequently, a better understanding of separation and purification processes, current and possible alternative and novel advanced methods is essential for achieving the overall technoeconomic feasibility and commercial success of sustainable biorefineries.

Low shear rate viscosity of bimodally dispersed suspensions

Abstract The low shear rate viscosity was determined experimentally for suspensions of bimodally distributed polymethyl methacrylate (PMMA) spheres dispersed in silicone oil over a large range of particle volume fractions. Analysis of the data showed that the concentration dependence of the reduced viscosity could be predicted by a modification of the empirical equation proposed by Eilers for monodispersed suspensions.

Transient moisture diffusion through paperboard materials

Abstract The performance of many products such as those made of containerboard or other paperboards is sensitive to moisture. Transient moisture profiles result in heterogeneities in mechanical properties and can often lead to catastrophic failure. Viewed as a composite porous medium the system comprises hygroscopic fibers and void spaces both of which are continuous. Furthermore, both phases conduct moisture by diffusion and for typical paperboards diffusion through the fiber matrix predominates at high moisture contents whereas vapor phase diffusion through the void space is dominant at lower values. Depending upon the external and initial conditions, competition between these two pathways and local sorption interplay and produce interesting effects, which can have significant impact on the mechanical performance of the composite medium. In order to delineate the different effects that can occur, we used a mathematical model for unsteady state diffusion to analyze the case of transient moisture transport through a paperboard exposed to differential humidity conditions on either side. Diffusion is assumed to occur along the thickness direction in the void space and in the fiber space. Local uptake of moisture is represented by the linear driving force approximation. A numerical solution of the mathematical model is sought. The dominant path for moisture transport can undergo a change from the fibers to the void spaces under large humidity differentials. A fiber conduction layer whose thickness develops with time and reaches a constant value at steady state is found. This raises the possibility of moisture response, which could depend on the direction of diffusion for significantly heterogeneous media. Another interesting feature is the development of minima in the moisture flux versus time curves, which are sensitive to the diffusion and local sorption parameters. Tracking such minima can provide a good method to tune the model parameters based on experimental flux data. The role of the interfacial region between the layers can affect the overall resistance and alter moisture content profiles significantly. Since similar ‘two-equation’ models have been proposed for other transport processes such as heat and momentum transfer, these results can have wider applicability. In particular, steady and unsteady profiles and fluxes can be exploited to yield information about the local and global transport coefficients for the heterogeneous medium under consideration.

Moisture Diffusion Inside Paper Materials in the Hygroscopic Range and Characteristics of Diffusivity Parameters

Abstract Some current models for moisture diffusion in paper in the hygroscopic range are analyzed. In general, two types of diffusion models can be recognized. Models of the first type treat paper as a homogeneous medium with moisture flux that is proportional to the gradient in moisture content. Although useful in some instances this approach fails frequently because it homogenizes the internal dynamics and relaxation processes occurring within the paper material. Recent studies have shown that a subtler approach which treats paper as a composite of fibers and void spaces is more successful at describing moisture transport dynamics. A review of these studies along with a generalization of this approach to three dimensions is considered here. The parameters appearing in such models can be identified with the physical processes of diffusion through the void space and through the fiber matrix. Diffusivities in these individual phases are supplemented by a local kinetic coefficient representing moisture flux interchange between the void and fiber phases. When the local moisture exchange coefficient takes on large values, the fibers and the void spaces are at local equilibrium with no net exchange of moisture. Under such conditions, the model reduces to the simpler Fickian diffusion model with nonlinear moisture diffusivity equivalent to earlier models.

Aerosol deposition in two-dimensional laminar stagnation flow

Abstract An analysis is presented for the deposition of aerosol particles in two-dimensional laminar stagnation flow. The mechanism for deposition is assumed to be a combination of inertial impaction, interception and gravity. Analytical expressions for particle trajectories were obtained by ignoring the boundary layer effect near the deposition plane. With the inclusion of the boundary layer effect, particle trajectory can only be determined numerically. In the limiting cases, the results are reduced to those reported by various previous investigators.

Separation of Lignocellulosic Hydrolyzate Components Using Ceramic Microfilters

Abstract Among the biorefinery processes based on wood resources that are under current development, hemicellulose extraction using aqueous media is one of the prominent ones. The hemicelluloses in wood extracts need to be separated from inhibitory compounds. We examined the separation of sugar maple (Acer saccharum) wood extracts by ceramic microfilters of two different pore sizes: 0.2 μm and 0.01 μm as a function of membrane fouling. This was the first separation step intended to prepare the permeate for subsequent nanofiltration where components will be separated. Cross-flow permeation fluxes were determined for different trans-membrane pressures and cross-flow velocities. Large declines in the permeate fluxes were observed, indicating extensive fouling. The kinetics of the flux decay appears to indicate pore blocking and the development of external fouling layers as the cause. Colloidal and particulate materials were separated from the extracts with turbidity reductions of 94 to close to 100% in most cases. No particulates were detected in the permeates. Significant separation of sugars assayed in xylose form was observed and could be correlated with the extent of fouling of the membranes. Since the membrane pores are much larger than the sizes of xylo-oligomers, fouling layers built up during filtration seem to affect oligomeric separations. Further, the conditions that favor build-up of thicker fouling layers (higher pore size, higher TMP, and longer time into filtration) seem to also aid the retention of more hemicellulose sugars. Under the most severe case of fouling, xylose concentrations in the permeate dropped to less than one-fifth of the feed values. Cleaning the membranes by soaking in alkali followed by alkali and de-ionized water rinse and back-flushing were effective in reversing most of the membrane fouling.

Role of basset force on particle deposition in stagnation flow

Abstract Deposition of nondiffusive aerosols in stagnation point flow was examined. The complete equations of particle motion, including the Basset and virtual mass effects for a particle in a linear velocity field with a constant body force acting on it, were solved, and a general solution was obtained from which particle trajectories and velocities may be predicted. It was found that Basset force generally tends to hinder deposition by keeping particles in flight for longer times than in their absence. However, when particle Stokes numbers are sub-critical Basset force can actually enhance particle fluxes. As a consequence, inertial enrichment of particle concentrations at the deposition plane was increased. An analytical solution for a particle coming to rest in a quiescent fluid was obtained and compared to numerical results of Pearcey and Hill [Pearcey, T. and Hill, G. W. (1956) Aust. J. Phys. 9, 19–30].

Separation and purification processes for lignocellulose-to-bioalcohol production

Abstract: The chapter begins by introducing the lignocellulosic biomass-to- ethanol biorefinery. It then reviews and discusses the separation methods and technologies in bioalcohol production, product separation and dehydration and removal of inhibitors in fermentation systems, including extractive distillation with ionic liquids and hyper-branched polymers, adsorption with molecular sieve and bio-based adsorbents, extractive fermentation, membrane pervaporation in bioreactors, and vacuum membrane distillation (VMD), etc. Key challenges and opportunities in separation technologies for the future biorefinery are also presented.