Moinuddin Malik

An Integrated Approach for Numerical Analysis of Coupled Flow and Heat Transfer in Co-rotating Twin Screw Extruders

Abstract The co-rotating twin screw extrusion process is widely employed in chemical industries for the processing of complex fluids including various polymers, suspensions, emulsions and gels. Here we propose a new integrated modeling strategy that is based on the numerical analysis of pressure-generating extrusion elements concomitantly with the pressure-losing extrusion elements of the co-rotating twin screw extrusion process for non-Newtonian fluids under nonisothermal conditions. The numerical analysis undertakes three-dimensional (3-D) finite element simulations of any multiple combinations of forwarding and reversing fully-flighted screw elements with other types of elements including kneading discs staggered in the forward or reverse configurations and the die. The abilities of the methodologies in simulating the coupled flow and heat transfer in industrially-relevant mixing sections or pressurization/die shaping are demonstrated with predictions of the degree of fill and typical velocity, deformation rate, stress magnitude, pressure distributions as functions of various operating parameters and basic twin screw extrusion geometries for a viscoplastic type generalized Newtonian fluid.

Evaluation of the treatment of chromite ore processing residue by ferrous sulfate and asphalt

The effectiveness of the treatment of chromite ore processing residue (COPR) with ferrous sulfate and encapsulation into asphalt were explored separately and in combination. The asphalt treatment was conducted by mixing COPR or ferrous sulfate pretreated COPR with varying amounts of asphalt. To assess the efficacy of the treatment, the leachability of toxicity characteristic leaching procedure (TCLP) total chromium (Cr) from all treated samples was determined for curing periods up to 16 months. X-ray absorption near edge structure (XANES) analyses were also performed to evaluate the Cr(6+) concentration in the selected samples. The combination treatment of ferrous sulfate and the encapsulation of the treated COPR into asphalt reduced the TCLP total Cr concentration to lower than the regulatory limit of 5mg/L for Cr contaminated soils, after 16 months. However, the Cr concentrations were still higher than the universal treatment standards (UTS) of 0.6 mg/L for hazardous waste. On the other hand, treatment with ferrous sulfate alone or the encapsulation of the COPR in asphalt failed to meet the TCLP total Cr concentration of 5mg/L, after 16 months. XANES analyses results showed that more than 75% Cr(6+) reduction was achieved upon pretreatment with ferrous sulfate.

Axial laminar flow of viscoplastic fluids in a concentric annulus subject to wall slip

The flow of non-Newtonian fluids in annular geometries is an important problem, especially for the extrusion of polymeric melts and suspensions and for oil and gas exploration. Here, an analytical solution of the equation of motion for the axial flow of an incompressible viscoplastic fluid (represented by the Hershel–Bulkley equation) in a long concentric annulus under isothermal, fully developed, and creeping conditions and subject to true or apparent wall slip is provided. The simplifications of the analytical model for Hershel–Bulkley fluid subject to wall slip also provide the analytical solutions for the axial annular flows of Bingham plastic, power-law, and Newtonian fluids with and without wall slip at one or both surfaces of the annulus.

Axial annular flow of a viscoplastic microgel with wall slip

The fully developed velocity distributions of a viscoplastic Carbopol microgel [0.12 wt. % poly(acrylic acid) in water] flowing axially in the annular gap between two long concentric cylinders were determined. The ratios of the length over the gap and the inner over the outer radii, κ, of the annulus were 442 and 0.78, respectively. The particle image velocimetry, PIV, measurements revealed that the viscoplasticity of the microgel in axial annular flow is manifested via the formation of plug flow and wall slip. The plug flow region spanned the entire or part of the annular gap depending on whether the absolute values of the shear stresses at the two walls were below or above the yield stress of the microgel, respectively. For all flow rates, the velocities at the two walls were similar indicating that the absolute values of the wall shear stress were also similar. Equality of the shear stresses at the two walls suggested that axial annular flow at a relatively high κ can be used as a viscometric flow for ...

Simulation of Co-Rotating Twin Screw Extrusion Process Subject to Pressure-Dependent Wall Slip at Barrel and Screw Surfaces: 3D FEM Analysis for Combinations of Forward- and Reverse- Conveying Screw Elements

Abstract Mathematical modeling and simulation of the coupled flow, deformation, heat and mass transfer, and rate of reactions occurring in the twin screw extruder allow the optimization of process parameters and the screw and barrel geometries. In mathematical modeling of the twin screw extrusion process the conventional flow boundary condition at the screw and barrel walls is the no-slip condition. However, most complex fluids, including polymers, polymeric suspensions and blends, exhibit wall slip, with the slip behavior depending on the intrinsic properties of the materials being processed, the operating conditions, the geometries of the barrel, screw and the die, and the properties of the solid surfaces. Typically, the slip velocity is specified to be a function of temperature, stress condition at the wall and the materials of construction. However, recent investigations have further revealed that the wall slip behavior can also be significantly affected by pressure. With an objective of considering the effects of wall slip on the dynamics of twin screw extrusion, fully-intermeshing co-rotating twin screw extrusion of a concentrated suspension is analyzed using three-dimensional finite element method, FEM, subject to the wall slip boundary condition. The wall slip boundary condition is first applied systematically to barrel and screw surfaces individually followed by the application of wall slip to both surfaces simultaneously. In an integrated fashion both the forward-conveying (pressure-generating) and reverse-conveying (pressure-losing) screw sections are considered. The effects of pressure on wall slip are also analyzed and elucidated.

Safety in Design and Manufacturing of Extruders Used for the Continuous Processing of Energetic Formulations

Extruders used for the continuous processing of energetic materials require various types of safety features and thus are differentiated from the extruders commonly available to civilian industries. Items of particular importance to the user include the in-process volume, control of the energetic material properties (especially temperature and pressure), the ability to quickly release pressure, reduction of metal-to-metal contact, and the control of electrical discharge. In this article, two novel extrusion platforms, the first one involving a flexible manufacturing platform and the second designed to process nanoenergetics, are described to illustrate the procedures necessary to design extrusion platforms for energetics manufacture. Particular emphasis is given to the safety features that need to be incorporated during the design stage, along with a detailed discussion of the flexibility and ease of use of extrusion equipment. The use of material-specific mathematical modeling in the design of the extrusion platforms is also elucidated as a first line of defense for safety and ease of use.

Numerically simulated flow characteristics of particulate beds in oscillating sectorial containers

Abstract In this work, the well-known Cundall–Strack discrete element simulation is employed to investigate the flow characteristics of unary and binary particulate beds in oscillating sectorial containers. The simulated flow characteristics of both unary and binary granular beds in the oscillating sectorial containers have similarities with those of the particulate beds subjected to combined vertical and horizontal vibrations in that the granular bed heaps toward one of the container walls. However, heaping in an oscillating container can be either active or stagnant. Active heaping occurs in lower bounded and upper unbounded frequency ranges and is characterized by a heap that keeps shifting alternately from one radial wall to another during an oscillation cycle. In case of stagnant heaping, which occurs in an intermediate frequency range, the particulate heap becomes almost fixed and does not change its orientation with respect to the oscillating container. It is found that in the stagnant regime, there is a critical frequency at which heaping becomes symmetrical. The asymmetric heaping patterns formed in both active and stagnant regimes at frequencies below and above the critical frequency are of opposite orientations. In a binary particulate bed of initially separate layers of two different size particles, continued oscillations of the container, at frequencies in active heaping regimes, lead to good mixing of the binary set of particles.

Dynamic assembly of anionic surfactant into highly-ordered vesicles

A highly-efficient dynamic assembly method for the transformation of the initial spongy lamellar structure of concentrated linear alkylbenzene sulfonate, LAS, incorporated with sodium silicate, into spherulitic vesicles is presented. A combination of drag and pressure flows, via twin screw extrusion, was used to mitigate the ubiquitous viscoplasticity and the wall slip behavior of the anionic surfactant paste and gave rise to the dynamic assembly of stable vesicular nanostructures within a narrow size range, that was not possible with either pure drag or pure pressure flows. Concomitantly with the structure transformation of the paste during assembly under the combination of pressure and drag flows, significant changes in its viscoelasticity, i.e., order of magnitude increases in storage and loss moduli and magnitude of complex viscosity, were observed. The demonstrated dynamic assembly of stable vesicular nanostructures, with vesicle diameters within the relatively narrow range of 300-600 nm, from a commodity surfactant is relevant to myriad templating and encapsulation applications, as well as shedding light on the mechanisms of the deformation-induced planar lamellar to vesicle transformation of concentrated amphiphiles.

Tangential annular (Couette) flow of a viscoplastic microgel with wall slip

The tangential annular or Couette flow of a viscoplastic microgel, i.e., 0.12 wt. % aqueous solution of poly(acrylic acid), Carbopol® 940, under isothermal and creeping flow conditions was investigated by simultaneous particle image velocimetry and rheometrical measurements (Rheo-PIV). A wide range of ratios of the inner over the outer radii of the annuli, i.e., κ = 0.329, 0.749, and 0.933, were used. The PIV measurements revealed the viscoplasticity of the microgel in Couette flow via the formation of plug flow (rigid body motion) and slip at the two walls. A procedure that relied on the characterization of the wall slip behavior was developed for the determination of the yield stress of the microgel, in turn leading to other parameters of the shear viscosity of the viscoplastic fluid. The wall slip velocity versus wall shear stress behavior of the microgel was overall consistent with the mechanism of apparent slip for all three gaps. However, the apparent slip layer thicknesses were dependent on the wal...

Artificial Neural Network (ANN) Model for Prediction of Mixing Behavior of Granular Flows

Mixing and segregation behavior of granular flows inside a particulate system comprising an oscillating sectorial container is predicted by an artificial neural network (ANN) model. By employing discrete element method (DEM), numerically simulated characteristics of a sectorial container, which is subjected to harmonic angular oscillations, are trained for the development of a neural network model. Binary system of particles is simulated and degree of mixing is estimated by varying different parameters, such as particle size ratio (1:1 to 1:3), frequency of oscillations (1 to 4 Hz), amplitude of oscillations (30 to 60°), volume filling fraction (0.04 to 0.24), and number of cycles (1 to 20). The learning of ANN is accomplished by feed forward back propagation algorithm. It is found that mean mixing concentration predicted by the neural network model developed in this work is in a good agreement with the simulated values. Percentage error predicted by ANN model is less than ± 8% for 82 out of the 90 data values. Development of the neural network model and its use for the prediction of the outcome of the system (especially in cases where several operating parameters, which determine the outcome of the system, have a non-linear relationship with each other) is believed to be an accurate and computationally inexpensive way of understanding the behavior of the system.