Christophe Henry

Towards a description of particulate fouling: from single particle deposition to clogging.

Particulate fouling generally arises from the continuous deposition of colloidal particles on initially clean surfaces, a process which can even lead to a complete blockage of the fluid cross-section. In the present paper, the initial stages of the fouling process (which include single-particle deposition and reentrainment) are first addressed and current modelling state-of-the-art for particle-turbulence and particle-wall interactions is presented. Then, attention is specifically focused on the later stages (which include multilayer formation, clogging and blockage). A detailed review of experimental works brings out the essential mechanisms occurring during these later stages: as for the initial stages, it is found that clogging results from the competition between particle-fluid, particle-surface and particle-particle interactions. Numerical models that have been proposed to reproduce the later stages of fouling are then assessed and a new Lagrangian stochastic approach to clogging in industrial cases is detailed. These models further confirm that, depending on hydrodynamical conditions (the flow velocity), fluid characteristics (such as the ionic strength) as well as particle and substrate properties (such as zeta potentials), particle deposition can lead to the formation of either a single monolayer or multilayers. The present paper outlines also future numerical developments and experimental works that are needed to complete our understanding of the later stages of the fouling process.

Numerical Study on the Deposition Rate of Hematite Particle on Polypropylene Walls: Role of Surface Roughness

In this paper, we investigate the deposition of nanosized and microsized particles on rough surfaces under electrostatic repulsive conditions in an aqueous suspension. This issue arises in the general context of modeling particle deposition which, in the present work, is addressed as a two-step process: first particles are transported by the motions of the flow toward surfaces and, second, in the immediate vicinity of the walls, the forces between the incoming particles and the walls are determined using the classical DLVO theory. The interest of this approach is to take into account both hydrodynamical and physicochemical effects within a single model. Satisfactory results have been obtained in attractive conditions but some discrepancies have been revealed in the case of repulsive conditions, in line with other studies which have noted differences between predictions based on the DLVO theory and experimental measurements for similar repulsive conditions. Consequently, the aim of the present work is to focus on this particular range and, more specifically, to assess the influence of surface roughness on the DLVO potential energy. For this purpose, we introduce a new simplified model of surface roughness where spherical protruding asperities are placed randomly on a smooth plate. On the basis of this geometrical description, approximate DLVO expressions are used and numerical calculations are performed. We first highlight the existence of a critical asperity size which brings about the highest reduction of the DLVO interaction energy. Then, the influence of the surface covered by the asperities is investigated as well as retardation effects which can play a role in the reduction of the interaction energy. Finally, by considering the random distribution of the energy barrier of the DLVO potential due to the random geometrical configurations, the overall effect of surface roughness is demonstrated with one application of the complete deposition model in an industrial test case. These new numerical results show that nonzero deposition rates are now obtained even in repulsive conditions, which confirms that surface roughness is a relevant aspect to introduce in general approaches to deposition.

Numerical Study on the Adhesion and Reentrainment of Nondeformable Particles on Surfaces: The Role of Surface Roughness and Electrostatic Forces

In this paper, the reentrainment of nanosized and microsized particles from rough walls under various electrostatic conditions and various hydrodynamic conditions (either in air or aqueous media) is numerically investigated. This issue arises in the general context of particulate fouling in industrial applications, which involves (among other phenomena) particle deposition and particle reentrainment. The deposition phenomenon has been studied previously and, in the present work, we focus our attention on resuspension. Once particles are deposited on a surface, the balance between hydrodynamic forces (which tend to move particles away from the surface) and adhesion forces (which maintain particles on the surface) can lead to particle removal. Adhesion forces are generally described using van der Waals attractive forces, but the limit of these models is that any dependence of adhesion forces on electrostatic forces (due to variations in pH or ionic strength) cannot be reproduced numerically. For this purpose, we develop a model of adhesion forces that is based on the DLVO (Derjaguin and Landau, Verwey and Overbeek) theory and which includes also the effect of surface roughness through the use of hemispherical asperities on the surface. We first highlight the effect of the curvature radius on adhesion forces. Then some numerical predictions of adhesion forces or adhesion energies are compared to experimental data. Finally, the overall effects of surface roughness and electrostatic forces are demonstrated with some applications of the complete reentrainment model in some simple test cases.

Vitronectin Receptor -αVβ3 Integrin- Antagonists: Chemical and Structural Requirements for Activity and Selectivity

αVβ3 integrin, a cell surface protein, has been targeted by a variety of natural and synthetic antagonists in the search for potential cancer and osteoporosis drug candidates. This review discusses chemical and structural requirements for activity and selectivity deduced from SAR studies and draws a tentative picture of the pharmacophore.

Microwave-Assisted Synthesis of Novel (5-Nitropyridin-2-yl)alkyl and (5-Nitropyridin-3-yl)alkyl Carbamates

A straightforward approach to novel (5-nitropyridin-2-yl)alkyl and (5-nitropyridin-3-yl)alkyl carbamate building blocks is presented in this study. Their construction is achieved by condensation of N-carbamate alpha- and beta-amino carbonyl derivatives with 1-methyl-3,5-dinitro-2-pyridone 1 under microwave irradiation. Judiciously chosen modifications in the nature of the parent carbonyl starting material has influenced the regiochemical outcome of the reaction and allowed an efficient access to novel nitrogen-containing scaffolds. Compounds sharing morphological similarities have been gathered in three libraries differing from each other in a single structural parameter.

A New Stochastic Approach for the Simulation of Agglomeration between Colloidal Particles

This paper presents a stochastic approach for the simulation of particle agglomeration, which is addressed as a two-step process: first, particles are transported by the flow toward each other (collision step) and, second, short-ranged particle-particle interactions lead either to the formation of an agglomerate or prevent it (adhesion step). Particle collisions are treated in the framework of Lagrangian approaches where the motions of a large number of particles are explicitly tracked. The key idea to detect collisions is to account for the whole continuous relative trajectory of particle pairs within each time step and not only the initial and final relative distances between two possible colliding partners at the beginning and at the end of the time steps. The present paper is thus the continuation of a previous work (Mohaupt M., Minier, J.-P., Tanière, A. A new approach for the detection of particle interactions for large-inertia and colloidal particles in a turbulent flow, Int. J. Multiphase Flow, 2011, 37, 746-755) and is devoted to an extension of the approach to the treatment of particle agglomeration. For that purpose, the attachment step is modeled using the DLVO theory (Derjaguin and Landau, Verwey and Overbeek) which describes particle-particle interactions as the sum of van der Waals and electrostatic forces. The attachment step is coupled with the collision step using a common energy balance approach, where particles are assumed to agglomerate only if their relative kinetic energy is high enough to overcome the maximum repulsive interaction energy between particles. Numerical results obtained with this model are shown to compare well with available experimental data on agglomeration. These promising results assert the applicability of the present modeling approach over a whole range of particle sizes (even nanoscopic) and solution conditions (both attractive and repulsive cases).

Efficient Syntheses of Methyl 2-Amino-2-Deoxy-3,4,6-Tri-O-Benzyl-α-D-Glucopyranoside and its 2-tert-Butoxycarbonylamino- and 2-Methylamino Derivatives from N-Acetyl-D-Glucosamine

ABSTRACT The synthesis of the title compounds started with N-acetylglucosamine which was converted into the corresponding methyl glycoside and O-protected with benzyl groups. Subsequent N-protection as its N-BOC-N-acetyl derivative and sequential removal of the N-acetyl group and of the BOC group led in good yield to the target compounds in multigram amounts.

A Stochastic Model for Particle Deposition in Turbulent Flows and Clogging Effects

Particle deposition in turbulent flows is a phenomenon which can lead to fouling and affect normal operating conditions of key components of industrial processes. To explain the deposition mechanisms and predict the deposition rate, several models have been proposed in the literature. The model presented in this paper is based on a stochastic Lagrangian approach, where each particle is explicitly tracked, and where the velocity of the flow seen by particles is modeled by a stochastic process which depends on the mean fluid properties at particle locations. The interactions between particles and near-wall coherent structures are taken into account. Recent developments have shown that the model is not only able to reproduce single-particle deposition and resuspension but can also be applied to simulate the formation and the growth of multilayer deposits. Such deposits result from the competition between particle–fluid, particle–surface, and particle–particle interactions. Different morphologies of the deposit (monolayer, dendrites, multilayer) can exist according to the chemical properties of the particles and wall. A porous medium approach is used to take into account the effect of the deposit formed on the flow to obtain more realistic evolution.

On the Origin of the Facial Selectivity of the Sharpless Asymmetric Dihydroxylation of Styrene Derivatives

Xylose/styrene‐based substrates were reacted with Sharpless asymmetric dihydroxylation reagents AD‐mix α and AD‐mix β. Unlike the previously reported xylose allyl ether, the saccharide unit did not affect the stereochemical outcome of the reaction. Asymmetric dihydroxylation using AD‐mix α or AD‐mix β of the chiral olefin gave mainly one diastereoisomer (de: 98%) with S and R configuration respectively. A modelling study directed at a rationalisation of the asymmetric dihydroxylation data is described and applied to diversely derivatised styrenes.