Partially wetted particles

If the thermodynamic properties of the thin liquid films formed between particles and the subsequent chemical reactions are ignored, the phenomena that oeeur when water is added to leady oxide can be explained simply as follows (a) initially, PbO particles are hydrated and their surface is covered by absorbed water (b) at the sites of eontaet between the particles a wedge of liquid is formed (c) this wedge forms water rings on the surface of partially wetted particles in the contact regions (d) these rings exert eohesion forees which hold the separate particles together in a rather loose system (Fig. 6.22) [27]. 

Figure 5.7. Attachment of spherical particles or droplets to (solid) surfaces, (a) h, no interaction (b) h of colloidal range interaction determined by the disjoining pressure across phase 2 (c) attachment (d) attachment of a rectangular particle (e) spreading of an attached drop until the contact angle is a (f) droplet deforms but does not wet (a = 180°) (g) complete wetting (h) partial wetting on a completely wetting film.

Figure 5.44. Positioning of a particle (hatched sphere) in an oil-water two-phase system on the basis of its wettability, in (a) the particle is hydrophobic, in (c) it is hydrophilic and in (b) it is partially wetted by oil and water.

This process involves extraction of fine particles from an aqueous phase into an oil phase. The effectiveness of this technique, as shown in Figure 2, is based on the stability of emulsion droplets with solid particles. If a particle is partially wetted by two immiscible liquids the particle will concentrate at the liquid-liquid interface. The thermodynamic criteria for distribution of solids at the interface of two immiscible liquids is the lowering in the interfacial free energy of the system when particles come in contact with two immiscible liquids. (12) If ygw, yWQ and ygp are the interfacial tensions of solid-water, water-oil and solid-oil interfaces respectively, and if ygQ > y + ygw then the solid particles are preferentially dispersed within the water phase. However, if ygw > ywq + ygQ, the solid is dispersed within the oil phase. On the other hand, if yWQ > ygQ + ysw, or if none of the three interfacial tensions is greater than the sum of the other two, the solids in such case will be distributed at the oil-water interface. 

Heterojlocailation between the oil droplets and suspension particles The latter may be partially wetted by the oil and may reside at the O/W interface (this is particularly the case if the oil droplets are much larger than the suspension particles). Heteroflocculation can also occur with suspension particles dispersed in a W/O emulsion. 

Heteroflocculation results from the competitive adsorption between the dispersant and emulsifier, particularly when these are not strongly anchored to the surfaces. The displacement of some or all of the dispersant by the emulsifier, and vice versa, may result in an attraction between the particles and droplets and the repulsive barrier is weakened in both cases. If the particles are partially wetted by the oil they may reside at the O/W interface if the oil droplets are sufficiently large. 

The catalytic liquid-phase oxidation of aqueous phenol solution, carried out in a variety of reactor systems, demonstrates that phenol can be transformed to nontoxic compounds at milder reaction conditions than used in the thermal processes. The present study indicates that it is advantageous to conduct the reaction in a trickle-bed reactor with partial wetting of catalyst particles, perhaps with cyclic operation, since a direct contact between the catalyst surface and gas-phase increases the concentration of active sites for phenol oxidation. Furthermore, the reaction selectivity in a trickle-bed reactor is higher than that in a slurry reactor. The main drawback of the investigated process is dissolution of metal ions into the liquid-phase, which calls for more stable catalysts. 

Catalyst particles are small, so less chance of diffusional resistance to mass transfer. (2) Better control of temperature (because of better heat transfer efficiency and high heat capacity of slurries), attractive for exothermic reactions. (3) No need to shut down for catalyst replacement or reactivation. (4) Partial wetting and need to maintain a coating film of liquid (as needed in the trickle bed) are not issues. (5) The space time yield is usually better in slurry reactors (under comparable conditions). 

Drying chambers are designed to realize co-current, counter-current, and mixed flow patterns of drying gas and droplets or wet particles (Fig. 7.58). Especially the counter-current and, to some extent, the mixed flow patterns increase the residence time of the descending solids in the drying chamber, create a certain amount of turbulence, and, therefore, result in collisions between partially dried solid tmits. These conditions lead to coalescense and to the formation of agglomerates. 

Particularly if dry powder is produced in a spray dryer plant from suspensions or slurries, agglomeration can be accomplished if the partially solidified but still wet particles are tumbled in an associated fluidized bed where, in most cases, final drying also takes place. Fig. 7.63 is the schematic flow sheet of a fluidized spray dryer (FSD). 

More common, however, is the spraying of concentrate onto particles in a fluidized bed of a down stream dryer. Of course, this is only possible if two or multi-stage dryers are used [6.4.1.1, B.97]. Growth agglomeration (Chapter 5) occurs after re-wetting the surfaces of the partially dried particles. 

Figure 8.13 Concentration profiles in catalyst particles (a) a partially wetted...

The principle of separation of wet and dry particles is applied in the design of a spin-flash dryer for sticky and pastelike materials (Figure 33.7). In the bottom of a cylindrical chamber a high-speed impeller disintegrates the wet feed in a rapid stream of the tangentially introduced heated gas stream. The swirling gas carries away dry particles, which are then separated in a cyclone. Partially dried particles fall back into the impeller zone and are disintegrated. This type of dryer is especially suitable for thick pastes as it can handle them without dilution. 

Most common in practice is a trickling flow that occurs at relatively low gas and liquid flow rates. Under these conditions in trickle-bed reactors (TBRs) complete liquid films around the particles would break up into partial films, rivulets and droplets and a so-called partial wetting trickling regime exist. It is easier for hydrogen to enter the no wetted or very thin wetted part of partially wetted catalyst pellets because the mass-transfer resistance between a gas and the particle sur-... 

The developed dynamic reactor model for the simulation studies of the unsteady-state-operated trickle-flow reactor is based on an extended axial dispersion model to predict the overall reactor performance incorporating partial wetting. This heterogeneous model consists of unsteady-state mass and enthalpy balances of the reaction components within the gas, liquid and catalyst phase. The individual mass-transfer steps at a partially wetted catalyst particle are shown in Fig. 4.5. 

Coated particle technique has been well employed to synthesize powders for transparent ceramics, especially for YAG. YAG powder retaining the morphology of AI2O3 powder has been synthesized by using a partial wet-chemical process, in order to form yttrium precipitate coated AI2O3 particles [208]. The formation of the so-called core-shell structure had two steps, including (i) direct precipitation of yttrium component at the surface of the AI2O3 particles and (ii) assembly of the yttrium precipitate. A spherical surface reaction process was illustrated. YAG phase... 

Gessinge GH, Fischmei HF, Lukas HL (1973) Influence of a partially wetting second-phase on sintering of solid particles. Powder MetaU 16 119-127... 

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