Adhesion and Agglomeration

Adhesion and agglomeration are vitally important in the gas-solid transport area although they have not received much attention in that respect. There have been numerous studies on these subjects relating the solid particles and surfaces to static conditions. As particles flow in a pipe, a certain number of the particles adhere to the soUd surface while others find their way into small crevices in the system. In the flow of a fine powder in a pipe, after the initial deposition of the particles to the pipe wall, the particles in suspension will interact with those adhering to the wall, causing different frictional and electrostatic behavior than would be anticipated from interactions of the particles with the pipe wall material alone, whether it is metal, plastic, or glass. 

It should be noted that the size of the adhesion force is indeed large on small particles. In many cases this force is several orders of magnitude greater than the gravity force on the particle. Because of these sizable adhesion forces, large forces are needed to remove the small particles from the adhering surfaces. 

The method considered here for determination of these forces is the modified Lifshitz macroscopic approach, considering the microscopic approach results. This approach uses the optical properties of interacting macroscopic bodies to calculate the van der Waals attraction from the imaginary part of the complex dielectric constants. The other possible approach—the microscopic theory—uses the interactions between individual atoms and molecules postulating their additive property. The microscopic approach, which is limited to only a few pairs of atoms or molecules, has problems with the condensation to solids and ignores the charge-carrier motion. The macroscopic approach has great mathematical difficulties, so the interaction between two half-spaces is the only one to be calculated (Krupp, 1967). 

As a model consider two solids, as shown in Fig. 2-1. The solids are separated from each other by a gap of width Zq. The internal electromagnetic fluctuation fields in solids set up an electromagnetic field F3, in the gap. The resultant electromagnetic field strengths 3 and 7/3 of the gap are used to determine the Maxwell electromagnetic stress tensor T. The force component parallel to the Z axis is 

After some mathematical effort, Lifshitz found the van der Waals adhesion pressure to be 

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W. Pietsch, Adhesion and agglomeration of solids during storage, flow, and handling—a survey, Trans, ASME, J, of Engng for Industry, 9, Ser. B, No. 2, 435-49 (1969). 

Fig. 7.75 is the photograph of the turbulently moving surface of a fluidized bed above which the atomizing nozzle for binder liquid is located. Other designs use atomization nozzles that are submerged in the bed to achieve more intimate contact between the liquid and the solids. Solids, fresh feed and recycle, are typically fed into the bed below its surface to allow collisions with other particles, adhesion, and agglomeration before they may be entrained in the off-gas leaving the apparatus. The fines are then removed from the gas, collected, and once more recirculated to the back-mixed fluidized bed. 

The dispersive component is associated with polymer-filler interaction and the specific component is associated with filler networking and agglomeration. The dispersive component of different fillers is more conveniently measured by inverse gas chromatography although it can also be measured by contact angle methods. The work of adhesion is given by the following equation, which has been modified to account for Fowkes theoiy. 

C. Interparticle Adhesive Forces in Static Powders and Agglomerates... 

C. INTERPARTICLE ADHESIVE FORCES IN STATIC POWDERS AND AGGLOMERATES... 

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