Solid-Gas Contact

The material on solids drying is divided into two subsections, Solids-Drying Fundamentals, and Sohds-Drying Equipment. In this introductory part some elementary definitions are given. In solids-gas contacting equipment, the solids bed can exist in any of the following four conditions. 

Sol-gel technique is particularly interesting due to possibility of the organic-inorganic composite material production, which porous structure ensures high solid<->liquid and solid<->gas contact surface. 

Solid-gas contact Liquid-gas contact No known charging provided solid or liquid does not break up and gas contains no free ions or suspended particles... 

The pulsed fluidized bed gives better mixing than a conventional fluidized bed, allows use of a wider range of particle sizes, and provides better solid-gas contacting. Instead of using an independent pulsator, the feed itself can be introduced through a pulse pump. In the use of this reactor, caution must be exercised in the use of proper range of pulsation parameters, and suitable baffles should be provided in the reactor. 

Excellent solids/gas contact Lower residence time than fixed bed gasifier... 

Entrained bed Handles all types of coal without pretreatment Low steam consumption Excellent solids-gas contact... 

Further, because of the nature of solids-gas contacting, which is usually by parallel flow and rarely by through circulation, heat transfer and mass transfer are comparatively inefficient. For this reason, use of tray and compartment equipment is restricted primarily to ordinary drying and heat-treating operations. Despite these harsh limitations, when the listed situations do exist, economical alternatives are difficult to develop. 

Condition of Solids In solids-gas contacting equipment, the solids bed can exist in any of the following four conditions. 

A more recent development is the use of formed plastics, especially opencell type resilient polyurethanes, for the separation and/or preconcentration of trace species from water samples. The main advantages of these materials are a quasimembrane structure which allows rapid solid/liquid or solid/gas contact in column operation and the ability to immobilize a variety of reagents as collectors and so act as a highly versatile solid phase support. The resilience of these foams permit their use in special modes such as automated batch pressure or pulsating column operation [10]. 

Evidently if S > 0 then k>+1. Were S>0 so that > o-, + a, this would imply that the solid-gas interface would immediately coat itself with a layer of the liquid phase and replace the supposedly higher free energy per unit area of direct solid-gas contact, cr g, by the supposedly lower sum of the free energies per unit area of solid-liquid and liquid-gas contacts, cr i + cr, thereby lowering the free energy of the system. However, in thermodynamic equilibrium this cannot be realized (Gibbs 1906, Rowlinson Widom 1982). Therefore, for a spreading film in thermodynamic equilibrium k = +1 S = 0), and locally there is a state of mechanical equilibrium at the contact line between the three phases. 

Refractories are meant to be exposed to high temperatures. These high temperatures cause many changes, which are the result of the refractories reactions with the environment. The environment can be in the form of solid, liquid, or gas. Reactions take place where the refractories and the environment make contact. Thus, we have solid-solid contact, solid-liquid contact, and solid-gas contact depending upon the solid refractories in contact with the environment in any of the three states of matter. 

Interfaces determine many properties and processes. At interfaces, the structure changes from one type to the next. Thus, we have interfaces between solids at solid-solid contact, between solid and liquid at solid-liquid contact, and between solid and gas at solid-gas contact. In order to know the mechanical properties, chemical phenomena, and electrical properties, we should understand the structure, composition, and properties of these contacts. 

There has been a far greater effort in the development of flTiidized bed processes for combustion and gasification of coal than corresponding efforts based on other modes of solids-gas contacting processes. This is due primarily to the inherent advantages that fliiidized beds offer (which are listed in Table III). 

Example 3 (Fig. 5.70c) illustrates a solid-gas contact, more precisely the contact between the n-type semiconductor Sn02 with O2 gas. At low temperatiures the adsorbed oxygen cannot penetrate the interior, but remains adsorbed on the surface. It traps electrons from the boundary layer and makes this positively charged, producing a depletion layer of increased resistance. This is the basic principle of the Taguchi sensor, which we will discuss later. Such effects must also be discussed at elevated temperatmes when phase equilibrium with oxygen is established. 

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