Mixing efficiency, solid-liquid reaction

Reactions of nanoscale materials are classified with respect to the surrounding media solid, liquid, and gas phases. In the solid phase, nanoscale crystals are usually connected with each other to form a powder particle (micron scale) or a pellet (milli scale) see Figure 14.1. Two or more materials (powder or pellet) are mixed and fired to form a new material. The nanosized structure is favored, due to the mixing efficiency and high reaction rate. Alloys (metals), ceramics (oxides), cement (oxides), catalysts (metals and oxide), cosmetics (oxides), plastics (polymers), and many functional materials are produced through solid reaction of nanoscale materials. One recent topic of interest is the production of superconductive mixed oxides, where control of the layered stracture during preparation is a key step. 

Thermal solid-state reactions were carried out by keeping a mixture of powdered reactant and reagent at room temperature or elevated temperature, or by mixing with pestle and mortar. In some cases, the solid-state reactions proceed much more efficiently in a water suspension medium or in the presence of a small amount of solvent. Sometimes, a mixture of solid reactant and reagent turns to liquid as the reaction proceeds. All these reactions are called solid-state reactions in this chapter. Solid-state reactions were found to be useful in the study of reaction mechanisms, since it is easy to monitor the reaction by continuous measurement of IR spectra. 

Speed-up of mixing is known not only for mixing of miscible liquids, but also for multi-phase systems the mass-transfer efficiency can be improved. As an example, for a gas/liquid micro reactor, a mini packed-bed, values of the mass-transfer coefficient K a were determined to be 5-15 s [2]. This is two orders of magnitude larger than for typical conventional reactors having K a of 0.01-0.08 s . Using the same reactor filled with 50 pm catalyst particles for gas/Hquid/solid reactions, a 100-fold increase in the surface-to-volume ratio compared with the dimensions of laboratory trickle-bed catalyst particles (4-8 mm) is foimd. 

Figure 6.15. The left panel illustrates laboratory data on the efficiency of the reaction between HC1 and CIONO2 for ice, nitric acid/water solid surfaces, and liquid sulfuric acid/water solutions. The right panel depicts the altitude variation of the temperature at which the efficiency of this reaction on liquid sulfuric acid/water solutions becomes greater than 0.3 for water vapor mixing ratios that can be observed in the lower stratosphere.

Many sulfonates are viscous liquids or solids. The use of reaction solvents is therefore often either essential or preferable to obtain efficient mixing, thereby ensuring uniform reaction. In many cases they function as suspending media, rather than as true solvents, since either or both of the reagents as well as the sulfonate product may be only slightly soluble. 

A trickle bed reactor (TBR) consists of a fixed bed of catalyst particles, where liquid and gas phases flow cocurrently downward through the bed. Although its wide application in chemical and petrochemical industry it is one of the most complicated type of reactor in its design and scale-up. Essencially, the overall rate can be controlled by one or a combination of the following processes mass transfer between interphases, intraparticle diffusion, adsorption and surface reaction. The hydrodynamics, solid-liquid contacting efficiency and axial mixing can also affect the performance of TBR. 

Some concerns direcfly related to atomizer operation include inadequate mixing of liquid and gas, incomplete droplet evaporation, hydrodynamic iastabiUty, formation of nonuniform sprays, uneven deposition of liquid particles on solid surfaces, and drifting of small droplets. Other possible problems include difficulty in achieving ignition, poor combustion efficiency, and incorrect rates of evaporation, chemical reaction, solidification, or deposition. Atomizers must also provide the desired spray angle and pattern, penetration, concentration, and particle size distribution. In certain appHcations, they must handle high viscosity or non-Newtonian fluids, or provide extremely fine sprays for rapid cooling. 

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