Counter-current mixing

The design of the spray drying chamber depends on the needed residence time (Table 23.12) as well as the type of atomizers nsed (Table 23.13). The mode of flow, e.g., concnrrent, counter-current, mixed flow, etc., depends on the desired characteristics of the product, as summarized in Table 23.14. Finally, Table 23.15 gives suggested spray dryer system layouts depending on the feedstock characteristics, e.g., presence of organic solvents, danger of fire or explosion, etc. 

To produce a moulding composition, aniline is first treated with hydrochloric acid to produce water-soluble aniline hydrochloride. The aniline hydrochloride solution is then run into a large wooden vat and formaldehyde solution is run in at a slow but uniform rate, the whole mix being subject to continuous agitation. Reaction occurs immediately to give a deep orange-red product. The resin is still a water-soluble material and so it is fed into a 10% caustic soda solution to react with the hydrochloride, thus releasing the resin as a creamy yellow slurry. The slurry is washed with a counter-current of fresh water, dried and ball-milled. 

Fluid Iron Ore Reduction (FIOR) is a process for reducing ore to iron with a reducing gas in a fluid bed. For thermodynamic efficiency, iron ore reduction requires counter current flow of ore and reducing gas. This is achieved in FIOR in a multiple bed reactor. Precautions are necessary to prevent significant back mixing of solids between beds, since this would destroy counter current staging. 

Multistage counter-current liquid-liquid extraction is a typical continuous process. Fig. 121 shows a schematic diagram of the process. The slope of the line depends on the volumetric ratio of the aqueous and organic phases. The aqueous and organic phases move through the system as two counter currents. The raffinate from a specific extraction stage moves on to the next stage, to be mixed with a relatively fresh portion of the extract, i.e. extractant that was previously mixed with a low-concentration aqueous solution. 

The reformer feeds and combustor air flow in a counter current manner as shown in Fig. 2. In order to transfer heat to the reformer evenly throughout the interface between reformer and combustor, the combustor is designed to feed the fuel through the holes distributed over the combustor. In this manner, the feed will mix with air incrementally and generate heat throughout the combustor plate evenly. The combustor plate is packed with a Pd catalyst and the reformer uses a Ni/Al203 catalyst. 

As discussed in Section 15.5.2, the separation of two or more sublimable substances by fractional sublimation is theoretically possible if the substances form true solid solutions. Gillot and Goldberger(10°) have reported the development of a laboratory-scale process known as thin-hlm fractional sublimation which has been applied successfully to the separation of volatile solid mixtures such as hafnium and zirconium tetrachlorides, 1,4-dibromobenzene and l-bromo-4-chlorobenzene, and anthracene and carbazole. A stream of inert, non-volatile solids fed to the top of a vertical fractionation column falls counter-currently to the rising supersaturated vapour which is mixed with an entrainer gas. The temperature of the incoming solids is maintained well below the snow-point temperature of the vapour, and thus the solids become coated with a thin film (10. im) of sublimate which acts as a reflux for the enriching section of the column above the feed entry point. 

The principle Zr ore, zircon (Zr silicate) is processed by caustic fusion or by direct chlorination of milled coke and zircon mixts. Washing of the Na fusion cake leave an acid soluble hydrated Zr oxide, whereas chlorination yields mixed Si and Zr tetrachlorides which are separated by distillation. Removal of the Hf from the Zr takes place through counter current liq-liq extraction (Ref 33), For this purpose the oxide or the tetrachloride is dissolved in dil hydrochloric acid to which ammonium thiocyanate is added as a complexing agent. The organic extracting phase is methyl isobutylketone... 

In liquid-liquid extraction one or more components are removed from a liquid mixture by intimate contact with a second liquid that is itself nearly insoluble in the first liquid and dissolves the impurities and not the substance that is to be purified. In other cases, the second liquid may dissolve i.e., extract from the first liquid, the component that is to be purified, and leave associated impurities in die first liquid. Liquid-liquid extraction may be earned out by simply mixing die two liquids widi agitation and dien allowing diem to separate by standing. It is often economical to use counter-current extraction, in which the two immiscible liquids are caused to flow past or dirough one another in opposite directions. Thus fine droplets of heavier liquid can be caused to pass downward through the higher liquid in a vertical tube or tower,... 

The Nippon Mining Co. refinery in Japan, which had previously used D2EHPA to extract cobalt from solutions obtained in the leaching of a mixed cobalt—nickel sulfide in sulfuric acid, changed in 1978 to the new extractant 2-ethylhexyl 2-ethylhexylphosphonate enabling a much-improved selectivity for cobalt over nickel to be obtained.8990 The process is carried out in three counter-current stages with an organic solution of phosphonic acid that has been converted to the ammonium salt by contact with aqueous ammonia ... 

The natural gas feed is depressurised again from 100 to 30 bar. The heated gas stream is saturated in a column by counter-current scrubbing with hot water. The saturated gas stream is heated further, before being mixed with additional steam to obtain the required steam to carbon ratio of 3.0. 

---