Countercurrent principle

The KEN-FLOTE column (11) is one of several column flotation processes based on a countercurrent principle. The feed slurry containing reagents is iatroduced iato the column just below the froth zone. Air is iujected at the bottom of the column via an air sparger. Wash water is sprayed within the froth zone to reject the entrained impurities from the froth. Test results on this column iadicate that a 6% ash product coal having a combustible-recovery of 75—80% can be obtained. A 70—80% pyrite reduction is also claimed. Figure 2 shows the operation of such a column. 

Unfortunately, most of Kitaibel s work was never published, but his manuscripts preserved at the Hungarian National Museum in Budapest show that he was an ingenious designer of chemical apparatus, such as a salt-evaporating pan which utilized the heat of the fuel gas on the countercurrent principle a device for the saturation of mineral water with carbon dioxide apparatuses for vacuum filtration and for the distillation of water and an improved lime kiln and brick kiln (12). 

Step 1). A 100 L quantity of urine is adjusted by the addition of acid (hydrochloric acid is preferred but not essential) to a pH of 4 and extracted with a suitable solvent such as n-butyl alcohol, benzol, chloroform or ether in a continuous extraction apparatus. By using the countercurrent principle we find that this volume of urine may readily be extracted during one day s time and the active fraction transferred completely to a 4 L volume of butyl alcohol. This alcoholic solution is chilled and filtered from salts and other insoluble matter. 

Oligohydridemethylsiloxane can also be produced by the continuous technique. In this case hydrolytic condensation should be carried out in a the flow circuit (pump - heat exchanger - hydrolyser) (see Fig. 36), in a shell-and-tube heat exchanger (see Fig. 58) or in common countercurrent spray towers with agitators. The neutralisation of the product of hydrolytic condensation should be conducted in sectional apparatus with agitators, which also use the countercurrent principle. 

An interesting rebirth of the countercurrent principle is the new ICI tube-cooled converter used in the LCA process. 

In order to increase the efficiency of such a batch process and to reduce the inconvenience of discontinuous operation when the extractor is decompressed and opened for replacing the spent material against fresh one the extraction volume is spread over three or four vessels. These are switched into the gas circulation in a battery-type sequence utilizing the countercurrent principle. This means the extractor containing already depleted material is first contacted with the fresh gas and the extractor with the fresh material containing the full extract concentration is contacted in the second or last position in order to benefit as much as possible from the dissolving capacity of the gas. 

This batch mode operation for solids even if quasi-continuous and according to the countercurrent principle affects the economics and restricts the application to products which provide a certain added value or to separation cases which caimot be solved otherwise. For this reason large-scale operations like the production of vegetable oils is still the domain of hexane extraction, but there are many other examples where the COj-process is highly competitive or offers new unique possibilities and solvent-free products. 

In the 16th century continuous condensation by water followed. The concentrating effect of a long vapour riser and of partial condensation (Fig. 7 a) were also recognized (1648). The modern counterpart is illustrated in Fig. 7 b. The countercurrent principle in condensation was introduced by Pissonnier in 1770, the same principle found today in the well-known Liebig conden.sers (Fig. 8b), which date back to Dariot (1533-1594 .see Fig. 8a). 

Liebig condenser using countercurrent principle (19th century)... 

Diagram of the multiplication of separating effects by the hairpin countercurrent principle of Kuhn... 

Small heating rates as they are characteristic for countercurrent systems in fixed-bed reactors will cause coal pyrolysis, and hence tar formation, to begin at about 350 °C. Since at this temperature the reaction of tar and carbon with steam proceeds very slowly, the concentration of intermediate products such as tar or other hydrocarbons which are liquid under normal conditions is very high. Owing to the countercurrent principle, these products are discharged from the reactor together with the gasification gas itself. 

More elaborate multiple extractors based on a countercurrent principle have been developed that are capable of giving very efficient separations in favorable circumstances. For separating amounts greater than about 40 g, a counterdoublecurrent apparatus is preferable. With this apparatus, continuous rather than batchwise operation is possible, if a solvent system can be found such that the distribution coefficient of the desired substance is on one side of unity and those of all impurities are on the other side. Due to the cost of the apparatus (about 8000), the limited quantity of material that can be used (a maximum of about 40 g of substrate), the time necessary to find a suitable solvent system, and the problems associated with running and cleaning the apparatus, countercurrent extractors are usually employed as a last resort, i.e., after distillation, recrystallization, simple extraction, and liquid chromatography have failed to effect a satisfactory separation. 

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