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CHAPTER XIX. ADDENDA.

The Yaryan Process of Evaporation.

This process, the invention of Mr. H. J. Yaryan, of Toledo, Ohio, U.S.A., has been considerably developed in America in its several applications, and as we have authentic information of its successful working for the recovery of soda in some of the largest paper-mills in the States, we think it deserving of mention. The principle of the process is that of “multiple effects,” upon which, indeed, all the modern methods of economic evaporation are based. The principle may be briefly explained as follows:—A liquid is converted into vapour under ordinary conditions of boiling, by overcoming the pressure of the atmosphere upon its surface. The quantity of heat required to vaporise, as also the temperature of the ebullition, will be less as the pressure to be overcome is less. Further, the vapour continuously driven off carries with it a quantity of heat, which is its heat of condition or latent heat. This heat it imparts to any colder body (e. g. a further quantity of the same liquid) with which it comes in contact, direct or indirect; if the quantity of the latter be relatively small, it will raise its temperature approximately to that of the ebullition of the first liquid. If now the pressure (atmospheric) on the surface of the latter be slightly reduced, by any means, it will boil. The vapour from this can be made to boil a third quantity of the liquid, under a further diminished pressure.

The successive effects in economic evaporation consist, therefore, in utilising the latent heat of a vapour given off from a liquid under a certain pressure (e. g. that of the atmosphere) to vaporise a further quantity of the liquid under a pressure maintained by mechanical means below that of the first. In the ordinary methods the vapour does its work in the successive effects by passage through systems of tubes, the liquid to be heated being in contact externally; in the Yaryan system, on the other hand, the arrangement {232} is reversed. The liquid to be evaporated traverses the system of tubes which are heated externally by the vapours. At the end of each effect, the liquid is caused to impinge, in a special chamber, upon a disc: in this way a complete separation of liquid and vapour is effected, each then passing on to the next effect, the former through the tube-system, the latter to the chamber inclosing these. The flow of liquid is maintained by a force-pump, and the diminished pressure by a vacuum-pump suitably disposed. This system differs from that described on page [180], in that the evaporation is continuous, the dilute liquors entering the apparatus and the highly concentrated liquors leaving it in an unbroken stream. The rate of flow is such that the evaporation of the caustic liquors from wood boiling from 8–10° to 80° Twaddell, in a quadruple effect, requires only a few minutes. At the latter concentration it is ready for the incineration process, which by means of a rotary furnace, such as that of Mr. J. W. Hammond, of the firm of S. D. Warren and Co., is also effected continuously. It is found, moreover, that the excess of heat available from this process is sufficient for the evaporation.

Ferric Oxide Causticising Process.

The principle of this process is the expulsion of the carbonic acid from sodium carbonate by ferric oxide at high temperatures; this oxide playing the part of a weak acid, entering into a loose combination with the soda, which is overcome by water in the subsequent lixiviation process, the ferric oxide being regenerated and a solution of caustic soda obtained.

The ferric oxide is therefore continuously available. The main features of the process, disregarding the question of commercial economy, are (1) that it leaves no by-products to be disposed of, (2) that it enables the manufacturer to produce directly, without evaporation, a highly concentrated caustic lye.

This process has been worked out under the auspices of Messrs. Brunner, Mond, & Co., and there is every reason to expect an extensive application of so simple a method.