A very efficient method of vacuum evaporation is that obtained
by the use of Multiple Effect Systems. In these greater economy of fuel for heating is secured. The apparatus consists usually of three or four simple vacuum pans, so joined together that the steam from the boiling liquid in the first pan is made to pass through the coils and jacket of the second pan, and the steam generated in the second pan goes through the coils and. jacket of the third, and so on through the system. The vacuum maintained in each pan of the series is greater than in the one preceding. Hence, notwithstanding its increased concentration, the boiling point of the liquid in the second pan is so low, that the steam from the first pan is sufficiently hot to boil it. Similarly the steam from the second pan is made toboil the liquid in the third, in which there is still less pressure, and so on to the fourth pan, in which the highest vacuum is maintained.
As a rule only four pans are used, for it is very difficult to sustain the vacuum sufficiently to work another pan in the series. In many plants only three pans (triple effects) are used.
An effective modification of this method is the apparatus known I as the Yaryan Evaporator (Fig. 4). It is made in triple and quadruple effects, and each pan is exactly like its neighbors. It consists of an outside shell of iron, within which is a system of small tubes (A, A), joined together in groups of five or six, each group constituting
a section or unit. The tubes in each unit are so connected at
the ends as to form one continuous coil. The liquor to. be evaporated is run through the several coils thus constructed in each pan. The tubes in the first pan are heated by steam, introduced into the shell directly from a boiler. As the liquid flows through the tubes, it is brought to boiling, and the steam generated mingles with it, converting the whole mass into foam, which runs through the coil and spurts against a baffle plate in the" separator" (B, B), which is an enlarged chamber at the end of the shell. The steam and liquid are separated, the liquid falling to the bottom and running off into the receiver (C), to be passed through the tubes of the next pan. The steam rises, passing through the steam dome and" catch-all" (D), and then into the shell of the next "effect," through the coils of which the liquid is passing under still greater vacuum, and so on through the system. The apparatus is very economical in its use of fuel, and as the liquid is exposed in thin layers to the action of the heat, the evaporation is very rapid; hence the liquid is subjected to a high temperature for only a short time. The apparatus is nearly
stopped and started very quickly, since it contains only a small
quantity of liquid at one time, and it occupies but little floor space when the several "effects" are placed one over the other.
The ordinary form of vacuum pan evaporates about 8~ lbs. of water per pound of coal, but it is said that the best forms of Yaryan apparatus evaporate from 23t to 25 lbs. of water per pound of coal in a triple effect, and 30t lbs. in a quadruple effect.
Organic Chemistry for the industry
Inorganic Chemistry for the industry
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