Evaporation

Evaporation- Inorganic Chemistry for Industries

Evaporation, in a technical sense, denotes the conversion of a
liquid into a vapor for the purpose of separating it from another
liquid of higher boiling point, or from a solid which is dissolved in
it. In the great majority of cases, the liquid evaporated is water.
If the liquid evaporated is to be recovered, the vapors are condensed, and the process then becomes one of Distillation.
'there are four general methods of evaporation:

1. Spontaneous evaporation in the open ail.
2. Evaporation by application of heat directly from a fire to
the vessel containing the liquid.
3. Evaporation by indirect application of heat from the fire, as
by means of steam, with or without pressure.
4. Evaporation under reduced pressure.

The first method, by spontaneous evaporation in the open air, is comparatively slow, and requires exposure of very large surfaces of liquid. The time necessary depends upon the temperature and humidity of the air, and the completeness with which the vapors are removed from the surface of the liquid; hot, dry weather, especially if a brisk wind is blowing, evaporates water quite rapidly.
This process is only used for the manufacture of salt from sea
water, or from natural brines. In certain warm countries considerable quantities of salt are thus prepared, and in this country some is made from a brine found near Syracuse, N.Y. Sometimes weak brines are allowed to trickle in fine streams over tall piles or "ricks" of brushwood in the open air. The liquid being so exposed in thin layers, to the air and wind, is concentrated to such a degree that it will pay to complete the evaporation by artificial heat.
The second method,* by direct application of heat from a fire, is very largely used in the arts. This may be done in two general ways:-
(a) The flames, or hot gases from the fire, are generally allowed to play directly on the bottom of the vessel containing the liquid;

or they may pass through flues or pipes, set into the vessel, so that the liquid surrounds them on all sides (Fig. 1). Such pans are often several yards in length, and may contain one large flue, or several small ones, according to the work desired; but this form of apparatus is expensive to build, and difficult to keep in repair.
(b) The flames and hot gases may be conducted over the surface of the liquid to be evaporated. This mode is only used for coarse and common products, or in the concentration or recovery of waste materials. But it has the advantage, that the bottom of the pan is less liable to be injured. by the crusting of a precipitate upon it.
Another point often in favor of surface heating, is that the liquid is evaporated in a reducing atmosphere. But as flue dust and ashes are liable to fall into the pans, the product is usually impure. Large shallow pans are used, which are generally arched over with brick, in order that the heat may be better utilized, through radiation from the brick walls.

There are various ways of setting the pans for this process; a simple method is shown in Fig. 2. A modification of this method is the use of a long cylinder, set at a slight incline, and revolving about its longitudinal axis (Fig. 3). The lower end is open for the entrance of the flames and gases from the grate (A), which pass through the cylinder (B), on their way to the
chimney (D). The hot gases are often passed through the flues of a boiler (C), to utilize the waste heat. The solution to be evaporated is fed into the cylinder at the upper end in a small stream, and comes in direct contact with the flame. The water is evaporated, and the solid matter is delivered into the pit or wagon (E) at the lower end of the furnace, in a dry and calcined state. Such furnaces are frequently used for evaporating waste liquors to recover

the salts which they contain; and for the treatment of sewage and other liquid refuse.

The third method of evaporation, by the use of steam heat, is
very often employed where there is danger of injury to the product by overheating.
(a) Jacketed pans or kettles maybe used. These are simply
double-walled vessels, the steam being admitted between the walls.
(b) The steam may be allowed to circulate through coils of pipe,placed inside the vessel, which is sometimes made of wood. The temperature of the liquid depends on the steam pressure; very often exhaust steam is employed.
The fourth method, evaporation in vacuo, is merely a modification of either the second 01' third method, but is considered separately for convenience. The boiling point of a liquid may be very materially lowered by reducing the pressure within the vessel.
Hence, solutions containing substances which would be injured by the heat necessary to boil them under the atmospheric pressure, or liquids boiling at very high temperatures, are evaporated in vacuum pans.
The different forms of apparatus used for vacuum evaporation
vary much in their details, but all depend on the principle of
reduced pressure. The essential parts of the plant are the vacuum pan or still, the pump for exhausting the air and steam from the pan and sending them to the condenser, and the heating apparatus. The vacuum pan is usually a globular copper or iron vessel, provided with a manhole, a pressure gauge, and a discharging valve. Very often a piece of heavy plate glass is set in the side to afford a view of the interior during evaporation. On the top of the pan is a dome or short tower, from which a pipe leads to a receptacle, called the "catch-all," that retains any liquid which may escape from
the pan. A small pipe returns this liquid to the pall, and a larger
one connects the "catch-all" with the vacuum pump, which is an ordinary double-cylinder air pump of large size, driven by an engine. An injector pump, which condenses the steam directly, may be used. The pan is generally heated by steam coils within it, or by a steam jacket, or by both.

continue to: Multiple Effect Systems vacuum evaporation

The above information on chemistry subjects is as they were described in the past centuries.