Evaporation is the method of concentrating a solution containing a non-volatile solute and a volatile solvent. Normally the solvent is water. In most cases, the residue or concentrate is the valuable product and the solvent is evaporated off.
Evaporation, like distillation mentioned earlier in this section, uses the input of heat to achieve separation. As evaporation results in the removal of only the solvent, any contaminants in the original solution remain in the concentrate, although, the evaporated solvent is pure. This principle is used in water "distillation" then pure water is produced from ordinary or salt water.
Practical evaporation arrangements can vary gently in design from large pans for the recovery of salt from brine to the common laboratory water still for pure or "distilled" water.
Garg Evaporation Units
The production of distilled water is one of the most common operations and is to be found in most laboratories. The inertness of borosilicate glass makes it an ideal material for such operations, and as a consequence of this pyrogen free water can be produced. The glass evaporation units shown show a typical evaporation arrangement followed by some standard units in which some property of the solution makes normal evaporation difficult. Batch evaporation is effected by the rotary film evaporator which can handle heat sensitive materials by operation under high vacuum conditions. The rotary film evaporator can also handle highly viscous liquids and is available in sizes which vary from 10 - 100 liters.
Continuous evaporation is effected by the climbing film evaporator. The climbing film evaporator can also handle heat sensitive materials and have the capacity to evaporate up to 200 litres per hour. This figure will vary depending on the nature of any operation.
Evaporation unit with thermosyphon loop
The arrangement shown is a general purpose evaporator which can be used for the evaporation of both high and low boiling solvents. For high boiling solvents, the unit is suitable for use under vacuum conditions.
Heating is carried out by means of a thermosyphon loop in which either glass or metal boilers can be incorporated. Metal boilers are preferable for relatively high throughputs or for the evaporation of solutions containing a high proportion of solids.
The unit shown includes an up and over condenser arrangement, a vent condenser and two vacuum receivers in parallel. The receivers can be alternately switched during operation to facilitate product removal.
Climbing film evaporator
General : In the pharmaceutical, chemical and food processing industries, the concentration of solutions by the evaporation of part of the solvent is a common problem.
When the material being handled is heat sensitive, the problem is made even more complicated by the need to avoid exposures of the material to high temperatures or to a long contact time with the heating surface.
The climbing film evaporator is often the best solution to this problem. High rates of heat transfer can be maintained without the use of high temperatures or long contact time.
Garg Scientific Systems have developed a standard climbing film evaporator which can be used as a small production unit, as a pilot plant or for instructional purposes. The evaporator is constructed from borosilicate glass which has almost universal resistance to corrosion and being inert, the risk of contamination of product is negligible.
Garg Scientific Systems have gained considerable experience in the construction of glass evaporation units and the design of the climbing film evaporator has been based on this know-how. the units can be used for many operations. These include the concentration of fruit juices, the production of evaporated milk and specialized pharmaceutical applications such as the concentration of bile salts and fish extracts.
The modular design of the unit means that it can be easily modified to suit changing requirements. It can be operated under reduced pressure conditions or at atmospheric pressure and can be used for both batch and continuous operations, evaporating in a single pass or with recycling of the concentrate.
The operation of the climbing film evaporator cn be best explained by the reference to the flow diagram shown in figure 1. The process liquid is gravity fed, via a flow meter, into the bottom of the vertical calandria tube until the liquid level in the calandria tube is initially about 300 mm higher than the end of the steam jacket. As steam is introduced, the liquid boils creating a much larger volume of vapour which rises at a high velocity, dragging with it a thin film of liquor, up the inside wall of the calandria tube into the cyclone. This is the principle of the climbing film evaporator. For a true climbing film effect, the ratio of length to nominal bore of the calandria tube is important. This figure should be between about 100:1 and 160:1 . The effect of this climbing film action is to maintain a thin turbulent film of liquid which is in contact with the heating surface creating conditions which are most favorable for heat transfer. These extremely favorable conditions ensure that the film is dragged at high velocity up the calandria tube. The consequent short residence time of the liquor in the evaporation zone is particularly attractive for heat sensitive materials.
In the cyclone, the liquid and vapour are separated. the liquid passes into two receiver vessels which are arranged to facilitate continuous removal of liquid under normal or reduced pressure conditions. Alternatively, the liquid can be recycled back into the calandria tube for further concentration.
The vapour from the cyclone is fed through a total condenser into two receiver vessels which are again arranged to facilitate continuous removal of the condensed liquid even under reduced pressure conditions.
Climbing film evaporators are normally operated under reduced pressure conditions. This reduces the boiling temperature of the solvent being evaporated and improves the temperature difference for heat transfer from steam jacket to the solvent. The principle of operation is the same when working at either atmospheric pressure or under reduced pressure conditions.
Instrumentation and Control:
Control Panel : The control panel is laid out and includes valves for water, steam, vacuum isolation and vacuum bleed. Steam pressure, vacuum and vapour temperature gauges are also incorporated in the panel.
Achievable vacuum : 100 torr
Power consumption : 0.25 kW
Power Supply : *240V 50Hz single phase
*Other supplies can be catered for on request.
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