It is convenient to classify evaporators on the basis of heating surface.
Horizontal Tube Evaporator
Norbert Rilliieux in Louisiana in 1843 for the first time built evaporators which received recognition. It was designed using horizontal tubes. The same was invigorated in 1879 into Wellner-Jelinek type from which evolved the most modern horizontal tube evaporator.
Heat is transferred through the horizontal tubes which are ducked in pool of liquid to be evaporated. The liquid just above the tubes gets heated and solvent molecules evaporate leaving behind the concentrated liquid which is collected from the bottom.
Short Vertical Tube Evaporator
Though not first to be built, it was the first to gain popularity. It was built by Robert, director of sugar factory at Seelowitz, Austria in 1850. It was known as Robert type. It became so popular in Europe that it was then later on called standard evaporator. Principle being, liquid which has to be evaporated is filled in the vertical tubes and steam is passed outside these tubes. Heat is transferred through the tubes and liquid inside the tubes gets heated. The rescue of the vapours and collection of the concentrated liquid is from the top and bottom, respectively.
Multiple Effect Evaporators
All of the above discussion is based on what is known as single effect evaporation. An alteration of this system is known as multiple effect evaporation for obtaining higher steam economies than is possible in single effect. In single effect evaporation described earlier, steam provides latent heat of vapourization to the liquid; the vapours so formed are given off to the condenser, where the heat is given to the cold water. Thus, a great deal of heat goes into the vane. Principle behind multiple effect evaporation is that the number of evaporator units are connected in such a way that it is only the first calandria that is heated by steam; vapours from this effect will then heat the calandria of the next effect. In other words, instead of using cold water, calandria of the next effect is used as a condenser and latent heat of vapourization is being utilized instead of being going into the drain. The vapours of the last effect the then taken to the condenser in the usual way.
Long Tube Evaporators
As the name suggests these are taller and slender tubes having the L/D ratio greater than 150. The evaporator may have the tubes about 60 mm in diameter and 7 m in length. Such longer sizes allow higher velocities due to violent pumping action of bubbles inside the tubes. Higher velocities are desirable because they tend to decrease the thickness of the viscous film and also assist in the quick removal of the steam bubbles as soon as they are formed. This improves the overall coefficient which is lower for the film on the boiling liquid side and comparatively higher for the condensing steam side. The first evaporator utilizing this effect was Kestner, which was patented in 1899.
Wiped Film Evaporator
Falling film evaporator suffers from the major drawback of large temperature differences and hot spots. In order to trounce it, evaporators are attached with wipers or other suitable rotary device. As with the falling film evaporator the liquid enters onto the heated wall from the top but a fast rotating wiper filament spreads it perfunctorily. The vapours produced flow in upward direction counter currently. The concentrated solution is haggard off at the bottom of the evaporator. The solution of viscosities up to 2,800–3,200 centipoises can be handled. Rotor speed is maintained between 200 and 400 rpm.
Rotary Evaporator
Rotary evaporator, often called as ‘rotovap’ is of great importance to the pharmaceutical industry. Vacuum is a key component of this device, and is used to aid in the evaporation of the solvent. Vacuum will lower the air pressure above a liquid, thereby lowering the boiling point of the liquid being heated. Use of a rotavap therefore allows liquid solvents to be removed without excessive heating. Rotary evaporation is most conveniently applied to separate low boiling solvents such as ethyl acetate or n-hexane from compounds which are solid at room temperature and pressure. Evaporation of high boiling hydrogen bond-forming solvents such as water is often a last recourse, due to the fact that in such solvents, the tendency to “bump” is accentuated.
The modern centrifugal rotary evaporator is particularly useful for high boiling solvents and when one has many samples to do in parallel. A centrifugal rotary evaporator comprises of a vacuum pump connected to a centrifuge chamber in which the samples are placed. The centrifugal force creates a pressure gradient within the solvent contained in the tubes, this means that the samples boil from the top down, helping to prevent bumping. The most advanced systems apply the vacuum slowly and run the rotor at speeds of 500 × gravity.
Reference:
- Khar, R.,Vyas, S., Ahmad, F., & Jain, G. (2016). Lachman/Lieberman’s The Theory and Practice of Industrial Industrial Pharmacy. New Delhi, ND: CBS Publishers & Distributors Pvt Ltd.
