This article has attempted to cover the basics of evaporators to give potential evaporator users some background knowledge of what evaporators are all about.The following description outlines the basics of evaporators and offers some guidance in choosing a suitable evaporator configuration
Evaporators come in many different shapes and sizes. Selecting the best evaporator for a givenapplication can sometimes be a confusing and even intimidating task. Technical terms like falling film, forced circulation and multiple effects can add to the challenge. In this brochure we will take a brief, not-too-technical look at the most common types of evaporators, how they work, and some of their applications.
The use of a multiple effect evaporator is key in implementing the Zero Liquid Discharge (ZLD) norms of pollution control board.
The basic task of an evaporator is simple: to remove water from a solution or slurry by evaporation. Evaporators are distinct from dryers in that the concentrate discharged from an evaporator is always in liquid form. The feed to an evaporator is always in liquid form and remains in liquid form even after the water is evaporated
The physical process of evaporation requires the input of energy in the form of heat to convert a liquid into vapor. Since all evaporators use the process of evaporation to remove water, every evaporator requires a source of heat to operate. The heat source for almost all evaporators is water vapor, either in the form of boiler steam or waste vapor from another process.
A second requirement for all evaporators is a means to transfer heat energy from the heat source into the evaporator liquid. Most evaporators use a tubular heater called a shell and tube heat exchanger for this purpose. In the heat exchanger shell, water vapor condenses on the outside of the tubes thus giving up its heat energy, called latent heat. The evaporator liquid, which is inside the tubes, absorbs the heat given up by the water vapor. This increase in heat causes the water in the evaporator liquid to boil. As the water in the evaporator liquid boils, it forms bubbles of water vapor in the liquid much like a pan cooking on a stove. As these bubbles reach the surface of the evaporator liquid and burst, the escaping water vapor carries some of the evaporator liquidwith it.
The final requirement for an evaporator, then, is a means of separating the evaporated water vapor from the evaporator liquid. These two main components, the heat exchanger and the vapor body, are connected together to form an evaporator. Almost all evaporators operate in the same way. Evaporator liquid is circulated through the heat exchanger tubes to absorb heat and then discharged into the vapor body to give up the water vapor which is boiled off. In most evaporators a centrifugal pump is used to circulate the evaporator liquid through the heat exchanger and vapor body.The circulating rate of the evaporator liquid depends on the type of evaporator and the evaporator liquid. The pressure in the vapor body of an evaporator determines the boiling point of the water in the evaporator liquid. If the pressure in the vapor body is atmospheric, the water will boil at 100oC. This requires the use of boiler steam as a heat source in the heat exchanger shell to achieve proper heat transfer
Falling film tubular
Like the rising film evaporator, the heat exchanger in a falling film evaporatoris vertically mounted. In this case, however, evaporator liquid is pumped to the top of the heat exchanger and flows in a downward direction through the tubes.Boiling of water in the evaporator liquid occurs as the liquid flows down through the tubes which help to force the liquid down and out of the tubes.
The falling film evaporator does have the advantage that the film is ‘going with gravity’ instead of against it. This results in a thinner, faster moving film and gives rise to an even shorter product contact time and a further improvement in the value of HTC.
It was developed for processing liquors which are susceptible to scaling orcrystallizing. Unlike the rising and falling film evaporators, this evaporator is specifically designed so that no boiling occurs while the evaporator liquid is in the tubes. The evaporator liquid is pumped by the circulating pump through the heat exchanger tubes where heat is absorbed. Liquid is circulated at a high rate through the heat exchanger, boiling being prevented within the unit by virtue of a hydrostatic head maintained above the top tube plate. As the liquid enters the separator where the absolute pressure is slightly less than in the tube bundle, the liquid flashes to form a vapor. The main applications for a forced circulation evaporator are in the concentration of inversely soluble materials, crystallizing duties, and in the concentration of thermally degradable materials which result in the deposition of solids. In all cases, the temperature rise across the tube bundle is kept as low as possible. This results in a very high recirculation ratio to that of water evaporated. These high recirculation rates result in high liquor velocities through the tube which help to minimize the buildup of deposits or crystals along the heating surface. Forced circulation evaporators normally are more expensive than film evaporators because of the need for large bore circulating pipework and large recirculating pumps. Operating costs of such a unit also are considerably higher.
Conservation of energy is one major parameter in the design of an evaporator system. The larger the evaporation duty, the more important it is to conserve energy
Multi-effect evaporation uses the steam produced from evaporation in one effectto provide the heat to evaporate product in asecond effect which is maintained at a lower pressure. In a two effect evaporator, it is possible to evaporate approximately 2 kgs of steam from the product for each kg of steam supply. As the number of effects is increased, the steam economy increases. On some large duties it is economically feasible to utilize as many as seven effects. Increasing the number of effects, for any particular duty, does increase the capital cost significantly and therefore each system must be carefully evaluated. In general, when the evaporation rate is above 1,000 kg/h, multieffect evaporation should beconsidered.
Thermo Vapor Recompression (TVR)
When steam is available at pressures in excess of 45 psig (3 barg) and preferably around 6 bar(g), it will often be possibleto use thermo vapor recompression In this operation, a portion of the steam evaporated from the product is recompressed by a steam jet venture and returned to the steam chest of the evaporator
A system of this type can provide a 2 to 1 economy or higher depending on the product the steam pressure and the number of effects over which TVR is applied. TVR is a relatively inexpensive technique for improving the economy of evaporation.
TVR can also be used in conjunction with multi-effect to provide evenlarger economies. Shown in Figure are the economies that can be achieved. Thermocompressors are somewhat inflexible and do not operate well outside the design conditions. Therefore if the product is known to foul severely, so that the heat transfer coefficient is significantly reduced, it is best not to use TVR. The number of degrees of compression is too small for materials that have high boiling point elevation.
The most economic evaporators utilize falling film tubular or plates, with either TVR or MVR. However with many duties, the required concentration of the final product requires a viscosity that is too high for a film evaporator. The solution is to use film evaporation for the pre-concentration and then a forced circulation finisher evaporator to achieve the ultimate concentration; e.g., a stillage or spent distillery wash evaporator.The material would typically be concentrated from 4% to 20% in a falling film evaporator andthen from 20% to 30% in a forced circulation evaporator. Usually the finisher would be a completely separate evaporator since the finisher duty is usually relatively low. In the duty specified above, almost 98% of the evaporation would take place in the high efficiency film evaporator.For cases where the finisher load is relatively high, it is possible to incorporate the forced circulation finisher as one of the effects in a multi-effect evaporator. However this is an expensive proposition due to the low coefficients at the high concentration.