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Table Of Contents:
1. Evaporation……………………………………………. 52. What an Evaporator Does?.............................................63. Working principle………………………………………64. Elements of evaporator………………………………...7
5. Evaporator Accessories……………………………...…76. Basis of classification of evaporators………………….87. Types of evaporators…………………………………..8a) Natural/forced circulation evaporator…………………8b) Falling film evaporator………………………………..11c) Rising film (Long Tube Vertical) evaporator…………13d) Plate evaporator……………………………………….15e) Multiple-effect evaporators…………………………..168. Troubleshooting………………………………………189. References……………………………………….
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Evaporation is the removal of solvent as vapor from a solution or slurry. For the overwhelming majority of evaporation systems the solvent is water. The objective is usually to concentrate a solution; hence, the vapor is not the desired product and may or may not be recovered depending on its value. Therefore, evaporation usually is achieved by vaporizing a portion of the solvent producing a concentrated solution, thick liquor, or slurry. Evaporation often encroaches upon the operations known as distillation, drying, and crystallization. In evaporation, no attempt is made to separate components of the vapor. This distinguishes evaporation from distillation. Evaporation is distinguished from drying in that the residue is always a liquid. The desired product may be a solid, but the heat must be transferred in the evaporator to a solution or a suspension of the solid in a liquid. The liquid may be highly viscous or a slurry. Evaporation differs from crystallization in that evaporation is concerned with concentrating a solution rather than producing or building crystals.
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What an Evaporator Does?
As stated above, the object of evaporation may be to concentrate a solution containing the desired product or to recover the solvent. Sometimes both may be accomplished. Evaporator design consists of three principal elements: heat transfer, vapor-liquid separation, and efficient utilization of energy. In most cases the solvent is water, heat is supplied by condensing steam, and the heat is transferred by indirect heat transfer across metallic surfaces. For evaporators to be efficient, the equipment selected and used must be able to accomplish several things:
Transfer large amounts of heat to the solution with a minimum amount of metallic surface area. This requirement, more than all other factors, determines the type, size, and cost of the evaporator system.
Achieve the specified separation of liquid and vapor and do it with the simplest devices available.
Make efficient use of the available energy. This may take several forms. Evaporator performance often is rated on the basis of steam economy-Pounds of solvent evaporated per pound of steam used.
Meet the conditions imposed by the liquid being evaporated or by the solution being concentrated. Factors that must be considered include product quality, salting and scaling, corrosion, foaming, product degradation, holdup, and the need for special types of construction.
The solution containing the desired product is fed into the evaporator and passes a heat source. The applied heat converts the water in the solution into vapor. The vapor is removed from the rest of the solution and is condensed while the now concentrated solution is either fed into a second evaporator or is removed.
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Elements of evaporator:-
Three principal elements are of concern in evaporator design:
Heat transfer Vapor-liquid separation Efficient energy consumption.
The units in which heat transfer takes place are called heating units or calandrias. The vapor-liquid separators are called bodies, vapor heads, or flash chambers. The term body is also employed to label the basic building module of an evaporator, comprising one heating element and one flash chamber. An effect is one or more bodies boiling at the same pressure. A multiple-effect evaporator is an evaporator system in which the vapor from one effect is used as the heating medium for a subsequent effect boiling at a lower pressure. Effects can be staged when concentrations of the liquids in the effects permits; staging is two or more sections operating at different concentrations in a single effect. The term evaporator denotes the entire system of effects, not necessarily one body or one effect.
Various types of equipment, both major and minor, must be supplied forevery evaporator in addition to the evaporator body itself. These include:(1) Condensers(2) Vacuum producing equipment(3) Condensate removal devices(4) Process pumps(5) Process piping(6) Instrumentation(7) Safety relief equipment(8) Thermal insulation(9) Equipment and pipeline tracing(10) Valves, manual and control(11) Process vessels(12) Electric motors and turbines(13) Refrigeration.
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Basis of classification of evaporators.
1. Heating medium separated from evaporating liquid by tubular heating
2. Heating medium confined by coils, jackets, double walls, flat plates etc.
3. Heating medium brought into direct contact with evaporating liquid.
4. Heating with solar radiation.
Types of evaporators
1. Natural/forced circulation evaporator.
2. Falling film evaporator.
3. Rising film (Long Tube Vertical) evaporator.
4. Plate evaporator.
5. Multiple-effect evaporators.
1.Natural/forced circulation evaporator:
This type of evaporator is generally made of long tubes (4-8 meters in length) which are surrounded by steam jackets. The uniform distribution of the solution is important when using this type of evaporator. The solution enters and gains velocity as it flows downward. This gain in velocity is attributed to the vapor being evolved against the heating medium, which flows downward as well. This evaporator is usually applied to highly viscous solutions so it is frequently used in the chemical, food, and fermentation industry.
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Forced circulation evaporator with external horizontal heating element
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Forced circulation evaporator with internal vertical heating element
2.Falling film evaporator.
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In this type of evaporator, boiling takes place inside the tubes, due to heating made (usually by steam) outside the same. Submergence is therefore not desired; the creation of water vapor bubbles inside the tube creates an accessional flow enhancing the heat transfer coefficient. This type of evaporator is therefore quite efficient, the disadvantage being to be prone to quick scaling of the internal surface of the tubes. This design is then usually applied to clear, non-salting solutions. Tubes are usually quite long (4+ meters); sometimes a small recycle is provided. Sizing this type of evaporator is usually a delicate task, since it requires a precise evaluation of the actual level of the process liquor inside the tubes. Recent applications tend to favor the falling film pattern rather than this one.
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LTV Falling-Film Evaporator
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3. Rising film (Long Tube Vertical) evaporator. In this type of evaporator, boiling takes place inside the tubes, due to heating made (usually by steam) outside the same. Submergence is therefore not desired; the creation of water vapor bubbles inside the tube creates an accessional flow enhancing the heat transfer coefficient. This type of evaporator is therefore quite efficient, the disadvantage being to be prone to quick scaling of the internal surface of the tubes. This design is then usually applied to clear, non-salting solutions. Tubes are usually quite long (4+ meters); sometimes a small recycle is provided. Sizing this type of evaporator is usually a delicate task, since it requires a precise evaluation of the actual level of the process liquor inside the tubes. Recent applications tend to favor the falling film pattern rather than this one.
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LTV Rising-Film Evaporator with Vertical-Tube Surface Condenser
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4. Plate evaporator. Plate evaporators have a relatively large surface area. The plates are usually corrugated and are supported by frame. During evaporation, steam flows through the channels formed by the free spaces between the plates. The steam alternately climbs and falls parallel to the concentrated liquid. The steam follows a co-current, counter-current path in relation to the liquid. The concentrate and the vapor are both fed into the separation stage where the vapor is sent to a condenser. Plate evaporators are frequently applied in the dairy and fermentation industries since they have spatial flexibility. A negative point of this type of evaporator is that it is limited in its ability to treat viscous or solid-containing products.
Spiral tube and bank assembly evaporators
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5. Multiple-effect evaporators.
Unlike single-stage evaporators, these evaporators can be made of up to seven evaporator stages or effects. The energy consumption for single-effect evaporators is very high and makes up most of the cost for an evaporation system. Putting together evaporators saves heat and thus requires less energy. Adding one evaporator to the original decreases the energy consumption to 50% of the original amount. Adding another effect reduces it to 33% and so on. A heat saving % equation can be used to estimate how much one will save by adding a certain amount of effects.
The number of effects in a multiple-effect evaporator is usually restricted to seven because after that, the equipment cost starts catching up to the money saved from the energy requirement drop.
There are two types of feeding that can be used when dealing with multiple-effect evaporators. Forward feeding takes place when the product enters the system through the first effect, which is at the highest temperature. The product is then partially concentrated as some of the water is transformed into vapor and carried away. It is then fed into the second effect which is a little lower in temperature. The second effect uses the heated vapor created in the first stage as its heating source (hence the saving in energy expenditure). The combination of lower temperatures and higher viscosities in subsequent effects provides good conditions for treating heat-sensitive products like enzymes and proteins. In using this system, an increase in the heating surface area of subsequent effects is required. Another way to proceed is by using backward feeding. In this process, the dilute products is fed into the last effect with has the lowest temperature and is transferred from effect to effect with the temperature increasing. The final concentrate is collected in the hottest effect which provides an advantage in that the product is highly viscous in the last stages so the heat transfer is considerably better.
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In spite of precautions taken during the design of evaporator systems, problems do arise during startup and operation. Parameters which cannot always be precisely determined make it nearly impossible to define all the problems during the design stage. These parameters include:
(I ) composition changes(2) Fouling(3) Validity of heat transfer data(4) Changes in product specification(5) Changes in raw materials(6) Changes in utilities.
Occasionally it becomes necessary to investigate the performance of an evaporator in order to evaluate its performance at other operating conditions or to determine why the system does not perform as expected. Problems encountered during the operation of evaporator systems can often be simply explained. Troubleshooting, therefore, often means checking for small details which have a great effect on the performance of the evaporator system. Of course, it is possible that a type of evaporator has been misapplied, the heat transfer surface that has been provided in not adequate for the intended service, or fouling is occurring. Discrepancies in performance may be caused by deviations in physical properties of fluids, flow rates, inlet temperatures, mechanical construction of the equipment. The troubleshooter should first check to see that compositions, flows, temperatures, and physical properties agree with those specified for design. He should then examine the equipment drawings to determine if the problem could be in the manner in which the equipment was constructed. The adequacy of the equipment installation should also be examined.Problems encountered in evaporators fall into four major categories:
(1) General operations(2) Vacuum(3) Steam economy(4) Relationship of evaporator performance to other parts of the plant,
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Typical operation problems involve:(1) Scale formation(2) Poor performance(3) Excessive entrainment losses(4) Mechanical failure of vessel internals.
These difficulties can often be traced to faulty operation, mechanical wear, or improper design. Problems arising in one or more of the system components will result in problems in other areas as well. If a detailed evaluation indicates that the basic design and installation is appropriate, then the task becomes to check specific things. Some of these will now be discussed. It should be emphasized that routine periodic inspections of evaporator systems be scheduled. Such inspections will uncover minor changes which appear harmless and are made for ease of cleaning and operation but which have seriously detrimental effects on the evaporator performance. Some items to be wary of:(1) Lack of venting(2) Open bypass valves(3) miscalibrated instruments(4) Lack of seal water on pumps(5) Leaking valves(6) Control valves not free to move.
CALANDRIASSpecific things to be checked for calandrias include:(1) Has the steam side been vented to remove air or other entrapped gases?(2) Has the steam control valve been adequately sized? What is the actual steam pressure in the steam chamber? (3) Has the steam trap been properly selected, sized, and installed?(4) Are the control valve and steam trap functioning correctly?(5) Is steam condensate flooding part of the surface? What is the temperature of the steam condensate? Is the condensate nozzle large enough? Is steam trap piping adequately sized?(6) Is the process liquid level maintained at the proper place? Are liquid level instruments calibrated? Are instrument leads plugged?(7) Is the liquid holdup adequate to prevent surging?
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(8) Are process compositions and temperatures equal to those used for design? Does the process material contain enough volatile to provide adequate boiling?(9) What is the temperature of the top head for natural circulation calandrias? A temperature higher than the liquid temperature may indicate inadequate circulation from some reason.(10) Is the available steam pressure equal to that used for design?
CONDENSERSSpecific things to be checked for condensers include:(1) Has a constant pressure vent system been provided? Has it been properly installed? lnerts should be injected downstream of the condenser, not upstream.(2) Is the vent system adequate?(3) Are condensate connections properly sized? Is liquid being entrained into the condenser? If horizontal, are the tubes levels (or sloped toward the outlet)?(4) Is all piping adequate?(5) Is the water side operating under a vacuum?(6) Are temperatures and composition equal to those used for design?(7) Is the water flow adequate? Properly vented?(8) Was all debris or other foreign objects removed from equipment and piping prior to startup?(9) If air-cooled, is the inlet piping adequate to effect good distribution?
VACUUM FAILS TO BUILDIf vacuum fails to build in the evaporator, the following guidelines will help to determine the cause.(1) Manhole cover and inspection ports not properly sealed(2) Check for other leaks(3) Liquid inlet or outlet valve not closed or seated properly(4) Packing glands not tight. No seal water flow.(5) Pump discharge check valves leaking(6) Safety valves not sealed(7) Barometric leg not sealed(8) Sight glasses leaking(9) Fouled condensers(10) Tube failure.
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Steam Jet Vacuum Systems(1) Steam pressure too low
steam valves not fully open strainer may be plugged boiler pressure too low
(2) Wet steam(3) Restriction in exhaust line from second-stage ejector(4) If two single-stage units are used, valves may be closed
(5) Barometric condenser flooded
water flows too high leak in barometric leg or barometric leg plugged discharge pump not adequate
(6) Not enough water to intercondensers to condense first-stage steam exhaust(7) Ejector steam nozzle plugged(8) Steam nozzles of different stages reversed by mistake
NO VACUUM IN STEAM CHEST(1) Vent valves not open(2) Steam valves open or not seated(3) Condensate pump seal leak or no seal water(4) Vent valve leak(5) Condensate pump check valve leak(6) Safety valve leak
VACUUM BUILDS SLOWLY(1) Air leaks(a) Fluid lines not tight(b) Covers and inspection ports leaking(2) Improper operation of first stage ejector(3) Wet steam(4) Steam pressure too low(5) Nozzle wear or plugging(6) Tube leak
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Hand book of evaporative technology By Paul E. MintonUnion Carbide CorporationSouth Charleston, West Virginia
APV Americas, Engineered Systems Separation Technologies
Parker, N. H., “How to Specify Evaporators”, Chemical Engineering, July 22, 1963,pp. 135-l 40.
Whiting Equipment Canada Inc.Swenson Evaporators
Evaporator - Wikipedia, the free encyclopediahttp://en.wikipedia.org/wiki/Evaporator
Shah,G.C., “Troubleshooting Distillation Columns, Chemical Engineering, July, 31,1978. pp. 70-78.
Shah, G. C., “Troubleshooting Reboiler Systems”, Chemical Engineering Progress, Ju I y1979. pp. 53-58.
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