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Thin Film Evaporation Recent experience indicates that mechanically aided thin film evaporators are neither as complex nor as expensive as was earlier thought
he principle of the mechanically aided thin film
Tevaporator is simple. A amount of liquid is continuously fed into the top of a still, usually from a small
number of openings. The liquid flows down the evaporator wall by gravity and, aided by mechanical wipers, forms a thin film that covers the evaporating surface. Thus only a thin film of liquid is heated at any given time. Products which are heat-sensitive or viscous, or have low thermal conductivity, are prime prospects for mechanically aided thin 6lm evaporators. Typical examples are fatty acids, amines, tall oil,esters, plasticizers, rocket fuels, urea, radioactive wastes, and fruit juices. The range of separation processes in which thin filmevaporators are used is also extremely broad and includes dehydrating, deodorizing, decolorizing, stripping, and concentrating. When should a product be processed by mechanically aided thin film techniques? As in selecting any process method, the choice depends first on the ability of the techniquea to do the job and then on their cost. Two misconceptions sometimes deter selection of mechanically aided thin 6lm evaporation by the p m =or. First, the c a t is often believed to be prohibitive. Second, the technique is considered too complex to be effective and reliable, due to moving parts and the high vacuum generally associated with it. While these opinions may have been partly true early in the develop-
Leonard E. Najder is Product Manager for Wiped Film Enaporatm sales for The pfaudler Co., a dimsion of pfnudler Permutit, Inc. He is an industrial engineer specializing in th chemical process industry and has been inuolved with thin film cuapmatm deign and applicationsfm 10 years. AUTHOR
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ment of mechanically aided thin film evaporation, they have practically no merit as a basis for assessing the value of thin film evaporation as it is today. Thin film evaporation is by no means an exotic art. The procedures used are relatively simple. There is no need to design a process around the evaporation step by trying to use other processing techniques such as stripping, bleaching, and absorption. In many applications, conventional approaches require so much sophistication that they often are more expensive than thin film evaporation. Examples of this are in removing color from plasticizers and when separating tall oil, rosin, fatty acids, and fatty amines. On a price per square foot basis, the cost of mechanically aided thin film units is higher than conventional evaporators. But the units become much more economical as their size increases. A 4-square-foot rnechanically aided thin film evaporator costs approximately $2000 per square foot, but a 100-square-foot unit costs only about $300 per square foot. Thin film evaporation should be considered when it is difficult or impossible to perform an operation in a calandria, natural or forced circulation, or other standard evaporating units. Mechanically aided thin film evaporation is almost mandatory in many cases. In handling plasticizers, for example, processors have found it the only suitable method for removing color to meet customer specifications. I t is virtually impossible to remove color from plasticizers by standard evaporation because of heat damage and decomposition resulting from the relatively long time the product must remain at temperature with this method. With a mechanically aided thin film evaporator, plasticizers can be exposed to higher temperatures for a relatively short period to remove color with no thermal degradation of the product. Some users find that the unit, though purchased for a specific, difficult process, can also be applied to conventional processes economically. The same thin film evaporator used for color removal with plasticizers can also be used to strip unreacted materials from alkyl phenol, for example. In fact, many products purified by stripping, bleaching, and adsorption can be economically handled by thin film evaporators. Process Consideration
As mentioned earlier, thin film evaporation should be given serious consideration whenever the processor is dealing with materials that are heat sensitive or viscous, or have low thermal conductivity. Defining the point at which a product falls into one, or all, of these categories, however, is by no means as simple as labeling the category. Compiling a complete formal set of criteria for application of the thin film evaporator is difficult. However, a good basic knowledge of the physical properties of the product may be all that is needed to decide whether or not thin film evaporation should be used.
Heat Transfer
Extremely high heat transfer rates are characteristic of mechanically aided thin film evaporators. In fact, over-all heat transfer coefficients as high a s 3750 B.t.u. per hour per square foot per degree Fahrenheit have been reported. Such values are atypical and result from controlled conditions with special experimental apparatus which has an extremely thin heat transfer shell. More common values range from 350 to 450 B.t.u. per hour per square foot per degree Fahrenheit for waterlike products in production equipment with a heavier gage wall that provides structural rigidity and strength to withstand high jacket pressure. The high heat transfer efficiency of mechanically aided thin film evaporators makes them ideal for handling products that have low thermal conductivity. With this type of product a thin film evaporator may prove to be more economical than a standard type evaporator, even though the latter may be able to handle it, since over-all heat transfer and ultimately the output will be greater in the thin film type. Examples of products having low thermal conductivity that can be handled by thin film equipment include fatty acids, ethanolamines, and urethane polymers, or, in general those materials whose thermal conductivity is 0.08 B.t.u. per hour per square foot per degree Fahrenheit per foot or less. High heat transfer is also the principal reason for the ability of mechanically aided thin film evaporators to handle heat sensitive products. Another reason is the rapid movement of the product over the heat transfer surface. In most thin film units the product remains in contact with the heat transfer shell from one second up to a maximum of five seconds, depending on the viscosity. Thus, time at temperature of the product is minimal, compared to conventional techniques. In fact with conventional evaporation, as in a calandria, for example, the time at temperature for a product can seldom be determined because of constant recycling of the material. Most thin film evaporation is a one pass operation. Vacuum also controls heat transfer rates in thin film equipment. Standard thin film evaporators usually operate at internal pressures from 0.3 to 20 millimeters of mercury absolute. In several types of thin film units (those with external condenser) there is little to be gained in terms of reducing boiling point by operating at pressures lower than approximately 3 to 5 mm. Hg absolute in the evaporator. This is because of the pressure drop which occurs from the external condenser to the evaporator. Units with an internal condenser, however, are not as limited in this respect. With an internal condenser, it is often desirable to distill many “fringe” products for which higher vacuum, if not actually needed, is more efficient and economical. With heat sensitive products, the time that a product can remain at a given temperature without degradation is usually known, or should be known. If laboratory work has been done on a product, the time-temperature relationships undoubtedly have been established. When normal evaporative methods exceed or approach this VOL. 5 6
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critical temperature, mechanically aided thin film evaporation should be investigated. I t must be remembered, however, that decomposition is a function of both time and temperature and, in many cases, a wellestablished (by normal standards) decomposition temperature is greatly exceeded by thin film processing without degradation. This, of course, is due to the short contact time with heat. A further benefit derived from higher temperature operation is a reduction in the viscosity of some materials. Viscosity
A great deal of work is being done with viscosities to develop criteria which can be used to determine if thin film techniques should be employed, but more data and experimentation are necessary. Whether or not a fluid is Newtonian or non-Newtonian-i.e., thixotropic, pseudoplastic, dilatant, etc.--should be taken into consideration. And shear rate us. shear stress values can be an aid in determining the need for a thin film unit. For a normal Newtonian fluid, thin film evaporators should always be considered whenever the viscosity of the residue approaches 20 to 30 centipoises and above, but a great many applications have proven to be economically feasible even with viscosities that are only 3.5 centipoises. Water or waterlike products are distilled by thin film equipment at maximum rates of about 50 pounds per hour per square foot. For low viscosity organics which have low latent heats of vaporization, distillate rates up to 100 pounds per hour per square foot have been obtained. Examples include desolventizing ethylene glycol or rocket fuels. On the other hand, high viscosity products (desolventizing urethane polymers, for example) exhibit distillate rates in thin film equipment as low as 10 to 20 pounds per hour per square foot. The more viscous the material, the greater the difficulty thinning, spreading, and depositing the film.
Nevertheless, mechanically aided thin film evaporation is probably the most effective method for evaporating highly viscous products. Materials whose residue viscosities are in the range of 100,000 centipoises, such as corn sirup, urethane polymers, and latex, have been processed with good results. Viscosity relates directly to the time-temperature relationship in thin film equipment. The more viscous the material, generally, the greater the contact time. Even materials of 100,000 centipoises, hoTvever, will pass through thin film units in about five seconds. Materials having a viscosity of 100 centipoises seldom remain in contact with the heat transfer surface more than one second, For a given product, viscosity varies at different points along the length of the mechanically aided thin film evaporator. At the top of the unit, fluid consistency is low, rate of evaporation is high. At the bottom, high product consistency and low evaporation rate result in a low film coefficient. Over-all film coefficients are the average of these extremes and intermediary points. Figure 1 shows over-all heat transfer coefficients obtained by processing New-tonian food products and water in a wiped thin film evzporator. Data are taken from tests conducted at University of California, Davis Agricultural Testing Center. Figure 2 shows heat transfer coefficient as a function of average viscosity for non-Newtonian fruit purees processed with the wiped film evaporator. Fruit concentrates of about 60% total solids were produced in one pass on the runs from which the above data were taken. Concentration of the fruit purees was limited by residue discharge problems and not by the evaporator's heat transfer efficiency. Heat damage to the sensitive purees was negligible. Methods
Mechanically aided thin film evaporators today can be categorized into two groups: molecular stills and
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VISCOSITY, CENTIPOISE Figure 7 . Effect of viscosity on heat transfer coeficient f o r Newtonian fluids. Data courtesy of L'niversity of California, Davis Center; Davis, Cahj. 28
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VISCOSITY, CENTIPOISE Figure 2. Effect of viscosity on heat transfer coejicient f o r nonNewtonian fluids. Data courtesy of University of California, Davis Center, Davis, Cd$.
low pressure evaporators refexred to herein as standard thin film units. Molecular distillation is a highly specialiied field in which units operate at extremely high vacuum, usually from 0.001 to 0.3 mm. Hg absolute. Two types of molecular stills are: (1) those where the turbulent thin film is produced by a spinning cone or disk; and (2) units which incorporate wiper blades and an internal condenser placed close to the evaporator surface, plus a diffusion pump to obtain the required high vacuum.
Figure 3. Om way to po&t a thin JXni on hcd transfm wfacts is with frue$?oating wipe bkuies, which are forced against fhc ewporatm wall with ccnfrifugalforce from a spinning rotm Standard thin film units are more widely used than molecular stills becauseof their simpler design and because the extremely high vacuum required in molecular stills is very difficult and expensive to maintain on a production basis. Degassers, for example, are required with high vacuum equipment, adding considerably to the cost. Although standard mechanically aided thin film units vary in construction, each type is designed to produce a thin film on the heat transfer surface irrespective of the viscosity, throughput, or evaporation ratio of the feed. One way this is accomplished is by wiping. Free floating wiper blades (Figure 3) are forced against the evaporator wall solely by centrifugal force created by a spinning rotor. The blades continuously wipe the product down the evaporator walls and distribute the material downward as well as circumferentially, thereby keeping the contact time of the product against the heated wall to a minimum. The wiper blades are slotted to prevent channeling of the product. High speeds for the rotor are not necessary since only enough centrifugal force to keep the wipers against the wall is required. There is room in this design for an internal U tube condenser as well as an integral entrainment separator. The basic unit can be modified for use as a molecular still. Because of the many unique processes to which thin film equipment is often applied, seldom are two units
exactly alike. Problem products, for instance, pose special difficulties which usually must be handled on a custom basis. Scale-up, thedore, is a custom job for which no general data have been published. Although some manufacturers have developed precise mathematical concepts and formulas for predicting scale-up requirements under given conditions, the data are proprietary for the most part. Size is a definite limiting factor in all thin film designs. Generally, the units will have no more than 200 square feet of heat transfer surface. Size is limited by mechanical design considerations relating principally to the rotor. As rotor size and speed increase, so do mechanical problems relating to critical speed, power drain, size of bearings, and shell concentricity. Feed rates vary with unit size, evaporator length to diameter ratio, jacket temperature, percentage of distillate desired from one pass, and physical properties of the feed. For example, minimum feed rate for a 12-inch diameter unit varies from approximately 50 to 100 pounds per hour average. Maximum feed rate for water in a 12-inch diameter evaporator varies in the range from 1500 to 2000 pounds per hour since care must be taken not to flood the unit. A general curve which indicates how evaporation rate varies with feed rate is given in Figure 4. At low feed rates there is insufficient wetting of the heated surface. This “dry wall” effect results in low evapration rates. As the feed rate is increased, the evaporation rate increases up to a maximum; when the feed rate is increased beyond this point, the film thickness increases and the evaporation rate decreases as a result. The maximum point of the curve generally can be shifted upward and to the right by increasing the temperature of the heating medium (or by operating a t higher vacuum). Typical System
In its simplest form, a thin film evaporator system consists of a feed tank, a gear-type feed pump, a rotameter or some other feed measuring device, a preheater
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and/or a degasser if necessary, a back pressure valve, the evaporator, a condenser, and a gear pump and receiving tank for the residue or concentrate (Figure 5). If the unit has an internal condenser, another gear pump and tank are required for recovery of the distillate. Vacuum system may be 3-, 4-, or 5-stage jet, depending on the absolute pressure level required in processing. A compound vacuum pump of the gas b a h t type may also be used. Special care must be taken to keep piping systems sealed when operating at 10 mm. Hg absolute p w u r e or lower. Feed, distillate, and residue pumps should be imme& in an oil bath to keep the temperature differential between process material and p u m p to a minimum. This prevents expansion of pump bearings. Instrumentation varies depending on work being done and extent of automation desired. Control of feed is sometimes necessary to prevent flooding of unit. In other cases, control of residue temperature helps to keep viscosity and/or product at an acceptable level. Control of heating medium temperature is also needed. Refinements to a basic system include a heating system for hot oil, Dowtherm, Aroclor, or possibly a cold water supply or refrigeration system for the condenser. It may be desirable to install 2- or 3-way valves in the feed line so that a solvent can be injected immediately if material being processed can solidify while in the unit. Conclusion
Manufacturers have demonstrated the importance of time as a controlling factor in the operation of thin film evaporators. Higher temperatures than once thought possible can be utilized because average retention time is low, due mainly to one pass operation.
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User understanding, however, of the relative importance of time us. temperature in thin film evaporators lags considerably behind demonstrated equipment capability. The need for better process methods should spur manufacturen to master the techniques for using thin film equipment more rapidly than in the past. Further impetus to adoption of thin film equipment will occur as news of successful applications become more widespread. Performance of thin film evaporators will continue to be improved. Both facton in the time-temperature relationship will be subjects of additional study. Heat transfer through the shell will undoubtedly receive much attention. Improvements here depend upon the advent of readily available and economical clad materials, such as c o p p e d a d stainless steel. Improved heat transfer through the shell will make the u8e of a lower temperature heating medium in the jacket practical, reducing the danger of local hot spots. Further work also must be done in jacket design to improve heat transfer. Use of spiral jackets is a step in the right direction. However, stiffening bars or some variation of dimpled jackets, etc., should eventually lead to use of thinner shells for better over-aU heat transfer in this type of equipment. Mechanical design considerations will continue to limit maximum size of standard thin film evaporaton to about 200 square feet. A major problem in this area is rotor design. Another limiting factor that will receive attention is concentrate removal. Advances in this area require development of improved pumps and new designs for outlets to keep highly viscous concentrates moving. Widespread use of thin film evaporaton is certain. For many problem products there is no effective alternative processing method.
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Figwe 5. SimpliJFcdpowchwtfor rypiCaI thin f l m cvaporafiongs&m
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