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Comment Processing & Handling

Evaporators: Design Concepts and Equipment Selection

By Alan Gabelman |

This primer provides guidance on key aspects to consider when designing and specifying evaporators, which are used in a diverse array of industrial sectors

Industrial evaporators are used to remove a solvent from a non-volatile solute to obtain a concentrated solution of the solute. This article focuses on evaporator heat-transfer fundamentals and other introductory concepts, as well as types of evaporators (Figure 1), including advantages, disadvantages and suitable applications of each.

In most applications, the heat source for evaporation is condensing steam. The concentrated product is aptly called concentrate, or thick liquor. The solvent that is removed is condensed to form process condensate, or simply condensate. With the exception of agitated thin-film units, the solvent is usually water, and this is assumed in most of the discussion that follows. The valuable discharge stream is almost always the concentrate, while the condensate is used for cleaning, recycled as process water, or simply discarded. The notable exception is the evaporation of seawater to obtain potable water, a process that is often part of a hybrid process with reverse osmosis. Evaporators are used in numerous industries, including food, chemicals, paper and textiles, and several significant applications are listed in Table 1.

Figure 1. Evaporators are used in a wide variety of process applications, and there are many styles of equipment in use today

Evaporator design

The challenge faced by the evaporator designer is to provide the amount and configuration of heat-transfer surface such that, when presented with the feed material at the intended process conditions, leads to efficient removal of water (or other solvent) at a rate that meets the production demand. Traditionally, an evaporator heat-transfer surface is tubular, but flat plates are also commonly used. Both types of heat-transfer surface are packaged in a variety of evaporator types, such as film evaporators, forced circulation evaporators and others. Sometimes these evaporators contain energy-saving features, such as multiple effects or vapor recompression. The preferred type of evaporator and, if appropriate, energy conservation technology, depends on numerous factors, including physical properties, required capacity, initial and final concentrations, foaming tendency, fouling tendency and heat sensitivity.

Important physical properties to consider for evaporator design are density, thermal conductivity, boiling point elevation (BPE), heat capacity, heat of vaporization and viscosity. To further complicate the design challenge, these physical properties can change significantly as the concentration increases. For instance, often the feed has a water-like viscosity, but the concentrate is a heavy syrup. The effect of temperature on physical properties must also be considered. Foaming can be a problem if organic compounds, such as protein (for example, in fermentation-derived products), are present. Chemical antifoams can help, along with extra headspace in vapor-liquid separators. Fouling may be caused by organic or inorganic species, with appropriate cleaning protocols available for each.

The evaporator design variable is heat-transfer area, which is specified to provide the required evaporation capacity, Q. Area (A) is calculated using the familiar capacity equation, shown in Equation (1), where U is the overall heat-transfer coefficient and ΔT is the temperature driving force:

A = Q/UΔT      (1)

In evaporators, ΔT is not as straightforward as a simple difference between two fixed temperatures, because temperature profiles are complex. BPE and pressure drop result in a change in temperature upon travel along an evaporator tube or plate. For purposes of design, ΔT is taken as the difference between the temperature of the condensing steam and the emerging concentrate, even though the actual ΔT varies along the length of the heat-transfer surface.

As illustrated in Figure 2, BPE erodes the temperature driving force, requiring more heat-transfer area for a given evaporation load. BPE can be estimated using Dühring’s rule, which says that the boiling point of a given solution is a linear function of the boiling point of pure water at the same pressure. An example is the Dühring plot for aqueous sodium hydroxide shown in Figure 3, which contains concentration as a parameter. Note that for concentrated solutions, the BPE is considerable.

Figure 2. Boiling point elevation (BPE) erodes the temperature driving force, requiring more heat transfer area for a given evaporation load


Figure 3. This Dühring plot for aqueous sodium hydroxide is used to determine the boiling point elevation. The parameter is mass fraction NaOH. Redrawn from data given in Ref. 3

An evaporator must remove a specified amount of water from the feed per unit time, bringing the solids content to some target level. This requirement, along with the thermal condition of the feed and its physical properties (heat capacity and heat of vaporization), determine Q. The temperature driving force is set by the steam temperature and the boiling point of the process material at the chosen operating pressure. There are a number of considerations in choosing the operating pressure, which is usually below atmospheric. A lower pressure (higher vacuum) results in a lower temperature, which is important with heat-sensitive products, such as fruit juices. Moreover, the lower temperature offers a greater temperature driving force, allowing a given capacity to be achieved with less heat-transfer area. On the other hand, vacuum generation is a cost, and the higher the vacuum, the higher the cost. Also, lower pressure means lower vapor density, and vapor piping needs to be larger to accommodate the corresponding increase in volume. For some viscous products, the minimum temperature is determined by the need to limit the viscosity and maintain a pumpable concentrate. Finally, the pressure must be high enough to allow a sufficient difference between the evaporation and cooling-water temperatures at the condenser.

Heat-transfer coefficients are well known for common applications, such as concentration of orange juice or aqueous sodium hydroxide. For less common situations, there are correlations that relate heat transfer coefficient to physical properties, tube or plate geometry and operating conditions [1, 2]. As an alternative to published correlations, equipment manufacturers are more likely to use their own proprietary methods. Heat-transfer coefficients can also be determined experimentally [2]. Values range from 100 to 1,000 Btu/h-ft 2-°F (570–5,700 W/m 2-ºC). In general, values are higher with lower viscosity and faster-moving fluids.

Figure 4. This simple schematic diagram provides a basis for development of evaporator mass and energy balances

 

Mass and energy balances

A simple schematic diagram of an evaporator is shown in Figure 4. The overall and component mass balances are given by Equations (2) and (3):

mF = mP + mV      (2)

xF  mF = xP mP      (3)

Equations (2) and (3) are combined to obtain the useful expression for the evaporation rate shown in Equation (4):

mV = mF  [1 – ( xF  / xP)]     (4)

The general form of the energy balance, assuming negligible heat loss, is given in Equation (5):

mS HS + mF HF = msHc + mV HV + mP H P      (5)

Note that the mass flowrates of steam and steam condensate are the same. Steam condensate should not be confused with process condensate, which is the water (or other solvent) removed from the feed during the evaporation process. It is convenient to rearrange Equation (5) to obtain the expression for the heat duty shown in Equation (6):

Q = mS ( HSHC) = mV HVmF HF + mPHP      (6)

For a situation with saturated steam and no condensate sub-cooling, HSHC is the latent heat of vaporization, λS, which is approximately 1,000 Btu/lb for water. More accurate values may be obtained from the steam tables, available either in handbooks or online. Steam superheat and condensate sub-cooling are usually much smaller than latent heat and can be neglected for estimating purposes. If more accuracy is desired, enthalpy of superheated steam is also given in the steam tables, and sub-cooled condensate enthalpy can be calculated using the familiar sensible heat equation, shown in Equation (7):

HC = CPC   ( TC – Tref)      (7)

Figure 5. This enthalpy-concentration diagram for aqueous solutions of sodium hydroxide can be used for evaporator heat-balance calculations. The parameter is temperature in °F. The dashed lines are extrapolated values. Redrawn from data given in Ref. 3

The reference temperature is taken as 32°F, the same as in the steam tables. The right-hand side of Equation (6) is the most general form of the heat balance, and it must be used for materials with significant heat of dilution. Examples include sodium hydroxide, sulfuric acid and calcium chloride. For some such systems, designers can use enthalpy-concentration diagrams, like the one shown in Figure 5 for sodium hydroxide. This diagram gives enthalpy as a function of the mass fraction of sodium hydroxide (NaOH), with temperature as a parameter. With no heat of dilution, the isotherms would be straight lines rather than the curves seen in the diagram. Note that the isotherms terminate at the saturated solution curve, although they could be extended to handle precipitated solids if desired [3]. For systems with negligible heat of solution, Equation (6) becomes the expression shown in Equation (8):

Q = mS (HSHC) = mF CPF  ( TF,sat – TF)+ mV λV     (8)

The first term on the right-hand side is the sensible heat needed to raise the temperature of sub-cooled feed (the typical case) to the saturation, or boiling, temperature. For the latent heat of vaporization, λV, the value in the steam tables at the evaporation pressure is usually used, although the actual value is slightly different when BPE is significant [ 3].

Equipment selection

As stated previously, the preferred type of evaporator depends on numerous factors, including physical properties, required capacity, initial and final concentrations, foaming tendency, fouling tendency and heat sensitivity. The following sections detail various types of evaporators and provide guidance for best practices in design and installation.

Batch evaporators

A batch evaporator consists of a vessel equipped with a heating jacket or internal coil, an overhead condenser, a condensate receiver, and usually, a source of vacuum. An example of a flowsheet for such an evaporator is shown in Figure 6. Feed is charged into the vessel, either by pumping, or more commonly, drawn in by vacuum. Heat is then applied and evaporated vapor is condensed overhead, while the contents of the vessel decrease in volume and increase in concentration of non-volatiles. Alternatively, in semi-continuous mode, additional feed is added through the feed-charging line as room allows. This way, at the end of the process, the vessel is full of product at the final concentration, rather than only partially full.

Figure 6. This flowsheet shows just one of the numerous ways to operate a batch evaporation process. Controls are mostly manual, but like any chemical engineering unit operation, the degree of automation can be totally manual, highly automated or anywhere in between

The heat source is usually steam, but sometimes hot oil or another heat-transfer fluid is used. Cooling fluid may be applied to reduce the temperature of the final concentrate prior to unloading. While some cooling is usually necessary for personnel safety, concentrate is often removed at an elevated temperature to avoid high viscosity and to ensure that the fluid will drain completely or is pumpable. City, well or cooling-tower water is typically used, but chilled water or aqueous ethylene or propylene glycol are also options.

Usually, the vessel is equipped with a mixer, which provides several benefits. The mixer facilitates liquid motion at the heat-transfer surface, resulting in a higher heat-transfer coefficient. In addition, any solids present initially or generated during the concentration are kept in suspension. Gradients in temperature or concentration are mostly eliminated, which typically reduces bump-over and allows for a smoother process. For semi-continuous operation, the mixer ensures that the incoming feed is quickly blended into the contents of the vessel, avoiding cold spots. However, while usually preferred, a mixer is not always essential because considerable movement results from the boiling itself. Disadvantages are increased maintenance, the need to seal against vacuum and higher capital cost, not only for the mixer itself, but also for the increased structural strength of the vessel needed to support it.

In Figure 6, condensed vapor is directed into one of two receivers. At any given time, one receiver is in service while the other is standing by. When full, the in-service vessel is taken offline, while the standby unit is evacuated then brought online. Without prior evacuation, a disruption in system vacuum would occur upon opening the valve connecting the standby receiver to the condenser. The vessel should be evacuated using an auxiliary vacuum source, rather than the same one used to generate the system vacuum, also to avoid a bump in the system vacuum. Once the fresh vessel is in service, air (or an inert gas such as nitrogen) is introduced into the full one to break the vacuum, then the vessel is drained and put on standby. Alternatively, a single receiver and a pump can be used in place of the two receivers and an auxiliary vacuum source.

The main advantages of a batch evaporator are simplicity, flexibility, relatively low cost and the ability to handle feeds containing undissolved solids — for example, jams and jellies with whole fruit [4]. On the other hand, there are several disadvantages, including low heat-transfer coefficients, low heat-transfer area per unit vessel volume, and the inherent loss of productivity with batch versus continuous operation. In addition, because of the extended residence time, batch evaporators are not suitable for heat-sensitive products. In spite of these disadvantages, however, batch evaporators are often chosen for specialty operations that require the flexibility to handle small volumes of multiple products, when the volume of any single product is not sufficient to justify the use of a continuous evaporator. When possible, similar products are run consecutively, so that extensive cleaning between products may not be necessary.

Short-tube vertical evaporators

The short-tube vertical evaporator, also known as a Roberts evaporator, is one of the oldest designs still in common use. The major application is concentration of sugarcane juice. In addition to their use as evaporators, these units are employed as evaporative crystallizers, primarily for the production of sugar.

A schematic representation of a short-tube vertical evaporator is shown in Figure 7. The shell-and-tube heat exchanger is situated inside of the evaporator vessel, near the bottom. As suggested by the name, the tubes are short relative to the height of the vessel. The heat exchanger, called the calandria, contains an open area at the center, known as the downtake. Process fluid circulates upward through the calandria tubes, against condensing steam on the shell side. The vapor formed travels to the top of the evaporator, where entrained liquid droplets coalesce on the mesh pad and fall back into the boiling liquid. Meanwhile, the liquid emerging from the calandria tubes travels downward through the downtake, then back up through the tubes for a subsequent pass. This circulation occurs by natural convection, as in a thermosyphon reboiler in a distillation column. That is, the lighter vapor-liquid mixture rises in the tubes as it is displaced by the heavier liquid coming from the downtake. As an alternative to the continuous operation depicted in Figure 7, these evaporators can be operated in batch mode, as is done in the sugar industry.

Figure 7. The short-tube vertical evaporator is one of the oldest designs still in common use

Why is the downtake needed? That is, why is it insufficient for the vapor-liquid mixture in the tubes simply to be replaced by the incoming feed? The reason is that the circulation rate is much higher than the feedrate, and without circulation, the linear velocity would be too low to obtain an adequate heat-transfer rate. To accommodate this relatively high circulation rate, the downtake area is about half that of the tubesheet. Typically, tubes are 2–3 in. in diameter by 4–6 ft in length, and the linear velocity through the tubes is about 3 ft/s. Generally, heat-transfer coefficients are highest when the liquid level, as indicated by an external sight glass, is about halfway up the tubes. Lower levels lead to incomplete wetting, and in turn, rapid fouling of the heat-transfer surface, a situation commonly known as burn-on. Higher levels are used for materials with a greater tendency to foul, or if the evaporator is also a crystallizer. A mixer is not needed, although the use of one can improve heat transfer, increasing capacity by as much as a factor of two. Use of a mixer may be preferred in crystallization service, not only for better heat transfer, but also to keep larger crystals from settling. However, there are many short-tube evaporator-crystallizers without mixers.

Short-tube vertical evaporators offer several advantages. Because the tubes are large in diameter and short in length, they are easily cleaned, and are well suited for materials that require mechanical descaling. Other advantages are low headroom requirement, proven designs based on many years of experience, and relatively low cost. Disadvantages include low heat-transfer coefficients, low heat-transfer area per unit volume, high floor-space requirement, large weight and high holdup of process material. In addition, use with corrosive materials is not advisable, because with the large evaporator body, a corrosion-resistant alloy would incur considerable cost.

Long-tube vertical-film evaporators

These evaporators operate with a thin film of liquid on the heat-transfer surface. As evaporation takes place, vapor fills the core of the flow channel, which thins and accelerates the film. The thin film and high fluid velocity lead to high heat-transfer coefficients, allowing a given evaporation capacity to be achieved with a relatively low Δ T. The low Δ T not only lowers the maximum temperature, but also reduces the occurrence of hot spots. Moreover, because the product exists as a thin film rather than filling the entire tube volume, liquid holdup, and in turn residence time, are low, minimizing heat exposure. Other advantages of film evaporators are low cost per unit area, simple construction, low floorspace requirement, ability to handle foamy liquids and ability to handle corrosive process streams.

Because of these advantages, film evaporators are preferred for many applications, and they represent more evaporation capacity than all other types combined. These evaporators are especially well suited for concentration of heat-sensitive materials, such as fruit and vegetable juices. On the other hand, some applications are not suitable — for example, high-viscosity fluids (>300–400 cP), because film formation is difficult. In addition, film evaporators are not recommended for materials with a high fouling tendency, or if solids are present or may form. These situations are usually handled with a forced circulation evaporator (discussed later).

A schematic drawing of a rising-film evaporator, the first film evaporator, is shown in Figure 8. Tubes are typically 1–2 in. in diameter, and their length ranges from less than 20 ft to more than 30 ft. Feed enters the bottom and is directed upward. Because the feed is usually subcooled, some portion of the heat-transfer area is needed to bring the temperature to the boiling point. Consequently, there is a hydrostatic head of liquid at the bottom of each tube that must be overcome. Afterward, the liquid film forms on the heat-transfer surface. The evaporated vapor accelerates the film, which becomes thinner as the two-phase mixture moves toward the top of the heat exchanger. Eventually, the high-velocity stream of vapor and concentrated liquid emerges at the top, impinges on a deflector plate, and then enters the vapor-liquid separator. The vapor goes to the condenser, while the concentrated liquid is removed as product. The design shown in Figure 8 provides for a portion of the concentrate to be recycled. This is necessary when the ratio of feed to concentrate volume is high, to ensure that there is sufficient liquid to keep the tubes wet.

Figure 8. In this long-tube rising-film evaporator, a film forms on the inside of the tubes. Evaporation occurs as the film rises, and the generated vapor causes the film to thin and accelerate

In a falling-film evaporator (Figure 9), the feed enters the top rather than the bottom of the heat exchanger. Unlike its rising-film counterpart, there is no static head of liquid caused by subcooled feed. Instead, assisted by gravity, the film forms immediately. Films are thinner and faster-moving than with the rising-film evaporator, leading to higher heat-transfer coefficients that make economic operation possible at Δ T values as low as 7°F. Moreover, the residence time is shorter than with rising-film units, typically only 15–30 s. This is attributable to the thinner film with the falling-film design, and also the absence of the column of liquid that occurs with rising-film units when the feed is subcooled.

Figure 9. Unlike its rising-film counterpart, this falling-film evaporator needs a distributor to ensure uniform distribution of liquid to each tube

For proper operation of a falling-film evaporator, the feed must be evenly distributed among the tubes. Tubes that receive insufficient feed may experience dry spots, localized overheating, increased fouling and reduced heat-transfer coefficients. Conversely, if too much liquid is delivered to a tube, film formation may be difficult, and the final concentration may be too low. To ensure adequate distribution, a distributor is placed over the top tubesheet. Perhaps the most common type is the orifice-type distributor, which is simply a metal plate with holes that direct the liquid flow. Another method is to use spray nozzles to direct liquid into each tube.

Forced-circulation evaporators

A schematic drawing of a forced-circulation evaporator is shown in Figure 10. Process fluid circulates from the vapor-liquid separator, also called the flash chamber, through the heat exchanger and back. The orifice plate (alternatively, a control valve or simply hydrostatic head may be used) applies enough back-pressure to prevent boiling in the heat exchanger. For this reason, this type of evaporator is also called a suppressed boiling evaporator.

Figure 10. In this forced-circulation evaporator, boiling does not occur in the tubes. Instead, vapor forms as the superheated process material flashes across the orifice plate

Only sensible heat (no latent heat) is transferred in the heat exchanger, and the process liquid exits the heat exchanger at a temperature above the boiling point at the prevailing pressure in the flash chamber. The pressure drop across the orifice (or other back-pressure device) leads to flashing as the liquid enters the flash chamber. The flashed vapor is directed to an overhead condenser, usually via a mesh pad to recover entrained liquid droplets. Meanwhile, the concentrated liquid makes another pass through the heat exchanger, with some new feed added, and some concentrate removed as product. The circulation rate is much higher than the feed and concentrate flowrates, as well as the evaporation rate. Typically, the circulation rate is 220–330 lb/h per 1 lb/h of evaporation [ 4], and the linear velocity in the heat-exchanger tubes is 6–15 ft/s.

Forced-circulation evaporators are less efficient than film evaporators. Heat-transfer coefficients are lower, meaning cost per unit heat-transfer area is higher. Cost is also driven up by the need for the large circulation pump and piping. The high linear velocity leads to high pumping and maintenance costs, and if abrasive solids are present, erosion problems. Moreover, the large flash chamber results in high process liquid holdup and residence time. Because of these disadvantages, a forced-circulation evaporator is chosen only when a film evaporator will not work. Such applications include viscous liquids, because these do not form a film easily. The forced-circulation design is also appropriate for products that have (or may have) crystals or other solids present, or are heavily fouling, because the high linear velocity keeps solids in suspension, hinders formation of a fouling layer and prevents plugging.

Although the discussion so far has focused primarily on tubular evaporators, flat plates are also used as the heat-transfer surface. A plate heat exchanger comprises a series of corrugated metal plates, separated by polymeric gaskets around the periphery of each plate. The plates are attached to a frame, then pressed together to form a series of flow channels. Steam and process fluid are directed to alternate channels, and heat is transferred across each plate from the steam side to the process side. Orifices, slotted openings and elastomeric seals are located to direct the various streams where they need to go, and to exclude them from places they do not belong. Baffles are employed to create a tortuous flow path for the process fluid that increases local velocity, and in turn, heat-transfer efficiency. Plate spacing is typically¼ to½ in. The corrugations allow the plates to contact each other at a number of points so that distortion is minimized. As with tubular evaporators, both film and forced-circulation designs are available, and the selection criteria are the same.

The rising-falling-film plate evaporator, developed by APV (now part of SPX Flow) in 1957, was the original plate evaporator, and is still in common use today. As shown in Figure 11, every other product channel is a rising pass, and the alternate product channels are falling passes. Feed, delivered through two parallel ports, is equally distributed to each of the rising-film passageways. The feed temperature is slightly higher than the saturation temperature. The resulting flash promotes turbulence, leading to more uniform distribution and better heat transfer. The liquid-vapor mixture rises to the top of the rising pass, then moves through a slot in the adjacent steam plate to the falling pass. There, gravity further assists film movement as the evaporation process is completed. In both the rising and falling passes, rapid movement of the thin film leads to low residence time and a high heat-transfer coefficient. The mixture of vapor and concentrate from all falling passageways flows through a rectangular duct to the vapor-liquid separator [ 4].

Figure 11. This rising-falling-film plate evaporator was first introduced in 1957, and is still in common use today [4 ]

Plate evaporators offer several advantages over tubular designs in certain applications. The corrugations and the tortuous flow path lead to turbulent flow at Reynolds numbers as low as 100 to 400. The resulting high heat-transfer coefficients allow a given evaporation rate to be reached at a lower ∆ T. This, along with the low holdup and short residence time, make the plate evaporator a good choice for heat-sensitive products. In addition to improved heat transfer, the high fluid velocity results in a lower rate of fouling when compared to tubular evaporators. Plate packs are easily disassembled for inspection and cleaning, hence the popularity of plate evaporators in food applications. Other advantages are the ability to add plates for additional capacity (limited by the size of the frame), low headroom and no need for supporting steelwork because the units are self-supporting.

A disadvantage of plate evaporators is the propensity to allow air leakage at higher temperatures. In addition, plate evaporators are not economical for high capacities. In general, rising-falling-film designs are suitable for water removal rates up to about 35,000 lb/h, while falling-film units, which have larger vapor ports, can handle up to 60,000 lb/h [ 4]. In plate evaporators, the maximum allowable pressure difference between the steam and process sides is about one bar, which may not provide adequate ∆ T if boiling point elevation is high. Finally, materials of construction are limited — for example, unlike tubular evaporators, graphite is not used.

Agitated thing-film evaporators

Unlike the film evaporators discussed above, agitated thin-film evaporators form the film mechanically, using a rotating blade situated near, or even contacting, the heat-transfer surface. As shown in Figure 12, the device comprises a mechanical rotor situated concentric to a cylindrical, jacketed body. Feed entering the top of the evaporator is distributed evenly by the rotor, then the process fluid spirals down the heated wall. Bow waves developed by the rotor blades, named for the waves that form at the bow of a ship as it moves through the water, generate high turbulence [ 5]. Consequently, heat-transfer coefficients are high, and volatile compounds evaporate rapidly.

Figure 12. Agitated thin-film evaporators use a rotating blade to form a film on the heated surface. The condenser can be either external (A) or internal (B)

Concentrate, usually called residue when referring to agitated thin-film evaporators (TFEs), exits the bottom, with flow of evaporated solvent either cocurrent or countercurrent. With the latter, vapor exits the top and proceeds to an external condenser, a setup that is called a wiped-film evaporator (WFE). For cocurrent flow, vapor travels from the heated surface to an internal condenser, situated concentric to the evaporator body, and the resulting condensate (usually called distillate) proceeds downward to its own exit at the bottom. Because the evaporator and condenser surfaces are physically close, this arrangement is known as a short-path evaporator or short-path still, and the evaporation process is called short-path distillation.

Unlike the other types of evaporators discussed in this article, the agitated TFE is typically used with organic rather than aqueous systems. Usually (but not always) the distillate is the product. Most rotors have a fixed clearance between the outer edge and the body wall, ranging from 1.25 mm for small evaporators up to 5 mm for large ones. Alternatively, the edge of the rotor can be fitted with an elastomeric material that actually touches the wall, but this is less common. Rotational speed may be as high as 40 ft/s, with the power requirement ranging from 0.25 to 2.5 hp per ft 2 of heat-transfer area. Units with up to 160 ft 2 of heat-transfer area are standard. The heat source is usually steam or hot oil, although smaller units are electrically heated [ 3, 5].

Agitated TFEs offer several advantages. Residence time is short (as low as a few seconds), and because the rotor action provides close to plug flow with little backmixing, the residence-time distribution is narrow. Surface renewal is rapid, which minimizes fouling, and holdup is low. Turbulence in the agitated film leads to high heat-transfer coefficients, which partially offset the considerable cost per unit heat-transfer area. High ratios of feed to residue flow are possible without circulation. The mechanical action allows thin-film units to handle feeds with viscosities up to 50,000 cP, or even higher with specialized designs.

Short-path units can be operated at very high vacuum, by virtue of the internal condenser. Because the distance traveled by the evaporated vapor is short, pressure drop is exceedingly low. Consequently, absolute pressures as low as 0.01 mm Hg are obtainable with a conventional vacuum pump, or even lower if a diffusion pump is included. Conversely, units with an external condenser incur a higher pressure drop between the evaporator body and the condenser. The achievable evaporation pressure is about 1 mm Hg, which is lower than most conventional evaporation and distillation systems, although not as low as with the short-path design. If lower pressure is not needed, an external condenser may be preferred because the simpler construction results in a lower cost, and the capacity limitation from the relatively low surface area of an internal condenser is avoided.

The high vacuum possible with a short-path unit allows recovery of heavy compounds with minimal heat damage, because boiling points are lower. The technique is often used to process residues obtained from conventional distillation processes, which do not operate at the low pressures achievable in short-path distillation. At the highest practical bottoms temperature, the residue from a conventional column may still contain significant levels of valuable low-volatility components. These can be recovered at a lower temperature in a short-path still, without thermal degradation. Moreover, such feeds are often highly viscous, and would be difficult to process without mechanical film formation. Such viscous materials usually require heat tracing on the piping, especially the residue, but also the feed, to avoid excessive viscosity. Even the distillate pipe may require heat tracing to keep the viscosity down, prevent precipitation or stay above the melting point. In some applications, such problems are avoided by using heated fluid on the condenser, rather than cold fluid as with conventional evaporators.

In summary, the low operating pressure, short residence time, low holdup and wiping action make agitated TFEs well suited for heat-sensitive, viscous, fouling or high-boiling liquids. Disadvantages are the increased complexity and maintenance incurred by the rotor, inability to handle particulate matter and costs that are 20 to 30 times higher than tubular evaporators of similar capacity. ■

References

1. Perry, R.H., Green, D.W., Maloney, J.O., eds., “Perry’s Chemical Engineers’ Handbook,” 7 th ed., McGraw-Hill, New York, Section 11, 1997.

2. Prost, J.S., González, M.T., Urbicain, M.J., Determination and correlation of heat transfer coefficients in a falling film evaporator, J. Food Engr., 73 (2006), pp. 320–326.

3. McCabe, W.L., Smith, J.C., Harriott, P., “Unit Operations of Chemical Engineering,” 7 th ed., McGraw-Hill, New York, 2005.

4. APV Evaporator Handbook, SPX Flow, Inc., 2009.

5. Glover, W.B., Hyde, W.L., Evaporation of Difficult Products, Chem. Proc., February 1997.

Author

Alan Gabelman is president of Gabelman Process Solutions, LLC (6548 Meadowbrook Court, West Chester, OH 45069; Phone: 513-919-6797; Email: alan.gabelman@gabelmanps.com; Website: www.gabelmanps.com), offering consulting services in process engineering. Gabelman’s over 40 years of experience include numerous separation processes and other engineering unit operations, equipment selection, sizing and design, process simulation, P&ID development, and process economics. He holds B.S., M.Ch.E. and Ph.D. degrees in chemical engineering from Cornell University, the University of Delaware and the University of Cincinnati, respectively. He is a licensed Professional Engineer and has served as an adjunct instructor in chemical engineering at the University of Cincinnati. Gabelman has edited a book on bioprocess flavor production, and he has authored several technical articles and a book chapter.

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