Feature Report
Heat Transfer in Wiped Film Evaporators Simulations demonstrate the complex, changing properties of materials within the evaporator. Such information can help in both designing and selecting the proper evaporator Daniel Bethge GIG Karasek
IN BRIEF
A
WIPED FILM EVAPORATORS HEAT TRANSFER EQUATIONS SIMULATION WIPING TECHNOLOGY SEPARATION EFFICIENCY
wiped film or agitated thin-film evaporator is very effective with difficult-to-handle materials. The evaporator consists essentially of a sealed cylinder that is provided with a heating jacket. The feed material is distributed on the inside as a thin film by means of a mechanical system. Due to the heating and the applied vacuum, the volatile components are evaporated and liquefied in an external condenser. In a short-path still, the condenser is fixed concentrically inside of the evaporator, so that a distillation at fine or even high vacuum is possible. A special version of the film evaporat evaporator or is the horizontal dryer, where the material is conveyed with special conveyor elements through the evaporator. The non-evaporative components are pumped as a residue or discharged as powdery solids. The evaporation process is mainly heat transfer controlled. For a simulation, the evaporator is advantageously divided into several zones. Using a database that is stored in simulation programs, heat transfer coefficients and temperature differences for each section can be determined. An accurate thermal design is the result, as will be explained with three examples.
Wiped film evaporators The basic setup of a wiped film evaporator is shown in Figure 1. The feedstock is fed at the top at a constant rate and is equally distributed on the inside of the cylinder as a thin film by means of a rotor equipped with wiping elements. Feedstock flows down due to gravity. The application of heat (in this case, steam) and vacuum causes the light volatile species to evaporate. The vapors are sucked through the vapor nozzle into a rectifier, or directly to a condenser where they are liquefied. The non-volatile components 44
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Motor
Vapors
Feed Steam Rotor
Condensate Heating jackets
Steam
Wiper Condensate
Residue FIGURE 1.
Shown here is a cross-section cross-section of of a wiped film/ agitated thin-film evaporator [2 ]
are discharged as residue. Wiped film evaporators are used for demanding evaporation processes, such as the recovery of valuable substances, the removal of solvents, the concentration of residues and so on. During the evaporation processes, the material changes its texture, for example, from a thin, watery w atery liquid to a pasty, or jelly-like, or even powdery consistancy. The heat transfer transfer changes accordingly accordingly..
Heat transfer equations •
The heat flow, Q , across the heated wipedfilm area, A, is given by Equation (1): •
Q A
=
U
T
(1 )
Where ∆T is the temperature difference between the heating medium and the film, and the heat-transfer coefficient ( U -value) -value) is deWWW.CHEMENGONLINE.COM
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A
B
850
0
830 K .
820
m / W , e u l a v -
810
2
U
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0
840
20 40
800
60
790 780
C ° , T 80 ∆
770
100
760
120
750 0
0.1
0.2
0.3
0.4 0.5 -axis, m
0.6
0.7
140
0.8
z
160 FIGURE 2.
These graphs show simulation results for Example 1. Graph A (left) illustrates how the calculated U -value varies along height, z , of the evaporator (top, z = 0 m; bottom, z = 0.75 m). Graph B (right) shows the calculated ∆T as a function of z
fined by Equation (2): 1
U
1 =
+
h
sw w
1 +
+
�L = heat transfer coefficient on the
R fouling
(2 )
product side Rfouling = loss of heat transfer due to fouling
scribed by Equation (3):
L
=
K
3
d i
L
nr
L
(3 )
3
L
Where: � h = heat transfer coefficient of the heating medium sw = thickness of metallic cylindrical wall with inside diameter, d ) i (d i >> sw �w = heat transfer coefficient of the wall
The heat transfer coefficient of Where: the heating medium, �h, usually K = equilibrium constant, dimensionamounts to at least 4,500 W/(m² .K). less For a cylinder made of stainless n r = rotor speed, rpm steel, �w =15 W/(m.K). The heat �L = thermal conductivity of the liquid transfer on the product side, �L, L = dynamic viscosity of the liquid is given by the law of Billet [ 1], de- Normally, K = 500 if d i is in millime-
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A
B
800 700
K . 2
m / W , e u l a v U
140 120
C ° , e r u t a r e p m e t m l i F
600 500 400 300 200
100 80 60 40 20
100 0 0
0.4
0.8 z -axis, m
1.2
1.6
0
0
0.4
0.8 -axis, m
1.2
1.6
z
FIGURE 3.
These graphs show simulation results for Example 2. Graph A (left) illustrates how the calculatedU -value varies along height, z , of the evaporator (top, z = 0 m; bottom, z = 1.6 m). Graph B (right) shows the calculated film temperature, T, as a function of z
ters, but in practice the observed val- value is Rfouling = 0.0002 m2.K/W. shows the U -value as function of the ues are smaller, so K = 250 is used The required evaporator size for a evaporator height ( z -axis; z = 0 at the in this study. given application can be roughly esti- top of the evaporator). In this exam The Rfouling term in Equation (2), mated because the U -value is mainly ple, close to 25% of the evaporation describes the loss of heat trans- determined by �w / sw . Mean values surface of about 0.5 m² (according fer due to fouling. Fouling occurs in for �L and L are generally used on to this calculation) is used for heating of the material and the evaporation foods, proteins and other applica- the product side. tions when hard layers form on the of light volatile materials. The U -value surface. The layers reduce the ef- Simulation increases mainly due to the temperaficiency of heat transfer. A typical A more precise calculation is pos- ture rise of the thin film (decreasing sible by using a simulation pro- ∆T , see Figure 2B). gram. The evaporator is subdivided Example 2. The second example is Drive into many sections, each consist- the drying of 100 kg/h of an aqueing of a heat exchanger and flash ous sludge for which at least – in this vessel. The flowsheet thus con- case – 1.8 m² of evaporator surface sists of many heat exchangers and might be required. After most of the flash vessels connected in series. water has been removed, the mate The operating pressure is set at rial becomes highly viscous and the the inlet. The heat duty of each ex- heat transfer decreases dramatically, changer has to be defined, for ex- as shown in Figure 3A. In practice, Vapor outlet ample, 10 MJ/h in the upper part of the material may not form a film anythe evaporator and 2 MJ/h in the more and thereby lose contact to lower part depending on the task. the heated wall so that further dryFeed After having selected the compo- ing stops. In a horizontal machine, inlet nents and estimated flowrate and however, the pre-dried material falls Rotation temperature of the feedstock, the to the bottom hot surface and further speed of simulation can be run. drying might be achieved. wipers in Wipers Evaporation zone 1 in zone 1 After preheating, the more volazone 1 tile components are removed step Wiping technology by step, whereby the film properties As already stated, the properties of change. The values for �L and L of the feed materials can vary a lot: very each section are used for calculat- thin fluids, such as water or ethanol; Rotation ing the local �L and thus U , and the viscous fluids, such as honey; or Wipers speed of Evaporation film temperature for estimation of doughy materials, sometimes conin zone 2 wipers in zone 2 zone 2 the local ∆T . From this, the area A sisting of two phases. Also, the propof each section can be determined. erties of the materials change during The sum of the areas of each sec- the process. The film might become tion gives the total required wiped sticky (and form layers), grainy or film surface area. powdery. To cope with these varying Example 1. As an example, let’s properties, different wiping systems Product or residue outlet consider the overhead distillation of are commercially available. FIGURE 4. This patented design has two evaporamethyl ester containing some glycFor drying, the best perfortion zones that can be heated separately. A coaxial mances can be achieved by using rotor makes it possible to operate the wipers in the erol and non-volatile solids, with a feedrate of 130 kg/h. Figure 2A two evaporators in series, one vertwo zones at different speeds 46
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A
% l o m
B
Flash
Film evaporation
100
100
10
10 Feed
1 200
300
400
500
600
Residue
% l o m
Feed
1 200
300
400
500
Vapor 0.1
0.01
0.01
FIGURE 5. These graphs show the simulation results for s eparation efficiencies of paraffin as used in the cosmetic industry. A better separation efficiency is observed for the wiped-film evaporator (B) when compared to a si mple flash evaporator (A). A more detailed simulation of the wiped-film evaporator (see text) shows that more heavy components are vaporized as well (C)
C
Residue Vapor
0.1
Molecular weight, g/mol
600
Molecular weight, g/mol
Simulation of the wiped film 100
10 tical and the other horizontal, each line) and the equipped with an appropriate wiper residue (red line) Feed % l 1 system. But to minimize investment as a function of o m Residue 200 300 400 500 600 and operating costs, users prefer to the molecular Vapor use only one evaporator. The cho- weight when it 0.1 sen wiper becomes a compromise. is treated in one The rotor can be equipped with dif- simple flash with 0.01 Molecular weight, g/mol ferent wiping elements. Best per- a vapor-to-liquid formances, however, are reached ratio (V/L) of 0.5. at different circumference speeds. Figure 5B For example, some commercial shows the distribution of the frac- longer reached. As can be seen from systems operate with wiper-blade tions after wiped film/agitated thin- Figure 5C, more heavy components speeds of about 2.5 m/s, and oth- film evaporation. As can be seen, can be vaporized as well. ers move at 8 m/s. the separation efficiency is sharper. To verify the layout and design The patented design [ 3] shown in The residue doesn’t contain any light of a wiped film or agitated thin-film evaporator, pilot testing should be Figure 4 has a coaxial rotor. It con- molecules. sists of a hollow shaft rotating at low The thin film might be simulated performed on the material. n speed and an inner shaft rotating at in more detail by using not only heat Edited by Gerald Ondrey high speed. On the hollow shaft are exchangers and flash vessels, but by End note fixed wiping blades, whereas on the distributing the material partly into This topic was first presented at the inner shaft are fixed pendulums. The the heat exchanger (the portion of CHEMCAD User Meeting, which two shafts are either driven individu- material that flows along the evap- was held in Berlin, Germany on Sepally by seperate motors, or, if only orator wall), partly in a bypass (the tember 16, 2016. one motor is used, by means of a portion that flows in the middle of planetary gear. the film) and partly through the flash Refences vessel (the portion flowing at the film 1. Billet, R., “Verdampfung und ihre technischen Anwendungen,” Verlag Chemie, Weinheim, Germany, 1981, p. Separation efficiency surface). Distributors and mixers 163. Paraffin wax is used in the cosmetic have to be included for that purpose 2. Same as Ref. 1, p. 164 industry, as well as in packaging and and optionally a multi-stream heat 3. Österreichisches Patent Nr. 516504, EP registration no. as fuel. For fractionation, wax is usu- exchanger with three sides that is 15455001.4-1371 ally distilled in a short path still. Let’s run through by material from the hot assume the feedstock consists of 10 wall-heat exchanger on the hot side, Author components (C16H34, C18H38, and by material from the middle section Daniel Bethge is head of R&D at GIG Karasek GmbH (Neusiedlerstr. so on) equally distributed in the feed, in the middle side and by material 15-19, 2640 Gloggnitz, Austria, each with 10 mol%. The evaporator that has seen the film surface on the phone: +43-2662-42780, Email:
[email protected]). He holds a is run such that half of the molecules cold side. Heat and mass transfer in degree in mechanical engineering are distilled overhead and half re- the thin film can be analyzed a little and a Ph.D. in material science main in the residue. The U -value is better this way. from the University (TH) in Karlsruhe (Germany). Since the 1990s quite constant because �L and L do Of course, such a simulation is he has worked in design, supply, not change much. somewhat arbitrary, but it shows construction and start-up of equipment and plants used Figure 5A shows the distribution the direction that the separation effi- for thermal processes. Since 2008 he has led the Reof the components in the feed (vio- ciency tends toward, in case the fee- search and Development Center of GIG Karasek with pilot plants for evaporation and distillation tests. let line), in the vapor/distillate (green drate is such that equilibrium is no CHEMICAL ENGINEERING
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