6832
Ind. Eng. Chem. Res. 2004, 43, 6832-6835
A Falling-Film Evaporator with Film Promoters Wilson M. Salvagnini and Maria E. S. Taqueda* Department of Chemical Engineering, Escola Polite´ cnica, University of Sa˜ o Paulo, Av. Prof. Luciano Gualberto, travessa 3, 380-Cidade Universita´ ria, CEP 05508-900, Sa˜ o Paulo, SP, Brazil
Falling-film evaporators have been used extensively in chemical processing plants due to their minimal fluid residence time and high heat transfer rates with low temperature differences. A cheaper alternative for achieving suitable liquid distribution and film stability was investigated here for solutions with low solids content and sufficiently low viscosity to allow for gravitydriven down-flow. The alternative consists of using a film promoter of thin stainless steel woven wire specified by mesh (opening number per linear inch), placed flush against the heating surface where the liquid film flows down. For very low liquid feed-flow rates per length of internal perimeter, such as 0.004 kg/(m‚s), the fluid can be scattered over the entire vertical surface of the evaporator in a very thin film. The use of this film promoter leads to 3-fold higher evaporation rates. Introduction Falling-film evaporators can operate at low temperature differences with high heat transfer coefficients and minimal fluid residence time.1 These features enable evaporators to evaporate liquids from heat sensitive solutions, exposing the liquids to short periods at temperatures close to their boiling point. This is a required condition in fruit juice processing plants. Recently, several sugarcane mills have prevented sugar inversion by introducing falling-film evaporators into their process.2 It should be kept in mind that the performance of this type of equipment is strongly affected by liquid distribution and film stability,3 factors that are critical in low feed-flow rates of liquids with high surface tension, such as water. Low feed-flow rates are used when the residence time must be kept at a minimum,4 as in systems that operate on a once-through basis. To avoid the effects of poor liquid distribution and film instability and to render the liquid scattering less critical, a low cost device was developed. In 1935 Hickman5 covered the entrance of a molecular distillation still with wire mesh in order to achieve good initial distribution. The wire mesh did not cover the entire internal surface of the tube. Our proposal, in contrast, involves covering the entire internal surface of the tube with a stainless steel wire mesh, which we call a film promoter. In this article, a comparison is made between the evaporation rates in a laboratory scale falling-film evaporator with and without the film promoter, using water as the liquid to be evaporated and air as the carrier gas. The gas kept the temperature of the film lower than the water boiling point at working pressure, simulating the vacuum operations usually employed in juice concentrating processes at low temperatures. Experimental Section The film promoter performance was evaluated using the apparatus illustrated in Figure 1. * To whom correspondence should be addressed. Tel.: 055 11 3818 2252. Fax: 055 11 211 3020. E-mail:
[email protected].
Figure 1. Experimental device used in the tests to check film promoter performance.
The falling-film evaporator consists of two concentric glass tubes. The film promoter (Figure 2) is applied on the internal surface of the inner tube (i.d. 0.051 m and length 1.5 m) inside which the liquid film flows down. The promoter consists of a thin stainless steel wire mesh which is specified by the number of openings per linear inch. The diameter of the wire is 0.25 mm. The stainless steel wire mesh is not fixed to the glass tube; instead, it is forced manually into the tube. The stainless steel wire mesh, resembling metallic gauze, is purchased as a flat woven mesh, cut into pieces of width equal to the internal perimeter of the tube and a maximum length of 30 cm. The wire mesh pieces are fashioned into a circular shape to fit the tube, as shown in Figure 3, and are carefully molded to the internal surface of the tube so that they lie flush against its glass surface, as shown in Figure 4. The circular wire mesh has a strong tendency to open like a spring, as shown in Figure 5, causing it to remain well fixed against the internal tube through attrition. Due to the effect of surface tension, the wire mesh causes the falling fluid to spread, producing a better and
10.1021/ie0307636 CCC: $27.50 © 2004 American Chemical Society Published on Web 09/09/2004
Ind. Eng. Chem. Res., Vol. 43, No. 21, 2004 6833
Figure 5. Top view of the wire mesh.
Figure 2. View of the juxtaposition of the stainless steel wire mesh at the tube wall.
stream was saturated. The temperature and pressure of this outgoing stream were important to consolidate the mass balance. The liquid inflow and outflow rates were measured using a stopwatch and an electronic scale with a precision of .1 g. The difference between these two flow rates was taken as the evaporation rate. The liquid film Reynolds number (Re) was calculated using eq 1, as recommended in refs 3, 6, and 7.3,6,7
Re )
Figure 3. Piece of stainless steel wire mesh.
m ˘f 4Γ ) η πηD
(1)
where η denotes the dynamic viscosity, m ˘ f is the mass inflow rate, and D is the internal diameter of the inner tube in the evaporator tube. A liquid flow rate of less than 0.35 g/s was found to cause the film to break, even when using the film promoter. This value was therefore adopted as the minimal flow rate for both series to compare the evaporator’s performance with and without the promoter under the same conditions. Results and Discussion
Figure 4. Position of the wire mesh in the internal tube.
more stable film. The stainless steel wire mesh must be positioned as close as possible to the heating surface so that the desired effect is obtained. Therefore, it is important that no gap be present between the promoter and the heat exchange surface. Using a peristaltic pump, the liquid to be evaporated was fed into the top of the internal tube through eight small holes located around its perimeter, forcing the liquid to flow down evenly on the tube’s inner surface. The liquid flow was scattered by the film promoter all over the surface. The inner surface of the internal tube was heated by steam (93, 32 kPa and 98 °C) fed to the annular space between the tubes. The steam was generated by a rheostat-controlled electric boiler. The carrier gas was supplied by a positive displacement compressor and its flow rate was measured using a 0.00001 m3/s precision rotameter. The vapor generated from the liquid film was carried along by the air and condensed before being released into the atmosphere, so that the outgoing gas
The film promoter’s performance was evaluated based on evaporation rate measurements. The evaporation tests were conducted in two series, both using water as the evaporating liquid and air as the carrier gas. The stainless steel wire mesh used was gauge 22 in all the tests with the film promoter. The first series, consisting of eight runs, was conducted to estimate the effect of the carrier gas on the evaporation rate. The classical approach of one-variableat-a-time was used, and the water flow rate was set arbitrarily at 0.79 g/s, with and without the film promoter. Table 1 and Figure 6 show the results of the first series of tests. As can be observed in Figure 6, within the range studied here, the evaporation rates with the film promoter were consistently higher than those without it. The difference in evaporation rates was greater at low carrier gas flow rates. However, the carrier gas flow rate contributes in an approximately linear and positive way (i.e., the higher the rate, the greater the evaporation, as shown in the figure), indicating that the highest evaporation rates (with or without the film promoter) occurred at the highest carrier gas flow rate of 2.44 × 10-3 m3/s. To verify the effect of the water flow rate, the carrier gas flow rate was fixed at 2.44 × 10-3 m3/s, and the second series of tests with 12 runs was conducted with and without the film promoter. The results of these tests are shown in Table 2 and Figure 7. The unusual deviation of the data at a water inflow rate of 3.37 kg/s (Re ) 84) probably resulted from difficulties in control-
6834
Ind. Eng. Chem. Res., Vol. 43, No. 21, 2004
Table 1. Test Results for the First Series, with Water Flow Rate Set at 0.79 g/s
test no. 1 2 3 4 a
carrier gas flow ratea QG (m3/s)
Re carrier gas
Without Film Promoter 6.44 × 10-4 93 126 8.71 × 10-4 1.66 × 10-3 240 2.26 × 10-3 328
evaporation rate Wevap (g/s)
test no.
0.09 0.16 0.20 0.24
5 6 7 8
carrier gas flow rate QG (m3/s)
Re carrier gas
evaporation rat Wevap (g/s)
With Film Promoter 6.53 × 10-4 95 8.71 × 10-4 126 1.66 × 10-3 240 2.37 × 10-3 328
0.28 0.31 0.36 0.41
Air inflow at 93.32 kPa and 21 °C.
Table 2. Test Results for the Second Series, with Carrier Gas Flow Rate Set at 2.44 × 10-3 m3/s
test no. 1 2 3 4 5 6
water inflow rate Win (g/s)
film Re number
Without Film Promoter 0.38 9 0.72 18 1.02 25 1.75 44 2.32 58 3.34 83
evaporation rate Wevap(g/s)
test no.
0.07 0.16 0.17 0.25 0.18 0.11
7 8 9 10 11 12
water inflow rate Win (g/s)
film Re number
evaporation rate Wevap (g/s)
With Film Promoter 0.39 10 0.72 18 1.02 25 1.72 43 2.49 62 3.37 84
0.33 0.50 0.55 0.49 0.37 0.46
Figure 8. Schemes A and B, film promoter action. Figure 6. Comparison of the evaporation rate in the falling film with and without film promoter.
Figure 7. Comparison of the evaporation rate in the falling film with and without film promoter, with carrier gas flow rate set at 2.44 × 10-3 m3/s.
ling the peristaltic pump, which increased the experimental error. Figure 7 confirms that the evaporation rates achieved when using the film promoter and a fixed carrier gas inflow rate of 2.44 × 10-3 m3/s were consistently higher than those without the promoter, although a maximum appears in each plot. The greatest variation in evaporation rates occurred at a Re of around 30 (around 1.2 g/s water flow rate). It was found that the closer the contact between the stainless steel wire mesh and the internal wall of the evaporator, the more stable the film. In other words,
the curvature of the mesh must be as similar as possible to that of the wall. Film stability is achieved by the liquid adhering to the mesh, which forces it to scatter, thereby filling all the openings of the mesh. The scheme A in Figure 8 shows how the liquid behaves when it is sprinkled over a smooth surface, while scheme B shows the liquid being forced to scatter over the mesh, inducing the formation of a film in response to the liquid’s surface tension. Therefore, the wire mesh promotes the formation and stabilization of a liquid film over the entire inside surface of the evaporator tube. Even if the liquid flow rate decreases, the film does not break down into separate paths. This behavior was confirmed visually during the tests. It should be noted that, because the experiment was carried out by the classical one-factor-at-a-time approach, no statistical analysis was necessary. Results indicating the mesh size that offers the best performance and a methodology to calculate the average overall heat transfer coefficient will be presented in a future paper. Conclusions The results of the evaporator’s performance indicated evaporation rates up to 3-fold higher with the film promoter than without it at water inflow rates of 0.721.00 g/s and a fixed carrier gas inflow rate of 2.44 × 10-3 m3/s. The difference in performance decreases at water inflow rates outside this range, but the increase in efficiency with the film promoter remains above
Ind. Eng. Chem. Res., Vol. 43, No. 21, 2004 6835
100%. The final effect of the film promoter was a marked increase in the effective evaporation area, causing a higher evaporation rate compared with that produced in the evaporator without the film promoter under the same operating conditions. Moreover, these results allow us to conclude that the film promoter is appropriate for efficiently handling very low infeed flow rates and offers a cheaper solution than structured surfaces. This means that, without the film promoter, the wet area of the falling film evaporator was small, as is clearly shown in Figure 8. The wet area probably increased about 3-fold with the use of the film promoter. These findings indicate that the use of a film promoter should be appropriate in all operations in which the interfacial area is an important factor affecting their performance, as in evaporation, absorption, distillation, etc. Literature Cited (1) Billet, R. Evaporation Technology: Principles, Applications, Economics; VCH: Weinheim, 1986.
(2) Bhagat, J. J. Falling film evaporation in the cane sugar industry: An Indian Experience. Sugar Technol. 1997, May, 101110. (3) Hewitt, G. F.; Shires, G. L.; Bott, T. R. Process Heat Transfer; CRC Press: Boca Raton, FL, 1994. (4) Cvengros, J.; Badin, V.; Polla´ik, S.; Polla´k, S. Residence time distribution in a wiped liquid film. Chem. Eng. J.1995, 59 (3), 259263. (5) Hickman K. C. D. Apparatus and Methods. Ind. Eng. Chem. 1937, 29 (9) 968-975. (6) Alhusseini, A.; Tuzla, K.; Chen, J. C. Falling Film Evaporation of Single Component Liquids. Int. J. Heat Mass Trans. 1998, 41 (12), 1623-1632. (7) Wadekar, V. V. Heat Transfer to Falling Liquid Films with High Prandtl Numbers. In 3rd European Thermal Sciences Conference, 2000 Edizione ETS, Pisa. (8) Chun, K. R.; Seban, R. A Heat Transfer to Evaporating Liquid Films. ASME J. Heat Transfer1971, 93, 391.
Received for review October 14, 2003 Revised manuscript received July 1, 2004 Accepted July 1, 2004 IE0307636
