Regeneration of Silica Gel Using High-intensity Ultrasonic under Low

Energy & Fuels 2009, 23, 457–463

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Regeneration of Silica Gel Using High-intensity Ultrasonic under Low Temperatures Ye Yao,*,† Shiqing Liu,‡ and Weijiang Zhang† Institution of Refrigeration and Cryogenics, Shanghai Jiao Tong UniVersity, Shanghai 200240, China, and Institute of Mathematics and Physics, Zhejiang Normal UniVersity, Jinhua, Zhejiang ProVince 321004, China ReceiVed January 24, 2008. ReVised Manuscript ReceiVed September 13, 2008

This paper presents the experimental study on the regeneration of silica gel using ultrasonics under lowtemperature conditions. Four kinds of regeneration temperatures, including 35, 45, 55, and 65 °C, were chosen to investigate the new regeneration technology. The temperature and humidity of ambient air were kept at 24 ( 0.5 °C and 75 ( 5%, respectively, during the experiments. The ultrasonics used for this study was of 40 W in power and 25.6 kHz in frequency. The actual regeneration period for each experimental condition was set as 160 min, during which the weight decrement of silica gel was recorded every 8 min. The experimental results showed that ultrasonics could contribute to improving the regeneration efficiency of silica gel under low-temperature conditions. The role of ultrasonics in the regeneration appears to become more important under lower regeneration temperatures and a higher moisture ratio in silica gel. The study indicates that ultrasonics can help decrease the regeneration temperature of silica gel and, hence, bring about chances of energy conservation in industrial applications.

1. Introduction Silica gels have been used for dehumidification processes in industrial and residential applications for their great pore-specific surface area and good moisture adsorption capacity. Generally, process air flows through the silica gel bed, and the moisture of the air will be absorbed. After the silica gel has become saturation with the moisture, the bed is required to be heated and purged of its moisture for regeneration. Currently, heating by thermal energy is a common way to make the regeneration of silica gel. The regeneration temperature of silica gel is usually as high as over 100 °C, to satisfy the demand of actual applications. However, such a high regeneration temperature may be the fatal disadvantage for silica gel to use the lowtemperature energy available extensively in the world. For example, when solar energy is used for regeneration, the efficiency of solar collectors will mostly decrease greatly if the collection temperature exceeds 100 °C.1 On the other hand, a higher regeneration temperature will lead to more energy losses because more cooling is required to cool the regenerated silica gel before it starts the dehumidification process. To sum up, the regeneration temperature will make a great influence on the amount of regeneration energy and, hence, a big impact on the operating cost of silica gel in the application of dehumidification. To decrease the regeneration temperature of silica gel, an innovative method of nonheating regeneration based on the technology of ultrasonics is presented in this paper. Ultrasonic waves consist of frequencies greater than 20 kHz and exist in excess of 25 MHz. Ultrasonic technology has been * To whom correspondence should be addressed. E-mail: [email protected]. † Shanghai Jiao Tong University. ‡ Zhejiang Normal University. (1) Kalogirou, S. A. Solar thermal collectors and applications. Prog. Energy Combust. Sci. 2004, 30 (3), 231–295.

used in many fields, e.g., nondestructive test,2 plastic welding,3 machining,4 medicine process,5 and industry cleaning.6 This is due to several special characters of high-intensity ultrasonics, such as good penetrability in the successive medium, causing local temperature rise, producing microvibration with highfrequency in the solid, inducing cavitations in the liquid, etc. Some researchers7-9 have announced that high-intensity ultrasonics could effectively improve the efficiency of food dehydration and reduce the dehydration time. They argued that ultrasonics permitted the removal of moisture content from solids without producing a liquid-phase change. This could complete the dehydration under low temperature, which would bring about energy conservation, compared to the heat-drying method. The regeneration of solid dehumidizers studied in this paper is actually the process of drying or dehydration. Similarly, (2) Rimlyand, V. I.; Kazarbin, A. V.; Dobromyslov, M. B. Active ultrasonic nondestructive testing of rotating parts and bearings. Res. Nondestr. EVal. 2004, 15 (1), 19–29. (3) Tsujino, J.; Uchida, T.; Ohkusa, K.; Adachi, T.; Ueoka, T. Transmission conditions of vibration stresses to welding specimens of ultrasonic plastic welding using various two-vibration-system equipments. Jpn. J. Appl. Phys., Part 1 1998, 37 (5B), 3001–3006. (4) Zhang, L.; Jia, C.; Lu, Y. Machining method on electroplated diamond wire saw with ultrasonic vibration. Binggong Xuebao 2006, 27 (5), 899–902. (5) Huang, K.; Li, J.; Liu, S. Kinetic model for ultrasonic enhancement of extraction process of Chinese traditional medicine. Huagong Xuebao 2004, 55 (4), 646–648. (6) Awad, S. B. Aqueous ultrasonic cleaning and corrosion protection of steel components. Met. Finish. 2004, 102 (9), 56–61. (7) Gallego-Juarez, J. A.; Rodriguez-Corral, G.; Galvez Moraleda, J. C.; Yang, T. S. New high-intensity ultrasonic technology for food dehydration. Drying Technol. 1999, 17 (3), 597–608. (8) de la Fuente Blanco, S.; Riera-Franco de Sarabia, E.; AcostaAparicio, V. M. Food drying process by power ultrasound. Ultrasonics 2006, 44 (Supplement 1), e523–e527. (9) Gallego-Juarez, J. A.; Riera, E.; de la Fuente Blanco, S. Application of high-power ultrasound for dehydration of vegetables: Processes and devices. Drying Technol. 2007, 25 (11), 1893–1901.

10.1021/ef8000554 CCC: $40.75  2009 American Chemical Society Published on Web 11/21/2008

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Figure 1. Silica gel container and ultrasonic shaker for this study.

power ultrasonics should be capable of regenerating the solid dehumidizers, a kind of porous medium. It is assumed that the special effect of microvibration with high frequency produced by ultrasonics during its transmission through porous medium would tear the agglomerate water in the pore into thousands of tiny drops of several micrometers in diameter that will be much easier to escape from the surface of the porous medium with the airflow. From this point of view, the ultrasonic regeneration is somewhat like a kind of mechanical process that directly removes the agglomerate water in the dehumidizer through high-frequency vibration. which will bring about energy saving because of the following reasons: (1) Moisture in the dehumidizer removed by ultrasonics is of no change of phase, which will save much latent heat of liquid. (2) Ultrasonics can complete the regeneration under lowtemperature conditions, which will overcome the disadvantage of high regeneration temperature of many dehumidizers, e.g., silica gel, and, hence, contribute to energy conservation. Meanwhile, the ultrasonic energy is partly absorbed by medium during the transmission that results in the temperature rising of the medium, which is the so-called heating effect of ultrasonics. In comparison to the traditional heating method that works by means of heat transfer, ultrasonic heating may be more effective and efficient because the whole body can be heated at the very beginning. Obviously, the regeneration will benefit from the microvibration and heating effects of ultrasonics. In the previous study,10 the feasibilities of ultrasonic regeneration for solid dehumidizers have been expounded theoretically and validated experimentally. The objective of this study is mainly to investigate the effect of ultrasonics on the regeneration of silica gel under different low-temperature conditions and how the moisture ratio in silica gel makes an influence on the ultrasonic regeneration. 2. Materials and Methods 2.1. Experimental Setup. The key component in the experimental setup is the silica gel container, as shown in Figure 1. The container is made of two steel cylinders and two round steel plates. Both cylinders are of about 95 mm in height and are placed concentrically between the two round steel plates. The section diameter of the inner cylinder and the exterior one is about 20 and 50 mm, respectively. Many small holes with a diameter of 2.5 mm were drilled in the surface of the cylinders. The silica gel was filled in the space enclosed with the two cylinders and the two round plates. The ultrasonic shaker was clung tightly to one of the two round plates, through which the ultrasonic could spread into the silica gel effectively. Another round plate was drilled with a hole with diameter of 20 mm in the center, through which the regeneration air can enter smoothly into the inner cylinder, and then passed through the silica gel and, hence, the external cylinder. The silica gel in this experiment is a kind of narrow pore spherical one, whose technical specifications are depicted as follows: diameter g 3.5 mm; specific surface area g 600 m2/g; pore diameter, pore volume, and bulk density are about 20-30 Å, 0.35-0.45 mL/g, and 750 g/L, respectively.

Figure 2. Schematic diagram for the experimental setup.

Figure 3. Photographs for the experimental setup and main instruments.

The schematic diagram and the photographs of the experimental setup are shown in Figures 2 and 3, respectively. The experimental setup includes an air duct, a fan, an electric heater with a power regulator, temperature and humidity sensors, an electronic balance, the silica gel container, the ultrasonic shaker, and the ultrasonic generator. The electric heater has a maximum input power of 400 W; the temperature and humidity sensors of type HMT100 have a measurement precision of (2% in humidity and (0.2 °C in temperature; and the least count of the electronic balance is of (0.1 g. The other instruments also include the humidifier for dehumidifying silica gel, a digital anemometer (measurement precision: (3% of the reading data) for the measurement of air flow in the duct, and the dry-wet bulb thermometer (measurement precision: (0.5 °C) for the ambient air temperature and humidity. The humidifier takes advantage of ultrasonics to produce many tiny water drops and spray them into the air, which can humidify the air rapidly under low-temperature conditions. The ultrasonic generator can produce high-energy ultrasonics with the maximum power of 300 W and at different frequency ranging from 16-100 kHz. The electric heater, used for heating the regeneration air, was placed in the upward stream of the air duct near the fan. The temperature and humidity sensors, connected with a high-precision data acquisition system Keithley 2700, were placed in the downward stream of the air duct near the silica gel container to monitor conditions of regeneration air. The humidifier was placed at the inlet of the fan. The inner cylinder of the container was actively connected to the air duct and so was the ultrasonic shaker with the ultrasonic producer, and hence, the silica gel container can be weighed separately. 2.2. Procedure. The container for the experiment was tightly filled with 145.6 g of dry silica gel. Initially, the inner cylinder of the container was connected with the air duct, and the silica gel was humidified by air with high humidity until the weight was reached up to 211.5 g. During the humidification process, the humidifier and fan were running continuously. After the silica gel

Regeneration of Silica Gel

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Figure 4. Change of the weight of silica gel with and without the ultrasonic effect at different regeneration temperatures.

was humidified, the container was disconnected from the air duct. The humidifier was switched off, and the electric heater was started. A certain regeneration temperature was then created through adjusting the power regulator. Four kinds of regeneration temperatures, i.e., 35, 45, 55, and 65 °C, were focused in this study. The regeneration experiments with and without ultrasonics did not start until the temperature of the regeneration air fluctuated within 0.2 °C of the target values. Until a certain regeneration temperature was adjusted, the container was wrapped by a plastic film to prevent the humid silica gel from proceeding mass exchange with ambient air and the weight of the silica gel was kept constant (at 211.5 g) before regeneration. The weight of the silica gel container was measured by the electronic balance for every 8 min during the regenerations, from which the variations in the weight of silica gel could be obtained. The regenerations, in fact, were carried out discontinuously. There was a very small break (about 20-30 s) between the 8 min continuous regeneration process, during which the silica gel container was disconnected from the duct, weighed, and then reconnected with the duct for the next 8 min regeneration. The actual regeneration time for different operating conditions was uniformly set as 160 min. The parameters of the ultrasonic producer were set as 40 W of power and 25.6 kHz of frequency. The temperature and humidity of ambient air were adjusted by an indoor air conditioner and were kept at 24 ( 0.5 °C and 75 ( 5%, respectively, and the air flow rate in the air duct was measured about 0.3 ( 0.02 m/s during the regenerations.

3. Results and Discussion Figure 4 shows the variations of the weight of silica gel under different conditions of regeneration. The labels of “Treg,U” and “Treg,NU” in the legends of all figures denote the temperature of regeneration with ultrasonics and that without ultrasonics, respectively. It can be seen from Figure 4 that the weight of silica gel with ultrasonics decreases more rapidly than that without ultrasonics under the same regeneration temperature. The curve of weight decrement of silica gel with ultrasonics under 35 °C is very close to that without ultrasonics under 45 °C, so is the case with ultrasonics under 45 °C to that without ultrasonics under 55 °C, and with ultrasonics under 55 °C to that without ultrasonics under 65 °C. From the experimental data, we can optimiztically suppose that the regeneration temperature of silica gel could decrease by about 10 °C with

the help of the ultrasonic effect at low-temperature conditions below 65 °C. To make better analysis on the effect of ultrasonics on the regeneration at different stages, the decrements of moisture in silica gel under different regeneration conditions are plotted in Figure 5a. It shows that ultrasonics would contribute much to the dehydration rate of silica gel in the beginning of regeneration. However, the contribution would decrease with the regeneration continuing. At the temperature of 35 °C, the contribution of ultrasonics was not obvious after 80 min. While under other temperatures (45, 55, and 65 °C), the time duration of obvious contribution was shorter, about 56 min. The phenomena may be explained by the reason that the moisture equilibrium is nearly formed in silica gel when the regeneration lasts for enough time under certain conditions, which leads to less decrease in the moisture of silica gel even under the ultrasonic radiation. As mentioned above, the precision of the measurement of the electric balance is 0.1 g. Therefore, the maximum absolute error for counting the decrease of moisture in silica gel for every 8 min will attain 0.2 g because 2 times of measurements are required for it. Thus, the relative error, Erelative, should be calculated using Erelative ) 0.2/∆D × 100%

(1)

where ∆D denotes the counted mass decrease of moisture in silica gel for every 8 min. Figure 5b presents the relative errors possible for the data plotted in Figure 5a. It can be seen from Figure 5b that smaller errors can be achieved (indicated as below 10% in the relative error) for counting the mass decrease of moisture in silica gel before 80 min in the regeneration. It is understood that the relative error will go up rapidly after 80 min of regeneration when the moisture decrease in silica gel becomes smaller and smaller. For quantitative analysis, the mean regeneration speed, which was defined as the mass decrement of moisture per minute in silica gel, was suggested in this paper to discuss the effect of ultrasonics on the regeneration of silica gel. As shown in Figure 6, the mean regeneration speed with ultrasonics (ab. MRSU) was obviously higher than that without ultrasonics (ab. MRSNU). For the first 8 min, the MRSU would be 150 mg/min higher than the MRSNU at the regeneration temperature of 35 °C and


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Figure 5. Change of the moisture content versus time in silica gel with and without the ultrasonic effect at different regeneration temperatures.

the gap between MRSU and MRSNU would enlarge at 45, 55 and 65 °C, which was about 112.5, 287.5, and 262.5 mg/min, respectively. However, with the regeneration going on, the effect of ultrasonics on the mean regeneration speed tended to decline. For example, under the temperature of 45 °C, the difference between MRSU and MRSNU in the first 16 min was about 100 mg/min, which would be lower than that in the first 8 min (112.5 mg/min) and higher than that in the first 24 min (79.1 mg/min). The effect of ultrasonics on the mean regeneration speed is likely to be related to the regeneration temperature. In this study, the best effect of ultrasonics occurred at the regeneration temperature of 55 °C when the difference between MRSU and MRSNU was obviously higher than those at the other temperatures. Figure 7 shows the moisture changes in silica gel with and without ultrasonics under different temperatures during 160 min regeneration. The percentage of water content, Rmoisture, in silica gel was defined as Rmoisture )

Mgel,wet - Mgel,dry × 100% Mgel,dry

(2)

where Mgel,wet and Mgel,dry denote, respectively, the mass of wet and dry silica gel. The tendencies of moisture changes in silica gel under different modes of regeneration are analogical to that of weight changes of silica gel. According to the curves in Figure 7, the trend line of moisture desorption of silica gel under different equilibrium conditions can be expressed as follows: (3) Rmoisture ) c + aebτ where a, b, and c denote empirical coefficients that are obtained by the experiment and τ denotes regeneration time. It can be inferred from Figure 7 that the coefficient b should be negative and the coefficients a and c should be positive. When τ ) 0, the initial moisture ratio in silica gel equals (a + c), and when τ f +∞, the coefficient c is actually the moisture ratio in silica gel under equilibrium conditions. Through nonlinear regression, the moisture equilibrium equations obtained from this study are presented as follows: 35oC with ultrasonics: Rmoisture ) 0.0753 + 0.3822e-0.0111τ (4)


35oC without ultrasonics: Rmoisture ) 0.0880 + 0.3680e-0.0081τ (5) 45oC with ultrasonics: Rmoisture ) 0.0664 + 0.3904e-0.0140τ (6) 45oC without ultrasonics: Rmoisture ) 0.0732 + 0.3833e-0.0120τ (7) 55oC with ultrasonics: Rmoisture ) 0.0481 + 0.4050e-0.0173τ (8) 55oC without ultrasonics: Rmoisture ) 0.0574 + 0.3885e-0.0131τ (9) 65oC with ultrasonics: Rmoisture ) 0.0228 + 0.4322e-0.0243τ (10) 65oC without ultrasonics: Rmoisture ) 0.0392 + 0.4160e-0.0183τ (11)


It is known from the above equations that the equilibrium moisture ratio in silica gel with ultrasonics will be reasonably higher than those without ultrasonics under an identical regeneration temperature. It means that ultrasonics can effectively enhance the degree of regeneration and contribute to improving the performance of absorption of silica gel. As shown in Figure 8, ultrasonics can obviously shorten the regeneration time. At 35 °C, ultrasonics could help to save about 46, 73, and 176 min to complete the regeneration that make the moisture ratio in silica gel change from the initial value (of about 45%) to 20, 15, and 10%, respectively. However, the time saving thanks to ultrasonics reduced significantly for higher temperatures above 35 °C, which indicates that lower temperature conditions may better highlight the role of the ultrasonic effect in the regeneration. Figures 9-12 show the effect of the moisture ratio on the mean regeneration speed of silica gel under different conditions. The labels “MRS” and “MR” denote, respectively, the mean regeneration speed that is calculated for every 8 min and the moisture ratio in silica gel. Clearly, under the same conditions,

Figure 6. Comparisons of the mean regeneration speed with and without ultrasonics at the beginning stage.

Figure 7. Change of the moisture against time in silica gel with and without ultrasonics at different regeneration temperatures.


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Figure 8. Comparisons of the regeneration time with and without ultrasonics for different equilibrium states of moisture in silica gel.

Figure 9. Effect of the moisture ratio on the mean regeneration speed with and without ultrasonics at the regeneration temperature of 35 °C.


the higher the moisture ratio in silica gel, the more rapid the mean regeneration speed will be. In addition, the moisture ratio in silica gel will impact the contribution of ultrasonics in the regeneration. The higher moisture ratio in silica gel will assist ultrasonics to play a more important role in the regeneration. It (10) Yao Y.; Liu, S. UltrasonicsA new regeneration technology for dehumidizer. The 4th International Conference on Cryogenics and Refrigeration, Shanghai, China, April 4-6, 2008; pp 984-990.



indicates that ultrasonics is more suitable in the beginning of regeneration when the moisture ratio in silica gel is relatively higher. 4. Conclusions The contribution of ultrasonics in the regeneration of silica gel under low-temperature conditions has been investigated through experimental studies. The experimental results showed that ultrasonics would significantly improve the regeneration efficiency of silica gel under low-temperature conditions. It


means that ultrasonics can help to decrease the regeneration temperature of silica gel during industrial applications and will save much energy, e.g., making low-temperature waste heat useful for regeneration and reducing a large amount of energy losses because smaller cooling energy is required to cool the regenerated silica gel. The experimental results showed that the contribution of ultrasonics in the regeneration efficiency of silica gel would be influenced by the regeneration temperature and moisture ratio. The contribution of ultrasonics may increase at lower regeneration temperatures or higher moisture ratios in silica gel. It is observed that ultrasonics is better used in the beginning of regeneration when the moisture ratio in silica gel is relatively higher and the temperature is lower.


Further study will be focused on the effects of other parameters of ultrasonics, e.g., frequency and power intensity, on the efficiency of regeneration. In addition, the optimal structure design for an ultrasonic regenerator will be carried out to make better use of this technology. Acknowledgment. This work was supported by the National Natural Science Foundation of China under contract number 50708057, the Specialized Research Fund for the Doctoral Program of Higher Education of China under contract number 2007024811, and the Natural Science Foundation of Zhejiang Province in China under contract number Y606238. EF8000554

Regeneration of Silica Gel Using High-intensity Ultrasonic under Low

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