Feasibility study on power ultrasound for regeneration of silica gel—A potential desiccant used in air-conditioning system

Applied Energy 86 (2009) 2394–2400

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Feasibility study on power ultrasound for regeneration of silica gel—A potential desiccant used in air-conditioning system Ye Yao a,*, Weijiang Zhang a, Shiqing Liu b a b

Institution of Refrigeration and Cryogenics at Shanghai Jiao Tong University, Shanghai 200240, China Institute of Mathematics and Physics, Zhejiang Normal University, Jinhua, Zhejiang Province 321004, China

a r t i c l e

i n f o

Article history: Received 14 December 2008 Received in revised form 22 March 2009 Accepted 1 April 2009 Available online 29 April 2009 Keywords: Power ultrasound Silica gel Regeneration Diffusivity Energy activation

a b s t r a c t A new regeneration method using power ultrasound was put forward to overcome the limitations of silica gel in air-conditioning applications, such as high regeneration temperature and low regeneration efficiency. The technical feasibility of the new method was validated experimentally and demonstrated in detail from different sides. The experiments were performed under different regeneration temperatures, i.e. 45 °C, 55 °C, 65 °C and 75 °C. The power and frequency of ultrasound applied in this experimental study was set as 40 W and 26 kHz, respectively. The three indicators, including the regeneration degree (RD), enhanced rate of regeneration (ER) and energy-saving rate (ESR), were suggested to evaluate the effect of power ultrasound in the regeneration. The Crank’s diffusion model was used for the calculation of the moisture diffusivity in silica gel, and the Arrhenius equation for the determination of energy activation of moisture desorption on silica gel. The analysis results prove that the introduction of high-intensity ultrasound to the regeneration of silica gel can help to improve the regeneration efficiency and reduce regeneration energy. The benefits should owe to the special ‘heating effect’ and ‘micro-vibration effect’ caused by power ultrasound that can enhance the moisture diffusivity in silica gel and lower the activation energy of moisture desorption on silica gel. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Dehumidification, as an important air-handling process, takes a large proportion of energy in air-conditioning systems that manage to maintain the indoor environment at desired thermal levels for people’s living or industrial production. Relevant literature [1] reported that the latent load due to dehumidification would account for over 40% of total air-conditioning load of civil buildings in hot and high-humidity climates. Traditionally, latent load and sensible load are treated in a coupled way, i.e. the air is firstly cooled to below the dew point temperature to reduce moisture, and then reheated to the supply temperature before it is delivered to the occupied spaces. The cooling method for dehumidification results in a poor energy efficiency of air-conditioners in which the evaporating temperature must be low enough to make the air temperature down to below the dew-point. Currently, there is an increasing trend in China to separate the treatment of sensible and latent load by using an independent humidity control system [2,3] that integrates liquid/solid desiccant devices with a conventional cooling system. Such system may bring about many chances of energy conservation, e.g. avoiding excess cooling and heating, utilizing waste heat rejected by machines [4] and solar energy [5] to accomplish the dehumidification. * Corresponding author. Tel.: +86 21 54744901. E-mail address: [email protected] (Y. Yao). 0306-2619/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2009.04.001

The desiccant system repeats the adsorption–regeneration–cooling cycle, in which the regeneration conditions will produce great influence on the performance of water vapor adsorption on dehumidizers [6]. Although higher temperature contributes to increasing the desiccant volume of dehumidizers, it may lead to poor energy efficiency of desiccant system because higher regeneration temperature will result in more energy dissipation during the cooling process. Moreover, the high regeneration temperature will make it difficult to utilize the low-temperature energy sources easily available. To solve these problems, some dehumidizers [7] of new types with lower regeneration temperature were developed to replace the traditional ones (e.g. silica gel) that demand relatively higher regeneration temperature. As a matter of fact, the limitation of high regeneration temperature for those traditional dehumidizers could be partly overcome by some non-heating methods, e.g. the pulsed corona plasma [8], for the regeneration. Currently, the new regeneration method using ultrasound (above 16 kHz in frequency) has been put forward by some researchers. They assumed that the acoustic cavitation caused by ultrasound could help enhance desorption of volatile organic compounds (e.g. trichloroethylene, phenol and 4-chlorophenol) from activated carbon and polymer resins [9–13]. It should be noted that the cavitation only occurs in liquid phase. When the ultrasound propagate in a solid medium, the sound wave causes a series of rapid and successive compression and rarefaction with rates depending on its frequency,

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which induces a rapid series of alternating contractions and expansions in the material, much like when a sponge is squeezed and released repeatedly. The ‘sponge effect’ helps to keep the microchannels for moisture movement unobstructed, and promote moisture migration in solid. In addition, the presence of acoustic streaming can reduce thickness of boundary layer near surface of solid, which will contribute to the enhancement of mass diffusion This mechanism, known as ‘‘rectified diffusion”, makes it possible for ultrasound to be applied in process where noticeable moisture diffusion takes place overall. In recent years, the application of ultrasound in food drying or dehydrating has been intensively focused on by some scholars. de la Fuente Blanco et al. [14,15] developed a multi-sample ultrasonic dehydration system in direct contact with a rectangular vibrating plate that is driven by a power ultrasonic transducer, working at a frequency of 20 kHz. They also used the system to study the influence of ultrasonic power (0 W, 25 W, 50 W, 75 W and 100 W) on the dehydration process of carrot. Garcia-Perez et al. [16] investigated the effect of power ultrasound (ultrasonic parameter: 21.8 kHz, 75 W) on the drying kinetics of different products, carrot, persimmon and lemon peel. The results showed that air velocity and raw material structure would play a role in drying kinetics assisted by power ultrasound. In their other work [17], the influence of power ultrasound on mass transfer process during the drying of a low porosity product was particularly assessed. They announced that the influence was only observed above an acoustic energy threshold from which a linear relationship could be found between the average effective moisture diffusivity or the mass transfer coefficient and the electric power applied to the transducer. All these studies give the hint that power ultrasound be also used for the regeneration of silica gel, a drying or dehydrating process in nature. Silica gel, a kind of desiccant, has found the potential application in the air-conditioning systems [18] and cooling systems [19]. It will make a significant sense if ultrasonic power could enhance desorption of moisture from silica gel during the regeneration. The enhancement will improve the regeneration efficiency of silica gel under lower regeneration temperatures and bring about energy saving. To validate the feasibility, the following issues are to be concerned about in this work:

Electric heater

copper-constant thermocouple Temperature & humidity sensor transducer

Fan silica gel bed Ultrasonic generator Humidifier

Power input regulator

Fig. 1. Schematic diagram for the experimental setup.

2. Experimental verification

orifices (about 2.5 mm in size) in the surface and two round steel plates. The two cylindrical shells, about 20 mm and 50 mm, respectively, in diameter, are concentrically placed and fixed by the two plates at both ends. The sample (i.e. silica gel) is then filled in the space enveloped by the two cylindrical shells and the two plates. The ultrasonic transducer is clung tightly to one plate through which the ultrasound propagates into the silica gel in the bed. There is one hole (about 20 mm in diameter) in the centre of the other plate for the hot air entering into the bed. During experiments, the hole is connected with the outlet of the air duct. The hot air from the duct firstly enters into the inner cylindrical shell, then passes through the silica gel in the bed and finally exhausts outside from the orifices of the outer cylindrical shell. The positive/negative electrode of the ultrasonic transducer is of active connection with the positive/negative output of the ultrasonic producer that can produce high-energy ultrasound with the power range of 0–300 W and different frequency ranging from 16 kHz to 100 kHz. The electric heater, which is used for producing different experimental temperatures of hot air, is installed in the upward stream of the air duct. A temperature-and-humidity sensor (type: HMT100; measurement precision: ±2% in humidity and ±0.2 °C in temperature) is placed at the outlet of the air duct to monitor the conditions of regeneration air during the experiments. A copper-constantan thermocouple (measurement precision: ±0.2 °C) is buried in the silica gel that locates in central position of the bed where the surface temperature of silica gel is monitored during the experiment. A humidifier used to wet the silica gel in the bed to the initial moisture ratio for the experiment, is placed at the inlet of the fan. The other instruments include an electronic balance (measurement precision: ±0.1 g) for measuring the moisture change in silica gel, a dry-wet bulb thermometer (measurement precision: ±0.5 °C) for monitoring the ambient air conditions and a digital anemometer (measurement precision: ±3% of reading data) for testing the airflow rate in the air duct.

2.1. Sample preparation

2.3. Procedure

The sample of silica gel used in this experimental study has the particle size distribution of 3.5 ± 0.5 mm in diameter. The physical properties, which are provided by the manufactory, mainly include the following aspects: specific surface area P 600 m2/g; pore diameter = 20–30 Å (angstrom); pore volume = 0.35–0.45 ml/g; and bulk density = 750 g/l.

Experiments were done at a series of regeneration temperatures, i.e. 45 °C, 55 °C, 65 °C and 75 °C, to investigate the effect of ultrasound in the regeneration. The acoustic power and frequency employed in this experimental study was set as 40 W and 26 kHz, respectively. During the experiments, the environmental conditions were kept basically stable, i.e. the air temperature and relative humidity was at about 28 ± 1 °C and 80 ± 5%, respectively. And the airflow rate in the air duct was checked as about 0.3 ± 0.05 m/s. The basic procedure of experiment was as follows: To begin with, certain amount of fresh silica gel (175.1 ± 0.1 g) was fully filled in the bed. The total weight of the bed together with silica gel and the ultrasonic transducer was measured and recorded.

(1) How on earth does the power ultrasound impact the regeneration of silica gel? (2) How does power ultrasound influence the moisture diffusivity in silica gel? And how about the energy of activation of moisture desorption of silica gel with or without ultrasonic radiation? (3) How about the energy consumption of regeneration assisted by ultrasound?

2.2. Experimental setup The experimental setup (see Fig. 1) mainly consists of the silica gel bed, the ultrasonic transducer, the ultrasonic generator, the fan, the duct and the electric heater with a power controller. As shown in Fig. 2, the bed is a cylindrical container with a height of about 95 mm. It is made of two steel cylindrical shells with numerous

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Air out

Air in

Ultrasonic transducer

silica gel bed

Air out

Small holes

Fig. 2. Schematic diagram for the silica gel bed.

Then, the bed was connected with the air duct through a funneled connection. The humidizer and the fan were started up to wet the silica gel in the bed till the experimental moisture ratio, In this study, the initial weight of the wetted silica gel before regeneration was identically 200.4 ± 0.1 g. And then, the bed was temporarily moved away from the air duct. The humidifier was shut off, and the heater was turned on. The experimental regeneration temperature (e.g. 45 °C) was created by adjusting the power controller that controls input power of the electric heater. The fluctuation within 0.2 °C in the target regeneration temperature was acceptable in this study. Afterwards, the bed was reconnected with the air duct and the experiment with or without ultrasound at the experimental temperature was performed. During the experiments of regeneration, the bed was weighed by the electronic balance for every 8 min in order to observe moisture changes of the silica gel inside. The total experimental time for each condition lasted until no measurable weight loss was observed in the sample. Finally, silica gel in the bed was fully dried to get the dry sample. It should be noted that the silica gel can not achieve the completely dry state no matter how it was dehydrated. In this study, the nearly dry sample, whose mass was measured as about 150.8 ± 0.1 g, was acquired by an electronic oven with the baking temperature of 300 °C. 2.4. Methods 2.4.1. Evaluation indicators To evaluate the possible benefits from power ultrasound, three indicators, including the regeneration degree, the enhanced rate of regeneration and the energy-saving rate, are suggested here. Regeneration degree (ab. RD) is determined as the ratio of the loss of moisture (Mloss, g/g dry sample) to the initial moisture ratio (Mini, g/g dry sample) in sample.

M loss RD ¼ Mini

ð1Þ

Enhanced rate of regeneration (ab. ER) assisted by ultrasound is evaluated by:

ER ¼

ðMRSÞU  ðMRSÞNU  100% ðMRSÞNU

ð2Þ

where MRS refers to the mean regeneration speed (g/[min (g dry sample)]), which is defined as the average mass decrement of moisture per minute in unit mass of dry sample in a period of regeneration time. The subscript ‘U’ and ‘NU’ denotes ‘Ultrasound’ and ‘No ultrasound’, respectively. Energy-saving rate (ESR) brought by power ultrasound is defined as:

ESR ¼

EU ðRDÞ  100% ENU ðRDÞ

ð3Þ

where E(RD) denotes the energy (MJ) used for the regeneration as certain RD of silica gel is achieved. 2.4.2. Calculation of moisture diffusivity and activation energy Since the pore diameter of silica gel is about 20–30 Å (angstrom), the kinetics could possibly be pore-diffusion controlled [20]. Meanwhile, power ultrasound does not change the mechanism of moisture diffusion in silica gel. Therefore, the diffusion model developed by Crank [21], as shown in Eq. (4), can be employed in this study.

MR ¼

 2 2  1 M  Me 6 X 1 n p De s ¼ 2 exp  Mini  M e p n¼1 n2 r2

ð4Þ

where De denotes effective diffusivity, m2/s, which is defined as ‘the amount of a particular substance that diffuses across a unit area in one second under the influence of a gradient of one unit’ [21]; M denotes the instant moisture ratio during the regeneration, g/g dry sample; s is the time, s; r is the radius of particle; the subscript ‘ini’ and ‘e’ stands for initial and equilibrium state, respectively. Eq. (4) is derived from the solution of Fick’s second law, in spherical geometry, non-steady state, and with constant surface concentration. Through fitting the experimental data, the moisture diffusivity of silica gel can be obtained by Eq. (4). Eq. (4) assumes that the effective diffusivity (De) be constant and no shrinkage occur in the spherical sample. When the time, s, is long, Eq. (4) could be simplified to a linear equation as [22]:

lnðMRÞ ¼ ln



6



 

p2

p2 D e r2



s

ð5Þ

The effective diffusivity (De) can be got according to a straight line of ln (MR) versus time that is plotted by the experimental data. Using the slope of the straight line, k, the effective diffusivity (De) can be calculated by:

De ¼

k  r2

ð6Þ

p2

In the following calculation of De, the particle radius, r, is assumed as 0.00175 m, the mean size of the sample in this experimental study. The temperature dependence on the effective diffusivity (De) is also subject to the Arrhenius equation [23]:

  Ea De ¼ Do exp  RT

ð7Þ

where, Do is the pre-exponential factor of Arrhenius equation (m2/ s); Ea is the activation energy (kJ/mol); T is the drying temperature (K) and R is the gas constant (kJ/mol K). Eq. (7) can be written as:

ln De ¼ ln Do 



 1 Ea RT

ð8Þ

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Thus, the activation energy (Ea) can be determined with respect to the slope of the straight line in which the values of ln (De) are plotted versus 1/T.

our other work [24], the higher moisture ratio will be more conducive for ultrasound to regenerate the silica gel. Therefore, the moisture ratio should be one of the important factors that influence the role of ultrasound in the regeneration of silica gel. In this experimental study, the invalid role of ultrasound (i.e. ER is close to zero or negative) occurs, respectively, after about 80 min of regeneration at 45 °C, 64 min at 55 °C, 56 min at 65 °C and 40 min at 75 °C. The determined values of effective diffusivity (De) of silica gel with and without ultrasound for different temperatures are plotted in Fig. 4. The results show that the De of silica gel is particularly enhanced after the ultrasound is applied. The De under no ultrasonic 2.10  1010 m2/s, radiation is about 1.41  1010 m2/s, 3.10  1010 m2/s and 4.00  1010 m2/s at 45 °C, 55 °C, 65 °C and 75 °C, respectively. While, it rises to about 2.07  1010 m2/s, 3.38  1010 m2/s, 4.10  1010 m2/s and 5.11  1010 m2/s, respectively, in the presence of ultrasound. To investigate how ultrasound impacts the activation energy of moisture desorption of silica gel, values of ln (De) are plotted versus 1/T, as shown in the right plot of Fig. 4. Thus, the activation energy can be calculated from the slope of the fitting line about the values of ln (De) against 1/T. It was found to be 27.93 kJ/mol and 33.13 kJ/mol, respectively, with and without ultrasonic effect. In the calculation, the gas constant of water vapor is given as 0.008298 kJ/(mol K), and the particle radius, r, is assumed as 0.00175 m. The result manifests that the application of ultrasonic energy can effectively decrease the activation energy of moisture desorption of silica gel. Less activation energy means lower temperature required for the regeneration. From this point of view, the regeneration temperature of silica gel should be cut down with the help of high-intensity ultrasound. Porous silica gel particles ab-

3. Results and discussion Fig. 3 shows the changes of RD against time under different regeneration modes. Obviously, the values of RD during the regeneration with ultrasound are higher than those without ultrasound. Since the indicator, RD, directly reflects how much proportion of moisture is removed from the silica gel during the regeneration, the higher RD achieved means the better effect of regeneration. Hence, the RD curves in Fig. 3 can well embody the effect of power ultrasound on the regeneration of silica gel. To investigate the specific role of ultrasound in the whole stage of regeneration, the values of ER assisted by ultrasound are calculated for every 8 min. As can be seen from Fig. 3, the ER tends to drop with the regeneration time. Particularly, there appears negative ER in the latter stage of the regeneration. The phenomenon can be explained by the reason that the comparisons between (MRS)U and (MRS)NU are made here in terms of different initial M (moisture ratio in the silica gel). As indicated from RD curves, the initial M for the calculation of (MRS)U is lower than that for (MRS)NU. Since the lower moisture ratio in sample will lead to the slower dehydrating rate, it is reasonable to see that the ER appears negative values toward the end of regeneration process. In spite of the relatively lower initial M, the (MRS)U is still distinctly higher than (MRS)NU in the early stage of the regeneration. The tendency of ER curves in Fig. 3 indicates that ultrasound will play a bigger role in the beginning stage when the silica gel is of relatively higher moisture ratio. As achieved in

RD or ER

2 RD Without ultrasonic RD With ultrasonic ER

Regeneration temperature: 45

1.5 1 0.5 0 -0.5 0

8

16

24

32

40

48

56

64

72

80

88

96 104 112 120 128 136 144 152 160

RD or ER

Time (min) 1 0.8 0.6 0.4 0.2 0 -0.2

Regeneration temperature: 55 RD Without ultrasonic RD With ultrasonic ER

0

8

16

24

32

40

48

56

64

72

80

88

96 104 112 120 128 136 144 152 160

Time (min)

RD or ER

1

Regeneration temperature: 65

0.8 0.6

RD Without ultrasonic RD With ultrasonic ER

0.4 0.2 0 -0.2 0

8

16

24

32

40

48

56

64

72

80

88

96 104 112 120 128 136 144 152 160

Time (min)

RD or ER

1 0.8

Regeneration temperature: 75

0.6

RD Without ultrasonic RD With ultrasonic ER

0.4 0.2 0 -0.2 0

8

16

24

32

40

48

56

64

72

80

88

96

104

112

120

144

Time (min) Fig. 3. Changes of RD and ER during the regeneration with and without ultrasound.

152 160

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Y. Yao et al. / Applied Energy 86 (2009) 2394–2400 - 20. 5

6. 00E- 10

Ln(De) NU

De NU De U

5. 00E- 10

Ln(De) U - 21

ln(De)

- 21. 5

2

De (m /s)

4. 00E- 10

3. 00E- 10

- 22 2. 00E- 10

- 22. 5

1. 00E- 10

0. 00E+00

- 23

45

55

65

75

0. 00287 0. 00296 0. 00305 0. 00314

Regeneration temperature ( )

-1

1/T (K )

Fig. 4. Comparisons of effective diffusivity between with and without ultrasound.

sorb water through the numerous small pores whose size ranges from sub nm to tens nm. The water molecules are clung to the surface of pores through hydrogen bonds [25]. Although the highintensity ultrasound can induce variation of pressure in medium and make the particles vibrate, it can not interfere with the adsorbate species on the molecular level, i.e. the mechanical force induced by ultrasound does not produce influence on the bonding force of hydrogen bonds existing in silica gel. Only if the frequency of the external mechanical vibration exceeds the critical value (about 1333 GHz) can it result in the breakage of such hydrogen bonds [26]. Hence, the drop of activation energy due to the ultrasound can only be explained by the fact that a part of the activation energy is substituted by the acoustic energy. In Rege’s study [9], the researchers owed the lowering of activation energy of phenol on macroreticular resin to the acoustic cavitation energy that exists in liquid. In solid medium like silica gel, however, the special ‘heating effect’ and ‘micro-vibration effect’ of power ultrasound may be responsible for the lowering of energy of activation of moisture desorption. As a result of specific absorption of acoustic energy by medium, a selective temperature increase will take place in the medium through which ultrasound propagates. This is so-called ‘heating effect’ of power ultrasound. It may originate from the following aspects: (1) part of ultrasonic energy is absorbed directly by the medium during the transmission of ultrasonic wave and (2) the sound wave causes a series of rapid and successive compression and rarefaction. In turn, the medium is subjected to a rapid series of alternating contractions and expansions, which results in energy

dissipation that will ultimately convert into heat. As the hydrogen bonds in the adsorbate will be weakened with the rising of temperature [27], the temperature rise in medium due to the ‘heating effect’ of power ultrasound can enhance the desorption process of moisture in silica gel. As shown in Fig. 5, the temperatures of silica gel were a little higher (about 1–2 °C) in the presence of ultrasound than that in the absence of it. However, the improvement of regeneration due to ultrasound does not benefit solely from the ‘heating effect’ of ultrasound. Only 2 °C in temperature rise can not bring about such good effect by ultrasound as indicated in Fig. 6. As far as the first 16-min stage of regeneration is concerned, it is apparent that the role of 40-W power ultrasound was at least equivalent to a temperature rise of 10 °C in the enhancement of regeneration. According to the experiments, the mean regeneration speed in the presence of ultrasound at 45 °C was faster than that without ultrasound at 55 °C. So is the same case for the presence of ultrasound at 55 °C (or 65 °C) to the absence of ultrasound at 65 °C (or 75 °C). The possible generalization of a new technology to the practice mainly depends, to a large extent, on the benefits brought by it. Usually, the environmental protection and energy conversation are the two factors to be particularly considered. With regards to the former factor, no relevant literature was found to warn people of the potential environment hazards caused by power ultrasound during the applications. Known to all, the energy of ultrasound will attenuate greatly as it propagates through the air. So, measures are easily taken to protect human from the harm done by ultrasound even if it does that. In the following section, the energy saving

95

Temperature (

)

85 75

RT=45

,NU

RT=45

,U

RT=55

,NU

RT=55

,U

RT=65

,NU

RT=65

,U

RT=75

,NU

RT=75

,U

24

32

65 55 45 35 25 8

16

40

48

56

64

72

80

88

96

104

112

Time (min) Fig. 5. Temperatures of silica gel during the regeneration with and without ultrasound.

120

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surface of medium so that the heat flux can only slowly transfer from the external surface to the inner of medium by means of heat conduction, which inevitably results in high energy loss and low energy utilization. Other than the heat transfer, the ultrasound at the very beginning penetrates into the medium and all the ultrasonic energy can be fully and rapidly utilized for the regeneration. Therefore, the energy efficiency of ultrasonic regeneration should be higher than that of the traditional heating method. The energy analysis is made to further specify the energy saving of ultrasonic regeneration. The electricity power for creating different regeneration temperatures are listed in Table 1. It can be easily found from Table 1 that more energy will be required for equal temperature rise when the regeneration temperature becomes higher. For example, about 47 kW more electricity power is consumed to make the regeneration temperature rise from 45 °C to 55 °C, while there needs about 116 kW more power for that from 65 °C to 75 °C. The phenomenon can be explained by the fact that higher temperature condition will result in more heat exchange between the regeneration air and the environment, and hence, more energy loss. Energy use for the regeneration is calculated with the energy power multiplied by the regeneration time. In ultrasonic regeneration, the energy power includes the power required to drive the ultrasonic generator. The regeneration time for achieving different RD of silica gel was estimated according to the experimental data, which is listed in Table 2. It can be seen that for the same RD achieved, the regeneration time is markedly shortened after the power ultrasonic is applied. The energy consumptions of regeneration under ‘U’ mode and ‘NU’ mode are plotted in Fig. 7. The results clearly manifest that less energy will be used when the ultrasonic technology is introduced to the regeneration of silica gel. It appears that the regeneration temperature may be one of the factors that produce great influence on the energy consumption of regeneration. As shown in Fig. 7, the energy consumptions at 55 °Care apparently less than that at other regeneration temperatures. The reason may be that

8.0

U: With ultrasonic

3.81

4.0 3.0 2.0

5.10

4.06

5.0

4.60

5.18

7.0 6.0

7.50

NU: Without ultrasonic 6.34

9.0

1.82

MRS (mg/g.dry sample)

10.0

1.0 0.0

45

55

65

75

Regeneration temperature ( ) Fig. 6. Comparisons of MRS in the first 16-min stage between with and without ultrasound.

Table 1 Electricity power for creating different-temperature air for the regeneration. Regeneration temperature (°C)

Current (A)

Voltage (V)

Electricity power (W)

45 55 65 75

0.6 0.8 1.1 1.6

228 230 226 228

136.8 184 248.6 364.8

brought by ultrasound in regeneration is particularly discussed about. Known from the experimental results, the temperature required for the regeneration of silica gel can be cut down in the presence of ultrasound. It is of great meaning because the lower regeneration temperature will promote the utilization of low-grade energy sources, e.g. waste heat and solar energy. On the other hand, the transfer of ultrasonic energy is greatly different from that of thermal energy during the regeneration. The hot air just acts on the

Table 2 Regeneration time required for certain RD achieved by different regeneration modes. Regeneration temperature (°C)

Regeneration mode

45

Regeneration time for different RD achieved (min)

NU U NU U NU U NU U

55 65 75

RD = 0.2

RD = 0.3

RD = 0.4

RD = 0.5

32.2 16.8 18.2 12.6 14.8 10.8 13.3 9.7

48.1 27.8 27.3 20.4 22.1 15.8 20.2 13.8

69.6 38.2 38.3 28.3 31.4 22.3 27.3 19.6

103.8 48.5 49.1 36.3 39.8 29.8 37.4 23.4

200.9 169.3

300 200

55

400

264.3 178.2

500

220.8 187.0 291.1 235.6

600

568.3

U

571.3 405.2 422.8 380.4 468.4 386.1 597.5 476.0

NU

394.8 294.9 301.4 274.2 329.6 273.6 442.1 335.2

800 700

45

Energy use (kJ)

900

514.5 542.1 487.9 593.7 516.0

1000

818.6

852.0

Note: ‘U’ and ‘UN’ stands for ‘Ultrasound’ and ‘No ultrasound’, respectively.

100

RD=0.2

RD=0.3

RD=0.4

Regeneration temperature (

75

65

55

45

75

65

55

45

75

65

55

45

75

65

0 RD=0.5

)

Fig. 7. Comparisons of energy use between with and without ultrasound at different RD achieved.

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Y. Yao et al. / Applied Energy 86 (2009) 2394–2400

Research Fund for the Doctoral Program of Higher Education of China under Contract No. 2007024811.

20% 10% 0%

45

55

65

Regeneration temperature (

19.1% 24.2% 20.3% 30.6%

30%

RD=0.3 RD=0.5

15.3% 17.0% 17.6% 13.1%

40%

RD=0.2 RD=0.4

15.7% 9.0% 10.0% 10.0%

50%

32.6% 25.3% 29.1% 39.6%

ESR (Energy Saving Rate)

60%

75

)

Fig. 8. Energy-saving rate brought by ultrasound under different regeneration temperatures.

the regeneration time required for certain RD of silica gel is too long at 45 °C, while the energy loss is too much at 75 °C. Fig. 8 shows the energy-saving rate (ESR) brought by ultrasonic under different regeneration temperatures. Similar regular patterns can be found that the higher ESR due to ultrasonic occurs at 55 °C in the regeneration temperature. 4. Conclusions In this work, the feasibility for the use of high-intensity ultrasound for the regeneration of silica gel has been demonstrated through experiments. The experimental results manifested that the efficiency of regeneration of silica gel could be significantly improved by the power ultrasound. The enhancement in moisture desorption rates brought by power ultrasound can be attributed to an enhancement of moisture diffusivity due to the special effects (i.e. ‘heating effect’ and ‘micro-vibration effect’) caused during the transmission of ultrasonic wave. The activation energy of moisture desorption on silica gel was calculated using the Arrhenius equation. The results showed that the activation energy under the action of power ultrasonic (about 27.93 kJ/mol) would be lower than that with no ultrasonic applied (about 33.13 kJ/mol). The reduction of activation energy in the presence of ultrasonic may owe to the fact that a part of energy of activation is substituted by acoustic energy that is presented in the form of ‘heating effect’ and ‘micro-vibration effect’. Lower activation energy means lower regeneration temperature, which will bring about chances of energy saving because more low-grade energy sources can be utilized in the regeneration of silica gel. The analysis of energy consumption also consolidates that less energy will be used for the regeneration after the power ultrasound is applied. However, further study is necessary with respect to the optimization of intensity and frequency of ultrasonic radiation to be applied, as well as the optimal structure design of ultrasonic transducers for the successful implementation of this process. Acknowledgements This work was supported by the National Natural Science Foundation of China under Contract No. 50708057, and the Specialized

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