effect of regeneration conditions on the adsorption_elsevier

Applied Thermal Engineering 24 (2004) 735–742www.elsevier.com/locate/apthermeng

Effect of regeneration conditions on the adsorptiondehumidification process in packed silica gel beds

Kuei-Sen Chang, Hui-Chun Wang, Tsair-Wang Chung *

Department of Chemical Engineering, Chung-Yuan Christian University, Chungli 320, Taiwan, ROC

Received 15 May 2003; accepted 14 November 2003

Abstract

The amount of energy consumption is considered to be responsible for the coefficient of performance

(COP) of most air-conditioning systems. The objective of this study is to evaluate the commercial silica gel

and the silica gel with improved transport properties used in the adsorption dehumidification process.

Effects of the regeneration temperature and the regeneration time for the specific moisture uptake in the

adsorption dehumidification process using the commercial and the modified silica gels were observed. The

dynamic adsorption experiments were conducted to measure the moisture breakthrough curves of the silica

gels, and the moisture uptakes on the silica gels were calculated from the breakthrough curves. A tem-

perature controlled heat source was used to carry out the regeneration of silica gels in this study. Since thesolar energy with low energy density was applied to regenerate the adsorption dehumidification systems

frequently, temperature of the heat source was controlled in less than 100 �C. Since the switch of theabsorption process to the regeneration process is selected at the breakpoint usually, the effective uptake

defined in this study is calculated from the initial to the breakpoint at a ratio of the effluent concentration to

the initial concentration of 0.15. Similar effective uptakes of the system using modified silica gels regen-

erated at 90 �C for 0.5 h and the system using commercial silica gels for 3 h regeneration were found.

Compared to the system using commercial silica gels, the system using modified silica gels has a good

energy saving in regeneration.� 2003 Elsevier Ltd. All rights reserved.

Keywords: Adsorption dehumidification; Silica gel; Regeneration; Breakthrough curve

* Corresponding author. Tel.: +886-3-2654125; fax: +886-3-2654199.

E-mail addresses: [email protected] (K.-S. Chang), [email protected] (H.-C. Wang), twchung@cycu.

edu.tw (T.-W. Chung).

1359-4311/$ - see front matter � 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/j.applthermaleng.2003.11.003

mail to: [email protected]

Nomenclature

DP average pore diameter (m)VP pore volume (m3/g)SBET specific surface area (m2/g)q water vapor uptake on the silica gel (g H2O/g silica gel)W total weight of adsorbent (g silica gel)F air flow rate (cm3/min)qa the density of inlet air (g air/cm3)C0 the humidity of inlet air stream (g H2O/g air)C the humidity of outlet air stream (g H2O/g air)

736 K.-S. Chang et al. / Applied Thermal Engineering 24 (2004) 735–742

1. Introduction

There are several desiccant dehumidifiers including solid packed bed, rotating horizontal bed,multiple vertical bed, and rotating honeycomb have been used for dehumidification [1]. Comparedto the commercial dehumidifiers, the process air of the solid packed bed and the rotating hon-eycomb can reach a very low dew point temperature ()60 �F), which can be widely applied indehumidification processes. The solid packed bed can deal with a great amount of moistureadsorption, while the rotating honeycomb gets more uniform humidity in process air. Bothdesiccant dehumidifiers have certain advantages and disadvantages. However, in this study thesolid packed bed is applied.Silica gels and zeolites have been utilized for dehumidification processes in industrial and

residential applications for their great pore surface area and good moisture adsorption capacity.In general, the regeneration temperature for silica gels is less than that of zeolites. In order todiscuss the system of low-temperature regeneration, the silica gel bed is used in this study. Processair flows through the silica gel bed, giving up its moisture to the silica gel. After the silica gel hasbecome saturation with the moisture, the bed is heated and purged of its moisture for regener-ation. Thermal energy that drives the regeneration of the silica gel is added to the process byheating the bed or the reactivation air stream. Engineers notice that the operating cost of thesetypes of systems is dependent on the amount of the regeneration energy. An economic analysis onthe operating cost of a silica gel bed was reported by Marciniak [2], and an adsorption perfor-mance analysis on the regeneration condition of the other adsorption process was reported byKamiuto and Ermalina [3].The rate of physical adsorption on a surface is generally high so that in a porous adsorbent the

overall rate of adsorption is always controlled by the mass or heat transfer resistance, rather thanby the intrinsic sorption kinetics [4]. Therefore, the modification of the silica gels in this study wasfocused on their mass transfer resistance in the adsorption of moisture. Modified silica gels withsmaller mass transfer resistance were made to compare with the adsorption performance of thecommercial silica gels in different regeneration conditions. There are several methods used toimprove the pore properties and/or reduce the mass transfer resistance of solid porous materials

K.-S. Chang et al. / Applied Thermal Engineering 24 (2004) 735–742 737

[5–7]. A proper control on the manufacturing variables is considered as a simple and direct wayto get a modified or improved porous material.Surface properties of silica gels such as the surface area, the pore volume, and the average pore

diameter were measured by the BET method [8]. The manufacturing variables of silica gelsincluding pH value, gelling and dehydration temperatures discussed in the previous study [7] wereapplied to prepare the modified silica gel in this study. Breakthrough curves of the silica gels weremeasured by a dynamic adsorption apparatus. The mass transfer resistance and the uptakewere discussed using the experimental breakthrough curves [9]. The influence of the regenerationtemperature and the regeneration time on the trends of breakthrough curves and the values ofuptake were observed in both commercial and modified silica gel beds.

2. Experimental section

The solid desiccants used in this study were commercial (Riedel-de Haen) and modified silicagels and their particle sizes were in the range 6–8 mesh. The starting materials for preparing themodified silica gel were tetraethoxysilane (TEOS), hydrochloric acid (HCl), ammonium hydroxide(NH4OH), and distilled water. In order to have a similar specific surface area with the commercialsilica gel, the manufacturing variables for preparing the modified silica gel were tested by theprocedure of the previous study [7]. After a series of experimental preparations, the proper molarratio of gel solution was TEOS: H2O: HCl: NH4OH¼ 1: 14: 0.02: 0.0155. The gelation temper-ature and the drying temperature were selected at 80 and 60 �C, respectively. The specific surfacearea, the pore volume, and the average pore diameter of samples were measured by nitrogenadsorption at the liquid nitrogen temperature 77 K in a BET sorptometer (ASAP2000, Mi-cromeritics). The silica gel sample was degassed at 200 �C in the vacuum before the nitrogenadsorption measurements. The specific surface area and the pore volume of the gels were mea-sured by the BET method. The average pore diameter was calculated from the surface area andpore volume.

Table

Surfac

Silic

Mo

Com

DP ¼ 4VP=SBET ð1Þ
The surface properties of the commercial and modified silica gels were listed in Table 1. Thespecific surface area of the modified silica gel is similar to that of the commercial silica gel. Thepore volume and average pore diameter are smaller in the modified silica gel. It should be notedthat the average pore size in the modified silica gel is in the range of micropore (average porediameter <20 �AA) and the commercial silica gel is close to the mesopore structure. The adsorbentwith micropore structure is good for water vapor adsorption and the chance of forming capillarycondensation of water vapor in the pores is less than that of the adsorbent with mesopore
1

e properties of the commercial and modified silica gels

a gel (SiO2) BET surface area (m2/g) Volume of pores (cm3/g) Average pore diameter (�AA)

dified SiO2 530 0.256 19.3

mercial SiO2 534 0.619 46.4

MFC

MFC

Box

MFCAir

Pump

HM

TC TC

Water Supply

Vent

HM

Glass Wool

Ultrasonic Vibrator

Air Filter

CompressedAir

TC

Adsorbents

Glass Beads

Vent

MFC : Mass Flow Controller TC : Thermocouple HM : Hygrometer

Vent

PackingTower

Fig. 1. Dynamic adsorption apparatus for this study.


structure. For water vapor adsorption in mesopores, the capillary condensation is accompaniedby a hysteresis phenomenon usually. This may result in a greater mass transfer resistance andincrease the area of mass transfer zone in the breakthrough curve. Therefore, a different masstransfer performance between the commercial and the modified silica gels is expected.A dynamic adsorption apparatus shown in Fig. 1 was used to obtain the breakthrough curves

of the silica gels. Three ultrasonic vibrators were placed in a 35 l stainless steal chamber with thecontrol of water supply to serve as a humidifier. Part of the inlet air stream controlled by the massflow controller was passed through the humidifier and then mixed with the main air stream toadjust the inlet air humidity. The adsorber was a packed bed with 1.2-in. diameter. The silica gelswere randomly dumped into the tower to about 10 cm height (25 g weight). This bed allowed theinlet moist air and silica gels to contact intimately. On the bottom of the tower, there were someglass beads with 2 mm diameter. The silica gels were placed on the top of the glass beads. Theglass beads could give the inlet air stream good distribution before contact with silica gels. Inregeneration operation, the bed with 25 g silica gels was placed in a temperature controlled waterbath to regenerate the adsorbent. The regeneration temperature was controlled in the range of 60–90 �C, and the regeneration time was selected in the range of 10 min to 3 h in this study. Thedynamic adsorption experiments were conducted after desorption of silica gels in differentregeneration conditions.

3. Results and discussions

As shown in Fig. 1 the inlet and effluent humidities of air stream were recorded by hygrometers(chilled mirror dew point meter, General Eastern). The analysis of the silica gel beds was based onthe effluent concentration–time curve, which was referred to as the breakthrough curve and ob-tained by flowing the moist air through the packed bed with the silica gels. The breakthrough

0 200 400 600 800 1000 1200 1400 16000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

60oC-2hr 70oC-2hr 80oC-2hr 90oC-2hr commercial silica gel (90oC-2hr)

C/C

0

Time (min)

Fig. 2. Comparison of moisture adsorption breakthrough curves of the commercial and the modified silica gels.


curve is a function of adsorber geometry, operating condition, and adsorption equilibrium. Themoisture adsorption breakthrough curves of the commercial and the modified silica gels wereshown in Fig. 2. As the solute first appeared in the effluent stream, the effluent concentration–timecurve started to go upward until the bed became saturation with the solute. This time periodcorresponded to the mass transfer zone in the bed and was related to the mass transfer resistanceof the adsorption process. The thinner mass transfer zone resulted in a smaller mass transferresistance and a larger mass transfer rate. The thinner mass transfer zone was found in themodified silica gel in Fig. 2. It means that the mass transfer resistance for the modified silica gel issmaller than that of the commercial silica gel. A higher moisture adsorption rate in the modifiedsilica gel was expected. It was observed that the breakthrough curve moved to right in a constantpattern when the regeneration temperature was increased from 60 to 90 �C. The breakthroughcurves of silica gels regenerated at 80 and 90 �C were almost coincident.The adsorption breakthrough curves of the modified silica gel regenerated by different time and

temperature were shown in Fig. 3. As mentioned earlier, the breakthrough curve moved to right ina constant pattern when regeneration temperature was increased. A similar observation was foundwhen regeneration time was increased. The breakthrough curves of the silica gels regenerated for2 and 3 h were almost coincident in Fig. 3. On the basis of this study, the regeneration at 80 �Cfor 2 h would be enough to regenerate the modified silica gel.The area behind the breakthrough curve represents the quantity of adsorbate retained in the

adsorbent. The calculation of moisture uptakes on silica gels from the breakthrough curves wasbased on the following equation [10].

q ¼ F � qa � C0W

Z 1

0

1

�� CC0

�dt ð2Þ

where q is the water vapor uptake on silica gel, W is the total weight of adsorbent, F is the air flowrate, qa is the density of inlet air, C0 and C represent the humidities of inlet and effluent airstreams. The values of uptake in Fig. 4 were calculated by Eq. (2). The uptake was increased whenthe regeneration time and/or regeneration temperature of the silica gel was increased. However,no significant increase on uptakes was found after 2 h regeneration. The variation of uptakes for

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.50.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

Inlet humidity= 16 gH2O/kg dry airAir flow rate= 1.5 L/min

60oC 70oC 80oC 90oCU

ptak

e (g

H2O

/g s

ilica

gel

)

Regeneration time (hr)

Fig. 4. Uptakes of the modified silica gel versus the regeneration time at various regeneration temperatures.

0 50 100 150 200 250 300 3500.0

0.2

0.4

0.6

0.8

1.0(d) 90oC

10 min 20 min 30 min 1 hr 2 hr 3 hr

Time (min)

0.2

0.4

0.6

0.8

1.0

(c) 80oC 10 min 20 min 30 min 1 hr 2 hr 3 hr

0.2

0.4

0.6

0.8

1.0

C/C

0

(b) 70oC 10 min 20 min 30 min 1 hr 2 hr 3 hr

0.2

0.4

0.6

0.8

1.0

(a) 60oC 10 min 20 min 30 min 1 hr 2 hr 3 hr

Fig. 3. Breakthrough curves of the modified silica gel at different regeneration time. (a) regeneration tempera-

ture¼ 60 �C. (b) regeneration temperature¼ 70 �C. (c) regeneration temperature¼ 80 �C. (d) regeneration tempera-ture¼ 90 �C.


silica gels regenerated at 80 and 90 �C for 2 h was negligible. This coincides with the characteristicsof breakthrough curves shown in Figs. 2 and 3.

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.50.00

0.02

0.04

0.06

0.08

0.10

0.12 Inlet humidity= 16 g H2O/kg dry air

Air flow rate= 1.5 L/min modified silica gel commercial silica gel

Upt

ake

(g H

2O/g

sili

ca g

el)

Regeneration time (hr)

Fig. 5. Comparison of the effective uptakes of the commercial and the modified silica gels.


There are two silica gel beds in the adsorption dehumidification system usually. When the silicagel in one bed has become saturated with moisture, the process air is diverted to a second bed andthe first bed is heated to regenerate. However, the switch time is advanced to the breakpoint inreal operations. The breakpoint, which is the time at the ratio of effluent concentration to initialconcentration of 0.15 in the breakthrough curve [8], is applied in this study. The upper limit of theintegration in Eq. (2) was changed to the time of breakpoint (at C=C0¼ 0.15) when calculated theeffective uptake in Fig. 5. The effective uptake is the real amount of moisture adsorption onthe silica gel in each adsorption-regeneration cycle of the adsorption dehumidification system.Similar effective uptakes of the system using modified silica gel regenerated at 90 �C for 0.5 h andthe system using commercial silica gel for 3 h regeneration were found in Fig. 5. Since the masstransfer resistance in the modified silica gel is smaller than that of the commercial silica gel (asshown in Fig. 2), its mass transfer rate should be faster than that of the commercial silica gel. Fora specific moisture uptake in the adsorption dehumidification process, the adsorbent with highermass transfer rate can reach this specific uptake in a lower degree of regeneration.

4. Conclusions

Effects of the regeneration temperature and the regeneration time on the moisture adsorption inpacked silica gel beds have been discussed in this study. As expected, the amount of adsorption inan adsorbent is affected by the degree of adsorbent regeneration. The breakthrough curve movedto right in a constant pattern when the regeneration temperature and/or the regeneration timewere increased in this study. It means that the mass transfer resistance is similar and the amountof adsorption is increased as the degree of regeneration increased. The commercial and themodified silica gels with similar specific surface area and different mass transfer resistance werecompared in this study to understand the role of the mass transfer resistance on the degree ofadsorbent regeneration. Compared to the commercial silica gel, it is not necessary to have a highregeneration temperature and a long regeneration time to reach a specific moisture uptake in theadsorption dehumidification process using the modified silica gel. Since the thinner mass transfer


zone was found in the breakthrough curve of the modified silica gel, its mass transfer resistancewas smaller than that of the commercial silica gel. Therefore, systems using modified silica gelhave a good energy saving in regeneration.

Acknowledgements

This project is supported by the National Science Council and Energy Council of the Republicof China through the grant NSC-91-2623-7-033-010-ET.

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