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Performance study on a structured packed liquid desiccant regenerator
Solar Energy 80 (2006) 1624–1631 www.elsevier.com/locate/solener
Performance study on a structured packed liquid desiccant regenerator Esam Elsarrag
*
Technical Studies Institute, Zayed Military City, Suwaihan, P.O. Box 39219, Abu Dhabi, United Arab Emirates Received 7 March 2005; received in revised form 17 November 2005; accepted 17 November 2005 Available online 2 February 2006 Communicated by: Associate Editor William Duff
Abstract The solution carryover in traditional desiccant regeneration towers is of serious concern in real applications especially when using triethylene glycol (TEG) as a desiccant. In this study, the cellulose rigid media pads are used as the structured packing. The packing arrangements have provided minimum carryover of TEG. A performance study of the simultaneous heat and mass transfer between air and desiccant in a structured packed-stripping tower is conducted. Through the study of the regenerator, important design variables are defined and the regenerator performance is compared with the previous studies. The effects of air and liquid flow rates, air humidity, desiccant temperature and desiccant concentration have been reported on the evaporation rate and humidity effectiveness of the column. It is found that high liquid flow rates do not have a significant effect on the performance variables. 2006 Elsevier Ltd. All rights reserved. Keywords: Desiccant; Packing; Regeneration
1. Introduction Liquid desiccant systems essentially consist of an absorber for dehumidifying the air, a regenerator for regenerating the solution and liquid–liquid heat exchangers for pre-cooling and pre-heating of solution. The degree of the absorptionndesorption depends on the concentration, temperature and characteristics of the hygroscopic solution because
*
Tel.: +971 507 160317; fax: +971 846 80127. E-mail addresses: [email protected], elsarrag69@yahoo. co.uk
of the vapor-pressure difference between the air and the liquid desiccant. Several liquid desiccants, including aqueous solutions of organic compound (e.g., triethylene glycol) and aqueous solutions of inorganic salts (e.g., lithium bromide), have been employed to remove water vapor from air. Unlike the hygroscopic salts the problematic nature of triethylene glycol (TEG) is that it evaporates slightly during operation into the process air and deposits on the walls and furniture. The process equipment utilized for liquid–gas contacting is generally falling film, spray or packed towers. The design theory of such absorbers and regenerators is available in standard texts and
0038-092X/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2005.11.005
E. Elsarrag / Solar Energy 80 (2006) 1624–1631
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Nomenclature m P Q w t X
mass flow rate (kg/m2 s) pressure (Pa) flow rate (m3/s) humidity ratio (kg water/kg dry air) temperature (C) desiccant concentration (kg desiccant/ kg solution)
Greeks e effectiveness U relative humidity
handbooks (Ramm, 1968; Treybal, 1981; Perry and Chilton, 1996). Different regenerator designs have been studied by researchers. For falling film, Jain et al. (1999) studied experimentally a falling film plate regenerator. The results were compared with the predictions from theoretical model. Howell (1987) modeled a regeneration chamber containing a finned heating coil, where hot water flowing through the coil to provide the heat of regeneration. It was found that by increasing the air to desiccant flow rate and the hot water flow rate gave a stronger desiccant leaving the chamber. For spray towers, Scalabrin and Scaltri (1990) analyzed a spray tower in which a stream of scavenging air comes into direct contact with the weak lithium chloride solution sprinkled over a tube bank heated internally by warm water. Chung et al. (1999) designed a U-shaped spray tower to prevent carry over and developed a mass transfer correlation for the air stripping process using triethylene glycol. Packed tower configurations have received more attention. Many researchers worked on packed regenerators and compared the results with theoretical models. Although random packed towers facilitate more mass transfer by providing larger area in a relatively smaller volume, the air pressure drop through the packing is generally high. Factor and Grossman (1980) compared the experimental and theoretical model of a packed regenerator using LiBr and pre-heated air. Etras et al. (1994) investigated the influence of the performance variables on a packed regenerator performance. Potniz and Lenz (1996) tested a packed regenerator using random polypropylene and structured packings. Experiments showed that the evap-
Subscripts a air db dry bulb e evaporation eq equilibrium i inlet or interface L liquid o outlet s solution v vapor wb wet bulb
oration rate was 130–300% greater in the randomly packed bed than the structured packing bed. Oberg and Goswami (2000) developed two novel performance correlations for the effectiveness of a packed bed liquid desiccant dehumidification/regeneration. A comparison to experimental results showed that the correlations presented correctly predict the influence of the design variables on the performance within 15% error. Fumo and Goswami (2002) assessed the regeneration process under the effect of the design variables of a packed regenerator using lithium chloride. Longo and Gasparella (2004) reported experimental tests on sorption/desorption liquid desiccant system using H2O/LiBr and H2O/ KCOOH. They compared between experimental and calculated values within 20% error. Gandhidsan (2005) investigated the influence of the heating source on the evaporation rate of a packed bed regenerator. From the literature review it can be noted that the studies on structured packed regenerators are rarely. This paper concerned with the operation of packed bed systems. Structured packings have shown excellent performance characteristics with a relatively low ratio of pressure drop to heat and mass transfer coefficient per unit volume (Bravo et al., 1985). Here, a structured packed air stripper has been developed and tested for desiccant regeneration using TEG. The packing material used in the column was cellulose rigid media pads, having a surface area of 440 m2/m3. This paper presents the moisture removal rate as calculated from experimental measurements. The results from experimental data are used to assess the previous performance studies found in the literature. The
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effects of air flow rate, liquid flow rate, air humidity, desiccant temperature and desiccant concentration have been reported on the evaporation rate and humidity effectiveness of the column. 2. Experimental set-up An experimental set-up was developed to carry out studies on a liquid desiccant regeneration system. As shown in Fig. 1, the test-section consists of a liquid regenerator which has a cross-sectional area of 0.09 m2. The total height of the packed column is 55 cm. The layers of the cellulose rigid media pads were placed on each other in different orientation so as to provide a zig-zag path for air flow thus providing uniform mixing and reducing carryover each having a height of 10 cm. The liquid and air flow angles changed from 30 and 60 to 120 and 150 from one layer to another. This arrangement minimized the solution carry over due to the change in the direction when air and solution are moving from a layer to another.
A circulating pump was used to circulate the hot weak desiccant through the heat exchanger and the regenerator. A copper distributor was fixed at the top of the packing to distribute the hot desiccant uniformly over the packing. A removable 2 cm layer of aluminium screen was placed above the packing section to eliminate and check (if any) TEG droplets being carried over by the air. An axial extract fan was fixed on the top of the regenerator section to extract the outdoor air from the bottom of the regenerator in counter manner to the desiccant flow. An electric water heater (50 l–1.2 kW) and an immersed type electric heater (2 kW) were used as the desiccant heating source. 3. Experimental procedure Before each experiment, 100 kg of TEG was stored in a tank and its temperature was adjusted by circulating the hot water through the heat exchanger. The TEG was allowed to re-circulate to remove any temperature and concentration gra-
Fig. 1. Experimental system schematic diagram.
E. Elsarrag / Solar Energy 80 (2006) 1624–1631
dients. The auxiliary immersed heater was used to raise the desiccant temperature when required. The desiccant temperature and concentration were measured before running the experiment; also a sample from the tank was taken for analysis. The dry bulb and wet bulb temperatures of the outdoor air were also measured before and during of desiccant experiments. The measurements were taken after allowing enough time for steady state readings. The measurements during the experiment are shown in Fig. 1. The following instruments were used to measure different parameters: (a) Digital RTD type thermometers, range of 0– 100 C and a resolution of 0.1 C, were used to measure the desiccant and air temperatures. (b) A portable digital vane anemometer was used to measure the air velocity. (c) Rotameters, range of 1–8 (US) gallon per minute, were used to measure the desiccant and water flow rates. The desiccant rotameter was calibrated under different desiccant temperatures. (d) A hand refractometer, range of 1.445–1.52 refractive index with a resolution of 0.001, was used to measure the desiccant concentration. The refractometer was calibrated using different types of materials with known refractive index.
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The evaporation rate can be calculated using the following equation: me ¼ ma ðwao wai Þ
ð1Þ
The evaporation effectiveness of a packed bed regenerator is defined as the actual change in air humidity ratio across the packed absorber divided by the maximum possible change: wao wai e¼ ð2Þ weq wai For counter flow in a packed bed, the maximum outlet achievable difference in air is obtained when the air is in equilibrium with the inlet desiccant solution (Pvo = Psi). 5. Results and discussion As shown in Fig. 2, the water evaporation rate increased with the increase of air flow rate. This is mainly because a higher flow rate rapidly removes the higher moist air from the interface, so reduces the humidity gradient between the interface and bulk air, maintaining a higher potential for mass transfer. The humidity effectiveness decreased with
The experiments were run in different weather conditions in humid and moderate seasons for about three months. The design variables for the study are the liquid flow rate, the air flow rate, the desiccant temperature, the desiccant concentration and the outdoor air humidity ratio (f(tdb, twb)). The similarity in weather conditions during the season (either humid or moderate) allowed running experiments in different air and liquid flow rates. The variation of weather conditions during day and night in the season allowed running experiments in the same liquid and air flow rates. 4. Performance parameters The objective of the regeneration process is to transfer the water vapor from the weak desiccant solution to the scavenging air stream whereas the water vapor is removed from the process air to the desiccant solution during the dehumidification process.
Fig. 2. Effect of air flow rate on evaporation rate and humidity effectiveness.
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the increase of air flow rate, as shown in Fig. 2. This is due to the reduction in the humidity gradient when increasing the air flow rate. The findings of the present study agree with the previous studies reported by Martin and Goswami (1999); Fumo and Goswami (2002) and Gandhidsan (2005). The effect of liquid flow rate on the regenerator performance is getting more attention. Some investigators used salt solutions and reported that the evaporation rate increased with the increase of solution flow rate such as Potniz and Lenz (1996) (mL = 0.8–1.5), Fumo and Goswami (2002) (mL/ ma = 4.7–6.8), Gandhidsan (2005) (mL/ma = 4– 7.3). However, Martin and Goswami (1999) found that the evaporation rate has only a slight dependency on TEG flow rate (mL/ma = 3.5–15). The present study did not contradict the findings obtained by the above studies. The evaporation rate and humidity effectiveness increased with the increase of the liquid flow rate to a certain limit (mL/ma 6 1.1), but no significant dependency on the evaporation rate was observed when high liquid flow rates were employed (mL/ma = 1.1–1.6), as shown in Fig. 3. However, it can be observed that the liquid flow rates employed on random packings
Fig. 3. Effect of liquid flow rate on evaporation rate and humidity effectiveness.
by Fumo and Goswami (2002); Gandhidsan (2005) and Martin and Goswami (1999) are very high compared to that employed on structured packings in the present study and Potniz and Lenz (1996). This may indicate that good wetting of structured packings can be achieved at lower liquid to air flow rate ratios compared to random packings. Hence, it can be concluded that in this study the performance variables increased with the desiccant flow rate to a limit where good wetting of the packing was achieved (mL/ma 6 1.1) and an additional increase in the liquid flow rate did not show a significant effect on the performance variables. The solution equilibrium humidity ratio depends on the inlet solution temperature and concentration (xe = f(Xi, tsi)). The lower the solution concentration is the larger amount of water contained in the solution. Therefore, a larger amount of water is able to evaporate from a solution of lower concentration. A sufficiently large increase in temperature causes a significant decrease of gas solubility in the liquid phase. Therefore, the stripping process reveals a higher driving force of mass transfer with increasing temperature. Hence, the water evaporation rate increased with the liquid temperature, as shown in Fig. 4. However, the effectiveness showed a slight dependency on the inlet desiccant temperature. This is because the highest possible humidity ratio that can be obtained at the air outlet is dependent on the desiccant temperature, making the effectiveness to some extent normalized with respect to the desiccant temperature (Eq. (2)). As shown in Fig. 5, the evaporation rate decreased with the increase of desiccant concentration. This is mainly because an increase in the desiccant concentration decreases the driving force of mass transfer so, lowers the evaporation rate. On the other hand, no significant change in the effectiveness was observed. This is mainly because the highest humidity ratio is dependent on the concentration and the humidity effectiveness is normalized with respect to the desiccant concentration (Eq. (2)). The influence of the inlet air humidity ratio on the stripper performance is shown in Fig. 6. An increase in the inlet humidity ratio increases the vapor pressure of the inlet air and decreasing the potential for mass transfer between the desiccant and the air. Hence, the evaporation rate and the humidity effectiveness decreased with the increase of the inlet air humidity ratio. This explains that in humid climates the driving force is lower than dry climates if the solution equi-
E. Elsarrag / Solar Energy 80 (2006) 1624–1631
Fig. 4. Effect of desiccant temperature on evaporation rate and humidity effectiveness.
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Fig. 6. Effect of air humidity ratio on evaporation rate and humidity effectiveness.
librium humidity is kept constant. Therefore, the desiccant regeneration in humid climates requires higher temperature to keep high driving force compared to dry climates. In this study higher evaporation rates were obtained from the structured packed regenerator compared to the previous studies. The reason behind this could be the high solution temperatures used in this study in addition to the zig-zag arrangements of the packing increased the height of the transfer units and provided good wetting of the packing at low liquid to air flow rate ratios. The effect of the process variables on the regenerator performance which are investigated in this study as well as those reported in the literature for a packed tower is summarized in Table 1. 6. Conclusions
Fig. 5. Effect of desiccant concentration on evaporation rate and humidity effectiveness.
In this study, the effect of different design parameters on the performance of a structured packed stripping column using TEG was investigated experimentally. The effects of air and liquid flow rates, air humidity, desiccant temperature and desiccant concentration were reported on the evaporation
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Table 1 Summary of the results Author
Desiccant
Elsarrag (present study)
TEG
Factor and Grossman (1980) Etras et al. (1994)
Performance parameters
mL (kg/m2 s) [ma = 1.2]
Fumo and Goswami (2002)
Gandhidsan (2005)
tsi (C)
Xi (%)
90–95 – +
0.64–1.1 + + mL/ma
1.2–1.6 + + Xi (%)
1–1.23 + tsi (C)
70–80 + + tai (C)
Xo
0.2–1 + Q (m3/s)
58–60 + Xi (%)
65, 80 + tsi (C)
40.5, 52 (+)
(18–38) · 105 +
33–43 + +
62–85 + + mL (kg/m2 s)
LiBr
CELD
LiBr me
Martin and Goswami (1999)
ma (kg/m2 s) [mL = 0.92]
me e
Xo tso Potniz and Lenz (1996)
Independent variables
0.8–1.5 + tsi (C)
wai gw/kga
18.1–24.2 Remarks
(+) Slight increase U (%) 60–80 +
TEG
mL (kg/m2 s)
ma (kg/m2 s)
me e LiCl
4.5–6.5 + + mL (kg/m2 s)
0.5–1.5 + ma (kg/m2 s)
me e
5.2–7.5 + + mL (kg/m2 s)
0.82–1.42 + ma (kg/m2 s)
tsi (C)
Remarks
4.5–8 +
0.82–1.42 +
68–72 +
Study compared with Fumo and Goswami (2002)
LiCl me
60–72 + +
wai gw/kga
tai (C)
92–95 + tsi (C)
Xi (%)
10–24 + Xi (%)
30–50 + + wai gw/kga
60–70 +
32.5–35
14–21
+: Coefficient increases with increasing variable. : Coefficient parameter decreases with increasing variable. +: Variable has no significant effect on coefficient.
rate and humidity effectiveness of the column. It was found that high liquid flow rates, (mL/ma = 1.1–1.6), do not have a significant effect on the performance variables. The results of the present study were compared with the previous studies. The cellulose rigid media pads were used as structured packing and different layers of the pad were oriented in a zig-zag manner so as to minimize TEG carryover. The desiccant regeneration requires a higher solution temperature in humid climates compared to dry climates. References Bravo, J.L., Rocha, J.A., Fair, J.R., 1985. Mass transfer in gauze packings. Hydrocarbon Processing 64, 91–105. Chung, T.W., Lai, C., Wu, H., 1999. Analysis of mass transfer performance in an air stripping tower. Separation Science and Technology 34 (14), 2837–2851.
Etras, A., Gandhidsan, P., Kiris, I., Anderson, E.E., 1994. Experimental study on the performance of a regeneration tower for various climatic conditions. Solar Energy 53 (1), 125–130. Factor, H.M., Grossman, G., 1980. A packed bed dehumidifier/ regenerator for solar air conditioning with liquid desiccants. Solar Energy 24, 541–550. Fumo, N., Goswami, D.Y., 2002. Study of an aqueous lithium chloride desiccant system: Air dehumidification and desiccant regeneration. Solar Energy 72 (4), 351–361. Gandhidsan, P., 2005. Quick performance predictions of liquid desiccant regeneration in a packed bed. Solar Energy 79 (1), 47–55. Howell, J.R., 1987. A survey of active solar cooling methods. Progress in Solar Engineering, 171–182. Jain, S., Dhar, P.L., Kaushik, S.C., 1999. Experimental studies on the dehumidifier and regenerator of a liquid desiccant cooling system. Applied Thermal Engineering 20, 253–267. Longo, G.A., Gasparella, A., 2004. Experimental analysis on chemical dehumidification of air by liquid desiccant and desiccant regeneration in a packed tower. Journal of Solar
E. Elsarrag / Solar Energy 80 (2006) 1624–1631 Energy Engineering, Transactions of the ASME 126 (1), 587– 591. Martin, V., Goswami, D.Y., 1999. Heat and mass transfer in packed bed liquid desiccant regenerators—An experimental investigation. Transactions of the ASME Journal of Solar Energy Engineering 121, 162–170. Oberg, V.M., Goswami, D.Y., 2000. Effectiveness of heat and mass transfer processes in packed bed liquid desiccant dehumidifier/regenerator. HVAC & Research, Ashrae 6, 22– 39. Perry, R.H., Chilton, C.H., 1996. Chemical Engineer’s Handbook, fifth ed. McGraw-Hill, New York.
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Potniz, S.V., Lenz, T.G., 1996. Dimensionless mass transfer correlations for packed-bed liquid-desiccant contactors. Industrial and Engineering Chemistry Research 35, 4185– 4193. Ramm, V.H., 1968. Absorption of Gases. Israel Program for Scientific Translations, Jerusalem. Scalabrin, G., Scaltri, G., 1990. A liquid sorption–desorption system for air conditioning with heat at lower temperature. ASME, Journal of Solar Energy Engineering 112, 70– 75. Treybal, R.E., 1981. Mass Transfer Operations, third ed. McGraw-Hill, New York.
Performance study on a structured packed liquid desiccant regenerator Esam Elsarrag
*
Technical Studies Institute, Zayed Military City, Suwaihan, P.O. Box 39219, Abu Dhabi, United Arab Emirates Received 7 March 2005; received in revised form 17 November 2005; accepted 17 November 2005 Available online 2 February 2006 Communicated by: Associate Editor William Duff
Abstract The solution carryover in traditional desiccant regeneration towers is of serious concern in real applications especially when using triethylene glycol (TEG) as a desiccant. In this study, the cellulose rigid media pads are used as the structured packing. The packing arrangements have provided minimum carryover of TEG. A performance study of the simultaneous heat and mass transfer between air and desiccant in a structured packed-stripping tower is conducted. Through the study of the regenerator, important design variables are defined and the regenerator performance is compared with the previous studies. The effects of air and liquid flow rates, air humidity, desiccant temperature and desiccant concentration have been reported on the evaporation rate and humidity effectiveness of the column. It is found that high liquid flow rates do not have a significant effect on the performance variables. 2006 Elsevier Ltd. All rights reserved. Keywords: Desiccant; Packing; Regeneration
1. Introduction Liquid desiccant systems essentially consist of an absorber for dehumidifying the air, a regenerator for regenerating the solution and liquid–liquid heat exchangers for pre-cooling and pre-heating of solution. The degree of the absorptionndesorption depends on the concentration, temperature and characteristics of the hygroscopic solution because
*
Tel.: +971 507 160317; fax: +971 846 80127. E-mail addresses: [email protected], elsarrag69@yahoo. co.uk
of the vapor-pressure difference between the air and the liquid desiccant. Several liquid desiccants, including aqueous solutions of organic compound (e.g., triethylene glycol) and aqueous solutions of inorganic salts (e.g., lithium bromide), have been employed to remove water vapor from air. Unlike the hygroscopic salts the problematic nature of triethylene glycol (TEG) is that it evaporates slightly during operation into the process air and deposits on the walls and furniture. The process equipment utilized for liquid–gas contacting is generally falling film, spray or packed towers. The design theory of such absorbers and regenerators is available in standard texts and
0038-092X/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2005.11.005
E. Elsarrag / Solar Energy 80 (2006) 1624–1631
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Nomenclature m P Q w t X
mass flow rate (kg/m2 s) pressure (Pa) flow rate (m3/s) humidity ratio (kg water/kg dry air) temperature (C) desiccant concentration (kg desiccant/ kg solution)
Greeks e effectiveness U relative humidity
handbooks (Ramm, 1968; Treybal, 1981; Perry and Chilton, 1996). Different regenerator designs have been studied by researchers. For falling film, Jain et al. (1999) studied experimentally a falling film plate regenerator. The results were compared with the predictions from theoretical model. Howell (1987) modeled a regeneration chamber containing a finned heating coil, where hot water flowing through the coil to provide the heat of regeneration. It was found that by increasing the air to desiccant flow rate and the hot water flow rate gave a stronger desiccant leaving the chamber. For spray towers, Scalabrin and Scaltri (1990) analyzed a spray tower in which a stream of scavenging air comes into direct contact with the weak lithium chloride solution sprinkled over a tube bank heated internally by warm water. Chung et al. (1999) designed a U-shaped spray tower to prevent carry over and developed a mass transfer correlation for the air stripping process using triethylene glycol. Packed tower configurations have received more attention. Many researchers worked on packed regenerators and compared the results with theoretical models. Although random packed towers facilitate more mass transfer by providing larger area in a relatively smaller volume, the air pressure drop through the packing is generally high. Factor and Grossman (1980) compared the experimental and theoretical model of a packed regenerator using LiBr and pre-heated air. Etras et al. (1994) investigated the influence of the performance variables on a packed regenerator performance. Potniz and Lenz (1996) tested a packed regenerator using random polypropylene and structured packings. Experiments showed that the evap-
Subscripts a air db dry bulb e evaporation eq equilibrium i inlet or interface L liquid o outlet s solution v vapor wb wet bulb
oration rate was 130–300% greater in the randomly packed bed than the structured packing bed. Oberg and Goswami (2000) developed two novel performance correlations for the effectiveness of a packed bed liquid desiccant dehumidification/regeneration. A comparison to experimental results showed that the correlations presented correctly predict the influence of the design variables on the performance within 15% error. Fumo and Goswami (2002) assessed the regeneration process under the effect of the design variables of a packed regenerator using lithium chloride. Longo and Gasparella (2004) reported experimental tests on sorption/desorption liquid desiccant system using H2O/LiBr and H2O/ KCOOH. They compared between experimental and calculated values within 20% error. Gandhidsan (2005) investigated the influence of the heating source on the evaporation rate of a packed bed regenerator. From the literature review it can be noted that the studies on structured packed regenerators are rarely. This paper concerned with the operation of packed bed systems. Structured packings have shown excellent performance characteristics with a relatively low ratio of pressure drop to heat and mass transfer coefficient per unit volume (Bravo et al., 1985). Here, a structured packed air stripper has been developed and tested for desiccant regeneration using TEG. The packing material used in the column was cellulose rigid media pads, having a surface area of 440 m2/m3. This paper presents the moisture removal rate as calculated from experimental measurements. The results from experimental data are used to assess the previous performance studies found in the literature. The
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E. Elsarrag / Solar Energy 80 (2006) 1624–1631
effects of air flow rate, liquid flow rate, air humidity, desiccant temperature and desiccant concentration have been reported on the evaporation rate and humidity effectiveness of the column. 2. Experimental set-up An experimental set-up was developed to carry out studies on a liquid desiccant regeneration system. As shown in Fig. 1, the test-section consists of a liquid regenerator which has a cross-sectional area of 0.09 m2. The total height of the packed column is 55 cm. The layers of the cellulose rigid media pads were placed on each other in different orientation so as to provide a zig-zag path for air flow thus providing uniform mixing and reducing carryover each having a height of 10 cm. The liquid and air flow angles changed from 30 and 60 to 120 and 150 from one layer to another. This arrangement minimized the solution carry over due to the change in the direction when air and solution are moving from a layer to another.
A circulating pump was used to circulate the hot weak desiccant through the heat exchanger and the regenerator. A copper distributor was fixed at the top of the packing to distribute the hot desiccant uniformly over the packing. A removable 2 cm layer of aluminium screen was placed above the packing section to eliminate and check (if any) TEG droplets being carried over by the air. An axial extract fan was fixed on the top of the regenerator section to extract the outdoor air from the bottom of the regenerator in counter manner to the desiccant flow. An electric water heater (50 l–1.2 kW) and an immersed type electric heater (2 kW) were used as the desiccant heating source. 3. Experimental procedure Before each experiment, 100 kg of TEG was stored in a tank and its temperature was adjusted by circulating the hot water through the heat exchanger. The TEG was allowed to re-circulate to remove any temperature and concentration gra-
Fig. 1. Experimental system schematic diagram.
E. Elsarrag / Solar Energy 80 (2006) 1624–1631
dients. The auxiliary immersed heater was used to raise the desiccant temperature when required. The desiccant temperature and concentration were measured before running the experiment; also a sample from the tank was taken for analysis. The dry bulb and wet bulb temperatures of the outdoor air were also measured before and during of desiccant experiments. The measurements were taken after allowing enough time for steady state readings. The measurements during the experiment are shown in Fig. 1. The following instruments were used to measure different parameters: (a) Digital RTD type thermometers, range of 0– 100 C and a resolution of 0.1 C, were used to measure the desiccant and air temperatures. (b) A portable digital vane anemometer was used to measure the air velocity. (c) Rotameters, range of 1–8 (US) gallon per minute, were used to measure the desiccant and water flow rates. The desiccant rotameter was calibrated under different desiccant temperatures. (d) A hand refractometer, range of 1.445–1.52 refractive index with a resolution of 0.001, was used to measure the desiccant concentration. The refractometer was calibrated using different types of materials with known refractive index.
1627
The evaporation rate can be calculated using the following equation: me ¼ ma ðwao wai Þ
ð1Þ
The evaporation effectiveness of a packed bed regenerator is defined as the actual change in air humidity ratio across the packed absorber divided by the maximum possible change: wao wai e¼ ð2Þ weq wai For counter flow in a packed bed, the maximum outlet achievable difference in air is obtained when the air is in equilibrium with the inlet desiccant solution (Pvo = Psi). 5. Results and discussion As shown in Fig. 2, the water evaporation rate increased with the increase of air flow rate. This is mainly because a higher flow rate rapidly removes the higher moist air from the interface, so reduces the humidity gradient between the interface and bulk air, maintaining a higher potential for mass transfer. The humidity effectiveness decreased with
The experiments were run in different weather conditions in humid and moderate seasons for about three months. The design variables for the study are the liquid flow rate, the air flow rate, the desiccant temperature, the desiccant concentration and the outdoor air humidity ratio (f(tdb, twb)). The similarity in weather conditions during the season (either humid or moderate) allowed running experiments in different air and liquid flow rates. The variation of weather conditions during day and night in the season allowed running experiments in the same liquid and air flow rates. 4. Performance parameters The objective of the regeneration process is to transfer the water vapor from the weak desiccant solution to the scavenging air stream whereas the water vapor is removed from the process air to the desiccant solution during the dehumidification process.
Fig. 2. Effect of air flow rate on evaporation rate and humidity effectiveness.
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the increase of air flow rate, as shown in Fig. 2. This is due to the reduction in the humidity gradient when increasing the air flow rate. The findings of the present study agree with the previous studies reported by Martin and Goswami (1999); Fumo and Goswami (2002) and Gandhidsan (2005). The effect of liquid flow rate on the regenerator performance is getting more attention. Some investigators used salt solutions and reported that the evaporation rate increased with the increase of solution flow rate such as Potniz and Lenz (1996) (mL = 0.8–1.5), Fumo and Goswami (2002) (mL/ ma = 4.7–6.8), Gandhidsan (2005) (mL/ma = 4– 7.3). However, Martin and Goswami (1999) found that the evaporation rate has only a slight dependency on TEG flow rate (mL/ma = 3.5–15). The present study did not contradict the findings obtained by the above studies. The evaporation rate and humidity effectiveness increased with the increase of the liquid flow rate to a certain limit (mL/ma 6 1.1), but no significant dependency on the evaporation rate was observed when high liquid flow rates were employed (mL/ma = 1.1–1.6), as shown in Fig. 3. However, it can be observed that the liquid flow rates employed on random packings
Fig. 3. Effect of liquid flow rate on evaporation rate and humidity effectiveness.
by Fumo and Goswami (2002); Gandhidsan (2005) and Martin and Goswami (1999) are very high compared to that employed on structured packings in the present study and Potniz and Lenz (1996). This may indicate that good wetting of structured packings can be achieved at lower liquid to air flow rate ratios compared to random packings. Hence, it can be concluded that in this study the performance variables increased with the desiccant flow rate to a limit where good wetting of the packing was achieved (mL/ma 6 1.1) and an additional increase in the liquid flow rate did not show a significant effect on the performance variables. The solution equilibrium humidity ratio depends on the inlet solution temperature and concentration (xe = f(Xi, tsi)). The lower the solution concentration is the larger amount of water contained in the solution. Therefore, a larger amount of water is able to evaporate from a solution of lower concentration. A sufficiently large increase in temperature causes a significant decrease of gas solubility in the liquid phase. Therefore, the stripping process reveals a higher driving force of mass transfer with increasing temperature. Hence, the water evaporation rate increased with the liquid temperature, as shown in Fig. 4. However, the effectiveness showed a slight dependency on the inlet desiccant temperature. This is because the highest possible humidity ratio that can be obtained at the air outlet is dependent on the desiccant temperature, making the effectiveness to some extent normalized with respect to the desiccant temperature (Eq. (2)). As shown in Fig. 5, the evaporation rate decreased with the increase of desiccant concentration. This is mainly because an increase in the desiccant concentration decreases the driving force of mass transfer so, lowers the evaporation rate. On the other hand, no significant change in the effectiveness was observed. This is mainly because the highest humidity ratio is dependent on the concentration and the humidity effectiveness is normalized with respect to the desiccant concentration (Eq. (2)). The influence of the inlet air humidity ratio on the stripper performance is shown in Fig. 6. An increase in the inlet humidity ratio increases the vapor pressure of the inlet air and decreasing the potential for mass transfer between the desiccant and the air. Hence, the evaporation rate and the humidity effectiveness decreased with the increase of the inlet air humidity ratio. This explains that in humid climates the driving force is lower than dry climates if the solution equi-
E. Elsarrag / Solar Energy 80 (2006) 1624–1631
Fig. 4. Effect of desiccant temperature on evaporation rate and humidity effectiveness.
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Fig. 6. Effect of air humidity ratio on evaporation rate and humidity effectiveness.
librium humidity is kept constant. Therefore, the desiccant regeneration in humid climates requires higher temperature to keep high driving force compared to dry climates. In this study higher evaporation rates were obtained from the structured packed regenerator compared to the previous studies. The reason behind this could be the high solution temperatures used in this study in addition to the zig-zag arrangements of the packing increased the height of the transfer units and provided good wetting of the packing at low liquid to air flow rate ratios. The effect of the process variables on the regenerator performance which are investigated in this study as well as those reported in the literature for a packed tower is summarized in Table 1. 6. Conclusions
Fig. 5. Effect of desiccant concentration on evaporation rate and humidity effectiveness.
In this study, the effect of different design parameters on the performance of a structured packed stripping column using TEG was investigated experimentally. The effects of air and liquid flow rates, air humidity, desiccant temperature and desiccant concentration were reported on the evaporation
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Table 1 Summary of the results Author
Desiccant
Elsarrag (present study)
TEG
Factor and Grossman (1980) Etras et al. (1994)
Performance parameters
mL (kg/m2 s) [ma = 1.2]
Fumo and Goswami (2002)
Gandhidsan (2005)
tsi (C)
Xi (%)
90–95 – +
0.64–1.1 + + mL/ma
1.2–1.6 + + Xi (%)
1–1.23 + tsi (C)
70–80 + + tai (C)
Xo
0.2–1 + Q (m3/s)
58–60 + Xi (%)
65, 80 + tsi (C)
40.5, 52 (+)
(18–38) · 105 +
33–43 + +
62–85 + + mL (kg/m2 s)
LiBr
CELD
LiBr me
Martin and Goswami (1999)
ma (kg/m2 s) [mL = 0.92]
me e
Xo tso Potniz and Lenz (1996)
Independent variables
0.8–1.5 + tsi (C)
wai gw/kga
18.1–24.2 Remarks
(+) Slight increase U (%) 60–80 +
TEG
mL (kg/m2 s)
ma (kg/m2 s)
me e LiCl
4.5–6.5 + + mL (kg/m2 s)
0.5–1.5 + ma (kg/m2 s)
me e
5.2–7.5 + + mL (kg/m2 s)
0.82–1.42 + ma (kg/m2 s)
tsi (C)
Remarks
4.5–8 +
0.82–1.42 +
68–72 +
Study compared with Fumo and Goswami (2002)
LiCl me
60–72 + +
wai gw/kga
tai (C)
92–95 + tsi (C)
Xi (%)
10–24 + Xi (%)
30–50 + + wai gw/kga
60–70 +
32.5–35
14–21
+: Coefficient increases with increasing variable. : Coefficient parameter decreases with increasing variable. +: Variable has no significant effect on coefficient.
rate and humidity effectiveness of the column. It was found that high liquid flow rates, (mL/ma = 1.1–1.6), do not have a significant effect on the performance variables. The results of the present study were compared with the previous studies. The cellulose rigid media pads were used as structured packing and different layers of the pad were oriented in a zig-zag manner so as to minimize TEG carryover. The desiccant regeneration requires a higher solution temperature in humid climates compared to dry climates. References Bravo, J.L., Rocha, J.A., Fair, J.R., 1985. Mass transfer in gauze packings. Hydrocarbon Processing 64, 91–105. Chung, T.W., Lai, C., Wu, H., 1999. Analysis of mass transfer performance in an air stripping tower. Separation Science and Technology 34 (14), 2837–2851.
Etras, A., Gandhidsan, P., Kiris, I., Anderson, E.E., 1994. Experimental study on the performance of a regeneration tower for various climatic conditions. Solar Energy 53 (1), 125–130. Factor, H.M., Grossman, G., 1980. A packed bed dehumidifier/ regenerator for solar air conditioning with liquid desiccants. Solar Energy 24, 541–550. Fumo, N., Goswami, D.Y., 2002. Study of an aqueous lithium chloride desiccant system: Air dehumidification and desiccant regeneration. Solar Energy 72 (4), 351–361. Gandhidsan, P., 2005. Quick performance predictions of liquid desiccant regeneration in a packed bed. Solar Energy 79 (1), 47–55. Howell, J.R., 1987. A survey of active solar cooling methods. Progress in Solar Engineering, 171–182. Jain, S., Dhar, P.L., Kaushik, S.C., 1999. Experimental studies on the dehumidifier and regenerator of a liquid desiccant cooling system. Applied Thermal Engineering 20, 253–267. Longo, G.A., Gasparella, A., 2004. Experimental analysis on chemical dehumidification of air by liquid desiccant and desiccant regeneration in a packed tower. Journal of Solar
E. Elsarrag / Solar Energy 80 (2006) 1624–1631 Energy Engineering, Transactions of the ASME 126 (1), 587– 591. Martin, V., Goswami, D.Y., 1999. Heat and mass transfer in packed bed liquid desiccant regenerators—An experimental investigation. Transactions of the ASME Journal of Solar Energy Engineering 121, 162–170. Oberg, V.M., Goswami, D.Y., 2000. Effectiveness of heat and mass transfer processes in packed bed liquid desiccant dehumidifier/regenerator. HVAC & Research, Ashrae 6, 22– 39. Perry, R.H., Chilton, C.H., 1996. Chemical Engineer’s Handbook, fifth ed. McGraw-Hill, New York.
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Potniz, S.V., Lenz, T.G., 1996. Dimensionless mass transfer correlations for packed-bed liquid-desiccant contactors. Industrial and Engineering Chemistry Research 35, 4185– 4193. Ramm, V.H., 1968. Absorption of Gases. Israel Program for Scientific Translations, Jerusalem. Scalabrin, G., Scaltri, G., 1990. A liquid sorption–desorption system for air conditioning with heat at lower temperature. ASME, Journal of Solar Energy Engineering 112, 70– 75. Treybal, R.E., 1981. Mass Transfer Operations, third ed. McGraw-Hill, New York.