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Air dehumidification by triethylene glycol desiccant in a packed column
Energy Conversion and Management 45 (2004) 141–155 www.elsevier.com/locate/enconman
Air dehumidification by triethylene glycol desiccant in a packed column Y.H. Zurigat a
a,1
, M.K. Abu-Arabi b, S.A. Abdul-Wahab
a,*
Department of Mechanical and Industrial Engineering, College of Engineering, Sultan Qaboos University, P.O. Box 33, Al Khoud 123, Muscat, Oman b Middle East Desalination Research Center, P.O. Box 21, Al-Khuwair 133, Oman Received 10 May 2002; received in revised form 27 November 2002; accepted 12 May 2003
Abstract The performance of an air dehumidifier using triethylene glycol (TEG) as desiccant under hot and humid conditions was investigated. The performance of the dehumidifier was evaluated and expressed in terms of the moisture removal rate and the dehumidifier effectiveness. A packed bed column (dehumidifier) was employed, with low packing density (77 m2 /m3 ), to provide direct contact between the air and the TEG. Two different structured packings were used, wood and aluminum. The experiments covered a wide range of parameter space that included the air and TEG flow rates, air and TEG inlet temperatures, inlet air humidity and inlet TEG concentration. The liquid flow rate investigated is much less than that covered in previous studies (<1 kg/m2 s). The trend of the dehumidifier performance was similar to that reported in the literature using high density and random packing. The results were compared to the Chung and Luo correlation, which over predicted the effectiveness. The Martin and Goswami correlation failed to predict the effectiveness under the conditions of this study. In the present study, it was found that the moisture removal rate increased with increasing inlet TEG concentration, TEG flow rate and air flow rate. This was seen for both the wood and the aluminum packings. In addition, the moisture removal rate is increased with increasing the inlet air temperature for the aluminum packing only. The effectiveness of the column was increased by increasing the TEG flow rate and inlet TEG temperature for the two packings. 2003 Elsevier Ltd. All rights reserved. Keywords: Desiccant; Triethylene glycol (TEG); Dehumidifier performance; Packed bed column
*
Corresponding author. Tel.: +968-515-350; fax: +968-513-416. E-mail address: [email protected] (S.A. Abdul-Wahab). 1 On sabbatical leave from University of Jordan, Amman, Jordan. 0196-8904/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0196-8904(03)00109-2
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Nomenclature A G L m p X Y z e p
cross-sectional area of dehumidifier column (m2 ) air mass flux (kg/m2 s) liquid desiccant mass flux (kg/m2 s) water condensation rate (g/s) vapor pressure (Pa) desiccant concentration (kg TEG/kg solution) air humidity ratio (kg water/kg dry) tower height (m) effectiveness (dimensionless) dimensionless vapor pressure difference
Subscripts equ equilibrium in inlet outlet outlet y air humidity ratio Abbreviation TEG triethylene glycol
1. Introduction Humidity control is important in many engineering applications, such as space air conditioning (AC), storage warehouses, process industries and many others. In space AC, where conventional vapor compression cooling is used, humidity control is often accomplished by cooling the air below its dew point. In hot and humid climates, this method is inefficient, since it should be followed by reheating the air to a safe temperature before it is introduced into the conditioned space. Hence, the evaporator in the vapor compression system operates at a lower temperature than is required to meet the sensible cooling load [1], leading to a lower coefficient of performance and, consequently, higher energy requirements. Likewise, the effectiveness of evaporative cooling relies heavily on the existence of low relative humidity conditions, being higher as the relative humidity decreases. Thus, the humidity puts an extra load on the vapor compression AC systems and renders the evaporative cooling systems ineffective. Furthermore, under high humidity conditions, energy efficient vapor compression systems that were designed to operate at higher evaporator temperatures have been found unable to maintain the indoor relative humidity within a comfortable range [1,2]. This calls for dehumidifying the air prior to entering the evaporator. Separating the control of humidity and temperature by means of desiccants could result in energy savings, as well as improved humidity control [1]. Desiccants are chemicals with great affinity to moisture. They may be used effectively as a supplement to conventional vapor compression systems to remove the latent part of the cooling load. That is, desiccants are efficient in handling latent loads (reducing the humidity), whereas the evaporator of the vapor compression
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system is efficient in handling the sensible cooling loads (lowering the air temperature). Davanagere et al. [3] listed the advantages of using desiccant based systems as follows: (1) very little electrical energy is consumed, and the sources of regenerating thermal energy can be diverse (i.e. solar energy, waste heat, natural gas); (2) they are particularly useful when the latent load is large in comparison with the sensible load; (3) a desiccant system is likely to eliminate or reduce the use of ozone depleting CFCs (depending on whether desiccant cooling is used in conjunction with evaporative coolers or vapor compression systems, respectively); (4) control of humidity can be achieved better than those cases employing vapor compression systems, since sensible and latent cooling occur separately; (5) the cost of energy to regenerate the desiccant is low when compared with the cost of energy to dehumidify the air by cooling it below its dew point; and (6) improvement in indoor air quality is likely to occur because of the normally high ventilation and fresh air flow rates associated with the higher evaporator temperature employed. Also, desiccant systems have the capability of removing airborne pollutants. Several different materials can be employed as desiccant, including both solid and liquid substances. Conventional solid desiccants include silica gel, activated alumina, lithium chloride salt, molecular sieves, titanium silicate and synthetic polymers. Liquid desiccants include lithium chloride, lithium bromide, calcium chloride and TEG. Liquid desiccants have several advantages over solid desiccants [7]. The pressure drop through the liquid desiccant is lower than that through a solid desiccant system; liquid desiccant can be pumped, and thus, several small units can be connected to meet the demands of large buildings; and liquid desiccant can be stored for regeneration by some inexpensive energy sources. More details about desiccant types, properties and the regeneration process are given by Kinsara et al. [8]. Liquid desiccants can absorb moisture from or add moisture to the air, depending on the vapor pressure difference between the surrounding air and the desiccant solution (i.e. the vapor pressure difference is the driving force for mass transfer). The equilibrium vapor pressure of the solution depends on its temperature and concentration. The higher the concentration and lower the temperature, the higher would be the moisture absorbed, but at high concentrations and low temperatures, there is a possibility of crystallization of the desiccant. Therefore, for different desiccants, operating concentration levels are different, e.g. 55–60% for lithium bromide and 95– 97% for TEG [9,10]. As cool and concentrated desiccant is brought in contact with air, water vapor in the air is absorbed by the desiccant, i.e. water condenses into the desiccant. During this process, heat is evolved due to the latent heat of condensation of the water and the heat of mixing. The earliest liquid desiccant system was suggested and experimentally tested by Lof [11] using triethylene glycol (TEG) as desiccant. Solar heated air was used for regeneration. Sheridan [12] investigated a solar operated liquid desiccant dehumidification-cooling system using lithium chloride as the working fluid. Reviews of the early work on the development of liquid desiccant systems can be found in Howell [7], Gandhidasan and Gupta [13], Lodwig et al. [14], Robison [15], Robison and Houston [16], Factor and Grossman [17], Peng [18], Grossman and Johannsen [19], and Johannsen [20]. In addition, more than 80 publications regarding liquid desiccants, liquid desiccant cooling systems, and the analysis of important system components were reviewed by € berg and Goswami [21]. O The performance of a packed bed absorption tower, operating as an air dehumidifier or a desiccant regenerator, is influenced by many design operating parameters and conditions [4]: desiccant fluid characteristics (viscosity, density and surface tension), packing type (shape, size
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and material), desiccant distribution over the packing, flow configuration (counter or co-current flow), tower height, fluids flow rates and the inlet conditions of the desiccant (temperature and concentration) and the air (temperature and humidity). To improve the effectiveness of the desiccant air dehumidification process, the impacts of these parameters have to be evaluated. Good system performance and energy savings can be achieved if the proper values of these variables are selected. A number of experimental studies are reported in the literature regarding this matter. For example, Chung et al. [4], Chen et al. [22], Patnaik et al. [23], McDonald et al. [24], Potnis and Lenz [25] and Ullah et al. [26] conducted experiments on € berg and Goswami [1], Chung packed bed dehumidifiers using salt solutions as the desiccants. O et al. [5], Martin and Goswami [6], Park et al. [27], Peng and Howell [28], Chebbah [29], Park et al. [30,31], Chung and Luo [32] and Chung and Wu [33] reported experimental results using TEG as the desiccant. Moreover, Chung [34] reported some experimental findings using both lithium chloride and TEG as the desiccants. A summary of the variables investigated by various researchers, the ranges and effects of these variables on moisture removal rate and dehumidifier effectiveness are presented in a table (see Table 2) in the results and discussion section of this paper. The objective of the present work is to study the effectiveness of air dehumidification using a liquid desiccant (TEG) in order to provide more design information on dehumidifiers. The work concentrates on conducting experimental work to cover a wider and more extended range of conditions than reported in the literature. Experimental analysis is conducted to study the impact of a number of design variables on the performance of the dehumidification process using a structured packing made of either wood or aluminum. The performance of this process will be evaluated in terms of the water condensation rate and the dehumidification effectiveness.
2. Experimental setup and procedures Fig. 1 shows the experimental setup used in this study for both air dehumidification and desiccant regeneration. In the air dehumidification mode, the air, conditioned to the required temperature and humidity at the AC unit, is introduced into the absorption tower from the bottom. The tower has a total height of 0.6 m with a structured type packing consisting of eight decks with seven plates per deck, resulting in a packing density of 77 m2 /m3 and a packing height of 0.48 m. Desiccant at the required temperature and flow rate is pumped into the top of the tower via the rotameter. The desiccant, flowing countercurrent relative to the humid air flow, is distributed over the packing and absorbs moisture as it comes into contact with the humid air. The diluted desiccant flows by gravity to the catch tank, where it is stored for regeneration. A uniform temperature and concentration of the desiccant are ensured by using an electric heater with temperature controller and the circulation loop shown in Fig. 1. The flow rate is monitored using a calibrated rotameter. The rotameter was calibrated under different desiccant temperatures. The dry bulb and wet bulb air temperatures are monitored using mercury thermometers (error of ±0.5 C) located at the inlet (bottom) and exit (top) of the absorption tower. Also, two more mercury thermometers are used to measure the inlet (tower top) and exit (tower bottom) desiccant temperatures. The air flow rate is monitored using an orifice flow meter (see Fig. 1).
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145
Fig. 1. Schematics of the experimental setup.
In any run, the inlet and exit temperatures and flow rates of both the desiccant and the air are measured after allowing enough time for steady state readings. Samples of the inlet and exit TEG solution are then collected. The TEG concentration is then determined using a calibrated refractive index meter. Before each run, the entire absorption column, including the packing, the thermometers and the droplet arrester, were cleansed using fresh water and then dried using warm and dry air supplied from the AC unit. The desiccant regeneration process used in this work consists of pumping the TEG solution collected during the experiments from the catch tank to the heater tank, where it is heated to about 55 C. The heated solution is then pumped to the absorption tower where the desiccant now comes into contact with dry and hot air supplied from the AC unit. In this mode, very low flow rates for the desiccant were used, while the air flow rate was at its maximum. The entire process described above may be repeated until the required desiccant initial concentration is restored.
3. Results and discussion The performance of the dehumidification system with low packing density was evaluated by conducting a series of runs with two packings, wood and aluminum. The parameters considered in this study included air flow rate, air inlet temperature and humidity, desiccant inlet temperature and concentration and desiccant flow rate. The performance of the dehumidifier was evaluated by calculating the column effectiveness and the moisture removal rate. The effectiveness is defined as the ratio of the actual change in moisture content of the air stream to the maximum possible change in its moisture content under a given set of operating conditions. Thus, the dehumidifier effectiveness, ey , can be expressed as:
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Table 1 Experimental data for TEG–air–water system in a column packed with wood packing (Panel A), aluminum packing (Panel B) Liquid flux (kg/ m2 s)
Air inlet temperature (C)
Air inlet humidity (kg H2 O/ kg dry air)
Air outlet humidity (kg H2 O/ kg dry air)
Liquid inlet temperature (C)
TEG concentration (wt.%)
Equilibrium humidity (kg H2 O/kg dry air)
Water condensation rate (g/s)
Effectiveness
Panel A 1.50 1.73 2.07 2.61 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07
0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.13 0.23 0.64 0.82 0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.23
30.5 31.0 31.4 31.4 30.4 30.4 30.4 30.4 30.4 30.4 29.9 29.9 29.9 29.9 30.4 30.4 30.6 31.0 31.4 25.4 27.0 30.0 33.4 39.2 44.0
0.0178 0.0206 0.0191 0.0172 0.0168 0.0169 0.0162 0.0190 0.0186 0.0185 0.0206 0.0207 0.0190 0.0169 0.0197 0.0204 0.0205 0.0205 0.0207 0.0168 0.0173 0.0166 0.0167 0.0178 0.0184
0.0140 0.0172 0.0159 0.0144 0.0143 0.0140 0.0129 0.0156 0.0148 0.0142 0.0160 0.0160 0.0140 0.0121 0.0165 0.0170 0.0170 0.0171 0.0178 0.0128 0.0134 0.0134 0.0140 0.0153 0.0160
33.5 31.0 31.7 31.7 31.0 31.0 31.0 31.0 31.0 31.0 31.5 31.5 31.5 31.5 28.2 31.2 35.2 43.3 45.5 31.5 32.0 32.0 32.0 33.0 34.0
95.6 95.6 95.6 95.6 93.0 94.0 95.0 95.6 97.0 98.0 95.6 95.6 95.6 95.6 95.6 95.6 95.6 95.6 95.6 95.6 95.6 95.6 95.6 95.6 95.6
0.0058 0.0058 0.0058 0.0058 0.0087 0.0083 0.0066 0.0058 0.0041 0.0025 0.0058 0.0058 0.0058 0.0058 0.0058 0.0058 0.0062 0.0125 0.0125 0.0058 0.0058 0.0058 0.0058 0.0058 0.0058
0.128 0.132 0.149 0.165 0.116 0.135 0.153 0.158 0.177 0.199 0.214 0.219 0.233 0.223 0.199 0.158 0.163 0.158 0.135 0.186 0.181 0.149 0.126 0.116 0.111
0.316 0.229 0.241 0.245 0.308 0.336 0.344 0.257 0.262 0.268 0.310 0.315 0.378 0.431 0.230 0.232 0.245 0.425 0.354 0.363 0.338 0.295 0.247 0.208 0.190
Panel B 1.50 1.73 1.96 2.61 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07
0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.13 0.23 0.32 0.49
30.5 30.6 30.3 30.6 30.4 30.2 30.8 30.4 30.3 30.5 30.6 30.3 30.4 30.6
0.0185 0.0218 0.0200 0.0160 0.0178 0.0161 0.0164 0.0190 0.0183 0.0185 0.0175 0.0171 0.0173 0.0173
0.0155 0.0188 0.0172 0.0124 0.0152 0.0130 0.0132 0.0154 0.0142 0.0140 0.0150 0.0140 0.0140 0.0134
31.3 31.0 31.0 31.5 31.2 31.5 31.5 31.0 31.5 31.0 31.4 31.2 31.7 31.6
95.6 95.6 95.6 95.6 93.0 94.0 95.0 95.6 97.0 98.0 95.6 95.6 95.6 95.6
0.0058 0.0058 0.0058 0.0058 0.0100 0.0083 0.0068 0.0058 0.0041 0.0026 0.0058 0.0058 0.0058 0.0058
0.101 0.116 0.124 0.156 0.121 0.144 0.149 0.167 0.191 0.209 0.116 0.144 0.154 0.181
0.236 0.187 0.197 0.352 0.332 0.396 0.332 0.272 0.289 0.283 0.213 0.274 0.286 0.338
Air flux (kg/m2 s)
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Table 1(continued) Liquid flux (kg/ m2 s)
Air inlet temperature (C)
Air inlet humidity (kg H2 O/ kg dry air)
Air outlet humidity (kg H2 O/ kg dry air)
Liquid inlet temperature (C)
TEG concentration (wt.%)
Equilibrium humidity (kg H2 O/ kg dry air)
Water condensation rate (g/s)
Effectiveness
2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07
0.64 0.82 0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.23
30.6 30.6 30.5 30.6 30.8 30.6 30.5 30.8 30.2 25.6 27.2 30.6 34.4 37.8 40.7
0.0175 0.0177 0.0189 0.0192 0.0190 0.0194 0.0192 0.0194 0.0205 0.0170 0.0174 0.0180 0.0216 0.0204 0.0208
0.0134 0.0134 0.0138 0.0145 0.0145 0.0150 0.0150 0.0152 0.0164 0.0126 0.0126 0.0130 0.0164 0.0150 0.0153
31.5 31.4 25.0 28.0 31.0 34.6 37.0 40.4 43.2 31.6 31.6 31.5 31.4 31.5 31.5
95.6 95.6 95.6 95.6 95.6 95.6 95.6 95.6 95.6 95.6 95.6 95.6 95.6 95.6 95.6
0.0058 0.0058 0.0034 0.0049 0.0059 0.0066 0.0083 0.0100 0.0116 0.0058 0.0058 0.0058 0.0058 0.0058 0.0058
0.191 0.200 0.237 0.219 0.209 0.205 0.195 0.195 0.191 0.205 0.223 0.233 0.242 0.251 0.256
0.350 0.360 0.330 0.329 0.342 0.344 0.384 0.445 0.463 0.392 0.413 0.409 0.328 0.369 0.366
mcond. (g/s)
Air flux (kg/m2 s)
0.28 0.24 0.2 0.16 0.12 0.08 0.04 0
(a)
y = 0.0343x + 0.0758 2
R = 0.981 y = 0.0482x + 0.0303 2
R = 0.994 1.0
1.5
2.0
2.5
3.0
2
G (kg/m -s) Wood 1.0
Aluminium
Best fit
(b)
εY
0.8 0.6 0.4 0.2 0.0 1.0
1.5
2.0
2.5
3.0
2
G (kg/m -s) Wood
Aluminium
Chung & Luo [32]
Fig. 2. The effect of air flux on condensation rate and dehumidification effectiveness.
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ey ¼
Yin Yout Yin Yequ
ð1Þ
where Yin and Yout are the absolute humidities of the air at the inlet and outlet conditions, respectively, and Yequ is the absolute humidity of the air at equilibrium with the TEG solution at the TEG inlet concentration and temperature. The rate of moisture removal from the air (water condensation rate) was calculated for the two packings from Eq. (2). m_ cond ¼ ðYin Yout Þ G A
ð2Þ
where G is the air mass flux, and A is the column cross-sectional area. The m_ cond and the column effectiveness calculated from the experimental data obtained in this study are presented in Table 1 (Panels A and B) and Figs. 2–6. Chung and Luo [32] developed the following correlation for predicting dehumidifier effectiveness. ( 0:6 h i ) 1
0:024
(
ey ¼ 1
Gin Lin
TG in TL in
exp 1:057
ðazÞ0:185 X 0:638
h
0:192 exp 0:615
TG in TL in
i )
ð3Þ
m cond. (g/s)
X 21:498
0.28 0.24 0.2 0.16 0.12 0.08 0.04 0
y = 0.0028x + 0.1434 2
R = 0.938 y = -0.0042x + 0.2834 2
R = 0.856
25
30 Wood
35 Ta, IN (º C)
40
Aluminium
45 Best fit
εY
1.0 0.8 0.6 0.4 0.2 0.0 25 Wood
30
35 Ta, IN (º C)
Aluminium
40
45
Chung & Luo [32]
Fig. 3. The effect of air inlet temperature on condensation rate and dehumidification effectiveness.
m cond. (g/s)
Y.H. Zurigat et al. / Energy Conversion and Management 45 (2004) 141–155
0.28 0.24 0.2 0.16 0.12 0.08 0.04 0
149
(a)
y = 1.7316x - 1.489 2
R = 0.984 y = 1.5886x - 1.3595 2
R = 0.989
0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99 XIN (kg TEG/kg solution) Wood
1.0
Aluminum
Best fit
(b)
εY
0.8 0.6 0.4 0.2 . 0.0 0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99
XIN (kg TEG/kg solution)
Wood Aluminium Chung & Luo [32] Fig. 4. The effect of TEG inlet concentration on condensation rate and dehumidification effectiveness.
where X is defined as a function of the vapor pressure depression of the desiccant solution to the vapor pressure of pure water ððPwater Psoln Þ=Pwater Þ, a is the surface area to volume ratio of the packing in m2 /m3 , z is the packing height in m, TGin and TLin denote the inlet temperatures of the air and the liquid, respectively. Gin and Lin represent the mass flux of air and liquid, respectively. The effectiveness calculated from their correlation, based on the values of the parameters used in this study, is also presented in Figs. 2–6. The effectiveness was also calculated by the correlation developed by Martin and Goswami [6]. However, their correlation failed to predict the effectiveness under the conditions used in this study, giving negative values. The reason behind this could be that their correlation was developed for high ratio of L=G (3.5–15.4), whereas the range of L=G in this work is 0.1–0.4. The effect of air flow rate on both the moisture removal rate and column effectiveness is shown in Fig. 2. For the two packings used in this study the condensation rate increased as the air flow rate increased. On the other hand, the effectiveness decreased somewhat as the air flow increased for both packings due to the reduced residence time of air in the dehumidifier. Increasing air flow
Y.H. Zurigat et al. / Energy Conversion and Management 45 (2004) 141–155
mcond. (g/s)
150
0.28 0.24 0.2 0.16 0.12 0.08 0.04 0
(a)
y = -0.0024x + 0.2883 2
R = 0.896
y = -0.0025x + 0.2534 2
R = 0.649
25
30
35
40
45
50
TL, IN (º C)
Wood
εY
1.0
Aluminium
Best fit
(b)
0.8 0.6 0.4 0.2 0.0 25
30
35 40 TL, IN (º C)
45
50
Wood Aluminium Chung & Luo [32] Fig. 5. The effect of TEG inlet temperature on condensation rate and dehumidification effectiveness.
rate results in a relatively higher water vapour pressure in the air. A larger water vapor pressure difference between the air and the solution leads to a better condensation rate as shown in Fig. 2(a). However, the dehumidified air still has a high humidity (Yout ), which results in lower effectiveness. This is in accord with the findings of Chen et al. [22]. They recommended that both the condensation rate and the dehumidifier effectiveness should be simultaneously considered when selecting air flow rates. As for the effect of inlet air temperature (Fig. 3), it is interesting to see that when aluminum is used, the mass of condensate increases as the air temperature increases. Aluminum has high thermal conductivity, so the plates of the column get heated as the air temperature increases, which reduces the viscosity of the TEG, thus causing it to spread more easily on the plates thereby making the desiccant more exposed to the air and, thus, providing more contact between the liquid and the air. On the other hand, the effectiveness does not change as the inlet air temperature is increased when aluminum packing is used but decreases when wood packing is used. Also, the Chung and Luo [32] correlation shows a decrease in the effectiveness with increasing inlet air € berg and Gaswami [35,36] experimental results (using polypropylene temperature. However, the O rings packing) as well as a finite difference model prediction indicated no change in effectiveness due to changing inlet air temperature.
mcond. (g/s)
Y.H. Zurigat et al. / Energy Conversion and Management 45 (2004) 141–155
0.28 0.24 0.2 0.16 0.12 0.08 0.04 0
(a)
y = 0.0177x + 0.214 R2 = 0.539
y = 0.1179x + 0.1127 2
R = 0.924
0
0.2
εY
Wood 1.0 0.8 0.6 0.4 0.2 0.0
151
0.4 0.6 0.8 2 L (kg/m -s)
1
Aluminium
Best fit
(b)
0
0.2
0.4 0.6 2 L (kg/m -s)
0.8
1
Wood Aluminium Chung & Luo [32] Fig. 6. The effect of TEG flux on condensation rate and dehumidification effectiveness.
The TEG inlet concentration has a significant effect on the moisture removal rate for both the aluminum and the wood packings (Fig. 4(a)), about 70% increase in removal rate when the TEG concentration increased from 93% to 98%. The column effectiveness (Fig. 4(b)) decreased slightly (less than 15%) for both packings. This decrease is due to the slightly higher inlet humidity for the last three experimental points, as shown in Table 1 (Panels A and B). If the inlet humidity is taken constant, then the first and second set of three experimental points show an increase in column effectiveness. This is in agreement with the Chung and Luo [32] correlation. There is a slight decrease in the condensation rate when the TEG inlet temperature is increased, about 10% in the case of the wood packing and 20% for the aluminum packing over the 20 C temperature rise (Fig. 5(a)). The effectiveness (Fig. 5(b)), however, increased by 50% in the case of the wood packing and by 40% in the case of aluminum. This behavior is due to the low TEG flow rate used in this study, which made the cooling effect by the air significant. The ‘‘cooling effect’’ influenced the values of Yout but had no influence on Yequ , since the latter is a function of the TEG inlet temperature and concentration only. As for the effect of the desiccant flow rate, it can be seen (Fig. 6(a)) that the rate of condensation increases as the desiccant flow rate increases for the aluminum packing, but it is only slightly increased for the case of wood packing. At low flow rate, there is quite a difference between the two packings. The attachment between the desiccant and aluminum could be responsible for this
Author
Desiccant
Present study
TEG
€ berg and Goswami [4] O
Patnaik et al. [23]
McDonald et al. [24]
TL;IN (C) 28–45 fl
G (kg/m2 s) 1.5–2.613 ›
Ta;IN (C) 25.4–44.0
mcond
XIN (kg/kg) 0.93–0.98 ›
ey
›
›
›
fl
›fl
ey KG a
L (kg/m2 s) 4.5–6.5 › ›
XIN (kg/kg) 0.94–0.96 › ›
XIN (kg/kg) 0.94–0.96 › ›fl
TL;IN (C) 25–35 fl ›fl
G (kg/m2 s) 0.5–2.0 › fl
Ta;IN (C) 25–35 ›fl ›fl
YIN (g/kg) 11–22 › ›fl
Z (m) 0.4– 0.8 › ›
Yes
ey
L (kg/m2 s) 4.5–6.5 ›fl ›fl
XIN (kg/kg) 0.3–0.4 ›
TL;IN (C) 26–39 fl
G (kg/m2 s) 0.2–1.2 ›
Ta;IN (C) 25–35 ›
YIN (g/kg) 17–22 ›
Z (m) 0.1– 0.6 ›
Yes
mcond
L (kg/m2 s) 0.1–1.0 ›
XIN (kg/kg) 0.45–0.58 ›
TL;IN (C) 24–32 ›fl
G (kg/m2 s) 1.3–1.9 ›fl
Ta;IN (C) 24–36 fl
YIN (g/kg) 9–22
mcond
L (kg/m2 s) 0.6–1.7 ›
XIN (kg/kg) 0.4–0.45 ›fl ›fl
TL;IN (C) 28–43
YOUT
L (kg/m2 s) 1.5–3.9 fl fl
Ta;IN (C) 31–36 ›fl ›fl
YIN (g/kg) 21–41 › ›
mcond
L (kg/m2 s) 0.8–2.5 › TL;IN (C) 27–30 fl
G (kg/m2 s) 1.0–2.0 fl
Ta;IN (C) 25–45 ›
Z (m) 0.4– 1.0 ›
No
ey
XIN (kg/kg) 0.4–0.45 ›
XIN (kg/kg) 0.94–0.96 › ›
TL;IN (C) 25–35 fl ›
G (kg/m2 s) 0.5–2.0 › fl
Ta;IN (C) 25–35 fl fl
Yes
mcond ey
L (kg/m2 s) 1.9–6.0 › ›
TEG
TEG
LiCl
LiBr
Mixture of LiCl and CaCl2
Ta;OUT Potnis and Lens [25]
Ullah et al. [26]
Chung [34]
Experiments
L (kg/m2 s) 0.13–1.00 ›
mcond
Chen et al. [22]
Independent variables
LiBr
CaCl2
TEG
Yes j Aluminum
› Wood
G (kg/m2 s) 0.5–2.0 fl ›
Yes
Yes
› Z (m) 0.5–1.48
Yes
›fl ›fl Yes
(›) Performance parameter increases with increasing variable; (fl) Performance parameter decreases with increasing variable; (›fl) Variable has no significant effect on the performance parameter.
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Chung et al. [2]
Performance parameter
152
Table 2 The effect of the process variables investigated in this study as well as those reported in the literature
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153
behavior. Aluminum has a smooth surface, which makes the desiccant less attached to the packing surface, providing less contact time and thus, requiring a higher flow rate of TEG to achieve the same rate of moisture removal. In terms of column performance, as the TEG flow rate increased, the effectiveness increased (Fig. 6(b)). The increase is about 40% and 70% for wood and aluminum packings, respectively. The effect of the process variables investigated in this study, as well as those reported in the literature for a packed tower, is summarized in Table 2. The table gives the parameters describing the performance, the process variables and the ranges covered. The effect of each variable on the performance parameter is listed under each variable by arrows as explained in the table. As shown, compared to previous studies, the desiccant flow rate used in this study is significantly lower, and the range of the other variables covered in this study, mainly the inlet temperatures of air and desiccant, is wider. Using structured packing in this study made it possible to work at low desiccant flow rates. Previous investigators who used random packing had a problem with ade€ berg and Gaswami [1] found that the performance has quate wetting of the packing. Actually, O no dependency on the liquid flow rate and, therefore, concluded that the high flow rate was only used to ensure maximum wetting. 4. Summary and conclusions In this study, the performance of a TEG liquid desiccant in a packed column dehumidifier was experimentally investigated. A dehumidifier with low density and structured packing was used in this work to study the effect of different design variables on its performance. The variables were air flow rate, inlet air temperature and humidity, inlet TEG concentration, inlet TEG temperature and TEG flow rate. The packings in the dehumidifier were made of wood or aluminum. It was found that the moisture removal rate increased with increasing inlet TEG concentration, TEG flow rate and air flow rate. This was seen for both the wood and the aluminum packings. In addition, the moisture removal rate is increased with increasing the inlet air temperature for aluminum packing only. For the two packings used the effectiveness of the column increased by increasing the TEG flow rate and inlet TEG temperature. Two correlations reported in the literature were examined to check their capability in predicting the dehumidifier performance under the conditions of this study. The results show that the Chung and Luo [32] correlation overpredicted the effectiveness, while that of Martin and Goswami [6] failed to predict the effectiveness under the conditions of this study. Acknowledgement The funding provided by Sultan Qaboos University (Project Number ENG/00/01) is gratefully acknowledged. References € berg V, Goswami DY. Experimental study of the heat and mass transfer in a packed bed liquid desiccant air [1] O dehumidifier. J Solar Energy Eng 1998;120:289–97.
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[2] Marsala J, Lowenstein A, Ryan WA. Liquid desiccant for residential applications. ASHRAE Trans 1989;95:828–34. [3] Davanagere BS, Sherif SA, Goswami DY. A feasibility study of a solar desiccant air-conditioning system-part I: psychometrics and analysis of the conditioned zone. Int J Energy Res 1999;23:7–21. [4] Chung TW, Ghosh TK, Hines AL. Dehumidification of air by aqueous lithium chloride in a packed column. Separat Sci Technol 1993;28:533–50. [5] Chung TW, Ghosh TK, Hines AL, Novosel D. Dehumidification of moist air with simultaneous removal of selected pollutants by triethylene glycol solutions in a packed-bed absorber. Separat Sci Technol 1995;30:1807–32. [6] Martin V, Goswami DY. Effectiveness of heat and mass transfer processes in a packed bed liquid desiccant dehumidifier/regenerator. HVAC&R Res 2000;6:21–39. [7] Howell JR. A survey of active solar cooling methods. In: Goswami DY, editor. Progress in solar engineering, 1987. p. 171–82. [8] Kinsara AA, Elsayed M, Al-Rabghi OM. Proposed energy-efficient air-conditioning system using liquid desiccant. Appl Therm Eng 1996;16:791–806. [9] Jain S, Dhar PL, Kaushik SC. Experimental studies on dehumidifier and regenerator of a liquid desiccant cooling system. Appl Therm Eng 2000;20:253–67. [10] Jain S. Desiccant augmented evaporative cooling: an emerging air-conditioning alternative. Indian Institute of Technology Delhi, Department of Mechanical Engineering, New Delhi, India, 2001. [11] Lof GOG. Cooling with solar energy. Congress on Solar Energy, 1955, Tucson, Arizona. p. 171–89. [12] Sheridan NR. Solar air-conditioning. J Inst Eng Australia 1961:47–52. [13] Gandhidasan P, Gupta MC. Sun powered air conditioning by absorption chemical dehumidification and evaporative cooling. In: Proceedings of the Conference on the Physics of Solar Energy. Tripoli: Arab Development Institute Special Publication; 1977. [14] Lodwig E, Wilke D, Bressman J. A solar powered desiccant air conditioner system. Solar collector heating and cooling systems. In: Proceedings of the Annual Meeting of American Section of ISES, vol. 1, 1977, Orlando FL. [15] Robison HI. Liquid sorbent air conditioner. In: Veziroglu TN, editor. Alternative energy sources. New York: Hemisphere Pub Corp; 1978. [16] Robison HI, Houston SH. Absorption/desorption solar cooling system performance. Fundamentals and applications of solar energy. AICHE Symp Ser 1980;76:139–43. [17] Factor HM, Grossman G. A packed bed dehumidifier regenerator for solar air-conditioning with liquid desiccants. Solar Energy 1980;24:541–50. [18] Peng CSP. The analysis and design of liquid absorbent/desiccant cooling/dehumidification cycles for low-grade thermal energy applications. PhD Thesis, The University of Texas, Austin, 1980. [19] Grossman G, Johannsen A. Solar cooling and air conditioning. Prog Energy Combust Sci 1981;7:185–228. [20] Johannsen P. Performance simulation of a solar air conditioning system with liquid desiccant. Int J Ambient Energy 1984;5:59–88. € berg V, Goswami DY. A review of liquid desiccant cooling. In: Advances in solar energy, vol. 12. American Solar [21] O Energy Society Publishers; 1998. p. 431–70. [22] Chen LC, Kuo CL, Shyu RJ. The performance of a packed bed dehumidifier for solar liquid desiccant systems. In: Proceedings of the 11th Annual ASME Solar Energy Conference, 1989, San Diego, California. p. 371–77. [23] Patnaik S, Lenz TG, Lof GOG. Performance studies for an experimental solar open-cycle liquid desiccant air dehumidification system. Solar Energy 1990;44:123–35. [24] McDonald B, Waugaman DG, Kettleborough CF. A statistical analysis of a packed tower dehumidifier. Drying Technol 1992;10:223–37. [25] Potnis SV, Lenz TG. Dimensionless mass transfer correlations for packed-bed liquid-desiccant contactors. Ind Eng Chem Res 1996;35:4185–93. [26] Ullah MR, Kettleborough CF, Gandhidasan P. Effectiveness of moisture removal for an adiabatic counterflow packed tower absorber operating with CaCl2 –air contact system. ASME J Solar Energy Eng 1988;110:98–101. [27] Park MS, Howell JR, Vliet GC, Peterson J. Numerical and experimental results for coupled heat and mass transfer between a desiccant film and air in cross flow. Int J Heat Mass Transfer 1994;37:395–402. [28] Peng CSP, Howell JR. Analysis and design of efficient absorbers for low-temperature desiccant air conditioners. J Solar Energy Eng 1981;103:401–8.
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[29] Chebbah A. Analysis and design of a solar-powered liquid desiccant air conditioners for use in hot and humid climates. PhD thesis, University of Florida, Gainesville, FL, USA, 1987. [30] Park MS, Howell JR, Vliet GC, Peterson J. Correlations for film regeneration and air dehumidification for a falling desiccant film with air in cross flow. In: Solar Engineering, Proceedings of the 13th Annual ASME Solar Energy Conference, vol. 2, 1995, Hawaii. p. 1229–38. [31] Park MS, Howell JR, Vliet GC, Peterson J. Correlations for regeneration of a falling desiccant film by air in cross flow. In: Solar Engineering, Proceedings of the 13th Annual ASME Solar Energy Conference, vol. 2, 1995, Hawaii. p. 1239–47. [32] Chung TW, Luo CM. Vapor pressures of the aqueous desiccants. J Chem Eng Data 1999;44:1024–7. [33] Chung TW, Wu H. Comparison between spray towers with and without fin coils for air dehumidification using triethylene glycol solutions and development of the mass transfer correlations. Ind Eng Chem Res 2000;39:2076–84. [34] Chung TW. Predictions of moisture removal efficiencies for packed-bed dehumidification systems. Gas Separat Purif 1994;8:265–8. € berg V, Goswami DY. Performance simulation of solar hybrid liquid desiccant cooling for ventilation air [35] O preconditioning. In: Proceedings of Solar Engineering International Solar Energy Conference, 1998, Fairfield, NJ. p. 176–82. € berg V, Goswami DY. Effectiveness of heat and mass transfer processes in a packed bed liquid desicant [36] O dehumidifier/regenerator. HVAC&R Res 2000;6:21–39.
Air dehumidification by triethylene glycol desiccant in a packed column Y.H. Zurigat a
a,1
, M.K. Abu-Arabi b, S.A. Abdul-Wahab
a,*
Department of Mechanical and Industrial Engineering, College of Engineering, Sultan Qaboos University, P.O. Box 33, Al Khoud 123, Muscat, Oman b Middle East Desalination Research Center, P.O. Box 21, Al-Khuwair 133, Oman Received 10 May 2002; received in revised form 27 November 2002; accepted 12 May 2003
Abstract The performance of an air dehumidifier using triethylene glycol (TEG) as desiccant under hot and humid conditions was investigated. The performance of the dehumidifier was evaluated and expressed in terms of the moisture removal rate and the dehumidifier effectiveness. A packed bed column (dehumidifier) was employed, with low packing density (77 m2 /m3 ), to provide direct contact between the air and the TEG. Two different structured packings were used, wood and aluminum. The experiments covered a wide range of parameter space that included the air and TEG flow rates, air and TEG inlet temperatures, inlet air humidity and inlet TEG concentration. The liquid flow rate investigated is much less than that covered in previous studies (<1 kg/m2 s). The trend of the dehumidifier performance was similar to that reported in the literature using high density and random packing. The results were compared to the Chung and Luo correlation, which over predicted the effectiveness. The Martin and Goswami correlation failed to predict the effectiveness under the conditions of this study. In the present study, it was found that the moisture removal rate increased with increasing inlet TEG concentration, TEG flow rate and air flow rate. This was seen for both the wood and the aluminum packings. In addition, the moisture removal rate is increased with increasing the inlet air temperature for the aluminum packing only. The effectiveness of the column was increased by increasing the TEG flow rate and inlet TEG temperature for the two packings. 2003 Elsevier Ltd. All rights reserved. Keywords: Desiccant; Triethylene glycol (TEG); Dehumidifier performance; Packed bed column
*
Corresponding author. Tel.: +968-515-350; fax: +968-513-416. E-mail address: [email protected] (S.A. Abdul-Wahab). 1 On sabbatical leave from University of Jordan, Amman, Jordan. 0196-8904/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0196-8904(03)00109-2
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Nomenclature A G L m p X Y z e p
cross-sectional area of dehumidifier column (m2 ) air mass flux (kg/m2 s) liquid desiccant mass flux (kg/m2 s) water condensation rate (g/s) vapor pressure (Pa) desiccant concentration (kg TEG/kg solution) air humidity ratio (kg water/kg dry) tower height (m) effectiveness (dimensionless) dimensionless vapor pressure difference
Subscripts equ equilibrium in inlet outlet outlet y air humidity ratio Abbreviation TEG triethylene glycol
1. Introduction Humidity control is important in many engineering applications, such as space air conditioning (AC), storage warehouses, process industries and many others. In space AC, where conventional vapor compression cooling is used, humidity control is often accomplished by cooling the air below its dew point. In hot and humid climates, this method is inefficient, since it should be followed by reheating the air to a safe temperature before it is introduced into the conditioned space. Hence, the evaporator in the vapor compression system operates at a lower temperature than is required to meet the sensible cooling load [1], leading to a lower coefficient of performance and, consequently, higher energy requirements. Likewise, the effectiveness of evaporative cooling relies heavily on the existence of low relative humidity conditions, being higher as the relative humidity decreases. Thus, the humidity puts an extra load on the vapor compression AC systems and renders the evaporative cooling systems ineffective. Furthermore, under high humidity conditions, energy efficient vapor compression systems that were designed to operate at higher evaporator temperatures have been found unable to maintain the indoor relative humidity within a comfortable range [1,2]. This calls for dehumidifying the air prior to entering the evaporator. Separating the control of humidity and temperature by means of desiccants could result in energy savings, as well as improved humidity control [1]. Desiccants are chemicals with great affinity to moisture. They may be used effectively as a supplement to conventional vapor compression systems to remove the latent part of the cooling load. That is, desiccants are efficient in handling latent loads (reducing the humidity), whereas the evaporator of the vapor compression
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system is efficient in handling the sensible cooling loads (lowering the air temperature). Davanagere et al. [3] listed the advantages of using desiccant based systems as follows: (1) very little electrical energy is consumed, and the sources of regenerating thermal energy can be diverse (i.e. solar energy, waste heat, natural gas); (2) they are particularly useful when the latent load is large in comparison with the sensible load; (3) a desiccant system is likely to eliminate or reduce the use of ozone depleting CFCs (depending on whether desiccant cooling is used in conjunction with evaporative coolers or vapor compression systems, respectively); (4) control of humidity can be achieved better than those cases employing vapor compression systems, since sensible and latent cooling occur separately; (5) the cost of energy to regenerate the desiccant is low when compared with the cost of energy to dehumidify the air by cooling it below its dew point; and (6) improvement in indoor air quality is likely to occur because of the normally high ventilation and fresh air flow rates associated with the higher evaporator temperature employed. Also, desiccant systems have the capability of removing airborne pollutants. Several different materials can be employed as desiccant, including both solid and liquid substances. Conventional solid desiccants include silica gel, activated alumina, lithium chloride salt, molecular sieves, titanium silicate and synthetic polymers. Liquid desiccants include lithium chloride, lithium bromide, calcium chloride and TEG. Liquid desiccants have several advantages over solid desiccants [7]. The pressure drop through the liquid desiccant is lower than that through a solid desiccant system; liquid desiccant can be pumped, and thus, several small units can be connected to meet the demands of large buildings; and liquid desiccant can be stored for regeneration by some inexpensive energy sources. More details about desiccant types, properties and the regeneration process are given by Kinsara et al. [8]. Liquid desiccants can absorb moisture from or add moisture to the air, depending on the vapor pressure difference between the surrounding air and the desiccant solution (i.e. the vapor pressure difference is the driving force for mass transfer). The equilibrium vapor pressure of the solution depends on its temperature and concentration. The higher the concentration and lower the temperature, the higher would be the moisture absorbed, but at high concentrations and low temperatures, there is a possibility of crystallization of the desiccant. Therefore, for different desiccants, operating concentration levels are different, e.g. 55–60% for lithium bromide and 95– 97% for TEG [9,10]. As cool and concentrated desiccant is brought in contact with air, water vapor in the air is absorbed by the desiccant, i.e. water condenses into the desiccant. During this process, heat is evolved due to the latent heat of condensation of the water and the heat of mixing. The earliest liquid desiccant system was suggested and experimentally tested by Lof [11] using triethylene glycol (TEG) as desiccant. Solar heated air was used for regeneration. Sheridan [12] investigated a solar operated liquid desiccant dehumidification-cooling system using lithium chloride as the working fluid. Reviews of the early work on the development of liquid desiccant systems can be found in Howell [7], Gandhidasan and Gupta [13], Lodwig et al. [14], Robison [15], Robison and Houston [16], Factor and Grossman [17], Peng [18], Grossman and Johannsen [19], and Johannsen [20]. In addition, more than 80 publications regarding liquid desiccants, liquid desiccant cooling systems, and the analysis of important system components were reviewed by € berg and Goswami [21]. O The performance of a packed bed absorption tower, operating as an air dehumidifier or a desiccant regenerator, is influenced by many design operating parameters and conditions [4]: desiccant fluid characteristics (viscosity, density and surface tension), packing type (shape, size
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and material), desiccant distribution over the packing, flow configuration (counter or co-current flow), tower height, fluids flow rates and the inlet conditions of the desiccant (temperature and concentration) and the air (temperature and humidity). To improve the effectiveness of the desiccant air dehumidification process, the impacts of these parameters have to be evaluated. Good system performance and energy savings can be achieved if the proper values of these variables are selected. A number of experimental studies are reported in the literature regarding this matter. For example, Chung et al. [4], Chen et al. [22], Patnaik et al. [23], McDonald et al. [24], Potnis and Lenz [25] and Ullah et al. [26] conducted experiments on € berg and Goswami [1], Chung packed bed dehumidifiers using salt solutions as the desiccants. O et al. [5], Martin and Goswami [6], Park et al. [27], Peng and Howell [28], Chebbah [29], Park et al. [30,31], Chung and Luo [32] and Chung and Wu [33] reported experimental results using TEG as the desiccant. Moreover, Chung [34] reported some experimental findings using both lithium chloride and TEG as the desiccants. A summary of the variables investigated by various researchers, the ranges and effects of these variables on moisture removal rate and dehumidifier effectiveness are presented in a table (see Table 2) in the results and discussion section of this paper. The objective of the present work is to study the effectiveness of air dehumidification using a liquid desiccant (TEG) in order to provide more design information on dehumidifiers. The work concentrates on conducting experimental work to cover a wider and more extended range of conditions than reported in the literature. Experimental analysis is conducted to study the impact of a number of design variables on the performance of the dehumidification process using a structured packing made of either wood or aluminum. The performance of this process will be evaluated in terms of the water condensation rate and the dehumidification effectiveness.
2. Experimental setup and procedures Fig. 1 shows the experimental setup used in this study for both air dehumidification and desiccant regeneration. In the air dehumidification mode, the air, conditioned to the required temperature and humidity at the AC unit, is introduced into the absorption tower from the bottom. The tower has a total height of 0.6 m with a structured type packing consisting of eight decks with seven plates per deck, resulting in a packing density of 77 m2 /m3 and a packing height of 0.48 m. Desiccant at the required temperature and flow rate is pumped into the top of the tower via the rotameter. The desiccant, flowing countercurrent relative to the humid air flow, is distributed over the packing and absorbs moisture as it comes into contact with the humid air. The diluted desiccant flows by gravity to the catch tank, where it is stored for regeneration. A uniform temperature and concentration of the desiccant are ensured by using an electric heater with temperature controller and the circulation loop shown in Fig. 1. The flow rate is monitored using a calibrated rotameter. The rotameter was calibrated under different desiccant temperatures. The dry bulb and wet bulb air temperatures are monitored using mercury thermometers (error of ±0.5 C) located at the inlet (bottom) and exit (top) of the absorption tower. Also, two more mercury thermometers are used to measure the inlet (tower top) and exit (tower bottom) desiccant temperatures. The air flow rate is monitored using an orifice flow meter (see Fig. 1).
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Fig. 1. Schematics of the experimental setup.
In any run, the inlet and exit temperatures and flow rates of both the desiccant and the air are measured after allowing enough time for steady state readings. Samples of the inlet and exit TEG solution are then collected. The TEG concentration is then determined using a calibrated refractive index meter. Before each run, the entire absorption column, including the packing, the thermometers and the droplet arrester, were cleansed using fresh water and then dried using warm and dry air supplied from the AC unit. The desiccant regeneration process used in this work consists of pumping the TEG solution collected during the experiments from the catch tank to the heater tank, where it is heated to about 55 C. The heated solution is then pumped to the absorption tower where the desiccant now comes into contact with dry and hot air supplied from the AC unit. In this mode, very low flow rates for the desiccant were used, while the air flow rate was at its maximum. The entire process described above may be repeated until the required desiccant initial concentration is restored.
3. Results and discussion The performance of the dehumidification system with low packing density was evaluated by conducting a series of runs with two packings, wood and aluminum. The parameters considered in this study included air flow rate, air inlet temperature and humidity, desiccant inlet temperature and concentration and desiccant flow rate. The performance of the dehumidifier was evaluated by calculating the column effectiveness and the moisture removal rate. The effectiveness is defined as the ratio of the actual change in moisture content of the air stream to the maximum possible change in its moisture content under a given set of operating conditions. Thus, the dehumidifier effectiveness, ey , can be expressed as:
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Table 1 Experimental data for TEG–air–water system in a column packed with wood packing (Panel A), aluminum packing (Panel B) Liquid flux (kg/ m2 s)
Air inlet temperature (C)
Air inlet humidity (kg H2 O/ kg dry air)
Air outlet humidity (kg H2 O/ kg dry air)
Liquid inlet temperature (C)
TEG concentration (wt.%)
Equilibrium humidity (kg H2 O/kg dry air)
Water condensation rate (g/s)
Effectiveness
Panel A 1.50 1.73 2.07 2.61 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07
0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.13 0.23 0.64 0.82 0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.23
30.5 31.0 31.4 31.4 30.4 30.4 30.4 30.4 30.4 30.4 29.9 29.9 29.9 29.9 30.4 30.4 30.6 31.0 31.4 25.4 27.0 30.0 33.4 39.2 44.0
0.0178 0.0206 0.0191 0.0172 0.0168 0.0169 0.0162 0.0190 0.0186 0.0185 0.0206 0.0207 0.0190 0.0169 0.0197 0.0204 0.0205 0.0205 0.0207 0.0168 0.0173 0.0166 0.0167 0.0178 0.0184
0.0140 0.0172 0.0159 0.0144 0.0143 0.0140 0.0129 0.0156 0.0148 0.0142 0.0160 0.0160 0.0140 0.0121 0.0165 0.0170 0.0170 0.0171 0.0178 0.0128 0.0134 0.0134 0.0140 0.0153 0.0160
33.5 31.0 31.7 31.7 31.0 31.0 31.0 31.0 31.0 31.0 31.5 31.5 31.5 31.5 28.2 31.2 35.2 43.3 45.5 31.5 32.0 32.0 32.0 33.0 34.0
95.6 95.6 95.6 95.6 93.0 94.0 95.0 95.6 97.0 98.0 95.6 95.6 95.6 95.6 95.6 95.6 95.6 95.6 95.6 95.6 95.6 95.6 95.6 95.6 95.6
0.0058 0.0058 0.0058 0.0058 0.0087 0.0083 0.0066 0.0058 0.0041 0.0025 0.0058 0.0058 0.0058 0.0058 0.0058 0.0058 0.0062 0.0125 0.0125 0.0058 0.0058 0.0058 0.0058 0.0058 0.0058
0.128 0.132 0.149 0.165 0.116 0.135 0.153 0.158 0.177 0.199 0.214 0.219 0.233 0.223 0.199 0.158 0.163 0.158 0.135 0.186 0.181 0.149 0.126 0.116 0.111
0.316 0.229 0.241 0.245 0.308 0.336 0.344 0.257 0.262 0.268 0.310 0.315 0.378 0.431 0.230 0.232 0.245 0.425 0.354 0.363 0.338 0.295 0.247 0.208 0.190
Panel B 1.50 1.73 1.96 2.61 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07
0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.13 0.23 0.32 0.49
30.5 30.6 30.3 30.6 30.4 30.2 30.8 30.4 30.3 30.5 30.6 30.3 30.4 30.6
0.0185 0.0218 0.0200 0.0160 0.0178 0.0161 0.0164 0.0190 0.0183 0.0185 0.0175 0.0171 0.0173 0.0173
0.0155 0.0188 0.0172 0.0124 0.0152 0.0130 0.0132 0.0154 0.0142 0.0140 0.0150 0.0140 0.0140 0.0134
31.3 31.0 31.0 31.5 31.2 31.5 31.5 31.0 31.5 31.0 31.4 31.2 31.7 31.6
95.6 95.6 95.6 95.6 93.0 94.0 95.0 95.6 97.0 98.0 95.6 95.6 95.6 95.6
0.0058 0.0058 0.0058 0.0058 0.0100 0.0083 0.0068 0.0058 0.0041 0.0026 0.0058 0.0058 0.0058 0.0058
0.101 0.116 0.124 0.156 0.121 0.144 0.149 0.167 0.191 0.209 0.116 0.144 0.154 0.181
0.236 0.187 0.197 0.352 0.332 0.396 0.332 0.272 0.289 0.283 0.213 0.274 0.286 0.338
Air flux (kg/m2 s)
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Table 1(continued) Liquid flux (kg/ m2 s)
Air inlet temperature (C)
Air inlet humidity (kg H2 O/ kg dry air)
Air outlet humidity (kg H2 O/ kg dry air)
Liquid inlet temperature (C)
TEG concentration (wt.%)
Equilibrium humidity (kg H2 O/ kg dry air)
Water condensation rate (g/s)
Effectiveness
2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07 2.07
0.64 0.82 0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.23
30.6 30.6 30.5 30.6 30.8 30.6 30.5 30.8 30.2 25.6 27.2 30.6 34.4 37.8 40.7
0.0175 0.0177 0.0189 0.0192 0.0190 0.0194 0.0192 0.0194 0.0205 0.0170 0.0174 0.0180 0.0216 0.0204 0.0208
0.0134 0.0134 0.0138 0.0145 0.0145 0.0150 0.0150 0.0152 0.0164 0.0126 0.0126 0.0130 0.0164 0.0150 0.0153
31.5 31.4 25.0 28.0 31.0 34.6 37.0 40.4 43.2 31.6 31.6 31.5 31.4 31.5 31.5
95.6 95.6 95.6 95.6 95.6 95.6 95.6 95.6 95.6 95.6 95.6 95.6 95.6 95.6 95.6
0.0058 0.0058 0.0034 0.0049 0.0059 0.0066 0.0083 0.0100 0.0116 0.0058 0.0058 0.0058 0.0058 0.0058 0.0058
0.191 0.200 0.237 0.219 0.209 0.205 0.195 0.195 0.191 0.205 0.223 0.233 0.242 0.251 0.256
0.350 0.360 0.330 0.329 0.342 0.344 0.384 0.445 0.463 0.392 0.413 0.409 0.328 0.369 0.366
mcond. (g/s)
Air flux (kg/m2 s)
0.28 0.24 0.2 0.16 0.12 0.08 0.04 0
(a)
y = 0.0343x + 0.0758 2
R = 0.981 y = 0.0482x + 0.0303 2
R = 0.994 1.0
1.5
2.0
2.5
3.0
2
G (kg/m -s) Wood 1.0
Aluminium
Best fit
(b)
εY
0.8 0.6 0.4 0.2 0.0 1.0
1.5
2.0
2.5
3.0
2
G (kg/m -s) Wood
Aluminium
Chung & Luo [32]
Fig. 2. The effect of air flux on condensation rate and dehumidification effectiveness.
148
Y.H. Zurigat et al. / Energy Conversion and Management 45 (2004) 141–155
ey ¼
Yin Yout Yin Yequ
ð1Þ
where Yin and Yout are the absolute humidities of the air at the inlet and outlet conditions, respectively, and Yequ is the absolute humidity of the air at equilibrium with the TEG solution at the TEG inlet concentration and temperature. The rate of moisture removal from the air (water condensation rate) was calculated for the two packings from Eq. (2). m_ cond ¼ ðYin Yout Þ G A
ð2Þ
where G is the air mass flux, and A is the column cross-sectional area. The m_ cond and the column effectiveness calculated from the experimental data obtained in this study are presented in Table 1 (Panels A and B) and Figs. 2–6. Chung and Luo [32] developed the following correlation for predicting dehumidifier effectiveness. ( 0:6 h i ) 1
0:024
(
ey ¼ 1
Gin Lin
TG in TL in
exp 1:057
ðazÞ0:185 X 0:638
h
0:192 exp 0:615
TG in TL in
i )
ð3Þ
m cond. (g/s)
X 21:498
0.28 0.24 0.2 0.16 0.12 0.08 0.04 0
y = 0.0028x + 0.1434 2
R = 0.938 y = -0.0042x + 0.2834 2
R = 0.856
25
30 Wood
35 Ta, IN (º C)
40
Aluminium
45 Best fit
εY
1.0 0.8 0.6 0.4 0.2 0.0 25 Wood
30
35 Ta, IN (º C)
Aluminium
40
45
Chung & Luo [32]
Fig. 3. The effect of air inlet temperature on condensation rate and dehumidification effectiveness.
m cond. (g/s)
Y.H. Zurigat et al. / Energy Conversion and Management 45 (2004) 141–155
0.28 0.24 0.2 0.16 0.12 0.08 0.04 0
149
(a)
y = 1.7316x - 1.489 2
R = 0.984 y = 1.5886x - 1.3595 2
R = 0.989
0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99 XIN (kg TEG/kg solution) Wood
1.0
Aluminum
Best fit
(b)
εY
0.8 0.6 0.4 0.2 . 0.0 0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99
XIN (kg TEG/kg solution)
Wood Aluminium Chung & Luo [32] Fig. 4. The effect of TEG inlet concentration on condensation rate and dehumidification effectiveness.
where X is defined as a function of the vapor pressure depression of the desiccant solution to the vapor pressure of pure water ððPwater Psoln Þ=Pwater Þ, a is the surface area to volume ratio of the packing in m2 /m3 , z is the packing height in m, TGin and TLin denote the inlet temperatures of the air and the liquid, respectively. Gin and Lin represent the mass flux of air and liquid, respectively. The effectiveness calculated from their correlation, based on the values of the parameters used in this study, is also presented in Figs. 2–6. The effectiveness was also calculated by the correlation developed by Martin and Goswami [6]. However, their correlation failed to predict the effectiveness under the conditions used in this study, giving negative values. The reason behind this could be that their correlation was developed for high ratio of L=G (3.5–15.4), whereas the range of L=G in this work is 0.1–0.4. The effect of air flow rate on both the moisture removal rate and column effectiveness is shown in Fig. 2. For the two packings used in this study the condensation rate increased as the air flow rate increased. On the other hand, the effectiveness decreased somewhat as the air flow increased for both packings due to the reduced residence time of air in the dehumidifier. Increasing air flow
Y.H. Zurigat et al. / Energy Conversion and Management 45 (2004) 141–155
mcond. (g/s)
150
0.28 0.24 0.2 0.16 0.12 0.08 0.04 0
(a)
y = -0.0024x + 0.2883 2
R = 0.896
y = -0.0025x + 0.2534 2
R = 0.649
25
30
35
40
45
50
TL, IN (º C)
Wood
εY
1.0
Aluminium
Best fit
(b)
0.8 0.6 0.4 0.2 0.0 25
30
35 40 TL, IN (º C)
45
50
Wood Aluminium Chung & Luo [32] Fig. 5. The effect of TEG inlet temperature on condensation rate and dehumidification effectiveness.
rate results in a relatively higher water vapour pressure in the air. A larger water vapor pressure difference between the air and the solution leads to a better condensation rate as shown in Fig. 2(a). However, the dehumidified air still has a high humidity (Yout ), which results in lower effectiveness. This is in accord with the findings of Chen et al. [22]. They recommended that both the condensation rate and the dehumidifier effectiveness should be simultaneously considered when selecting air flow rates. As for the effect of inlet air temperature (Fig. 3), it is interesting to see that when aluminum is used, the mass of condensate increases as the air temperature increases. Aluminum has high thermal conductivity, so the plates of the column get heated as the air temperature increases, which reduces the viscosity of the TEG, thus causing it to spread more easily on the plates thereby making the desiccant more exposed to the air and, thus, providing more contact between the liquid and the air. On the other hand, the effectiveness does not change as the inlet air temperature is increased when aluminum packing is used but decreases when wood packing is used. Also, the Chung and Luo [32] correlation shows a decrease in the effectiveness with increasing inlet air € berg and Gaswami [35,36] experimental results (using polypropylene temperature. However, the O rings packing) as well as a finite difference model prediction indicated no change in effectiveness due to changing inlet air temperature.
mcond. (g/s)
Y.H. Zurigat et al. / Energy Conversion and Management 45 (2004) 141–155
0.28 0.24 0.2 0.16 0.12 0.08 0.04 0
(a)
y = 0.0177x + 0.214 R2 = 0.539
y = 0.1179x + 0.1127 2
R = 0.924
0
0.2
εY
Wood 1.0 0.8 0.6 0.4 0.2 0.0
151
0.4 0.6 0.8 2 L (kg/m -s)
1
Aluminium
Best fit
(b)
0
0.2
0.4 0.6 2 L (kg/m -s)
0.8
1
Wood Aluminium Chung & Luo [32] Fig. 6. The effect of TEG flux on condensation rate and dehumidification effectiveness.
The TEG inlet concentration has a significant effect on the moisture removal rate for both the aluminum and the wood packings (Fig. 4(a)), about 70% increase in removal rate when the TEG concentration increased from 93% to 98%. The column effectiveness (Fig. 4(b)) decreased slightly (less than 15%) for both packings. This decrease is due to the slightly higher inlet humidity for the last three experimental points, as shown in Table 1 (Panels A and B). If the inlet humidity is taken constant, then the first and second set of three experimental points show an increase in column effectiveness. This is in agreement with the Chung and Luo [32] correlation. There is a slight decrease in the condensation rate when the TEG inlet temperature is increased, about 10% in the case of the wood packing and 20% for the aluminum packing over the 20 C temperature rise (Fig. 5(a)). The effectiveness (Fig. 5(b)), however, increased by 50% in the case of the wood packing and by 40% in the case of aluminum. This behavior is due to the low TEG flow rate used in this study, which made the cooling effect by the air significant. The ‘‘cooling effect’’ influenced the values of Yout but had no influence on Yequ , since the latter is a function of the TEG inlet temperature and concentration only. As for the effect of the desiccant flow rate, it can be seen (Fig. 6(a)) that the rate of condensation increases as the desiccant flow rate increases for the aluminum packing, but it is only slightly increased for the case of wood packing. At low flow rate, there is quite a difference between the two packings. The attachment between the desiccant and aluminum could be responsible for this
Author
Desiccant
Present study
TEG
€ berg and Goswami [4] O
Patnaik et al. [23]
McDonald et al. [24]
TL;IN (C) 28–45 fl
G (kg/m2 s) 1.5–2.613 ›
Ta;IN (C) 25.4–44.0
mcond
XIN (kg/kg) 0.93–0.98 ›
ey
›
›
›
fl
›fl
ey KG a
L (kg/m2 s) 4.5–6.5 › ›
XIN (kg/kg) 0.94–0.96 › ›
XIN (kg/kg) 0.94–0.96 › ›fl
TL;IN (C) 25–35 fl ›fl
G (kg/m2 s) 0.5–2.0 › fl
Ta;IN (C) 25–35 ›fl ›fl
YIN (g/kg) 11–22 › ›fl
Z (m) 0.4– 0.8 › ›
Yes
ey
L (kg/m2 s) 4.5–6.5 ›fl ›fl
XIN (kg/kg) 0.3–0.4 ›
TL;IN (C) 26–39 fl
G (kg/m2 s) 0.2–1.2 ›
Ta;IN (C) 25–35 ›
YIN (g/kg) 17–22 ›
Z (m) 0.1– 0.6 ›
Yes
mcond
L (kg/m2 s) 0.1–1.0 ›
XIN (kg/kg) 0.45–0.58 ›
TL;IN (C) 24–32 ›fl
G (kg/m2 s) 1.3–1.9 ›fl
Ta;IN (C) 24–36 fl
YIN (g/kg) 9–22
mcond
L (kg/m2 s) 0.6–1.7 ›
XIN (kg/kg) 0.4–0.45 ›fl ›fl
TL;IN (C) 28–43
YOUT
L (kg/m2 s) 1.5–3.9 fl fl
Ta;IN (C) 31–36 ›fl ›fl
YIN (g/kg) 21–41 › ›
mcond
L (kg/m2 s) 0.8–2.5 › TL;IN (C) 27–30 fl
G (kg/m2 s) 1.0–2.0 fl
Ta;IN (C) 25–45 ›
Z (m) 0.4– 1.0 ›
No
ey
XIN (kg/kg) 0.4–0.45 ›
XIN (kg/kg) 0.94–0.96 › ›
TL;IN (C) 25–35 fl ›
G (kg/m2 s) 0.5–2.0 › fl
Ta;IN (C) 25–35 fl fl
Yes
mcond ey
L (kg/m2 s) 1.9–6.0 › ›
TEG
TEG
LiCl
LiBr
Mixture of LiCl and CaCl2
Ta;OUT Potnis and Lens [25]
Ullah et al. [26]
Chung [34]
Experiments
L (kg/m2 s) 0.13–1.00 ›
mcond
Chen et al. [22]
Independent variables
LiBr
CaCl2
TEG
Yes j Aluminum
› Wood
G (kg/m2 s) 0.5–2.0 fl ›
Yes
Yes
› Z (m) 0.5–1.48
Yes
›fl ›fl Yes
(›) Performance parameter increases with increasing variable; (fl) Performance parameter decreases with increasing variable; (›fl) Variable has no significant effect on the performance parameter.
Y.H. Zurigat et al. / Energy Conversion and Management 45 (2004) 141–155
Chung et al. [2]
Performance parameter
152
Table 2 The effect of the process variables investigated in this study as well as those reported in the literature
Y.H. Zurigat et al. / Energy Conversion and Management 45 (2004) 141–155
153
behavior. Aluminum has a smooth surface, which makes the desiccant less attached to the packing surface, providing less contact time and thus, requiring a higher flow rate of TEG to achieve the same rate of moisture removal. In terms of column performance, as the TEG flow rate increased, the effectiveness increased (Fig. 6(b)). The increase is about 40% and 70% for wood and aluminum packings, respectively. The effect of the process variables investigated in this study, as well as those reported in the literature for a packed tower, is summarized in Table 2. The table gives the parameters describing the performance, the process variables and the ranges covered. The effect of each variable on the performance parameter is listed under each variable by arrows as explained in the table. As shown, compared to previous studies, the desiccant flow rate used in this study is significantly lower, and the range of the other variables covered in this study, mainly the inlet temperatures of air and desiccant, is wider. Using structured packing in this study made it possible to work at low desiccant flow rates. Previous investigators who used random packing had a problem with ade€ berg and Gaswami [1] found that the performance has quate wetting of the packing. Actually, O no dependency on the liquid flow rate and, therefore, concluded that the high flow rate was only used to ensure maximum wetting. 4. Summary and conclusions In this study, the performance of a TEG liquid desiccant in a packed column dehumidifier was experimentally investigated. A dehumidifier with low density and structured packing was used in this work to study the effect of different design variables on its performance. The variables were air flow rate, inlet air temperature and humidity, inlet TEG concentration, inlet TEG temperature and TEG flow rate. The packings in the dehumidifier were made of wood or aluminum. It was found that the moisture removal rate increased with increasing inlet TEG concentration, TEG flow rate and air flow rate. This was seen for both the wood and the aluminum packings. In addition, the moisture removal rate is increased with increasing the inlet air temperature for aluminum packing only. For the two packings used the effectiveness of the column increased by increasing the TEG flow rate and inlet TEG temperature. Two correlations reported in the literature were examined to check their capability in predicting the dehumidifier performance under the conditions of this study. The results show that the Chung and Luo [32] correlation overpredicted the effectiveness, while that of Martin and Goswami [6] failed to predict the effectiveness under the conditions of this study. Acknowledgement The funding provided by Sultan Qaboos University (Project Number ENG/00/01) is gratefully acknowledged. References € berg V, Goswami DY. Experimental study of the heat and mass transfer in a packed bed liquid desiccant air [1] O dehumidifier. J Solar Energy Eng 1998;120:289–97.
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