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Ind. Eng. Chem. Res. 2002, 41, 1378-1385
Heat and Mass Transfer of the New LiBr-Based Working Fluids for Absorption Heat Pump Sung-Bum Park† and Huen Lee* Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, 373-1 Kusung-dong, Yusung-gu, Taejon 305-701, South Korea
The heat and mass transfer characteristics of water vapor absorption into the LiBr-based working fluids were experimentally investigated using both static pool and falling film type absorbers. The working fluids treated in this study are the commonly used LiBr + water and two newly developed ones with LiBr + 1,3-propanediol + water and LiBr + LiI + 1,3-propanediol + water solutions. To enhance the water vapor absorption rate, two surfactants of n-octanol and 2-ethyll-hexanol were tested over a wide range of concentrations. The absorption behaviors of three working fluids observed from two different types of absorber were discussed in connection with the thermophysical and transport properties of each working fluid. Considered in the aspect of operating range and heat and mass transfer characteristics, the LiBr + LiI + 1,3-propanediol + water solution with 2-ethyl-1-hexanol could be a promising working fluid-additive pair as an alternative to the LiBr + water solution. Introduction Recently, the heavy demand on electricity in the summer season and environmental problems have made the absorption heat pump cycle more attractive for both residential and industrial applications. The absorption heat pump primarily consists of four major components: a generator, a condenser, an evaporator, and an absorber. The overall performance of an absorption heat pump is greatly affected by the characteristics of heat and mass transfer in an absorber, where the refrigerant vapor is absorbed into the absorbent. In particular, a variety of mechanical and physicochemical methods have been widely adopted for the improvement of absorber performance. One of the possible mechanical ways is to bring up the wavy motions in the vertically falling film absorber. At a low flow rate, no wavy motions in the falling film occur. With an increase in the film flow rate, any disturbance initiates a wave on the surface by hydrodynamic instabilities. This wave motion can break the slowly developed diffusion boundary layer to enhance the mixing process. Many experimental and theoretical studies on wavy film and its effect are available in the literature. The 50-200% increase in terms of the mass transfer coefficient by the wavy motion were reported experimentally1-4 for the LiBr + water solution. Morioka and Kiyota5 investigated the effect of waves on heat and mass transfer by numerical analysis. Benzeguir et al.6 performed the experimental study to determine the heat and mass transfer rates in the falling film of the LiBr + water system and compared the experimental results with the numerical calculations considering the wave model. On heat and mass transfer, Kim et al.7 examined the effect of several factors such as film flow rate, solution * To whom correspondence should be addressed. Phone: 8242-869-3917. Fax: 82-42-869-3910. E-mail: hlee@ mail.kaist.ac.kr. † Present address: Energy & Environmental Research Team, SK Corporation, 140-1, Wonchon-dong, Yusung-gu, Taejon 305-712, South Korea.
temperature and concentration, coolant temperature, and absorber pressure. Moreover, the addition of a certain amount of proper surfactants to working fluids has been known to greatly enhance heat and mass transfer by a vigorous surface instability named Marangoni convection. Kashiwagi8 investigated the heat and mass transfer phenomena of the LiBr + water solution with several alcohol additives and first speculated a surface turbulence mechanism related to the surface and interfacial tensions of solution and alcohol droplets. Hozawa et al.9 reported both the experimental and theoretical investigations on Marangoni convection induced by n-octanol. Elkassabgi and Perez-Blanco10 also performed an experimental study on the effect of several alcohol additives on heat and mass transfer by measuring the pressure decay history and solution temperature in a stagnant pool absorber. Hihara and Saito11 carried out absorption rate experiments with the LiBr + water solution and 2-ethyl-1hexanol by using an inclined flat plate absorber and obtained a 4-5-fold enhancement. Jung et al.12 used four alcohols (n-heptanol, n-octanol, 3-octanol, and 2-ethyl-1-hexanol) as additives with the LiBr + water solution in a falling film type miniabsorber and found that n-heptanol was the most effective for mass transfer enhancement and that 3-octanol was the worst. Kim et al.13 investigated the mechanism of mass transfer enhancement with a vertical falling film absorber at various conditions and reported that the Marangoni instability is responsible for absorption enhancement. They also compared the experimentally measured onsets of mass transfer enhancement with those calculated from various surface tension gradients. A numerical study on the additive-induced enhancement of heat and mass transfer was performed by Koenig et al.14 and Daiguji et al.15 using the static pool type absorber. Kang et al.16 visualized Marangoni convection by several alcohols in a water-ammonia system. They measured surface tensions and interfacial tensions with additives more or less than the solubility limit. They concluded that the temperature gradient of surface tension should
10.1021/ie010596e CCC: $22.00 © 2002 American Chemical Society Published on Web 02/02/2002
Ind. Eng. Chem. Res., Vol. 41, No. 5, 2002 1379
Figure 1. Schematic diagram of experimental apparatus for static pool type absorption.
not be a criterion for Marangoni convection in a waterammonia system and that the magnitude of interfacial tension did not affect the occurrence of Marangoni convection. Recently, Kulankara and Herold17 developed the vapor surfactant theory to describe the enhancement of heat and mass transfer in the absorber. They performed both the absorption/condensation of water vapor in the falling film apparatus and surface tension measurements of LiBr + water with 2-ethyl-1-hexanol and finally concluded that surfactant transported to water vapor by the boil-off plays a dominant role in inducing Marangoni convection. Although many experimental and theoretical works are reported in the literature, most of them are limited to the LiBr + water solution. Few studies treated the newly suggested working fluidadditive pairs for an absorption heat pump. Furthermore, each result was greatly dependent on the types of apparatus and experimental method, which made it difficult to produce systematic conclusions. In these connections, the overall objective of this work was focused on providing the pertinent experimental information on heat and mass transfer that occurred in the LiBr-based working fluids and investigating the enhancement of heat and mass transfer by adding surface-active additives to the new working fluids. However, as the preliminary attempt of this study, the absorption experiments in the static pool type were performed for the LiBr-based working fluid-additive pairs. Second, the experimental apparatus of a minisorber for investigating the characteristics of heat and mass transfer in the falling film absorption was specifically designed and constructed for testing water vapor absorptions into the LiBr-based working fluids with and without surfactants. The working fluids treated in this work are LiBr + water, LiBr + 1,3-propanediol + water (LiBr/1,3-propanediol ) 3.5 by mass), and LiBr + LiI + 1,3-propanediol + water (LiBr/LiI ) 4 by mol and (LiBr + LiI)/1,3-propanediol ) 4 by mass). These solutions were previously suggested as the new working fluids for the absorption chiller in our laboratory.18,19 The absorption behavior drawn from two different experiments was discussed in connection with the transport properties of each working fluid. Experimental Section Static Pool Type Absorption. The experimental apparatus for static pool type absorption was introduced in the previous study.20 The apparatus schematically shown in Figure 1 is a stagnant pool type and mainly consists of vapor-generating and absorbing parts, including a movable cell. In general, the mass transfer enhancement occurring at the absorber is strongly affected by the geometric shape of the absorber and, particularly, by the shape of interface between solution
Figure 2. Schematic diagram of experimental apparatus for falling film type absorption.
and water vapor. In this work, various shapes of absorption cells were preliminarily examined to give vigorous convection. The amount of sample solution and the time of total absorption were also predetermined to give a clear difference in the absorption rate among various samples. Sample solutions containing a desired relative amount of additive were prepared prior to each experiment. For each run, the 18 cm3 of sample solution was introduced into the cell and was placed in the absorption chamber after the total mass was accurately weighed. The apparatus, including the vapor-generating part, was slowly evacuated to a slightly higher pressure than the vapor pressure of sample solution. By adjustment of the valves, the vapor-generating part was then completely evacuated, and a small amount of pure water was introduced to the vapor-generating vessel, stirred with a magnetic stirrer. The temperature of vaporgenerating part was controlled by a constant-temperature bath. The absorption of water vapor into the absorbent began by opening a valve between the absorber and the vapor-generating vessel, and the absorption continued for a specified time period. During the absorption process, surface convection in the sample solution was observed through a sight glass in the absorption chamber. After the absorption experiment was completed, open air was introduced into the apparatus. The cell containing the sample solution was carefully taken out to weigh the amount of vapor absorbed with an uncertainty of (0.002 g. Falling Film Type Absorber. The experimental apparatus for falling film absorption was constructed in a similar way to that of Kim et al.13 The schematic diagram of the apparatus is presented in Figure 2. The major compartments of the system consist of an absorber, an evaporator, a cooling water system, a circulation system, a vacuum-generating part, a measuring device, and a sampling device. The apparatus was constructed for only the batch-mode experiments. The detailed scheme and solution path for the absorber appear in Figure 3. The main part of absorber was constructed using two concentric tubes: an inner stainless steel tube and outer glass tube. Inside the inner tube, the cooling water was circulated to remove the heat of absorption in a countercurrent manner during vapor absorption on the outer wetted tube surface. The strong solution (before absorption) was fed to the top of the absorber using the solution pump. Through the distributor at the top of the absorber, the strong solution fell down the outside of the inner stainless steel tube. The outer glass tube enabled the visual observation of
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Figure 3. Detailed solution and coolant path in the absorber.
the falling liquid. The top and bottom of the absorber made by stainless steel was connected to the glass tube by a rubber O-ring. The strong solution line through the top of the absorber was adjusted to be located near the center of the inner stainless steel tube. Centering was carefully done for the strong solution from the top of the inner tube to wet the outer surface uniformly over the inner tube. No special treatment was done on the surface of the stainless tube for heat and mass transfer improvement. The evaporator was constructed using Pyrex glass to supply the water vapor for the absorption. It was maintained in an acrylic water bath of which the temperature was controlled by an external bath circulator within ( 0.1 K. A K-type thermocouple with digital thermometer (Barnant, 692-8010) was inserted to measure the temperature of the evaporator. The electrical heating inside the evaporator was provided to treat the heat of vaporization during vapor generation. The cooling water to remove the heat of absorption was circulated by an external bath circulator (Lab camp, RBC-20). It can provide a flow rate of up to 2 kg/min, which is sufficient for the designed system in this work. The circulation system consisted of a strong solution chamber, a collecting tank, a flow meter, and a circulation pump. To provide the proper heat to the strong solution, an electrical heater equipped with a power supply was inserted indirectly in the solution chamber. The temperature in the solution chamber was maintained within ( 0.1 K. The solution pump to deliver the strong solution to the top of the absorber was purchased from Cole Parmer company (P-07083-30). A flow meter from Dwyer Instrument (56-170563-00) was connected between the solution pump and the top absorber unit. The volumetric flow rate was measured within ( 1% and converted to a mass flow rate using the density of the solution. The weak solution (after absorption) through bottom of the absorber passed the line with a sampling device and thermocouple and finally went down to the collecting tank. The liquid solution after absorption was extracted through a pre-evacuated syringe. The vacuum gauge from Kurt J. Lesker Co.
(KJL-912011) was attached on top of the absorber unit. The accuracy in the pressure measurement was within ( 0.01 Torr. The vacuum pump from Korea KIVAC (GHP-150) was use to evacuate the system to 10-4 Torr. The experimental procedures for falling film absorption are as follows. The working fluids (strong solution) were introduced to the solution chamber with an electrical heater and a controller. The solution chamber, evaporator, sampling device, and reservoir were evacuated to the required vacuum level. The water vapor was generated by controlling the evaporator temperature and pressure. The temperature of the inlet solution and cooling water was adjusted to the desired values. After the outside surface of the stainless steel tube was wetted by opening the flow meter and solution pump, the solution flow rate was adjusted to a set value. A steady state was assumed when the system pressure, outlet cooling water, and solution temperature did not change severely for at least 2 min. The outlet cooling water and solution temperature were recorded, and the weak solution after absorption was withdrawn by a sampling device. Data Reduction for Falling Film Absorption. The measured raw data and specified inlet condition are denoted as follows: Ms in is the inlet solution flow rate, Cs in and Cs out are the inlet and outlet concentrations, Ts in and Ts out are the inlet and outlet solution temperatures, and Tcw in and Tcw out are the inlet and outlet cooling water temperatures, respectively. The average driving potential for heat and mass transfer must be as a priori defined in case that local gradients are not considered. The data reduction procedures are same as those of Kim et al.13 From the absorbent mass balance, the absorption rate, Mabs, was calculated by
Mabs ) Ms out - Ms in ) Ms in(Cs in/Cs out - 1)
(1)
The total heat transfer to the cooling water, Q, can be calculated from the energy balance
Q ) Ms inhin - Ms outhout + Mabs Ha
(2)
where hin and hout are the inlet and outlet solution enthalpies at the corresponding temperature and concentration conditions, respectively, and Ha is the heat of absorption. The enthalpies of each working fluid were determined from the necessary data available in the literature.18,19,21 The heat of absorption was directly calculated from the heat of dilution.22 The overall heat transfer coefficient, UO, was determined from
Q ) UOA∆T
(3)
where A and ∆T are the total heat transfer area and the driving potential for heat transfer, respectively. The log mean temperature difference (LMTD) defined in eq 4 was used for ∆T
LMTD )
∆Tin - ∆Tout ln(∆Tin - ∆Tout)
(4)
where ∆Tin and ∆Tout are defined as follows:
∆Tin ) Ts in - Tcw out
and
∆Tout ) Ts out - Tcw in (5)
The film heat transfer coefficient is more meaningful because the overall heat transfer coefficient represents
Ind. Eng. Chem. Res., Vol. 41, No. 5, 2002 1381 Table 1. Experimental Condition and Test Solution-Additive Pairs for Static Pool Type Absorption Experimental Conditions initial pressure of absorber solution temp vapor-generating temp absorption time
10 mmHg 298.15 ( 0.3 K 293.15 ( 0.1 K 3 min
Solution-Additive Pairs solution
additives
LiBr + water, 50 mass % LiBr + water, 60 mass % LiBr + 1,3-propanediol + water, 54.5 mass % LiBr + 1,3-propanediol + water, 54.5 mass % LiBr + 1,3-propanediol + water, 67.9 mass % LiBr + 1,3-propanediol + water, 67.9 mass % LiBr + LiI + 1,3-propanediol + water, 60.7 mass % LiBr + LiI + 1,3-propanediol + water, 60.7 mass % LiBr + LiI + 1,3-propanediol + water, 69.9 mass % LiBr + LiI + 1,3-propanediol + water, 69.9 mass %
2-ethyl-l-hexanol 2-ethyl-l-hexanol n-octanol 2-ethyl-1-hexanol n-octanol 2-ethyl-1-hexanol n-octanol 2-ethyl-1-hexanol n-octanol 2-ethyl-1-hexanol
the combination of cooling water heat transfer, wall conduction, and film heat transfer. As usual, the film heat transfer coefficient for the flow in a circular tube can be expressed as follows:
1 hf ) x DO 1 w DO UO km D Dihi
(6)
L
where hi, xw, km, DL, DO, and Di are the cooling water heat transfer coefficient, pipe thickness, thermal conductivity of wall, log mean diameter, outside diameter, and inside diameter, respectively. For the determination of hi, an empirical equation for the heat transfer coefficient at the solid wall in turbulent flow by McAdam23 was used. The experimental results were expressed in terms of the film Reynolds number defined as follows:
Ref )
4ΓO µ
(7)
where ΓO is the mean mass flow rate of film and µ is the dynamic viscosity of a working fluid. Because the film thickness is extremely difficult to measure, the characteristic length for a vertical plate was determined using the kinematic viscosity of the working fluids
Lch )
() ν2 g
1/3
(8)
The film Nusselt number for a falling film was defined by the following expression:
Nuf )
hfLch k
(9)
Actual thermal conductivity data of falling film, k, were provided from the previous study.24 Results and Discussion Static Pool Type Absorption. The absorption experiments were carried out for 10 pairs of solutionadditive mixtures, which are listed in Table 1 together with the experimental conditions. All of the absorbent concentrations were chosen for each working fluid to have the same vapor pressure of 50 and 60 mass % of
Figure 4. Absorption of water vapor into 50 and 60 mass % LiBr + water solution with 2-ethyl-l-hexanol: (O) 50 mass %, (b) 60 mass %.
the LiBr + water solution. The experiments were carried out near the range of additive concentration from 25 to 1000 ppm. The additive concentrations were all expressed in mass ppm. The total amount of water vapor absorbed for 3 min was plotted against the additive concentration in log scale. For all of the solutions with additives, a vigorous surface turbulence was observed through the sight glass directly after the mass transfer enhancement began. The results of water vapor absorption in 50 and 60 mass % LiBr solutions with 2-ethyl-1-hexanol were shown in Figure 4. The mass transfer driving potential during absorption is the difference between the vapor pressure of the refrigerant (water vapor) and the vapor pressure of the absorbent at the concentration and temperature when absorption takes places. The vapor pressure of water at the evaporator temperature is 17.53 mmHg. The vapor pressures of the each absorbent at the absorber temperature are 6.07 and 1.98 mmHg, respectively. Because of the lower vapor pressure of a 60 mass % LiBr solution, the absorbed amount of water without additives was about three times larger than that of 50 mass %. As the concentration of additives increased up to a critical concentration, the absorbed amount of water into solution also increased. The solubility limit of 2-ethyl1-hexanol in a 60 mass % LiBr + water solution is known to be positioned near 200 ppm. Over the solubility limit, the absorbed amount of water vapor became almost constant without any noticeable increase. The general absorption behavior confirms that the 2-ethyll-hexanol effect on mass transfer enhancement is induced below 10 ppm of additive concentration. For the LiBr + 1,3-propanediol + water solution, two kinds of additives, n-octanol and 2-ethyl-l-hexanol, were used, and the overall absorption results were presented up to near 1000 ppm of additive concentration in Figures 5 and 6. With an increase in the absorbent concentration from 54.5 to 67.9 mass %, the absorbed amount of water increased by twice as that when any additives were not included. At both absorbent concentrations, the absorption capacity of the proposed working fluid was found to be a little lower than that of the LiBr + water solution. In the static pool type absorber used in this work, the solution temperature increased because of the heat evolved during water vapor absorp-
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Figure 5. Absorption of water vapor into 54.5% LiBr + 1,3propanediol + water solution with additives: (O) n-octanol, (b) 2-ethyl-l-hexanol. Figure 7. Absorption of water vapor into 60.7% LiBr + LiI +1,3propanediol + water solution with additives: (O) n-octanol, (b) 2-ethyl-l-hexanol.
Figure 6. Absorption of water vapor into 67.9% LiBr + 1,3propanediol + water solution with additives: (O) n-octanol, (b) 2-ethyl-l-hexanol.
tion. This temperature rise led to increasing absorbent vapor pressure and finally to reducing the driving potential for absorption. Judging from experimental data of heats of dilution, the heats of absorption of the LiBr + water solution appear to be higher than those of the LiBr + 1,3-propanediol + water solution,22 which resulted in more reduction of the driving potential for the LiBr + water solution during water vapor absorption. The low absorption capacity of the LiBr + 1,3propanediol + water solution might be expected from its higher viscosity and lower water diffusivity than for the LiBr + water solution at the same vapor pressure conditions.18 Accordingly, the diffusivity and viscosity seemed to be more influential on the absorption capacity than on the heat of absorption. At 54.5 mass % of absorbent concentration, the maximum amount of absorbed water vapor was obtained around 200 ppm for n-octanol and 500 ppm for 2-ethyl-1-hexanol. However, for a 67.9 mass % solution, both n-octanol and 2-ethyl1-hexanol showed the maximum absorption near 500 ppm. Around 200 ppm of additive concentration, the tiny surfactant drop phase was observed and became larger with an increase in its concentration. It must be again realized that the addition of surfactant over its solubility limit did not noticeably improve the absorption rate. In other aspects, the surface tension effect on absorption rate needs to be carefully examined. Over the whole
Figure 8. Absorption of water vapor into 69.9% LiBr + LiI +1,3propanediol + water solution with additives: (O) n-octanol, (b) 2-ethyl-l-hexanol.
surfactant concentration range treated in the present absorption experiments, the surface tensions of the solution with 2-ethyl-l-hexanol were found to be higher than that with n-octanol.24 In cooperation with the results of the absorption experiments, it can be concluded that the lowest surface tension does not provide the necessary condition for maximum enhancement. This similar explanation can also be found in the study by Beutler et al.25 Over the whole concentration of additives, 2-ethyl-1-hexanol rather than n-octanol showed better enhancement. The overall absorption behavior of the LiBr + LiI + 1,3-propanediol + water solution with two surfactants is basically similar to LiBr + 1,3-propanediol + water solution, as shown in Figures 7 and 8. The maximum absorption rate of the LiBr + LiI + 1,3-propanediol + water solution was higher than that of the LiBr + 1,3propanediol + water solution but somewhat comparable with that of the LiBr + water solution. It is worthwhile to note that the absorbent concentration for each solution was selected to maintain the same driving
Ind. Eng. Chem. Res., Vol. 41, No. 5, 2002 1383 Table 2. Operating Conditions for Falling Film Absorption Experiment without Additive case I
case II
case III
absorbent LiBr, LiBr + LiBr + LiI + concn (Cs in) 60 mass % 1,3-propanediol, 1,3-propanediol, 67.9 mass % 69.9 mass % absorbent flow 30-90 10-70 20-90 rate (Ref) absorbent temp 40 °C (Ts in) absorber 7.0 Torr pressure cooling water 30 °C temp absorber 0.854 m length
potential for water vapor absorption. By considering absorbent solubility, the newly suggested working fluids can be used at the higher concentration with the higher driving force.18,19 Therefore, the LiBr + LiI + 1,3propanediol + water solution can be a promising candidate as a new working fluid from the viewpoints of both thermophysical properties and heat and mass transfer characteristics. Falling Film Type Absorption. The operating conditions for falling film absorption considered in this study are presented in Table 2. Three working fluids, 60 mass % LiBr + water, 67.9 mass % LiBr + 1,3propanediol + water, and 69.9 mass % LiBr + LiI + 1,3-propanediol + water, were treated with a close analogy to the static pool type absorption. The absorber pressure, absorbent temperature, and cooling water temperature are considered to be the typical operating variables generally adapted in the absorption cooling machine. The LiBr + water solution was chosen as the reference working fluid because its absorption behavior was well understood in the literature.7 The experimental results for each working fluid are graphically presented in Figures 9 and 10 in terms of the mass transfer rate and the film Nusselt number. Considering the experimental reproducibilities and uncertainties during the measuring of raw data, the uncertainties in the mass transfer rate and the film Nusselt number appeared to be ( 5% and 7%, respectively. With an increase in the absorbent flow rate, the concentration difference gradually decreased because of a short contact time. However, the corresponding mass transfer rate increased up to about Ref ) 40 because the high absorbent flow rate provided a large amount of Ms in in eq 1. However, the film heat transfer coefficient gradually increased with the absorbent flow rate because of the increase of heat evolved during absorption. These effects of absorbent flow rate on key process variables were found to be quite similar to those of the literature. In the study by Kim et al.,7 the mass transfer rate did not increase noticeably above about Ref ) 60 of the absorbent flow rate. This phenomenon seemed to occur because of the increase in film thickness with the absorbent flow rate. Furthermore, the developed falling film could act as a resistance to heat and mass transfer. Other working fluids suggested in this study (cases II and III) showed a similar trend for absorbent flow rate. The relatively high viscosity of cases II and III can develop a film to be thicker than that made from the LiBr + water solution18,19 and make the mass transfer rates of new working fluids to be a little lower than that with 60 mass % of the LiBr + water solution. The critical absorbent flow rate, where the increase of mass transfer rate was terminated, was observed near or less than Ref ) 40 for both cases II and III. As can
Figure 9. Effect of absorbent flow rate on the mass transfer rate: (O) case I, (0) case II, (2) case III.
Figure 10. Effect of absorbent flow rate on the film Nusselt number: (O) case I, (0) case II, (2) case III.
be seen in Figure 9, the case III solution showed a little higher heat and mass transfer rate when compared with case II. This low heat and mass transfer rate of case II can possibly be attributed to high solution viscosity and low water diffusivity. As discussed also in static pool type absorption, the absorption capacity of the case III working fluid was confirmed to be quite comparable to that of case I. With the danger of crystallization avoided, the LiBr + LiI + 1,3-propanediol + water solution is expected to have a potential possibility for the promising working fluid. As stated here, the optimum flow rate was found to be about Ref ) 40 when the proper additives were not used. Although the optimum additive concentration could be roughly determined through static pool type absorption, the final conditions must be applied to falling film type absorption. At this stage, it is necessary to examine the optimum surfactant concentration at which the maximum enhancement of mass transfer rate occurs. Three solutions at the fixed flow rate of Ref ) 40 were treated by varying the additive concentrations. All other conditions such as evaporating temperature, inlet solution temperature, and cooling water temperature are the same as those presented in Table 2. When 2-ethyl-1-hexanol was added to the absorbent solution, the solution surface tension fell remarkably24 and led to easier wetting when compared with the solutions without additives. During the absorption experiment with an additive, a uniform wetting at the outer surface was clearly observed. The additive effects on the mass transfer rate for each working fluid are graphically presented by Figures 11-13. For all solutions, 2-ethyll-hexanol was added from 10 to 1000 ppm. A surfactant
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Figure 11. Effect of 2-ethyl-l-hexanol on the mass transfer rate in case I solution.
Figure 12. Effect of 2-ethyl-1-hexanol on the mass transfer rate in case II solution.
Figure 13. Effect of 2-ethyl-1-hexanol on the mass transfer rate in case III solution.
island on the liquid surface was visualized over 200500 ppm. As expected, with an increase in additive concentration, the mass transfer rate increased for all working fluids treated in this work. Similarly to the results from static pool type absorption, no enhancement of the mass transfer rate by surface convection was found for the solutions without an additive. The experimental results for 60 mass % of the LiBr + water solution seemed to be somewhat similar to those given by Kim et al.7 in that the maximum enhancement appeared before the solubility limit. Kim et al.7 experimentally showed that the optimum 2-ethyl-1-hexanol
concentration for 60 mass % of the LiBr + water solution was about 20-30 ppm but that, above this concentration, no further enhancement occurred. However, the optimum value for 60 mass % of the LiBr + water solution was found to be about 100 ppm. This discrepancy might be attributed to surfactant clogging on the walls of the experimental apparatus through the absorbent path. The case II solution showed the lowest enhancement of mass transfer rate compared with cases I and III. However, the case III solution showed nearly the same degree of enhancement as that in case I, where the optimum concentration of additive was about 250300 ppm. Overall, the heat and mass transfer enhancement for the newly developed working fluids in the falling film absorption started before the solubility limit, and the maximum performance appeared near the solubility limit of the additive. As discussed in static pool type absorption, the case III solution can be used in high absorbent concentration, while the case I solution has the danger of crystallization at the specified absorbent concentration. On the basis of the experimental results of water vapor absorption in a falling film type absorber and operating range in terms of the absorbent solubility, it can be concluded that case III could be a promising alternative to the LiBr + water solution. For a complete development of design equation for the new working fluids, a detailed study on the effect of other operating variables, such as the absorbent concentration and temperature, cooling water temperature, and evaporating temperature, and others, must be also provided. Conclusion The absorption experiments in static pool and falling film type absorbers were performed for the LiBr-based working fluids with and without surfactants. Three different working fluids of the LiBr + water, LiBr + 1,3propanediol + water, and LiBr + LiI + 1,3-propanediol + water solutions were chosen and closely examined for comparison. Among these three working fluids, the LiBr + water solution without surfactants showed the best absorption rate for static pool type absorption. However, the LiBr + LiI + 1,3-propanediol + water solution was confirmed to have a better absorption capacity than the LiBr + 1,3-propanediol + water solution. With an increase in surfactant concentration, the absorption rate increased up to the solubility limit of surfactants in the treated working fluids. It was experimentally verified that the surfactant island was not a necessary condition for absorption enhancement. Besides surface tension, the heat of absorption and viscosity and water diffusivity must be also provided to analyze heat and mass transfer enhancement behavior. In the falling film absorption without surfactants, the heat and mass transfer coefficients increased with the film Reynolds number by wavy motion. Because of resistance by the developed film and a short contact time, a further increase in flow rate did not show better enhancement in the mass transfer rate. The LiBr + LiI + 1,3propanediol + water solution might be a promising candidate for an absorption chiller, considering the operation range, because its heat and mass transfer characteristics were comparable with the LiBr + water solution. Acknowledgment This work was supported by Grant No. 20006-30704-2 from the Cooperation Research Program of Korea
Ind. Eng. Chem. Res., Vol. 41, No. 5, 2002 1385
KOSEF and German DFG and also partially by the Brain Korea 21 project. Nomenclature A ) total heat transfer area (m2) C ) absorbent concentration (mass %) DL ) log mean diameter of pipe (m) DO ) outer diameter of pipe (m) Di ) inner diameter of pipe (m) g ) gravity acceleration (m s-2) h ) enthalpy of solution (kJ kg-1) Ha ) heat of absorption (kJ kg-1) hf ) film heat transfer coefficient (kW m-2 K-1) hi ) cooling water heat transfer coefficient (kW m-2 K-1) k ) thermal conductivity of falling film (kW m-1 K-1) km ) thermal conductivity of pipe (kW m-1 K-1) Lch ) characteristic length (m) M ) solution flow rate (kg s-1) Mabs ) absorption rate (kg s-1) Nuf ) film Nusselt number Q ) total heat transfer to cooling water (kW) Ref ) film Reynolds number T ) temperature (K) ∆T ) driving potential for heat transfer (K) UO ) overall heat transfer coefficient (kW m-2 K-1) xw ) pipe thickness (m) Subscripts s in ) inlet solution s out ) outlet solution cw in ) inlet cooling water cw out ) outlet cooling water Greek Letters ΓO ) mean film flow rate (kg m-1 s-1) µ ) dynamic viscosity (kg m-1 s-1) ν ) kinematic viscosity (m2 s-1)
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Received for review July 12, 2001 Revised manuscript received December 3, 2001 Accepted December 13, 2001 IE010596E
