Choice of Low Temperature Salt for a Resorption Refrigerator

Apr 22, 2010 - refrigerate a 33 L cold box, with two temperature zones. When the ambient ... librium drop (ΔTequ) for each experiment was calculated,...
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Ind. Eng. Chem. Res. 2010, 49, 4897–4903

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Choice of Low Temperature Salt for a Resorption Refrigerator H. S. Bao,† R. G. Oliveira,‡ R. Z. Wang,*,† and L. W. Wang† Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong UniVersity, Shanghai 200240, China, and Centro de Tecnologia, UniVersidade Federal do Pampa, Alegrete 97546-550, Brazil

Three bench-scale resorption systems were compared with respect to the degree of conversion, the specific cooling power, and the influence of the working condition on the temperature equilibrium drop. The mass of refrigerant that reacted during specified periods of each working condition studied was assessed gravimetrically. The three systems had MnCl2 in the main reactor, and they were designed to provide a cooling effect at 0 and at -5 °C. However, each system had a different salt (BaCl2, NaBr, or NH4Cl) in the reactor employed for cold production. The system with NH4Cl was the most suitable for cold production at the temperatures indicated above, whereas the system with BaCl2 was not suitable to operate at these levels of cooling production. Thus, we designed and manufactured a larger resorption system and assessed the feasibility of using this system to refrigerate a 33 L cold box, with two temperature zones. When the ambient temperature was 30 °C, the air temperature in the bottom of the cold box could be kept below 0 °C for 5 h, while the temperature of the upper zone remained below 6 °C for about 2.5 h. 1. Introduction Chemisorption refrigerators and heat pumps have been widely researched1 and may be considered environmentally friendly systems, because the refrigerants normally used neither deploy the ozone layer nor contribute to the green house effect. Resorption system is a type of chemisorption refrigerator similar to the conventional one, except that in the former system, the condensation and evaporation processes are replaced by synthesis and decomposition reactions. Thus, the basic resorption system is composed of at least two reactors, and each one contains a different reactive salt. These salts can react with the same refrigerant, but they have different equilibrium temperatures under the same pressure. The salt with the high equilibrium temperature is named a high-temperature salt (HTS), whereas the other salt is referred as a low-temperature salt (LTS). Examples of resorption systems containing different pairs of high- and low-temperature salts were presented elsewhere.2-8 Each experimental work about the resorption system previously reported in the literature dealt with only one working pair. The LTSs studied were BaCl2,3,5 PbCl2,4 NH4Cl,6 and NaBr,7,8 whereas the HTSs studied were MnCl24-8 and NiCl2.3,5 To allow a direct comparison of resorption systems with different LTSs, we conducted experiments with three bench-scale resorption prototypes. Although each system had a different LTS, all of them use MnCl2 as HTS and ammonia as refrigerant. The LTSs used were BaCl2, NH4Cl, and NaBr. Each prototype was tested at several conditions of constraint temperatures. During each phase of the cycle, the mass of ammonia transferred between the reactors of each prototype was assessed gravimetrically, and used to calculate the specific cooling power (SCP). Moreover, the mean temperature equilibrium drop (∆Tequ) for each experiment was calculated, and its influence on the reaction conversion was assessed. Once the most suitable LTS was identified, we designed and manufactured a 33 L cold storage box and assessed the feasibility of using a resorption system with NH4Cl and MnCl2 to refrigerate this box. * To whom correspondence should be addressed. E-mail address: [email protected]. † Shanghai Jiao Tong University. ‡ Universidade Federal do Pampa.

2. Materials, Experimental Procedures, and Analysis Method 2.1. Description of the Resorption Cycle. A complete single-effect resorption cycle consists of two phases: high pressure phase (HPP) and low pressure phase (LPP), which are shown in the Clausius-Clapeyron diagram of Figure 1. The lines assigned as S/G 1 and S/G 2 represent the solid-gas equilibrium conditions of the reactions in the LTS and the HTS, respectively. As can be seen in Figure 1, during the high pressure phase of an ideal cycle, reactors with the LTS and the HTS are at pressure Ph. The HTS reactor is subjected to the temperature constraint Th, the LTS reactor is subjected to the temperature constraint Tm, and they undergo decomposition and synthesis, respectively. In the low pressure phase, the direction of the reaction is reversed, and the HTS and LTS reactors are subjected to the temperature constraints Tm and Tl, respectively. During the low pressure phase, the cold effect can be obtained at the equilibrium temperature of the LTS at Pl, because the decomposition reaction is endothermic. When ammonia is the refrigerant, the following reactions will occur inside the different reactors if the operation and equilibrium conditions are different:

Figure 1. Clapeyron diagram of a single-effect resorption system.

10.1021/ie901575k  2010 American Chemical Society Published on Web 04/22/2010

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MnCl2 · 2NH3 + 4NH3 T MnCl2 · 6NH3 + 4∆HMnCl2 NaBr + 5.25NH3 T NaBr · 5.25NH3 + 5.25∆HNaBr BaCl2 + 8NH3 T BaCl2 · 8NH3 + 8∆HBaCl2 NH4Cl + 3NH3 T NH4Cl · 3NH3 + 3∆HNH4Cl The difference between the operating condition and the thermodynamic equilibrium is known as equilibrium drop pressure equilibrium drop (∆Pequ) or temperature equilibrium drop (∆Tequ), depending on the variable of interest.9-13 The equilibrium drop is the driving force for the chemical reaction and, thus, influences the reaction rate and the specific thermal power of the system. Although higher equilibrium drop results in higher specific cooling power, it may also result in lower coefficient of performance (COP) due to the larger demand of energy.13 Thus, the choice of the operation conditions and, hence, the equilibrium drop should be such that neither the specific cooling power nor the COP would be greatly compromised. 2.2. Comparison of the Bench-Scale Resorption Prototypes. 2.2.1. Experimental Setup. The salts used in the reactors were impregnated in expanded graphite (EG) and compressed to form blocks, as described elsewhere.17 Expanded graphite was used to avoid agglomeration of the salt,20 and enhance both the mass transfer11,18-20 and the heat transfer.11,21-23 Table 1 shows some properties of the sorbent blocks. The mass ratio between salt and EG was 13/7, and the dimension of the sorbent blocks are shown in Figure 2. The scheme of the experimental setup for the bench-scale resorption prototypes is shown in Figure 2a. The HTS and LTS reactors were connected by a pipe, in which a piezoelectric pressure sensor with an uncertainty of (2.5 kPa was installed. Thermo-resistances Pt1000 were installed at two different radial positions of each sorbent block. One of the sensors was placed close to the gas channel in the center of the reactor, whereas the other one was placed near the reactor wall, as presented in Figure 2b. The uncertainty in the temperature measurement was (0.3 °C. The temperature of the sorbent blocks (Tblock) was assumed as the weighted mean of the temperature measured near the gas channel (TGC) and that measured near the wall (TW). The temperatures in the blocks and in the baths and the pressure in the system were recorded every 10 s, with a datalogger. The value of the pressure (Psys) was used to obtain the temperature equilibrium drop of each salt, according to the following equation. ∆Tequ ) Tblock - Tequ(Psys)

∆H ∆S - R0 · ln(Psys)

Table 1. Characteristic of Composite Sorbents

MnCl2 BaCl2 NaBr NH4Cl

(2)

The values of ∆H and ∆S for the reactions with MnCl2 and BaCl2 were reported by Touzain,14 who collected information published previously by other researchers.15,16 ∆H and ∆S of the reactions with NH4Cl and with NaBr were presented by Oliveira et al.6,7 In each experimental condition tested, the temperature equilibrium drop was calculated every 10 s because neither the pressure of the system nor the temperature of the blocks was constant. The mean temperature equilibrium drop (∆Tequ) for

mass of salt (g)

density (kg/m3)

55.5 45.9 34.6 31.5

306 303 299 299

Table 2. Constraint Temperatures (Heat Source and Heat Sink Temperatures)

a

(1)

Tequ and Psys were correlated by the following modified form of the Clausius-Clapeyron equation: Tequ )

Figure 2. (a) Scheme of the experimental test rig. (b) Internal structure of the reactor and the arrangement of the temperature sensor inside the sorbent blocks: (1) gas channel; (2) composited consolidated sorbent blocks; (3) stainless steel reactor.

experimental condition

constraint temp for the HTS (°C)

constraint temp for the LTS (°C)

HPP01 HPP02 HPP03 HPP04 HPP05 HPP06 LPP01 LPP02 LPP03 LPP04

145 155 165 145 155 165 30 30 35 35, a30b

30 30 30 35 35 35 0 -5a, 5b 0 -5, a10b

MnCl2/NaBr and MnCl2/NH4Cl. b MnCl2/BaCl2.

each phase of an experimental condition are the values presented in this work. 2.2.2. Experimental Procedures. During the HPP, the reactor exchanged refrigerant for 30 min, whereas in the LPP, the reactors exchanged refrigerant for 50 min. The prototypes operated at six conditions of constraint temperature in the HPP, and at four conditions in the LPP, as shown in Table 2. The constraint temperatures imposed to the reactors were controlled with thermostatic bathes. The mass of the reactors was measured with an electronic balance (Sartorius AG BS2202S with an uncertainty of 0.01 g) at the beginning and at the end of each phase of the cycle, and the mass of ammonia exchanged between the reactors was assumed as the difference between these two consecutive measurements.

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Figure 3. ∆Tequ and heat source temperature during the HPP. Heat sink at (a) 30 °C and (b) 35 °C.

Each reactor was designed to exchange up to 30 ( 0.3 g of refrigerant (mc,max). The initial mass of refrigerant in the reactors undergoing synthesis was 0.0 ( 0.3 g and in the reactors undergoing decomposition was 30 ( 0.3 g. Each experimental condition was repeated four times, and results presented in this paper are the mean value and the standard deviation of each experimental condition. 2.3. Method to Comparison. The relative mass of refrigerant exchanged between the reactors (mc) during each phase was expressed as gram of ammonia per gram of LTS (g/g), and the degree of reaction conversion (x) was dimensionless and described as x)

mc mc,max

(3)

The specific cooling capacity (Qcooling, LTS) and specific cooling power (SCP) of the LTS were calculated with the following equations: Qcooling,LTS ) [∆H · mc + (m · cp · ∆T)salt + (m · cp · ∆T)EG + (m j · cp · ∆T)NH3 + (m · cp · ∆T)reactor]LTS(4) SCP )

Qcooling, LTS t

(5)

where t is the length of time of the LPP, which contributes to the cooling effect. The coefficient of performance (COP) of a single stage resorption system can be expressed as Qheat,HTS ) [∆H · mc + (m · cp · ∆T)salt + (m · cp · ∆T)EG + (m j · cp · ∆T)NH3 + (m · cp · ∆T)reactor]HTS(6) COP )

Qcooling,LTS Qheat,HTS

(7)

where Qheat, HTS represents the heat energy consumption of the HTS reactor. Because the LPP and the HPP were studied separately, mc values in these phases were not the same. But, in a machine operating continuously, the cycled mass converges to the smallest value, and we used the smallest mc between the value obtained in the low pressure phase and that obtained in the high pressure phase. 3. Results and Discussion 3.1. Equilibrium Drop of LTSs. The relation between the mean temperature equilibrium drop (∆Tequ) and heat source

Figure 4. ∆Tequ and temperature of cold production and heat sink.

temperature during the HPP is presented in Figure 3. In the system with NH4Cl and in that with NaBr, the increment of ∆Tequ was more pronounced when the heat source was below 155 °C and at the heat sink temperature of 35 °C. The reaction rate is proportional, among other factors, to the equilibrium drop. However, each reaction is affected differently by the equilibrium drop, and although the system with BaCl2 had the highest equilibrium drop, it had the lowest reaction rate. For the period of HPP chosen, the gain in the increase of the reaction rate for the systems with NH4Cl or with NaBr will be larger if the temperature is raised in the range between 145 and 155 °C than in the range between 155 and 165 °C. The opposite occurred in the system with BaCl2, which would have more gain in the increase of reaction rate if the heat source had its temperatures increased above 155 °C. Figure 4 shows that the equilibrium drop was practically insensitive to the temperature of cold production when the heat sink was 35 °C, and the reactions with NH4Cl and with NaBr occurred with similar ∆Tequ’s. When the heat sink temperature was 30 °C, the reaction with cold production at 0 °C occurred with an equilibrium drop at least 15% higher than that of reaction with cold production at -5 °C. This result indicates that the specific cooling power of a resorption machine operating with cold production at 0 °C could be at least 15% higher than that operating with cold production at -5 °C. The differences between the degrees of conversion during the HPP of the studied systems were smaller than those during the LPP (Figure 5). In the LPP, the highest and the lowest degree of conversions were obtained in the system with NH4Cl and BaCl2, respectively, when both systems had similar ∆Tequ’s.

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Figure 6. Variation of the SCP with cooling production temperature. Figure 5. Degree of conversion and ∆Tequ of LTS.

The conversion in the HPP was always higher than that in the LPP even though both the ∆Tequ and the reaction time were smaller in the HPP. The slopes of the curves in Figure 5 indicate that during the HPP, each increase of 1 °C in the ∆Tequ increased 0.049-0.081 the final degree of conversion, depending on the LTS used in the system. Moreover, during the period of time chosen for the HPP, a ∆Tequ of 5 °C was enough to practically complete the synthesis reactions of the NH4Cl and NaBr ammoniates. In the LPP, the final degree of conversion of the reaction in the system with NH4Cl was insensitive to the equilibrium drop in the range studied and reached 88% of the maximum possible conversion. In the systems with NaBr and BaCl2, the final degrees of conversion in the LPP were also smaller than those of the HPP, but they increased with the increase of the ∆Tequ. The results above indicated that BaCl2 in pair with MnCl2 was unsuitable for the cold production under the conditions studied in this work due to its low conversion in the LPP, even though the system with this working pair had the highest ∆Tequ in this phase. The pressure in the LPP was below 0.8 bar in any experimental condition, whereas in the HPP, it was at least 5 bar, which was high enough to avoid any mass transfer limitation.21 Hence, the lower conversion obtained in the LPP can be partly explained by the mass transfer limitation; and thus, the equilibrium drop in the LPP must be far higher than that of the HPP, if one intends to achieve similar conversion in both phases, during similar periods of reaction. The results above indicated that for the operating conditions studied, the critical conditions of temperature and pressure were those of the LPP. 3.2. Comparison of the Specific Cooling Power and the Coefficient of Performance. The variations of the SCP of the systems with NH4Cl and NaBr at the heat sink temperatures (THSN) of 30 and 35 °C and that of the system with BaCl2 at the heat sink temperatures of 30 °C are presented in Figure 6. The system with NH4Cl had the highest SCP. The system with BaCl2 had extremely low SCP, which was 3 W/kg, when the heat sink temperature was at 35 °C, and the cold production was 0 °C. The SCP depends on the reaction rate, which is related not only to ∆Tequ but also to the difference between the sorbent temperature and the constraint temperature. The former temperature in the reactor with NH4Cl became practically constant and approached the constraint temperature between 25 and 35 min, depending on the experimental condition; thus, it can be expected that the SCP in the system with NH4Cl would be higher

than the values presented in Figure 6, if the mass of ammonia exchanged between the reactors were measured when the reaction had almost halted. The uncertainties in the calculated SCP of the systems with NH4Cl, NaBr, and BaCl2 were, respectively, between 7.9 and 9.5%, between 6.7 and 8.5%, and between 20.8 and 23.4%. The uncertainty for the system with BaCl2 was particularly large, because the uncertainty in the ∆H reported by Touzain14 was large, and the adsorbed mass of the refrigerant was small, in any operation condition. Due to the low SCP obtained with the system using BaCl2, only the COP of the systems with NH4Cl and NaBr were calculated and presented in Figure 7. The highest COP was achieved when the heat source temperature was at 155 °C, regardless of the heat sink temperature. This result indicates that when the heat source temperature increased from 145 to 155 °C, the increase in desorbed mass, which resulted in an increase of cooling capacity, was higher than the increase in sensible heat. Above the temperature of 155 °C, there was little gain in the increase of desorbed mass, and the COP decreased, because the sensible heat load increased more than the cooling capacity. The system with NaBr had COP between 2 and 13% higher than that obtained with the system with NH4Cl. The biggest difference in the COP of these systems occurred when the heat source temperature was 165 °C, the heat sink temperature was 30 °C, and the cooling production was at 0 °C, whereas the smallest difference occurred at the same condition of heat source temperature, but heat sink temperature at 35 °C, and cold production at -5 °C. In the former operation condition, both systems had almost complete conversion in the HPP; thus, they consumed practically the same amount of heat. However, the cooling production of the system with NaBr was higher because the enthalpy of the reaction between this salt and NH3 is higher than the enthalpy of the reaction between NH4Cl and NH3. In the latter condition, the conversion of the system with NH4Cl was higher; thus, the consumption of energy of this system was 8% higher than the consumption of the system with NaBr. However, due to the lower reaction enthalpy in the system with NH4Cl, the cooling production was only 6% higher The uncertainties in the calculated COP of the systems with NH4Cl and NaBr were, respectively, between 11.2 and 13.1% and between 10.7 and 12.6%. 4. Description of a 33 L Cold Box Coupled to Resorption Prototype Because NH4Cl led to the highest SCP, which was also less sensitive to the operation condition in the LPP, we used this

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Figure 7. COP of the resorption systems with NH4Cl and with NaBr. Heat sink at (a) 30 °C and (b) 35 °C.

reactors (NH4Cl+EG) was 1.22 kg. The reactors with LTS were divided into two groups with four reactors each. One group had reactors with external annular fins and was placed at the back of the box, whereas the other group was placed near the bottom of the box, thus creating two cooling zones. A temperature sensor was installed in the central position of the each cooling zone. Three cylindrical stainless steel reactors with external longitudinal fins comprised the group of the HTS reactors, and they were filled with 2.73 kg of consolidated compound sorbent of MnCl2 and expanded graphite. In both composite sorbents, the proportion between salt and EG was 4:1 and the density of the blocks was 460 kg/m3. During the HPP, each of the HTS reactors was heated by a 300 W electric heater, placed axially in their radial center, and the box with these reactors was completely closed to reduce heat losses to the surrounding. In the LPP, the top cover of the box was removed, allowing a natural convection of air flow to cool the HTS cylinders. The setup temperature of the electric heater was 180 °C to ensure that the temperature in any position of the sorbent block was at least 155 °C. Because the amount of mass exchanged between the reactors was not directly measured, the reaction was assumed to halt when the pressure of the system became constant, which occurred 3 h after the beginning of the HPP. Then the reactors were cooled by natural convection to the ambient temperature for a period of 20 h. Once all reactors had their temperatures similar to the ambient temperature, the valve that connected the group of HTS reactors to the groups of LTS reactors was opened, and the cooling effect was produced in the cold box. Figure 8. Resorption system coupled with cold box. (a) Resorption system prototype with two main parts. (b) External appearance of the real equipment: (1) LTS box; (2) HTS box. (c) Top view of the real equipment, two well-insulated boxes containing HTS reactors and LTS reactors, respectively: (3) connection valve between the two main parts; (4) valve for filling refrigerant; (5) HTS reactor; (6) insulation materials surrounding the HTS reactors. (d) Cold box containing LTS reactors: (7) temperature display; (8) LTS reactors; (9) electric heater switch; (10) pressure gauge.

salt in a resorption system designed to refrigerate a 33 L cold box. This system, which is shown in Figure 8, had one group of HTS reactors with MnCl2 and two groups of LTS reactors with NH4Cl (Figure 8a). These groups were placed in separated insulated boxes (Figure 8b) and were connected with a single valve (shown as Figure 8c). The box for LTS reactors was similar to a small refrigerator (Figure 8d). The total mass of composite sorbent in the LTS

5. Cooling Production with the Resorption Prototype Coupled to the 33 L Cold Box Figure 9 shows the variation of air temperature inside the cold box. When the ambient temperature was 30 °C, the air temperature in the bottom of the cold storage box was cooled to 0 °C in the first hour and remained between 0 and -6 °C for almost 5 h, but the air temperature in the upper zone dropped to only 5 °C. When the ambient temperature was 26 °C, the temperature of the air in the two zones reached almost the same minimum value. At this condition, it took only 0.5 h for the air temperature in the bottom zone to become 0 °C, and the air temperature reached the minimum value about 1 h earlier than when the ambient temperature was 30 °C.

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Figure 9. Temperature variation inside the cold box. Subscripts: a, 30 °C; b, 26 °C; 1, bottom zone; 2, upper zone.

sorbent bed during the low pressure phase indicates that the SCP could be higher than the values presented in this paper, because the reaction was practically halted before the period used in the calculation of the SCP. The COP of the systems with NaBr and with NH4Cl reached maximum values when the heat source temperature in the high pressure phase was 155 °C, because between this temperature and 165 °C, the sensible heat load increased more than the desorbed amount. The system with NaBr had COP between 2% and 13% higher than those obtained with the system using NH4Cl because, for the same amount of mass desorbed, both systems had the same consumption of energy in the HPP, but the system with NH4Cl had a smaller cooling capacity, due to its smaller reaction enthalpy. Among the resorption systems tested, the one using NH4Cl was the most suitable for applications requiring cooling production below 0 °C, due to its higher SCP, which was less sensitive to the cold production temperature and heat sink temperature. The system with NaBr would be the second option, because its COPs were the highest ones. The system with BaCl2 had the worst performance, with SCP of 3 W/kg, when the heat sink temperature was at 35 °C, and the cold production occurred at 0 °C. Thus, the combination of NH4Cl as LTS and MnCl2 as HTS was used in a larger resorption system that was used to refrigerate a 33 L cold storage box. This system had 1.22 kg of composite sorbent with NH4Cl and 2.73 kg of composite sorbent with MnCl2. When the ambient temperature was 30 °C, the temperature of the air in the bottom zone of the cold box could be kept below 0 °C for about 5 h, while in the upper part was below 6 °C for a period of 2.5 h. Acknowledgment

Figure 10. Temperature variation at the wall and inside the block of the LTS and HTS reactors. Subscripts: a, 30 °C; b, 26 °C; 1, at the wall of reactors; 2, inside the blocks.

As can be seen in Figure 10, the temperatures were similar inside the blocks and at the wall, which indicates that the blockwall heat transfer coefficient was not a limitation to the reaction heat dissipation. The air inside the cold box was cooled faster when the ambient temperature was smaller due to the higher external heat transfer dissipation in the HTS reactors. The better heat dissipation of the HTS reactors affected the system pressure and consequently increased the equilibrium drop of the LTS. Therefore, when the ambient temperature was 26 °C, the temperature of the LTS reactors reached the minimum value faster, and which was about 5 °C lower than the temperature reached when the ambient temperature was 30 °C. 6. Discussion and Conclusions Three low-temperature salts (LTS) for the resorption system were compared at selected operation conditions, regarding the amount of conversion during synthesis and decomposition reactions, the specific cooling power (SCP), and the influence of the operation condition on the mean temperature equilibrium drop. The conversion in the system with NH4Cl was the highest among the systems studied regardless of the working conditions studied in this work, whereas the lowest conversion was obtained with the system that used BaCl2. The SCP of the NH4Cl system was the highest among the systems studied, at any operation condition, and the analysis of the temperature profile of the

This work was supported by the Key project of the Natural Science Foundation of China under the contract No. 50736004.

NOMENCLATURE cp COP EG HPP HTS LPP LTS mc,max Ph Pl Psys Qcooling,

specific heat, J g-1 K-1 coefficient of performance expanded graphite high pressure phase high temperature salt low pressure phase low temperature salt maximum mass of refrigerant exchanged between reactors, g g-1 high pressure, Pa low pressure, Pa pressure of the whole system, Pa cooling production of the LTS reactor, J

LTS

Qheat, HTS R0 SCP Tequ Th Tl Tm THSN

heat energy consumption of the HTS reactor, J universal gas constant, J g-1 K-1 specific cooling power, W kg-1 equilibrium temperature, K high temperature, K low temperature, K middle temperature, K heat sink temperature, °C

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∆H ∆S

-1

reaction enthalpy, J g reaction entropy, J g-1 K-1

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ReceiVed for reView April 8, 2009 ReVised manuscript receiVed March 20, 2010 Accepted April 12, 2010 IE901575K