Absorption of water vapour into lithium bromide-based solutionswith additives using a simple stagnant pool absorber

Jin-Soo Kima, Huen Leea,* , Sun Il Yub

aDepartment of Chemical Engineering Korea Advanced Institute of Science and Technology, 373-1 Kusung-dong, Yusung-gu, Taejon 305-701,South Korea

bGas Utilization Division, R & D Center, Korea Gas Corporation, 638-1 Il-dong, Ansan-city, Kyunggi-do 425-150, South Korea

Received 14 April 1998; received in revised form 2 September 1998; accepted 21 October 1998

Abstract

Absorption of water vapour into the 50 mass % lithium bromide solution with four eight-carbon alcohol additives such as n-octanol, 2-octanol, 3-octanol, and 2-ehtyl-1-hexanol were investigated by using a simple stagnant pool absorber. Four solutionsof the 60 mass % lithium bromide1 water, 68 mass % lithium bromide1 ethylene glycol1 water (LiBr/HO(CH2)2OH �4.5 by mass), 60 mass % lithium bromide1 lithium iodide 1 water (LiBr/Lil � 4 by mole), and 70 mass % lithium bromide1 zinc chloride 1 water (LiBr/ZnCl2 � 1 by mass) containing the 2-ethyl-1-hexanol additive were also considered toexamine the additive effect on mass transfer of water vapour into the different types of absorbents. The experimental apparatuscould be used with good confidence and accuracy particularly for studying mass transfer enhancement over the effective rangeof additive concentration which, in this work, is mostly between 10 and 500 ppm. A vigorous interfacial turbulence wasobserved during absorption process using additives. The water vapour absorption rate remarkably increased with increasingthe additive concentrations up to about 200 ppm and then stopped increasing above 200 ppm for all the systems considered. Theonset additive concentrations for enhancing mass transfer were located between 5 and 8 ppm for all systems except two systemsof the 50 mass % lithium bromide solution with 3-octanol and 70 mass % lithium bromide1 zinc chloride solution with 2-ethtyl-1-hexanol for which the corresponding concentrations were 2.5 and 35 ppm, respectively.q 1999 Elsevier Science Ltdand IIR. All rights reserved.

Keywords:Absorption; Water; Lithium bromide

Absorption de la vapeur d’eau par des solutions a` base de bromurede lithium al’aide d’un absorbeur simple de type a` surface

liquide stagnante

Resume

On a etudie, utilisant un absorbeur simple de type a` surface stagnante, l’absorption de la vapeur d’eau par une solution debromure de lithium a` 50% m/v contenant des additifs aux alcools a` 8 carbones tels n-octanol, 2-octanol, 3-octanol et 2-e´thyl-1-hexanol. Quatre solutions ont e´galement e´te etudiees afin de de´terminer l’effet additif du transfert de masse de la vapeur d’eaudans divers absorbants : une solution de bromure de lithium (60%) dans l’eau, l’une de bromure de lithium (68%) dans de

International Journal of Refrigeration 22 (1999) 188–193

0140-7007/99/$20.00q 1999 Elsevier Science Ltd and IIR. All rights reserved.PII: S0140-7007(98)00061-9

* Corresponding author. Tel.:182 42 869 3902; fax:182 42 869 3910.E-mail address:hlee@hanbit.kaist.ac.kr (H. Lee)

l’ethylene glycol et de l’eau (LiBr/HO(CH2)2OH� 4,5 en termes de masse), une solution de bromure de lithium (70%)1 duchlorure de zinc1 de l’eau (LiBr/ZnCl2� 1 en termes de masse) contenant l’additif 2-e´thyl-1-hexanol. L’appareil utilises’estavere precis, particulierement dans l’e´tude de l’augmentation du transfert de masse dans la plage de concentration d’additifefficace (de 10 ppm a` 500 ppm dans cette e´tude). On a observe´ de la turbulence marque´e a l’interface pendant le processusd’absorption utilisant des additifs. Le taux d’absorption de la vapeur d’eau a augmente´ fortement avec des concentrationscroissantes d’additifs jusqu’a` la concentration de 200 ppm, puis a cesse´ d’augmenter au-de´la de cette concentration pour tousles syste`mes e´tudies. La concentration seuil d’additif donnant lieu a` une augmentation du transfert de masse e´tait situee entre 5et 8 ppm pour tous les syste`mes sauf deux : la solution de bromure de lithium a` 50% m/v avec le 3-octanol comme additif et lasolution de bromure de lithium a` 70% m/v 1 du chlorure de zinc avec du 2-e´thyl-1-hexanol comme additif. Pour ces deuxsolutions, les concentrations seuils d’additif donnant lieu a` une augmentation du transfert de chaleur e´taient de 2,5 et de35 ppm respectivement.q 1999 Elsevier Science Ltd and IIR. All rights reserved.

Mots cles : Absorption ; Eau ; Bromure de lithium

1. Introduction

One of the most important considerations for enhancingthe performance of an absorption chiller using water as arefrigerant is to improve the performance of the absorber.The working fluid in the absorber should be maintained nearequilibrium state for a heat pump to be operated under themost efficient condition. The equilibrium state can be,however, achieved by the effective mass transfer of watervapour into solution. Eight-carbon alcohols such as n-octa-nol and 2-ethyl-1-hexanol were commonly used as the effec-tive additives for enhancing the heat and mass transferefficiency of the water1 lithium bromide solution whichis one of the typical refrigerant and absorbent pairs. Whenthe water vapour is absorbed into the lithium bromide solu-tion containing small amount of alcohol additive, a vigoroussurface turbulence called Marangoni convection resultsfrom the presence of surface tension gradient which playsa key role in enhancing mass transfer. Several experimentalworks were carried out by using two different types ofabsorber. The stagnant pool-type absorber [1–3] is

commonly used for both the preliminary additive-effecttest and the theoretical mass transfer enhancement investi-gation. The falling film-type absorber [4–7] is used for themore practical purpose such as generating the design databecause this type of absorber is more closely related to thatof real machine. Kashiwagi [1] investigated the heat andmass transfer phenomena of the lithium bromide solutionwith several alcohol additives and firstly speculated amechanism of the surface turbulence related to the surfaceand interfacial tension of the solution and alcohol droplet.Hozawa et al. [2] reported both of the experimental andtheoretical investigations Marangoni convection inducedby n-octanol. Elkassabgi and Perez–Blanco [3] alsoperformed an experimental study on the effect of severalalcohol additives on heat and mass transfer by measuringthe pressure decay history and the solution temperature in astagnant pool absorber. Hihara and Saito [4] carried out anabsorption rate experiment with lithium bromide solutionand 2-ethyl-1-hexanol by using a inclined flat plate absorberand found that 4–5 folds of enhancement occurred. Jung et al.[5] used four alcohols (n-heptanol, n-octanol, 3-octanol, and

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Fig. 1. Schematic diagram of experimental apparatus: (1) solution cell; (2) sight glass; (3) absorption chamber; (4) vapour generating flask; (5)stirrer; (6) constant temperature bath; (7) water introducing port; (8) pressure gauge; (9) vacuum pump; (10) circulator; (11) valve 1; (12) valve2; (13) valve 3; (14) valve 4.Fig. 1. Sche´ma de l’appareil utilise´ : (1) cellule contenant la solution ; (2) vitre de visualisation ; (3) chambre d’absorption ; (4) ballon danslequel la vapeur est produite ; (5) agitateur ; (6) bain maintenu a` une tempe´rature constante ; (7) orifice d’entre´e d’eau ; (8) manome`tre ; (9)pompe a` vide ; (10) circulateur ; (11) de´tendeur 1 ; (12) de´tendeur 2 ; (13) de´tendeur 3 ; (14) de´tendeur 4.

2-ethyl-1-hexanol) as additives with lithium bromidesolution in a falling film type mini-absorber and foundthat n-heptanol was the most effective for mass transferenhancement and 3-octanol the worst. Kim et al. [6] madea research on the mechanism of mass transfer enhancementwith a vertical falling film absorber at various conditionsand reported that the Marangoni instability is responsiblefor absorption enhancement. They [7] also compared theexperimentally measured onsets of the mass transferenhancement with those calculated from various surfacetension gradients. Most of works related to mass transferenhancement was done for the lithium bromide and watersolution, and even theoretical approaches were limited onlyto the specific pairs of solution and additive. Moreover,thermodynamic behavior between salt solutions and alcoholsurfactants has not yet been reported in detail and thereforemore clear understanding for this part should be undertakenin order to find the more efficient additives for mass transferenhancement. In this work a simple stagnant pool absorberwas newly designed and constructed. The basic designconcept of apparatus was mainly focused on easy handlingand reliable data collection. By using this apparatus theabsorption rate experiments were carried out for a varietyof solution and additive pairs.

2. Apparatus and procedure

The schematic diagram of the apparatus for the measure-ment of absorption rate is shown in Fig. 1. The apparatus is astagnant pool-type and mainly consists of vapour generatingand absorbing parts including a movable cell. The vapourgenerating part consists of a glass flask (500 cm3), a constanttemperature bath, a stirrer, a vacuum gauge (VRC 902016)

capable of reading to 0.01 mmHg, and a water inlet port.The vapour absorbing part consists of small absorption cell(L 103 × W 14 × H 16 mm3) made of polystylene and achamber (ID 133, H 48 mm) assembled with two pieces ofstainless steel, a sight glass, and two o-rings for sealing. Ingeneral, the mass transfer enhancement occurring at theabsorber is strongly affected by the geometric shape of theabsorber and particularly by the shape of interface betweensolution and water vapour. In this work various shapes ofabsorption cell were examined preliminary. Then the shapewas determined to give a vigorous convection. The amountof sample solution and the time of total absorption were alsopredetermined to give a clear difference in absorption rateamong various samples. Sample solution containing adesired relative amount of additive was prepared prior toeach experiment. For each run the 18 cm3 of sample solutionwas introduced into the cell and was placed in the absorptionchamber after the total mass was accurately weighed. Theapparatus including the vapour generating part was slowlyevacuated to slightly higher pressure than the vapour pres-sure of the sample solution. During this procedure the valves1 and 2 were opened and the others were closed. When thepressure reached a predetermined set value, the valve 1 wasclosed. The vapour generating part was then more evacuatedup to 0.1 mmHg and the valve 2 was also closed. A littleamount of pure water was introduced through the valve 3and was stirred well with magnetic stirrer. The temperatureof vapour generating part was controlled by a constanttemperature bath. The absorption of water vapour intoabsorbent began by opening valve 1 and the absorptioncontinued for a specified time period. During the absorptionprocess the phenomena occurring on the surface of thesample solution was observed through the sight glass inthe absorption chamber. After the absorption experiment

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Table 1Experimental condition and test solution-additive pairsTableau 1Conditions expe´rimentales et couples solution examine´e/additif

Experimental Condition Initial evacuating pressure of absorber Initial solutiontemperature Vapour generating temperatureAbsorption time

10.0 mmHg 26.5 (0.5)8C 20.0 (^ 0.1)8C3 min

Solution AdditivesSolution-Additive Pairs LiBr 1 H2O 50 mass % n-octanol

LiBr 1 H2O 50 mass % 2-octanolLiBr 1 H2O 50 mass % 3-octanolLiBr 1 H2O 50 mass % 2-ethyl-1-hexanolLiBr 1 H2O [9] 60 mass % 2-ethyl-1-hexanolLiBr 1 HO(CH2)2OH 2 H2O[10, 11]

68 mass % 2-ethyl-1-hexanol

(LiBr / HO(CH2)2OH � 4.5by mass)LiBr 1 Lil 1 H2O [12] 60 mass % 2-ethyl-1-hexanol(LiBr / Lil � 4 by mole)LiBr 1 ZnCl2 1 H2O [10] 70 mass % 2-ethyl-1-hexanol(LiBr / ZnCl2 � 1 by mass)

was completed, the open air was introduced into the appa-ratus by opening the valves 4, 2, and 1, sequentially. The cellcontaining the sample solution was carefully taken out toweigh the amount of vapour absorbed.

3. Results and discussions

The absorption experiments were carried out for eightpairs of the solution and additive mixture which are listed

in Table 1 together with the experimental condition. Foursystems consist of the same solution (LiBr1 H2O 50 mass%) and different additive pairs and the other four systemsthe different solutions and same additives (2-ethyl-1-hexa-nol). For four systems having different solutions and sameadditive, the absorbent concentrations at which all foursystems have the same vapour pressure were used tocompare each other at the same operating conditions. Allthe experiments were carried out in the range of additiveconcentration from 10 to 500 ppm. In that range, the results

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Fig. 2. Absorption of water vapour into 50% LiBr1 H2O solutionwith n-octanol.Fig. 2. Absorption de la vapeur d’eau dans la solution de LiBr (a`50% m/v) 1 H2O avec du n-octanol.

Fig. 3. Absorption of water vapour into 50% LiBr1 H2O solutionwith 2-octanol.Fig. 3. Absorption de la vapeur d’eau dans la solution de LiBr (a`50% m/v) 1 H2O avec du 2-octanol.

Fig. 4. Absorption of water vapour into 50% LiBr1 H2O solutionwith 3-octanol.Fig. 4. Absorption de la vapeur d’eau dans la solution de LiBr (a`50% m/v) 1 H2O avec du 3-octanol.

Fig. 5. Absorption of water vapour into 50% LiBr1 H2O solutionwith 2-ethyl-1-hexanol.Fig. 5. Absorption de la vapeur d’eau dans la solution de LiBr (a`50% m/v) 1 H2O avec du 2-e´thyl-1-hexanol.

showed very distinct and continuous trend of absorptionrate. The overall experimental results for all the systemstreated in this study were presented in Figs. 2–9. The totalamount of water vapour absorbed for 3 minutes was plottedagainst the additive concentration in log-scale. Each figurealso includes the linear-scale plot. The maximum amount ofwater vapour absorbed into the solution with additive, theamount of water vapour absorbed into the correspondingsolution without additive, and the estimated onset ofenhancement were denoted by arrows. The onset point ofmass transfer enhancement was estimated by extrapolatingthe linear line regressed by a least-squares representation.

The data regression was carried out only for the concentrationregion below 200 ppm because the saturated solubilities ofall additives in the solutions used in this study are known tobe near 200 ppm [8]. A vigorous surface turbulence wasobserved through the sight glass directly after the masstransfer enhancement began. The water vapour absorptionrate remarkably increased with increasing the additiveconcentrations up to about 200 ppm and then stoppedincreasing above 200 ppm for all the systems considered.The amount of absorbed water vapour increased up to 4–4.5folds for four systems having the same 50 mass % lithiumbromide solution and different alcohol additives. The

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Fig. 6. Absorption of water vapour into 60% LiBr1 H2O solutionwith 2-ethyl-1-hexanol.Fig. 6. Absorption de la vapeur d’eau dans la solution de LiBr (a`60% m/v) 1 H2O avec du 2-e´thyl-1-hexanol.

Fig. 7. Absorption of water vapour into 68% LiBr1 HO(CH2)2OH1 H2O solution with 2-ethyl-1-hexanol.Fig. 7. Absorption de la vapeur d’eau dans la solution de LiBr (a`68% m/v) 1 HO(CH2)2OH 1 H2O avec du 2-e´thyl-1-hexanol.

Fig. 8. Absorption of water vapour into 60% LiBr1 LiI 1 H2Osolution with 2-ethyl-1-hexanol.Fig. 8. Absorption de la vapeur d’eau dans la solution de LiBr (a`60% m/v) 1 LiI 1 H2O avec du 2-e´thyl-1-hexanol.

Fig. 9. Absorption of water vapour into 68% LiBr1 ZnCl2 1

H2O solution with 2-ethyl-1-hexanol.Fig. 9. Absorption de la vapeur d’eau dans la solution de LiBr (a`68% m/v) 1 ZnCl2 1 H2O avec du 2-e´thyl-1-hexanol.

n-octanol, 2-octanol, and 2-ethyl-1-hexanol additives showeda similar pattern of mass transfer enhancement. The onsetadditive concentrations of enhancing mass transfer were allfound to be between 5 and 7 ppm. However, the mass trans-fer enhancement in the solution using 3-octanol additiveincreased a little mildly with additive concentration whencompared with the solutions using other additives. The onset3-octanol concentration of enhancing mass transfer wasfound to be between 2 and 3 ppm. While the 3-octanolexhibited to have such an enhancing effect in stagnantpool-type absorber, no noticeable effect was observed infalling film-type absorber [5]. For the other four systemshaving the same additive, all the solution showed similarlevel of absorption rate without additive and showed 2.5–3folds of enhancement with additive. The lithium bromide1water, lithium bromide1 ethylene glycol 1 water, andlithium bromide 1 lithium iodide 1 water solutionscontaining the 2-ethyl-1-hexanol showed a similar trend ofmass transfer enhancement and the onset additive concen-trations of enhancement for these three systems were foundto be between 5 and 8 ppm. The lithium bromide1 zincchloride 1 water solution showed a little lower enhance-ment than the other three solutions and the onset additiveconcentration of enhancement was found be between 30 and40 ppm. The Marangoni convection seemed to be inhibitedby the density and viscosity increase resulting from the highsalt concentration in the lithium bromide1 zinc chloride1 water solution. The Marangoni convection doesn’tappear, of course, in the solutions having no additive andtherefore the vapour absorption rate is greatly influenced bythe solution vapour pressure, but to little extent by the solu-tion density and viscosity. The vapour absorption rate of the60 mass % lithium bromide solution was found to be muchhigher than that of the 50 mass % lithium bromide solutionwhen both solutions were used without the 2-ethyl-1-hexa-nol additive as shown in Figs. 5 and 6. Marangoni convec-tion must not be, of course, generated for these non-additivesolutions. The maximum mass transfer rate of the 50 mass% lithium bromide solution with the 2-ehtyl-l-hexanol addi-tive was found to be about four time higher than that withoutthe additive, while that of the 60 mass % lithium bromidesolution increased by two and a half times.

4. Conclusions

The absorption experiments of water vapour into thelithium bromide-based solutions containing the eight-carbon alcohol additives were carried out using a newstagnant pool absorber in order to examine their effect onmass transfer enhancement. Four systems consisted of the50 mass % lithium bromide solution and four differentadditives and the other four systems the 2-ethy-1-hexanoladditive and four different solutions. The simply-designedexperimental apparatus used in this study was found to bevery efficient for investigating the effect of alcohol additives

on the absorption of water vapour into the absorbent solu-tion. All the additives including 3-octanol showed positiveeffect on the mass transfer enhancement in the lithiumbromide 1 water solution. Other solutions excluding thelithium bromide 1 zinc chloride 1 water also showed asimilar trend of mass transfer enhancement by adding 2-ethyl-1-hexanol additive. The lithium bromide1 zincchloride 1 water solution showed a little lower level ofmass transfer enchantment with 2-ethyl-1-exanol additiveand the onset concentration of additive for the enhancementwas very high. The experimental apparatus used in thisstudy can be easily and accurately applied to absorptionexperiments using the various absorbent-additive pairs.When compared the overall experimental results of Kim etal.’s [7] with those of this study, the performance character-istics of both the simple stagnant pool-type absorber and thefalling film-type absorber revealed to be almost equivalent.However, the surface tension gradient of each solution-addi-tive pair should be precisely investigated for the betterunderstanding of the surfactant-based absorption behavior.

References

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[7] Kim KJ, Berman NS, Wood BD. Absorption of water vapourinto LiBr solutions with 2-ethyl-1-hexanol. AICHE J1996;42:884–888.

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[9] McNeely LA. Thermodynamic properties of aqueous solutionof lithium bromide. ASHRAE Trans 1979;85(1):413–434.

[10] Inoue N. H2O/LiBr 1 C2H2(OH)2 system and H2O/LiBr 1

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[12] Lyoki S, Iwasaki S, Uemura TT. Vapour pressures of thewater-lithium bromide-lithium iodide system. J Chem EngngData 1990;35:429–433.

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