Bubble columns are gas-liquid contactors often used in industry. Although
they are used primarily for mass transfer purpose, since gas and liquid
phases are fed to the column at different temperatures, direct contact
heat transfer becomes important as well. This research is about the heat
transfer characteristics of bubble columns. Experiments were carried out
using a plexiglass column with dimensions 160x160x1180 mm. Water was
first put into the column to the height of 1 m and heated to the desired
temperature, and then experiments were performed by introducing ambient
air to the column and recording the variation in temperature of water and
inlet and outlet air temperatures and humidities. Models developed for the
convective heat transfer from the liquid interface to the gas in the bubble
were used together with the experimental data to determine the heat and
mass transfer coefficients. The volumetric heat transfer coefficient (hGa)
was found to vary over the range 0,3-16 kW/(m3·K) with gas flow rate and
the volumetric mass transfer coefficient (kGa) in the range 0,35-7,49 kmol/
m3·s·atm.Mjehurasti stupci su dodiri između plina i kapljevine koji se često koriste u
industriji.. Iako ih se primarno koristi s ciljem prijenosa mase, budući da se
plinska i kapljevita faza razdvajaju u stupce pri različitim temperaturama.
tada njihov izravni dodir postaje važan za prijenos topline.. Ovo se
istraživanje bavi karakteristikama prijenosa topline mjehurastih stupaca.
Eksperimenti su napravljeni koriste cilindar od plexiglasa dimenzija
160x160x1180 mm. U cilindar je prvo ulivena voda do visine od 1 m i koja
je zagrijavana do željene temperature, nakon čega se u tu vodu uvodio zrak
okolišnje temperature, pri čemu se mjerila promjena temperature vode, kai
i ulazna i izlazna vlažnost zraka. Modeli razvijeni za konvektivni prijenos
topline sa slobodne površine kapljevine na mjehur su korišteni zajedno
sa eksperimentalnim podacima za određivanje koeficijenata prijenosa
topline i mase.. Iznađeno je da je volumetrički koeficijent prijenosa topline
varirao u području 0.3-16 kW/(m3·K) s protokom zraka i volumetričkim
koeficijentom prijenosa mase u području 0.35 – 7,39 kmol/(m3·s·atm

Bekir Zühtü UYSAL

Duygu UYSAL

Serdar DEMIR

Özkan Murat DOĞAN

English

HRČAK - Portal of Croatian Scientific and Professional Journals

Strojarstvo 52 (5) 543-548 (2010) B. Z. UYSAL et. al., Heat Transfer in Bubble Columns 543CODEN STJSAO ISSN 0562-1887 ZX470/1475 UDK 66.021.4Original scientific paperBubble columns are gas-liquid contactors often used in industry.  Although they are used primarily for mass transfer purpose, since gas and liquid phases are fed to the column at different temperatures, direct contact heat transfer becomes important as well.  This research is about the heat transfer characteristics of bubble columns. Experiments were carried out using a plexiglass column with dimensions 160x160x1180 mm. Water was first put into the column to the height of 1 m and heated to the desired temperature, and then experiments were performed by introducing ambient air to the column and recording the variation in temperature of water and inlet and outlet air temperatures and humidities.  Models developed for the convective heat transfer from the liquid interface to the gas in the bubble were used together with the experimental data to determine the heat and mass transfer coefficients.  The volumetric heat transfer coefficient (hGa) was found to vary over the range 0,3-16 kW/(m3·K) with gas flow rate and the volumetric mass transfer coefficient (kGa) in the range 0,35-7,49 kmol/m3·s·atm.Prijenos topline u mjehurastim stupcimaIzvornoznanstveni članakMjehurasti stupci su dodiri između plina i kapljevine koji se često koriste u industriji.. Iako ih se primarno koristi s ciljem prijenosa mase, budući da se plinska i kapljevita faza razdvajaju u stupce pri različitim temperaturama. tada njihov izravni dodir postaje važan za prijenos topline.. Ovo se istraživanje bavi karakteristikama prijenosa topline  mjehurastih stupaca. Eksperimenti su napravljeni koriste cilindar od plexiglasa dimenzija 160x160x1180 mm. U  cilindar je prvo ulivena voda do visine od 1 m i koja je zagrijavana do željene temperature, nakon čega se u tu vodu uvodio zrak okolišnje temperature, pri čemu se mjerila promjena temperature vode, kai i ulazna i izlazna vlažnost zraka. Modeli razvijeni za konvektivni prijenos topline  sa slobodne površine kapljevine na mjehur su korišteni zajedno sa eksperimentalnim podacima za određivanje koeficijenata prijenosa topline i mase.. Iznađeno je da je volumetrički koeficijent prijenosa topline varirao u području 0.3-16 kW/(m3·K) s protokom zraka i volumetričkim koeficijentom prijenosa mase u području 0.35 – 7,39 kmol/(m3·s·atm)Bekir Zühtü UYSAL, Özkan Murat DOĞAN, Duygu UYSAL and Serdar DEmiRGazi University, Faculty of Engineering, Department of Chemical Engineering and Clean Energy Research and Application Center, (CERAC-TEMENAR), Maltepe,  06570 Ankara, TurkeyKeywords Bubble column Heat transfer Mass transferKljučne riječi Mjehurasti stupac Prijenos mase Prijenos toplineReceived (primljeno): 2009-11-30 Accepted (prihvaćeno): 2010-08-20Heat Transfer in Bubble Columnsbzuysal@gazi.edu.tr1. IntroductionBubble columns have a significant field of application in absorption processes or as liquid-gas reactors in chemical, biochemical and energy technologies. They have two-phase systems involving gas and liquid or three-phase systems involving gas, liquid and solid phases. Although the general objective of usage of bubble columns is to create a medium for reaction or mass transfer, due to the temperature difference between phases, direct-contact heat transfer occurs simultaneously and this case must be considered in modeling.  There are plenty of researches done to observe the heat transfer between two-phase or three-phase and wall or an immersed surface [6]. But, the researches about heat transfer between phases are very few [5, 7]. In this work, a path similar to humidification [9] and direct-contact evaporation [2, 5] processes was been developed in order to determine the analytical model of direct-contact heat transfer and with the experimental data, the heat transfer  coefficient between phases and mass transfer coefficient have also been determined.2. Heat transfer in bubble columnsA simple demonstration of the model is shown in Figure 1. This model is used to determine the heat transfer 544 B. Z. UYSAL et. al., Heat Transfer in Bubble Columns Strojarstvo 52 (5) 543-548 (2010)Symbols/Oznakea - interphase area, m2/m3  - međufazna površinaC - density of water in air leaving the column, kg/m3  - gustoća vode u zraku koji napušta stupacCs - heat capacity of humid air, kJ/(kg·K)  - specifični toplinski kapacitet vlažnog zrakaet - dimensionless number, ∆TG/(TL-TGi)  - bezdimenzijski brojG - gas flow rate, kg dry air/s   - maseni protok suhog zrakahg - heat transfer coefficient, kW/(m3·K)  - koeficijent prijenosa toplinehga - volumetric heat transfer coefficient, kW/(m3·K)  - volumetrički koeficijent prijenosa toplineHG - height of transfer unit, m   - visina prijenosne jediniceHX - liquid enthalpy, J/kg water   - specifična entalpija kapljevite vodeHy - gas enthalpy, J/kg dry air   - specifična entalpija suhog zrakakg - gas phase mass transfer coefficient, kmol/(atm·m2·s)  - koeficijent prijenosa mase plinske fazekga - gas phase volumetric mass transfer coefficient,     kmol/(atm·m3·s)  - volumetrički koeficijent prijenosa mase plinske     fazeky - gas phase mass transfer coefficient, kmol/m2·s  - koeficijent prijenosa mase plinske fazeKy - gas phase overall mass transfer coefficient,      kmol/(m2·s)  - ukupni koeficijent prijenosa mase olinske fazeMA - molecular weight of water, kg/kmol  - molarna masa vodeMB - molecular wight of air, kg/kmol  - molarna masa zrakaNA - molar flux, kmol/(m2·s)  - gustoća količinskog tokaNG - number of transfer unit  - broj prijenosnih jedinicapT - total pressure   - ukupni tlakqc - convective heat flux, kW/m2   - konvektivna gustoća toplinskog tokaqb - heat flux corresponding to latnet heat, kW/m2   - gustoća toplinskog toka koja odgovara latentnoj     topliniS - surface area of air bubble, m2   - površina mjehuraTi, TL - water temperature, K   - temperature vodeTGi - temperature of gas at the exit of column, K   - temperatura plina na ulazu u stupacTGf - temperature of gas at the exit of column, K   - temperatura plina na izlazu iz stupcay - gas phase molar ratio   - molni udio plinske fazeZT - total column height, m   - ukupna visina stupcaλo - latent heat, kJ/kg   - latentna toplinacoefficient used in the calculation of direct-contact heat transfer between gas and liquid in bubble columns. Herein, a bubble can be chosen representatively and a heat transfer model can be developed based on this bubble.At first, the following assumptions can be done;  Batch bubble column is used.• The heat transfer resistance at the liquid phase is • neglected. [1]The value of liquid phase heat transfer coefficient is • assumed to be very high. [3]The interphase temperature is assumed to be equal to • liquid temperature.The temperature is assumed to be constant during • the stay of bubble in the column. [5]Figure 1.  Schematic demonstration of bubble columnSlika 1. Shematski prikaz stupca mjehuraBy choosing a control volume element with a height “dz” in the column shown in Figure 1, equation of energy considering the heat transfer from hot liquid to bubble phase can be written as follows; Strojarstvo 52 (5) 543-548 (2010) B. Z. UYSAL et. al., Heat Transfer in Bubble Columns 545. (1)Some part of the heat transferred to bubble is transferred convectively from interphase to bubble gas, and the other part of this heat is used to evaporate some of the water molecules at the interphase thus increasing the humidity of bubble gas.   (2)Convective heat flux can be written as follows. (3)The heat flux corresponding to the latent heat used for the evaporation of water is, (4)By using the definition of mass transfer flux, the expression of molar ratio related to humidity, the relation between the mass transfer coefficients; (5) (6) (7)the expression for mass transfer flux can be written as; (8)When Equation (8) is used, Equation (4) would be; (9)If Equation (3) and (9) is substituted into Equation (2), (10)is obtained.  By using this expression and Equation (1), the following equation is obtained. (11)Here, using the humid heat capacity, (12)the equation is reduced to the following.  (13)  (15)This expression can be integrated between the column inlet and exit. (16)If assumptions are considered; integration gives the following result. (20)This can be rephrased as the well known expression stating that the height of the column is the product of  the height of transfer unit and the number of transfer units. (21)Writing explicitly at the interphase, and gas enthalpies at the inlet and exit would be written as follows, (26) (27)The humidity values at the entrance and at the exit can be determined experimentally and used in these equations. If Ti=TL assumption (Equation 17) is not made, modeling can be done as follows.  For gas phase; (28)or,(14)(17)(18)(19)(22)(23)(24)(25)546 B. Z. UYSAL et. al., Heat Transfer in Bubble Columns Strojarstvo 52 (5) 543-548 (2010) (29)For liquid phase; (30)or in terms of specific liquid enthalpy, (31)By summing up Equations (29) and (31), (32)the above equation is obtained.  By using the definition of volume based overall mass transfer coefficient, (33) (34) (35) (36)and upon integration one can get (37) (38)In the case when the mass transfer is negligibly small, the following approach can be followed. If it is considered that the heat transferred from liquid to gas phase through the interphase is used to increase the temperature of gas rising up through the column in the form of bubbles, the heat balance can be written as, The integration of this expression along the entire height of column gives the following.  (40)When the left-hand side of this expression is reorganized,(39) (41)an expression for for  (hGa) is obtained. (42)Using the definitions of  (43)the equation takes the following form, (44)3. ExperimentalExperiments were carried out in a plexiglass column, shown in Figure 2, which had the dimensions of 16x16x118 cm.  Water entered the column at the bottom through four nozzles.  Gas was introduced co-currently through one nozzle.  Firstly, the column was filled with water up to the level of 100 cm.  The experiment was performed as batch system.  With the help of a heater immersed into the water, water was initially heated and then gas was let into the column. Regarding the durability of plexiglass, the water was heated up to maximum 40 oC. The flow rate of air in column was changed between 28 and 340 L/min. With the thermocouples at the top and middle of plexiglass column, temperature measurement was done throughout the experiment.   Figure 2. Experimental setupSlika2. Eksperimentalni uređajStrojarstvo 52 (5) 543-548 (2010) B. Z. UYSAL et. al., Heat Transfer in Bubble Columns 5474. Results and discussionThe experimental data, obtained in the  experiments performed using the method explained above, were used to evaluate the volumetric heat transfer coefficient (hga) and volumetric mass transfer coefficient (kga). Their graphs are drawn in Figure 3 and 4 to show their dependencies on the air flow rate. Gas phase holdup was estimated with the following equation, obtained at similar operating conditions [4]; (45)Figure 3. Variation of volumetric gas phase heat transfer coefficient with air flow rateSlika 3. Ovisnost volumetričkog koeficijenta prijenosa topline o volumenskom protoku zrakaFigure 4. Variation of individual gas phase mass transfer coefficient with air flow rateSlika 4. Ovisnost koeficijenta prijenosa mase pojedinačne plinske faze o volumenskom protoku zrakaAs seen in Figures 3 and 4, as the air flow rate increases, the gas phase volumetric heat transfer coefficient and the gas phase volumetric mass transfer coefficient also increase. This is an expected result. With the increasing gas rate, both the turbulence inside the column and number of bubbles formed the increase. These two factors make volumetric heat and mass transfer coefficients increase. The change in overall mass transfer coefficient with different gas rates is shown in Figure 5. Kya, defined with Equation 33, expresses the reciprocal of the sum of gas and liquid side individual resistances. It increases with a higher gas rate at a slower rate than individual coefficients as expected.     Figure 5. Variation of gas phase overall heat  transfer coefficient with air flow rateSlika 5. Ovisnost ukupnog koeficijenta prijenosa topline na strani plinske faze o volumenskom protoku zrakaWith the experimental data given in Figure 5 and Equation 32, liquid side individual heat transfer coefficient can also be calculated.  The results are shown in Figure 6.Figure 6. Variation of volumetric liquid phase heat transfer coefficient with air flow rateSlika 6. Ovisnost volumetričkog koeficijenta prijenosa topline na strani kapljevite faze o volumenskom protoku zrakaThe comparison of Figure 5 and 6 is interesting; both of them show linear change with gas rate. This shows the inevitable and profound effects of the liquid side individual resistances on the overall transfer coefficient.5. ConclusionsSimultaneous heat and mass transfer phenomena were modeled for bubble columns involving direct contact between gas and liquid phases and volumetric heat and mass transfer coefficients were calculated by using the heat transfer experimental data. Use of the volumetric transfer coefficients as presented in this paper is easier and more practical in design calculations. The assessment of the results leads to the conclusion that the magnitude of the resistance at the liquid side has a significant effect on the overall transfer coefficient.548 B. Z. UYSAL et. al., Heat Transfer in Bubble Columns Strojarstvo 52 (5) 543-548 (2010)REFERENCES[1] ALABOWSKII, A. N.: Thermal Interaction at the Gas-Liquid Interface in an Apparatus with an Immersed Burner, Inz. Fiz. Zhurn., 22 (1972), 117-122.[2] BHARATAN, D.: Direct Contact Evaporation, in Direct Contact Heat Transfe, F. Kreith and R. F. Boehm, ed., Hemisphere Pub. Corp., New York, 1988.[3] BIRD, R.B.; STEWART, W.E. and LIGHTFOOT, E.N.: Transport Phenomena (2nd Ed.), J. Wiley & Sons, New York, 2007.[4] DEMIREL, S, A.; ÇULHA, F. F. AR and UYSAL, B. Z.: İki ve Üç Fazlı Taşkın Yataklarda Gaz Tutulması ve Kütle Transferi, UKMK-1 Kongre Bildiri Kitabı, Ed. A. Tiryaki, 1. Cilt, s. 104, Ankara, 1994 (in Turkish). [5] GUY, C.; CARREAU, P.J. and PARIS, J.: Heat and Mass Transfer Between Bubbles and a Liquid, Can. J Chem. Eng., 70 (1992), 55-60.[6] HULET, C. CLEMENT, P.; TOCHON, P.; SCHWEICH, D.; DROMARD, N. and ANFRAY, J.: Literature Review on Heat Transfer in Two- and Three Phase Bubble Columns, Int. Jr. Chem. Reac. Engrg., 7 (2009), 1-94.[7] KREITH, F. and BOEHM, R.F.: Direct Contact Heat Transfer, Hemisphere Pub. Corp., New York, 1988.[8] SIDEMAN, S.: Direct Contact Heat Transfer Between Immiscible Liquids, Adv. Chem. Eng., 6 (1966), 207-286.[9] TREYBAL, R. E.: Mass Transfer Operations, McGraw Hill, New York, 1980.

Prijenos topline u mjehurastim stupcima

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