علياء خضير مجيد38-30 Al-Khwarizmi Engineering Journal Al-Khwarizmi Engineering Journal, Vol. 7, No. 3, PP 30 - 38 (2011) Effect of Solid Loading on Carbon Dioxide Absorption in Bubble Column Alyaa Khadhier Mageed Department of Chemical Engineering/ University of Technology Email: Aliaamce_1980@yahoo.com (Received 3 May 2011; accepted 30 Jun 2011) Abstract In the present work experiments were conducted to study the effect of solid loading (1,5 and 9 vol.%) on the enhancement of carbon dioxide absorption in bubble column at various volumetric gas flow rate (0.75, 1 and 1.5 m3/h) and absorbent concentration (caustic soda)( 0.1,0.5 and 1 M ). Activated carbon and alumina oxide (Al2O3) are used as solid particles. The Danckwerts method was used to calculate interfacial area and individual mass transfer coefficients during absorption of carbon dioxide in a bubble column. The results show that the absorption rate was increased with increasing volumetric gas flow rate, caustic soda concentration and solid loading. Mass transfer coefficient and interfacial area were increased with increasing volumetric gas flow rate, and solid loading. Keywords: Carbon dioxide absorption, bubble column, danckwerts method, mass transfer coefficients and interfacial area. 1. Introduction Slurry bubble columns are intensively used as a multiphase contactors in the chemical, biochemical and petrochemical industries where heterogeneous gas-liquid or gas-solid reactions take place, particularly, in which the liquid phase controls mass transfer process due to the relation with solubility of gases[1, 2]. Important applications of three phase bubble column are in hydrogenation, oxidation, and waste water treatment and in biochemical applications [2]. The rate of acid gas absorption such as carbon dioxide absorption in a gas-liquid or gas-liquid- solid contactor may be enhanced considerably by the presence of particles in the liquid –phase. To be effective, the particles have to be considerably smaller than the gas-liquid film thickness and need to have a high affinity for the component to be transferred. Enhancement of the gas absorption rates due to the presence of small particles is explained by the so-called grazing or shuttle mechanism [3-6]. Sharma and Mashelkar (1968) [7] were the first to report an increase in the mass transfer in a bubble column by small particles. Similar effects were found later by Wimmers and Fortuin (1988) [8], Beennackers and van swaaij (1993) [6] and Marius et al (2007) [9]. Lindner et al (1988) [10] and Kluytmans et al (2003) [11] found that in non-coalescing liquids, e.g., concentrated salt solutions, fine activated carbon particles may hinder bubble coalescence and significantly increase the specific interfacial area. Vandu and Krishnal (2004) [12] observed that addition of solids and high solid concentrations caused reduced values of mass transfer coefficients due to increased large bubble size. Sumin et al (2007) [13], studied the absorption of carbon dioxide in carbonate solution (K2CO3) in the presence of activated carbon particles and found that the absorption rate enhanced significantly, and the maximum enhancement factor was 3.7. The present work aimed to study the absorption rate of carbon dioxide in caustic soda (NaOH) solution 0.1, 0.5 and 1M at different volumetric gas flow rate 0.75, 1 and 1.5 m3/h and carbon dioxide concentration of 10% by volume mailto:Aliaamce_1980@yahoo.com Alyaa Khadhier Mageed Al-Khwarizmi Engineering Journal, Vol. 7, No. 3, PP 30 - 38 (2011) 31 with and without solid particles (activated carbon and alumina oxide (Al2O3) (1, 5 and 9 vol.%), keeping other variables constant as temperature of (20±2)oC, and atmospheric pressure. 2. Experimental Work Experiments of absorption of carbon dioxide from gaseous mixture (10% carbon dioxide - air) has been carried out by using aqueous solution of (0.1, 0.5 and 1M) NaOH. This has been performed in a conventional slurry glass cylindrical bubble column of 7.5 cm inside diameter, 100 cm height over a wide range of gas flow rate of 0.75, 1 and 1.5 m3/h (as velocity 0.0117-0.0235 m/sec), solid loading (1, 5 and 9vol.%) and different types of solid particles (Activated carbon, Alumina oxide) to study the effect of these parameters on the fractional conversion, absorption rate and mass transfer coefficient of carbon dioxide. Schematic diagram of experimental set up is shown in Figure (1).Table (1) shows the characteristics properties of Activated carbon, Alumina oxide. Perforated plate sparger was used as a gas distributor 104 hole of 1mm diameter and placed between the column and distributor chamber which has a drain at the bottom and gas inlet at the side. Fig.1. Experimental Set Up for Carbon Dioxide Absorption. Pressurized CO2 Source One way val ve 2 One way valve 1 Pump Compressor Rotameter 1 Rotameter 2 Tank Gas outlet Liquid Outline Thermocouple Sampling Liquid Inlet Bubble Column Gas Inlet CO2 cylinder Alyaa Khadhier Mageed Al-Khwarizmi Engineering Journal, Vol. 7, No. 3, PP 30 - 38 (2011) 32 Table 1, The Characteristics Properties of Activated Carbon, Alumina Oxide. Activated Carbon (A.C) Surface area (m2/g) 1122 Bulk density (g/m3) 0.44 Porosity ( - ) 0.46 Particle diameter (m) 1.1*10-3 Min. fluidizing velocity (m/s) 0.000329 Particle terminal velocity (m/s) 0.00309 Alumina oxide (Al2O3) Surface area (m2/g) 300 Bulk density (g/m3) 0.56 Porosity ( - ) 0.51 Particle diameter (m) 1.5*10-3 Min. fluidizing velocity (m/s) 0.00537 Particle terminal velocity (m/s) 0.0521 3. Procedure of Experiment In all experiments the volume of liquid has been set constant and equal to 1.5 liter, 30 cm height above the sparger. The gas flow of carbon dioxide and air were measured by two calibrated rotameters separately, then entered the bottom of the bubble column. The samples were taken from the side of the bubble column every 3 min. . The temperature was measured periodically by a thermocouple until the end of the experiment. All experiments were performed at constant temperature of (20±2)oC and atmospheric pressure. 3.1. Chemical Reactions When carbon dioxide is absorbed into aqueous sodium hydroxide solutions, the following two reactions should be considered [14, 15]: CO2 + OH- = HCO3- …(1) HCO3- + OH- = CO3- 2 +H2O …(2) At 30oC and at infinite dilution reaction (1) is practically considered irreversible and second order, i.e. first order with respect to both carbon dioxide and OH- ions. Reaction (2) is a proton transfer reaction and has a very much higher rate constant than reaction (1), thus this reaction can be regarded as an instantaneous reversible reaction[14, 15]. In strong hydroxide solutions, the equilibrium concentration of HCO3- ions can be neglected and the overall reaction is: CO2 + 2OH- = CO3- 2 + H2O …(3) 3.2. Physcio - Chemical Properties 1. Diffusivity (DCO2-NaOH) of carbon dioxide in aqueous caustic soda solution was estimated as follows[16 ]:- DA = 1.833*10-6T -4.717*10 -4 – 1.042*10-5N …( 4 ) 2. Reaction rate constant (k2) was estimated as follows[9, 17]:- 2 2 016.0221.0 2895 985.11log cc IIT k −+−= …(5) Where Ic is the ionic strength and can be estimated as follows: iic CZI 2 2 1 ∑= …(6a) [ ]3*2*12 1 COOHI c += …(6b) Let ( ) ( )231 bCOandbOH == Then m=b1+2b2 my =b2 mI c 2 1 =∴ …(6c) 3. Solubility of carbon dioxide in the liquid phase was calculated using the Henry's law. Henry's law constant for CO2-Na2CO3 system was determined as follows[ 10]:- ∑−= ico hIH H log …(7) Where gi hhhh ++= +− h_ for OH- =0.061 liter/g ion h+ for Na+ =0.094 liter/g ion hg for CO2 =-0.017 liter/g ion Where Ho is Henry's law constant for CO2 in water and can be obtained as follows[13]:- 2520 10*8857.710*9044.51229.9log TTH −− +−= …(8) 4. [ ] [ ] = inputNaOH reactedNaOH naconversioCausticsod % …(9) Alyaa Khadhier Mageed Al-Khwarizmi Engineering Journal, Vol. 7, No. 3, PP 30 - 38 (2011) 33 3.3. Interfacial Area A and Individual Mass Transfer Coefficient kL Danckwerts method was used to calculate the interfacial area (a) and the individual mass transfer coefficient (kL) as follows [18, 19]:- 2 2 * 2 L bulk BAOC kCkDaCN += …( 10) [ ]222 2 * 2 akCkDa C N L bulk BA OC +=         …( 11) Interfacial area a and individual mass transfer coefficient kL are obtained by plotting 2 * 2         OCC N vs. bulkBA CkD 2 , the relation is straight line of slope a2 and intercept [ ]2akL . Figure (2) shows Danckwerts plot for estimated interfacial area (a) and mass transfer coefficient (kLa). This method is used when the concentration of absorbent is not constant with time. Fig.2. Danckwerts Plot: a) Free Solid Concentration; b) 1% vol. Al2O3 Solid Concentration at Different Volumetric Gas Flow Rate. Fig.3. Caustic Soda Concentration vs. Time at 0.75 m3/h Volumetric Gas Flow Rate. Alyaa Khadhier Mageed Al-Khwarizmi Engineering Journal, Vol. 7, No. 3, PP 30 - 38 (2011) 34 This implies that a part of the chemical reaction between carbon dioxide and hydroxyl ions is carried out at interface and the other part in the bulk of the liquid. Therefore; the surface renewal theory developed by Danckwerts is satisfied for estimating interfacial area a and mass transfer coefficient kLa[19 ]. Figure (3) shows the caustic soda concentration during carbon dioxide absorption at a given condition. It can be seen that the caustic soda decreases with time. 4. Results and Discussion 4.1. Influence of Superficial Gas Velocity Figures (4 to 6) show the effect of volumetric gas flow rate on absorption rate, mass transfer coefficient and interfacial area. It can be seen that at a given solid loading the absorption rate increased with increasing volumetric gas flow rate. This is attributed to the fact that the rate of breakup of bubble increased. In addition, higher superficial gas velocity gives smaller bubbles. The smaller bubble of lower rising velocity leads to form large residence time and consequently higher gas – liquid interfacial, mass transfer coefficient and absorption rate .These results are in agreement with previous work [20, 21]. Fig.4. Absorption Rate vs. Total Volumetric Gas Flow Rate (CO2+air): a) A.C Solid Loading , b) AL2O3 Solid Loading. Fig.5. Interfacial Area vs. Total Volumetric Gas Flow Rate (CO2+air). Alyaa Khadhier Mageed Al-Khwarizmi Engineering Journal, Vol. 7, No. 3, PP 30 - 38 (2011) 35 Fig.6. Mass Transfer Coefficients vs. Total Volumetric Gas Flow Rate (CO2+air). Fig.7. Absorption Rate vs. Total Volumetric Gas Flow Rate (CO2+air) As a Function of Absorbent Concentration and Solid Loading: a) A.C Solid Loading; b) AL2O3 Solid Loading. Alyaa Khadhier Mageed Al-Khwarizmi Engineering Journal, Vol. 7, No. 3, PP 30 - 38 (2011) 36 4.2. Influence of Absorbent Concentration Figure (7) shows the effect of absorbent concentration on absorption rate at a given volumetric gas flow rate and solid loading. The results show that the absorption rate increases with increasing absorbent concentration. Increasing of absorbent concentration will cause the zone of reaction approach the gas-liquid interface rapidly which leads to increase the driving force (∆C) and decreases the thickness of the liquid film through the solute due to increase OH concentration in solution that reacts with carbon dioxide which leads to increase the reaction controlling step (equation 2). These results are in agreement with previous work [22, 23, 24]. 4.3. Influence of Solid Loading Figures (4 to 7) show the effect of solid loading on absorption rate, mass transfer coefficient and interfacial area. It can be seen that the absorption rate, mass transfer coefficient and interfacial area increase with increasing solid loading. Enhancement of the gas absorption rates due to the presence of small particles is explained by the so-called grazing or shuttle mechanism. It is assumed that the particles travel between the stagnant liquid mass transfer layer and the bulk of the liquid. Near the interface, the adsorptive particles are loaded with solute and the solute concentration in the liquid mass transfer layer decreases. The concentration gradient of the solute in the mass transfer layer increases leading to enhance gas absorption. After a certain time in the liquid side mass transfer layer, the particles returns to the bulk of the liquid where the gas- phase component is desorbed and the particle regenerated [3-6, 13, 25]. Due to the hydrophobic properties of active carbon, the concentration of particles in the mass transfer zone is much higher than in the bulk of the suspension, leading to higher absorption rates and mass transfer coefficients [25]. Also, the surface area of activated carbon was higher than the alumina. This leads to that the absorption rate, mass transfer coefficient and interfacial area with activated carbon particle is higher than that when alumina particle was loaded. 5. Conclusions The following points are concluded from the present work:- ß The absorption rate increased with increasing volumetric gas flow rate and absorbent concentration within the conditions used. ß The absorption rate, mass transfer coefficient and interfacial area increase with increasing solid loading according to grazing or shuttle mechanism. ß Danckwerts method was used to calculate interfacial area (a) and individual mass transfer coefficient (kL). Nomenclature Symbol Unit Definition a m2/m3 Interfacial area CB kmol/m 3 Concentration of liquid reactant (B) in the bulk * 2OC C kmol/m 3 Concentration of carbon dioxide at equilibrium Ci kmol/m 3 Concentration of ions AD m 2/s Diffusivity of carbon dioxide in caustic soda solution H atm.m3/kmol Henry's constant Ho atm.m3/kmol Henry's constant in pure water hi L/g ion Parameters of equation 7 of h+, h- and hg respectively, hi = h- + h+ + hg h+ L/g ion Parameter of cation h- L/g ion Parameter of anion hg L/g ion Parameter of gas Ic m 3/ kmol Ionic strength kL.a 1/ s mass transfer coefficient Lk m/s Liquid side mass transfer coefficient k2 m 3/kmol.s Reaction rate constant N kmol/m3.min. Absorption rate m kmol/m3 Molarity T K Temperature y ( - ) Fractional conversion Zi ( - ) Valance of ion 6. References [1] Lye, G.J., stuckey, D.C., "Extraction of erythromycin-A using colloidal liquid aphrons: Part II. Mass transfer kinetics", Chem. Eng. Sci., 56 (2001) 97-108. [2] Nigar, K., Faher, B., Kutlu, O.U., "Bubble Column Reactors", Process Biochemistry, 40 (2005) 2263-2283. Alyaa Khadhier Mageed Al-Khwarizmi Engineering Journal, Vol. 7, No. 3, PP 30 - 38 (2011) 37 [3] Chandrasckaran, K., Sharma, M.M., "Absorption of oxygen in aqueous solutions of sodium sulfide in the presence of activated carbon as catalyst", Chem. Eng., Sci., 32 (1977) 669. [4] Quicker, G., Alper, E., Deekwar, W.D., "Effect of fine activated carbon particles on the rate of carbon dioxide absorption", A.I.Ch.E. J., 33 (1987) 871. [5] Vinke, H., Hamersma, P.J. Fortuin, J.M.H., "Enhancement of the gas-absorption rate in agitated slurry reactors by gas-absorbing particles adhering to gas bubbles", Chem. Eng. Sci., 48 (1993) 2197. [6] Beenackers, A.A.C.M., van Swaaij, W.P.M., "Mass transfer in gas-liquid slurry reactors", Chem. Eng. Sci., 48 (18) (1993) 3109. [7] Sharma, M.M., Mashelkar, R.A., "Absorption with reaction in bubble columns", Institute of Chemical Engineers Symposium Series, 28 (1968) 10-21. [8] Wimmers, O.J., Fortuin, J.M.H., "The use of adhesion of catalyst particles to gas bubble to achieve enhancement of gas absorption in slurry reactor part", Chem. Eng. Sci., 43 (1988) 119-313. [9] Marius, R., Anita, M., Aydin, K., Adrian, S., "Surfactant adsorption on to activated carbon and its effect on absorption with chemical reaction", Chem. Eng. Sci., 62 (2007) 7336- 7343. [10] Lindner, D., Werner, M., Schumpe, A., "Hydrogen transfer in slurries of carbon supported catalysts (HPO process). A.I.Ch.E.J., 34 (1988) (10) 1691-1697. [11] Kluytmans, J., van Wachem, G., Kuster, B., Schouten, J., "Mass transfer in sparged and stirred reactors: influence of carbon particles and electrolyte", Chem. Eng. Sci., 58 (2003) 4719-4728. [12] Vandu, C.O., Krishna, R., "Volumetric mass transfer coefficients in slurry bubble columns operating in churn-turbulent flow regime", Chem. Eng. Process, 43 (2004) 987-95. [13] Sumin, L.U., Youguang, M.A., Chunying, Z.H.U., Shuhua, S.H.E.N., "The enhancement of carbon dioxide chemical absorption by potassium carbonate aqueous solution in the presence of activated carbon particles", Chin. Chem. Eng. J., 15(2007) (15)842-846. [14] Hikita, H., Asia, S., Takatsuka, T., "Absorption of carbon dioxide into aqueous sodium hydroxide and sodium carbonate- bicarbonate solutions" Chem. Eng. J., 11 (1976) 131. [15] Danckwerts, P.V., "Gas-liquid reactions", Mc-Graw Hill, New York, 1970. [16] Wales, C.E., "Physical and chemical absorption in two-phase annular and dispersed horizontal flow", AIChE.J., 12,6 (1966) 1166. [17] Astarita, G., Savage, D.W. and Bisio ,A.," Gas treating with chemical solvent " , Wiley, New York, 1983. [18] Zhao Wei-rong, Shi Hui-xiang, and Wang Da-hui, "modling of mass transfer characteristics of bubble column reactor with surfactant present", Journal of Zhejiang University Science ,5,6(2004) 714-720. [19] Navaza, J.M., Gomez-Diaz, D., and Dolores Le, R...,"Removal process of CO2 using MDEA aqueous solutions in a bubble column reactor", Chem.Eng.Sci. , 146, (2009), 184-188. [20] Mouza, A.A., Dalakoglou, G.K, Paras, S.V., “Effect of liquid properties on the performance of bubble column reactors with fine pore spargers”, Chem. Eng. Sci., 60 (2005) 1465 – 1475. [21] Maalej, S., Benadda, B., and Otterbein, M.,"Interfacial area and volumetric mass transfer coefficient in a bubble column reactor at elevated pressure" , Chem. Eng. Sci., 58 (2003) 2365 – 2376. [22] Yih, S.M., and Sun, C.C., “Simultaneous absorption of hydrogen sulphide and carbon dioxide into potassium carbonate solution with or without amine promoters" Chem.Eng.J., 34(1987)65. [23] Maceiras, R., Alvarez, E., and Cancela, M.A., "Effect of temperature on carbon dioxide absorption in monoethanolamine solutions", Chem. Eng. J., 138 (2008) 295- 300. [24] Lin, S. H., Tung, K.L., Chen, W.J., and Chang, H.W., "Absorption of carbon dioxide by mixed piperazine–alkanolamine absorbent in a plasma-modified polypropylene hollow fiber contactor", J.Membr.Sci., 333 (2009) 30-37. [25] Dagaonkar, M.V., Heeres, H.J., Beenackers , A.A.C.M., and Pangarkar , V.G., " The application of fine TiO2 particles for enhanced gas absorption" , Chem. Eng. J., 92 (2003) 151–159. )2011( 30 - 38، صفحة 3، العدد 7مجلة الخوارزمي الھندسیة المجلد علیاء خضیر مجید 38 غازدراسة تأثیر تحمیل مادة الصلب على عملیة امتصاص ثنائي اوكسید الكربون في العمود الفقاعي علیاء خضیر مجید التكنولوجیة ةالجامع/ قسم الھندسة الكیمیاویة الخالصة معدل كربون في العمود الفقاعي فياوكسیدالعلى عملیة امتصاص غاز ثنائي )نسبة حجمیة%٩, ٥, ١(ھذا البحث تضمن دراسة تأثیر تحمیل الصلب م) ١و٥, ١ ,٠و٧٥((التدفق الحجمي للغاز م\كیلومول ١, ٠و٥, ٠و١)(اوكسید الصودیوم(وتركیز المادة الماصة ) ساعة\ ٣ المنشط الكربون تم استعمال ).٣ امل انتقال الكتلة اثناء عملیة امتصاص غاز ثنائي اوكسیدالكربون في طریقة دانكویرتز استعملت لحساب المساحة البینیة ومع. كمواد صلبة واوكسید االلمنیوم . العمود الفقاعي كما تبین .حمیل الصلبومقدار ت) اوكسید الصودیوم(تركیز المادة الماصة,اظھرت النتائج ان معدل االمتصاص یزداد بزیادة معدل التدفق الحجمي للغاز . الصلب الحجمي للغاز ومقدار تحمیل ان معامل انتقال الكتلة والمساحة البینیة یزدادان بزیادة معدل التدفق