OXYGEN ABSORPTION INTO SODIUM DITHIONITE SOLUTION Susumu FUKUSHIMA, Atsuo UYAMA, Yoshiro YAMAGUCHI,Em TSUJI and Seiichi MEZAKI Department of Chemical Engineering, Kansai University, Suita 564 Absorption rates of pure oxygen gas into sodium dithionite solution with sodium hydroxide were measured at 20 and 30°C in a laminar jet. Absorption rates were also measured under 0.315 to 4.0 atm of oxygen pressure in a continuous stirred tank. From analysis of the data on the basis of chemical absorption theory, the rate equation of chemi- cal reaction for molecular oxygen A was obtained, where the molar ratio of sodium dithionite B to sodium hydroxide is lower than 1/2, as where *Of i.9=(3.7±0.7)xl010 exp (-1.20xWfRT) [(//g-mole)° 9 sec 1] The absorption for the bubble stirred tank showed that the reaction orders for respective chemical species are reasonable. Furthermore, the homogeneous reaction assay in the continuous stirred tank established that this rate equation of chemical reaction is reasonable. where the pH was approximately 13. Introduction There are few reports of kinetic data on the basis To obtain the inter facial area and liquid-phase mass of chemical absorption theory. Jhaveri and Sharma3) transfer coefficient without chemical reaction on the discussed, from an analysis of pure oxygenabsorption basis of chemical absorption theory it is desirable to into dithionite solution in a laminarjet, oxygen absorp- perform absorption experiments in a reaction system tion through free surface into solution in a continuous in which the real rate equation of chemical reaction stirred tank and oxygen absorption into solution in a is already known in a wide concentration range of packed column at 33°C. They reported the first chemical species. order for CBand the zero order for CAwhere the CBis Sharma and co-workers3"5'7>8>15) have reported data smaller than 8xlO~2M, and the second order for of inter facial areas and gas- and liquid-phase mass CBand the zero order for CAwhere the CBis larger transfer coefficients in various gas-liquid contactors than 8xlO"2M. for oxygen absorption with dithionite solution at 33°C. The present work was attempted to find the real rate Since the study of reaction between dithionite and equation of chemical reaction between dithionite and molecular oxygen in aqueous solution was conducted molecular oxygen from an analysis of absorption data by Meyer10) in 1903, various kinetic data have been for a laminar jet and two types of stirred tanks at 20 reported. Rinker et al.12) have shown that the chemi- and 30°C on the basis of chemical absorption theory. cal reaction is a half order for dithionite concentra- Further, this rate equation was established by oxygen tion CBand the first order for molecular oxygen CA monitored assay for a homogeneoussystem in a con- from oxygenabsorption experiments in a batch stirred tinuous stirred tank. tank. The initial CB was varied from 5xlO~3 to 1. Theory 2xlO~2M in 0.1N alkaline solution. Morello et al.U) have also reported the first order of CBand the The following reactions take place in excess sodium zero order of CAin stopped-flow experiments at 37°C. hydroxide solution. The feed concentrations were varied from 8 x 10~5 to Na2S2O4+O2+H2O >NaHSO3+NaHSO4 (1) 4.8X10~4M for CB and 10"4 to 4.8x10"4M for CA9 NaHSO3+NaOH >Na2SO3+ H2O (2) Received December 12, 1977. Correspondence concerning this article should be addressed to S. Fukushima. NaHSO4+NaOH --*Na2SO4+H2O (3) VOL ll NO. 4 1978 283 Thus tion, and then 30 cm3 methyl alcohol was added to the Na2S2O4+O2+2NaOH >Na2SO3+Na2SO4+H2O flask to avoid solidification of leuco-compound. (4) The ampoule was broken by a glass rod under bubbling 1. 1 Chemical absorption nitrogen gas at about 12 cm3/min through glass tubing closed by the bottom of the flask. Whenthe solution The theoretical reaction coefficient of gas absorption is still blue, the dithionite content is less than that of in the (m+#)-th order irreversible reaction, /3, is given methylene blue because the methylene blue is quantita- by Hikita and Asai2) as follows: tively reduced with dithionite on an equimolecular basis from an intense blue color to an almost colorless leuco-form. Thus the content of dithionite in the where sample was 93.4 to 94.0% as determined from the weight of sample at which the solution just became 7 ktl(m+i)m-nAAtJ L(^--i) J colorless. (6) 2) Iodimetric titration12) A glass ampoule was put and in a flask containing 30 cm3 of excess 0.05 N iodine CBoDB\ \DA solution with acetic acid. A nitrogen gas cylinder n /i I ^Bo-^B \ -L^A (H\ was connected with glass tubing that reached close to the flask bottom. This first flask was connected with When the value of j' is larger than 6, Eq. (5) becomes a second flask containing 0.05 N thiosulfate solution and with two other, flasks containing water in the same or fashion. Under bubbling nitrogen gas at about 12 cm3/min, 20 cm3 of 37%formalin was added into the VDA-VDA~~l(m+lfm-nCAi j LT/3--1) SeJ solution in the first flask and then the glass ampoule was broken by a glass rod. All the solutions were (9) mixed and the total content of dithionite and thio- Thus, the reaction orders of m and n are determined sulfate in the sample was determined by back- from the analysis of absorption data by Eq. (9), titration with 0.01 N thiosulfate solution. The total where y'>6. content of dithionite, sulfite and thiosulfate was also 1. 2 Homogeneousreaction identified by the same procedure without formalin. The oxygensolution wasfed into a continuous stir- Thus the total content of dithionite and thiosulfate and red tank filled with aqueous solution. Whenthe tracer the content of sulfite in the sample were found to be of dithionite solution is instantaneously added into the 93.0 to 94.0 wt% and 6.0 to 8.0wt%, respectively. stirred tank, the material balance of oxygen and The former values agree well with the aforementioned dithionite are given as follows : data on dithionite content. For oxygen balance 2. 2 Oxidation products of dithionite sample in chemi- l -A=km,nC?TlCS ,tAmBnrl+(dAldd) (10) cal absorption For dithionite balance In a flask, 0.05 to 0.1 g of dithionite sample was -B=km,nC7inC^Aá"BnTl+(dBldd) (1 1) added into 5 cm3 of 5 N NaOHsolution and oxidized The boundary conditions are written as under bubbling pure oxygen gas at 30°C until no reduc- tion of methylene blue was identified. Apart of this #=0, A=\ and £=1 (12) solution was acidified with acetic acid and titrated The time dependences of A and B are obtained from with indirect iodimetry to determine the total content these equations. of sulfite and thiosulfate. The sulfite was thus the 2. Experimental sum of the impurity in dithionite sample and oxida- tion products. The major part of the solution was 2. 1 Purity of dithionite sample further oxidized in the presence of 10"4M CoCl2 The sodium dithionite (Wako Pure Chemical In- catalyst at pH 8 and 30°C. In this step, the thiosulfate dustries) in this work was used without purification. was stable. Thus sulfite was identified as one of the Sample purity was checked by two different titration oxidation products. The total content of sulfate in methods, as follows, in order to be strictly accurate. the final oxidation products was determined by titra- The sample was sealed in a glass ampoule under dry tion of 10"2 N BaCl2 solution with THQX)as an indi- nitrogen gas. cator. 1) Methylene blue titration12) A glass ampoule It was thus found that the sample contained no containing the sample was put into a flask containing thiosulfate or sulfate and that the dithionite in the 30 cm3 of 10"3 N oxygen-free methylene blue solu- sample was oxidized at excess NaOH according to 284 JOURNAL OF CHEMICAL ENGINEERING OF JAPAN Table 1 Physical properties of dithionite solutions Series p cAi [°C] [105 Dacm2/sec]DB [g/cm3] [102g/cm[105 cm2/sec] - sec] [104 g-mole//] 1 A 1.0M NaOH-0.5 M Na2S2O4 20 1.10 60 .61 0.806 5.61 1 30 1.10 27 .09 1.05 1 5.12 B 0.5 M NaOH-0.5 M Na2S2O4 30 1.08 ll .31 1.16 6.21 C 0.5 M NaOH-0.3 M Na2S2O4 20 1.06 1.26 92 0.960 8.42 30 1.06 1.01 .48 1.24 7.40 D 0.5 M NaOH-0.25 M Na2S2O4 20 1.05 1.26 .92 0.964 8.81 30 1.05 0.987 ,52 1.26 E 0.1 M NaOH-0.5 M Na2S2O4 30 1.08 1.10 2.36 1.18 7.24 Eq. (4). without chemical reaction, kf, were also estimated 2. 3 Chemical absorption from the following equation on the basis of penetration 1) Laminarjet The nozzle was 0.18 cm in diameter theory. and the Scriven14)-type receiver for the laminar jet was 0.2 cm in diameter. The liquid jet length was varied ki~V"ir~i^rv f; J (13) from 7.07 to 12.3 cm by adjusting nozzle position and Continuous stirred tank The 18Ni-8Cr-stainless the liquid flow rate was varied from 3.6 to 5.2 cm3/sec. steel tank with coil was 8.5 cm in diameter and 13.8 cm To measurethe laminar jet diameter accurately, in height, mounted with 4 baffles, 0.8 cm in width.