The Simultaneous Adsorption of Sulphur Dioxide and Carbon Dioxide by Y Zeolites GIGI DRAGAN 1 * 1 Colegiul Naþional,,A.D.Ghica, 78 Viitorului Str., Alexandria, Romania Y zeolites proved their efficiency in the individual adsorption of sulphur dioxide and carbon dioxide. When testing the Y zeolites as adsorbents for a mixture of these two dioxides, a different behaviour was noticed showing the increasing of adsorption capacity over that measured in the case of single adsorption. This behaviour depends on the sodium content of the zeolites and on the sulphur dioxide gas content which favours both the carbon dioxide adsorption. A reaction mechanism has been proposed to explain this Y zeolites comportment. Keywords: Y zeolites, sulphur dioxide, carbon dioxide, adsorption, adsorption mechanism The polluted residual gases, which result mainly from burning processes, contain mostly carbon dioxide and much smaller amounts of sulphur dioxide [1-9]. The possibility to use zeolites as sorbents for these gases depolution gives the reason of studying the adsorption of the carbon dioxide and sulphur dioxide individually and as mixture on some commercial zeolites [10]. In the case of simultaneous SO 2 and CO 2 sorption on specified zeolite the aim is to identify how these two oxides influence each retention on the solid sorbent. The influence of the temperature at which the adsorption occurs was taken into consideration, as well as the partial pressures of the mixture s components and the way the adsorption is achieved. Experimental part The experimental device and the s Y zeolites here used are the same with those used in the study of the individual adsorption [10]. So the zeolites namely the HY-928 and NaY-1633 were used to study the simultaneous adsorption of the carbon dioxide and sulphur dioxide. The adsorption was accomplished in a tubular reactor made of quartz with a firm base of zeolite and inert. The desorption is achieved by gradually heating to 400 o C in a nitrogen flux. Maintaining 400 o C in the apparatus for 30 min the thermal regeneration of the zeolite is considered accomplished. The desorbed gases are retained in the bubble flasks. Sulphur dioxide is absorbed in 50 ml solution of Na 2 [HgCl 4 ] 0,1n and carbon dioxide is absorbed in 50 ml solution of NaOH 2n. In the desorption process, the order of the oxides absorption was established in accordance with the data from the literature[11,12]. Although the mercury complex solution retains sulphur dioxide selectively, even from mixtures that contain carbon dioxide as well [13], the position of the bubble flasks was also sustained by the fact that the two oxides are soluble in dissimilar measures, in the work conditions [14]. The measurement of sulphur dioxide content is obtained iodometrically and that of carbon dioxide is accomplished by using the Warder method [15]. Results and discussions The Influence of Temperature on Adsorption With the aim to establish zeolites adsorption capacities at SO 2 and CO 2 simultaneous adsorption when the parameter temperature is changed a synthetic gas which contains 1.83% sulphur dioxide, 8.6% carbon dioxide and 89.57% N 2 (volume), at temperatures between 25 and 120 o C, flows, with a flow rate of 131 ml/min (fictive flow velocity of 0.6 m/s), through an established fixed bed (net weight of fixed bed of 3 g) of HY-929 and NaY-1633 zeolite. The experimental results are, graphically represented, in figure 1 where for correspondence is given also the zeolites comportment in single gas sorption. Fig. 1. The temperature influence on sulphur dioxide adsorption by NaY zeolite( individual adsorption ( ), mixture adsorption (o) and by HY zeolite (individual adsorption ( ), mixture adsorption (Δ) * email: gigidragan56@yahoo.com; tel.0752088792 REV. CHIM. (Bucharest) 61 Nr. 11 2010 http://www.revistadechimie.ro 1071
Fig. 2. The temperature influence on carbon dioxide adsorption by NaY zeolite ( individual adsorption ( ), mixture adsorption (o) and by HY zeolite (individual adsorption ( ), mixture adsorption (Δ) Fig. 3. The influence of sulphur dioxide concentration on the sulphur dioxide adsorption capacity at flow velocity of 0.6 m/s, total CO 2 and SO 2 concentration of 8.6% v/v and a temperature of 25 o C Fig. 4. The influence of carbon dioxide concentration on the carbon dioxide adsorption capacity at flow velocity of 0.6 m/s, total CO 2 and SO 2 concentration of 8.6% v/v and a temperature of 25 o C Comparing the data obtained for the gaseous mixture to those obtained for the individual adsorption of every component of the mixture [14], the following can be noticed: -when referring to sulphur dioxide, the HY zeolite temperature, practically having the same values as in the individual adsorption, while the NaY zeolite adsorption capacity decreases with the increase in temperature, registering values 2-4 units lower than in the individual adsorption; -when referring to carbon dioxide, the HY zeolite 1072 http://www.revistadechimie.ro temperature, registering values 50-100 units lower than in the individual adsorption, while the NaY zeolite temperature, practically having the same values as in the individual adsorption (fig. 2). The Influence of the Partial Pressures of the Mixture Components If we have gaseous mixtures of sulphur dioxide, carbon dioxide and nitrogen which contain sulphur dioxide and carbon dioxide at a constant total concentration (the partial pressure of the sulphur dioxide and carbon dioxide mixture is constant) no changes can be noticed in the adsorption REV. CHIM. (Bucharest) 61 Nr. 11 2010
Fig. 5. The influence of sulphur dioxide concentration on the sulphur dioxide adsorption capacity at flow velocity of 0.6 m/s, 8.5 % v/v constant CO 2 concentration and temperature of 25 o C Fig. 6. The influence of sulphur dioxide concentration on the carbon dioxide adsorption capacity at flow velocity of 0.6 m/s, 8.5 % v/v constant CO 2 concentration and temperature of 25 o C capacities of sulphur dioxide and carbon dioxide, regardless of the value of the sulphur dioxide/ carbon dioxide molar ratio. The main goal of this new set of experiments was to keep the sum of volumetric concentrations of sulphur dioxide and carbon dioxide (8.6%) constant, with the variation of each component concentration: sulphur dioxide from 0.9 to 5.03% and carbon dioxide: from 7.7 to 3.57%. In these experiments the gas flow rate of the supplying gas mixture was 131mL/min (fictive flow velocity of 0.6 m/s) and the temperature was 25 o C. The obtained results, presenting the state of sorbent capacity, for each gas component, are shown in figures 3 and 4. It can be noticed that, compared to the individual adsorption of sulphur dioxide and carbon dioxide, when referring to the adsorption of the mixture of these two gases, the adsorption capacities remain constant even if there is an increase in the concentration of the mixture components, which is not the case with the individual adsorption, where the adsorption capacities rise with the increase in gas concentration. For sulphur dioxide concentrations smaller than 1% v/v in the gaseous flux, the presence of carbon dioxide leads to an increase in the adsorption capacity, while sulphur dioxide concentrations is over 1%v/v, the carbon dioxide concentration does affect the sulphur dioxide adsorption capacity of the two zeolites. For the adsorption of the carbon dioxide and sulphur dioxide mixture, the adsorption capacity remains constant when the mixture concentration rises, reaching a higher value than the values of the adsorption capacities for the individual adsorption. In another set of experimental determinations we kept the carbon dioxide concentration constant at 8.6%v/v while gradually increasing the sulphur dioxide concentration from 0.90 to 5.03%. For all these experiments we keep, as in the above cases, the total flow rate at 131 ml/min and the temperature at 25 C. As shown in the figure 5, increasing the sulphur dioxide concentration, the HY zeolite s capacity of its adsorption increases slowly, having higher values every time, compared to the individual adsorption, while for the NaY zeolite it remains constant and lower than the ascending adsorption capacities obtained at the individual adsorption. When maintaining the carbon dioxide partial pressure constant and gradually increasing the sulphur dioxide partial pressure for the same set of determinations, a decrease in the carbon dioxide adsorption capacity of the HY zeolite and a slight increase in it for the NaY zeolite was noticed (fig. 7), compared to the individual adsorption capacity. These changes that occur in the adsorption capacity of the two gases can be explained if it is accepted that, when they are adsorbed by the zeolite, there are interactions which modify the level and type of some active centres acidity. Taking into consideration these interactions, it can be anticipated that, as shown by the determinations of zeolites acidity, the carbon dioxide adsorption by the NaY zeolite is slightly influenced by the presence of sulphur dioxide, probably through the adsorbed sulphur dioxide creating new Lewis centres[16], ones which favour carbon dioxide adsorption. This, however, is not a certain phenomenon for the HY zeolite, as proven by the curves contour in figure 6. REV. CHIM. (Bucharest) 61 Nr. 11 2010 http://www.revistadechimie.ro 1073
Table 1 SULPHUR DIOXIDE AND CARBON DIOXIDE ADSORPTION CAPACITIES FOR DIFFERENT ACCOMPLISHING METHODS OF ADSORPTION PROCESS Fig. 8. Possibilities of interaction between sulphur dioxide and the zeolite Fig. 7. The mechanism of carbon dioxide adsorption by newly created Lewis acid centres through sulphur dioxide adsorption The Influence of the Method of Accomplishing the Adsorption When we study the adsorption of a gaseous mixture by various adsorbents, a very interesting aspect is the influence of the way in which the adsorption process is accomplished on the quantity of the adsorbed gases. When the gaseous mixture contains sulphur dioxide and carbon dioxide, the adsorption process can be realized as follows: individual, successive and simultaneous adsorption. The answers are shown in table 1 were we compare to the individual adsorption of sulphur dioxide and carbon dioxide. We can notice regarding the mixture (simultaneous) adsorption that the sulphur dioxide adsorption capacity slightly decreases and that of carbon dioxide increases. As it was expected in the case of the successive adsorption, when sulphur dioxide is initially adsorbed, the sulphur dioxide adsorption capacity is identical to that of the individual adsorption, while the carbon dioxide adsorption capacity increases. It seems that the presence of sulphur dioxide next to carbon dioxide or prior to it favours carbon dioxide adsorption through two mechanisms. The coadsorption of carbon dioxide on the same type of adsorption centres on which sulphur dioxide is adsorbed, a hypothesis supported by the increase in the carbon dioxide adsorption capacity when the mixture of carbon dioxide + sulphur dioxide is adsorbed compared to the individual adsorption. The creation of new adsorption centres, probably Lewis acid centres [16,17], derived from the interaction between sulphur dioxide and the zeolite s surface, favourable to carbon dioxide adsorption. A mechanism of carbon dioxide adsorption on newly created Lewis acid centres through sulphur dioxide adsorption is presented in figure 7. The fact that the carbon dioxide adsorption capacity is identical both in the case of the sulphur dioxide and carbon dioxide mixture adsorption, and in the case of the successive adsorption of sulphur dioxide and carbon dioxide could suggest a saturation of the zeolite in carbon dioxide which leads to the idea that, along with the two mechanisms presented earlier, there is also a phenomenon which blocks the carbon dioxide adsorption through an effect, which can be called the umbrella effect. This means that when the adsorption process occurs, the sulphur dioxide molecule with its angular structure can block the active centres for carbon dioxide adsorption, as the drawing in figure 8 suggests. Blocking the access of carbon dioxide to the adsorption centres through competitive sulphur dioxide adsorption is probable, because sulphur dioxide s kinetic diameter (3.6 Å) favours it compared to carbon dioxide kinetic diameter (4.26 Å) [14,18]. The data obtained from the successive adsorption of carbon dioxide and afterwards sulphur dioxide when the carbon dioxide adsorption capacity has, as expected, the 1074 http://www.revistadechimie.ro REV. CHIM. (Bucharest) 61 Nr. 11 2010
same value as in the case of individual adsorption, and the sulphur dioxide adsorption capacity is lower than in the individual adsorption but is equal to that of the mixture of sulphur dioxide and carbon dioxide, supports the mechanism through which the adsorbed sulphur dioxide creates new Lewis acid centres favourable to the carbon dioxide adsorption (fig. 8). There still remains to explain the slight decrease in the sulphur dioxide adsorption capacity when the mixture of the two gases is adsorbed or in the case of the successive adsorption of carbon dioxide and afterwards sulphur dioxide compared to the individual adsorption. For the HY zeolite, as can be seen in table 1, there are differences between the adsorption capacities but they are smaller. The increase in the carbon dioxide adsorption capacity when it is adsorbed together with sulphur dioxide or successively after sulphur dioxide supports the fact that the presence of sulphur dioxide favours carbon dioxide adsorption by creating new adsorption centres. It is interesting to notice that the increase in the difference between the carbon dioxide adsorption capacities in the presence of sulphur dioxide or following its adsorption compared to the individual adsorption by the two zeolites is proportional to the increase in the sulphur dioxide adsorption capacity by the two zeolites, hence the following ratios are approximately equal: Ca sulphur dioxide by NaY / / Ca sulphur dioxide by HY ~ ~ ΔCa carbon dioxide by Na Y / ΔCa carbon dioxide by HY Therefore, as was expected, based on the two mechanisms, there are smaller differences in the case of the HY zeolite than in the NaY zeolite. This behaviour can be directly correlated with the presence of Na in the NaY zeolite structure, which leads to changes in the chemical structures of the surface and therefore changes in the chemical nature of the active centres involved in the adsorption process. It seems in any case that the presence of sodium is favourable to sulphur dioxide adsorption and unfavourable to carbon dioxide adsorption. Conclusions The experiments carried out with the purpose of studying the adsorption of the carbon dioxide and sulphur dioxide mixture led to the following conclusions: -the adsorption capacity of sulphur dioxide from the mixture decreases with the increase in temperature and is slightly influenced by the presence of carbon dioxide, just in the case of the NaY zeolite; -the adsorption capacity of carbon dioxide from the mixture decreases with the increase in temperature for both of the zeolites and is negatively influenced by the presence of sulphur dioxide in the case of the HY zeolite; -increasing the sulphur dioxide concentration, its adsorption capacity increases in the case of the HY zeolite, having higher values compared to the individual adsorption, and is practically constant for NaY zeolite but lower than the adsorption capacity in the individual adsorption; -from the types of adsorption (individual, mixed and successive) it can be concluded that the presence of sulphur dioxide favours carbon dioxide adsorption both in a mixture and in the successive adsorption following sulphur dioxide; -the presence of sodium in high amounts, within the NaY zeolite, favours sulphur dioxide adsorption. 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