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Issue 2 - 2001/02 |
ISSN 1311-8978 |
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STUDY OF THE DRYING ABILITY OF FIBROUS SORBENTS Dimitar T.Gogov University “Prof.Dr.A.Zlatarov”, Bourgas Received 25.11.2003; Cited 22.12.2003
Abstract The drying ability of fibers with high content of silicon dioxide obtained by leaching sodium-silicate glass fibers was studied. The absorbton ability of the sorbents for water was increased due to adjustment of their porous structure by thermal treatment of the fibers at 773 K for 3, 5, 7 and 24 h and chemical treatment with solutions of ammonium fluoride and hydrochloric acid. Under dynamic conditions, the sorbents were found to dry gaseous flows as deep as 209 K. The fibrous sorbents obtained were used also as carriers of drying agents. In this case, dipotassium hydrogenphosphate, sulfuric acid and sulfur trioxide were used as modifying agents. The sorption abilities of the samples were compared to these of other dryers and modified porous materials. Key words: adsorption, porous fibers, dew point, deep drying..
Introduction The presence of moisture in gaseous and liquid technological flows usually leads to deterioration of the qualities of the reagents and the end product, operation regime of production lines, catalyst poisoning, decrease of the fire gases caloricity, etc. [1]. This stipulates investigations aimed to obtain new and more effective sorbents of water. Porous glasses are special kind of glass [2]. Due to their high specific area, they can be successfully used as adsorbents and carriers of various chemical reagents - pigments, ferments, catalysts [3,4]. Special attention is paid to sorbtion-active fibrous forms of the porous glasses. They allow to produce sorbents in forms of cloths, bands, non-woven materials, paper, etc. [5]. These forms predetermine higher effectiveness of the adsorption-desorption processes allow optimizations of the equipment design. The aim of the present work is to study the drying ability of fibers with high content of silicon dioxide and modified fibers obtained from sodium-silicate glass fibers.
ExperimentalVery well dried and homogenized mixture of silica sand and ammonia soda ash was melted in silite oven at temperature of 1700 K. The upper thermal limit of crystallization of the sodium silicate glass was determined by the method of mass crystallization and was found to be 1240 K. This value determined the low temperature at which the fibers were drawn - ca.1300 K. At this temperature and at drawing speed of 2800 m/min, the average thickness of the fiber drawn was ca.14 mm. The solution of the unstable phase of the fibers (leaching) was carried out with 10% solution of sulfuric acid at temperature of 323 K for 90 min. The ratio between the liquid and solid phases was 30:1. The sulfur-acidous solution was decanted, the fibers were washed with demineralized water on a Buchner funnel until neutral reaction and dried at 390 K - sample 1. To adjust the parameters of the porous structure of the fibers with high content of silicon dioxide,the initial sodium silicate glass fibers were treated at 773 K for 3 to 24 h to obtain: n Sample 2 - continuous sodium silicate fibers treated at 773 K for 3 h. The leaching was carried out as for sample 1; n Sample 3 - prepared as sample 2 but the thermal treatment was 5 h; n Sample 4 - as sample 2 but with thermal treatment of 7 h; n Sample 5 - as sample 2 but with thermal treatment of 24 h An attemt was made to modify the porous structure of the fibers with high content of silicon dioxide. They were treated with 4% aqueous solution of ammonium fluoride for 10 min (sample 6) and then with 10% solution of hydrochloric acid for 15 min (sample 7). The fbers were washed with demineralized water and dried at 393 K. The structural characteristics of the fibrous adsirbents were determined by a high vacuum adsorption apparatus. Like porous glasses, the fibrous adsorbents with high content of silicon dioxide are suitable for use as carriers of various reagents. Therefore, the fibers of sample 1 were modified by the method of impregnation with 5, 10, 20 and 50% aqueous solutions of dipotassium hydrogenphosphate (samples 8, 9, 10 and 11) and concentrated sulfuric acid (sample 12). Samples 13 and 14 were prepared by adsorbing sulfur trioxide onto fibrous sorbents. For this purpose, fibers with high content of silicon dioxide (sample 1) were treated at 453 K for 3 h, then a gaseous mixture containing 0.416% sulfur trioxide was flown through them. The gaseous mixture was obtained by bubbling dry gaseous nitrogen (218 K) through 8% oleum with temperature of 353 K - sample 13. Sample 14 was obtained as sample 13 but the preliminary thermal treatment was carried out at393 K. The drying ability of the samples of fibrous sorbents with high content of silicon dioxide obtained was determined under static and dynamic conditions. For these experiments, specially designed sorption devices equipped with quartz balances and hygrometer type “Lena” for determination of the dew point of gaseous flows.
Results and DiscussionThe adsorption isotherm of the fibers with high content of silicon dioxide (FCS) obtained - sample 1 (Fig.1-1) was of first classic type according to [6]. Its shape proved their
Fig.1. Adsorption isotherm for water vapor of adsorbents obtained from sodium silicate glass fibers: 1 - sample 1; 2 - sample 7. micrporous structure: microporous volume 0.188 cm3/g, structural constant B=8.06x10-7, characteristic energy of adsorption 7.1 kJ/mol and pore radius ~0.5 nm. These results showed that the FCS obtained were adsorbents of molecular sieve type. Therefore, they should be especially effective for deep drying (below 245 K) of process flows containing low quantities of moisture. That is why, kinetic studies were carried out at normal temperature (298 K±0.5) and low relative pressure of the adsorbate (water vapor) - P/Ps=0.11. Despite that at this pressure the water vapor pressure in the system was by an order lower than that of saturated vapor, the FCS sorbed significant amount of water ~ 450 mg/g (Fig.2).
Fig.2. Dependence of the quantity of adsorbed substance at temperature of 298 K and relative pressure P/Ps=0.1 on time.
The drying ability of the fibrous adsorbents under dynamic conditions was determined using nitrogen with moisture content 10730 mg/m3 (dew point 285 K) and 3508 mg/m3 (dew point 269 K). The results are presented in Fig.3.
Fig.3. Dependence of the drying ability of fibrous adsorbents on time, preparation conditions and water content of the gas flow:1 - sample 1; 2 - sample 4; 3 - sample 3; 4 - sample 5; 5 - sample 3; 6 - sample 6; 7 - sample 7.
Despite the quite high water content of the inflowing gas (10730 mg/m3, or dew point 285 K), very deep drying was achieved with fibrous adsorbent sample 1 - dew point 235 K, which means that the absolute humidity of the outflowing gas was 148 mg/m3. The break-through time which depends on the adsorbent layer thickness and the gas flow velocity in the adsorber was assumed to be 180 min, despite that up to the 30th minute the increase of the dew point value registered was very low - 237 K or 182 mg/m3. The thermal treatment of the sodium silicate glass fibers had positive effect on the sorption (and, respectively, drying) ability of the fibers with high content of silicon dioxide (samples 2-5). The dew point decreased from 235 to 223 K, i.e. humidity from 148 mg/m3 to 38.1 mg/m3. Probably, the thermal treatment resulted in rearrangement of the glass fibers unstable phase and formation of larger pores after the leaching. Taking into account that the facilities for deep drying process gaseous flows with comparatively low humidity (from 3500 to 2500 mg/m3), the dehydration ability of the adsorbents synthesized was determined for gases with low humidity. Gas flow with dew point of 269 K was subjected to drying. The specific shape of the curves at the beginning of the drying process (up to the 10th minute) was due to the physical adsorption of water and breaking of siloxane bridges formed during the regeneration. As a result from the rebuilding of the hydrate coating, the sorption capacity (and, respectively, drying ability) of the fibrous adsorbents was found to increase. An attempt was made to improve the fibrous sorbents porous structure by treatment with ammonium fluoride and hydrochloric acid. As can be seen from Fig.3, dew point of 239 K (226 mg/m3) was achieved after the first stage of the treatment for sample 6 and 219 K (23.5 mg/m3) after the second stage - sample 7. The lower drying ability of sample 6 can be explained with the nucleophilic substitution of the surface hydroxyl groups with fluorine, which further was partly hydrophobized. The treatment of the fluorinated sample with hydrochloric acid produces hydrogen fluoride. It attacks the silicon-oxygen carcass and increases pore sizes and mainly pore openings. It should be noted here that this treatment preserved the microporous structure of the adsorbent. The shape of the adsorption isotherm (Fig.1) and the values of the structural characteristics calculated for sample 7 (Table 1) can prove it. Table 1. Structural characteristics of fibrous adsorbent after chemical treatment with ammonium fluoride and hydrochloric acid (sample 7)
As a result from the chemical treatment, biporous microstructure was formed with total pore volume higher than that of the initial fibers with high content of silicon dioxide - sample 1. It is the newly formed biporous microstructure which preserved the molecular sieve effect and the higher sorption capacity (higher drying ability) of the fibers with high content of silicon dioxide in sample 7. To obtain effective driers, an attempt was made to use the fibrous adsorbents as carriers of drying agents. Potential modifiers suitable for this purpose were considered to be dipotassium hydrogenphosphate and sulfuric acid. Fig.4 shows the results for the absorption capacity of some of the most frequently used driers. As cna be seen, P2O5 appeared to be one of the most effective drying agents. However, it has a major disadvantage - it rapidly “melts” during the drying process. A syrup-like layer of polymethaphosphoric acid was formed above the unused P2O5 which significantly decreased the drying effect.
Fig.4. Dependence of the amount of adsorbed water on time and absorbent nature: 1 - CaCl2; 2 - concn.H2SO4; 3 - P2O5; 4 - siccacide; 5 – siccapent.
Besides, the use of P2O5, CaCl2 and H2SO4 as drying agents is connected with some technical and technological difficulties like large-sized devices and enhanced corrosion. These disadvantages can be avoided by using”siccacide” and “siccapent”, both products of Merck. According to commercial information, these materials have been prepared by deposing P2O5 and H2SO4 onto a carrier. Hence, after preliminary thermal treatment of fibers with high content of silicon dioxide, they were modified with solutions of dipotassium hydrogenphosphate, sulfuric acid and sulfur trioxide. The studies of their sorption ability for water were carried out under static conditions at 298 K and relative water vapor pressure P/Ps=1. The results are presented in fig.5
Fig.5. Dependence of the amount of sorbed water on the concentration of the modifying agent (K2HPO4) and the temperature of thermal treatment: 1 - sample 9 - 623 K; 2 - sample 8 - 623 K; sample 10 - 623 K; 4 - sample 11 - 623 K; 5 - sample 1 - 393 K; 6 - sample 11 - 393 K.
The effect of modifyer concentration on adsorption ability of the fibers was studied. The highest static sorption capacity for water was observed for the fibers modified with 10% solution of dipotassium hydrogenphosphate. Real equilibrium was established after 24 h and maximal sorption capacities were within the range from 226 to 350 mg/g. However, a qualitative estimation of the properties of the samples studied should be based on the maximal capacity but on the amount of substance sorbed during the initial period of the process. Practically, during the first 4-6 h, the fibrous adsorbent modified with K2HPO4 (sample 9) sorbed about 12-22% more water than the adsorbents siccapent, siccacide, CaCl2 and H2SO4 (Fig.5). The influence of the thermal treatment after the modification was investigated. The highest sorption capacities were found to have the samples treated at 623 K. At this temperature, probably, the dipotassium hydrogenphosphate deposed within the pores was dehydrated to produce pyrophosphate and then P2O5: K2HPO4 à K4P2O7 à P2O5
Fig.6. Dependence of the amount of sorbed water on time and carrier type: 1 - pumice; 2 - silicagel; 3 – zeolite.
Fig.7. Dependence of the amount of sorbed water on time: 1 - sample 12, 2 - sample 13; 3 - sample 14.
The sorption ability of the fibers containing sulfuric acid was also determined under static conditions at 298 K and relative water vapor pressure P/Ps=1. As can be seen from the results obtained and shown in Fig7, samples 12, 13 and 14 showed high sorbtion ability. The following surface configuration was probably formed on the surface of the fibers with high content of silicon dioxide which had adsorbed sulfur trioxide:
This configuration can bind water vapor and, in this case at about the 7th hour of the experiment (Fig.6), provides a sufficient value of the sorption capacity - 102-105 mg/g. Higher capacity (~111 mg/g) was observed for these fibers with high content of silicon dioxide in which sulfur trioxide had been deposited. Thus, by both methods of preparation of samples, the disadvantages of the use of concentrated sulfuric acid as drier were avoided - enhanced corrosion, large-sized devices, high hydrodynamic resistance and risk of labor accidents during operation. The studies carried out showed that fibrous adsorbents were obtained on the basis of sodium-silicate glass fibers. It was found that their structure-sorption characteristics (drying ability) was improved after deliberate thermal and chemical treatments. The modification of the fibers with high content of silicon dioxide with chemical reagents makes them an excellent replacement of the classic driers and avoids their disadvantages. Therefore, the set of fibrous sorbents obtained can be successfully used where fluids must be dried.
References1. Keltsev N.V., Basics of adsorption technology, Moscow, Khimiya, 1984, 296. 2. Aslanova M.S., Myasnikov A.A., Glass state, Leningrad, Nauka, 1971. 3. Berezin I.V., N.L.Klyachko, A.V.Levashev, K.Martinek, V.V.Mozhaev, Yu.L.Khmelitski, Immobilized ferments, Moscow, Vysshaya Shkola, 1987. 4. Toldrla F., N.B.Yansen, G.T.Tsao, Int.Sugar Y.G., Vol.87, Iss. 7, 2000, 321. 5. Zverev M.P., Chaemosorption fibers, Moscow, Khimiya, 1981. 6. De Boev I.H., The structure and properties of porous materials, London, Butterworth, 1958. 7. Gregg S.I., K.S.W.Sing, Adsorption, surface area and porosity, London, Academic Press, 1982.
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