Oxygen Enrichment of Air on Zeolites Modified By Li + and Zn 2+

Transcription

1 Research Paper Oxygen Enrichment of Air on Zeolites Modified By Li + and Zn 2+ Hüseyin Karaca and Murat Teker Department of Chemistry, Sakarya University, Sakarya, Turkey Tel.: ; Fax: * karaca@sakarya.edu.tr Abstract Turkey is one of the world s leading manufacturers of natural zeolite clinoptilolites. The effectively usage of reserves of natural zeolites in the oxygen production process can provide better designing for systems requiring oxygen in industry. The main goal of this paper was to determine the adsorption and oxygen production capabilities of natural zeolites modified by different metal ions. In this study, separation of nitrogen and oxygen of air by adsorption on the clinoptilolite which is natural zeolite and modified with zinc and lithium salts was investigated. Modified natural zeolites were activated at 400 o C thermally. Variation of nitrogen adsorption and oxygen enrichment was investigated by changing pressure, feeding rate and period of gas, cation type on modification. Keywords: Adsorption, Oxygen Enrichment, Zeolite, Zeolite Modification, Clinoptilolite 1. Introduction In numerous chemical processing refineries, metal production and other industrial applications, high purity gas streams are frequently employed for a variety of purposes. For example, high purity oxygen is used in various industries such as chemical processing, steel mills, paper mills and in lead and glass production (Kimberly & Schaub, 1994). The used oxygen was being supplied previously by cryogenic systems (Ruthven, 1984). Investigators have aimed easier, cheaper methods and the production of oxygen at site where it used because the cost of equipment is expensive and oxygen gas needs to transport and to store that is resistant to pressure (Teker, 1991). Most studied and applied method is the adsorption of nitrogen and oxygen of air on various solids by pressure swing adsorption (PSA). In such PSA systems designed for the recovery of high purity oxygen product as the less readily absorbable component of air, such adsorbent bed commonly contains an adsorbent material capable of selectively adsorbing nitrogen as the more readily absorbable component of air. Adsorbed nitrogen is desorbed and removed from bed upon reduction of bed pressure to the lower desorption pressure, as the cyclic adsorption/desorption sequence is carried out in the PSA system (Kimberly & Schaub, 1994). In oxygen production applications, it is desirable to minimize design, fabrication, transportation and equipment costs in order to achieve lower capital costs. Accordingly, it is desirable to use PSA systems and processes that minimise the number of operating components, such as adsorbent vessels, adsorbent inventory, related tanks, valves, compression equipments, process lines. So the costs are minimised by the use of processes that require lower power per unit of oxygen produced (Kimberly & Schaub, 1994). PSA is a process for separating gas mixtures. PSA system has a number of beds (commonly two or three) filled with solid adsorbents. Carbon molecular sieves (Haruna & Hayashi, 1987 and Schroter et al, 1994) and zeolites (Minato & Tamura, 1976 and Earls & Long 1980) have been used as adsorbent to fill the beds. Maximum oxygen product reco- Available online at Page 71

2 very was achieved with reduced size of the compressor by using small beds After the PSA processes developed by Skarstrom (1960 and 1972), it found widespread application on air separateon. Many scientific researches and patented applications for nitrogen and oxygen separation of air were issued (Lee & Stahl, 1973; Smith & Armond, 1974 and Sircar & Zondlo, 1979). Furthermore many mathematical models on gas separation by PSA were also studied (Shendalman & Mitchell, 1972; Mitchell & Shendalman, 1973; Flores-Fernandez & Kenney, 1983 and Hassan et al, 1986). Many countries are applying adsorption processes for oxygen obtaining. The studies of different processes to produce oxygen by adsorption are going on. In this study, it is aimed to produce oxygen by adsorption using clinoptilolites from Bigadiç, Turkey. Zeolites are available plenty in Turkey and should be used as raw material for industrial processes. 2. Material and Methods 2.1. Zeolite Modification Process West Anatolia clinoptilolites-the natural zeolites, were used for this purpose mesh clinoptilolites were chosen from grounded and sieved ones. One portion of natural zeolites were interacted with 2 M ZnCl 2 solution while another portion of natural zeolites were interacted with 2 M LiCl solution in a wash-bottle for 6 hours. After these, zeolites were washed with distilled water, the moisture of zeolites then were eliminated at 120 o C in a stove for 24 hours. They were activated thermally at 400 o C for 4 hours and cooled in a desiccator. Consequently modified zeolites with Zn 2+ and Li + were obtained Zeolite Characterisation The characterisation of zeolites was realized with X-ray diffraction (XRD). XRD analysis was carried out to identify the phases using a Rigaku X-Ray Diffractometer D/ max-2200 Ultima/Pc. It can be clearly seen from Figure 1 that the modification process of zeolite did not damage the zeolitic structure of natural clinoptilolite SEM-EDS Analysis of Zeolites The elemental composition of zeolites was also monitored with Jeol JSM-5410 Scanning Electron Microscope. The zeolite was characterised by scanning electron microscopy (SEM) using energy dispersive spectroscopy (EDS) analysis after the thermal activation at 400 o C and cooled in a desiccator. EDS analysis shows the elemental percentage of the zeolites and proves the ion-exchange in the zeolite cavity (Figure 1) BET Surface Area N 2 adsorption-desorption isotherms at 77 K were measureed by using a Micromeritics Tristar 3000 sorptometer to determine textural properties. Surface area was calculated by using the B.E.T. (Brunauer-Emmett-Teller) equation. Table 1 gives the calculated surface areas according to B.E.T Column Experiments of Oxygen Enrichment The length of adsorption column is 337 mm and inner diameter is 24 mm. A compressor and flow meters were used for supplying air and controlling the rate respectively. Figure 1. On the Left XRD Patterns { > Clinoptilolite-(Na 0.52K 2.44Ca 1.48) (Al 6.59Si 29.41O 72)(H 2O) (100.0%)} and on the Right EDS Spectra of (a) Natural (b)li + and (c)zn 2+ Modified Zeolites Available online at Page 72

3 After the adsorption of nitrogen, air enriched by oxygen from column was sent to a graduated burette for analysis. The modified zeolites used as adsorbent was activated by a vacuum pump between operations. A manometer which is mounted to column was used for measuring the initial gas pressure and the pressure reached after adsorption of nitrogen. Table 1. B.E.T. Surface Areas of Natural (Unmodified), Li + and Zn 2+ Modified Zeolites Zeolites B.E.T. Surface Area (m 2 /g) Natural Zeolite Li 1+ - Zeolite Zn 2+ - Zeolite column is lower than oxygen enriched air by nitrogen adsorption in the column. The optimum pressure for oxygen enrichment was found in this study is 4 atm and it is agreed with literature (Minato & Tamura, 1976 and Teker, 1991). Table 2. Calculated Adsorption Capacity and Oxygen Enrichment Rates Zeolites Adsorption Capacity (cm 3 N 2 /g ) Rate of Oxygen Enrichment (%) Li Zn All the experiments for oxygen enrichment were achieved at room temperature. Initial gas pressure in column, cation type (M 2+ or M + ) on modification, air rate and feeding period were investigated on experiments of separating nitrogen and oxygen of air. For investigating the pressure effect on adsorption, dried air was sent to column in the fixed rate until pressure reached to 1, 2, 3, 4, 5 and 6 atm after the column was filled with activated zeolite. The adsorption of nitrogen of air sent to column by pores of zeolite causes the decrease of pressure. After the pressure decreased, a gas sample in the column was sent to a graduated burette for analysing oxygen percentage. The same method was used for the investigation of effect of air rate and feeding period. Feeding periods at the fixed rates until pressures mentioned above were determined by a chronometer, the percentage of oxygen was then measured in column. 3. Results and Discussion 3.1. The Effect of Initial Gas Pressure to Oxygen Enrichment All the results are shows in Tables 2 and Figures 1-3. The increase of oxygen ratio from air as increasing pressure can as be seen from Figure 2 at the studies of oxygen enrichment of air on the clinoptilolites modified by Li + and Zn 2+ respectively. The increase of nitrogen adsorption and oxygen enrichment of air can be seen till 4 atm of initial gas pressure in the column. The decrease of oxygen ratio as column was filled with more than 4 atm of initial pressure doesn t mean a decrease of nitrogen adsorption. Oxygen enrichment reaches to its maximum value at 4 atm. Also adsorption capacity of zeolite reaches to its maximum at 4 atm. The incoming air into column makes the column diluted after 4 atm. Because oxygen ratio of air coming into Figure 2. The Percentage of Obtained Oxygen on (a) Li + and (b) Zn 2+ Modified Zeolite at Fixed Feeding Period and Different Initial Gas Pressures 3.2. The Effect of Feeding Rate of Air into the Column It can be seen from Figure 3 (a & b) for oxygen enrichment on clinoptilolites modified by Li + and Zn 2+ respectively that the oxygen ratio decreases with increase in feeding rate of air into the column. While produced oxygen ratio is maximum at feeding rate of about ml/sec. The diffusion of nitrogen molecules into the pores of zeolite and the adsorption of air is decreasing at higher feeding rate of air. More efficient oxygen production is realized at lower feeding rate of about ml/sec and at 4 atm of initial gas pressure in column Determination of Adsorption Capacity of Zeolites This study was carried out at 4 atm of pressure and 110 ml/sec of feeding rate of air into column in order to determine adsorption capacity of zeolites which are clinoptilolites from Bigadiç modified by Li + and Zn 2+. For this purpose, g Li + and g Zn 2+ modified zeolites were put into the columns separately. When the pressure in the column reached to the 4 atm, upper valve in Figure 1 was opened providing 4 atm of pressure fixed. Gas samples were then taken to analyse oxygen ratio until oxygen ratio reached to the 21% in the column. Figure 3(c) shows results for Li + and Zn 2+ modified zeolites. Available online at Page 73

4 Figure 3. The Effect of Feeding Rate of Air for (a) Li + and (b) Zn 2+ Modified Zeolite at Fixed Initial Gas Pressures and Different Feeding Rates. (c): Obtained Data for Adsorption Capacity Investigation both Li + and Zn 2+ Modified Zeolites A regression analysis was realized by applying data on Figure 4 to y=aln(x)+b equation. The equations of y = Ln(x) (R 2 =0.9912) and y= Ln(x) (R 2 =0.9959) were found for Zn 2+ and Li 1+ modified zeolites respectively. Adsorption capacities of modified zeolites were calculated by integrating these equations. The obtained data is given in Table Conclusion This study investigated the oxygen enrichment of air by using Li + and Zn 2+ modified natural zeolite. Ion-exchange method is used for changing the ions in the zeolite cavity and ion changing is demonstrated with EDS spectra. Also XRD patterns show that the zeolite structure is not damaged by thermal activation at 400 o C. The results obtained from the column experiments as 4 atm of initial air pressure, ml/sec of air feeding rate are best results in this study. In addition, oxygen enrichment capacity is determined for both modified zeolites (Table 2). The possible reason for being higher of adsorption capacity at Zn 2+ -modified zeolite is that two of Li + ions have taken place instead of one Zn 2+ ion in inner cavity. Consequently Zn 2+ modified zeolite has been adsorbing more N 2 than Li 1+ modified zeolite. Acknowledgement The authors thanks to Prof Dr Murat TEKER for his continuous support in his work. References Earls, D.E., and Long, G.N. (1980) Multiple Bed Rapid Pressure Swing Adsorption for Oxygen. Us Patent 4, 194, 891. Flores-Fernandez, G., and Kenney, C.N. (1983) Modelling of The Pressure Swing Air Separation Process. Chem. Engng. Sci., 38, p Haruna, K., and Hayashi, S. (1987) Process for Producing High Concentration Oxygen by a Pressure-Swing- Adsorption Method. US Patent 4, 661, 125. Hassan, M.M., Ruthven, D.M., and Raghavan, N.S. (1986) Air Separation by Pressure Swing Adsorption on a Carbon Molecular Sieve. Chemical Engineering Science, 41(5), pp Kimberly, A.S., and Schaub, H. (1994) Single Bed Pressure Swing Adsorption System and Process. US Patent 5, 370, 728. Lee, H., and Stahl, D.E. (1973) Oxygen-rich gas from air by pressure using adsorption process. AIChE Symp. Ser., 134, 69, pp Minato, H., and Tamura, T. (1976) Production of Oxygen and Nitrogen with Natural Zeolites. Natural Zeolites, Mitchell, J.E., and Shendalman, L.H. (1973) Study of Heatless Adsorption in the Model System CO 2 in He: Part II. A.I.Ch.E. Symp. Ser., 69(134), p. 25. Ruthven, D.M. (1984) Principles of Adsorption and Adsorption Processes. John Wiley and Sons. Schroter, H.J., Berndt, A.S.-S., Heinbach, H., and Tarnow, F. (1994) Process for Obtaining Nitrogen from Air or Nitrogen-Containing Gases by Pressure Swing Adsorption on Carbon Molecular Sieves. US Patent 5, 275, 640. Shendalman, L.H., and Mitchell, J.E. (1972) A Study of Heatless Adsorption in the Model System CO 2 in He: Part I. Chem. Engng. Sci., 27, p Available online at Page 74

5 Sircar, S., and Zondlo, J.W. (1979) Fractionation of Air by Adsorption. US Patent 4, 013, 429. Skarstrom, C.W. (1960) Method and Apparatus for Fractionating Gaseous Mixtures By Adsorption. US Patent 2, 944, 627. Skarstrom, C.W. (1972) Heatless Fractionation of Gases Over Solid Adsorbents. Recent Developments in Separation Science, 2, pp. 95, CRC Press. Smith, K.C., and Armond, J.W. (1973) Adsorption as a Technique for Gas Separation. In: Prod. Use. Ind. Gases. Conf. Proc. Cryotech. PC Sci. Techn. Press, p Teker, M. (1991) Katı Maddelerde Adsorpsiyon Yoluyla Havanın Oksijen ve Azotunun Ayrılması, Ph.D. Thesis, Istanbul, Yıldız Univ. FBE. Available online at Page 75