Gördes klinoptilolit rezervinin NH4+ değişimi ve CO2 adsorbsiyonu yardımıyla karakterizasyonu
Başlık çevirisi mevcut değil.
- Tez No: 46507
- Danışmanlar: PROF.DR. AYŞE ŞENATALAR EMİNE
- Tez Türü: Yüksek Lisans
- Konular: Kimya Mühendisliği, Chemical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1995
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 52
Özet
ÖZET Zeolitler son yıllarda endüstride iyon değiştirici, adsorban ve katalizör olarak yaygın bir şekilde kullanılmaya başlanmışlardır. Günümüzde 40 'a yalan doğal ve 200 civarında da sentetik zeolitin varlığı bilinmektedir. Genelde sentetik zeolitler daha fazla kullanılmaktadırlar. Doğal örneklerin zeolit içerikleri ile içerdikleri zeolitin fiziksel ve kimyasal özellMerinin rezervden rezerve ve hatta aynı rezerv içinde bile değişebilmesi, yaygm olarak kullanımlarını engellemektedir. Fiziksel ve kimyasal özellilerinin karakterize edilmesi ve gruplanduılması ise hem maliyeti arttırmakta hem de uzun ve titiz bir çalışma gerektirmektedir Bu çalışma da temelde bu nokta üzerinde kurulmuştur. Gördes yöresinden çıkarılmış olan farklı oranlarda klinoptilolit içeren numunelerin iyon değişimi ve C02 adsorpsiyon kapasiteleri belirlenmiş, klinoptilolit içeriği, iyon değişimi ve adsorpsiyon kapasiteleri arasındaki ilişkiler incelenmiştir. Adsorpsiyon deneyleri, hazırlanan Na ve H formlarına uygulanmıştır. Elde edilen izotermlere Dubinin-Astakhov (D-A) model denklemi uyarlanmış ve bu denklemin karakteristik parametreleri hesaplanmıştır. Klinoptilolit içeriği ve adsorsiyon kapasiteleri arasındaki üişkinin kuvvetli olduğu dolayısıyla güvenilirliğin yüksek olduğu gözlenmiştir. Aynı şekilde iyon değişimi ve klinoptilolit içeriği arasındaki ilişkide de çok yüksek korelasyonlar elde edilmiş ve kullanılan bu iki metodun zeolit içeriğinin saptanması için uygun olduğu belirlenmiştir. Mayk Mıgırdiç Dindisyan Haziran, 1995
Özet (Çeviri)
CHARACTERIZATION OF GÖRDES CLINOPTILOLITES BY NH4+ EXCHANGE AND C02 ADSORPTION SUMMARY The term 'Zeolite“ was created by Cronstedt (1756) from the Greek 'to boil”and“stone”words, for minerals which expel water when heated and hence seem to boil. They are very common and well known as fine crystals of hydrothermal genesis in geodes and fissures of eruptive rocks, or as micro-crystalline masses of sedimentary origin. All commercial applications of natural zeolites make use of one or more of several physical or chemical properties, including (1) ion exchange, (2) adsorption and related molecular sieve properties, (3) dehydration and rehydration, and (4) siliceous composition. These properties, of course, are functions of the specific crystal structure of each individual zeolite species and of their framework and cationic compositions. A natural zeolite is framework alumino-silicate whose structure contains channels filled with water and exchangeable cations; ion exchange is possible at low temperature (100 °C at most) and water is lost at about 250 °C and reversibly readsorbed at room temperature. As is well known, the primary building units (PBU) of the structures of silicates are the T04 tetrahedra, where T is mainly Si. In framework silicates, these PBU are linked so as to form a 3-dimensional framework and (nearly) all oxygens are shared by two tetrahedra; the sharing coefficient is hence 2 or a little smaller. If all tetrahedra were centered by Si, and the sharing coefficient were 2, the chemical formula of the framework would be Sin02n; if there is some Al in tetrahedra the formula is [AlmSin_m02n]m\ the m negative charges being balanced by the extraframework cations, usually K, Na, Ca, less frequently Li, Mg, Sr and Ba. Natural zeolites contain a mixture of elements in the framework, as exchangeable cations and as impurities. Extensive ion exchanges were VIperformed on clinoptilolite in solutions of alkali-metal or alkaline-earth-metal cations and radioactive Cesium (Cs137). Ion exchange isotherms for binary systems have been determined for several cation combinations. The following selectivity series for binary systems with clinoptilolite was evident: 2+~XTo-N>\,f“2+ K+>NH4+>Ca2+sNa+>Mg: Most of the above studies utilized either chloride or nitrate solutions of the cations in concentrations ranging from 0.05 to 1.0 N at temperatures ranging from 25 to 65 °C. Full exchange was reported to occur within a few days at these conditions. Some results, however, indicate far less success at similar conditions. Crystalline zeolites are also unique adsorbent materials. Under normal conditions, the large central cavities and entry channels of zeolites are filled with water molecules forming hydration spheres around the exchangeable cations. If the water is removed, usually by heating to 350 °C or 400 °C for a few hours or overnight, molecules having effective cross-sectional diameters small enough to pass through the entry channels are readily adsorbed in the dehydrated channels and central cavities. Molecules too large to pass through the entry channels are excluded, giving rise to the well-known ”molecular sieving“ property of most zeolites. Because of the uniform size of the rings of oxygens in their framework structures, zeolites have relatively narrow pore-size distributions, in contrast to the wide range of pore sizes of other commercial adsorbents, such as silica gel, activated alumina, and activated carbon. Many zeolites can therefore act as selective adsorbents or ”molecular sieves“ separating gaseous molecules on the basis of size, shape, and surface selectivity. The surface area available for adsorption ranges up to several hundred square meters per gram, and some zeolites are capable of adsorbing up to about 30% of their dry weight. Most of the surface area is found within the zeolite structure and represents the inner surface of dehydrated channels and cavities. Only about 1% is contributed by the external surface of the zeolite particle. VllIn addition to their ability to separate gas molecules on the basis of size and shape, the unusual charge distribution within the dehydrated void volume due to the presence of cations, hydroxyl groups, and field gradients generated by the substitution of aluminum for silicon in the framework, allows many species with permanent dipole moments to be adsorbed with a selectivity unlike that of almost any other adsorbent. Thus, polar molecules such as H20, C02, and H2S are adsorbed preferentially over non-polar molecules, and adsorption processes have been developed using natural zeolites to remove C02 and other contaminants from impure natural gas and other methane streams to give almost pure CH4 products. Adsorption in the microporous channels of clinoptilolite is strongly affected by the internal electrostatic fields that envelop the entire pore space. The contributions of the framework ions, charge-balancing cations, and adsorbate molecules to these internal fields are all important and interdependent. The theory of volume filling of pores is consistent with this pore-adsorbate interaction concept. The theory of volume filling is ideally suited to gases and vapors adsorbed at temperatures between the normal boiling and critical points. Model parameters determined from a measured isotherm within this region can be used to predict isotherms at other temperatures equation : The model isotherm is given by the Dubinin-Astakhov (D-A) q=q0exp[-(A/E)n] The model parameters are the limiting adsorption value (q0), the characteristic free energy of adsorption (E), and the structural parameter (n). The parameters E and n are temperature invariant for a given sorbent/gas combination. As was mentioned before, zeolites have obtained widespread applications in industry, agriculture and environmental protection. Synthetic zeolites of ZSM-5, A, X, Y, L, Omega, Zeolon (Mordenite) and natural minerals such as mordenite, chabazite, erionite, phillipsite and Vlllclinoptilolite are the zeolites which are commercially utilized, primarily as adsorbents, catalysts and ion exchangers. Although several zeolite minerals such as clinoptilolite, mordenite and phillipsite are available in mineable deposits having relatively high purity, extensive application has not yet been achieved. This problem arises from the fact that zeolite ores contain a variety of minerals such as unreactive volcanic glass, quartz, feldspar, calcite and clay minerals. Moreover, zeolitic rocks vary in mineralogical composition and particle size from deposit to deposit. In addition, the zeolites themselves may vary chemically from deposit to deposit and even in the same deposit. Thus, before a particular use, a zeolite reserve must be thoroughly characterized with respect to its chemical composition, crystal structure and other physical and chemical properties that will be useful in the proposed applications. Estimation of the zeolite content, which is the most important measure of the performance of a natural sample is especially important. The most commonly used method for the estimation of zeolite contents of natural zeolitic samples is the semi-quantitative XRD technique. In a previous study, the ion-exchange and adsorption capacities of a set of samples taken from the Bigadiç reserve were shown to be related to the clinoptilolite contents linearly. The purpose of this study is to characterize the Gördes clinoptilolite deposit via the investigation of the variation of ammonium exchange and CO2 adsorption capacities of a set of samples with different zeolite contents and to relate the capacities to the zeolite content. Selected natural samples with different zeolite contents taken from the Gördes reserve were ground to 90-180 pm and were washed with water in a soxhlet extractor for 24 hours for the removal of soluble impurities. Repeated ion exchanges with 1 M NaCl solutions of 80 °C were performed, followed by exchange with 1 M NH4CI solutions. The samples were then analyzed for their NH/ contents by Kjeldahl method. To investigate the dependence of adsorption capacities on clinoptilolite content, Na and H forms of the samples were used. C02 adsorption capacities were measured by using a volumetric glass adsorption system. Prior to the experiments, the samples were activated by heating at 400 IX°C and 10”5 mbar for 6 hours. The sorption isotherms were then measured up to 100 kPa. Dubinin- Astakhov model was applied to the isotherm data. The correlation coefficient of the relationship between the ion exchange capacities and zeolite contents was observed to increase with the number of ion exchange treatments. The coefficient obtained after the fifth treatment was r = 0.9645, which is quite high. Stronger relationship was observed between the adsorption capacities and zeolite contents for the samples in their H forms (r = 0.9512) than for those in their Na forms (r = 0.9156). Finally linear relationships were developed for the estimation of the zeolite contents of the samples from Gördes reserve, from the measurement of their ion exchange and adsorption capacities, which can be used with a reasonable confidence level. Mayk Mıgırdiç Dindisyan June, 1995
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