H-25M-S katalizörlerle MTBE sentezi
MTBE synthesis with H-25M-5 catalysis
- Tez No: 46509
- Danışmanlar: PROF.DR. AYŞE ERDEM ŞENATALAR
- Tez Türü: Yüksek Lisans
- Konular: Kimya, Chemistry
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1995
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 63
Özet
Benzine oktan yükseltici olarak eklenen kurşunlu bileşiklerin, yarattıkları çevre sorunları nedeniyle kullanımları giderek kısıtlanmaktadır. Bunların yerine oksijenli hidrokarbonların, özellikle de metil tersiyer-bütil eterin (MTBE) kullanımı yaygın olarak kabul görmekte ve MTBE talebi hızla artmaktadır. MTBE ticari olarak, asidik bir iyon-değiştirici reçine katalizör üzerinde izobüti- len ve metanolün reaksiyonu ile üretilmektedir. Reçine katalizör kullanımının, 90°C'm üzerindeki sıcaklıklarda kararsız olması, korozyona yol açması ve seçicilik problemleri gibi çeşitli sakıncaları bulunmaktadır. Bu sakıncaları ortadan kaldıracak zeolit katalizörler ile yapılan tarama çalışmasında, HZSM-5 zeolitinin MTBE sentezi için aktif ve seçici olduğu görülmüştür. Bu çalışmada, MTBE üretiminde Amberlyst-15'e alternatif olarak kullanılabilecek ZSM-5 tipi katalizörler incelenmiştir. Bu amaçla, Amberlyst-15'in yanı- sıra, Si/Al oranları farklı bir seri ZSM-5 katalizörü ile 60-90°C aralığında seçilen farklı sıcaklıklarda MTBE sentez deneyleri gerçekleştirilmiş, Si/Al oranı ve sıcaklığın reaksiyonun yürüyüşü üzerindeki etkileri araştırılmıştır. Denenen ZSM-5 tipi katalizörlerden bir tanesi ile, besleme akımmdaki reaktanlarm oranları değiştirilerek yapılan bir seri deney yardımıyla, reaksiyon hızının re- aktan konsantrasyonları ile değişimi de incelenmiştir. Çalışılan tüm katalizörlerin aktiviteleri termodinamik sınırlamaların olmadığı koşullarda, sıcaklık ile artmıştır. SiO2/AI2O3 oranının 50'den 30'a düşürülmesi ile aktivitede gözlenen artış, asit merkez konsantrasyonunun da artmış olması ile açıklanabilmektedir. SiO2/AI2O3 oranı 80 olan HZSM-5 katalizör ile ise beklenenin çok üzerinde dönüşme değerleri elde edilmiş, bu katalizörün aktivitesinin Amberlyst-15'inkine yakın olduğu belirlenmiştir. Asit merkez konsantrasyonu daha düşük olan bu katalizör için, diğer iki ZSM-5 katalizöre kıyasla, çok daha yüksek TOF (asit merkez başına reaksiyon hızı) ve düşük aktivasyon enerjisi değerleri hesaplanmıştır. Asit merkezlerin kuvvetindeki bu artışın, aralarındaki mesafelerin artmış olmasıyla ilgili olduğu düşünülmektedir. Reaksiyon hızının reaktan konsantrasyonları ile değişiminin incelendiği deneyler de ise reaksiyon hızının isobütilenin kısmi basıncı ile arttığı, metanolün kısmi basıncı ile azaldığı gözlenmiştir. Elde edilen verilerden SiO2/AI2O3 oranı 50 olan katalizör için reaksiyonun hız ifadesi r=k PjfreOH " ^5° olarak çıkarılmıştır.
Özet (Çeviri)
The environmental legislation around the vvorld has forced the use of oxyge- nates (alcohol and tertiary ethers) for gasoline blending to phase out lead additives and to reduce reactive evaporative and exhaust emissions. The Clean Air Act Amendments of 1990 have increased severity of emissions li- mits of vehicles and require the manufacture of reformulated gasoline (RFG). According to the program of the Clean Air Act Amendments of 1990 gasoline composition has been changed to include oxygenates. Also the amount of olefins, aromatics and volatile compounds are being reduced. The content of heavy hydrocarbons have also been reduced to meet the performance specifications for ozone-forming tendency and for release of toxic substances such as benzene, formalehyde, 1,3-butadiene ete. The oxygenates commonly used in gasoline blending are methanol, ethanol, methyl tertiary butyl ether (MTBE), ethyl tertiary butyl ether (ETBE) and tert- amyl methyl ether (TAME). The attention vvithin the petroleum refining industry is focused on the question of vvhich ether ör alcohol ör their mixture ör combination of ethers can do the best job of handling reformulated gasoline (RFG) blending vvithin the refinery complex and at low cost. Of ali the oxygenates tertiary ethers are preferred to lighter alcohols because of their lower Rvp and lower latent heats and their full fungibility to petroleum refining and distribution systems. Among ali of the ethers MTBE is the most commonly used ether. Because of lower methanol prices MTBE is a more attractive option than ETBE. The use of MTBE as gasoline additive has been grovving sharply during the past decade in particular since the realization of the Clean Air Act Amendments of 1990. It's reported that in 1993, MTBE was the second largest volume organic chemical produced in the U.S. after ethylene. Some estimates are that by 1995 demand for MTBE could increase at a rate över 25% per year. For a variety of reasons, mainly economical, MTBE has overvvhelmingly become the oxygenate of choice. MTBE demand is forecasted to be tripled (29 million metric tons) by the year 2000.MTBE has lovver blending Reid vapor pressure and lovver vaporization latent heats. it avoids phase separation in the presence of water, vvhich accounts for its full compatibility in the petroleum refining and distribution system. Commercialy, MTBE is produced by the direct addition of methanol to isobutylene in the presence of a strong - acid ion - exchange resin. Bentonites are also knovvn as suitable catalysts for MTBE synthesis. But, the most commonly used catalysts are macroporous ion-exchange resins. The catalyst is knovvn as sulfonated polystyrene divinyl benzene copolymer vvhich is knovvn as Amberlyst-15 ör Dowex commercial names. The most commonly used production method for MTBE is based on the liquid-phase chemical reaction betvveen methanol and isobutene at a temperature in 40-80°C range and a pressure of 1.5 MPa. The reaction is given belovv. CH3OH + (CH3)2 = CH2(CH3)3 COCH3 The reaction is exothermic and the heat of reaction is equal to -37 kJ/mol. Since the reaction is reversible, the maximum conversion is determined by the thermodynamic equilibrium constant. Being an exothermic epuilibrium reaction, at high temperatures rate of reaction increases vvhile at low temperatures high isobutylene conversions are obtained. in the reaction media, temperature should be controlled so that the by products that form at high temperatures can be prevented. The possible side reactions that can take place can be listed as foliovvs: 1-The reaction of isobutylene with vvater to give tertiary-butyl alcohol (TBA). 2-isobutylene dimerization resulting in diisobutylene (DIB). 3-Reaction of methanol with another methanol molecule to give dimethyl ether and vvater. 4-The methanolysis reaction of linear butenes with methanol to give methyl secondary-butyl ether. Another reason why the temperature of the reaction medium should be kept lovv is the instability of the resin catalyst at high temperatures. The sulphonic and sulphuric acids released are corrosive. High methanol/isobutene ratios are necessary vvhich then increases the cost of circulation of methanol. The resin catalyst also has some selectivity problems.Thus, current operating temperatures appear to be limited by three factors: 1 - The instability of the resin catalyst at temperatures above 90°C. 2-Poor selectivity due to dimer formation above 90°C. 3-Equilibrium conversion limitations. Because of the limitations of the resin catalyst given above, a new kind of catalyst group should be developed for MTBE synthesis. For this purpose, zeolites seem to be the most attractive group of catalysts because of their high thermal stability, high catalytic activity, high MTBE selectivity, no dimer selectivity and non corrosive character. Since it will be possible to work at higher temperatures and high space velocities with high MTBE selectivities, use of stable zeolite catalysts will also reduce the reactor size. From the results of the catalyst screening work carried out previously, ZSM-5 was seen to be an effective catalyst for MTBE synthesis. ZSM-5 is a medium pore size zeolite having a three dimensional channel geometry. The Si/Al ratio ranges f rom 10 to infinity. it shovvs a different kind of shape selectivity than other zeolites. Adsorption and catalytic experiments show that crystallographic pore radius is not constant, so molecules having critical diameters greater than 5.5A° can also penetrate into the ZSM-5 channels at catalytically relevant temperatures. HZSM-5 has favorable catalytic and sorbent properties like high thermal stability, high adsorption capacity, adjustable concentration of superacidic sites and pore dimensions well fitted to hydrocarbon reactions. For the reasons given above HZSM-5 is used in petrochemical industry in different processes such as catalytic cracking and ethylbenzene production. The aim of this study was to examine in more detail the performance of HZSM-5 catalyst in MTBE synthesis, vvhich can be used instead of Amberlyst-15. For this purpose, beside Amberlyst-15, HZSM-5 catalysts having different SiO2/AI2O3 ratios (30,50,80) were tested in the temperature range of 60°-90°C at the methanol/isobutylene ratio of 1.00, and the influence of Si/Al ratio on the catalyst performance was investigated. With öne of the catalysts (SiO2/AI2O3=50), the effect of reactant concentrations on the reaction rate expression was also studied. During this study, instead of püre isobutylene, isobutylene in C4 cut was used. The C4 cut contained butene-1 in significant amounts beside isobutylene.The reactions were carried out in a stainless-steed reaction system. C4 and N2 gases were fed from pressurized tanks vvhile methanpl was fed by a dosage pump. The flow rates of the C4 and N2 gases were controlled by fine metering valves. Prior to the reactor (catalyst bed), the reactants entered a preheating section full of quartz, in which the methanol was vaporized, and a homogenized mixture of methanol, C4 and N2 was obtained. Then the reactants entered into the catalyst bed, follovved by a second quartz bed. Ali of the beds (vaporizer, reactor and second quartz bed) were heated with the same furnace. The inside-outside temperature gradient of the reactor was measured by means of two Ni-CrNi thermocouples, the first öne inserted axially in the center of the catalyst bed and the second öne located at the reactor outer surface close to the catalyst bed. The inside and outside temperature gradients were not higher than 2-3°C. The reactor effluent was analyzed by means of a“United Technologies Packard 439”gas chromatograph equipped with a flame ionization detector (FID). A constant volume product sample was injected to the column by a 6-port sampling valve. The column used was Porapak Q having an outer diameter of 0.0032 m and a length of 4 m. As was told before, HZSM-5 catalysts having different SiO2/AI2O3 ratios were compared with the commercial catalyst Amberlyst-15, in this study. Conversions and reaction rates in the absence of thermodynamic limitations, increases with temperature. Amberlysî-15 was seen to be the most active catalyst among the catalysts studied. The HZSM-5 catalyst having a SiO2/AI2O3 ratio of 50 shovved the minimum activity. When SiO2/AI2O3 ratio was decreased from 50 to 30, conversions to MTBE increased in accordance with the higher number of acid sites in the structure. The turnover frequencies (TOF) for these two catalysts were similar. The HZSM-5 catalyst with a SiO2/AI2O3 ratio of 80 gave a much higher conversion and a higher TOF value than the other two ZSM-5 catalysts. For zeolites, activity is a function of acidity. On the other hand, it is knovvn that acidity depends on the acid site concentration (extensive parameter) and the strength of the acid sites (intensive parameter). Hence, the difference in the activities shovvn by the HZSM-5 catalysts with SiO2/Al2O3 ratios of 30 and 50 could be explained by the difference in their acid site concentration, since the strengths of the sites were similar for both catalysts. On the other hand, the low number of acid sites present and the high TOF values obtained indicate that the active sites present in this HZSM-5 with SiO2/AI2O3 ratio of 80 catalyst are stronger than those in the other two HZSM-5 catalysts. This increase in active site strength may be related to theincrease in the distances between the sites, decreasing their interaction or to other structural parameters such as the presence of extra-lattice aluminum species in the catalyst pores. Activation energies of the zeolite having Si02/Al203 ratios of 30 and 50 were in the range of 40-60 kJ/mol, while the activation energy of the catalyst with a Si02/Al203 ratio of 80 was 13 kJ/mol, which is much closer to the activation energy of the commercial catalyst Amberlyst-15, 8 kJ/mol. The rate of MTBE synthesis reaction was observed to increase with the partial pressure of isobutylene and to decrease with the partial pressure methanol in the feed stream. For the catalyst with the Si02/Al203 ratio of 50, the power law rate equation was obtained as r=k P^oh * p°b5° -xii -
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