Mezo-gözenekli MCM-41'in sıcak gazlardan kükürt giderme sorbentlerinde inert taşıyıcı olarak kullanılması

Gases from gasification processes mostly contain H2S, which is a very corrosive and poisonous gas, causes serious problems in applications where they are used. Thus, in many cases it is necessary to reduce the H2S concentration in these gases from percent to ppm level. For example, the allowable H2S concentration for the combine cycle power plants is 20-100 ppm while it is required to be lower than 1 ppm for the fuel cell systems. Removal of H2S form gases is a serious problem and needs to be solved for the fuel cell applications. A lot of studies have been devoted to this subject. In these studies, commonly metals and metal oxides were investigated as adsorbent material for this purpose. It was found from the studies that the suitable metals are Ca, Co, Cu, Fe, Mn, Mo and Zn. Cu and Zn, with their superior properties, are classified as most favorable metals. The pure metal oxides used as adsorbents, however, suffer from evaporation, loss in the surface area and porosity, sintering and mechanical disintegrations that affect their performance and life time adversely. For example, there is a loss problem due to evaporation for metallic Zn at elevated temperatures or there is a surface area loss problem for porous Cu. In order to overcome these problems, pure metal oxides or their admixture are loaded to inert carriers/supports with high surface areas. These are composite materials that consist of an active metal oxide(s) and an inert carrier. The main properties required for carrier materials are to be inert, high surface area, channels of large diameters, good mechanic strength, no phase change during sulfidation-regeneration process and hydrophobicity. Some inert oxides such as Al, Ti, Zr oxides and high silica zeolites are the most commonly used carriers. In this study, an adsorbent was prepared by using copper and mesoporous silica material for the H2S removal from hot gases. CuO was used as the active component and mesoporous MCM-41 was used as the carrier materials. CuO/MCM-41 type adsorbent was prepared by wet impregnation method, in which copper was loaded to the MCM-41 mesoporous material. The silica based carrier was also synthesized as a part of the study. By using wet impregnation method, one CuO/MCM-41 type adsorbent sample was prepared. The adsorbent sample contains 37.46% Cu by weight. The fresh and sulfided adsorbent samples were characterized by using X-ray diffraction (XRD), nitrogen adsorption, chemical analysis, thermal gravimetric analysis (TGA), H2-chemisorption. The H2S removal characteristics of the adsorbent sample from hot gases were investigated by using a laboratory scale fixed bed reactor of 14 mm diameter. The experimental set up consisted of three major sections: 1. gas supply, 2. reactor, and 3. gas analysis unit. Experiments were carried out as sulfidation-regeneration cycles at a temperature range of 823-873K. In these experiments, a gas mixture of H2S (2000 ppm), H2 (20%) and N2 (balance) was fed into the reactor in the sulfidation steps. Sulfidation steps were ended, when the H2S concentration of the reactor effluent reached 20 ppm which was selected as the breakthrough point. Regeneration of the sulfided adsorbent samples were carried out by gas mixtures composed of (5-10%) and N2 (balance). The results of the nitrogen adsorption analysis showed that, the adsorbent sample that was prepared by wet impregnation method, has lower BET surface areas and lower pore volumes than that of the original carrier material, MCM-41. This result indicates that the loaded CuO, partly plugs pores and pore inlets of the carrier. The decrease in the BET surface area in the pelletized samples was approximately 66% for CuO/MCM-41. The drop in pore volume was measured to be 72% for the same sample. The sample was tested for three consecutive sulfidation-regeneration cycles. The average breakthrough point sulfur capacities of the CuO/MCM-41 type adsorbent sample was determined to be 1.89 g S/100 g adsorbent, after three cycles. The XRD and nitrogen adsorption analysis indicated that the structure of the sample was distructed to some extend during the sulfudation-regeneration processes. In spite of this fact, change in the sulfur up take was not comparable and remained rather limited. Keywords: Desulfurization, MCM-41, wet impregnation.Yüksek oranda kükürt içeren fosil yakıtların gazlaştırılmasıyla elde edilen hidrojence zengin gaz karışımları birçok safsızlık içerir. Bu safsızlıkların türü ve miktarı gazlaştırılan fosil yakıtın kaynağına ve türüne göre değişiklik gösterir. Gazların içerdiği H2S, özellikle yakıt hücresi gibi uygulamalarda yakıt hücresi elektrotlarını zehirleme gibi ciddi sorunlara yol açmaktadır. Bu çalışmada, bu tür sıcak gazlardan hidrojen sülfürün (H2S) uzaklaştırılması amacıyla kullanılabilecek yenilenebilir sorbentlerin geliştirilmesi hedeflenmiştir. Yapılan çalışmalar periyodik tabloda yer alan geçiş metallerinin birçoğunun bu amaçla kullanılabileceğin göstermiştir. Ancak,  bu metallerin tek başlarına kullanılmaları durumunda, kükürt tutma kapasitesinin süreç içinde önemli ölçülerde azalması, yüzey alanlarının küçülmesi, termal ve mekanik özelliklerinin yetersiz kalması gibi sorunlarla karşılaşılmaktadır. Bu sorunları çözmek için başvurulan yöntemlerden bir tanesi de bir taşıyıcının kullanılmasıdır. Bu çalışmada, mezo-gözenekli bir malzeme olan MCM-41’ın taşıyıcı olarak kullanıldığı CuO/MCM-41 tipi sorbent geliştirilmiştir. Kullanılan MCM-41 yine çalışma çerçevesinde sentezlenmiştir. Taşıyıcı malzemeye, bakır nitrat kullanılarak yaş emdirme yöntemi ile CuO yüklenmiştir. Numunenin kükürt tutma özelikleri, 2000 ppm H2S içeren, azot hidrojen karışımı kullanılarak incelenmiştir. Çalışmalar sırasında H2S için eşik nokta (breakthrough point) derişimi 20 ppmv olarak seçilmiştir. Deneysel çalışmada elde edilen sonuçlar MCM-41’in sıcak gazlardan kükürt gidermek amacıyla kullanılacak sorbentlerin hazırlanmasında taşıyıcı malzeme olarak kullanılabileceğini göstermektedir. %37.46 oranında Cu içeren sorbentin eşik nokta kükürt tutma kapasitesi 3. kükürt tutma-yenilenme döngüsü sonunda yaklaşık olarak 1.89 g S/100 g sorbent, kapasite kullanma oranı ise yaklaşık %10 olarak ölçülmüştür. Kükürt tutma-yenilenme süreci, sorbentin yapısında bazı değişmelerin meydana gelmesine neden olmakta, ancak sorbent işlevini sürdürmeye devam edebilmektedir.  Anahtar Kelimeler: Sıcak gazlar,  H2S giderme sorbenti,  MCM-41