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بررسي تاثير ارتقاء دهنده¬هاي قليايي خاكي بر پايداري كاتاليستي و تشكيل كربن كاتاليست¬ نيكل مورد استفاده در فرآيند احياء مستقيم آهن

كارشناسي ارشد

مهندسي شيمي

1398

1401/06/30

دكتر مهران رضايي - دكتر سيد مهدي علوي

مهندسي شيمي

در اين پژوهش اثر فلزات قليايي خاكي بر روي¬¬ پايداري كاتاليستي و تشكيل كربن كاتاليست نيكل مورد استفاده در فرآيند احياء مستقيم آهن بررسي شده است. يكي از چالش¬هاي اين فرآيند، نشست كربن بواسطه واكنش بودوارد مي¬باشد كه طي اين واكنش، گاز CO تجزيه شده و كربن تشكيل شده از اين واكنش با مسدود كردن سايت¬هاي فعال كاتاليست؛ باعث كاهش عملكرد كاتاليست مي¬شود. از آنجايي¬كه گاز CO2 خاصيت اسيدي دارد؛ مي¬توان با استفاده از ارتقاء دهنده¬هاي قليايي و افزايش خاصيت بازي كاتاليست، جذب CO2 را بر روي سطح كاتاليست افزايش داد و در نتيجه از پيشروي واكنش تجزيه CO ممانعت بعمل آورد. به همين منظور در مرحله اول اين پژوهش، كاتاليست 10%Ni-Al2O3 بدون ارتقاء دهنده و همچنين همراه با 5 درصد وزني از هر كدام از فلزات قليايي خاكي MgO، CaO، BaO و SrO و با ميزان بارگذاري ثابت NiO (10 درصد وزني) سنتز گرديد. نتايج آناليز TPR نشان¬دهنده بهبود احياپذيري كاتاليست¬هاي ارتقاء يافته نسبت به كاتاليست بدون ارتقاء دهنده بود. با توجه به نتايج آناليز TPO و تست عملكرد كاتاليستي، نمونه با ارتقاء دهنده MgO، با ميزان 19/61 درصد تبديل متان در دماي °C700 بهترين عملكرد را داشت و به عنوان نمونه مناسب انتخاب گرديد. نتايج آناليز TPR حاكي از اين موضوع بود كه افزايش ميزان بارگذاري منيزيم، باعث كاهش احياپذيري كاتاليست¬ها شده است. نتايج TPO و SEM نشان داد كه با افزايش ميزان بارگذاري MgO، نشست كربن با كاهش همراه مي¬باشد. نتايج تست راكتوري نيز نشان داد كه كاتاليست 10%NiO-45%MgO-Al2O3، با ميزان 57/66 درصد تبديل متان در دماي °C700 بالاترين تبديل كاتاليستي را دارد و در نتيجه به عنوان مناسب ترين نمونه در اين مرحله انتخاب گرديد. در مرحله آخر اين پژوهش نيز ميزان بارگذاري¬هاي مختلف نيكل (5/2، 5، 10 و 15 درصد وزني) بررسي گرديد. با توجه به نتايج تست عملكرد كاتاليستي، نمونه¬ها با ميزان بارگذاري 10 و 15 درصد وزني NiO بالاترين ميزان تبديل متان را داشتند. نتايج آناليز TPO و SEM نيز نشان داد كه با افزايش ميزان بارگذاري NiO، ميزان تشكيل كربن افزايش يافته است. در نهايت تست عملكردي اين كاتاليست در شرايطي كه CO2 و H2O بصورت همزمان در واكنش حضور داشته باشند نيز بررسي گرديد و نتايج نشان دادند كه در اين شرايط؛ كربن دي اكسيد در محدوده °C600-500، به دليل واكنش جابجايي آب-گاز هيچگونه تبديل كاتاليستي ندارد. همچنين به علت واكنش متان با بخار آب و با توجه به اينكه در اين واكنش هيدروژن بيشتري نسبت به كربن مونوكسيد توليد مي¬شود؛ در نتيجه نسبت H2/CO بيشتر از يك مي¬باشد.

1401/09/22

Investigation of the effect of alkaline earth promoters on the catalytic stability and coke formation of nickel-based used in the direct reduction process of iron

9/21/2023 12:00:00 AM

The present study seeks to investigate the effect of alkaline earth metals on the catalytic stability and the coke formation of nickel-based catalyst in the direct reduction process of iron. One of the challenges of this process is the carbon deposition due to Boudouard reaction that during this reaction, CO is decomposed and the carbon formed from this reaction by blocking the active sites of the catalyst, reduces the catalyst performance. Since CO is acidic, it is possible to increased CO2 adsorption on the catalyst surface by using alkaline promoters and increasing the catalyst's basicity and as a result, it prevented the progress of the CO decomposition reaction. For this purpose, in the first stage of this research, 10%Ni-Al2O3 catalyst without promoter and also with 5% by weight of each of the alkaline earth metals MgO, CaO, BaO, SrO and with a fixed amount of NiO loading (10%), was synthesized. According to the results of TPO analysis and catalytic performance test, the sample with MgO promoter had the best performance with 61.19% methane conversion at 700°C and was selected as a suitable sample. Then, in the next step, the amount of different loadings of MgO (25, 45, 65 and 85%) with a fixed amount of loading of NiO (10%) was investigated. The results of the reactor test showed that increasing the loading rate up to 25% was almost ineffective and increasing the loading rate up to 45 and 65%, increased the methane conversion, but increasing it up to 85% has the opposite result and has reduced methane conversion. According to the diagrams, the 10%NiO-45%MgO-Al2O3 catalyst had the highest catalytic conversion. Also the stability test of this catalyst was checked and the results showed that all the samples except the 10%NiO-85%MgO-Al2O3 catalyst had good stability. The results of TPO and SEM also showed that with increasing the amount of MgO loading, Carbon deposition is accompanied by a decrease. The results of the reactor test also showed that the 10%NiO-45%MgO-Al2O3 catalyst has the highest catalytic conversion with 66.57% of methane conversion at 700°C, and as a result, it was selected as the most suitable sample at this stage. Then in the next step, the amount of different loadings of NiO (2.5, 5, 10 and 15%) and considering the ratio of one to one for MgO and Al2O3 was investigated. The catalytic activity of the samples showed that the catalyst with 2.5% NiO loading had the lowest catalytic activity and the samples with 10 and 15% NiO loading had the highest methane conversion. The results of TPO and SEM analysis showed that with increasing the amount of NiO, due to the increase in the crystal size of nickel and the decrease in its dispersion, the amount of carbon formation has increased. According to the mentioned results, 10%NiO-45%MgO-45%Al2O3 catalyst was selected as the most suitable sample of this research. Finally, the catalytic activity of this sample was also investigated in the condition that CO2 and H2O are present in the reaction at the same time. And the results showed that in this situation, Carbon dioxide in the range of 500-600°C has no catalytic conversion due to the water-gas shift reaction (in this temperature range, carbon dioxide is produced due to the progress of the water-gas shift reaction in the forward direction).also due to the reaction of methane with water vapor and considering that more hydrogen than carbon monoxide is produced in this reaction; As a result, the H2/CO ratio is greater than one.

كاتاليست، گاز سنتز، ريفرمينگ خشك متان، كربن، ارتقاء دهنده قليايي، احياء مستقيم

Catalyst, Synthesis gas, Dry methane reforming, Carbon, Alkali promoters,

Omid Sattari

Dr Mehran Rezaei - Dr Seyyed Mahdi Alavi