Semester

Spring

Date of Graduation

2011

Document Type

Thesis

Degree Type

MS

College

Statler College of Engineering and Mineral Resources

Department

Chemical and Biomedical Engineering

Committee Chair

Dady B Dadybujor

Abstract

Production of hydrogen via the water-gas shift (WGS) reaction normally requires multiple reaction steps followed by CO2 separation. However, in-situ removal of CO2 via the carbonation reaction between CaO and CO2 provides the opportunity to combine WGS and separation reactions into a single unit. Consequently, the need for heat exchangers between catalyst beds as well as the absorption and stripping units for CO2 removal can be eliminated.;In this study, a commercial high-temperature-shift (HTS) catalyst, Fe 2O3/Cr2O3/CuO is used. Calcium oxide (CaO) and calcium oxide incorporated with calcium aluminate (Ca12Al 14O33) are used as sorbents for CO2 capture. The CaO/Ca12Al14O33 sorbent is synthesized in 75/25 and 50/50 ratios.;Initially, the performance of HTS catalyst alone is studied. The results indicate that the CO conversion increases as the temperature and H2O/CO ratio increase. From these experiments, the forward and reverse kinetic parameters are calculated. The activation energy and pre-exponential factor are found to be 79.01 KJ/mol and 2.44x103 mol/gcatalyst/sec/atm 2 respectively.;Subsequently, the activity of sorbents towards the WGS reaction is measured. The sorbents exhibit minute activity towards WGS and the CO conversions obtained are negligible.;Finally, the combined shift and carbonation reactions are conducted. Conditions are 500°C and 1 atm with a H2O/CO ratio of 2. Two different modes are used: catalyst and sorbent particles well mixed (sorption-enhanced WGS, SEWGS), and catalyst and sorbent particles separated by quartz chips. The SEWGS will proceeds in three phases: sorbent enhancement, sorbent breakthrough, and post-breakthrough. For the HTS-CaO, no CO2 is detected in the reactor outlet till after the 6th minute. This is the sorbent enhancement period. For HTS-CaO/Ca12Al 14O33 (75/25), the sorbent enhancement period lasts for 5 mins. For HTS-CaO/Ca12Al14O33 (50/50), this period lasts for only 4 mins. The sorbents CaO, CaO/Ca12Al 14O33 (75/25), and CaO/Ca12Al14O 33 (75/25) reach their post-breakthrough period at approximately 35, 29, and 20 minutes respectively. The sorbents CaO and CaO/Ca12Al 14O33 (75/25) show better performance in the SEWGS than does the CaO/Ca12Al14O33 (50/50) sorbent.;The presence of any sorbent (CaO, CaO/ Ca12Al14O 33 whether 75/25 wt % or 50/50 wt %) in the reactor as a separate bed does not affect the CO conversion, and the values obtained are same as that of HTS catalyst alone. The CaO, CaO/Ca12Al14O33 (75/25), and CaO/Ca12Al14O33 (50/50) adsorb all the CO2 formed for the first 7, 6, and 3 mins respectively, and finally reach their post-breakthrough period at approximately 45, 36, and 28 minutes respectively.;Sorption capacities are calculated based on both the total weight and on a CaO basis. The CO2 sorption capacities of CaO, CaO/Ca 12Al14O33 (75/25), and CaO/Ca12Al 14O33 (50/50) sorbents on the total weight basis in the separated mode are 8.27, 6.33, and 3.92 mol/Kg respectively. In the mixed mode, the corresponding values are 8.54, 6.36, and 4.16 mol/Kg respectively. The sorption capacity is found to remain relatively constant for each sorbent in the two different modes. On the CaO basis, the CO2 capture capacities of CaO/Ca12Al14O33 (75/25) and CaO/Ca12 Al14O33 (50/50) in the separated mode are 8.43 and 7.84 mol/Kg respectively. In the combined mode, the corresponding values are 8.48, 8.31 mol/Kg respectively. Hence, the CO2 capture capacities of all the sorbents are found to be approximately the same on the CaO basis. From these results, it is concluded that the calcium aluminate (Ca12 Al14O33) acts as a binder only and does not necessarily participate in the carbonation reaction.

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