CO Oxidation in Catalytic Hollow Fibres and Granular Catalysts:

: A Comparative Investigation of Catalyst Effectiveness

Abstract

This research focuses on the development of novel catalytic hollow fibres, specifically aiming to demonstrate that catalytic reactions that have mass transfer limitations in the structure of the catalyst support can in many cases be incorporated into a hollow fibre structure with a higher activity, per unit volume and/or per unit mass of catalyst, than packed beds containing the same catalyst. Additionally, the hollow fibre configuration provides additional potential benefits relative to a packed bed such as lower reactor pressure drop, lower rates of catalyst attrition, and in many cases simpler reactor designs as catalyst retention is integral in the hollow fibre geometry and therefore retention screens and a means of providing a preload are eliminated.

Catalytic reactions investigated in this research dealt specifically with airborne contaminants of concern to the aerospace community, specifically ambient temperature oxidation of carbon monoxide and formaldehyde as it applies to closed systems such as spacecraft. However, the application of catalytic hollow fibre technology can be used for any reaction with internal mass transfer limitations and/or where pressure drop is a major consideration.

Candidate catalysts were selected based on a literature review, and the results of catalyst screening trials. Catalyst performance was compared to the baseline platinum on carbon catalyst used by the National Aeronautics and Space Administration (NASA) for trace contaminant control in existing closed loop life support systems. Based on the results of the catalyst screening trials, and research identified in the literature, a catalyst of platinum on titania, manufactured by the author, was selected for evaluation and development of catalytic hollow fibres. A commercially available catalyst, consisting of gold on titania and considered state of the art for the ambient temperature oxidation of carbon monoxide, was similarly investigated for comparison.

The selected catalysts were crushed to produce particles less than 50 microns in diameter and were mixed with various polymer solutions and subsequently used to produce the catalytic hollow fibre structures using common hollow fibre spinning techniques. Polymers used in this research included polyethersulfone and polyvinylidenefluoride which were dissolved in 1-methyl-2-pyrrolidone. Precipitation of the polymer/catalyst spinning mixture was accomplished in a water bath for all samples. Catalyst concentrations investigated ranged from 70-90% (wgt.) in all fibres with the balance of the fibre consisting of polymer. Bore fluid compositions investigated included, 0, 70, 80, and 85% (wgt.) 1-methyl-2-pyrrolidone, in distilled water. The most significant increase in the catalyst effectiveness factor resulted when using a high solvent concentration in the bore fluid, with a corresponding increased porosity of the inner surface of the catalytic hollow fibre.

Multiple catalytic hollow fibres were combined to produce a catalytic hollow fibre module and tested in a differential reactor configuration to determine the internal effectiveness factor of each sample. The experimentally determined effectiveness factors of the catalytic fibres ranged from 0.001 to 0.062. The measured effectiveness factors for the original granular form of the gold and platinum catalysts were 0.058 and 0.040, respectively. In all cases, the internal effectiveness factors for the gold catalytic fibers were considerably lower than the granular form, ranging from 0.001- 0.034, due to degradation and/or poisoning of the gold catalyst during storage and manufacturing. The effectiveness factors measured for the platinum catalytic fibres ranged from 0.016 to 0.062, resulting in a maximum increase in the measured effectiveness factor of approximately 50% relative to the granular form.

In a comparative test of Pt/TiO2 catalytic hollow fibres and granular catalysts with similar internal effectiveness factors conducted at a gas hourly space velocity of 110,400 hr—1, the conversion of the granular form of the platinum catalyst was approximately 20% higher than the catalytic fibres. In the same test, the reaction rate per unit mass of the catalytic fibres was approximately 260% greater than the granular form. These results indicate that the catalytic hollow fibres result in a more efficient use of the catalyst which is expected due to the lower internal effectiveness factors of the granular catalyst for the ambient temperature oxidation of carbon monoxide.

Date of Award	9 Mar 2011
Original language	English
Awarding Institution	University of Bath
Supervisor	Semali Perera (Supervisor)