Date of Award

1-1-2010

Document Type

Campus Access Thesis

Department

Chemical Engineering

First Advisor

Michael A. Matthews

Abstract

There were many economical and environmental reasons to develop safe method for storing hydrogen in a compact and light weight manner. It was known that complex chemical hydrides were a means to store hydrogen in the solid state at near ambient temperatures and pressures. Hydrides had potential to overcome the technological challenges of storing hydrogen. NaBH4 was the primary focus of this work because it had a lower cost and better stability than other complex chemical hydrides. NaBH4 reacted with water vapor to generate hydrogen and sodium metaborate according to the hydrolysis reaction shown in Eq.(1):

NaBH4(s) + (2+x) H2O(l, g) 4 H2(g) + NaBO2*xH2O(s) + heat. Eq.(1)

Prior studies on the hydrolysis of NaBH4 were mostly limited to low temperatures and pressures. The most critical issue was that the reaction, if conducted in the aqueous phase, requires a large excess of water. Ideal hydrolysis represented by x = 0, but, in practice, the solid by-product (NaBO2*xH2O(s)) can exist with varying degrees of hydration, depending on the reaction conditions. Therefore, the goal of this work wass to decrease x, the excess hydration factor.

Because the hydrolysis of NaBH4 with water vapor had not been studied in depth previously, this thesis provided the first fundamental experiment that conduct the reaction with the different molar ratio of NaBH4 to H2O at elevated temperature in order to improve water utilization (by decreasing the excess hydration factor).

The hydrolysis of NaBH4 with water vapor varying with the molar ratios was run at elevated temperatures (150 ºC, 180 ºC and 200 ºC) and elevated pressure (30 psi) for 180 min. As the temperatures profiles showed, temperatures increased with increasing time and heat source and were controlled by temperature control system. The experimental pressure was a function of the amount of water input, reaction temperature and time. The final experimental pressure increased with x* and time, but it did not increase with increasing temperature. Also, the final experimental pressure corresponded to the theoretical pressure between x=1/3 and x=2 except when the limited reaction water (i.e. x*=0.5).

NaBO2*1/3H2O appeared to be the favored product in all cases with varying amounts of NaBO2*2H2O present. Relative amounts of x=1/3 to x=2 varies as a function of reaction temperature and x*. x* was the value of x if water in the feed was incorporated into the structure water of a hydrated metaborate. Also, the conversion of NaBH4 was up to 99.5 % and the hydrogen generation capacity was up to 7.7 wt% closely related to x* and reaction temperature. For the same T and different x*, the conversion of NaBH4 increased with increasing x*. For the same x* and different T, the conversion of NaBH4 was increasing with decreasing T.

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