SORPTION PUMPING OF HYDROGEN BY CRYODEPOSITS--DYNAMIC PUMPING CHARACTERISTICS
Final rept. Apr-Sep 1969
ARNOLD ENGINEERING DEVELOPMENT CENTER ARNOLD AFB TN
Pagination or Media Count:
The sorption for cryodeposited frosts for hydrogen has been investigated both analytically and experimentally. Most of the experimental tests were carried out with carbon dioxide frost at temperatures between 12 and 22K as the cryosorbent and hydrogen at 300K as the sorbate. Dynamic pumping characteristics of the sorbent-sorbate combination were systematically measured for chamber pressures between 10 to the minus 7th power and .001 torr. Additional experimental results were obtained with sulphur dioxide and methyl chloride cryosorbents. An initial hydrogen pumping speed of 30 literssq cm-sec was measured for 12K carbon dioxide frost which was equivalent to an initial effective capture probability of about 0.7. The initial pumping speed appears to be governed by the sorbent, sorbate-surface interaction and may be increased somewhat by forming the surface in a manner to make it smoother but at the consequence of an attendant loss of sorption capacity. The pumping speed of a frost cryosorbent decreases with increasing amount of gas sorbed. A model of the sorption dynamics was formulated. An approximate closed-form solution was obtained in the limit of rather compact frosts whose sorption behavior would be limited by ability of the molecules to penetrate and diffuse into the frost. Comparisons between theory and experiment indicated that the diffusion constant for hydrogen in a variety of cryodeposits varied from 10 to the minus 8th power to 10 to the minus 15th power sq cmsec. Also, it was noted that carbon dioxide and sulphur dioxide frosts deposited at temperatures below 30K appear to undergo some crystal transition if their temperature is increased to above 30K.