Vapour residence time
The effect of vapour residence time on organic liquid yield is relatively well understood although the interaction of temperature and residence time is less understood. Studies by Diebold et al., Liden et al and Knight et al. have attempted to interlink both primary liquids formation and secondary cracking, but one essential component which is neglected is the variation of water yield with temperature and residence time. It is believed that at temperatures below 400*C, secondary condensation reactions occur and the average molecular weight of the liquid product decreases. Boroson et al. have demonstrated that the average molecular weight decreases with the degree of secondary reaction, i. e. increasing residence time and temperature (10).
For chemicals, it is considered necessary to "freeze” the process at the appropriate time-temperature point in the envelope to maximise yield. In one case this has led to a commercial reactor design where vapour residence times down to 90 ms are claimed.
Fuels have less specific process requirements and most work has focused on maximising liquid yield rather than product quality. The window for fuel production requires more R&D to better understand the processes and match the product quality requirement to process parameters. There is no definition of product quality in terms of physical or chemical properties or composition, and this area will need to be addressed as more applications are tested and alternative supplies of biofuel-oil become available.
Long vapour residence times and high temperatures (> 500*C) cause secondary cracking of primary products reducing yields of specific products and organic liquids. Lower temperatures (< 400 eC) lead to condensation reactions and the subsequent formation of lower molecular weight liquids which can also react.