New expressions for the maximum spoutable height and spout pressure drop ratio are developed for fine particles spouted in water. The maximum spoutable height in both water and air systems is found to be dependent upon two dimensionless parameters, β and ALim. For fine particles spouted with water, the dimensionless pressure drop ratio depends only upon the voidage, ϵ at the bottom of the spout.
Supercritical
CO2 (sCO2) removes both water
and extractives from wood chips and flakes at 60 °C. The water
appears to be mostly displaced by sCO2 because its nominal
concentration in sCO2 exceeds its solubility limit. SEM
imaging and contact angle measurements show no major differences in
surface properties between sCO2-treated and thermally dried
flakes, which suggests that their interaction with resin should be
similar. An economic analysis for the removal of water and extractives
from pine flakes for the manufacture of oriented strand board shows
that sCO2 treatment is potentially much more cost-effective
than thermal drying from both capital and operational perspectives.
The main reasons are that the water is removed by displacement rather
than through evaporation, environmental control costs are drastically
reduced, and the extractives removed represent a value stream instead
of pollutants whose emissions need to be controlled. Because the sCO2 is largely recirculated, the process is greener than conventional
direct dryers that generate CO2 from the combustion of
wood fines used as a fuel.
This work explores the use of carbon dioxide, water, and their mixtures as solvent for the precombustion beneficiation of raw coal without using any toxic mineral acids in the temperature range of 200–400 °C. The fluid polarity, ionic constant, and supercritical point can be adjusted by H2O/CO2 ratio and temperature. Adding carbon dioxide to hydrothermal fluid also increases the ionization by forming carbonic acid. Extractions with supercritical fluids have several benefits including enhanced mass transport, ease of separation and recycle, wide range of extractive capability and tunability, better inherent safety, and in the case of carbon dioxide and water–low cost. A semi-continuous extraction system was designed and built in which pressure, temperature and the relative flow rates of CO2 and H2O can be controlled. Coal powder is kept in a packed bed and the extraction is carried out at 143 bar pressure. Using sulfur as a model heteroatom, extractive efficiency is examined as a function of the temperature, fluid composition, fluid flow, and extraction time. The results indicate that carbon dioxide, water, and supercritical water-carbon dioxide (ScWC) all can effectively extract about 50% of total sulfur from bituminous coal in 1 h. Extraction above 350 °C decreased effectiveness, and extraction above the supercritical point of pure water caused hydrothermal carbonization. ScWC extraction may provide necessary control to prevent organic dissolution while removing sulfur.
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