2.4.2. Experimental procedure
After the desired operating temperature inside the ovens is
reached, the heating systems of both the feed pump and compressor
are regulated to a temperature value equal to that inside
the oven. The pressure inside the hydrodynamic apparatus is then
increased to 7 MPa by means of CO2 addition. The pressure will
increase until the point where the desired temperature inside the
ovens returns to its original value. If the final pressure is greater
than the desired operating pressure, then CO2 must be released to
the atmosphere, otherwise carbon dioxide is pumped into the system
until the desired operating pressure inside the hydrodynamic
apparatus is reached. The liquid mixture (≈1200 ml) is fed into the
equilibrium cell. The system is agitated by means of the stirrer and
recycled in countercurrent mode using the gear pumps for approximately
1 h. The intensive mass transfer between both gas and liquid
phases causes a decrease in the pressure of the system, because
the carbon dioxide dissolves in the liquid phase. Thus, fresh carbon
dioxide must be added to the system until the desired state
conditions are reached. Vapor–liquid equilibrium is the necessary
condition to decouple mass transfer from hydrodynamic process.
Once vapor–liquid equilibrium at the desired state conditions is
reached, the mass flow rates of the coexisting liquid and gas phases
are regulated by means of precision potentiometers. The flooding
point is determined as follows: the mass flow rate of the supercritical
phase is held constant while that ofthe liquid phase is increased
until the flooding point is reached. The flooding point was defined
as the point at which the developed liquid layer over the top of the
packing surface reaches 5 cm in height. In addition, a digital camera
has been used to observe the flooding phenomena and to compare
the experimental results with the method that associates flooding
to the point at which the pressure drop starts to have an infinite
slope