Simulated ILDF Permeate
Before fully applying the above equations to continuous ILDF systems,
aspects of the permeate stream should be considered. Figure 3D showed
the amount of permeate collected with each pass for the various initial
ethanol concentration runs (not all are show for visual simplicity). As
initial ethanol concentration decreased, the amount of permeate
collected increased. Additionally, as the ethanol concentration
decreased with each pass, the amount of permeate collected increased and
plateaued, but did not reach the same level as the plateaus for other
initial concentration runs. For example, 24% initial ethanol
concentration started at 4.5kg/pass and plateaued at approx. 8kg/pass,
15% initial ethanol concentration started at 7kg/pass and plateaued at
approx. 10kg/pass, and 7.5% initial ethanol concentration started at
10.5kg/pass and plateaued at approx. 12kg/pass.
Figure 3E shows the permeate flow rates versus the retentate ethanol
concentration for each pass for various initial ethanol concentration
runs (not all are show for visual simplicity). Similar to the permeate
collected in Figure 3D, flow rates increased as ethanol concentrations
decreased and peak flow rates increased with decreasing initial ethanol
concentration. Specifically, 24% initial ethanol concentration started
at 4kg/min permeate flow and peaks at approx. 9kg/min while starting
with 7.5% ethanol gave 10.7kg/min and peaked with approx. 13kg/min. It
would be expected that a given ethanol concentration would yield a given
flow rate, similar to the ethanol concentration/permeate flow rate
results in Figure 2A (Permeate Flow vs. Ethanol concentration). Instead,
when the initial ethanol concentrations of 24%, 15%, and 7.5% were
reduced to 5%, for example, the flow rates were 7.5, 9.0 and
11.3kg/min, respectively.
This indicates that hollow fiber permeability is not only impacted and
dependent on ethanol concentration during the process but is set by the
initial ethanol concentration exposure and only able to recover a
limited amount of performance. Stated another way, when the ethanol
concentration levels between runs are equivalent in the retentate, it
does not correlate to a specific permeability, but rather permeability
is set by the initial ethanol concentration and will only improve a
relative amount. This aligns with the other plot in Figure 2A (Permeate
flow rate vs. DV). Figure 2A captures the plateau effect, but not the
effect of the initial ethanol concentration since there is only one
initial concentration.
Once the hollow fiber was exposed to the initial concentration in the
Figure 1D set up, the permeability was set and overall performance
limited. Therefore, the Figure 1D set up is not a fully valid
representation of a continuous ILDF process. The Figure 1D set up used
the same single hollow fiber to facilitate all passes/stages for each
run, whereas a continuous ILDF process like Figure 1C would have
individual hollow fibers for each pass/stage. More appropriately, the
permeate values for only the initial ethanol concentration passes should
be used and extrapolated; these values being accurate representations of
the initial ethanol exposures of some of the independent hollow fibers
in a continuous ILDF set up (Figure 1C).