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).