Article Cite This: Ind. Eng. Chem. Res. 2017, 56, 14651−14661
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Insight into Liquid Interactions with Fibrous Absorbent Filter Media Using Low-Field NMR Relaxometry. Prospective Application to Water/Jet Fuel Filter−Coalescence Korhan S. Osman† and Spencer E. Taylor*,†,‡ †
Department of Chemistry and ‡Centre for Petroleum and Surface Chemistry, University of Surrey, Guildford, Surrey,GU2 7XH, U.K. S Supporting Information *
ABSTRACT: We describe a low-field NMR relaxometry study of water and aviation turbine fuel (jet fuel) in contact with samples of fibrous media removed from an unused aviation fuel filter cartridge. This media was studied both in its as-received state and after modifying its wettability. The interaction of liquid droplets with fibers is the basis for fibrous coalescence widely used industrially to separate different types of liquid−liquid dispersion. Here, we are specifically concerned with interactions that potentially occur during the removal of entrained water from jet fuels, an application that is of particular importance on safety grounds, primarily to prevent ice formation in aircraft fuel tanks which could otherwise lead to fuel-line filter blockage and fuel starvation to the engines. Jet fuel is treated using various filtration systems, including cartridge-type microfilters and filter−coalescers, at different points between refinery and aircraft fuel tank in order to remove dirt (particulates) and water and ensure that contamination levels always meet the required specifications. The main objective of the present study was to apply proton NMR relaxometry to provide insight into the water coalescence process, specifically by probing water environments on fibrous media. On the basis of T2 (spin−spin) relaxation distributions, it has been confirmed that the water environments depend on the media wettability, with three environments being identified: fiber surface adsorbed water,; interfiber absorbed water, and unassociated (largely bulk) water. The use of such NMR techniques may therefore offer potential for monitoring or improving filter−coalescer performance.
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which will have been subject to several filtration/coalescence processes by the time it reaches the aircraft. In view of the diversity of petroleum source and refinery processing (e.g., straight-run, hydroprocessed, Merox-treated), the compositional complexity of jet fuels is considerable, and it is therefore important that the methods to mitigate contamination, including filtration/coalescence, are as effective as possible. Moreover, postproduction additization is often necessary for jet fuels to meet international specifications. This includes adding, for example, lubricity improver, antioxidant, electrical conductivity improver, metal deactivator, and icing inhibitor, most of which exhibit some activity at fuel−water and fuel−solid interfaces.16,17 Water Separation from Jet Fuels. The presence of water in jet fuel is potentially detrimental to the safe operation of aircraft.18 Water-contaminated jet fuel at freezing temperatures can result in ice being formed in the fuel systems, which on very rare occasions, with extreme levels of contamination, can result
INTRODUCTION Fibrous media have been used extensively for many years in the commercial separation of flowing emulsions, both water-in-oil and oil-in-water.1−9 In the case of “secondary emulsions”, comprising 1 s. The respective relaxation times reflect the increasing entropy of water molecules within the different regions, as seen in the peak positions in Figure 6a. Water Addition to Wettability-Modified Media. Figure 7 contains a comparison of the effects of surface treatment on the water T2 relaxation distributions. Increasing hydrophilicity introduced by treatment with 1000 ppm Petronate L solution is seen to result in the coalescence of peaks W1 and W2, possibly indicating an increased water exchange between these two environments, in contrast to the untreated media discussed above. For the hydrophobized media, however, distinct contributions from all three environments produce a trimodal (W1 + W2 + W3) relaxation distribution. Peak W1 for the hydrophobic surface appears at a shorter relaxation time (0.040 s) than for the untreated surface (0.059 s), suggesting stronger water−surface interactions in the former case. However, the most significant effect is the greatly reduced
In Figure 4, increasing JF addition is seen to result in a steady transition between peak positions for imbibed and bulk states.
Figure 4. Positions of the J1 (circles) and J2 (squares) peaks shown as a function of the volume of jet fuel added to the fibrous media.
Linear extrapolation of the lines in Figure 4 indicates that the limiting “imbibed” J1 and J2 T2 relaxation times are ∼0.03 and ∼0.10 s, respectively, and that the bulk JF distribution is attained upon the addition of 300 μL of JF. However, the concentration dependence of each component is different. The suppression of the aliphatic peak is apparently strongest at low concentrations on the media, such that it is overshadowed by the aromatic signal when surface effects are important. When ∼60 μL of JF is added to the media, the aliphatic signal apparently becomes more dominant in the relaxation distribution, as the aromatic peak approaches a limiting contribution to the total area. Thereafter, further additions show up mainly as contributions to the aliphatic peak. Notwithstanding these uncertainties in the individual contributions from the J1 and J2 peaks in Figure 3a and plotted as integrated area contributions in Figure 3b, it is also evident in Figure 3b that the total integrated area of each relaxation distribution correlates linearly with the volume of JF added to the media. The T2gm values, as shown in Figure 3c, have been found previously to be a good indicator of the overall proton environment.44 Here, it is apparent from Figure 3c that, together with the total area, this parameter also correlates linearly with the volume of jet fuel added to the media. Water Addition to Untreated Media. On adding increasing volumes of water to the fibrous media, the two initial water peaks mentioned above change their positions and relative intensities, each gradually moving to longer relaxation times. This is shown in Figure 5 for two different experiments in which different incremental water volumes are added to the media. In Figures 5a,b, peaks W1 and W2 apparently coalesce at ∼100−120 μL of water addition, the differences in the peaks in this concentration region reflecting inevitable variations in liquid distribution on the media. This region also closely corresponds to the appearance of the W3 peak, indicating saturation of the media. In these relaxation distributions, the position and intensity of W1 are found to be more variable than 14656
DOI: 10.1021/acs.iecr.7b03837 Ind. Eng. Chem. Res. 2017, 56, 14651−14661
Article
Industrial & Engineering Chemistry Research
Figure 5. (a) T2 relaxation distributions for incremental addition of water to the fibrous media (run 1), with the three peak positions W1, W2, and W3 indicated. (b) T2 relaxation distributions for larger incremental volume additions of water to the fibrous media (run 2). (c) Total relaxation distribution peak area as a function of the water volume added. (d) Geometric mean T2 relaxation times, T2gm, as a function of the water volume added. Lines are added to (c) and (d) to guide the eye.
uncertainty in individual measurements to be approximately ±10%. Surprisingly, under these conditions approximately twice as much water is apparently associated with fiber surfaces than is retained within the fibrous structure itself. However, as discussed in connection with JF and water addition to untreated media, the intensity of the faster relaxing peaks, closer to the echo time used, may be subject to more uncertainty. In this case, this applies particularly to the addition of 10 μL of water. Hydrophilic Media. The behavior of media treated with different concentrations of Petronate L is entirely different from the preceding hydrophobic situation. Indeed, the treated media remain highly absorbent toward water and, as shown in Figure 9, produce bimodal (W1 + W2) relaxation distributions for all Petronate L concentrations studied. By comparing data given in Figures 5, 7, 8, and 9, for 20 or 40 μL water additions to untreated, hydrophilic, and hydrophobic media, the total integrated area data shown in Figure 10 have been compiled. This comparison shows that the integrated areas are influenced by the media wettability, and decrease in the order hydrophilic > untreated > hydrophobic. For the media treated with Petronate L, it is also seen that the measured area is highest for 100 and 1000 ppm Petronate L
W2 peak and together with the appearance of a W3 peak suggests that water ingress is particularly restricted into the interfiber region (W2 peak) of the hydrophobized media. The treated media will be considered in more detail below, especially in relation to the mechanism of filtration− coalescence. Hydrophobic Media. With reference to Figure 8, it appears that a significant proportion of the initial water (10 μL) becomes strongly bound to fiber surfaces, which are presumably areas of the media where the hydrophobic treatment may be incomplete. Unlike the usual symmetrical log-normal appearance of ILT-generated relaxation peaks, the asymmetry observed in the W2 peaks in Figure 8 suggests that water in this predominantly hydrophobic environment may be slow to equilibrate, an effect that is not seen in the untreated media. Multiple T2 relaxation distributions (Figure S2, Supporting Information) were obtained following 40 μL water addition to the hydrophobic media in order to provide a measure of the repeatability of the method. Average values (in arbitrary units) for the areas under the three peak regions (with their standard deviations) for four replicates were (9.38 ± 1.24) × 10−4 (W1), (5.35 ± 0.58) × 10−4 (W2), and (4.31 ± 0.46) × 10−4 (W3). On the basis of the standard deviations, we estimate the 14657
DOI: 10.1021/acs.iecr.7b03837 Ind. Eng. Chem. Res. 2017, 56, 14651−14661
Article
Industrial & Engineering Chemistry Research
Figure 8. T2 relaxation distributions for addition of different volumes of water to a sample of hydrophobic media.
Figure 6. (a) T2 positions for peaks W1, W2, and W3 as a function of the water volume added to the fibrous media. The dashed line is the bulk water relaxation T2. (b) Corresponding plots of the respective peak areas, with the dashed line representing the total peak area from Figure 5c. Solid lines on both plots are added to guide the eye. Error bars are obtained from nonlinear curve fits of the deconvoluted data.
Figure 9. Bimodal T2 relaxation distributions for addition of water (20 (light gray) and 40 μL (dark gray)) to media treated with different amounts of Petronate L.
Figure 7. T2 relaxation distributions for 40 μL water in contact with untreated fibrous media and media treated to increase hydrophilic and hydrophobic character.
Figure 10. Total integrated areas for different wettability media in the presence of 20 and 40 μL water (hydrophilic, blue; hydrophobic, red; untreated, gray). 14658
DOI: 10.1021/acs.iecr.7b03837 Ind. Eng. Chem. Res. 2017, 56, 14651−14661
Article
Industrial & Engineering Chemistry Research coatings. These latter concentrations also correspond to a shift to shorter relaxation times for both 20 and 40 μL additions (Figure 9), representing greater water affinity for the media, but also leading to greater uncertainty in the respective integrated area values. Increased affinity for water in the presence of Petronate L has been seen previously, when water condensed and spread over similarly treated coalescer fibers upon cooling in a saturated atmosphere inside an environmental scanning electron microscope.53 By comparison, untreated media showed discrete water droplet formation which exhibited a wide range of contact angles on the fibers, typical of droplet−fiber contacts.54 Liquid Addition to Prewetted Media. Water on JetFuel-Wetted Media. In the practical situation of jet fuel filter− coalescence, the media will be principally wetted by the excess hydrocarbon phase. Dispersed water droplets must therefore interact with fiber surfaces through a jet fuel wetting layer. In Figure 11 are shown T2 relaxation distributions for the addition
Figure 12. T2 relaxation distributions for addition of jet fuel to media presoaked with water (60 μL).
appearing on addition of jet fuel, with the series of relaxation distributions mainly serving to illustrate the degree of variability of individual T2 measurements, perhaps arising from an uneven distribution of liquid phases on the media. However, the overall behavior is the opposite of that found when water was added to the media, and suggests that jet fuel is entering adsorbed and absorbed phase regions. The results of these latter coaddition experiments seem to suggest that the initially applied liquid dictates the effects of subsequently added liquids. Implications for Water Coalescence. As we have already discussed to some extent, wettability is an important factor for the efficient removal of finely dispersed water droplets from hydrocarbons. In the present study, low-field NMR has been able to recognize surface (adsorbed) and interfiber (absorbed) sites for water. Excess liquids behave approximately as bulk materials. Changing the wettability has been shown to cause a redistribution of water between the different environments. In addition, the respective relaxation time distributions suggest that the strength of the interactions may also change. These results provide some confirmation of certain mechanistic aspects reported over the years. For example, the presence of petroleum sulfonate surfactants has been shown by Hazlett to influence the release process from the coalescer, such that socalled “graping” occurs, in which the water is released into the fuel stream as very fine droplets.55 This occurs through an excessive water spreading within the fibrous structure, rather than the existence of discrete (but enlarged) droplets. On the other hand, the hydrophobic case described here showed evidence for a small residual interaction with the fiber surfaces, but with much-reduced incorporation within the interfiber absorption region. Excess water is identified as a bulk water peak. It thus appears that the hydrophobizing treatment was not 100% effective, but in this regard, it fortuitously provided evidence for the coalescence mechanism of media of this type. In particular, it demonstrated that the hydrophobized media is able to retain water without becoming saturated, which could offer advantages as a model coalescer. Future work could therefore involve designing and evaluating the partial silanization of fibrous media as a practical coalescer system.
Figure 11. T2 relaxation distributions for addition of water to media presoaked with jet fuel (180 μL): no water, solid black curve; 5 μL of water, dashed blue curve; 20 μL of water, solid blue curve. The relaxation distribution is also shown for 20 μL of water on untreated media (red solid curve).
of water to media prewetted with jet fuel. Here, it can be seen that 5 and 20 μL of water cause little effect on the magnitude and position of the jet fuel J1 and J2 peaks, with the only notable change being the appearance of the W3 excess water peak at ∼1.6 s for the larger water volume. This indicates that a small volume of water remains associated with the inherently hydrophilic fibers, only slightly affecting the relaxation distribution which largely retains the JF characteristics, but suggests a slight increase in overall intensity. On the other hand, the further addition of 15 μL of water results in the formation of a bulk water peak at ∼1.6 s, indicating that it remains unassociated with the media which would otherwise be expected to produce the red distribution in Figure 11. Jet Fuel on Water-Wetted Media. In contrast to the foregoing, in Figure 2c it has already been seen that the effect of introducing jet fuel (30 μL) to the equivalent volume of water already present on the media is to increase the intensity of the existing water peaks W1 and W2. Thus, in Figure 12, where the media had been prewetted with water to bring the media close to saturation, we also see no evidence for peaks J1 and J2 14659
DOI: 10.1021/acs.iecr.7b03837 Ind. Eng. Chem. Res. 2017, 56, 14651−14661
Industrial & Engineering Chemistry Research
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CONCLUSIONS
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ASSOCIATED CONTENT
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REFERENCES
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In this work, the use of low-field NMR relaxometry has been shown to provide relevant information relating to the behavior of water and jet fuel in contact with fibrous filtration media. When the liquids are applied to such media, three relaxation environments are discernible, each of which is characterized by a different T2 relaxation time. Peaks exhibiting the shortest relaxation times are characteristic of liquid molecules associated with fiber surfaces. Slightly longer relaxation times are a result of liquid present within the fiber network. The slowest relaxation times are due to bulk liquid that is external to the filter medium. Depending on factors such as media wettability, when liquids enter a particular region, the corresponding peak will increase in amplitude (up to a saturation point). Additionally, when the phase entered is the adsorbed phase, the corresponding peak has been observed to shift to a slower relaxation time. When jet fuel is applied to media which has been pretreated with water, the corresponding peak is seen to decrease in amplitude; when the region is the fiber surface, the corresponding peak also shifts to a shorter relaxation time. The use of fibrous filters as water coalescence media to separate water from oil gives additional significance to the observations in this study, for example in condition monitoring. Using this knowledge, it would be possible to observe changes in the relative amounts of water present in the filter media over time, since the relaxation distribution is sensitive to the amount of water present in different environments. It has thus been shown that there is evidence for saturation of the media, which in practice could represent a “disarmed” condition of a filter− coalescer. Undoubtedly, more work is necessary to explore fully the application being considered here in order to understand its relevance to wettability determination in filter/coalescer systems. This work should extend to the influence of the properties of the media itself, including the role of surface relaxation. Additionally, the results of this study have suggested that filtration media could be modified by partial hydrophobization. In the past, other workers have used mixtures of mixed wettability fibers,56 but controlled (partial) silanization may be a convenient route for certain applications.
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b03837. Decay curves for several different CPMG experiments for water on fibrous media; replicate measurements made after 40 μL water addition to hydrophobized filter media (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Spencer E. Taylor: 0000-0001-5528-2103 Notes
The authors declare no competing financial interest. 14660
DOI: 10.1021/acs.iecr.7b03837 Ind. Eng. Chem. Res. 2017, 56, 14651−14661
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Industrial & Engineering Chemistry Research
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DOI: 10.1021/acs.iecr.7b03837 Ind. Eng. Chem. Res. 2017, 56, 14651−14661
