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Dispersed Water and Particulates in Jet Fuel: Size Analysis under Operational Conditions and Application to Coalescer Disarming Alisdair Q. Clark Air BP Ltd., Sunbury Business Park, Sunbury-on-Thames, Middlesex, TW16 7LN, United Kingdom
Alastair G. Smith and Steve Threadgold Shell Global Solutions (U.K.), Shell Technology Centre, Thornton, PO Box 1, Chester, CH1 3SH, United Kingdom
Spencer E. Taylor* Chemical Sciences Division, Faculty of Health and Medical Sciences, University of Surrey, Guildford, Surrey GU2 7XH, United Kingdom
bS Supporting Information ABSTRACT: Jet fuel cleanliness, in terms of dispersed water and dirt, is of paramount importance to ensure aviation safety. In this study, a process image analyzer has been used to determine size distributions of dispersed water droplets and real and standard test dust particulates in jet fuel, both in the laboratory and under representative full-scale operational conditions. The technique is also applied to monitoring water droplet coalescence in a filter-water separator in the presence of a surfactant known to cause coalescer disarming, again under simulated operational conditions. The measured water and dirt count (number) distributions are exclusively log-normal, whereas corresponding volume size distributions show deviations from log-normal behavior as a result of contributions from the relatively small number of larger particles or aggregated droplet clusters, highlighting the importance of volume-based size analyses. Distinguishing between dispersed water droplets and solid particles has been demonstrated quantitatively, using a cosolvent to solubilize the contribution from free water droplets.
1. INTRODUCTION Global jet fuel specifications mandate a high level of product cleanliness on safety grounds. Therefore, efficient and effective removal of heterogeneous contaminants from jet fuel is of paramount and ongoing concern to the aviation industry. The measures taken during the various stages of handling jet fuel are designed to remove suspended water droplets and dirt particles and include filtration/coalescence, filtration/absorption, and microfiltration. However, notwithstanding the importance of minimizing fuel contaminant levels, relatively little is known about the size distribution of dispersed water and particulates in jet fuel under operational conditions. Only recently have attempts been made to quantify contamination in jet fuel in terms of particle or droplet number concentrations, using automated particle counters developed for hydraulic oil cleanliness monitoring. The pseudo-logarithmic number scale given by ISO 4406 has been adopted as the standard reporting format.1 Although size distribution information can be extracted from the ISO-based methods, detailed information required for a more fundamental understanding is limited, and this is the principal focus of the present study. Water present in hydrocarbon fuels is described as either dissolved or “free”. The solubility of water in jet fuel is typically in the range of 50100 ppm at 25 °C, but is highly dependent on temperature and fuel composition.2 If the water concentration exceeds the solubility limit at any given temperature, it necessarily exists as a separate phase and is classed as free water. Free water can be present at much higher concentrations than dissolved water. In r 2011 American Chemical Society
turn, free water can either exist as fine droplet “hazes” generated by nucleation processes, for example, by cooling water-saturated fuel,3 or as coarser droplets resulting from dispersion of bulk water in the fuel as a result of turbulence generated, for example, in pumps or valves during handling. The filtration and coalescence processes mentioned above remove the free water, leaving dissolved water unaffected. In general, the relative concentrations of each form of water will increase in the order dissolved < nucleated < dispersed. This is the same order for the respective size scales of the different forms, ranging from molecular through nanometer to micrometer;4 however, specific information in the literature pertaining to the size of dispersed water droplets in jet fuel is limited, especially under operational conditions. Using a centrifugal pump, back-pressure regulator and cooling heat exchanger in a recirculating system, Bitten and Fochtman5 produced fine dispersions in the concentration range of 190990 ppm water in JP-5 fuel with median droplet diameters ≈1 μm (100% < 8.9 μm, although the analytical method for determining the droplet size was not given). On the other hand, Grilc et al.6 produced larger droplet size dispersions in the range of 1356 μm by mixing water and various solvents, including kerosene, in a baffled tank. Stakeholders such as fuel suppliers and filter manufacturers Received: December 20, 2010 Accepted: March 29, 2011 Revised: March 29, 2011 Published: March 29, 2011 5749
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Industrial & Engineering Chemistry Research consider that “secondary dispersions”,7 with diameters less than 3040 μm, are representative of dispersed water in jet fuel, based on this range being the smallest visible with the naked eye. This is one reason why some methods currently employed in jet fuel specifications, that is, visual inspection for “clear and bright”, are not universally regarded by the industry as being reliable measures. Alternatives have been developed for suspended solids, involving filtration of a standard volume of fuel through a 0.8 μm Millipore filter, such as line sampling for gravimetric or colorimetric analysis (ASTM D2276) and laboratory gravimetric analysis (ASTM D5452). However, these alternatives do not discriminate between suspended particulates and water and have been criticized regarding precision and the inability to provide the user with quantification of the number and size of the particles.1 Particulate matter in jet fuels can take different forms. Passage through pipelines in the presence of water inevitably results in the accumulation of metallic corrosion products, typically iron oxides and sulfides. The presence of water interfaces promotes microbiological growth leading to the build up of organic particulates and slime.8 Additionally, exposure to the atmosphere potentially leads to the incorporation of natural and anthropogenic particulates, such as silica, aluminosilicates, pollen grains, and soot particles,9 which can contribute to fuel contamination. In an attempt to introduce improved quantification to the measurement of suspended matter, proposals have been made for the introduction of particle counting methods, such as those used for the condition monitoring of hydraulic fluids, in which particulate levels are also critical to performance.1 It has been argued that the incorporation of particle counting sensors will enable continuous real-time fuel monitoring, which will improve data quality and thereby increase safety and operational performance of fuel systems.1 In addition, strategically positioned particle monitoring units will provide condition monitoring of vital filtration equipment, in order that change-outs are made at precisely the correct times, potentially adding economic as well as safely benefits. In the present study, an image analysis-based instrument, the Visual Process Analyzer (ViPA), has been used to determine water and particulate size distributions in conjunction with a fullscale aviation jet fuel hydrant test rig. As described more fully below, the ViPA uses image capture and analysis to obtain size parameters for individual particles: in this sense, it operates as a particle counter, but it provides more information about the particles than other types of particle counters. Its main applications to date have been based on water analysis (for dispersed oil and particulates), and this is believed to be the first large-scale demonstration of an oil-based application. After describing the experimental approaches, the present paper addresses four objectives: • To demonstrate continuous online particle sizing of jet fuel in an operational context, generating size distributions for dispersed water droplets and standard test dust particulates. A systematic series of experiments is described to evaluate the online particle sizing of a jet fuel stream under representative operational conditions by assessing the effects of fuel flow rate, and water or particulate concentrations. This was achieved using the Shell Hydrant Rig (SHR), a full-scale test rig capable of producing flow rates and pressures typical of those seen in major airport hydrant systems, located at the Shell Thornton Technology Centre, Thornton. • To monitor changes in water droplet size distributions during the surfactant disarming of a filter-water separator
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coalescer element. The effects on the water droplet size distribution during surfactant-induced disarming of a new coalescer are quantified in the SHR, together with subsequent reactivation by water washing. • In a laboratory study, to measure the size distribution of real particulate deposits removed from a spent jet fuel microfilter. • To demonstrate the use of a polar cosolvent to solubilize water in conjunction with the ViPA to distinguish between dispersed particulates and water droplets. This involves laboratory ViPA particle size measurements of (i) dirt particulates extracted from a previously in-service microfilter; (ii) ultrasonically dispersed water in jet fuel; (iii) mixtures of i and ii; and (iv) iii to which 2-propanol has been added to solubilize the free water component.
2. EXPERIMENTAL SECTION 2.1. Materials. 2.1.1. SHR Materials. Approximately 24 000 L of a Merox-treated Jet A-1 fuel was used in this study. As will be described in the following section, it was subject to extensive clean up to remove traces of contaminants, including surfactants, water and particulates. Conductivity was restored to the Jet A-1 specification using 0.5 mg/L Stadis 450 (Innospec Inc., DE). Contaminants used were A2 fine silica test dust obtained from Powder Technology, Inc., South Burnsville, MN, and deionized water. Although the silica test dust is not truly representative of the solid contaminants generally found in jet fuel, it is widely used for qualifying jet fuel filtration systems owing to its particle size characteristics. However, this study has included a brief additional laboratory analysis of real contaminant particles taken from a jet fuel filter. The surfactant used in the disarming study was Petronate L, a fuel-soluble sodium petroleum sulfonate (Sonneborn Refined Products, NJ). 2.1.2. Laboratory Materials. A generic specification F34 jet fuel used for the laboratory test program was supplied by Qinetiq plc, Farnborough, U.K. Dodecane and 2-propanol were obtained from Fisher Scientific. Samples of real particulates were obtained from a 6-in. Faudi microfilter element (0.5.71093/1), which had previously been in service at a U.K. oil terminal supplying a major airport. For comparison as a baseline, an identical unused element was also available. 2.2. Methods and Equipment. 2.2.1. Fuel Analysis: Water and Particulate Concentrations and Surfactancy. Standard industry test methods were used for the determination of water and particulate concentrations in the fuels, and to ensure that the fuel was free from surfactant contamination. Total water concentrations were determined by Karl Fischer titration according to ASTM D6304/IP 438. The presence of free water was indicated by two methods: (i) the reaction between water and a fluorescein-impregnated absorbent pad (Aqua-Glo, Gammon Technical Products, Inc., NJ) according to ASTM D3240, in which the fluorescence intensity is proportional to water concentration (in the approximate range of 048 ppm free water, depending on fuel volume used. Method precision is reduced at higher water concentrations/smaller fuel volumes) and (ii) the visual reaction produced between water and Shell Water Detector capsules, a “Go/No-go” device for determining the presence of finely dispersed free water in jet fuel at concentrations lower than those normally detectable by visual examination.10 A visual calibration chart, not intended for field use, gave an approximate indication of free water content, albeit of lower precision than the preceding method. Particulate concentrations were determined 5750
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Figure 1. General configuration of the Shell Hydrant Rig showing the relative positions of the ViPA unit and bulk sampling point.
gravimetrically by filtering a known volume of fuel through a 0.8 μm Millipore filter, according to IP423/ASTM D5452. Results are reported as mg/L. The surfactancy of the SHR fuel was determined using the Microseparometer (Emcee Electronics, Venice, FL) method, according to ASTM D 3948 in which, under standard conditions, a small volume of water is vigorously dispersed into the fuel and immediately pumped through filtration media. The turbidity of the filtered fuel compared with the original fuel is expressed as a percentage; a value of 100 therefore signifies the ideal case of highly efficient water removal and therefore low surfactancy. 2.2.2. Shell Hydrant Rig (SHR). The SHR, shown schematically in Figure 1, is designed to emulate fuel throughput and filtration capabilities representative of real jet fuel supply operations. Fuel is capable of being delivered at rates up to 4500 L/min through 4-in. lines, giving linear flow velocities up to 9.6 m/s, and turbulent flow regimes (NRe > 4000) above approximately 35 L/min. The fuel storage tank holds up to 30 000 L, and there are two pumps available to cover different flow requirements. The smaller pump is a fixed speed 3000 rpm pump compliant with the requirements of API/EI aviation filtration test protocols, while the larger pump is typical of airport hydrant facilities, with the addition of a variable speed facility to allow adjustment of flow rates. Before any test, this fuel is recirculated at 600 L/min through a filter-water separator (FWS, incorporating fifth Edition API/EI 1581-compliant elements) to remove dirt and free water, and through an optional (attapulgus clay) treatment vessel to remove surfactants. During the test program, the clay vessel was bypassed. Contaminants are introduced into the fuel immediately upstream of the test section fuel pump to ensure maximum dispersion. Samples of the resultant fuel are taken downstream of the pump. The location of the ViPA unit with respect to test circuit is also shown. 2.2.3. Visual Process Analyzer (ViPA). The ViPA is an image analysis-based system manufactured by Jorin Ltd., Leicester, UK.11 Its position with respect to the SHR flow line is shown in Figure 2, which also indicates the configuration of the sample withdrawal pipe perpendicular within the fuel flow. Frames are captured and analyzed at a rate of approximately 10 frames per second (fps). All particles or droplets contained within a certain “focal volume” of each image volume (quoted as 6.315 107 μm3 by the manufacturer) are analyzed in terms of their
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Figure 2. Position of the ViPA sensing unit and SHR main flow line. The inset indicates the sampling system (not to scale).
perimeter and cross-sectional area. The ViPA software then computes size and shape parameters for each imaged particle and determines the corresponding concentration based on the focal volume. The flow rate through the ViPA was maintained within the range 2030 mL/min by adjusting the needle valve on the outlet of the ViPA unit. This ensured that particles did not produce blurred images, miss being counted altogether (flow rate too high), or were counted multiple times (flow rate too low). The quoted particle size is the average Feret diameter (Di, defined as the longest chord in a projection of the object at a given angle, in this case 0°, 45°, 90°, and 135°). The shape factor (SF) is given by SF = 4πA/P2, where A and P are, respectively, the particle cross-sectional area and perimeter. As examples, the respective SF values for spheres, squares and equilateral triangles are 1, 0.785, and 0.605. Fibers with aspect ratios of 0.1 have an SF of 0.26. For each frame containing an imaged particle or droplet, the concentration is calculated as described above and expressed in visual ppm units, ppmvis. In order to determine the mean contaminant concentration in the present study, the summation of the ppmvis data for each image, j, is averaged over the total number of frames, n (some of which may not contain recorded particles) in batchwise mode (see below), according to m
concentration ¼
∑1 cj n
ð1Þ
where m is the total number of imaged particles. Data collection can be made either in batchwise or periodic modes. In batchwise collection, images are captured continuously at ∼10 fps (typically, 7100 frames were imaged in a 10 min period, which has subsequently been used as a conversion factor for converting frame number to time), allowing the collection of up to a maximum of 50 000 particles per test. Periodic collection is best used for trend analysis, especially where particle numbers are high, since the frequency of data capture and the duration over which it is collected can be set. This restricts the size of data files produced, and after each period of collection, average parameters (e.g., diameter and shape factor) are accumulated. The majority of the data in this study has been collected in the batchwise mode. 2.2.4. Analysis of ViPA Particle and Droplet Size Data. The particle size data can be analyzed and presented in different 5751
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ways. The present paper is concerned with several aspects of the particle and droplet data, namely (i) the number concentration (“count”) of particles/droplets and the related size distributions; (ii) the corresponding volume-based results; (iii) the consequent effects of changing experimental conditions on particle and droplet size parameters. Some tests generated a substantial amount of raw data (size, shape and concentration information for numerous particles/ droplets). For particle size distribution analyses, histograms were produced (using Microsoft Excel) using convenient bin sizes, which were optimized largely by trial-and-error (particularly for size data). A size interval of 2 μm was chosen, increasing arithmetically, for generating particle size distributions. For time profiles, particle sizes/distributions were averaged over suitable time intervals. For a given set of conditions, batch data were accumulated for typically 10 min; these data have been treated by histogram analysis using 20s bins, and, for comparative purposes, are therefore reported as “counts per 20 seconds”. Particle counts can also be expressed using “ISO codes” as defined by ISO 4406. Each frame analyses a volume of 6.315 107 μm3 as indicated above; therefore, to sample a 1 mL volume (i.e., 1012 μm3), requires 15,834 frames to be accumulated. Average particle counts per 20s (equivalent to 236 frames) can then be converted into counts per mL and then to ISO codes; where this has been done, range numbers R4, R6, R10, R14, R21, R25 and R30, corresponding, respectively, to >4, >6, >10, >14, >21, >25 and >30 μm, have been used for consistency with the jet fuel specification,12 although the use of larger size ranges is outside the normal ISO 4406 three digit size system (>4, >6, >14 μm). There are numerous ways to express statistical mean values of size distributions,13 and in this paper we use arithmetic mean (D h4,3) diameters, calculated h1,0) and volume-weighted mean (D from the raw data according to the respective equations m
D1 , 0 ¼
∑i ni Di m
∑i ni
ð2Þ
and m
D4 , 3 ¼
∑i ni D4i m
∑i
ð3Þ
ni D3i
Throughout, we have adopted the systematic nomenclature for mean sizes detailed by Alderliesten.13 2.2.5. Test Protocol for Online Sizing of Water Droplets and Particulates in the SHR. The main focus of this part of the study involved establishing the response of the ViPA to different levels of water or particulate contamination in the systematic program set out in Table 1. The SHR was configured as shown in Figure 1, in which two fuel flow rates were selected, 1000 and 3000 L/min. At each flow rate condition, two concentrations each of water and silica test dust were introduced in turn, followed by mixtures of the two contaminants. Water was introduced into the fuel line via a peristaltic pump immediately upstream of the main fuel pump. The test dust was introduced as a suspension (22.5 g dust in 150 L jet fuel) which had been thoroughly premixed by vigorous
Table 1. Summary of the Experimental Program Used for Online Size Analysisa test no.
a
nominal flow rate
[dirt] (mg/L)
[water] (ppmv)
(L/min)
(A2 fine silica test dust)
(distilled)
1.1 1.2
1000 1000
0 0
0 15
1.3
1000
0
45
1.4
1000
0
15 (rpt)
1.5(baseline)
3000
0
0
1.6
3000
0
15
1.7
3000
0
45
1.8(baseline)
1000
0
0
1.8 1.9
1000 T 3000 1000
0.15 f 0.3 0.3 f 0.15
0 0 f 15
1.10
1000
0.15
1.11(baseline)
3000
0
15 0
1.11
3000
0 f 0.15
0
1.12
3000
0.3
0
1.13
3000
0.15 f 0
0 f 15 f 0
Arrows indicate changes of conditions during the indicated test.
stirring in a hopper vessel. Again, this contaminant was also introduced at the appropriate rate via a dosing pump immediately upstream of the main test section fuel pump. Before the start of a series of tests the stored fuel was passed through the filter/water separator (FWS) and clay treatment vessel to ensure that it was substantially free from surfactants, water and particulates. After restoring the conductivity to the Jet A-1 specification as indicated earlier, the suitability of the test fuel was then confirmed by standard Microseparometer, Karl Fischer titration, Aqua-Glo, Shell Water Detector, and Millipore analyses (see section 2.2.1.). The tests were of two types. For a given set of conditions, particle size data were collected batchwise typically over a 10-min period. Additionally, the response to condition changes was investigated by continuous data acquisition, in sequence tests. 2.2.6. Effects of Surfactant-Induced Coalescer Disarming on Water Droplet Size. A different SHR configuration was necessary to evaluate the effects of surfactant addition on the droplet size characteristics, and the modified experimental arrangement is shown in Figure 3. A FWS vessel containing a new API/EI 1581 (5th edition) compliant filter/coalescer element and hydrophobic separator element was positioned immediately upstream of the ViPA sampling point. A different pump was used to produce a fuel flow rate of 200 L/min through the system, this being lower than the previous rate owing to design constraints of the FWS vessel. The FWS was then disarmed by flowing surfactantcontaining fuel through the system in a single-pass operation, the fuel passing into a segregated storage tank to be cleaned up at a later stage to avoid surfactant contaminating the whole system. 2.2.7. Laboratory Particle and Droplet Size Analyses (a). Real Jet Fuel Particulates. This aspect of the study focuses on real particulates extracted from a 6-in. Faudi microfilter element (0.5.71093/1), which had previously been in service at a U.K. oil terminal supplying a major airport. Samples were taken from a position approximately 10 cm above the base of the filter (which was mounted vertically in vessel when in service). Comparisons were made with a new element sampled in the same way. Microfilter elements of this type comprise outer and inner filtration layers. Samples (1 cm 1 cm squares, weighing 5752
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Figure 3. Configuration of the Shell Hydrant Rig used in coalescer disarming tests.
∼40 mg) of each layer were taken from both new and used elements and subjected to the same protocol. The filter media squares were suspended in dodecane (40 mL) in clean 50 mL glass jars (previously tested and found to be free from significant particle counts, i.e.
