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Ind. Eng. Chem. Res. 2003, 42, 636-645
Detection and Monitoring of Oil Foams Using Raw Capacitance Data Daniel Pacho*,† and Graham Davies Department of Chemical Engineering, UMIST, P.O. Box 88, Manchester M60 1QD, U.K.
Electrical capacitance measurements have been used to detect the presence of oil foams and study their behavior, aiming for a better understanding of these systems and leading to the development of an improved foam control methodology. With the use of electrical capacitance, it was possible to identify the air/foam and foam/liquid interfaces. It was also demonstrated that accurate monitoring of the liquid content in the foam was possible. Data from capacitance measurements were processed to obtain information on the 2-D distribution of liquid in the foam as a function of time and also to recognize the main processes taking place during foam collapse. The application of capacitance measurements to the study of foam collapse has yielded promising results that encourage its further exploitation in the development of a novel methodology of foam control. 1. Introduction The formation of stable oil foams is a significant industrial problem in the production of oil and gas. During primary separation of petroleum, severe foaming could impose serious operational restrictions. Stable oil foams complicate the liquid level control inside the separators, require longer residence times, and could result in liquid being carried over into the gas stream, which presents potential damage to downstream equipment such as compressors. In many distillation and absorption processes, unwanted foams can cause the loss of throughput, reduction of separation efficiency, and consequent decrease in product quality. Many crude units in refineries use preflash drums to improve the performance of distillation columns, but severe foaming can take place and produce significant losses in distillate products. Foaming has been identified as the second leading cause of tower flooding in refineries,1 and foaming problems have also been reported in crude-oil process plants in the U.K.,2 Kuwait,3 and the U.S.4 Poindexter et al.5 pointed out that, depending on the nature of the oil crude and the type of separation scheme used, foaming problems can restrain crude separation and lead to unexpected process shutdowns. To address this problem, different techniques have been tested, but most of them have relied on extensive use of antifoam agents, and so their success has been limited because of the lack of detailed studies of oil foam dynamics. In recent years, important research efforts have been focused on the development of multiphase flow meters (including foam detectors) suitable for use in industrial environments. However, as new progress has been made to meet current needs, more demanding goals have being set. Through these developments, it has been observed that higher efficiency levels required better process units as well as improved instrumentation. Then, to monitor and control processes at the high * To whom correspondence should be addressed. † Current address: Facultad de Ingenierı´a Quı´mica UADY Av Jua´rez 421 Cd Industrial 97288 Me´rida Yucata´n, Me´xico. E-mail:
[email protected].
standards set by the industry, new techniques had to be investigated, and as a result, more accurate and detailed measurement techniques have been introduced. Under these circumstances, the use of tomographic techniques, originally developed for medical purposes, has proved valuable in the monitoring of multiphase systems. This is mainly because such techniques, in particular electrical methods such as capacitance and conductance methods, offer reliability and versatility at a relatively low cost, especially if they are compared to other techniques such as magnetic resonance imaging (MRI),6 nuclear magnetic resonance (NMR) spectroscopy,7 or X-ray- and γ-ray-based techniques. Capacitance measurements are not new in the oil industry, as they were used to determine local concentrations of solids in fluidized systems in the early 1950s. Similarly, capacitance techniques have been used for more than 30 years to measure water content in oil.8 However, the application of capacitance techniques to the detailed monitoring of processes requires fast and precise measurements of minor capacitance changes, yet the measurement of capacitance at very low levels (picofarads or femtofarads) and high sampling rates is not a simple task. Thanks to recent advances in circuitry design and improved measurement procedures, these techniques have been used more frequently to study different process systems.9-11 Because of their nature, electrical techniques in process monitoring and control applications offer important opportunities that are yet to be exploited at a large scale in the process industry. 2. Sensor Design for Multiphase Systems and Foam Detection There are several references in the literature about the detection of foams and froths, but almost all of them are devoted to aqueous systems.10-14 The fact that the permittivity of water is very different from that of a gas simplifies the problem of foam detection in aqueous foams. In oil systems, the permittivity of the foams is very similar to that of empty space; thus, foam detection is complicated because the difference between dry foam and air is very slight. This problem is accentuated on the gas/foam interface because the first stage in detect-
10.1021/ie020387+ CCC: $25.00 © 2003 American Chemical Society Published on Web 01/10/2003
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ing foam is the correct identification of its boundaries, i.e., the gas/foam and liquid/foam interface positions. 2.1. Detection of Interface Positions in Multiphase Systems. Important efforts have been made in the application of electrical measurements to detect interface positions.13-20 In most cases, the systems used a variety of sensors distributed in columns in a probelike fashion, making it easier to detect interface level positions. Devices designed for single-level detection are commonly known as level transducers. Such devices normally use capacitance, optical, pressure, displacement, ultrasonic, or radiation techniques to perform the measurements. Level transducers based on the capacitance measurement principle have been applied in industry for many years,15 and many capacitance transducers available in the market are intended for singlephase capacitance detection. However, the application of single-level capacitance sensors to multiphase detection leads to ambiguous readings. This problem can be eliminated by using multiple capacitance sensors forming vertical sets of identical electrodes distributed along the whole measuring length of the system. The main problems associated with such designs are external electromagnetic noises, fringe effects, and stray capacitance. To reduce external electromagnetic noises, Huang et al.16 proposed an extension of the outside guard between the electrodes, and although this configuration required more manufacturing effort, it did guard the electrodes more effectively. Fringe effects can be reduced with electrode guards surrounding the sensing area. Although stray capacitance cannot be eliminated, it can be minimized, and the easiest way to do so is to keep the cable length to a minimum and to improve the circuitry design. Different electrode configurations can be used to detect interface levels. Wang et al.17 proposed a segmented capacitance sensing array that contained thin electrode strips in a comb-shaped configuration that was suited for monitoring interfaces in sludge/water/oil/air systems. In this design, electrodes were used either as source electrodes or as detection electrodes in alternate fashion. A high-frequency signal was applied to the source electrodes while a capacitance to the voltage detector transformed the capacitance measurement into a voltage signal that was enhanced before being fed to a computer for processing. The number of electrodes had to be larger than the expected number of interfaces, and in fact, the higher the number of electrodes, the higher both the accuracy of the measurement and the resolution obtained. The experimental results presented showed that this array was effective in detecting different phases and interface levels. The interfaces sludge/water and water/oil were well-defined, whereas the interface gas/oil was also visible but produced only slight signal variations. The authors did not provide any information about foam detection. Hutzler et al.13 used multiple capacitance sensors forming vertical sets of identical cells to monitor drainage in aqueous foams. In this array, the identification of each phase was simply made by interpretation of individual capacitance values. The pairs of sensors were equally distributed along all phases, and the resulting set of output signals allowed for capacitance profiles to be obtained. Yang et al.18 proposed a multi-interface level measurement system for oil separators that consisted of 64 segmented source electrodes and a single detection electrode. The source electrodes were arranged
in a single column and were mounted in a printed circuit board (PCB) facing a single detection electrode. The signal from the detection electrode consisted of two parts. One part was a stationary signal (DC) representing the absolute capacitance between the source electrode and the detection electrode. The second part was a fluctuation signal (DC) related to the existence of the foam. The analysis of data showed that the different phases coexisting in the separator could be detected. The foam layer produced a fluctuating capacitance signal that was different from other fluctuating signals such as circuit noise. Hence, the foam layer could be identified, but no other characteristics of the foam could be learned from this fluctuating signal. Shi et al.19 designed a sensor array that was also applied to multi-interface systems containing foam. A multiplexer was used to drive the electrodes sequentially, so in each measuring cycle, every electrode had three different roles, either as a source or detection electrode or as a guard if connected to ground. Only one electrode at a time was used as a source with the adjacent electrode being set as the detection electrode; all remaining electrodes were grounded. The interfaces were detected by comparing the reading obtained at each level with the maximum possible change for the system. The instrument used a system that produced two signal components, so the interface level detection was done using the steady-state component while the dynamic capacitance measurements were used for foam detection. When the dynamic signal from an electrode just above the air/foam interface was analyzed, the AC measurement produced a markedly high signal. The experimental data showed that the foam layer produced a characteristic signal as a result of the unstable nature of the foam. However, the data obtained were not specifically correlated to obtain information about the foam itself. 2.2. Detection of Foam Structure Using Capacitance and Resistance Measurements. Several applications of capacitance-based systems are aimed at detecting bubble size distributions in bubble columns and flotation systems. Bennet et al.20 demonstrated that electrical capacitance tomography (ECT) could be used to identify flow regime in bubble columns and that bubble size detection was possible in homogeneous flows. Thus, the capability of the technique to provide relevant measurements for monitoring and control of low-capacitance systems was analyzed. Wang and Cilliers14 used electrical resistance tomography (ERT) to visualize the internal foam structure. They showed that ERT was a suitable method for the monitoring of areas of nonuniform foam density caused by differences in bubble size distribution. Corlett21 evaluated the performance of an ECT in flow pattern and void fraction determination in multiphase flows containing oil and water. Their study showed that ECT was suitable for flow pattern identification at low water levels and that slug and stratified regime detection was possible even at higher water contents. In this study, the identification of bubbly flows was not accurate and only bubble flows of low water fraction could be visualized. Schu¨ller et al.22 developed a probe to detect multiple interphase level in oil applications that is commercially available. They presented a system that relies on single-electrode detection for each measurement, and in so doing, they avoided short circuits that can occur when conductive water-continuous dispersions are present. The evalua-
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tion of the probe under controlled conditions showed that it operated effectively in high-pressure hydrocarbon/ water systems and that level positions could be controlled on the basis of its readings. Jaworski et al.23 developed a portable probe to monitor the dynamics of multiphase flow inside multiphase separators. Their probe performed measurements on the basis of a chargetransfer principle originally developed for tomography measurements. The probe included 24 electrodes embedded into a metal case and protected with resin to insulate the electrodes from the fluid media. The probe was tested at a production site and placed into a phase separator to monitor level positions. The measurements obtained matched well with the readings from a level controller installed at the site and demonstrated a potential for use in control for industrial applications. In recent years, interesting developments have occurred in the study of foam behavior, and electrical measurements have been used regularly.9,12,13 As a result of these developments, laboratory equipment such as FOAMSCAN, which uses, among other techniques, electrical measurements to study foam dynamics, is now available in the market. However, the inclusion of foam sensors in control applications require a more flexible structure. In this paper, a simple yet practical approach is presented in which capacitance measurements can be employed to detect and monitor foam occurrence in low- or nonconducting media of industrial relevance such as oil foams. 3. Theory of Foam Stability and Drainage Foams are dispersed gas bubbles separated by a network of thin liquid films, called lamellae, and capillaries, called plateau borders (PBs). Foams are created when a liquid and a gas are processed together and the bubbles formed remain trapped in the liquid by the stabilizing action of a surface-active agent (SAA) present in the liquid. Oil foams can remain stable for long periods depending on the type of stabilizing agent present in the liquid and on the method of foam production, but in general, very stable foams require the presence of good surfactants and small bubble sizes.24 When the mechanism that generates the foam (gas sparging, gas condensation, stirring, etc.) ceases to operate, the foam starts to decay. Foams degenerate mainly because the liquid in the foam drains out as a result of the combined action of many forces acting on the foam at different times and under different circumstances. The phenomena that induce liquid to drain are gravity, the tearing of liquid films, and bubble coalescence. Meanwhile, these phenomena are counterbalanced by the stabilizing action of PB suction, the opposition of the Gibbs-Marangoni effect, the bulk and surface viscosity of the liquid, and other interfacial forces.25 In newly created foam, the bubbles are rounded and separated by a network of capillaries, which are thick and contain most of the liquid available in the system. As time passes, the liquid drains out, and the capillaries thin, thus allowing the bubbles to come closer and eventually touch each other. Hence, the original rounded shape of the bubbles changes to polyhedral as the bubbles grow by coalescence and diffusion, and the initially fast liquid flow slows and finally ends. The liquid continues to drain by gravity until the capillary pressure in the foam becomes strong enough to coun-
Figure 1. Distributions of the sensors in the sensing panel. The separation between columns in the same row was 1 mm. The distance between consecutive rows was 11 mm.
terbalance the gravitational pull.26 When a critical PB thickness is reached, the capillary pressure halts the liquid drainage, and then the gas can diffuse through the lamellae and the bubbles can start to grow. During this period, the larger bubbles grow at the expense of the smaller ones, and so, the stress on the lamellae increases. The lamellae can stretch only as long as there is enough liquid to extend the surface, but the continuous stress on the thinned lamellae would eventually break them, allowing the bubbles to coalesce. When a cluster of bubbles coalesces, the mechanical equilibrium in the foam is modified, and widespread coalescence is favored, thereby inducing the collapse of the whole foam structure. 4. Experimental Work 4.1. Design of Capacitance Sensors for Foam Monitoring. The design of sensors for foam monitoring constituted the first stage of the experimental work. Because of size restrictions on the production machinery, the dimensions of the boards were set as maximum 500 mm × 250 mm. A guard electrode of 50 mm was kept around the sensing area to isolate the electrodes from external disturbances. Previous designs have demonstrated that guard electrodes with dimensions similar to those of the sensing electrodes were generally good enough for this purpose.27 The remaining area was divided into equal-sized electrodes distributed in rows and columns so as to obtain information from both vertical and horizontal directions. A double-layer printed circuit board (PCB) was used to minimize electrical interactions between connections and sensors. Tracks leading to individual sensors were placed behind the sensing board and terminated at the edge of the board to provide soldering points for the connection of signal wires. Figure 1 shows the sensor distribution in the sensing board. A copper sheet of 2-mm thickness with dimensions equal to those of the sensing board was used as the source electrode. This board was divided into electrode stripes of equal size to promote a more uniform electrical field distribution. Each source was excited with a square-wave excitation operating at a switching frequency of 500 kHz. Guard electrodes for the backsides of both source and sensing boards were manufactured
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Figure 2. Simulation of the electrical field inside the capacitance sensors. The equal-potential lines and the field distribution are shown for an oil medium.
using copper sheets of 0.5-mm thickness. Both guard electrodes were connected to the sensing and source boards using single connections at the top of the boards. As a complementary step in the design work, a simulation was carried out using the Maxwell Field Simulator from ANSOFT, to estimate the electrical field distribution likely to be obtained in the PCB. A number of simplifying assumptions were introduced to perform the simulation. One of the main assumptions was to consider the electrical field within the boards as two-dimensional. This assumption was possible because the cross section of the board consisted of segmented detectors facing a common source electrode. With this assumption, a simplified model of the system could be built, so that relatively fast numerical simulations of the electrical field were possible. Under these conditions, three different scenarios were analyzed. The first case considered the situation of an empty cell. For this scenario, the background material was set as air (low-permittivity medium, r ) 1). The second case corresponded to a capacitor filled with oil (r ) 2.5), where both the detection electrode and the source electrode were in contact with thin oil layers. The analysis of the simulation showed that the equalpotential lines were evenly distributed inside the capacitor plates and that the electrode guards eliminated most of the distortions (Figure 2). The field distribution indicated that, closer to the center of the capacitor plates, the field lines became almost vertical, whereas near the edges of the plates, the lines showed slight distortions. A simplified model of wet foam was the last case simulated. The foam was simulated as gas bubbles
arranged to form a closely packed tessellation in a rhomboidal dodecahedron arrangement. In this way, the empty space between the bubbles had the properties of the background (already set as oil), and so, a structure equivalent to wet foam would fill the capacitor. The representation of the foam by this model offered one main advantage: because the largest distortion on the electrical field was expected from foam at its wettest point (i.e., when the PBs were thicker and the gas bubbles more rounded), the results of the simulation would be more meaningful at wet conditions. The results of this simulation showed that the equalpotential lines were again evenly distributed inside the capacitor plates. Even though the electrical field was slightly more distorted in this case compared to the previous cases, it was still homogeneous. This analysis introduced a better understanding of the field distribution and the capacitance measurements expected, and so, it helped to identify possible areas for improvement. With this knowledge, it was easier to improved the design previous to its manufacture. 4.2. Experimental Methods. Figure 3 shows a schematic diagram of the rig used to carry out measurements of oil foam dynamics. Foam stability measurements were performed using electrical capacitance sensors placed inside a low-pressure cell. This cell was made of 6-mm-thick Perspex with general dimensions of 580-mm height × 260-mm width × 50-mm thickness. During the design stage, the capacitance calculations considered a gap of 20 mm between the sensors and the source electrode. This distance was selected because it made possible the generation of foam beds wide enough to minimize wall effects on drainage and not so narrow
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Figure 3. Schematic diagram of the experimental rig.
that they consisted of just a few bubble layers. For this reason, the average initial bubble size in the foams was kept smaller than 3 mm. To achieve this condition, a sparger was made using sintered glass no. 4 (16-40µm pore size), and the gas flow rate (up to 5 L/min) was adjusted to suit the experiments using a flow meter. For the creation of the oil foams, Catenex oil (a kerosene-based solvent from Shell International); light crude oil, 30.4 °API, kinematic viscosity 7.3 cst (supplied by BP-Amoco), and heavy crude oil 25.3 °API, kinematic viscosity 32 cst (supplied by Petrobras S.A.), were used. The surface-active agent (SAA) selected was Fluorad FC-740 (50% w/w) supplied by 3M (Manchester, U.K.). For gas production, nitrogen (oxygen-free) and CO2 from BOC, U.K., were used. Capacitance measurements were conducted using a modular capacitance measuring unit (MCMU) manufactured by Process Tomography Ltd. (Chesire, U.K.). Aiming to produce foams of different characteristics, both the gas flow rate and the SAA concentration in the oils were varied. Solutions of oil with added SAA were prepared to cover a wide range of concentrations near and far from the critical micelle concentration (CMC), which was determined using a Processor Tensiometer KL2 from Kru¨ss GmbH, Hamburg, Germany. Solutions at 0.025, 0.05, 0.075, 0.10, 0.15, and 0.20% w/w concentrations were prepared, and surface tension was measured sequentially following the ring method using 50 mL of solution. With the meter, it was possible to carry out series of measurements; thus, for each sample, the surface tension was measured five times, and the average value is reported. Following this method, the CMC for the system Catenex/SAA was found to be 0.075%. For the system light crude oil/SAA, the CMC was found to be 0.040%, and for the system heavy crude oil/SAA, the CMC was found to be 0.030%. To obtain different foam types, the size of the bubbles and the SAA concentration were changed. To do this, the flow rate was adjusted by means of a flow meter, to produce three different bubble sizes for each SAA concentration. Table 1 presents the conditions used for foam creation for Catenex/N2 experiments. The influence of the SAA on the oil capacitance was measured by comparing the capacitance of solutions containing
SAA conc (% w/w) 0.05 0.05 0.05 0.10 0.10 0.10 0.15 0.15 0.15 0.20 0.20 0.20
gas flow rate approximate (L/min) bubble size (mm) 1.4 1.0 2.0 3.0 3.8 8.0 1.2 1.0 1.8 3.0 3.5 8.0 1.2 1.0 1.8 3.0 3.5 8.0 1.2 1.0 1.6 3.0 3.2 8.0
SAA concentrations below and above the CMC with the capacitance measured for pure oil. In all cases, the capacitance detected was the same; thus, no contribution to the total capacitance was observed for the presence of surfactant. A computer program to control the data acquisition and to provide the user with visual information on the process of foam collapse was written. One of the main features of the program was the capability of controlling the multiplexer module of the MCMU to perform sequential readings. Two possible sequences for reading data were available: a vertical sequence and a horizontal sequence. The horizontal sequence allowed data to be collected for up to two rows and served to calibrate sensors. Calibration was done in a row-wise fashion aiming to monitor foams of known liquid content. To do this, the oil inside the cell was adjusted to a level just below the row of sensors under calibration, and then, small foam beds were created to cover only one row of electrodes. Archimedes’ law provided a simple procedure to estimate the liquid content of all foams created. The change in the liquid level produced as a consequence of foam formation was related to the total volume occupied by the foam, as expressed by the equation
φl )
liquid going into the foam volume occupied by the foam
(1)
Taking into account the geometry of the cell, the equation can be simplified to yield
φl )
∆h hf
(2)
where ∆h measures the decrease in the liquid level during foam generation and hf is the final height of the foam. For this reason, to quantify liquid content, the only requirement was to record the liquid level in the cell at two points, just before and just after foam production. The vertical sequence was used during normal operation; in this way, capacitances were read vertically, in ascending order starting with the right column (electrodes 1-6), continuing with the central column (electrodes 7-12), and ending with the right column (electrodes 13-18). Figure 4 shows the appearance of the software interface for vertical scanning during one experimental run. 5. Results and Discussion 5.1. Experimental Relation between Foam Capacitance and Liquid Content. The key idea in this
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Figure 4. Computer interface used to control the capacitance readings in a draining foam. In the experiment shown, the bottom row of sensors (S6, S12, and S8) is submerged in oil, while rows R2-R5 are facing foam (different colors denote variation of foam wetness). The topmost row of electrodes (S1, S7, and S13) is in contact with air.
work is that the value of the capacitance measured is proportional to the liquid content in the foam. To correlate the capacitance measured to the actual liquid content in the foam, the capacitance needed to be corrected to account for the presence of oil layers on the surface of the boards. These oil layers formed when foam came into contact with the sensing board, especially during foam collapse and for very viscous oils. The sensors detected this layer as the presence of foam, and for this reason, its contribution needed to be subtracted from the total capacitance to avoid overprediction of foam wetness. A correct calculation of the expected capacitance for different foam wetnesses required the construction of the equivalent capacitor formed by the presence of foam inside the sensors. To do this, the relationship between the capacitance and the oil content in the foam as predicted by the Bruggeman equation28 for the permittivity of nonuniform mixtures was used. Taking into account the size of the sensors as well as the contribution of the oil films to the total capacitance, the theoretical capacitance could be estimated. When comparing the capacitance calculated with the capacitance measured, some differences were detected, especially at low liquid contents, where the Bruggeman equation is less valid. Despite these differences, the theoretical capacitance values were close to the experimental results and provided a good guide to the estimation the liquid content in the foam. The experimental capacitances measured had to be mormalized if meaningful data were to be obtained. Normalization was done in the following way
C f - Ca C ˆ ) C o - Ca
(3)
where C ˆ is the normalized capacitance, Cf is the capacitance measured in a given sensor, Ca is the capacitance of the empty cell (0% liquid), and Co is the capacitance of the same capacitor completely filled with oil (foam with 100% oil). It was observed that the
Figure 5. Calibration curve for the relation of normalized capacitance to liquid content in oil foam. The maximum foam wetness reached was 11%.
normalized capacitance accounted for the same change in the foam capacitance measured, even if the readings were taken from different electrodes. Thus, it was found that, for any electrode in any row i or column j, the variation in the capacitance measured ∆(Cij - Ca) over a given time interval, when normalized, was constant; that is
(
)
∆(Cij - Ca) ) constant Co - Ca
(4)
∀ i such that 0 < i e 6 and j such that 0 < j e 3 When the normalized capacitance was plotted against the corresponding liquid level calculated, a clear relation was found. The calibration curve is presented in Figure 5. All points in the curve include corrections due to the presence of the oil layer. 5.2. Drainage Profiles of Oil Foams. Figure 6 shows a typical drainage profile obtained through electrical capacitance measurements. The drainage profiles in this figure present the capacitance readings
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Figure 6. Drainage profiles from standing foam made of Catenex and CO2.
obtained in a single sensing column in a free drainage experiment. The profiles shows all of the stages in the lifetime of the foam, as well as several characteristics of the collapse process. From this figure, the time taken by the rising foam to reach the top of the sensing board as well as the time elapsed until the collapse of the foam bed can be determined. In Figure 6, the profile that shows the highest capacitance values corresponds to sensor number 12, which was situated just above the liquid interface. As expected, it was in this zone that liquid concentrated in larger amounts, and so, the capacitance detected was higher. Correspondingly, electrode 1 gave lower signals, as it was in this zone that the foam was at its driest point. During foam generation (between times 0 and 220 s), it can be seen that, as the foam bed was rising, the liquid
tended to drain out because of gravity; however, this tendency was counterbalanced by the upward flow of the gas. When the gas flow rate was shut down (at t ) 230 s), the liquid started to drain out freely, allowing the profiles to acquire their characteristic shape. 5.3. Influence of Production Conditions on Foam Stability. The influence of the production conditions on foam stability can be appreciated from Figure 7, where four different drainage profiles are shown. All profiles correspond to readings from the electrodes situated at the central column of the detection board and were obtained in experiments 2, 3, 5, and 6 as described in Table 1. The profiles in Figure 7a and b were produced using the same surface-active agent (SAA) concentration but different flow rates. Because the flow rate used for foam creation was lower, the foam in Figure 7a had a smaller bubble size; hence, its liquid content was higher, and its collapse time was longer. This trend was seen again in the two profiles shown at Figure 7c and d, where the SAA concentration was maintained while again the flow rate was varied to increase the bubble size. The total collapse time was once again longer for the case of the foam of smaller bubble size (Figure 7c). The influence of surfactant concentration on foam collapse is also illustrated in this figure. By comparing the profiles, we can observe that, for the same bubble size (Figure 7a and c), the collapse time increased when the SAA concentration increased. For each case, higher concentrations of surfactant induced longer collapse times. Figure 8 present the results obtained for the whole set of experiments Catenex/N2. Collapse times are reported for experiments having similar gas flow rates (Vg). This figure shows that, for solutions near the
Figure 7. Drainage profiles for foams under different production conditions: (a) experiment 6, (b) experiment 5, (c) experiment 2, (d) experiment 3. (For production conditions, see Table 1.)
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Figure 8. General trends for the whole set of experiments described in Table 1.
Figure 9. Variation of liquid level during foam generation observed during calibration of sensors in rows 5 and 6.
critical micelle concentration (CMC), there was very little change, if any, in the collapse time. 5.4. Detection of Position Variations on the Foam/Air Interface Level. During calibration experiments, it was noted that small liquid level variations were clearly recorded with electrical capacitance (Figure 9). In this figure, the profile at the top corresponds to oil capacitance as recorded by electrode 12 in the 6th row of the sensing row. Similarly, the profile at the bottom of the figure shows the foam rising and later collapsing as recorded by electrode 11 in the 5th row. Thus, this figure shows that, while the foam was rising, the liquid level was decreasing exactly at the same rate. It must be considered that, in that experiment, the liquid level lowered just 2 mm. Therefore, it can be said that detection of the foam/liquid interface within a short range could be performed with this technique. An additional point of interest is the speed of the readings: even though the period of foam formation was short (it lasted for only 50 s), it was still possible to track the liquid level variation. The position of the interface foam liquid was varied to observe the response of the technique to changes in the rate of interface variation. Liquid was poured into the cell until the first row of electrodes was covered; at that point, the recording of capacitance started. More liquid was poured into the cell using different flow rates thus changing the liquid/air interface position at different speeds until the next row of electrodes was covered. As shown in Figure 10, throughout the experi-
Figure 10. Variation of interface position at different rates as detected through capacitance.
ment, capacitance readings showed a continuous change of interface position as indicated by the variations in the capacitance value. It can be observed that, at one point, the addition was suspended for a few seconds so the capacitance detected was constant. Afterward, the addition of liquid was restarted at a certain rate and later decreased to observe how the technique detected the variation. As shown in Figure 10, all of these variations were clearly recorded through capacitance readings. It should be noted that the total variation in interface position was 58 mm, while the change occurred in a time interval of approximately 75 s. As can be observed, this technique allows for a variety of measurements and so offers interesting possibilities for control applications. 5.5. Integration of Capacitance Readings in Foam Draining Images. Data from electrical capacitance experiments could be reinterpreted to produce an image describing the liquid distribution within the foam. The liquid hold-up corresponding to the measured capacitance at each detection electrode in the sensing board was calculated from the experimental data and given a color depending on its value. The data were then displayed as a picture of colored squares, which varied depending on capacitance readings. A computer program was written to transform each capacitance datum into a colored square within the picture and to display the images sequentially to identify flow patterns inside the foam. An example of these frames is presented in Figure 11. The scale at the bottom of this figure shows the relation of liquid content corresponding to a color code (the darker the color, the higher the liquid concentration in the foam). As can be appreciated, these images provide information about the spatial distribution of liquid in the foam. Because the position of the detection electrodes in the sensing board is known, it is possible to distinguish changes of liquid content as a function of time and position. This sort of information opens opportunities for foam control, because, in this way, a region of the foam showing high stability could be readily spotted and, if necessary, a localized corrective action (in the form of antifoam agents) could be taken. Further work is required to study the feasibility of incorporating this sort of information into a control methodology. However, the simplicity of the tasks involved in generating these images, along with their apparent accuracy, suggests that this kind of imaging could be successfully modified for foam monitoring purposes.
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Figure 11. Spatial liquid distribution during oil foam formation. The color scale indicates the variation of local liquid hold-up as measured by each electrode in the sensing board.
6. Conclusions The feasibility of using electrical capacitance for detecting and monitoring oil foams of industrial relevance was demonstrated. An important parameter in the success of the technique is the proper construction of the sensors. A well-insulated unit able to minimize external disturbances is vital for monitoring lowpermittivity systems such as oil foams. Through the application of electrical capacitance, a simple relation to correlate capacitance measurements with liquid content in foam was found. With this technique, foam drainage profiles were obtained, and specific features of the foam collapse process were identified. The analysis of the data showed that different drainage conditions inside the foam could develop simultaneously at different rates and in different sections of the foam. Electrical capacitance techniques proved very useful in the study of foam dynamics with sufficient detail. On the basis of these features, the potential of this technique for applications in foam detection and monitoring has been highlighted. Acknowledgment The authors acknowledge the financial support provided by Consejo Nacional de Ciencia y Tecnologia (CONACYT) Mexico and Advance Research Partnership, Manchester, U.K., that made it possible to carry out this research. Notation CMC ) critical micelle concentration C ˆ ) normalized capacitance Ca ) capacitance of empty cell (pF) Cf ) foam capacitance (pF) Cij ) capacitance at any row i and column j (pF) Co ) capacitance of oil (pF) ∆h ) decrease in the liquid level during foam generation
hf ) final height of the foam MCMU ) modular capacitance measuring unit SAA ) surface-active agent Greek Letters φl ) liquid content in the foam ) permittivity
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Received for review May 24, 2002 Revised manuscript received December 2, 2002 Accepted December 6, 2002 IE020387+
