Article pubs.acs.org/IECR
Optical Fiber Reflectance Probe for Detection of Phase Transitions in Multiphase Systems Yujian Sun, Sean G. Mueller,† Boung W. Lee, and Milorad P. Dudukovic* Chemical Reaction Engineering Laboratory (CREL), Department of Energy, Environmental & Chemical Engineering, Washington University in St. Louis, St. Louis, Missouri 63130, United States ABSTRACT: An optical fiber reflectance probe is developed for fast, in situ detection of phase transitions between heterogeneous and homogeneous states in multiphase systems. Experiments with two binary systems, methanol−cyclohexane and carbon dioxide−methanol, are conducted to demonstrate the capability of the reflectance probe to capture the phase transitions in both liquid−liquid mixtures and gas expanded liquids (GXLs). The instantaneous revelation of transition in the latter from the gas−liquid two-phase state to a single phase at sufficiently high pressures provides important information for use and further investigation of this fully expanded regime. The current probe can withstand high pressures up to 100 bar and temperatures 100 °C, which suggests its applicability in many practical processes.
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INTRODUCTION Many industrially important multiphase systems consist of immiscible fluids between which distinct phase boundaries exist. However, as temperature or pressure changes, the solubility of one phase in the other may increase until the two phases become completely miscible and the phase boundaries cease to exist. For example, at high enough pressure in a constant temperature system the gas phase may completely dissolve in the liquid. Such homogeneous states of multiphase systems, especially for supercritical fluids (e.g., scCO2) and carbon dioxide expanded liquids (CXLs), are gaining increasing interest as suitable substitutes for organic solvents for separations, extractions, reactions, and other applications.1−7 Being able to detect the transition to a single phase in situ is important, as it has ramifications for transport rates compared to the original two-phase system and for application of the automatic control needed to maintain the homogeneous state. Although these phase transitions can be observed by the naked eye in vessels with fully transparent walls, they are difficult to detect in most reactors with steel walls unless expensive view cells are provided. High temperatures and high pressures are typically employed in mixed autoclaves for processes where these transitions are expected to occur at harsh operating conditions. Thus, it is desirable to develop a reliable approach for fast, in situ detection of the phase transitions for various processes practiced commercially. A recent review8 summarized several experimental techniques that have been developed in laboratory settings for detecting phase transitions in supercritical mixtures, such as the acoustic method9 and the fiber optic reflectometer.10 Xue et al.11 used a four-point optical probe with conical tips to determine bubble dynamics in a bubble column, and Mueller and Dudukovic12 used a single-point one to measure gas holdup in a stirred tank. Continuing the previous work of Mueller et al.,13 who measured the volumetric expansion of CXLs using the same conical-ended probe, in this study another type of probe is developed. This optical fiber reflectance probe has a flat end with a mirror attached and can be used to detect the phase transitions for both liquid− © XXXX American Chemical Society
liquid mixtures and gas expanded liquids. Different from the reflectometer10 mentioned above which detects the light reflected by the fiber−medium interface, our probe detects in addition the light reflected by a mirror placed a short distance from the flat end of the fiber through which the light is introduced and collected. This combined light signal, in which reflected light from the mirror is dominating, reveals remarkably accurately the disappearance of phase boundaries associated with the phase transition from heterogeneous to homogeneous state, as demonstrated in this work. We illustrate first the changes in the light signal captured during in situ detection in the methanol−cyclohexane (liquid−liquid) mixture as the system temperature is raised from below to above its critical solution temperature at fixed pressure. We then illustrate the signal changes with pressure in the CO2− methanol (gas−liquid) system at different temperatures. These examples demonstrate the applicability of the optical fiber probe in the detection of the phase transitions for various types of multiphase systems.
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EXPERIMENTAL SECTION Probe Setup. The optical probe system used in this work consists of three optical fibers (I, II, and III), as shown in Figure 1. One polished end of fiber II (i.d. = 1035 μm) is coupled closely with fibers I and III (i.d. = 630 μm), which are connected to the laser source (Thorlabs S1FC635PM) and the light detector (Thorlabs PDA36A), respectively. The laser wavelength used in the experiments is 635 nm. The main body of fiber II is sealed into 1/8 in. o.d. (outside diameter) stainless steel tubing by Epoxy (Loctite, General Purpose) for protection. Attached to and supported by the tubing is a small piece of mirror (about the size of the tubing), which is about 2 mm away from the other polished end of fiber Received: October 2, 2013 Revised: November 27, 2013 Accepted: December 13, 2013
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dx.doi.org/10.1021/ie403253c | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 1. Schematic of the optical fiber reflectance probe (“---⪫”, light path).
II, as shown in Figure 1. This probe end with the mirror attached is then immersed in the fluid being studied. In this way, the light emitted from the laser source can be transmitted to the fluid through fibers I and II and then reflected by the mirror back to fiber II and sent to the light detector through fiber III. The light signal can be converted to an analogue signal by the detector, which can then be transferred to the data acquisition system (PowerDAQ PD-BNC-16) and recorded by the computer. The sampling frequency is 100 Hz for each run. Rationale. The light signal acquired by the detector consists of the light reflected by the fiber−medium interface and the mirror, and scattered light from the medium. The mirror is attached close to the probe end to ensure a significant contribution to the probe recorded signal of the mirrorreflected light on its passage through the medium to the probe. Because the reflected light travels a certain distance (twice the distance between the fiber end and the mirror) through the medium being studied, the detected signal depends on the scattering properties of the medium including the presence or absence of phase boundaries. When a mixture is at a heterogeneous state, there are distinct interface boundaries between the two immiscible phases. Under sufficient stirring, these interface boundaries are broken down into small fragments which are distributed randomly throughout the heterogeneous mixture, resulting in strong fluctuations of the light signal. At phase transition, when interphase boundaries disappear, the mixture turns to a homogeneous transparent state; thus the fluctuations of the light signal are greatly reduced at the same stirring rate. This disappearance of the fluctuations distinguishes the homogeneous state from the heterogeneous one and thus can be used as the criterion to determine the transition conditions. Procedure. For the methanol−cyclohexane system, as it is stirred and gradually heated, the temperature change is recorded by video at the same time with data acquisition of the light signal. When the temperature exceeds the critical solution temperature for the specific composition, there is a signal change which coincides with the transition. From the data obtained, the exact time of this signal change is determined, and the temperature is simultaneously recorded with time to obtain the critical solution temperature. For the CO2−methanol system, the experimental setup is shown in Figure 2. CO2 is pumped into the 3 in. i.d. (inner diameter), 1L autoclave. Pressure is controlled via a Tescom 4000 back-pressure regulator and Validyne pressure transducer. Temperature control is achieved using Omegalux heating tape wrapped around the autoclave, an Omega thermocouple inserted into the vessel’s thermowell, and an Omega CN132 temperature controller.13 The end of the reflectance probe is immersed in the liquid (methanol) to detect the phase transition. The details of how the probe is sealed into the
Figure 2. Experimental setup of the autoclave (adapted from Mueller et al.13 Copyright 2007 American Chemical Society.).
autoclave are described elsewhere.14 As CO2 is gradually pumped into the autoclave, in which a certain amount of methanol has been initially added, the pressure increases and the signal reflects a two-phase system until the transition point at the specific temperature, which results in a signal change that can be captured and analyzed. It should be noted that in preparation of CXLs in various applications exactly the same procedure is used. The pressure of the system is raised through carbon dioxide addition, while maintaining a constant temperature, until the solvent is expanded to the desired extent.6
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RESULTS AND DISCUSSION Methanol−Cyclohexane (L−L). As the liquid−liquid mixture of methanol and cyclohexane is heated to reach its critical solution temperature, the stirred mixture turns to the homogeneous transparent state, which is clearly captured by the disappearance of light intensity fluctuations. Figure 3 shows in
Figure 3. In situ light signal (upper) and temperature ramp (lower) for the methanol−cyclohexane mixture during the miscibility transition (xMeOH = 50%).
situ data collected at 50% mole fraction of methanol. The reflectance probe can also detect the reverse transition, i.e., the transition from homogeneous to heterogeneous state with decreasing temperature, as show in Figure 4. This property is important, because in practical applications one may lose the homogeneous state if conditions drift, and it is advantageous that this probe is capable of detecting such changes rapidly. In the same way, the critical solution temperatures of methanol−cyclohexane mixtures are determined at different compositions, and all agree well with previously reported data by Jones and Amstell,15 as shown in Figure 5. Since in each of B
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fluctuations during pressurization of the CO2−methanol system by addition of CO2 as the state transition is approached, as shown in Figure 6. The recurrence of significant fluctuations
Figure 4. In situ light signal for the methanol−cyclohexane mixture during reverse transition (xMeOH = 50%).
Figure 6. In situ light signal (upper) and pressure ramp (lower) for CO2−methanol during pressurization at 25 °C. (The subscript “t” stands for “transition”.)
during depressurization, after crossing back into the multiphase region, can also be sensitively detected by the probe, as shown in Figure 7. The signal profiles at other temperatures are similar in trend and thus not displayed here.
Figure 5. Critical solution temperatures of methanol−cyclohexane at different compositions.
our methanol−cyclohexane experiments the mass of our system and its composition are fixed, the transition results can be plotted on the usual phase diagram of Figure 5 with the twophase region lying below the curve. As seen above in Figure 3, at the transition point we always find an abrupt rise in the signal. The reason for this is as follows. In the heterogeneous methanol−cyclohexane mixture, the heterogeneity results in opaqueness, as shown in the left photograph of the vessel in Figure 3. This opaqueness increases the light scattering in the liquid mixture and reduces the reflectance part of the probe signal since the more light is scattered away by the medium, the less light can be received back by the probe. The scatter is largely due to the randomness of the number and orientation of phase boundaries along the signal path. During the transition to complete miscibility, the mixture turns from the heterogeneous opalescent to the homogeneous transparent state, as shown in the right photograph in Figure 3. The reflected light is enhanced, since the scattering is reduced; thus the entire signal increases sharply at the transition. There is another interesting phenomenon worth noting. The signal intensity continuously and gradually goes down as the transition to the homogeneous state is approached until it reaches the minimum before rising sharply during the transition. Since the same phenomenon regarding the detected light intensity cannot be observed for pure methanol or for pure cyclohexane, this has to be related to the interactions between the two phases. It seems to indicate that interphase boundaries decrease in length scale but increase in number during heating up, which is responsible for scattering more reflected light and results in decreased total signal intensity. Whether this can be quantified may merit a future investigation. CO2−Methanol (G−L). In examining the evolution of the CO2−methanol (G−L) system with pressure at constant temperature, as practiced in achieving a CXL state, the reflectance probe captured a significant reduction in signal
Figure 7. In situ light signal for CO2−methanol during depressurization at 25 °C.
The significant reduction of signal fluctuations indicates disappearance of phase boundaries, i.e., a transition to a homogeneous state. The isothermal phase diagram of CO2− methanol at 25 °C (see Figure 8) suggests that this is a
Figure 8. Phase diagram of CO2−methanol at 25 °C (adapted from Brunner et al.16 Copyright 1987 Elsevier.).
transition point reached when the p−xCO2 trajectory (pressure vs total mole fraction of CO2), depicting approximately the history of our system during the pressurization experiment, intersects the upper VLE (vapor−liquid equilibrium) curve. This trajectory starts from below the VLE curve in the twophase region and ends above it in the single-phase region. The C
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lower VLE curve demarcating the single-phase region from the two-phase region is unavailable in the literature and is not necessary for interpretation of our data in the range of pressures and temperatures used in our experiments. It would lie very close to the abscissa. Recall that in our experiments pressurization is achieved by adding liquid CO2 which then spreads through the two-phase system as it continues to dissolve in liquid methanol, until at high enough pressure transition to the homogeneous state is reached. Beyond the transition point additional CO2 continues to mix with the now homogeneous state of the system. During our experiments, the pressure in the system is increased by addition of CO2, and therefore, the mole fraction of carbon dioxide in the system increases together with the pressure. The pressurization trajectory (p−xCO2) depends on the initial amount of methanol charged to the autoclave. Therefore, the transition pressures detected by the probe indicate when the CO2−methanol mixture at hand turns into the homogeneous state. These transitions from two phases to a single one and from single to two phases can be readily detected in situ by the probe, as demonstrated by Figures 6 and 7. The transition pressure detected in Figure 6 (about 51 bar, point A in Figure 8) while pressurizing the system is lower than the transition pressure detected in Figure 7 (about 55 bar, point B in Figure 8) during depressurization. This is to be expected as during pressurization the transition pressure, pt,A, is reached at some mole fraction of CO2, xCO2,tA, which corresponds to point A on the VLE curve in Figure 8. Continuous addition of CO2 beyond this point, until the final pressure pF is reached, further increases the mole fraction of CO2 in the homogeneous state to xCO2,F (point F in Figure 8). The line AF indicates the approximate path of the p−xCO2 trajectory. During depressurization amounts of the homogeneous state with the composition xCO2,F are gradually discharged from the system to lower the pressure. The depressurization path is thus a vertical line FB on the phase diagram. The composition of the single-phase system in the autoclave remains unchanged while pressure decreases until the transition to the two-phase region occurs at the upper VLE curve (point B in Figure 8). This occurs at a higher mole fraction of CO2 than when the homogeneous state was first formed by pressurization (xCO2,tB = xCO2,F > xCO2,tA), so the obtained transition pressure is higher in depressurization than during pressurization (ptB > ptA). In another set of experiments with a smaller initial amount of methanol than used in obtaining the results shown above, a higher transition pressure (about 57 bar) at 25 °C was detected as expected. This is because during our experiment, the p−xCO2 trajectory shifts to the right due to the larger volume that the gas phase can occupy. Thus, such trajectory intersects the VLE curve at higher xCO2,t and yields a higher transition pressure pt. In addition, the increase of transition pressure with temperature is clearly captured by the probe as shown in Figure 9. This is expected because the VLE curve, within the temperature range from 25 to 40 °C used in our experiments, moves up with the temperature.16 As previously discussed, the transition pressures at a given temperature are process-dependent; i.e., they depend on where the actual p−xCO2 trajectory during pressurization intersects the upper VLE curve. It is obvious that such transitions can occur anyplace (pt, xCO2,t) along the curve. Clearly, the transition
Figure 9. Phase transition pressures of CO2−methanol at different temperatures.
pressure, pt, increases with the mole fraction of carbon dioxide at transition, xCO2,t, along the bubble point line until it reaches the highest point, for example, (pc,xCO2,c), where the bubble point line and the dew point line intersect. At 25 °C, which is below the critical temperature of pure CO2 (31.1 °C), this point lies on the right boundary of the phase diagram (i.e., xCO2,c = 1);16 therefore pc is the vapor pressure of pure CO2 at that temperature. Since the vapor pressure of pure CO2 is not defined above its critical temperature, the highest point (pc,xCO2,c) on the VLE curve of CO2−methanol above 31.1 °C lies in the vicinity to, but not exactly on, the right boundary of the phase diagram.16 Such points at given temperatures are defined as the critical points of CO2−methanol. Yoon et al.17 have determined the critical pressure of CO2−methanol at 40 °C by locating the highest point on the VLE curve. In view of this it is evident that the maximum transition pressure our probe can ever detect in our CO2−methanol experiment is the critical pressure at the given temperature if the temperature is above 31.1 °C. Figure 9 clearly shows that the transition pressures detected by our probe at different temperatures are always bound by the reported critical pressures.17,18 Figure 9 also illustrates that transition to a homogeneous state occurs at transition pressures at all temperatures. One significant advantage of CXLs over ScCO2 is that the physicochemical properties of the solvent such as solubility of various gases, viscosity, and diffusivity, etc., can be enhanced during solvent expansion at pressures much lower than the critical pressure.6 This work shows that the developed probe can detect the phase transition into a single phase (homogeneous state) where, due to a large amount of CO2 dissolved in the solvent and the disappearance of the gas−liquid interphase boundaries, the transport properties may be further enhanced at pressures lower than the critical pressure as shown in Figure 9. To capitalize on this improvement and explore its potential merits further investigation.
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CONCLUSION An optical fiber reflectance probe is developed which detects the reflected light signal change during phase transitions of both methanol−cyclohexane and CO2−methanol mixtures. The probe readily detects any transition between the heterogeneous and homogeneous states. The probe signal change can be coupled to the temperature or pressure recorder to determine the transition conditions in situ. The developed probe can be used as a phase transition detector and thus a process control tool in the various applications of gas expanded liquids (e.g., CXLs), supercritical fluids (e.g., ScCO2), and other processes where enhanced D
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(13) Mueller, S. G.; Werber, J. R.; Al-Dahhan, M. H.; Dudukovic, M. P. Using a fiber-optic probe for the measurement of volumetric expansion of liquids. Ind. Eng. Chem. Res. 2007, 46, 4330−4334. (14) Mueller, S. G. Optical measurements in gas−liquid stirred tanks. Dissertation, Washington University in St. Louis, MO, USA, 2009. (15) Jones, D. C.; Amstell, S. The critical solution temperature of the system methyl alcohol-cyclohexane as a means of detecting and estimating water in methyl alcohol. J. Chem. Soc. 1930, 1316. (16) Brunner, E.; Hültenschmidt, W.; Schlichthärle, G. Fluid mixtures at high pressures IV. Isothermal phase equilibria in binary mixtures consisting of (methanol + hydrogen or nitrogen or methane or carbon monoxide or carbon dioxide). J. Chem. Thermodyn. 1987, 19, 273−291. (17) Yoon, J. H.; Lee, H. S.; Lee, H. High-pressure vapor−liquid equilibria for carbon dioxide + methanol, carbon dioxide + ethanol, and carbon dioxide + methanol + ethanol. J. Chem. Eng. Data 1993, 38, 53−55. (18) Yeo, S.-D.; Park, S.-J.; Kim, J.-W.; Kim, J.-C. Critical properties of carbon dioxide + methanol, + ethanol, + 1-propanol, and + 1butanol. J. Chem. Eng. Data 2000, 45, 932−935.
physicochemical (e.g., solubility) and transport properties resulting from heterogeneous to homogeneous state are crucial. One main advantage of the this probe is that it can be used at many industrially relevant conditions (e.g., vessels with opaque steel walls, high pressures, and so on). It could be simply inserted into the vessel through a port to detect phase transitions. This is a less expensive and more practical modification of the vessel than addition of expensive view cells. Moreover, the detection of phase transitions of CO2− methanol to a single liquid phase suggests further investigation into the potential application of this fully expanded regime.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Address †
S.G.M.: Honeywell UOP, 25 E Algonquin Rd, Des Plaines, IL 60016 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported in part by the grant from the National Science Foundation (Grant CBET 0933780). Industrial support to the CREL MRE (multiphase reaction engineering) program is also acknowledged. We acknowledge the useful discussions with B. Subramaniam and other faculty at the CEBC at The University of Kansas that motivated this work.
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