Pressure-Drop Method for Detecting Bubble and Dew Points of

Article pubs.acs.org/jced

Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Pressure-Drop Method for Detecting Bubble and Dew Points of Multicomponent Mixtures at Temperatures of up to 573 K Gurbuz Comak,†,‡ Christopher Wiseall,† James G. Stevens,† Pilar Gomez,† Jie Ke,† Michael W. George,*,†,§ and Martyn Poliakoff*,† †

School of Chemistry, University of Nottingham, University Park, Nottingham, NG7 2RD, U.K. Department of Biotechnology, Faculty of Science, Konya Necmettin Erbakan University, Meram, Konya 42090, Turkey § Department of Chemical and Environmental Engineering, University of Nottingham Ningbo China, 199 Taikang East Road, Ningbo 315100, China ‡

S Supporting Information *

ABSTRACT: Filled or empty tubular reactors have been at the heart of many chemical processes in academia and industry. Understanding the phase behavior in such reactors is essential to improving the conversion and selectivity of a given chemical transformation and to minimizing energy consumption. This study shows that the pressure-drop method is a simple and effective technique for measuring vapor−liquid phase equilibria at temperatures of up to 573 K. The basis of the pressure-drop method is flowing the fluid through a capillary with a relatively small inner diameter. The pressure drop between the inlet and outlet of the capillary depends on the phase state of the fluid (gas and/or vapor). In this article, pure propan-2-ol and the binary system propan-2-ol + water have been investigated to validate the method at high temperatures for these fluids. The binary system water + acetonitrile was then measured to demonstrate that the phase equilibrium of a thermally reactive mixture can also be determined by using the pressure-drop method. We have modeled the experimental pipeline pressure-drop results with the Process Systems Enterprise gPROMS ProcessBuilder 1.1.0 modeling environment using the Peng−Robinson equation of state and the superTRAPP algorithm for transport properties, and we find that the theoretical calculations are in good agreement with the experimental results.

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system.15 However, the relationship between the pressure drop and the phase behavior of a fluid in a fixed bed reactor was mentioned by Harröd et al.,16 who carried out the continuous hydrogenation of fatty acid methyl esters in supercritical propane, and subsequently the correlation of the pressure drop with the phase state, as a method for phase equilibrium studies of highpressure fluids, was reported.17 This method allows the detection of phase transitions for low-temperature systems with acceptable accuracy in a fixed bed system. Despite this, the detection of both the dew and bubble points for a given binary mixture has been a challenge for this approach.18 Until now, the method has remained less effective for low-molecular-weight organic compounds at temperatures above 423 K. This is due to pressure “noise” generated by various components of a typical flow system being comparable to the change in the pressure drop.17 In this study, we have reduced the problem of noise by using a long empty capillary tube. This long tube approach is successfully applied to pure fluid propan-2-ol (IPA) and binary system IPA + water at temperatures above 423 K. Because of

INTRODUCTION High-temperature continuous reactions occurring in either fixed bed or empty tubular reactors are of great importance in academia and the chemical and petrochemical industries. For example, naptha premixed with steam is cracked at high temperatures in empty tubular reactors, or ethylene is oxidized in fixed bed reactors filled with silver catalyst supported on carbon black.1,2 A range of these methods for studying the phase equilibria of high-pressure fluids for sub- and supercritical conditions have been developed in our laboratory,3−6 and we have recently focused on the development and application of methods related to transportation problems associated with carbon capture and storage (CCS).7−12 However, each of our methods has strengths for particular fluids under the temperature and pressure conditions studied and limitations for other systems. For example, attenuated total reflectance Fourier transform infrared spectroscopy is an effective method for phase behavior studies of high-pressure and high-temperature water systems, and the holey-fiber method is sensitive only to organic components due to the use of a flame ionization detector.13,14 The pressure-drop method is based on the pressure difference across a tubular pipe or reactor. The pressure difference relates to the density, viscosity, and flow rate, etc. For a packed column, this is expressed by the Ergun equation for a single-component © XXXX American Chemical Society

Special Issue: In Honor of Cor Peters Received: August 24, 2017 Accepted: January 31, 2018

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DOI: 10.1021/acs.jced.7b00755 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. Chemicals Used in This Worka

a

chemical name

formula

MW/(g mol−1)

source

purity (mass fraction)

acetonitrile propan-2-ol ethanoic acid acetamide

CH3CN C3H7OH CH3COOH CH3CONH2

41.05 60.10 60.05 59.07

Sigma-Aldrich, Dorset U.K. Sigma-Aldrich, Dorset U.K. Sigma-Aldrich, Dorset U.K. Acros Organics.

>99.9% 99.8% ≥99% 99%

None of the chemicals were purified further. All analysis was carried out with gas−liquid chromatography.

Figure 1. Schematic of the automated high-pressure fluid continuous flow apparatus. The components are labeled as follows: BPR, back pressure regulator; CT, capillary tube; GLC, gas−liquid chromatography; HP-SL, high-pressure sample loop; OP, organic pump; P, pressure transducer; PC, computer; S, sample reservoir; T, thermocouple (shaded for logging temperature); V, check valve; and W, waste container. Dotted lines indicate data flow.

Rig Design and Experimental Procedure. The measurement method is based on monitoring the pressure difference between the inlet and outlet pressure transducers separated by a 3-m-long stainless steel tube (see details below). The stainless steel tube is coiled around an aluminum heating block and surrounded by a heating jacket. Automated sampling using a highpressure sample loop is used to analyze the composition of potentially reactive mixtures of CH3CN + H2O at high temperatures and pressures. A high-pressure liquid chromatography (HPLC) pump (Jasco PU-980) and a back pressure regulator (BPR, Jasco BP-1580-81) are used to deliver fluid samples at a controlled flow rate and pressure. The pressure readings were carried out by means of a model TJE pressure transducer (RDP Electronics), which was calibrated against a reference pressure transducer with a maximum

the small internal volume of the stainless steel tube, we have also been able to investigate the phase behavior of a potentially reactive mixture, acetonitrile + water, since this mixture would normally react in the time required for conventional phase measurements. Thus, the aim here is detecting phase transitions without significant reaction occurring during the transit of the fluid through the tube.

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EXPERIMENTAL SECTION Materials and Methods. All chemicals were used without further purification and were purchased from commercial sources; chemical sample descriptions are given in Table 1. Ultrapure water (H2O) was supplied by a Milli-Q Advantage A10 unit with a resistivity of 18.2 MΩ cm at 298 K and a total organic carbon (TOC) content of less than 5 ppb. B



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working pressure of 34.48 MPa (5000 psi), obtained from the MENSOR Corporation. The estimated accuracy according to the manufacturer’s specification is ±0.01% at the upper pressure limit, leading to a transducer uncertainty of 0.003 MPa. Fluctuations in pressure arise from the use of the BPR, which leads to an uncertainty of 0.05 MPa for the pressure measurements. The calibration of the K-type thermocouple for low temperatures was carried out using a calibrated Pt100 platinum sensor, that is placed into a refrigerated circulator (model no. 1162A, working over the temperature range of −20 to 100 °C) that monitors the temperature to a precision of ±0.01 K. However, due to the very small internal diameter of the stainless steel tube used, temperature monitoring for the phase measurements was carried out with the thermocouples placed between the outer wall of the stainless steel tube and the heating jacket, which surrounds the stainless steel coil. The estimated uncertainty for temperature measurements is 0.5 K Gas−Liquid Chromatography. To probe whether the composition of the CH3CN + H2O mixture was altered by reaction inside the hollow tube during the measurements, an online analysis of the fluid samples downstream of the stainless steel tube was carried out by means of a Shimadzu gas−liquid chromatograph (GLC) fitted with a flame ionization detector. For these measurements, regular samples were taken with a highpressure sample loop, as shown in Figure 1. The GC column used in the experiments was HP5 with helium as the carrier gas. The GLC was calibrated for starting material (acetonitrile), intermediate (acetamide), and product(s) (acetic acid). Stainless Steel Tube. The key component is the coil of stainless steel capillary tubing across which the pressure drop occurs. The specifications of the tube are 1.59 mm outer diameter, 0.10 mm inner diameter, and 3 m length. The pressure rating of the tubing is 38.6 MPa. The pressure drop was calculated as the difference in pressure measured before and after the tube (inlet and outlet pressures, respectively, in Figure 1): ΔP = Pinlet − Poutlet

A surface roughness of 0.015 mm was assumed for stainless steel.22 The pressure-drop model ignored the heat-up stage of the process and assumed that the tube was adiabatic and the fluid was pumped at the experimental pressure and temperature rather than starting from ambient temperature and pressure. The maximum pressure drop calculated was matched to the pressure drop observed for the pure IPA system using an adjustment factor of 0.2. The adjustment factor is a linear multiplier of the Fanning friction factor and was needed because the initial predictions overestimated the pressure drop. This model defined by the IPA pressure drop was then used for all subsequent calculations.

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RESULTS AND DISCUSSIONS Single-Component Isopropanol (IPA). Pressure-drop values for a single-component system of IPA were measured for a range of pressures, and the reproducibility of the measurements was investigated. Figure 2a shows an example of how to locate transition points by increasing the temperature isobarically at 1.64 MPa.

(1)

The general procedure used to record pressure-drop values was as follows. A sample of known composition was pumped through the rig, as shown in Figure 1. The pump and BPR were set to the elevated flow rate and pressures. The temperature controller was set to a temperature lower than the expected transition temperature. The rig was allowed to equilibiriate, and a temperature ramp of 0.5 or 1 K/min was applied for the collection of transition points. The pressure readings between the inlet and outlet of the stainless steel tube were recorded and plotted as a function of temperature. The pressure drop depends, to a great extent, on the velocity of the fluid. Therefore, several flow rates in the range from 0.02 mL/min to 1 mL/min were tested to establish the effect of fluid flow rate on the pressure-drop values. These flow rates were chosen to minimize fluctuations caused by the BPR. As explained below, transition points were identified by a distinct change in pressure-drop values. Modeling. The Process Systems Enterprise gPROMS ProcessBuilder 1.1.0 modeling environment was used for the pipeline pressure-drop calculations. The Peng−Robinson equation of state was used with the superTRAPP algorithm for transport properties, unless otherwise stated.19,20 This computational method was required to complement the experimental method due to the limited literature values under the conditions in question. The computational model is based on an empty tube using the Haaland equation to calculate the Fanning friction factor.21

Figure 2. (a) Pressure-drop measurements in pure IPA with a flow rate of 0.07 mL/min at 1.64 MPa. The continuous black line shows the experimental recording, the black dashed line shows the computational prediction, and the black arrow indicates the transition point from the liquid to the vapor phase determined experimentally. (b) Measured P−T diagram of IPA from a range of isobars (○, experimental value; ▲, computer model; more details can be found in the Modeling section) compared to the literature data (−).23 A ramp of temperature at a rate of 1 K/min was used for each isobaric measurement. C



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A transition temperature can be deduced from the pressuredrop data by distinct changes in ΔP at the transition point. The temperature was ramped at +1 K/min. At 451.5 K, ΔP increases rapidly due to the increase in velocity in response to the decrease in density of the component, as it converts into vapor. The bubble point can therefore be identified as 451.5 K. The selected flow rate affects the phase transition in terms of the pressuredrop values as well as maintaining a constant pressure using a BPR. The small internal diameter of the stainless steel tube allows a pressure difference comparable to that in the filled tubular reactor to build up across the length of the tube.17 Friction is the underlying principle of the theory behind the pressure-drop method. In the capillary case, it was assumed that friction could exist between the internal surface of the capillary and a component flowing through the stainless steel tube. A series of transition points, at several pressures, were taken to validate the method. Figure 2b shows a comparison of our data with the literature data reported by Barr-David and Dodge.23

A difference of ca. 2 K is observed between the data measured by the pressure-drop method and the literature data over the pressure range of 1.5 to 3.5 MPa. This could be due to the indirect temperature recording of the system or the accumulation of error from pressure fluctuations and thermocouples as well as any error in identifying the transition point. As mentioned in the Experimental Section, the thermocouple is placed between the heating jacket and the coil of the stainless steel tube because the internal diameter of the tube, 0.1 mm, is less than the diameter of the thermocouple itself. In addition, a larger deviation is observed for higher pressures, and it is thought that these pressure fluctuations are due to the needle

Figure 5. Pressure drop for a 0.15 mole fraction of IPA in water at 2.09 MPa with a temperature ramp of 0.5 K/min: (a) bubble-point determination with a 0.06 mL/min flow rate and (b) dew-point determination with a 0.02 mL/min flow rate. Two short dashed lines indicate the beginning of phase transitions and the bubble and dew points. Figure 3. Pressure-drop variation of 0.4 mole fraction of IPA in H2O at 7.05 MPa, along with a temperature ramp of 1 K/min with a flow rate of 0.11 mL/min. Two short dashed lines indicate the beginning of phase transitions and the bubble and dew points. Between the lines, the system is in a biphasic region (liquid and vapor).

Table 2. Bubble and Dew Points for the Mixtures of IPA + H2O Obtained with the Pressure-Drop Method Transition conditions Transition temperatures (K) at given pressuresa IPA mole fraction, xIPA

bubble point

dew point

p, MPa

0.40

461.0 480.0 495.0 507.5 516.0 527.0 507.5 514.5 466.0 486.0 501.0 515.0 468.5 491.5 508.5 523.0 534.0

464.0 483.0 499.0 511.0 519.5 530.5 512.0 519.0 476.5 500.0 516.0 527.0 478.5 500.0 520.0 532.5 547.0

2.03 3.06 4.06 5.07 6.03 7.05 5.06 5.97 2.09 3.09 4.05 5.10 2.07 3.08 4.07 5.05 6.06

0.15

0.05

Figure 4. Comparison of the bubble (○) and dew points (●) for the binary mixture with xIPA = 0.40. The symbols and the line represent our work and the literature data, respectively.23

a

The standard uncertainties are u(T) = 0.5 K, u(p) = 0.05 MPa, and u(xIPA) = 0.002. D



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movement of the BPR. However, there are a number of other possible sources of pressure fluctuations such as thermal expansion and enthalpy of vaporization. The pressure was recorded at 1 s time intervals, along with the applied temperature. An average of the pressures recorded throughout the whole temperature ramp was taken to be the pressure of the system for the value of an isobar. A pressure difference between two points, e.g., 1.61 and 1.64 MPa, could unsurprisingly lead to a difference in the transition temperature, as seen in Figure 2b. This system was also modeled using the tube diameters and conditions described above. There was a slight difference between observed and predicted behavior with the maximum predicted pressure drop of ca. 0.8 MPa. A frictional adjustment factor was used to match the maximum pressure drop observed in the model. The calculated pressure drop was minimal in the liquid phase at ca. 0.02 MPa, which was treated as the experimentally observed ca. 0.12 MPa pressure-drop baseline. The 0.12 MPa difference is assumed to be due to the pressure drop between the end of the tube and the pressure transducer fitting. This 0.12 MPa was subsequently used as a baseline for the pressuredrop calculations. Binary System of IPA and Water. A binary mixture of IPA + H2O with xH2O = 0.4, 0.15, and 0.05 was used to validate the pressure-drop method for the detection of both bubble and dew points. The identification of bubble and dew points differs slightly depending on the alcohol concentration in the mixtures. Figure 3 shows the general trend in ΔP for a binary mixture of IPA + H2O. The detection of a bubble point in a binary mixture, as shown in Figure 3, is relatively simple since the pressure drop shows a large increase. One can recognize from Figure 3 that ca. 527 K is the transition temperature where the first bubble appears. The pressure-drop value continues to increase as more liquid converts to the gas phase. The second change, which is the appearance of a discontinuity in the pressure-drop values, was assigned to the transition point from the biphasic region to gas phase. This transition point is identified as the dew point and is

shown as the second dashed line in Figure 3. Similar behavior, namely the appearance of a discontinuity in the pressure-drop values, was also observed by Akien et al. for pure methanol in the packed bed.17 As the temperature increases, the fluid starts boiling and two phases (liquid and vapor) coexist inside the capillary, shown as LV in Figure 3. The pressure drop increases due to the increase in friction with the increased velocity of the vapor phase. The fluctuations in pressure drop show a smoother trend indicating that the system is again in the single-phase state. For this particular pressure of IPA + H2O, the bubble and dew points are taken to be 527 and 531 K, respectively. A range of pressures, from 2 to 6 MPa, were studied, and Figure 4 shows a comparative P−T diagram of the results for 0.4 mole fraction of IPA. Our results are in good agreement with the literature data.23 The reference data shown in Figure 4 was interpolated from the P−x phase diagrams reported by David and Dodge.23 Two other mixtures with IPA mole fractions of 0.15 and 0.05 were also studied to test the applicability of this method at low IPA concentrations. The change in pressure-drop values became less easily identifiable at low concentration, making it more

Figure 6. Comparison of the bubble (unfilled symbols) and dew points (filled symbols) for 0.05 and 0.15 mole fractions of IPA in H2O at various pressures compared to the literature data.23 (○, ●) are the points measured for the 0.15 IPA mole fraction and (□, ■) are the points measured for the 0.05 IPA mole fraction. The solid line and dashed line represent the literature values for the 0.15 and 0.05 IPA mole fractions, respectively.

Figure 8. Pressure-drop variation with temperature for CH3CN + H2O with xCH3CN = 0.1 at various pressures and a flow rate of 0.06 mL/min for each isobar: (1) 2.12 MPa, (2) 3.14 MPa, (3) 5.61 MPa, (4) 8.35 MPa, (5) 10.29 MPa with ○ and ● representing the bubble and dew points, respectively. The temperature ramp rate is 1 K/min. The dashed lines represent the computational prediction for the pressure drop for each isobar.

Figure 7. GLC chromatogram of the outlet at 8.35 MPa. Two injections were made automatically for each pressure, in which the temperature for a specific injection was between the initial and end point of the ramp temperature.

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Reactive Mixture of CH3CN + H2O. Phase behavior studies of a reactive mixture in a continuous systems was the primary aim for the development of the pressure-drop method.18 Here, this method is applied to the mixture of CH3CN + H2O, a reaction which has previously been used for kinetic studies.24 The challenge was to obtain phase data before significant hydrolysis of the CN group occurred. To monitor hydrolysis, the apparatus was connected to the GLC for direct quantitative measurements of the starting material, intermediates, and products if reaction occurs. Continuous online sampling was carried out using an automated high-pressure sample loop. Monitoring by GLC. During our phase-behavior measurements, there was no indication of the formation of either acetamide or acetic acid. The chromatogram collected during an

difficult to locate dew points. Thus, a different approach was followed to detect the dew points. The temperature ramp rate was decreased from 1 K/min to 0.5 K/min for lower mole fractions of IPA. For each pressure, two experiments were carried out, one with high flow rates for the bubble point and another with low flow rates for the dew point. Figure 5 shows an example of data collected for an IPA mole fraction of 0.15 at a single pressure. At a high flow rate (Figure 5a), no peak shape was observed, but better accuracy was observed for the bubble-point transition. At a lower flow rate (Figure 5b), a peak shape was observed in the pressure-drop values which was used for dewpoint determination. This approach works with reasonable accuracy, as can be seen from a comparison of our results with the literature data in Figure 6.23

Figure 9. (a) Plot of the bubble (○) and dew points (●) obtained for CH3CN + H2O with xCH3CN = 0.10 using the experimental pressure-drop method. (b) Plot of the bubble (△) and dew points (▲) with xCH3CN = 0.10 obtained using the computational pressure-drop method. The dashed and solid black lines represent smoothed curves which join the four reported data points for either bubble or dew points.31 F



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Although a clear indication of the phase transition could be obtained using the pressure-drop method, the deviation between the two methods is larger for the IPA + H2O system. Both the experimental and computational method results were compared with the previously reported results, as shown in Figure 9. The bubble and dew points for the experimental method showed absolute average deviations of ±0.53 and ±0.38 MPa, respectively. For the computational prediction, the bubble and dew points showed absolute average deviations of ±1.02 and ±0.37 MPa, respectively. The GLC results show that there is no significant amount of reaction occurring during the collection of a point, at the given pressure for a specific mole fraction. The pressure-drop behavior predicted using the computer model became more accurate at higher pressures, which can be seen in Figure 10. The bubble and dew points observed and predicted as per the pressure-drop method are also summarized in Table 3.

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CONCLUSIONS The pressure-drop method using a long length of smalldiameter stainless steel tube provides a simple and rapid means of detecting phase transitions of multicomponent mixtures at temperatures of up to 573 K. The method also works well for a single-component system. The agreement of the experimental results with both the computational prediction and literature is promising as this proves the method could be used for further studies with systems such as high-temperature water. The results also show that the phase behavior of a reactive mixture could be studied, as the residence time is short enough to avoid significant degradation of the reactants. These results also show that the pressure-drop method can also be used for empty tubular reactors, and hence the pressure-drop method can be a valuable tool for understanding phase behavior in tubular reactors, regardless of whether the tube is filled with catalyst. Furthermore, it is able to operate under high temperatures for both reactive and nonreactive systems with simplicity.

Figure 10. Comparison of the relative difference between the experimental value for the bubble point (○) and the computational prediction (△) for the bubble point and previously reported data for the 0.1 mole fraction CH3CN + H2O system.13

isobaric pressure-drop study for a 0.1 mole fraction of acetonitrile is shown in Figure 7. The lack of intermediate or product is probably due to the small volume of the tube, and the residence time of only ∼24 s at a flow rate of 0.06 mL/min is insufficient for the reaction to occur to any extent. This residence time is much shorter than in the work published on the hydrolysis reaction.17 Acetonitrile Mixture. Most of the published work on the phase behavior of a mixture of CH3CN and H2O has been restricted to low temperature and pressure, around 423 K and 0.49 MPa.25−30 Here we chose a solution of 0.1 mole fraction of CH3CN in H2O since there were some previous data31 based on a fiber optic reflectometer method for this composition. As can be seen in Figure 8, one can easily distinguish the phase changes in this mixture, which is displayed as the open and solid circles for bubble and dew points, respectively. When higherpressure isobars are run, a change in the peak shape is observed. See traces 4 and 5 in Figure 8. The computational model was then used to predict the behavior of the 10% CH3CN and H2O system at the five pressures used experimentally. The computer model accurately predicted the pressure drop at the five different pressures. The bubble and dew points can be calculated from the transitionary zone before and after the large pressure-drop increase, respectively. The dew point has a lower error compared to the bubble point, with predictions of both dew and bubble points increasing in accuracy at higher pressures.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.7b00755. Phase-transition points for pure IPA obtained with the pressure-drop method (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: martyn.poliakoff@nottingham.ac.uk. Tel: +44 (0)115 951 3520.

Table 3. Bubble and Dew Points for the Mixtures of CH3CN + Water Obtained with the Pressure-Drop Method Transition conditions Transition temperatures (K) at given pressuresa experimental

a

computational

CH3CN mole fraction, xCH3CN

bubble point

dew point

bubble point

dew point

p, MPa

0.1

481 500 528 553 569

493 512.5 538 562 578

461 483 519 548 565

483 503 537 562 575

2.12 3.14 5.61 8.35 10.29

The standard uncertainties are u(T) = 0.5 K, u(p) = 0.05 MPa and u(xCH3CN) = 0.002. G



Article

ORCID

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Christopher Wiseall: 0000-0003-2217-6325 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Professor Cor Peters for his sustained contribution to the field of phase equilibria over many years. We thank the EPSRC for supporting this research. We thank Mark Guyler, Peter Fields, and Richard Wilson for technical assistance, Dr. Alfredo Ramos and Process Systems Enterprise, Ltd. for their provision of gPROMS, and the EPSRC Centre for Doctoral Training in Carbon Capture and Storage and Cleaner Fossil Energy (CCSCFE).

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