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Flow Toolkit for Measuring Gas Diffusivity in Liquids Jisong Zhang, Andrew Teixeira, Haomiao Zhang, and Klavs F. Jensen Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b05396 • Publication Date (Web): 20 Feb 2019 Downloaded from http://pubs.acs.org on February 23, 2019
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Analytical Chemistry
Flow Toolkit for Measuring Gas Diffusivity in Liquids Jisong Zhang,[a,b] Andrew R. Teixeira, [b,c] Haomiao Zhang [b] and Klavs F. Jensen*[b] [a]
The State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China [b]Department
of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, United States,
E-mail:
[email protected] [c]
Department of Chemical Engineering, Worcester Polytechnic Institute 100 Institute Road, Worcester, MA 01609
ABSTRACT: Precise knowledge of gas diffusivity in liquids is critical for describing complex multiphase reaction systems. Here we present a high-throughput flow concept to measure gas diffusivity in liquids. This strategy takes advantages of the tube-in-tube reactor design whereby semipermeable Teflon AF-2400 tubes facilitate fast mass transfer between gas and liquid without directly contacting the two fluids. Coupled pseudo-steady-state flux balances over the gas and liquid describe the gas dissolution rate and corresponding diffusivity with the aid of a single gas flow meter and a continuously ramped liquid flow rate. This in situ method demonstrates excellent accuracy in diffusion coefficient measurements, with less than 5% deviation from established techniques.
Gas-liquid processes are ubiquitous: CO2 capture involving absorption of CO2 into aqueous alkanolamines; hydrogenation of organic substances; waste water treatment.1,2 A detailed knowledge of gas diffusivity data in liquids is essential for developing new and efficient chemical processes.3 Diffusion of CO2 into solvent mixtures of varying compositions is an attractive systems for carbon capture.1,4 From a reaction engineering perspective, accurate knowledge of diffusion coefficients is required for proper reactor design, optimization, and control.5 The precise quantification of the diffusion coefficients across wide parameter spaces of gases, liquid compositions, temperatures, and pressures is arduous by classical equilibrium methods. Thus, it would be useful to develop a robust method that is capable of rapidly measuring diffusion coefficients at process conditions. There are several classical and non-traditional methods to measure gas diffusivity in liquids, including the pressure decay method6,7 laminar jet method,8 wetted wall column method,9 constant/decrease bubble size method,10 microfluidic approach5,11, chronocoulometry12,13, electrochemical mass spectrometry 14,and methods based on the use of Planar Laser-Induced Fluorescence (PLIF)15,16. The widely used methodology is pressure decay method, where the measurement is performed in a cell reactor by recording the pressure decay over time with no analysis of the liquid phase (Figure 1 a).6 This method is rather simple to operate and widely applied. However, experiments typically require substantial equilibration times of 4~5 h
for each data point and reloading operation are the major limitations for gas diffusivity determination. The laminar jet, wetted wall column and microfluidic approaches partially solve this problem, but at the expense of increased complexity and potential deviation from process conditions. In addition, poor resolution and control of the interfacial hydrodynamics (e.g. rippling on the liquid surface) can lead to erroneous results. The review by Vaidya provides additional details of those techniques.17
Figure 1. Schematic illustration of two methods to determine the gas diffusivity. (a) Classical pressure decay method whereby gas is slowly diffused into a liquid at autogenous pressures; (b) Tube-in-tube reactor where rapid transport leads to rapid equilibration of the two phases at user-defined pressures.18
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Figure 2. Automated tube-in-tube system for measuring gas diffusivity in liquids.
Recently, “tube-in-tube” reactors (Figure 1 b) constructed with a semipermeable fluoropolymer tubing (Teflon AF2400) encased by a concentric non-permeable gas tubing (i.e. PTFE or steel), have opened new and exciting opportunities for gas-liquid reactions in flow.2,19-27 Teflon AF-2400 tubing is a commercially available product that demonstrates selective gas permeability but is nonpermeable to liquids. This tube is used to continuously supply gas from the outer isobaric shell, through the semipermeable inner tube and into the flowing liquid without physically contacting the gaseous and liquid streams. The gas-liquid interface is well-defined by the physical constraint of the inner tubing wall. The reactor is simple to fabricate and has excellent chemical compatibility owing to the inert characteristics of the fluoropolymer. The short diffusional length scales of the internal fluid flow path (inner diameter = 0.6 mm) leads to rapid saturation times in the range of 10~30 seconds.18 A high-sensitivity gas sensor using Teflon AF tubing was first reported for absorbance detection.28 More recently, Ley and co-workers measured H2 and CO dissolution into a flow stream using an automated “bubble counting” technique21 and in-line FTIR spectrometer20, respectively. O'Brien developed an automated colorimetric titration technique to determine CO2 concentrations in solvent streams using a Teflon AF-2400 tube-in-tube design.29 Several publications report potential applications of this reactor geometry in gas-liquid reactions, including ozonolysis (O3),23 hydrogenation (H2)24 and oxidation (O2)25,30. The tube-in-tube reactors also provide a simple, rapid, and robust approach to measure the gas diffusivity in liquids. In our previous work, we developed a fully automated strategy utilizing the tube-in-tube reactor platform for fast in situ measurement of gas solubility in liquids.18 Here we propose introducing dynamics in a new strategy to determine the gas-liquid diffusivity. Ramping of the liquid flow rate adjusts the flow at a constant rate of change to
generate a continuous time-series of gas transfer rates. A mathematical model describing the mass transfer resistances in this tube-in-tube is successfully developed. By fitting the model to the measured gas transfer rate data, gas diffusivity in liquids can be obtained. Several gas-liquid systems have been tested, demonstrating less than 5% deviation from established literature values. Finally, the effect of ramping rate on the measured results is delineated.
Experimental Section Hydrogen (99.999%, pure), nitrogen (99.999%, pure) and carbon dioxide (99.99%, pure) were all supplied by Airgas (Salem, NH, USA). Methanol, cyclohexanone and toluene were all obtained from Sigma-Aldrich with the purity of 99.9%, 99% and 99+%, respectively. De-ionized water was obtained from VWR meeting ASTM type II specifications. The construction of the tube-in-tube reactor has been described extensively in literature.2,23,24 Figure 2 gives the schematic overview of the automated flow platform, which is the same one as used in our previous work on gas solubility determination.18 A mass flow meter was used to continuously measure the gas uptake rate through the membrane into the flowing liquid stream. This approach enables direct measurement of gas dissolution and is the basis for gas diffusivity determination in this work. To ensure the solvent was initially void of dissolved gases, the liquid was degassed prior to experiments by evacuating the headspace for at least 1 h.31 The purged liquid was then saturated with the analyte by sparging in a ventilated tank with the pure gas at atmospheric pressure. This means the liquid entering the inner tube of this reactor is not the degassed solvent but saturated liquid at one atmospheric pressure of analysis gas.
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Analytical Chemistry
Figure 3. Gas diffusivity determination scheme based on the model and experimental data.
The system design was carefully considered to minimize liquid demand, requiring
