Measurement of diffusion coefficients and estimation of viscosity using carbon dioxide expanded liquids

Abstract

Introduction: The knowledge of the diffusion coefficient of a solute in neat carbon dioxide (CO2) and CO2 mixed with organic solvents is of importance as it correlates to the efficiency of chromatography and extraction processes.
Background: Several studies have been found in literature for diffusion coefficients measurements of different solutes in neat carbon dioxide, but to our knowledge rather few can be found using CO2-expanded liquids/binary mixtures.
Aim(s): The aim of this project was to modify and validate a home-built chromatography set-up by measuring the diffusion coefficient of toluene in neat CO2 and compare the obtained values with data available in literature. When the equipment set-up had been validated, the aim was to measure the diffusion coefficient of toluene in CO2-expanded liquids. The estimation of the viscosity for neat CO2 and CO2-expanded liquids using theoretical models was also evaluated.
Methods: The Taylor dispersion method was used for the measurements of diffusion coefficients. It consists of injecting a quick pulse of solute into a laminar flow of solvent running through a long, hollow capillary. As the solute moves through the capillary, its peak broadens due to convection effects along the peak and molecular diffusion in the radial direction.
Results: Four different flow rates were examined to see how the flow rate affected the magnitude of the diffusion, to which the conclusion was drawn that a too low flow rate does give a statistically significant impact on the diffusion coefficient. Three injection volumes were also examined, and it was determined that the magnitude of the diffusion coefficient was not affected by the injection volume in the range examined. The configuration of the detector (UV/Vis CCD) was also evaluated. The system was validated and compared to data available in literature, giving deviations between 15-30%, and well-known theoretical models were used to estimate the viscosity of the solvent, giving deviations between 5-33%, depending on what equation was used and the pressure/temperature conditions. Measurements of the diffusion coefficient and viscosity of toluene in 4% ethanol in CO2 were then done and compared to literature, giving the mean diffusion coefficient of 1.06*10-8m2/s showing deviations between 8.5-10% in regards to the viscosity depending on which equation was used. Finally, measurements of the diffusion coefficient of toluene in solvents of CO2 and ethanol and CO2 and ethyl lactate with different molar ratios were performed, showing how the viscosity of the solvent changes with the addition of CO2.
Conclusion: Modifications were made to an already existing chromatographic set-up, in an attempt to measure diffusion coefficients of toluene which would align with values reported in literature, and then use these diffusion coefficients to estimate the viscosity of the solvent using theoretical equations. Even though the measured diffusion coefficients deviated from the literary values quite a lot, the deviations were less for the estimated values for the viscosity for neat CO2 and 4% of ethanol in CO2. Diffusion coefficient measurements of toluene was thereafter made in solvents with the molar ratio of 0.1, 0.3 and 0.5 CO2 in ethanol and ethyl lactate, and through exponential regression, the conclusion was drawn that the addition of CO2 affected the viscosity of ethyl lactate more than the viscosity of ethanol.

Popular Abstract

When mixing juice, one takes concentrated, flavourful juice extract and blend it with water. The two liquids fuse, creating a homogenous mixture where the flavour and colour of the concentrated juice extract have been diluted in the water. Scientifically speaking, the molecules of the juice extract have spread out, or diffused, in the water and gone from a high concentration to a lower one.

This phenomenon of diffusion has been studied in this work, except that the juice extract has been exchanged with toluene and the water with neat carbon dioxide or carbon dioxide mixed with an organic solvent. When mixed, carbon dioxide and organic solvents are however not in the normal gas or liquid phase in which we are used to find them, but in a subcritical phase which is achieved by adding a certain amount of pressure and heat to a solvent. In this subcritical phase, the solvent has a density similar to that of a liquid but the viscosity closer to a gas. The solvent can also be called carbon dioxide expanded liquid. The viscosity of solvents and diffusion coefficient of solutes are important properties to investigate, as mass transfer rates are crucial when designing, scaling up or optimizing extraction and chromatography processes.

In this work, a home-built chromatography like set-up was modified to be able to measure diffusion coefficients of toluene in neat carbon dioxide which correlated to those already available in literature at two different temperatures (40 and 60 °C) and two different pressures (150 and 250 bar). When comparing the results with values reported in literature, they deviated between 15-30%.
The measured diffusion coefficients were thereafter used to estimate the viscosity of the CO2, giving deviations between 8-13% for the modified versions of the Wilkes-Chang, Schiebel and Lusis-Ratcliff equation, and 5-7% for the Lai-Tan equation. The diffusion coefficient of toluene was thereafter measured in 4% ethanol in CO2, and the estimated viscosities was compared to literature, giving deviations between 8-10% depending on what equation was used. Finally, the diffusion coefficient of toluene was measured in solvents of 0.1, 0.3 and 0.5 of CO2 in ethanol and ethyl lactate under the same temperature and pressure conditions as before, and the viscosities were once again estimated. Using exponential regression, the conclusion was drawn that there is a larger impact on the viscosity of ethyl lactate with the addition of CO2 than there is for ethanol.

• author: Torell, Tea
• supervisor: Larissa Cunico; Margareta Sandahl; Charlotta Turner
• organization: Department of Chemistry
• course: KEMR30 20201
• year: 2020
• type: H2 - Master's Degree (Two Years)
• subject: Chemistry
• keywords: Diffusion coefficients, CO2-expanded liquids, Carbon dioxide, Taylor dispersion method, Viscosity, Analytical chemistry, Analytisk kemi
• language: English
• id: 9019354
• date added to LUP: 2020-06-18 13:43:05
• date last changed: 2020-06-18 13:43:05