Diffusion coefficients of toluene in supercritical carbon dioxide

Abstract

Introduction: Knowing diffusion coefficients in supercritical carbon dioxide is important from an efficient process in chromatography and extraction.

Background: Several papers have estimated diffusion coefficients in pure supercritical carbon dioxide, but rather few have investigated the effect of adding a cosolvent.

Aim(s): Building and validating a system for estimating diffusion coefficients in supercritical carbon dioxide and in supercritical carbon dioxide with a cosolvent.

Methods: Taylor dispersion technique using a long hollow capillary.

Results: Diffusion coefficients of toluene was estimated at 40 °C and 60 °C at the pressures 158 bar and 180 bar. The diffusion coefficients were estimated to be between 6.4*10-9 (± 2.0*10-9) and 8.2*10-9 (± 3.4*10-9) m2/s. The diffusion coefficients varied inside a series of injections. Ethanol was not added into the carbon dioxide due to time limitations.

Conclusion: The large variance in the estimations of diffusion coefficient means that no conclusion of the effects of pressure and temperature could be made.

Popular Abstract

From high concentration to low concentration
Imagine dropping a single drop of red food colouring in the middle of a glass of water. The red colour spreads slowly from the middle of the glass out to its sides. Eventually the whole glass is filled with a red solution. Or when you are in your kitchen cooking food, the fragrance is the most pronounced right at the stove. But slowly the aroma is spreading into all the rooms in your house, and soon the whole family comes rushing to the kitchen starving. At the source, when the red drop just mixed into the water glass or at the stove, the concentration of colour and aroma is the highest. But as time goes, the colour and aroma spread out to places of lower concentration, the rest of the water glass or the rest of your house. Both colour and aroma consist out of different molecules. The spreading of these molecules is due to something called diffusion and occurs every time there is a difference in concentration between two points.

The aim of this study is to measure molecular diffusion but not in media like water in your water glass or in the air you breathe, but in a medium that is called supercritical carbon dioxide. If we take carbon dioxide gas, released by cars for example, purify it from other substances that are also abundant in exhaust from cars and put it under pressure we get carbon dioxide as a liquid. It is even possible to make solid carbon dioxide by applying pressure and lowering the temperature, something we call dry ice (which can be used in movies to create a dense fog when mixed with water). If we increase the pressure and temperature of the carbon dioxide enough, we get something that behaves both like a liquid and a gas, called supercritical carbon dioxide (a supercritical fluid).

We can use this supercritical carbon dioxide to separate or extract different compounds from a sample, called supercritical fluid chromatography and supercritical fluid extraction. Think of the red drop being dropped into viscous honey instead of water, and the increase in time for the same spreading. The aim of this study is to estimate diffusion in supercritical carbon dioxide, under various temperature and pressure conditions, meaning different viscosities.

In this study, diffusion rates were estimated in supercritical carbon dioxide at 40 °C and 60 °C at two different pressures, 158 bar and 180 bar.

• author: Steen, Emil
• supervisor: Margareta Sandahl
• organization: Department of Chemistry
• course: KEMK06 20191
• year: 2019
• type: M2 - Bachelor Degree
• subject: Chemistry
• keywords: Supercritical carbon dioxide, Diffusion, Viscosity, Taylor, Toluene, Analytical chemistry, Analytisk kemi
• language: English
• id: 8984749
• date added to LUP: 2019-07-04 13:24:02
• date last changed: 2019-07-04 13:24:02