Ionic liquids for gas separation:

Measurement of selective gas solubilities of the gas-mixture CH4/CO2

Introduction

Ionic Liquids (IL) are molten (liquid) salts with a very low melting point. Many IL are liquids at room temperature. Specific advantages of these new substances for chemical processes are that they have almost no vapour pressure, possess good dissolution characteristics for many substances and that they are not flammable or toxic. The technical application of IL is interesting because of their good thermal, mechanical and chemical stability as well as favourable densities and viscosities. Besides, the required specifications can be adjusted easily by the large number of possible different combination of anions and kations when formulating the IL. In technical processes, ionic liquids are among others used as chemical solvents, catalyst, electrolytes in fuel cells as well as for gas-separation and –storage by absorption.

Particularly for the absorption or solubility of gases the characteristics of IL are of advantage. Many IL show for example a high solubility for carbon dioxide (CO2) and therefore they are promising substances for gas separation processes in which CO2 shall be separated from a mixture. An example for such a separation is the reduction of the CO2 concentration in natural gas – which contains up to 10% CO2, depending on the source area. As essential data for using IL in such gas-separation processes the solubility of methane (CH4) as main component of natural gas and carbon dioxide (CO2), as well as their mixtures was measured in high pressure range in two IL.

Measurement method

Sorption processes and gas-solubilities can be detected very precisely with the so called gravimetric measuring method. In doing so, the mass of the sample and the mass change which is caused by sorption or dissolution of gas is measured by a high-resolution balance. Conventional gravimetric analysers are limited to low pressures (up to a few bars). However, many technical processes require much higher pressures and therefore cannot be measured with the conventional gravimetric analyzers.

The separation of CH4/CO2 mixtures is an example for such a process which is carried out at higher pressures. Magnetic suspension balances make it possible to weigh samples contactless under nearly all measuring conditions. Instead of hanging directly at the balance (conventional gravimetric apparatus) the sample to be investigated is linked to a permanent magnet. An electromagnet, which is attached at the weighing hook of a balance, maintains the freely suspended state of the suspension magnet. Using this magnetic suspension coupling the measuring force is transmitted contactless from the measuring chamber (in which the sample is located under technically realistic conditions like high pressure) to the microbalance which is located outside the chamber under ambient conditions. Consequently, this set up eliminates almost all restrictions which are inherent to conventional gravimetric measuring instruments, and a general application of this extremely accurate and direct measuring method in various scientific research and development fields becomes both possible.

By selecting suitable metallic materials for the measurement chamber a high-precise and reproducible weighing of the sample in a large pressure (high-vacuum up to 500 bar) and temperature range (-196°C up to 1100°C) can be realized.

With such a magnetic suspension balance the solubilities of CO2 and CH4 and mixtures of these two gases were measured in two Ionic Liquids. The measurements were conducted at 50°C in a pressure range up to 130 bar.

Pure-gas-measurements

The measurement of gas-solubilities started after the IL was pre-treated by evacuation for removing dissolved gases before the measurement. The gas pressure in the measurement chamber of the magnetic suspension balance was increased stepwise at constant temperature. After every pressure increase it was waited until an equilibrium was reached. The sorption or solubility equilibrium is reached, when the balance records a constant mass of the IL-sample. After the equilibrium data were recorded, the pressure was increased to the next higher value. Thus, a so called sorption isotherm is measured.

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