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Thermophysical Properties and Solubility of CO2/CH4 in Aqueous Alkanolamine Solutions and Ionic Liquids

Muhammad, Ayyaz Muhammad (2009) Thermophysical Properties and Solubility of CO2/CH4 in Aqueous Alkanolamine Solutions and Ionic Liquids. PhD thesis, UNIVERSITI TEKNOLOGI PETRONAS.

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Abstract

Natural gas is the most useful and vital source of energy which could be considered as one of the cleanest and safest fossil fuel as compared to oil and coal. Regardless of the natural gas (NG) importance, it is usually contaminated with acid gases such as carbon dioxide (CO2). The percentage of CO2 in natural gas varies widely depending on the geological locations of gas fields. In some of the Malaysian gas fields in Peninsular and Sarawak, the average amount of CO2 in natural gas has been found as high as 46 and 72 % respectively (Darman and Harun, 2006). The presence high CO2 contents in NG decreases its heating value and also create corrosion and blockage problems during transmission. In many industrial applications, the selective separation of gas from mixture is usually performed by absorption process using glycol ethers and aqueous solutions of monoethanolamine (MEA), diethanolamine (DEA), N-methyldiethanolamine (MDEA) etc. Most recently, ionic liquids (ILs) have been introduced as novel solvents with unique properties such as negligible vapor pressure, high thermal stability and high gases selective solubility. The solvent characteristics play an extremely important role in the separation of gases at operating conditions of the absorption unit. The solubility measurements of gases in potential solvents are essential for generating reliable gas liquid equilibrium (GLE) data which provide basic information for the design and development of industrial absorption processes. In the present work, a fundamental study is carried out to systematically investigate the potential application of imidazolium based ILs for bulk removal of CO2 from NG to replace the existing alkanolamine solutions. For this purpose, initially thermophysical properties (density, isobaric thermal expansion coefficients, viscosity, surface tension, and refractive index) of aqueous MDEA/PZ solutions and ILs were measured and correlated. Thermal stability of all the studied solvents was also explored using a thermogravimetric analyzer (TGA). The density and viscosity values for all the studied ILs were found to be higher than all the studied amine solutions. The effect of pressure on densities of ILs was found to be insignificant up to 100 bar. The IL: [C6mim][PF6] was found to be highly viscous as compared to other studied ILs i.e., 431 mPa.s at 303.2 K. The surface tension of solvents was found to be in decreasing order of (PZ + water), (MDEA + PZ + water), (MDEA + water) and ionic liquids. The studied imidazolium based ILs showed very high thermal stability with their onset temperature values reaches three times higher than the amine solutions. A theoretical understanding has been proposed to develop a relationship between solvent physical properties and solubility behavior which is based on the experimental conclusions of physical properties of solvents. The solubility measurements for pure CO2 and its binary mixtures with CH4 are performed in all the studied solvents at temperature range of (303.15 to 333.15) K and pressure up to 90 bar. The solubility experiments were performed in a specially designed high pressure gas solubility cell with auto data logging and acquisition system and an online gas chromatograph (GC). The CO2 solubility in 4 molal and 8 molal aqueous MDEA solutions reached to 5.138 (moles.kg-1) and 9.132 (moles.kg-1) at P ≈ 54 bar and at T = 303.15 K. The CO2 solubility in ILs with different anions was found to be in decreasing order of [C6mim][Tf2N], [C6mim][PF6] and [C6mim][BF4] at P ≈ 60 bar and at temperature range from 303.15 K to 333.15 K. The effect of ILs’ anion appeared to play the most significant role in determining the CO2 solubility as the IL with [Tf2N] anion showed highest CO2 solubility (5.483 moles.kg-1) than [PF6] and [BF4] i.e., 3.428 and 3.872 (moles.kg-1) respectively. The solubility measurements for CO2/Methane in fresh solvents and recycled ILs were performed at exploration conditions of upstream NG with a perspective to explore potential capabilities of ILs to replace alkanolamines. The amount of CO2 dissolved in recycled ILs was found to be similar to that obtained for ILs with maximum deviation of 6.8% obtained in case [C6mim][PF6]. The solubility of CO2 in all the studied solvents significantly decreased due to presence of methane in gaseous mixtures in comparison to its solubility as pure carbon dioxide. The gas liquid equilibrium (GLE) data is correlated with an extended Henry’s law constant using Peng- Robinson Equation of State (PR-EOS) approach and other important thermodynamic parameters (Enthalpy, Gibbs free energy and Entropy) of solutions were investigated.

Item Type: Thesis (PhD)
Subject: UNSPECIFIED
Divisions: Engineering > Chemical
Depositing User: Users 5 not found.
Date Deposited: 04 Jun 2012 10:12
Last Modified: 25 Jan 2017 09:44
URI: http://utpedia.utp.edu.my/id/eprint/2906

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