Methanol Water Separation

Methanol is used as an alternative energy resource to reduce the use of fossil fuels. Methanol blended with gasoline improves the thermal efficiency of engines and reduces the emission of exhaust gas. Another application of methanol is in fuel cells to directly convert chemical energy to electric energy and offers clean energy conversion. In addition, methanol is important in chemical industrial processes.

Methanol (CH3OH) is water-soluble and readily biodegradable, comprising four parts hydrogen, one part oxygen, and one part carbon, and is the simplest member of a group of organic chemicals called alcohols. Methanol is a clean-burning, biodegradable fuel. Increasingly, methanol’s environmental and economic advantages make it an attractive alternative fuel for powering vehicles and ships, cooking food, and heating homes.

As renewable energy, methanol can be produced by fermentation of biomass, such as corn, sugarcane, sorghum, and microalgae. In the production processes, separation of methanol from aqueous solution is required. Conventionally, methanol is separated from aqueous solution by distillation, but this process consumes large amounts of energy. Some alternative methods have been developed and applied, such as pervaporation, adsorption to zeolite, gas stripping using ionic liquids, and filtering with nanotubes. However, it is still a challenge to develop an innovative technique to more effectively separate methanol from aqueous solution.

Water-Methanol separation using carbon nanotubes and electric fields

Conventionally, separation of methanol from aqueous solution is by distillation. However, this method consumes a large amount of energy; hence development of a new method is needed. In this work, molecular dynamics simulations are performed to investigate the effect of an electric field on water–methanol separation by carbon nanotubes (CNTs) with diameters of 0.81 to 4.07 nm. Without an electric field, methanol molecules fill the CNTs in preference to water molecules. The preference of methanol to occupy the CNTs over water results in a separation effect. This separation effect is strong for small CNT diameters and significantly decreases with increasing diameter. In contrast, under an electric field, water molecules strongly prefer to occupy the CNTs over methanol molecules, resulting in a separation effect for water. More interestingly, the separation effect for water does not decrease with increasing CNT diameter. Formation of water structures in CNTs induced by an electric field has an important role in the separation of water from methanol.

In the recent years, carbon nanotubes (CNTs) have shown promise as separation membranes for gas, desalination, gas–water, and organics–water separation. Moreover, the study of the separation of methanol from aqueous solution with CNTs has attracted considerable attention. Under a chemically potential gradient, methanol molecules flow through CNTs in preference to water molecules. Modification of CNT hydrophobicity by attaching carboxyl acid (COOH) groups on the inner wall of the CNTs slightly increases the selectivity of methanol molecules over water molecules. When CNTs are immersed in water–methanol solution, methanol molecules preferentially fill and occupy the CNTs over water molecules, resulting in a separation effect. However, the selectivity significantly decreases with increasing CNT diameter. Immersing CNTs in methanol and other alcohol solutions shows that the selectivity for alcohols over water in occupying CNTs also depends on the number of alcohol–carbon atoms.

Water confined in nanoscale space under an electric field has many interesting properties, which is very important for nanotechnology and biological science. A significant number of studies concerned with the effects of an electric field of the order up to 8 V nm⁻¹ on water confined in CNTs and between two-plates were reported recently. The strength of that field is still comparable with the typical field in biological transmembrane channels, which is 0.06 to 0.3 V nm⁻¹. Moreover, various strategies to explore the field effects for developing nanofluidic devices have been reported as well. Nano pumping can be performed by employing a time-dependent, vibration, and rotating electric field. Introducing an electric field along the CNT can enhance reverse osmosis for water purification. The axial field can control the dynamics of water molecules in the CNT to induce flow. 

In this work, we investigated the separation of water and methanol from water–methanol solutions with CNTs with and without an axial electric field using molecular dynamics (MD) simulations. We investigate the effect of an electric field on the selectivity of a water–methanol mixture flowing into (6, 6) to (30, 30) CNTs. In the absence of an electric field, methanol molecules preferentially enter and occupy the CNTs over water molecules, resulting in a separation effect for methanol. However, the separation effect for methanol is only strong for small CNT diameter and considerably decreases with increasing CNT diameter. In contrast, when an electric field is applied, water molecules strongly prefer to enter and occupy the CNTs over methanol molecules, which produce a separation effect for water. More importantly, the selectivity for water molecules does not depend on the CNT diameter, indicating a strong separation effect for water.

 

Separation of the Methanol-Ethanol-Water Mixture Using Ionic Liquid

Vapor pressure data for the binary systems (water/methanol/ethanol + 1-ethyl-3-methylimidazolium acetate ([EMIM]+[Ac]−)) and the ternary systems (methanol + water + [EMIM]+[Ac]−, ethanol + water + [EMIM]+[Ac]−, and methanol + ethanol + [EMIM]+[Ac]−) were measured by a modified equilibrium still. For the above systems, the maximum average relative deviation between experimental data and the UNIFAC-Lei model predicted values was 7%, confirming the prediction accuracy of the UNIFAC-Lei model. Thus, this model was further used to predict the isobaric vapor–liquid equilibrium (VLE) data at 101.3 kPa for the methanol + water + [EMIM]+[Ac]−, ethanol + water + [EMIM]+[Ac]−, and methanol + ethanol + [EMIM]+[Ac]− systems at a fixed mole fraction of ionic liquid (IL) (xIL = 0.2). It was demonstrated that the ionic liquid [EMIM]+[Ac]− was an appropriate entrainer to separate the methanol–ethanol–water mixture. On this basis, the extractive distillation process was simulated using the rigorous equilibrium (EQ) stage model. The results showed that the entrainer consumption, the heat duty of total reboilers, and the heat duty of total condensers decrease by 25, 6, and 6%, respectively, when [EMIM]+[Ac]− replaces the conventional entrainer ethylene glycol (EG). Furthermore, the σ-profiles and excess enthalpies obtained by the COSMO-RS model provided theoretical insights into the separation mechanism at the molecular level.

McCabe-Thiele Method for Methanol-Water Separation

The McCabe–Thiele graphical solution method for binary distillation is used to determine the number of equilibrium stages needed to achieve a specified separation in a distillation column. This method assumes:

1. The distillation column is adiabatic.

2. Constant molar overflow (CMO), which means that for every mole of vapor condensed, one mole of liquid is vaporized. This results in constant liquid and vapor flow rates between stages (the exception being the flow rates of the stages above and below the feed stream, which are not equal). This assumption requires that:

2a. Specific heat changes are small compared to latent heat changes

2b. Heat of vaporization is the same for both components and thus independent of concentration.

3. Heat of mixing is negligible. 

The equilibrium curve was calculated using the modified Raoult's law. The operating lines for the rectifying and stripping sections are used to determine the number of stages.

Selective separation of methanol-water mixture using functionalized boron nitride nanosheet membrane: a computer simulation study

The separation of alcohol-water mixture from each other is one of the significant subjects for scientists in the pharmacy and engineering fields owing to economic savings. In this research, the separation of methanol-water mixture was investigated using molecular dynamics (MD) simulations method. The MD results explain the mechanisms of solvent separation from each other in the atomic-scale perspective. As a separator membrane for separation of methanol from water, boron nitride nanosheets (BNNS) with two various functionalized pores was applied. In these systems, in the normal conditions, solvation separation phenomenon did not occur. Therefore, external pressure was applied to the simulation box. Each of methanol and water molecules passed through a specific functionalized pore of BNNS, so that these pores acted as a selective membrane to separate them from each other. Results were confirmed with the calculation of potential of mean force for each solvent in both pores. The separation of the methanol-water mixture using functionalized BNNS was dependent on the amount of applied pressure and the pore size and chemical group on the edge pores.

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Blog by- Utkarsha Mandlik | SejaL Nikhade | Uday Sonwane | Yugal Urkude | Tanmay

 Khiratkar| Department of Chemical Engineering | Vishwakarma Institute of Technology, Pune.