Molecular sieve membrane dehydration technology
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As a new type of inorganic separation membrane, molecular sieve membranes can be used under harsh conditions (high temperatures and organic solvents), and they possess advantages such as high flux and high stability. Therefore, they hold great promise for application in pervaporation, particularly in the field of vapor pervaporation, and have become a focus of research and development for pervaporation membrane materials both domestically and internationally. Introduction: Permeative vaporization and steam permeation are separation processes driven by a degree gradient of component differentiation in a mixture, utilizing the differences in the dissolution (or surface adsorption) and diffusion rates of various components within the membrane. Among them, feeding in liquid form is called pervaporation, while feeding in vapor form is called vapor permeation. The permeate is transferred and collected on the permeation side through vacuum condensation (or gas purging, adsorption). Separation mechanism: The more widely accepted models are dissolution diffusion and screening. Application scope: 1. Removal of small amounts or trace components from mixtures, such as the removal of small quantities of water from organic substances; since only some of the components vaporize, energy consumption is significantly reduced ； 2. The separation of near-boiling and azeotropic substances is not constrained by vapor-liquid equilibrium; there is no need to use entrainers (in entrained distillation) or extractants (in extractive distillation), which reduces energy consumption and is more environmentally friendly ； 3. Simultaneous separation of multiple components, such as simultaneous dehydration of multiple components ； 4. Separate heat-sensitive substances at lower temperatures ； 5. Coupled with the reaction process, it takes advantage of its high separation coefficient, excellent single-stage separation performance, and ability to operate continuously to selectively remove reaction products, thereby promoting the shift of the chemical equilibrium. Application systems: Esters, including methyl acetate, ethyl acetate, butyl acetate, dimethyl carbonate, methylethyl carbonate, diethyl carbonate, vinyl carbonate, etc.; Alcohols, including methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, tert-butanol, pentanol, cyclohexanol, benzyl alcohol, alkynols, polyols, etc.; Ketones, including acetone, butanone, methyl isobutyl ketone (MIBK), N-methylpyrrolidone (NMP), N-ethylpyrrolidone (NEP); Ethers, including methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE), diisopropyl ether (DIPE), tetrahydrofuran (THF), methyltetrahydrofuran (METHF); Aromatic compounds, including benzene, toluene, ethylbenzene, chlorobenzene, etc.; Hydrocarbons, including dichloromethane, dichloroethane, trichloromethane, tetrachloromethane, tetrachloroethylene, propane, etc.; Others, including acetonitrile, N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAC), dimethyl sulfoxide (DMSO), pyridine, fusel oils, etc. While it seems to have a wide range of applications, it has many limitations: first, the conductivity must be less than 10, the pH must be between 6 and 9, and it must not contain pigments or solid particles
Here, I’d like to add some domestic manufacturers of molecular sieve membranes: Jiangsu Jiutian High-Tech, Wuhan Zhihong Sibo, Ningbo Xinyuan Membrane Industry, Zhejiang Huiyong New Materials, Hubei Zhongzhou Environmental Protection, and Shandong Lanjing Membrane Technology, etc. They are mainly located in the southern part of China; there is only one such manufacturer in the north.
There are still quite a few restrictions, making it difficult to promote it
Yes, this needs to be coupled with processes such as distillation or extraction; the preliminary treatment stage is used to remove impurities
This post was last edited by zyp0009928 on 2021-9-13 09:01. It’s a bit difficult to keep the conductivity below 10; in our workshop, the conductivity of water produced by reverse osmosis is controlled at just 15. pH value – it’s easier to deal with solid particles. What about the role of key materials and molecular sieves, as well as phosphorus pentoxide in dehydration? How many tons of dichloromethane are produced per year? In such cases, using membranes is more suitable than using molecular sieves or phosphorus pentoxide.
Conductivity is a factor that directly affects the lifespan of membrane tubes. A higher conductivity allows water to pass through the membrane, but it also reduces its service life. I do have a typical case here of deep dehydration of dichloromethane, reducing the concentration from 5000 ppm to 200 ppm. As for the appropriate amount to use, it depends mainly on the specifications of dichloromethane. Membranes are more stable than pressure swing adsorption, and they reduce energy consumption compared to other technologies, thus saving energy.
This post was last edited by zyp0009928 on 2021-9-13 at 15:53. We use dichloromethane as a solvent here; in the final step, we add water and distill off the dichloromethane. After being cooled by two stages of frozen brine, the distilled dichloromethane is separated from water, and dehydration is carried out using phosphorus pentoxide or molecular sieves; the required water content is 400 ppm, although 200 ppm is even better. I remember that in the past, using molecular sieves could remove moisture from bromoethane and bromobutane; in winter, the moisture content could be reduced to 100 ppm, and in summer to 200 ppm. On the other hand, when using sodium carbonate, the moisture content could be reduced to 400 ppm in winter, but only to 800 ppm in summer. The moisture content of the layered dichloromethane has not been measured; the annual circulation volume of dichloromethane is around 15,000 tons. Since dichloromethane is distilled, its electrical conductivity has never been measured. In a few days, I’ll measure the conductivity of the distilled water. This can only measure water, right? It seems that the conductivity meter cannot measure the conductivity of solvents.
After washing with an organic solvent, the layers are separated and the conductivity of the aqueous phase is measured. If molecular sieves or other materials are used, wear may be greater and regeneration may be more complicated, but the technology is well-established. The membrane has been in use for over 2 years now; its concentration generally fluctuates around 200 ppm. This value can be reduced if the membrane area is increased. The only factors that affect his water content are temperature and vacuum level
Currently, molecular sieve membrane technology has been successfully applied to the dehydration of various solvents such as ethanol, isopropanol, **furan, ethyl acetate, acetone, acetonitrile, dichloromethane, and dichloroethane; it continues to operate stably to this day. Currently, the major industry trend is bioethanol and the dehydration of fuel ethanol.
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Personally, I think it’s fine for use on a small scale, but for industrial use, more efforts are still needed!