Asking about reposting? Solvent dehumidification
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I posted a request for help in another discussion forum, but I haven’t received any replies from anyone so far. I would like to post my request in a different forum – is that allowed? Could the moderator please delete the original request post? Thank you. Original post: Solvent dehydration – Dimethyl carbonate reacts with ethanol to produce ethyl methyl carbonate. At present, the purity of the other components in the product is within acceptable limits; it’s only the water content that is too high, currently around 200–300 ppm. The required level for water content is below 20 ppm. What suggestions do you have for reducing the water content? First, thank you all. My reply is below, on floor 1: No one has answered, so I’m posting this to stimulate further discussion and hope to get the correct answer. I’ve looked into many methods, but I haven’t put most of them into practice; therefore, I hope those who have practical experience can share some insights, as well as relevant data and methods. Aqueous solvents can generally be treated using the following methods: 1. Add water-absorbing substances to the solvent, such as anhydrous copper sulfate, anhydrous magnesium sulfate, anhydrous sodium sulfate, anhydrous calcium chloride, anhydrous magnesium chloride, calcium oxide, and phosphorus pentoxide; these substances react with the water in the solvent to form crystalline water, which is then removed. This method is very effective in the laboratory – I always use it, and it allows for the removal of most of the water, with levels dropping to around 100 ppm. 2. If the solvent contains a high amount of water, salts such as sodium chloride, sodium carbonate, and sodium sulfate can be added to dissolve the water in the solvent, forming a saltwater solution. This saltwater solution will separate from the solvent; generally, the saltwater solution has a higher density and remains in the lower layer, while the solvent is in the upper layer, and separation can then be achieved by decanting. 3. Based on the properties of the solvent, azeotropic distillation is carried out: water and some of the solvent vaporize, while the pure solvent remains in the reactor; subsequently, product isolation can be achieved through distillation based on boiling point differences. 4. If the efficiency of azeotropic distillation is too low, the operating conditions can be modified by applying pressure or reducing it, raising or lowering the temperature, using a third component in azeotropic distillation, or adding such a third component for extractive distillation, in order to separate water from the solvent. Subsequently, the azeotrope-forming agent, also known as the entrainer, or the extractant can be recovered. 5. Molecular sieves are used for dehydration; generally, the 3A, 4A, and 5A types are employed. I always use them in the laboratory for dehydration purposes, and they can be used to remove trace amounts of water from materials used in Grignard reactions and diazotization processes. However, one thing is certain: when using molecular sieves for dehydration, it is essential not to contaminate the solvent or introduce any impurities. 6. Magnesium powder, magnesium strips, sodium wire, calcium hydride, sodium hydride, lithium aluminum hydride are used to react with the trace amounts of water in the solvent in order to achieve complete water removal. However, these methods are quite dangerous; extreme caution must be exercised when using them. Measures such as speed control, temperature control, inert gas shielding, reflux, ice baths, and pre-purification are necessary to prevent explosions. 7. In some cases, the solvent can be vaporized and then dehydrated by passing it through molecular sieves, silica gel, or alkali columns (solid columns of sodium hydroxide or potassium hydroxide, on whose surfaces water can undergo deliquescence). 8. Freeze dehydration? I’m just guessing; I’ve never seen it or tried it, but water is special, and under certain circumstances this method might work. 9. Osmotic vaporization: separates water from solvents using membrane separation methods, but lacks industrial data and has limited applications. 10. An inert gas is used to reduce the equilibrium partial pressure of water vapor, thereby enabling the removal of water.
1. Adding water-absorbing substances to the solvent: It is not very useful for industrial applications. It is not cost-effective; the product is prone to contamination, and it is difficult to separate the organic phase from the aqueous phase. 2. High water content in the solvent: This is not the case for your property. In practical operations as well, it’s not possible to distinguish clearly. 3. Azeotropic distillation is carried out based on the properties of the solvent: Azeotropic distillation is a commonly used unit operation, but it does not seem to be suitable for such extremely low concentration ranges. It seems that ordinary distillation will suffice. However, processing the purchased raw materials in this way requires a lot of energy. 4. Same as above, energy consumption is too high. 5. Using molecular sieves for dehydration (in liquid state): Yes, it is possible. But the effect depends on the circumstances. 6. Using magnesium powder, magnesium strips, sodium wire, calcium hydride, sodium hydride, lithium aluminum hydride to react with trace amounts of water in the solvent: has little industrial significance. 7. In some cases, it is possible to vaporize the solvent and then use molecular sieves, silica gel, or alkali columns (solid columns of sodium hydroxide or potassium hydroxide, on whose surfaces water can undergo deliquescence) to achieve dehydration: this is an application used in industry today, but molecular sieve gas-phase adsorption is generally employed. 8. Freeze dehydration? I’m just imagining it: that’s true indeed. For the methanol-water system, it is technically possible; it is estimated that freezing to below minus 30 degrees is required for solid water (ice) to appear. However, the melting point of alkyl ethyl esters with a certain carbon count is too close to that of water. 9. Hydrophilic pervaporation: a membrane separation method used to separate water from solvents, but it lacks industrial data and has limited applications; this is the direction being pursued currently. There are already many examples of applications. 10. Using inert gases to reduce the equilibrium partial pressure of water vapor in order to remove water: this is not feasible for industrial applications, nor even in the laboratory. Although in this extremely rare case the activity coefficient of water is very high, and the relative volatility of water/ethyl methyl carbonate is much greater than 1, its partial pressure is still much lower than that of the organic solvent. If this operation is feasible, there is significant solvent loss. In fact, the industrial process that utilizes this property is distillation. Recommended procedure: 1. The final step in production should be distillation. Therefore, increasing the reflux ratio, raising the tower height, and using more efficient packing can **reduce the water content, although it may not necessarily meet the specified standards. 2. Adsorption using gas-phase molecular sieves. This is the most widely used in industrial applications at present, such as the dehydration of 95% alcohol. 3. Penetrative vaporization. There is definitely a dehydrating effect, but whether it can be reduced to below 20 ppm depends on thermodynamic calculations or experiments.
Thank you to the person in floor 1 for the in-depth analysis and for providing comments on the method I described. I hope this will be useful to everyone, and I also hope that more people will come forward to offer further and more detailed comments on said method. Many production processes may seem simple and can be analyzed in detail, but it is the actual results obtained from production that are the most reliable. Therefore, I hope that by combining theory with practice, you can help me and others who face similar difficulties in solving these problems.