Doubts regarding the renovation of the company’s methanol distillation unit
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I would like to ask the experienced colleagues and fellow industry professionals at HaiChuan: our company currently intends to modify the existing batch-type, atmospheric-pressure single-effect methanol distillation unit and convert it into a continuous-type, negative-pressure distillation system. Our material to be distilled is an aqueous solution of methanol at 40% concentration and formaldehyde at 5% concentration. The distillation heat source is saturated steam, with a steam pressure of 3 to 4 kilograms. The overhead stream is required to be a methanol aqueous solution at 93% concentration, while the bottom stream is to be a formaldehyde aqueous solution at 10% concentration. The produced fluid must be recovered and reused. The temperature of the product at the top of the tower ranges from 65 to 70 degrees, while the temperature at the bottom of the tower is around 80 to 90 degrees. Our condenser uses circulating water for cooling, and its heat exchange area is approximately 100 square meters. Our column reactor is a 3-t capacity unit, meaning it can perform batch distillation on 3 tons of material at a time; it currently takes 3 to 4 hours to complete the distillation process for one batch. The column is approximately 14 meters tall and uses corrugated packing. Today, the boss brought a technician to provide guidance. In his view, the normal-pressure distillation of methanol is slow and resource-intensive, and it is completely unnecessary; especially since the methanol we distill is subject to rough distillation, where a high degree of separation is not required – only ensuring the desired concentration of the product is sufficient. Using negative pressure distillation **improves efficiency, increases production, and reduces steam consumption – it’s perfect. I read in books that the production capacity of vacuum distillation is lower than that of atmospheric and pressurized distillation, but from the perspective of these technicians, it seems to make sense as well. After all, the boiling point of methanol isn’t that low; cooling it with water to below 60 degrees turns it into a liquid. Also, our tower is assembled by tightening multiple bolts together; the connection method is similar to that of flanges, with gaskets in between. Could anything go wrong if this type of tower is directly modified into a negative-pressure tower? Methanol is flammable and explosive; I think the current sealing of the tower simply cannot ensure no leaks. If a leak occurs, it would represent a serious safety hazard. Is such a modification reasonable? I’m asking everyone for help – could you please assist me in making a decision? I’m a bit hesitant to make any changes.
By changing from atmospheric pressure to negative pressure, the tower throughput decreases. What he meant was that after switching from batch mode to continuous mode, the reflux ratio becomes smaller; there is no need for loading or for removing residual liquid, etc. Overall, the production volume increases while energy consumption decreases. Whether to reduce the pressure or not is debatable; lowering it too much is of little use, while raising it too much requires replacing the cooling medium at the top of the tower
The continuous negative-pressure distillation process is excellent for saving energy and reducing emissions while increasing efficiency. The issue is that the tower equipment in this system is connected using segmented flanges, which can lead to leakage points; moreover, the vacuum pumping system presents some difficulties. However, this isn’t a major problem – considering modifications to the tower equipment could be an option. Gland seals can be added to the flanges of the tower equipment, and as long as there are no leaks during vacuum pumping, no problems will arise. Another option is to replace the tower equipment. This is just for reference.
Production may not necessarily increase; I suddenly realized that our distillation volume will have to rise significantly. The gas drawn out by the vacuum pump cannot be completely condensed, so the original total condenser becomes a partial condenser. The gas drawn out must be absorbed by water behind the pump, and the liquid resulting from this absorption then needs to be further distilled. The specific production volume and energy consumption require detailed calculation. I’m thinking about seeing if there’s any software I can use to simulate it...
To determine whether vacuum distillation reduces energy consumption, it is not only necessary to consider steam usage and circulating water; I believe material loss should also be taken into account. If there is a large amount of material loss, it represents waste; if the material drawn away by the vacuum pump is reused, this increases the distillation capacity. Intuitively, it may seem that the time required to process 2 tons of material has decreased, but the amount of material obtained might be similar to that obtained when processing 1 ton; of course, it could also not be that much of a difference. It’s so difficult to decide. It seems that while it’s possible to reduce emissions through modifications, going to another manufacturer’s facility to take a look at their equipment and then making adjustments to one’s own old equipment involves quite a few complexities.
1. You need to consider whether the current evaporation rate of the tower has reached the designed value If it is far below the design value, negative pressure distillation can be considered to increase the evaporation rate ； 2. With the increased evaporation rate, can the cooling capacity provided by the top condenser meet the requirements? ； 3. When the evaporation rate is high, the cooling capacity of the top condenser is sufficient; however, it is necessary to check whether the tower height meets the requirements. If the tower is too short, the reflux ratio will be high, resulting in low efficiency of the tower ； 4. It is recommended that you think it through carefully and calculate the operating costs.
I agree with the opinion from the sixth floor. You need to calculate the costs of renovation as well as the operating costs.
Thank you for your advice. I’m switching fields, and I’ve only been working in the chemical industry for less than a year. If I want to be able to calculate these things, what books should I read to gain more knowledge? Please give me a few recommendations!
Well, I also think that specific data are the only way to get accurate information. The problem is that I’m not majoring in chemistry; I haven’t even studied college-level chemistry. I want to calculate these things, and I need some relevant knowledge for that; where can I find such knowledge in books? Don’t tell me Baidu Wenku...
This post was last edited by Desert Fish on 2016-1-24 at 10:22. To determine whether a change is possible, one needs to consider the device itself first, followed by the manufacturing process; feel free to contact me if needed!
Is vacuum distillation more energy-efficient than atmospheric distillation? That’s a false proposition. The latent heat of vaporization remains relatively constant, and energy consumption is similar as well. What’s more frustrating is that multiple cooling stages are required after condensation to reduce volatile gases; otherwise, the vacuum pump will draw in a large amount of volatile gases. Negative pressure feeding can reduce the need for feed pumps; compared to the power required by vacuum pumps, the power needed for feed pumps is negligible. Of course, the type of pressure to be used depends primarily on the relative volatility and boiling point; for substances with high boiling points that cannot be vaporized, negative pressure must be used, and there’s nothing else to say about that.