Which is more cost-effective: putting a distillation tower into operation or the energy consumption costs?
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This post was last edited by WoAiMaoMaoRan on 2015-12-19 at 16:53. May I ask if there are any principles for determining the height of a distillation column? In my opinion, when designing a tower, it is necessary to take into account both the costs associated with equipment investment and the energy consumption after the tower is put into use. However, equipment investment is a one-time cost, while energy consumption is indeed permanent. Suppose the cost of a tower is 1 million, and the reboiler at the bottom of the tower is heated by steam; the cost for that as well is 1 million per year. If the height of the tower is increased to twice its current value, making it taller and slimmer, although the cost of equipment used increases to 2 million, the amount of steam required decreases by 200,000 per year. Over time, this cost will be offset. It’s still quite cost-effective. Am I thinking correctly? What confuses me is: if the tower height is increased while the specifications of the distillation products remain unchanged, can steam consumption really be reduced?
In my humble opinion, this metaphor is not good. Steam is determined through the heat enthalpy calculation of the material, and it has little to do with raising its level. In actual production, the final amount of steam used may vary due to the surface area of the tower, heat dissipation, and heat transfer capacity, but this is not the fundamental factor in the calculations. (Although we also consider Q loss)
Thank you for your reply. It’s not about heat dissipation; for something like a stripping tower, does increasing its height improve the stripping effect? Then wouldn’t the steam reduce the usage?
The impact won’t be very significant. You can use two methods to calculate it: 1. Heat introduced into the tower + heat introduced by the reboiler + heat introduced by the reflux = Heat carried away from the top of the tower + Heat carried away from the bottom of the tower + Q_loss. 2. Heat introduced into the tower + heat introduced by the reboiler = Heat carried away at the outlet + Heat recovered in the condenser + Heat carried away from the bottom of the tower + Q_loss. In fact, all these variations are reflected in Q_loss, and I think the changes won’t be very significant. The height of the tower is primarily determined by separation requirements, with energy consumption not being a key factor.
Factors such as the gas velocity and gas-liquid ratio were used to determine an empirical height-to-diameter ratio for the single plate； To achieve the desired separation effect, the required number of theoretical plates is calculated; multiplying these two values gives the necessary tower height. It is not advisable to make the calculated tower more slender. If the tower diameter remains unchanged and it is heightened, some steam can be reduced. Since the number of plates increases, it is easier to achieve the desired separation effect; this allows for a reduction in reflux, thereby decreasing the amount of steam consumed
Well, if the tower diameter is reduced, it doesn’t necessarily mean there will be savings, right?
If the tower diameter is reduced, the gas velocity must be increased, which can lead to problems such as flooding; to ensure normal operation, the production volume has to be reduced
Well, I’ll study it more carefully. Thank you for your answer!
When considering the economics of a tower system, it is not enough to focus only on the tower itself; the size of the reflux tank heat exchanger also needs to be taken into account, as well as architectural and structural support issues. For utility systems, steam, electricity, circulating water, and instrument air also need to be considered. First and foremost, what needs to be considered is whether it is possible to meet the product requirements or the separation precision. Second, the tower becomes taller and slimmer; it may be necessary to change the internal components of the tower, and a too-small tower diameter makes it unsuitable for plate towers. Third, the tower becomes tall and slender, and wind conditions also need to be taken into account; if it is too tall, the upper trays may become ineffective. Fourth, the tower is thick and heavy; the steel structure of the two-story platform needs to bear additional loads ； The tower is tall and needs to be secured; should additional fixing platforms be added? Is it necessary to provide extra platforms at the location of the heat exchangers as well? Does the installation cost need to be increased? Fifth, how much the reflux ratio changes due to variations in tower diameter. Can the existing reboilers and condensers meet the requirements? Does the reflux tank need to be changed? If the backflow rate changes, does the pump also need to be changed? Sixth, steam, circulating water volume, electricity, and instrument air are all changing. Although the prices of the other three utility services are not as high as that of steam, they still need to be taken into consideration. Seventh, the general lifespan of a device is 14 years. Calculate the reduction in utility costs over 14 years to see in how many years it will be sufficient; there’s no need to go into too much detail, as prices will change after all. Eighth, **encourage the reduction of energy consumption. It’s better to calculate whether the diameter of your tower can be changed; use hydraulic software for the calculations and then make adjustments. It’s not as extreme as you think – there aren’t that many variations.
Increasing the height of the tower mainly involves increasing the number of trays to improve the distillation efficiency. By saving steam while ensuring that the quality of each product remains unchanged, the investment can be recovered within ten years; what’s left over is profit. The payback period for the investment is a bit long.
The answers above are quite comprehensive! I learned it! Personal opinion! The tower we have calculated is based on its separation capacity; of course, when determining the results it is also necessary to consider the energy consumption. By changing the height and diameter of the tower and then re-evaluating the energy consumption, it can be seen that energy consumption does change as the tower diameter changes, but not as significantly as one might expect. After all, it is still necessary to vaporize the cold material and carry away that heat in the form of high-temperature gas ； Optimizing equipment based on energy consumption can indeed be effective, but careful calculation is required before implementing it ； After all, increasing the tower diameter infinitely much won’t result in a significant reduction; when reducing the tower diameter, it’s necessary to take into account the risk of flooding! Of course, if energy consumption is reduced, other utility costs will naturally decrease as well! This is different from the one on the 9th floor ； In fact, energy consumption reduction can be achieved from various perspectives, not just this one: pressure reduction, pressure increase, pretreatment, etc. Of course, if pressure reduction is involved, then the cost of cold coal must be taken into account!