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Comment PDF Processing & Handling

Designing for distillation

By Chemical Engineering |

I read your article “Flooded Condenser Controls: Principle and Troubleshooting” [ Chem. Eng., January 2016, pp. 37–49]. The article was very informative. Thank you for sharing your knowledge and experience.

I need your guidance related to Figure 4b. In the article you have stated: “A common design practice is to introduce the liquid from the top of the drum via a slotted or perforated pipe,… especially in situations with a high degree of sub cooling, such as during cold winter nights or low-rate operation.”

In this article you have not given the method/design of the slotted pipe or perforated pipe for liquid inlet. Can you please share your experience and knowledge on how to size the slotted or perforated pipe so that system will operate with less trouble over the entire operating range and ambient condition?

I have faced this problem (i.e., hammering due to vapor collapsing, pressure fluctuation) while running the system at turndown condition during winter nights when ambient temperature reached to around 5C. At that time to avoid hammering, we have operated the system above turndown condition by diverting the entire flow to one system and closing the second parallel system and the system worked fine. Thank you

– Mehul Gandhi

Author’s response:

Many thanks for your kind words about my article.

As you could have inferred from the article, the line that you quoted, “this method is better than introducing all the liquid into the vapor space, but is not as good as introducing all the liquid near the bottom of the drum” means exactly what it says. My experience is that the best method is to introduce all the liquid near the bottom of the drum, no slots. When we had to deal with an existing pipe with slots that hammers at turndown, we just blanked the slots or replaced the slotted pipe with one that has no slots. In Figure 4 we used a solid pipe that went down to about 6 in. above the drum floor, no slots. Recently we experienced two incidents of hammering similar to yours. Both had slotted pipes that were designed by others. In one, we blanked the slots — no more hammering since. In the other, we did not have a turnaround yet. Reducing the condensate subcooling (by switching off fans or cutting on cooling water) had been effective in both and other cases, but switching off fans or throttling cooling water generates its own problems. I also like your solution of increasing the loads to get the system out of turndown — neat.

Normally the pipe size is usually dictated by the maximum flowrate, typically designed for no more than 3–4 ft/s liquid velocity at maximum rate to avoid incurring excess turbulence.

-Henry Kister
Fluor, Aliso Viejo, CA, 92698
 

Editor’s note: Some excerpts were removed from the above letters to fit to page. The full letters can be found on our website at www.chemengonline.com

 

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