5 replies to this topic

[Josie]

Would the reduction in feed rate to my distillation tower affect the relationship that properties have on each other? For example in normal operation, the increase in overhead pressure usually results in my bottom impurities to increase. However, in the case of a reduction in the feed rate to the tower, would there be a possibility that this relationship will not hold? I have tried trending PHD data and somehow this might be true but could anyone enlighten me on the theory aspect of this? Thanks in advance.

 

P.S. sorry for my poor english because it is not my first language.

[Pilesar]

There is a possibility the operating relationships may not hold. Lowering feed rate can affect the efficiency of the separation. In real equipment, the flow regime can change appearances such as spray, mist, or bubbles. The flow pattern can change such that there is recycling of liquid inside the column or bypassing resulting in worse vapor-liquid mixing. Reducing feed will also reduce the tendency of the vapor to entrain the liquid higher in the column. The separation may get better or worse depending on whether you are moving toward or away from the conditions of best efficiency. So the relationships you notice may continue over a certain range, but should not be extrapolated too far.

[Pilesar]

In a private message, Josie requested I explain my answer further. I answer publicly because I want others to correct me if I am wrong or have not properly understood the question.

 

Distillation columns (trays or packing) have an operating range. There are many parameters that change with flow rate. Google "distillation turndown" for general background information. You are familiar with plots of stages versus reflux? The Gilliland chart is good example: http://www.hyper-tvt.../gilliland2.gif Reducing feed rate changes the efficiency which changes the number of stages in the column. To keep the product separation the same, you would need a different reflux ratio than would be predicted by a linear relationship.

[Josie]

Hi Pilesar,

 

I thank you for your efforts and dedication to this thread, I really appreciate it. I understand your point on a lower feed rate causing lower efficiency which changes the number of stages in a column hence affecting separation.

 

However, I'm still perplexed as to how the inter-relationship between overhead pressure and bottom impurity will change.

 

In normal operation, an increase in the overhead pressure would result in more impurities being pushed down the column to our bottom products. However, in a low feed rate operation, the increase in overhead pressure surprisingly results in lesser impurities being pushed down to our bottom products. This is more of a question of the relationship between two properties (overhead pressure and bottom impurities) for optimization and not so much as to how separation changes with low feed rates. 

[Pilesar]

In your initial query, your question was "would there be a possibility"? and you ask to be enlightened about "the theory aspect of this." My answer is "yes, there is a possibility" and I gave some theoretical descriptions of how changing feed rate can affect operation. Now you say you have actually seen the relationship in real life. Why did you ask "would there be a possibility" if you had already witnessed the very thing happening? Now this thread is changing from "theory" to discussing the performance of an actual column for which you alone know any details. Unfortunately, my ability to read minds is at one of its periodic nadirs and I will not offer you further insight on this matter. Thank you for expressing your appreciation of my participation and I hope others might be more suited to help you. Please reveal the answer you are seeking when you discover it as I am interested in how you find resolution.

[Josie]

I apologize if I had caused any confusion with my choice of words. I was trying to draw the link closer between lower feed rates and relationship between inputs.

 

Yes real plant data has roughly shown such a twist in the relationship but because it is not concrete enough it might just be a coincidence at that time where probably other factors came into play. That was why I felt that I needed some theory to draw the link closer between lower feed rates and this twist in the relationship. I myself am not very sure about this whole thing because this trend might or might not even exist. What I can do now is probably extend the trending to more time periods to see if it is really true.

 

Nevertheless, I am still grateful for your valuable insights and hope that I have not offended you in any way. 

 

Cheers.