7 replies to this topic

[Olaniyi]

Hi all
First of all many thanks from the helpful ideas I've recieved and read here over the years. I have a situation under consideration and would appreciate any theoretical and practical advice/experience.

A client is considering removing the insulation on a deethanizer column due to issues with corrosion under insulation. The main concern is that the site ambient can get as low as minus 15degC, and the client is wondering as to how this would affect the duty requirements in the column, meeting of product specs and the column temperature profile.

I've attached 2 unisim models which I roughly developed for the column, one each for the insulated and non-insulated case. For the non-insulated case I used the heat loss functionality in unisim (under column environment) and used a typical U value of 50W/(m2K). The feed data is as specified in the column feed in the model, entering at 25 bara and 25 degC and containing N2, CO2 and C1 up to C10. The column spec is 2% C3 in C2 top product which is achieved by setting a tray temperature (theoretical stage 2) at 80degC.

My questions are as follows:
(i) Does this model work as I have built it? I can see from the model that my heat loss (hence increase in reboiler duty required) is minimal - less than 5%.Is this correct, or is there a better way to go about this?
(ii) My column temperature profile seems not to have changed almost at all.. I was expecting a slight change in the column temperature profile and this gives me some concern. Is there something I'm missing?
(iii) Are there other considerations before taking this kind of action? I'm wondering for example if the airside temperature of -15 degC will not cause any condensation on the inside of the column of components that could cause corrison which would normally not condense when the column wall temperature is hot due to the insulation?
(iv) Any other ideas or considerations from people's personal experiences both theoretical and practicalon uninsulated columns in cold ambient environment

Many thanks!
Ogeds

Attached Files

•  deethanizer mode.zip   489.82KB   35 downloads

[pavanayi]

Ogeds,
My observation regarding question (ii) :
The column temperature profile depends only on two parameters: The composition of hydrocarbon on a particular stage, and the pressure at the particular stage. The temperature will be fixed according to the vapor liquid equilibria. As long as the composition or pressure does not change,I would not expect the temperature to change.

[Olaniyi]

Thanks Pavanayi!

[Zauberberg]

The temperature may change - and that is all about - depending on the temperature difference between process fluid and the atmosphere, and also depending on the overall heat transfer coefficient. Simply stating that temperature will not change because pressure and composition are fixed, is not true. Why? Because, if there is any additional heat transfer between the system and the ambient, it will affect equilibrium conditions inside the column. That is why we insulate equipment.

To the OP: not sure how Unisim calculations are reliable with respect to convective heat transfer with ambient, but maybe you can try to do it manually - by knowing superficial velocity inside the column, and ambient conditions (temperature, wind velocity) outside the tower. It doesn't need to mean that you will be 100% correct but it is always good to verify software results with something that arises from heat transfer fundamentals.

[pavanayi]

Zauberberg,
My line of thought was like this:
As you said, there will be heat loss to the atmosphere due to the lack of insulation, which means it will affect the equilibrium conditions. My bottom product specification will not match the original composition because lighter components will be more. To achieve my original specification, I will have to increase the boil up (Heat input to the system): Which will lead to the conclusion: The new heat loss from system due to lack of insulation will have to be matched by additional heat input through the reboiler to achieve my original column specifications; which should mean my temperature profile along the column should be the same before I removed my insulation.

Since my feed composition is not changing, neither is the top/bottom product qualities and the column pressure, I concluded that at steady state, the temperature profile should not change.

[Zauberberg]

Yes, that is true - speaking strictly from thermodynamics point of view. The true question here, in my opinion, is what will be the effects of removed insulation, i.e:

- Is additional reboiler duty available, and
- Will the tower flood as the reboiler will need to provide more heat to the bottom section of the column?

Cold ambient will act as a heat sink (in essentially the same way the overhead condenser does), and this will in reality change the overall temperature profile. Hysys cannot tell us that; it only sees that the heat lost to ambient will be compensated by additional reboiler duty, and the profile will remain unchanged. This may not be true in reality, depending on how much heat is lost to the surrounding atmosphere.

[Olaniyi]

Thanks guys for the helpful comments. There is additional reboiler duty available to meet the extra heat load required. However Zauberbergs comments do give me pause - as he said the simulation software still maintains the column profile. Any idea how one may go about trying to simulate how the column temperature profile may actually change in reality..?

Without having looked at the models, composition would change if the external reflux has been fixed. If the compositions at both ends are fixed, the external reflux should adjust itself (get lowered) since internal reflux has increased and you may perhaps end up with a similar reboiler duty.