8 replies to this topic

[distillationman]

Hi friends!

I've just completed a simulation (assumed equilibrium stages) of a column wich separates water from methanol.
The second stage temperature (numbered from bottom to top) is 86ºC, and the the first stage temperature (wich represents the reboiler) is 98ºC. I intend to use saturated steam at 6,5 barg as a energy source for the reboiler. I need now to roughly estimate the reboiler size. To do this, i am considering the following temperatures to compute LMTD:

T1 = T2 = 168ºC ( saturation temperature of 6,5 barg steam )
t1 = 86ºC (second stage temp)
t2 = 98ºC (reboiler temp)

Am I considering the right temperatures? Or am I making a mistake?


I am assuming an overall heat transfer coefficient of 100 Btu/h ft2 F. Is this an adequate value for estimating the required area?

[Root]

Hi,
1^st you need to calculate required heat duty and then calculate required area.and for LMTD you need T1,t1,T2,t2 but in your thread no T2 given therefore 1^st learn about LMTD calculation and then go for other calculation work.
Toor

[Shivshankar]

distillationman,

http://www.cheresour...__fromsearch__1

Check this thread.
Hope this will help you.

Regards
Shivshankar

[katmar]

If the 6,5 barg is your steam supply pressure then you need to allow some pressure drop across the control valve - assuming you are controlling on the steam side. In this particular case if you allow a typical 0,25 to 0,5 bar across the valve it makes very little difference to the temperature but I point it out just for the sake of highlighting the correct procedure. So I would take T1 = T2 = 165°C.

On the process side you should not take the second stage temperature into account. The temperature on that stage in lower than the bottom because there is colder (strictly - richer in methanol) liquid flowing down from the 3rd stage to the second stage. This has no influence on the reboiler. In fact t1 may be very slightly higher than t2, but in a thermosyphon reboiler there is lots of liquid circulating and the temperature will remain very close to the saturation temperature. I would take t1 = t2 = 98°C. This will make the LMTD lower than in your proposal and it is therefore the more conservative option. I have never calculated a reboiler using anything other than t1 = t2.

[paulhorth]

First, I would say that Toor is wrong, not for the first time.

but in your thread no T2 given

but distillationman clearly gives T2 = T1 (at 168 C), which is correct for condensing steam.

Now. I agree with Katmar's points if a recirculating thermosyphon reboiler is use.However, if a kettle type reboiler is used, drawing from the bottom tray, then there will be a change in composition from the reboiler feed to the reboiler liquid outlet,and this will give a differrence in temperature. In Hydrocarbon service (where the fluids are multicomponent mixtures) there can be significant differences between t1 and t2.
But Katmar's proposal, to use the outlet temperature, is the most conservative, so is a sound basis.
The heat transfer coefficient of 100 BTU/hr ft2 F is probably rather low for condensing steam and boiling water, you could consider 120.

Paul

[distillationman]

I thank you all for the answers! they helped me a lot!

But there's still a doubt: Once t2=t1=t and T2=T1= T , it's not possible to compute LMDT.
In this case the temperature difference to be considered would be simply: T - t ?

Mr. Katmar, I can't figure out how t1 could be (even slighter) higher than t2 (reboiler outlet). t1 is reboiler inlet ( column bottoms to reboiler).

Thanks a lot!

[katmar]

You are right - I got t1 and t2 the wrong way around. t1 will always be less than or equal to t2.

LMTD is just (T - t) when there are no temperature changes. I remember proving this in college, but my calculus is a bit rusty for that now!

[S.AHMAD]

I agree with Katmar that the driving force is the temperature difference between condensing steam and boiling liquid. So your energy equation should be
Q = U A (T-t)

Edited by S.AHMAD, 23 April 2012 - 08:47 PM.

[distillationman]

Very good! Thanks again for the help!