8 replies to this topic

[mnancej]

Hello,

I am designing a thermosyphon for a fatty acid distillation column with HTRI.

As I have the required duty for this reboiler based on my chemcad simulation, i am achieving only a max of 5% vapor at the outlet of the thermosyphon. As the duty is fixed, this means that the amount of vapour is also fixed (am I right?)

I have tried reducing the inlet and outlet piping to achive a lower recirculation. However, I have limited the outlet velocity to about 70m/s.

Any idea on how I can increase this ratio of vaporization.

Also, there is a message that is warning me of choked flow velocity at the outlet of the thermosyphon. But if I increase the size of the outlet piping (which by the way is 800 mm for a 1500mm shell size) this warning is still existent but then my rho-v2 is too small for a good vaporization.

Any ideas anyone?

Thanks in advance.

[chemsac2]

Try reducing static head (driving force equal to difference in height of liquid level and reboiler inlet). This input is available in reboiler input tab in HTRI. This change would be more effective than changing resistance i.e. changed line sizes.

Your pipe routing and equipment layout should consider this static head.

For a given duty, amount of vapour (i.e. kg/hr of vapour) would be constant and set by column design. Recirculation would just decide %vapour in reboiler outlet. % vaporization of about 10-40% is common (depends on operating pressure of column).

Regards,

Sachin

Edited by chemsac2, 13 September 2010 - 11:34 AM.

[mnancej]

Thanks for the info Sachin.

However, my liquid static head is already at minimum. As I am taking the top of the tubes at equal level as my liquid level in my column, the static head is already equivalent to the length of the tubes which is 3m. In reality, my reboiler will be operating at a higher circulation rate because I am basing my liquid level on the low level instead of the normal operating level.

At the moment my reboiler is about 50% overdesign but I still cant get the choke flow problem to go away.

[millne123]

Try increasing the static head of the inlet pipe to the thermosyphon reboiler. Typically the static head at the inlet must be able to overcome the inlet pipe losses, the pressure drop through the reboiler and the losses in the return pipe. For good design the static head should be approximatle 2 - 3 times the losses in the pipe. Also the outlet velocity seems very high. Typically any outlet velocity above 21 ft/s should suffice for the thermosiphon reboiler. The high outlet velocity is contributing to high head loss in the return pipe, thus contributing to the choke flow warning you are observing

[chemsac2]

Does your system have high viscosity products operating under vacuum?

Vacuum would lower density, increase velocity and increase chances of choked flow.

If possible furnish Final Results sheet of HTRI output fur further analysis.

Attached is extract from Introduction to Process Engineering and Design By Thakore/bhatt, page 247.

See if this is applicable to your system.

Regards,

Sachin

Attached Files

•  Vacuum_Reboiler.doc   123KB   135 downloads

Edited by chemsac2, 14 September 2010 - 09:52 PM.

[mnancej]

Try increasing the static head of the inlet pipe to the thermosyphon reboiler. Typically the static head at the inlet must be able to overcome the inlet pipe losses, the pressure drop through the reboiler and the losses in the return pipe. For good design the static head should be approximatle 2 - 3 times the losses in the pipe. Also the outlet velocity seems very high. Typically any outlet velocity above 21 ft/s should suffice for the thermosiphon reboiler. The high outlet velocity is contributing to high head loss in the return pipe, thus contributing to the choke flow warning you are observing

Thanks for your advice.

However, wouldn't increasing the static head mean I am increasing my flow and hence the choke flow would be higher.

[mnancej]

Does your system have high viscosity products operating under vacuum?

Vacuum would lower density, increase velocity and increase chances of choked flow.

If possible furnish Final Results sheet of HTRI output fur further analysis.

Attached is extract from Introduction to Process Engineering and Design By Thakore/bhatt, page 247.

See if this is applicable to your system.

Regards,

Sachin

Hi Sachin,

Yes I am operating under vacuum, from about 27mbars to 16mbars, but i think my viscosity is still considerably low (0.2cP) because my temperature is very high at about 233-257.

I will try to attach relevant files.

The major thing is that the output says that my thermosyphon system is stable. And from what I have read, choke flow amounts to the fact that I am able to increase my mass flow.

I have decided to reduce this mass circulation rate because as my duty is fixed, hence my vapor flow is fixed. By increasing my liquid mass flow, the percentage of vapor to liquid drops to about only 1 percent.

Is my design acceptable?

[Jenergy]

Dear mnancej:

I think you are misunderstaning about driving force of thermosyphon reboiler.

It is the difference between hydrostatic head(density x height) of feed line and return line. So you must take base line of the heights on the lower tube sheet, because the densities of the two lines are different above this point.

Though you say that you've lowered feed line liquid level (... taking the top of the tubes at equal level as my liquid level in my column...), it has no relation with the static head, because driving force is the difference of 2 heads. - it is already at the maximum driving force, isn't it?

If you raise the return line exit, the driving force will be weakened, and velocity slowed.

Regards,

Jenergy

[sheiko]

Which percent vaporization do you want/need to reach?
I believe it will also depend on the composition of the reboiler feed. Check if the percent vaporization required is in adequation with the amount of heavy components (which increase vapor pressure). Don't overlook thermodynamics.

And, will this thermosyphon reboiler be "once-through" or "circulating" type?

The basic approach for designing thermosyphons is to:
1/ Estimate the fraction vaporized
2/ Determine the circulation rate from the piping layout and pressure drop
3/ Calculate the heat transfer rates
The calculations are repeated until the estimated vapor fraction and calculated fraction converge.

You may consider reading the HTRI Design Manual - Design of Vaporization Equipment by Bell K.

Edited by sheiko, 16 October 2010 - 05:12 PM.