reboiler control query

[hswang2]

Reboiler controls can be classified into flow controls and temperature controls as
shown in Fig. 4-9 (a) and Fig. 4-9 (b).(please see the attachment)
Which should be used depends on the purpose and conditions of the tower.
The problem here is the flow measuring point and the location of the control valve.
If flow measurement is conducted downstream of the reboiler, should we adopted a small size orifice, or any measurement difficulties will arise in this case.

Meanwhile, if the control valve is located-downstream on the condensate side, I don’t quite undersatnd the following merits we can be obtained
(1) If the control valve is located downstream of the reboiler (on the condensate
side), could the required valve size be be smaller than the case where the control
valve is located upstream of the reboiler?
(2) As the reboiler steam pressure can be kept at the same pressure as the steam
supply pressure (the pressure before pressure reduction in the valve), the
temperature difference can be effectively utilized.
(3) The pressure on the return side can be maintained at a fixed level
(4) If the control valve is located before the reboiler, probably the reboiler
pressure, namely the condensate pressure will lower excessively, depending on the
reboiler heat transfer area.

Could you anyone kindly explain to me? Thank you so much for your help.

 

[ione]

This is how I interpret it.

1. In temperature controlled operation, the control valve should be placed upstream the reboiler on the steam side, and managed in a modulating manner in order to control process temperature fluctuations. The reboiler is basically a heat exchanger, fed on the hot side with steam. Downstream the reboiler you have a steam trap, sized to handle the steam flow required by the process. The steam trap act as an automatic valve which is open to discharge the condensate formed, drain the reboiler and prevent the risk of flooding and those bothering problems such as reduction in reboiler’s performance and corrosion issues. No need to control discharge operation. Steam is kept within the heat exchange until it turns into condensate.
2. The parameter which plays the bigger role in steam fed reboiler is the latent heat, that is the heat realised during the heat transfer process when steam turns into condensate. The higher the steam pressure the lower the latent heat. So, keeping in mind that the driving force of a heat transfer process is always the log mean temperature difference, the lowest the steam feeding pressure, the higher the latent heat available for the process.
3. Steam traps are characterized by a discharge capacity which is related to the differential pressure existing over the steam trap itself. Neglecting flash steam formation, which is an issue, steam traps discharge saturated condensate at its saturation temperature. The back pressure on the condensate line depends on condensate flow, size of the condensate pipe, elevation, fittings.
4. It is understood that you have to take into account the pressure drop caused by your control valve fitted upstream the reboiler, together with the other pressure drop in the line from the steam source to the reboiler. If underestimate the pressure drop and so the actual pressure of steam available in your heat exchanger, you’ll never be capable to size the reboiler the right way.

 

[ione]

Well, this is one of the aspects of this site I like most: it forces you to make questions and extend your knowledge. Before reading this thread I wasn’t really aware about reboiler operated on condensate side contol.

I will now attempt to give an answer to your questions based on formation I have gathered :

1. In such type of reboilers the control valve is actually placed on the condensate side, and it has consequently a smaller size than that of the valves located on steam/vapour side (it is consequently cheaper and does not ask for a steam trap unit). It is smaller because steam has a higher specific volume compared to condensate. This implies that a steam mass flow rate flowing through given valve determines a higher pressure drop than that produced by the same mass flow rate of condensate through the same valve. As drawback you can have cavitation or flashing issue (control valve manufactured with a specifically conceived trim are available to handle flashing fluids).
2. Steam supplied to the reboiler has the maximum pressure (no throttling on the steam side) and saturation temperature is the highest possible as well. Consequently the deltaT appearing in the heat rate formulation (q = U*A*deltaT) is not changed, as it would happen with a control valve on the steam side. On the other hand in such type of reboilers a variable portion of the surface is kept flooded. They are operated varying the surface involved in the heat exchange process. As the condensate level inside the reboiler varies, the most “active” surface (A), I do mean the one in touch with the condensing vapour, where latent heat can be exploited, varies as well.
3. With a control valve on the steam inlet the steam pressure changes. As condensate removal capacity of a steam trap depends on the pressure difference established over the steam trap itself, this could lead to heat exchanger backing up (this is more likely to happen when heat exchanger is over-sized). This should also give you a hint on your fourth question.

 

[25362]

Upon assuming the steam is saturated, the superheat after a CV would be moderate and not affect the heat transfer coefficient, there are other factors to keep in mind such as the type of reboiler (vertical, horizontal, etc.), the fluid being reboiled whether extremely fouling, reactive or not, widely changing process loads, the critical heat flux, etc. See, for example,

Chemical Engineering articles:
Performace of steam heat-exchangers by Jimmy Mathur, Brown & Root, Inc., Sept. 3, 1973.
Operating performance of steam-heated reboilers by Albert E.Helzner, Badger america Inc., Feb. 14th, 1977.

Hydrocarbon Processing: Controlling steam heaters by W.C. Driedger, Colt Engineering, November 1996.

thread124-105064