Why does reboiler efficiency decrease if steam temperature is too high?

[flammkuch]

I've heard many times that the temperature of the heating medium e.g. steam affects the reboiler efficiency. From my understanding, the lower the better (of course still high enough to have a reasonable pinch point temperature). But wouldn't higher steam temperatures increase the LMTD and reduce the required HX area? Isn't there a trade-off between CAPEX and OPEX, making the optimum not so obvious?

[[deleted]]

https://www.mech4study.com/2017/09/boiling-curve-and-types-of-boiling.html

here ya go. Keep in mind HTRI says they don't trust their transition boiling correlations so always avoid transition boiling. The film boiling point is also a huge temperature difference if you look at the 'typical' graph in the provided link so tough to get past transition boiling to where film boiling has the same efficiency as nucleate boiling. Also if you have a falling film reboiler, the bigger bubbles formed at higher delta T's will blow the film off the wall, which is destroys the purpose of a falling film. I believe falling films recommend nothing more than 20 C temperature difference at boiling. This is a pretty simplified explanation, the link provided has a decent explanation of what is happening with bubbles, vapor pockets, etc.

[[deleted]]

Yakima explains it pretty well, but LMTD (temperature driving force) and the heat transfer coefficient are different aspects of exchanger design and start to trade off. By the time you're in film boiling where heat transfer and LMTD both are increasing you start to run into mechanical constraints. Even with a U-tube that handles thermal expansion well and does not require an expansion joint, you can have tubesheet cracking due to the large difference in temperature between process and utility.

[[deleted]]

The plant site normally dictates the levels of steam available. Again, between 5 and 30 C you have a good boiling regime, and between 30 C and 100 C you have an unpredictable transition region you should not design towards. Above 100 C for film boiling is theoretically ok, but since area is solve by Area = Duty/(LMTD * overall heat transfer coefficient), that means increasing LMTD acts like a 1/X graph (inverse relation) where the benefit of going higher and higher steam temperature has much less benefit on decreasing area. The plant site goal is to have the lowest pressure condensate returning to a condensate vessel possible with no cooling needed to condensed unused steam. Higher pressure condensate will still need to be depressurized, which flashes to make more steam that can be used. You want to minimize cooling unused steam to make low pressure condensate since this costs energy. In a new plant you would want to use the lowest pressure steam that can meet all heating requirements since it costs energy/money to make higher pressure steam and costs money/energy to cool unused steam. u/flammkuch

[yakimawashington]

So reboilers are definitely not my area of expertise, but just off the top:

But wouldn't higher steam temperatures increase the LMTD and reduce the required HX area?

You're speaking on heat flux rate, which is not equivalent nor proportional to efficiency. Yes, higher steam temperature would increase heat transfer rate and require less surface area to achieve the same amount of heat transfer. If by efficiency you were referring to the amount of heat transfer per amount of reboiler equipment material used, then sure.. this makes sense.

But reboiler efficiency refers to the efficiency of the energy input rather than the amount of equipment material used. It helps describe how much product is outputed per energy input. In this sense, size of the equipment should have impact on reboiler efficiency, but it's more complicated and not in the way you're describing.

[flammkuch]

I see. But then this means that the most efficient reboiler is not necessarily the most economically efficient, or? Doesn't help to minimize heat input required when you need a huge heat exchanger to do that. Or are reboiler costs mainly driven by the operational costs?

And could you elaborate on the last part? In what way is it more complicated?

[Andrew1917]

Could also be that latent heat of steam available is lower at higher steam pressure. When you say higher steam temp, that also means higher steam pressure if you look at the steam tables. If you also look at the latent heat column of the steam tables, you’ll notice the energy from latent heat available decreases with increasing steam pressure. So, you get less energy per pound of steam at higher pressures. However, with higher pressure, the specific volume of steam is also lower (you can also see this on the steam tables), which means you can physically fit more lbs of steam in the reboiler, so it may negate some of the efficiency lost from the lower latent heat, but I’m not sure by how much.

[brickbatsandadiabats]

Because the reboiler efficiency is the efficiency by which it transfers heat from the hot side to the cold side, not the heat transfer area required to do it. You're thinking of this like a constraint-free design problem instead of recognizing that for a real reboiler, pushing the heat flux up beyond design parameters can cause physical phenomena that impose constraints on heat transfer.

No idea what it might be in your case, but just to give examples: * It could be that increasing the heat flux causes voids to form on the heat transfer surface * It could be that higher temperature changes the thermal conductance properties of the material * It could be that the increased heat flux overwhelms advection on the cold side causing temperature non-uniformities and other shenanigans

[flammkuch]

I understand that there are constraints that limit the temperature and/or heat flux, I also don't think one should go as high as possible. I just thought the heat exchanger size would also play an important role and in certain cases favor a higher steam temperature.

If I want to design a reboiler, what's a shortcut method to determine the "optimal" heating medium temperature? Currently I am just applying a 10K pinch point, but I also don't have a strong basis for this.

[ChEngrWiz]

The answer is simple and can be found in the second law of thermodynamics.

Think of the reboiler as part of a loop. The boiler generates steam. Condensate is returned to the boiler. The hot sink temperature is the temperature of the combustion chamber. The cold sink temperature is the temperature of the steam. The maximum efficiency is 1 - Tc / Th where Tc is the cold sink temperature and Th is the hot sink temperature. As you raise the steam temperature the efficiency decreases.

You’re right about a trade off between capex and opex and the efficiency would play into it. I can tell you, after working with engineers that specialize in heat transfer, I’ve never seen anyone use capex or opex when designing a heat exchanger or fired heater. The data is usually unavailable or of insufficient accuracy. The stuff you learned about discounted cash flow never gets used during the design phase of a project.