Dehydrator Reboiler Input Heat Requriement

[zdas04]

I took a vessel design class in 1981 and I made a marginal note in my book that "it takes 6,500 BTU to cook 1 lbm of H2O from the glycol" of an atmospheric reboiler on a TEG dehydrator. I've been using that number for nearly 30 years without questioning it. Last week I had to figure a reboiler capacity and wondered where that number came from. The steam tables get me much closer to 1,000 BTU/lbm (based on the change in enthalpy) than 6,500. I could accept 25% efficiency and get 4,000 BTU/lbm, but I'm having a hard time justifying the 6,500 BTU/lbm that I've used forever. Can anyone explain it, or should I just not rely on my un-footnoted marginal notes from earlier decades?

David

 

[dcasto]

that 6500 BTU / lb H20 looks like a ciculation ration of 2.5 gallons/lb H2O removed, which is on the shy side.

Anyway, over 70% of the energy goes to heating the glycol from 200F to 390F. the rest is heating water up, vaporizing it and some for the reflux in the heat balance. So, the 1000 BTU/lb heat of vaporization is about right.

 

[zdas04]

Thanks, I knew I was missing something. Adding the mc[Δ]T of the glycol gets it into the ballpark.

David