# Thermodynamics questions

I was hoping someone could help me out here.

Delta T refers to the difference in between your vapor temp and evaporation temp doesnt it?

Example above: (tell me if I’m wrong)

If I’m at 130 torr and my ethanol evaporates at 37c, since delta T is 30, I’d be raising the temp of my ethanol vapor to 67c correct?

The reason I’m asking about this is for pre heating, I’m trying to figure out a way to prevent boiling my ethanol in the plate pre heaters when using the hot vapor to pre heat. I think I could do this by using a plate heat exchanger to cool the vapor down into a liquid before running it though the plate pre heaters that pre heat the crude.

What do you guys think?

1 Like

Delta T ” is the most common use of the word delta in the HVAC industry, meaning temperature difference. If the temperature before a cooling coil is 75F and the temperature after the cooling coil is 55F, subtract 55F from a 75F to find a delta t of 20F.

3 Likes

you are unable to boil ethanol with only ethanol vapor without creating multiple pressure zones.

Even if the vapor is above its boiling point by 30 degrees?

1 Like

Either I’m really stoned or that spreadsheet is really tough to follow. Probably both. Uuuuuuunnnnniiittttssss

3 Likes

It’s super confusing at first, you can change the values though and it gives you heating and cooling numbers for how much ethanol you want to evaporate in an hour.

If you look at the spread sheet theres 2 different heat calculations

Evaporation and delta T.

If I can remove enough heat after evaporation but before the plate pre heaters I should be able to prevent evaporating ethanol in the plate pre heaters.

OR

I could create 2 different zones of pressure using a one way check valve.

If I have the preheater plates under pressure you wouldnt be able to boil the ethanol till it went under vac

I’m nervous though it would instantly vaporize when sprayed into the tube and shell under vacuum.

I know I’m crazy but I really think this design could work, and the efficiency on it would be pretty good if I could get it dialed in like you said

you’re a little off track with your vapor temperature assumptions

1 Like

First, DM me and I’ll give you my phone number and explain.

But let me try to clarify—you need to calculate heat required to raise the temperature of your solvent from T1 to its boiling point T2. This is where heat capacity comes in—heat capacity is a function of temperature, but let’s assume it stays constant for the time being.

T2-T1 is delta T.

HEAT 1 = massheat capacitydelta T

The heat required to boil the ethanol, which is now at its boiling point requires using the “Latent Heat of Vaporization”. Which is a measure of how much energy is required to boil a unit mass of ethanol.

HEAT 2 = Latent Heat of Fusion*mass

Heat 1 + Heat 2 = Total Heat Requirement.

You can get that value in Joules or BTU.

Divide by the efficiency of the heater (safely assume 0.6 - 0.8) and that’s the size of a heater you’ll need.

Terp is right, the pressure zones created will greatly complicate those calcs. You need to find how temperature/pressure affect heat capacity of ethanol vapor and then you can start calculating required heat addition/removal based on a more accurate heat capacity.

6 Likes

If I’m running the whole thing under vacuum though then theres only 1 pressure isnt there? As long as I dont have a check valve or a pump to create pressure on the crude side of course, if I put a pump in between the evaporator and pre heaters then I’d have different pressures wouldnt I?

I was told by someone smarter than me that if you want to do the 2 pressure zones youd need to treat them as seperate enteties then integrate them.

draw a picture of your proposed system. post it or dm it to me.

2 Likes

Will do, I’ll send it on IG if you dont mind

1 Like

I’d love to jump in but I still have no clue what I’m looking at😂

The rows and columns follow no order. I see units of density as “kW,” units for moles as “mol/L,” etc…

3 Likes

Yeah the units columns got screwed up with that somehow. I must have deleted a couple of cells at some point and missed the unit shift. The math should still be correct.

The top kW and BTU should be removed from C17 and C18. I think the rest are correct at first glance.

@Kingofthekush420 I built that sheet. If you want a hand with it feel free to DM me.

4 Likes

If I’m understanding your OP question correctly, you’re asking about how to prevent crude laden ethanol from boiling in your preheater(s), I’ll make an assumption this is regarding the input to the FFE you posted pics of in the “seems to be the cheapest falling film” thread?

Assuming that’s the case, and you are using a spray nozzle on your infeed side you will have two different pressure zones on either side of the spray nozzle as there is a restriction point there. Past the spray nozzle there will be a pressure gradient that is highest near the spray nozzle that gets lower as the vapor cools and finally condenses on your cold side, this pressure gradient is what keeps vapor moving from the evaporation side to the condensing side. This is true regardless of whether or not there is vacuum on the vapor path of the FFE.

IMO the easiest way to keep your input fluid from boiling is to plumb your inlet side like this
holding tank → pump → heat exchanger → spray nozzle.

Probably put a tee at the spray nozzle with a gauge too so you can see what pressure is occurring there

And figure out what your strategy will be to regulate/maintain pressure. A liquid held under positive pressure will take higher temperature in order to reach boiling temps, the reverse is also true (which is why water boils at 95C near Denver and even lower temps at higher altitudes)

1 Like

Can someone post this spreadsheet again?
I cant find it for the life of me right now…

1 Like

@hambread do you have it somewhere?
Lost it a few months back

Wanna bet it’s behind Search results for '@Lincoln20XX .xls' - Future4200

Or you could just pretend you’re @Kingofthekush420 and ask the guy that posted it…

1 Like

I’m quite certain that there’s a web version linked somewhere in my post history.

I found it and sent it to cannachem already

4 Likes