simple (?) question regarding tank headspace vapour

[leakyseal]

Hi Folks,

I am having a real Chem Eng 101 Brainfreeze. My embarrassment at having to ask this question, however, is outweighed by my desire to get things straight in my mind.

I have an essentially pure solvent in a storage tank (relatively high boiling, benign solvent, smallish, in plant process tank). The tank is being vented through a condenser by virtue of a downstream blower and I am trying to estimate the solvent load to the condenser.

Sadly quite rusty on equilibria calcs. I approached the calc (baby steps!) like this:

- assume exhausted vapour is always at equilibrium with the liquid (conservative)
- partial pressure of solvent in tank is equal to vapour pressure at tank temp (pp = vp * x, where x is liquid molar fraction of solvent, 1.0 in this case for pure solvent)
- liquid mole fraction y = pp / Ptotal, where Ptotal = 1 atmosphere

So now I know the mole fractions of solvent vapour and air in the exhaust (under equilibrium). Knowing mole weights I can then determine mass fractions, and knowing vapour densities I can then determine volume fractions. I need this because I only know my exhaust flow in volumetric units (scfm).

This part gets a little sketchy because I can only estimate the vapour density with SG based on MW ratios. Also the air density is not known accurately because I don't know the RH. Doesn't matter, I can wave my hands at it and get close enough. So I calculate about 1.2 lb of solvent every minute in my exhaust (2000 SCFM) which seems high to me, conceptually...so I try to corroborate it.

Which is where my confusion starts. I start fiddling with EPA TANKS 4.09, which calculates its vapour density according to:

density = molecular weight * vapour pressure / RT

This produces a much lower density than my prior estimate (like, a couple of orders of magnitude) and hence a lower overall vented mass. I recognize conceptually the validity of the equation (from IG Law etc) but can't figure out where I misstepped in my first approach to get such a different answer. (As I write this I wonder if I just miscalculated the volume fractions...but I checked them several times...but it was late last night...and the calcs are on the desk in my office while I am now sitting at home...but I can't get this off my poor mind!)

Assuming you've slogged through this, any insight?

thanks in advance,
Leaky

 

[Montemayor]

Leaky:

Quite frankly, I almost fell asleep reading all about your struggle to find equilibrium values and such things as that. In your second paragraph you state that you are using a "downstream blower and I am trying to estimate the solvent load to the condenser". The answer to your quest is the capacity of the blower! Someone, somehow, somewhere selected or picked this blower. The capacity should be stamped on it or the proper documentation stored in any respectable engineering department should have it.

What is going to be the fun part is that knowing the capacity of the blower is not enough. What you need is a flow RATE and you also fail to tell us HOW LONG this operation is going to take place (this is a BATCH OPERATION, as described). Obviously you are not going to be taking vapors from a pure solvent 100% of the time - that is, UNLESS there is some steady source causing the tank vapor pressure to increase. Otherwise, something doesn't make sense in this operation or you are not telling ALL of the story. We have to presume that the basic reason you are taking vapors from this tank is because it is increasing in vapor pressure - possibly to a point close to the MAWP of the tank - correct?

If the above is correct, then we also have to presume that the reason the vapor pressure is increasing is because the tank is receiving heat from an external source - correct? That is probably due to the vapor pressure characteristics of the stored solvent and the state (or lack) of the tank's insulation. Normally, the heat leak into the tank would determine the vapor pressure generation RATE and the ability to vent the excess vapor generated to atmosphere. But you probably don't want to do that for environmental or economic reasons. Therefore, the need to boost the pressure of the excess vapors in order to send them to a recovery unit - a condenser. I would wager that the capacity of the blower is based on the maximum tank heat leak PLUS a contingency amount.

After assuming all the above basic data, the answer is:

Size the condenser based on the capacity of the blower (if you want to make sure) or of the calculated tank heat leak (if you want to be "tight").

I hope I guessed all the basic data right.

 

[leakyseal]

Hi Montemayor,

Thank you for your reply. I don't blame you for having a hard time staying awake!

You are correct that I did not give you the whole story. Tank is heated to 80 degC to facilitate dissolution of solids in the process vessel, via hot water dimple jacket. The tank vent is planned to be fitted with a cannister filter housing and fan, to maintain a slight negative pressure and draw up any free dusts. The fan is therefore sized for linear velocity to produce good dust capture, with no consideration being given to heat input or vapour generation. (Er, I framed the problem as a condenser because I thought it would make it "simpler"...*sheepish grin*).

I was working elsewhere in the facility and made the drive-by comment "has anyone considered whether any free solvent vapours are going to get pulled into this device, and either condense or end up back in the plant?" (since the filter currently vents back into the indoor production space). They shrugged and asked me to look into it...so I blew the dust off my basic VLE notes and set out to estimate the vapour load and see if we even had a problem.

At root I am trying to sort out why I can't get agreement between two basic methods for a simple VLE calc that I should be able to do in my sleep.

thanks
leaky

 

[BigInch]

Then this problem should be solved shortly.

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