Bobo!

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jmh80 said:
Sorry Sin - DC clearance is the distance between the bottom of the DC and the tray below.

Think about the path of the liquid and vapor on each tray. That should give you an idea exactly what will likely happen.

(This scenario was encountered last week near the bottom of a tower....)
Click to expand...

If the level is so far above the downcomer, that it's touching the DC from the above tower while the Column is down for inspection, I don't see how it wouldn't fill up and flood the above tray after startup.

The molar balance would have to satisfy this inequality (below), thus implying unsteady state: you are filling something up.
LXin + VYin > LXout + VYout
 
thesinner; said:
...
LXin + VYin > LXout + VYout
Click to expand...

Why not stay flexible:

LXin + VYin >=< LXout + VYout
 
Uuuhhhh....



 
matthew76; said:
I like the way you think, SM...
Click to expand...
Thanks, Matthew! Just wanted to suggest some flexibility. With TheSinner's permission, my expansion of his inequality will retain validity.
 
thesinner said:
If the level is so far above the downcomer, that it's touching the DC from the above tower while the Column is down for inspection, I don't see how it wouldn't fill up and flood the above tray after startup.

The molar balance would have to satisfy this inequality (below), thus implying unsteady state: you are filling something up.
LXin + VYin > LXout + VYout
Click to expand...

But why would the tray with the improper DC clearance flood?

And what type of flood?
(There are 2 kinds - if I give you the names, you'll figure out the why.)
 
strategicmove said:
Why not stay flexible:

LXin + VYin >=< LXout + VYout
Click to expand...

For a tray to flood - it must be out of material balance.

Sinner and I are talking immediately upon commissioning and start-up of said distillation column (i.e. a momentary point in time - unsteady state).

Further along - once the tray is flooded, it's flooded (to the tray above, that is). Then - the tray is in material balance.
What liquid is flowing to it from the downcomer of the tray above it is flowing down it's own downcomer to the tray below.

It just happens to be full of froth (about 2 feet worth) due to improper tray installation by idiot contractors that just worked at McDonald's (and a poor chemical engineer who just supposedly inspected said tower and didn't force the idiot contractors to fix said tray).
 
jmh80 said:
But why would the tray with the improper DC clearance flood?

And what type of flood?
(There are 2 kinds - if I give you the names, you'll figure out the why.)
Click to expand...

All I can think of at the moment is:
-The tray is too cold, and vapors are condensing at abormal rates.
-Feed Rate is too high, or there is an improper feed state
-There's a clog in the tray, or a design error (perferations have to high porosity, or DC is too small or mislocated.)

But my brain's a li'l fried right now, studying for finals.
 
Ok - I'll help some.

On a tray in the middle of a tower (let's say tray 20) - the weir for that tray is set up at a certain height above that tray. Let's use 2" in this case.

It's goal is to hold a liquid level on that tray - right? So that mass transfer can take place with the vapor come through it's sieve holes (or whatever type of tray it is).

The distance between tray 20, (where the liquid exits to the tray in discussion) and tray 21's downcomer MUST be lower than the tray 20 weir height.

Why?

Think about where the vapor could go if this were not the case.
(I've already established we have sufficient weir height to have a liquid level on tray 20.)
 
jmh80 said:
Ok - I'll help some.

On a tray in the middle of a tower (let's say tray 20) - the weir for that tray is set up at a certain height above that tray. Let's use 2" in this case.

It's goal is to hold a liquid level on that tray - right? So that mass transfer can take place with the vapor come through it's sieve holes (or whatever type of tray it is).

The distance between tray 20, (where the liquid exits to the tray in discussion) and tray 21's downcomer MUST be lower than the tray 20 weir height.

Why?

Think about where the vapor could go if this were not the case.
(I've already established we have sufficient weir height to have a liquid level on tray 20.)
Click to expand...


The DC needs to be lower than the weir(new term) so it goes directly into the liquid. If it goes into the vapor and "rains" down, you risk it drawing condensed vapor down with it (or vice versa depending on liquid and vapor conditions).

That makes sense. We never went over tray design in my sep's class; hence, the weir not above the DC in my drawing. I might have to have a chat with the CHEE deptartment's chairman.
 
thesinner said:
The DC needs to be lower than the weir(new term) so it goes directly into the liquid. If it goes into the vapor and "rains" down, you risk it drawing condensed vapor down with it (or vice versa depending on liquid and vapor conditions).

That makes sense. We never went over tray design in my sep's class; hence, the weir not above the DC in my drawing. I might have to have a chat with the CHEE deptartment's chairman.
Click to expand...

It's called a liquid seal of the downcomer. (Only liquid in the bottom of the DC - the settling area.)

If you don't seal the downcomer - the vapor WILL flow into the above tray's downcomer creating what is called a downcomer fill flood.
(Pushing the liquid up onto the tray above.)

The other type of flood is called jet flood. More of a Stoke's Law effect. The vapor velocity is sufficient to push the liquid all the way up to the tray above (through the sieve holes).

I came upon this situation near the bottom of a tower. I asked the contractors to fix it.
Took me an hour of arguing as to why. I finally gave up and said "look - you aren't paid to think. I am. Now - go do it!"



Mullet - I don't wonder. I've stated reasons in threads in here in the past. Doesn't have sh*t to do with chemical engineering.
 
jmh80 said:
Mullet - I don't wonder. I've stated reasons in threads in here in the past. Doesn't have sh*t to do with chemical engineering.
Click to expand...

Not being able to take a joke?

:stick:
 
Sin - hope that helps. You don't get taught that stuff in ChE undergrad.

I learned that from the grey beards once I got to industry.
 
jmh80 said:
My face is so incredibly ugly it's not even funny.

I look like the love child of Sloth and Mini Me.
(Except that I don't have Slothy strength, or Mini Me's tripod.)

:sad:
Click to expand...

Beauty is but a light switch or a bottle of bourbon away my little snugglepuss.
 
It's in the eye of the beholder. And you, jimmy ho, are being way too self-deprecating at the moment. Remind me to punish you for this.
 
jmh80 said:
Sin - hope that helps. You don't get taught that stuff in ChE undergrad.

I learned that from the grey beards once I got to industry.
Click to expand...

Yeah. There's a lot of stuff we aren't taught in college. It's actually been aggrevating me in class because everything being taught is so incredibly ideal

I started working parttime for a small cosmetic plating manufacturer (think of a set of shiny 22's) as one of their chemistry experts. It's....well...it's been interesting. :rolleyes:

What do you suppose happens if you strip zinc in the same acid tank as you pre-soak before going into the chrome plating bath? :study:

That was last weeks highlight.
 
Just remember - above all, you are there to learn to think/reason like a chem. engineer.

Not the equations or whatever else.
 
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