# Mixing Question



## Fudgey (Sep 28, 2006)

So I ate and drank a little too much the other night, and my stomach got really full. Eventually, I barfed a couple of times.

I was observing my ejecta as I was hurling away, and I noticed something.

The human digestive system behaves more like a plug flow reactor than a continuously flowing stirred tank reactor. (CFSTR)

The first thing I noticed was the creamy remnants of some ice cream and apple cobbler I had for dessert after I got home from eating.

Then I some of the second plate I ate at a Chinese buffet for dinner. A hint of zucchini (from pepper beef with zucchini), a little bit of boneless rib, etc.

Then as I continued making chunky, I saw some of the earlier stuff from the first plate. A bit of broccoli from a tofu and broccoli dish, and a little shrimp from a seafood dish.

The last thing I saw was a bit of yellowtail from the sushi I grabbed for an appetizer.

I then realized that the digestive tract is best modeled by a PFR arrangement. Discrete objects moving along without mixing, entering and exiting at a certain time. Based on peristaltic velocity in this case I guess?

If it were a CFSTR set-up, I think I would have seen everything mixed together in a homogenous pile of slop.


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## JasonT33 (Sep 28, 2006)

Was there any backmixing? Variations in the exit velocities of your chunkies?

If that was the case, I believe you are dealing with Dispersed Plug Flow

Regards

JT


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## FusionWhite (Sep 28, 2006)

The assumption in chemical kinetics is that CSTR's are homogenous throughout. This assumption isnt always valid if the tank isnt well stirred (last time I checked most people dont have mixing vanes installed in their stomachs). In addition for CSTR's flow in = flow out which is most certainly not the case for your stomach. Your stomach is more of a batch reactor with a very low rate of reaction value. Stuff goes in, mixes very very slowly, reacts very very slowly and then comes out (one way or another). Ill have to bust out my copy of Perry's ChE Handbook and see if I can calculate the reaction constant for your stomach.


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## JasonT33 (Sep 28, 2006)

Simple batch reactor

View attachment 185


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## Fudgey (Sep 28, 2006)

Wel maybe you are onto something. I just kinda hung out after dinner because I didn't feel so good.

But I bet if I ran around a lot and did some sommersaults, I coulda mixed it.

I think a batch reactor with a very low 'k' value for the reaction rate might be the best way to model it.

But, a batch reaction has no intake or outflow of materials. It's a closed system.

Maybe the best way to model something like this would be a low-tech lagoon. Low flows in and out, some detention, low reaction rate, odor issues, etc.


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## petergibbons (Sep 28, 2006)

You guys are definetly on to something here. :thumbsup:


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## FusionWhite (Sep 28, 2006)

Even if your stomach and intestines are a PFR the velocities required for plug flow would be so high the food you ate would be splattering in your boxers before you could say "pass the salt".

Batch reactor with phase change and gas withdrawl (which would explain the belching and bad smell).


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## Fudgey (Sep 28, 2006)

> Simple batch reactor


A typical fudge pump.


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## GTScott (Sep 28, 2006)

Will this be on the exam? I can't find this covered to any great degree in the CERM.

-GT


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## RIP - VTEnviro (Sep 28, 2006)

Ah, I see you've already been introduced to Fudgey.

No one's sure if he's actually an engineer or taking the PE or what.

He's generally harmless, just don't take his sandwich.


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## Fudgey (Sep 28, 2006)

> Even if your stomach and intestines are a PFR the velocities required for plug flow would be so high the food you ate would be splattering in your boxers before you could say "pass the salt".


Sometimes I do!



> Batch reactor with phase change and gas withdrawl


Well I think it'd be more like off-gassing than gas withdrawl. If you collect some waste gas to treat or use later, like using the methane from digested sludge, that's gas withdrawl.

If you're just exhausting it, that's off-gassing. And since there aren't any tubes in my butt right now that I know of, I think it's off-gas.


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