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Quench Tower/absorber Tower


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#1 CRF

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Posted 20 April 2010 - 02:23 AM

Hello there,

I am having some trouble designing a quench tower... actually, i am not sure how to start it. Should I consider it like an absorber tower? Can I simulate it in ASPEN PLUS like Radfrac, to predict the amount of water? what about NUT and HUT?

the goal is to exchange heat with a stream gas coming from a reactor, from 230 C to 90 C, at 19 bar... then, the water vapor in the stream will condensate and also some co2 will be scrubbed, even tough its not the objective.


ah, just one more thing... can i use metal packing ( steel carbon), in a absorber tower with gasoline and glp, at 50 C? I thinks theres no problem right, since gasoline is not corrosive and metal is the cheapest option


thanks folks ;)

#2 abhi_agrawa

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Posted 20 April 2010 - 09:11 AM

Carlinha,

I think that you have a hot stream coming out of a reactor at 230 deg C and you intend to cool it to 90 deg C, by contacting with water. So your tower will have the hot gas feed at the bottom, water will be fed at the top. The vapor from the top of the tower is to be at 90 deg C, the bottom product will be circulating water + condensed water. You also mentioned about condensed gasoline components. These should also be taken out from the bottom of the tower.

Now if my understanding is correct, this tower has the following functions:
- Cool the reactor effluent
- Condense gasoline components from reactor effluent
- Absorb some CO in water (incidental)

You can simulate this tower in ASPEN PLUS as a Radfrac column. Please be sure to check the Absorber option "ON" for easier convergence. In this tower, you have no reboiler or condenser, so the degrees of freedom is "0".

I do not know what you mean by "NUT" and "HUT". If you implied "Number of transfer Units" and "Height of transfer unit", then for number of transfer unit (NTU), you will have to do a rigorous heat transfer calculation. The height of transfer unit (HTU) will depend on:
- The type of packing
- Size of packing

Please search literature for HTU. Once you have the HTU, assume some heat transfer coefficient and do a heat balance across each simulation stage. The temperature of vapor/liquid in and from each stage is known, hence the total heat transfer is known. This will give you the duty that can be extracted from that stage. From this duty you can estimate the water condensation in that stage.

I have seen and deigned several such towers, and invariably the material of packing is Carbon Steel. Usually there is no corrosion problem (unless the reactor effluent can form corrosive compound after dissolving in water), also carbon steel packing will have much better strength than plastic or ceramic packing.

Hope this helps,
Abhishek

#3 CRF

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Posted 20 April 2010 - 10:09 PM

Abhishek,

Thanks for the help!
Actually, I know how a quench works, but I was not sure how to design it... the diameter is a function of liquid and vapor flows, but I was wondering how to calculate HTU and NTU, since the main objective is heat transfer, not mass transfer.
The objectives are:

-Coolling the gas stream - reactor efluent;
-Condensating the water of the gas stream;
-Scrubbing part of co2 of the gas stream.

I was thinking about using a packed tower, but I ve been reading and I think that´s not the best option... I will use a spray tower. In this case, I can design it as a flash tank with some spray nozzles(atomizers)...I am not sure how to calculate the diameter of the atomizers and how many ...it sure will be function of water flow, but i ve got no references about atomizers


The gasoline will be absorbed in other tower.


Thanks again ;)

#4 abhi_agrawa

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Posted 21 April 2010 - 07:58 AM

Carlinha,

For this service I have never designed a spray tower, and can not help you in that.

I have designed several such tower that used packing. Please note that the contacting efficiency in a spray tower will be significantly less than that of a packed tower.

Abhishek

#5 siretb

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Posted 05 May 2010 - 08:40 AM

Hello.

You have the two options, as stated.
For a packed column (metal or ceramic packing) the HTU will be reasonably well calculated by ASPEN, RATEFRAC.
You will have the risk of a dry/wet transition zone, if the gas is moving upwardsa and if the wetting rate is low.
I see you operate at 19 bars. At what temperature do you have the so called adiabatic temperature?
As for the amount of water necessary, you may use , if you want to go for the ASPEN option, a simple flash coupled with a design spec. You increase the water flow until some liquid phase apperas in the liquids of a Flash2. You also get the temperature.

Many a time a conditioning tower thta will spray water to cool gas down, from temperatures of some 300-400°C down to 150°C will be high (some 15m to give a flavor). For 19 bars and 90°C, this will change a bit.

You do have a third option, depending where the dewpoint is, and if the 90°C you mention is critical: That would be to spray a large amount of liquid, (quench like design) where you would spray 6 to 10 times the flow that will be evaporated (and already calculated (ASPEN, FLASH2). And you recirculate to the quench the water that is left. Such unit should be much smaller in size.
At any rate, be extremely cautious of areas where you may have dry/wet transitions (corrosion, fouling).




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