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Topic: Adiabatic flash  (Read 30753 times)

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Offline mbeychok

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Adiabatic flash
« on: May 30, 2006, 07:26:56 PM »
When the pressure of a saturated liquid is decreased by the liquid flowing across a throttling valve or other device to a lower pressure, a part of the liquid immediately "flashes" into vapor. Both the flashed vapor and the residual liquid are cooled to the saturation temperature of the liquid at the lower pressure. The flashing is an isenthalpic (i.e., constant enthalpy) process and is often referred to as an "adiabatic flash". It is also sometimes referred to as "auto-refrigeration" and is the basis of most conventional vapor compression refrigeration systems.

If the liquid is a single-component liquid (i.e., liquid propane or liquid ammonia), the following equation derived from a simple heat balance around the throttling valve or device is used to predict how much of a single-component  liquid is vaporized.



( Note: The words "upstream" and "downstream" refer to before and after the liquid passes through the throttling valve or device.)

Multi-stage flashing is used in the desalination of ocean water. The water is heated and then routed into a reduced pressure flash evaporation "stage" where some of the water flashes into steam which is subsequently condensed into salt-free water. The residual salty liquid from that first stage is introduced into a second flash evaporation stage at a pressure lower than the first stage pressure where more water is flashed into steam which is also subsequently condensed into more salt-free water.  This sequential use of multiple flash evaporation stages is continued until the design objectives of the system are met. A large part of the installed desalination capacity worldwide uses multi-stage flashing and typically such plants may have up to 24 or more stages of flash evaporation.
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Offline Donaldson Tan

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Re: Adiabatic flash
« Reply #1 on: May 30, 2006, 07:55:43 PM »
Isn't this the Joule-Thomson effect?

I am curious of viscosity has an effect of slowing down the flash process, ie. we don't get as much vapour as expected.
« Last Edit: May 31, 2006, 08:48:56 PM by geodome »
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Offline eugenedakin

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Re: Adiabatic flash
« Reply #2 on: May 30, 2006, 10:46:03 PM »
Hi mbeychok,

Thats a great explanation.

Just for the fun-of-it, could you provide a simple calculation with your posted formula... lets say: I have a natural gas stream with 40 % propane, and 60 % ethane.  I want to design a process to remove the ethane only.  Lets say the pressure going into the facility is 7000 kPa.  Feel free to assume other numbers to help aid in a calculation.

Sincerely,

Eugene
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Offline mbeychok

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Re: Adiabatic flash
« Reply #3 on: May 31, 2006, 01:52:08 AM »
Eugene:

The adiabatic flash calculation equation I presented applies only to a single-component liquid as I was careful to state in my posting.

The flashing of a multi-component liquid is much more complex and requires an iterative, trial and error solution involving the Rachford Rice equation.  If you are truly interested in how to calculate the flash of a multi-component liquid, please see http://en.wikipedia.org/wiki/Flash_evaporation.

Also note that a single equilibrium flash stage will not separate only ethane from an ethane-propane liquid  mixture. The flashed vapor will be rich in ethane but it will still contain propane.  To separate only ethane from an ethane-propane liquid mixture would require distillation.
« Last Edit: May 31, 2006, 02:24:56 AM by mbeychok »
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Offline mbeychok

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Re: Adiabatic flash
« Reply #4 on: May 31, 2006, 02:23:11 AM »
Geodome:

In my 40+ year experience in the process design of petroleum refineries and natural gas processing plants, I have never heard of viscosity having any effect on the adiabatic flashing of a liquid. That is not to say that it has no effect ... it only means that I have no no knowledge of that happening.

As for your question "Is this the Thomson effect?", did you mean the Joule-Thomson effect? if that is what you meant, than the answer is:

No, the adiabatic flash of a liquid is definitely not the same as the Joule-Thomson effect which applies only to gases.   The Joule-Thomson effect is such that under certain conditions,  gases will  undergo either a decrease in temperature or a rise in temperature when their pressure is reduced. The Joule-Thomson coefficient which determines the magnitude of the temperature change and whether the temperature will decrease or increase with reduction in pressure varies from gas to gas and with the pressure and temperature of any specific gas.

The Joule-Thomson effect deserves a posting of its own in this forum.  If you would like to have such a posting, let me know and I will develop and submit a posting on that subject.
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Offline Donaldson Tan

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Re: Adiabatic flash
« Reply #5 on: May 31, 2006, 12:10:03 PM »
As for your question "Is this the Thomson effect?", did you mean the Joule-Thomson effect? if that is what you meant, than the answer is:

Oh yeah... I tend to associate adiabatic flashing with gases, thus I came up with the term "Joule-Thomson Effect". You are right (again) to say it is only applicable to gases, and not liquids. I can't help but to see the similarity in both process. They are isoenthalpic, so the lever rule is applicable. Joule-Thomson effect refers to the free expansion of gas that results in the rapid drop of temperature. The temperature drop coupled with heat exchanger for more cooling results in liquidification of the gas. Such process includes the Linde Process and the Claude Process.

Although I have not learned liquid adiabatic flashing in school, it reminds me of the distillation process. This is because the feed into the distillation column is usually at saturated condition, and the pressure in the column relative to the feed entering the column is low because of the high vapour velocity inherent in the column (Bernoulli's Effect). However, in derriving the McCabe-Thiele Plot, we were supposed to assume that the saturated feed is not partially vapourised upon entry. Instead, it becomes part of the liquid phase flowing down the column into the reboiler. Vapourisation takes place enroute downstream towards the reboiler, and not upon entry, which is suggested by liquid adiabatic flashing.

Please address this discrepancy.
"Say you're in a [chemical] plant and there's a snake on the floor. What are you going to do? Call a consultant? Get a meeting together to talk about which color is the snake? Employees should do one thing: walk over there and you step on the friggin� snake." - Jean-Pierre Garnier, CEO of Glaxosmithkline, June 2006

Offline mbeychok

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Re: Adiabatic flash
« Reply #6 on: May 31, 2006, 12:42:45 PM »
Geodome:

It has been my experience that even with fairly large pressure drops, the partial vaporization that takes place in the adiabatic flashing of a liquid is relatively small ... 20 to 30 % or less.  Thus, the amount of flashing that occurs as the feed enters a distillation column is probably quite small indeed in most cases.  Thus, it is conservative to assume that no flashing takes place.  After all, an adiabatic flash is simply one theoretical equilibrium step and, in most cases, if we can calculate the number of theoretical stages required in a distillation column to that accuracy (i.e., plus or minus 1 stage), we are quite happy. 

In some cases, if a quite large pressure drop exists between the distillation feed liquid and the distillation column operating pressure, the feed is flashed in a pressure drum ahead of the distillation column.  The resulting separated liquid and vapor stream are then fed into the distillation column at two separate entry points.

I hope this is helpful to you.

« Last Edit: June 07, 2006, 12:45:56 AM by mbeychok »
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Offline Donaldson Tan

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Re: Adiabatic flash
« Reply #7 on: June 03, 2006, 11:02:18 AM »
After all, an adiabatic flash is simply one theoretical equilibrium step and, in most cases, if we can calculate the number of theoretical stages required in a distillation column to that accuracy (i.e., plus or minus 1 stage), we are quite happy.

Are you suggesting that the theoretical number of plates needed for distillation is the therotical number of stages derrived from the McCabe-Thiele Plot minus one for the reboiler and minus one for the entry point?

Also taking in account of imperfect behavior in real system, when we used the murphy efficiency to evaluate the actual number of plates required, do we use the theoretical number of plates or the theoretical number of stages?


Cheers
geodome
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Offline mbeychok

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Re: Adiabatic flash
« Reply #8 on: June 03, 2006, 11:38:27 AM »
Geodome:

Quote
Are you suggesting that the theoretical number of plates needed for distillation is the therotical number of stages derrived from the McCabe-Thiele Plot minus one for the reboiler and minus one for the entry point?

No, I didn't mean to suggest that at all.  I was trying to say that, in those cases, where the feed might flash upon entry to a column that it can be handled by an external flash drum or, if the amount of flash is fairly small, it can probably be ignored.

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Offline ehernaba

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Re: Adiabatic flash
« Reply #9 on: March 24, 2010, 06:06:41 AM »
Someone knows how to calculate how much liquid chlorine held in a one ton container would vaporize if this container happened to explode? I have read that in order to design emergency gas scrubbers, the assume a vaporization speed of 800 lb/min for the first minute, and then of about 200 lb/min until it is empty. Does anyone know how they make these assumptions? Is it possible to calculate those numbers from the equations displayed in the first post? Thank you very much in adavance for your help.

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