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Hi guys, I'm fairly green at ASPEN, however I've been trying to use it seriously for about 4 months now.
I have a RadFrac distillation column that is being fed with 3.952 kmol of fluid containing 1wt% triolein, 2.9wt% methanol, 0.1wt% glycerol and 96.1wt% methyl oleate (biodiesel) at 60 deg C.
All I want to do is take the methanol out of the stream and if you look at the vapour-liquid equilibrium diagrams of methanol and any of the other components in the stream you'll see that methanol literally jumps right out of the stream (it's very insoluble in any of the components). So naturally I assumed a distillation with 3 theoretical stages only should be able to easily distill the methanol in pure form.
This is true to only to an extent though... I used the design spec and vary options on my column in order to make it recover (mol basis) 99.9% of the methanol in the distillate and have a distillate mass purity of 99.9% methanol. Now I can achieve a 99% methanol purity and recovery in the distillate stream but not a 99.9% mol recovery (mass purity is achievable) at a reflux ratio of 1.52859136, a distillate rate of 0.84097257 kmol/hr and a column pressure of 32 mmHg (vacuum operation).
Now in trying to tweak the column to get a 99.9% mol recovery of methanol from the feed into the distillate, I am encountering very little flexibility.
Now I thought that increasing the number of stages might get recover more methanol into the distillate, so I increased the theoretical stages to 5 and set the feed stage to 3. However, this causes column stage 1 (top stage) to have zero vapour or liquid flowrate???
Changing my distillate rate or pressure to anything else (higher) also causes column stage 1 to have the same effect...
So I'm guessing my column is flooding because my top column has no flow (I'm guessing no liquid flow...). How do I stop this flooding? Changing the distillate rate or reflux ratio or number of stages or pressure individually, always causes zero flowrate?
Is it that my column simply can't recover 99.9% of the methanol into the distillate stream and I am trying to do the impossible? Are there any references on how to stop the flooding? I know that by preheating the feed a bit I can eliminate flooding to a certain point..
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Aspen Vapour Of Liquid Flow Of Stage X = 0
Started by Guest_Shen_*, Jan 09 2009 07:11 PM
2 replies to this topic
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#2
astro
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Gold Member
Posted 12 January 2009 - 12:26 AM
Shen
Good to see that you've improved on the info that you're supplying to us to help you out, compared with your previous post:
Previous query
Firstly, a point about significant figures. A reflux ratio of 1.52859136 is about 1.5 in my book. Trust me, I doubt whether anyone out there (including your lecturer) is that concerned about an RR quoted to 8 decimal places.
Down to business, on first pass you seem to be getting pretty high performance from your design compared with some typical results out there:
Methanol recovery from biodiesel
A triple effect vacuum flash evaporator appears to be a proven technology:
Multiple effect evaporator
So, I'd agree, increasing the number of distillation stages would seem to have limited value.
Without having the time to analyse your data in detail, it appears to me that you're trying to over constrain the distillation problem. You're targetting a high purity product with a high level of recovery. If you reduce your specs, you might find the column simulation becomes more stable.
As I mentioned previously, flooding &/or dumping is a hydraulic effect based on internal specifics. You haven't got to that level of detail yet, so I suggest that you don't confuse matters with that thinking. Get the mass transfer / separation working reliably and then worry about improving performance &/or the equipment mechanical detail.
If you can't achieve the purity / recovery target you're after, then you can provide this discussion in your assignment report. You'll get more out of delivering a workable design than burning a lot of hours in frustration pushing for an answer that may be impossible to achieve.
Engineering's about delivering an optimum of cost and performance for a set of constraints. One of the constraints is the time budget to do the work. You need to draw your own line for setting out when you've done enough analysis/simulation. Leave time to write up your work and hand in a coherent report and get used to sometimes not achieving a gold plated result.
Good to see that you've improved on the info that you're supplying to us to help you out, compared with your previous post:
Previous query
Firstly, a point about significant figures. A reflux ratio of 1.52859136 is about 1.5 in my book. Trust me, I doubt whether anyone out there (including your lecturer) is that concerned about an RR quoted to 8 decimal places.
Down to business, on first pass you seem to be getting pretty high performance from your design compared with some typical results out there:
Methanol recovery from biodiesel
A triple effect vacuum flash evaporator appears to be a proven technology:
Multiple effect evaporator
So, I'd agree, increasing the number of distillation stages would seem to have limited value.
Without having the time to analyse your data in detail, it appears to me that you're trying to over constrain the distillation problem. You're targetting a high purity product with a high level of recovery. If you reduce your specs, you might find the column simulation becomes more stable.
As I mentioned previously, flooding &/or dumping is a hydraulic effect based on internal specifics. You haven't got to that level of detail yet, so I suggest that you don't confuse matters with that thinking. Get the mass transfer / separation working reliably and then worry about improving performance &/or the equipment mechanical detail.
If you can't achieve the purity / recovery target you're after, then you can provide this discussion in your assignment report. You'll get more out of delivering a workable design than burning a lot of hours in frustration pushing for an answer that may be impossible to achieve.
Engineering's about delivering an optimum of cost and performance for a set of constraints. One of the constraints is the time budget to do the work. You need to draw your own line for setting out when you've done enough analysis/simulation. Leave time to write up your work and hand in a coherent report and get used to sometimes not achieving a gold plated result.
#3
curious engineer
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- Members
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Brand New Member
Posted 18 March 2009 - 09:40 PM
Article entitled Biodiesel production from waste cooking oil : 1. Process design and technological assessment by Y. Zhang et al might help
They used Aspen Hysys to simulate the biodiesel plant with different types of production
They used Aspen Hysys to simulate the biodiesel plant with different types of production
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