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Designing A Light Ends Column

design distillation column project light heavy ends ethylene oxide process

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#1 C.Eng.Student

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Posted 16 February 2017 - 07:15 AM

Hi Guys, 

 

Firstly, I would just like to say this is my first post, so apologies if I've made any mistakes!

 

I've been doing a project on the production of Ethylene Oxide (oxygen based) and now have to delve further into a specific component.

 

Let me give a little information on the process

  • Three main sections; Reactor, CO2 Removal and the Purification section
  • Assumptions have been made based on literature and research that affect the mass balance, I would type them here but there are quite a few. (If you would like them to assist with the answers, please let me know and I will provide)

The component I have chosen to base my project on is the Light Ends Column which can be found in the Purification section. My Understanding is that here, the light ends (i.e. the impurities) will leave as through the vent in the column and a solution of EO and Water will enter the final distillation column. There will be trace amount of impurities entering the DC and I have accounted for that in the mass balance.

 

My Questions:

 

  1. If there is a vent present in the column, does this mean the pressure inside the column will be atmospheric?
  2. Does the Mass Balance look okay? (notice it's in kmol/hr)
  3. How would I go about working out the temperature?
  4. What would you recommend doing in terms of starting the design?

Thanks so much for reading and any answers or suggestions you give are much appreciated.

 

 

Attached Files



#2 pavanayi

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Posted 17 February 2017 - 03:36 AM

1. If there is a vent present in the column, that does not mean the pressure in side the column is atmospheric. The vent can be from the net off-gas from the condenser. The off gas line will have a control valve that will help maintain the pressure of the upstream section (column) to whatever desired pressure you wish it to be. If the column is indeed atmospheric (or near atmospheric), then you will need a compressor to take the off-gas and deliver it to wherever it should go. It does not just vent to atmosphere. Search and read a bit more about column pressure control. 

 

2. I can't make head or tail of your mass balance as it is very difficult to follow and understand. Probably better (and a bit more professional) to sketch the PFD out and number the streams, and provide the material balance for the streams in a separate table with reference to the stream numbers.

 

3. What temperature? be specific. If you are talking about the column temperatures, keep in mind that the column pressure will be defined (or determined) usually by the condensing media availability. For example, if you have cooling water at 20°C as your condensing media, then your condensing temperature should be 30°C (temperature of the condensing utility + an approach temperature). Since you already have the composition of your overhead product and the temperature of your condenser, the pressure of the system can be back calculated using VLE. 

 

4. I believe the above points can give you a starting point. Obviously a better PFD (showing the columns with condensers, overhead products (if it is liquid or vapour), reflux drums, control valves etc) will look more professional. Also please re-do your material balance in the way I pointed out. 



#3 C.Eng.Student

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Posted 17 February 2017 - 06:04 AM

Thanks so much for your detailed response, I have attached the PFD I made for this process along with the stream tables.

 

  1.  Okay, I will read more into column pressure control, thanks for tip!
  2. I have attached the PFD and stream table, hopefully this is easier to follow.
  3. Sorry, I should've been more specific. I was talking about the temperature in the column itself, I am not sure how to work it out

Light Ends Column:

This is my understanding of what happens in this column.

  • There is a stream coming in (liquid phase) containing Ethylene Oxide, Water and Impurities.
  • The column separates the impurities from the EO and Water solution which will then pass through to the final distillation column
  • Obviously, trace amount of impurities will enter the distillation column too but for now I've considered them to be negligible. 
  • The impurities (according to literature and talking to people) will leave as a vent gas which I'm assuming is open to atmosphere, but reading what you've put I'm now thinking that does mean the pressure within the column is atmospheric? 

I'm not sure I understand what you mean by condensing media, in this column, there is no additional stream entering other than the inlet (which is coming from the Ethylene Oxide Stripper (desorber)). I am separating the impurities from the EO/Water solution (or trying to anyway).

 

My Questions:

 

  1. Based on the information I've given (hopefully it's enough this time), how can I go about calculating the Pressure and Temperature of the column

 

I will start to read about column pressure control and other things in the mean time.

 

Thanks so much for your help!

 

 

Attached Files



#4 pavanayi

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Posted 17 February 2017 - 04:32 PM

CEng Student,

Please note, I'm focusing only on the light ends column (streams 16, 17 and 18), though certain aspects may be applicable to the whole material balance as well. You have assumed 100% recovery of EO and Water into the bottom stream and 100% recovery of the other components into the overhead stream. This is not a realistic scenario (which I trust you are already aware).

 

Regarding the column itself, I hope you are using a simulation tool to help you in your calculation? 

Condensing media is the stream that you use in the column overhead condenser on the non-process side, which will absorb the heat given off by the process.

The operating pressure of any distillation column is defined by the utility available to be used in the overhead condenser. For example, since you are in the UK, you can assume to have cooling water available at 20°C. This water will be used in the overhead condenser as your cooling utility or cooling media. If you had a refrigeration circuit in your plant, then maybe you have a cooling media available at -20°C. If you were in a desert with low water availability, your condensing media might be air at 40°C etc. I hope you got the gist. 

 

Going back to the basics of VLE, to define a saturated system, you need to define only 2 parameters out of Temperature, Pressure and Composition. You might have come across the TPx diagrams of binary systems in your thermodynamic lessons. Coming back to your problem, you have decided the T at the overhead. (if your condensing media is at 20°C and you assume an approach temperature of 10°C on the heat exchanger, the process side should be at 30°C). You already have defined the composition of your off-gas from your overhead reflux drum (from your material balance). Since you have defined 2 out of 3 parameters, your pressure can be back calculated using a simulation software. You can run a few case studies to arrive at a suitable operating condition of your column.

 

Just a few pointers to help you on your way: The overhead condenser will be a partial condenser due to presence of non-condensables. You might struggle to find an answer with your assumption of 100% recovery. 


Edited by pavanayi, 17 February 2017 - 04:37 PM.


#5 C.Eng.Student

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Posted 20 February 2017 - 05:23 AM

ChE Plus Subsriber, thanks so much for your responses, it is really helping me.

 

Yes, I assumed 100% recovery for the preliminary mass balance, this now needs to be corrected and a more realistic mass balance has been calculated (see 'Updated Stream Table' file).

 

I've been struggling to find literature and reports commenting on the %s of components that go through to each section, this has meant assumptions had to be made using very little data.

 

Question:

  1. Is there something I can do (finding literature/reports or doing calculations) to make the accuracy of my mass balance higher?

 

Yes, I've been using AspenPlus, it's new to me so am still trying to figure it out, but it's getting there.

 

I have a question about the phases, as I said the literature and reports I have come across have not mentioned phases of inlet and outlet streams. Would the phase of the inlet (into the Light Ends Column) be a gas or liquid? The stream is coming from the top of the EO Stripper (desorber) and passes through a heat exchanger before entering the LEC. 

 

Thanks to all that reply, you really do help me a lot!

 

 

 

Attached Files






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