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3 Components Flash Vessel


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#1 BenHarwood

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Posted 06 February 2015 - 12:02 PM

Hi,

 

I am trying to design a flash vessel which reduces the pressure of a stream containing Glucose, Water and dissolved Carbon Dioxide from 240 bar to around 2 bar. The feed has a temperature of 190 Celcius.

 

I have the following parameters for the feed

 

Pressure: 240 bar

Temperature: 190 degrees Celcius

Total flow rate: 46000kg/h   

Component flow rates: 38,950 kg/h Water   ;   4350 kg/h Glucose      ;    2700 kg/h Carbon Dioxide

 

I have calculated the amount of steam that will be produced from the enthalpy to be roughly 43%, so around 16,500kg, and so far assumed that the Carbon Dioxide will come out of solution due to the pressure reduction, as well as assuming that glucose is inert to the pressure reduction. So I know the flow rates of liquid and vapour streams to be 26,700kg/h and 18,300kg/h respectively.

 

I have done some very basic calculations assuming a binary mixture of water and carbon dioxide.

 

Although, from here I wasn't sure how to do this design, as I'm not sure if the system being 3 components effects the design as I haven't seen any 3 component flash systems (even though I'm assuming the glucose to be inert and not vapourise). Would I need any phase diagrams to start the design? and what basic principles would be best to start designing this unit?

 

Many thanks,

 

Ben



#2 Zauberberg

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Posted 06 February 2015 - 12:45 PM

Pressure letdown from 240 bar (gauge or absolute?) to 2 bar (gauge or absolute?) is a serious operation. This can probably be achieved via series of restriction orifices and a control valve upstream of the receiver vessel, rather than through a single valve - although not impossible.

 

What you have there is an isenthalpic process, so either by using process simulators or relevant phase diagrams, you have arrived to the liquid-vapor equilibrium conditions as you calculated. I assume the letdown temperature is known as well - otherwise it would be very hard to estimate the amount of steam generated by pressure reduction.

 

Which elements of the letdown system are you actually designing? Are you starting from a vessel or piping containing the mixture at 240 bar (a/g), or there is more to it? And what should be covered with the design? Is it just a Process Flow Diagram, or you are supposed to provide all relevant deliverables (P&ID, Datasheets, Piping Classes etc.)?



#3 BenHarwood

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Posted 07 February 2015 - 10:26 AM

I understand this would be a hard process with the pressure drop, all pressures absolute. I have tried using Pro II to simulate the process, however it didn't have any thermodynamic data for Glucose.

 

Let down temperature was from 190 degrees to 60 degrees (celcius).

 

The vessel is being fed from an upstream reactor, so starting with a pipe containing the mixture at these conditions.

 

This is for my design project, I have a basic PFD for it at the moment with a small amount of control (such as level and pressure) a P&ID needs to be made, which I'm in the process of doing now. I am doing a mechanical design so every aspect needs to be covered really. 

 

Do you know what phase diagrams I would need to start the process of design here?

 

Thanks for the reply 


Edited by BenHarwood, 07 February 2015 - 10:29 AM.


#4 Zauberberg

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Posted 07 February 2015 - 01:39 PM

Carbohydrates are really far away from my domain so I'm afraid I cannot suggest any reference. Simply by using common sense approach, I would look for available data on glucose-water solutions (because glucose is fully soluble in water, if I remember well) and see if it is possible to find any enthalpy-temperature diagrams for the initial solution concentration. Perhaps it is even possible to observe the initial mixture as a combination of fully saturated solution of glucose in water, plus the extra pure water, and this would make the calculations even easier since you would have two very specific compounds - a saturated solution plus pure water. And not to forget the CO2. But again, this is so far from anything I have worked with in my career that I cannot suggest anything else than to consult with a subject matter expert if this is to be done right.

 

What confused me a bit is how did you come up with the exact figures of each component flow after the isenthalpic flash, if you did not have any thermodynamic model for calculations? You state there is significant amount of steam after flashing, but at 2 bara and 60 degC there should be virtually no steam in the system - apart from what is stripped out of the liquid phase with the aid of CO2.



#5 BenHarwood

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Posted 07 February 2015 - 04:31 PM

Ok, thank you! I will have a look at them suggestions.

 

I calculated the amount of steam that would vapourise from the following equation

 

(Initial liquid enthalpy - Final liquid enthalpy)/Enthalpy of evaporisation = Ratio of condensate to vapour produced. 

 

and assumed the Carbon dioxide will come out of solution due to depressurisation and glucose is inert. The 2 bar 60 degrees stated is the conditions in the vessel and the level required for liquid stream.






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