Jump to content



Featured Articles

Check out the latest featured articles.

File Library

Check out the latest downloads available in the File Library.

New Article

Product Viscosity vs. Shear

Featured File

Vertical Tank Selection

New Blog Entry

Low Flow in Pipes- posted in Ankur's blog

How To Maintain Partial Pressure Over Different Conditions

partial pressure reaction pressure mass flow controller nitrogen water steam reactor thermogravimetric analyser tga

This topic has been archived. This means that you cannot reply to this topic.
4 replies to this topic
Share this topic:
| More

#1 metalnonmetal

metalnonmetal

    Brand New Member

  • Members
  • 2 posts

Posted 09 October 2016 - 08:12 PM

I need to do a reaction at a steam partial pressure of 3 atm. The  balance is nitrogen gas.  If I use 300 ml/min of N2 setpoint from a Mass Flow Controller (MFC) at the start of my experiment. and then when my reactor temperature reaches 500C, I pressurize the reactor to 6 atm using N2.

 

Now what should be my water flow rate be in a pump so that I get 3 atm partial pressure of steam. I am using a preheater to generate steam at 300C.  and it mixes with the nitrogen aso at 300 in the preheater being fed to the reactor. 

 

I am confused. Some people before me were calculating the volumetric flow of water corresponding to 300C and 3 atm and 300ml/min of steam. Is it correct. Will not the volumetric flow of Nitrogen also change?

 

Please help!



#2 Bobby Strain

Bobby Strain

    Gold Member

  • Members
  • 3,529 posts

Posted 09 October 2016 - 10:36 PM

I am confused, too.

 

Bobby



#3 sgkim

sgkim

    Gold Member

  • ChE Plus Subscriber
  • 289 posts

Posted 10 October 2016 - 02:09 AM

To maintain a certain level of partial pressure of steam in the reactor, the molar fraction of water should be maintained.

3 atm of steam absolute "partial" pressure in total 7 atm absolute in the reactor is equal to 3/7 mole fraction in the gas phase.

 

If steam is consumed in the reactor,  steam(or water) shall be injected continuously to the reactor to maintain the molar concentration of steam.

For no-steam consuming reactor,  3 mole of steam for every 4 mole of nitrogen shall be injected and maintain the total pressure under 7 atmosphere absolute.

 

Remember the equation:  

(Partial pressure of steam, atm absolute) = (mole fraction of steam in the system)*(Total absolute pressure of the system, atm absolute) ...(1)

 

Stefano Kim


Edited by sgkim, 10 October 2016 - 02:10 AM.


#4 metalnonmetal

metalnonmetal

    Brand New Member

  • Members
  • 2 posts

Posted 10 October 2016 - 01:10 PM

I think my total absolute pressure is 6 atm and I want to maintain 3 atm of steam and 3 atm of N2 partial pressure. My steam is reacting and so I am adding steam continuously. I want to know at what temperature and pressure I should do my calculations to calculate the moles of water I should set in my syringe pump before starting the steam injection if I have set a flow of 300ml/min of N2 from a MFC since the start. 



#5 sgkim

sgkim

    Gold Member

  • ChE Plus Subscriber
  • 289 posts

Posted 11 October 2016 - 10:21 PM

(1) Calculate first the number of moles of nitrogen feed, and feed the same amount of steam to maintain the partial pressure of steam to meet the molar concentration 0.5 mol/mol for the case when no steam is consumed. But if steam is consumed by the reaction, additional steam shall be fed together.  So far as the molar concentration is maintained at 3 bar, the partial steam pressure will be kept steadily under 6 atm of total absolute pressure.      

 

Molar feed rate of N2 can be calculated from the ideal gas equation:    n =(V*P)/(RT)      

Where, V= 300 ml/min, P=absolute pressure of N2, R=ideal gas constant, T=absolute temperature;  P and T is measured at the flow meter condition.   

 

N2 feed rate, n mg-mole /min = (P atm)*(300 ml/min)/(R*T)  ..........(1)  

 

Ideal gas constant,  R = (22.414 ml * 1 atm)/(1 mg-mole*273.15 K) = 0.082057 ml-atm/mg-mole/K

Then, water rate, w mg/min =  (18.02 mg/mg-mole)*{(n mg-mole water/min) + (water consumption rate, m mg-mole/min)} ....(2)

 

(2) In case that the reaction system does not have any discharge of either liquid or gas phase, the first nitrogen fed to build up pressure will stay in the reactor. Then no additional nitrogen is required, feed steam only to make up for the consumption.

 

(3) At the beginning of the reactor start up, both nitrogen and total water to build up absolute pressure at 6 atm and 500℃ can be estimated as follows:

 

Reaction system volume:   Vr  [m3]

Initial pressure (purged by N2): Po = 1 [atm abs.]  

Initial temperature of N2 in the reactor:  20℃, To =295.15 [K]   

Initial number of moles of N2 in the reactor:  No =  (Vr*Po)/(R*To)  .................(3)

 

Reactor temperature after feeding, heating up and pressurizing: Tr = 773.15 K

Final reactor pressure after feeding and heating up:  Pr 6 atm abs.

Total number of moles of N2 and steam required in the reactor:   Nt  = Nn + Ns =  (Vr*Pr)/(R*Tr).......(4)

Molar fraction of steam N2 in the reactor:  Ns/Nt = 3 atm/ 6atm = 0.5;  

- ditto - of N2:  Nn/Nt = (Nt-Ns)/Nt =1- Ns/Nn =1-0.5 = 0.5

 

Nitrogen required for pressurizing  = Nn - No  = Nt/2 - No ....(5)    <---initially charged N2 in the reactor to be deducted. 

Steam required for pressurizing = Ns = Nt-Nn = Nt/2.............(6)

 

After heating up and pressurizing, the same amount of steam as N2 plus consuming steam additionally shall be fed to the reactor. 

 

Stefano


Edited by sgkim, 13 October 2016 - 12:15 AM.





Similar Topics