14 replies to this topic

[uae_ramesh]

Dear Gentlemen,
Please let me introduce myself, I am a mechanical Engineer, with experience in Rotating Equipment.
I am trying to understand the procedure for Centrifugal compressor performance assessment.

In the process I came across one message in this site as follows:

".....  For engineering applications related to gas compression and expansion the useful engineering entity is specific heat ratio (k) also known as isentropic expansion coefficient and is defined as:

k = Cp/Cv

Specific heat at constant pressure (Cp) of pure gases is readily available from a wide source available in physics, chemistry or thermodynamics textbooks and even on the internet as a free resource. One such free, recognized and reliable resource can be found as below:

http://webbook.nist.gov/chemistry/

If you are dealing with a gas mixture then the Cp of the gas mixture can be found out by the law of proportion based on the mole fraction of the individual pure components of the gas. The Cp of the gas mixture of say A,B & C components thus would be:

mole fraction of A*Cp of A + mole fraction of B*Cp of B + mole fraction of C*Cp of C"

 

The NIST webbook requires two inputs - Pressure & Temperature

 

My question is for a total gas pressure 200 psia, and 115 F, do I need to use partial pressure of gas A, B and C to obtain Cp values from NIST webbook or the full pressure (200 psia in this case)?

Please advise.

Regards,

Ramesh

[staffel]

1) for mole numbers use mole fractions, for mass numbers use mass fractions

 

2) if you wish to calculate cp at some operating conditions, in your case 200 psia, use that value

 

3) cp (and cv) are not constant over a range of temperatures and pressures,

    in most cases one has to solve (iteratively) an operation at specified enthalpy

    or emtropy or volume etc.

 

4) for a polytropic stage there are direct  methods as  ASME PTC 10

    and iterative (more accurate) solutions  applicable to both

    gas and vapor+liquid

Edited by staffel, 14 June 2014 - 05:01 AM.

[PingPong]

My question is for a total gas pressure 200 psia, and 115 F, do I need to use partial pressure of gas A, B and C to obtain Cp values from NIST webbook or the full pressure (200 psia in this case)?

For a gasmixture that behaves ideal (so Z = 1.0) Cp and Cv do not depend on pressure and the mixture Cp can easily be calculated from the gas composition and the individual Cp's while Cv = Cp - R.

But in the real world gases seldom behave ideal, especially not if they are saturated (that is: at their dewpoint), and Cp and Cv cannot be calculated so simple anymore, and if accurate values are required one needs to use an Equation Of State (EOS), which is best done using a software like Hysys, Aspen Plus, PRO/II, ProMax, Prode, ......

 

What is the composition of your gas mixture?

[Bobby Strain]

What you need is a good collegue who is a chemical engineer. Chemical engineers understand these things. There are numerous good correlations for natural gas. I seem to remember that Cameron has some pretty good centrifugal compressor, (but my memory is unreliable. It may be reciprocating compressors), software that you can download for free. Ariel has free software, too, but for reciprocating compressors. But it includes gas properties, too. If you are serious about compressor performance assessment, you need a process simulator. Or a good friend who has access to one and is willing to help you. Try this site. http://checalc.com/

 

Bobby

[uae_ramesh]

Gentlemen,

Let me repeat my question

 

I have a gas mixture

Methane - 20%

Ethane - 25%

Propane - 50%

N Butane - 5%

 

Mole wt = 35.6.

 

Conditions : Pressure : 650 psia

Temperature 115 F

 

I need to find the Cp of Gas mixture

 

as per gas mixture properties:

 

Cpm = (m1 * Cp1 + m2 * Cp2 + m3*Cp3 + m4*Cp4)

           --------------------------------------------------------

                         m1+m2+m3+m4

 

Cp1 corresponds to - Methane

Cp2 - Ethane

Cp3 - Propane &

Cp4 - N Butane

 

Referring to NIST webbook i enter the value to Temperature 115 F and pressure 650 psia

 

I get value of Cp1 as 9.6755  Btu/lbm - R for Methane

 

Calculating the partial pressure of methane in the gas mixture and use 115 F and at its partial pressure 58.31 psia

 

I get value of Cp1 as 8.8112 But/lbm-R for methane.

 

Which is the correct method?

 

Regards,

 

Ramesh

Edited by uae_ramesh, 15 June 2014 - 08:50 AM.

[PingPong]

I have a gas mixture

Methane - 20%

Ethane - 25%

Propane - 50%

N Butane - 5%

 

Mole wt = 35.6.

Judging from the mole wt 35.6 the composition is in mole %. Right?

However a stream with this molar composition at 115 ^oF and 650 psia will be mainly liquid.

Double check your composition, temperature and pressure.

And why did you earlier mention 200 psia and now 650 psia?

Or do you mean compressor suction is 200 psia @ 115 ^oF and compressor discharge is 650 psia at a much higher temperature?

[Bobby Strain]

Peng-Robinson EOS gives Cp  = 17.184 for the mixture and Cp/Cv of 1.240 and Z = 0.86754 at 200 psia and 115 F. With the conditions you listed above at 650 psia, the fluid is two phases. I have assumed that you give the composition in Mol %. You should always state your composition units of measure.

 

Bobby

[uae_ramesh]

Mr. PipnPong, Bobby,

You are right the gas conditions are as follows

 

Suction:

200 Psia, 115 F

 

Discharge:

650 psia, 224.8 F

 

However my question still remains on usage of NIST webbook:

When I use the NIST webbook for calculation of Cp of gas mixtures, I have to specify the temperature and pressure,

should I use the partial pressure of the gas or Full pressure.

 

Regards,

 

Ramesh

[Bobby Strain]

I gave you the mixture value. So, see which one comes closest.

 

Bobby

[staffel]

1)

for a mixture (not behaving as ideal fluid) you should calculate density, cp, cv for the mixture with a EOS,

if you wish to sum up the contributes of individual components, calculate values at specified conditions

(Suction 200 Psia, 115 F,  Discharge 650 psia, 224.8 F)

for example for CP at 200 psia 115 F

0.2*20.494 (kJ/kmol-R) + 0.25*33.428 +....

see points 1) & 2) in my post #2

 

you may expect some errors with this method as the volume (and derived properties) depends from all components in mixture

while you consider individual components.

 

2)

from your posts #5 & #8

assuming a *MOLAR* composition

Methane  20%

Ethane  25%

Propane  50%

N Butane  5%

with

Suction:

200 Psia, 115 F

Discharge:

650 psia, 224.8 F

 

for a *Polytropic* stage

Peng Robinson model + Huntigton method in Prode Properties

returns about 1.09 as efficiency

which is not possible (in real applications),

assuming 0.8 as efficiency with the same procedure

you should expect a dicharge temperature about 245 F,

 

you can calculate values from NIST with the method discussed in point 1)

the solve with the methods discussed in GPSA or other pubblications

and compare values

[PingPong]

When I use the NIST webbook for calculation of Cp of gas mixtures, I have to specify the temperature and pressure,

should I use the partial pressure of the gas or Full pressure.

Calculating mixture Cp and Cv from Cp's and Cv's of the components only works for ideal gas mixtures, so forget about it in your case.

[uae_ramesh]

Dear Mr. Bobby,

 "Peng-Robinson EOS gives Cp = 17.184 for the mixture and Cp/Cv of 1.240 and Z = 0.86754 at 200 psia and 115 F. With the conditions you listed above at 650 psia, the fluid is two phases. I have assumed that you give the composition in Mol %. You should always state your composition units of measure."

 

Could you guide me in calculating Cp from PREOS spread sheet?

 

Rgds,

Ramesh

[Bobby Strain]

I have no idea what PREOS spreadsheet is. Maybe someone else can help.

 

Bobby

[PingPong]

That PREOS spreadsheet can only handle single components, not mixtures, so it is pretty useless in the real world.

Moreover it is difficult use, so that is another good reason to ignore it.

 

The checalc website that Bobby mentioned earlier has an EOS online calculator: http://checalc.com/calc/gaseos.html

however it seems to me that the Cp and Cp/Cv from that calculator are not correct. They seem independent of pressure or selected EOS.

 

That is always the problem with software that one finds on the internet: one can never be sure that it is accurate.

Edited by PingPong, 17 June 2014 - 03:40 PM.

[uae_ramesh]

Dear PingPong,

"That PREOS spreadsheet can only handle single components, not mixtures, so it is pretty useless in the real world"

How to calculate for a gas component using Preos sheet.

 

"The checalc website that Bobby mentioned earlier has an EOS online calculator: http://checalc.com/calc/gaseos.html"

I shall check and revert to you.

Thanks,

 

Ramesh