4 replies to this topic

[Said Salim]

Hello everybody.

I am calculating a 4-stage CO2 compressor polytropic efficiency. I have used the designed inlet and outlet conditions of the flow same as the datasheet and the ploytropic efficiencies for the first three stages were matching. However, the 4th stage was different i.e. instead of 57.1% I got 80%.

Here are the inlet and outlet condition for the 4th stage:

 

Ts = 50 DEGC

Td = 110 DEGC

Ps= 84.48 Bara

Pd= 158 Bara

In order to have 57.1% polytropic efficiency I had to have Cp/Cv = 1.18389 i.e. Cp(mean)= 53.507 kJ/kg which is impossible with any mole fraction distribution of the three flow constituents (CO2, air & water).

Cp_m(CO2)= 40.671 kJ/kg

Cp_m(H2O)= 33.921 kJ/kg

Cp_m(air)= 29.471kJ/kg

 

the formula I used is Cp_m/R = A+B*T_am+C/3*(4*T²_am - T_s*T_d)+D/(T_s*T_d) ((Smith & Van ness book page 111))

where T_am= (T_s+T_d)/2

Cp (mean)= y_CO2 *Cpm(CO2)+ y_H2O *Cpm(H2O)+ y_air *Cpm(air).

Can any one help regarding these calculation did I miss anything.

 

 

Attached Files

•  Polytropic Effiency.docx   1.24MB   32 downloads

[ankur2061]

Salim,

 

CO2 behaves as a supercritical fluid at the temperatures and pressures mentioned by you. The physical properties are not well defined at supercritical conditions and even process simulators like Aspen HYSYS provide odd results for heat capacity ratio (Cp/Cv) at the mentioned temperatures and pressures.

 

Refer the following link to understand the implications on heat capacity ratio for CO2 at these high pressures and temperatures:

 

http://www.cheresour...io-compilation/

 

Regards,

Ankur.

[twell]

as said by Ankur at such high pressures some models
do not work reliably,

I have read that REFPROP (from NIST)
is accurate for CO2 and some other pure
components,

also I have obtained good results with the
Extended Peng Robinson in PRODE PROPERTIES,
there is a free version available at
www.prode.com

it includes a Excel page for solving
polytropic stages,

it allows you to calculate directly
the efficiency given inlet and outlet
conditions,

you may try and report the results.

Edited by twell, 28 October 2013 - 02:49 AM.

[RockDock]

I've worked on a number of CO2 compression projects. Hysys is definitely not reliable at all for these calculations, often more than 200% off. Most of these types of simulations are done with ProMax. Their CO2 capabilities are very good. When we used it, the percent error from operating data was less than 5%.

 

I'd also be interested to know your experience with Prode. This is the first I've heard of it.

[twell]

RockDock,
I have read good comments about ProMax but I have not access to that simulator
(as well as Hysys) and I can't compare results,
instead I use Excel with PRODE library
(you may get a copy from Prode as said above)
the library includes a model for solving a polytropic stage,
as you and Ankur noticed a problem with supercritical CO2
could be the accuracy of the model, the Peng Robinson model in
PRODE includes 5 parameters per each fluid in the databank
and the developer gives the values for specific applications
as supercritical CO2
(I don't know how accurate the free
version is for this application
but you can try or contact the developer
for the parameters to utilize).

Edited by twell, 28 October 2013 - 10:06 AM.