A question about standard metric cube

exergetic

Hi everybody,
I'm having difficulties sizing a compressor for natural gas.
First,my assumption is to take natural gas as mixture of ideal gases.The compression must be between 9 and 32 bar and it starts at 45 °C.
I know that volumetric flow rate must be 85000 Sm3 per day ,where I assume 15°C and 1 bar as Standard Conditions.
Now how do I think about volumetric flow rate considering conditions of the fluid inside the compressor?
Do I need to change unit of measure of the volumetric flow rate from (Sm3 per day) to (m3 per day) using ideal gas' law?
Because I must find the power required by comrpessor.

Thanks in advance.If I haven't been clear about something tell me.

Skrambles

Since the gas is being compressed, the volume entering is going to be larger than the volume exiting. Think about it in terms of mass flow rate instead of volumetric flow rate. The same amount of mass has to leave the compressor as came into it.

Also, you shouldn't just assume something is an ideal gas.

exergetic

Right,it was just what I was thinking about.My assumption of natural gas as ideal gas was just for approximate its thermophysical proprieties knowing composition of this natural gas.
About this I have also another question:in evaluating power of compression I also need to find the specific work of compression but I should know first the specific heat capacities Cp and Cv (and so them ratio k).
Would it be correct to evaluate Cp and Cv of natural gas as average of the heat capacities of all the component gases weighted on molar fraction?
For example,the mean temperature and pressure of compression are 70°C and 16 bar and natural gas is composed in:75% CH4,15% H2S,10% CO2(in volumes,so in moles).
Is it correct to write:

Cp_{natural gas(70°C,16 bar)}=Cp_{CH4(70°C,16 bar)}*0.75+Cp_{H2S(70°C,16 bar)}*0.15+Cp_{CO2(70°C,16 bar)}*0.10

and in same way for Cv?