CBEN 408 Quiz #5
Name (1 point): ___________________________________
April 6, 2016
1. (3 points) A natural gas has an ideal gas specific gravity of 0.62. What is its molecular
weight?
2. (3 points) You want to treat 100 MMscfd gas containing 20 mol% CO2. You remove
75% of this CO2 in an amine absorber. What is the CO2 content in the treated gas (in
mol% to at least three significant digits)?
3. (3 points) You want to compress 100 MMscfd gas (0.65 specific gravity, γ=1.316) from
1 psig & 87°F to 205 psig. You need to keep the compression ratio in each stage of the
compressor to 3 or less. How many stages of compression (with interstage cooling)
should you use?
Solution
1. Based on the air molecular weight of 28.9625:
M   0.62 28.9625  17.96
2. There are 80 MMscfd hydrocarbon & 20 MMscfd CO2 in the inlet gas. You will removed
75% of the 20 MMscfd CO2, so there is 5 MMscfd left. The mole fraction of CO2 in the
outlet (i.e., treated) gas is:
yCO2 
5
 0.0588 .
80  5
3. The overall compression ratio in absolute units is:
R
Pout 205+14.7 psia

 13.99 .
Pin
1  14.7 psia
The number of stages to give a maximum compression ratio of 3 is:
m
ln  R 
ln  Rmax 

ln 13.99 
ln 3
So 3 stages are needed.
 2.40