Question

A 20-ft³ tank contains natural gas with the composition given below. After removing 200 SCF from the tank, the pressure and temperature drop to 1800 psia and 80^oF. Determine the how many moles of gas were in the tank originally.

	Component	Mole %
	C1	89.50
	C2	4.50
	C3	3.50
	iC4	0.70
	nC4	0.60
	iC5	0.40
	nC5	0.20
	C6	0.10
	C7+	0.50
MC7+= 110		ϒ C7+= 0.75

Answer 1

We know that Standard Cubic Foot = one cubic foot of gas at 21°C or 70°F and 101.35 kilopascals pressure or 14.7 psai = 760 mmHg

Moles of gas = 1.2 moles at SCF OR 0.002 lbs moles

Let initial moles = x

so decrease in moles = x-0.002

So new gas equation will be

PV = nRT

R = gas constant = 998.9 ft3 mmHg K−1 lb-mol−1

P = 1800psai = 93086.8 mmHg

V = 20 ft^3

n = x-0.002

T = 80⁰F = 299.8 K

x-0.002 = 93086.8 X 20 / 998.9 X 299.8 = 6.216

x = 6.218 lb moles = 2819 moles