Page 254 - Pipeline Rules of Thumb Handbook

Gas—General 241

Sample calculation 2 Mol fraction of water vapor = y = P w P (4)

Mols of components are given: P w = partial pressure of water vapor. It is exactly equal to the
saturation pressure if the gas is saturated.
Ethane 400mols/hr
Propane 4,800mols/hr MW mix = yMW mix + (1 - y) ¥ MW dg (5)
n-Butane 900mols/hr
Total Flow 6,100mols/hr where w = water vapor
dg = dry gas
Mol % ethane = 400/6,100 = 6.56%
m total = m dg[1 + ( MW w ¥ P w ) ( MW dg ¥ P dg )] (6)
Table 3
Saturated pressure of water vapor at 100°F is equal to
Component Mol/hr Mol %
0.95psia, and since the mixture is saturated, the partial pres-
Ethane 400 6.56 sure is also equal to 0.95psia.
Propane 4,800 78.69 Calculate mol % of water vapor using Equation 4.
n-Butane 900 14.75
y = 096 16 = 5 94 mol % water vapor
.
.
Totals 6,100 100.00
Calculate MW mix using Equation 5.
The mol % values listed in Table 4 can be used to calcu-
late the properties of the mixture following the procedure MW mix = 0 0594 (18 02. ) + (1 - 0 059 ) ¥ 44 01.
.
.
detailed in Sample Calculation 1.
.
= 42 48
Sample calculation 3 Calculate the mass flow of the mix using Equation 6.
Mass flow of components is given. M total = 1 000 [1 + (18 02 ¥ 0 95 ) (44 01. ¥ (16 - 0 95 ))]
.
,
.
.
,
= 1 026 lbm min
Table 4
Mass Flow Mol flow
Component (kg/h) Mol wt (kmol/h) Mol %
Ethane 15,000 30.070 499 11.08
Propane 150,000 44.097 3,402 75.55
n-Butane 35,000 58.123 602 13.37
Totals 4,503 100.00

Mol flow of ethane = 15,000/30.07
= 499
Mol % of ethane = 499/4,503
= 11.08%

When a dry gas is saturated with water, it will be necessary
to consider the effects of the water on the molecular weight
of the mixture. The water will also affect the total mass flow
of the mixture.

Sample calculation 4

Given: Compress 1,000lbm/min dry CO 2 , which is initially
water saturated. MW = 44.01, P inlet = 16psia, T inlet = 100°F. Figure 1. Compressibility chart for very low values of reduced
Calculate the molecular weight of the mixture and the total pressure. Reproduced by permission of Chemical Engineering,
mass flow for the required 1,000lbm/min of dry CO 2 . McGraw Hill Publications Company, July 1954.