Question

Compute the composition of the vapor and liquid when the wellstream feed is flashed to 500 psia and 120o F in a separator. Calculations should be made to at least 3 significant digits (i.e compositions should add to 1.000). All work must be shown for credit. You can use the equilibrium ratios from problem 5.

Separator (500 psia, 120o F)

Comp Feed Liquid Gas

C1 0.5

C2 0.15

C5 0.15

C6 0.1

C7 + 0.1

for more info, please go to sample test (it's question # 6)

http://www.pe.tamu.edu/barrufet/public_html/PETE310/sampleexams/Sample-Exam3b.pdf

thanks

Answer 1

K = Y / X = constant

Separator		(500 psia, 120 F)
Comp	Feed	Liquid (X)	Gas (Y)	K (Y/X)
C1	0.5618	0.1188	0.7749	6.523
C2	0.1011	0.07	0.1161	1.659
C3	0.0787	0.106	0.0655	0.618
C4	0.0674	0.1397	0.0326	0.233
C5	0.0337	0.0871	0.008	0.092
C6	0.0225	0.0643	0.0024	0.037
C7+	0.1348	0.414	0.0005	0.001

Separator
Comp	Feed	K (Y/X)
C1	0.5	6.523
C2	0.15	1.659
C5	0.15	0.092
C6	0.1	0.037
C7+	0.1	0.001

Let F be the feed flow, G the vapor product flow and L the liquid product flow

Let G/F = Gf, and L/F = Lf

Mass Balance:

F = G + L

1 = G/F + L/F

1 = Gf + Lf … eqn (1)

Component Balance:

F * X_F = G * Y + L * X

For C1:

F * 0.5 = G * Y1 + L * X1

0.5 = K1 * X1 * G/F + X1 * L/F

0.5 = X1 (K1 Gf + Lf)

Similarly for C2:

0.15 = X2 (K2 Gf + Lf)

For C5:

0.15 = X5 (K5 Gf + Lf)

For C6:

0.1 = X6 (K6 Gf + Lf)

For C7+:

0.1 = X7 (K7 Gf + Lf)

ΣXi = 1

X1 + X2 + X5 + X6 + X7 = 1

Σ X_Fi / (KiGf + L) = 1

Using eqn (1) we get

Σ X_Fi / (KiGf + 1 - Gf) = 1

Σ X_Fi / (1 + Gf (Ki - 1)) = 1

0.5 / (1 + Gf (6.523 – 1)) + 0.15 / (1 + Gf (1.659 – 1)) + 0.15 / (1 + Gf (0.092 – 1)) + 0.1 / (1 + Gf (0.037 – 1)) + 0.1 / (1 + Gf (0.001 – 1)) = 1

Solving we get Gf = 0.574

Lf = 1 – 0.574 = 0.426

X1 = 0.5 / (K1 Gf + Lf)

= 0.5 / (6.523 * 0.574 + 0.426)

= 0.12

Y1 = X1 * K1 = 0.12 * 6.523 = 0.782

Similarly we can generate results as:

Separator
Comp	Feed, XF	K (Y/X)	Liquid	Gas
C1	0.5	6.523	0.120	0.782
C2	0.15	1.659	0.109	0.181
C5	0.15	0.092	0.313	0.029
C6	0.1	0.037	0.224	0.008
C7+	0.1	0.001	0.234	0.000

ΣY = ΣX = 1