Question

Assuming normal heptane and normal octane to form ideal solutions, do the following:

d) Calculate the dew pressure of the solution at 40 ◦C.

e) Calculate the bubble temperature at 1 bar.

f) Calculate the dew temperature at 1 bar.

g) Calculate the amount and composition of vapor and liquid when a solution with z1 = 0.45 is flashed to 40 ◦C, 0.065 bar. The saturation pressures of the two components are given by the Antoine equation:

ln Pi sat = Ai − Bi/(Ci + T)

where P sat is in mm-Hg, t is in Kelvin, and the parameters

Ai , Bi , Ci , are

nC8: A = 15.9426 B = 3120.29 C = –63.63

nC7: A = 15.8737 B = 2911.32 C= –56.51

Answer 1

Given sata

xf = 0.45

Mixture of n-heptane and n-octane

n-heptane is more volatile than n-octane

Antonie constants are given in question

For n-heptane

A: 15.8737

B: 2911.32

C: -56. 51

For n-octane

A: 15.9426

B: 3120.29

C: -63. 63

ln Pi= A-(B/(T+C) )

P is in mmHg and T is in K

D) dew pressure at 40°C

T = 40°C = T = 313 K

At dew point x = 1

P = 44.0541 mmHg

Dew pressure = 44.9541 mmHg = 0.0599 bar

E)

Bubble temperature at 1 bar

At bubble point

Subsituting P = 1 bar

1 bar = 750.062 mmHg

Solving we get

T = 383.6 K

Bubble temperature = 383.6 K = 110.45°C

F) dew temperature at 1 bar

At dew point

Solving we get

T = 388.3 K = 115.15°C

g)

P = 0.065 bar

T = 40 °C

Dew pressure at T = 40°C is 0.0599 bar

Bubble pressure at 40°C =

Bubble pressure = 58.433 mmHg = 0.07790 bar

Operating pressure = 0.065 bar

(V/L) =( 0.065-0.0599) /(0.07790-0.065)

V/L = 0.3953

Basis

F = 100 mol

Material balance

F = V+ L

V/L = 0.3953

V = 0.3953 L

100 = V+L

100 = 0.3953L+L

L = 71.669 mol

V = 28.330 mol

Applying Raoults law

yP = x(V.P)

For heptane

V. P for heptane at 40°C = 92.129 mmHg = 0.1228 bar

y(0.065) = x(0.1228)

y = 1.889x

Doing component balance

100(0.40) = V(y) +L(x)

100(0.40) = 28.330(1.889x) +71.669(x)

x = 0.3195

y = 1.889(0.3195) = 0.6035

Composition of liquid

L = 71.669 mol

Heptane- 31.95%

Octane -68. 05%

Composition of vapor

V = 28.330

Heptane - 60.35%

Octane- 39.65%

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