Question

Nitrogen is bubbled through a liquid mixture that contains initially equimolar amounts of benzene (B) and toluene (T). The system pressure is 3atm and the temperature is 80 degrees Celsius. The nitrogen flow rate is 10 standard liters per second. The gas leaving the bubbler is saturated with B and T vapors.

(a) Estimate the initial rated (in mol/min) at which B and T leave the bubbler.

(b) How will the mole fraction of B and T in the liquid change with time (Increase or decrease or remain the same)? explain your answer.

(c) How will the mole fraction of B and T of the exiting the gas change with time (Increase or decrease or remain the same)? explain your answer.

Answer 1

(a)

22.4 standard liters of gas = 1 mol

Nitrogen flow rate = 10 L/min

= 1*10/22.4 mol/min = 0.446 mol/min

Let the flow rate (mol/min) of B and T leaving the bubbler be Mb and Mt respectively.

By mass balance we can write:

Mb+Mt=0.446 mol/min (1)

From vapor pressure data at 80 degrees Celsius, vapor pressure of:

B= 760 mmHg

T= 291.5 mmHg

Partial pressure = Vapor pressure * Liquid mole fraction

Partial pressure of:

B= 760*0.5= 380 mmHg (Pb)

T= 291.5*0.5= 145.75 mmHg (Pt)

Ratio of vapor mole fraction (X) = Ratio of partial pressures (P)

Therefore, Xb/Xt= Pb/Pt

Xb/Xt= 380/145.75 = 2.607

Initial rated flows of B and T will be in the ratio of their vapor mole fractions (X) which gives:

Mb/Mt= 2.607 (2)

Solving equation 1&2, gives

2.607Mt + Mt = 0.446 mol/min

or Mt = 0.124 mol/min

and Mb = 0.322 mol/min

(b)

X-Axis: Time, Y Axis: Liquid mole fraction.

As seen from above graph, mole fraction of B in liquid will decrease with time as B is more volatile component. Mole fraction of T in liquid will increase with time as T is relatively less volatile.

(c)

Initial mole fraction of B&T:

Xb/Xt = 2.607

Xb+Xt = 1

2.607Xt + Xt = 1

Therefore Xt= 0.277 and Xb= 0.723

X Axis: Time, Y Axis: Vapor mole fraction

As seen from above graph, mole fraction of B in vapor will decrease with time as its partial pressure which depends on liquid mole fraction also decreases with time. Mole fraction of T in vapor will increase with time as its partial pressure which depends on liquid mole fraction also increases with time.