Question

A gas mixture containing 80 mole% nitrogen and the balance n-hexane flows through a pipe at a rate of 90 m3/h. The gauge pressure is 1 atm and the temperature is 95 oC.

a) Is the gas saturated or superheated? Find its degrees of superheat assuming this operation runs at constant pressure.

b) To what temperature would the gas have to be cooled at constant pressure in order to condense 80% of the hexane?

Answer 1

A gas mixture contains 80 mole% nitrogen and 20% n-hexane flows through a pipe at a rate of 90m³/h.

Temperature of gas inside the pipe = 95⁰C

Pressure of gas inside the pipe = 2 atm (absolute pressure)

a)

At 95⁰C, vapor pressure of n-hexane =2.1 atm

At 95⁰C, vapor pressure of n-hexane > 34 atm

Hence,

Therefore the gas is superheated.

Now, the gas mixture to be saturated, followig condition must be followed

At temperature, T = -186.6⁰C, above equation changes to

Hence at this temperature, the gas mixture will be saturated.

Hence degree of superheat = 95-(-186.6)

= 281.6⁰C

b)

At 95⁰C, vapor pressure of n-hexane =2.1 atm

Now, 80% of n-hexane is to be condensed, therefore

By Antoine equation, at this vapor pressure

Temperature will be around 48.23⁰C