Question

Solids soaked with liquid hexane are dried by being contacted with nitrogen at an elevated temperature. The gas stream leaving the dryer is at 80°C, 1 atm absolute and 50% relative saturation.

(a) One of several possibilities for recovering the hexane from the gas is to send the stream to a cooling condenser. The gas stream leaving the condenser would contain 5.00 moles of hexane (out of 100 total moles), and hexane condensate would be recovered at a rate of 1.50 kmol/min. The condenser would be operated at a pressure of 1 atm absolute. Calculate the temperature to which the gas must be cooled and the required flow rate of fresh nitrogen to the dryer in standard cubic meters per minute (SCMM).

(b) In an alternative arrangement, the gas leaving the dryer would be compressed to 10.0 atm and the temperature would simultaneously be increased so that the relative saturation remains at 50%. The gas then would be cooled at constant pressure to produce a stream containing 5.00 mole of hexane. Calculate the final gas temperature and the ratio of volumetric flow rates of the gas streams leaving and entering the condenser. State any assumptions you make.

(c) What would you need to know to determine which of processes (a) and (b) is more cost-effective? Solids soaked with liquid hexane are dried by being contacted

Answer 1

a)The temperature can be calculated by taking into account the amount of gas that has been sent to stream and the temperature prevailing at the cooling condenser.

Gas stream leaving the dryer at a temp of 80. Total =100moles. Hexane content recovered =1.50kmol per minute.

Therefore the temperature=(80*5)/100=4degree celcius.

Rate of flow in standard cubic meters per minute=1.50*10^3*1(pressure)=1500

b)final gas temperature=80*50/100(80 is the temperature and saturation remains 50%)

=40 degree celcius

Ratio of volumetric flow=5/100=0.05(as five moles are obtained from total of 100 moles)

c)Process b would be cost effective as only the final gas temperature is taken into account while in process a the initial temperature is also taken into account.