A liquid stream containing half n-hexane and half n-pentane at 80°C and 5 atm is fed at a rate of 100 mol/hr to a flash evaporator operating at pressure (P) and 65°C. The decrease in pressure causes a lot of the liquid to vaporize. The liquid and vapor exit streams are in equilibrium with each other at the same temperature and pressure as in the evaporator. The liquid exit stream contains 59% hexane.a. Calculate the evaporator pressure (P), the fraction of hexane in the vapor stream, and the flow rates of the exiting liquid and vapor streams.b. Calculate the rate of heat flow to or from (specify which, to OR from) the evaporator.c. Usually we think of evaporators as units that increase temperature to vaporize species. Explain how this flash evaporator is able to vaporize the feed with a decrease in temperature.

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Asked Apr 4, 2019
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A liquid stream containing half n-hexane and half n-pentane at 80°C and 5 atm is fed at a rate of 100 mol/hr to a flash evaporator operating at pressure (P) and 65°C. The decrease in pressure causes a lot of the liquid to vaporize. The liquid and vapor exit streams are in equilibrium with each other at the same temperature and pressure as in the evaporator. The liquid exit stream contains 59% hexane.
a. Calculate the evaporator pressure (P), the fraction of hexane in the vapor stream, and the flow rates of the exiting liquid and vapor streams.
b. Calculate the rate of heat flow to or from (specify which, to OR from) the evaporator.
c. Usually we think of evaporators as units that increase temperature to vaporize species. Explain how this flash evaporator is able to vaporize the feed with a decrease in temperature.
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Expert Answer

Step 1

Part (a)

The Antoine equation to be used is written below.

Here, Pi* is the vapor pressure of the component i.

A, B, and C are the constants for component i.

T is the temperature at which Pi* is  to be calculated.

Antoine equation constants for hexane and pentane are written in the table shown below.

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Step 2

Now, calculate the value of vapor pressure of hexane and pentane at 65°C as:

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Step 3

Now, use Raoult’s law as it is given that the vapor and liquid phases ar...

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