Question

Asked Jan 24, 2019

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The analysis of pesticides for environmental extracts using chromatography typically requires a solvent exchange from dichloromethane (DCM) to a chromatograph-compatible solvent such as hexane. If DCM and hexane can be separated after the procedure, they can be recycled to reduce waste. A mixture of liquid hexane and DCM (50wt% hexane) is fed to an evaporator resulting in a vapor containing 70wt% hexane and liquid containing 10wt% hexane.

a. Draw and label a flowchart for a continuous and steady-state process with a feed rate of 200.0 kg/min. Write out the general mass balance equations for total mass and hexane. Explain why you are discarding any of the terms. Solve the equations to find the mass flow rates of the vapor and liquid streams leaving the evaporator.

b. Draw and label a flowchart if the process is performed in a closed container filled initially with 200.0 kg of the 50:50 mixture. Write out the integral mass balances for total mass and hexane. Explain why you are discarding any of the terms. Solve the equations to find the masses of the final vapor and liquid phases.

Step 1

Part (a).

Let the basis of the process be 200 kg/min of the feed. Hexane is represented by ‘H’ and dichloromethane by ‘DCM’. The process unit is an evaporator.

It is given that the mass percentage of hexane in feed is 50 wt%. So, the mass fraction of hexane in the feed stream is, *x _{f}* = 0.500 kg H/kg

Now, the mass flowrate of the vapor stream is *ṁ _{v}* kg/min and the mass flowrate of the liquid stream is

The mass percentage of hexane in vapor stream is 70 wt%. So, the mass fraction of hexane in the vapor stream is, *x _{v}* = 0.700 kg H/kg

The mass percentage of hexane in liquid stream is 10 wt%. So, the mass fraction of hexane in the liquid stream is, *x _{l}* = 0.100 kg H/kg

The flowchart of the given process is drawn below as:

Step 2

Now, the general equation of mass balance is:

Input + Generation = Output + Consumption + Accumulation

The given process is assumed to be at steady state, thus, Accumulation = 0.

There is no reaction taking place in the process, thus, Generation = Consumption = 0.

Therefore, the final mass balance equation which is applicable for the given process is:

Input = Output

Now, apply total mass balance on the process as in equation (1) and,

Apply mass balance on hexane present in the process as in equation (2).

Step 3

Solve the two equations s...

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