Question

A liquid mixture containing ethanol (55.0wt%) and the balance water enters a separation process unit at a rate of 90.5 kg/s. A technician draws samples of the two product streams leaving the separator and analyzes them with a gas chromatograph, obtaining values of 86.2wt% ethanol (product stream 1) and 10.9% ethanol (product stream 2). The technician then reads a manometer attached to an orifice meter mounted in the pipe carrying product stream 1, converts the reading to a volumetric flow rate using a calibration curve, and converts that result to a mass flow rate using the average density of ethanol and water. The result is 54.0 kg/s. Finally, the technician calculates the mass flow rate of the second product stream using a material balance and reports the calculated product stream flow rates and compositions to you. You examine them, do some calculations, and reject them.

(a) Draw and label a flow chart of the separation process.

(b) Carry out the calculations that led you to reject the submitted results and explain how you knew the values were wrong.

(c) List up to five possible reasons for the incorrect results. For each one, briefly state how you might determine whether it was in fact a cause of error and what you might do to correct it if it was.

Answer 1

F= S1+S2 ( where S1= stream 1 flow rate and S2 = Stream 2 flow rate)

Stream 1(S1) flow rate is 54 kg/s and contains 86.2 wt% ethanol

Ethanol in the stream 1= 54*86.2/100=46.528 kg/s (as per analysis )

S2= 90.5-54=36.5 kg/s, ethanol in S2= 36.5*10.9/100=3.975

Total flow rate of ethanol = that leaving S1 + that leaving S2= 46.528+3.975 kg/s=50.5265 kg/s

While entering is only 49.775 kg/s as per the balance

1. c) One is sample is not collected under steady state conditions. That is clearly shown in the balance equation
2. Temperature correction factor is not applied for the mass flow rate of vapor stream
3. The volume of mixture is less than the individual volumes which can lead to incorrect density measurement.
4. The orifice meter liquid has to be mercury. If any other liquid is used, the deflection may not be read correctly.
5. The mixture forms Azetrope at 95.63% ethanol and water and actual value is less than this.