EXPERIMENT 7: EVAPORATOR – MASS BALANCE

Introduction

An evaporator is a unit operation that is typically used to concentrate a solution by evaporating some of the solvent. In most cases, the solvent is water. To achieve temperatures above the boiling point, steam is used for heating purposes. Evaporation can take place at atmospheric pressure or under vacuum. When vacuum is present, the temperature at which the solvent will boil and evaporate is significantly less than at atmospheric pressure. The pressure of the steam used for evaporation greatly affects the rate of evaporation; the higher the steam pressure (hence higher the temperature) the more water can be evaporated over a given length of time. The amount of water that has evaporated can readily be calculated by a simple mass balance. It should be noted that when the evaporation process occurs, it is assumed that only water is evaporating – any solute is left in the concentrate.

While the evaporation process itself is quite simplistic, the evaporator is much more complex. Typically, the water evaporated needs to be recovered so as to not cause condensation problems throughout the plant. Also, because steam is used, a number of precautions need to be taken to minimize the problems of overheating equipment. In our lab, condensers and steam traps help to reduce some of these hazards.  Below is an image of the evaporator in E030 with key components pointed-out.

Evaporator in E030

The driving force for the evaporation process is the temperature difference, ΔT, between the heating steam, Tsteam, and the boiling solution, Tliquid:

ΔT = Tsteam – Tliquid

This temperature difference can be increased by increasing the heating steam pressure (hence, the steam temperature) and/or by reducing the temperature of the boiling solution by applying a vacuum.  Both Tsteam and Tliquid can be calculated from steam tables once the pressure of the supplied steam or of the boiling solution are known.  The course tables can be used or web-sites as https://www.engineeringtoolbox.com/boiling-point-water-d_926.html for pressures higher than atmospheric and https://www.engineeringtoolbox.com/water-evacuation-pressure-temperature-d_1686.html for pressures below atmospheric.

The evaporator in E030 has been automated and most functions are controlled through an “operator station”.  The user-interface of the operator station is provided in the image below, where most user functions are explained.

Purpose

The purpose of this experiment is to concentrate a dilute solution of methylene blue  in water by evaporating some of the water; and, then to do a mass balance on the evaporation process.

Procedure

  1. Make up 30 liters of 5 ppm methylene blue solution in a tank. This is done by diluting a concentrated, 1000 ppm solution of methylene blue.  Below is an example of a dilution calculation. 

  2. Drain the evaporator and the condensate drum and dispose of any liquid in them.
  3. Load your 5 ppm solution into the evaporator.  This might not be such a simple process.
    1. Close all valves except A, B, and C as marked on the evaporator.
    2. Pump the solution into the evaporator. This can be done either using the pump or creating a vacuum by turning on the after – condenser steam. Often you will need to do both in order to fill the evaporator.  One challenge here is that the pump, being a centrifugal one, needs to be “primed” (i.e. needs to be filled with liquid in order to manage to pump more liquid).  If you properly drained the evaporator (step 2 above), the pump will be dry and hence unable to pump!! Turning on the after-condenser steam will help by creating a vacuum that will draw some liquid into the pump housing and prime the pump.  The created vacuum will also ease the pumping operation.
    3. The evaporator is full when the water level is near the top of the sight glass on the vapour liquid separator. Stop the pump, and quickly close valve A.
    4. Turn off the after-condenser steam, if used.
    5. If any solution is left in the bucket, weigh this so as to obtain the mass of solution actually put into the evaporator.
  4. Take a sample of the solution in the evaporator through the sampling port on the side of the unit.
  5. Open the (service) condenser water via the operator station
  6. Turn on the steam via the operator station and note the starting time of the run. Set the steam pressure to 25 psig.
  7. While the evaporator is warming up, make 3 standard solutions of 2, 5, and 10 ppm of methylene blue. Use the concentrated 1000 ppm methylene blue solution, as you did for making the 5 ppm solution that is now in the evaporator.
  8. Start a timer when the solution starts boiling. Boiling can be assessed two ways:
    1. A constant solution temperature inside the evaporator.  That temperature should be close to 100°C, but not exactly there.
    2. Water dripping from the condensate drum (provided that the solenoid valve above the drum is open)
  9. Take a sample from the evaporator every five minutes during the run (after it reaches the boiling point).  Weigh your sample (this can be easily done by first weighing the container in which the sample if taken).  Keep the sample until the end of the experiment.
  10. Stop the experiment after 30 minutes.  Turn off the steam and the condenser water.
  11. Collect the condensate and weigh it.
  12. Drain the evaporator contents (concentrated solution) via the inlet hose into a bucket, weigh and take a sample from the concentrated solution to determine its concentration.
  13. Use the spectrophotometer to determine the Absorbance of your seven samples (at time 0, 5, 10, 15, 20, 25, and 30 minutes) and of the standard solutions.  You might also need pure water for calibrating the spectrophotometer (check with your instructor)

Check with your lab instructor if the following need to be done:

  1. Rinse the evaporator by pumping tap water into the evaporator via the operator station.
  2. Run the evaporator steam at 30 psig for only 3 minutes to force the rinse to circulate through the evaporator.
  3. Drain the evaporator via the inlet hose into a bucket. Weigh the collected solution, take a sample from it and measure its absorbance.

Check with your instructor is you need to repeat the experiment.

Report (Long Report):

  • Create a table with ALL your measurements:
    • Absorbance of the standard solutions
    • Mass and, if measured, absorbance of the original solution fed into the the evaporator, all samples taken through time, condensate and any other possible solution or sample. Make sure your table has units
  • Use the absorbance values of your standard solutions to create an absorbance / concentration plot.  Your plot must have proper axes labels with units.  Run a trendline (best-fit line) through your data, making sure, through, that your trendline crosses through the origin (0,0), as zero absorbance corresponds to zero concentration.  The video below walks you through how to create the plot.  Make sure your plot has proper axis titles with units

  • Use the trendline equation to convert your absorbance readings to concentrations. Report in a Table and do not forget to include units on the column or row titles.
  • Plot the concentration of methylene blue in the evaporator vs time.  If you performed multiple runs, plot all runs on the same plot.
  • Use the concentration and mass of all samples to perform (for each run, if multiple runs were performed):
    • A total mass balance of the process: is the amount of solution initially fed into the evaporator equal to all the samples removed in the end?
    • A mass balance on methylene blue:  Is the amount of methylene blue initially fed into the evaporator equal to methylene blue exiting the unit?  Recall that methylene blue exits through the samples taken at 5-minute intervals and the final solution collected from the evaporator.
  • Comment on:
    • The plot of concentration vs time
    • Was the evaporation effective?
    • The two mass balances your performed.  Is there any gain or loss?  If so, why?
    • If performed multiple runs, comment on any differences between the runs.

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PROCTECH 2CE3 Lab Manual Copyright © by Kostas Apostolou. All Rights Reserved.

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