Distillation of a Methanol - Water Solution

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The purpose of this experiment is to introduce students to elementary aspects of the most widespread separation technique used in the chemical industry, distillation. The experiment allows the important concepts of reflux and reboil to be demonstrated and to test how these variables influence the purity of the distillate and bottoms streams. Because the column is fitted with view ports on many trays, hydrodynamic effects, such as flooding and weeping can also be studied visually.

The parameters which can be varied in this experiment are :

1) feed flow rate,

2) feed tray,

3) reflux rate,

4) steam heating to the reboiler

The following effects can be studied :

1) purity of the distillate and bottoms streams

2) tray efficiency

3) tray temperatures

4) visualization of the column hydrodynamics

A 13-tray bubble cap column with an equilibrium reboiler and total condenser is available. The reboiler is heated using a steam coil. Tanks are available for distillate and bottoms collection. Feed can be introduced on any of five trays, and most trays are fitted with liquid and vapor sample ports, thermocouples and view ports. Feed rate, reflux rate, reboiler steam pressure, and distillate and bottoms feed rates ( not all these variables are independent) are computer controlled. The computer also provides data acquisition for the tray temperatures. Compositions are measured using either a refractometer or a gas chromatograph.

Flow Charts for Distillation System

• Overhead System
• Column System
• Tank System

• Distill a MeOH-H2O solution (20 mole percent ) under total reflux and determine the tray efficiency using published liquid-vapor equilibrium data for system MeOH - H2O and experimental measurements of the composition of the liquid on each tray and the vapor leaving each tray. Tray efficiency is a function of the size and nature of the trays, column loading, vapor and liquid, as well as local composition.

   

• Develop a program to characterize the tray efficiency and propose a significant sub-project. Experimental groups after the first should use all prior data in developing and analyzing the tray efficiency problem.

• Check the composition of the feed. If it is much lower than 20 mole %, ask the laboratory technicians to add MeOH.

   

• For the preliminary report calculate the proper flow rate settings for the column. Assume a 50% tray efficiency. Refer to Treybal, pages 423-426 for the use of the McCabe - Thiele method incorporating tray efficiency. Calculate the various flow rates (rotameter and valve settings) for conditions corresponding to 50% of flooding. See Treybal, pages 161-176.

• Use the following values of constants for bubble-cap trays (Treybal, 2nd ed.) in Equation 6.29 and 6.30.

  • a = 0.0041t + 0.0l35
  • b + 0.0047t + 0.068
  • t = tray spacing inches

• Column characteristics:

  • Diameter = 8 in.
  • Tray spacing = 12 in
  • No. of trays = 13

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