Engineering 435 Lab Report
November 30, 1998
Table of Contents
The function of distillation is to separate, by vaporization, a liquid mixture of miscible and volatile substances into individual components or, in some cases, into groups of components. The separation of a mixture of alcohol and water into its components; of liquid air into nitrogen, oxygen, argon; and of crude petroleum into gasoline, kerosene, fuel oil, and lubricating stock are examples of distillation. Suppose there are two components, A and B. Both of these components are found in both phases. There are four variables: pressure, temperature, and concentrations of component A in the liquid and vapor phases (the concentrations of component B are unity less those concentrations of A). If the pressure is fixed, only one variable, e.g., liquid-phase concentration, can be changed independently and temperature and vapor-phase concentration follow. In practice, distillation may be carried out either of two principal methods. The first method is based on the production of a vapor by boiling the liquid mixture to be separated and condensing the vapors without allowing any liquid to return to the still. There is then no reflux, which is called batch distillation. The second method is based on the return of part of the condensate to the still under such conditions that this returning liquid is brought into intimate contact with the vapors on their way to the condenser. Either of these methods may be conducted as a continuous steady-state distillation process, including single-stage partial vaporization without reflux (flash distillation) and continuous distillation with reflux (rectification), for systems containing only two components.
The distillation column experiment is a purification experiment to separate a binary mixture of methanol and water. After becoming familiar with the column and its setup, several experiments were conducted to observe the operation and performance of the column. The holding reservoir, which contained the methanol-water mixture, was analyzed to determine the composition of the liquid. We performed a boiling point test on the mixture to determine the unknown composition of the liquid. The data obtained from the boiling point test allowed us to determine the composition using an x, y, T data plot for methanol-water. Two experiments were performed during the four-week period. A batch distillation was performed first. The distillation column was monitored until the reboiler started to boil. The time was recorded and the volume change was monitored in the reboiler. The second experiment involved a continuous distillation experiment. The column was allowed to reach steady state. Once steady state was obtained the time was recorded and the compositions of each tray were determined during a specific time interval. The collection of this data would allow us to determine the tray efficiency and plot the separation line on the equilibrium diagram. The diagram of our system is given on the following page as Figure 1.
Figure 1. Distillation Column
4. Do composition analysis on the distillate (boiling point test).
3. Perform calculations with the data obtained to determine tray efficiency.
The batch distillation data results are given in Table 1 below.
Table 1. Batch Distillation Data
The continuous distillation data is given in Table 2, which is provided below.
Table 2. Continuous Distillation Data
Discussion of Results
The data provided in Table 1 for the batch distillation experiment illustrates that we may have a problem with the column or the batch distillation spreadsheet. The spreadsheet shows that the volume in the reboiler should be changing at a much higher rate than the column actually performed. Since the spreadsheet had been successfully used on previous occasions, it is likely that the error is in the column. Attached on the following pages are the results of the batch distillation experiment (spreadsheet solution and graphical solutions).
The data provided in Table 2 for the continuous distillation experiment illustrates that trays 1 through 6 had temperature readings slightly below the boiling point of methanol. This is likely due to the RTD's not being calibrated correctly. This sheds some doubt on the accuracy of the results for the remaining trays, which are given in Table 2 provided above. The liquid compositions shown in Table 2 were obtained by using a methanol-water x, y, T diagram. The useable data was plotted on a x, y diagram to determine the individual tray efficiencies. The plot illustrating the individual tray efficiencies is attached on the following page.
During experimentation of the distillation column the group noticed distillate being formed even during total reflux. This could indicate a problem with the reflux valve. This could also explain the problem with the reboiler volume during batch distillation experiments. The behavior of the reflux valve should be monitored to determine whether it is functioning correctly.
Being able to perform experiments on the distillation column was very useful task as it provides practical experience in using the theoretical knowledge acquired in the Unit Operations class throughout the semester.