CL-333 Manual MT 303: Batch Distillation Batch Distillation Equipment Operating Panel Refrectometer 1
CL-333 Manual MT 303: Batch Distillation Objectives: To determine the height equivalent to number of theoretical stages and the number of transfer units for a packed column. Theory: Distillation is the most widely used separation technique in the chemical and petroleum industry and is performed in either tray or packed columns. In a packed column, HETP (Height Equivalent to a Theoretical Pate) or HTU (Height of a Transfer Unit) are used to relate the column height with the number of theoretical stages obtained by standard design methods such as McCabe-Thiele or Ponchon-Savaritt. Distillation operated at total reflux yields maximum possible separation. The number of theoretical stages provided by the column is the minimum number of theoretical stages for the top and bottom composition. Minimum number of stages may be calculated using graphical methods or by Fenske s equation given by (1). N M xd 1 xw log( ) 1 xd xw + 1 = logα av (1) α = α Xα (2) av top bottom N M = number of theoretical stages at total reflux, and α av = average relative volatility Experimental procedure: 1. Prepare a calibration chart of RI (Refractive Index) vs. mole fractions of components A and B. 2. Fill approximately 2/3 rd volume of the reboiler kettle with a mixture of A and B. 3. Start water circulation through the condenser. 4. Close the bottom sample draw out valve to prevent the overflow of liquid through the sample line during the experiment. Switch on the heating mantle and allow the flask temperature to rise. 5. When the vapors rise to the top of the column, increase the heating rate if necessary. After about 30 min., note the temperatures along the length of the column. If some milky liquid appears in the reflux line, drain it in a small glass vessel and discard. This could be 2
due to the formation of emulsion of the organic liquid with moisture already present in the system. 6. When the temperature readings along the column length become steady (based on four successive readings taken at 10 min time interval) it indicates that the system has attained steady state and the samples may be withdrawn. 7. 2 to 4 ml of top sample is collected in a sample bottle after purging off some quantity. The bottle should be immediately closed to avoid losses. Determine the RI of the sample. 8. Step 7 should be repeated now for the bottom sample. 9. Count the number of actual plates in the column. HETP may be calculated by (3) once the number of theoretical stages (N M ) provided by the column is known. Height of the column HETP = (3) N M HTU can be calculated from (4) once NTU (Number of Transfer Units) for the column is known. NTU for a packed column is determined using (5). Height of the column HTU = (4) NTU NTU y D dy = (5) y * y yb # Please read standard text books (e.g. Treybal) on mass transfer operations to understand the difference between HTU and HETP. Observations Data from the literature 1. Molecular weights of A and B. 2. Boiling point of A and B. 3. Refractive indices of A and B at ambient temperature. 4. Densities of A and B at ambient temperature. 5. Height of Column. 3
Calibration data for mole fraction vs. RI S. No. Mole fraction of A RI Figure 1: Set-up for Batch Distillation Observations 1. Number of actual stages in the column. 2. Top temperature. 3. Bottom temperature. 4. RI of top and bottom samples. 4
5. Mole fraction of top sample (x D ). 6. Mole fraction of residue or bottom sample (x B ). Calculations, Results and Discussion 1. Calculate the number of theoretical stages by McCabe Thiele method and calculate the HETP using equation (3). 2. Calculate the number of stages by Enthalpy-Composition (Ponchon-Savaritt) method and calculate the HETP using (3). 3. Find the number of stages by Fenske s equation (1) and compare the result with that obtained from Step 1 above. 4. Determine the NTU using (5). 5. Comment on the precaution and sources of error. 5