UNIVERSITY OF MINNESOTA DULUTH DEPARTMENT OF CHEMICAL ENGINEERING ChE CONTINUOUS BINARY DISTILLATION

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1 UNIVERSITY OF MINNESOTA DULUTH DEPARTMENT OF CHEMICAL ENGINEERING ChE CONTINUOUS BINARY DISTILLATION OBJECTIVE The objective of this experiment is to determine the overall column efficiency for separating a binary mixture of methanol and water. INTRODUCTION A bench-top glass tray distillation column is used to distill methanol from water. A schematic of the column is shown in Figure 1. The column consists of six perforated trays numbered from one to six up the column. The liquid feed is to the fourth tray. A constant reflux ratio distilling head is mounted at the top of the column. This distilling head is designed to give a constant reflux ratio regardless of distillation rate. This unit is designed to give reflux ratios of 1 to 1, 3 to 1, 5 to 1 and 11 to 1. It is normally set for a reflux ratio of 1 to 1 The feed and bottoms streams are pumped with peristaltic pumps. The distillate is gravity fed back to the storage bottle, mixed with the bottoms product and recycled back to the column. The temperatures of the bottoms, feed and vapor in the condenser are measured with type K thermocouples. The scanning digital thermometer cycles through the temperature measurements; 1 = bottoms, 2 = feed, 3 = vapor. Liquid samples of the streams are collected from stop-cock three-way valves. The composition of the streams may be determined from the refractive index, measured with a refractometer, available in the laboratory. REFERENCES ChE 3230 Textbook: McCabe, et al, Unit Operations of Chemical Engineering. 1

2 Figure 1. Schematic of continuous distillation apparatus 2

3 EQUIPMENT 1. Refractometer 2. Continuous distillation apparatus ml volumetric flasks ml graduated pipet 5. Screw capped test tubes 6. Pasteur pipets CHEMICALS/MATERIALS 1. Methanol 2. Deionized Water SUGGESTED EXPERIMENTAL PROCEDURES 1. Generate a standard curve of Index of refraction versus mole fraction of methanol. See SAMPLE ANALYSIS below for details 2. Turn on the water circulation to the condensers. 3. Turn on the variac at 100% power. Allow the bottoms liquid to begin boiling before pumping feed to the column. 4. Stir the methanol/water feed mixture with the magnetic stirrer. 5. Set the feed and bottoms pumps to the setting of Monitor the vapor-liquid mixing on each tray through the non-silvered strip down the front of the column. Control the variac and feed pump settings to avoid column flooding, while ensuring that all trays are wetted. 7. Adjust the bottoms pump setting to maintain a steady-state level in the reboiling flask. 8. Monitor the temperatures until steady-state operation is reached. 9. Note the feed flow rate on the rotameter. Calculate the volumetric feed rate based on the composition of the feed and the rotameter calibration with water, located in Appendix A. 3

4 10. Draw samples of the feed, bottoms, and distillate streams. Measure the refractive index of each sample. 11. Turn off the variac and pumps. Allow the boiler to cool until boiling stops. 12. Turn off the cooling water. 13. Calculate the overall column efficiency. SAMPLE ANALYSIS Prepare 6-10 ml mixtures of methanol and water ranging from 0.0 to 1.0 mole fraction methanol to generate a standard curve for the analysis of your samples. To prepare the mixtures, weigh a 10 ml volumetric flask and its stopper as accurately as possible. With a 10 ml pipet dispense a known volume of methanol into the 10 ml volumetric flask. Stopper the flask and reweigh it. Make up the volumetric flask short of the mark by about 5 mm with laboratory deionized water. Stopper the flask and mix its contents by inversion. Now, make up the flask to the mark, stopper, mix and reweigh it. Using a refractometer determine the index of refraction of each standard and also of pure methanol and pure water. The operating instructions for the refractometer are located in the laboratory. The refractive indices of each standard and pure liquid are plotted against the compositions of the solutions expressed in mole fractions of methanol. Measure the index of refraction of your samples. The composition of your samples can then be determined by interpolation from the graph plotted above. SAFETY NOTES 1. Before starting the experiment, review the Material Safety Data Sheet (MSDS) on methanol (CH 3 OH). This sheet can be found in the MSDS notebook located in the laboratory. 2. Personal protective equipment shall include safety glasses. Disposable nitrile gloves should be worn when handling the CH 3 OH solutions. 3. Make sure the safety shield is in front of the boiling flask and distillation column. 4. Pumping of flammable hazardous chemicals will be conducted in this experiment. Check all hoses and connections before starting the experiment. If a spill occurs, turn off the pumps (if possible without injury) and immediately get in touch with the Laboratory Services Coordinator or faculty member. 4

5 WASTE DISPOSAL PROCEDURES Collect all waste from the standards in the container marked DISTILLATION WASTE - STANDARDS. Place the remainder of the samples in the container marked DISTILLATION RECYCLE. 07/98 5

6 APPENDIX A The float in the rotameter is constructed from glass. The factory calibration of the rotameter is provided in Table A.1. At larger Reynolds numbers, the relationship between the flow rate of different fluids through the rotameter is 1 : ν ν A B = ( ) ( ) ρ ρ ρ ρ ρ ρ f A B f B A where í i and ñ i are the volumetric flow rate and density of fluid I, and ñ f is the float density. Table A.1 Rotameter Calibration using water at 70EC. (Float Density: 2.53 g/ml) Scale Readings Flow (ml/min) Bennet and Meyers, Momentum, Heat and Mass Transfer, 3rd ed., McGraw-Hill, New York, p. 84,

7 Department of Chemical Engineering Stockroom Checkout slip Continuous Binary Distillation ChE 4211 Name: Group No.: Date: (print name) Lab No.: Lab 1 Tuesday Thursday Lab 2 Tuesday Thursday (circle one) Equipment Out In Equipment Out In 10 ml graduated pipet 6-10 ml vol flasks 6 - screw capped test tubes 2 pasteur pipets Rubber bulb Test tube rack Name: (Signature) 7