CHE 4115 Chemical Processes Laboratory 2 Experiment 1 Batch Distillation BACKGROUND Distillation is one of the most commonly used unit operations in chemical engineering. In general, a distillation operation may be designed as either a batch or a continuous process and either with or without reflux. In this experiment, batch distillation with total reflux will be studied. OBJECTIVE To evaluate the efficiency of a plate column for batch distillation. PROCEDURE 1. Measure volume of each component (500 ml of water, 1000 ml of acetic acid). Calculate mole fractions for the initial mixture. [Note: the specific gravity of acetic acid = 1.049] 2. Mix together and charge into pot. Caution: get an instructor to help you with the next step. Slip glass joints together until they align and seal. Close distillate stopcock. 3. Turn on condenser cooling water to a moderate rate. 4. Turn on low heat. Gradually increase until slow boil occurs. 5. Adjust heat upward gradually until water vapor can be seen rising up into the condenser. If too much heat is applied, the column will flood with liquid and make a rattling noise. Turn off heat immediately until noise stops, then gradually increase heat until there is vapor in the condenser but not flooding occurs. CAUTION: An excessive amount of heat could cause the condenser to receive more vapor than it can condense. If this happens, vapor may be released into the room. Shut off the heat immediately and increase rate of condenser cooling water. 6. When steady state occurs, take samples (approx. 50 ml each) of liquid and vapor in stillpot, and of distillate product (close stopcock again). Measure both the volume and the weight of each sample. Record temperatures in the stillpot and the top of the distillation column. Measure the input and output temperatures and 4 1
the flow rate of the cooling water. When taking a vapor sample, submerge the receiving flask in an ice bath to ensure that the entire sample is condensed. [Note: if the technique used to analyze sample composition is temperature dependent (see Appendix), allow samples to cool to approximately room temperature before analysis.] 7. Repeat step 6 a few minutes later: Collect all samples and apply all measurements, as above. ANALYSIS 1. Using the equilibrium data provided below, construct the y-x equilibrium curve and the T-x-y plot of the system. 2. The given y-x equilibrium values were calculated using a constant value of relative volatility in α x A y A = (11.8) 1+ ( α 1) x Plot 1/y versus 1/x to calculate that value from the slope or intercept of the plot. 3. Using the McCabe-Thiele graphical method, determine the theoretical number of stages and the overall column efficiency, given that the column is equipped with 27 plates, and operated under nearly total reflux. 4. Using Equation (11.51) in the text, calculate another value of the number of theoretical stages at total reflux. Compare the values and discuss any differences. A n x A x log x x logα B d A s + 1 = (11.51) av B 5. Calculate the water mole fraction (x s1 ) and total moles (S 1 ) in the initial stillpot mixture (from the given volumes of water and acetic acid used to prepare it). 6. Calculate the ratio of final to initial moles in the stillpot, S 2 /S 1 a. from the measured values of x d av, x s1, x s2 b. from the measured x s1, x s2 and assuming x d =k x s (since x s varies very little) x av d x = xs1s1 xs 2S2 S1 S S1 S1 dxs ln = 2 S 2 x x d xs S 2 4 2
Both methods should give the same result. Use it to derive S 2, compare the result to S 2 =S 1 -D b (D b : total moles of collected distillate), and discuss. 7. From the measured flow rate and temperature change of the cooling water, calculate the heat removal in the condenser. Write a heat balance on the overall system (column, stillpot, heater, and condenser) and use it to calculate the net heat supplied to the system by the heater (i.e., heat supplied by the heater minus any heat losses to the environment). EQUILIBRIUM DATA The following data has been calculated for the equilibrium mole fractions of water in the liquid (x A ) and vapor (y A ) phases for the water (A)/acetic acid (B) binary system: T (ºC) x A y A 118.3 0.000 0.000 110.6 0.188 0.306 107.8 0.308 0.447 105.2 0.450 0.597 104.3 0.520 0.658 103.5 0.582 0.711 102.8 0.675 0.780 102.1 0.726 0.824 101.5 0.795 0.867 100.8 0.879 0.919 100.5 0.913 0.941 100.2 0.958 0.971 100.0 1.000 1.000 REFERENCE Coulson, J.M., and Richardson, J.F., Chemical Engineering, Vol. 2, 4 th Ed., Pergamon (1991). 4 3
APPARATUS The glass distillation column is a versatile apparatus, which can be used in several modes to determine useful data. The vacuum jacketed column has multiple plates with disc bubble caps and liquid downcomers. Two still pots are available each with a corresponding size electric mantle heater. On top, a condenser and flow splitter complete the apparatus. All connections (joints) are ground glass and usually need no grease for sealing, but must be handled in a careful way so that only vertical and no bending forces are imposed on the joints. Since most liquids of interest are flammable and/or toxic, safety precautions must be observed to prevent skin contact or emission to the room. Figure 4.1. Distillation Unit and Gas Chromatograph 4 4
1 2 3 6 7 4 5 8 Figure 4.2. Distillation Unit 1 - Condenser 5 - Heating mantel 2 - Distillate 6 - Still pot vapor sample 3 - Distillation column 7 - Still pot liquid sample 4 - Still pot 8 - Temperature readout 4 5
Figure 4.3. Distillation Unit Closer View 4 6
APPENDICES A. SAFETY PRECAUTIONS Heater: Avoid spilling liquid on the heater. Always heat liquids slowly to prevent rapid boiling and overflow. Stillpot: The pot is under pressure. A sudden opening may permit hot liquid to flow out rapidly, potentially causing a burn or a fire. Handle the connections with care. Do not run the flask dry, keep at least 200 ml of liquid at all times. Do not force glass materials. (See the instructor.) Column: Any blow or bending motion will break the column and your experiment will be over. Please use extreme care in handling the column. Rotate ground glass joints slowly to free them. Lift slightly while rotating. Do not force them. Condenser: This is delicate glass. When adjusting the condenser or any glassware, be careful and concentrate fully on what you are doing. Cooling Water: Be absolutely sure this is on at all times during the experiment. Thermocouples: Be sure that they are set firmly in their connectors. Sampling: (a) liquid from stillpot- open the stopcock slowly and have a cold flask in position ready to catch liquid. Cover sample immediately after drawing it. (b) Vapor from stillpot: this is under pressure, so open valve slowly and catch sample with flask in ice bath. Either of these samples may be hot enough to burn your skin. Cool all samples to room temperature before attempting analysis. Liquid or Vapor Spills: Be sure the lab door is open. Wipe up the spill with sponge and flush down a sink. Wash hands with lots of water. Avoid excessive inhalation of vapors. B. ph METER A ph meter may be used to measure the molarity of an acetic acid solution assuming that an equilibrium constant may be defined as K eq = [H + ][Ac - ]/[HAc]. You may assume K eq = 1.75 x 10-3 mol/l. Note: the accuracy of this measurement is limited, especially at high concentrations (low ph). That should not affect the ability to perform the calculations required for the experiment. It may, however, affect the values of calculated parameters and thus be worth discussing in the final report! 4 7
Gas Chromatograph Operating Instructions These are the steps that you should follow to start the GC properly. If this is your first time, you may want to read the next section, Choosing the GC Settings. 1. Turn the helium cylinder valve so that the gas is fed to the GC. 2. Adjust the helium flow rate to approximately the desired value using the bubble flow meter. 3. Turn on the power supply to the GC. 4. Set the oven, detector and injector temperatures. 5. Set the detector current and attenuation. 6. When all the settings have reached their set point values, adjust the helium flow rate again (when the GC oven temperature changes, so does the flow rate). Turn on the integrator. Wait for about 5 minutes before injecting a sample to let the system stabilize. 7. Check that the integrator baseline does not drift, and then inject the sample. 8. When you are ready to turn the GC off, set the oven, detector and injector temperatures and the detector current at their minimum. Let the GC cool down for at least 20 minutes; if you turn it off when the oven temperature is still high, the packing will melt and the column will have to be replaced. 9. Turn off the power supply and stop the helium flow rate. 4 8
Choosing the GC Settings Helium flowrate The standard flow rates are 30 cc/min for 1/8 columns and 60 cc/min for 1/4 columns. You can change these flow rates according to your needs. For example, if two key components are overlapped you may want to use a lower flow rate, but keep in mind that the peaks are not going to be as sharp as before. Conversely, if you want to reduce the run time, you can increase the flow rate, although the peaks may overlap after doing so. Oven Temperature You should try to use the oven temperature that the manufacturer of the column recommends. If this does not yield satisfactory results, you may change it taking in mind the following: The maximum oven temperature that the GC can handle is 400 ºC. All column packings have a maximum permissible temperature; never use an oven temperature higher than that. Increasing the temperature reduces the elution time and produces sharper peaks (and vice versa). It is not recommended to operate the oven at a temperature more than 10 ºC below the boiling point of any of the components of the sample because the elution time will become too long. Injector and Detector Temperature The injector and detector temperatures are usually chosen to be 5 to 15 ºC and 20 to 40 ºC above the oven temperature respectively. Detector Current Changing the detector current changes the sensibility of the apparatus. The maximum current that can be used depends on the temperature and the carrier gas used. You should check the GC manual to find the maximum allowed current for your temperature and carrier gas. When more sensitivity is needed, it is better to reduce the attenuation than to increase the detector current; this will prolong the detector s life. 4 9
Attenuation The attenuation has the same effect as the detector current. Using the lowest attenuation without getting noise in the chromatogram will enable you to use the smallest detector current and make it last longer. Gas Chromatograph Parameters Column Temperature Tc = 170 C Injector Temperature Ti = 180 C Detector Temperature Td = 185 C Detector Current Setting 85 ma Attenuation 64 Sample size 2 µm Volume and Mol Percentage versus GC Area Percentage for Acetic Acid/Water Volume Percentage = 1.0831+ 1.3664 X 3.7666 10 AA 3 X 2 AA Mol Percentage = 1.5197 + 0.24127 X AA + 7.3534 10 3 X 2 AA 4 10