ABSTRACT Distillation column is used to determine pressure drop for various boil-up rates in batch distillation. The degree of foaming on trays for each power increment also can be determined. In order to study the pressure drop of the column, the power was set to 0.5 kW and then 0.75, 1.00, and lastly 1.25 kW. For each power, the sample was collected until it reached 90mL and then the procedures were repeated for each increment. The samples collected were tested for its Refractive Index. It is observed that degree of foaming increases when the power was increased. The degree of foaming at 0.5 kW was gentle as opposed to the others, where the foaming starts to become vigorous. Finally, the refractive index for pure MCH and toluene was determined in order to match the mixture obtained from the sample.
INTRODUCTION Distillation is a process of separating a mixture into two or more products that have different boiling points, by partial vaporization of a liquid mixture and/or by partial condensation of a gas mixture thereby rendering liquid phase richer in less volatile (with higher boiling point) component and the vapor phase is richer in more volatile (with lower boiling point) component. Distillation probably accounts for 90% of all separation processes in the chemical industry, and is also a significant user of energy due to the necessary heating involved. This process is conducted by using the Distillation Column apparatus. The column is consisted of bubble cap trays, reboiler, product and feed tank, reflux splitter, and condenser. Each bubble cap allows vapor to pass upward from the tray below and condense while allowing liquid to pass back. Two heat exchangers are located above the column. The second exchanger acts as a condenser. It condenses the vapor leaving the first exchanger using cooling water. The condensate flows into the reflux tank. Then it can be refluxed back to the top of the column or directed to the product tank.
Figure 2.1: Schematic Diagram of Distillation Column
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There are two principal methods to carry out distillation process. The first one is producing a vapor by boiling the liquid mixture to be separated and condensing the vapors without allowing any liquid to return to the still. That means, there is no reflux, and this process is called batch distillation. The second method is part of the condensate returns to the still under such conditions that this returning liquid is brought into close contact with the vapors on their way to the condenser. For systems containing only two components, distillation can be conducted by using either of these methods as a continuous steady-state distillation process, including single-stage partial vaporization without reflux (flash distillation) and continuous distillation with reflux (rectification). Examples of distillation process in the industry are: separation of a mixture of liquid air into nitrogen, oxygen, argon; the distillation of crude fermentation broths into alcoholic spirits; and the fractionation of crude oil into useful products, such as gasoline and heating oil. In the organic lab, distillation is used to purify solvents and liquid reaction products.
OBJECTIVES The purposes of this experiment are: 1. To determine the pressure drop of the distillation column for various boil-up rates in batch distillation. 2. To plot the curve relating the pressure drop and the boil-up rates. 3. To observe the degree of foaming on trays for each power increment. 4. To determine the refractive index for unknown concentration of methylcyoclohexane/toluene from the distillation column for each power increment.
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THEORY The total pressure drop across each tray is the sum of that caused by the restriction of the holes in the sieve tray, and that caused by passing through the liquid (foam) on top of the tray. As the velocity of the vapours passing up the column increases then so does the overall pressure drop. The velocity is controlled by varying the boil-up rate which is done by varying the power input to the boiler. Under condition with no liquid present, the sieve tray will behave like an orifice in that pressure drop will be proportional to the square of velocity. Due to the fact that there is a liquid head however, this square relationship does not become apparent until the head of liquid has been overcome and foaming is taking place. In a graph of pressure drop vs boil-up rate (log/log), at low boil-up rates the pressure drop will remain fairly constant until foaming occurs when the pressure drop would be expected to rise sharply for unit increases in boil-up- rate. For the system methylcyclohexane/toluene, mixture of known concentration can be made up and their refractive index measured. The refractometer measure the critical angle of the liquid under test and each concentration will show a different critical angle. From this, the Refractive Index can be found.
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APPARATUS
a
b g f
c d e
Figure 5.1: Distillation Column Apparatus 1. Distillation Column a. Condenser b. Electromagnet (reflux control) c. Reboiler d. Cooler e. Bottom f. Distillate g. Feed 2. Refractometer 3. Refractor 4. 1- 250ml Measuring Cylinder graduated in mls 5. 1- 1000ml Beaker graduated in mls 6. Stopwatch / timing device
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MATERIALS 1. Methlcyclohexane 2. Toluene
PROCEDURE 6.1 General Start-Up Procedures 1. It is made sure that all valves are closed. 2. The power for the control panel is turned on. 3. A 30-40 L mixture containing methylcyclohexane and toluene at the desired composition is prepared. The unit is charged as follows: i)
The charge port cap at the reboiler B1 is opened, and the reboiler is filled up to the level of the internal overflow height.
ii)
The remaining methylcyclohexane-toluene mixture is poured into the feed vessel B2 through the feed charge port.
4. The reflux divider KFS-101 is set to total reflux. This is to prevent any distillate from escaping when starting up the distillation column. 5. The cooling water is let to flow into the condenser W2 and product coolers W3 and W4 by opening valves V13, V14 and V15. 6. The tip of the level switch is made sure to be located below the reboiler equator, which is at the liquid inlet from the cartridge heaters. 7. The cartridge heaters at W1 are switched on. The liquid in the reboiler is allowed to boil. 8. The vapor rising from the boiling liquid into the distillation column is observed. When the vapor reached the condenser, it condensed and flowed back into the column as liquid. Mass transfer took place between this liquid and vapor phase in the column. 9. The condenser is checked if it had sufficient cooling water flow to minimize escape of vapor into the vent. 10. The distillation column is run until a stable condition is observed. The stable condition is reached when all temperature indicators on the column give a constant reading with an allowable fluctuation of ± 0.2°C.
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6.2 General Shut-Down Procedures 1. The reflux divider KFS-101 is set to total reflux. 2. The cooling water is kept running through the condenser W2 and product coolers W3 and W4. 3. The cartridge heaters W1 are switched off. The temperature drop in the reboiler is monitored until it cooled down to less than 50°C. 4. The cooling water valves V13, V14 and V15 are closed. 5. All liquid from the reboiler B1 and product vessels B3 and B4 are drained by opening valves V6, V9 and V12. The liquid in the pipelines are also drained by opening valves V5, V8, V10 and V11. The liquid can be re-mixed and recycled for future use. 6. The power for the control panel is turned off.
6.3 Experiment A: Determining Column Pressure Drop in Batch Distillation 1. The general start-up procedures as in Section 6.1 are performed. 2. The power is set to 0.5 kW. 3. The distillation column is let to reach stable condition after 15 minutes. 4. Valves V6 and V7 are opened. The pressure drop of the distillation column is recorded. 5. Valves V6 and V7 are closed. 6. To measure for the boil-up rate, valve V3 is opened, the time is started as sample is collected by using a 100 mL measuring cylinder. 7. The time is stopped after 90 mL of sample is collected, valve V3 is closed. 8. About 5 drops of the sample is taken to measure for the refractive index in Section 6.4, while the balance is put into the conical flask provided. 9. The degree of foaming on the trays is observed. 10. All steps above are repeated with power of 0.75, 1.0, 1.25, and 1.5 kW. 11. Graph of pressure drop against boil-up rate is plotted.
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6.4 Experiment B: Determining Unknown Concentration
a) Finding Refractive Index using Refractometer 1.
The refractive index value is taken by using automatic digital refractometer for each power sample of product at each power increment.
2.
The refractometer is run: i. The surface is cleaned using distilled water. ii. The power is switched on. iii. ZERO setting is pressed (SWL key). iv. Start key is pressed after placing the sample. The refractive index value is then recorded. v. It is cleaned and the power is switched off.
b) Non-distillation Mixing of Methylcyclohexane and Toluene 1. 10 mL of solution is assumed equal to 10 mol%. 2. 0 mol% methylcyclohexane is mixed with 100 mol% toluene using conical flask. 3. The solution is carefully stirred. 4. Steps 2 and 3 are repeated with : i.
25 mol% methylcyclohexane + 75 mol% toluene
ii.
50 mol% methylcyclohexane + 50 mol% toluene
iii.
75 mol% methylcyclohexane + 25 mol% toluene
iv.
100 mol% methylcyclohexane + 0 mol% toluene
5. The refractive index reading for each mixture is obtained using the refractometer. 6. Graph of refractive index against percentage concentration of methylcyclohexane is plotted to determine the unknown concentration.
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RESULTS
Experiment A: determining column pressure drop
Power (kW)
Boil-up rate
Pressure drop
(mL/s)
(cm H2O) (Top Bottom Overall)
Degree of forming on tray
Refractive Index (R.I)
0.50
0.692
64
Gentle
1.43950
0.75
2.8125
67
Flooding
1.43958
1.00
2.922
189
Flooding
1.44374
1.25
3.913
147
Flooding
1.45317
1.50
6.429
143
Stable
1.46168
pressure drop (cmH2O)
Pressure drop vs boil-up rate 200 180 160 140 120 100 80 60 40 20 0
y = 13.487x + 76.769 R² = 0.264
0
1
2
3
4
5
6
7
boil-up rate (mL/s)
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Experiment B: determining mixture compositions
Concentration
Refractive Index (R.I)
100% MCH
1.42312
100% toluene
1.49676
25% MCH
1.47276
50% MCH
1.45245
75% MCH
1.42743
SAMPLE CALCULATION
For 25 mol percent methylcyclohexane and 75 mol percent toluene Molecular weight methylcyclohexane = 98.19 Molecular weight toluene = 92.15
Density of methylcyclohexane = 0.774 g/ml Density toluene = 0.867 g/ml
25 = Thus, 25 =
3=
x
x
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=3x
=3x
x
x
= Thus for 100 ml of mixture, quantities required will be:
Methylcyclohexane =
x 100
= 71.4286 ml
Toluene =
x 100
= 28.5714 ml
DISCUSSIONS
From the graph plotted, as the power increases, the from 0.50 kW to 1.50 kW, the boil-up rate increases from 0.692 ml/s to 6.429 ml/s. for pressure drop, at the initial of the experiment, the pressure is increases at boil-up rate of 0.692 until 2.922 ml/s, but then the pressure drop from 189 cm H2O to 147 cmH2O and 143 cmH2O. The degree of forming on trays was gentle at first but then it was flooding at the middle of the experiments and then become stable again at the end of the experiment. At the initial of the experiments, the pressure drop is not really occur, because the heat generated from the power that is used to boil up the solutions is not so high. Therefore, the pressure was needed to boil-up the solutions so that the required distillate can be collected. But, when the power was set up at higher value, the boil-up rate getting higher and the pressure start to drop as the boil- up rate reached its needed rate. At this point, the pressure start to drop drastically as the distillate was collected.
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At first, the degree of forming on tray was gentle, this is because, the solutions still not reached its boiling point and the boil-up rate was quite low during this time, but then, in the middle of the experiment, the solutions on the tray start to flooding, this is because, the power is continuous increases and the boil-up rate was getting higher and at the end of the experiment, the degree of forming on tray getting stable again. Every time the distillate collected, its refractive index (RI) was tested using refrectometers to test the mixture compositions. The value for the mixture can be compared with experiment B. In experiment B, the RI of pure MCH, pure toluene, 25 % MCH, 50 % MCH and 75 % MCH was tested. From the result, it can be clearly seen that, the higher the percentage of MCH, the lower the refractive index. This is because; the density of toluene is higher than MCH. From experiment B, we know that the higher the boil-up rate, the higher the percentage of toluene being distillate.
CONCLUSION From the experiment, the objective was to determine the pressure drop over the distillation column for various boil-up rates in batch distillation. From the data collected, graph of pressure drop versus boil-up rate was plotted and the RI also calculated to determine the mixture compositions. In conclusion, the experiment was a success because the pressure drop was successfully determined with various boil-up rates. Therefore, the objective was successfully determined.
RECOMMENDATIONS These are some recommendations that should be done while carrying out this experiment: 1) Avoid direct contact with the distillation column because it is hot. 2) Always re-cap the vials quickly after collecting samples to avoid excessive evaporation of ethanol before analysis. 3) While measuring the volume of methylcyclohexane, use a glove to hold the measuring cylinder. 4) Before using the refractometer, make sure to clean the surface first using distilled water. 5) Collect the unused samples of mixture in a conical flask so that it can be reused. 6) For next experiment, the procedures should be repeated another two times, so that we can compare the results and the values that we get are accurate.
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REFERENCES
There are a few sources used as references in the making of this report: 1) Geankoplis, C.J. (2003). Transport Processes and Separation Process Principles (includes Unit Operations): 4th Edition. Pearson Education International. 2) Treybal, R.E. (1980). Mass Transfer Operations: 3rd Edition. McGraw-Hill, New York. 3) McCabe & Smith, J. (1976). Unit Operations of Chemical Engineering. McGrawHill, New York. 4) O’Shea, S.M. (2007). Distillation Analysis. Retrieved December 2, 2012, from http://www.wpi.edu/Pubs/E-project/Available/E-project-081607132906/unrestricted/MQPDDB2371Final.pdf 5) Tham, M.T. (2009). Distillation. Retrieved December 2, 2012, from http://lorien.ncl.ac.uk/ming/distil/distil0.htm
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APPENDIX
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