1.0 ABSTRACT
This experiment is conducted to study the effect of feed temperature on the number of theoretical plates in a continuous distillation operating at constant reflux 2:2. The SOLTEQ Bubble Cap Distillation Unit (Model: BP 681-80) was used to commence the experiment. Generally, this unit was used for the separation of mixtures at atmospheric pressure in a continuous operation. The desired feed temperature for this experiment is 25 ⁰C and 27 ⁰C and the reflux ratio used was at 2:2. When the unit achieved the desired temperature, samples from distillate and bottom product was taken. Refractive index (RI) readings are used to determine the composition of ethanol-water mixture. Calibration curve of RI obtained is directly proportional to the composition for the whole range of mixture. For distillation at reflux ratio of 2:2, the refractive index obtained for distillate and bottom are 1.3380 and 1.3298. The molar composition for distillate (X D) and bottom (X B) obtained are approximately 17.97 and 4.76. From the Figure 2 in the result, it is shown that the theoretical plate obtained is approximately 1. For distillation at reflux ratio of 2:2, the refractive index obtained for distillate and bottom are 1.3356 and 1.304. The molar composition for distillate (X D) and bottom (X B) obtained are approximately 14.58 and 0. From the Figure 3 in the result, it is shown that the theoretical plate is approximately 1.5. As for conclusion, The feed temperature at 25 ⁰C and 27 ⁰C does affect the number of theoretical plates in a continuous distillation operating at constant reflux 2:2 where the theoretical plate obtained is approximately 1. The recommendations for improvement of this study are, firstly, use lower temperature as a variations for data collection as the unit took quite some time to calibrate and achieved desired feed temperature. Secondly, make sure the beakers are properly labeled when taking samples from the unit to avoid error in data analysis. Thirdly, take more than one reading during data collection in order to get the optimum result. 2.0 INTRODUCTION
Distillation is a liquid-liquid separation process and can be carried out in a continuous or batch system. Removal of heat is used to exploit differences in relative volatility in distillation. The components with lower boiling points and higher volatility will vaporize and leaving less volatile components as liquids (“Distillation Columns,” n.d). High relative volatilities mixtures are easier
to separate. Separations of close-boiling and azeotropic feeds will be difficult so special distillation techniques is needed to separate these mixtures. Binary and multi-component mixtures can be separate using distillation. Variables such as column pressure, temperature, size, and diameter can be determine by the properties of the feed and the desired products. Continuous distillation is where the liquid mixture of two or more miscible components is continuously fed into the process and physically separated into two or more products by preferentially boiling the more volatile, which has lower boiling point components out of the mixture. In a large scale, continuous distillation is used in the chemical process industries where Page | 1
large quantities of liquids have to be distilled. For example, as in petroleum refining, natural gas processing, petrochemical production, hydrocarbon solvents production, coal tar processing, and the liquefaction of gases such as hydrogen, oxygen, nitrogen, and helium. Continuous distillation happens when a liquid mixture is heated until it boils. The evolved vapor will have a higher concentration of the more volatile, lower boiling point components than the liquid mixture from which it evolved (“Continuous Distillation,” n.d). The less volatile
components tend to condense in a greater proportion than the more volatile components when a vapor mixture is cooled. Figure 1 shows a schematic illustrate about what happens in a distillation column. column. A liquid mixture is fed into the distillation column. The heated feed is partially vaporized and rises up on entering the column. As it rises, it is cools by contacting the descending cooler liquid and partially condenses. While part of vapor continues to flow upward, the condensed portion is enriched in the less volatile component and flows downward. As the vapor continues to flow upward, it undergoes partial condensation a few times and each time it becomes richer in the more volatile component. Parts of the feed liquid that did not vaporize on entering the column will flows downward and is heated by contacting the upward flowing hot vapor until it is partially vaporized. The resulting vapor flows upward and the residual liquid is enriched in the less volatile component and flows downward. As the liquid continues to flow downward, it undergoes partial vaporization a few times and each time it becomes richer in the less volatile component.
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Figure 1: Schematic diagram of a continuous binary distillation column
3.0 OBJECTIVE
To study the effect effect of feed temperature on the number of theoretical plates in a continuous distillation operating at constant reflux.
4.0 THEORY
Various vapor and liquid contacting methods are uses to provide the required number of theoretical equilibrium stages in distillation columns (“Continuous Distillation,” n.d) . The devices are known as “plates” or “tray”. Each of these plates or trays have different temperature and
pressure. The tower bottom stage has the highest temperature and pressure. The temperature and pressure decreases as the height of the tower increases. The vapor-liquid equilibrium for each feed component in the tower are adjust to the different pressure and temperature conditions at each of the stages so that each component establishes a different concentration in the vapor and liquid phases at each stages and will results in the separation of the components. Figure 2 shows example of trays.
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Figure 2: A distillation column with a bubble-cap trays
Reboiler acts as an additional theoretical equilibrium stage. The number of physical trays needed for a given separation is equal to the number of theoretical equilibrium stages or theoretical plates if each physical tray or plate were 100% efficient. Usually, a distillation column needs more actual plates than the required number of theoretical vapor-liquid equilibrium stages. The Murphree Tray Efficiency, E M is based on a semi-theoretical models that assumes the vapor between trays is well-mixed, has uniform composition and the liquid in the downcomers is well-mixed, has uniform composition and the liquid on the tray is well mixed and has the same composition as the liquid in the downcomer leaving the tray. It is defined for each
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tray according to the separation achieved on each tray. It is based on either the liquid phase or the vapor phase. A given component is equal to the change in actual concentration in the phase, divided by the change predicted by equilibrium condition. The Murphree Tray Efficiency can be calculated by using the equation below:
− ∗ − − EML = ∗ −
Vapor phase: EMV = Liquid phase:
+1 +1
+1 +1
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5.0 APPARATUS
Figure 1: Process flow diagram for the bubble cap distillation unit.
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Column trays
Distillate sampling
Feed Vessel for continuous Outlet vessel for
s stem.
distillate product
Reboiler
Bottom sampling
Outlet vessel for bottom product
Figure 2: Bubble cap distillation column unit.
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Figure 3: Field unit controller. Figure 1 shows the process flow diagram of bubble cap distillation unit (BP 681-80). The flow diagram has been state clearly about the process of distillation unit. There are reboiler (B1) which contain 20 kW electrical cartridge heaters. The unit also contain bubble cap column (K1) which is 10 plates, silver coated and vacuum jacketed. The unit also contain top condenser (W2) with cooling water connected to the unit. The feed vessel (B2) was installed to the unit for the continuous process.
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Figure 3 show the field unit controller of bubble cap distillation unit. The TT-101 until TT110 indicate the tray temperatures. The TIC-111 are f or reboiler temperature controller. The TT112 indicate the distillate temperature and TIC-113 are indicate the feed temperature control for this system. 6.0 PROCEDURE
Start-up Procedure Pump and Pressure. 1. All valves were closed. 2. The power on control panel was turned on. 3. 20L of mixture containing ethanol and water were prepared. 4. The reflux divider (KFS-101) was set to total reflux which is no. 2. 5. Let the cooling water flows into the condenser. 6. The cartridge heaters was switched on and allowing the ethanol solution to boil. 7. Let the distillation column to stabilize. Continuous Process Procedure 1. The system was allowed to be stabilized at temperature approximat ely 0.2 ⁰C. 2. The pump P1 was turned on and valve V2 was opened. 3. The reflux ratio was set to 1:1 (No. 3) and constant throughout the process. 4. The heater W5 was turned on to preheated the feed of ethanol-water to desired temperature which is 25 ⁰C. Page | 9
5. Valve V7 was opened partially to let the flow out of the system. 6. The system was let until the inlet temperature are constant at desired temperature. 7. Sampling Procedure was conducted. 8. The Process was repeated with different inlet temperature which is 27 ⁰C. Sampling Procedure 1. Distillate sampling: 1. The remaining liquid in the sampling valve V11 was drained. 2. A vial was placed below valve V11. 3. The reflux divider (KSF-101) was set to distillate off take (No. 1) for a few seconds. 4. Valve V11 was opened and 5-10 ml of samples was collected and the valve was closed. 5. The reflux divider was switched back to original setting (No. 3). 2. Bottom sampling: 1. Stagnated liquid remain in the valve V8 was drained. 2. A vial was placed below valve V8. 3. Valve V8 was fully opened and V7 was partially opened, 5-10 ml samples were collected and both valves are closed. 4. The samples was cooling before analyzed.
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Composition Analysis of Ethanol in Concentrated samples procedure. 1. 10 ml of measuring cylinder was placed on a weighing scale and the scale was set to zero. 2. By using a dropper, 1 to 2 g of samples were transferred into the measuring cylinder and the sample weight was recorded. 3. Water was added into the measuring cylinder to dilute the sample and obtaining a total solution weight in the range 5 to 10 g. The solution weight was recorded. 4. The diluted solution was mixed well before measuring its refractive index (RI). 5. The diluted solution’s composition was obtained from the ca libration curve. 6. The actual sample’s composition was calculated.
Preparation of Calibrating Curve Procedure. 1. A set of mixtures containing ethanol and water was prepared within a specified range of composition between pure water and pure ethanol. 2. The refractive index reading was obtained for each mixture using a refractometer. 3. The calibrating curve of RI versus composition for the whole range of mixtures was plotted. 4. The calibrating curve was used to determine the diluted samples solution’s c omposition of the
distillation process. 7.0 RESULT
A: Calibration Calibration curve of of refractive index (RI) versus versus wt% ethanol ethanol Purity of ethanol: 96% Page | 11
Density of ethanol: 789 kg/m 3 Density of water: 1000 kg/m 3 Amount of of ethanol (mL) 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0
Amount of of water (mL) 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0
Ethanol composition wt% mol% 0 0 8.0601 3.3153 16.4444 7.1627 25.2615 12.1545 34.4692 17.0635 44.1029 23.5832 54.2020 31.6435 64.8011 41.8637 75.9384 55.2463 87.6558 73.5276 100 100
Refractive index (RI) 1.3303 1.3363 1.3406 1.3455 1.3505 1.3469 1.3572 1.3588 1.3597 1.3600 1.3593
Refractive Index (RI) versus Ethanol composition (wt%) 1.365 1.36 1.355 ) I R ( 1.35 x e d n I e 1.345 v i t c a r 1.34 f e R
1.335 1.33 1.325 0
2
4
6
8
10
12
Ethanol composition (wt%)
Figure 1: Graph for RI versus Ethanol composition (wt%)
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B: Number of theoretical plates at constant reflux using x-y equilibrium diagram Reflux ratio: 2:2 @ temperature : 25 ⁰C Diluted solution’s Temperature Sample Solution Refractive Tray (°C) weight (g) weight index composition (g) (wt%) Distillate
37.3
1
54.6
2
42.2
3
31.3
4
30.6
5
31.1
6
49.0
7
50.5
8
49.7
9
50.8
10
51.2
Bottom
89.0
Actual sample’s
composition (wt%)
1.207
8.237
1.3387
16.67
17.96
1.390
8.200
1.3316
16.71
4.76
x-y equilibrium diagram 100 90 ) y ( r o p a v n i l o n a h t e % e l o m
80 70 60 50 40 30 20 10 0 0
10
xB
20
xD
30
40
50
60
70
80
90
100
mole % ethanol in liquid (x)
Figure 2: Graph of ethanol-water at reflux ratio 2:2 Page | 13
Reflux ratio: 2:2 @ temperature : 27 ⁰C Diluted
Actual
solution’s
sample’s
composition (wt%)
composition (wt%)
Tray
Temperature (°C)
Sample weight (g)
Solution weight (g)
Refractive index
Distillate
76.4
1.255
8.388
1.3385
16.81
14.58
1
85.6
2
85.3
3
85.8
4
85.6
5
86.0
6
79.1
7
78.0
8
78.9
9
78.0
10
78.3
Bottom
86.0
1.230
5.991
1.3332
15.86
0
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C. Murphree efficiency for each tray in the column x-y equilibrium diagram 100 90 80 ) y ( r o p a v n i l o n a h t e % e l o m
70 60 50 40 30 20 10 0 0
10
xB
20
30
40
50
60
70
80
90
100
mole % ethanol in liquid (x)
xD
Figure 3: Graph of ethanol-water at reflux ratio 2:2 1. Reflux ratio 2:2 x-y equilibrium diagram 100 ) y ( r o p a v n i l o n a h t e % e l o m
yn+1 yn
90 80 70 60 50 40 30 20 10 0 0
10
xB
20
xD
30
40
50
60
70
80
90
100
mole % ethanol in liquid (x)
Figure 4: Graph of ethanol-water at reflux ratio 2:2 Page | 15
Em =
− ( ∗ − (
+1 ) +1 )
It is assumed that y n=y*n as the data is insufficient to locate the y* n. Em =
− (5) 18 − (5) 18
Em = 1
D. Temperature and composition profile versus tray number Reflux ratio 2:2 x-y equilibrium diagram 100 90 ) y ( r o p a v n i l o n a h t e % e l o m
80 70 60 50 40 30 20
yn+1
10
yn
0 0
xB
10
20
xD
30
40
50
60
70
80
90
100
mole % ethanol in liquid (x)
Figure 5: Graph of ethanol-water at reflux ratio 2:2 Em =
− ( ∗ − (
+1 ) +1 )
It is assumed that y n=y*n as the data is insufficient to locate the y* n. Em =
− (1) 17 − (1) 17
Em = 1
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8.0 SAMPLE CALCULATIONS
1. Weight percentage of ethanol:
Weight of ethanol g
= amount amount of ethanol ethanol mL × densit density y
= 1.0 1.0 mL × 789 789
kg m3
×
1000 1
×
kg m3
3 1000 1
= 0.0007 0.000789 89 kg kg Weight of water g = amount amount of water water mL × densit density y( ) mL
= 9.0 9.0 mL × 1000 1000
kg m3
×
1000 1
×
3 1000 1
= 0.00 0.009 9 kg kg
Weight percent of ethanol
=
=
weight of ethanol weight weight of ethanol ethanol + weight weight of water water
× 100% 100%
0.000789 0.000789 kg
100% 0.000789+0.009 kg × 100%
= 8.06 8.0601 01% %
2. Mole percent of ethanol:
Mole of ethanol mol =
=
weight of ethanol (kg) kg molecular weight ( mol)
0.000789 0.000789 kg g 46.07 ( mol )
×
1000 1
= 0.0171 0.01713 3 mol mol
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weight of water (kg) kg molecular weight ( mol)
Mole of water mol =
=
0.009 kg g 18.02 ( mol )
×
1000 1
= 0.49 0.499 9 mol mol Mole percent percent of ethan ethanol ol = =
mole mole of etha ethano noll mole mole of ethan ethanol ol +mol +mole e of wate waterr
×100%
0.01713 mol
0.01713+0.499 mol ×100%
= 3.31 3.3153 53%
3. XD calculations
− 16.4444 = 1.3380 − 1.3370 25.2615 − 16.4444 1.3428 − 1.3370 = 17.9 17.96 6 4. XB calculations calculations
− 16.4444 = 1.3380 − 1.3370 25.2615 − 16.4444 1.3428 − 1.3370 = 17.9 17.96 6 9.0 DISCUSSION Continuous distillation is an ongoing separation process in which a liquid mixture of two or more miscible components is continuously fed into the process and physically separated into two or more products by preferentially boiling the more volatile components out of the mixture. In this particular experiment, experiment, this process is operating at constant feed temperature at 25 ⁰C and 27 ⁰C as we need to study the effect of constant reflux ratio on the number of theoretical plates
in the distillation column.
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Based on the experiment A, refractive index (RI) readings are used to determine the composition of ethanol-water mixture. Figure 1 shows that the calibration curve of RI is directly proportional to the composition for the whole range of mixture. For distillation at reflux ratio of 2:2, the refractive index obtained for distillate and bottom are 1.3380 and 1.3298. The molar composition for distillate (X D) and bottom (X B) obtained are approximately 17.97 and 4.76. From the Figure 2, it is shown that the theoretical plate obtained is approximately 1. For distillation at reflux ratio of 2:2, the refractive index obtained for distillate and bottom are 1.3356 and 1.304. The molar composition for distillate (X D) and bottom (X B) obtained are approximately 14.58 and 0. From the Figure 3, it is shown that the theoretical plate is approximately 1.5. The Murphree efficiency for both reflux ratio is the same unfortunately. This is due to insufficient amount of data to allocate the value of y* n. Besides, the temperature and composition profile versus tray number also cannot be plotted as we just achieved only one plate.
10.0 Conclusion
The feed temperature at
25 ⁰C and 27 ⁰C does
affect the number of theoretical plates in a
continuous distillation operating at constant reflux 2:2 where the theoretical plate obtained is approximately 1. The objective of this experiment is achieved. 11.0 Recommendation
The recommendations of this study are: 1) Use lower temperature as a variations for data collection as the unit took quite some time to calibrate and achieved desired feed temperature. 2) Make sure the beakers are properly labeled when taking samples from the unit to avoid error in data analysis. 3) Take more than one reading during data collection in order to get the optimum result.
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12.0 REFERENCES
1. Chang, H., Lin, T., Chan, H., Ho, C., & Cheng, T. (2017). Experimental and optimization studies of diabetic membrane-based distillation columns. Journal Of The Taiwan Institute Of Chemical Engineers , 73, 75-85. http://dx.doi.org/10.1016/j.jtice.2016.03.026
2. Chen, H., Zhang, L., Huang, K., Yuan, Y., Zong, X., Wang, S., & Liu, L. (2016). Reactive distillation columns with two reactive sections: Feed splitting plus external recycle. Chemical Engineering And Processing: Process Intensification , 108 , 189-196. http://dx.doi.org/10.1016/j.cep.2016.08.008 3. Delgado, J., Águeda, V., Uguina, M., Sotelo, J., García-Sanz, A., &García, A. (2015). –water liquid mixtures by adsorption on BPL activated carbon with air Separation of ethanol –
regeneration. Separation And Purification Technology , 149, 370-380. http://dx.doi.org/10.1016/j.seppur.2015.06.011 4. Dhole, V., &Linnhoff, B. (1993). Distillation column targets. Computers & Chemical Engineering , 17 (5-6), (5-6), 549-560. http://dx.doi.org/10.1016/0098-1354(93)80043-m
5. García-Ventura, U., Barroso-Muñoz, F., Hernández, S., & Castro-Montoya, A. (2016). Experimental study of the production of high purity ethanol using a semi-continuous extractive batch dividing wall distillation column. Chemical Engineering And Processing: Process Intensification , 108 , 74-77. http://dx.doi.org/10.1016/j.cep.2016.07.014
6. Springer, P., Baur, R., & Krishna, R. (2003). Composition Trajectories for Heterogeneous Azeotropic Azeotropic Distillation in a Bubble-Cap Bubble-Cap Tray Column. Column. Chemical Engineering Research And Design, 81(4), 413-426. http://dx.doi.org/10.1205/026387603765173682
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7. Zhao, L., Lyu, X., Wang, W., Shan, J., &Qiu, T. (2017). Comparison of heterogeneous azeotropic distillation and extractive distillation methods for ternary azeotrope ethanol/toluene/water separation. Computers & Chemical Engineering , 100 , 27-37. http://dx.doi.org/10.1016/j.compchemeng.2017.02.007 8. Seader, J.D & Henley, E.J. (1998). Distillation of Binary Mixtures. John Wiley & Sons Inc., Separation Process Principles. United States of America. Page 355.
9. Introduction to Distillation . (2001). Retrieved June 4, 2013 from http://faculty.kfupm.edu.sa/CHE/binoushousam/files/Introduction%20to%20distillation.pd f 10. The Separation of a Binary Water/Ethanol Solution via a Continuous Feed Distillation Column as a Function of Feed Stage Location and Reflux Ratio. (2015). International Journal Of Science And Research (IJSR) , 4(12), 807-812.
http://dx.doi.org/10.21275/v4i12.nov152084 11. Geankopolis C. J. (2014). Transport Process & Separation Process Principles (includes Unit Operations). Fourth Edition. Pearson New International Edition. England. 12. Distillation Columns. (n.d). Encyclopedia of Chemical Engineering Equipment. Citizendium, n.d. Continuous Distillation. http://en.citizendium.org/wiki/Continuous_distillation (accessed 7.04.2017).
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13.0 APPENDICES
Murphree efficiency equation Em =
Where,
− ∗ − +1
+1
is the average actual concentration of the mixed vapor leaving the tray n is the average actual concentration of the mixed vapor entering tray n ∗ the concentration of the vapor that would be in equilibrium with the +1
liquid of
concentration leaving the tray to the downcomer
Figure 1: Use of Murphree efficiency to determine actual number of trays.
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