Transcript
(Data Reporter) (Leader)
(Inspector) Expt. #1: Rectification
Section 021, Group 1
October 18, 2006.
Ryerson University
Department of Chemical Engineering
CHE 415 Unit Operations II
Lab Report
Experiment #1: Rectification
_______________________________________________
Experiment Performed on: October 12, 2006
Report Submitted to
Dr.
By Group # 1 Section # 021 1. (Leader)
2. (Inspector)
3. (Data Reporter)
Date Report Submitted: October 18, 2006.
Marking Scheme
Formatting Answer to all 6 questions in each Report Section / 10
General Appearance; Grammar and Spelling / 5
Complete and Informative Tables and Graphs / 15
Contents Accuracy and Precision of Results / 20
Comparison with Literature Data / 10
Influence of Procedural Design on Results / 10
Logic of Arguments / 20
Sample Calculations / 10
_____
Total: / 100
Table of Contents
Introduction 3
THEORETICAL BACKGROUND 4
Technical procedure 6
Results and discussion 8
Error Analysys 11
Conclusion and recommendations 12
REFERENCES 13
APPENDIX
RAW DATA 14
Sample Calculations 15
List of Figures
Figure 1: Vaopr-Liquid Equilibrium Curve 4
Figure 2: Distillation Column Apparatus 7
Figure 3: Graph of Vapour Liquid Equilibrium for Methanol-Water system 9
Figure 4 :Vapour and Liquid Equilibrium Mole Fractions for Methanol 9
Figure 5: Pycnometer Values for Initial Solution, Final Distillate and Final Pot Liquid 14
Figure 6: Temperature Recordings to Reach Equilibrium 14
INTRODUCTION
This experiment was performed in order to assess certain aspects of a separation column, which contained six stages distillation stages in total and had a boiling pot volume of approximately 75 liters. The aspects under investigation were the boilup rate, the maximum overall separation efficiency and the thermal efficiency.
A methanol-water solution was used as the starting liquid. A sample was taken initially to determine the density of the liquid. The mixture was then brought to boil. At this time, the liquid was allowed to vaporize and condense within the six stages which it was built to have. After reaching an equilibrium point, which was done by monitoring the temperatures until they remained constant, distillate was collected. This distillate was measured in a pycnometer in order to determine its density.
The density of the distillate was found to be 0.7910 g/ml, and was used in order to calculate for boilup ratio. The boilup ratio was found to be 2.69 g/s. The maximum overall separation efficiency was found to be 250% and the thermal efficiency of the heat source was found to be 29.67%.
Theoretical Background
Distillation is a separation process in which a mixture of two or more substances is separated into their individual components by the application and removal of heat. This process is dependent upon the distribution of the substances between a gas and a liquid phase. Distillation is the most common separation technique and is based on the differences in boiling points of the components of a solution. For a successful distillation, it is crucial to have knowledge of the equilibrium relationship that exists between the vapor and liquid phases of a mixture. Vapor-liquid equilibrium data of a binary mixture can be represented as a plot of vapour mole fraction (y) versus liquid mole fraction (x),
Figure 1: Vaopr-Liquid Equilibrium Curve
The equilibrium line describes the composition of the vapor and liquid in equilibrium at a constant pressure.
Reflux is the vapors from the top of the distillation column that are condensed in the condenser by cool water to provide contacting liquid. The reflux ratio is defined as
R = L/D (1)
where L is the liquid entering the top stage and D is the distillate rate. If the reflux ratio is increased, the slope of the operating line for the rectification section (the section above the feed entry in the column) approaches a value of one. When this occurs, better separation is achieved with less number of trays. Under total reflux conditions (R = ?), minimum number of trays are required.
The liquid at the bottom of a distillation column passes through a reboiler and is heated by condensing steam to provide contacting vapor called boil up. The vapor rate leaving the reboiler is called the boil up rate and can be calculated using the following equation,
B = (V * rdistillate) / t (2)
where V is the volume of distillate collected at time t and rdistillate is the density of the
distillate.
The maximum overall separation efficiency of the distillation column can be
calculated by the following equation,
Efficiency = (number theoretical plates / actual number plates) * 100 (3)
The number of theoretical stages can be determined by plotting an operating line along with the equilibrium curve and the 450 line.
The thermal efficiency of the distillation column can be calculated by taking the sum of the heat added to the reboiler and the heat used to boil the distillate
Efficiency = (Q distillate + Q Reboiler) * 100 (4)
where Q distillate is the heat supplied to distillate and Q Reboiler is the heat supplied to the reboiler. The heat supplied to the distillate can be calculated by,
Q distillate = (Volume * ? distillate / time) * (xA * ?A + xB * ?B) (5)
where xA and xB are the liquid mole fractions within the distillate and, ?A and ?B are the heat of vaporization.
TECHNICAL PROCEDURE
The apparatus used for this experiment was a distillation column, before beginning the experiment it was checked to make sure all valves on the distillation column were closed. For this scenario there was already a prepared solution within the boiling pot. A sample of the pot liquid was taken to measure the density with a pyncometer. The density will be used to determine the composition of the pot liquid. The valves for the cold water were opened to allow cold water to enter the condenser. The reboiler was then turned on and the heat was turned to 70%. Temperature readings were taken every five minutes at the four points within the column. The column was then allowed to reach equilibrium while adjusting the heat to ensure that weeping or entrapment does not occur. If weeping occurred the heat was turned up, if entrapment occurred the heat was turned down. Once equilibrium was reached a volume of the distillate was collected for 30 seconds and its density was measured. The heat to the reboiler was then shut off to allow the system to cool, the condenser was left running to ensure no methanol vapours escaped in to the lab. After sufficient time had passed and the system was no longer bubbling the condenser was shut off.
EXPERIMENTAL APPARATUS
Figure 2: Distillation Column Apparatus
Results and discussion
An experiment was performed to assess the performance of a batch distillation column. A mixture of methanol and water was distilled to determine the boil up ratio, separation efficiency and the thermal efficiency. The initial charge into the boiling pot was already provided for this experiment. A sample was taken with a pycnometer of the initial so as to determine the composition of said pot liquid.
The first objective studied in this experiment was to determine the boil up rate. The boil up rate is a measure of the rate of methanol vapour that is travelling up the column. The boil up rate was found to be 2.69 g/s. This means that 2.69 grams of methanol was evaporated and moved up the column every second. This property can be limited by how much heat can be added to the boiler, because if entrainment occurred then the heat has to be turned down.
The second objective of this lab was to determine the separation efficiency of the rectifier. It was determined by finding the number of actual stages by visually counting them. Six stages were observed and this was compared to the theoretical value of 15 stages. This was calculated by drawing the following on a graph of vapour VS liquid mole fractions; a 45 degree line, an operating line starting from a point on the y axis equal to xdistallate / (R + 1), found to be 0.495, and finishing at the value on the 45 degree line that corresponds to the mole fraction of methanol in the distillate, xdistiallate. This was found to be 0.99 at the condenser temperature of 64.56 oC,. This can is shown in the following graph:
Figure 3 : Graph of Vapour Liquid Equilibrium for Methanol-Water system
And the graph depicting the theoretical stages:
Figure 4 :Vapour and Liquid Equilibrium Mole Fractions for Methanol
This yields 250% efficiency, this value is not feasible value for use in an industrial setting. But consider that the purity of the distillate is so high, 99% methanol by composition, that although the calculated value is absurd, the actual separation ability is rather remarkable near 100%. The reason for such a high separation efficiency is due to the fact the distillate was near 100% and the calculated theoretical number of stages will require a higher number of stages due to the fact that the operating line is so close to the vapor liquid equilibrium curve.
The last characteristic to be examined was the thermal efficiency of the column. This is the ratio of energy consumed in the process, and energy supplied by the reboiler. The heat consumed by the process was calculated by using enthalpies of vaporization of the distillate and the mole fraction, in this case just methanol. The energy used in the system was found to be 3 647.18 J/s and the energy provided by the reboiler was 8 000 J/s. The thermal efficiency was calculated to be 53.2%. This suggests that the remaining 46.8% was energy that was lost to the surroundings. Based on these results we can say that this rectification process was not efficient enough for a real industrial setting unless some measures were taken to preserve the heat input into the system.
ERROR ANALYSIS
Overall, this experiment preformed quite well, but there were a few errors involved. Firstly, the heat source did not perform up to expectations. It took long for it to heat up and once it did, it was difficult to control the boiling. The efficiency was only 29.67%, which showed that it worked poorly.
The maximum overall efficiency was found to be 250%, which was theoretically impossible. The number of actual stages did not match the number of theoretical stages, which produced the overage of efficiency.
Also, methanol odors were observed by one of the participants, which not only was a hazard to all in the laboratory, but also suggested that the concentration in the distillate was not accurate. This further suggested that the column may not have been working as accurately as observed.
CONCLUSION AND RECOMMENDATIONS
This experiment was preformed in order to determine the three major aspects of a distillation column. These aspects included the boilup rate, the maximum overall separation efficiency and the thermal efficiency.
Overall, this experiment preformed well. There was little to no entrainment or weeping within the column, which allowed for proper distillation of methanol from the solution. The participants were very effective in controlling the entrainment within the distillation stages. The heat source did not perform effectively, since it was found that the thermal efficiency was 29.67%. But, the column itself worked very well.
The recommendations for this experiment would have to be to change the heat source under the boiling pot, since it is not very accurate in its performance. Also, to allow for different reflux ratios by fixing the control panel so that it is possible to work on settings other than either no reflux or total reflux.
REFERNECES
Treybal, Robert E., Mass-Transfer Operations, McGraw-Hill Book Company, USA, 1980.
Geankopolis, Christie J., Transport Processes and Unit Operations, Prentice Hall, New Jersey, 1993.
_http://lorien.ncl.ac.uk/ming/distil/distil0.htm_
APPENDIX
Data Tables
Mass of Pycnometer (g) Mass of Pycnometer Plus Solution (g)
Initial Solution 14.847 38.895
Final Distillate 15.342 35.118
Figure 5: Pycnometer Values for Initial Solution, Final Distillate and Final Pot Liquid
Time (min) T.C. 1 (condenser) (F) T.C. 2 (sixth stage of column) (F) T.C. 3 (third stage of column)
(F) T.C. 4 (reboiler) (F)
0 67 67 67 67
5 67.4 68.0 67.2 105.6
10 67.2 67.8 67.2 126.6
15 67.3 67.7 67.2 143.7
20 67.2 67.6 67.2 159.9
25 67.2 67.6 67.2 175.0
30 67.4 67.6 118 186.4
35 151.6 150.0 169.8 186.9
40 150.7 149.5 161.6 187.7
45 149.6 148.7 158.4 188
50 149.2 148.4 156.9 188.2
55 148.6 148.0 156.0 188.2
60 148.4 147.7 155.4 188.2
65 148.2 147.5 155.1 188.0
70 148.2 147.5 155.0 188.0
75 148.2 147.5 155.0 187.9
Figure 6: Temperature Recordings to Reach Equilibrium
Sample Calculations
For Boil up ratio:
Assume a total reflux due to the setup of the system, where all the overhead vapour is being condensed and is then returned to the top stage.
Therefore,
R = L/D = infinity
Thus, D = 0
L/V = 1 or L = V
B = (V * rdistillate) / time
B = (102 ml * 0.7910 g/ml)/30 sec
B = 2.69 g/s
Where,
B is the boil up ratio
L is the moles of liquid in the still
V is the moles of vapour in the still
D is the distillate flow rate from still
R is the reflux ratio
rdistillate is the density of the distillate liquid
For Maximum overall separation efficiency:
From the vapor-liquid equilibrium curve for methanol and water, the mole fraction of methanol in the distillate xdistillate, can be calculated from the temperature in the condenser. The temperature in the condenser was 64.56 0C, so xdistillate = 0.99
From this data we will be able to determine the y intercept of the rectifying line using:
xdistallate / (R + 1) = 0.99 / (1 + 1)
=0.495
The number of theoretical stages can be determined by plotting these 2 data points on an x-y diagram along with the 450 line
# stages = 16 – 1 (from reboiler) = 15 stages
Efficiency = (number theoretical plates / actual number plates) * 100
= (15/6) * 100
= 250%
Thermal efficiency:
This can be calculated by adding the heat added to the reboiler and heat used to boil distillate.
Efficiency = (Q distillate / Q Reboiler) * 100
Where, Q distillate is the heat supplied to distillate and Q Reboiler is the heat supplied to the reboiler.
Q distillate = (Volume * ?distillate / time) * (xA * ?A + xB * ?B)
With, xA and xB equal to the liquid mole fractions within the distillate and, ?A and ?B are the heat of vaporization with A being methanol and B being water.
Q distillate = ((102 mL * 0.7910 g/ml) / 30 sec) * (0.99 * 1.101 KJ/g + 0.01 * 2.257 KJ/g)
= 2.99 KJ/sec
Efficiency = (2.99 KJ/sec / 10.08 KJ/sec) * 100
= 29.67%
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