HACETTEPE UNIVERSITY DEPARTMENT OF CHEMICAL ENGINEERING
KMU 402-23 CHEMICAL ENGINEERING LABORATORY II
3- DISTILLATION ADVISOR: Assoc. Prof. Dr Hülya YAVUZ ERSAN ASSISTANTS: Aslıhan BAYRAKTAR -Dicle -Dicle DEMIR REPORT SUBMISSION DATE: 09.05.2011 09.05.2011 REPORT RUN DATE
: 17.05.2011
GROUP: P4 EBRU DOĞANGÜN
20622627
DUYGU GÖKÇE
20622665
NEŞE KAYNAK
20622737
VİCDAN PINARBAŞI
20622847
1
17/05/2011 Dear Assoc.Prof. Dr. Hülya YAVUZ ERSAN, The experiment which is named as
‘Distillation
was completed on 9th of May, 2011 by group P4
within the scope of ‘Basic Measurements Laboratory’. These group’s members who are Duygu Gökçe, Ebru Doğangün, Neşe Kaynak and Vicdan Pınarbaşı performed the experiment. The goals of the experiment were to have knowledge about distillation processes, to learn types of distillation such as batch and continuous, to understand the working principle of distillation methods in practice for both batch and continuous processes and to observe the effect of power and reflux ratio on efficiency of distillation tower. And also it was aimed to understand review basic concepts of distillation such as reflux, reflux ratio, sections of a distillation column, deriving operating lines for these sections and finding theoretical number of trays by using McCabe-Thiele Method. For these purposes the experiment was performed at three section. In the first part of the experiment, it was aimed that to find optimum power corresponding to maximum efficiency of distillation tower in batch operation. So that, the amounts of ethanol were determined ay bottom and overhead stream by using refractometer. The aim of the second part of the experiment was to observe the effect of reflux ratio on the efficiency of tower in continuous operation. Reflux ratios were adjusted as 2, 10 and 15 respectively. Finally the third part of the experiment was purposed that to observe the effect of time the amount of ethanol at bottom and overhead overhead stream at same reflux ratio and power in continuous operation. operation. In the batch distillation part, firstly the optimum power was determined 1 kW and then by using the optimum power rate at continuous distillation, the efficiency of the column was calculated around 9 %. In continuous distillation part, the column efficiency was calculated as approximately 37.5 % and experimental plates of enriching section was determined as 3 by using Mc-Cabe Thiele Diagram.
The assistants of the experiment who are Aslıhan Bayraktar and Gökçe Dicle Demir contributed to our work and we as a group spent totally twenty hours to write our report. Yours faithfully,
Duygu GÖKÇE
Ebru DOĞANGÜN
Neşe KAYNAK
Vicdan PINARBAŞI
2
Contents 1 THEORY ........................................................................................................................................ 6
1.1
1.1.1
Types of Distillation ........................................................................................................ 7
1.1.2
Main Components of Distillation Columns ..................................................................... 9
1.2
2
3
DISTILLATION ..................................................................................................................... 6
BASIC OPERATION AND TERMINOLOGY ...................................................................10
1.2.1
McCabe-Thiele Design Method..................................................................................... 10
1.2.2
Vapour Flow Conditions ................................................................................................ 15
EXPERIMENTAL PROCEDURE ............................................................................................... 17 2.1
Aim of the Experiment .......................................................................................................... 17
2.2
Description of Apparatus ...................................................................................................... 17
2.3
Experimental Procedure ........................................................................................................ 18
2.3.1
Continuous Operation .................................................................................................... 18
2.3.2
Batch Operation .............................................................................................................18
RESULTS AND DISCUSSION................................................................................................... 20 3.1
RESULTS FOR BATCH DISTILLATION.......................................................................... 20
3.1.1
Weeping and Flooding Point .........................................................................................20
3.1.2
Calculation of the Number of Theoretical Plates........................................................... 21
3.1.3
Determination of the Variation of Top and Bottom Composition with Time ............... 23
3.2
Continuous Distillation ......................................................................................................... 25
4
CONCLUSION ............................................................................................................................ 27
5
NOMENCLATURE ..................................................................................................................... 28
6
REFERENCES ............................................................................................................................. 29
3
LIST of TABLES
Table3.1 Pressure As A Function Of Boil-Up Point........................................................................... 20 Table3.2 Minimum Number of Plates and Column Efficiency........................................................... 21 Table3.3 Vapor Densities, Liquid Densities, Linear Gas Velocities and F f actors............................. 22 Table3.4 The Change Of The Mole Fractions With Time.................................................................. 23 Table3.5 Calculated values of number of plates and % efficiencies by using Mc-Cabe Thiele Method.................................................................................................................................................26
LIST of FIGURES
Figure1. 1 A Simple Distillation Apparatus.......................................................................................... 7 Figure1. 2 Fractional Distillation Apparatus....................................................................................... ..8 Figure1. 3 Distillation column ................................ ............................................................................9 Figure1. 4 McCabe-Thiele Diagram with an enriching section.......................................................... 11 Figure1. 5 McCabe-Thiele Diagram with an stripping section........................................................... 12 Figure1. 6 McCabe-Thiele Diagram with operating and equilibrium section.................................... 13 Figure1. 7 McCabe-Thiele Diagram with Number Of Stages......................................................... ... 14 Figure1. 8 Feed Lines.......................................................................................................................... 15 Figure2. 1 Experimental Set-Up Of Distillation Process.................................................................... 17 Figure2. 2 Refractometer.....................................................................................................................18 Figure3. 1 Effect Of Boil-Up Rate On Pressure Drop For Batch Distillation..................................... 21 Figure3. 2%Column Efficiency versus F Factor................................................................................. 23 Figure3. 3 Mole Fractions (XB) Versus Time.................................................................................... 24 Figure3. 4 Mole fractions (XD) versus time....................................................................................... 24
4
SUMMARY
The objectives of the experiment are;
to have information about distillation processes such as batch and continuous distillation,
to understand the difference between batch and continuous distillation,
to be informed of Raoults’ law and their usage,
to learn the working principle of distillation column,
to study on operating charecteristics of sieve trays,
to observe foaming at different power values,
to learn how we use McCabe-Thiele Method and to find the number of trays needed by using McCabe-Thiele Method.
to observe the effect of power and reflux ratio on the distillation and system efficiency.
For these purposes, a mixture containing ethanol & water was distilled in a distillation column both by continuous and batch processes. In the batch part, for a batch distillation bottom and overhead mole fractions were obtained for various power values. Then optimum power which is at the highest mole fraction of distillate product was determined. In the second part of continuous part, system was fixed in this optimum power value and reflux ratio. Then, from data effect of reflux ratio is observed. In the continuous part, time dependency of a continuous system and the effect of the reflux ratio on the products were investigated. During the experiment, refractive index of each sample which was taken from the overhead and bottom products was measured. 0
Distillation experiment was performed at 25 C, 690 mmHg. The system includes a distillation column and a computer connected to it. The computer was used to set reflux ratio and to measure temperature values of the system. The same equipment were used
for batch and continuous
operation of experiment. To sum up, maximum mole fraction of ethanol in distilled stream was determined when the heater value was run as 1.00 kW. So this value was accepted as the optimum power for the batch system. Moreover, for continuous system number of tray numbers were found as 3, 2 and 3 for reflux ratios 2, 10 and 15 with using McCabe-Thiele Method. The efficiency of the column was calculated as 27,5% and 37,5% .
5
1
THEORY
1.1
DISTILLATION
Distillation is a physical method of assorting mixtures depending upon the difference in the boiling point of the component substances. The working principal of distillation is to heat a mixture at a specific temperature, collect the hot vapors and condense to separate the component substance. In simpler terms, a highly volatile compound is separated from a less-volatile or non-volatile compound by using distillation. As per evidences, the principle of distillation has been used since ancient [1]
times.
The successful application of distillation techniques depends on several factors. These include the difference in vapor pressure (related to the difference in the boiling points) of the components present, the size of the sample, and the distillation apparatus. Distillation relies on the fact that the vapor above a liquid mixture is richer in the more volatile component in the liquid, the composition being controlled by Raoult’s law:
[2]
In an ideal solution the partial pressure (P A) of component A at a given temperature is equal to the vapor pressure P
o
A
of pure A multiplied by the mole fraction of A (xA) in solution.
Consider an ideal solution of A and B:
(1)
[2]
[2]
(2)
(3)
nA and nB represent the number of moles of components A and B.
(total
(4)
[2]
(5)
vapor pressure)
(6)
This relationship, derived from Raoult’s law, is capable of describing the boiling point behavi or of compound A in a mixture of compounds under a variety of different circumstances. The boiling point of the solution is reached when P T is equal to the pressure applied to the surface of the solution.
[2]
6
1.1.1 Types of Distillation There are several types of distillation depending on the procedure and the instrument setup. Each of the distillation type is used for the purification of compounds having different properties. Following are the common types of distillation:
[3]
Simple Distillation: Simple distillation is practiced for a mixture in which the boiling point
of the components differ by at least 70°C. It is also followed for the mixtures contaminated with involatile particles (solid or oil) and those that are nearly pure with less than 10 percent contamination. Double distillation is the process of repeating distillation on the collected liquid in order to enhance the purity of the separated compounds.
[3]
Figure1. 1 A Simple Distillation Apparatus [4 ]
Fractional Distillation: Those mixtures, in which the volatility of the components is nearly
similar or differs by 25°C (at 1 atmosphere pressure), cannot be separated by simple distillation. In such cases, fractional distillation i s used whereby the constituents are separated by a fractionating column. In the fractionating column, the plates are arranged and the compound with the least boiling point are collected at the top while those with higher boiling point are present at the bottom. A series of compounds are separated simultaneously one after another.
7
[4 ]
Figure1. 2 Fractional Distillation Apparatus
Vacuum Distillation: Vacuum distillation is a special method of separating compounds at
pressure lower than the standard atmospheric pressure. Under this condition, the compounds boil below their normal boiling temperature. Hence, vacuum distillation is best suited for
separation of compounds with higher boiling points (more than 200°C), which tend to decompose at their boiling temperature. Vacuum distillation can be conducted without [3]
heating the mixture, as usually followed in other distillation types.
Batch Distillation: The column is charged with a 'batch' and then the distillation process is
carried out. When the desired task is achieved, a next batch of feed is introduced. A condenser to turns the vapors back to a liquid so they can be easily collected. Batch distillation comprises of tall column that is desirable to improve the separation. Finally, by using different collection vessels, the original mixture can be separated into fractions. Batch distillation is labor intensive
[5]
Continuous Distillation :Continuous columns process a continuous feed stream. No interruptions
occur unless there is a problem with the column or surrounding process units. They are capable of handling high throughputs. A continuous column operates under steady conditions where the composition at a given location does not change over time. The composition only depends upon the position in the column and so additional product steams can be easily tapped at different heights and 8
each tray will have a different composition of compounds.
[5]
1.1.2 Main Components of Distillation Columns Distillation columns are made up of several components, each of which is used either to tranfer heat energy or enhance materail transfer. A typical distillation contains several major components:
[6]
a vertical shell where the separation of liquid components is carried out column internals such as trays/plates and/or packings which are used to enhance component separations a reboiler to provide the necessary vaporisation for the distillation process a condenser to cool and condense the vapour leaving the top of the column a reflux drum to hold the condensed vapour from the top of the column so that liquid (reflux) can be recycled back to the column The vertical shell houses the column internals and together with the condenser and reboiler, constitute a distillation column. A schematic of a typical distillation unit with a single feed and two product streams is shown below:
[6]
Figure1. 3 Distillation column
[6 ]
9
1.2 BASIC OPERATION AND TERMINOLOGY The liquid mixture that is to be processed is known as the feed and this is introduced usually somewhere near the middle of the column to a tray known as the feed tray. The feed tra y divides the column into a top (enriching or rectification) section and a bottom (stripping) section. The feed flows down the column where it is collected at the bottom in th e reboiler. Heat is supplied to the reboiler to generate vapour. The source of heat input can be any suitable fluid, although in most chemical plants this is normally steam. In refineries, the heating source may be the output streams of other columns. The vapour raised in the reboiler is reintroduced into the unit at the bottom of the column. The liquid removed from the reboiler is known as the bottoms product or simply, bottoms. The vapour moves up the column, and as it exits the top of the unit, it is cooled by a condenser. The condensed liquid is stored in a holding vessel known as thereflux drum. Some of this liquid is recycled back to the top of the column and this is called the reflux. The condensed liquid that is removed from the system is known as the distillate or top product. Thus, there are internal flows of vapour and liquid within the column as well as external flows of feeds and product streams, into and out of the column.
1.2.1 McCabe-Thiele Design Method The McCabe-Thiele approach is a graphical one, and uses the VLE plot to determine the theoretical number of stages required to effect the separation of a binary mixture. It assumes constant molar overflow and this implies that: molal heats of vaporisation of the components are roughly the same heat effects (heats of solution, heat losses to and from column, etc.) are negligible for every mole of vapour condensed, 1 mole of liquid is vaporised
10
1.2.1.1
Operating Line for the Rectification Section
The operating line for the rectification section is constructed as follows. First the desired top product composition is located on the VLE diagram, and a vertical line produced until it intersects the diagonal line that splits the VLE plot in half. A line with slope R/(R+1) is then drawn from this intersection point as shown in the diagram below.
Figure1. 4 McCabe-Thiele Diagram with an enriching section
[6 ]
R is the ratio of reflux flow (L) to distillate flow (D) and is called the reflux ratio and is a measure of how much of the material going up the top of the column is returned back to the column as reflux.
[6]
11
1.2.1.2
Operating Line for Stripping system
The operating line for the stripping section is constructed in a similar manner. However, the starting point is the desired bottom product composition. A vertical line is drawn from this point to the diagonal line, and a line of slope L s /Vs is drawn as illustrated in the diagram below.
[6]
Figure1. 5 McCabe-Thiele Diagram with an stripping section
[4 ]
Ls is the liquid rate down the stripping section of the column, while V s is the vapour rate up the stripping section of the column. Thus the slope of the operating line for the stripping section is a ratio between the liquid and vapour flows in that part of the column. 1.2.1.3
[6]
Equilibrium and Operating Lines
The McCabe-Thiele method assumes that the liquid on a tray and the vapour above it are in equilibrium. How this is related to the VLE plot and the operating lines is depicted graphically in the diagram on the right.
[6]
12
Figure1. 6 McCabe-Thiele Diagram with operating and equilibrium section
[6 ]
A magnified section of the operating line for the stripping section is shown in relation to the corresponding n'th stage in the column. L's are the liquid flows while V's are the vapour flows. x and y denote liquid and vapour compositions and the subscripts denote the origin of the flows or compositions. That is 'n-1' will mean from the stage below stage 'n' while 'n+1' will mean from the stage 'n' while 'n+1' will mean from the stage above stage 'n'. The liquid in stage 'n' and the vapour above it are in equilibrium, therefore, x n and yn lie on the equilibrium line. Since the vapour is carried to the tray above without changing composition, this is depicted as a horizontal line on the VLE plot. Its intersection with the operating line will give the composition of the liquid on tray 'n+1' as the operating line defines the material balance on the trays. The composition of the vapour above the 'n+1' tray is obtained from the intersection of the vertical line from t his point to the equilibrium line. 1.2.1.4
[6]
Number of Stages and Trays
Doing the graphical construction repeatedly will give rise to a number of 'corner' sections, and each section will be equivalent to a stage of the distillation. This is the basis of sizing distillation columns using the McCabe-Thiele graphical design methodology as shown in the following example.
[6]
13
Given the operating lines for both stripping and rectification sections, the graphical construction described above was applied. This particular example shows that 7 theoretical stages are required to achieve the desired separation. The required number of trays (as opposed to stages) is one less than the number of stages since the graphical construction includes the contribution of the reboiler in carrying out the separation.
Figure1. 7 McCabe-Thiele Diagram with Number Of Stages
[4]
The actual number of trays required is given by the formula: Actual number of trays=(number of theoretical trays)/(tray efficiency)
(7)
Typical values for tray efficiency range from 0.5 to 0.7 and depend on a number of factors, such as the type of trays being used, and internal liquid and vapour flow conditions. Sometimes, additional trays are added (up to 10%) to accommodate the possibility that the column may be under-designed. 1.2.1.5
The Feed Line (q line)
The diagram above also shows that the binary feed should be introduced at the 4'th stage. However, if the feed composition is such that it does not coincide with the intersection of the operating lines, this means that the feed is not a saturated liquid. The condition of the feed can be deduced by the slope of the feed line or q-line. The q-line is that drawn between the intersection of the operating lines, and where the feed composition lies on the diagonal line. Depending on the state of the feed, the feed lines will have different slopes. For example
14
q = 0 (saturated vapour) q = 1 (saturated liquid) 0 < q < 1 (mix of liquid and vapour) q > 1 (subcooled liquid) q < 0 (superheated vapour
Figure1. 8 Feed Lines
[4 ]
1.2.2 Vapour Flow Conditions Adverse vapour flow conditions can cause
foaming
entrainment
weeping/dumping
flooding
Foaming: Foaming refers to the expansion of liquid due to passage of vapour or gas.
Although it provides high interfacial liquid-vapour contact, excessive foaming often leads to liquid buildup on trays. In some cases, foaming may be so bad that the foam mixes with liquid on the tray above. Whether foaming will occur depends primarily on physical properties of the liquid mixtures, but is sometimes due to tray designs and condition. Whatever the cause, separation efficiency is always reduced Entrainment: Entrainment refers to the liquid carried by vapour up to the tray above and is
again caused by high vapour flow rates. It is detrimental because tray efficiency is reduced: lower volatile material is carried to a plate holding liquid of higher volatility. It could also contaminate high purity distillate. Excessive entrainment can lead to flooding. Weeping/Dumping: This phenomenon is caused by low vapour flow. The pressure exerted 15
by the vapour is insufficient to hold up the liquid on the tray. Therefore, liquid starts to leak through perforations. Excessive weeping will lead to dumping. That is the liquid on all trays will crash (dump) through to the base of the column (via a domino effect) and the column will have to be re-started. Weeping is indicated by a sharp pressure drop in the column and reduced separation efficiency. Flooding: Flooding is brought about by excessive vapour flow, causing liquid to be
entrained in the vapour up the column. The increased pressure from excessive vapour also backs up the liquid in the downcomer, causing an increase in liquid holdup on the plate above. Depending on the degree of flooding, the maximum capacity of the column may be severely reduced. Flooding is detected by sharp increases in column differential pressure and significant decrease in separation efficiency.
16
2
EXPERIMENTAL PROCEDURE
2.1 Aim of the Experiment The aim of the experiment was to learn working principles of distillation column and to have knowledge about distillation methods such as continuous and batch distillation. To l earn how we use McCabe-Thiele Method and to find the number of trays needed by using McCabe-Thiele Method. Additionally, to observe the effect of power and reflux ratio on distillation and system efficiency.
2.2 Description of Apparatus
Distillation column
Figure2.1 Experimental Set-Up Of Distillation Process [7 ]
17
Refractometer: A refractometer is a laboratory or field device for the measurement of an index of refraction.
[8]
Figure2. 2 Refractometer [8 ]
2.3 Experimental Procedure In the experiment a mixture containing ethanol & water was distilled in a distil lation column both by continuous and batch processes.
2.3.1 Continuous Operation 3
At continuous section of the experiment, feed rate was adjusted 20 cm /min and the power was adjusted 1 kW. Reflux ratio was adjusted 2 by using computer program. Then system was run and system was waited as 10 min for the steady state conditions. After 10 min, samples were taken from overhead and bottom products. Sample taken from overhead was diluted by half (2 mL sample+2mL water). Refractive index of samples was analyzed using refractometer. Also temperature values (T 1, T2, T3) were read on computer screen. After waited 5 min, the same procedure was performed with same reflux ratio. The same procedures were applied as reflux ratio 10 and 15, respectively. For this part, nB(refractive index of bottom product), nD(refractive index of distillate product), T 1 (at reboiler section), T2 (at distillation column), T3 (at condensate section) values were recorded on appropriate table.
2.3.2 Batch Operation At batch section of the experiment, to obtain optimum power, different power values were adjusted as 0,25kW, 0,5kW, 0,75kW, 1kW, 1,25kW, respectively. Firstly, system was run while power value
18
is 0,25kW and 10 min was waited to reach steady state conditions. After 10 min, boil up rate (ml/min)(at top product tank and for filling 10 mL), pressure drop(cm H2O), degree of foaming, o
temperature of column at different section( C) and the refractive index of overhead and bottom products were taken. To obtain the refractive index of samples, the refractometer method was used. Sample taken from overhead was diluted by half, again. Every 5 min interval, all of these data was taken. Each increment of power, same procedures was performed and values were recorded on appropriate table. Second part of batch distillation, reflux ratio was adjusted as 0,01 and optimum power value which is 1 kW was set. To reach steady state conditions, 10 min was waited. For this part, nB(composition of bottom product), n D(composition of distillate product), T1 (at reboiler section), T2 (at distillation column), T3 (at condensate section) values were recorded on appropriate table at every 5 min intervals.
19
3
RESULTS AND DISCUSSION
3.1
RESULTS FOR BATCH DISTILLATION
3.1.1
Weeping and Flooding Point
In first part of the experiment distillation of ethanol-water mixture was performed. Power applied to the system from 0.25 Kw to 1.25 Kw with 0.25 increments and then change of pressure was measured by using the manometer in cm H 2O. Temperature measurements were obtained for three different points and samples were taken from the top and bottom and refractive indexes were measured for each sample in refractometer. Finally, Measured and calculated values were used to form Table 3.1. Table 3.1 Pressure Drop as a Function of Boil-up Rate
Power(kW)
Boil-up Rate
log(Boil-up Rate)
∆P(cm H2O)
log ∆P
(mL/min)
0.25
1.7900
0.253
43
1.633
0.50
38.625
1.586
46
1.663
0.75
74.516
1.872
51
1.707
1.00
86.957
1.939
61
1.785
1.25
84.530
1.927
82
1.914
As seen in Table 3.1, as applied power increases, pressure difference increases, too. Depending on this increment, we expect an increase in boil-up rate because the energy in system increases. This increment was observed for first 5 power values; however there was a decrease when power was 1.25 kW. The reason of this may be a time measurement error due to increasing speed of filling the tank with increasing power. Relation between change of pressure drop and change in boil-up rate in log-log graph in addition the weeping and flooding points are shown in Figure 3.1.
20
Figure 3.1 Effect of boil-up rate on pressure drop for batch distillation
As seen in Figure 3.1, pressure difference (∆P) increases with the increment of boil -up rate. As mentioned earlier, there is a deviation in our final data due to an experimental error. Flooding was observed at high boil- up rate and ∆P. This behavior was observed during the experiment, too. 3.1.2
Calculation of the Number of Theoretical Plates
Power applied to the system, mole fractions of liquid and gas phase which were calculated by using refractivity and equilibrium diagram of ethanol-water system, minimum number of plates which
were calculated by using Frenske’s equation and column efficiency are shown in Table 3.2. Table 3.2 Minimum Number of Plates and Column Efficiency Power
XB
YB
XD
YD
αAB
(kw)
Frenske
%
(Nmin+1)
Column Efficiency
0.25
0.0444
0.320
0.210
0.670
8.837
0.805
8.944
0.50
0.0286
0.220
0.210
0.670
8.554
0.932
10.356
0.75
0.0570
0.790
0.430
0.760
6.584
0.818
9.088
1.00
0.0570
0.790
0.472
0.790
6.591
0.819
9.100
1.25
0.0570
0.790
0.469
0.785
6.533
0.815
9.056
21
As seen in Table 3.2, x D values are higher than x B values, as expected. The reason of this is ethanol is more volatile than water and ethanol was distillated in our system. For the same reason, bottom products have to include in large fractions of least volatile product. Maximum mole fraction of ethanol in distilled stream was obtained when heater power was run wit h 1.00 kW so this value was accepted as the optimum power for the system and this power value was used in the other parts of experiment. The lowest efficiency was obtained when the column was worked with the 0, 25 kW. Efficiency increases with increasing power up to a point then begin to decline. The reason of this is boiling-rate increases with in increasing power and tower begins flooding. Realization of flooding prevents the distillation and the efficiency of column decrease. Average of three temperature values, which were obtained from three different locations of the column, were calculated and water densities at these temperatures were found from the tables. Ethanol-water vapor was assumed to be ideal and density of gas for every average temperature values were calculated by using distilled ethanol mole fractions. Then, by using water and gas densities, boil up rate and diameter of column, linear gas velocity was computed. Finally, F-factor calculation was done and Table 3.3 , which shows all these data, was formed. Table 3.3 Calculated Values of Vapor Densities, Liquid Densities, Linear Gas Velocities and F factors Power (kw)
Tav( °C)
ρgas (g/L)
ρliquid (g/L)
Ut(m/min)
Fgt 3 0,5
(m/min)(kg/m )
0.25
82.73
0.743
970.23
1.190
1.030
0.50
79.33
0.750
972.38
25.20
21.82
0.75
78.50
0.946
972.96
45.54
42.29
1.00
82.90
0.971
969.90
69.12
68.11
1.25
82.20
0.970
970.39
44.60
43.92
As seen in Table 3.3, as power increases, boiling-rate increases and according to this ri sing linear gas velocity increases too. As mentioned above, this behavior is not seen in recent data because of the experimental error. Increasing in linear gas velocity causes increase in gas phase kinetic energy correlation factor (Fgt).
22
The relationship between % column efficiency and F-factor is indicated in Figure 3.2.
% Column Efficiency versus F factor 10,500
% y10,000 c n e i c 9,500 i f f E 9,000 n m u l o 8,500 C 8,000 1,03
21,82
42,29
68,11
43,92
Fgt(m/min)(kg/m3)0.5
Figure 3.2 % Column Efficiency versus F-factor
As seen in Figure 3.2, at high rates of boil up causes a loss of efficiency. 3.1.3
Determination of the Variation of Top and Bottom Composition with Time
In this part of the experiment, column was worked at minimum reflux ratio which is 0,01 and optimum heater power. The change of the mole fractions of bottom and overhead products were observed with the time and Table 3.4 is formed by using these data. Table 3.4.The Change of Mole Fractions with Time t(time)
XB (mole %)
XD (mole %)
0
0.0552
0.472
5
0.0444
0.344
10
0.0396
0.344
15
0.0396
0.336
20
0.0396
0.336
As seen in Table 3.4, when compared the variation with time in each value, x B decreases by the time as expected but x D decreases as not expected. Because, distillated product should have richer volatile
23
component than bottom product. The reason of this error may be experimental. For example, system steady state might not work or the measurements might be wrong. Variation of xB value is shown in Figure 3.3.
Mole Fractions (XB) versus time 0,06 0,05 0,04
XB
0,03 Mole Fractions (XB) versus time
0,02 0,01 0 0
5
10
15
20
25
t(min)
Figure 3.3 Mole Fractions (X B) Versus Time
As seen in Figure 3.3, bottom product fraction decreases by the time as expected. The reason of this is amount of distilled ethanol increases. The amount of methanol away from the water increases and mole fraction of methanol in water decreases with time. Variation of xD value is shown in Figure 3.3.
Mole Fractions (XD) versus time 0,5 0,4 0,3
XD
0,2
Mole Fractions (XD) versus time
0,1 0 0
5
10
15
20
25
t(min)
Figure 3.4 Mole Fractions (X D) Versus Time 24
As seen in Figure 3.4, ethanol fraction in overhead product decreases by the time. It is unexpected result for our system because distillation increases with time and amount of distillated ethanol should increase with decreasing bottom product fraction. The reason of this error may be experimental as mentioned above. For example, system steady state might not work or the measurements might be wrong. 3.2
Continuous Distillation
The aim of this part changing reflux ratio how affects the column performance in continuous
distillation that’s why three different reflux ratios was adjusted (2, 10 and 15). Then, mole fractions of top and bottom which are X B and XD were calculated with helping of the refractive indexes of samples. Firstly, refractivity of the samples were found and these values were converted to weight percent from refractivity table. Mole fraction was required for finding number of plates and efficiencies so that weight percentages were divided to molecular weights of ethanol and water. With using Mc-Cabe Thiele Diagram for ethanol-water system, number of plates was obtained for three 3
different reflux ratio when feed rate was 20 cm /min and power was 1 kW in short optimum conditions. Actual number of plates were 8 plates and when efficiencies were calculated for each reflux ratio, minimum number of plates which were calculated from Mc-Cabe Thiele Diagram, were divided to actual number of plates. There was a trick when measuring refractivity of the distillate that was the concentration of distillate was not low enough to measure. Therefore, the distillate was diluted with water then its refractivity could be measured during the calculation weight percent of distillate was multiply with two because of 1:1 dilution.
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Table 3.5. Calculated values of number of plates and % efficiencies by using Mc-Cabe Thiele Method
RD
XB (mole%)
XD (mole%)
Number of plates
Efficiency %
2
0,054
0,41
3
37,5
10
0,057
0,369
2
25
15
0,054
0,42
3
37,5
Reflux ratio which can affect the separation efficiency, is too important for distillation columns. Increasing reflux ratio increases up to a certain degree the quality of the distillation. High rate of the reflux ratio causes to reduce the temperature and reduces amount of distillate. During the experiment efficiency of the column was not increase with increasing reflux ratio and normally, increasing reflux ratio results less number of theoretical stages required. However, it could not be obtained from data.
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4
CONCLUSION
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5
NOMENCLATURE
B : Flow rate of heavy or bottom product, mol/h D : Flow rate of light or overhead product, mol/h F : Feed rate, mol/h 3 0.5
Fgt :F-factor, (m/sec)(kg/m ) L
: Flow rate of liquid, in general or in rectifying section, mol/h
N : Number of ideal plates; Nmin, minimum number at total reflux RD :Reflux ratio, L/D
T : Temperature, °C V: Flow rate of vapor, in general or in rectifying section, mol/h; Vn, from plate n x :Mole fraction of more volatile component in liquid phase; xB, in bottoms; x D, in overhead; x F, in feed; xm, xn, on plates m and n y
:Mole fraction of more volatile component in vapor phase; ym, yn, from plates m and n
αAB :Relative volatility, dimensionless; α AB, of component A relative to component B in binary system
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6
REFERENCES
[1] http://www.buzzle.com/articles/types-of-distillation.html [2] http://swc2.hccs.edu/pahlavan/2423L7.pdf [3] http://www.buzzle.com/articles/types-of-distillation.html [4] http://swc2.hccs.edu/pahlavan/2423L7.pdf [5] http://www.satyamindustries.com/distillation_column%20.html [6] http://lorien.ncl.ac.uk/ming/distil/distil0.htm
[7] Hacettepe University Basic Measurement Laboratory‟s Handbook [8] http://www.bs-ltd.com/ltd/refractometers.html
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