1. Abstract/summary Regarding to the experiment objectives, which is to determine the pressure drop of the distillation column for various boil-up rates in batch distillation, to observe the degree of forming on trays for each power increment, to plot the curve relating pressure drop and boilup
rate,
to
determine
the
refractive
index
for
unknown
concentration
of
methylcyclohe methylcyclohexane/t xane/toluene oluene from the distillation distillation column for each power increment. increment. We do this experiment using distillation column provided. To determine the pressure drop, we setup different power which is 500W, 750W, 1000W, 1250W and 1500W. From the data collected, we plot the graph of pressure drop versus boil-up rate. The graph shows that the pressure drop is directly proportional to the boil-up rate. During experiment progress we observe that there are three level of degree of forming which is gentle, flooding and forming. We collect the data of refractive index for each power increment and put it in the observation table. To determine the refractive index for unknown concentration, we mix the mixture of toluene and meth methyl ylcyc cyclo lohe hexa xane ne in appro appropr pria iate te volu volume me that that we assu assume me,, and and then then we meas measur uree the the refractive index using refractometer. From the data, we plotted the graph of refractive index versus percentage of toluene to find the percentage of toluene in the unknown concentration.
1
2. Introduction Distillation is defined as a process in which liquid or vapor mixture of two or more substances is separated into its component fractions of desired purity, by the application and removal of heat. Distillation is based on the fact that the vapor of a boiling mixture will be richer in the components that have lower boiling points, therefore when this vapor is cooled and condensed, the condensate will contain more volatile components. At the same time, the original mixture will contain more of the less volatile material. There are many types of distillation column, each designed to perform specific types of separation, and each differs in terms of complexity; I.
Batch column: in batch operation, the feed to the column is introduced batchwise. That is, the column is charge with a `batch` and then the distillation process is carried out. When the desired task is achieved, a next batch of feed is introduced.
II.
Continuous column: in contrast, continuous columns process a continuous feed stream. No interruptions occur unless there is a problem with the column or surrounding process unit. They are capable of handling high throughputs and are the most common of the two types, trays column and packed column; a. Binary column – feed contains only two components b. Multi-component column – feed contains more than two components c.
Tray column(internal) – where trays of various design are used to hold up the liquid to provide better contract between vapor and liquid
d.
Packed column – where instead of trays `packings` are used to enhance contact between vapor and liquid.
2
Distillation columns are made up of several componen ts, each of which is used either to transfer heat energy or enhance material transfer. A typical distillation contains several major components: I. A vertical shell where the separation of fluid components is carried out II. Column internals such as trays/plates or packings which are used to enhance components separations III. A reboiler to provide the necessary vaporization for the distillation process IV.
A condenser to cool and condensed the vapor leaving the top of the column
V.
A reflux drums to hold the condensed vapor from the top of the column so that liquid (reflux) can be recycle back to the column.
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 tray divides the column into a top (enriching) section and bottom (stripping) section. The feed flows down the column where it is collected at the bottom in the reboiler. Heat is supplied to the reboiler to generate vapor. The source of heat input can be any suitable fluid, although in most chemical plants this is normally stream. In vapor raised in the reboiler is re-introduced into the unit at the b ottom of the column. The liquid removed from the reboiler is known as the bottom product. The vapor 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 the reflux 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 vapor and liquid within the column as well as external flows of feeds and product streams, into and out of the column.
3
4
3. Aims/Objectives
The objectives of this experiment: 1. To determine the pressure drop of the distillation column for various boil-up rates in batch distillation. 2. To observe the degree of forming on trays for each power increment 3. To plot the curve relating pressure drop and boil-up rate 4.
To determine the refractive index for unknown concentration of methylcyclohexane/toluene from the distillation column for each power increment.
5
4. Theory Distillation is a process of separating two or more miscible liquids by taking advantage of the boiling point differences between the liquids. To understand how distillation works consider the mixture of toluene and methylcyclohexane for this distillation experiment. Heat is added to the mixture of toluene and methylcyclohexane and eventually the most volatile component (in this case methylcyclohexane) begins to vaporize. As the methylcyclohexane vaporizes it takes with it molecules of toluene. The methylcyclohexane-toluene vapor mixture is then condensed and evaporated again, giving a higher mole fraction of methylcyclohexane in the vapor phase and a higher mole fraction of toluene in the liquid phase. This process of condensation and evaporation continues in stages up the column until the methylcyclohexane rich vapor component is condensed and collected as tops product and the water rich liquid is collected as bottoms product. F igure below, illustrates the distillation column used in the experiments.
6
To understand distillation, first consider what happens upon heating a liquid. At any temperature, some molecules of a liquid possess enough kinetic energy to escape into the vapor phase (evaporation) and some of the molecules in the vapor phase return to the liquid (condensation). Equilibrium is set up, with molecules going back and forth between liquid and vapor. At higher temperatures, more molecules possess enough kinetic energy to escape, which results in a greater number of molecules being present in the vapor phase. If the liquid is placed into a closed container with a pressure gauge attached, one can obtain a quantitative measure of the degree of vaporization. This pressure is defined as the vapor pressure of the compound, and can be measured at different temperatures.
7
5. Apparatus
• • •
• • • • • •
Distillation column Methylcyclohexane 50mol% Toluene 50mol% Computer interface Automatic digital refactometer Distilled water Measuring cylinder Conical flask Stop watch
6. Procedure 8
a) Distillation column
1. Make sure all valves on the equipment are fully closed. 2. Open valve 10 (V10) 3. Fill the boiler with 10L of mixture consist of 50 mol% methyhcyclohexane and 50 mol% toluene which will be distilled. Make sure the filler cap was replaced. 4. Switch on the control panel power 5. Set the temperature selector switch to T9 6.
Turn the power controller for the reboiler heating element fully anti-clockwise and switch it on.
7.
Adjust the power to 0.5kW; let it stabilized for 10 minutes.
8. Observe the degree of forming on the trays 9. Open valve 7 (V7) and valve 6 (V6) then record the pressure drop value using manometer tube. 10. Open valve 3 (V3), collect 90ml of upper product volume (methylcyclohexane) and at the same time record the time taken for volume to reach 90ml. 11. Repeat step 7 until 10 for 0.75kW, 1.0kW, 1.25kW and 1.5kW of power increment.
a) Finding refractive index using Refractometer
1. Take the refractive index value using automatic digital refractometer for each power sample of product at each power increment. 2. Running the automatic digital refactometer: i.
Clean the surface using distilled water
ii. Switch on the power iii.
Press ZORE setting (SWL key)
iv. Press start key after placing the sample, record the refractive index value v. Clean and switch off.
a) Non distillation mixing of methylcyclohexane and toluene 9
1. Assume 10ml of solution equal to 10 mol% 2.
Let say sample A, mix 0 mol% methylcyclohexane with 100 mol% toluene using conical flask.
3. Stir the solution carefully 4. Repeat step 2 until 3 for; i.
B (20 mol% methylcyclohexane, 80 mol% toluene)
ii. C (40 mol% methylcyclohexane, 60 mol% toluene) iii. D (60 mol% methylcyclohexane, 40 mol% toluene) iv. E (80 mol% methylcyclohexane, 20 mol% toluene) v. F (100 mol% methylcyclohexane, 0 mol% toluene) 5. Record the refractive index value for all sample using automatic digital refractometer 6. Plot the pressure drop versus boil-up rate graph. 7. Determine the unknown concentration from refractive index versus percentage of methylcyclohexane graph.
10
7. Result A. Using distillation column Power (watt)
Collection time (s)
Boil-up rate (ml/s)
Pressure drop (mm H2O)
Refractive index
500
85.2
1.056
68
1.43846
750
20.4
4.41
89
1.43911
1000
12.6
7.14
115
1.44028
1250
3.00
30
219
1.44708
1500
2.88
31.25
223
1.44785
B.
Power (watt)
Observation
Concentration of unknown (toluene mol %)
500
Gentle
21.4
750
Gentle
22.7
1000
Flooding
24.9
1250
Flooding
39.8
1500
Foaming
40.9
Without using distillation column ( finding refractive index from mixing of solution)
Sample
Mixture percentage (mol %)
Refractive index
Methylcyclohexane
Toluene
A
0
100
1.49685
B
20
80
1.48110
11
C
40
60
1.46800
D
60
40
1.44726
E
80
20
1.43754
F
100
0
1.42310
Column temperature: 102 ºC Process temperature: 104.9 ºC
Pressure drop versus boil-up rate graph
Refractive index versus percentage of toluene graph
12
8. Discussion Distillation is a commonly used method for purifying liquids and separating mixtures of liquids into their individual components. Familiar examples include the distillation of crude fermentation broths into alcoholic spirits such as gin and v odka, and the fractionation of crude oil into useful products such as gasoline and heating oil. In the organic lab, distillation is used for purifying solvents and liquid reaction products. In this experiment we have to determine the pressure drop of the distillation column for various boil-up rates in batch distillation, to observe the degree of forming on trays for each power increment, to plot the curve relating pressure drop and boil-up rate, to determine the refractive index for unknown concentration of methylcyclohexane/toluene from the distillation column for each power increment. After completing preparing and setup the equipment, we run the experiment. This experiment can be divided into three stages. First is distillation at the distillation column, second is finding refractive index using refractometer and third is non distillation mixing of methylcyclohexane and toluene. For distillation using the distillation column provided we need to determine the pressure drop at each of power increment. Firstly we switch on the control panel and setup the power value at 500 watt and wait for 10 minutes, this is done so that the solution of toluene and methylcyclohexane stable, ready for the distillation process and the amount of product produce is enough for the experiment. Although there is often slightly shift of power value at the control panel, but we manage to get the constant value which we needed. This slight shift of power value may affect the pressure drop and the condition in the distillation column. After 10 minutes, we open the valve and start to estimate the pressure drop. At the same time we also collect 90ml of upper product or in other name is distillate which is methylcyclohexane and records the time taken to collect the sample. The sample was brought to refractometer to measure its refractive index.
13
To use refractometer, clean the lens of refractometer using distilled water and set the first value by pressing the ZERO setting key, then after placing the sample start key was pressed. The refractometer will indicate the refractive index for the sample automatically. We repeat the same step for 750 watt, 1000 watt, 1250 watt and 1500 watt of each power increment. For non distillation mixing of methylcyclohexane and toluene, we assume 10 ml solution equal to 10 mol%, and then we setup 6 samples which have different amount of methylcyclohexane and toluene but overall is 100ml. The samples were brought to the refractometer to estimate its refractive index. All the data we tabulate in the table at the result section. In this experiment, methylcyclohexane become the upper product or distillate because it has lower boiling point than toluene thus more volatile (boiling point for methylcyclohexane is at 101 °C meanwhile for toluene is at 110.6 °C). We observe that there are 3 levels of vapor flow condition in the distillation column during the distillation process which is firstly gentle, secondly flooding and third is foaming. Gentle flow refers to the flow of vapor which in steady condition and no sudden abrupt rise occurs. Flooding refers by excessive vapor flow, causing liquid to be entrained in the vapor up the column. Forming refers to the expansion of liquid due to passage of vapor or gas. Although it provides high interfacial liquid-vapor contact, excessive foaming often leads to liquid buildup on trays.
We plotted the graph of pressure drop versus boil-up rate and graph of refractive index versus percentage of toluene. From the graph it can be said that the pressure drop is directly proportional to the boil-up rate. The second graph is used to find the percentage of toluene in a unknown concentration.
During the experiment there are several precaution steps that need to be alert. Avoid direct contact with distillation column because it is hot, use glove to hold the measuring cylinder during taking the volume of methylcyclohexane, clean the refractometer surface using distilled water before use that apparatus, after finish the experiment, collect back the sample of mixture because it can be reused, use spectacles provided during collecting the sample and lastly the solution is flammable, avoid it from the fire. 14
Fractionating columns are widely used in the chemical process industries where large quantities
of
liquids
have
to
be
distilled. Such
processing, petrochemical production, natural
industries gas
are
the petroleum processing, coal
tar processing, brewing, liquefied separation, and hydrocarbon solvents production and similar industries but it finds its widest application in petroleum refineries. In such refineries, the crude oil feedstock is a very complex multicomponent mixture that must be separated and yields of pure chemical compounds are not expected, only groups of compounds within a relatively small range of boiling points, also called fractions and that is the origin of the name fractional distillation or fractionation. It is often not worthwhile separating the components in these fractions any further based on product requirements and economics.
9. Conclusion
15
At 500 watt the pressure drop in distillation column is 68 mmH2O, at 750 watt the pressure drop is 89 mmH2O, at 1000 watt the pressure drop is 115 mmH2O, at 1250 the pressure drop is 219 mmH2O and at 1500 the pressure drop is 223 mmH2O. 2. The degree of forming at 500 and 750 watt is gentle, at 1000 and 1250 watt the flooding occurs and at 1500 watt the forming in the distillation column trays occurs. 3. According to the graph plotted, the pressure drop is potentially proportional to the boil-up rate. 4. According to the graph plotted, concentration of toluene (mol%) at 500 watt is 21.4 mol %, at 750 is 22.7 mol% at1000 watt is 24.9 mol%, at 1250 is 39.8 mol% and at 1500 is 40.9 mol%. 1.
10. Recommendations
•
Avoid direct contact with distillation column because it is hot 16
• •
• • • •
Use glove to hold the measuring cylinder during taking the volume of methylcyclohexane Clean the refractometer surface using distilled water before use that apparatus. After finish the experiment, collect back the sample of mixture because it can be reused. Use spectacles provided during collecting the sample. The solution is flammable, avoid it from the fire. Make sure the eye is directly to the meniscus at the manometer ruler during taking the pressure drop value.
11. References
•
http://chem.engr.utc.edu/webres/435f/dist/DIST-96.HTML http://en.wikipedia.org/wiki/Distillation_column http://www.armfield.co.uk/pdf_files/uop3.pdf
•
http://www.scribd.com/doc/12276923/Distillation
• •
•
•
•
Working Guide to Process Equipment, McGrawhill 2nd edition, Norman p.Lieberman, Elizabeth t.Liberman. Unit Operation of Chemical Engineering, McGrawhill 7th edition, Warren L Mccabe, Julian C.Smith, Peter Harriot. Process unit operation,CPE521 lecture notes (semester 3).
17
12. Appendices
18
