Ashiq Dungarwalla
Packed Distillation Column PC Introduction: Packed still column distillation is a type of separation process that involves the distillation principles which is based on relatively easy separation of the components involved as a result of differences in their volatilities or boiling points. In the packed column there exist true countercurrent systems in which vapour flows steadily up and the reflux steadily down the column.Packing can comprise of saddles, rings or other shaped particles which provides a large interfacial area for the transfer. In this type of distillation technique, the use of knitmesh structured packing are found in the column of limited diameter, which is further lagged with a polymer plastic foam in order to prevent heat loss to the surroundings (this is more suited for distillation processes at constant reflux and its inexpensive nature). he aim of the experiment is to determine the estimated height equivalent to a theoretical plate (!"P). !"P values are generally within the range of #.$ % #.&m depending on the height height of the packing. packing. I.e. the smaller smaller the packing, packing, the lower the !"P value, value, which in this case is sometimes less than #.$'m. !owever achieving this involves the use of two chemicals undergoing distillation with different compositions. he results obtained from the process are then interpreted into a cabe*hiele graph, in which the number of stages is achieved. +lternatively the number of theoretical stages could be obtained using the enske-s equation. It should be noted that entire distillation process occurs at total reflux hence the theoretical stages is calculated. rom the calculated theoretical stages, the !"P could be obtained through the formula given his particular process involves a liquid*liquid separation, combining the toluene and heptane into a mixture and then placing it into a distillation column. nce the mixture was added/ it is heated with steam and the more volatile liquid turns into a gas and floats to the top of the column, it is then condensed using water and collected for sampling. he column contains packing inside which is used to increase the contact area between the liquid and the gas so that separation can occur. nce a top product i.e. more volatile substance/ is gathered it is sampled and the same goes for the bottom product i.e. less volatile substance.
Hazard Analysis here are a number of ha0ards which must be taken into consideration whilst carrying out this experiment. he first ma1or ha0ard is the toluene and heptane which they are2 •
oxic by inhalation, ingestion or by absorption through skin. o prevent this gloves must be worn, if however some do get in contact with your hands, wash it off345.
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6erious irritant if they get into eyes. his ha0ard can be minimi0ed by wearing goggles, also if some toluene or heptanes do get in contact with your eye, call your supervisor immediately so your eye could be washed clean.
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Ashiq Dungarwalla
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"xperimental teratogen. herefore make sure chemicals are not consumed since teratogens at large amounts cause defects in cells.
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7oth chemicals are highly flammable, therefore make sure that you are not near any flames or no one plays with lighters. In case of a fire call your supervisor and go to the nearest fire exit immediately 385.
ther ha0ards that should be taken under consideration are2 •
9alves can be extremely hot, consequently when opening the steam valve insulated gloves have to be worn to prevent burning your hands. If your skin does get burnt soak the burn in cool water for at least : minutes. he cool water helps reduce swelling by pulling heat away from the burned skin. Inform your supervisor.
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inally care must be taken when climbing up the ladders to place the mixtures into the distillation column. he correct footwear should be worn to enable better grip when climbing up to carry out the experiment.
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ake sure there are no spillages that can cause you to slip. If a spillage occurs it should be promptly mopped up.
herefore taking all these factors in to account, to perform a safe experiment PP" should be worn at all time. It is also recommended to return the bottles of the chemicals back as soon as they are no longer required to avoid spillages.
Apparatus •
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;istillation olumn ondenser
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6tep >adder hermometers Pipettes +bbe
abels 6oft issues
Procedure
Experiment 1 (40 toluene! "0 n#Heptane$
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Ashiq Dungarwalla
4) irstly ensure the correct PP" was worn throughout the duration of the experiment. 8) he appropriate quantities to be added to the reboiler were measured. 6ince A:# ml will be added to reboiler, the composition will be '#B oluene ($## ml) and &#B n* !eptane (':#ml) $) he water supply to the condenser was then switched on and the valve was opened until the desired speed was achieved as indicated by a float which acted as a flow meter. ') "nsured that both collection taps were closed. :) he A:#ml solution was poured into the top of the column using a funnel and a step ladder to get to the top of the column. "nsured the step ladder was held in place by a colleague to prevent any accidents. &)
%e&ractometer 1) he water bath was pre*set to 8: o by the lab technician. 2) o prevent contamination, the housing prism was lifted and the prism surface was 3) 4) 5) 6)
covered with a few drops of de*ionised water using a clean pipette. he housing prism was lowered and then lifted again and the de*ionised water was wiped away using soft tissues. @hilst the housing prism is raised, the prism surface was covered with the sample using another clean pipette. he prism was lowered and the refractive index was allowed to stabilise for 8#*$# seconds. hereafter recorded the
Experiment ' ("0 oluene! 40 n#Heptane$
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Ashiq Dungarwalla
4) he experiment was repeated for the reverse composition/ &#B oluene (':# ml) and '#B n*!eptane ($## ml). 8) he rig, including the water supply, is now ready to be switched off and the necessary calculations can be made.
Dia)ram
%esults %e&racti*e index results &or top and +ottom products &rom +ot, experiments:
ample 6ample 4
Experiment 1 op Product 4.$C&C8
ottom Product 4.'&$48
Experiment ' op Product 4.$C:?8
ottom Product 4.''C&:
-ole .raction o& op / ottom Products:
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Ashiq Dungarwalla
Experiment 1:
op product (2 D$: 03''
ottom product (2 5$: 03'16
Experiment ':
op product (2 D$: 034'
ottom product (2 5$: 037"
olumn inside diameter E :#mm E 0306m !eight of knitmesh structured packing in column E :&cm E 036"m Pressure steady at 8: Psi Fsing the cabe hiele ethod2 "xperiment 4 emperature at steady state2 A: o Gumber of theoretical stages (appendix :) E 4# * 4 E 8 !"P E
!eight of olumn Packing (m) Gumber of heoretical 6tages (G)
E
#.:& E 030"'m C
"xperiment 8 emperature at steady state2 A$ o Gumber of theoretical stages (appendix :) E C*4 E !"P E
!eight of olumn Packing (m) Gumber of heoretical 6tages (G)
E
#.:& E 0309m ?
Discussion he experiment was carried out and the values obtained for the !"P were 030"'m for experiment 4 and 0309m for experiment 8. +ccording to oulson and
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Ashiq Dungarwalla
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alculation errors/ human errors (such as misreading the value for temperature, time or amount of liquid).
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It could also be due to contamination as during the experiment, some of the equipments used weren-t in their purest state, even after a thorough wash.
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ne factor was the inaccuracy of the measurements as the measuring cylinder used was 4###cm $. o overcome this factor a more accurate measuring cylinder could be used such as using a burette which is much more accurate.
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ther errors such as the parallax error, ( which occurs when trying to get the meniscus of the liquid to the level required) could have aided to the inaccurate results. herefore when measuring the amounts of solution ensure readings are taken at eye level to prevent human errors.
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+nother apparatus error is the thermometer. he thermometers had a measurement error of J 4K. o eliminate this error a more accurate thermometer could be used.
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@hen recording the refractive index reading the value kept fluctuating for a few seconds indicating inaccuracy within the
here were also procedural errors that affected the results obtained. @hen transferring the solutions to the column, there was loss of solution as not all the mixture transferred to the column. @hen the system was cooling down, we did not allow it to cool down for a sufficient amount of time (initial temperature in 8 nd experiment * $CK) therefore this meant that n* !eptane was heated a lot faster which could mean that it wasn-t as pure as it could have been. o avoid this in the future the rig should be shut down completely and cooled, to prevent the unreliability of the system. +nother error is the fact that when the first mixture was being drained/ and the next mixture was added/ the composition would not have been completely accurate as more oluene or n*!eptane might have been present. o avoid this, the column should be drained completely and cleaned/ before the new mixture is added. urther, the error in the !"P value in experiment 8 compared with the published values may have been down to systematic error in that the graph given to obtain the mole fraction of n*!eptane was poorly calibrated and therefore had big margins for error when reading values. +s a result, the !"P value was inaccurate and unreliable as the theoretical stages were inaccurate. If a better graph could have been obtained, the results could have been more reliable and accurate. Improvements can be made to this experiment by recording the temperature at every $# seconds for the first few minutes. +ll of this could lead to a more agreeable result in which the same efficiencies are obtained for both the experiments. @hen measuring the column performance and calculating the !"P value alternative methods can be used such as the enske equation as with the cabe hiele method there will be room for human error when drawing the steps on the 9>" diagram and the enske method can be used and compared with the cabe hiele method to obtain more reliable results.
%e&erences 345 http2LLlorien.ncl.ac.ukLmingLdistilLdistil#.htm 385 http2LLchemistry.about.comLcsL:LfLbldistillation.htm
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Ashiq Dungarwalla
3$5 http2LLgisceu.netLP;LF88#.pdf 3'5 Page &'# *
omenclature ;nits Explanation !"P o G Psi N; N@ ml min B mm cm cm$
!eight "quivalent to a heoretical Plate emperature measurement in ;egree elsius Gumber of Plates Pound per square inch ole fraction of the top product ole fraction of the bottom product easurement of volume in millilitre inutes Percentage illimetres entimetres entimetre cubed
Appendices +ppendix 4 * ;ata sheet with additional notes +ppendix 8 *
Appendix 7 Calculations: Packing height of knitmesh structured in column E :&.: cm (#.:&:m) emperature at steady state E A' o
cabe*hiele ethod2
Calculations &or "0 oluene / 40 Heptane composition Fsing the average refractive index values obtained from the for top and bottom product the mole fraction of n*heptane is obtained.(using figure 4) ole ractions (n*heptane)2
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Ashiq Dungarwalla
op product N @ * #.$& 7ottom product N ; % #.?'8 hen using the cabe*hiele method the number of theoretical stages is calculated. Gumber of Plates E G % 4 G E 4: number of stages herefore2 G % 4 E C % 4E ? Gumber of theoretical stages, n E ? Fsing the below equation the !"P can be calculated.
!"P E #.:&: L ? E 0309m
Calculations &or 40 oluene / "0 Heptane composition Fsing the average refractive index values obtained from the for top and bottom product the mole fraction of n*heptane is obtained.(using figure 4) ole ractions (n*heptane)2 op product N @ * 03'' 7ottom product N ; 03'16 hen using the cabe*hiele method the number of theoretical stages is calculated. Gumber of Plates E G % 4 G E 4# number of stages herefore2 G % 4
E 4# % 4E C
Gumber of theoretical stages, n E C Fsing the below equation the !"P can be calculated.
!"P E #.:&: L C E 030"'m
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Ashiq Dungarwalla
o achieve the ideal number of theoretical stages2 rom the information gathered from oulsan D
9
