Report Distillation columns Sieve tray column - NTNUfolk.ntnu.no/andersty/3. Klasse/Felleslab (SepTek og RekTek)/SepTek... · Report Distillation columns Sieve tray column Lene Tapio

ReportDistillation columnsSieve tray column

Lene [email protected]

Pia [email protected]

Room K4-408Gr B14

Performed 18. september 2012

Contents

1 Theory 31.1 Floodingpoint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.2 Weepingpoint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.3 McCabe-Thiele method . . . . . . . . . . . . . . . . . . . . . . . . . 31.4 Gas chromatography . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2 Experimental 52.1 Column startup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.2 Exercise 1: Time required for reaching steady state condition . . . . 62.3 Exercise 2: Efficiency vs. vapor velocity . . . . . . . . . . . . . . . . 62.4 Column shutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3 Results 63.1 Time required for reaching steady state condition . . . . . . . . . . 63.2 McCabe-Thiele Diagram . . . . . . . . . . . . . . . . . . . . . . . . 73.3 Total column efficiency vs vapor velocity . . . . . . . . . . . . . . . 13

4 Discussion 14

5 Conclusion 15

A Laboratory safety 16

B McCabe-Thiele calculations 19

C Calculations 21C.1 Water-ethanol mixture . . . . . . . . . . . . . . . . . . . . . . . . . 21C.2 Calculating the mole fraction of ethanol . . . . . . . . . . . . . . . 22C.3 Calculating vapor velocity . . . . . . . . . . . . . . . . . . . . . . . 23C.4 Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

D McCabe-Thiele 27

1

E Assignment calculations 28E.1 Mole fractions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28E.2 Vapor velocity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29E.3 McCabe-Thiele . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

2

1 Theory

1.1 Floodingpoint

In a distillation column vapour flows up and liquid flows down. The floodingpoint is referred to as the vapour velocity when liquid begins to accumulate inthe column. This happens when the vapour velocity gets to high, resulting in thepressure exerted by the vapour flowing upwards balances the gravitational forcesthat pull the liquid down. The flooding point can be observed by a sudden increasein the pressure drop or when liquid starts flowing over the top of the column.[1].

1.2 Weepingpoint

At steady state conditions the vapour velocity through the perforations preventsthe liquid from dropping down to the next stage through the perforations. Whenthe vapour velocity gets too low the liquid is able to weep down through these per-forations. This gives us less vapour/liquid contact, which result in lower efficiencyoff the column[1].

1.3 McCabe-Thiele method

The McCabe-Thiele method was presented by MaCabe and Thiele in 1925. This isperhaps the simplest and most instructive method for analysis of binary distillation[1].

Assumptions[3]:

• the molar heats of vaporization of the feed components are equal

• for every mole of liquid vaporized, a mole of vapour is condensed

• heat effects such as heats of solution and heat transfer to and from the dis-tillation column are negligible.

3

The MaCabe-Thiele diagram can be used to determine the theoretical number oftrays in a distillation column. To construct the diagram the mass balance and VLE(vapor-liquid-equilibrium) data for the lower boiling point component, componentA, is used. The x-axis in the diagram represents the mole fraction of A in gasphase, and the y-axis represents the mole fraction of A in liquid phase. Plottingthe VLE data gives the equilibrium line, and the material balance gives the twooperating lines [1]:

In the enriching section the mole fraction of A in gas phase, yn+1 is given by

yn+1 =R

R + 1xn +

xDR + 1

(1.1)

where R is the reflux ratio, xn is the mole fraction of A in liquid phase at step nand xD is is the mole fraction of A in liquid phase in the distillate.

In the stripping section the mole fraction of A in gas phase, ym+1 is given by

ym+1 =LmVm+1

xm − WxWVm+1

(1.2)

where Lm is liquid flow from tray m, Vm+1 is vapor flow from tray m+1, W is bot-toms flow, xm and xW is mole fraction of liquid phase in tray m and the bottomsflow.

The operating line will be equal to the x=y line when it is total reflux.

The parameter q represents the condition of the feed and is defines as follows

q =heat needed to vaporize 1 mole of feed at entering conditions

molar latent heat of vaporization of feed(1.3)

And the q-line in the diagram is the line with slope q/(q-1) through the cross

4

point between the operating lines as well as the line x = y. To determine thetheoretical number of stages in a distillation steps are made in the McCabe-Thielediagram, starting at the top tray where x = xD. The steps are made by makingstraight lines, alternately horizontal and vertical between the equilibrium line andthe operating lines, all the way down to x = xB, where xB is the mole fractionof A in liquid phase at the bottom. These steps represent the theoretical traysneeded in the distillation[1].

1.4 Gas chromatography

Gas chromatography (GC) is used in analytical chemistry for separating andanalysing compounds that can be vaporized without decomposition. GC can beused to test the purity of a particular substance, in this experiment, the purity ofethanol after distillation[1].

2 Experimental

2.1 Column startup

We started the experiment with turning on the computer and the cooling water,and made sure that the bottom and distillate tap lines were closed. Then wecharged the column with mixture(5800 ml) of water(3954 ml) and ethanol (1846ml). Then we sat the column to total reflux by using the control box and satT=105◦C. We also had to set the boilerńs power duty to 40% when the top tem-perature substantially changed or the pressure difference over the column risedquickly. Then we waited until the vapor stream started condensing(t=0).

In case of flooding conditions, we switched the heater off immediately, and inany emergency situation we did turn off the heater but leaved the cooling wateron.

5

2.2 Exercise 1: Time required for reaching steady state con-

dition

We sat the reboiler to a desired value(30 %) and waited until the column startedto give condensate. Then we took samples from the top every 5 minutes within1 hour, and 1 sample after 1 hour from the bottom. In the end we analyzed thesamples with GC.

2.3 Exercise 2: Efficiency vs. vapor velocity

We run the column with total reflux for five different boiler powers. Each timewe sat the power to the desired in the reboiler. Then we waiter until the columnreached the steady-state, we assumed this was after the time found in Exercise 1.After that we took samples from the top and from the bottom, 20 ml each sample,and wrote down the reflux rate at the time. Then we analyzed the samples withGC and calculated the efficiency of the column and the theoretical number ofstages using the McCabe-Thiele method. We also calculated the vapor velocity.In the end we identified the flooding and weeping points and wrote down the powerof the boiler at these two points.

2.4 Column shutdown

When we were finished with the exercises we turned the heater off and let thecolumn cool down. Then we emptied the column and turned off the cooling waterand the rest of the equipment.

3 Results

3.1 Time required for reaching steady state condition

The time required to reach study state condition was approximately 30 minutes,see Fig. 1.

6

Figure 1: The volum percent, %vol ethanol, plottet as a function of Time.

3.2 McCabe-Thiele Diagram

Making McCabe-Thiele diagrams, see Figure 2-6, for the different power used in theexperiment gave a total number of stages, including the reboiler, shown in Tabel1. In the same table is also the theoretical stages in the column and referring tothe McCabe-Thiele diagrams for the different reboiler powers. The equilibriumline in the McCabe-Thiele diagram is yet plotted as a function of xet, with valuesfrom table 2 in appendix B [5].

It was impossible to make an McCabe-Thiele to reboiler power equal to 25% be-cause this result was over the azeotropic point between ethanol and water.

Weeping point was observed to be when the reboiler power was 18 %, and theflooding point at 45 %.

7

Table 1: Total number of stages and theoretical stages in the column at the dif-ferent powers.

Reboilerpower

Total numberof stages

Theoreticalstages in thecolumn

McCabe-Thiele Di-agram

20% 10 9 Figure 225% - - -28% 6 5 Figure 330% 6 5 Figure 432% 4 3 Figure 535% 5 4 Figure 6

8

Figure 2: Red line is equilibrium line, green line is x ethanol = y ethanol, blueline is number of stages. x ethanol is the mole fraction of ethanol in liquid phase,y ethanol is mole fraction of ethanol in gas phase.

9


10


11


12


3.3 Total column efficiency vs vapor velocity

Total column efficiency is plotted as a function of vapor velocity in Figure 7.

13

Figure 7: Total column efficiency plotted as a function of vapor velocity.

4 Discussion

The results presented is the results another group got on the Sieve-tray column.This is because this column leaked when we run the experiment, which gave uswrong results. After looking on the GC results from the first exercise we sawthat they were not as expected, and the system never reached steady-state whenwe started exercise two. The temperature in the column was over 100 ◦C, whichmeans that the ethanol had vapoured out of the system.

As we can see from Figure 1 it took approximately 30 minutes for the concen-tration of ethanol to stabilize and the column to reach steady state conditionsin the first part of the experiment. When the system reached steady state, andthe system runs with total reflux, the concentration of the distillate will remainconstant.

The volume percent we got from the GC in the first part of the experiment seemsreasonable. After the column reached steady state, the volume percent of the dis-

14

tillate was stable around 93-94%, which was expected from the column. In thesecond part the volume percent of the distillate varied a bit more, from 90-97%.To get 97 vol% is impossible for this distillation, because this result is over theazeotrope of a mixture between ethanol and water. There must have been an errorin the analysis of this sample. This error may come from bad measurements of thesolutions that were mixed before analyzing the sample with GC.

When the column efficiency was calculated for the different power duties, theMcCabe-Thiele diagram was used to find theoretical number of trays needed forthe distillation. For the sample that showed 97 vol% the McCabe-Thiele diagramcould not be drawn, because it was over the azeotropic point in the diagram. Inthe plot of column efficiency vs. vapour velocity it seems that the column efficiencydecreases when the vapour velocity gets higher. It seems that the column is mostefficient right above the weeping point. This makes sense because this is the pointwhen there is most contact between vapour and liquid.

One possible error source in this experiment can be some flooding. Floodingwill give the wrong volume fraction of ethanol in the distillate when undistilledsolution mixes with the distillate. Another error source could be weeping, whichwill give lower contact surface between vapor and liquid phase. This will result ina lower mole fraction of ethanol in the distillate. Un mixed samples tested withGC because of too little shaking, will give wrong results of the volume percentethanol in the sample. During the experiment 12 samples were taken. Because theconcentration in these samples were high, this could have altered the concentrationof ethanol in the system. When the samples were taken the stabilization of thesystem were interrupted, and could also lead to wrong results. When the samplesfrom the bottom flow were taken the system were not totally homogeneous becauseof lack of stirring. This may also affected the results.

5 Conclusion

The time needed to reach steady state condition was approximately 30 minutes.The total column efficiency decrease with increasing vapor velocity, so at the lowest

15

vapor velocity the the column is most efficient . Weeping point was at 18 % oftotal reboiler power, and flooding point at 45 %.

References

[1] Felles lab: Distillation Columns, 2012. Viewed 13. September 2012.http://www.nt.ntnu.no/users/preisig/Repository/TKP4110FellesLab/experiment%20descriptions/DistInstruct.pdf

[2] Gordon Aylward, Tristan Findlay, SI chemical data 6th edition, 2008

[3] Geankoplis, C. J., 2003. Transport Processes and Separation Process Principles. 4thed. Prentice Hall. pp. 706-717.

[4] Green, D. W. and Perry, R. H., 2008.Perrys chemical Engineers handbook. 8th ed. McGraw-Hill. pp. 13-18-13-19,14-36.

[5] Gmelin Handbook of Inorganic and Organometallic Chemistry p. 171

A Laboratory safety

16

NTNU

Norges teknisk-naturvitenskapelige

universitet

Kjemikaliekort

STOFFNAVN Etanol

FORMEL C2H5OH

GENERELL HELSEFARE Ingen utpreget helsefare.

FYSIKALSKE DATA

Sm.p/f r.p Kokepkt. fl.p. Molar masse

Ekspl.gr.

BRANNFARE Meget brannfarlig

-117° C 79°C 425°C 46,07 g/mol 3-19 % REAKSJONSFARE -

VERNETILTAK

Alt arbeide med farlige kjemikalier skal utføres i avtrekkskap eller i godt ventilert og godkjent rom. Nøddusj og mulighet for øyeskylling skal finnes på arbeidsplassen. Skift forurensede klær. Vask hender og ansikt etter arbeid med produktet. Åndedrettsvern (type A) skal anvendes ved forekomst av damper/aerosoler. Ved risiko for direkte kontakt eller sprut skal øyebeskyttelse benyttes. Bruk ikke kontaktlinser ved håndtering. Vernehansker skal benyttes ved fare for direkte kontakt og sprut.

ADMINISTRATIV NORM -

HELSEFARE Innånding Svelging Hud Øyne

Vil irritere slimjimmene I nese og øvre luftveier. Vil kunne føre til brekninger, kvalme, absorpsjon av toksiske kvantiteter kan gi symptomer som ved alkoholpåvirkning. Eufori. Svimmelhet. Bevisstløshet. Åndedrettstopp. Lett irritasjon. Ved langvarig hudkontakt med etanol kan føre til avfetninga av huden og hudbetennelse. Hudsprekker. Irriterer øyne.

BRANNSLUKKING Slukkes med pulver, skum, karbondioksid eller vann.

MERKNADER -

FØRSTEHJELP

ØYNE Skyll lenge med rennende vann. Dersom irritasjon vedvarer gå til spesialist. HUD Ta av forusenset tøy og skyll godt med vann.

SVELGING Drikk mye vann. Kontakt lege. Gi aktivt kull om mulig. Fremkall ikke brekninger. Ikke melk/matolje. Hold luftveier åpne. INNÅNDING Frisk luft og hvile. Gi kunstig åndedrett ved behov.

SPESIELLE MERKNADER

SPILL/ RESTER

Destrueres etter lokale forskrifter. Håndter forurenset forpakning på samme måte som substansen. Klassifisert som farlig avfall. Avfallsgruppe: Organiske løsemidler uten halogen. Forhindre utslipp til kloakk: Eksplosjonsrisiko.

LAGRING

Forpakningen oppbevares lukket og tørt ved romtemperatur (+15 °C til +25 °C). Holdes vekk fra antennelseskilder.

B McCabe-Thiele calculations

In table 2 are the equilibrium values for mole fraction of ethanol in gas phase, x1,and mole fraction of ethanol in liquid phase, y1[5].

Table 2: Values for mole fraction of ethanol in gas phase, x1, and mole fraction ofethanol in liquid phase, y1 at equilibrium.

19

%empirical data for ethanol xe_e=[0 0.0051 0.0069 0.0162 0.0316 0.0823 0.1065 0.1368 0.1450 0.1770 0.4034 0.5733 0.7152 0.7715 0.8160 0.8180 0.8386 0.8780 0.9167 0.9910 1]; ye_e=[0 0.0486 0.0752 0.1442 0.2429 0.3985 0.4513 0.4812 0.4805 0.5095 0.6120 0.6849 0.7607 0.7961 0.8246 0.8322 0.8450 0.8789 0.9117 0.9892 1]; %uses the function polyfit to find the coeffesients for the polynom fitting the data= eq. line %uses polyval to find the polynom x_poly = 0:0.001:1; n=10; p = polyfit(xe_e,ye_e,n); f = polyval(p,x_poly); %Our polynom, with the coeffesients p g =@(x,y)(p(1)*x^10+p(2)*x^9+p(3)*x^8+p(4)*x^7+p(5)*x^6+p(6)*x^5+p(7)*x^4+p(8)*x^3+p(9)*x^2+p(10)*x^1 - y); %Plotting the curves %The eq. line plot(x_poly,f,'r'); hold on; % operating line set(line([0 1],[0 1]),'Color',[0 1 0]); % distillate, xd, and bottom, xb, mole fractions are defined from % experimental data xd=0.744; xb=0.033; %The figure figure(1); hold on; axis([0 1 0 1]); %Labeling the axis xlabel('x ethanol'); ylabel('y ethanol'); %Drawing the steps i=1; xp(1)=xd; %start x-value yp(1)=xd; % start y-value y=xd; while (xp(i)>xb), %gÂr xp(bare x verdien som varierer)helt til vi kommer til xb(nederst pÂ eq.line) xp(i+1)=fzero(g,0.5,[],y); set(line([xp(i) xp(i+1)],[yp(i) yp(i)]),'Color',[0 0 1]); yp(i+1)=xp(i+1); y=yp(i+1); if (xp(i+1)>xb) set(line([xp(i+1) xp(i+1)],[yp(i) yp(i+1)]),'Color',[0 0 1]); end i=i+1; end % Displaying the (integer) number of necessary steps. disp('Number of steps:'); disp(i-1); hold off;

C Calculations

Physical data used in the calculations is found in SI[2], unless other is stated.

C.1 Water-ethanol mixture

The mole fraction of ethanol, xet, is given by

xet =net

net + nw(C.1)

where net is the number of moles of ethanol and nw is the number of moles of water.

Moles of ethanol, net, is given by

net =Vet · ρetMet

(C.2)

where Vet is the volume of ethanol and Met is the molar mass of ethanol.

Moles of water, nw, is given by

nw =(Vtot − Vet) · ρw

Mw

(C.3)

where Vtot is the total volume, ρw is the density of water and Mw is molar mass ofwater.

Substituting Eq. C.2 and Eq. C.3 into Eq. C.1 gives the amount of pure ethanol

xet =

Vet·ρetMet

Vet·ρetMet

+ (Vtot−Vet)·ρwMw

(C.4)

where ρet is the density of ethanol. With the values xet = 0, 12, ρet = 791 gm3 ,

21

Met = 46, 07 gmol

, ρw = 998 gm3 , Mw = 18, 02 g

mol, Vtot = 5800ml and by using Wol-

framalpha we get

Vet=1772 ml

The amount of ethanol needed when we use ethanol with 96% purity, Vet96%, isgiven by

Vet96% =Vet0, 96

= 1846ml (C.5)

Water, Vw, needed in the mixture is given by

Vw = Vtot − Vet96% = 3954ml (C.6)

C.2 Calculating the mole fraction of ethanol

All calculations for the distillate is done with values from Table 4 and calculationsfrom the bottom of the column is done with values from Table 9. Both tables arein Appendix C.4.

Calculating number of moles from volume percent, %volet, ethanol by

net =%volet · V · ρet

Met

(C.7)

Number of moles of water is given by

nw =(1−%volet) · V · ρw

Mw

(C.8)

Then the mole fraction of ethanol, xet is given by

xet =net

net + nw(C.9)

22

C.3 Calculating vapor velocity

The vapor velocity, v, is given by

v =V̇gasA

=V̇gasπ4·D2

(C.10)

where V̇gas is the volume of the sample in gas phase, A is cross section area ofcolumn and D is the inner diameter of the column.

V̇gas is given by

V̇gas =ngas ·RT

P(C.11)

where R is the universal gas constant, T=79 ◦C is the temperature, P=1 bar isthe pressure and ngas is the total number of moles.

ngas is given byngas = ntot = net + nw (C.12)

C.4 Values

Number of theoretical stages in the column and the reflux at the different powersare listed in Table 3, and there is also referred to which McCabe-Thiele Diagrambelonging to them.Values used in the calculations for the distillate are listed in Table 4, and valuesused in the bottom calculations are listed in Table 9.

23

Table 3: Total number of stages and theoretical stages in the column at the dif-ferent powers.

Reboilerpower

Reflux Theoreticalstages in thecolumn

McCabe-Thiele Di-agram

20% 10 9 Figure 225% - - -28% 6 5 Figure 330% 6 5 Figure 432% 4 3 Figure 535% 5 4 Figure 6

Table 4: Values used in the calculations for the distillate and vapor velocity. Ris the reflux, topget is gram ethanol, Topvol% is the volume percent of ethanol,gvann is gram water, xet is the mole fraction of ethanol, ngas is total number ofmoles, the first V gas is volume gas, the second V gas is volume gas, and vgas isthe vapor velocity. All values are for the distillate.

24

Figure 8: Values used in the calculations for the bottom of the column.Bottom vol% is the volume percent ethanol, Bot mlet is mL ethanol, Bot getis mass ethanol, Bot net is number of moles ethanol, gw is the mass of water, nwis the number of moles of water, xbot, et is the mole farction of ethanol. All valuesare for the bottom of the column.

25

26

D McCabe-Thiele

Figure 9: Hand drawn McCabe-Thiele diagram on mm-paper.

27

E Assignment calculations

E.1 Mole fractions

The volume of the pycnometer, Vi is given by

Vi =Wi,H2O −Wi

ρH2O

(E.1)

where Wi,H2O is the weight of the pycnometer with water, Wi is the weight of theempty pycnometer and ρH2O is density of H2O.The density of a sample, ρsample is given by

ρsample =Wi,sample −Wi

Vi(E.2)

where Wi,sample is the weight of the pycnometer with the sample. V1=4,988 cm3

and V2=4,999 cm3

ρ1 = 0, 8237 g/cm3 gives a weight percent of 87, 8% ethanol [4], and ρ2 = 0, 9847

g/cm3 gives a weight percent of 8, 0% ethanol.The mole fraction of ethanol is given by

xet =netntot

=net

net + nw=

met

Met

met

Met+ mw

Mw

=wet%Met

wet%Met

+ (100−wet%)Mw

(E.3)

where ntot is the total number of moles in the sample, met and mw is the massesof ethanol and water, and wet% is the weight percent of ethanol in the sample.This gives

xet,1 = 0, 738

xet,2 = 0, 033

which is approximately the same result we get from GC.

28

E.2 Vapor velocity

The vapor velocity, v, is given by

v =V̇gasA

=V̇gasπ4·D2

(E.4)

where V̇gas is the volume of the sample in gas phase, A is cross section area ofcolumn and D is the inner diameter of the column.

V̇gas is given by

V̇gas =ngas ·RT

P(E.5)

where R is the universal gas constant, T=105 ◦C is the temperature, P=1 bar isthe pressure and ngas is the total number of moles.

ngas is given by

ngas = ntot =mtot

Mtot

=ρ1 · Vtot

xet ·Met + (1− xet) ·Mw

(E.6)

where mtot is the total mass, Mtot is the total molar mass and ρ1 is the den-sity of the distillate.

By solving Eq. E.4-E.6 with the values D=0,05 m, Vtot=0,883 ml, and valuesfor ρ1, xet and the molar masses from section E.1 we find

v=0,232 m/s

E.3 McCabe-Thiele

Values for x ethanol and y ethanol is found in Gmelin [5]

29


Number of stages: 5

30

Report Distillation columns Sieve tray column - NTNUfolk.ntnu.no/andersty/3. Klasse/Felleslab (SepTek og RekTek)/SepTek... · Report Distillation columns Sieve tray column Lene Tapio

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