research article microbubble distillation for ethanol...

Research ArticleMicrobubble Distillation for Ethanol-Water Separation

Atheer Al-yaqoobi,1 David Hogg,2 and William B. Zimmerman1

1Department of Chemical and Biological Engineering, University of Sheffield, Mappin Street, Sheffield S1 3JD, UK2Rejuvetech Ltd., The Sheffield Bio-Incubator, 40 Leavygreave Road, Sheffield S3 7RD, UK

Correspondence should be addressed to Atheer Al-yaqoobi; [email protected]

Received 1 January 2016; Accepted 14 March 2016

Academic Editor: JunwuWang

Copyright © 2016 Atheer Al-yaqoobi et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

In the current study, a novel approach for separating ethanol-watermixture bymicrobubble distillation technologywas investigated.Traditional distillation processes require large amounts of energy to raise the liquid to its boiling point to effect removal of volatilecomponents.The concept of microbubble distillation by comparison is to heat the gas phase rather than the liquid phase to achieveseparation.The removal of ethanol from the thermally sensitive fermentation broths was taken as a case of study. Consequently theresults were then compared with those which could be obtained under equilibrium conditions expected in an “ideal” distillationunit. Microbubble distillation has achieved vapour compositions higher than that which could be obtained under traditionalequilibrium conditions. The separation was achieved at liquid temperature significantly less than the boiling point of the mixture.In addition, it was observed that the separation efficiency of the microbubble distillation could be increased by raising the injectedair temperature, while the temperature of the liquid mixture increased only moderately. The separation efficiency of microbubbledistillation was compared with that of pervaporation for the recovery of bioethanol from the thermally sensitive fermentationbroths. The technology could be controlled to give high separation and energy efficiency. This could contribute to improvingcommercial viability of biofuel production and other coproducts of biorefinery processing.

1. Introduction

Distillation columns are one of the oldest andmost importantpieces of equipment used in the chemical and petrochemicalindustries. In addition to the large capital costs required forthe installation of the plant, the operational costs are alsohigh due to the energy intensive nature of the procedure, inparticular the reboiler and the condenser units requiring thelion’s share of the energy input of the overall process.

The simple distillation unit is a fundamental module ofcomplex multicomponent separations by distillation. Basi-cally, in this process the vapour rising as a result of boilingof the liquid in the still is richer in more volatile componentthan the residual liquid. The composition of vapour leavingthe liquid phase is in thermodynamic equilibrium with theliquid phase [1]. The vapour composition is determined bynet effect of combination temperature, pressure, and liquidcomposition [2].

For design purposes, thermodynamic equilibrium isassumed to be established between the vapour and liquid

leaving each stage. Practically, in actual operation, bothphases leave the stage without achieving equilibrium [3].Usually, stage efficiency is used to estimate the real vapourcomposition that leaves the tray which is typically betweenone-third and one-half of the theoretical change possible thatthe equilibrium composition could achieve. The deviationof the tray efficiency from the theoretical stage can becompensated through an increase in column height andreflux ratio. To achieve this improvement, additional capitaland operating cost is required which includes provision forlarger heat exchangers and pumps, leading to a significantincrease in energy demand.

Distillation is commonly used in the biorefinery process-ing of bioethanol production. This adds a significant energyload on the production process. The estimation showed thatmore than half of the total production energy is consumed bydistillation [4]. Consequently, tomake bioethanol productionprocessing commercially competitive with the fossil fuelindustry, reducing the production costs is of the utmostimportance. In addition, accumulation of bioethanol in

Hindawi Publishing CorporationInternational Journal of Chemical EngineeringVolume 2016, Article ID 5210865, 10 pageshttp://dx.doi.org/10.1155/2016/5210865

2 International Journal of Chemical Engineering

the fermentation bioreactor inhibits the activity of microor-ganisms. An ethanol concentration of approximately 63 g/Lcan cause damage to themembrane of an organism’s cell wall.At higher concentrations of around 95 g/L, the cell growth ofyeast is inhibited by ethanol [5]. One of the ways to overcomethis problem is to remove the products from the fermentationbioreactor by a continuous separation unit, where the ethanolis extracted from the broth during the fermentation process[6]. That method was found to be effective in preventingthe inhibition of microorganisms and increases ethanolproductivity markedly [4]. The microorganisms used infermentation processes are thermally sensitive; consequently,the overheating of the fermentation broth media in theseparation unit used in above-mentioned scenario should beconsidered [7].

Recently, the evaporation dynamics of hot microbubblesin air-water systemshave been studied [8].The computationalmodel presents the concept of nonequilibrium evaporationby heating the gas phase rather than the liquid phase. Intraditional distillation the liquid is boiled to achieve themaxi-mum equilibrium vapour pressure.The computationalmodelhypothesis takes into consideration the internal gas dynamicsof the bubble. The model predicts the impact of microbubblefeatures on the mechanisms of sensible heat transfer andevaporation.Themodel shows that, by controlling the bubbleresidence time in the liquid phase,maximumevaporation canbe achieved. The maximum absolute humidity is obtaineda short time after bubble detachment. Thereafter, if thebubble was still hotter than the surrounding liquid, thecontact between the bubble and the liquid phase promotessensible heat transfer from the bubble to the liquid. Thissensible heat transfer cools the bubble and is accompaniedby recondensation of vapour back into the liquid phase.

The microbubble humidification-dehumidification mod-elling and experimental work showed a potentially novelapproach for applying the technology to the separation ofliquid mixtures.The use of microbubble distillation was usedin the separation of binary mixtures effectively as a firststep towards using the technology in potentially complexmixtures.

The aim of the current work was to investigate theperformance of microbubble distillation of a binary mixture,with the exemplar of ethanol-water separation. A secondobjective was to compare the efficiency of a microbubbledistillation unit with the simple distillation unit as a potentialreplacement for traditional distillation. Microbubble distil-lation was expected to achieve separation higher than thatachieved by traditional distillation due to the nonequilibriumconditions imposed by injecting hot microbubbles into acolder liquid. Consequently, the vapour-liquid compositionof microbubble distillation was compared with the vapour-liquid composition achieved under equilibrium conditions,since the traditional distillation was based on achievingthe equilibrium operating conditions. In addition, the effectof air temperature on the stripping of ethanol from thewater/ethanol mixtures and the performance of microbubbledistillation was also investigated. The study highlighted thepossible feasibility of microbubble distillation technologyfor the recovery of ethanol from fermentation broths. All

(1) Separation tank(2) Flowmeter(3) Fluidic oscillator(4) Air process heater

(5) Thermometer(6) Condenser(7) Bleeding valve

62

73 4

5

1

T

T

Figure 1: Schematic diagram of the experimental rig setup.

experiments were conducted with the use of energy efficientfluidic oscillation to produce microbubbles.

This paper is organised as follows: Section 2 is dedicatedto the experimental methodology, the experiment setup,equipment andmaterials used in the experimental procedure,and the analysismethod. Results anddiscussion are presentedin Section 3. Finally, Section 4 summarises conclusions.

2. Experimental Work

2.1. Experimental Apparatus and Material. The microbubbledistillation experiments were performed using a semibatchsystem. Liquid mixtures were charged into the separationtank initially, but there was a continuous flow of gas throughthe liquid via microbubble injection. The off-gases werecollected by use of a suitable condenser. The schematicdiagram of the experimental setup is shown in Figure 1.The separation tank was made of glass fixed on a 145 ×80mm stainless steel base. The base was provided with amicroporous diffuser of 60mm × 125mm dimensions and5mm thicknessmanufactured by Point Four Systems Inc.Theair was heated by using a heatermade by RSComponents Ltd.with power of 750W.The air streamwas heated and fed to thefluidic oscillator and then to the separation tank.The desiredflow ratewasmeasured from the tank outlet and controlled bythe use of bleeding valves, which vented the excess air fromthe outlet of the oscillator.Theoff-gaswas connected to a glasscondenser of 85mmdiameter and 350mmheight as shown inFigure 2. Conical appendages weremade on the inner surfaceof the condenser to increase the contact with the availablesurface area of the condenser. The vapour entered from thetop of the condenser and split towards the condenser wallsby using a Y-joint splitter. Appendages formed in this wayguaranteed that the drops of liquid condensed on the surfaceand then drained to the bottom of the condenser, wherethe condensed liquid was collected. The off-gases whichwere permanent gases exited from the top of the condenser.The sample of the condensed liquid was collected from thebottom of the condenser. An ice and salt mixture was used as

International Journal of Chemical Engineering 3

Air outlet

Vapour inlet

Sampling port

Figure 2: Schematic diagram of the condenser.

a coolant to maintain the condenser at −19∘C to −20∘C. Thesystem was insulated by using glass wool thermal insulation.

The K-type (Ni Cr+/Ni Al−) thermocouples were used tomeasure the temperature in the system. The entire pipeworkwas made from steel in order to bear the high workingtemperatures. The fluidic oscillator was constructed fromaluminium.The binary mixture used in the experiments wasdistilled water and ethanol (HPLC grade) with purity of≥99.8% provided by Sigma-Aldrich, UK.

The initial volume of the liquid in the tank was preparedaccording to the required liquid level above the diffuser. Theapproximate volume was calculated as follows:

𝑉required = Area of the tank base ∗ 𝐻, (1)

where (𝐻) is the required liquid level.The desired liquid mixture was prepared and mixed well

in a cold water bath to maintain the liquid temperaturearound 20∘C.The air was supplied to the heater at a flow rateof 80 litres/min. The liquid mixture of ethanol and water waspoured into the tank, after the desired air temperature wasreached.

Samples of the liquid from the condenser and residueliquid were taken periodically. The temperature reading wastaken every ten minutes.

2.2. Bubble Measuring. The bubble sizing was conducted viaa Spraytec system (Malvern Instrument, UK). The methodwas based on the diffraction in the laser beam to infer bubblesizes of equivalent spheres. The Spraytec system has twoarms where the apparatus was placed between the arms. Thelaser beam emitted from the light source was fixed on oneof the arms which then passed through the apparatus andwas then received by series of detectors fixed on other arms.The laser beam was scattered by the bubble cloud duringits path through the apparatus; consequently, the scatteredlight is focused by lens onto the detector. Thereafter, the data

10 20 30 40 50 60 70 80 900Time (min)

05

1015202530354045

Liqu

id co

mpo

sitio

n (w

t.%)

Figure 3:The change in liquid composition at initial liquid compo-sition of 40% and air temperature of 150∘C.

from the scattering laser beam was processed by the Spraytecsoftware to calculate the size distribution of bubbles via aninverse method for consistency with Fraunhofer diffractionpattern.

2.3. Analysis. All the samples were analyzed by a BrukerALPHA FTIR-ATR spectrometer. The resolution was set to4 cm−1 and the spectrum was taken with a scan region from350 to 4500 cm−1. OPUS version 6 software was used for thesample spectrum analysis. The calibration curve was madebased on the ethanol peak that appeared at 1045 cm−1. Eachsample was repeated three times.

3. Results

The previous study of hot microbubble humidificationshowed that the liquid level had a significant effect on thevapour-liquid composition in microbubble distillation. Thelower liquid levels showed a higher vapour composition thanthat achieved with higher liquid level under the same experi-mental conditions [8, 10]. Accordingly, this set of experimentswas conducted at a liquid level of 3.5mm to study the vapour-liquid composition obtained from microbubble distillation.Bearing this inmind, the liquid level was estimated accordingto the volume of liquid present in the tank. The injected airtemperature was maintained at 150∘C and four liquid mix-tures of 10%, 25%, 40%, and 60% were chosen to investigatethe vapour-liquid composition obtained with microbubbledistillation.

Figure 3 shows the change in liquid composition duringthe bubbling process at an initial liquid concentration of 40%.The liquid composition reduced sharply with time due to thehigh evaporation of ethanol at the low liquid level.

The vapour composition which is represented by theliquid accumulated in condenser during the experimentaltime is shown in Figure 4. The results indicated that thevapour composition was influenced by the reduction in theliquid concentration. The first fraction that accumulated inthe condenser collected after 20min was 81.7%; concurrently,the liquid composition decreased to approximately 32%.


76

77

78

79

80

81

82

Liqu

id co

mpo

sitio

n (w

t.%)

10 20 30 40 50 60 70 80 900Time (min)

Figure 4: The concentration of the liquid accumulated in thecondenser with time at liquid level.

Subsequently, the vapour phase composition decreased as theliquid phase composition decreased.

However, to study the vapour-liquid composition ofmicrobubble distillation at different liquid concentrations,the first fraction that accumulated in the condenser wasconsidered as the vapour composition at the initial liq-uid composition, even though the liquid composition wasreduced to a lower value during the sampling time.

Figure 5 shows the vapour-liquid composition ofmicrobubble distillation at different liquid concentrations.The compositions of the vapour phase obtained frommicrobubble distillation were 57.8%, 75.5%, 81.7%, and84.9% for initial liquid composition of 10%, 25%, 40%, and60%, respectively. The average liquid temperature at allliquid compositions for most of the experimental time wasaround 36–38∘C as shown in Figure 6. However, the liquidtemperature profile showed that the liquid temperatureincreased as the liquid mixture concentration decreased. Theaverage liquid temperature at a liquid concentration of 10%was around 38.5∘C, and the average liquid temperature at aliquid concentration of 60% was around 37∘C.

The mechanism of microbubble distillation is proposedby a computational model developed by [8]. Later on theconcept was tested with a binary system of ethanol and water[10]. After the injection of hotmicrobubbles in the cold liquidmixture, the components flash to a maximum concentrationwhich is proportional to the bubble temperature. An addi-tional nonequilibrium driving force came from the fact thatthe bubble injected was dry which provided additional masstransfer impetus for ethanol to join the vapour phase. Themodel also showed that the microbubble had an isothermaltemperature profile inside the bubble. The microbubble hadvigorous internal mixing due to the high surface area tovolume ratio, which enhanced the ability for ethanol to reachthe maximum concentration in such a short time.

The microbubble generated by the fluidic oscillator hasa smaller heat transfer coefficient in comparison with biggerbubble. This is due to the laminar pattern of the microbubblethrough the liquid which gently detached from the pores ofthe diffuser [8, 11–13]. This advantage gives the preferencefor evaporation to dominate over the sensible heat transfer

10 20 30 40 50 60 700Liquid composition (wt.%)

50

55

60

65

70

75

80

85

90

Vapo

ur co

mpo

sitio

n (w

t.%)

Figure 5: The vapour-liquid composition of microbubble distilla-tion with different liquid composition at air temperature of 150∘C.

10%25%

40%60%

20 40 60 800Time (min)

26

28

30

32

34

36

38

40

Liqu

id te

mpe

ratu

re (∘

C)

Figure 6: Liquid temperature profile with different liquid concen-tration at air temperature of 150∘C.

during the bubble rising through liquid until the maximumconcentration is achieved. Furthermore, Zimmerman et al.[12] pointed out that themicrobubbles cloud has high surfacearea to volume ratio which resulted in high interfacial area.This results in a high mass transfer flux, which is improvedby a high interfacial area.

It is known that a very large amount of energy is requiredto raise the temperature of the mixture to the boiling pointin a traditional distillation unit, which makes the operationa very energy intensive technology. On the other hand, theessential advantage of the microbubble distillation is that theseparation was achieved at a temperature significantly belowthe boiling point of themixture.The comparison between thetraditional distillation process and the microbubble distilla-tion technology was based upon the differences between thethermal and separation efficiencies. The liquid temperatureand the composition of vapour phase obtained in bothmethods are highlighted in the following section.


The boiling point of the liquid mixture can be estimatedin terms of Raoult’s law and Dalton’s law as follows:

𝑃 = 𝑃∗

eth ⋅ 𝑥1 ⋅ 𝛾1 + 𝑃∗

𝑤

⋅ 𝑥2⋅ 𝛾2, (2)

where𝑃 is the total pressure, 𝛾𝑖is the activity coefficient of the

component, and𝑃∗ is the vapour pressure of pure ethanol andwater estimated from the following Antoine equations:

ln𝑃∗𝑤

= 74.475 −

7164.3

𝑇

− 7.327 ln (𝑇) + 3.13 ∗ 10−6

∗ 𝑇2

,

ln𝑃∗eth = 72.55 −7206.7

𝑇

− 7.1385 ln (𝑇) + 4.05 ∗ 10−6

∗ 𝑇2

.

(3)

The activity coefficient of each component can be estimatedaccording to the NRTL model:

ln (𝛾1)

= 𝑥2

2

[𝜏21⋅ (

𝐺21

𝑥1+ 𝑥2

)

2

+ 𝜏12⋅

𝐺12

(𝑥2+ 𝑥1⋅ 𝐺12)2

] ,

ln (𝛾2)

= 𝑥2

1

[𝜏12⋅ (

𝐺12

𝑥2+ 𝑥1

)

2

+ 𝜏21⋅

𝐺21

(𝑥1+ 𝑥2⋅ 𝐺21)2

] ,

(4)

where

𝐺𝑖𝑗= exp (−0.3 ⋅ 𝜏

𝑖𝑗) , 𝜏

𝑖𝑗=

𝐴𝑖𝑗

𝑅𝑇

. (5)

The binary interaction parameters of ethanol and water at atotal pressure of 101.3 KPa are [14]

𝐴12= −633,

𝐴21= 5823.1.

(6)

The boiling temperature of the liquid mixture was calculatedby solving the nonlinear equation result from substitutionequations (3) and (4) in (2).

The equilibrium vapour composition for the liquid mix-ture is estimated according to modified Raoult’s law:

𝑦𝑖=

𝑃∗

𝑖

⋅ 𝛾𝑖⋅ 𝑥𝑖

𝑃

. (7)

Figure 7 shows the estimated boiling temperature andvapour composition according to the equilibrium conditions.The 𝑇-𝑥𝑦 diagram shows that the boiling point of liquidmixture is varied with the mixture composition.The estimateboiling point of the mixture of 10% is 91.6∘C and the boilingpoint of the mixture declines with increasing of the ethanolpercentage in the mixture, where the boiling point decreasedto 82.4∘C at liquid composition of 60%.However, in compari-sonwithmicrobubble distillation, a high vapour composition

Liquid phase compositionVapour phase composition

0.2 0.4 0.6 0.8 10x, y (wt.%)

80

82

84

86

88

90

92

94

Tem

pera

ture

(∘C)

Figure 7: 𝑇-𝑥𝑦 diagram for the ethanol and water mixture atequilibrium condition.

is achieved at a liquid temperature significantly below theboiling point ofmixture, where the liquid temperature rangedfrom 36 to 38∘C as shown in Figure 6. It is worth mentioningthat the presence of a noncondensable gas in the gas streamallows the evaporation of the mixture components below itsboiling point.

The estimated equilibrium vapour composition and thevapour composition achieved with microbubble distillationat different liquid compositions are presented in Figure 8. Asis evident from Figures 7 and 8 vapour composition obtainedfrom microbubble distillation is higher than vapour com-position obtained from boiling the mixture at equilibriumconditions. The separation of higher purity of ethanol can beachieved by microbubble distillation processing compared toa traditional distillation stage which is limited at best to theequilibrium composition. Bear in mind that the equilibriumcomposition was never achieved in the conventional distilla-tion unit, where the efficiency fraction is less than unity.

3.1. Effect of Air Stream Temperature. The studies [9, 15, 16]on the isothermal vapour-liquid equilibrium compositionof ethanol and water mixtures at different temperaturesshowed that increasing the liquid temperature leads to lowerconcentrations of ethanol in the vapour phase, as shown inFigure 9.

However, it was demonstrated that, by increasing theair temperature in microbubble distillation, different trendswere obtained from that of equilibrium operation.The resultspresented for the effect of air temperature on vapour-liquidcomposition achieved by microbubble distillation showedthat by an increase of the air temperature the vapour com-position increased as the liquid composition decreased.

The effect of air temperature on the microbubble distilla-tion was studied at air temperatures of 120∘C and 150∘C forliquid compositions of 25% and 40% and a liquid level of3.5mm. Figure 10 shows the composition of the vapour phaseat different air temperatures with initial liquid concentrations


Vapour-liquid at equilibriumVapour-liquid from microbubble distillation

48

53

58

63

68

73

78

83

88

Vapo

ur co

ncen

trat

ion

(y) (

%)

10 20 30 40 50 60 700Liquid concentration (x) (%)

Figure 8: Comparison between the vapour-liquid compositionsat equilibrium and the vapour-liquid composition result from themicrobubble distillation at air temperature of 150∘C (current work).

30∘C90∘C

0.2 0.4 0.6 0.8 10x (mol%)

00.10.20.30.40.50.60.70.80.9

1

y (m

ol%

)

Figure 9: Vapour-liquid equilibrium composition at temperature of30∘C and 90∘C [9].

of 40% and 25%. The vapour phase composition increasedfrom 78.8% at an air temperature of 120∘C to around 81.7%at an air temperature of 150∘C with a liquid concentrationof 40%. Likewise, the vapour phase increased from 70.5%to 75.5% when the air temperature increased from 120∘C to150∘C at a liquid concentration of 25%.

Accordingly, the residual liquid concentration showsobvious effects of air temperature on the separation per-formance as shown in Figures 11 and 12. With a liquidconcentration of 25%, the liquid concentration dropped toaround 6.5% at an air temperature of 150∘C, whilst at an airtemperature of 120∘C the final liquid concentrationwas 11.8%.Similar behaviour was observed with a liquid concentrationof 40% with a different air temperature. The final liquid

40 25

120∘C150∘C

(%)

65

67

69

71

73

75

77

79

81

83

Figure 10: The vapour phase composition with air temperature of120∘C and 150∘C at liquid composition of 40% and 25%.

T = 120∘CT = 150∘C

20 40 60 80 1000Time (min)

0

5

10

15

20

25

30

Liqu

id p

hase

conc

entr

atio

n (w

t.%)

Figure 11: Liquid level profile with liquid concentration of 25% atair temperature of 120∘C and 150∘C.

concentration decreased from 22% to 8.5% by increasing thetemperature from 120∘C to 150∘C.

The temperature of the liquid mixture increased as theair temperature increased in both liquid mixtures as inFigure 13. Although the trend of the liquid temperature wasthe same for both liquid concentrations, the increase in liquidtemperature at a lower liquid concentration of 25% is higher.The sources of heat transfer to the liquid phase came from thebubbles and the gas distribution, assuming the heat loss to theperiphery was low due to efficient insulation [17]. The resultshowed that the vapour concentration increased as the liquidconcentration increased. Consequently, the required latentheat increased to satisfy the maximum vapour concentrationrequired, where the latent heat was drawn from the systementhalpy and resulted in a decrease in the system temperature.


05

1015202530354045

Liqu

id co

ncen

trat

ion

(wt.%

)

20 40 60 80 1000Time (min)

T = 120∘CT = 150∘C

Figure 12: Liquid concentration profile with liquid concentration of40% at air temperature of 120∘C and 150∘C.

23

25

27

29

31

33

35

37

39

41

Liqu

id te

mpe

ratu

re (∘

C)

20 40 60 80 1000Time (min)

T = 120∘C (25%)T = 150∘C (25%)

T = 120∘C (40%)T = 150∘C (40%)

Figure 13: Liquid temperature profile for liquid concentration of25% and 40% at air temperature of 120∘C and 150∘C.

The reason for the increase of the vapour composi-tion with the increasing of air temperature is due to thenonequilibrium driving force in microbubble distillation.The entropy of ethanol vapour is 282.7 J⋅mol−1⋅K−1, and theentropy of the water vapour is 188.8 J⋅mol−1⋅K−1 at 298.15 K.Therefore, for the same quanta of enthalpy, the evaporationof one mole of ethanol achieves a higher increase in entropythan for the equivalent molar quantity of water. Maximumentropy is achieved by the selective evaporation of ethanol.Consequently, the vapour phase enrichment with ethanolin microbubble distillation was higher than the equilibriumproportion of the vapour composition. Similarly, increasingthe air temperature yields preferential ethanol evaporationabove the equilibrium proportion as the entropy increaseswith temperature.

This result clearly showed that microbubble distillationcould achieve a higher separation efficiency of ethanolthan with traditional distillation at certain liquid levels and

10

0.8

0.2

0.7

0.3

0.6

0.4

0.5

0.9

0.1

0.45

0.65

0.35

0.75

0.25

0.85

0.15

0.95

0.05

0.55

x (mol%)

00.05

0.10.15

0.20.25

0.30.35

0.40.45

0.50.55

0.60.65

0.70.75

0.80.85

0.90.95

1

y (m

ol%

)

Figure 14:Number of ideal stages equivalent to separation efficiencyof microbubble distillation batch system.

air temperatures. Also the separation efficiency could beincreased by an increase of the air temperature at the sameliquid concentration. Furthermore, the microbubble distil-lation system showed a vapour-liquid composition higherthan the theoretical distillation column stage “ideal plates,”while the single stage of commercial distillation columnhas an efficiency lower than that of the theoretical stage inwhich the vapour and liquid left the stage in thermodynamicequilibrium.

Microbubble distillation can mimic traditional distilla-tion by assuming the distillation column works on a totalreflux operation, and the overhead vapour concentration ofthe distillation column is the last fraction accumulated in thecondenser and the residual liquid concentration is the finalliquid concentration after the end of the experiment period.Consequently, theminimumnumber of theoretical stages canbe estimated for a distillation column equivalent to that forthe microbubble distillation unit.

Figure 14 shows the equivalent number of stages formicrobubble distillation with a liquid concentration of 40%and an air temperature of 150∘C at a liquid level of 3.5mm.The last fraction accumulated in the condenser after 80minwas 76.35% (55.8mol%) and the residual liquid concentrationwas 8.9% (0.03mol%). The distillation unit required a mini-mum of two stages to obtain the separation achieved by themicrobubble distillation. It is worthwhile to mention that anactual distillation column requires a greater number of stagesthan theminimumnumber of the theoretical stages estimatedby total reflux methods.

Although microbubble distillation shows the advantageof improving the separation efficiency over the traditionaldistillation, microbubble distillation is also more energy effi-cient in several respects when compared with traditional dis-tillation. Microbubble distillation requires energy for heatingthe gas phase, while the liquid phase is heated in traditionaldistillation. The quanta of heat required for heating the gasphase are much lower than the quanta of heat required forraising the liquid temperature to the same level. The reasonbehind that is difference in the density and heat capacity ofthe air (𝜌cp) to that of the liquid (ethanol and water mixture


0

2

4

6

8

10

12

14

Volu

me (

%)

11.5

–13.

5

135–

155

155–

185

255–

295

215–

255

185–

215

115–

135

100–

115

295–

350

13.5

–16

16–1

8.5

16–2

1.5

10–1

1.5

21.5

–25

6.5–

7.5

7.5–

8.5

4.5–

5.5

3.5–

4.5

5.5–

6.5

85–1

00

8.5–

10

45–5

539

–45

55–6

5

29–3

525

–29

75–8

565

–75

35–3

9

Bubble size (𝜇m)

Figure 15: Bubble size distribution size under oscillatory flow at airflow rate of 1.5 L/min.

in current study).The quanta of energy consumed on heatingthe gas phase to a certain temperature will achieve higherseparation if the same energy is used to heat the liquid phase.In addition, the mechanism of microbubble generation by anenergy efficient fluidic oscillator reduced the energy requiredin the process. Reference [12] has reported that the fluidicoscillator has no moving parts and capable of producinga microbubble cloud without need of external energy. Thegeneration of microbubbles by a fluidic oscillator only needsan air blower at an offset pressure that is slightly higher thanthe head of water [18]. By comparison, any other way ofproducingmicrobubbles leads to additional operational coststo the process.

3.2. Bubble Size Analysis. The bubble size distribution formicrobubbles generated by the fluidic oscillator is shown inFigure 15. The bubble size measurement was conducted withflow rate of l.5 L/min and liquid mixture concentration of40%. The result shows a single peak bubble size distribution,indicating monodispersed and noncoalescent bubbles. It wasshown that 50% of the bubbles that existed in the testedvolume had a bubble size of 91 𝜇m and the Sauter meanbubble size was 78𝜇m as listed in Table 1.

The mechanism of generation microbubbles with usingfluidic oscillator is based on releasing the bubble during theearly step of growing. Also, the presence of ethanol in theliquidmixture reduced the surface tension of the liquidwhichenhanced the reduction of bubble size formed [19, 20].

3.3. Comparison with Pervaporation. The results of themicrobubble distillation achieved a high separation efficiencywith a minimum increase of liquid temperature. This fea-ture of the microbubble distillation technology could meetthe pertinacious limitations in the separation of thermallysensitive mixtures. Microbubble distillation of ethanol-watermixtures could be applied for the recovery/production ofbioethanol from fermentation broths. Bioethanol is widelyused in the production of beverages and as an importantsource of renewable fuels.

Table 1: Bubble size analysis.

𝐷[3, 2] 𝐷V(90) 𝐷V(50) 𝐷V(10)78 𝜇m 164𝜇m 91 𝜇m 47 𝜇m

Several techniques have been performed to recoverethanol from the fermentation broths, and these methodstake the thermal sensitivity of the fermentation broths intoconsideration [21]. Gas stripping is an established techniquefor continuous ethanol removal during fermentation pro-cesses. Different gases such as nitrogen and CO

2have been

widely used in a wide variety of processes as they do notadversely affect culture media [5, 22].

A number of studies were carried out in order to test theextent to which pervaporation could serve as a viable methodof recovering ethanol from the fermentation medium [23–25]. The concept of the pervaporation is selective separationof volatile products from the liquid feed mixture by utilizingmembranes. The liquid feed mixture is heated to a certaintemperature to bring the components into a vapour phaseat sufficient vapour pressure. The vapour comes into contactwith the membrane, which allows selective separation of thecomponents. A pressure differential made across the feedside and permeate side results in a flux of vapour containinga higher concentration of the desired component passingthrough to the condenser.

Selectivity is a measure of the ability of the mem-brane to allow the permeation of a specific component. Adimensionless quantity (𝛽) known as the separation factorrepresents the membrane selectivity which is dependent onthe concentration of the desired component in feed andpermeate:

𝛽 = (

𝑥𝑖𝑝

1 − 𝑥𝑖𝑝

)(

1 − 𝑥𝑖𝑓

𝑥𝑖𝑓

) , (8)

where 𝑥𝑖𝑝and 𝑥

𝑖𝑓are the fractions of selective component in

the feed and permeate, respectively.In order to compare between the separation of the ethanol

andwatermixture bymicrobubble distillation and the separa-tion by pervaporation, the dimensionless separation factor isreformatted to fit the microbubble distillation outcomes in away that maintains the concept and the function of the factorwhich indicates the efficiency of the separation:

𝛽𝑚= (

𝑥𝑖𝑐

1 − 𝑥𝑖𝑐

)(

1 − 𝑥𝑖𝑓

𝑥𝑖𝑓

) , (9)

where 𝛽𝑚is the separation factor of the microbubble distilla-

tion and 𝑥𝑖𝑐and 𝑥

𝑖𝑓are the fractions of the ethanol in vapour

phase and in initial concentration of the solution.Figure 16 shows the separation factor for microbubble

distillation as a function of the feed concentration. Theseparation factor for microbubble distillation unit was 12.3 ata liquid concentration of 10% and decreased by increasing theethanol concentration to reach 6.69 at a liquid concentrationof 40%.

The type of the membrane used in the pervaporationplayed an important role on the separation efficiency and


0

2

4

6

8

10

12

14

𝛽m

10 20 30 40 50 60 700Ethanol in feed (%)

Figure 16: Separation factor for microbubble distillation at airtemperature of 150∘C.

the separation factor. The separation factor was around 11for a feed concentration of 10%, and the separation factordecreased by increasing the concentrationwhen different cel-lulose acetate membranes are used in the pervaporation [6].A similar result was shown with silicone rubber membranes;the separation factor was around 8.5 for a feed concentrationof 10% for a PPMS membrane, where the separation factordecreased by increasing the concentration [26]. However, theseparation factor increased to around 60 with an ethanolconcentration of 10% by using a silicate membrane at a 30∘Cfeed temperature [24].

One of the general drawbacks of the pervaporationprocess is the low permeation rate, which is increased bymaximizing the feed concentration and temperature [27].However, fermentation broths are thermally sensitive andthe concentration of the ethanol is relatively low. Anotherdisadvantage of the membrane process is the membranes’thermal and chemical stability [28]. This factor limits thelifetime of the membrane in long term. Although the energyconsumption of pervaporation process is low compared tothat of distillation processing, the energy required to createthe vacuum used in the latter stages of the process may add asignificant energy load.

The recovery of ethanol from the broth by distillationis a complex process and consumed more than half of thetotal energy required to produce the bioethanol by fermen-tation [4]. Overall, microbubble distillation was shown tobe a promising technology to recovery of bioethanol fromfermentation broths. Microbubble distillation showed a highenergy and separation efficiency over traditional distillation.The liquid level and the air temperature could be optimizedto find the best conditions to achieve a higher separation andprevent overheating of the medium.

4. Conclusion

In the current study, microbubble distillation for ethanoland water was investigated. The comparison between themicrobubble distillation and traditional distillation showsthat the microbubble distillation can be controlled to achievehigher separation efficiency. The vapour-liquid composition

results from the microbubble distillation at air temperatureof 150∘C were higher than that of equilibrium vapour-liquid composition. In addition, effect of air temperature wasinvestigated with liquid concentration of 25% and 40% andthe results showed that the efficiency of separation can beincreased by an increase in the air temperature. Althoughthe liquid temperature increased with the increase of the airtemperature, it was significantly less than the boiling pointof the mixture. Furthermore, traditional distillation requiredmore than two ideal stages to equal the separation efficiencyof a batch microbubble distillation unit. The enhancement ofthe separation efficiency inmicrobubble distillation is accom-panied by improvement in energy consumption achievedby the method of generating the microbubble and theconcept of heating the air stream in microbubble distilla-tion. Microbubble distillation can be applied to recovery ofbioethanol and achieve a separation factor of 12.3 at a liquidconcentration of 10%. The pertinent demand for enhancingthe energy and separation efficiency has been achieved withmicrobubble distillation. Finally, microbubble distillation is anew technology that shows a great deal of potential usewithinbioethanol recovery from fermentationmedia and separationof thermally sensitive mixture.

Competing Interests

The authors declare that they have no competing interests.

Acknowledgments

William B. Zimmerman thanks Innovate UK and EPSRC fora grant under the IB Catalyst industrial research scheme,EP/N011511/1. Atheer Al-yaqoobi thanks the Iraqi Ministry ofEducation for a doctoral scholarship.

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