Gas-Liquid Mass Transfer Characteristics in a Gas-Liquid-Solid Bubble Column under Elevated Pressure and Temperature☆

Haibo Jin*,Suohe YangGuangxiang HeDelin LiuZemin TongJianhua Zhu

Fluid Dynamics and Transport Phenomena

Gas-Liquid Mass Transfer Characteristics in a Gas-Liquid-Solid Bubble Column under Elevated Pressure and Temperature☆

Haibo Jin1,*,Suohe Yang1,Guangxiang He1,Delin Liu1,Zemin Tong1,Jianhua Zhu2

1Department of Chemical Engineering,Beijing Institute of Petrochemical Technology,Beijing 102617,China2Faculty of Chemical Science and Engineering,China University of Petroleum,Beijing 102249,China

A R T I C L EI N F O

Article history:

Mass transfer characteristic

Bubble column

Elevated pressure

Elevated temperature

The volumetric mass transfer coeff i cient kLa of gases(H2,CO,CO2)and mass transfer coeff i cient kLon liquid paraff i n side were studied using the dynamic absorption method in slurry bubble column reactors under elevated temperature and elevated pressure.Meanwhile,gas-holdup and gas-liquid interfacial area a were obtained. The effects of temperature,pressure,superf i cial gas velocity and solid concentration on the mass transfer coeff icient were discussed.Experimental results show that the gas-liquid volumetric mass transfer coeff i cient kLa and interfacialareaaincreasedwiththeincreaseofpressure,temperature,andsuperf i cialgasvelocity,anddecreased with the slurry concentration.The mass transfer coeff i cient kLincreased with increasing superf i cial gas velocity and temperature and decreased with higher slurry concentration,while it changed slightly with pressure.According to analysis of experimental data,an empirical correlation is obtained to calculate the values of kLa for H2(CO,CO2)in the gas-paraff i n-quartz system in a bubble column under elevated temperature and elevated pressure.

?2014TheChemicalIndustry andEngineeringSocietyofChina,andChemicalIndustryPress.Allrightsreserved.

1.Introduction

Bubble column reactors(BCR)have been widely used in chemical, petrochemical,coal chemical,chemical-metallurgical industries and environmentengineering,suchasoilfractionhydrogenation, desulphurization and denitrif i cation of coal,oxidization of dimethyl benzene,Fisher-Tropsch(FT)synthesis,synthesis of methanol,synthesis of dimethyl ether,wet oxidization of wastewater containingorganic materials and soon[1,2].They offer many advantages over other multiphase reactors—simple construction,no mechanically moving parts, goodmasstransferproperties,highthermalstability,lowenergysupply and hence low construction and operation costs understood.However, the design and scale-up of reactors for the industrial processes require a lot of basic data,such as reaction kinetics,thermodynamics,hydrodynamics,and characteristics of heat transfer and mass transfer under industrial conditions.

Inrecentyears,alotofresearchworkshavebeencarriedoutin mass transfer characteristics of SBCR(slurry bubble column reactor).Most of the research work focused on mass transfer characteristics in such reactors at atmospheric conditions and with water as liquid phase. There are only few data obtained on volumetric mass transfer coeff icient at high temperature and high pressure.In recent studies,Yang et al.studied the gas-liquid mass transfer behavior of syngas components(H2and CO)in a three-phase bubble column reactor at industrial conditions,and proposed the empirical correlations to predict kLand a values for H2and CO in liquid paraff i n/solid particles slurry bubble columnreactors[3,4].Sehabiagueetal.reportedkLavaluesinalarger-scale SBCR operating under high pressure(0.17-3.0 MPa)and temperature (298-453 K)in Fischer-Tropsch molten wax and showed the impact of operating temperature,superf i cial gas velocity,and solid concentrations and temperatures on these parameters[5].Lemoine et al.described the effects of temperatures and solid concentrations on kLa values under high pressure(0.17-3.0 MPa)in organic liquid mixtures [6].VanduandKrishnastudiedtheeffectsofliquidpropertiesandslurry concentration[25%(by volume)]on kLa in churn-turbulent f l ow regime under atmospheric condition[7].Han and Al-Dahhan showed the effects of pressure(0.1-1.0 MPa)and high superf i cial gas velocity(up to 0.6 m·s?1)on kLa values,but without temperature effect[8].Also Chaumat et al.studied the mass transfer in a bubble column with water and organic media in industrial conditions and the connection with mass transfer and hydrodynamics[9].Schaaf et al.and Chilekar etal.studiedgas-liquidmasstransferina0.15mdiameterslurrybubble columnwithapressurefrom0.1to1.3MPa,andpresentedacorrelation of mass transfer coeff i cient[10,11].Lemoine et al.developed theempirical and neural network correlations for predicting gas holdups,volumetric liquid-side mass transfer coeff i cients,Sauter mean bubble diameters,gas-liquid interfacial areas and liquid-side mass transfer coeff i cients for total,small and large gas bubbles in BCRs and SBCRs[12].

A series of liquid-phase technologies have been developed rapidly,such as Fisher-Tropsch,methanol to gasoline,liquid phase methanol,and liquid phase di-methyl ether processes.These processes adopt syngas components as reactants to produce liquid fuel.Understanding and improvement of the mass transfer process for syngas components in bubble columns with organic liquid under industrial conditions has important guiding and reference signif i cance.This paper presents the volumetric mass transfer coeff i cient kLa of gases (H2,CO,CO2)and mass transfer coeff i cient kLof liquid paraff i n surface using a dynamic absorption method under elevated temperature and elevated pressure.

2.Experimental

2.1.Apparatus

The bubble column had a 0.1 m internal diameter and a 1.25 m height and was made of stainless steel.The gas distributor is a perforated plate with holes having an inner diameter of 8 mm and an opening fractionof2.56%.Thereactorisheatedbycirculatingheatingoilthrough the wall jacket and the temperature of the reactor is controlled by a temperature control system as shown Fig.1.

The system pressure is controlled by both a pressure regulator and a backpressure regulator.Gas f l ow rate is regulated by a mass fl ow controller.Gas components of the inlet and outlet are analyzed by means of gas chromatograph(H2)and infrared analyzer(CO, CO2).The holdups and the rising velocity for the small bubble swarm and large bubble swarm are determined using the differential pressure method and dynamic gas disengagement(DGD)method[3, 13].

In all the experiments,system temperature varied from 293 to 473 K,systempressurevaried form 1.0to 3.0MPa,super fi cialgasvelocity varied from 0.03 to 0.1 m·s?1and slurry concentration varied from 0 to 20%(by mass).

Fig.1.Schematic diagram of experimental set-up.1—nitrogen bomb;2—compressor; 3—pressureregulator;4—massf l owcontroller;5—preheater;6—reactor;7—condenser; 8—gas—liquid separator;9—back pressure regulator;10—mass f l ow meter;11—gas chromatograph or infrared analyzer;12—computer;13—temperature.

2.2.Experimental conditions

The range of experimental condition is summarized in Table 1 and gas and liquid properties are shown in Tables 2 and 3,respectively.

2.3.Experimental procedure

The liquid phase volumetric mass transfer coef fi cient kLa was determinedbymeansofa dynamicabsorptiontechnique[3,13].Firstthesystem temperature and system pressure were elevated to given values. Then,the slurry was stripped out by an inert gas(N2)until the concentration of H2(or CO,CO2)in the slurry was 0.Then the nitrogen supply wasturned offand the compressed H2/N2is fed simultaneously into the reactor under certain volumetric fl owrate NGat t0=0 s.Themole fraction of H2(CO,CO2)in exit gas yG(ti)is measured at an equal interval with 2 s by gas chromatography.At the same time,the corresponding fl ow rate of the exit gas NG(ti)is recorded until the component of the exit gas is invariable.

Inlet and outlet gas samples for H2were collected and analyzed using a GC14C gas chromatograph(Shimadzu,Shanghai,China)with a thermal conductivity detector,which were quanti fi ed by calibration curves.Gas samples for CO and CO2components were analyzed by means of infrared analyzer GXH-3011(Institute of BeijingHuayun AnalyticalInstrumentCo.,Beijing,China).Thesedataobtainedfromanalysis are weight fraction,which are later converted to mole fraction.

2.4.Experimental data reduction

The overall mass-transfer coeff i cient,KL,is the addition of mass transfer coeff i cients in the gas and liquid sides,kGand kL,respectively, and Henry's constant,H,as follows:

For gas components(H2,CO etc.)having low solubility in the liquid phase(H2O,hydrocarbon),the gas-side mass transfer resistance is negligible.So,the liquid phase mass transfer coeff i cient determines overall mass transfer characteristics in a gas-liquid-solid bubble reactor.

According to the mass conservation principle and the results obtained by GC,the kLa can be estimated from the slope of the curve of?ln[1?CG(t)/C*G]versus time as in Refs.[3,13].

Thebubbleswarmconsistsofboth‘small’and‘large’bubblesoperated in the churn-turbulent regime in bubble column reactors[14],the small bubbles are in thesizerange of 3 to6 mm andare either spherical orellipsoidalinshapedependingoftheliquidproperties.Thelargebubbles are typically in the range of 20-80 mm and these bubbles undergo frequent coalescence and breakup[15].Gas-holdup and bubble rising velocity were measured by differential pressure-sampling technology [16,17].The large and small bubble gas-holdup and rising velocity were measured and calculated respectively.

Table 1Experimental conditions

Since the gas-liquid interfacial area a was closely related with bubblesize,bubblesizecanbeestimatedfromtheknowledgeoftheterminal rise velocity by using an appropriate correlation.The bubble formation and bubble characteristics are reviewed in Ref.[18].Several correlations are selected to calculate the relationship between terminal rise velocity and bubble size(Table 4).

Therefore,we used the correlations proposed by Mendelson[19] andJamialahmadi[20]forsmallbubbles.Thesegeneralizedcorrelations can be used with a wide variety of liquids and are based on experimental measurements with bubbles in several liquids.The correlations proposed by Mendelson[19],Lehrer[21]and Talaia[22]were used to estimate large bubble size.These correlations are based on extensive dataforbubblesdispersedinliquidsandarevalidforbubblesizesgreater than 3 mm.So,the bubble size can be estimated from correlations in Table 4 based on the rise velocity obtained from DGD.It is important to use the bubble size obtained from this procedure with some caution.In some instances,correlations are unable to predict bubble size from known values of rise velocities.Therefore,we have a mean value from data obtained by these correlations as shown in Table 5.

Table 2Physical property of gas

Table 3Physical property of paraff i n

Table 4Correlations for estimating bubble size from bubble rise velocity

The bubbles'Sauter diameter dScan be calculated with Eq.(2)[17]:

The gas-liquid interfacial area a and transfer coeff i cient kLare then given by

3.Results and Discussion

Table 5Results from different correlations(T=298 K)

3.1.Inf l uence of pressure

Fig.2 shows the variation of(a)volumetric mass transfer coeff i cient kLa,(b)gas-liquid interfacial area a,and(c)mass transfer coeff i cient kLwithsystempressurePinH2-paraff i n,CO-paraff i nandCO2-paraff i nsystems.The higher gas holdup at elevated pressures is partially due to the change in liquid properties.The gas solubility in the liquid phase increases with pressure,which results in decreasing liquid viscosity and surface tension[3,23].On the one hand,higher gas density at elevated pressures also enhances bubble breakup and suppresses bubble coalescence,which further promotes the formation of smaller bubbles.Meanwhile,the small bubble size is the major contributor to the increase in gas holdup in the churn-turbulent regime,and in turn,a factor in the dramatic increase in the interfacial mass-transfer area[24].On the other hand,the rising velocity of the large bubble swarm decreases and the rising velocity of the small bubble swarm increases slightly with an increase of liquid viscosity[16],and consequently a relatively short contact time.Since the mass transfer coeff i cient is inversely proportionaltothesquarerootofthecontacttimeaccordingtotheHigbie's penetration theory[25],kLwill decrease.Moreover,the decrease of liquid viscosity will increase the mass transfer coeff i cient,because kLis inversely proportional to the liquid viscosity[3,26,27].So,the overall effect of pressure leads to the slight variation of kLvalues with the increase of pressure.

3.2.Inf l uence of temperature

Many researches had studied the effect of liquid properties(surface tension,viscosity)on kLa[7,28,29].kLa increased with an increase in temperature from Fig.3(a).Since the gas diffuse coeff i cient in liquid andliquidpropertieswerestronglyaffectedbythesystemtemperature, the effect of temperature on mass transfer coeff i cient was an integrated result of different factors mentioned above.Gas diffuse coeff i cient increased with increasing temperature and resulted in the increase of kLmainly in Fig.3(c).Gas-liquid surface tension and liquid viscidity decreased with temperature.On theonehand,thereduction of liquid surface tension will befavorable for theformationofsmall gas bubbles andfor gas-liquid interfacial area a.Moreover,the increase of operating temperature led to the increase of the gas holdup.On the other hand, the decrease of liquid viscidity force will result in a slight increase of bubble rising velocity and consequently a relatively short contact time. In overall,gas-liquid interfacial area a increased with system temperature as shown in Fig.3(b).

Fig.2.Inf l uence of pressure on interfacial area and mass transfer coeff i cient.

Fig.3.Inf l uence of temperature on interfacial area and mass transfer coeff i cient.

3.3.In fl uence of super fi cial velocity

The effect of the super fi cial gas velocity on the kLa,a and kLvalues at 1.0MPa can be seen in Fig.4.It is well known that gas-holdupincreased withgasvelocity,andsmallbubbleswarmincreasedinchurn-turbulent fl owregime,sodoesthegas-liquidinterfacial area.Theliquidside mass transfer coef fi cient kLincreases as UGincreases in the gas velocity rangeasFig.4(c)showed.Thegasvelocityeffectfoundinthisworkisinagreement with the previous studies[30-32].

Fig.4.Inf l uence of superf i cial velocity on interfacial area and mass transfer coeff i cient.

Fig.5.Inf l uence of slurry concentration on interfacial area and mass transfer coeff i cient.

3.4.Inf l uence of slurry concentration

In this work,quartz sand was used as solid phase.The slurry mass concentrationCSvariedfrom 0%to20%.Fig.5summarizesthemeasured interfacial area and mass transfer data for H2(CO,CO2)-paraff i n slurries in the column.The addition of solid particles caused a signif i cant reductioninthekLa,aandkLvaluesasshowninFig.5.Thestrongdecreasecan be attributed to theincrease in thesize of fast rising large bubbles in the churn-turbulentregimeandthedecreaseofgas-holdupwithhighslurryconcentration[7].On the other hand,with slurry concentration increasing,theapparent liquid viscosity increased,and the surfacerenewal and mobility decrease which results in reduced kL[33].This prevented the gas diffusion into liquid phase and the kLvalues decreased.

3.5.Correlation of kLa

Considering the effects of pressure,temperature,superf i cial velocity and slurry concentration on volumetric mass transfer coeff i cient of different gases in paraff i n systems,an attempt was made to correlate kLa valuesintermsofthephysicalpropertiesofthegas-liquid-solidsystem andtheoperatingvariablesusedandthefollowingempiricalcorrelation was obtained:

with an overall average deviation of±30%in the range of 0≤Cs≤20%, 0.03m·s?1≤UG≤0.10m·s?1,15×10?5Pa·s≤μL≤100×10?5Pa·s, and 1.0 MPa≤P≤3.0 MPa.It can be seen that kLa is a function of gas density to the power of 0.524.Dewes and Schumpe[34]studied the effect of gas density(up to 18.8 kg·m?3)on kLa in slurry bubble columns andfoundthata powerof 0.46.Inaddition,Behkishet al.[35]presented the effect of pressure and solid concentration on kLa and found that kLa wasa functionof gas density to the power of 0.49,whichis veryclose to the value obtained in this study.

4.Conclusions

The experimental results show that system pressure,gas velocity and concentration of the solid are major factors in volumetric mass transfer coeff i cient.It increases with an increase in system pressure, temperature and gas velocity,and decreases with an increase in slurry mass concentration up to 20%.According to the experimental data,a correlation for volumetric gas-liquid mass transfer of H2,CO,and CO2inliquidparaff i nisobtained.Basedonthemeasurementsofgasholdups and risingvelocity for small bubble swarm and large bubble swarm,the Sauter bubble diameter(ds)and gas-liquid interfacial area are derived. It is found that system pressure,superf i cial gas velocity and solid concentration affected strongly gas-liquid interfacial areas(a),but mass transfer coeff i cient(kL)is only affected by superf i cial gas velocity,temperature,solid concentration,and slightly by pressure.The hydrodynamic and mass transfer characteristics of bubble columns are mainly controlled by small bubble behaviors.

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1 November 2012

☆Supported by the National Natural Science Foundation of China(20776018),and the Importation and Development of High-Caliber Talents Project of Beijing Municipal Institutions(CIT&TCD20130325).

*Corresponding author.

E-mail address:jinhaibo@bipt.edu.cn(H.Jin).

http://dx.doi.org/10.1016/j.cjche.2014.06.019

1004-9541/?2014 The Chemical Industry and Engineering Society of China,and Chemical Industry Press.All rights reserved.

Received in revised form 2 September 2013

Accepted 20 September 2013

Available online 28 June 2014