Mass transfer characteristics in a rotating packed bed with split packing☆

Youzhi Liu*,Deyin Gu,Chengcheng Xu,Guisheng Qi,Weizhou Jiao

Shanxi Province Key Laboratory of Higee-oriented Chemical Engineering,North University of China,Taiyuan 030051,China

Research Center of Shanxi Province for High Gravity Chemical Engineering and Technology,North University of China,Taiyuan 030051,China

Keywords:Rotating packed bed(RPB)Split packing Mass transfer Physical absorption Chemical absorption

A B S T R A C T The rotating packed bed(RPB)with split packing is a novel gas–liquid contactor,which intensifies the mass transfer processes controlled by gas-side resistance.To assess its efficacy,the mass transfer characteristics with adjacent rings in counter-rotation and co-rotation modes in a split packing RPB were studied experimentally.The physical absorption system NH3–H2O was used for characterizing the gas volumetric mass transfer coefficient(k y a e)and the effective interfacial area(a e)was determined by chemical absorption in the CO2–NaOH system.The variation in k y a e and a e with the operating conditions is also investigated.The experimental results indicated that k y a e and a e for counter-rotation of the adjacent packing rings in the split packing RPB were higher than those for co-rotation,and both counter-rotation and co-rotation of the split packing RPB were superior over conventional RPBs under the similar operating conditions.

1.Introduction

Rotating packed bed(RPB)is a new high-gravity gas–liquid mass transfer equipment,which produces a centrifugal force up to several hundred times the gravity[1].Fig.1 shows the schematic of RPB,in which liquid enters the RPB through the liquid distributor and is splashed onto the inner edge of the rotor packing,and then it moves outwardly by centrifugal acceleration and contacts counter currently with gas,producing a large gas–liquid interfacial area,causing intensive mixing and resulted in higher mass transfer efficiency[2–6].

The characteristics of mass transfer in RPBs have been studied extensively.The liquid-side mass transfer coefficient in RPBs has been declared to be several times higher than that of conventional packed columns[7–12].For example,Ramshaw[7]conducted water-oxygen absorption experiment and found that the liquid-side mass transfer coefficient was 27–44 times higher than that of conventional packed columns.Jiao et al.[8]reported the liquid-side mass transfer coefficient was 1–2 order of magnitude higher than that of conventional packed beds.How ever,Sandilya et al.[13]reported the gas-side mass transfer coefficient in RPBs was in the same range as that of conventional packed columns,because the gas involves a large frictional drag in RPBs by the packing,so that the gas undergoes a solid-body-like rotation in the rotor.Rao et al.[14]reported that there has been no enhancement in the gas-side mass transfer coefficient in RPBs,because the tangential slip velocity(tangential velocity difference between gas and packing)may be negligible.For the purpose of enhancing the tangential slip velocity,namely the gas-side mass transfer coef fi cient,Chandra et al.[15]proposed a novel rotating packed bed with split foam-metal packing in w hich the packing is split into annular rings with gaps betw een them,so that when the alternate rings rotate in the counter-direction mode,a large tangential slip velocity can be achieved.

Many researchers adopted absorption of SO2in aqueous NaOH to study the intensification of gas-side mass transfer in a split packing RPB.For example,Reddy et al.[16]measured the gas volumetric mass transfer coefficients(kyae)using the SO2–NaOH absorption system in a split packing RPB,and the results show that gas volumetric mass transfer coefficients are 2 orders of magnitude higher than those reported with a single packing[13,17].Shivhare et al.[18]determined the gas side mass transfer coefficient(ky)via absorption of SO2in aqueous NaOH in the split-packing and conventional single-block RPB,and the results indicate that split-packing RPB was superior to conventional single-block RPB for gas-side mass transfer resistance controlled processes.These previous researches strongly indicate that the gas mass transfer coefficient can be enhanced in a split packing RPB.How ever,Guo et al.[19]used a physical absorption system NH3–H2O and a chemical absorption system SO2–NaOH to study the mass transfer characteristics in a cross- flow RPB,and the results show that the gas volumetric mass coefficient for SO2absorption is higher than NH3absorption by 70%.The enhancement is possibly due to the effect of chemical reaction,and there may be a chemical acceleration in the mass transfer process.Therefore,more experimental studies on other systems are desired to evaluate the mass transfer and hydrodynamic behavior in the split packing RPB.

Fig.1.The sketch of rotating packed bed.

The effective interfacial area(ae)and the gas volumetric mass transfer coefficient(kyae)are two important parameters for characterizing the mass transfer process.In this work,kyaeand aefor counter-rotation and co-rotation modes in a split packing RPB were studied experimentally.The gas volumetric mass transfer coefficient was characterized by a physical absorption system NH3–H2O,which is a typical gas-side mass transfer resistance controlled system,where the liquid-side mass transfer resistance is negligible.The effective interfacial area was determined in the chemical absorption CO2–NaOH system which is used in many works[20,21].

2.Experimental Setup

Fig.2 is a rotor schematic illustration of a split packing RPB which is made in the laboratory.The rotor of split packing RPB consists of a pair of disks,and three stainless-steel concentric annular rings of different radius are fixed onto each disk.Each annular ring consists of two stainless-steel sheets(2 mm thick each),which sandwich wire gauze packing in between.The diameter of gauze packing wire is 0.4 mm.There are 3 rows of 8 mm holes drilled on each stainless-steel sheet as the fluid pathway,and a small gap(8 or 9 mm)remains between the adjacent concentric rings when the two disks are brought together.The split packing has a specific surface area of 1027 m2·m?3and bed porosity of 0.9,and the axial length of the rings is 30 mm.The two disks can rotate in either co-rotation or counter-rotation mode at the control of a switch.When the two disks rotate in counter-direction,a large tangential slip velocity between the gas and the packing is achieved.Table 1 show s the geometric details of the concentric rings of split packing RPB.

Fig.2.Rotor schematic illustration.

Table 1Details of the packing rings

Fig.3 shows a schematic diagram of experimental setup.Air from a roots blower and CO2from carbon dioxide cylinder respectively flowed into the split packing RPB by pressure difference through different gas flow meters.The aqueous NaOH from liquid storage tank measured by a liquid flow meter flow ed into the split packing RPB by of a liquid pump.The aqueous then splashed onto the inner edge of the packing ring through a liquid distributor,moved outwardly by centrifugal acceleration,and counter current contacted with mixed gas, finally moved out of the RPB after collected by the casing.The absorption of NH3into H2O was conducted similarly in the same operating conditions.The CO2concentration of inlet gas and exit gas was analyzed by CO2analyzer(PGM-54,RAE,USA),the NH3concentration of inlet gas and exit gas was analyzed by NH3analyzer(GT-2000,KEERNUO,China).In this study,the CO2concentration of inlet gas was maintained 15700 mg·m?3and the NH3concentration of inlet gas was maintained 380 mg·m?3.

2.1.Effective interfacial area

In this experiment,the effective interfacial area aewas measured by chemical absorption of CO2in aqueous NaOH.In order to guarantee the absorption of CO2in aqueous NaOH to be a first order fast reaction in the liquid film,the experimental conditions were such that the Hatta numberand the initial concentration of NaOH solution was 1 mol·L?1in all experimental runs.The effective interfacial area was determined from[22]

The effective interfacial area can be obtained through the integration of above equation from the outer radius to the inner radius.

Here,the non-dimensional number,Ha,is the ratio of chemical reaction rate and physical absorption ratein liquid film;kLis the local liquid side mass-transfer coefficient,D is the solute diffusivity;k is the pseudo first-order reaction rate constant;G is the gas flow rate;yinand youtare the CO2concentration of gas at inlet(r=r1)and exit(r=r2);h is the axial length of the packing rings;r1and r2are the inner and outer radius of the packing rings;cA?is the solute concentration at th e liquid interface.The expressions for cA?,D,kLand k are well established in the literature[22].

2.2.Gas volumetric mass transfer coefficient

The gas volumetric mass transfer coefficient,kyae,can be obtained from the physical absorption of NH3into H2O,and the gas volumetric mass transfer coefficient was determined from

w here yinand youtare the NH3concentration of inlet gas and exit gas,respectively.

3.Results and Discussion

3.1.Effects of the gas velocity on ae and kyae

Fig.3.Experimental setup schematic.1—Roots blower,2—CO2 cylinder,3—NH3 cylinder,4,5,6—gas flow meter,7,8—sampling port,9—split packing RPB,10—liquid flowmeter,11—liquid pump,12,13—liquid storage tank.

Fig.4.Variation of measured a e and k y a e with gas velocity.(Liquid velocity=0.02 m·s?1,rotational speed=695 r·min?1).

Fig.4 plots the increase of aeand kyaealong with the gas velocity.The tendency of experimental data is in agreement with Reddy et al.[16]and Rajan et al.[22],aeand kyaefor counter-rotation are larger than those for co-rotation.This phenomenon may be explained by the deduction that with the increasing gas velocity,the radial velocity at the inner periphery of the packing rings increase,it may lead to a stronger disturbance of gas–liquid phases and a better dispersion of the liquid,which result in an increase in kyaeand ae[23].When the two adjacent rings rotate in opposite directions,a larger tangential slip velocity between the gas and packing and a better liquid distribution onto the packing rings are obtained than those in corotation mode,which are beneficial to the mass transfer process[16,22].

3.2.Effects of the rotational speed on ae and kyae

Fig.5 show s that aeand kyaeincrease with the increase of the rotational speed,and aeand kyaefor counter-rotation are larger than those for co-rotation.This may be explained by the fact that with the increasing the rotational speed,the shearing force imposed on the liquid by the packing enhances,causing better liquid distribution onto the packing rings[23].When the two adjacent rings rotate in opposite direction,a larger gas–liquid turbulence and tangential slip velocity are obtained than those of co-rotation[16,22],which lead to enlarge the relative velocity between gas and liquid and the contact area of gas–liquid[20,21],these factors are conducive to the mass transfer process.

3.3.Effects of the liquid velocity on ae and kyae

Fig.6 shows that aeand kyaeincrease with the increase of liquid velocity,and aeand kyaefor counter-rotation are higher than those for co-rotation.The reasons for such changes may be that more tiny liquid droplets,small threads and thin films are produced with the increasing liquid velocity[20,23],and the wetted packing fraction increases,too[22].The combination of these factors is conducive to the mass transfer process.As discussed above,in counter-rotation,a better distribution of liquid on every packing ring and a lager tangential slip velocity between the gas and packing can be obtained compared with those for corotation[18,22],which result in higher aeand kyaefor counterrotation than those for the co-rotation mode.

3.4.Comparison with literature data

Fig.5.Variation of measured a e and k y a e with rotational speed.(Liquid velocity=0.02 m·s?1,gas velocity=2.1 m·s?1).

Fig.6.Variation of measured a e and k y a e with liquid velocity.(Gas velocity=2.1 m·s?1,rotational speed=695 r·min?1).

Fig.7.Comparison of a e,k y a e of present work with literature data.

Fig.7 compares the effective interfacial are a and gas volumetric mass transfer coefficient of conventional RPBs with those of the split packing RPB investigated in this work.The interfacial area for counter-rotation and co-rotation in as plit packing RPB is larger than that of a conventional RPB studied by Munjal et al.[24]which the specific packing area is 1134 m2·m?3.This may be due to that higher gas–liquid turbulence is obtained in a split packing RPB than that of a conventional RPB[18],which leads to a better liquid distribution onto the packing rings than that of a conventional RPB[22].The gas volumetric mass transfer coefficient for counter-rotation and co-rotation in a split packing RPB is larger than that of a conventional RPB studied by Sun[25],probably because a higher gas–liquid turbulence and a larger tangential slip velocity between the gas and packing are obtained in a split packing RPB than those of a conventional RPB[22].