Hydrodynamics and Adsorption Mass Transfer in a Novel GasâLiquid

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Hydrodynamics and Adsorption Mass Transfer in a Novel Gas-Liquid-Solid Circulating Fluidized Bed Adsorber Tan Lin,† Liu Mingyan,*,†,‡ and Hu Zongding† † ‡

School of Chemical Engineering and Technology, Tianjin University, 92 Weijin Road, Nankai District,Tianjin 300072 China State Key Laboratory of Chemical Engineering, 92 Weijin Road, Nankai District, Tianjin 300072 China ABSTRACT: A novel gas-liquid-solid circulating fluidized bed adsorber for the separation of natural products or traditional Chinese medicines from extracts was successfully developed in this work. The hydrodynamics, axial dispersion, and mixing characteristics of phases and the adsorption and desorption of Ginkgo flavonoids in macroporous resin particles with prepared extract were investigated. The results showed that with the increase of downer superficial gas velocity, the deviation degree of flow pattern from plug flow, and the liquid backmixing in the downer were enhanced because the liquid Pe number of the downer became smaller while its axial dispersion coefficient got larger. Similar tends were found for the riser as the riser superficial gas velocity climbed. Compared with that of the downer, the flow in the riser was more close to plug flow. Investigations on the adsorption kinetics of macroporous resin particles showed that the adsorption separation of Ginkgo flavonoids from extract was controlled by the surface film mass transfer and intraparticle diffusion. Experimental results of adsorption separation of Ginkgo flavonoids in the three-phase circulating fluidized bed indicated that the air bubbles introduced in a liquid-solid circulating fluidized bed could enhance the adsorption and desorption processes, and smaller particles, lower superficial feed velocity, and adsorbate concentration were favorable for the three-phase circulating fluidized bed adsorption separation. The adsorption model of the downer and the desorption model of the riser were built, and they predicted the adsorption and desorption processes well.

1. INTRODUCTION Ginkgo flavonoids, the main active components of Ginkgo biloba Linn (G. biloba L.), have functions of antioxidation, reducing blood fat, improving immunity, and antitumor, etc.1-5 The particles of macroporous resin have been utilized in recent years to separate and purify the Ginkgo flavonoids from the extract of Ginkgo leaves due to the high selectivity, large adsorption capacity, and high desorption rate.4-6 The adsorption separation of pharmaceutical products by using the particles of macroporous resin is commonly processed in a fixed bed or a packed bed, a type of liquid-solid contacting device. However, certain solid impurities in the raw extract always block the flow channels between the particles in the fixed beds, resulting in a much higher bed pressure drop. And some feed liquid pretreatment processes such as solid sedimentation and filtration are needed to remove the particulates from the extract. The fluidized bed is another type of liquid-solid contacting device to carry out the operation of adsorption separation.7-9 It can overcome the drawbacks of the fixed beds. But a new problem of using the conventional fluidized bed is the serious back-mixing. Thus, the expanded bed adsorption (EBA) was developed to reduce the back-mixing degree with a relatively small fluidized velocity, wide size distribution, and high density of solid particles.10-14 Li and Chase13 assessed the suitability of the use of macroporous adsorbent Amberlite XAD7HP in expanded bed adsorption processes for the isolation of flavonoids from crude extract of G. biloba L. The expansion and hydrodynamic properties of expanded beds were investigated. Residence time distributions (RTDs) were studied using acetone as a tracer. Three measures of liquid-phase dispersion (the height equivalent of theoretical plate, Bodenstein number, and axial distribution coefficient) were investigated and compared to values previously r 2011 American Chemical Society

obtained with commercial EBA adsorbents developed for protein purification. The results demonstrated that Amberlite XAD7HP should be suitable for expanded bed adsorption of flavonoids from crude extract of G. biloba L. They14 also systematically compared three techniques, including liquid-liquid extraction, packed bed adsorption, and expanded bed adsorption for the purification of flavonoids from the leaves of G. biloba L. For the method using expanded bed adsorption with Amberlite XAD7HP as the adsorbent, unclarified crude extract could be loaded directly into the column, and relative higher recovery yield and shorter average process time to obtain the same amount of product were found. This suggests that the adoption of EBA procedures can greatly simplify the process flow sheet and in addition reduce the cost and time to purify flavonoids from G. biloba L. Even though the expanded beds can achieve a stable classification of macroporous resin particles under low bed pressure drop and back-mixing, the operations of adsorption, washing, elution, and resin regeneration have to be switched frequently.8 The magnetically stabilized fluidized bed can suppress the dispersion and backmixing of particles greatly and has been used as an adsorber. But the magnetic bed needs special magnetic adsorbent particles and is also a kind of batch operation equipment for adsorption separation, which limits its wide applications. To reach a continuous adsorption separation operation, a liquidsolid circulating fluidized bed (LSCFB) is a good choice. Studies on the hydrodynamics in the LSCFB can be found in recent literature.15-17 Ito et al.18 used the LSCFB adsorber to extract uranium from seawater. Lan et al.19-21 introduced a concept of Received: October 18, 2010 Accepted: January 24, 2011 Revised: December 25, 2010 Published: February 21, 2011 3598

dx.doi.org/10.1021/ie102113u | Ind. Eng. Chem. Res. 2011, 50, 3598–3612

Industrial & Engineering Chemistry Research

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Table 1. Physical Properties of AB-8 Macroporous Resin Particles property

property ivory white, opaque particles

average pore size ( 10-9m)

diameter range ( 10 m)

0.3-1.25

porosity (%)

42-46

specific surface area (m2/kg)

480 000-520 000

wet superficial density (kg/m3)

1050-1090

appearance -3

LSCFB ion exchanger system for continuous recovery of biochemical products and studied the effect of operating conditions on the hydrodynamics and developed the successful application for the continuous recovery of bovine serum albumin (BSA). Feng et al.22 described the cesium separation using a continuous ion exchange circulating fluidized bed. Liu23 investigated the flow and masstransfer characteristics of LSCFBs with macroporous resin particles as solid phase and Ginkgo flavoniods extract as liquid phase. Gaikwad et al.24 developed a mathematical model to study the performance of the LSCFB countercurrent adsorber. The model predicted the adsorption zone length and height equivalent to theoretical plates (HETPs) and illustrated the comparative performances of adsorption in a conventional packed bed, expanded bed, and LSCFB in terms of HETP values. The cation exchange resin was used as adsorbent. A 2% (w/v) nickel sulfate (NiSO4) solution was used as a loading solution. A commercial grade hydrochloric acid (2% (v/v)) solution was used for elution of bound nickel ions. At lower velocities such as 0.0035 m/s, HETPs of the LSCFB, packed bed, and expanded bed were 5, 12, and 18 cm, respectively. Very recently, Mazumder et al.25 utilized an experimentally validated mathematical model to study a multiobjective optimization of the LSCFB system for the continuous protein recovery at the design stage, simultaneously considering several objectives such as the production rate and recovery of protein as well as the amount of ion-exchange resin requirements. Elitist nondominated sorting genetic algorithm with its jumping gene adaptation (NSGA-II-aJG) was used to solve a number of two- and three-objective function optimization problems. The performance of the system was further improved at the design stage optimization as changes in physical dimensions of the LSCFB system can provide better performance than would have been possible by adjusting only the operating parameters. Eldyasti et al.26 established a pilot-scale LSCFB to study the reliability and commercial viability of leachate treatment, and anoxic and aerobic columns were used to optimize carbon and nutrient removal capability from leachate. The LSCFB achieved COD, nitrogen, and phosphorus removal efficiencies of 85, 80, and 70%, respectively, at a low carbon-to-nitrogen ratio of 3:1 and different nutrients loading rates. It is known that gas bubbles play a key role in determining the flow and mass-transfer behavior in a gas-liquid-solid fluidized bed (GLSFB) system and the wakes of air bubbles can enhance the liquid-solid mass-transfer coefficients.27 Hence, if the inert air is introduced into the LSCFB system, adsorption mass transfer should be improved due to the effects of the bubble wakes. With this kept in mind and on the basis of the knowledge of the LSCFB adsorber, a novel gas-liquid-solid circulating fluizied bed (GLSCFB) adsorber (GLSCFBA) was proposed and established to separate Ginkgo flavoniods from the extract of Ginkgo in this work. The GLSCFB itself is not a new concept ,and its hydrodynamics and phase dispersion behavior have been investigated to certain degree, especially in the riser for the glass beads. The research results on this area28-32 indicated that the flow pattern in the GLSCFB is more closed to an ideal plug flow than that in a traditional GLSFB and gas bubbles in the GLSCFB are small and uniform in size, which is beneficial for better phase contact and sufficient mass transfer

13-14

between phases. The axial and radial diffusion coefficients of the GLSCFB are small, and the back-mixng degree is lower than that of the traditional GLSFBs.33,34 Compared to the LSCFB adsorber, the GLSCFBA should have a higher mass-transfer rate because of a decrease in the thickness of the boundary layer on the surfaces of the resin particle due to the interactions between the wakes of gas bubbles and the particles. However, until now no related fundamental work on the GLSCFBA is available in the open literature. In this work, the investigations on the hydrodynamics, phase dispersion characteristics, adsorption kinetics of macroporous resin particles, and adsorption and desorption processes of Ginkgo flavonoids from prepared extract with macroporous resin particles in the GLSCFB were conducted to establish a potential method and a theoretical basis for the separation and purification of traditional Chinese medicines or similar natural products using the GLSCFBA. The adsorption of Ginkgo flavonoids by macroporous resin particles belongs to a physical adsorption, which is generally controlled by the particle diffusion or film diffusion. Only when the adsorption process is the film diffusion control could the adsorption efficiency be promoted effectively by changing the facility, operation conditions, or liquid velocity. The adsorption kinetics of Ginkgo flavonoids on the macroporous resin particles was investigated with the batch operation modes. The adsorption was carried out in a downer, and the desorption was conducted in a riser with low-density macroporous resin particles as solid phase. To our knowledge, no such systematical research was reported in the open literature both from the point of view of hydrodynamics and from that of adsorption and desorption mass transfer.

2. MATERIALS AND EQUIPMENT 2.1. Materials. Ginkgo biloba L. extract (GBE) (total Ginkgo flavone glycosides, 24-48%; terpene lactones, 6-18%; bilobol and Ginkgoic acid e 0.005%; heavy metals e 0.02%) was produced by Tianjin Yangcheng High-Tech Natural Product Co,. Ltd. The control substance of rutin (analytical pure) was purchased from Tianjin Institute of Drug Control. The properties of AB-8 macroporous resin particles are listed in Table 1. Hydrodynamic studies were conducted using deionized water (25 °C or 298.15 K; conductivity, (1.81-4.1) 10-4S/m) as liquid phase. Other reagents (analytical purity grade) including ethanol absolute (99.7%, m/m), aqueous ethanol solution (95%, m/m), sodium hydroxide, sodium nitrite, aluminum nitrate, and potassium chloride were supplied by Tianjin Kewei Industrial Co., Ltd. 2.2. Experiment Apparatus. The major components of the GLSCFBA system are shown in Figure 1. In the experiments of GLSCFB adsorption separation, Ginkgo flavonoids solution was pumped into the inlet of the distributor of feed in the bottom of the downer at a stable flow rate. Simultaneously ethanol solution was pumped into the riser through the primary and auxiliary liquid flow entrances for the desorption of Ginkgo flavonoids adsorbed on the macroporous resin particles. The superficial velocity of feed in the downer was kept lower than the terminal velocity of resin particles (Ut) to allow the particles to fall down, contacting countercurrently with the feed. The superficial velocity of desorption liquid in the riser (Ulr) was maintained higher than Ut to let the particles rise when 3599

dx.doi.org/10.1021/ie102113u |Ind. Eng. Chem. Res. 2011, 50, 3598–3612


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closed; the declined height for the calculation of the corresponding volume of gas phase in the beds was recorded. The packing volumes of resin particles were measured for the calculation of solid holdups in the downer and riser. 3.1.2. Circulation Rate of Resin Particles in the GLSCFB. The circulation rate of resin particles, Gs, was measured via a butterfly valve which was fixed in the downer of the GLSFCB. In the normal operations, the butterfly valve was full open. However, when Gs was measured, the valve was closed to accumulate the resin particles to a certain height at given time. Gs is calculated according to Gs ¼

Figure 1. Schematic diagram of the gas-liquid-solid circulating fluidized bed adsorption: 1, riser; 2, downer; 3, separator; 4, primary liquid flow entrance; 5, auxiliary liquid flow entrance; 6, distributor of auxiliary liquid flow; 7, gas distributor of the riser; 8, test section in the riser; 9, sampling port in the riser; 10, upper circulating pipe; 11, butterfly valve; 12, sampling port in the downer; 13, test section in the downer; 14, gas distributor of the downer; 15, distributor of feed liquid in the downer; 16, lower circulating pipe; 17, distributor of primary liquid flow; 18, personal computer; 19, high-speed camera.

making the adsorbed Ginkgo flavonoids eluted from the particles. In the separator, the regenerated particles were separated from effluent containing Ginkgo flavonoids and continuously returned to the downer. The gas phase could either be introduced into the downer through the downer gas distributor to enhance the mass transfer of adsorption process or into the riser through the riser gas distributor to enhance the mass transfer of desorption process or into both of them.

3. MEASURING AND CALCULATING METHODS OF FLOW AND ADSORPTION PROCESS PARAMETERS 3.1. Flow Parameters. 3.1.1. Phase Holdups in the GLSCFB. The gas holdup in the riser was obtained by using the developed charge coupled device (CCD) image measuring system,35 which included a CCD camera, an OK-M20H type black and white image card, a personal computer, and a software system. The CCD images could be processed with the developed software named Chemical 2.0 and Photoshop programs to estimate the gas holdup of the riser.36 The gas bubble separation from the gas-liquid-solid images should be finished before the gas holdup calculation. A typical isolation process of gas bubbles from the images was shown in Figure 2. The estimations of phase holdups with the CCD image measuring system were only performed in the cases in which the bubbles or particles are dilute in the three-dimensional fluidized bed and the relative error is about (5%. Hence, a simple but accurate bed collapse method was used to measure the high gas holdup in the downer and high solid holdups in the riser and downer. Under stable operating conditions, the height of liquid level in the downer was first recorded; then the valves of gas flow, liquid flow, and circulation pipe were

hAð1 - εp ÞFs hð1 - εp ÞFs ¼ At t

ð1Þ

3.1.3. Residence Time Distribution and Axial Mixing Behavior. The RTD of the liquid phase was measured by pulse method with KCl solution as tracer. The standard curve of KCl solution showed a linear function relationship between KCl concentration and conductivity of KCl solution. The solution conductivity was measured with the DDS-307 type conductivity meter. The technical parameters of the conductivity meter are as follows: measurement values of conductivity range from 0 to 1 105 μS/cm (10 S/m), electrode constants range from 0.01 to 10, values of compensation temperature change from 15 to 35 °C with the basic temperature of 25 °C, and equipment accuracy is 1% full-scale. The maximum absolute errors of the experimental data of the solution conductivity and the KCl concentration are 4.45 μS/cm and 0.03 mmol/L (10-3 mol/L), respectively. A linear regression equation was obtained by the least-squares method as CKCl = 73G - 0.0107 with correlation coefficient R = 0.9998. KCl solution was quickly injected into the sampling port at the bottom of the downer or the riser. The conductivity of liquid that flowed out of the sampling port at the top of the beds was simultaneously measured by the electrode of conductivity meter. KCl concentration and thus the RTD curves of the liquid phase were determined on the basis of the conductivity data. Liquid axial dispersion coefficient Daxl and dimensionless Peclet number (Pe) which characterize the degree of liquid axial mixing were calculated on the basis of the average residence time and residence time variance according to eqs 2-5.37 Higher Pe number and lower Daxl mean lower axial mixing degree. N

average residence time : t ¼

∑ ti CAi i¼1

ð2Þ

N

∑ CAi

i¼1

N

residence time variance : σt 2 ¼

∑ ti 2CAi i¼1 N

∑ CAi

- t2

ð3Þ

i¼1

Using dimensionless time θ =t/t and close-open boundary conditions, Peclet number : Pe ¼ 3600

Ul L εl Daxl

ð4Þ



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Figure 2. Separation process of gas bubbles from the original CCD image in the riser: (a) original image; (b) image after removing background noise; (c) separated gas bubbles; (d) final image of gas bubbles.

Figure 3. Determination of solid particle RTD in GLSCFB by the method of solid particles tracer: (a) injector of solid particles tracer; (b) typical gasliquid-solid image in the GLSCFB.

dimensionless residence time variance : σ θ 2 ¼

σt 2 2 1 ¼ þ 3ð Þ2 Pe Pe t2 ð5Þ

Yellow macroporous resin particles adsorbing Ginkgo flavonoids were used as the tracer of solid phase, and the image acquisition method was applied to determine the concentration of tracer particles and RTD of solid particles. The tracer particles were put into a vertical small column in a three-way valve injector and were injected into the beds with a pulse by pressured tap water, as shown in Figure 3a. The measuring position in the axial of the riser is Zr = 1.0 m and that of the downer is Zd = 0.4 m (0.9 m from the outlet of the top circulating pipe). The concentration of the particle tracer was calculated from the obtained images by counting the number of the tracer particles in the images. A typical image is shown in Figure 3b. The axial dispersion coefficient of solid particles, Daxs, and Pe number of solid particles can also be obtained such as those of the liquid phase. For axial dispersion coefficient of solid particles, eq 4 becomes Pe = (Us/ εs)(L/Daxs) = (L/t)(L/Daxs). The linear velocity of solid particles was calculated from the average residence time.38

3.2. Adsorption Process Parameters in the GLSCFB. Using rutin as the control substance39 and 5% NaNO2 and 10% Al(NO)3 as chromogenic reagents, the Ginkgo flavonoids concentration could be measured by the spectrophotometry method with a 722B type visible spectrophotometer (Tianjin Precise Instrument Co., Ltd.) at detected wavelength of 510 nm (5.1 10-7 m). The technical parameters of the visible spectrophotometer are as follows: the values of the wavelength range from 340 to 1000 nm, absorbance values change from 0 to 2, wavelength precision is (2 nm, spectral bandwidth is 4 nm, transmittance precision is e(0.5%T, and stray light is e0.5%T. All measurements were obtained at room temperature (about 25 °C), and the relative errors of the concentration or luminosity change from 1.36 to 7.73% for absorbance values range from 0.39 to 0.03. The linear regression equation was built on the basis of the least-squares method as C = 0.0965A þ 0.0003 with R = 0.9996. During the adsorption, Ginkgo flavonoids liquor was sampled at different axial positions of the GLSCFB at different time with needle tube sampler (0.3 mm i.d. 32 mm). Adsorption ratio, desorption ratio, and overall yield of Ginkgo flavonoids were calculated as follows: adsorption ratio : Xd ¼ 3601

Qd Cod - ½Qd Ced þ Vd ðCZ1 þ CZ2 þ CZ3 Þ=3 ð6Þ Qd Cod


Industrial & Engineering Chemistry Research overall yield : X ¼

Qr Cer Qd Cod

desorption ratio : Xr ¼

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ð7Þ

qor - qer qor

¼

Qr Cer Qd Cod - ½Qd Ced þ Vd ðCZ1 þ CZ2 þ CZ3 Þ=3

¼

X Xd

ð8Þ

To obtain more accurate calculation results, the residual amount of Ginkgo flavonoids in the downer after adsorption experiments was also taken into account in eq 6. 3.3. Adsorption Kinetics of Ginkgo Flavonoids on the Macroporous Resin Particles. In Ginkgo flavonoids adsorption kinetics experiments, the macroporous resin particles with weight of 1 10-3 kg in a certain diameter were added into a beaker filled with 1 10-4 m3 Ginkgo flavonoids solution in a concentration of 1 kg/m3. Then the liquid-solid mixture was stirred at constant speed in a certain temperature. The Ginkgo flavonoids concentration in the beaker was sampled and analyzed at specific time interval. The adsorbed amount of Ginkgo flavonoids on the macroporous resin particles at time t, qt (mg/g or 10-3 kg/kg), was calculated by eq 9. The static adsorption capacity of the macroporous resin particles, qe, was thus obtained when adsorption equilibrium was reached. ðC0 - Ct ÞV ð9Þ W The initial rapid adsorption process was fitted by a Lagergren first-order rate equation (eq 10)40 and Kannan-Sundaram41 intraparticle diffusion equation (eq 11). The adsorption rate constant Kad. and intraparticle diffusion constant kp were estimated. qt ¼

Lagergren equation: logð1 - FÞ ¼ -

Kad: t 2:303

ð10Þ

where F = qt/qe, meaning the adsorption fraction. Kannan and Sundaram intraparticle diffusion equation: ð11Þ qt ¼ kp t 1=2 þ C When the adsorption process is controlled by the liquid film diffusion, 1og(1 - F) should be a linear function of t, passing through the coordinate origin; when the particle diffusion is the control step of the adsorption process, qt should be a linear function of t1/2.42

4. RESULTS AND DISCUSSION 4.1. RTD and Phase Mixing Characteristics of GLSCFB. 4.1.1. Liquid-Phase RTD and Mixing Characteristic in the GLSCFB

4.1.1.1. Downer. During liquid-phase RTD measurements in the downer, 2 mL (10-6 m3) of KCl solution with concentration of 5 mol/L was used as pulse tracer. The curves of liquid-phase RTD, the values of the Pe number, and liquid-phase axial

Figure 4. Liquid-phase RTD, Pe number, and liquid-phase axial dispersion coefficient Daxl in downer of GLSCFB under different Ugd, when dp = (0.6-0.9) 10-3 m, Ugr = 1.699 10-3 m/s, Um = 4.24 10-3 m/s, Ua = 5.66 10-3 m/s, Uld = 0.428 10-3m/s, and Gs = 0.104-0.092 kg/(m2 s): (a) liquid-phase RTD; (b) Pe and Daxl.

dispersion coefficient Daxl in the downer of GLSCFB at different downer superficial gas velocities are shown in Figure 4. The calculated parameters are shown in Table 2. Compared with the liquid-phase RTD curves in the downer of LSCFB,27 the peaks of liquid-phase RTD curves in the GLSCFB come earlier and the trailing was longer as shown in Figure 4a. With the increase of downer superficial gas velocity, average RTD and peak time decreased and peak values increased (see Figure 4a). The main reason was that with the introduction of gas phase into the downer and the increase of downer superficial gas velocity, liquid-phase mixing, and liquid entrainment were intensified due to the disturbance of bubbles and their wakes. It is well-known that, in the plug flow reactor, dimensionless residence time variance is 0, and in the mixed flow reactor, it is 1.0. In the downer of GLSCFB, the dimensionless residence time variance changing from 0.2 to 0.4 as shown in Table 2 indicated that the flow pattern in the downer of GLSCFB deviated from the plug flow and underwent a liquid back-mixing to a certain degree. Research on the liquid mixing in the LSCFB23 showed that the flow pattern in the downer of LSCFB was close to the plug flow pattern because its dimensionless residence time variance varied from 0.1 to 0.2. The flow pattern difference between LSCFB and GLSCFB systems resulted mainly from the addition of the gas phase in GLSCFB. The irregular movement of air bubbles made 3602



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Table 2. Liquid-Phase RTD Related Parameters of Downer under Different Downer Superficial Gas Velocities Ugd (10-4 m/s)

3.54

5.90

8.26

11.80

peak values (mol/m3)

0.4839

0.4960

0.5008

0.5185

average resident time, t (60 s)

20.05

17.06

15.27

13.21

dimensionless variance of liquid resident time, σθ2

0.2416

0.2932

0.3211

0.4129

the radial distributions of phase holdups and flow in the GLSCFB more uneven than in LSCFB.33 The estimated values of liquid-phase Pe number and axial dispersion coefficient Daxl in the downer of GLSCFB also supported the above analyses, as shown in Figure 4b. With the increase of downer superficial gas velocity, liquid-phase Pe number reduced from 9.6 to 6.0 and Daxl increased from 7.9 10-5 to 12.6 10-5 m2/s, indicating that liquid back-mixing was intensified and the flow pattern deviated further from the plug flow. 4.1.1.2. Riser. During liquid-phase RTD measurements in the riser, 5 10-6 m3 of KCl solution with the concentration of 5 103 mol/m3 was used as pulse tracer. Liquid-phase RTD curves, the values of Pe number, and Daxl in the riser of the GLSCFB under different riser superficial gas velocities are shown in Figure 5. The calculated parameters are shown in Table 3. The average resident time of the liquid phase in the riser was much shorter than that in the downer of GLSCFB, and the peak time of RTD curves varied little with the increase of riser superficial gas velocity. The explanations are as follows. The superficial liquid velocity in the riser was much higher than that in the downer, so the effect of gas phase on the liquid resident time was relatively small. The bubbles were small, and the distribution of bubble size was narrow when gas, liquid, and solid flows were cocurrent in the riser. Higher superficial liquid velocity could reduce the amount of the liquid detention in the dead space in the beds. As shown in Table 3, dimensionless variance of liquid resident time in the riser was in the range of 0.1-0.3, indicating that the flow pattern in the riser of GLSCFB was much closer to plug flow than that in the downer ranging from 0.2 to 0.4. The Pe number decreased from 17.6 to 8.1 and Daxl increased from 1.4 10-3 to 3.0 10-3 m2/s with the increase of the riser superficial gas velocity, which means that riser liquid back-mixing was intensified and the flow pattern deviated further from plug flow. The wet density of AB-8 macroporous resin particles is close to that of water, unlike the density of glass beads. So the gas and liquid velocities were relatively low to reach the stable circulating operation compared to fluidized beds of glass beads. The stability of GLSCFB was higher than the traditional fluidized bed, and the flow pattern was closer to the plug flow.23 4.1.2. RTD and Mixing Characteristics of Solid Phase. Measured RTD curves and estimated mixing parameters of solid particles in the riser and downer are shown in Figure 6 and Table 4, respectively. As shown in Figure 6, the peak of solid RTD curve of the riser was high and steep, but a long trailing was found after about 4 min. This related to the existence of back-mixing of solid particles due to the annual-core flow structure in the GLSCFB, in which solid particles flowed upward in the center region but moved downward in the wall region. A single peak and a long trailing of RTD curve of solid particles in the downer were observed due to the solid particle mixing in the downer caused by the countercurrent flow of solid phase and gas-liquid phase, which could lead to obvious inner-circulating movement of solid particles in the downer. The upward entrainment of solid particles by gas-liquid phase enhanced the back-mixng degree in the downer. The deviation of RTD estimation mainly related to the error of the image acquisition and treatment method

Figure 5. Liquid-phase RTD, Pe number, and liquid-phase axial dispersion coefficient Daxl in the riser of GLSCFB under different Ugr when dp = (0.6-0.9) 10-3 m, Ugd = 5.90 10-4 m/s, Um = 4.24 10-3 m/s, Ua = 5.66 10-3 m/s, Uld = 0.428 10-3 m/s, and Gs = 0.102-0.108 kg/(m2 s): (a) liquid-phase RTD; (b) Pe and Daxl.

since this method could only obtain the two-dimensional movement information of solid particles in an image measured near the bed wall and could not reflect the movement information of solid particles in the three-dimensional inner beds. As shown in Table 4, under the same operation conditions, the average residence time of the liquid phase in the downer was 17.06 60 s, and in the riser 3.87 60 s, and the average residence times of the solid phase in the two beds were both about 2 60 s. Resin particles in the GLSCFB could circulate and be regenerated rapidly to ensure high dynamic adsorption capacity. During RTD measurement of the solid phase, the number of tracer particles was counted. When the solid holdup was low (this was common in the riser), the traced particles could be accounted accurately and no large concentration variation of tracer particles existed between inner bed and wall. However, when the solid holdup was high (this is the case in the downer), 3603



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Table 3. Liquid-Phase RTD Related Parameters of Riser under Different Superficial Gas Velocities in Riser Ugr (10-4 m/s)

5.10

8.49

11.90

16.99

peak values (mol/m3)

3.078

3.134

3.224

3.264

average resident time, t (60 s)

4.31

3.97

3.75

3.87

dimensionless variance of liquid resident time, σθ2

0.1230

0.1890

0.2242

0.2914

Figure 6. Curves of solid-phase residence time distribution in the riser and downer when Ugr = 1.699 10-3 m/s, Ugd = 5.90 10-4 m/s, Um = 4.24 10-3 m/s, Ua = 5.66 10-3 m/s, Uld = 0.428 10-3 m/s, dp = (0.6-0.9) 10-3 m, and Gs = 0.102 kg/(m2 s).

Table 4. Average RTDs and Back-mixing Parameters of Solid Particles in the Riser and Downer solid holdup

t (60s)

σθ2

Pe

Daxs

riser

0.0345

1.76

0.4238

5.916

1.60 10-3

downer

0.0644

1.78

0.209

10.886

6.98 10-4

GLSCFB

the experimental error was unavailable. In the experiments of obtaining solid-phase RTD by pulse method under different conditions, the relative error was less than 20%. 4.2. Adsorption Kinetics of Macroporous Resin Particles. 4.2.1. Static Adsorption Capacity of Macroporous Resin Particles. The effects of size of macroporous resin particles and stirring speed on the static adsorption capacity of Ginkgo flavonoids in AB-8 resin particles are given in Figure 7. It can be seen from Figure 7 that the static adsorption capacity of macroporous resin particles increased with the reducing of particle size and the increasing of stirring speed. The smaller the particle size, the lower the intraparticle diffusion resistance. Low intraparticle diffusion resistance is beneficial to adsorption. High stirring speed could reduce the film diffusion resistance of particles and promote the adsorption process. 4.2.2. Adsorption Kinetics of AB-8 Resins for Ginkgo Flavonoids. Lagergren and Kannan-Sundaram equations were used to determine the adsorption rate of Ginkgo flavonoids on the macroporous resin particles. As shown in Figure 7, the adsorption process in the first 90 60 s was the fast initial adsorption stage. Adsorption rate constant Kad and intraparticle diffusion constant kp were estimated according to eqs 10 and 11 on the basis of the data of the rapid initial adsorption stage at high stirring speed. And the results are shown in Figures 8 and 9 and Tables 5-7. 4.2.2.1. Effect of Particle Size on the Adsorption Kinetics of Resin Particles. Figure 8a shows the relationship between log(1 - F) and t, and the adsorption rate constants and related

Figure 7. Static adsorption capacity of macroporous resin particles with different sizes and stirring speeds of magnetic stirrer at room temperature.

parameters are shown in Table 5. Figure 8b expresses the relationship between qt and t1/2. The constants of intraparticle diffusion rate and related parameters are shown in Table 6. It can be found that the correlation coefficients R of the Lagergren and the Kannan-Sundaram equations were all greater than 0.98, which indicated that the two linear equations could describe the adsorption process kinetics within the range of present experiment conditions. The intercepts of each line in the Lagergren plot were all close but not equal to 0 (see Table 5), which showed that the liquid film diffusion process was not the only control step of adsorption process.42 The linearity of qt versus t1/2 being good and the intercepts of each line being all not equal to 0 in Kannan and Sundaram plot indicated the existence of intraparticle diffusion resistance but was not the only control step of adsorption process.41 For different resin particle size, Kad. and kp decreasing when the size of resin particles increased meant that the adsorption rate decreased with the increase of the particle size. The correlation coefficients of the two equations were both higher than 0.98, showing that both the liquid film diffusion on the surfaces of resin particles and the intraparticle diffusion influenced the adsorption process. 4.2.2.2. Effect of Stirring Speed on Adsorption Kinetics of Resin Particles. The results in Figure 9 and Table 7 further showed that the liquid film diffusion and intraparticle diffusion were both important. From Table 7 it is found that Kad. and kp both increased with the increase of the stirring speed, which showed that the adsorption mass-transfer process was enhanced due to the improved contacts and mixings of liquid and solid phases. That was why the adsorption process of Ginkgo flavonoids on resin particles was enhanced when gas phase was introduced into the liquid-solid circulating fluidized bed system. Many tiny bubbles could be observed when the gas phase was introduced into the up-flow bed (riser) and the down-flow bed (downer). As a result, the liquid and solid phases were mixed greatly by bubbles and trailing vortex. Mass transfer between liquid and solid phases was 3604



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promoted, and the adsorption process of Ginkgo flavonoids was also enhanced. 4.3. Adsorption and Desorption of Macroporous Resin Particles in the GLSCFB. 4.3.1. Effect of Superficial Gas Velocity of Downer on Adsorption and Desorption. To investigate the effect of downer superficial gas velocity on the adsorption and desorption processes, the gas phase was only introduced into the downer. The concentration of Ginkgo flavonoids in the downer and that in the riser with the running time was measured at one sampling port of the downer (Zd = 1.4 m) and one of the riser (Zr = 2.0 m). Figure 10 is the gas-liquid-solid adsorption and liquid-solid desorption curves under different downer superficial gas velocities in the GLSCFB. With the increase of downer superficial gas velocity, the concentration of Ginkgo flavonoids decreased at first and then increased at the outlet of the downer (Figure 10a) at given time, but in contrast at the outlet of the riser (Figure 10b). The results showed that the adsorption and desorption processes were improved in the first place

and then weakened with the increase of downer superficial gas velocity. The reason of the enhancement on adsorption mass transfer between liquid phase and resin particles under proper superficial gas velocity in downer was that the thickness of the mass-transfer boundary layers on the particle surfaces decreased due to the interaction between the particles and the wakes of bubbles and thus reduced the mass-transfer resistance.43 However, when the superficial gas velocity of downer further climbed, the entrainment amount of solid resin particles in the downer by the bubble wakes increased and solid back-mixing became obvious, which could reduce the circulation rate of solid resin particles. Hence, higher superficial gas velocity became a disadvantage for the adsorption processes. The intensification or the deterioration of the desorption process in the riser mainly resulted from the transfer of concentration variation trends from the downer. No gas phase was introduced into the riser in this case. 4.3.2. Effect of Riser Superficial Gas Velocity on Adsorption and Desorption. Similarly, the gas phase was only introduced

Figure 8. Adsorption of Ginkgo flavonoids on AB-8 resin particles at different particle sizes with initial concentration of Ginkgo flavonoid of 1.0 kg/m3: (a) Lagergren plot; (b) Kannan and Sundaram plot.

Figure 9. Effect of stirring speed on the adsorption of Ginkgo flavonoids on the resin particles with particle diameter of (0.45-0.6) 103 m and initial concentration of Ginkgo flavonoid of 1.0 kg/m3: (a) Lagergren plot; (b) Kannan and Sundaram plot.

Table 5. Adsorption Rate Constants and Related Parameters kinetic model Lagergren equation log(1 - F) = -Kad.t/2.303

parameters

dp = (0.45-0.6) 10-3 m

dp = (0.6-0.9) 10-3 m

dp = (0.9-1.2) 10-3 m

Kad. R2

0.0153 0.9922

0.0102 0.9903

0.0083 0.9855

intercept

0.0046

-0.1057

-0.1502

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Table 6. Constants of Intraparticle Diffusion Rate and Related Parameters kinetic model Kannan and Sundaram equation qt = kpt1/2 þ C

parameters

dp = (0.45-0.6) 10-3 m

dp = (0.6-0.9) 10-3 m

dp = (0.9-1.2) 10-3 m

kp

4.807

3.160

R2

0.9831

0.9896

0.9920

intercept

-8.813

-1.970

0.0776

2.253

Table 7. Constants of Adsorption and Intraparticle Diffusion Rates under Different Stirring Speeds in the Adsorption of Ginkgo Flavonoids on the Resin Particles with Diameter of (0.45-0.6) 10-3 m kinetic models Lagergren equation: log(1 - F) = -Kad.t/2.303

Kannan and Sundaram equation: qt = kpt1/2 þ C

parameters

low stirring speed

high stirring speed

Kad.

0.0116

R2

0.9973

0.9960

intercept

-0.1107

-0.0620 4.807

0.014

kp

4.218

R2

0.9881

0.9831

intercept

-8.947

-8.813

Figure 10. Adsorption and desorption curves at different downer superficial gas velocities when Ugr = 0 m/s, Ugd = (0-11.80) 10-4 m/s, Um = 4.24 10-3 m/s, Ua = 5.66 10-3 m/s, Uld = 0.224 10-3 m/s, dp = (0.45-0.60) 10-3 m, Gs = 0.043-0.110 kg/(m2 s), and Cod = 1 kg/m3: (a) gas-liquid-solid adsorption in the downer; (b) liquid-solid desorption in the riser.

Figure 11. Adsorption and desorption curves at different riser superficial gas velocities when Ugr = (0-1.699) 10-3 m/s, Ugd = 0 m/s, Um = 4.24 10-3 m/s, Ua = 5.66 10-3 m/s, Uld = 0.224 10-3m/s, dp = (0.45-0.60) 10-3 m, Gs = 0.140-0.175 kg/(m2 s), and Cod = 1 kg/m3: (a) liquid-solid adsorption in the downer; (b) gas-liquid-solid desorption in the riser.

into the riser for investigating the effect of riser superficial gas velocity on the adsorption and desorption processes. Sampling positions for the concentration measurements were the same as those of downer. Figure 11 shows the liquid-solid adsorption

and gas-liquid-solid desorption curves under different riser superficial gas velocities in the GLSCFB. As shown in Figure 11a, when the riser superficial gas velocity was low (Ugr = 5.10 10-4 m/s), the outlet concentration of the 3606



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Figure 12. Adsorption and desorption curves of riser-and-downer-aired GLSCFBA when Ugr = 8.4910-4 m/s, Ugd = 3.5410-4 m/s, Um = 4.24 10-3 m/s, Ua=5.6610-3 m/s, Uld = 0.22410-3 m/s, dp = (0.45-0.60) 10-3 m, Gs = 0.104 kg/(m2 s), and Cod = 1 kg/m3: (a) gas-liquid-solid adsorption in the downer; (b) gas-liquid-solid adsorption in the riser.

downer was high which indicated that the adsorption efficiency was relatively low. High riser superficial gas velocity (1.699 10-3 m/s) indirectly improved the adsorption process in the downer, but the adsorption efficiency at middle riser superficial gas velocity (Ugr = 8.49 10-4 m/s) was the highest. Investigations on hydrodynamics in the GLSCFB with low solid holdup of resin particles showed that increasing riser superficial gas velocity could increase the solid particle circulation rate to a certain degree, which increased the downer adsorption capacity. On the other hand, desorption was enhanced with the climbing of superficial gas velocity in the riser due to the reducing of desorption mass-transfer resistance caused by air bubbles, as shown in Figure 11b. 4.3.3. Effects of Both Riser and Downer Superficial Gas Velocities on Adsorption and Desorption. The gas phase was introduced into both the downer and the riser simultaneously in this case. The GLSCFBA in this operation condition may be called bi- or riser-and-downer-aired GLSCFBA. Experimental results are shown in Figure 12. Figure 12 showed that the concentration of Ginkgo flavonoids in the liquid phase varied obviously at different axial positions in the downer but changed slightly in the riser, which indicated that the desorption rate was fast in the riser and designed desorption bed height was longer than the desorption process needed. 4.3.4. Effect of Superficial Feed Velocity on Adsorption and Desorption. With the increase of superficial feed velocity at a

Figure 13. Effect of feed velocity on the adsorption and desorption processes when Ugr = 1.699 10-3 m/s, Ugd = 5.90 10-4 m/s, Um = 4.24 10-3 m/s, Ua = 5.66 10-3 m/s, Uld = (0.224-0.428) 10-3 m/s, dp = (0.45-0.60) 10-3 m, Gs = 0.102 kg/(m2 s), and Cod = 0.52-1.00 kg/m3: (a) gas-liquid-solid adsorption in downer; (b) gas-liquid-solid desorption in riser.

constant feed concentration of Ginkgo flavonoids, the concentrations both in the adsorption raffinate and in the desorption solution increased. Though higher concentration of final eluent and production capacity with the increase of Uld, the loss of Ginkgo flavonoids also increased because higher concentrations in the adsorption raffinate indicated a higher loss amount of Ginkgo flavonoids after adsorption and lower overall yield. To individually explore the effect of superficial feed velocity on the adsorption and desorption, experiments were also carried out at a specific total mass flow rate of Ginkgo flavonoids. That is to say the product of the concentration and feed velocity was a constant for each run. Results are shown in Figure 13. It can be seen from Figure 13b that, at given total mass flow rate of Ginkgo flavonoids, the higher superficial feed velocity (at the same time feed concentration decreased to keep the same mass flow rate of Ginkgo flavonoids) gave a higher concentration of the Ginkgo flavonoids desorbed from the resin. However, with regard to the 3607



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Table 8. Typical Results of LSCFB and GLSCFB Adsorption and Desorption of Ginkgo Flavonoids adsorption ratio

desorption ratio

stable adsorption time, 60 s

stable desorption time, 60 s

LSCFB

78.51

79.19

47

52

GLSCFB

83.20

86.36

42

48

adsorber

adsorption in the downer, Figure13a cannot tell whether the lower concentration of the Ginkgo flavonoids at higher superficial feed velocity was caused by the increase of the feed superficial velocity or by the reduce of the feed liquid concentration itself (higher superficial feed velocity corresponded to the lower Ginkgo flavonoids concentration). The true reason which caused the decrease of the concentration of the Ginkgo flavonoids in the downer can be found by analyzing the variation of the mass flow rate of Ginkgo flavonoids. The mass flow rates of Ginkgo flavonoids adsorbed in the resin particles under three different feed superficial velocities (from 0.000 224 to 0.000 428 m/s) are 6.19 10-7, 6.16 10-7, and 6.12 10-7 kg/s. Hence, smaller superficial feed velocity was beneficial for the mass transfer between liquid and solid phases under present experimental conditions. Meanwhile, the superficial feed velocity in the downer should be maintained lower than the terminal velocity of the resin particles to let the particles flow down successfully. Experimental investigations on adsorption and desorption processes of different particle sizes at given packing volume of resin particles, LSCFB, riser-aired GLSCFB, downer-aired GLSCFB, and riser-and-downer-aired GLSCFB were also done. The main results are as follows. Smaller diameter particles were good for adsorption and desorption at given packing volume of resin particles; the GLSCFB was better than the LSCFB in adsorption and desorption of Ginkgo flavonoids due to the introduction of gas phase (for typical results see Table 8); riser-and-downer-aired GLSCFB was better than other beds for the adsorption and desorption of Ginkgo flavonoids. The orthogonal tests of four factors and three levels on the adsorption and desorption of Ginkgo flavonoids with the macroporous resin particles in the GLSCFB were done, and optimal conditions were obtained. The optimal test conditions were validated by the adsorption and desorption experiments, which were carried out three times under the same conditions. The average adsorption ratio of the three experiments was 84.4%, which was higher than those of the orthogonal tests.

(5) Adsorbate on the liquid-solid interface was in a dynamic equilibrium, which could be described with the Langmuir adsorption isotherm equation.8 (6) Mass-transfer rate of the liquid-solid phase was calculated using mass transfer rate equation of external diffusion,45 and relevant parameters were obtained by the batch experiments. 5.2. Adsorption Process of Ginkgo Flavonoids in the Downer. The mass balance equation of liquid phase was deduced according to the principle of mass conservation: Dcd D2 cd Uld Dcd 3kf F0 ðcd - cf Þ ¼ Daxl R Dt Dzd 2 εld Dzd

ð12Þ

The initial condition and boundary conditions of eq 12 were as follows: initial condition : at t ¼ 0; cd ðzd ; 0Þ ¼ 0

for 0 e zd e L

ð12aÞ

boundary condition 1 : at zd ¼ 0; cd ð0; tÞ -

Daxl εld Dcd ¼ cod Uld Dzd

for t > 0 ð12bÞ

boundary condition 2 : at zd ¼ Ld ;

Dcd ¼0 Dzd

for t > 0

ð12cÞ

Taking the solid-phase axial dispersion coefficient into account, the concentration distribution of Ginkgo flavonoids in the solid phase in downer was Dqd D2 qd Usd Dqd 3kf F0 ðcd - cf Þ ¼ Daxs þ R Dt Dzd 2 εsd Dzd

ð13Þ

5. MASS-TRANSFER MODELS OF ADSORPTION AND DESORPTION IN THE GLSCFB

The initial condition and boundary conditions of eq 13 were as follows:

5.1. Axial Dispersion Model. On the basis of the axial dispersion model,10,44 the adsorption in the downer and the desorption in the riser of GLSCFB of Ginkgo flavonoids were modeled, including hydrodynamics, phase mixing characteristics, and adsorption kinetics of macroporous resin particles. Model assumptions are as follows:10,44 (1) Hydrodynamics could be described by the axial dispersion model. (2) Compared with the axial concentration gradient, the radial concentration gradient could be neglected in the riser and downer. (3) The diameter of the resin particles was uniform, and average particle diameter was used. (4) The axial dispersion coefficient in the whole beds was maintained constantly.

initial condition: at t ¼ 0, qd ðzd , 0Þ ¼ 0

for 0 e zd e L

ð13aÞ

boundary condition 1: at zd ¼ 0,

Dqd ¼0 Dzd

for t > 0

ð13bÞ

boundary condition 2: Daxs εsd Dqd ¼ qod for t > 0 ð13cÞ Usd Dzd When the adorption could be described with a Langmuir isotherms equation and the effect on the adsorption rate caused at zd ¼ Ld , qd ð0, tÞ -

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Figure 14. Adsorption and desorption experimental data and simulation results in the GLSCFB when Um = 4.24 10-3 m/s, Ua = 5.66 10-3 m/s, Uld = 0.428 10-3 m/s, Ugd = 5.90 10-4 m/s, Ugr = 1.699 10-3 m/s, Cod = 0.52 kg/m3, dp = (0.6-0.9) 10-3 m, and Gs = 0.102 kg/m2s: (a) adsorption in downer; (b) desorption in riser.

by the diffusion resistance inside particles was neglected, the diffusion rate outside the particles was calculated in accordance with the batch experiments and Mckay equation:46 Ct 1 mkL 1 þ mkL ln kf Ss t ð14Þ ¼ ln C0 1 þ mkL 1 þ mkL mkL

Ginkgo flavonoids in the solid phase with the height of the riser:20 Usr Dqr ¼ Vsr ð16Þ εsr Dzr The following equation was deduced from eqs 15 and 16: Usr Dqr ¼ - kqr εsr Dzr

where kL ¼ KL qe Ss ¼

6W dp Fs ð1 - εp Þ

ð14aÞ ð14bÞ

The liquid film mass-transfer coefficient kf was obtained from the slope and intercept of the straight line of eq 14. 5.3. Desorption Process of Ginkgo Flavonoids in the Riser. In most cases, the desorption of Ginkgo flavonoids from macroporous resin particles was very fast and could be described as a first-order reaction equation.23 The desorption kinetic equation of Ginkgo flavonoids in the riser was expressed as Vsr ¼

dqr ¼ - kqr dt

ð15Þ

At a given riser superficial liquid velocity, the total desorption rate in the riser was the rate change of the concentration of

ð17Þ

boundary conditions: Zr ¼ 0

qr ¼ qor ¼ qed

Zr ¼ Lr

qr ¼ qer

ð17aÞ ð17bÞ

The concentration of Ginkgo flavonoids in the solid phase of the riser was εsr kzr ð18Þ qr ¼ exp qor Usr The corresponding desorption equation was deduced Dcr D2 cr Ulr Dcr εsr kzr ¼ Daxl 2 - k exp qor Dt Dzr εlr Dzr Usr 3609

ð19Þ



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Initial and boundary conditions of eq 19 were as follows: initial condition: at t ¼ 0, cr ðzr , 0Þ ¼ 0

for 0 e zr e L

ð19aÞ

boundary condition 1: at zr ¼ 0, cr ð0, tÞ -

Daxl εlr Dcr ¼ c0r Ulr Dzr

for t > 0

ð19bÞ

boundary condition 2: at zr ¼ Ld ,

Dcr ¼0 Dzr

for t > 0

ð19cÞ

5.4. Validations of Adsorption and Desorption Models with Numerical Simulations. The previous partial differential

equations were solved by using pdepe() tools of Matlab software. Simulation and experimental results of adsorption and desorption processes are shown in Figure 14. The concentrations of Ginkgo flavonoids in the liquid varied with bed heights of the riser or downer. Experimental conditions were as follows: Um = 4.24 10-3 m/s, Ua = 5.66 10-3 m/s, Uld = 0.428 10-3 m/s, Ugd = 5.90 10-4 m/s, Ugr = 1.699 10-3 m/s, Gs = 0.102 kg/(m2 s), Cod = 0.52 kg/m3, and dp = (0.6-0.9) 10-3 m. It can be seen from Figure 14 that experimental data and simulated results were in general good agreement when the operation was stable. However, simulation values were slightly higher than the experimental data of adsorption. Athough the adsorption process in the downer and desorption process in the riser of Ginkgo flavonoids with macroporous resin particles in the GLSCFB were well-modeled by the established axial dispersion models, some model limitations still exist: the models could only be applied to the macroporous resin-water-air system at relative narrow operating range; the models could mainly describe the mass-transfer process of liquidsolid phase, and the effect of gas phase on the processes was only reflected on the experimental parameters, not in the models. The models need further improvement on the universality and accuracy.

6. CONCLUDING REMARKS A novel GLSCFB adsorber was first built and the hydrodynamics, phase mixing properties, adsorption kinetics of macroporous resin particles, and adsorption and desorption behaviors were studied experimentally and theoretically. The results provide some new basic understandings on the GLSCFB adsorber. Considering the complexity of the gas-liquid-solid fluidization system and the difficulties of the multiphase flow measurements, the experimental data were very invaluable for both science and technique areas. The main concluding points are as follows: When increasing the downer superficial gas velocity, the deviation degree of downer flow pattern from plug flow and the liquid back-mixing intensified because the liquid Pe number of downer reduced from 9.6 to 6.0 and liquid axial dispersion coefficient of downer increased from 7.9 10-5 to 12.6 10-5 m2/s. For the riser, the same variation tendency was found since the liquid Pe number in the riser fell from 17.6 to 8.1 and liquid axial dispersion coefficient increased from 1.4 10-3 to 3.0 10-3 m2/s with the increase of riser superficial gas velocity.

Hence, the liquid back-mixing was intensified and the flow pattern deviated further from plug flow with the increase of superficial gas velocity. The typical solid Pe number of the riser was 5.9 and that of the downer was 10.9. Correspondingly, the solid axial dispersion coefficient of the riser was 1.6 10-3 m2/s and that of the downer was 7.0 10-4 m2/s. These experimental data are valuable for further study and industry applications. Investigations on the adsorption kinetics of macroporous resin particles showed that the adsorption process of Ginkgo flavonoids was controlled by film diffusion and intrapore diffusion. Higher stirring speed enhancing the mass-transfer rate indicated that the adsorption process could be enhanced by the positive disturbance of air bubbles in the gas-liquid-solid circulating fluidized beds. Ginkgo flavonoids were continuously separated from the extract by using the GLSCFB adsorber with macroporous resin particles. The adsorption and desorption processes were improved as the gas phase was bubbled into the adsorber. It was preferable for the GLSCFB adsorption and desorption with smaller particle size, lower feed velocity, and diluted Ginkgo flavonoids concentration. On the basis of the experiments results, an adsorption model of the downer and a desorption model of the riser were developed. Both models could predict adsorption and desorption processes well. Further work is needed to build more general but more accurate models under detailed investigations.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The authors gratefully acknowledge the financial support of the National Nature Science Foundation of China (Grant No. 20576091), Technique Gallery of Separation and Purification of Medicines of Comprehensive Drug Discovery and Development Technical Major Platform Project of National Eleventh Five-Year “Significant Drug Discovery” Science and Technology Major Project (Grant No.2009ZX09301-008-P-11), and the Open Foundation of State Key Laboratory of Multiphase Complex System, Institute of Process Engineering, Chinese Academy of Sciences. The authors are also grateful to Dr. Xu Jing for her valuable suggestions on this paper given when one of the authors visited the University of Western Ontario. ’ NOMENCLATURE A = adsorbance C, c = concentration of Ginkgo flavonoids or constant, kg/m3 CAi = concentration of tracer at time i, mol/m3 Ced = Ginkgo flavonoids concentration in the raffinate after adsorption in the downer, kg/m3 Cer = Ginkgo flavonoids concentration in the raffinate after desorption in the riser, kg/m3 CKCl = concentration of KCl solution, mol/m3 C0 = initial concentration of Ginkgo flavonoids, kg/m3 Ct = Ginkgo flavonoids concentration at time t, kg/m3 CZi (i = 1-3) = Ginkgo flavonoids concentration in the height of Z1 being 40 cm, Z2 being 80 cm, or Z3 being 140 cm of downer after adsorption, kg/m3 Daxl = liquid dispersion coefficient, m2/s Daxs = solid dispersion coefficient, m2/s Db = riser or downer diameter, m dp = particle diameter, m 3610


Industrial & Engineering Chemistry Research E = residence time distribution dimension F = qt/qe Fo = εs/εl G = conductivity, S/m Gs = solid particle circulation rate, kg/(m2 s) h = particles accumulation height, m k = adsorption rate constant, s-1 Kad. = adsorption rate constant, s-1 kf = liquid film mass-transfer coefficient, m/s kL = constant in Langmuir equation defined as KLqe, m3/kg KL = Langmuir constant, m3/mol kp = intraparticle diffusion constant, 10-3 (kg/kg)/s1/2 L = height of riser or downer, m m = mass quality of adsorbent per unit volume of adsorption solution, kg/m3 Qd = flow rate of feed, m3/s Qr = flow rate of elution of ethanol solution, m3/s qe = static adsorption capacity, 10-3 kg/kg qer = Ginkgo flavonoids concentration in macroporous resin particles in solid-liquid separator, kg/m3 qor = Ginkgo flavonoids concentration in macroporous resin particles in the inlet of riser kg/m3 qr = riser concentration of Ginkgo flavonoids in the solid phase kg/m3 qt = adsorption amount at time t, 10-3 kg/kg R = radius of particle, m Ss = specific surface area of macroporous resin particles, m-1 t = time, s T = transmittance ht = average residence time, s U = superficial velocity of flow, m/s Ua = superficial liquid velocity of auxiliary stream, m/s Ugd = superficial gas velocity of downer, m/s Uld = feed liquid velocity of downer, m/s Ulr = superficial gas velocity of riser, m/s Um = superficial liquid velocity of primary stream, m/s Up = superficial solid velocity, m/s V = volume of liquid, m3 Vd = adsorption volume of downer, m3 Vsr = desorption rate of Ginkgo flavonoids in the riser, 10-3 kg/(kg s) W = weight of macroporous resin particles, kg X = overall yield Xd = adsorption ratio Xr = desorption ratio Z = height of riser or downer from the bottom, m Greek Letters εg = gas holdup εl = liquid holdup εp = porosity of the resin particles, 0.42-0.46 εs = solid holdup φ = diameter, m θ = dimensionless time Fs = density of solid particles, kg/m3 Fsa = wet packing density of solid particles, =Fs(1 - εp), kg/m3 σθ2 = dimensionless residence time variance σt2 = residence time variance, s2 Subscripts a = auxiliary liquid flow d = downer

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e = end, adsorption equilibrium f = particle surface g = gas phase l = liquid phase m = main or primary flow n = n = 1, 2, 3 o = beginning r = riser s = solid p = particle

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