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Hydrodynamic Characteristics and Expansion Behavior of Beds Containing Single and Binary Mixtures of Particles Krishna S. V. S. R. Bandaru,† Lars Christian Kessler,‡ Michael W. Wolff,§ Udo Reichl,§ Andreas Seidel-Morgenstern,‡ and Subramaniam Pushpavanam*,† Department of Chemical Engineering, IIT Madras, Chennai-600036, India, Physical and Chemical Foundations of Process Engineering Group, Max Planck Institute for Dynamics of Complex Technical Systems, Sandtorstrasse 1, Magdeburg 39106, Germany, and Bioprocess Engineering Group, Max Planck Institute for Dynamics of Complex Technical Systems, Sandtorstrasse 1, Magdeburg 39106, Germany
In this work, the hydrodynamic behavior of expanded beds was studied experimentally and theoretically. The transient response and steady-state behavior of the bed height at different flow rates were measured. The applicability of the Richardson-Zaki equation to predict the steady-state behavior is demonstrated, and a correlation is proposed to determine the dependence of the transient response of the bed height on particle and fluid properties. We have also determined the axial size distribution in the expanded state under steady conditions for four different kinds of particles. We have extended the investigation to analyze the behavior of the bed experimentally for a binary mixture of different particles. In the experiments with binary mixtures, a distinct segregation of these particles could be achieved. Size distribution analysis indicates a clear axial stratification of the two chosen particles, leading to a maximum in average particle diameter as we move along the bed in the upward direction. The data obtained in this study showed that it is possible to estimate the steady-state bed height for a single particle type as well as for mixtures of two particles. The number of empirical parameters needed to characterize the system is only two. The results of this study can be used to design expanded-bed setups using two differently functionalized particle types. 1. Introduction Chromatography is widely used as a method for the isolation of high-value products in many industries. While various forms exist, the most prominent one is packed-bed chromatography.1 One of the disadvantages of this mode of operation is its inability to process fluids containing solid particles, as this would lead to a blockage of the column. One possible way to combine the resolution power of chromatography with an increased robustness toward particulate matter is to use so-called expanded beds.2 Here, the stationary phase is not packed but fluidized by a flow in the upward direction. This effectively increases the porosity and allows components, which would otherwise block the flow, to pass through the bed without disturbing the adsorption of target substances on a functionalized matrix. After adsorption, the bed is washed and the flow direction is reversed as desorption is carried out in a conventional packed-bed mode. One of the most important areas of the application of expanded beds is downstream processing in biotechnology. Here, cultivation broths and raw cell lysates have to be filtered or centrifuged in order to remove suspended solids. Using expanded-bed adsorption (EBA), this step can often be skipped, leading to reduced process time.2,3 Therefore, a number of studies on the expansion behavior of beds have been motivated by applications in bioseparation. Chang and Chase4 investigated adsorption characteristics of lysozyme for different degrees of bed expansion, viscosities, and flow rates to determine optimal operating conditions of the bed. Ghose and Chase5 studied the * To whom correspondence should be addressed. E-mail: spush@ iitm.ac.in. Fax: +914422570545. Tel.: +91-44-2257 4161. † IIT Madras. ‡ Physical and Chemical Foundations of Process Engineering Group, Max Planck Institute for Dynamics of Complex Technical Systems. § Bioprocess Engineering Group, Max Planck Institute for Dynamics of Complex Technical Systems.
dependence of the hydrodynamic behavior of expanded beds on the column diameter for three different column diameters (0.5, 1, and 5 cm). They found that all three beds exhibited a similar degree of expansion at steady state and also gave similar breakthrough curves for the adsorption of lysozyme. Streamline SP particles were used in both studies. Bruce and Chase6 investigated axial variations in particle size, bed voidage, liquid dispersion, and dynamic capacity for Streamline SP and DEAE, respectively. Tong and Sun7 carried out individual hydrodynamic experiments for two stationary phases, a Streamline quartz base matrix and an agarose-coated steel particle, differing in size distribution characteristics. They found that the volume-weighted mean diameter of both particles decreases linearly with axial height. Yun et al.8 studied the axial dispersion along the bed height in expanded-bed columns using Upfront Fastline SP particles and found that the axial dispersion coefficient decreases with increasing bed height. Thelen and Ramirez9 conducted experiments on bed dynamics using step changes (increases and decreases) in liquid velocity in a liquid-solid fluidized bed for Streamline DEAE particles. They developed a distributed-parameter model based on particle and fluid properties. The model predictions were accurate for small step changes, while for large step changes, a considerable deviation was observed between the model predictions and the experimental data. Thelen and Ramirez10 applied two-phase theory for predicting the bed dynamics in Stokes regime on expanded beds. They studied the dynamic behavior of expanded beds experimentally during step changes in fluidization velocity for different fluid properties. Streamline DEAE particles were used in this study. Rasul et al.11 used fluidization as a technique for separating lead glass, coal, sand, and rutile particles of different densities and diameters and obtained a mixing/separation regime map in terms of density ratios and diameter ratios. Asif12 studied the overall bed expansion of binary particle mixtures, consisting
10.1021/ie061580x CCC: $37.00 © 2007 American Chemical Society Published on Web 05/16/2007
Ind. Eng. Chem. Res., Vol. 46, No. 13, 2007 4687 Table 1. Particle Characteristics for the Four Media Used in This Study particle
size rangea (µm)
densitya (kg/m3)
avg sizea (µm)
avg size measuredb (µm)
avg size measuredc (µm)
Streamline SP Streamline DEAE Streamline Direct CST Q Hyper-Z
100-300 100-300 80-165 40-105
1200 1200 1800 3200
200 200 140 75
173.4 196.2 134 71.3
165 187.8 131.6 65.7
a
Provided by manufacturer. b Experimentally measured (volume-weighted mean diameter, vwmd). c Experimentally measured (d50).
of PET particles and sand, in a liquid-solid fluidized bed. The overall bed expansion was predicted using four models, namely, (1) a property-averaging model, (2) a serial model, (3) a packing model, and (4) a voidage-average model. Escudie et al.13 studied the effect of particle size, shape, and density on mixing and segregation of a binary mixture of particles in liquid-solid fluidized beds. For this, poly(tetrafluoroethylene) (PTFE) particles with an identical volume were used. The main aim of the work reported here is to understand the influence of various operating parameters on the hydrodynamic characteristics of an expanded bed containing a single type of particle as well as a mixture of two different particle types (hereafter called a two-particle mixture). The studies have been carried out using four types of particles, differing in particle diameter, size distribution, and density (see Table 1). All these materials are commercially available chromatographic stationary phases, showing particle-specific size distributions. We have investigated the transient as well as the steady-state behavior under different conditions of fluid velocity and initial bed height. The Richardson-Zaki equation14 is used to estimate bed porosity, thus enabling us to theoretically predict the steadystate height of the expanded bed. The variation of the bed height with time and the dependency of the expanded-bed height on flow rates for different kinds of particles are also studied. An empirical relation is proposed to determine the variation of the axial bed height with time for step changes in flow rate. The axial particle size distribution is also investigated for both the single particle types and the two-particle mixtures. The study presented in this contribution is a necessary first step toward using two types of chromatographic materials, preferably with different functionalizations, in expanded-bed chromatography. The formation of two distinct layers offers, for example, the possibility to selectively adsorb one component in a layer (say, the bottom layer), for example, using an affinity ligand. This is useful if the adsorbed component exhibits similar binding behavior toward the second layer, which could make a highpurity separation difficult. After a selective removal of the bottom layer, good separation can be achieved more easily. The results presented here can be used to determine whether the described segregation of layers is possible for a given combination of particles. 2. Experimental Section 2.1. Materials. The stationary phases used in this study are Streamline SP, Streamline DEAE, and Streamline Direct CST (GE Healthcare, Uppsala, Sweden) and Q Hyper Z (Ciphergen Biosystems, Fremont, CA, U.S.A.). The physical particle properties for all these systems are shown in Table 1. Additional physicochemical information on the particles is given in Table 2. Bed expansion experiments were conducted in a Streamline 25 column (24.8 mm i.d., 1 m height) using an AEKTA Basic system (both from GE Healthcare, Uppsala, Sweden). For determination of particle axial size distribution, a custom-made Plexiglas tube (25.7 mm i.d., 1 m height) with nine side ports was used (Figure 1). As shown in Figure 1, these side ports are
Table 2. Features of the Particles (Details Are Presented as Provided by the Manufacturers) (1) Streamline SPa composition flow rate range binding capacity pH stability (operational) CIP stability (short term) (2) Streamline DEAEa composition flow rate range binding capacity pH stability (operational) CIP stability (short term) (3) Streamline Direct CSTa composition flow rate range binding capacity pH stability (operational) CIP stability (short term) (4) Q Hyper Zb composition flow rate range binding capacity pH stability (operational) CIP stability (short term)
macroporous cross-linked 6% agarose containing a crystalline quartz core 200-400 cm/h 60 (mg of lysozyme)/(mL of adsorbent) 3-13 3-14 macroporous cross-linked 6% agarose containing a crystalline quartz core 200-400 cm/h 40 (mg of BSA)/(mL of adsorbent) 2-13 2-14 macroporous cross-linked 4% agarose containing stainless steel material 200-400 cm/h 30 (mg of BSA)/(mL of adsorbent) 3-13 2-13 zirconium oxide 250-500 cm/h 40 (mg of lysozyme)/(mL of adsorbent)
a GE Healthcare, Uppsala, Sweden. b Ciphergen, Biosystems, Fremont, CA, U.S.A.
made of Plexiglas with an outside thread. A silicone seal is placed on top and fastened using a metal cap with a small (1.5 mm) aperture in the center. The ports are located at heights of 1.75, 5, 10, 15, 20, 25, 30, 35, and 40 cm above the flow distributor at the bottom of the column. This allows us to collect samples for size distribution analysis. All experiments were performed with high-purity water from a Direct-Q 5 system (Millipore Corporation, Billerica, MA, U.S.A.). Measurements of particle sizes were carried out in a Cilas 1180 laser particle size analyzer (Cilas U.S., Inc., Madison, WI, U.S.A.) and in a more sensitive Mastersizer 2000 system (Malvern Instruments, Malvern, U.K.) in the case that lower sample quantities are obtained for the measurements of axial distribution. 2.2. Experimental Procedures. The column was loaded with varying amounts of material, resulting in different initial bed heights H0. For all the particles, an estimated initial porosity (0) of 0.39 was obtained by draining the liquid volume from the given volume of sample. This value is very close to that reported in the literature.1,15 In this study, H0 was varied from 10 to 30.7 cm. For a defined flow rate, the beds expanded to a certain level. If no further expansion was observed, we assumed that steady state had been reached. Bed height was then measured visually using a scale fixed to the column. The choice of initial bed height range was based on the investigations of Hjorth et al.,16 who used a similar range for
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n ) 4.4Re-0.1 for 1 < Ret < 500 t n ) 2.4 for 500 < Ret where ul is the superficial liquid velocity, ut is the particle terminal velocity, is the bed porosity at the superficial liquid velocity of ul, Ret is the terminal particle Reynolds number, and n is the RZ parameter. In current EBA operations, however, terminal particle Reynolds numbers are >0.2. The particle terminal velocity (ut) is calculated using Stokes terminal velocity equation (eq 2).
dp2(Fp - Fl)g ut ) 18µl
(2)
In this equation, Fp and Fl are the particle and liquid densities, g is the gravitation constant, and µl isthe liquid viscosity. We use these equations to predict the bed porosity at a given superficial liquid velocity. This is then used to predict the corresponding final bed height Hf from
(1 - ) ) (1 - 0)H0/Hf Figure 1. Schematic representation of the experimental setup for the determination of particle size distribution.
expanded-bed experiments with monoparticles. For graphical representation, the height of the bed was scaled with the initial height for each experiment. Consequently, all curves originate at a relative bed height of unity. Each experiment was repeated three times to verify reproducibility. For these experiments where we determined bed height, the commercially available Streamline 25 column was used. For the determination of axial particle size distributions, we used the Plexiglas column. Liquid (2 mL) containing small samples of stationary phase was carefully collected at the column center from each port using a needle (0.9 mm i.d.) and a 2 mL syringe. Here, the focus was to avoid disturbing the bed; therefore, withdrawal was very slow, with a sampling time of approximately 20 s each. The total amount of the solids removed from the bed during sampling was
