The preparation and filtration characteristics of Dextran–MnO2 gel particles

doi:10.1016/j.powtec.2005.07.006

Powder Technology

Volume 161, Issue 1, 10 January 2006, Pages 41-47

https://doi.org/10.1016/j.powtec.2005.07.006 Get rights and content

Abstract

Different kinds of Dextran–MnO₂ gel particles are prepared in different conditions, depending on variables such as the molecular weight of dextran and formation temperature. Some physical properties and filtration characteristics of these gel particles are measured and discussed. Although the mean sizes of these gel particles are very close to each other, their filtration characteristics are far different due to their mechanical strength and compressibility. A typical filtration curve of gel particles can be divided into three regions, and a retardation cake compression during the filtration can be observed from the curve. The particle formation temperature has a trivial effect on their mechanical strength. An increase in formation temperature leads to only a slight decrease in particle size. On the other hand, gel particles are formed by using dextrans with three different molecular weights – 70,000 (sample A), 500,000 (sample B) and 2,000,000 Da (sample C) – and are used in filtration experiments. The results show that the molecular weight of dextran plays a major role in determining particle mechanical strength; the sequence of particle hardness is sample B > sample A > sample C. The dynamic analysis method proposed by Hwang and Hseuh [K.J. Hwang, C.L. Hseuh, J. Membr. Sci. 214 (2003) 259] is employed to estimate the local cake properties in a filter cake, e.g., solid compressive pressure, porosity and specific filtration resistance. Because sample C has the highest compressibility, it constructs a cake with the lowest porosity and the highest specific filtration resistance; and consequently, the lowest filtration rate. Although the mechanical strength of sample B is higher than that of sample A, its wider particle size distribution results in lower cake porosity and higher filtration resistance, as well as a lower filtration rate. It could be said that the filtration rate decreases with an increase in the molecular weight of dextran.

Introduction

Gel filtration is a reliable and efficient method for purification of enzymes, polysaccharides, proteins and other biological macromolecules. In a gel filtration, bio-materials are separated according to their size differences as they pass through a column packed with chromatographic gel particles. In general, gel particles are porous materials in nature; the range of their pore sizes plays the major role in determining the selectivity and filterability of a gel filtration.

The gel particles used in gel filtration may be formed by many different methods depending on the designed particle size, chemical–physical stability, and pore sizes. In practice, a three-dimensional network can be constructed to form a gel by cross-linking of a single polymer, e.g. dextran. However, the mechanical strength and the deformable nature of these soft gels should be noted if a high-pressure filtration is carried out. In some manufacture processes, a second polymer or other composite material can be grafted onto a pre-formed polymeric matrix to combine their valuable properties and to enhance gel strength. This is the reason why we use MnO₂ as the second material.

When deformable or soft particles are filtered, their highly compressible behavior may result in a compact “skin layer” next to the membrane surface [2]. The thickness of this layer is about 20% of the whole cake; however, almost the total filtration resistance exists in this layer. Tanaka et al. [3] found that the specific filtration resistance of cakes formed by microorganisms, highly compressible cells, was highly dependent upon the filtration pressure. In a previous experiment, Meeten [4] carried out the constant pressure filtration of a kind of gel particle, Sephadex. He found that the time dependence of the cumulative filtrate volume was in accordance with the Ruth relationship under an extreme low applied filtration pressure (< 10.3 kPa). The compressibility of Sephadex cake varied with the magnitude of the applied pressure. However, it can be concluded that the dt / dv versus v filtration curve no longer forms a straight line if a deformable or soft particulate matter is filtered [1], [5], [6], [7]. Hwang et al. [6] studied the microfiltration mechanisms in deformable submicron particles. They found that the tangents of the filtration curves might change slopes due to particle compression and deformation. Hwang and Hseuh [1] studied the filtration characteristics of Dextran–MnO₂ gel particles. The filtration curves were analyzed to understand the effect of filtration pressure on the cake properties, such as porosity and specific filtration resistance.

Based on the important results of previous researches [1], [5], [6], [7], the filtration characteristics of gel particles can be summarized as following:

(1)
The filter cake is highly deformable and compressible,
(2)
A creeping (or retardation) effect of cake compression may occur depending on the particle mechanical strength,
(3)
Some flow models, such as the Kozeny equation, can no longer be used to relate the filtration rate and the applied pressure, since cake porosity may be as low as 0.1–0.2,
(4)
A typical dt / dv versus v filtration curve is shown in Fig. 1 [1]. The filtration curve can be divided into three regions. The first region can be regressed into a straight line similar to that indicating filtration of rigid particles because of the retardation effect of cake compression. The second region of the filtration curve suddenly increases its tangent slope and shows a concave–convex trend. The increase of the local tangent slope is dependent on the deformability and the compressibility of the gel particles. A compact cake may be constructed during this period. Because most of the solid compressive pressures rivet on the compact structure next to the membrane surface, a slight compression on the newly formed cake can be observed. The slopes of the local curve tangents therefore become smaller and remain constant in the third stage.

As soft and porous particles are filtered, the filter cakes always reveal a viscoelastic behavior because of the rearrangement and compression of the particles. Therefore, the packing structure and the degree of cake compression are major effects on the filtration resistance. In order to grasp the filterability of a gel particle, it is essential to first understand the cake properties under various conditions. Based on some empirical equations and known conditions, Hwang and Hseuh [1] proposed a dynamic analysis method for estimating the porosity and specific filtration resistance of cake during a constant pressure filtration. The major equations included the basic filtration equation, the cake compression model and Tiller empirical equations [8]. With only a set of t versus v data, the distributions of local cake properties could be estimated.

In this study, four kinds of Dextran–MnO₂ gel particles were prepared at different formation temperatures using dextrans with different molecular weights. The size distributions of the prepared particles were measured and compared. Experiments of constant pressure filtration were carried out using these gel particles. The filtration curves of these gel particles were compared, and the cake properties were analyzed and discussed using the dynamic analysis method proposed by Hwang and Hseuh [1].

Section snippets

Particle preparation

Dextran–MnO₂ gel particles were prepared under four different conditions listed in Table 1. Dextran, a kind of polysaccharide with three different molecular weights (70,000, 500,000 and 2,000,000 Da) manufactured by SIGMA® Co., was dissolved in a liquid ethanol. The solution was then slowly added to a 1.5 wt.% KMnO₄ aqueous solution. The solution was well stirred continuously at a given temperature (said formation temperature) until the chemical reaction was complete. In the reaction process,

Results and discussion

Fig. 5 shows the filtration curves of sample D under various filtration pressures. The trends of these curves are all the same as that in Fig. 1. The whole process of a filtration can be divided into three regions according to the compression statuses of the cake and particles. This figure shows that the lower the applied filtration pressure, the higher the curve will be located. Since the vertical coordinates represent the reciprocal of filtration rates, an increase in filtration pressure

Conclusion

Four kinds of Dextran–MnO₂ gel particles have been prepared at different formation temperatures using dextrans with different molecular weights. The prepared gel particles had almost the same mean sizes, original void volume fractions in particles and intrinsic densities. However, their filtration characteristics were far different due to their mechanical strength and compressibility. The results showed that the formation temperature had trivial effect on the particle mechanical strength, while

List of symbols

cake thickness, [m]

P_s

local solid compressive pressure, [N/m²]

filtration time, [s]

t_c

the filtration time at transition point from region I to region II, [s]

T_s

particle formation temperature, [°C]

received filtrate volume per unit area, [m³/m²]

distance from the membrane surface, [m]

Greek letters.

specific filtration resistance of cake, [m/kg]

porosity of cake, [–]

ΔP

filtration pressure, [N/m²]

Subscript.

average cake properties

Acknowledgments

The authors wish to express their sincere gratitude to the National Science Council of the Republic of China for its financial support.

References (8)

K.J. Hwang et al.
J. Membr. Sci.
(2003)
G.H. Meeten
Chem. Eng. Sci.
(2000)
W.M. Lu et al.
J. Membr. Sci.
(2002)
F.M. Tiller et al.
A.I.Ch.E. J.
(1973)

There are more references available in the full text version of this article.

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