Local solidosity of microcrystalline cellulose during dead-end filtration and sedimentation

https://doi.org/10.1016/j.cherd.2013.01.007Get rights and content

Abstract

Solid–liquid separation by filtration and sedimentation are important operations used in a wide range of industries. One important characteristic of both the filtration and sedimentation processes is the solidosity of the filter cake/sediment that is formed, affecting the efficiency and design of the separation. In this study local solidosity was investigated using a γ-attenuation method during both filtration and sedimentation experiments for microcrystalline cellulose, a highly crystalline cellulose with particles of about 2–80 μm in diameter. Constitutive relationships for the solidosity were investigated using both filtration (i.e. cake build-up and expression) and sedimentation data for experiments at different pH and suspension concentrations. The sedimentation behaviour under these different conditions was also investigated. It was found that a three parameter empirical model could be used to describe the constitutive relationship between local solidosity and local solid compressible pressure for the sediment formed and the filter cake after both cake build-up and expression. This correlation worked well for the material investigated even at low solid compressible pressures.

Highlights

► An empirical model could be used to describe the pressure dependence of solidosity. ► Sedimentation was useful when investigating compressible behaviour at low pressures. ► Using both sedimentation and filtration data was found to be advantageous. ► The sedimentation rate for the MCC was lower at high pH.

Introduction

Solid–liquid separation is an important operation used in a wide range of processes. While such separation can be achieved by drying, mechanical methods are often preferred due to the low energy demand, i.e. low operational cost. Two operations commonly used for solid–liquid separation are dead-end filtration and sedimentation. In the former, a pressure difference drives the suspension towards the filter medium that is used as separating agent, allowing only the liquid to pass through. In the latter the solid particles, which are of higher density than the liquid, are allowed to settle, forming a sediment containing a high concentration of solids.

Sedimentation is often characterised as being either free or hindered. Free sedimentation is often assumed for suspensions with a low solid concentration: that the particles settle independently of each other. A solid concentration of 0.5 volume% has been suggested as an upper limit for this assumption (Svarovsky, 2000). A simple, often used, correlation for a sphere under laminar conditions is the Stokes equation:vsed=dp2(ρsρl)g18μwhere vsed is the sedimentation velocity of the particle, dp the diameter of the particle, ρs the density of the particle, ρl the density of the liquid, g the acceleration of gravity and μ the viscosity of the fluid.

Richardson and Zaki (1954) suggested the following extension of the Stokes equation to account for interactions between particles during sedimentation:vsed=dp2(ρsρl)g18μ(1ϕ)4.65where ϕ is the solidosity.

For hindered sedimentation, a widely accepted theory was proposed by Kynch (1951). The Kynch theory is based on several assumptions, an important one being that the local sedimentation velocity is a function of the local solid concentration only; using this approach, a particle flux curve for hindered sedimentation may be constructed. Buscall and White (1987) proposed a model for consolidation of concentrated suspensions considering the flocculated suspensions as a body that possesses a concentration-dependent yield stress. The approach considering the compressive yield stress has been applied to both sedimentation, e.g. Green et al. (1996) and filtration, e.g. Usher et al. (2001).

Darcy's work with respect to flow through porous beds (Darcy, 1856) is often referred to as the starting point for a more fundamental understanding of dead-end filtration. The Darcy equation for flow and pressure drop over a stationary porous bed can be used to construct models for growing cakes; it forms the basis of the classical filtration equation, based on the average properties of the cake being formed (e.g. Ruth, 1935).The classical filtration equation is derived from the assumption that the filter cake formed is incompressible, i.e. the local solidosity (volume fraction of solids) does not increase as the solid compressible pressure increases. Modelling of dead-end filtration where a compressible cake is formed requires that local filtration properties are considered. Local properties, such as the local solidosity and the local solid compressible pressure, can be related to each other by the use of constitutive relationships. One approach employed often is the use of semi-empirical equations (e.g. Tiller and Leu, 1980).

Whilst local properties can be estimated indirectly, direct measurement is to be preferred. Several different techniques for doing this can be found in the literature. The local pressure during cake build-up has been studied using pressure probes mounted in different configurations in several studies (e.g. Chase and Willis, 1992, Fathi-Najafi and Theliander, 1995, Johansson and Theliander, 2003, Mattsson et al., 2011, Okamura and Shirato, 1955, Shirato et al., 1971). The local solidosity of both sediments and filter cakes has been investigated using X-rays and γ-attenuation techniques (e.g. Bergström et al., 1992, Bierck et al., 1988, Johansson and Theliander, 2003, Mattsson et al., 2011, Tiller et al., 1995). Been and Sills (1981) used this type of technique to measure the solidosity of the sedimentation of soil in water while simultaneously measuring the pressure using pressure probes. Several other methods have also been used to measure local solidosity, including measuring conductivity (e.g. Chase and Willis, 1992, Holdich and Sinclair, 1992, Rietema, 1953, Shirato et al., 1971) and 1H NMR (e.g. La Heij et al., 1996). Both of these methods are non-destructive and can be used directly during the build-up cake. Cake dissection, which is a destructive method, has also been used (e.g. Dell and Sinha, 1964, Meeten, 1993, Smiles and Rosenthal, 1968, Yim and Song, 2008).

The material investigated in this study, microcrystalline cellulose (MCC), is highly crystalline and is currently used as a food additive as well as for tableting in the pharmaceutical industry. MCC is an interesting model material for novel materials, such as nanofibrillated cellulose, of which dewatering is known to be very challenging (Liu and Berglund, 2012). Earlier studies of the filtration behaviour of MCC show that this material forms compressible cakes with an average filtration resistance of about 1010 (m/kg) (Mattsson et al., 2012a). Indications of skin formation during filtration of MCC has also been found (Mattsson et al., 2012b).

The cake formation of MCC during filtration has already been studied, whereas its behaviour during sedimentation has still to be investigated. The local solidosity of sediments formed by MCC at lower solid compressible pressure is also poorly investigated. To the knowledge of the authors, no modelling of constitutive relationships based directly on measured local solidosity, where data from both sedimentation and filtration (i.e. cake build-up and expression), can be found in the literature.

This study investigates the behaviour of MCC particles during solid–liquid separation using filtration and sedimentation. Local solidosity was investigated using a γ-attenuation method in both filtration and sedimentation experiments. Local hydrostatic pressure was also measured in the filter cakes formed during the filtration operation. Two slurry concentrations were investigated: 5 and 10 volume%. Surface interactions between the MCC particles were altered by changing the pH of the slurries; the three pH levels used were 2.9, 6.3 and 9.3.

Section snippets

Materials

The material investigated, a commercially available microcrystalline cellulose known as Avicel® PH-105, is produced by FMC BioPolymer. This type of MCC is prepared by a strong acid hydrolysis of delignified wood fibre. After spray-drying, porous particles of 10–50 μm in size are formed, consisting of highly crystalline cellulose microfibrils held together by strong hydrogen bonds (Iijima and Takeo, 2000, Levis and Deasy, 2001).

Methods used for characterisation

Several characteristics of the MCC particles were investigated using

Characterisation

The MCC was characterised using the methods described in Section 2.2. The material was found to have a BET surface area of 2.1 m2/g and a solid density of 1560 kg/m3.

The chemical composition of the particles was determined after hydrolysis of the MCC (Theander and Westerlund, 1986). The MCC was completely dissolved using acid hydrolysis, thereby indicating a negligible content of lignin. Glucose was found to form 97% of the carbohydrate monomers and small amounts, of about 1.5% each, of xylane

Conclusions

A methodology where the local solidosity was investigated for a wide range of local solid compressible pressure using both sedimentation and filtration experiments were developed.

A three-parameter empirical model could be used to describe the constitutive relationship between the local solidosity and the local solid compressible pressure of the sediment and the filter cake formed after both the cake build-up and expression phases. This correlation worked well for the material investigated, even

Acknowledgements

The authors are indebted to the Knut and Alice Wallenberg Foundation and the Swedish Research Council for their financial support.

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