Abstract
Microcrystalline cellulose (MCC) is a purified partially depolymerized nonfibrous form of cellulose, a crystalline powder composed of porous particles. In this study, the drying of MCC was investigated with two different solids content MCCs by using three different drying methods: a high-velocity cyclone dryer, a spray dryer, and a fluidized bed dryer. The effects of the different drying techniques on the geometrical dimensions and morphology of the dried MCC particles and aggregates were researched. Based on the results, the dried MCC particle morphology is highly dependent on the used raw material properties as well as the liquid removal mechanism during drying. The preserving of the morphology of the raw material MCC was mostly achieved by fluidized bed drying, which facilitated MCC to dry as discrete particles with high surface area and lesser aggregation when using the MCC with 45% dry consistency as raw material. Spray drying was observed to produce small, most circular particles with the most homogenous size distributions and high velocity cyclone largest, most heterogenous and irregular shape particles and aggregates. All results have been presented as such after drying without screening.
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Effect of Drying Methods on the Particle Morphology of Microcrystalline Cellulose
Annina Lähdeniemi * and Olli Dahl
Microcrystalline cellulose (MCC) is a purified partially depolymerized nonfibrous form of cellulose, a crystalline powder composed of porous particles. In this study, the drying of MCC was investigated with two different solids content MCCs by using three different drying methods: a high-velocity cyclone dryer, a spray dryer, and a fluidized bed dryer. The effects of the different drying techniques on the geometrical dimensions and morphology of the dried MCC particles and aggregates were researched. Based on the results, the dried MCC particle morphology is highly dependent on the used raw material properties as well as the liquid removal mechanism during drying. The preserving of the morphology of the raw material MCC was mostly achieved by fluidized bed drying, which facilitated MCC to dry as discrete particles with high surface area and lesser aggregation when using the MCC with 45% dry consistency as raw material. Spray drying was observed to produce small, most circular particles with the most homogenous size distributions and high velocity cyclone largest, most heterogenous and irregular shape particles and aggregates. All results have been presented as such after drying without screening.
DOI: 10.15376/biores.20.4.9524-9541
Keywords: Microcrystalline cellulose; MCC; Cellulose drying; Cyclone dryer; Spray dryer; Fluidized bed dryer; Cellulose powder; Never-dried MCC
Contact information: Department of Bioproducts and Biosystems, Aalto University School of Chemical Technology, PO Box 16300, Vuorimiehentie 1, Espoo, Finland;
* Corresponding author: annina.lahdeniemi@aalto.fi
INTRODUCTION
Microcrystalline cellulose (MCC) is a purified partially depolymerized nonfibrous form of cellulose derived mostly from wooden plants and cotton with mineral acids. It is an amorphous, odorless, tasteless, thermally stable crystalline powder composed of porous particles. MCC is capable of forming stable colloidal suspensions, creating a three-dimensional network, and maintains these properties as well at high temperature as in low pH. Consequently, MCC is highly applicable in products that require solution stabilizers, bulking agents, material texture modifications, compaction stability, dry processability, compatibility, and other material modification properties (Battista and Smith 1962; Vasiliu-Oprea and Nicoleanu 1993; Krawczyk et al. 2009; Tuason et al. 2012).
The first products from MCC were developed and patented for the food and pharmacy industries during the 1960’s. At present, these industry sectors are yet today MCC’s most common application areas with cosmetics. MCC has been widely used in industrial applications as a suspension stabilizer and reinforcing agent for final products. Since MCC is inert for humans, it can be widely applied in pharmaceuticals, where it is used currently as bulk, binder, compression aid additive for tablets and thickening, and as a structural agent for healing creams. Even today, it is considered as most possibly the best dry filler-binder currently available and remains the major excipient for the preparation of pharmaceutical pellets by extrusion/spheronization (Balaxi et al. 2009). Additionally, MCC presents a high dilution potential, making it suitable for the formulation of low dose and potent drugs. Possessing about 35% market share, it has been estimated that the pharmaceutical sector is the biggest application area for MCC (Transparency market research 2015).
The physical properties of MCC differ plenty based on the used application. The average particle size of commercial MCC products ranges from 20 to 250 μm, although larger agglomerate products between 1000 and 1400 μm are available as well (FMC 2015; JRS 2015; MingTai 2015). Different processing conditions affect the properties of MCC in terms of surface area, porosity, crystallinity, molecular weight, moisture content, and shape. These differences also affect the powder and compaction behavior of the MCC. By changing particle size and aspect ratio, porosity, surface charge, adding functional groups, or by changing the drying circumstances and creating bundles of several single MCC particles, the properties of the processed tablets can be adjusted (Mills 2007; Kamel et al. 2008; Thoorens et al. 2014).
Because MCC drying requires high energy input, it is desirable to have a simple and economical dryer. Commercially, spray drying is one of the most commonly used techniques in the industry for drying MCC products into powder form. In the past, different fluidized bed dryers, spin flash dryer as well as lyophilization have been among the available options for MCC drying (Beck et al. 2012; Peng et al. 2012). One promising future drying technology that has been investigated, but not yet been used on MCC, is the high velocity cyclone or vortex type dryer (Nebra et al. 2000). The different drying methods have dissimilar and distinct effects on the particle morphology, size, porosity and aggregation, thus affecting the characteristic of the produced dried product and its post-processing procedures, such as screening and milling.
The main advantages of spray drying are the extremely short contact time of the suspension with the heat source and elimination of post-processing stages for fiber separation, such as grinding and screening. Additionally, spray drying increases the sterility of the product and different high-quality powders can be produced by engineering the size, shape, and morphology of particles by optimizing the operating conditions. The disadvantages of the process are clogging the spraying systems with above-scale material and the possible formation of particle clusters. Most MCC dried by the spray drying imparts a high inter-particulate porous and aggregate nature to the dried powder (Walton and Mumford 1999; Cal and Sollohub 2010; Peng et al. 2012; Amin et al. 2014; Paudel et al. 2013).
Fluidized bed drying (FBD) facilitates powders to dry as discrete particles with a low inter particulate void (Nwachukwu and Ofoefule 2020). The advantages of fluidized bed drying include high rate of moisture removal, high thermal efficiency, easy material transport inside the dryer, straightforward control, and low operating and maintenance costs (Mujumdar and Devahasthin 2003). Challenges of this technology are the entrainment of fine particles, attrition or pulverization of particles, along with agglomeration of fine particles as well as achieving a suitable gas-solid contact, bed stability and uniform product quality for certain types of FBDs (Daud 2008; Liu et al. 2014; Zimmermann et al. 2016).
The results of high velocity cyclone drying depend on the equipment design, but as a non-thermal separation technology, cyclones have the advantages of a high separation efficiency, low energy consumption due to the relatively low temperatures used, and simple maintenance (Grimm et al. 2017; Dehdarinejad and Bayareh 2023).
Typically, a highly desirable feature of a good drying technique is the preserving of the properties and morphology of the MCC particles. It is well-established knowledge that drying has a permanent effect on the cellulose structure. Upon the drying of cellulose, the water removal leads to the formation of internal hydrogen bonds due to the presence of hydroxyl groups on the cellulose surface, which promotes an attraction among the cellulose fibers. This phenomenon, which promotes the formation of fiber agglomerates and the loss of their original dimensions in the nanometric scale, is commonly known as hornification (Spence et al. 2010). Hornification is known by the literature as an irreversible or partially reversible bond between the hydroxyl groups present in the cellulose and it has been reported that hornification decreases MCC’s binding ability during tableting as well as swelling and disintegration (Balaxi et al. 2009, Diniz et al. 2004). New studies made by Sjöstrand et al. (2023) shows that the hornification process begins already at a level of dryness of 20 percent and at temperatures as low as 40 °C. Nevertheless, the biggest changes in fiber structure are seen at temperatures above 100 °C.
Considering MCCs multiple current applications and future potential ones, it is of interest to investigate the possibility of producing MCC with different shape in cross-section to increase the surface area as it is of great importance for specific applications. To the authors’ knowledge, there has been no research data published of drying MCC with high velocity cyclone. Additionally, no research have been made of the drying of never-dried MCC produced with AaltoCell™ technique that varies from the commercially available MCC manufacturing methods with its low chemical consumption and lower production costs (Vanhatalo 2017). It is expected that the dried MCC powder quality and particle morphology are highly dependent on the used raw material as well as the liquid removal mechanism and the liquid-MCC interaction. Therefore, a study of different drying methods including their qualitative evaluation and analysis of the produced powders is thought to give an insight into the processing of the materials involved.
In this study, unmodified, never-dried microcrystalline cellulose MCC (produced with patented AaltoCell™ method, PURA™ MCC, Nordic Bioproducts Group) as a raw material was dried with three different drying methods; fluidized bed, high velocity cyclone and spray drying. The main aim was to discover how the various drying methods affect physical properties of the dried MCC powder. Due to the absence of additives and nano-size particles, the manufactured powders should be highly applicable for food additives and drug delivery. The effects of the drying actions on the crystalline structure, morphology, and geometrical dimensions of the manufactured dried MCC powders were investigated.
EXPERIMENTAL
Cellulose Raw Materials and the Production of MCC (AaltoCell™)
The cellulose raw material RA45% was never-dried microcrystalline cellulose manufactured with the AaltoCell™ method, as described in Vanhatalo and Dahl (2014). In brief, bleached softwood kraft pulp was hydrolyzed with a 1.5% dosage of sulfuric acid at 160 °C at 10% consistency for 110 min using a tube-like reactor with volume of 2.5 dm3. After the hydrolysis, the reactor was cooled, and the MCC material was washed with distilled water in a Büchner funnel until the washed filtrate conductivity was below 5.0 μS/cm. The washed MCC material was centrifuged at 4500 rpm with a filter bag to the dry consistency of 45%. The other MCC raw material, RA60%, was pre-dried from RA45% MCC with a heated Lödige Ploughshare® Laboratory Mixer (Gebrüder Lödige Maschinenbau GmbH, Germany) to the dry consistency of 60%.
Experimental Equipment
High-velocity cyclone dryer
The two MCC samples (45% and 60%) were processed with a cyclone dryer as described in Grimm et al. (2017). A schematic flow sheet of the process is presented in Fig. 1. An electric motor-powered fan creates a high-velocity airstream guided into an air channel and mixed with the processed material. The processed raw material is fed into the inlet airstream with a tubular disc conveyor rod. From the air channel, the mixed material flows into a cyclone-formed device in which the changes in radial velocity and pressure produce heavy vibrations in the cellulose material, causing them to dehydrate and disintegrate. Fluids and the finest fraction of the processed material (reject) escape from the cyclone through the upper part of the cone and are sent into a separate baghouse filter unit. The dried product comes out from the bottom of the cyclone.
Fig. 1. Simplified illustration of the high velocity cyclone drying process. Obtained from the Industrial Mechanical separators and vessel’s internet site. (Source: Industrial Separation GmbH and Co, 2025)
In test trials, the intake air was heated from 12 to 60 ℃ with a heat exchanger. During the experiment, the fan and the conveyors were set to run at approximately the same time that the intake air was preheated. The feeding rate was 4 kg/h, and the temperature inside the cyclone was 49 ℃. None of the parameters or functions of the drying process (such as input air temperature, material input rate, depth of the upper shutter, etc.) were optimized or maximized during the experiment.
Fig. 2. Simplified scheme of the Spray drying process obtained from the manufacturer’s internet site. (Source: Toption Decive Ltd and Co, 2025)
Spray dryer
The produced MCC was converted to dry powder by spray drying (Niro Mobile Minor, Niro Atomizer Ltd., Copenhagen, Denmark), using inlet and outlet air temperatures of 350 and 90 °C, respectively. Both MCC samples (45% and 60%) were diluted into 5% solid feed consistency before the drying. The spray drying process, presented in Fig. 2, can be characterized by three central stages: (a) in the first phase, the fluid, consisting of the water suspension and cellulose, is dispersed by spraying in the form of droplets; (b) in the second phase, the suspension droplets becomes in contact with the warm air stream as the heat transfer begins; and (c) in the third step, the solvent evaporation and formation of solid particles (Oliveira and Petrovick 2009).
Fig. 3. Illustrated picture of the particle fluidization in the Retsch fluidized bed laboratory dryer obtained from the manufacturer’s internet site. (Source: Retsch GmbH and Co, 2024)
Fluidized bed drying
A fluidized-bed dryer (rapid dryer model TG 100, Retsch GmbH and Co., Germany), with bed column of cylindrical shape approximately 18 cm in diameter and 22 cm high, was used for the drying experiments. The two raw materials, MCC 45% and MCC 60%, were dried at a temperature of 60 ± 1 ºC and inlet air of 30 m3/min for 3 h. The fluidized drying process in the used laboratory device is displayed in Fig. 3. In fluidized bed drying, hot air is blown trough the perforated plate, where the hot air gets distributed. The air pressure causes the fluidized particles to float. Heat and mass are exchanged effectively between particles and the hot air due to the large surface area of the particles. Violent mixing of the particles takes place in the fluidized layer because of gas bubbling (Hovmand 2020). The hot air stream extracts moisture from the particles and then exits through the filter bag in the cover of the equipment.
Test Standards and Methods
The particle size distribution, particle circularity, as well as the aggregate sizes and shapes of the samples were measured with Mastersizer 2000 equipped with a Hydro 2000MU dispersion and insight units (Malvern Instrument Ltd, Worcestershire, UK) that uses ultrasonication to disperse and separate the particles. The size distribution results and circularity plots as well as the particle agglomerates’ shape images were obtained with the Mastersizer 2000 software version 5.12. The measurement was conducted as published (Vanhatalo and Dahl 2014). Particle size distribution d(0.1), d(0.5), and d(0.9) values were reported to describe physical particle sizes. All particle size distribution and circularity measurements were done in triplicate.
The structural morphology of the CelRaw and hydrogel samples were observed with scanning electron microscopy (Zeiss Ʃigma-VP, Jena, Germany) at 1.5 keV, equipped with an Everhart-Thornley detector. Prior to imaging, the samples were sputter coated (Emitech K-100X, Lewes, UK) with a 10-nm gold layer to obtain proper conductivity.
RESULTS AND DISCUSSION
Particle Morphology with SEM Micrographs
SEM investigation was used to inspect the particle surface morphology. The SEM micrographs of the dried samples and raw materials are shown in Figs. 4 and 5. The micrographs show that the two raw materials displayed dissimilar shapes and apparent size differences. The never-dried raw material with the dry consistency of 45% presented clear crystalline cellulose structure with sizable, aggregated particles with more specific surface area. The pretreated raw material with the dry consistency of 60% appeared to have already less crystalline, more separated, spherical particles with reduced accumulation and smooth surfaces.
With the never-dried raw material, it seems that the crystalline structure of the particles and the specific surface area is maintained only with fluidized-bed drying, whereas spray drying and high velocity cyclone produced larger circular particles with less available surface area. In addition, it appears that the formation of aggregates was greater with the high velocity cyclone and spray drying than with the fluidized bed. These observations from the SEM micrographs are consistent with those reported by other researchers in past.
Fig. 4. SEM micrographs with two different magnifications (100x and 500x) of the never-dried MCC raw material a) and b), and the samples dried from it with spray dryer (Spray 45%) c) and d), fluisized bed dryer (FDB 45%) e) and f), and high-velocity cyclone dryer (HVC 45%) g) and h)