Agitated Drying Research Articles

Scale-up of granular material flow in an agitated filter dryer

A challenge in agitated drying is to take information from the laboratory scale and use it to scale up the process to the manufacturing scale. In this study, we focus on scale-up of the granular flow in an agitated dryer by measuring the impeller torque which can be related to the particle shear stress. The shear stress can in turn be related to particle breakage. The impeller torque was independent of rotation rate and increased linearly with material bed height, and the particle size had an impact on the torque. The bulk friction coefficient and material cohesiveness were computed from a linear correlation between normal stresses and shear stresses in the particle bed. Torque predictions from the bulk friction coefficients exhibited good agreement with torque measurements. This work serves as an effective way to predict the granular flow on a larger scale using data from the laboratory scale.

Development of a Novel Screening Methodology for the Assessment of the Risk of Particle Size Attrition during Agitated Drying

A material-sparing screening methodology has been developed for assessing the risk of particle size attrition of active pharmaceutical ingredients (APIs) during agitated drying using a single-ball ...

Investigating particle-level dynamics to understand bulk behavior in a lab-scale Agitated Filter Dryer (AFD) using Discrete Element Method (DEM)

Agitated filter drying (AFD) is a complex physical-thermal separation process which involves isolating solutes from its mother liquor. In agro-chemical and pharmaceutical industry, filter-dryers are used for sequestering active ingredients (AIs) and key intermediates from the wet cake after the crystallization step. During the agitated drying phase, the mechanical agitation of the wet cake, implemented to enhance heat and mass transport, has been commonly observed to result in formation of undesired agglomerates that require further processing. Only relatively few experimental and computational studies of the effects of operating parameters and material properties on the drying and agglomeration growth kinetics have been described in the literature. In absence of robust predictive models, the go-to solution in order to avoid the agglomeration behavior of AIs has been to use minimal agitation which is not only suboptimal but also significantly increases the drying times.The simulation of drying and agglomeration behaviors in AFD is particularly challenging because the agitated drying processes are mechanistically governed by simultaneous heat, mass and momentum transfer equations. In addition, the behavior of agglomeration growth and drying pathway varies significantly with the physical properties of the residual solvents in the cake as well as the operating conditions of the agitated dryer. A comprehensive modeling approach to simulate both drying and agglomeration behavior in AFDs through implementation of mechanistic Discrete Element Modeling (DEM) simulations with coupled granular liquid bridge cohesion model, heat conduction model and evaporation kinetics is presented. Additionally, in-depth analysis of particle scale behavior which is responsible for drying and agglomerate growth kinetics are also studied with respect to different scaling criteria is also presented.

Effect of calcium oxide (CaO) and sawdust on adhesion and cohesion characteristics of sewage sludge under agitated and non-agitated drying conditions

Stickiness phenomenon is widely observed in sewage sludge drying practices. This paper is aimed at demonstrating and comparing the sticky properties of sewage sludge through non-agitated and agitated drying tests specially designed for sewage sludge. Special attentions were paid to the effects of additives, i.e. CaO, fine sawdust (FSD) and coarse sawdust (CSD), on the adhesive and cohesive characteristics of sewage sludge. The results indicated that the sticky properties of the sludge were markedly different under the different testing methods, and was also greatly influenced by CaO or sawdust addition. For instance, in the non-agitated drying tests, CaO can significantly enhance the maximum adhesive and cohesive stresses of the sludge, whereas in the agitated drying tests, the torque of agitation, which strongly correlated with the cohesive stress of the sludge, was lowered by CaO addition. During agitated drying process, sludge lump with CaO addition started to break up at higher moisture content than that of original sludge. On the other hand, sawdust also affected the sticky properties of sludge in a way that was totally different with CaO. After sawdust addition (at 5–10%WS (wet sludge basis)), the cohesive stress of the sludge was markedly increased due to strengthening of mechanical interlocking inside the sludge, whereas the adhesiveness of the sludge was lowered by sawdust addition. The influencing mechanisms of CaO and sawdust under the different testing methods were detailedly discussed in the paper.

Dynamic agglomeration profiling during the drying phase in an agitated filter dyer: Parametric investigation and regime map studies

One of the serious challenges with the operation of an agitated filter dryer (AFD) is the issue of undesired agglomeration which often plagues the unit operation during the agitated drying cycles. Experimental analyses of the lumped powder formation at various operating conditions are now being used to gain insight into minimizing such activity; however, there is still a lack of understanding into the fundamental processes prompting such behavior and the effectiveness of suggested mitigation measures. This study aims to investigate the underlying agglomeration mechanisms induced during the agitated drying phase of an AFD operation. Through parametric study and granule growth regime map analysis, the dynamic drying and agglomeration profiles were studied for a model cohesive powder, micronized acetaminophen, at various process conditions. The granule growth regime map analysis showed that induction and nucleation type of granule enlargement where the dynamics of wetting dictates the formation of granules is predominant during agitated drying. Depending on the amount of time the drying experiment remains in the specific growth regime, the dynamic agglomeration profile differs drastically. In addition, through parametric investigation, the choice of wash solvent and the presence of critical residual moisture content at the start of the drying cycle showed dominant influence on agglomerate formation. Wash solvents which have high fluid viscosity seem to form stronger agglomerates that retain their structure through the drying process. In terms of the amount of residual moisture content, a critical moisture content range is noticeable for different wash solvents where the agglomerates formed possess superior strength. For prevention of agglomerates, such critical moisture levels and use of high viscous solvents must be avoided.

Scale‐up of agitated drying: Effect of shear stress and hydrostatic pressure on active pharmaceutical ingredient powder properties

Scale‐up of agitated drying processes to minimize particle size changes in active pharmaceutical ingredients (API) can be challenging. Particle agglomeration or attrition problems due to agitated drying are often discovered on the initial scale‐up from the lab to the plant. Traditional laboratory drying equipment has not successfully reproduced the degree of agglomeration or attrition observed at scale. This discrepancy may be attributed to the ability of particulate solids, such as crystalline API's to transfer stresses from the normal direction into the shearing direction. As batch size increases during scale‐up, the compressive and shearing forces experienced by the API increase. To overcome this limitation, a modified laboratory setup was constructed which reproduces the range of hydrostatic pressures observed during scale‐up. This work highlights the use of the modified setup to characterize the propensity for particle attrition to occur at different stages of the drying process by measuring impeller torque. Torque measurements of the API powder at different hydrostatic pressures revealed a behavior consistent with Coulomb's law of friction. The torque data obtained from these measurements were used to determine the bulk friction coefficient for API powder beds at different liquid content. Additionally, the amount of work done by the impeller blades was correlated to the degree of particle attrition observed. A workflow for assessing risk of API attrition at scale is described. © 2014 American Institute of Chemical Engineers AIChE J, 61: 407–418, 2015

Experimental study on agitated drying characteristics of sewage sludge under the effects of different additive agents

Drying experiments of dewatered sewage sludge (DSS) were conducted on a agitated paddle dryer, and the effects of additive agents, i.e., CaO, pulverized coal (PC), heavy oil (HO), and dried sludge (“DS” through back mixing) on the agitated drying characteristics of DSS were investigated. The results indicated that CaO can significantly increase the drying rate of DSS. The drying rate at CaO/DSS (mass ratio)=1/100 was 135% higher than that of CaO/DSS=0. Pulverized coal has no obvious effect on drying rate, but the increase of PC/DSS can promote breaking up of sludge lump. Heavy oil was found to be slightly effective in improving the drying rate of DSS in the examined experimental range of HO/DSS=0–1/20. It is also found that HO can reduce the torque of the dryer shaft, due to its lubrication effect. Back mixing of DS was found to be effective in alleviating the unfavorable effect of the lumpy phase by improving the mixing effect of the paddle dryer. There was a marked increase of drying rate with an increase of the DS/DSS in the experimental range of DS/DSS=0–1/3.

Development of New Laboratory Tools for Assessment of Granulation Behavior During Bulk Active Pharmaceutical Ingredient Drying

Approximately 30% of active pharmaceutical ingredients (APIs) experience agglomeration, granulation, and breakage during agitated drying. Currently, there is no small-scale bench tool to help assess and observe granulation behavior of APIs in the laboratory and subsequently lead to the development of a robust drying method. As a result, more conservative drying methods are usually used at scale and much longer drying times are needed. In this work, we build on work reported in the literature and demonstrate that a mixer torque rheometer (MTR) can be a useful small-scale tool to flag potentially problematic granulation behavior of APIs under different conditions. The results from the MTR were confirmed using a second new tool involving the use of an acoustic mixer to verify and observe the granulation behavior on a small scale. We also show consistency between the data collected at the laboratory and the pilot plant scales.

Baker's Yeast Behavior during Vacuum Agitated Contact Drying

AbstractDrying is considered as an intensive operation that consumes large quantities of energy. Usually, baker's yeast is obtained using freeze drying or fluidized‐bed drying, both of which are considered as expensive technologies. So, exploring other techniques such as contact drying could limit this disadvantage. In addition, no work dealing with contact drying of baker's yeast has been accomplished yet. Therefore, here, the behavior of baker's yeast during vacuum agitated contact drying is presented. The results show that the drying process can be divided into three phases: the pasty phase, the lumpy phase, and the granular phase. The influence of the drying parameters, such as the temperature, the impeller velocity, and the initial mass, was also studied. It was found that the wall temperature and the impeller velocity have a positive effect on the drying kinetics, as their increase allows a reduction in the drying time. Nevertheless, an increase in the pressure level or the initial mass of the product caused the drying time to increase.

Investigation of factors affecting isolation of needle-shaped particles in a vacuum-agitated filter drier through non-invasive measurements by Raman spectrometry

The effects of pressure filtration and vacuum agitated drying on cellobiose octaacetate (COA) particles in methanol slurries were studied by making Raman measurements through the glass wall at the side of a filter drier beneath the oil jacket. The change in intensity of methanol peaks in the spectra allowed the removal of the solvent from the particle bed to be monitored. Also, drying curves for COA generated from the Raman measurements gave an indication of the changing physical status of the particle bed during continuous or intermittent agitation. The intensity of the Raman signal for COA depended on the bulk density of the particle bed, which changed due to aggregation and attrition that occurred during solvent removal and particle motion induced by agitation during vacuum drying. Loss on drying (LOD) measurements of samples removed at the end of the pressure filtration and vacuum agitated drying stages established the degree of wetness and confirmed the end point of drying (<0.5% w/w solvent), respectively. Dynamic image analysis confirmed that minimum attrition of COA was achieved when (a) the majority of the methanol was removed during pressure filtration at 0.5bar N2 and (b) intermittent agitation was applied during the vacuum drying stage.

Influence of measurement cell size on predicted attrition by the Distinct Element Method

During agitated drying and mixing processes, particle beds are exposed to shear deformation. This leads to particle attrition, the extent of which is dependent on the prevailing stresses and strains in the bed. The distributions of shear stresses and strain rates within the bed are highly non-uniform, requiring attention to localised conditions. Therefore a narrow angular sector of the bed is divided radially and vertically into a number of measurement cells, within which the stresses and strain rates are calculated throughout one rotation by the Distinct Element Method. These are then used in an empirical relationship of material breakage to predict the extent of attrition due to agitation. Here we investigate the influence of the measurement cell size on the estimated stresses and strain rates, and the subsequent effect on the predicted attrition. The measurement cell size is altered by varying the measurement sector size and the number of radial and vertical divisions within it. The median particle size is also varied to establish its influence on the predicted attrition. An increase in the average number of particles in a given cell, by varying the particle size or measurement cell dimensions, leads to a reduction in the estimated stresses and strain rates, and therefore a reduction in the predicted attrition. Comparison of the predicted attrition with the experimental breakage in the agitated vessel shows that the prediction method is accurate when the cell dimensions are comparable to the width of a naturally occurring shear band.

Laboratory methods for assessing API sensitivity to mechanical stress during agitated drying

Two distinct laboratory methods have been developed to assess the propensity of active pharmaceutical ingredients to undergo particle breakage during agitated drying operations. In the first method, mechanical stress is applied to particles by mechanical agitation of powders compressed under an applied normal force. For the second, particles experience stresses as they are carried within a pressurized gas stream. These methods are simple, relatively compound-sparing, and are used to rank materials according to a quantitative breakage classification scale as hard, medium, or easy to break. Based on the results obtained using these methods at laboratory scale, recommendations and precautions for processing at larger scale are made. In this paper, these methods are described in detail, and the results, obtained for several pharmaceutical compounds, are presented.

Impact of agitated drying on the powder properties of an active pharmaceutical ingredient

In this study, the impact of agitated drying on the physical and bulk powder properties of an active pharmaceutical ingredient (API) is presented. The effects of different agitated drying conditions such as agitation rate, drying temperature, drying time pre-agitation, drying time during agitation and number of solvent wash cycles on the bulk density, millability, flow and specific surface area is reported. The crystal morphology is altered from fibrous needles to agglomerates when switching from tray to agitated drying. An increase in bulk density and specific surface area was evident when using agitated drying compared to tray drying as hard coarse granules were produced with an increase in the number of fine particles < 10 μm. The bulk density was found to increase with an increase in agitation speed, drying time and number of solvent wash cycles used during filtration. Controlling both fine and coarse particle size of the granules for this API during agitated drying was difficult to achieve due to the fibrous crystal habit. However, the increase in the bulk density observed has the potential to facilitate improvements in the ease of drug product development. In the case of this system further particle size control was required through the use of dry milling.

The effect of agitated drying on the morphology of l-threonine (needle-like) crystals

Experiments have been carried out to study the behavior of l-threonine (needle-like) crystals during agitated drying. For an l-threonine/water system the morphology of the crystals was monitored using light microscopy and image analysis. Analysis of the transient behavior of the crystal size and shape distribution showed that attrition and agglomeration took place simultaneously during the process. The variation of the operating conditions (temperature, agitation and vacuum) revealed that attrition dominated the drying process when the drying rate was low and/or the shear rate was high. For high drying rates and low shear rates, agglomeration became dominant. This study suggests that crystal redissolution had no significant impact on crystal morphology. It was also found that due to their needle-like shape, large l-threonine crystals were very sensitive to attrition. When attrition controlled the drying process, most of the particle size reduction took place below a critical moisture content of 4%. When agglomeration controlled the drying process, most of the crystal size enlargement occurred below a moisture content of 6%.

Impact of agitated drying on crystal morphology: KCl–water system

An experimental procedure was developed to study agitated drying of crystals. The procedure quantifies the impact of drying conditions (temperature, agitation speed and vacuum) on crystal properties (size and shape distribution). KCl was used as a model compound. The morphology of the crystals was determined using light microscopy and image analysis. The analysis of the transient behavior of the crystal size distribution showed that there is a permanent competition between attrition and agglomeration during drying. However, these two phenomena only had a significant effect on the crystal size distribution when the moisture content was below a critical value. In particular, it was found that attrition dominates the drying process when the drying rate is low and/or the shear rate is high. Under these conditions, the drying time is long enough for particles to encounter many particle-impeller and particle–particle collisions that can produce small crystal fragments. For higher drying rates, which are obtained at high temperature and low pressure, the number of ‘collisions’ decreases, stronger agglomerates are formed and thus agglomeration becomes more dominant. Other physical phenomena, which can take place during drying, such as crystal redissolution, had no significant impact on crystal morphology.

HEAT, MOMENTUM, AND MASS TRANSFER MEASUREMENTS IN INDIRECT AGITATED SLUDGE DRYER

ABSTRACT Whereas indirect agitated drying has been extensively studied for granular materials, little is known in the case of pasty products. We describe an experimental set up specially designed for the investigation of drying kinetics, of heat transfer coefficient evolution, and of the mechanical torque necessary for stirring. This device was applied to municipal sewage sludge. Preliminary experiments were performed to investigate the influence of ageing of sludge on the drying kinetics. It appears that ageing does have no effect except for the first two days. The influences of the wall temperature, the stirrer speed, the dryer load and the location of the stirrer against the heated wall were studied. Three different rheological behaviors were observed during sludge drying. In particular, the sludge goes through a “glue” phase, and high levels of mechanical strain are recorded when the compact mass begins to break up. A critical stirrer speed is found within the range 40–60 rpm. To shorten the drying time, it seems better to adjust the wall temperature in accordance with the moisture content of the sludge.