Compression Line Research Articles

Unified elastoplastic framework for the volumetric behavior of unsaturated soils during drained, undrained, and static compaction tests

When unsaturated soils are subjected to drained or undrained compression tests, they approach the saturated compression line with different slopes. During compression, four main phenomena occur in these materials: first, with the reduction in volume, the degree of saturation increases; second, with the reduction in the size of pores, the soil-water retention curve shifts on the axis of suction; third, these two phenomena produce an increase in the suction stress; and finally, this increase in suction stress produces a certain amount of collapse on the sample. In this paper, a unified elastoplastic framework is developed and coupled to a hydraulic model to simulate the volumetric behavior of unsaturated soils under different stress paths. This approach is based on a single equation with singled values for the compression and recompression indexes, resulting in a unified compression theory for soils. The comparison between experimental and numerical results shows the pertinence of the model.

Tracking raw material flow through a continuous direct compression line. Part II of II: Predicting dynamic changes in quality attributes of tablets due to disturbances in raw material properties using an independent residence time distribution model

Continuous manufacturing of pharmaceuticals promises many advantages regarding economics and quality. However, tracing deviating material in such processes is much more challenging than in batch processes due to axial back-mixing. The literature has proven the traceability of disturbances in the active pharmaceutical ingredient (API) by residence time distribution (RTD) models. Nevertheless, pharmaceutical quality attributes (QAs) and disturbances are not limited to the API content. The present study investigates different disturbances affecting the particle size in a direct compression by recording various QAs. The application of a tracer-based model demonstrates the generalizability of RTD models regarding tracing the propagation of nonconforming material. The model applied may predict the appearance of deviating material in the tablets accurately up to 60 s. However, nonlinear and dynamic effects complicate predicting values of the QAs. Due to dynamic effects, tablet masses deviate up to 2.5 percentage points from values measured in steady-state, at an acceptable total deviation of 7.5 %. Consequently, if other QAs are critical, the transfer of control concepts developed for the API content will be challenging due to such effects different from API-based disturbances. Additionally, higher tolerances for errors will be required considering those deviations.

Evaluation of a new grain breakage factor based on the single grain crushing strength

In granular soils subjected to high-level shear stress, a micro-damage occurs in the soil's internal structure, referred to as grain breakage. The main factors controlling grain breakage are the applied stress level and the single grain crushing strength of the individual grains (σf). This paper presents a laboratory study on ballast grains' single grain crushing behavior. Double-plate grain crushing tests were carried out on dolomite, calcerous, and basalt origin grains, having sizes ranging from 2.00 to 37.50 mm. Factors affecting the single grain crushing strength, such as grain shape, loading rate, and grain size, were investigated. Individual grains' single grain crushing strength was observed in three phases related to granular soils' 3-staged one-dimensional compression behavior. An equation that considers grain shape in terms of grain sphericity (form) has been proposed to determine the single grain crushing strength. One-dimensional compression tests were performed on uniformly graded soil samples consisting of calcerous and basalt origin grains. The single grain crushing strength of the calcerous origin soil sample’s mean grain-sized grains was determined to be 12.7 MPa, and this value was determined to be 25.6 MPa for the sample consisting of basalt origin grains. One-third/fourth of this single grain crushing strength value was obtained as the virgin compression line (VCL) initial stress of the soils’ one-dimensional compression curve (4.3 and 6.0 MPa for calcerous and basalt origin soil samples, respectively). The final stress of the VCL was determined to be close to 1–1.25 times of this specified single grain crushing strength (16 and 26 MPa for calcerous and basalt origin soil samples, respectively). The change in the mean grain size of the soil samples (d50) during one-dimensional compression was defined as an exponential function. A new grain breakage factor depending on the single grain crushing strength of the soils’ mean grain-sized grains has been proposed to determine the grain breakage that develops under one-dimensional compressional conditions.

Impact of Vertical Blender Unit Parameters on Subsequent Process Parameters and Tablet Properties in a Continuous Direct Compression Line.

The continuous manufacturing of solid oral-dosage forms represents an emerging technology among the pharmaceutical industry, where several process steps are combined in one production line. As all mixture components, including the lubricant (magnesium stearate), are passing simultaneously through one blender, an impact on the subsequent process steps and critical product properties, such as content uniformity and tablet tensile strength, is to be expected. A design of experiment (DoE) was performed to investigate the impact of the blender variables hold-up mass (HUM), impeller speed (IMP) and throughput (THR) on the mixing step and the subsequent continuous manufacturing process steps. Significant impacts on the mixing parameters (exit valve opening width (EV), exit valve opening width standard deviation (EV SD), torque of lower impeller (TL), torque of lower impeller SD (TL SD), HUM SD and blend potency SD), material attributes of the blend (conditioned bulk density (CBD), flow rate index (FRI) and particle size (d10 values)), tableting parameters (fill depth (FD), bottom main compression height (BCH) and ejection force (EF)) and tablet properties (tablet thickness (TT), tablet weight (TW) and tensile strength (TS)) could be found. Furthermore, relations between these process parameters were evaluated to define which process states were caused by which input variables. For example, the mixing parameters were mainly impacted by impeller speed, and material attributes, FD and TS were mainly influenced by variations in total blade passes (TBP). The current work presents a rational methodology to minimize process variability based on the main blender variables hold-up mass, impeller speed and throughput. Moreover, the results facilitated a knowledge-based optimization of the process parameters for optimum product properties.

Tracking raw material flow through a continuous direct compression line Part I of II: Residence time distribution modeling and sensitivity analysis enabling increased process yield

Continuous manufacturing (CM) offers advantages in quality and space–time yield compared to common batch manufacturing. However, higher yield losses due to the start-up procedure make a broad application uneconomical. This work discusses the possibility of reducing yield losses by adjusting the degree of back-mixing. Back-mixing of nonconforming material from disturbances or start-up will result in the contamination of subsequent material. Therefore, higher degrees of back-mixing cause the discharge of additional material. Choosing an advantageous setting of operational parameters may be a simple way to change the degree of back-mixing. Based on direct compression, this work demonstrates the identification of promising parameters. Therefore, step-change experiments using color-marked material in the feeder, blender, and tablet press quantify the impact of three operational parameters per device. Models for the devices and the entire process result from those measurements. Subsequently, a global variance-based sensitivity analysis identifies the most influential parameters. As a result, adjusting the minimal filling level of the feeder and the rotational feed frame speed of the tablet press reduces back-mixing by more than 30 %. At high costs of the raw materials, the resulting savings can significantly improve the economic performance of CM compared to batch manufacturing.

In-depth analysis of the long-term processability of materials during continuous feeding

This study determined the feasibility of long-term continuous powder feeding and its effect on the overall process performance. Additionally, quantitative relationships were established between material properties, process settings and screw feeding responses during gravimetric feeding. Twelve divergent raw materials were processed over longer periods using a GEA Compact Feeder integrated in a continuous direct compression line (ConsiGma™ CDC-50). The resulting gravimetric feeding responses were combined with the material properties and process settings into an overall PLS model. The model elucidated the impact of the material descriptors for density; powder flow; particle size; compressibility; permeability and wall friction angle on the feeding process. Furthermore, long-term processing of the materials exhibited challenges related to the processability and refill consistency where a significant impact of the compressibility and cohesive/adhesive properties of the materials was observed. Overall, this approach provided insights into the feasibility of long-term continuous feeding which is not possible through ‘short-term’ feeding trials. Additionally, throughout this study, the need for material-specific adjustments of the feeding and refill equipment was highlighted.

Continuous direct compression: Development of an empirical predictive model and challenges regarding PAT implementation

In this study, an empirical predictive model was developed based on the quantitative relationships between blend properties, critical quality attributes (CQA) and critical process parameters (CPP) related to blending and tableting. The blend uniformity and API concentration in the tablets were used to elucidate challenges related to the processability as well as the implementation of PAT tools. Thirty divergent ternary blends were evaluated on a continuous direct compression line (ConsiGma™ CDC-50). The trials showed a significant impact of the impeller configuration and impeller speed on the blending performance, whereas a limited impact of blend properties was observed. In contrast, blend properties played a significant role during compression, where changes in blend composition significantly altered the tablet quality. The observed correlations allowed to develop an empirical predictive model for the selection of process configurations based on the blend properties, reducing the number of trial runs needed to optimize a process and thus reducing development time and costs of new drug products. Furthermore, the trials elucidated several challenges related to blend properties that had a significant impact on PAT implementation and performance of the CDC-platform, highlighting the importance of further process development and optimization in order to solve the remaining challenges.

On compression behavior and particle breakage of carbonate silty sands

Carbonate soils are high in compressibility and low in particle strength, and therefore need special treatment for many geotechnical applications. This paper presents comprehensive one-dimensional compression tests to investigate the compression behavior and particle breakage of carbonate soils with various initial particle size distributions (PSDs) and a wide range of initial void ratios at a high effective vertical stress of 32 MPa. A clear shape transition in terms of the void ratio against effective vertical stress was observed with varying silt fraction and void ratio. The deformation mechanism for the samples with less silt fraction and higher void ratio is mainly dominated by the particle-breakage-induced unlocking effect. Samples with more silt fraction and lower void ratio will experience less particle breakage and their deformation mechanism is mainly attributed to interparticle locking effect. A ‘transitional’ mode of behavior without a unique normal compression line was identified for the samples with silt fraction ranging from 1.6% to 33%. It was also found that carbonate soil samples with both ‘transitional’ and ‘non-transitional’ modes of compression behavior may both experience considerable amount of particle breakage and show different trends in the evolution of particle shape.

Investigation of Bursting Stress and Spalling Stress in Post-tensioned Anchorage Zones

Over the past decades, considerable efforts have been made to quantify the bursting forces in the post-tensioned anchorage zones based on the simplified model or fitting formulas, however reproducing the transverse stress distribution is still a challenging topic, which is also important to detail the reinforcing details in the anchorage zones, especially for cracking control. To address this issue, this paper is devoted to seeking an elasticity solution for transverse stresses in the anchorage zones, and providing a more rational equation for transverse distribution in anchorage zones. The sum function of normal stresses is employed to solve the stresses filed in the anchorage zones with concentric load and two eccentric loads. The bursting stresses in the concentric anchorage zones and spalling stresses in the eccentric anchorage zones are verified by the photoelastic tests. The transverse stresses along the symmetry axis of the eccentric anchorage zones can be handled as a concentric single anchorage zone with equivalent bearing plate width. Moreover, according to the concept of force stream tube, the profiles of isostatic line of compression (ILCs) are determined and validated, which confirms the existence of ILCs.

Characteristic zones for initial state of sand under undrained shearing

A new line for characterising the initial state of sand in the e-lnp plane is firstly introduced in this note, which is defined as the normal compression line (NCL) for sand. Together with a series of existing characteristic state lines, four characteristic zones are subsequently established in the e-lnp plane, namely, liquefaction zone; strain softening zone; quasi-steady state zone and strain hardening zone. Under undrained shearing, saturated sands with the initial state located at the same zone can exhibit similar type of stress–strain relation denoted by the name of zone. A good agreement has been found between the proposed zones and existing experimental results, revealing the ability of proposed zones in characterising the initial state of sand. Finally, the significance of the proposed characteristic zones is demonstrated in the applications associated with sand.

Soil Fabric and Transitional Behavior in Completely Decomposed Granite: An Example of Well-Graded Soil

Unlike sedimentary soils, limited studies have dealt with completely decomposed granite (CDG) soils, even though they are plentiful and used extensively in several engineering applications. In this paper, a set of triaxial compression tests have been conducted on well-graded intact and disturbed CDG soils to study the impact of the fabric on soil behavior. The soil behavior was robustly affected by the soil fabric and its mineral composition. The intact soil showed multiple parallel compression lines, while a unique isotropic compression line was present in the case of disturbed soil. Both the intact and disturbed soils showed unique critical state lines (CSL) in both the e-log p′ and q-p′ spaces. The intact soil showed behavior unlike other transitional soils that have both distinct isotropic compression lines ICLs and CSLs. The gradient of the unique ICL of the disturbed soil was much more than that of the parallel compression lines of the intact soil. In the intact soil, the slope of the unique CSL (M) in the q-p′ space was higher than that of the disturbed soil. The isotropic response was present for both the intact and disturbed soils after erasing the inherited anisotropy as the stress increased with irrecoverable volumetric change. Soil fabric is considered the dominant factor in the transitional behavior and such a mode of soil behavior is no longer restricted to gap-graded soil as previously thought.

Limits of lateral expansion in two-dimensional materials with line defects

The flexibility of two-dimensional (2D) materials enables static and dynamic ripples that are known to cause lateral contraction, shrinking of the material boundary. However, the limits of 2D materials' lateral expansion are unknown. Therefore, here we discuss the limits of the intrinsic lateral expansion of 2D materials that are modified by compressive line defects. Using thin sheet elasticity theory and sequential multiscale modeling, we find that the lateral expansion is inevitably limited by the onset of rippling. The maximum lateral expansion ${\ensuremath{\chi}}_{\text{max}}\ensuremath{\approx}2.1\phantom{\rule{0.16em}{0ex}}{t}^{2}{\ensuremath{\sigma}}_{d}$, governed by the elastic thickness $t$ and the defect density ${\ensuremath{\sigma}}_{d}$, remains typically well below 1%. In addition to providing insight to the limits of 2D materials' mechanical limits and applications, the results highlight the potential of line defects in strain engineering, since for graphene they suggest giant pseudomagnetic fields that can exceed 1000 T.

Determination method and strain-attribute interact in the calculation of precompression stress from soil compression curves

Determination of soil precompression stress (σp) from compression curves has markedly changed in recent decades, with several available methods of varying complexity. The graphical procedure proposed more than eight decades ago remains a standard, though it is now emulated by statistical and numerical procedures. We tested nine determination methods based on linear, polynomial, nonlinear and spline regressions. Four attributes were used to represent soil deformation during compression (strain-attributes): bulk density (ρb), void ratio (e), total porosity (ϕ) and strain (ε). Undisturbed soil samples equilibrated either at 10 or 100 kPa water tension were subjected to confined, drained uniaxial compression tests. The experimental factors (determination method, strain-attribute, and water tension) and their interaction affected the determination of σp. Sigmoid equations converged for 84%–95% of the compression curves, failing for samples at 100 kPa with with steep virgin compression lines. Methods based on fourth-degree polynomials resulted in 10-94% of the values outside the range 10–1550 kPa, depending on the strain-attribute. Although ε and e are linearly related, the former suits better the application of methods based on polynomial and spline regression, also fitting well for methods based on sigmoid equations with samples at 10 kPa. The bi-linear intersection methods presented relaistic σp values for bi-linear curves and should be solely applied to ρb. The spline method resulted in σp values clustered along the loads applied during the compression tests.

A Constitutive Model for Overconsolidated Structured Soils Using Structural Variable

Due to the influence of soil structure, structured soils exhibit significantly different mechanical behavior compared to the reconstituted soils having the same material. In this work, a theoretical analysis focusing on the mechanical behavior of structured soils is presented. Based on the mechanical behavior of the structured soil, a newly defined variable structural index was used as a measurement of the integrity of soil structure based on the concept of intrinsic compression line of intact structured soils. Furthermore, a novel correlation for the variation in volume of structured soils is established using effective stress and newly defined structural index as the constitutive variables. The novel correlation provided interpretation about the mechanism of compression behavior of the structured soils. Afterwards, the proposed correlation for the variation in volume was extended to triaxial stress state in the framework of subloading surface to include the effect of overconsolidation. Comparisons between the predictions and experimental results validated the proposed constitutive model for structured soils.

Hydromechanical behavior of unsaturated soils: Interpretation of compression curves in terms of effective stress

This state-of-the-art paper on the hydromechanical behavior of unsaturated soils focuses on the interpretation of the compression curves of unsaturated soils in terms of effective stress, with the goal of understanding the relative impacts of suction on the effective stress, net yield stress, effective yield stress and slope of the virgin compression line (VCL) during a monotonic increase in net stress. A database of compression curves was compiled for both high and low plasticity fine-grained soils under a wide range of suctions, isotropic or oedometric stress states, drainage conditions (constant suction or constant water content) and preparation techniques (impact compaction, static compaction, consolidation from slurry). Most of the compression curves plotted in terms of effective stress revealed a consistent hardening response with increasing suction and a slight suction dependency on the slope VCL. Interpretation of the compression curves in terms of effective stress led to load-collapse curves with a similar shape for a wide range of soils. Most soils evaluated had a greater rate of increase in effective yield stress with suction than the rate of increase in suction stress with suction, implying that these compacted soils may be susceptible to collapse upon wetting. Inconsistent trends were observed in some studies, which were attributed partially to natural variability but also experimental issues and limitations on the range of conditions investigated. Accordingly, recommendations are provided for future studies on the compression curves of unsaturated soils to ensure that results can be clearly interpreted in terms of effective stress.

Powder composition monitoring in continuous pharmaceutical solid-dosage form manufacturing using state estimation – Proof of concept

In continuous solid-dosage form manufacturing, the powder feeding system is responsible for supplying downstream the correct formulation of the drug product ingredients. The composition of the powder delivered by the feeding system is inferred from the measurements of powder mass flow from the system feeders. The mass flows are, in turn, inferred from the loss in weight measured in the feeder hoppers. Most loss-in-weight feeders post-process the mass flow signal to deliver a smoothed value to the user. However, such estimated mass flows can exhibit a low signal-to-noise ratio. As the feeders are critical elements of the control strategy of the manufacturing line, better instantaneous estimates of mass flow are desirable for improving the quality assurance. In this study, we propose a model-based approach for monitoring the composition of the powder fed to a continuous solid-dosage line. The monitoring system is based on a moving-horizon state estimator, which carries out model-based reconciliation of the feeder mass measurements, thus enabling accurate composition estimation of the powder mixture. Experimental datasets from a direct compression line are used to validate the methodology. Results demonstrate improvement with respect to current industrial solutions.

Real-Time Monitoring of Powder Mass Flowrates for Plant-Wide Control of a Continuous Direct Compaction Tablet Manufacturing Process

While measurement and monitoring of powder/particulate mass flow rate are not essential to the execution of traditional batch pharmaceutical tablet manufacturing, in continuous operation, it is an important additional critical process parameter. It has a key role both in establishing that the process is in a state of control, and as a controlled variable in process control system design. In current continuous tableting line operations, the pharmaceutical community relies on loss-in-weight feeders to monitor and understand upstream powder flow dynamics. However, due to the absence of established sensing technologies for measuring particulate flow rates, the downstream flow of the feeders is monitored and controlled using various indirect strategies. For example, the hopper level of the tablet press is maintained as a controlled process output by adjusting the turret speed of the tablet press, which indirectly controlling the flow rate. This gap in monitoring and control of the critical process flow motivates our investigation of a novel PAT tool, a capacitance-based sensor (ECVT), and its effective integration into the plant-wide control of a direct compaction process. First, the results of stand-alone experimental studies are reported, which confirm that the ECVT sensor can provide real-time measurements of mass flow rate with measurement error within -1.8 ~ 3.3% and with RMSE of 0.1 kg/h over the range of flow rates from 2 to 10 kg/h. The key caveat is that the powder flowability has to be good enough to avoid powder fouling on the transfer line walls. Next, simulation case studies are carried out using a dynamic flowsheet model of a continuous direct compression line implemented in Matlab/Simulink to demonstrate the potential structural and performance advantages in plant-wide process control enabled by mass flow sensing. Finally, experimental studies are performed on a direct compaction pilot plant in which the ECVT sensor is located at the exit of the blender, to confirm that the powder flow can be monitored instantaneously and controlled effectively at the specified setpoint within a plant-wide feedback controller system.

Relationships between gradation and deformation behavior of dense granular materials: Role of high‐order gradation characteristics

AbstractThis paper numerically investigates deformation behavior of coarse‐grained granular materials with emphasis on the effect of grain size distribution (GSD). Biaxial compression tests are simulated in two dimensions using distinct element method (DEM) on a set of samples composed of uncrushable spheres following a wide variety of GSDs. The results prove that high‐order characteristics of gradation, such as the shape of the GSD curve, significantly affect deformation behavior of widely‐graded materials. Two grading indices are proposed to consider the combined effects of the spread and the shape of the GSD curve, and they are highly correlated with the parameters of the materials’ internal fabric. Strong correlations are found between the proposed grading indices and deformation parameters, such as those of the normal compression line and the critical state line (CSL) in the e‐p space, and the deformation modulus. In contrast, using the more conventional coefficient of uniformity cannot reach effective gradation‐dependent relations for those deformation parameters due to the deficiency in characterizing high‐order features of gradation. This study suggests the need for incorporating high‐order characteristics of gradation into a refined measure of size polydispersity in order to establish gradation‐dependent relations of deformation parameters widely effective for polydisperse granular materials.

Soft sensor for real-time estimation of tablet potency in continuous direct compression manufacturing operation

One of the critical quality attributes of the solid oral dosage forms produced in continuous direct compression operations is the tablet potency. A novel soft sensor comprising of a combination of first principle-based and empirical models has been developed to enable real-time monitoring of blend and tablet potency, and concentrations of other excipients at various stream levels along the direct compression line. The soft sensor model has only three adjustable parameters, primarily associated with the equipment design and operation, so the model is product agnostic which is key to enable flexible manufacturing. The estimation accuracy of the soft sensor is demonstrated through a series of real time experiments which include steady state and dynamic transitions of potency during the runs, compared with offline analytically tested tablet cores. The results indicate that the proposed soft sensor can be utilized as a robust tool for real-time monitoring of potency, suggesting an extension of its utilization to higher levels of control. Two potential applications of the soft sensor are: 1. An element of a control strategy for product diversion; 2. A predictive model for advanced process control strategy to minimize the variability in tablet composition.

Empirical Compressibility Index Equations for Artificial Remolded Clay Mixtures

A series of one-dimensional consolidation tests was conducted on reconstituted soil mixtures with different mineral compositions. Test groups were prepared by mixing various proportions of kaolinite, quartz, and montmorillonite in which the initial water contents were equal to the liquid limits of the soils. The focus was on the compression behavior of reconstituted clay mixtures, including the rarely reported triple mixtures, as well as establishing a link between the liquid limit and the clay mineral ratio of the specimens. The intrinsic compression indexes of the soils were highly dependent on the composition of the soils and the initial water contents. New equations were proposed to estimate the compression index based on the clay mineral content, liquid limit, plasticity index, and initial void ratio. A triangular chart was proposed to estimate the compression indexes of reconstituted soils. A simple way to determine the virgin compression lines of reconstituted specimens is described.