ARR - Annual Report

The focus of the report is on development of a contact model for usage in CFD-DEM simulations. Great effort is placed in this step since it provides a basis for all future results upon which the continuum model will be built.

We present a contact model able to capture the response of interacting adhesive elastic-perfectly plastic particles under a variety of loadings. The model is built upon the Method of Dimensionality Reduction which allows the problem of a 3D axisymmetric contact to be mapped to a semi-equivalent 1D problem of a rigid indenter penetrating a bed of independent Hookean springs. Plasticity is accounted for by continuously varying the 1D indenter profile subject to a constraint on the contact pressure. Unloading falls out naturally, and simply requires lifting the plane indenter out of the springs and tracking the force. By considering the incompressible nature of this plastic deformation, the contact model is also able to account for the nonlocal effects of neighboring contacts, including formation of secondary contacts from outward displacement of the free surface. JKR type adhesion is recovered easily by simply allowing the springs to ‘stick’ to the 1D indenter’s surface. Additionally, we account for the rapid stiffening in the force-displacement curve under high confinement (e.g. during powder compaction) by triggering a superimposed bulk elastic response based on a simple criterion related to contact area.

Given that the model arises from rigorous contact mechanics formulations and simple geometric arguments only a few physical inputs are needed to run the model. Namely, the average radius of the particles Ro, Young’s modulus E, Poisson ratio ν, yield stress Y, and effective surface energy Δγ. An outline of the numerical implementation of the model is included. Every aspect of the contact model is validated, either through comparison to finite element simulations or in the case of adhesion directly to the JKR theory of adhesion. These comparisons show that the proposed contact model is able to accurately capture plastic displacement at the contact, average contact stress, contact area, and force as a function of displacement under a variety of complex loadings. This gives us confidence in the predictive capability of the contact model and its ability to reflect reality when used in the upcoming CFD-DEM simulations.

This annual report presents key advances made during year 2 of which, a major component is a concise treatise on our key advances in model-guided dry coating-based enhancements of poor flow and packing of fine cohesive powders, included in Appendix A. The report also includes a review of the available particle contact models for both smooth and rough particles and presents a database of industry relevant materials and their key properties. In terms of IFPRI Member interactions, we have held regular update meetings and worked with a member company on their powder characterization device. We plan to prepare a manuscript on that and include that in the report next year.

Major Accomplishments

Major accomplishments include a review of the existing van der Waals force-based particle contact models to elucidate the main mechanism of flow enhancement through silica dry coating. Our multi-asperity model explains the effect of the amount of silica, insufficient flowability enhancements through conventional blending, and the predominant effect of particle surface roughness on cohesion reduction. Models are presented for the determination of the amount and type of guest particles, and estimation of the granular Bond number, used for cohesion nondimensionalization, based on particle size, particle density, asperity size, surface area coverage, and dispersive surface energy.

Processing Conditions

Selection of the processing conditions for LabRAM, a benchmarking device, is presented followed by key examples of enhancements of flow, packing, agglomeration, and dissolution through the dry coating. Powder agglomeration is shown as a screening indicator of powder flowability (Appendix B). The mixing synergy is identified as a cause for enhanced blend flowability with a minor dry coated constituent at silica <0.01%. The analysis and outcomes presented in this paper are intended to demonstrate the importance of dry coating as an essential tool for industry practitioners.

Objectives:

• To establish a predictive criteria
• To identify the key factors
• To establish a test methodology

The original aim and objectives of the project remained unchanged: to design a diagnostic tool to determine if a powder formulation will stick to the punch-face during tablet production.

Timeline

Work on the project started effectively in August 2021 with the arrival of PhD student Ahmad Ramahi. In February 2022 PhD student Vishal Shinde joined the project. The first two objectives have been completed and work on the third objective is ongoing aiming to complete by the AGM in June 2023.

Approximately 20 characterisation techniques were employed or explored at different levels of detail as described in this report. Measurement of the % area of the punch covered by the sticking powder was selected as the main method to quantify sticking. Following regular monthly project meeting with the IFPRI advisory group, given the complexity involved in the sticking phenomena (summarised in Section 1), the focus was maintained on empirical studies to identify the relevant mechanisms for the materials of interest and on creating a database of approximately 20 powder materials (including sticking and non-sticking APIs, sticking and non-sticking excipients, powder formulations and lubricated formulations), and delve into the science of each mechanism in the follow-on proposal.

The database contains material properties including chemical information (formula, structure, molecular weight), physical characteristics (particle size distribution, density, shape, morphology of the particles, bulk density of powders), mechanical properties of particles (Young’s modulus, Poisson’s ration, yield strength), interaction properties between particles (friction coefficient between particles, surface energy), thermal properties (conductivity, heat capacity, thermal expansion coefficient) and humidity related properties (water adsorption isotherms, water activity).

The sticking behaviour of powders during single compression events is characterised considering 4 processing parameters: compaction pressure, temperature, humidity, and compaction rate. Work is ongoing, only 4 materials were characterised so far. The compression tooling used was provided by iHolland (B tooling). Long term sticking (multiple compaction events) are planned.

The deliverable the end of the 3 years is a predictive toolkit comprising of the database analysed using Principal Component Analysis to extract functional relationships between material properties, process parameters (compaction pressure and rate) and environmental conditions (temperature and RH) and finally assign a risk for sticking.

A proposal is being developed for a follow-on project to extend the database for new materials to further validate the predictive capability, establish the science base to understand the underlying mechanisms, link molecular level information to sticking behaviour and develop mitigating strategies for sticking at early stage product development. A collaboration with Professor Jerry Heng at Imperial College is proposed to cover the Chemistry/Chemical Engineering aspects.

In the current funding period we have made advances on three fronts:

1. use of kinetic Monte Carlo (kMC) simulations to predict the morphology of organic crystals grown from solution for cases where the solution is pure and also when it contains growth inhibitor molecules,
2. completed incorporating three new all-atom force fields into ADDICT in order to test the sensitivity of our morphology predictions with respect to different estimates of intermolecular interaction energy, and
3. developed a new crystal growth model for asymmetric organic molecules with two molecules in the unit cell. This is a precursor to developing a more general model with many molecules in the unit cell.

We report progress on all three topics.

We have continued to develop our kinetic Monte Carlo (kMC) modeling codes to predict the morphology of organic crystals grown from solution both with and without the inclusion of impurity molecules on the crystal surfaces. We have used these codes to make morphology predictions for naphthalene grown from ethanol solvent at increasing supersaturations in impurity-mediated solutions. The results were quite satisfactory and are reported in more detail later in the report.

In addition to the Generalized Amber Force Field (GAFF) which is already included in ADDICT, we have added three new all-atom force fields to ADDICT. They are the Coulomb-London-Pauli (CLP) force field, the Consistent Force Field (CFF), and the Universal Force Field (UFF). Each has its own specific advantages and limitations.

• CLP is the most general with over 90 atom types (similar to GAFF).
• CFF is specific to organic molecules containing intermolecular hydrogen bonds, especially carboxylic acids and amines.
• UFF has one atom type for every element in the periodic table - it is very general but with only one atom type for each element it is not very accurate.

Later in the report we describe these force fields in more detail and show new results for predicting the morphology of five organic crystals using the CLP force field. We continue to develop a new crystal growth model for asymmetric organic molecules under the restriction of that the unit cell contains exactly two molecules. This allows us to make a big leap from essentially one (symmetric) molecule per unit cell to two asymmetric molecules. Once this theory is fully tested and validated on real molecular systems it will lead to further extensions to 4 molecules in the unit cell, and then many.

• Design of particle-free fall SDD measurement system.
• Adaptation of the existing equipment to the project requirements.
• Measurements of rheological properties of aqueous solutions of selected materials.
• Selection of suitable experimental media and determination of quality criteria.

The research program presented for IFPRI assumes finding the relationship between the rheological properties of the solution and the drying speed on the morphology of the particles obtained by the spray drying method. For this purpose, in the first year of the project, the following tasks were assigned to be implemented:

Spray drying is a complex process. Many process parameters affect not only the operation of drying towers but also the final properties of the product. This complexity makes modelling of spray drying difficult, and the developed mathematical models are usually dedicated to the specific processes and materials (Filkova et al., 2015). Even though the spray drying process has been used for over a century in the industry, mathematical models are still elaborated to predict the physicochemical properties of powders, based on drying process parameters and rheological properties of the sprayed solution.

Spray drying is one of the basic techniques for the formation of particles from liquid solutions, suspensions or slurries. The process involves spraying a solution in the stream of a hot drying medium, usually air. As a result of the rapid spray extension, intensive moisture evaporation takes place without a significant increase in dried material temperature. Short drying time and low product temperature in spray drying are beneficial in many industries, from pharmaceuticals, through food, to chemicals, especially for drying heat-sensitive materials.

of granule structures.

measurement will be combined on an significant set of granules, representing a variation

In the last part of this first period the two aspects of structure description and property

between structural and physical measures is the next step that was performed.

The combination of both kinds of measures to evaluate if there are any significant correlations

between differently structured granules.

The structural as well as the physical parameters provide results that are suitable to distinguish

granules are dispersed in an optical glass cuvette and measured continuously.

measurement of particles size distribution over time in a laser diffraction system. The

beaker test. The second method to measure dissolution behaviour contains the

of granules containing salt within the binder phase can be determined in a standard

Regarding dissolution and disintegration behaviour of the structured granules, the conductivity

positions of granules on a sample container and crushes these granules consecutively.

single granules has to be measured. An experimental set-up was designed that scans

To achieve statistically relevant results for the single particle crushing, a high number of

behaviour by conductivity measurements and online particle sizing respectively.

mechanical strength is measured by single particle crushing and the dissolution or disintegration

For the determination of physical properties three different methods were chosen. The

and covariance function.

were evaluated include volume and surface fraction, star volume, chord length distribution

and used for further calculations of structure descriptors. The structure descriptors that

Three dimensional X-ray micro tomography images of the model granules were recorded

coarse limestone particles was chosen and combined with a polyethylene glycol binder.

to achieve different internal structures. A bimodal particles size distribution of fine and

Model granules were generated containing different size distributions of primary particles

correlation to structure parameters.

this year was about the determination of physical parameters of model granules and their

While the last period of work was focused on the implementation of structure descriptors

Executive Summary

This project year focused on the air classification step of the circuit. Trials in two air classifiers, in laboratory and industrial scales, were conducted. It was compared which aspects of this process are influenced by grinding aid and which are determined by machine design. The main conclusions of the work are as follows:

a.

The classifier corrected cut-size (when no bypass of fines to the course outlet occurs) is not affected by the use of grinding aids, being a result of wheel speed and air volume flow rate;

b.

Grinding aids promoted a reduction in bypass of fines to the course outlet for both classifier scales, although with smaller intensity at very small cut-sizes;

c.

In the industrial scale classifier, it was not observed reduction on powder caking on the chamber walls with grinding aids tested. This fact seems to be due to the chamber design and formation of air stream dead zones, but more studies are required.

d.

The fish-hook effect for ultra-fine particles can be reduced by grinding aids, but is also very affected by the machine design.

e.

Grinding aids promoted an increase in separation sharpness for coarser size classes, reducing coarser residue on the fine product.

In the previous year of the project (first year) batch grinding tests and powder flowability measurements of the product were conducted in order to assess grinding aid contribution to the breakage aspect of milling, without powder transport. It was also conducted initial open-circuit milling trials to study the effect of grinding aids on powder transport, mill holdup and process dynamics and stabilization. It was observed that additives promoted a more stable and efficient process. Although it was also observed that excessively high powder flowability can result in process ineffectiveness by reduction of mill residence time and no reduction of product particle size.

In this project, dry grinding of the materials alpha alumina and calcium carbonate is studied. Three substance classes were adopted as grinding aids: An Alcohol, a Carboxylic acid and a Glycol. For the experiments, a 47-liter batch-wise or continuously operated ball mill as well as a reflector-wheel air classifier are selected.

• Amount of material coated on equipment surfaces.
• Total mass of product inside the mill and residence time
• Powder flowability;
• Tendency of fine particle agglomeration;
• Product fineness after grinding;

This project aims in developing a system engineering approach for understanding, optimizing and scaling industrial dry grinding processes, with a special focus on the manipulation of the material properties and, thus, the grinding and classification efficiency by grinding aids. Grinding aids are defined here as liquid or dry substances that are added to the process in order to increase the product throughput, decrease the specific energy consumption and/or to reach a certain product fineness. During milling operations, grinding aids impact powder material mainly in:

Abstract

In this report, we quantitatively assess the effectiveness of discrete element method (DEM) calibration methods utilised by 8 industrial DEM practitioners for a number of differing experimental geometries, particulate media, and combinations thereof. The accuracy of the methods is assessed by comparing the outputs of simulations performed following the procedures of the 8 participants with detailed experimental data produced using Positron Emission Particle Tracking (PEPT), a technique which allows the dynamics of particulate systems to be imaged, in three dimensions, with sub-millimetre spatial resolution and sub-millisecond temporal resolution. Strikingly, of all the participants surveyed, no two institutions adopted the same practices, highlighting the need for a more standardised approach and best practice. Our results show that while most contemporary calibration methods are able to successfully capture the dynamics of simple, free-flowing, spherical particles under low-shear conditions, the vast majority of procedures tested were unable to correctly reproduce the behaviours of smaller, more cohesive particles, or higher-shear environments. For the latter case, though qualitative agreement and visual similarity between simulated and experimental systems could be observed, deeper and more quantitative analysis using PEPT revealed significant disparities. A number of methodologies were able to successfully capture the dynamics of aspherical, highly-angular particles, but no advantage was observed in the implementation of complex and computationally-intensive geometric models over the simpler and more efficient rolling-friction method for the materials and systems explored. Of the calibration methods examined, the most effective – indeed the only one to consistently reproduce the experimentally-measured dynamics of the cohesive systems tested – involved the combination of both static and dynamic powder characterisation tests, suggesting this to be the best practice for multi-parameter DEM calibration.

Executive Summary

Project ARR-98 focused on mixing rules for powder mixing in rotating drum flows. The first phase of the project explored the viability of PEPT measurements to yield the desired measurements for validating and complimenting a granular flow model of rotating drum flows. In this regard, coarse graining strategies proved integral to the success of the project. A useful outcome was the observation of the reverse Brazil nut effect observed for a binary mixture (by size).

Phase two built upon the serendipitous finding of a Peclet-based rheology that underpinned most of the granular flow phase space. ~200 DEM simulations across a wide range of flow configurations was used to successfully validate the new rheology.

Paper II related to year 3 deliverables is under correction and will be submitted soon.

Year 3 Deliverables

• (2) Empirical models able to predict reconstitution times from powder physicochemical characteristics.
• (1) Fitting of reconstitution kinetics followed by granulometry;

The third year of the PhD work was dedicated to modelling. On one hand, a new approach of descriptive modelling of food powders reconstitution kinetics followed by granulometry was investigated. The developed model allows to describe the different reconstitution steps by successive first-order indicial responses, thus allowing to calculate characteristic times and rates of each step. On the other hand, it was tried to develop a predictive model for reconstitution times in reconstitution conditions employed in year 1 based on physicochemical properties of powder characterized in year 1. The deliverables were achieved in November 2021:

This part, focused on AFM, will be continued during the 4th year of IFPRI project (Feb. 22 – Feb. 23) by a post doc or a technician working on the project.

Year 2 Deliverables

• (2) Effect of surface modifiers (quantities to cover the surface, distribution at the particle surface, minimal quantity necessary to improve wetting, etc.).
• (1) Surface chemical mapping and nanoindentation to establish correlations with powder wettability;

The second year of the PhD work was focused on a powder presenting a low wettability (i.e. whey protein powder), which was coated with sugars to improve its wetting behavior. In agreement with IFPRI partners, five sugars (i.e. sucrose, lactose, glucose, fructose, and galactose) were chosen for their wide range of physicochemical properties: solubility, chain length, structure, glass transition temperature, hydrophilicity, etc. Links between powder wetting and sugar nature, surface modification, quantity, and coating depth were thoroughly investigated. The deliverables were achieved at the end of July 2021:

Paper I has been published in Powder Technology and is related to year 1 deliverables. https://doi.org/10.1016/j.powtec.2021.01.056.

Year 1 Deliverables

• (2) First statistical correlations between the various powder characteristics and their wettability and reconstitutability.
• (1) Powder classification according to their reconstitution behavior;

The first year of the PhD work dealt with the systematic physicochemical analysis of powders and their classification according to their reconstitutability. It was achieved at the end of January 2020 with the following deliverables:

(PhD duration: 1st February 2019 – 31th January 2022)

Executive summary until October 2021 – year 3