Measuring and Understanding Powder Behaviour

© Freeman Technology Ltd, 2018

Measuring and Understanding Powder Behaviour

Rajeev DattaniApplications Specialist

Analytica 2018


• Powders are used extensively throughout a range of industries.

• Thousands of formulations, hundreds of processes.

• Powder characteristics will influence manufacturing performance and the properties of final product.

• Powder behaviour is still poorly understood which results in stoppages, downtime, out of spec product or scrapped material.

• Expensive and inefficient use of resources.

• Comprehensive understanding of powder properties is essential for efficient processing.

• What parameters describe flowability in a process?

Introduction


Particles are complex, and variable

Each particle defined by a set of physical and chemical properties

• Elasticity

• Plasticity

• Porosity

• Potential for electrostatic charge

• Hygroscopicity

• Hardness / Friability

• Amorphous content

• Particle Size & Distribution

• Shape

• Surface Texture

• Surface Area

• Density

• Cohesion

• Adhesion

Each will contribute to how the powder behaves!

The Nature of Powders


Powders are bulk materials, made from: -

• Solids (the particles)

• Liquid (water on the surface of the particle, in the particle or in the air between particles)

• Gas (normally air, between particles)

Bulk powder “behaviour” is complex and will depend on how these three phases interact

Multiple Phases


Powder behaviour = fn (size) + fn (shape) + fn (stiffness)

+ fn (porosity) + fn (surface texture) + fn (density)

+ fn (cohesion) + fn (adhesion) + …….

… but also the environmental conditions the powder is exposed to: -

• Consolidation• Aeration• Humidity level• Extent of Shear / Strain• Equipment surface material……

• No mathematical way of predicting behaviour from primary properties.

• If there are 12 variables, each with just 4 permutations, this gives over 16million combinations!

What Influences Powder “Behaviour”?



• Same powder can be fluid like at low stress, or a single solid entity ifconsolidated (or anything in between)

• Particles have same physical properties (size, distribution, shape, etc) but thepowder can behave very differently

The Influence of Packing State (Aeration and Consolidation)


Understanding the Processing Environment


Process Flow for continuous tablet manufacture (wet and dry granulation)


Understanding Powder Properties and Process Conditions

In both batch and continuous process, a detailed understanding of thepowder properties and processes employed is important.

The relationship between material properties and process conditionsdetermines the quality of the finished product.

For example, in a tableting process: -

COMPRESSIONMILLGRANULATEFEEDRAW MATERIAL

COATINGMIX (Mg.St)DRYMIXDISPENSE


Powder Rheology


A powder rheometer measures the resistance that the powder exerts on the blade, as the blade forces its way through the sample.

This resistance is expressed as “Flow Energy”, which is calculated from the direct measurements of Torque and Force.

Dynamic Testing


Non-Cohesive Powder

Cohesive Powder

AIR IN

Aeration


Changes in volume due to: -

• Processing• Direct compression• Roller compaction• Screw feeding

• Transportation• Storage

• Hoppers• Kegs

Compressibility


Powder behaviour during: -

• Aerosolisation / DPI

• Hopper Flow

• Direct compression

• Pneumatic transfer

Low Air Pressure Drop means air can escape easily

Permeability


Shear in consolidated powder occurs duringflow in a hopper or IBC, or during forcefeeding through augers or transfer chutes.

• Shear Cells measure the onset of flow, the transition from static to dynamic.

• Good for understanding behaviour in hoppers.

• Flow largely dependant on mechanical properties like size, distribution, morphology, surface texture, adhesion due to binders.

Shear Cell


Case Study

Wet Granulation & Tablet Production


To investigate the change in material properties of both wet and drygranules as a function of a variation in formulation and process configuration…..

……and to relate these material properties to tablet characteristics

Purpose of this study


Wet Granulation

• Converts fine powders into larger granules. Benefits include: -

• Improved flow

• Reduced segregation

• Better content uniformity

• Improved compression properties

• Reduced dusting

• Granulation via high shear can be a batch or a continuous process

• In both cases, water is introduced whilst the powder is sheared

• Process variables: -

• Amount of water added

• Screw speed (continuous)

• Powder feed rate (cont.)

• Impeller and chopper speed (batch)

• Granulation time (batch)

• Water addition rate (batch)


GEA ConsiGmaTM 1 Continuous High Shear Wet Granulator and Drying

System

(photo courtesy of GEA Pharma Systems)

Process Variables (granulator)

• Water content

• Screw Speed

• Powder Feed Rate

• Barrel Temperature

Process Variables (dryer)

• Time

• Air Velocity

• Air Temperature


Formulation

Two types of formulation were considered in this study: -

1) APAP – 90% API

2) DCP – 90% API

Process Variables

• Water content varied to produce granules from under-granulated to over-

granulated, (determined visually).

• Screw speed varied to investigate the influence on granule properties.

• Dry powder feed rate also varied for some samples. Settings reduced from

25 kg/hr (equivalent to ConsiGma 25) to 20 and 15 kg/hr.


Changes in Bulk Material Flow Properties of Wet Granules as a function of Water Content & Screw Speed


Data for wet granules of DCP formulation showing how granules of similar properties can be manufactured using different process settings


Data showing how the flow properties of granules from each “Condition” change through the process (wet, dry, milled, lubricated)


GEA Modul™ S Tablet Press

Tooling 7mm Round

Pre-Compression

Upper Position2.15mm

Pre-Compression

Lower Position4.82mm

Compression

Upper Position2.29mm

Compression

Lower Position4.29mm

(photo courtesy of GEA Pharma Systems)

Tablet Hardness Tester


Data showing the relationship between granule properties and tablet hardness for each granule stage (wet, dry, milled, lubricated)


• Powders are complex materials.

• The multiple process steps in a continuous tableting line subject raw materials and intermediates to a range of different environments.

• Each process operation provides the opportunity to adjust settings in order to improve process efficiency and / or to alter the properties of the material leaving that stage of the process.

• With sufficient understanding of the relevant material properties and critical process parameters, it is possible to employ a QbD approach to continuous tablet manufacture.

• Powders have many characteristics, so single number characterization, or even a single technique is not going to thoroughly describe powder behaviour in every process – a multivariate analysis is required.

• Each stage of the process, from initial feeding to final compression needs to function efficiently in order that product of the desired properties can be can be manufactured. Problems at any stage have the potential to translate downstream, ultimately affecting tablet properties.

Summary


Drag Force Flow

(In-line Measurement)


Drag Force Flow Measurement System

• Probe (DFF - immersion, RealShear mounted flush with the wall)

• Optical detection principle

• Temperature compensation

• Directional measurement

• Measurement system – Interrogator


Advantages

• Direct, real-time measurement

• In-line operation with no disruption of process

• Small footprint (~3 mm diameter)

• Stainless steel construction - sterilizable and chemically resistant

• Sensitivity from 0.4 Pa to 100MPa

• Can measure viscosity (using known flow parameters)

• Free from electromagnetic interference

• No ignition hazard

• Self-calibrated for temperature variations

• Fast measurement rate (more than 1 kHz) for detailed analysis of dynamic flows

• Turn-key operation when coupled with a Lenterra optical interrogator and software


The principle - Fiber Bragg Grating (FBG)

FBGs are periodic structures of varying refractive index embedded in optical fibers.

• FBG is attached to the cantilever.When the cantilever bends inresponse to shear stress, the FBG isstrained which shifts its opticalspectrum

• By interrogating FBG with a lightsource, this strain can be measuredby tracking the shift in the resonantwavelength

Force

Δλ ~ Force

λ1λ2

Δλ

P


• Mounted on a stationary base, the thin DragForce Flow (DFF) “needle” contains two fiber-optic gauges which are connected to acontroller via a fiber-optic cable.

• When immersed in a liquid, powder or granules,the DFF bends under the force of the flow,measuring the drag force.

• The amount of bending is measured in-line inreal-time.

• Allows the user to determine immediatelywhether or not any adjustments are necessary,eliminating the need to stop the operation foroffline measurement.

Lenterra Drag Force Flow (DFF) Sensor



Case Study

High Shear Wet Granulation


Three mixtures of MCC, anhydrous lactose, sodiumcroscarmellose and different amounts of HPC werewet granulated with 40% wt/wt water in a GEAPharmaConnect™ high shear wet granulator.

For each formulation, changes in Force PulseMagnitude (FPM) during the granulation step, weremonitored in-line using the Lenterra DFF Sensor

Flow


Change in FPM (from DFF Sensor) as a function of time

• Force Pulse Magnitude (FPM) as afunction of time was relatively consistentfor the three formulations until the pointof water addition.

• An increase was observed as the granulesdeveloped and began to increase in size.

• A maximum FPM value occurred soonafter the end of the water addition period.

• This is consistent with conventionalunderstanding that wet granulation endpoint is achieved shortly after the end ofwater addition.

• The peak position suggests that whilst ahigher HPC binder content increases thetime taken to achieve granulation endpoint it also potentially results in strongergranules.


Similarities are observed between the

Basic Flowability Energy (BFE) and the

results from the DFF sensor:

• There is an increase and

subsequent decrease in BFE as a

function of time after the start of

water addition.

• Higher levels of binder result in

higher BFE values, indicating

stronger, denser, larger granules.Change in BFE (from FT4 Powder Rheometer®) as a function of time


Summary

• Highly sensitive, capable of measuring small differences in the powder / granulate properties.

• Suitable for a range of applications / unit operations:

• Mixing

• Granulation

• Conveying

• Provides real-time and continuous monitoring of powder processing operations:

• End-point determination / blend uniformity

• Irregular or problematic flow

• Measures powder / granule properties in conditions relevant to the unit operation.

• Results correlate with off-line measurements.


Thank you for your attention!

Please visit us in Hall A2 Stand 423.

www.freemantech.co.uk

Date post:	03-Oct-2021
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Measuring and Understanding Powder Behaviour

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