Erik Börjesson (firstname.lastname@example.org), Food Colloid Group, Department of Food Technology, Engineering and Nutrition, Lund University, Sweden
The formation of lumps during recombination of powders is a common problem during industrial mixing. The formed lumps results in incomplete recombination which affects composition as well as behavior (rheology, stability, functionality) of the final product.
The entire process of powder lump formation during recombination currently is studied in a research project founded by the Swedish research Council at the Department of Food Technology, Lund University, Sweden. The aim of the project is to grasp the entire process and has therefore demanded construction of experimental equipment with which it is possible to study the interaction between solvent and powder at different scales.
The dissolution of powder is studied at:
– The scale of individual powder particles or individual agglomerates (scale of µm).
– The scale of the powder bed, where a portion of powder made up of large amounts of individual particles and agglomerates are considered as one porous medium. This portion, can be situated at the surface of the solvent, but can also have been sucked down below the surface of the solvent as a result of the use of industrial mixing equipment (scale of mm).
If sufficient mixing is applied during a powder recombination process, the solvent-powder interaction quite rapidly starts to occur at the powder bed scale, which also is the scale where the formation of lumps occurs. At this scale, the wetting of the porous powder portion is governed by capillary forces. Only if a complete wetting of the powder portion is achieved, the individual powder particles will disperse which would result in a lump free dissolution.
The wetting of porous media is governed by Darcy’s law:
Where A and L is the cross section area and length of the porous media respectively, ΔP is the pressure drop, µ is the viscosity of the flowing liquid and k is the permeability constant of the porous media. For a powder bed the permeability constant is governed by the porosity (void space), ε, and the specific surface, M [m2/m3], of the bed. There are many models deriving a permeability constant from ε and M, but generally the permeability is considered to increase with increasing porosity of the bed whilst the opposite relation is true for the specific surface.
Both the porosity and specific surface of a powder bed is affected by the average particle size as well as by the width of the size distribution of the powder. Powder consisting of mono-sized particles of a size larger than 100 μm is believed to have constant porosity no matter the particle size. The specific surface area, however, will increase with decreased particle size and decrease with increased particle size, see Table 1.
When the size distribution widens for powder containing only particles larger than 100 μm, then the porosity is thought to decrease since smaller particles will be able to fit in between larger particles. In the same way, the specific surface area is thought to increase. Narrowing of the size distribution would then have the opposite effect, see Table 1.
Table 1. Change in porosity and specific surface area for powder consisting of a collection of mono-sized spheres (>100 μm) and for a powder consisting of a collection of spheres of mixed size (>100 μm), when altering the particle size and size distribution respectively.
|Particle size||Width of Size distribution||Porosity||Specific surface area|
|Size > x||→||↘|
|Size < x||→||↗|
For powders with particles sizes smaller than 100µm, adhesive forces are increasing between the particles with decreasing particle size. This will result in increasing aggregation of particles of smaller size classes which affect both porosity and the specific surface of the bed.
Based on this information, theoretical predictions of the wetting rate of a liquid into powder beds with varying ε and M could be performed. When this was done for skim milk powder (SMP) samples, a quite rapid wetting rate was predicted, see Figure 1. Two powder that have been investigated in this study are called SMP and SMPX and is distinguished by different degrees of agglomeration.
Figure 1. Comparison between experimental and theoretical penetration rates of water on SMPs as a function of penetration depth.
Such a theoretical model could be validated by monitoring the penetration rate of a droplet of liquid into a powder bed. The penetration rate of water into the two SMP samples, however, proved significantly slower than predicted by theory as evident in Figure 1. In reality the water droplets shows a very slow wetting rate of both powders, which after some time ceases completely.
If the same drop penetration test of the skim milk powder samples is remade with ethanol, however, the result is completely different. Ethanol very rapidly wets both samples with flow velocities in line with what is predicted from theory. Therefore there must be phenomena going on at the scale of individual SMP particles when subject to water which does not occur when subject to ethanol.
These phenomena have been investigated in a new type of flow cell, allowing for microscopy studies of the dissolution of individual particles in flowing solvent. The difference in behavior of SMP particles in flowing water and ethanol is illustrated in Figure 2.
Figure 2. The behaviour of individual SMP particles in flowing water (left) and in flowing ethanol (right)
As seen in Figure 2, the SMP particles are unaffected by the flowing ethanol, whilst the water have a significant effect on the individual particles. Obviously from Figure 2, SMP particles start to dissolve when subjected to water. If the entire dissolution process is recorded and the resulting movies are subjected to image analysis, the interaction between SMP particles and water can be determined.
The results from such an analysis indicate that the individual particles swell quite rapidly during the initial wetting, due to water uptake from the bulk. In addition, the particles rapidly start to dissolve which increases the concentration of dissolved powder in the liquid phase. This behavior of individual particles can be considered general for powders consisting of macromolecules which form viscous solutions when dissolved (e.g. sodium caseinate, skim milk powder, whole milk powder). If this behavior of individual particles is transferred to a close packed powder bed during capillary wetting, the result will be a concentration gradient within the void space, which would cause a gelling of the bed if the concentration becomes high enough, see Figure 3.
Figure 3. The concentration gradient rapidly occurring during water penetration of a SMP bed. The concentration increase eventually results in a gelling of the bed and a complete stop of further wetting.
The water is penetrating into the individual particles, causing them to swell. It might also cause a transition of the solid particle to a gel which fill out the voids of the bed and further enhances the clogging of the porous media.
This is why lumps are formed during industrial recombination. An insufficient wetting at the powder bed scale causes a gelling of the powder which prevents dispersion of the individual particles. Instead a gelled lump is formed, possibly with an inner region that still is dry.
For SMP powders most of the wetting problems that occur during industrial recombination are solved with agglomeration of the powders during manufacture as well as by treatment of the particles with lecithin (a surfactant that is found in e.g. egg yolk) which increases the wetting rate. However, some of the powders governed by the same principles (like sodium caseinate) are considered very troublesome to dissolve by the industry.
In order to increase the understanding of lump formation of powder during industrial recombination a simulation tool describing the capillary wetting of porous media and how this is affected by the behavior (dissolution, swelling, water uptake) of individual powder particles is under construction at the Department of Food Technology, Lund University. The aim of the tool is to be able to provide information to the industry that facilitates the optimization of dissolving processes of difficult powders.
Börjesson E, Innings F, Trägårdh C, Bergenståhl B, Paulsson M, The dissolution behavior of individual powder particles, 2013, Dairy Science and Technology, doi:10.1007/s13594-012-0098-x
Hellborg D, Bergenståhl B, Trägårdh C (2012), The influence of powder properties on the imbibation rate. Colloid Surf. B-Biointerfaces:108-115. doi:10.1016/j.colsurfb.2011.12.023