Dilute phase pneumatic conveying of whey protein isolate powders: Particle breakage and its effects on bulk properties

Highlights

• Effects of operating condition, bend radius and initial particle size on powder attrition were explored.

• Breakage level could be greatly reduced by decreasing conveying velocity.

• Powder integrity could be greatly improved by selecting particles with a smaller initial size.

• Flowability is not significantly affected by the breakage of powder particles.

• Tapped bulk density increased while wettability decreased due to the greater breakage.

Abstract

Breakage of dairy powder during pneumatic conveying negatively affects the end-customer properties (scoop uniformity and reconstitution). A dilute phase pneumatic conveying system was built to conduct studies into this problem using whey protein isolate powder (WPI) as the test material. Effects of conveying air velocity (V), solid loading rate (S_L), pipe bend radius (D), and initial particle size (d) on the level of attrition were experimentally studied. Four quality characteristics were measured before and after conveying: particle size distribution, tapped bulk density, flowability, and wettability. The damaged WPI agglomerates after conveying give rise to many porous holes exposed to the interstitial air. V is the most important input variable and breakage levels rise rapidly at higher airspeeds. The mean volume diameter D[4,3] decreased by around 20% using the largest airspeed of 30 m/s. Powder breakage is also very sensitive to particle size. There appears to be a threshold size below which breakage is almost negligible. By contrast, S_L and D show lesser influence on powder breakage. Reflecting the changes in particle size due to breakage, tapped bulk density increases whereas wettability decreases as a result of an increase in conveying air velocity. However, breakage does not show a significant effect on powder flowability as powder damage not only decreases particle size but also changes the particle’s surface morphology.

Introduction

In dairy processing, pneumatic conveying is typically selected for transporting dairy powders either from a fluidized bed or directly from the spray dryer to the storage silo or bag filling stages of the manufacturing process. Industrial pneumatic conveying systems can be classified as being either lean phase or dense phase and each, in turn, can be operated with positive pressure (‘pushing’) or vacuum (‘pulling’). In the dairy sector, all systems use stainless steel piping for sanitation and corrosion resistance. Powder particles undergo multiple impacts on the pipe wall (especially at the bends) during pneumatic transportation [1], which will cause breakage of powder after conveying. Attrition of dairy powders changes the size, shape, and structure of the powder [2]. This negatively affects the handling of the product (wettability and flowability) and end-customer properties (scoop uniformity, reconstitution, etc.) [3], [4]. Wettability is a measure of how readily a powder can absorb water. Poor wetting powders tend to float on the surface of still water and sink very slowly into the water, which will directly affect the rehydration of dairy powders which is a very important functional property of dairy powders [5]. Powders that have undergone large amounts of breakage, resulting in a smaller particle size tend to be less easy to wet.

There are several studies that are devoted to (food) powder breakage in pneumatic conveying systems [6], [7], [8]. Many factors are found to play a role in powder attrition. Kalman [9] and Aked [10] confirmed the considerable influence of the number of bends on the breakage level. Kalman [11] also demonstrated the influence of vibration of bends on powder attrition. Konami et al. [12] focused the investigation on changes in the size of granules in the attrition process during repeated pneumatic transport. With the aim of identifying the basic breakage mechanisms, some experiments [13], [14] based on well-defined conditions in simple set-ups were carried out. In addition to experimental methods, some simulation approaches combined Computational Fluid Dynamics (CFD) with Discrete Element Method (DEM) [15], [16], [17], [18], [19] to explore the degradation and dynamics of particles in conveying systems.

In contrast to the considerable effort that has been made to examine the pneumatic conveying of food or pharmaceutical powders, there are few works have been carried out in the specific area of transporting dairy powders. Hanley et al. [20], [21] investigated the effects of conveying conditions and conveying mode (dense or dilute phase) on attrition of infant formula in a lab-scale modular pneumatic conveying rig with a diameter of 25 mm. They found that mode of conveying, conveying air velocity and a number of passes all had a statistically significant effect on bulk density. For mean volume diameter (D [4,3]) and wettability, the mode of conveying was the only significant factor, while none of the factors had a statistically significant influence on particle density. Boiarkina et al. [22] focused research on the conveying of Instant whole milk powder (IWMP) at two industrial plants with different transport systems; a pneumatic system and bucket elevator. They evaluated the importance of breakdown of the final product properties given different conveying methods and suggested that producing powders of the right agglomerate size and bulk density prior to transport can compensate for the inevitable particle breakage.

The dairy industry conveys a huge range of powder through pneumatic lines. In general, fine dairy powders (average diameter smaller than 500 µm) have unique properties due to the complex components, especially the fat content, lactose, and protein content [23]. This makes the conveying behavior of dairy powder considerably different from ordinary food powders which normally have a relatively much bigger size. Whey protein isolate (WPI) is the purest form of whey (protein content ≥ 90%) and is a complete protein. It contains all the essential amino acids that the human body needs to repair muscle after a workout. In this work we conducted experimental research on pneumatic conveying of whey protein isolate (WPI) powders in a laboratory-scale dilute conveying system with positive pressure, and probe the effects of conveying operating conditions (conveying airspeed and solid feeding rate), bend radii, and initial particle size on the breakage level of WPI. To quantify their influences on the change in powder properties, particle shape, and surface analysis is employed and some quality characteristics are measured before and after conveying: particle size distribution, tapped bulk density, wettability, and flowability. This is the first study to comprehensively examine the pneumatic conveying of dairy powder in an experimental unit that relates to industrial practice.