Recent advances in Agglomeration during spray drying

By Ejnar Refstrup, Niro A/S, Copenhagen, Denmark

Vibrating fluid bed for agglomeration of food powders.

The quality requirements to Instant Food Powders in general and Instant Milk Powders in particular are constantly getting stricter as an increasing number of properties must be optimized simultaneously - to the extent that this is possible - and controlled within still narrower limits in order to obtain a high quality product with the highest degree of uniformity, fulfilling the requirements set by customers, organizations and industrial and/or legislative standards.

This situation is very demanding for powder production people, powder technologists and others responsible for product quality, and a thorough knowledge of powder technology and an understanding of all processes or unit operations taking place simultaneously in a spray dryer are required.

The present paper describes qualitatively some processes and unit operations and their interaction in different drying processes and the importance of certain components on agglomerate structure and thereby on the final product quality.

Agglomeration during spray drying

Agglomeration is often defined simply as the association of smaller particles into clusters or agglomerates.

Generally the process of agglomeration involves:

Assuming that agglomeration occurs as a series of binary interactions the rate of agglomeration of i and j-type particles can be expressed by:

K_ij is primarily determined by certain Fines Return design parameters, and E_ij is to a large extent determined by the chemical composition of the particles and in particular their moisture content which affect the adhesiveness of the particles, n_i and n_j represent f. inst returned fines particles and primary spray particles, respectively.

The concentration of primary spray particles at a given location in the vicinity of the atomizing device is determined by the flow of feed to the atomizer (plant capacity) and atomization efficiency (spray particles size distribution).

The concentration of returned fines particles is somewhat more complicated to estimate as it depends on a number of secondary unit operations, shown in figure 1, which is schematic presentation of the primary drying processes, here defined as the different subprocesses comprising the total spray drying process, the secondary unit operations as defined below, the state of the product (agglomerates or fines, where fines are often arbitrarily defined as particles < 125 μm), and the fines return system.

Figure 1 Unit operations during spray drying / agglomeration.
Primary drying process	Secondary unit operation	State of powder	Fines Return System
Atomization Primary drying   Secondary drying Cooling	Agglomeration Separation Attrition Classification

The secondary unit operations are defined as follows:

Agglomerate structure and powder properties

Agglomerate structure: Onion Raspberry Compact grape Loose grape Particle moisture content at collision: High Low Mechanical stability: High Low Bulk density (no attrition): High Low Bulk density (after attrition): High Low High Slowly dispersible particles: Many Few Dispersibility (after attrition): Poor Good Poor

Figure 2 Agglomerate structure / powder properties relationship.

Depending on the design and adjustment of the Fines Return system - particularly the location of the introduction of the fines in relation to the atomization device - different agglomerate structures result, which influences certain powder properties (Pisecky and Hansen, 1990), such as Bulk Density, Mechanical Stability, Dispersiblity and Slowly Dispersible Particles.

The relation between agglomerate structure and certain powder properties is illustrated in Fig. 2.

If the fines are introduced close to the atomizing device the moisture content of the primary spray particles is high and thereby their plasticity and stickiness, and the fines particles may penetrate primary particles or be completely covered by concentrate (Fig. 3.). Such agglomerates have been termed 'onion'-structured. When collision takes place at a progressively longer distance, away from the atomizing device, less compact agglomerate structures are obtained. Such structures have been termed 'raspberry'-and 'grape'-structures in decreasing order of compactness.

Figure 3 Scanning electron micrograph of 'onion'-structured agglomerate.

Figure 4 Scanning electron micrograph of 'grape'-structured agglomerate.

Onion'-structured agglomerates are characterized by a high Mechanical Stability and a high Bulk Density, but they will often appear as Slowly Dispersible Particles after reconstitution. They may also be collected during the different Dispersiblity tests in use and jeopardize the general quality evaluation of the product.

With progressively looser agglomerate structures the Bulk Density and Mechanical Stability decrease gradually and the overall instant properties improve. However, if a 'lose grape'-structure is eventually obtained, the Mechanical Stability may be so low that the powder becomes very susceptible to attrition resulting in deteriorated instant properties. A 'compact grape'-structure (Fig.4) is regarded as the ideal compromise where the powder has simultaneously good instant properties and sufficient mechanical strength to enable necessary transport and packing.

New flexible fines return systems

Agglomeration processes in conventional spray dryers or agglomeration as separate, consecutive process have been reviewed elsewhere (Jensen, 1975). Following the development of the new generation of spray dryers (the Multi-Stage Dryer and the COMPACT DRYER™, (Pisecky, 1983)) the need for improved and more flexible agglomeration systems appeared, resulting in the development of two quite different systems for the two types of dryers.

A. Spray dryer chamber 1. Feed 2. Drying air 3. Fines return 4. VIBRO-FLUIDIZER® 5. Ambient air 6. Dehumidifying air 7. Final product outlet 8. Exhaust air Figure 5 Multi-Stage Dryer

Multi-stage dryer (fig. 5)

Due to the special air flow pattern a considerable spontaneous, secondary agglomeration takes place in this type of dryer. For production of high quality Instant Whole or Skim Milk Powder this spontaneous agglomeration suffices and the fines are just returned to the integrated fluid bed. However, the agglomeration may be further enhanced by forced, primary agglomeration (collision of sprays from different nozzles in a multi-nozzle atomization unit) and/or by returning the fines to the atomization zone (forced, secondary agglomeration). Further flexibility can be gained by designing the atomization unit in a way that allows the distance between the single nozzles or between the nozzles and the fines return tube to be altered (Fig. 6).

Figure 6 Fines Return to Multi-Stage or Tall-Form Dryer.

A similar atomization unit and Fines Return system can also be used in Tall-From Dryers, which are often chosen for manufacture of baby food and protein powders.

COMPACT DRYER™ (Fig. 7)

For this type of dryer, operating with rotary atomization, a new, flexible Fines Return apparatus called FRAD (Fines Return Air Disperser) (Fig. 8) was developed and a patent applied for (Pisecky and Hansen, 1990). In this apparatus the fines stream is split up in usually four streams, each passing through its own FRAD tube on which ends are mounted turntable powder deflectors. By adjusting these deflectors the point of collision between fines and primary particles can be controlled within certain limits and thus the powder properties varied.

Figure 7 COMPACT DRYER™

A. Spray dryer chamber 1. Feed 2. Drying air 3. Fines return 4. VIBRO-FLUIDIZER® 5. Ambient air 6. Dehumidifying air 7. Final product outlet 8. Exhaust air

Figure 8 Fines Return Air Dispenser (FRAD)

The FRAD can also in most cases be adopted to conventional dryers, where it can replace the traditional Fines Return tube in the centre of the chamber. Even though this old Fines Return system has worked quite well over the years, it has in many cases been a nuisance from an operational point of view and has often been a source of lumps because of the accumulation of powder deposits on it.

Obtaining desired product properties

Today's strict quality requirements to tailor-made products where an increasing number of properties must be met simultaneously, are very demanding. Although theoretical studies of agglomeration processes may provide inspiration and a better understanding of how to influence and improve powder properties, agglomeration theory is still in its infancy and cannot yet be applied in process control.

Consequently experience and industrial scale experimentation - particularly when new products and/or drying equipment are being implemented - are still essential in achieving and controlling desired powder properties.

References:

Hogg, R. Agglomeration models for process design and control, Powder Technology, 69 (1992) 69-72.

Jensen, J. Due. Some Recent Advances in Agglomerating. Instantizing, and Spray Drying. Food Technology, June 1975, 60-71.

Pisecky, J. and S. P. Hansen. Process and spray drying apparatus for producing stable agglomerates. Eur. Patent Appl. 0 378 498, 1990.

Translation of reprint No. S-351 from "Zeitschrift Für Lebensmitteltechnologie" - ZFL, October 1992