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Spray Dryer Scale-Up: From Laboratory to Production

Rudolf J. Schick and

Kathleen BrownSpray Analysis and Research Services

Spraying Systems Co.Wheaton, IL 60187 USA

As presented at: PITTCON 2004 Annual Conference and Exposition for Laboratory Science, Chicago, IL, March 2004

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Spray Dryer Scale-Up: From Laboratory to Production

Objective

A successful transition from the development stage to the production stage is vital in bringing a product to market. A method for optimizing spray nozzle performance in a large-scale production dryer based on data obtained from an optimized small-scale laboratory spray dryer is crucial to a successful transition.

A spray dryer mixes a heated gas with an atomized liquid stream within a drying chamber to achieve evaporation through a direct contact, adiabatic process. Evaporation rate and droplet size are the most critical parameters for spray dry applications. The evaporation rate and air temperature determine the amount of air required. This in turn determines the sizing of all other components. The particle size requirements affect the evaporation rate and finished product quality (Figure 1).

The process of generating drops is called atomization. Atomization is the most critical step in the spray drying process. The extent of atomization of the solution determines drop size diameter related to the

surface area of the drops. This drop size notation is called the Sauter Mean Diameter (SMD or D32). This quantity is used to calculate the efficiency and mass transfer rates.

Figure 1. Spray Dry Setup

General Requirements

Spray drying is the ideal option for the preparation of fine particles in the pharmaceutical industry. The spray dryer must be able to produce very fine drops by means of a uniquely designed spray nozzle. In this particular study, the spray characteristics of a laboratory-scale system needed to be obtained to determine appropriate nozzles for the increased capacity required for production-scale. For this particular application, a scale up from 21 to 85 ml/min is required. The spray characteristics must remain within 10% of the small-scale spray characteristics at similar operating conditions. This is required to maintain the integrity of the finished product.

Experiment

This paper will develop and validate a method for optimizing spray nozzle performance, and determine the effects of scale-up. The optimization method

consists of experimentally obtaining all spray characteristics of the spray nozzles used in the laboratory spray dryer. The data of particular interest will be related to the spray pattern uniformity, drop size and velocity. Using this data as a benchmark, a series of large-scale spray nozzles will be developed and evaluated based on reproducing this data with higher throughput nozzles.

Two types of nozzles (low and high flow rate) were tested under laboratory conditions.

Drop size data was collected at a constant spray height of 1 inch and in the center of the spray throughout testing.

A TSI Phase Doppler Particle Analyzer (PDPA-2D) was used for all measurements of drop size and velocity. A photo and schematic of the PDPA is shown in Figure 2.

The nozzles were placed inside a mist collection chamber to simulate process conditions.

EQUIPMENT & METHODS

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Spray Dryer Scale-Up: From Laboratory to Production

Figure 2. 2D PDPA Setup

Testing was performed with Acetone, 5% polymer, and 10% polymer. This demonstrates the variation of drop size and velocity due to physical properties of the liquid (i.e. density, viscosity, surface tension).

The D32 Sauter Mean Diameter was used to evaluate drop size data.

Test Nozzles

Two Spraying Systems Co. nozzles and a production scale nozzle were used in this test.

An 1/8JJ-SS+SU1A-SS and 1/8JJ-SS+SU3-SS air atomizing nozzles were evaluated for the low flow condition. These nozzles are internal mix two-fluid atomizers. A photo of this type of nozzle is shown in Figure 3.

Figure 3. Laboratory Scale Nozzle

The production-scale atomizer is a two-fluid internal mix nozzle. It features coaxial air and liquid inlets, wherein the air and liquid mix internally to produce a finely atomized spray. A sample two-fluid production scale nozzle is shown in Figure 4.

Figure 4. Production Scale Nozzle

EQUIPMENT & METHODS

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Spray Dryer Scale-Up: From Laboratory to Production

Effect of Flow Rates

The effects of liquid flow rate on D32 were evaluated at 5, 8, 13, 17 and 21 ml/min for the Spraying Systems Co. nozzles and 34, 51, 68 and 85 ml/min for the production-scale atomizer.

D32 increased with an increase in liquid flow rate at a constant air pressure (Figures 5 – 6).

This trend was observed for both low and high flow rate nozzles throughout the operating range.

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Sauter Mean Diameter Vs. Air Pressure

Acetone

SMD

(µm

)

Air Pressure (psi)

1/8JJ-SS+SU1A-SS (SSCo.) 5 ml/min 8 ml/min 13 ml/min 17 ml/min 31 ml/min

Production Scale Nozzle 34 ml/min 51 ml/min 68 ml/min 85 ml/min

Figure 5.

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30

40

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60

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Average Velocity Vs. Air PressureAcetone

1/8JJ-SS+SU1A-SS (SSCo.) 5 ml/min 8 ml/min 13 ml/min 17 ml/min 31 ml/min

Production Scale Nozzle 34 ml/min 51 ml/min 68 ml/min 85 ml/min

V AVG (

m/s

ec)

Air Pressure (psi)

Figure 6.

Effects of Atomizing Air Pressure

In all tests using Acetone and the polymer variations, an increase in atomizing air pressure reduced the D32 (Figures 7 – 8).

Average Velocity (Vavg) values increased with an increase in air pressure.

These trends were observed at all test conditions and are attributed to the increased break-up caused by the high velocity atomizing air.

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Sauter Mean Diameter Vs. Air PressurePolymer - 5%

SMD

(µm

)

Air Pressure (psi)

1/8JJ-SS+SU1A-SS (SSCo.) 5 ml/min 17 ml/min 21 ml/min

Production Scale Nozzle 51 ml/min 68 ml/min

Figure 7.

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15

20

25

30

35

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Average Velocity Vs. Air PressurePolymer - 5%

V AVG (

m/s

)

Air Pressure (psi)

1/8JJ-SS+SU1A-SS (SSCo.) 5 ml/min 17 ml/min 21 ml/min

Production Scale Nozzle 51 ml/min 68 ml/min

Figure 8.

RESULTS & DISCUSSION

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Spray Dryer Scale-Up: From Laboratory to Production

Conclusions

A method for optimizing spray nozzle performance in a large-scale production dryer based on data obtained from an optimized small-scale laboratory spray dryer was presented in this work. This method includes baseline testing optimization of the small-scale spray dryer, establishing a correlation between the large-scale production nozzle and the small-scale laboratory nozzle. The effects of “normal” operating parameters: liquid flow rate, atomizing air pressure and solution variations on drop size and velocity were evaluated. The following trends were observed and verified with collected data.

The effects of liquid flow rate on D32 and Vavg were evaluated at various flow rates using Acetone, 5% polymer solution and the 10% polymer solution. At a constant atomizing air pressure, an increase in flow rate increased both D32 and Vavg values. This trend was observed for all test conditions and is consistent with the expected trends.

In all tests using Acetone and the polymer variations, an increase in atomizing air pressure reduced the D32 and increased the Vavg values. This trend was observed for all test conditions and is due to the increased break-up caused by the high velocity atomizing air.

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Sauter Mean Diameter Vs. Air PressureAcetone Vs. Polymer - 5%

SMD

(µm

)

Air Pressure (psi)

1/8JJ-SS+SU1A-SS (SSCo.)Acetone: 5 ml/min 17 ml/min 21 ml/minPolymer: 5 ml/min 17 ml/min 21 ml/min

Figure 9.

Figure 9 provides a drop size comparison of Acetone and the 5% polymer solution. This figure shows that there is very little difference in drop size when spraying these two different solutions. An increase of about μm is seen when comparing Acetone to the 10% polymer solution at the 51 ml/min flow rate over most of the atomizing air pressure (Figure 10). At 68 ml/min flow rate this difference drops to only about 2 μm to 3 μm.

In general it can be said that there is an increase in droplet velocity as atomizing air and liquid flow rates are increased.

The extent of these effects and the significance of these factors in a spray dry application is dependent on several other variables related to the spray dryer. Other factors that interact to affect the overall spray dry application are the amount of heat, the amount of air, the direction of air flow, the collection technique, and mixing chamber properties. These variables were not included in this study. More work is needed to explore the effects of these aspects and their relationship with the atomization process within a spray dryer.

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Sauter Mean Diameter Vs. Air PressureAcetone Vs. Polymer - 10%

SMD

(µm

)

Air Pressure (psi)

Production Scale NozzleAcetone: 51 ml/min 68 ml/minPolymer: 51 ml/min 68 ml/min

Figure 10.

RESULTS & DISCUSSION