Atomization of fruit juice with fibres as drying aid : nozzle : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Chemical and Process Engineering at Massey University, Palmerston North, New Zealand
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Date
2019
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Massey University
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Abstract
Spray drying of fruit juices is desirable as it produces dry powders which extend the shelf-life, reduce storage and transport costs, and produce a free-flowing powder which makes it easier to blend as an ingredient. Commercially, maltodextrin is added to the juices as a drying aid to increase the efficiency of the spray drying process. In this project, pomace fibres were investigated as an alternative drying aid. The main attraction of pomace fibres as a drying aid is the pomace fibres are originally derived from the fruits itself. This study explores the rheological behaviour of juice-fibre suspensions inside the spraying device, specifically the nozzle, to ensure high efficiency powder production by enabling atomization of the mixtures. This study also sought to determine the type of nozzle and operating conditions for efficient atomization of the juice-fibre suspensions inside the spray dryer.
Flow-fields inside a nozzle consists of shear and extensional flows. Previous studies on the shear rheology of fibre suspensions revealed the addition of fibres creates a non-Newtonian shear-thinning liquid. The studies on extensional rheology of fibre suspensions, however, were absent. It is widely known the atomization of liquids with both shear and extensional resistances, require additional energy for atomization when compared to Newtonian liquids or viscous non-Newtonian liquids of a similar intrinsic viscosity.
In this work, four types of fibres with different aspect ratios were investigated. Some of the
significant and notable methods achieved during the study include 1) the use of capillary viscometer to examine the shear rheology of fibre suspensions at shear rates up to 20 000 s⁻¹, which represents the calculated shear rate experienced during atomization, 2) the building of a portable capillary breakup extensional rheometer to accurately characterize the extensional rheology of the fibre suspensions at high extensional strain rates and 3) the use of flash photography technique to capture the atomization patterns. Important findings from this work include:
• Fibre suspension is a non-Newtonian shear thinning liquid with shear viscosity dependents on the fibre aspect ratio. The shear thinning behaviour continued until the shear rate of 25 000 s⁻¹ and a plateau occurred at 25 000 s⁻¹ shear rate. The plateau is independent of the fibre aspect ratio.
• The fibre suspension exhibited extensional resistance. The extensional rheological
properties of the fibre suspensions were dependent on fibre aspect ratio. When comparing between the shear and extensional rheology of a specific fibre suspension, the transient extensional viscosity, 𝜂⁺𝐸 of the fibre suspension was relatively greater than its corresponding shear viscosity.
• The entrance pressure drop into the nozzle was significantly increased with the addition of fibre. This made the use of a pressure nozzle inefficient. It was advised by personal discussion with industry experts that rotary atomizers usually fail in atomizing extensional liquids, so its application was not explored in this study.
• Spray visualization showed the extensional resistance of the fibre suspensions significantly affected the atomization behaviour and pattern (droplet size distribution) by forming filament structures connecting successive droplets
together. This pattern was absent in Newtonian atomization.
• Successful atomization of fibre suspensions was achieved by using a two-fluid nozzle at high atomizing air velocities and at air-to-liquid ratio above 0.25. At an atomizing air velocity of 150 m/s, the atomization performance is dependent on the fibre aspect ratio, but this effect was reduced at higher atomizing air velocities. At the highest tested atomizing air velocity of 240 m/s, all fibre suspensions yielded a volume-based average droplet size, D(v,50), of 200 – 250 µm.
• Using a two-fluid nozzle at high atomizing air velocities, the droplets sizes of the fibre suspensions insignificantly reduced when the temperature of the inlet fibre suspensions was increased. No change was observed to the atomization performance when an ultrasonic mechanism was added to the two-fluid nozzle as
an attempt to improve the atomization performance.
• A method of predicting the atomization performance of a given two-fluid nozzle from its dimensionless numbers, for actual spray dryer applications, was demonstrated and the results showed that the two-fluid nozzle can be used inside a spray dryer if the air-to-liquid ratio exceeds 0.25.
Overall, the methodology used in this thesis provides a systemic means of investigating the
suitability of an atomization method for the spraying of a non-Newtonian fluid with
extensional rheological properties.
Description
Permission was obtained to re-use the following Figures: 2.3 & 2.4, 2.5, 2.6, 2.9, 2.11 & 2.12, 2.13 & 2.14, 2.15, 2.19 & 2.20, 2.23 and 3.9.
Keywords
Fruit juices, Drying, Spray drying, Plant fibers, Atomization, Nozzles, Non-Newtonian fluids