Basic steps in a spray drying process

The various stages in a standard spray-drying process have been extensively described in a number of previous studies (Gharsallaoui et al., 2007; Lebrun et al., 2012; Woo & Bhandari, 2013; Costa et al., 2015), but will be briefly summarised in the following section. Figs. 2.22 depicts the basic functional principles and components of a typical spray-drying system for illustrative purposes.

Figure 2.22 Functional principles and components of (a) typical spray-drying system and (b) spray-drying sample feed and dispersion system (Büchi, 2009).

a)

79 2.4.2.1. Atomisation

The first step involves the conversion of the feed material to small aerosolised droplets by the application of centrifugal energy or pressure. In the model system depicted in Fig. 2.22, atomisation is achieved by the application of compressed air to the feed solution, which is then forced through the narrow orifice of the spray nozzle and dispersed inside the spray cylinder in aerosolised form. This increases the available surface area for heat transfer and mass transfer between the drying air and the solvent (Patel et al., 2015). The greater the amount of energy applied in the atomisation process, the finer the resultant particles. An increase in the feed rate or viscosity of the feed material at the same atomisation energy will result in larger particles (Woo & Bhandari, 2013; Costa et al., 2015).

2.4.2.2. Air-droplet contact

Air-droplet contact initiates the drying stage and occurs during atomisation. Concurrent spray-drying refers to a set-up in which the liquid feed is sprayed in the same direction as the hot air current, which is typically at a temperature in the range 150 to 220 °C, causing instantaneous evaporation of moisture and exposure of the dried particles to moderate temperatures within the range 50–80 °C (Gharsallaoui et al., 2007). This is the desired configuration when heat-sensitive compounds are present within the core. Countercurrent spray-drying, which involves the spraying of the liquid feed against the flow of hot air, exposes the dried product to higher temperatures and thus limits its use, although it has been noted that countercurrent configurations consume less energy (Woo & Bhandari, 2013; Patel et al., 2015). Fig. 2.23 illustrates the basic drying process of an individual atomised droplet in a typical spray drying set-up. The heating of an atomised droplet is driven by the temperature gradient between the droplet surface and the drying air. The temperature gradient within the droplet itself is usually negligible due to the small particle sizes and the high rates of heat transfer, and the droplet itself is usually considered uniform in temperature (Chen, 2005; Patel & Chen, 2008). The heating of a droplet can be roughly described by the following equation:

∆𝑇

∆𝑡 = ℎ𝐴(𝑇𝑎𝑖𝑟− 𝑇𝑑𝑟𝑜𝑝𝑙𝑒𝑡) − ∆𝑚

∆𝑡 ∆𝐻𝑒𝑣𝑎𝑝

where ∆T/∆t is the rate of temperature change (K.s-1_{), h is the convective heat transfer coefficient}

(W.m-2_.K-1_{), A is the droplet surface area (m}2_{), (Tair - Tdroplet) is the temperature difference (K), ∆m/∆t}

is the drying rate (kg.s-1_{) and ∆Hevap is the latent heat of evaporation (kJ.kg}-1_{) (Woo & Bhandari,}

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Figure 2.23 Simplified diagram of basic spray drying process (Woo & Bhandari, 2013).

The drying rate is also driven by the difference between the droplet surface vapour concentration and the drying air vapour concentration (Patel et al., 2009). The humidity of the drying air will therefore significantly affect the rate of moisture loss of the droplet. Mathematically, it can be expressed as follows:

𝑑𝑚

𝑑𝑡 = ℎ𝑚𝐴(𝜌𝑑𝑟𝑜𝑝𝑙𝑒𝑡,𝑠𝑢𝑟𝑓𝑎𝑐𝑒− 𝜌𝑎𝑖𝑟)

where ∆m/∆t is the rate of moisture loss (kg.s-1_{), hm is the convective mass transfer coefficient (m.s}-1₎

and (ρdroplet,surface - ρair) is the vapour concentration difference (kg.m-3) (Woo & Bhandari, 2013).

2.4.2.3. Evaporation of moisture

As soon as the aerosolised droplets are exposed to the heated drying air, temperature and partial vapour pressure balances are established between the gas and liquid phases. Heat transfer is then carried out from the drying air towards the product due to a temperature gradient, and water is transferred from the droplet to the surrounding air along a pressure gradient (Ré, 1998). Fig. 2.24 describes the basic mass and heat transfer principles which affect the formation of solid particles in spray-drying.

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Figure 2.24 Convective heat and mass transfer and particle formation in spray-drying (Woo & Bhandari, 2013).

The fundamental theory of drying states that three distinct phases or steps are present (Gharsallaoui et al., 2007): (1) the transfer of heat increases the droplet temperature to a constant level, at which point (2) the evaporation of moisture occurs at constant temperature and partial water vapour pressure. It is generally assumed that the rate of water diffusion from the droplet center to the surface is constant and equal to the rate of evaporation from the surface (Cal & Sollohub, 2010). Once the droplet moisture content (MC) falls below a material-specific critical limit, a dry crust forms at the surface which then limits the rate of diffusion of water to the environment. The drying stage ends (3) when the temperature of the dried particle is equal to that of the air. The duration of these steps may differ based on the operating parameters and the nature of the product. For instance, when the air inlet temperature is high, the formation of the dry crust occurs rapidly due to the high rate of evaporation. In addition, the large surface to volume ratio of the aerosolised particles increases the rate of heat transfer (Patel et al., 2009; Costa et al., 2015).

The dry product is usually separated from the air by means of a cyclone system with an aspirator pump, which improves the product yield by diverting the fine particles into a collection vessel and reduces losses to the environment and clogging of outlet filters (Woo & Bhandari, 2013).