Monodisperse drying

Patel et al. reviewed US patents to identify recent developments in spray drying of food, flavour and pharmaceutical applications (Patel, Patel, & Chakraborty, 2014). They showed that recent developments in these application fields have been focusing on microencapsulation and other product properties rather than energy saving purposes. An innovation with energy saving potential is the monodisperse-droplet atomiser, a concept which is already mentioned in the 90’s for ink-jet printing (Brenn, Helpiö, & Durst, 1997). Compared to conventional systems, monodisperse-droplet atomisation yields droplets with identical shape and size (Deventer et al., 2013), resulting in powder particles with the same size, density, porosity, nutrient and moisture content. Identical droplets require the same drying time, and therefore no energy is lost due to overheating of smaller droplets. Enabling to control the right drying time for every droplet makes this technology interesting for thermosensitive products like milk. Uniform droplet drying can, furthermore, reduce the drying time with 50% under severely moist drying conditions, which results in either a smaller dryer and/or an equivalent increase in production capacity (Kosmodem’yanskii, Fokin, & Planovskii, 1968).

Monodisperse droplets are generated by gravitational and inertial forces based on the Rayleigh breakup of liquid streams. Additional electrical or mechanical forces in the nozzle speed-up the droplet breakup, and allows droplet size regulation. Most promising results are achieved by nozzles with piezo-electric materials transducers, where the vibrating force influences the droplet breakup (Wu, Patel, Rogers, & Chen, 2007). In recent years several results are published on monodisperse-droplet atomisers in spray drying systems (Deventer et al., 2013; Fu et al., 2011; Liu, Duo Wu, Selomulya, & Chen, 2012; Rogers, Fang, Qi Lin, Selomulya, & Dong Chen, 2012).

Figure 3 shows the difference in particle size rage between conventional and the monodisperse- droplet atomiser, before and after drying. Due to the absence of fines there is no powder in the exhaust air and the latent heat in the exhaust air can be recovered via air dehumidification. The dehumidified exhaust air can be recirculated, saving energy which is further discussed in section 4. The absence of fines also implies an increased product yield, as no powder is lost via the exhaust air through the filters.

Figure 3. Comparison of particle size range between a conventional atomiser system and monodisperse- droplet atomiser. Both show particle size range direct after atomisation, and after drying. Drying enlarges the size distribution after monodisperse droplet generation, but shows to be out of the range of fines (highlighted area in the figure).

Monodisperse-droplet atomisers also have the potential to process fluids with high viscosities and enable to atomise milk concentrates with total solids contents in the range 50 – 60%. As concentration methods like evaporation and membrane distillation are more energy efficient compared to drying, a further reduction of energy consumption can be realised.

Feed flows between 100 and 250 L/h for monodisperse-droplet atomisers are mentioned in literature (Brenn et al., 1997; Deventer et al., 2013; FMP Technology Gmbh, 2011) which fit to the requirements for specialty products (for example infant formulas, flavourings, and microencapsulation’s in the food and pharmaceutical industry). For capacities applied in the production of commodity products, like milk powder, multi-nozzle systems are required. 3.3.2 Air dehumidification

Applying a monodisperse-droplet atomiser results in a minimal number of fines, and fines are no longer a restriction in reusing the exhaust air. The exhaust air from the drying tower has a temperature between 60 – 95°C, but contains too much water vapor to be reused. Consequently, the air has to be dehumidified. For air dehumidification two systems are suitable: 1) a membrane contactor using liquid adsorbents (see next section), and 2) a contact sorption system with adsorbents like zeolite or silica gel. The adsorbents remove water from the dryer exhaust air while releasing heat of condensation/adsorption. The released heat benefits the operation of other units in the system. During dehumidification, the adsorbent is gradually saturated with water and needs to be regenerated. The energy for regeneration increases the energy consumption, nevertheless up to 50% energy recovery can be realised by exploiting surplus/waste heat from the regeneration system (Boxtel, Boon, Deventer, & Bussmann, 2012). The proposed closed-loop drying system is depicted in Figure 4.

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Figure 4. Proposed configuration of a closed-loop one-stage spray dryer with air dehumidification, including cooling or heating and regeneration of the adsorbent. The energy content of the exhausted regeneration medium can be used for heat integration with other processes like membrane distillation. In order to prevent accumulation of gasses in the system some air can be added to and exhausted from the air loop.

Membrane contactor

Membrane contactors are industrially used for degassing, for example the separation of CO2 from gas streams (Li & Chen, 2005). More recently membrane contactors have become of interest for air dehumidification due to the energy efficiency, simple equipment without rotating parts, no direct contact between air and desiccant, and continuously operation mode (Yang, Yuan, Gao, & Guo, 2013). Most research is focused on dehumidification in air conditioning systems at ambient air temperatures, showing energy efficiency improvements up to 60% compared to conventional air conditioning systems (Bergero & Chiari, 2001; Isetti, Nannei, & Magrini, 1997; Jain, Tripathi, & Das, 2011; Kneifel et al., 2006). Despite published results as a proven technology for flue gas treatment (Li & Chen, 2005), air dehumidification with a membrane contactor at air temperatures in the range of 60 – 95°C is still limited.

The principle of the membrane contactor is the same as for membrane distillation; water vapor from the air passes a hydrophobic membrane and is adsorbed by a desiccant solution. The water vapor partial pressure difference between the moist air on one side of the membrane and desiccant solution on the other side is the driving force. Most used desiccants are lithium chloride (LiCl), lithium bromide (LiBr), magnesium chloride (MgCl2), calcium chloride (CaCl2) and triethylene glycol (TEG), or a combination of these (Abdel-Salam, Ge, & Simonson, 2013).

In the membrane contactor latent heat from the vapor in air is converted to sensible heat (raised temperature) in the desiccant solution, which can internally be recovered by a heat exchanger, generating hot water. Regeneration of the desiccant solution can be realised by using steam in an evaporator or hot air in an additional membrane contactor module. Energy recovery from the vapor of the regenerating evaporator is essential for the feasibility of the membrane contactor in the perspective of energy efficiency. Further research should focus on this as no literature is yet available.

Contact sorption system

Contact sorption systems make use of solid adsorbents with a high affinity for water, like zeolite or silica gel (Srivastava & Eames, 1998). The advantage of zeolite compared to silica is the ability for dehumidification and regeneration at elevated temperatures (Boxtel et al., 2012). Silica performs better at lower temperatures. The exhaust air of a spray dryer has temperatures in the range of 60 – 95°C, and therefore zeolite is preferred. Although additional energy is needed for regeneration, an energy reduction for regeneration with hot air of about 30 – 50 % is realised with zeolites (Boxtel et al., 2012; Djaeni, Bartels, Sanders, Straten, & Boxtel, 2007). The exhaust air from the regenerator has a temperature around 150°C, and allows heat application for other processes in the production chain. To increase the water loading capacity, the zeolite has to be cooled after regeneration. The energy obtained from cooling can be used to preheat ambient air that is used as regeneration medium.

Regeneration with superheated steam is an alternative for hot air regeneration. The advantage of this system is that the steam after regeneration has more options to be used at the production site. According to Bussmann et al. (Patent No. US 20060010713 A1, 2006) the energy costs for a dryer system with zeolite and a regeneration system with superheated steam, can be reduced up to 70% compared to a conventional dryer. Van Boxtel et al. (2012) showed that spray drying with air dehumidification by zeolite requires 96 kJ/kg air of which 46 kJ/kg air is recovered as steam, resulting in a 50% reduction compared to a conventional system requiring 92 kJ/kg air. Note that the humidity of the drying air will affect drying behaviour and consequently the product quality. Not all products will benefit from dry-air use in the spray dryer. For these products the dehumidified air can be mixed with a bypass of the non-dehumidified or ambient air.