This art icle was downloaded by: [ Tno - Mep] On: 06 August 2013, At : 01: 20 Publisher: Taylor & Francis I nform a Lt d Regist ered in England and Wales Regist ered Num ber: 1072954 Regist ered office: Mort im er House, 37- 41 Mort im er St reet , London W1T 3JH, UK Drying Technology: An International Journal Publicat ion det ails, including inst ruct ions f or aut hors and subscript ion inf ormat ion: ht t p: / / www. t andf online. com/ loi/ ldrt 20 New Atomization Nozzle for Spray Drying Henk van Devent er a a , René Houben b & Robin Koldeweij b TNO , Zeist , The Net herlands b TNO , Eindhoven , The Net herlands Published online: 10 Jun 2013. To cite this article: Henk van Devent er , Ren Houben & Robin Koldeweij (2013) New At omizat ion Nozzle f or Spray Drying, Drying Technology: An Int ernat ional Journal, 31: 8, 891-897, DOI: 10. 1080/ 07373937. 2012. 735734 To link to this article: ht t p: / / dx. doi. org/ 10. 1080/ 07373937. 2012. 735734 PLEASE SCROLL DOWN FOR ARTI CLE Taylor & Francis m akes every effort t o ensure t he accuracy of all t he inform at ion ( t he “ Cont ent ” ) cont ained in t he publicat ions on our plat form . However, Taylor & Francis, our agent s, and our licensors m ake no represent at ions or warrant ies what soever as t o t he accuracy, com plet eness, or suit abilit y for any purpose of t he Cont ent . Any opinions and views expressed in t his publicat ion are t he opinions and views of t he aut hors, and are not t he views of or endorsed by Taylor & Francis. The accuracy of t he Cont ent should not be relied upon and should be independent ly verified wit h prim ary sources of inform at ion. 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Term s & Condit ions of access and use can be found at ht t p: / / www.t andfonline.com / page/ t erm s- and- condit ions Drying Technology, 31: 891–897, 2013 Copyright # 2013 Taylor & Francis Group, LLC ISSN: 0737-3937 print=1532-2300 online DOI: 10.1080/07373937.2012.735734 New Atomization Nozzle for Spray Drying Henk van Deventer,1 René Houben,2 and Robin Koldeweij2 1 TNO, Zeist, The Netherlands TNO, Eindhoven, The Netherlands Downloaded by [Tno - Mep] at 01:20 06 August 2013 2 A new atomization nozzle based on ink jet technology is introduced for spray drying. Application areas are the food and dairy industry, in the first instance, because in these industries the quality demands on the final powders are high with respect to heat load, powder shape, and size distribution. The ink jet nozzles can atomize fluids with a higher viscosity than conventional spray-drying nozzles, leading to energy savings. The produced monodisperse droplets undergo less shear force in the nozzle and all droplets experience the same heat load during drying. The monodisperse droplets lead to monodisperse powder particles, a completely different product compared to conventional atomization nozzles that deliver particles with a wide spread in size and form. The ink jet technology, the state of the development, and the integration in existing spray dryers are elucidated in this article. Advantages with respect to energy savings, emission reduction, and increased possibilities for energy recovery are discussed. Attention will be given to the quality aspects of the resulting powders. Keywords Energy saving; Ink jet nozzle; Monodisperse particles; Spray drying INTRODUCTION Different powder production systems exist; most of them are based on the production of powder from a liquid that contains the ingredients for the dry powder.[1] To produce powder from this fluid or slurry requires atomization of the fluid and a certain drying method to produce the dry powders. One commonly used powder production system is a so-called spray dryer. A schematic representation is visualized in Fig. 1. An atomizer on top of the dryer generates small droplets with a certain size distribution (Fig. 2). By creating a heated airflow past these droplets inside the drying chamber, the solvent, mostly water, evaporates from the droplets and powder particles are formed. Different sizes and shapes are formed and powder particles might stick together, or agglomerate, to larger agglomerates. The powder is subsequently removed from the bottom of the drying tower or is removed from the drying air by means of a cyclone or filter. Correspondence: Henk van Deventer, TNO, P.O. Box 360, Zeist 3700, AJ, The Netherlands; E-mail: henk.vandeventer @tno.nl The commonly used atomizers create a rather broad and uncontrolled size distribution of droplets, resulting in a large size distribution of particles as well. Large and small droplets are created simultaneously and the drying tower must be designed to be able to dry even the largest particles, resulting in overdrying and overheating of the smaller particles. The very small droplets that are formed as well, resulting in so-called ‘‘fines’’ or extremely small powder particles, are very difficult to separate from the airflow. These fines make air cleaning indispensable and obstruct recirculation of drying air and heat recovery because of fouling of the heat exchangers. Also, potentially hazardous situations can occur due to the increased risk of dust explosions. The high pressures and high shear rates in the atomizers can affect sensitive ingredients. A more controlled way of creating the initial droplets is desirable. A solution came from developments in rapid manufacturing, where ink jet nozzles are applied to print molten plastic droplets that, after hardening, are used to build up in layers any arbitrary construction. These printers are able to cope with high viscosities and produce monodisperse droplets. This possibility, the usage of high viscous ink jet technology for spray drying, is patented[2] and developed for applications in the food industry. This ink jet technology can replace the atomization head in spray dryers.[3,4] The principle of the droplet formation is based on Stimulated Rayleigh break-up of the fluid stream.[5,6] The viscous fluid, viscosity up to 500 milliPascal second, mPa.s, is pushed through a well-defined hole with a periodically changing speed due to a vibrating piezoelectric element, with frequencies of about 20,000 Hz. This results in monodisperse droplets emitted in lines. Droplet size and frequency of droplet shake-off depend on the opening size, the fluid pressure, the fluid viscosity, and the piezoelectric frequency. The development started with a single-hole nozzle (see the generated droplets in line in Fig. 3). The print head design is based on the pressure independent set-up as described in WO2009151332: ‘‘Pressure independent droplet generation.’’[7] Several experiments are performed with the single nozzle.[8] 891 892 VAN DEVENTER ET AL. Downloaded by [Tno - Mep] at 01:20 06 August 2013 FIG. 1. Schematic representation of a spray dryer. A typical industrial spray dryer has a drying capacity in the order of tons of solvent evaporation per hour. A single-nozzle print head has a material throughput of 0.15 kg=hour only. Therefore scale-up is necessary to achieve a commercial applicable system. The system is scaled up towards a multi-nozzle system, with 500 holes and one actuator, a piezoelectric element capable of processing up to 100 kg=hour material feed. This makes the system applicable at pilot-scale spray-drying facilities. Figure 4 shows a schematic representation where, next to the pressure independent design, the nozzle orientation is shown; when the holes in the nozzle are positioned under an angle with respect to the nozzle plate itself, a conical spray pattern will result. Figure 5 shows the multi-nozzle print head in action. Figure 6 shows a detail of the holes in the printing head. Several experiments were performed with the multinozzle print head[9] at a pilot spray dryer to investigate FIG. 2. online). Conventional process (swirl flow nozzle) (color figure available FIG. 3. Stroboscopic picture of a monodisperse droplet chain from a single-hole nozzle. the performance of the system. The results can be categorized on the one hand by the effects on the system’s energy performance and, on the other hand, by the effects of the process on the powder properties (see Figs. 7 and 8). Further scaling up is progressing by increasing the number of holes and=or by putting multi-hole nozzles in parallel. FIG. 4. Schematic representation of the multi-nozzle print head (WO2009151332[7]). NEW ATOMIZATION NOZZLE FOR SPRAY DRYING Downloaded by [Tno - Mep] at 01:20 06 August 2013 FIG. 5. online). 893 Multi-nozzle print head droplet pattern (color figure available Energy Efficiency High Dry Matter Concentration The spray nozzles used in up-to-date spray towers, either rotating wheel or high-pressure nozzles, are to their limits with respect to the maximal viscosity, 300–350 mPa.s, which means for milk a dry matter of about 55%. The limited dry matter content means that relatively large amounts of water have to be evaporated during spray drying. With ink jet nozzles, the maximal viscosity is up to 500 mPa.s, therefore the product can be fed with a higher dry matter concentration; consequently, less water needs to be evaporated during the spray-drying phase. For example, for one commercial creamer the dry matter concentration can be increased from 62% to 67%. In this creamer, palm fat is replaced by sunflower oil in order to maintain a fluid at room temperature. For a commercial high-protein product, an increase in dry matter content from 25% to 35% is possible.[8] For product streams FIG. 6. Detail of the holes in the printing head. FIG. 7. Powders from conventional swirl nozzle after spray drying, with size distribution (color figure available online). containing a lower percentage of water, the water has to be subtracted from the feed before spraying. This, however, can be done using multi-stage evaporators with a much higher energy efficiency compared to the spray-drying step itself. When one is able to remove more water during pre-concentration, this results in a more energy-efficient processing. Monodispersity Energy saving can also be achieved due to the fact that monodisperse droplets are generated. The drying process can then be optimized towards this specific droplet size. This is contrary to the current dryers, which are designed to be able to dry even the largest droplets, wasting energy on overheating and overdrying of the other droplets. To examine the energy advantage of the monodispersity, a calculation based on the model of Langrish[10] was performed for lactose[11] comparing a monodisperse and polydisperse droplet distribution. The reaction engineering approach, stipulating threshold energy to start evaporation, was used as a correction factor.[12] The model calculates droplet trajectory, momentum, and heat equations. A Rosin Rammler distribution was chosen to represent the polydisperse spray, which is defined as a cumulative mass distribution in the form.[13] 894 VAN DEVENTER ET AL. d n Fm ðd Þ ¼ 1
e
ðdÞ Where d [m] is a droplet diameter, d and n are empirical constants with n chosen as 3, and d¼ dmM 1 0:693n With dmM the mass median diameter, similar to the monodisperse droplet diameter: Downloaded by [Tno - Mep] at 01:20 06 August 2013 d ¼ 110 lm The polydisperse flow is discretized using droplet size steps of 5 mm. Other parameters chosen can be found in Table 1. Figures 9 and 10 show the difference between drying a monodisperse and polydisperse droplet stream under similar conditions. Monodisperse drying is clearly more efficient due to a better area to volume ratio. In most drying situations the capacity of the drying air is not optimally used. By having a monodisperse distribution, this can easily be optimized. Absence of Fines Although the absence of fines cannot directly be seen as an energy reduction, it results in a more efficient usage of materials. With conventional systems, percentages of material are lost as fines and for the removal of these fines out of the airflow measures have to be taken, from cyclones to complex filter systems. The absence of fines therefore results in a higher yield and less investment and operational costs for the filtration system. A second advantage in the absence of fines lies in the possibility of using a heat exchanger to recover excess heat from the outlet air. In the usual drying system, fines stick to the heat exchanger surface and are condensation nuclei, thus limiting the possibilities in heat exchangers and representing the danger of FIG. 8. Powders from ink jet nozzle (top), after spray drying. Bottom left: Monodisperse size and shape of the particles; bottom right: highdensity, spherical particles (color figure available online). (bio) fouling, especially if recovery of latent heat is applied. With monodisperse atomization, this problem is overcome. Powder Properties Monodispersity As mentioned as an advantage for energy efficiency, the monodispersity of the generated powders can also be seen as an advantage for the powder properties. When the amount of dying energy is more in line with the particle size, there is less overheated material, increasing the overall powders’ performance. Figure 11 shows the increase in monodispersity. The monodispersity of a powder can be TABLE 1 Parameters for drying calculation Parameter Property Value A [m2] L [m] U [m3=s] Ti [K] vmi [g=kg] Ui [m=s] h [0] Td [K] vdi [%] / [m3=hr] Tower area Drying length Air flow Initial air temperature Initial air moisture content Initial droplet velocity Droplet spray angle Initial droplet temperature Droplet solid content Fluid flow 5 15 10 383 (110 C) 10 8 27 313 (40 C) 0.5 1 FIG. 9. online). Dry mass percentage to drying height (color figure available NEW ATOMIZATION NOZZLE FOR SPRAY DRYING 895 FIG. 12. Powder generated by means of the printing process (color figure available online). Downloaded by [Tno - Mep] at 01:20 06 August 2013 FIG. 10. Moisture content drying air (color figure available online). characterized by the relative span, S, of the particle size distribution: S¼ D90
D10 D50 From Fig. 11 we can determine that the relative span of the particle size distribution, 1–2 for a conventional powder, can be reduced to 0.6 and 0.7, respectively, for two different ink jet powders. The D10, D50, and D90 values are respectively 80, 105, and 144, and 103, 150, and 206.[8] Structure Since the droplets are generated in a very controlled manner, the droplets contain no air-entrapments, resulting FIG. 11. Reduced span of an ink-jet-printed powder.[8] in solid particles. Also, the generated powder consists of spherical particles (Figs. 12 and 13), in contrast to the variety in shapes and morphologies that can be seen with conventional powders (Figs. 14 and 15). This round particle shape results in an excellent flow ability of the dried material. One has to keep in mind that, although the absence of air entrapment is an advantage for the powder properties, it might be more difficult to dry a fully dense particle compared to a particle with air entrapment. Therefore longer drying times could be required for massive droplets, depending on their size. Density For a model material (maltodextrin) a 50% density increase is measured; for a real product, a creamer, a density increase of 30% is realized. Mild Processing Compared to high-pressure or swirl-nozzle, the print head handles the materials quite gently with low shear stresses on the slurry, resulting in the ability to process FIG. 13. SEM image of maltodextrin generated by means of the printing process. 896 VAN DEVENTER ET AL. FIG. 14. Powder generated by means of a swirlflow nozzle (color figure available online). Downloaded by [Tno - Mep] at 01:20 06 August 2013 sensitive emulsions. Figure 16 shows a comparison of emulsion droplet sizes before and after ink jet and conventional atomization.[8] Discussion The results show several advantages for the use of ink jet technology in spray-drying applications. Two challenges, however, remain: 1. Maintaining monodispersity: In conventional spray drying, a lot of small droplets are formed, and due to the random nature of the process, collisions between droplets occur, leading to the mean particle size required (with a relatively large span around it). With the ink jet printing process it is possible to generate identical droplets and to keep control over the droplet paths as well, preventing droplet collision, resulting in monodisperse powders. Unfortunately conventional spray towers are not designed for this purpose, requiring a whole new view on tower design. 2. Drying the generated droplets: The ink-jet-based process is a significantly different process compared to the conventional atomizers. To maintain monodispersity, the initial droplet formed should lead to the final FIG. 15. SEM image of maltodextrin DE32 generated by means of a swirlflow nozzle. FIG. 16. Emulsion droplet sizes before and after processing (color figure available online). powder particle, contrary to the conventionally formed agglomerated particle consisting of a large number of smaller initial droplets. Since it takes more time to dry large droplets than small droplets, the use of ink jet technology (although in principle resulting in a much higher production efficiency) requires a drying tower with higher residence time to achieve dry powder. Thus, the printing process shows a lot of advantages; however, to really prove the benefits the spray drying tower needs to be adjusted as well. Therefore, within TNO FIG. 17. Spray tower under construction at TNO Eindhoven (color figure available online). NEW ATOMIZATION NOZZLE FOR SPRAY DRYING Downloaded by [Tno - Mep] at 01:20 06 August 2013 construction has started on a test facility specifically for drying experiments on ink-jet-printed droplets. A 15 m high drying tower will be constructed to further optimize the process (Fig. 17). CONCLUSIONS It can be concluded that the ink-jet-based print head initially developed for rapid manufacturing applications has a much broader range of application areas. In the field of powder processing, the use of the technology allows monodisperse powders to be created. The monodisperse nature of the droplets not only results in increased powder properties, it also enables more exact dosing of drying air, resulting in an increase in energy efficiency. The print head is able to process material feed containing higher concentrations of solids, resulting in an even higher overall system performance. The absence of fines results in a more economic process due to a reduction in the intensity of exhaust air filtration. The created powders reveal a different nature compared to conventional agglomerated powders containing relatively large amounts of enclosed air. The resulting powders are spherical without air enclosures, resulting in high-density powders. The process, on the other hand, handles the materials with care, allowing processing of sensitive emulsions. ACKNOWLEDGMENT Part of the experiments on pilot scale were performed at the pilot spray dryer of Royal Friesland Campina in The Netherlands. 897 REFERENCES 1. Masters, K. Spray Drying Handbook, 4th ed.; John Wiley: New York, 1985. 2. Poortinga, A.T.; Houben, R.J. Method and apparatus for spray drying and powder produced using said method. World Patent WO 2008069639, 2008. 3. Patel, K.C.; Chen, X.D. Production of spherical and uniform-sized particles using a laboratory ink-jet spray dryer. Asia-Pacific Journal of Chemical Engineering 2007, 2, 415–430. 4. Wu, W.D.; Patel, K.C.; Rogers, S.; Chen, X.D. Mono disperse droplet generators as potential atomizers for spray drying technology. Drying Technology 2007, 25, 1907–1916. 5. Rayleigh, F.R.S. On the instability of jets. Proceedings of the London Mathematical Society 1878, 10(4), 4–13. 6. Eggers, J.; Dupont, T.F. Drop formation in a one-dimensional approximation of the Navier–Stokes equation. Journal of Fluid Mechanics 1994, 262, 205–221. 7. De Vreede, F.J.M.; Aulbers, A.P.; Houben, R.J. Pressure independent droplet generation. World Patent WO 2009151332, 2009. 8. Nouws, P.F.J. Printing Powders: Inkjet-Based Droplet Generation to Improve Powder Properties and Enable Processing of Highly Viscous Materials in Spray Dryers, Final design project report; Stan Ackermans Instituut: Eindhoven, The Netherlands, 2006. 9. Ribeiro, E.J.F.P. Printing Powders Scale-Up, Design project report; Stan Ackermans Instituut: Eindhoven, The Netherlands, 2007. 10. Langrish, T.A.G. Degradation of vitamin C in spray dryers and temperature and moisture content profiles in these dryers. Food and Bioprocess Technology 2007, 2, 400–408. 11. Lin, S.X.Q.; Chen, X.D. A model for drying of an aqueous lactose droplet using the reaction engineering approach. Drying Technology 2006, 24, 1329–1334. 12. Chen, X.D.; Lin, S.X.Q. Air drying of milk droplet under constant and time-dependent conditions. AIChE Journal 2005, 51(6), 1790–1799. 13. Crowe, C.T. Multiphase Flow Handbook; CRC Press: Boca Raton, FL, 2006.