Gaukel, V.

Kleinhans, A.

Schröder, J.

Schuchmann, H. P.

Stähle, P.

English

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Effervescent atomizationAuthors: Stähle, Philipp; Schröder, Jewe; Kleinhans, Agnes; Schuchmann Heike Petra; Gaukel, VolkerA new atomizing technique for the energy-efficient spray drying of food liquids with high viscosityIDM | Technology/IT26  · 8 2013 | international-dairy.comSpray drying is widely used for the produc-tion of food powders. Prior to the drying itself, the liquid feed has to be atomized into fine spray droplets. The smaller the droplets, the bigger the surface of the liquid and thus the drying can be accomplished in a shorter time [1]. Unfortunately, it is very hard to create spray droplets with a uniform size. As a result, there are always particles which are either ex-posed to unfavorably high temperatures (smaller droplets) or are not completely dried (bigger droplets). The latter may lead to serious stick-ing with consequent problems in the handling of the particles. Additionally, the width of the spray droplet size distribution influences other impor-tant powder properties such as its bulk density, flowability, dusting characteristics and instant properties (wettability, sinkability, dispersibility, solubility). To achieve a high bulk density a broad droplet size distribution is favorable. Small parti-cles tend to dust and to float on the water sur-face when reconstituted by the consumer.The energy consumption of spray dryingConcerning its energy consumption spray dry-ing is a very expensive process. Published data of spray drying plants in the U.K. indicate a consump-tion of 4.88 GJ per ton of evaporated water [2]. Unfortunately, there are not many possibilities to save or regain energy. Regarding a typical milk production line, calculations show that it is not possible to save more than 20 % of the energy used via heat exchanger devices [3]. A heat flow balance highlights the dry matter content of the feed as a parameter of outstanding importance regarding the energy consumption of a spray drying plant [4]. However, the practical implemen-tation is limited as a reduction of the water con-tent is normally linked to a higher feed viscosity and consequently to a worse atomization. In gen-eral, different atomization techniques exist [1]. To meet the economical interests, the best atom-izer should be able to atomize a feed liquid with elevated viscosity into fine spray droplets with an energy input as low as possible.Atomizing techniques and their applicability in saving spray drying energyThe easiest way to distinguish the different at-omizing techniques is to look at the energy used for atomization:•  Kinetic energy transferred via centrifugation ➡ Rotary atomizers•  Energy of pressurized liquid ➡ Pressure nozzles•  Kinetic energy transferred via a gaseous phase ➡ Two fluid or pneumatic nozzlesThe following statements are valid for all types of atomizers: With a higher energy input the spray droplet size decreases as well as the size of the dried particles. Furthermore, with increasing liquid viscosity and surface tension, the spray droplet size increases. In rotary atomizers (Figure 1, left side) the liquid is centrally distributed on a fast rotating disc. Because of the high rotational speed the liquid is accelerated to velocities up to 180 m/s. The liquid spreads into a thin film and is atom-ized at the edge of the disc. One may use guide grooves or increase the friction between the liquid and the rotating disc in order to minimize the slip between the liquid and the geometry. Rotary atomizers are suitable to atomize abra-sive as well as particle loaded fluids and do not tend to become blocked. The average spray droplet size is usually within the range of 20 to 200 microns whereas the spray shows an inter-mediate size distribution [1]. To ensure a satisfy-ing atomization with rotary atomizers the liquid viscosity must be in a low to moderate range. Thus, they are not suitable to save drying en-Figure 1: Schematic drawing of a rotary at-omizer (left) and a pressure nozzle (right).Figure 2: Schematic drawing of a pneu-matic nozzle with internal (left) and ex-ternal (right) mixing.ergy when drying highly viscous liquids.In pressure nozzles (Figure 1, right side) the liquid is pumped under pressurized conditions (2 to 30 MPa) to an exit orifice with a small diam-eter. Here, the pressure energy is transformed into kinetic energy. A thin liquid film is formed which is subsequently atomized into fine spray droplets. Additionally, the liquid can be set into ro-tary motion upstream the exit orifice to enhance the atomization. With a pressure nozzle average spray droplet sizes of 50 to 400 microns and an intermediate size distribution width is achievable [1]. Because of the small diameter of the exit orifice, pressure nozzles are of limited use for the atomization of abrasive and particle loaded liq-uids. In addition, they are prone to clogging. The atomization pressure is directly linked to the liquid viscosity which must consequently be in a low to moderate range. Again, with pressure nozzles it is also hard to save spray drying energy when dry-ing highly viscous liquids.In twin fluid nozzles the liquid is fed under am-bient conditions to the atomizer. Subsequently, it is mixed with a fast moving gaseous phase either inside or outside the nozzle (see Figure 2). The required kinetic energy is applied by the compres-sion and subsequent relaxation of the gaseous phase. If air is used as gaseous phase the atomizer is called pneumatic nozzle. Since there is a huge difference between the velocity of the air and the liquid, large shear and elongational forces act on the liquid`s surface. As a result, the liquid is atomized into fine droplets. The used amount of air for atomization is expressed in relation to the amount of the liquid by mass via the Air-to-Liquid-Mass-Ratio (ALR) as in Equation 1:  The mean spray droplet size decreases with increasing ALR. Compared to pressure nozzles, twin fluid nozzles are equipped with exit orifices of larger diameters. Thus, they show a decreased susceptibility to clogging and are also suitable for the atomization of particle loaded and abrasive liquids. With this technique it is possible to atomize a liquid with high viscosity into spray droplets with an average diameter of 5 to 75 microns. How-ever, the main drawback of this technique is the huge amount of compressed air which is needed for the atomization. Because of the high energy consumption of the air compressors, the atom-izing technique of the twin fluid nozzle becomes uneconomic at a point.25Equation 1FILTECHOctober 22 – 24, 2013Wiesbaden – GermanyThe Filtration Eventwww.Filtech.de“We exhibit at FILTECH for manyyears now and have continuallybeen impressed with the quality ofleads generated. FILTECH is indeeda excellent event. It is well orga ni zedand good crowd to meet. In 2011several people commen ted as visitorshow busy it was. FILTECH providesan excellent arena for us to presentour products and services to keydecision makers, resulting in somenew business opportunities.FILTECH stands out in terms ofquality of visitors as well as theorganisation of the show itself. Wefeel that it is headed to be the singlemost important show in the mem-brane based technology in Europe.We were very pleased to attendFILTECH this year again!”Peter Bolduan, Managing Directoratech innovations gmbh Hall 5 Stand P3Phone: +49 2132 9357 60 · e-mail: info@filtech.dePure Innovation for the DairyIndustry56%InternationalParticipantsProjekt1_Layout 1  16.06.13  17:17  Seite 1An approach to solving this problem is to change the way in which the atomizing air and the liquid are brought in contact e.g. by effer-vescent atomization.Effervescent atomizationFigure 3 shows a schematic drawing of an ef-fervescent nozzle. A targeted mixing of the air into the liquid in a mixing chamber upstream the exit orifice distinguishes this nozzle from con-ventional pneumatic nozzles. A two fluid flow of air bubbles and liquid is formed. At suitable pro-cess parameters the liquid forms a thin sheet at the wall of the exit orifice with air occupying the core. The liquid leaves the exit orifice in this shape and disintegrates into fine spray droplets [5]. The main benefits of that atomizing tech-nique are: Good atomization even at an elevated viscosity, small gas flow rates, low air and liquid pressures and alleviated clogging problems.Figure 4 shows the Sauter mean diameter x1,2 of the spray droplets produced by an effervescent and a conventional external mixing pneumatic nozzle in relation to the ALR [6]. The Sauter mean diameter represents the diameter of a droplet whose ratio of volume to surface area is equal to that of the spray as a whole. With decreasing x1,2 the specific surface area is increased and thus the drying time is reduced.From the data in Figure 4 it is evident that the effervescent atomization technique requires only 30 to 50 % of the gas mass of the con-ventional pneumatic technique to achieve alike Sauter mean diameters [6]. Thus, with an ef-Figure 3: Schematic drawing of an effer-vescent atomizer (according to [5])28  · 8 2013 | international-dairy.comIDM | Technology/ITFigure 4: Impact of ALR on the Sauter mean diameter of the spray droplets atomized by an external mixing pneumatic nozzle and an effervescent atomizer at constant liq-uid flow (liquid viscosity: 25.5 mPas) [6]fervescent atomizer it is possible to decrease the main drawback of the pneumatic nozzles: Even for liquids with elevated viscosities, a good atomization can be achieved at a relatively low energy consumption of the air compressors. In comparison to pressure nozzles, the much lower liquid pressures are also worth mentioning.In addition, if a liquid feed based on an oil-in-water-type emulsion such as milk or other dairy educts is to be spray dried, the effer-vescent atomization technique can save ho-mogenizing energy [7, 8]. Because of the ef-fervescent atomization process a breakup of the dispersed oil droplets can occur (Figure 5). With an increasing energy input (ALR) of the atomization process the resulting oil droplet size decreases. This indicates that the energy used for the atomization process not only re-sults in the breakup of the liquid bulk volume into spray droplets, but can also result in a change of the inner structure of the feed.Our research currently deals with the poten-tial application of the effervescent atomization technique in the spray drying of food based products. The dried products are extensively characterized with respect to important prod-uct properties by consideration of particle size, particle density, bulk density, amount of free fat, oxidation stability and instant properties.Contact: Philipp Stähle, Institute of Process Engineering in Life Sciences, Section I: Food Figure 5: Oil drop size distribution of an oil in water emulsion before (continuous line) and after atomization (discrete data points) by an effervescent atomizer at two dif-ferent ALR [7].Process Engineering, Kaiserstraße 12, 76131 Karlsruhe Germany, Phone.: +4972160848586, E-Mail: philipp.staehle@kit.eduList of references:[1]:  Masters K., Spray-Drying in Practice, Spray-DryConsult International ApS, Charlotten-lund, 2002[2]:  Baker C.G.J., McKenzie, K.A., Energy Con-sumption of Industrial Spray Dryers, Drying Technology, 23 (1-2 SPEC. ISS.) 365-386, 2005[3]:  Atkins M.J., Walmsley M.R.W., Neale J.R., In-tegrating heat recovery from milk powder spray dryer exhausts in the dairy Industry, Applied Thermal Engineering, 31 (13) 2101-2106, 2011[4]:  Kemp I. C., Reducing Dryer Energy Use by Process Integration and Pinch Analysis, Dry-ing Technology, 23 (9-11) 2098-2104, 2005[5]:  Sovani S.D., Sojka P.E., Lefebvre, A.H., Effer-vescent atomization, Progress in Energy an Combustion Science, 27 (4) 483-521, 2001[6]:  Schröder J., Charakteristika der „efferves-cent atomization” rheologisch komplexer und mehrphasiger Flüssigkeiten, Disserta-tion, Dr. Hut, München, 2011[7]:  Schröder J., Werner F., Volker G., Schuchmann H.P., Impact of effervescent atomization on oil drop size distribution of atomized oil-in-water emulsions, Procedia Food Science, 1, 138-144, 2011[8]:  Schröder J., Kleinhans A., Serfert Y., Drusch, S., Schwarz K., Schuchmann H.P., Gaukel,V., Viscosity ratio: A key factor for control of oil drop size distribution in effervescent at-omization of oil-in-water emulsions, Journal of Food Engineering, 111(2) 265-271, 2012

Effervescent atomization: A new atomizing technique for the energy-efficient spray drying of food liquids with high viscosity

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Effervescent atomization: A new atomizing technique for the energy-efficient spray drying of food liquids with high viscosity

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