Drying spray

Spray drying is a technique which is used to produce spherical materials. It can be used as an intermediate process for drying and producing free-flowing powders, or as a real shaping technique producing attrition-resistant spherical particles with a diameter between 10 and 100 pm. 

Advantages and disadvantages of different spray drying atomization systems 

Rotating disc High feed rates Larger diameter dryer required 

Pressure nozzle Large agglomerate capability Downtime due to part wear and 

The process itself is more art than science. There is hardly any information in the open scientific literature. Most publications have to do with spray drying of ceramic powders (ref. 15 and references therein, ref. 16). There are also some standard books about spray drying [17]. Important process parameters are the viscosity of the liquid, the solids content of the suspension, the film-forming characteristics, the type of atomizer, the temperature, the rotation speed of the wheel, gas velocity, etc. 

Spray drying processes can be batch or continuous depending on production needs and the stability of the solutions to be spray dried. Because of reduced product manipulation, microbial and particulate burden can be reduced. Normally there is a solution vessel, a filtration system with prefilters and sterile filters, a pressure vessel to feed the spray dryer at a controlled rate, the spray dryer itself, and bulk containers. 

The air used for product drying should be HEPA filtered. When designed with silicone gaskets, the system will withstand sterilization temperatures. The atomizing device can be either a spray nozzle or a high speed centrifugal device. 

Spray dried products are typically temperature sensitive, therefore, air temperature should be controlled and as low as possible. Design of the atomizing device should ensure that product will not adhere to vessel walls. Surface drying and depyrogenation can be done in a continuous operated tunnel or batch oven. The former method is preferred since it minimizes the potential of particulate contamination during loading. 

Spray drying is a technique with many existing and potential applications in the food industry. In spray drying the incoming feed material is atomised to form a spray (Masters, 1991). Evaporation takes place as the droplets in the spray come into contact with warm air in the dryer. Moisture is lost from the surface and replaced by water migrating from the centre of the droplet. Eventually, a dry shell is formed around the droplet and the loss of moisture slows. The dried particle is then removed from the air stream. 

Spray drying has been used to prepare pharmaceutical products from solid meat by-products such as liver, intestine and stomach. The solid material is first ground and then homogenised in a colloidal mill (Masters, 1991) prior to spray drying. This technology could be adapted to prepare food ingredients from solid offals, trimmings and MSM. 

The spray drying equipment used for the production of dry flavorings is essentially the same as is used for the production of dry milk. We, therefore, And substantial equipment available for the manufacture of spray dry flavorings. Equipment availability has contributed toward making this process the dominant method for the production of dry flavorings. 

Gum Arabic (acacia) is the traditional carrier used in spray drying. It is a good, natural emulsifier and rates well on the other alteria used in evaluating a flavor carrier. Since beverage applications account for a large proportion of dry flavorings used, emulsion stability in the finished product is one of the most important criteria in carrier selection. 

Gum Arabic is a natural exudate from the trunk and branches of leguminous plants of the family Acacia [58]. There are several hundred species of Acacia, however, only a few are gum producers, and these are located in the sub-desert region of Arabic. The main gum-producing countries are Sudan, Senegal, Mali, and Nigeria. Gum Arabic is collected during the dry season, which runs from November to May. The trees are tapped (injured) by villagers, exudation occurs, and then the exudate is hand collected. It is of interest that each tree will produce only about 300 g of gum/year. 

Modified starches — Cost and limited supply of gum Arabic have led to the development of alternative carriers for spray drying. The cheanically modified starches most closely reproduce the functional properties of gum Arabic. Native starches impart 

Flavor carrier choice has been shown to influence volatile retention during spray drying by numerous authors [19,62-66]. This influence can be indirect in the sense that some carrier materials become very viscous at relatively low solids contents. For example, Dronen [67] has shown that a soy protein concentrate infeed is limited to ca. 15% infeed solids or its viscosity prohibits effective atomization. Low solids means poor flavor retention. Dronen [67] found an average retention of a model volatile mixture to be only 13.1% when spray dried in a soy concentrate but 24.7% when dried in a whey protein isolate carrier (30% infeed solids). We assume that the two proteins would act reasonably similarly in terms of retaining volatiles and the difference in retention is primarily due to the low infeed solids of the soy protein infeed. 

Powder particle characteristics can be controlled by controlling the various parameters of the spray-drying process—the droplet size solution 

Nozzle atomizer using mixed-flow conditions. 

This chapter is devoted to the other ceramic powder fabrication processes not easily classified in the solid, liquid, and gas phase sjmthe-sis schemes of Chapters 5, 6, and 7. These methods include spray drying, spray roasting, freeze drying, metalorganic decomposition, sol-gel sjmthesis, and melt and fiux solidification. Each of these techniques is described in various levels of detail. 

Spray drying is an industrial process used very often in the generation of dry powders from liquids or suspensions. A basic description of the preferred techniques can be found in Spray Drying Handbook by Masters [1]. A guide to spray diyer performance evaluation is given 

Chapter 8 Other Ceramic Powder Fabrication Processes 

Heat and evaporate the liquid and heat the vapor to the exit temperature, 

Provide the energy required for the heat of the reaction or crystallization, if applicable, 

Precipitation from a concentrated solution of cations can be performed by solvent evaporation. To ensure that the particle size remains small, the concentrated solution may be atomised at high pressure into fine droplets of 100-500 pm diameter the solvent is rapidly evaporated by an upward stream of hot gas. The particles obtained, which can be as small as 100 nm, are compacted and calcined to produce the ceramic. A schematic representation of the spray-drying process is shown in Fig. 3.4. Several alternative methods are currently under development they are known as aerosol synthesis, aerosol pyrolysis or mist pyrolysis, depending on the specific technique to produce the gaseous suspension of fine particles aerosols are produced in high pressure nozzles and mists are obtained by means of nebulisers. YIG particles (0.25 pm) have been obtained by mist pyrolysis (Matsumoto et ai, 1991) by nebulising an aqueous solution of 

Mn-Zn ferrites have been prepared by calcination at 900 °C of spray-dried particles (Jain, Das Avtar, 1976). The solution was prepared by mixing sulphates. The formation of the Mn-Zn ferrite involved decomposition of the FeS04 to a-FejOj, which reacted with the ZnSQ to form Zn ferrite. MnS04 decomposed last to react with the Zn ferrite leading to the desired Mn-Zn ferrite. 

An alternative method that has been used to prepare ferrite particles in a single step is the evaporative decomposition of the solutions, or spray-roasting (Ruthner, 1977). The solutions were mixed and atomised, and the droplets fell through a reaction chamber at 900-1050 °C. The solvent evaporated and the salts decomposed to oxides. The process took 3-5 s. By using a roasting furnace for industrial production, agglomerates of 40-200 pm containing 1 pm ferrite particles were obtained. The furnace feedstock was an aqueous suspension of the oxides, carbonates or hydroxides of the desired composition. However, the residence time was insufficient and complete transformation to the desired ferrite was not achieved. 

Examples include the manufacture of rapidly dispersible forms of concentrated food products (clusters, agglomerates) and of press feed for various ceramics applications. 

Drug is first dispersed in an aqueous acidic solution of chitosan. The next step is the addition of a suitable crossUnking agent. This solution is then atomized in a stream of hot air. This leads to the formation of small droplets from which solvent evaporates, leading to the formation of free flowing particles [72], 

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Figure C2.11.3. A scanning electron micrograph of tire spherical alumina granules produced by spray drying a ceramic slurry. The granules are comprised of individual alumina particles, sintering additives, and an organic binder.

Spray drying Spray lay-up Spray painting Spray pyrolysis Spray recrystallization Sprays... 

Product from melt or suspension treatment is obtained directly as emmb or powder. Polymer recovered from solution treatment is obtained by precipitative cooling or spray drying. Polymer with now stable end groups may be washed and dried to remove impurities, especially acids or their precursors, prior to finishing operations. 

CHIYODA thoroughbred 121 process forced oxidation lime spray drying... 

Theie aie only a few fat replacement products based on protein. LITA is a com protein—polysaccharide compound the role of the polysaccharide is to stabilize the protein (zein). The final product is 87% protein and 5% polysaccharide. The mixture, spray dried after processing, claims to look like cream on rehydration. It is low in viscosity, flavor, and lubricity, and is stable to mild heating. The protein particle size is 0.3—3 p.m (55). 

Compounded Flavors. Liquid or dry blends of natural or synthetic flavor compounds are called compounded flavors. Most commercial preparations are available as water- and oil-soluble Hquids, spray-dried and plated powders, emulsions, and carbohydrate-, protein-, and fat-based pastes. Compounded flavors are used throughout the food industry in confections, baked goods, snack foods, carbonated beverages, and processed foods (53). 

In California, Spirulina sp. grown in paddle-wheel-agitated open ponds with CO2 is harvested through stainless steel screens, with recycling of the nutrient-rich water to the ponds. The wet Spirulina is spray-dried at 60°C for a few seconds to yield a food-grade product (47). 

The protein precipitate is washed with water, redispersed at pH 7, and then spray dried. Typical commercial soy protein isolates contain greater than 90% cmde protein, dry wt basis. 

A modification of the conventional soy protein isolate process has been investigated on a small pilot-plant scale. It is based on the absorption of water from the aqueous protein after extraction at pH 8.5 using temperature-sensitive polyisopyropylacrylamide gels, followed by spray drying to give a 96% protein isolate (111). 

In Du Pont patents (116) the catalyst is prepared by spray-drying a mixture of colloidal siUca or other carriers and Pt/Pd salts. Aqueous hydrogen peroxide solutions up to 20 wt % ate reported for reaction conditions of 10—17°C and 13.7 MPa (140 kg/cm ) with 60—70% of the hydrogen feed selectively forming hydrogen peroxide. 

Reaction times can be as short as 10 minutes in a continuous flow reactor (1). In a typical batch cycle, the slurry is heated to the reaction temperature and held for up to 24 hours, although hold times can be less than an hour for many processes. After reaction is complete, the material is cooled, either by batch cooling or by pumping the product slurry through a double-pipe heat exchanger. Once the temperature is reduced below approximately 100°C, the slurry can be released through a pressure letdown system to ambient pressure. The product is then recovered by filtration (qv). A series of wash steps may be required to remove any salts that are formed as by-products. The clean filter cake is then dried in a tray or tunnel dryer or reslurried with water and spray dried. 

The vegetable-tanning materials are commercially extracted using hot water. The extraction is normally done in countercurrent extractors that permit the final removal of the extracts with fresh water. The dilute extracts are then evaporated to the desired concentration in multiple effect evaporators. Some extracts may be further dried by spray drying or any other means that proves effective without overheating the extract. Extract preparation depends on the type of extract, the si2e of the operation, and the desired concentration of the final product. 

The spray dried MgCl2 powder is melted ia large reactors and further purified with chlorine and other reactants to remove magnesium oxide, water, bromine [7726-95-6], residual sulfate, and heavy metals (27,28). The molten MgCl2 is then fed to the electrolytic cells which are essentially a modification of the LG. Farben cell. Only a part of the chlorine produced is required for chlorination, leaving up to 1 kg of chlorine per kg of magnesium produced. This by-product chlorine is available for sale. 

Fluidized-bed reactor systems put other unique stresses on the VPO catalyst system. The mixing action inside the reactor creates an environment that is too harsh for the mechanical strength of a vanadium phosphoms oxide catalyst, and thus requires that the catalyst be attrition resistant (121,140,141). To achieve this goal, vanadium phosphoms oxide is usually spray dried with coUoidal siUca [7631-86-9] or polysiUcic acid [1343-98-2]. Vanadium phosphoms oxide catalysts made with coUoidal sUica are reported to have a loss of selectivity, while no loss in selectivity is reported for catalysts spray dried with polysUicic acid (140). 

Spray Drying. Spray-dry encapsulation processes (Fig. 7) consist of spraying an intimate mixture of core and shell material into a heated chamber where rapid desolvation occurs to thereby produce microcapsules (24,25). The first step in such processes is to form a concentrated solution of the carrier or shell material in the solvent from which spray drying is to be done. Any water- or solvent-soluble film-forming shell material can, in principle, be used. Water-soluble polymers such as gum arable, modified starch, and hydrolyzed gelatin are used most often. Solutions of these shell materials at 50 wt % soHds have sufficiently low viscosities that they stiU can be atomized without difficulty. It is not unusual to blend gum arable and modified starch with maltodextrins, sucrose, or sorbitol. 

Fig. 7. Flow diagram of a typical spray-dry encapsulation process.

The second step is to disperse the core material being encapsulated in the solution of shell material. The core material usually is a hydrophobic or water-knmiscible oil, although soHd powders have been encapsulated. A suitable emulsifier is used to aid formation of the dispersion or emulsion. In the case of oil core materials, the oil phase is typically reduced to a drop size of 1—3 p.m. Once a suitable dispersion or emulsion has been prepared, it is sprayed into a heated chamber. The small droplets produced have a high surface area and are rapidly converted by desolvation in the chamber to a fine powder. Residence time in the spray-drying chamber is 30 s or less. Inlet and outlet air temperatures are important process parameters as is relative humidity of the inlet air stream. 

Several parenteral microencapsulated products have been commercialized the cote materials ate polypeptides with hormonal activity. Poly(lactide— glycohde) copolymers ate the sheU materials used. The capsules ate produced by solvent evaporation, polymer-polymer phase separation, or spray-dry encapsulation processes. They release their cote material over a 30 day period in vivo, although not at a constant rate. 

Liquid food ingredients encapsulated are typically oil-soluble flavors, spices (see Flavors and spices), and vitamins (qv). Even food oils and fats are encapsulated (63). These core materials normally are encapsulated with a water-soluble shell material appHed by spray drying from water, but fat shell formulations are used occasionally. Preferred water-soluble shell materials are gum arabic, modified starch, or blends of these polymers with maltodextrins. Vitamins are encapsulated with 2ero bloom strength gelatin by spray drying. 

Dmm-dried products ate more affected by heat than spray-dried products. Drying ia a vacuum chamber decreases the temperature and thus the heat effect on the product, although the atmospheric dryers are used more widely. 

Ultrafiltration. Membranes are used that are capable of selectively passing large molecules (>500 daltons). Pressures of 0.1—1.4 MPa (<200 psi) are exerted over the solution to overcome the osmotic pressure, while providing an adequate dow through the membrane for use. Ultrafiltration (qv) has been particulady successhil for the separation of whey from cheese. It separates protein from lactose and mineral salts, protein being the concentrate. Ultrafiltration is also used to obtain a protein-rich concentrate of skimmed milk from which cheese is made. The whey protein obtained by ultrafiltration is 50—80% protein which can be spray dried. 

Casein is used to fortify flour, bread, and cereals. Casein also is used for glues and microbiological media. Calcium caseinate is made from a pressed casein, by rinsing, treating with calcium hydroxide [1305-62-0], heating, and mixing foUowed by spray drying. A product of 2—4% moisture is obtained. 

Casein hydrolyzates are produced from dried casein. With appropriate heat treatment and the addition of alkaHes and enzymes, digestion proceeds. FoUowing pasteurization, evaporation (qv), and spray drying, a dried product of 2—4% is obtained. Many so-called nondairy products such as coffee cream, topping, and icings utilize caseinates (see Dairy SUBSTITUTES). In addition to fulfilling a nutritional role, the caseinates impart creaminess, firmness, smoothness, and consistency of products. Imitation meats and soups use caseinates as an extender and to improve moistness and smoothness. 

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